NAVTRADEVCEN 1440-1 SOLID STATE IMAGE INTENSIFIERS

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78B04770A002200060010-2
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 107
• Document Creation Date: December 28, 2016
• Document Release Date: March 23, 2005
• Sequence Number: 10
• Publication Date: March 1, 1965
• Content Type: REPORT

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Technical Report: NAVTRADEVCEN 1440-1
Declass Review by NGA
SOLID STATE IMAGE INTENSIFIERS
Phase Ill
Annual Report
A Broad Continuing Program for the Application
of Light and Image Intensification Techniques
to Military Training
Project LIT
March 1965
FORT V~MG?a~ G;l 0 ~] ~~O o N.V
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NAVTRADEVCEN 1440-1
SOLID STATE IMAGE INTENSIFIERS
ABS TRACT
AT
AT
Technical Report NAVTRADEVCEN 1440-1 describes the continuation
of the work performed under contract and reported in technical
report NAVTRADEVCEN 562-1a The objectives of the program were: (1) to
improve the characteristics of the low resolution (40 lines/inch) photo-
conductive-electroluminescent (PC-EL) type image intensifier panel; (2)
to construct a high resolution (1000 lines/inch) PC-EL type image intensifier
(3) to build an experimental model compact optical projection system,
incorporating the image intensifier panel.
During the first year of this program the work was concentrated
toward the first and second objectives. Regarding the first objective,
work proceeded on the following lines: (1) a parameter study of the
PC-EL circuit was made with the aid of a digital computer; (2) a study
schedule of fundamental semiconductor characteristics of the PC material
was set up, for helping to improve the characteristics of the PC material;
(3) CdSe sintered layers have been analyzed and improved; (4) an
extensive program on plastic embedded CdSe PC powder has been carried out
by the during the last half year; (5)
an exp new rL, materials, such as ZnSe, As2Se and In2S3
has started; (6) improved image quality of the image intensifier panel
'.as been obtained, although the reproducibility of the preparation method
!s quite lows
Panels with the best uniformity, least graininess and widest half-
Zone range reproducing capability had maximum standard luminous gains
(measured with a 2870 K tungsten light source) of 20 foot- Lambert per
foot-candle and maximum output brightnesses of 30 foot-Lamberts at 200
Volts and 2000 cps. Response times between 50 and 500 mseconds were
measured.
For the development of the hi h resolution image intensifier panel
(1) a large effort was made by the
to improve the evaporated films, and resulted in the
preparation of EL films with longer life and better reproducibility; (2)
some effort was made for obtaining useful characteristics of evaporated
PC layers; (3) experiments for combining PC and EL films started and gave
some sign of success.
Work is being continued toward improving the speed, spectral
response, gain, and life characteristics of PC layers, developing the
high resolution image intensifier panel, and building a compact optical
projection system with the improved image intensifier panels.
IR prod ctioffi off thlo pub1;lcatfform
in, whole or In part io permitted
for any puzPaGe of the United
Mateo Cava Gat
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FOREWORD
This report describes the first year's work performed under NAVTRADEVCEN
Contract Ito continue
the development of a solid state image intensa ier. is program was
initiated under NAVTRADEVCEN Contrac and reported in Technical
Report NAVTRADEVCEN 562-1 dated November 1963.
The objective of this work is to produce image intensifier panels with
improved characteristics. Materials are being developed to overcome
the existing limitations on speed of response, gain, output brightness,
and life.
Some success has been achieved as a result of the work now in progress.
Improved panels have been produced with better uniformity, less graininess,
increased gray scale reproducing ability, increased luminous gains and
faster response times.
This report is being released prior to completion of the program to make.
the information available to others working in the same field as early
as possible, and thereby prevent duplication of effort.
U. S. Naval Training Device Center
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NAVTRADEVCEN 1440-1
Section
Page
I
INTRODUCTION . . . . . . . . . . . . . . . .
.
1
I
OBJECTIVES . . . . . . . . . . . . . . .
.
3
III
METHOD OF APPROACH .
5
Computer Study of PC-EL Light Intensifiers .
.
5
Study of PC Materials . . . . . . . . . . .
.
8
Techniques of Measurement. . . . . . . . . .
..
9
IV
EXPERIMENTAL. WORK. . . . . . . . . . . .
.
14
Photoconductive Materials. . . . . . . . . .
.
14
Sintered PC Layers . . . . . . . . . . . .
.
14
PlastUc Embedded PCPowders . . . . . . . .
.
23
Evaporated PC Films . . . . . . . . . . .
.
27
Evaporated EL Thin Films . . . . . . . . .
.
28
Image Intensifier Construction Problems. . .
.
34
Light Amplifier Elements . . . . . . . . .
.
34
Low Resolution Image Intensifier . . . . .
.
35
High Resolution Image Intensifier . . . .
.
36
Data of Best Image Intensifier Panels. . .
.
36
V
DISCUSSION, PLANS FOR FOLLOWING YEAR . . . .
.
38
V I
REFERENCES . . . . . . . . . . . . . . . .
.
40
Vii
APPENDIX
A. Mathematical Analysis of PC-EL Light
Amplifiers . . . . . . . . . . . . . . .
.
42
B. Computer Program in FORTRAN Language for
PC-EL Light Amplifier.. . . . . . . .
.
46
C. Basic Equations of PC Cells. . . . . . .
.
47
D. Mathematical Analysis of Bridge Circuit. .
51
E. Illustrations. . . . . . . . . . ... .
.
54
VIII
GLOSSARY . . . . . . . . . . . . . . .
.
88
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fvAVTRADEVCEM 1440-1
LIST OF TABLES
Table
1. Target Characteristics of Image Intensifier
Pa
Panels . . . . . . . . . . . . . . . . . . . 3
2. Target Characteristics of PC Layers for Sandwich
Type Image Intensifiers . . . . . . . . . . . . . .
3. Contrast factor9 at Different Capacitance Ratios
.e sen co = 4 tt 10-5 and C2 = lO-9 F . . . . . . . . 7
C Calculated fmatt values for Transfer Characteristics
of Figure 7 . . . . . . . . . . . . . . . . . . . 7
g. Calculated Data of a S intajed CdSe PC Call (fro. 02-
/IIYA-l=8) taking = 10 cm /volt sec. . . . . o . 1o
6. Data Calculated for Coplanar CdSe Cell with is =
30 cm2/volt sec . . . . . . . . . . . . . . . . . . 20
7. Measured and Calculated Characteristics of an
Acetic Acid t7ashed Sintered CdSe Sandwich Cell
(top +, d = 100 volts) . . . . . . . . . . . . . . 22
0. Spectral Analysis of High Purity CdSe Powders. . . 22
9. Sensitivities of CdSe Plastic Embedded Cells,
Containing f1H4Cl e . . . . . . . . . . . . . . C . 26
to. Sensitivities of CdSe Plastic Embedded Cells,
Containing CdC12 . . . . . . . . . . . . . . . . 0 26
it. Stability of CdSe.Cu.Cl Powder Cell in Sandwich
Construction (Thickness 6 mils). . . . . .. . . . G 26
12. Effect of Aging CdSe Powder Sandwich Cells . . . . 27
13. Optimum Brightness-Voltage Data of Evaporated EL
14. Characteristics of E1. Films Combined with a C7gF2
Film in Different Constructions and Processings
(0 insulating layer, F firing, CG cyanide vmsh). . i1
lg. Constructional and Electrical Characteristics of
Best Sandwich Type Image Intensifiers . . . . , . 37
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MAVTADEVCEOil 1440=1
LOST OF OLLUSTRAT00MS
Figure
0. Omage Ontensifier in Sandwich Construction . 0 0 0 0
2. Basic Circuit of Light Ontensifier . . 0 . . . 0 . 0
3. Source Program in FORTRAN Language for PC-EL Light
page
54
55
Intensifiers s o . . . . . 0 0 0 0 . . . . 0 . . . 56
4. Computed Data as Typed by Computer . . . . . . . 0 . 57
5. Calculated Transfer Characteristics of Light
Ontensifier o. 0 0. 0 0 0 0, o a o o a, a o 0 0 58
6. Calculated Transfer Characteristics and Gains of
Light Ontensifiers having PC Eleuints with 3
Different Sensitivities . 0.. 0. 0 0= o.. a o 59
7. Voltage Dependence on PC and EL Elements of a
Light Intensifier . . . . . . . . . . . . . . . . . 60
8. Transfer Characteristics and Gains of Light Ontten-
sifiers having PC Elements with Square Root, Linear
and Square Current vs. Light Ontensity Dependence. 60
9. Effect of Dark Current of PC Element on Transfer
Characteristics of Light Amplifiers . . . . . . . . 62
00. Effect of Power Factor of EL Element on Transfer
Characteristics of Light Amplifier o. o 0 0 0 0 0 63
11. Transfer Characteristics of Light Amplifiers with
non-ohmic PC Elements . . 0 . . 0 . 0 . . . . . . . 64
02. Gains of Light Amplifiers with non-ohmic PC Elements 65
03. Structural Details and Dimensions of PC Cell
82-71XA=1-8 . . . . . . . . 0 . 0 . . 0 0 0 0
14. Formulas for Calculating Fundamental Characteristics
of PC Cells from Voltage, Current, Light Ontensity
and Time Constant Measurements . . . , 0 . 0 . 0 0 . 67
15. Same Formulas as in Figure 14 with Numerical Values
of PC Cell of Figure 13 Substituted . . 0 . . . . . 68
06. Bridge Circuit o 0 . . . . . . 0 0 . 0 . 0 . 0 . 69
17. Scope Patterns in Bridge Measurements . 0 0 . . . . 70
18. Electrode Structure of a Hall=Sample . . . . . . . 70
19. Circuit for Hall-Effect Measurements . . . . . 0 . 72
20. Current-Voltage Curves of a Sintered 6dSe Cell
(No, 82-71XA=l=8) with Coplanar Electrodes. . . 0 . 73
21. Current-Voltage Curves of Sintered CdSe Cell
(Moo 82-71XA-1-8) in Sandwich Construction o . . . 74
22. Gain vs. Light Ontensity of a Sintered CdSe Cell
(moo 82-71XAm1-8) . . . . . . 0 . . . . . . . 0 0 0
v
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NAVTRADEVCEN 1440-1
LIST OF ILLUSTRATIONS (Continued)
Figure Page
23. M-Factors of a Sintered CdSe Cell (No. B2-71XA-1-8) 76
24. Trap Distribution in a Sintered CdSe Cell (No. 82-
71 XA-1-8) . . . . . . . . . . . . . . . . . . . . . . 77
25. Characteristics of an Acetic Acid Washed Sintered
CdSe Cell . . . . . . . . . . . . . . . . . . . 78
26. Thermally Stimulated Current of a Sintered CdSe
Coplanar Cell. Rate of Temperature Change: 27.6*C/
min. . . . . . . . . . . . . . . . . . . . . . . . . 79
27. Life Test Measurements on EL Films at AC Voltage . 80
28. Life Test Measurements on EL Films at DC Voltage . . . 81
29. Life Test Measurements on EL Films at DC Voltage
with Constant Current . . . . . . . . . . . . . . . . . 82
30. Life Test Measurements on EL Films with Constant
Brightness . . . . . . . . . . 83
31. Calculated and Experimental Transfer Characteristic
of a Light Amplifier Element . . . . . . . . . . . . . 84
32. Calculated and Measured Gains of the Light Amplifier
Element' of Figure 31 . . . . . . . . . . . . . . . . 85
33. Transfer Characteristics of Image Intensifier No. 221 86
34. Intensified Image of a Projected Slide Picture on
Image Intensifier No. 252, V = 200 Volts, f = 2000 cps,
L = 1 ft-c, B = 8 f t-L . . . . . . . . . . . . . . . . 87
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NAVTRADEVCEN 1440-1
AT
AT
Work on photoconductive-electroluminescent (PC-EL) id
state image intensifiers at the
was started in April 1959 with the
suppor S. Naval Trai g Device Center, Port Washington, AT N. Y. The results of the four years of
work on this contract are described in the Final Reportl edited in
March 1963.
The above work was divided into two parts: (1) the construction
of a low resolution (40 lines/inch) Image intensifier and (2) the
construction of a high resolution (1000 lines/inch) image intensifier.
The greatest effort was placed in the development of the low resolution
image intensifier and in that of the evaporated EL film, necessary for
the high resolution panel.
In meeting the first objective, three new types of construction were
developed. PC materials had to be prepared with different character-
istics for these different constructions, and the development of these
PC materials constituted the main part of the effort In this program.
PC layers in three forms were developed: (1) evaporated films, (2)
sintered layers, and (3) plastic embedded powders. The material,
considered first, was CdS, but as the required response time could not
be obtained from CdS, most of the work was concentrated in developing
the necessary characteristics in sintered CdSe.
Among the construction types of the image intensifier, the sandwich
construction (Figure 1) proved to be the best solution because of its
simplicity and inherent highest resolution. Sandwich type image
Intensifiers have been built in sizes up to 6" x 6", with maximum standard
luminous gains (measured with a 2870'K color temperature light source) .
of 100 ft-L per ft-c, with maximum output brightness of 30 ft-L and with
resolution higher than 100 lines per inch.
For the high resolution image intensifier, the most important
achievement was the development of evaporated thin EL films, with
characteristics well suited to the construction of high resolution
sandwich type image intensifiers. A high voltage variety of EL film
gave about 100 ft-L brightness at around 120 Volts and 400 cps, and a
low voltage variety gave the same brightness at around 25 Volts (400 cps).
The film of the high voltage variety was more reproducible and its
preparation method was better suited for making larger area panels
uniformly. However, the efficiency of these films was lower by a factor
of up to ten, and the half life was lower by a factor of ten to one
hundred, than those of the corresponding powder EL layers.
Although the development of the low resolution sandwich type image
intensifier could be considered as an important achievement, there were
some characteristics, such as (1) the picture quality (uniformity and
contrast), (2) the speed of response, (3) the gain (primarily for shorter
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HAV7RADEVCER1 1'440-D
wavelengths), (4) output brightness and (5) Die, which needed improve-
ments.
The purpose of the present contract is to make an accelerated
effort for improving the achieved characteristics. For the low resolution
image intensifier, this necessitates the development of improved PC
materials, and this has been the most important goal of the program.
For the high resolution image intensifier, improvement in the efficiency
and in the life of the evaporated EL film was needed along with more
work toward the construction of the panel. The third requirement was
to build a compact optical projection system which will incorporate
the image intensifier panel. No special effort was planned for this
last requirement in the first year of the program reported here.
?
?
is
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The subject contract is the continuation of NAVTRADEVCEN Contract
AT ](see Reference 1). The work had to be directed principally
o t e same objectives as specified in the previous contract.' with an
accelerated program to develop solid state image intensifier panels with
target characteristics shown in Table 1.
Type of Image-intensifier
Low resolution
panel
High resolution
panel
Resolution
40
1000
TV lines/inch
Max. Brightness
50
100
foot-Lamberts
Light Amplification
100
100
f t-L/f t-c
Persistence at 0.1 ft-c
20
20
milliseconds
illumination
Size
square inches
Table 1 - Target Characteristics of Image Intensifier Panels
As it was pointed out in Reference 1, the low resolution Image
Intensifier has to be suitable for making large area observation screens.
Therefore, It has to be simple to fabricate and should not be costly.
It was obvious that the high resolution panel will not possess these
characteristics; its construction will be considerably more delicate
and more expensive than that of the low resolution panel. Consequently
its use for the observation screen could not be considered.
To meet the objectives of Table 1, the program had to be directed
at the development of large area polycrystalline PC layers with the
characteristics listed in Table 2.
The derivations in Appendix C show that under certain conditions
the theoretical maxi e~owe~ a In of a low resolution' Image Intensifier
incorporating the PC layer with the characteristic values of Table 2 Is
hher than a thousand and that of a high resolution Image Intensifier
Is higher than a hundred, satisfying the requirements of table 1.
The objectives for the evaporated EL film were: better than 1 lm/
watt efficiency and more than one hundred hours half-life time.
3
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For Type of Ima Intensifier
Specific conductivity
at 0.1 ft-c illumination
Dark conductivity
Breakdown field strength
Speed of response at
0.1 ft-c illumination
Layer thickness
Type of PC layer
considered
Low resolution High resolution
10-7 mhos/cm 10-7 mhos/cm
10-8 mhos/cm
105 Volts/cm
20 msec
Sintered or
Plastic Embedded
Table 2 - Target Characteristics of PC Layers for
Sandwich Type Image Intensifiers
at 50 Volts
for low
resolution
and 25 Volts
for high
resolution
at 200 Volts
for low
resolution
and 100 Volts
for high
resolution
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NAVTRADEVCEN 1440-1
SECTION III
METHOD OF APPROACH
COMPUTER STUDY OF PC-EL LIGHT INTENSIFIERS
It was thought that a computer could be a useful tool for a para-
meter study of the image intensifier. By feeding in the characteristics
of the PC and EL layers, the computer could calculate the transfer
characteristics, the luminous.gain, and the voltages on the PC and EL
elements under various driving conditions. This method gave a fast
answer for the proper choice of the components and showed the way for
the improvement of the image intensifier of a given construction.
A mathematical analysis of the ~asic circuit of the image intensifier
(Figure 2) was given by Diemer et al for the special case where three
assumptions were made: (1) negligible dark current of the PC layer, (2)
negligible conductance of the EL layer, and (3) ohmic behavior of the PC
layer (i.e. linear current-voltage dependence). In Appendix A a very
general mathematical analysis is described, without any of the above
restrictions. The derived equations served for the computer program.
The computer program was written in FORTRAN language (see Figure 3),
which is widely used for engineering problems. Appendix B describes this
program. At the beginning an IBM 1620, later an IBM 1401 digital
computer was used.
Data was fed to the computer on four cards. The first contained
the voltage and frequency of the driving power (Vo, f), the second
contained the constants of the PC element (m, n, co, cd), the third the
constants of the EL element (c(, Bo, A, C2, cosf ) and the fourth
contained the capacitance ratio (p). Characteristics such as the
conductance of the PC element (gl), voltage on the PC element (V1),
voltage on the EL element (V2), output brightness (B), and gain X
were calculated for 13 input light intensities and generally all
characteristics for four groups of capacitance ratios.
Having nonlinearities in the circuit elements raises the question
how well the formulas of Appendix A describe the light intensifier,
because these equations implicitly assumed sinusoidal voltages and
currents. The problem is that the nonohmic characteristic of the PC
element brings non sinusoidal voltage to the EL element. The experi-
mental fact that the EL element is not sensitive to distorted voltages,
simplifies the problem and brings the possibility of agreement between
calculated and experimental characteristic despite the nonlinearities.
. Because of the limited storage capability of the IBM 1401 computer,
a separate program was set up for the calculation of the maximum
contrast of the transfer characteristic (inflexion point). This
calculation was made only for the simplified case where (1) ohmic PC
element, (2) negligible dark current and (3) negligible conductance of
the EL element were considered.
Figures 5 to 12 show some characteristic curves drawn from the
5
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WAVTt DEVCEW l 440-l
calculated values for specially chosen driving conditions with given PC
and EL elements. The data given in these drawings are in four lines,
following the scheme:
Driving power: V0 f
PC element: m n co cd
EL element: at So A C2 cosc
Capacitance ratio: p
The meaning of the symbols is explained in Appendix A. The
numerical values chosen are the practical data of average elements of
1 cm2 area.
Figure 5 shows one group of transfer characteristics with.the
capacitance ratio as parameter. One can see the influrence of this
parameter on the minimum output brightness (dark output).
Figure 6 shows the transfer characteristics and the gains (ft-L per
ft-c) in the case of p=5, with the cell sensitivity as parameter.
Figure 7 shows the voltage dependence of the PC and EL elements as
a function of the input light intensity for the circuit with the elements
of Figure 6 when co = 4xl0?5 mhos/ft-c.
Figure 8 shows the transfer characteristics and the gains for light
intensifiers having PC elements with sublinear, linear and superlinear
current vs. light intensity characteristics (n = 0.5, 1 and 2). One sees
how well the gamma decreases when the exponent n is decreased. The
conductivity of the PC cells was taken as equal at 1 ft-c illumination.
Figures 9 and 10 show how much effect the dark current and the power
factor have on the transfer characteristics.
Figures 11 and 12 show the effect of superlinear voltage-current
characteristics (non-ohmic) of the PC element. Exponents of m = 1, 2,
3 and 4 were chosen for these calculations. The sensitivities of the
PC material were taken as equal at V = 100 Volts in each case, and the
dark currents were supposed to be negligible (cd = 0). It Is surprising
at first that the nonohmic PC element does not deteriorate the character-
istics of the light amplifier, on the contrary improves It; makes
possible a wider useful Input light range, and diminishes the contrast
slightly, as the exponent increases, For this reason, plastic embedded
PC layers could be used advantageously, because they have an exponent
m = 4.
Equation (23) of Appendix A gives an idea how the contrast of the
Image (gamma) depends on the different characteristics of the elements.
So of this equation depends only on the capacitance ratio p and the
conductance to capacitance ratio of the PC element. In Table 3 to
values are given for different capacitance ratios with CO tt lO?5
and C2 = lOm F.
6
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5
10
20
0.44
0.60
Table 3 - Contrast Factors o at Different .
Capacitance Ratios, when co = x i0?5
and C2 = 10?9 F
One can see from Table 3 that lower capacitance ratio gives lower
contrast. But one cannot use capacitance ratios much less than p = 5
because the output brightness with dark input becomes disturbing.
The formula shows that p~eax is proportional to the onent n and
to With A = 3.4 x l0 Colts and V? = 200 Volts = 4.124,
quite large factor. This factor could be diminished by using EL elements
with smaller A constants and by applying higher voltages. Both have
practical limitations and a sublinear PC element (in current vs. light
intensity, i.e.: n < 1) would be requi ed for approaching unit gamma.
Table 4 gives the maximum contrasts Q max) for the transfer character-
istic curves of Figure 8, showing the ependence on exponent n.
at
L-f t-c
0.5 1.772
8.88 x
10-4
1.0 3.545
2.98 x
10-2
2.0
7.090
1.73 x
10-1
Table 4 - Calculated max Values for Transfer
Characteristics of Figure 7
One can see from Figure 8 that the lower n not only diminishes the
contrast, but extends the useful. range of light.
The frequency of the driving voltage does not influence the
contrast for this simplified condition; It changes only the value of
the input light intensity, where the maximum is reached.
7
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NAVTRADEVCEN 1440-1
STUDY OF PC MATERIALS
Since the limitations in the characteristics of the image intensifier
panel were caused mostly by those of the PC material, the prime effort of
the program was directed toward improving the characteristics of the PC
materials. An improvement in sensitivity, speed,, slope of current vs.
light intensity characteristic, homogeneity, graininess, stability and
life was necessary for meeting the requirements.
It was realized that the.wanted improvement could be achieved only
by a "break-through" over the present state of the art. Therefore a
systematic study was planned for understanding the conduction mechanism
of the PC effect in the materials selected.
The characteristics which had to be improved are interdependent, so
that a variance of one processing parameter generally changed several
characteristics of the PC material. Therefore, it was not planned to
separate the line of approach for improving individually-the different
characteristics.
The work program was generally the following: a series of experi-
ments was made by changing one parameter in the processing of the PC
material and all the important characteristics of the PC cell were
measured. After a theoretical analysis, according to the derivation in
Appendix C, was completed the results were evaluated and the direction
for the. next step was considered.
Measuring equipment was set up for fast measurements and recordings
of the following characteristics:
(a) Current vs. DC voltage curves, illumination as parameter.
Light source: tungsten filament lamp, with color
temperature of 2870 K.
(b) Same as (a) with AC voltage at different frequencies.
(c) Time constant measurements at given DC voltage,
illumination as parameter (2870 *K color temperature
light ource).
(d) Spectral response curves
(e) Hall-effect measurements at different illuminations.
(f) Recording of thermally stimulated currents.
(g) Bridge for resistance and capacitance measurements.
(h) Recording of field strength stimulated currents.
Both surface and volume conduction was analyzed, PC cells being
made in coplanar (or lateral) and in sandwich construction, according
to Figure 13.
From these measurements and from the geometrical data of the PC
cell more fundamental characteristics of the material were calculated
mostly by the equations of Figure 14, as: resistance, specific
conductivity, capacitance, relaxation time, gain, M-factor, carrier
?
0
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NAVTRADEVCEN 1440-1
density, life time, trap density, -trap distribution,. Mobility etc.
On Figure 15 the same formulas are listed with the numerical values of
the PC cell of Figure 13.
List of symbols used In equations l to 13 of Figures 14 and 15:
RL - resistance of PC cell. at L ft-c Illumination In ohms
L - illumination (light intensity) of PC cell in ft-c.
I - current of PC cell in amperes
V - voltage on PC cell In Volts
L - conductivity of PC material at L ft-c illumination in mhos-cm
()Q - conductance per square of PC layer (for coplanar cells) In
mhos/square
C - capacitance of PC cell in Farads
trel - relaxation time constant of PC cell In seconds
G - gain of PC cell
M - trap-constant of PC cell
nL - density of electrons (cm-3)
? - mobility of electrons in cm2/Volt sec.
fl - number of photons absorbed by PC cell per second
F - number of photons absorbed in unit volume per second
-3
(
-~)
cm
sec
life-time of electrons in seconds
~d - decay time constant of electrons in seconds
ntr - trap density per eV (cm-3/eV)
Etr - trap depth below conduction band in eVolts
TECHNIQUES OF MEASUREMENT
Some details of three special circuits and instruments used for
the measurement of capacitance and resistance, mobilities, and thermally
stimulated currents are described below.
(1) BRIDGE. The purpose of the bridge circuit is to measure the
capacitance and resistance of either a PC or an EL cell. In the case
of the latter the power factor and the efficiency may be calculated
from the measured data.
Figure 16 shows the bridge circuit. The resistance and
capacitance values, given b were chosen so that the
bridge Is the most appropriate for the range of impedances to be measured
in this program.
Considering the unknown cell as a resistance and capacitance in
parallel and assuming R1 and R3 to be pure resistance and having no
ST
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NAVTRADEVCEI 1440-1
capacitive component*, the balance point of.the bridge is given by
(1)
where Zx and Z2 are the impedances of the unknown and the second
branch respectively. From equation (1),
1 _ RI 1
Zx R3 Z2
(2)
Expressing Zx and Z2 in terms of the resistances and susceptances,
equation (2) becomes
R1 F
R + J Cx = R + J WC
3 2
2
Equating real and imaginary parts,
R2
R R
x = R1 3
C2
Cx = R1 R3
The actual values substituted, one has simply:
R = a2 Megohms (106 11 )
Cx = R1 picofarads (10-12 F)
(3)
(4)
(5)
(6)
(7)
The balance point is achieved by adjusting R1 and R2 for nullifying
the voltage drop on the oscilloscope and eliminating the phase difference
set up by the capacitances. Two different cases can occur: (1) if
the components are ohmic (linear voltage-current relation), the scope
pattern at the balance point will be a straight line, as shown in
Figure 17a; (2)? if the components are not linear, the scope pattern
will be as in Figure l7b.
* See Appendix D for the more general case.
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NAVTRADEVCEN 1440-l
(2) HALL-EFFECT MEASUREMENTS. A very important tool in the study
of conduction processes is the Hall effect. It consists of the appearance
of a voltage difference (called Hall-voltage) in a semiconductive (or
conductive) layer due to the effect of a magnetic field, perpendicular
to the plane of the semiconductive layer. From the measurement of the
Hall-voltage, the speed of the carriers (or the mobility, which is the
speed in unit field), and the density of the carriers can be calculated.
Figure 18 shows the electrode structure of a sample, used for
Hall-voltage and conductivity measurements. The general equation
describing the Hall effect is:
(B x v) = Eh (8)
Where B is the magnetic field's vector, v is the speed-vector of the
carriers, th is the Hall-field vector, c is the speed of light and e is
the charge of the carriers. Equation (8) applied to the case of the
sample on Figure 18, gives in practical units
By=dh 108
where Vh is the measured Hall voltage. Expressing the speed of the free
carriers with the mobility it and the electric field (V is the voltage
between electrodes l and 2):
vV (10)
Substituting (10) in (9), one obtains the expression for the mobility
Vh J, 108
? - V d B
(9)
Measuring the primary current (1), the density of free carriers
(n) can be calculated using the equation
l'= en ? (12)
n = IB 10-8
h
(13)
where t is the thickness of the PC layer.
Considering scattering phenomena, a correction factor comes in the
formulas, but this factor is generally nearly unity. Another factor has
to be considered, if I is smaller than 2. Therefore, Hall samples are
made generally with values higher than 2.
Figure 19 shows the circuit used to measure the Hall-voltage (Vh)
with S1 switch in position 6 or 7 (depending on the polarity) and to
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
NAVTRADEVCEN 1440-1
measure conductance in-switch positions 4 (when S3 is in position 2)
or 5 (when S3 is in position 1). The primary current can be measured
with S switch i.n position 1 or 2 (switch S In position 1 or 2 respec-
tively) and primary voltage is measured witF S11 in position 3 and S3
in position 1. The polarity of the compensating voltage can be changed
with switch S2.
Since the measured Hall voltages are generally low in PC layers,
care has to be taken to minimize electrostatic pick-up. Therefore, the
instrument was carefully shielded. And because the sample resistances
were high, measurements were made generally under illumination. Even
so, additional leakage between electrodes had to be carefully eliminated.
The illumination of the sample was made by guiding the light
of a tungsten lamp through a flat microscope slide. The sample was
placed on the far end of this slide, where the glass was etched and
homogeneously scattered the light to the PC sample. In this way the
light source was outside of the magnetic field, enabling the use of
small magnetic pole distances, and the illumination was not changed,
when the sample was moved in and out of the magnetic field. A permanent
magnet was used which gave a field of about 1000 Gauss.
The noise level of the measured Hall voltage being generally
high, because of high noise of the samples and some interference noises,
a recorder was used to make possible the evaluation of Hall voltages
when the signal to noise ratio was smaller than one.
Also, some thought has to be given to the limited value of
these Hall effect measurements. Since the electrodes on the Hall sample
are in coplanar structure, the calculated mobility and the carrier
density (when the proper thickness is considered) relates to the surface
current and can be different from those of the volume current. The
construction of a Hall sample, where the volume current gives rise to
the Hall voltage, was considered, but could not be made successfully.
(3) THERMALLY STIMULATED CURRENTS. Another useful tool for the
study of semiconductors is the measurement of thermally stimulated
currents. When the traps of a PC cell are filled up at low temperature
by illuminating it at a high light intensity and then the PC cell is
heated at a constant rate in dark, to free these trapped carriers, the
current-temperature curve provides data on the density and energy
distribution of the traps. If it is assumed that the Fermi-level Ef is
located at the energy-level of the trapping center Et, when the current
vs. temperature curve has a maximum, Et can be calculated from equations
(12) and
n = Nc exp(-Ef/kT) (14)
Et = kTl n (Nce?/O' )
where Nc is a constant.
(15)
12
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NAVTRADEVCEN 1440-1
According to the process described above, PC cells were cooled
in a Dewar bottle containing liquid nitrogen and the thermally stimulated
current was recorded on an X-Y recorder during the heating period. An
approximate constant heating rate was obtained by the proper heating of
a heating element in the Dewar bottle.
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NAV1RADEVCEN 1440-1
SECTION IV
EXPERIMENTAL WORK
PHOTOCONDUCTIVE MATERIALS
Uniform large area PC layers, required for the image intensifier
panels, can be prepared in different ways, as by (1) sintering a sprayed
or settled powder layer, (2) spraying or settling in plastic embedment
already sensitized PC powder, (3) vacuum evaporation, (4) vapor
deposition from the elements, (5) sputtering, (6) chemical spraying or
(7) chemical deposition. Methods (4), (5) and (6) are applied using
heated substrates, the others, excepting (2), need a subsequent heat
treatment for obtaining sensitive PC layers. Our work in this program
was limited to methods (1) to (3), because they seemed.to offer the most
adequate, though compromised, characteristics. The other methods were
not out of our interest, and the result of work accomplished by others3.4
was followed with the prospect of an eventual application for the con-
struction of image intensifier panels.
Sintered PC Layers
A considerable effort was spent in the program on the development
of improved sintered PC layers for the application to the low resolution
sandwich type image intensifiers. Most of the work was done with
cadmium selenide (CdSe), and some time was devoted to exploratory
experiments with other materials, as zinc selenide (ZnSe), arsenic
sulfide (As2S3), and indium sulfide (in2S3).
Cadmium Selenide
(a) An unfavorable characteristic of the low resolution image
intensifier panels, made with CdSe sintered layers, was its low picture
quality caused by optical graininess of a size of 10 mils diameter or
smaller, and by a very high contrast. It was evident that for both of
these characteristics the PC layer was responsible. Long series of
experiments were carried out to find the cause of the graininess and to
diminish the high contrast.
Regarding the. graininess, it was thought that the sensitivities
of the individual grains are different and longer baking times or higher
temperature baking could improve the homogeneity. Experiments made with
this Idea seemed to give some improvement, but more improvement was
needed. The idea that finer powder, obtained by longer ball milling,
would diminish the graininess, did not result in complete satisfaction.
An other idea was that the possible cause of the inhomogeneity
could be in the nonuniform bonding of the particles to each other or to
the substrate or both. One of our group J Ibrought attention
to the creation of cadmium oxide and cadmium oxych ors a during the
preparation of CdSe powders. Acetic acid was used to wash out these
compounds from the powder. Similarly we tried to wash a sintered CdSe
layer in 5% acetic acid. Surprisingly, the CdSe layer, which was strongly
bonded to the tin-oxide coated Pyrex glass substrate, came off the glass
in one piece, so that the bond among the powder grains was not disturbed.
i
ST
?
14
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HAVTPAOFVCFN 1 o-0-l
Furthermore, a pattern was visible on the substrate, seemingly having
the same distribution as the.inhomogeneous light pattern on the image
intensifier. On the other hand, previous water wash and boiling, which
have been used to diminish the dark current of the sintered layer by
dissolving the superfluous CdC12, did not dissolve the bond between the
CdSe layer and the substrate.
From these experiments, one could arrive at the conclusion
that the binding between the glass and the sintered layer is caused
neither by CdC121 nor by the CdC12-CdSe melt, which supposedly binds the
powder particles together. Therefore, it is thought that during the
prebaking and the sinterin process - in the air bake - a cadmium
oxychloride (xCdO. y CdC12) compound is synthetized, and it is this
cadmium oxychloride which is responsible for the binding of the PC layer
to the glass substrate. This compound is similar to the "Sorel-cement",
which is magnesium oxychlorlde.
It is thought that the cadmium oxychloride does not form a
continuous uniform layer between the glass and the CdSe, and this causes
the grainy structure on the image intensifier. To prove this assumption,
the following experiment was made: The CdSe.CdC12.CuC12 eowder was
washed in acetic acid (5%) after a prebaking at about 500 C for 60
minutes. The washed powder was deposited on a tin-oxide coated glass
substrate and baked at 500 C for 35 minutes. The CdSe layer was not very
strong, but it seemed that an image intensifier can be completed on it.
After this was done, the panel showed a very good homogeneity. But the
next day, before measurements were made, the sandwich layer separated
from the glass.
After these experiments the following tests have been performed
to complete the understanding of the bonding mechanism of sintered
cadmium selenide layers and to prove the assumptions described above.
(i) Pure ZnC12 or CdC12 was baked on both uncoated and tin-
oxide coated silica substrates at 5300C, both in air and nitrogen
atmosphere. In air, on an uncoated silica substrate, three materials
were present: (1) Water soluble unreacted ZnCl2 or CdC12. (2) Water
insoluble, but acetic acid soluble zinc oxychloride (x ZnO.y ZnC12) or
cadmium oxychlorlde (x CdO.yCdCl2). These compounds were not
stoichiometric. (3) Water insoluble and acetic acid insoluble zinc
or cadmium silicates. These layers were highly insoluble probably
because of excess silica being present; however, in the case of the
zinc compound, the presence of zinc metasilicate (ZnSiO3) was confirmed
by the characteristic orange fluorescence under ultra-violet illumination.
In the case of the tin-oxide coated silica substrate, only the first two
materials were formed.
(ii) Pure CdO or ZnO was baked foe- thirty minutes on both
uncoated and tin-oxide coated silica substrates at 530 C, both in air
and in nitrogen atmosphere. In air or in nitrogen, on uncoated or
tin-oxide coated silica, only superficial, if any attack was noted. A
water wash resulted In an unetched clean substrate in any case.
(ill) A mixture of ZnO plus ZnCI2 or CdO plus CdC12 was baked
15
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NAVTRADEVCEN 1440-1
AT
under the same conditions as (i) and (ii) above with results as in (1)
above. However, similar materials were formed even in the nitrogen
atmosphere. This indicates the necessity for the presence of oxygen
or ZnO or CdO for the reactions to proceed.
(iv) A mixture of CdSe plus either ZnCl2 or CdC12 was baked
at 530~C for 30 minutes on uncoated and on tin?-oxidde coated silica
substrates both in air or in nitrogen atmosphere. When sintered in
air, strong binding was observed. A water wash removed a small'quantity
of CdC12. A dilute acetic acid rinse removed intact the sintered CdSe
layer from the substrate, whether uncoated or tin-oxide coated silica.
In the case of the uncoated silica substrate, a third highly insoluble
layer was noted. When ZnC12 was used as the flux, this third layer
displayed the characteristic orange fluorescence of zinc metasilicate.
When the samples were baked in nitrogen, only Zn or Cd chloride was
present, and no bonding was observed. This confirms the necessity of
oxygen or ZnO or CdO.
Since pure cadmium selenide oxidizes very slightly, if at
all, in thirty minutes of baking at 5300C, one might question how to
explain the formation of quite large quantities of the cadmium oxide
needed for the formation of cadmium oxychioride. It is thought that
this is caused by selenium's high affinity for chlorine as compared to
oxygen. Cadmium or zinc chloride, which has a high vapor pressure at
530 C; supplies the chlorine with which selenium from the'cadmium
selenide reacts, forming volatile selenium chlorides or oxychlorides.
This leaves the elemental cadmium free to oxidize to CdO. The CdO
thus formed then reacts with more of, the ZnC12 or CdC12 present, to
produce the zinc or cadmium oxychioride. In the case of an uncoated
silica substrate, the cadmium or zinc oxychloride then attacks the
substrate resulting in the formation of a quantity of the silicate.
It is a well known fact in the phosphor industry that chlorides are
good fluxes for the low temperature formation of metasilicates.
Although definite proof is very difficult to obtain when
many simultaneous reactions take place, it is felt that the preceeding
tests and observations indicate the correctness of the assumptions and
explanations which were made f our group.
Summarizing: In the bonding of CdSe sintered layers on glass
substrates, three distinct mechanisms have to be considered.
First: Particle to particle bonding, caused by the solubility
of CdSe in CdC12, followed by the evaporation of excess CdC12 at high
temperature.
Second: Layer to substrate bonding, caused by oxidation of CdSe
to CdO, followed by a solid state reaction between CdO and CdC12.
This results in a hard layer of xCdO.yCdCl2, which creates a strong
bond to the substrate. This layer also is responsible for the
inhomogeneity of the PC layers.
Third: A layer resulting from a reaction between CdC12 and Si02
16
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NAVTRADEVCEN 1440-1
from the substrate. This layer consists of CdSiO3 which probably
contains excess Si02. This layer does not exist on tin-oxide coated
glass, but is present, when CdSe is sintered directly on glass substrates,
giving an increased bond to it.
Reisman and Berkenblit discussed the same problem in a paper
published in the Journal of the Electrochemical Society5 and concluded
that in the preparation of sintered CdSe layers on silica-based
substrates two processes must be considered (the first and third
mechanisms above). Our analysis gives a more complete description of
this mechanism.
(b) The conduction mechamism was studied in sintered CdSe layers
with coplanar and sandwich electrodes, and the fundamental semiconductor
characteristics calculated, as described in Section III under "Study of
PC Materials". As an example Figures 20 to 24 and Tables 5 and 6 show
some of the most interesting characteristics of a PC cell, processed
as described in Technical Report NAVTRADEVCEN 562-1.
Examination of Figures 20 and 21 and of the corresponding
numerical. data in Table 5 shows a large difference between the
characteristics of coplanar and sandwich construction (surface and
volume currents) of the same material. While the coplanar cell has
high dark current and nearly linear current vs. voltage relation (ohmic
electrodes), the sandwich cell shows linearity only below 10 volts, and
becomes highly superlinear above 50 volts, when the top electrode is
positive. When the top electrode is negative this superlinearity
starts at lower voltages. In both cases (top - and +) the super-
linearity becomes evident at lower voltages when the illumination is
lower. Considering the field strengths, one calculates 1.33 x 103
volts/cm with 100 volts on the coplanar cell. The same field strength
is reached with 11.6 volts on the sandwich cell. It is true that
besides this difference the electrodes in the two cases are somewhat
different. In the coplanar cell both electrodes are tin-oxide, while
in the sandwich cell the bottom electrode is tin oxide and the top
electrode is gold. The tin oxide electrodes are present during the
high temperature baking of the cell, while the gold electrode is
evaporated after the sintering.
For comparing the coplanar and sandwich cells, the
conductivities must be considered. The data In Table 5 reveal a large
difference; more than one hundred times higher conductivity of the
coplanar cell, than that of the sandwich cell at I ft-c illumination,
and higher factors at lower Illuminations. This Increasing conductivity
ratio at decreasing Illumination is caused by sublinearity in the
current vs. light intensity characteristics that prevails for the
coplanar cell, and the superlinearity that exists for the sandwich
cell.
One tries to explain these variances by the differences
between the crystal structure of a surface layer and that of the
Inside of the material. However, another reason for the difference
is the absorption of high energy photons (X < X ) in a very thin
layer of the CdSe. This does not present any proems for the coplanar
cell if the light enters on the electrode side. But if the light
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r"de
L
ft-c-
I
eres
R
Ohms
CrL
Mho rZ fm
-rd
sec
'Vrel
sec
0
M
nL
cm-3
F
cm-3sec
It
a
ntr
cm-3 V-1
Etr
V
10
6.4x10-4
1.5x104
3.9x10-3
x10?3
1.8x10-9
4.2x102
1.3x10-4
2.5x1015
1017
2.5x10 2
2.3x1016
2.1x10?1
1
8.2xl0?5
1.2x105
4.9xl0 4
.1x10 2
1.4x10-8
5.4x102
3.8x10 4
3.1x1014
1016
3.1x10 2
8.Oxl&l5
2.6x10?1
Coplanar
10?1
1.4x10-5
7.0x105
8.4x10-5
.5x10-2
8.4x10?8
9.2x102
1.2x10?3
5.3x1013
1015.
5.3x102
2.5x1015
3.1x10?1
10-2
7.5x10-6
1.3x106
4.5x10?5
-
1.6x10-7
-
-
2.8x1013
1014
?
?
-
0
xl 06
1. x106
3.9x.10-5
-
1.8x10-7
-
-
2. xlo13
-
-
10
1.5x10?3
6.6x103
5.3x10?5
1.5x10-2
1.5x10?
3.7x103
3.8x10?3
3.3x1013
1.15x1016
2.9x10 3
6.5x1015
3.2x10?1
Sandwich
1
4.0x10?5
2.5x105
1.4x10?6
.0x10?2
5.8x10?7
1.0x103
1.1x10-2
8.8x1011
l.15x1015
7.500
2.3x1015
4.1.10-'
10
Top +
10?1
9.5x10?7
1.1x107
3.1x10-8
xl0-1
2.5x10-5
2.4x102
2.8x10?2
2.1x1010
?1.15x1014
1.8x10'
9.0x1014
5.1x10?1
Volts
10?2
7.0x10?8
1.4x108
2.5x10-9
x10-1
3.2x10-4
1.7x102
1.6x109
1.15x1013
1.3x10
3.6x1014
5.7x10?1
0
.0x10-9
x109
1.1xl0-1
-
.6x10-3
6.6x107
-
-
10
2.Ox10?3
5.0x103
7.0x10-5
xl0-2
1.1x10-8
5.0x103
1.1x10-3
4.4x1013
1.15x106
3.8)00-3
2.2x1016
3.1x10?1
Sandwich
1
7.0x10?5
1.4x105
2.5x10-6
1.4x10-1
3.2x10-7
1.7x103
4.000-3
1.6x1012
1.15x1015
1.300 3
6.3x1015
3-900- 1
10?1
4.3xl0?6
2.3x106
1.5x10 7
.0x10?1
5.3x10-6
1.1x103
1.0x10-2
9.200 10
1.15x10 14
8.000-4
2.7x1015
4.7x10?1
op'-
10?2
7.Ox10?8
1.4x108
2.5x10-9
.7
3.2x10-4
-
-
1.6x109 .
1.15x1013
1.4xl0?4
1.2x1015
5.7x10?1
SAXID-8
8
J-- BY- A
1.900-9
-
-4
9
Coplanar
1
4.6x10?4
1.1x105
5.5xl0-4
1.300-8
3.0x103
1.9x10 3
3.4x1014
3.400?2
8.2x1015
2.6x10?1
10?1
8.0x10?5
6.2x105
.5x10-5,
7.4x10-8
.3x103
6x10-3
6.Oxio13
000?2
6
2
5
1015
0
10?1
3
.
.
x
x
.
1
4.6x10?
l.lxlO5
3.200 ?6
Same
2.5x10-7
1.1x10+
5.5x10?2
2.0x10.2
Same
t.7xl0
2.2x10 5
3.9x1 0
50 S
andwich
10-1
4.3x106
1.2x107
3.0x10-8
as
2.8x10-5
1.1x103
1.5x10-l
1.9x1010
as
1.6xl 04
9x1014
5.1x10?1
Volts
op +
10?2
WO -7
1.3x108
2.8x1 09
with
3.0x10 4
1.0x103
3.7x10?l'
1.7x109
with
1.5xlO-
3.6x1014
5.6010?1
0
1x1 0 7
.0x108
.0x10-1
10
1.100-3
-
-
10
_
-
1
4.0910-
1.3x105
2.8x10?6
volts
3.0x10 7
1.0x10
2.1x10?2
l.7x102
volts
1.5x1 03
6.3xl0 5
3.9x1 0
andwich
10?1
4.6x10 5
1.1x1 0
3.2x10 7
2.5xl0-6
1.1x104
4.6x10-2
2.0x1011
1-700-4
2.7x1015
4.5x10?1
op -
10?2 .
7.OxTO-6
7.0x106
4.9x10 8
1.6x10 5
-
-
3.1x1010
2.600
1.2x1015
4.9xl0?1
0
4. xlo?6
1.1x107
.2x10?8
2.500-5
e
Table 5. Calculated Data ~f a Sintered CdSe PC Cell (No. B2-71XA.l-8)
taking ? = 10 cm N sec.
0
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NAVTRADEVCEN 1440-1
enters on the other side, the shorter wavelengths are absorbed faster
than the longer ones and the spectral response curve is transferred to
longer wavelengths (see Figure 20 in Reference No. 1). In the case
of sandwich construction the current carrier electrons must be trans-
ported through the layer from the negative electrode to the positive
electrode. There is no complication when the light enters on the
negative electrode ("top +10 in the measurements), only the excited
electrons have to be diffused toward the positive electrode. But when
the light enters on the positive electrode ("top -" in the measure-
ments), the other (negative) electrode has to supply the electrons.
This can happen if the negative electrode has an ohmic contact to the
PC material. The mechanism is similar to that of the coplanar
conduction; an electron arriving to the positive electrode is replenished
with a new electron emitted from the negative electrode. The shift in
the spectral response curve (see Figure 22 in Reference No. 1) proves
that the excited electrons are primarily responsible for the photo-
current.
The increasingly superlinear dark current at increasing
field strengths is explained by the space charge limited currents. An
inertia phenomena with changing field strength is connected to this
mechanism.
Comparing the decay and rise time constants of the cells, one
sees that the response time of the sandwich cell is slower than that
of the coplanar cell by a factor of approximately five. The rise time
constant is about three times higher than the decay time constant in
both cases. In the calculations only the decay time constants were
considered.
For the gain-calculation, it was supposed that there are
about 1013_photons/cm2 sec in I ft-c of light intensity, and the
quantum efficiency is 1. The curves in Fiure 22 show that the gain
is higher with higher voltage, reaching 109 electrons/photons at 50 V.
The positive or negative slope of these curves corresponds to super-
linear or sublinear current vs. light intensity characteristics
respectively.
The M-factor* is connected to the maximum gain available with a
PC material. The maximum gain is limited by the field strength where
space charge Limited currents enter and in this case M Is equal or
lower than one. In some special cases M can be higher than one
representing a PC material with superior characteristics. The M factor
of.our photocells (Figure 23) show that they are in the general
category, where M < 1.
For calculating the electron density n1, it was assumed that the
mobility is equal 10 cm2/volt sec. It was assumed furthermore that
* Appendix C
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the absorption and conduction mechanism in the coplanar cell takes
place in a thickness of about 10 micron (t*)*.. In the case of the
sandwich cell the current goes through the total thickness. The life
time of the carrier, calculated with these assumptions, is-acceptable
for the sandwich cell, but are in contradiction with decay time
constant measurements for coplanar cell. A larger life time value
than the decay time is obtained, which indicates that some of the
assumptions are not valid. It was probable that the assumed mobility
of 10 cm2/volts sec. was too low for the surface conduction, but this
value could be reasonable for volume currents. Indeed, when Hall
effect measurements were made on similar cells, a value of ? = 30 cm2/
volt sec. was obtained. Since this measurement relates to surface
currents, data for the coplanar cell were recalculated with this
mobility and are presented on Table 6.
The obtained lifetimes now are lower than the decay 'times,
excepting at the 10 ft-c illumination. Considering the errors in
the measurements and in the other assumptions, this can be regarded as
an acceptable agreement. But shorter lifetimes (2.5 to 5 milliseconds)
were calculated, when u = 100 cm /volt sec. was assumed.
Voltage
Volts
_,__....L_~.
ft-c
Q
Mhos/square
nL
cm'3sec-
~G
sec
ntr
cm-3v-1
Etr
eV
10
10
3.9x10'6
8.3x1014
8.3x10-3
2.34x1016
0.237
1
4.9x10-7
1.1x,1014
1.1x10'2
8.2x1015
0.288
10-1
8.4x10'8
1.76x1013
1.76x10-2
2.54xlOl5
0.334
50
1
5.5x10'7
1.13x1014
1.13x10'2
8.2x1015
0.287
10'1
9.5x10'8
2x1013
2x10'2
2.54xl015
0.33
Table 6. Data Calculated for Coplanar CdSe Cell
with ? = 30 cm2/volt sec.
Figure 24 shows the calculated trap distributions for both
type of cells. Curves with ? = 10, 30 and 100 cm2/volt sec are
displayed for the coplanar cell. It is seen that the trap distribution
curves for the volume (top +) and surface layer match well, when the
mobility are 10 and 100 cm2/volt sec respectively for the two cases.
But this matching is not necessary, since it is well possible that the
surface trap structure is much different from that of the volume
structure. As it was pointed out, efforts to make Hall effect measure-
ments on volume currents were not successful.
(c) Series of experiments were made with varying prebaking and
sintering temperatures and times and impurity concentrations as para-
meters. The objectives of these experiments were (1) to improve the
* Appendix C
?
i
?
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NAVTRADEVCEN 1440-1
sensitivity, (2) to increase the speed of response, (3) to obtain
sublinear current-illumination dependence, (4) to approach linear
current voltage dependence, and (5) to increase the response for
shorter wavelengths. It was found that the highest sensitivities -
higher than 10-7 mhos/cm specific conductivity at 0.1 ft-c illumination
in sandwich construction - could be obtained with a preparation
schedule as follows:
(1) Composition of powder
100 g CdSe
4 g CdC12
0.01 g CuC12
(2) Prebaking at 480?C for sixty minutes
(3) Sintering at 505'C for thirty minutes
(4) Aging at 190*C for 16 hours
With higher baking temperatures, some improvement was obtained
in objectives (3) and (5).
It was obvious that the usually measured electro-optical
characteristics still do not give a full picture for the usefulness
of the PC material in the image intensifier. Inhomogeneities, spottiness
and graininess comes in, which could be examined by a complicated
scanning device or on the finished image intensifier panel. Not having
a scanning instrument available, experimental image intensifier panels
were built, and the PC layer was judged by the image quality of the
intensified picture. It was found that the most sensitive PC materials
showed more spottiness and graininess and the search to find the cause
of this graininess led to the problem of bonding, discussed in a
previous part of this section. The findings of the bonding experiments
resulted in the preparation of a material which showed improved char-
acteristics in objectives (1), (3) (see Figure 25) and (4) and gave
improved picture quality (less graininess) on the image intensifier
panel. The starting material had the same composition, but the
following schedule was used for the preparation of the sintered layer:
(1) Prebaking at 505?C for 60 minutes
(2) Washing the powder in 5% acetic acid
(3) Sintering at 505?C for 30 minutes
(4) Aging at 1900C for 16 hours
Increased prebaking temperature gave higher sublinearity in
the current vs. light intensity curve (objective 3), and higher sinter-
ing temperature gave higher sensitivity (objective 1). No improvement
could be obtained in the speed of response, and the spectral response
curve of these layers showed higher long wavelength response.
Figure 25 shows the current-light intensity characteristics
of such a PC cell in lateral and in sandwich construction measured with
DC and AC voltages (100 Volts, RMS in AC case). Other characteristics,
measured and calculated, are in Table 7. A mobility of 10 cm2/V sec
21
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AT
L
ft-c
(Y
mhos/cm
Td
sec
Trel
-sec
G
M
nL
cmsec
sec
n
cn-3V-l
E t r
ev
10
7x10-6
3x1002
1.lxlO-7
3.5x103
1.3x10-2
3.1012
3.7x10-4
9.4x1015
0.38
1
9xl0?7
9x10-2
8.8x10-7
4.5x103
4.4xl0-2
4x1011
4.8x10-4
3x1015
0.43
10-1
1.5x]0-7
3x10-1
5.3x10-6
6.5x103
1.1x10-1
6.6x1010
8x10-4
1x1015
0.47
10-2
4x10-8
1.5
2x10-5
1x104
1.35x10-1
1.75x1010
2.2x10-3
4.8x1014
0.51
0
2x10?8
4xlo-5
-
-
-
-
-
-
TABLE 7. Measured and Calculated Characteristics of an Acetic Acid
Washed Sintered CdSe Sandwich Cell (top +, V = 100 Volts)
CdSe
In purities in m
Powder
Ag
Al
B
Ca
Cr
Cu'
I Fe
Mg
Mn
Ni
Pb
Si
Zn
1
1
1-10
1-10
1-5
1-10
1-10
1-10
1-5
1-5
1
1-10
1-5
10-50
1-10
1-10
-
1
-
1-10
1-5
1-10
10-100
1-10
10-100
-
The following elements were sought, but not detected: As,Ba,Be,Bi,Co,Ge,Mo,P,Sb,
Sn,Sr,Ta,Th,Ti,W. Detection limits of these elements range from 1. to 1000 ppm.
TABLE 8. Spectral Analysis of High Purity CdSe Powders
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NAV TJtAD EV CE I 11440-1
Comparing the trap distribution of this cell to that in
Figure 24, one sees that the trap densities are higher in the vicinity
of the conduction band, but the slope of the curve is steeper and the
trap density is lower above 0.47 eV.
(d) From the spectral analysis of the CdSe powders (see Table 8),
which were used in the above experiments, it was obvious that the purity
of these materials was not sufficient. It seemed that there is some
probability of obtaining improvements in the speed and spectral response
curve by using purer CdSe. Synthesis of CdSe from the available very
pure elements would be one way for obtaining better material and
purification of the available CdSe is the other possibility. Work in
the second direction has been started. Some preliminary experiments
in zone-refining the CdSe were not successful. Therefore, another
method, the purification by sublimation, has been cho9en. First
experiments, made according to the work of Bube et al (see also Ref.
7), yielded enough purified material for preparing PC layers from it.
These layers, prepared in the usual schedule, showed lower conductivities
than those made from the unpurified material. A series of experiments
are being carried out to find the proper processing schedule for the
purified material. A spectral analysis is also being made on the
sublimed powder.
Other Materials
Other materials with a band gap between 2 and 3 eVolts were
considered also for application of PC-layers in the image intensifiers
hoping that they could show some better characteristics than those. of
the CdSe. Though the state of the art offered only the CdS and CdSe,
which had acceptable conductivity for the present application, some
theoretical considerations and experimental results (see Ref. 8 to 11)
gave a basis for further work with other materials.
An appreciable effort was made toward the preparation of ZnSe
sintered PC layers. A long series of experiments to sinter the ZnSe
powder brought negative results. The presence of zinc oxide in the
ZnSe powder was suspected for the difficulties and a purification of
the ZnSe by the sublimation method was made. It was assumed that this
purification removes ZnO together with other impurities. Experiments
made with the sublimed powder failed to give any perfect sintering.
Work is being continued with mixtures of ZnSe and CdSe10.
Some experiments, started with As2S3 and In2S3, are being continued.
A series of experiments, using the prebaked CdSe powder in
plastic embedment, showed promising PC characteristics. Therefore it
was decided to spend a larger effort for the development of plastic
embedded PC powders. The work in the last six months was carried out
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AT
and w i l l be continued tor another six month period. The it-poi;
program follows.
The experimental work may be divided into two phases, in the first
of which firing was carried out in two stages - an initial firing at
400 *C for 30 minutes, followed by a second firing at 6500C. In the
second phase a single firing was made at around 500*C.
Much experimental work was carried out in the first phase, but the
final conclusions drawn were that the conductivities of the samples were
below those of samples fired at the lower temperature. However, many
of the experimental techniques developed on the 650-C powders have been
adapted and used in the 500 C firings.
Copper has been used as activator and halogens as coactivators.
In the early work the halogens were added as ammonium halides, later
cadmium chloride was used. After firing, all powders were tested by
mixing them with a solution of HD27 plastic and "doctor blading" them
to give dried layers about 100 ? thick. The final dried layers were
found to be optimized when they contained about 1-2% plastic. Using
these layers, cells were made up in both gap and sandwich configurations.
1. High Temperature Firing
The mix of raw materials was fired first at 400?C in order to
reduce the rate of volatilization of the ammonium halide, then the
temperature was raised to 6500C for 30 minutes. Initial firings were
carried out in argon, so that oxygen might be introduced into the
lattice at a later stage and in a controlled fashion.
Long series of experiments were carried out, systematically
examining the effect of copper and of different halogen concentrations.
Data of optimum copper, ammonium chloride, cadmium chloride, ammonium
iodide, halogen to copper ratio were obtained for the special processing
conditions used in these experiments where firing was made in argon
atmosphere. Oxygen was then introduced into argon fired PC powders by
a number of different techniques.
Several lots of powders were prepared, each of which contained
0.01% Cu and 2% chlorine. A comparison was made between oxygen built
into the photoconductor by (1) air baking after argon firing, (2) including
magnesium oxide in the charge, and (3) firing the charge in static air.
The inclusion of the MgO experiment in this series was of
particular interest since this addition has been used to prevent
excessive sintering of powder charges, thus achieving powders having
fine grain sizes. In this particular experiment 5% wt/wt of MgO was
added to the charge before firing, and the mixture was fired in argon.
After cooling, each of the charges in this series of
experiments was washed .with acetic acid to remove excess MgO, soluble
chlorides, and oxychlorides. After. water washing, the powders were dried.
24
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The grain size of the MgO fired powder was very much less
than that of the other powders, and no sintering at all took place
during firing.
The overall conclusion of the experiments was that the.air
bake after argon firing produced the greatest increase in the light to
dark ratio consistent with only a small decrease in overall sensitivity
and that the use of MgO lead to a very fine and non-sintered powder.
Experiments were carried out to study the effect of prefiring
the CdSe raw material at high temperatures in argon (10500C) or in
H2S (9000C) in order to remove volatile impurities and fill selenium
vacancies with sulphur. The conductances of the powders after this
treatment were very different, and in addition, the current-voltage
characteristic of the argon fired material was strongly super-linear,
whereas the H2S fired material was nearly voltage-independent. The
argon fired material showed breakdown at about 200 Volts AC, whereas
the H2S fired material did not break down at 400 Volts.
If the H2S fired material was mixed with CdC12 and refired in
argon, it had a low conductance and low breakdown potential (100 Volts
DC). If the CdCl2 firing was again carried out in H2S however, the
conductance remained low but the breakdown potential was higher than
400 Volts DC. Finally, if the H2S prefired material was refired in a
50-50% H2S-HC1 atmosphere, the conductance increased by four orders but
the breakdown potential remained high.
2. Low Temperature Firing
General.results of the above work seemed to indicate that at
a 6500C firing temperature the powders prepared had limiting conductances.
From early experiments on materials of composition CdSe.0.005% Cu
fired with 4-5% CdCl2 at 5050C, it became apparent that fairly better
conductances could be obtained, but that the charges had to be fired
in static air rather than in argon, if acceptable voltage breakdown
figures were to be obtained. In addition, samples fired with CdC12
at the low temperatures did not break up readily in acetic acid. As
with most materials of,this type, mechanical crushing of the hard
cakes prior to washing was found to greatly reduce the conductance and
increase the dark current.
However, the incorporation of 5% MgO in the charge during
firing again led to much reduced sintering and to materials which broke
up completely in dilute acetic acid, so avoiding the necessity for
mechanical grinding.
Samples were prepared containing either 4% CdC12 or 2.3% NH4C1
to maintain a constant chlorine content (1.55% Cl wt/wt), and all
samples contained 0.005% Cu. Parallel specimens were prepared with and
without MgO. Results, illustrated in Tables 9 and 10, show that the
samples fired with CdCl2 were much better than those fired with NH4Cl.
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Currents A 100 V DC
Illumination
Lateral Cell 0.75 mmu39mm
Sandwich Ce
ll lcm 6 l thick
ft-c
No MgO
5% MgO
No Mg(1
5% MgO
33
2.1
6.4
30.0
17.0
10
0.7
2.0
3.5
5.6
1.0
0.057
0.2
0.13
0.19
0.1
0.012
0.033
0.023
0.04
0.01
0.005
0.013
0.01()
0.016'
0
0.003
0.0086
0.0065
0.010
Table 9. Sensitivities of CdSe Plastic Embedded Cells,
Containing N114C1
Currents in A at 100 V DC
Illumination
Lateral Cell .75 x 39 mm
Sandwich Ce
ll lcm 6 mil thick
ft-c
No MgO
5% MgO
No MgO
5% MgO
33
130
160
640
300
10
48
38
250
60
1.0
7
2.5
40
3
0.1
1.5
0.28
10
0.3
0.01
0.6
0.084
5
0.05
o
0.46
0.060
4
0.03
Table 10. Sensitivities of CdSe Plastic Embedded
Cells, Containing CdCl2
3. Stability of Plastic Embedded Powders and Cells
It has previously been noticed that a P.C. powder sample had
decreasing conductance when measured repeatedly, indicating that the
selenide powders are not stable after preparation. To see if this was
also true with the CdSe.Cu.Cl.(MgO) powders, cells were made up, tested,
and the remainder of the powder samples stored for 12 days before
further cells were made up. Results are shown in Table 11 and indicate
that the instability is indeed present.
Illumination
Currents in A at 100 V DC
ft-c
Tested June 1st
Tested June 12th
33
300
80
10
60
18
1.0
3
1.3
0.1
0.3
0.22
0.01
0.05
0.13
0
0.03
0.11
Table 11. Stability of CdSe.Cu.C1 Powder Cell in
Sandwich Construction (Thickness 6 mils)
It was known that the overall sensitivities of thin film CdS
photoconductors may be greatly altered by heating the layers and then
is
0
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cooling them in light of different intensities. In addition, it has
also been shown that postbaking or 10aging1? of photoconductors at
temperatures lower than 200 C can improve their conductances, reduce
their dark currents, and stabilize them.
Accordingly, a number of cells were made up in sandwich
construction (approx. 6 mils thick) using the powder which had previously
been kept for 12 days after preparation. After preparation, all the
cells gave substantially the same current figures when tested at 100 V
DC.
The cells were then heated to 1600C in air for 30 minutes,
and then allowed to cool slowly (20-30 minutes) in light of different
intensities.
Results of the experiments are shown in Table 12. It will be
seen that the heating itself caused a marked increase in. the conductance
of the cells at high light levels and a lesser increase in dark current,
whereas the intensity of light in which the cell was cooled had little
effect on the high light level conductance, but high Intensity
illumination increased the dark current markedly.
Currents A/cm2 at 100 V DC
Illumination
12 day old
Aged and
Aged and
Aged and
ft-c
sample
cooled in
cooled in
cooled in
unheated
dark
room light
bright light
33
80
2600
3000
3000
10
18
800
900
goo
1
1.3
50
60
60
0.1
0.22
5.0
4.0
10
0.01
0.13
0.9
0.8
4.0
0
0.11
0.5
0.4
3.0
Table 12. Effect of Aging CdSe Powder Sandwich Cells
Work is being continued for improving sensitivity, homogeneity,
speed and spectral response of the plastic embedded PC cells.
Evaporated PC Films
A small effort was devoted to the preparation of evaporated PC
films, for their possible use in the construction of the high reso-
lution image intensifier panel.
Four general problems present themselves for the evaporated film
in this application:
(1) the sandwich construction
(2) the need for high thickness (about 20 ?)
(3) the sensitization, without poisoning, or being poisoned by,
the EL film on which the PC film Is deposited
(4) uniformity of photosensitivity
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AT
Problem (1) i.e. the sandwich construction, in contrary to the
coplanar construction, brings in the necessity of eliminating any
pinholes in the film, which can cause short-circuit of the PC cell.
The high thickness is necessary for two purposes: first for diminishing
breakdown at the high voltages used for the panel, second for having
low capacitance, necessary for the sandwich construction. Problem (3)
arises because the PC film has to be evaporated on top of the EL film,
and has to be heat treated for sensitizing. In this baking process
there is an interdiffusion of impurities between the two layers, and
the characteristics of both will be modified by it. Problem (4) is
the nonuniformity of photosensitivity, found generally in evaporated
polycrystalline films.
A series of experiments was carried out with evaporated CdS
films for finding the lowest temperature, at which highly sensitive
sandwich cells can be made. Temperatures of 300* and 3500C with
different sensitizing powders (using embedment in the powder) did not
result in useful sensitivity. Increasing the temperature to 4300C and
using a sensitizing powder of 0.1% CuC12 and 4% CdC12, gave good
conductivity and appropriate characteristics for the planned appli-
cation. The film thickness was about 6 a and the number of pinholes
was not very high. The work Is being continued to eliminate pinholes,
make thicker layers and to improve uniformity.
EVAPORATED EL TH I N FILMS
Plastic embedded EL layers were used exclusively for the con-
struction of the low resolution image intensifiers, prepared as
described in Appendix E of reference 1. Practically no development
work was done with EL layers of this type, but an appreciably large
effort for improving evaporated EL films for the applica ion in the
high resolution image intensifier was carried out in the
This work was planned for one year period and besides the principal
goals of
(1) improving the efficiency
(2) improving the life
of the EL films, the following other objectives were set up at the
beginning
(3) to produce low voltage (30 volt) films In areas large enough
to be useful In a light amplifier
(4) to find a protective insulating coating for the EL film
which would allow deposition (and firing) of the photo-
conductive layer onto the EL layer without harm
(5) to experiment with a buffer, also insulating, layer between
the EL film and the conducting glass on which It is formed;
this appeared to be the most likely method of significantly
improving the efficiency (lumens per watt) of the EL films
(6) to produce films, with the best characteristics possible, in
larger areas: namely, 601 x 6", 410 x 4" and 2" x 2" dimensions
was the final objective.
?
?
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Work was done toward all six of these objectives and is described
in this section.
1. Low Voltage Yellow-Emitting Films (ZnS:Cu,C1,Mn)
Much of the time at the beginning was spent on the problem of
producing lar a area films of uniformly high sensitivity (100 ft-L at
30 volts, say); throughout this period, the characteristics of small
areas of the films continued to improve, that Is, brightness at'a given
voltage improved slowly, as did the luminous efficiency; but we were
not successful in producing large area films with uniform characteristics
across the area. Occasionally a film would be uniformly sensitive
across a three inch dimension, but reproducibility was very poor.
Initially, the finished films showed pinholes which led to
poor electrical strength. A new cleaning procedure, Involving-ultrasonic
wash in detergent and treatment in isopropyl alcohol vapor, was used.
Also, a shaped baffle between the heated boat carrying the ZnS powder
charge to be evaporated and the glass substrate, apparently eliminated
bombardment of the film during deposition by particles decrepitating
from the boat, without interfering with deposition of the ZnS. These
two measures apparently were responsible for alleviating the pinhole
problem.
The next problem to be recognized was warping of the substrates
following the tin-oxide coating step, and this was eliminated by slow
cooling of each batch of glass after being coated. The warping was
completely eliminated (that is, a five inch dimension of glass
substrate was flat to about 0.0005"). Nevertheless, the nonreproduci-
bility of large areas persisted. The next problem to be recognized
was warping of the glass substrate during cooling of the film on the
substrate after the firing step and while still embedded in the powder
phosphor. Although it is hard to understand how an appreciable
temperature gradient can be set up through a 1/8" thick glass plate
buried in a powder which completely fills a thick-walled, covered
crucible, the warping did in fact occur. It was eliminated by more
gradual cooling, and better firing temperature uniformity, but this
also failed to improve reproducibility of the film characteristics
in large areas.
Thus the elimination of a number of successive problems,
each of which was expected to be the culprit, failed to lead to
successful large area high sensitivity films, even though other
aspects of the films (optical uniformity, transparency, freedom from
pinholes, sensitivity in small areas, and efficiency) all showed
improvement. At this point, after thirty-four evaporations and
complete processing of 140 films, the major effort on low voltage
films was halted.
Some optimum brightness voltage data are listed in Table 13.
Some of the adjacent pairs of values were observed on the
same film, and some were not.
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EL Film
Brightness
ft-L
Volts
a. Low voltage films, AC
10
11
120
23
b. High voltage films, AC
200
95
300
130
c. Low voltage films, DC
220
22
500
47
600
55
960
.65
Table 13. Optimum Brightness Voltage Data of
Evaporated EL Films
2. Insulating Layers
The next aspect studied was the effect of an insulating film
on either or both sides of the luminescent ZnS film. Two materials
were used, SiO and MgF2. These phosphor films were about 3.5 ? in
thickness and the insulating films were made about 0.5 ? thick. An
insulating film deposited on the tin oxide coated glass, and followed
by the ZnS film, will be referred to as the "base" film. An insulating
layer deposited on top of the ZnS film will be called a "cover" film.
A. Yellow Emitting Films
SiO
SiO base films (at least in thicknesses around 0.5
reduce adherence of the ZnS if the sequence is SnO-SiO-ZnS-fire. Some
areas adhered well enough to measure. However, if the sequence is
SnO-Si0-ZnS-SiO-fire, the SiO cover largely prevents activation of the
ZnS during firing. This implies that SiO is promising as a buffer
between ZnS and the photoconductor during the sensitizing process of
the latter in image intensifier fabrication. These layers of SiO
(about 0.5 ?) were much too thick; they absorbed an excessive fraction
of the applied voltage and they were light amber in color. In addition,
SiO base films seem to react during firing with the SnO conducting
layer and become optically more dense; this does not occur where the
SID film is on bare glass. Adherence of ZnS to glass (without SnO
coating) appears to be better with an SlO base coat between them.
Results of electrical measurements are: (1) At equal
light output, SlO base films under ZnS do not affect efficiency but
increase voltage by a large factor (5X) for equivalent brightness.
(2) Unfired SiO cover films over ZnS Increase efficiency at low
brightness by 15X but do not affect efficiency at high brightness;
this agrees with earlier results with plastic overcoatings. (3) SiO
over ZnS, then fired, reduces efficiency by 100X (prevents activation).
(4) 0.5 ? SiO layers are far too thick. (5) SiO fired in ZnS phosphor
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is 20X more lossy than unfired S10,; its resistivity is only 25X
greater than the ZnS phosphor.
MgF2
Some sequences (SnO-MgF2-ZnS-fire) peeled and some did
not, but it seemed certain that fired ZnS adheres less well to 0.5 ?
MgF2 than to tin-oxide on glass. In the sequence SnO-MgF2-CN wash,
the CN wash was effective and so either penetrated the MgF2 cover, or
removed it - probably the latter. MgF2 cover prevents activation of
ZnS and survives the firing step at least in some instances. In the
sequence SnO-MgF2-ZnS-MgF2-fire-CN wash, the fluoride layer appears to
have survived the CN wash. An MgF? base film caused no visible differ-
ence in the appearance or photoluminescence of the ZnS layer. The
MgF2 layers are extremely transparent. The results of electrical
measurements on samples with MgF2 layers are presented in Table 14,
where the sequence of operations in making the film is symbolized by I
(a 0.5 ? insulating film of MgF2), ZnS (the evaporated layer of zinc
sulfide), F (the firing or activation step), CN (the cyanide wash always
used to remove excess copper).
Brightness
ft-L
Near Breakdown
Voltage (400 cps)
Range of Efficiency*
(lumens per watt)
Sample
rms volts
low
high
voltage
voltage
ZnS-F-CN (Standard)
80
90
.01
.10
I-ZnS- F-?CN
8
140
.06
.06
ZnS-I-F-?CN
75
25
.01
.15
ZnS-F-CN-I
50
165
.12
.15
ZnS-F-I-CN
50
80
.01
.07
I-ZnS--F-CN-1
20
200
.10
.10
Table 14. Characteristics of EL Films Combined with a MgF2
Film in Different Constructions and Processings
(I insulating layer, .F firing, CN cyanide wash)
B. Green Emitting Films
Green emitting films were made from a standard green
emitting powder phosphor which shows a maximum efficiency at 400 cps
of 2 lumen/watt at 1-10 ft-L (200-400 V), and at 4,000 cps of 1.1
lumen/watt at 25-100 ft-L (200-500 V). Green emitting films from this
phosphor show a maximum efficiency of about 1 lumen/watt at about
1 ft-L at 20 V, 400 cps, which is about half the efficiency at a tenth
* Peak efficiencies of yellow emitting films go as high as 0.25 to 0.3
lumens per watt. Since the peak efficiency of a commercial lamp made
of yellow emitting EL phosphor powder was measured to be 0.4 lumens
per watt, this film efficiency cannot be expected to go much higher
at this time.
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the voltage of the powder phosphor. The same films show: 0.3 lumen/
watt at 10 ft-L, 40 V, 400 cps; 0.4 lumen/watt at 10 ft-L,30 V. 4000
cps; 0.1 lumen/watt at 80 ft-L, 85 V, 4000 cps. All of these films
were overcoated with 0.2 micron MgF2. The 80 ft-L at 85 V,.4000 cps
figure is probably the best brightness voltage value yet obtained from
a green emitting film.
There seems to be a small probability of increasipg
efficiency of green emitting films since an MgF2 layer acting as a
buffer between the tin-oxide and phosphor films does not improve but
rather reduces the efficiency by about a factor of three. Apparently
we were mistaken in assuming the tin oxide badly contaminates the
phosphor film. An MgF2 layer deposited on the evaporated ZnS film
before firing also has a deleterious effect on efficiency, although
with the yellow emitting films an increase in both sensitivity and
efficiency resulted from the same procedure.
3. Lifetime Improvement
A. AC Operation
Improvement of lifetime of finished EL films was the next
objective. Evidently we had allowed the halide content of our embedding
material to become depleted, since firing the lb tter with additional
halide improved the lifetime of the films subsequently fired in the
material. Twenty new films were made by firing in embedding phosphor
which had been treated by ammonium or copper halides (chloride,
bromide or iodide). Six groups of films were life tested in dry air.
Only ammonium chloride treatment gave much improved maintenance. The
results are shown in Figure 27, in which curves A, B, C are yellow,
emitting ZnS:Cu, Mn, Cl films about 2.5 microns in thickness with
0.4 micron MgF2 coating. Sample A is a standard, made similarly to all
recent films; i.e., with embedding material which had not been treated
with halide. Sample B was made with twice used NH4C1 treated embedding
phosphor and Sample C was the third firing in this powder. Half-lives
of A, B, C increase in the ratio 1:10:50 approximately. Sample B' was
identical to Sample B, but the MgF2 coating was omitted. These life-
times, in view of the 1000 cps operating frequency, are as good as any
we have produced previously.
Efficiency was measured on films, A, B, C and found to
increase slightly in that order; i.e. best lifetime associated with
best efficiency. Breakdown brightness of Sample C is 100 ft-L at
105 volts rms.
Maintenance of emission of EL powders is usually increased
if iodine or gallium is used for coactivation instead of chlorine.
Accordingly, batches of powder phosphor were prepared according to the
following recipes:
(1) Chlorine Coactivation
200 gm ZnS
3 gm Cu acetate
0.3 gm NH4C1
8 gm MnCO
10 gm purified sulphur
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(2) Iodine Coactivation
200 gm ZnS
3 gm Cu acetate
1.8 gm NHlI
8 gm MnC03
10 gm purified sulphur
(3) Gallium Coactivation
200 gm ZnS
3 gm Cu acetate
0.28 gm Ga2203
8 gm MnCO
10 gm purilied sulphur
All were fired 100 minutes at 1000'C in flowing N2, except the gallium
lot which was fired in flowing H2S. Films were made from each type,
but no significant difference in lifetime or efficiency was found.
Apparently in the case of films, only an increase in chloride content
results in lifetime improvement.
B. DC Operation
Some data on direct current, constant voltage lifetime of
one yellow emitting film appear in Figure 28. This behavior is perhaps
least favorable since the film was operated in room air, with no
protection from moisture, it was chlorine coactivated but probably was
deficient in chlorine, the film had no insulating layer on either
surface (such as those which markedly improve AC lifetime). Note
that efficiency increases by about a factor of three; power input
decreased by a factor of eight during the five hours of the run.
Electrically, the major effect is an increase in impedance with operation,
and this is characteristic of all EL phosphors, powder or film, AC or DC
operated.
C. Other Aspects of EL Lifetime
The usual method of defining lifetime of emission of
any lamp is to run the lamp at constant voltage and to measure the
time required for the emission to decrease to a minimum tolerable
level; this was the procedure, for example, in obtaining the data of
Figures 27, 28.
These EL films are excitable by direct current and it is
a simple matter, with a constant current supply, to determine the
maintenance characteristics at constant current instead of constant
voltage; some results are shown in Figure 29. Although this general
behavior is quite typical of constant-current operation of films, the
increase in brightness and efficiency with operating time is usually
not so great. Although constant-current operation is not useful where
the EL film is used in a conventional light amplifier, the curves of
Figure 29 remind one that constant voltage operation is not the ideal,
nor even a reasonable, way to realize best lifetime.
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Improvement in usable lifetime is obtained by operating
the device at "constant brightness", that is, increasing voltage (or
current) slightly and continuously to correct for normal decrease in
output with operating time. Figure 30 is taken from old data on
electroluminescent lamps, and indicates that a sizeable increase in
usable lifetime is realized by "constant brightness" operation.
It is hereby suggested that provision be made for adjust-
ment by gradually increasing the overall voltage on the completed
light amplifier; a range of 10% should be sufficient. Such operation
would put the useful life time on an entirely different basis, and
results in many more operating hours.
4. Large Area Films
Large area EL films will, when excited over their entire area,
require a highly conducting transparent electrode. For the smaller
areas we have tolerated 50 ohms per square before firing and 100-200
ohms per square after firing, but spent much time on lowering this
resistivity. Both substrate temperature and spraying procedure were
varied, and as a result many films have been made which show about
6 ohms per square before firing and perhaps 10-30 ohms per square
after firing. The coating temperature of the PPG No. 6695 glass is
now held at about 815?C. The low value of about 6 ohms per square is
now consistently obtained for all glass sizes from the usual 1 1/2"
square to 6" x 6" pieces.
A new furnace, large enough to take the 6" x 6" plate in its
crucible has been arranged. The temperature cycle at the film, during
firing (activation) of the film, was measured first for a 1 1/2" x
1 1/2" film in the small furnace and then for a 6" x 6" film in the
large furnace and power adjusted so both ultimate temperature and
heating rate are similar.
Evaporation conditions for the large area films have
apparently been successfully worked out. Both 4" x 4" and 6" x 6"
sizes have been evaporated to a uniform thickness (by appearance) and
fired to uniform activation (also by appearance); they are not tested
and finished yet at the.time of writing this report.
IMAGE INTENSIFIER CONSTRUCTION PROBLEMS
Light Amplifier Elements
It was mentioned in Section III that an agreement between the
calculated characteristics and the experimental data of the image
intensifier should not necessarily be found when the PC or EL elements
have nonlinear voltage current characteristics. Nevertheless an
agreement was expected, because of the nondiscriminatory character
of the EL cell regarding the shape of the voltage curve. To see if
truly an agreement exist, light amplifier units with separated PC and
EL elements have been tested. In one experiment the PC cell, in an
other the EL element had non-ohmic characteristics. Figure 31 and 32
0
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show the result for the first case., where a sintered PC CdSe cell and
a plastic embedded EL element were combined. The current of the PC
cell had a quadratic dependence on the voltage and the measured values
of the transfer characteristic agree within the error of the measure-
ments with the calculated curve. In a second case an evaporated.EL
film with non-ohmic characteristic was combined with an ohmic PC cell
and again the calculated and measured transfer characteristic agreed
within the error range.
After this proof, the theoretical considerations (see Section III -
Computer Study) could serve for guiding the choice of the electro-
.optical characteristics of the PC and EL elements in the construction
of the image intensifier.
It is true that the characteristics of the PC as well as those of
the EL elements in a completed image intensifier panel were somewhat
different from those of separated cells, and from measurements of
separated elements one could not have a complete picture of the image
quality of the intensifier panel. This was found in the case of both
the low and high resolution panel.
Low Resolution Image Intensifier
Large number of image intensifier panels in sizes from 2" x 2"
to 6" x 6" have been fabricated to test the changes in electrical
characteristics and in the image quality, when changes in the pre-
paration of PC layers were made.
The most effort, in improving the characteristics of the image
intensifier panel, was placed in the aim to improve the image quality,
i.e. eliminate the very disturbing graininess and inhomogeneity and
decrease the very high contrast of the intensified image.
As a result of this effort the gamma was decreased so that image
intensifier panels were built showing five to six half tones. The
maximum $ of the transfer characteristic was diminished from he usual
six to only about two to three, but the distribution of the through
the useful illumination range was improved so that a less contrasty
picture was obtained.
For decreasing the graininess, a study on the bonding mechanisms,
described in the Photoconductive Materials part of this Section, seemed
to indicate the cause of the graininess. Using the conclusion of this
study some experimental panels were made, showing highly improved
pictures. However, the reproducibility of the fabrication technique
was not good.
It seemed at the end of the one year work period that there could
exist a very critical contact inhomogeneity between the PC and EL sand-
wich or the PC layer is modified by the EL spraying process so that a
grainy structure results from it. The study is being continued to
clear up this problem and to be able to reproduce the fabrication of
panels with improved picture quality.
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High Resolution Image Intensifier
Evaporated EL films were connected to separate PC elements and
characteristics of the light intensifiers studied. Plastic embedded,
and sintered CdSe cells and evaporated CdS films were used as PC
elements in these experiments. With all three variety of PC cells
maximum gains between unity and ten were measured.
Building an available plastic embedded CdSe PC layer on the top
of the EL film showed a gain of around unity and a highly grainy
structure. Improvements in the plastic embedded PC powder program have
to be achieved before continuing more experiments with this type of
PC layer.
Construction with sintered or evaporated PC layers demand a
high temperature baking of the EL and PC film together'for sensitizing
the PC layer. During this baking both layers are modified by the
other and as experiments made in the previous contractl on image
intensifiers, both layers showed highly deteriorated characteristics.
The higher the baking temperature is, the worse the deterioration can
be. Some experiments indicated that a thin SID layer between the
PC and EL films can diminish the interaction during the baking process.
Experiments for finding the lowest sensitizing temperature on
CdS evaporated films showed that acceptable sensitivities can be
obtained with a 4300C baking temperature. Consequently CdS layers
were evaporated on the top of a 2 ? thick SiO coated EL film and
sensitizing of the PC film was made without much deterioration of the
photoluminescence of the EL film.
Problems of depositing intermediate electrodes, necessary of
achieving some gain, are being worked out. After finding the
appropriate electrode material (probably gold) which does not deter-
iorate the characteristics during the heat treatment, small samples
of image intensifiers in this construction will be built.
Another problem, increasing the thickness of the PC layer, has
to be solved also, because the low breakdown voltage and shorts on
the 6 ? thick CdS films presents some difficulties. As the low
voltage variety of EL films was not developed for larger areas than
1" x 1", the PC film has to withstand voltages as high as 130 volts.
More than 13 micrn thick films will be needed to keep the field
strength below 10? volts/cm.
Data of Best Image Intensifier Panels
Figure 33 depicts the transfer characteristics of an image
intensifier panel, prepared at the end of the working period. The
measurements were performed with a 2870*K color temperature tungsten
light source.
Figure 34 shows a photograph of an intensified image. Comparing
this figure with Figures 67 to 73 of reference 1, one sees the improved
uniformity; the almost complete elimination of the dense dark and
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bright spottiness and a finer grain structure. The most important
constructional and electrical characteristics of the best panels are
listed in Table 15.
Characteristics
Data
Construction type
Sandwich
Size
6" x 6"
PC layer
Sintered CdSe, 4
mils thick
EL layer
Plastic embedded
ZnS, 2 mils
Voltage
100-200 volts
Frequency
60-2000 cps
Input light intensity range
10-2 to 2 ft-c
Output brightness range
10-4 to 30 ft-L
Maximum standard luminous gain
30 ft-L per ft-c
Spectral response max. of PC layer
7600 A
Spectral emission max. of EL layer
5300 A
Decay time constant
100 msec
Rise time constant
100 to 1000 msec
Resolution
100 TV lines per
inch
Brightness uniformity among elements
Better than I to
2 in average
Table 15. Constructional and Electrical Characteristics
of Best Sandwich Type Image Intensifiers
Some explanation is needed to see the meaning of the last line
in Table 15. When one illuminates the image intensifier panel with
a homogeneous light intensity, one sees some kind of nonuniform
graininess, which results from the unequal emission of the elements.
This nonuniformity is generally the most disturbing at medium illumination
levels (in the center of the transfer characteristics, where (is
maximum) and disappears at the saturation of the transfer character-
istics. It is caused probably by the not well defined contacts
between electrodes and PC particles. In panels made before an improved
preparation method was found (see Section IV), the brightness ratio
between two elements could go up to 50 for a very large number of
elements. The panels prepared lately show a large improvement in this
respect, and by this fact offer a less grainy structured, better image
quality.
37
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DISCUSSION, PLANS FOR FOLLOWING YEAR
The work in the beginning of this program was progressing at
a slower pace than it was planned. The full effort was started only
after the first half of the year had passed.
The work proceeded along five principal lines. The first line of
work was directed toward improvements in sintered PC materials. The
second line was directed toward construction problems and improvements
in the low resolution image intensifier panels. The third line of
work was performed during the last half year at the
F_ -77
AT
AT with the goal of developing plastic
embedded powders for the image intensifier application. The fourth
line of work was performed during the last ten months at.the
AT
AT I Iwi e goal o improving the characteristics ot
evaporated films and the fifth line of work was directed toward
construction problems of the high resolution image intensifier panel.
Regarding the sintered PC materials most of the effort was devoted
to CdS.e. Improvements in sensitivity, current light intensity character-
istics and graininess have been achieved. Some problems of conduction
mechanism have been examined and understood better. Based on the above
study the image quality of the low resolution image intensifier panel
has been improved, achieving better homogeneity, less graininess and
lower contrast. A parameter study of image intensifier characteristics
executed by a digital computer helped to clarify the electro-optical
behavior of the image intensifier circuit. In the PC powder program
the sensitivity of the CdSe plastic embedded powder has been increased
to a practically useful level. In the evaporated EL film program a
method was developed which gave films with reproducible life character-
istics. The solution of the difficult problem of building a sensitive
PC film on the EL film is progressing satisfactorily; CdS evaporated
films have been sensitized oh the top of EL films having an intermediary
SiO layer, without seemingly deteriorating the EL film.
For the following year all the activities will continue with
goals of fulfilling the requirements of the program (see Section 11).
(1) Work with new PC materials, which started already, will
continue, with the main effort directed toward achieving
an improvement in speed of response without loss of high
conductivity.
(2) Some construction problems of the low resolution image
intensifier panel have to be cleared and the reproducibility
AT (3) of the fabrication will be improved. ill continue the
powder PC program for the next half year for developing a
plastic embedded PC powder approaching the goal character-
istics.
?
ST
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(4) No further work for the improvements of the evaporated EL
film is planned. A number of EL film panels are being
delivered by o us, and these panels
probably should satisfy our needs for the construction of
high resolution image intensifier panel. Nevertheless, if
the need arises to resume EL film preparation, it could be
scheduled in could give
some assistance.
(5) A larger effort will be directed toward the construction of
the high resolution image intensifier.
(6) A compact optical projection system, incorporating the image
intensifier panel, will be built.
ST
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5. Reisman, A.-Berkenblit, M.: Impurity Incorporation into CdSe and
equilibria in the System CdSe-CdCI2.
Journ. Electrochem. Soc., vol. 109, p. 1111-1113, 1962
6. Bube,, R. H. et al: Maximizing the Performance of Photoconductors.
Scientific Report No. 2, Report AFCRL-62-158, pages 24-26
7. Vecht A.-Ely B. W.-Apling A.: Purification of CdS by Partial
Sublimation in a Gas Flow.
Journ. Electrochem. Soc., vol. 111, p. 666-668, 1964
8. Bube, R. H.: Cross Section Ratios of Sensitizing Centers in
Photoconductors
J.A.P. vol 32, p. 1707-1709, 1961
9. Bube, R. H.-Lind, E. L.: Photoconductivity of Zinc Selenide
Crystals and a Correlation of Donor and Acceptor Levels in II-VI.
Photoconductors.
Phys. Rev. vol. 110, p. 1040-1049, 1958
10. Kolomiets, B. T.-Chun-ming Lin: Spectral Distribution of the
Internal Photoe4ectrit Effect in the ZnSe-CdSe System.
Soviet Physics - Solid State Physics vol. 2, p. 154-156, 1960
11. Bube R. H.-McCarrol W. H.: Photoconductivity in Indium Sulfide
Powders and Crystals.
Journ. Phys. Chem. of Solids, vol. 10, p. 333-335, 1959
12. Gilles J. M.-Hatwell H.-Offergeed G.-Van Cakenberghe J.: Photo-
conductivity in ln2S3.
Physica Status Solidi vol. 2, p. K73-77, 1962
13. DeVore, H. B.: Gains, Response Times, and Trap Distributions in
Powder Photoconductors.
RCA Rev. vol. 20, p. 79-91, 1959
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14. Rose, A.-Bube, R. H.: The Role of Space-Charge Limited Currents in
Light Amplifiers.
RCA Rev. vol. 20, p. 648-657, 1959
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SECTION VII
APPENDIX A
MATHEMATICAL ANALYSIS OF PC-EL LIGHT AMPLIFIERS
A general formula of the transfer characteristic for the basic
circuit of the PC-EL light amplifier (Figure 1) Is derived here.. The
dark current and the nonlinear current voltage (non-ohmic) dependence
of the PC element, as well as the conductance of the EL element were
considered in this derivation.
The formula, giving the output brightness (B) of the EL cell is
given by
B = B0fO exp - 4 V L22
(1)
where B , A and CX are constants, f Is the frequency and V2 Is the RMS
value o? the applied sinusoidal voltage on the EL cell (the amplitude
of the voltage could be used as well, but the value of A would be V-2 -
times higher in this case).
Fo.r the circuit of Figure 1, V2 can be calculated from the equation
_ ) Z 2 1 V2 - Vo IZl + Z2 (2)
Where Vo is the RMS value (or amplitude) of the applied voltage, Z1
and Z2 are the complex impedances of the PC and EL cells respectively.
If gl and g2 are the conductances and sl = W C1 and s2 = W C2 are
the susceptances of the PC and EL elements respectively
1 91 - Jsi
Zl = g1 + jsl =g12 + 512
1 _ 92-js2
Z2 g2 + js2 92 2 + S 2 2
The absolute values of these impedances are:
Z 112_ 1 2
g12 + S1
(3)
(4)
(5)
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(912 + S 12) (922 + S22)
Substituting equations (6) and (7) in (2), one has
(91 + g2)2 + (S I + S2
Introducing the capacitance ratio of the two cells as
(6)
(7)
(8)
(9)
and making some modifications, the voltage on the EL cell can be
expressed as follows:
V2=Vo
2
91 + 1
WC
P(91 + 92) 2 + (l + p)2
C2
Similarly, the voltage on the' PC cell is given by:
P92 p2
WC2
?o P(91 + 92) 2 2
C + (l + P)
2
and the voltage ratio
V2
VI
I Z Z 12 l9l
1+ =
g22 + S22
a212 1
2
2
p
(10)
(12)
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Equations (10)to 02) contain, besides constants of the circuit
elements (p, 92, C2) and data of driving conditiion (Vo,W) the
conductance of the PC element, which can be expressed as:
gl co (cd + Ln) V1m-1
(13)
where co, cd, n, and m are constants of the PC element, L Is the Illum-
nation and VI is the voltage on the PC element. Substituting equation
(11) in equation (13) one has
91 = f(g1)
(14)
type of equation, which can be solved by the Newton iteration method,
using repeatedly the formula
* 91 - f(91)
91 = 91 1 - f1(91)
WC2 P921 P92 2 2
LI +C 2 + (1+p)
(15)
(16)
Starting with an arbitrary gl and continuing the application of
equation (15), one can generally approach the solution with any precision
desired. Substituting this gl value in equation (10), one obtains the
voltage on the EL element and through equation (1) the output brightness
of the light amplifier for the given input light intensity L.
d log B
j1 = d (log L)
In the case of an ohmic PC element (m It 1) one has:
FAC a2C0Ln La2glg2 (g1 + 92) + p (p + 2) 91 - g2
2 (a 2 g2 + 1)2 3/4
1 T (91)
P91 pg2
f1 (9 ) _- m - 1 1) WC2 evC2
1
The luminous gain X is calculated to
X= L (17)
The contrast ( (gamma) of the image intensifier can be calculated
(18)
(19)
?
0
?
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NAVTRADEVCEN 1440-1
a2 (91 + 92)2 + (1 + p)2
1 (91) = 2 2
a 91 + 1
(20)
If the conductance of the EL element and the dark current of the
PAC'element are negligible (g2 = cd = 0), one has a simpler equation for
a a -
2 2 2 2
np (a + 2) FA a 91 a 91 + 1 (21)
0 2 (a2 gl2 + 1)2 a2 912 + (1 + p)2
Calculati g rmax. of the transfer characteristic, one must solve
the equation ir = 0 for L. Carrying out the calculations, one has the
following quadratic equation for g12:
4a4(912)2 + p (p + 2) a2912 - 4 (p + 1)2 = 0 (22)
By solving this quation for gl and substituting it in equation (21),
one will have max.
With a given PC element fmax can be expressed in a simple form,
max ?to n N Vo
where to depends only on the capacitance ratio.
(23)
45
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NAVTRADEVCEN 1440-1
COMPUTER PROGRAM IN FORTRAN LANGUAGE FOR PC-EL LIGHT AMPLIFIERS
A computer program in FORTRAN language was written for calculating
the transfer characteristic (output brightness as a function of input
illumination) and other data of a light amplifier.
Equations (1), (10), (13), (15), (16) and (17) of Appendix A were
used in the program for calculating the conductance of the PC element
(g1), the voltages on the PC and on the EL element (V1 and Vz), the
output brightness (B) and the luminous gain (X). The FORTRAN program
for the IBM 1401 computer is shown in Figure 3.
The data of the circuit are fed to the computer on four cards.
The first card contains the voltage and the frequency of the power sup ly
(V0,f), the second contains the constants of the PC element (m,n,c,cdJ,
the third card the constants of the El, element (cC ,60,A,C , cost' o).
As it is seen, the power factor (cos c) ) is given here, whbch has more
practical meaning, than the conductance. The connection between the two
quantities is expressed by
60C2 cost
92= 1 m co
(1)
The fourth data card contains the capacitance ratio (p), for which
the calculation has to be done. Generally four p values were used and
data were calculated for 13 Input light Intensities for each capacitance
ratio. A part of the result (half for one set of data), as it was
printed by the computer, is shown in Figure 4. The calculation and
printing of these data required less than two minutes.
The calculation of the contrast could not be included in the same
program because of the limited storage capacity of the IB4 1401 computer.
Therefore a separate program was written for the calculation of the
maximum contrast ( rrx) only. The program was based on equations (21)
and (22) of Appendix A. As it is seen, these equations are limited for
the case where the PC element has linear voltage current dependence
(ohmic characteristic, m = 1), and the dark current of the PC element
and the conductance of the EL element are negligible (cd = 92 = 0).
The work done using these programs is discussed in Section Iii,
and some calculated data in graphical form are shown on Figures 5 to 12.
?
?
?
46
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NAVTRADEVCEN 1440-1
APPENDIX. C
If one supposes, as it is the case with CdS and CdSe, that the
PC current is transported only by one type of carrier and the other
type can be neglected, the conductivity his expressed by the basic
formula
0-= n e ?
(1)
where n is the density of the free carriers, e is the charge of the
electron, and ? is the mobility of the carriers.
If the light creates F electrons per second per unit volume of
the PC material, and the life time of the carriers is the 'carrier
density will be:
n = FT
Substituting (2) in (1), one has for e':
a'= F T e u
(2)
(3)
Multiplying both sides with the thickness t* In which the Incident
light is absorbed (t* is generally smaller than the thickness of the
layer):
t* = F t* ft e ? (4)
Ft* is the number of electLgns created in a unit area and O *t* is the
conductance per square ( 013 ) of the PC layer. If the photoconductive
layer absorbs all the incident photons and the quantum efficiency of
the electron creation in the PC layer is unity, Ft* is the number of
photons falling on the unit area (cm2) of the PC per second: Nph/cm2
and (4) can be written as
Nph/cm 2
(5)
Here the left side is a measure of the sensitivity of the PC layer and
is related to the fundamental characteristics: life time and mobility
of the material.
If one divides equation (5) by e:
e Nph/cm 2
= t Il
(6)
47
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NAVTRADEVCEN 1440-1
Here ea is the number of carriers per square per unit voltage, and
thus equation (6) states that the number of carriers per square per
unit voltage divided by the number of photons per cm2 is equal to the
product of the life time and mobility.
O~A , the conductance per square, has meaning for lateral cells
only, but equation (4) still can be applied to sandwich cells as well.
Another quantity, the so called "specific sensitivity" can be
derived by specifying the geometry of the photocell. For this purpose
one has to discuss separately the lateral and the sandwich type PC cell.
For a lateral cell, where the direction of the light is perpen-
dicular to the direction of the carriers, the conductance is:
V=or dt* (7)
where I is the current, V is the applied voltage and G , d, t* are
the cell dimensions, as shown in Figure 13. From (7) and (3)
1 d
V= F t* d e. ?= F t*- e't u (8)
If thl quantum efficiency of the photon-electron process is unity,
F t* d is the number of photons absorbed in the cell per second.
The power E of the incident light is
E=hI F t* e d
(9)
where h is the Planck constant, c is the speed of light, and N is the
equivalent wavelength of the incident light. Substituting F t* I d
from (9) into (8) one has
S V E2 = h c 0.8 J1. ? (10)
where I is measured in microns.
S is called the "specific sensitivity" of the PC material, and
is obtained by multiplying the photoconductance by the square of the
electrode spacing and dividing by the absorbed radiation power. The
specific sensitivity is an intrinsic property of the material, being
proportional to the carrier life time and mobility.
Similarly to the above, one can derive for a sandwich cell
(Fig. 13) the following equation:
S - I t?_ X e
V E - h c (11)
48
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NAVTRADEVCEN 1440-1
This shows that the "specific sensitivity" is independent of any
geometrical factors.
If the number of primarily created electrons per second is
F t* I d in the photocell, and the number of free carriers per second
is e, the ratio of the two quantity is called the gain of the photocell:
From equation (8), this gain is equal:
to V
G d2
(13)
The gain may also be expressed as the ratio of the life time of the
free carriers to the transit time (Tr) of the carriers
(14)
As the transit time is given by
d2
Tr V
equations (13) and (14) are identical.
(15)
The decay and rise time constants, 't d and 'C r, of the PC cell
are generally longer than the life time. This behavior is caused by
the high number of traps. If the trap density is n and the free r carrier density is n, the decay time constant will be
ntr + n
Ld = n
(16)
If the trapped electrons are in thermal equilibrium with th
free carriers, one can calculate the energy level of the traps asI3:
N
Etr = ke I n -- eV
(17)
where k Is the Boltzmann constant and Nc is the density of the states
in the conduction band.
Another important equation, expressing the maximum available
G Ed M
rre1
(18)
49
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
NAVTRADEVCEN 1440-1
where rel is the relaxation time of the photocell ('trel = R x C)
and M is a factor, depending on the distribution of the recombination
centers.
The theoretically maximum gain of the image intensifier, according
to Rose and Bube14 can be expressed by
2V2uT_f
GL = 5 t2 G
(19)
where E is the efficiency of the EL element. The gain practically
cannot be increased indefinitely with increasing voltage, because of
the appearance of the highly superlinear space-charge-limited currents.
The maximum voltage, applicable to the PC cell is given by
i/l. n 2 f7. M
Vmax ? ,Ud
where f is the frequency of the applied voltage.
(20)
?
?
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
NAVTRADEVCEN 1440-1
Since, practically, there are some capacitances present on arms
1 and 3 of the bridge circuit (Figure 16), it was interesting to see
what is the error in the measurement, introduced by their presence.
Assuming a capacitance Cl and C3 in parallel with Rl and R3
respectively, the balance condition is given by:
?x _ z
Z2 Z1
where Z represents the impedance.
From equation (1):
(1)
1 Z1 (2)
Zx Z2 Z3
Expressing the admittances (Z) with the conductances (g = 1/R) and
susceptances (s =WC)
(g2 + js2) (g3 + js3)
9x+Jsx gl+Jsl
After equating real and imaginary parts, one has:
Rx = (RR + eu2C12R1R2R3)/(1-W2C2C3R2R3+W2C1C3R1R3+UIC1C2R1R2)
1
(3)
Cx = R1\ R2 + R CRRR + w 2C1C2C3R1)/(1 + CV 2R12C12) (4)
3 2 23
Case (A)
If C1 = C3 = 0; the simple equations are obtained
Cx R
(5)
(6)
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
NAVTRADEVCEN 1440-1
Case (B)
Let us assume that Cl and C3 are not equal to zero but have a
value of the order of a few pico-farads and other components have
values shown in Figure 16. With these data, one can see the validity
of the following inequalities
1 > C12 ; C3R3>> C1R1 ; C2R2>> C1R1 (7)
1
and one has the simplified expression for R x :
Rx = R R / (1 - W 2C2C3R2R3)
1
(8)
In the case, where Cx is constant R3/R1 is a constant.
If R3/R1 = K and R) = (A) 2C2C3R3)?1 equation (8) becomes:
R
1/Rx KR (1 - 1) (9)
2 R
If C3 = 1 pF, R2 = 103 ohms and the frequency is 1000 c/s
R 1 (1 - 0.04)
Rx - KR2
Thus the 1 pF capacitance in the third arm causes a 4% error. A
10.pF capacitance would cause a 40% error. It can be seen that if the
value of the unknown capacitance is kept constant, the percentage
error in the value of the unknown resistance increases with increasing
unknown resistance.
Similarly, for the expression of the unknown capacitance
(equation 4) one can consider the following inequalities
1
2
;
3
1
1
6UC
R
R2 3 3
and have the simplified formula
C L6)
Cx _ RI R2 + CR3 22
(10)
?
0
W2C
2
C
R
>> WC1R1
>> C
R
;
,
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NAVTRADEVCEN 1440-1
where
Cx = R1 R2 (1 + R11/R2)
3
C3R3 = R11
If C3 = 1 pF, R2 = 103 ohms and the frequency is 1000 cps
C2
Cx _ R1 R3 (1 + 0.001)
Thus a 0.1% error in capacitance measurement is caused when a I pF
capacitance is in the third arm of the bridge and a 1% error is brought
in by 10 pF. It can be seen that, if the value of the unknown capacitance
is kept constant, the percentage error in its value decreases with
increasing unknown resistance.
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
Bout
?' ?e ???' ? ?e o ? ? ? \ \ ' ' a
?
%???\% ??` ? ?-? ? \, N
`? v %
%
L.in
Transparent u layer
layer
Black layer
layer
Tin oxide coating
Glass
irrage intensifier in Sandwich Constr_,ctio-
?
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0
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NAVTRADEVCEN 1440-1
Figure 2. Basic Circuit of Light Intensifier
55
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NAVTRADEVCEN 1440-1
C IMAGE INTENSIFIER 8
DIMENSION P(4)
11 READ I,V,F,R,S,U,UD,ALF,D,A,C,PF
READ 2, (P(I ), I=1 ,4)
PRINT 3, V,F,R,S,U,UD,ALF,D,A,C,PF
DO 15 1=1,4
PRINT 4, P(I)
PRINT 41
Y2=P(I)*PF/SQRTF(1.-PF*PF)
T=1.E-04
DO 15 J=1,13
G=U*(UD+T* S)*((V/4.1 . ))
12 Y 1=P (I) ,,G / (6.28*F*C )
Y=Y 1+Y2
))
Z=(1.+P(I ))*(1 .+P (I
V1=V'SQRTF((Y2*Y2+P(I)kP(I ))/(Y*Y4Z))
G1=U* (UD+T*>,S )*(Vl l . ))
D I FG=(R-1 . )%;P (I )'G l*Y/ (6.28*F*C* (Y*Y+Z) )
G2=G-(G-G1)/(1.+DIFG)
TEST=ABS F ((G2-G)/G2)
IF(TEST-0.01) 14,14,13
13 G=G2
GO TO 12
14 .Y3=P (I.) kG2/ (6.28*F -C )
VE=V%SQRTF((Y3*Y3+1 . )/ ((Y3+Y2)%', (Y3+Y2)4Z) )
B=D%; (F *ALF) /EXPF (SQRTF (A/VE) )
X=B/T
PRINT 5,T,G2,V1,VE,B,X
15 T=TSQRTF(10.0)
GO TO 11
1 FORMAT (2F7.1/2F4.1,2E10.3/F4.1,4E10.3)
2 FORMAT (4F6.1)
3 FORMAT (1H1//1X,2F7.1/1X,2F4.1,1P2E11.3/1X,OPF4.1,1P4EI1.3)
4 FORMAT (/6H P=,F4.1//)
5 FORMAT (1X,1P6E11.3)
410 FORMAT (67H L-FT.-C. GI-MHOS VI-VOLTS V2-VOLTS R-FT.-L.
1 GAIN
END
Equivalence of Characters:
FORTRAN
V
F
R
S
U I
UD
AL
D
A
C
PF
P
T
G,GI,G2
Vi
VE
B
X
Equations
V
f
m
n
co
cd
of
B
A
C2
Cv$'
p
L
gl I
V,I
V21
BI
X
Figure 3. Source Program in FORTRAN Language for PC-EL
Light Intensifiers.
56
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
NAB .R.:+DE'ICEN p440=i
200.0 2000.0
1.0 0.5 4.000L-05 0.000E-01
1.0 3.000C
00 3.4001
03 1'.0001-09 1.000E-01
L-FT.-C.
G1-MHOS
V1-VOLTS
V2-VOLTS
B-FT.-L.
GAIN
1.000E-04
4.000E-07
1.500E 02
4.998E 01
1.5711 00
1.571E 04
3.162E-04
7.113E-07
1.497E 02
5.0571 01
1.622E 00
5.128E 03
1.000E-03
1.265E-06
1.491E 02
5.1o5E 01
1.7071 00
1.797E 03
3.162E-03
2.249E-06
1.475E 02
5.5115E 01
2.402E 00
7.5951 02
1.000E-02
4.000E-06
1.438E 02
6.597F 01
4.575E 00
4.575E 02
3.162F-02
7.113E-06
1.348F 02
8.816E 01
1.205F 01
3.8121 02
1.000E-01
1 .265E-05
1.100E 02
1.224C 02
3.0851 01
3.085E 02
3.162E-01
2.249E-05
6.6P6E 01
1.574E 02
5.753L 01
1.8101 02
1.000E 00
4.000E-05
5.669E 01
1.8C6F 02
7.833E 01
7.8331 01
3.1621 00
7.113E-05
3.3981 01
1.918E 02
9.904E 01
2.816E 01
1.000F 01
1.265E-04
1.959E 01
1.965E 02
9.363E 01
9.363E 00
3.162E 01
2.249E-04
1.113E 01
1.984E 02
9.554E 01
3.021E 00
1.000E 02
4.000E-04
6.286E 00
1.9')2F 02
9.6371 01
9.637E-01
L-FT.-C.
G1-MHOS
V1-VOLTS
V2-VOLTS
B-FT.-L.
GAIN
1.000E-04
4.000E-07
1.665E
02
3.3515E
01
2.548E-01
2.548E 03
3.162E-04
7.113E-07
1.661E
02
3.435E
01
2.867E-01
9.066E 02
1.000E-03
1.265E-06
1.652E
02
3.6elF
01
4.019E-01
4.019E 02
3.162E-03
2.249E-06
1.631E
02
4.358E
01
8.750E-01
2.767E 02
1.000E-02
4.000E-06
1.581E
02
5.917E
01
3.063E 00
3.063E 02
3.1621-02
7.113E-06
1.464E
02
8.750E
01
1.177E 01
3.723E 02
1.000E-01
1.265E-05
1.231E
02
1.257E
02
3.311E 01
3.311E 02
3.162E-01
2.249E-05
8.974E
01
1.609E
02
6.050E 01
1.913E 02
1.000E 00
4.000E-05
5.747E
01
1.825E
02
8.007E 01
8.007E 01
3.162E 00
7.113E-05
3.414E
01
1.925E
02
8.974E 01
2.838E 01
1.000E 01
1.265E-04
1.963E
01
1.967E
02
9.387E 01
9.387E 00
3.162E 01
2.249E-04
1.1.14E
01
1.985E
02
9.562E 01
3.024E 00
1.000E 02
4.000E-04
6.287E
00
1.992E
02
9.640E 01
9.640E-01
L-FT.-C.
G1-MHOS
Vi-VOLTS
V2-VOLTS
B-FT.-L.
GAIN
1.000E-04
4.000E-07
1.899E
02
1.120E
01
1.630E-04
1.630E 00
3.162E-04
7.113E-07
1.893E
02
1.423E
01
1.163E-03
3.676E 00
1.000E-03
1.265E-06
1.880E
02
2.103E
01
1.805E-02
1.805E 01
3.162E-03
2.249E-06
1.8501
02
3.422E
01
2.815E-01
8.901E 01
1.000E-02
4.000E-06
1.778E
02
5.703E
01
2.660E 00
2.660E 02
3.162E-02
7.113E-06
1.616E
02
9.142E
01
1.348E 01
4.261E 02
1.000E-01
1.265E-05
1.317E
02
1.321E
02
3.760E 01
3.760E 02
3.162E-01
2.249E-05
9.292E
01
1.656E
02
6.464E 01
2.044E 02
10000E 00
4.000E-05
5.828E
01
1.81471
02
8.219E 01
8.2191 01
3.162E 00
7.113E-05
3.431E
01
1.933E
02
9.056E 01
2.864E 01
1.000E 01
1.265E-04
1.966E
01
1.970E
02
9.415E 01
9.415E 00
3.162E 01
2.249E-04
1.114E
01
1.985E
02
9.571E 01,
3.027E 00
1.000E 02
4.000E-04
6.288E
00
1.993E
02
9.643E 01
9.643E-01
Figure 4. Computed Data as Typed by Computer
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NAVTRADEVCEN 1440-1
L ft-c ?-P
Figure 5. Calculated Transfer Characteristics of Light Intensifie,
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NAVTRADEVCEN 1440-1
200 V 1000 cps
1 c 0
3 34x103 10-9 0
f4 x 10-5 A l 10-6 ?4 ~ 10-7
102
A 10-5
x 10J6
k 10-7
10 10-2 10-
Figure 6. Calculated Transfer Characteristics and Gains of Light Intensifiers
having PC Elements with 3 Different Sensitivities
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
NAVTRADEVCEN 1440-1
200 V 1000 cps
1 4x10-5 0
3 3.4x10? 10-9 0
p = 5
10-2 10- ' 1 10 10
L - f t-c 0
Figure 7. Voltage Dependence on PC and EL Elements of a Light Intensifier
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i
?
?
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NAVTRADEVCEN 1440-1
1o2
10-4
200 V 1000 cp
1 n 4 x 10-5
0
1 3 3.4 x 103
10-9 0
p = 10
7T
n =0.5
1
2
/100,
5
?
1
21
10-4
1o-3 10-2 10-
1 - ft-c.
103
102
101
Figure 8. Transfer Characteristics and Gains of Light Intensifiers having
PC Elements with Square Root, Linear and Square Current.vs.
Light Intensity Dependence.
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NAVTRADEVCEN 1440-1
102
200 V 1000 PS
1 1 4x 105cd
1 3 3.4 x
p - 10
03 10-9 10-1
10-3
10-3 10-2 10-1 1 10
Figure 9. Effect of Dark Current of PC Element on Transfer Characteristics
of Light Amplifiers.
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NAVTRADEVCEN 1440-1
1?2 1?I
L- Foot Candles
Figure 10. Effect of Power Factor of EL Element on Transfer
Characteristics of Light Amplifier.
63
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NAVTRADEVCEN 1440-1
1o2
10 1
10-4
200 V 1000 cp
m 1 4 x 10-4
x 10-2m 0
1 3 3.4 x 10
10-9 to-1
p 10
C - 4x1
'12
O
4
10 10
4
10 8
-6
m.4 3 2
1
10-3
10-2
102
Figure 11. Transfer Characteristics of Light Amplifiers with non-ohmic
PC Elements.
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 ?
102
10 10
Figure 12. Gains of Light Amplifiers with non-ohmic PC Elements
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0
200 V 1000 cps
m 1 4 x 10-4 x l
-2m 0
1 3 3.4 x 103 I
V l0 l
p - 10
/4)
3 2 1
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NAVTRADEVCENI 1440-1
Coplanar-cell
1 = 1.27 cm
d = 7.5 x 10-2 cm
a = 1 x d = 10-1 cm2
t = 8.7 x 10-3 cm
1' = 1.6 x 10-1 cm
d' - 1.6 x 10-1 cm
cm
as = 1' x d' 2.5 x 10 2 2
t' = 8.7 x 10-3 cm
Figure 13. Structural Details and Dimensions of PC Cell B2-71XA-1-8.
--OIL d'~
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NAVTRADEVCEN 1440-1
Coplanar Cell
RL = V ohms
Sandwich Cell
(1) RL = ohms
(7L = V It* mhos/cm
Tn = 1 mhos/square -L V
C _ it >t 10-12 F
4 n d
'Erel = R L C sec
G = 1 = 10-13 1
of1 el-a
M_ G1 -re' = 10-25 V t 6
Td 4nd2eL'Cd
mhos/cm
(2) ~L V Id
(3)
(4)
C=a 10-12 F
4nt
(5) Irrel = R L C sec
(6) G = f = 10-13 I
(7)
"L - _ I TT-17e-ti electrons/cm3 sec. (8) nl = 6 = I at electrons/cm3 sec.
e4 V
(9) f1 = 1013 La photons/sec.
F = 1013 t* photons/cm3 sec.
(10) F = 1013 t photons/cm3 sec.
sec.
FL = 10-13 V1Id ?L
_e ?d 13e
"tr = kT "L = 10 TT
L
* traps/cm3 V
Etr = e In c e Volts n L
(11) ~= FL = Vae?2L sec.
(12)
e I e a
M = G 'rrel 10-25 V T -I = 4nteL d
_e 'td 13 e
"t r = TT "L 7 = 10 TT
L
d 3
(13) Etr = -LT in is e Volts e L
Figure 14. Formulas for Calculating Fundamental Characteristics of
PC Cells from Voltage, Current,'Light Intensity and Time
Constant Measurements.
67
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f1 = 1013 La photons/sec.
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NAVTRADEVCEN 1440-1
RL = ohms (11) RL = V ohms
Or = 5.9 x 10 V mhos/cm (2') O = 3.5 x 10-1 1 mhos
= 5.9 x 10-2 V mhos/square
C = 1.2 x 10-13 F
(3')
(4') C = 2.3 *x 10-12 F
sec.
~rel = 1.2 x 10-13 sec. (5') rel = 3.2 x 10-12 1 V
G=6.6x 1061
M = 8 x 10-7 V
-
(6') G = 2.5 x 107 L
(7')
nl = 3.7x1020 1 electrons/cm3 sec.(8')
fl = 1012 L photons/sec.
F = 1016 L photons/cm3 sec.
t = 3.7 x 104 V? sec.
ntr = 3.89x10 nL =
3.89 x 1017 L Td traps/cm3 V
19
Etr = 5.8x10-2 log 10 e Volts
L
(9')
(10')
(11')
M = 5.7 x 10-5 L-V -
nL = 2.18x1018 1 electrons/cm3 sec.
fl = 2.5 x 1011Lphotons/sec.
F = 1.15x1015 L photons/cm3 sec.
= 1.9 x 103--L sec.
?
ntr = 3.89x10 n1Td =
( 1 2 ' ) 4.5 x 1016 L T d traps/cm3 V
19
(13') Etr = 5.8x10-2 log 10 e Volts
L
Figure 15. Same Formulas as in Figure 14 with Numerical
Values of PC Cell of Figure 13 Substituted.
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 ?
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50 Mn
WAN
Oscilloscope
RU= 0 - I03ohms
R2=0- IO4ohms
R3=106 ohms
C2aIpF
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Figure 16. Bridge Circuit
0
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NAVTRADEVCEN 1440-1
?o
b.
Unbalanced
Balanced
Figure 17. Scope Patterns in Bridge Measurements
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NAVTRADEVCEN 1440-1
P. C. Layer
Substrate
1-2 - Electrodes for Primary Current
3-4 - Electrodes for Hall Effect Measurement
4-5 - Electrodes for Conductivity. Measurement
Magnetic Field Perpendicular to the Plane
of the Layer.
Figure 18. Electrode Structure of a Hall-Sample
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(SI)
(S3)
(S2)
Z:5 elded Box -
Figure 19. CIRCUIT FOR HALL EFFECT MEASUREMENTS
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Volt
Meter
0
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NAVTRADEVCEN 14 0-1
103
104
L'IOFI.C.
II?
00
I0
Io2
Figure 20. Current-Voltage Curves of a Sintered CdSe Cell (No. B2_71XA_1-8)
with Coplanar Electrodes.
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NAV TRADEVCEN 1440-1
00
V -Volts
I0
V- Volts ------- ~
Figure 21. Current-Voltage Curves of Sintered CdSe Cell (No. B2-71XA-1-8)
in Sandwich Construction.
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NAVTRADEVCEN 1440-1
Sandwich +SO V
Coplanar 50V
Sandwich -10 j
00, ole
i.00 /
1 00,
ar IO V
Copian
4
/',Sandwich +IOV
e
/
.0000,
/
1000,
i0
L - Foot Candles ---
Figure 22. Gain vs. Light Intensity of a Sintered CdSe Cell (No. B2-71XA-1-8)
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NAVTRADEVCEN 1440-1
10-2 10-1
Figure 23. M-Factors of a Sintered CdSe Cell (No. B2-71XA-1-8)
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NAVTRADEVCEN 1440-1
7
101
101
30 cm2/V
sec
2
Coplanar
ell, p = 1
/Vsec
cm
Coplana
r sell
u = 10
m /Vsec -~
\
N
~
?
~
~1R
San
ich cell
top -
?
asurements
ade at 10
Sa
ndwich cell
x
of
50
top +
101
Figure 24. Trap Distribution in a Sintered CdSe Cell (No. B2-71XA-1-8)
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NAVTRADEVCEN 14401
10
Figure 25. Characteristics of an Acetic Acid Washed Sintered CdSe Cell
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NAVTRADEVCEN 1440-1
103
102
_Inn -rn
Temperature -C 11
Figure 26. Thermally Stimulated Current of a Sintered CdSe Coplanar
Cell. Rate of Temperature Change: 27.60C/min.
I I' I I I 1 1 1 1 I 1 I 1 1 1
0 50
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
100r.
0L
0.1
30\ 125
BI
1.0 10
RUNNING TIME AT 1000 CPS (HOURS)
Figure 27. Life Test Measurements on EL Films at AC Voltage
100
0 Approved For Release 2005/05/OW IA-RDP78BO477OA002200060010-2 0
 ?
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0
0.1
. 1.0
TIME (HOURS)
Figure 28. Life Test Measurements on EL Films at DC Voltage
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0
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2
TIME (HOURS)
Figure 29. Life Test Measurements on EL Films at DC Voltage
with Constant Current?
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82
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NAVTRADEVCEN 1440-1
VOLTAGE V MAINTAINED
U)
U)
W
z
1.0
W O.8V
co
Q
0 0.6V
C' 04V
J
a
a
a O.2V
20 40 60 80 100 120 140
TIME (HOURS)
^ . a ,
20 40 60 80 100 120 140
TI (HOURS)
Figure 30. Life Test Measurements on EL Films with Constant Brightness
(a)
15 FOOT LAMSERTS MAINTAINED
83
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NAVTRADEVCEN 1440-1
102
103
4 L
1? 1?3
1?a . 10 1
L Foot C?otd1?i?~.
Figure 31. Calculated and Experimental Transfer Characteristic of a Light
Amplifier Element.
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NAVTRADEVCEN 1440-1
?
?
103
102
I0
'c
0
Co
lol
102
lol
L- Foot Candles
Figure 32. Calculated and Measured Gains of the Light Amplifier Element
of Figure 31.
85
0
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102
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Image Intensifier Panel 61339-03-4
L - foot-candles
Figure 33. Transfer Characteristics of Image Intensifier No. 221
86
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NAVXRADEVCEN 1440-1
0
?
Figure 34. Intensified Image of a Projected Slide Picture
on Image Intensifier No. 252, V = 200 Volts,
f = 2000 cps, L = 1 ft-c, B = 8 ft-L.
S
87
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
Brightness - Measure of visual sensation of luminous intensity. It is
often used instead of luminance or luminous emittance.
Brightness gain - The ratio of the luminance of a selected portion of
the radiating area in foot-Lamberts to the illumination of
the corresponding irradiated input area measured in foot-
candles. The input spectrum, the output phosphor and the
input or output level must be specified.
Discrimination ratio - The brightness ratio of an EL lamp at a given
voltage to the brightness at the j-th part of the voltage
(j = 2,3,4...) and designated by Rj. The commonly used
discrimination ratio is R2 (j = 2). The discrimination
ratio is dependent on the brightness and frequency.
Efficiency - Measure of power conversion of a light source, given
generally in units of lumens/watt. A light source
converting completely the input power to light power would
have an efficiency of about 680 lm/w, if radiating at a
wavelength of 5550 A.
Electroluminescence (EL) - Luminescence excited by an electric field
or current.
Foot-candle (ft-cl - Unit of illumination. Equal to the luminous flux
(in lumens) received on 1 square foot area of the illuminated
surface.
Foot-Lambert (ft-L) - Unit of brightness (more correctly luminous
emittance) of a radiating source. Equal to the luminous
flux (in lumens) emitted by 1 square foot area of the
light source.
FORTRAN (FOR-mula TRAN-slation) - Symbolic language for writing mathe-
matical or engineering programs for digital computers.
Gain - Measure of amplification in (1) photocells and (2) Image inten-
sifier panels.
(1) The gain of a photocell is equal to the ratio of
the number of the free carriers per second to the
number of the primarily created electrons per
second.
(2) The gain of the image Intensifier is the ratio of
the output to the input light intensity. Depending
on the units and on the input illumination source
used, one defines the following gains: luminous
gain, brightness gain, radiant power gain, standard
luminous gain, standard brightness gain, and
?
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
NAVTRADEVCEN 1440-1
Optical Train - A compact optical system, for the demonstration of image
intensifier panels. It contains: (1) a given projection
system (as simple projection of transparencies or opaque
images, projection of cathode ray tube displays, superposed
projection of two separate Images, and two stage cascaded
projections), (2) power supplies and (3) image. intensifier
panels.
Optical Bench - Equipment for enabling precise positioning of lenses
and other optical instruments, providing great versatility
for the realization of any optical system.
Peak Radiant Power Gain - The same as radiant power gain, when the input
flux Is essentially a monchromatic radiation at the wave-
length yielding maximum radiant power gain.
Photoluminescence - Luminescence excited by electromagnetic radiation
as ultraviolet or visible light, and x-rays.
Radiant Power Gain - The ratio of the total radiant output flux in
absolute units (e.g. watts) to the corresponding input
flux measured in the same units. The spectral distribution
of the input and output flux, and the input or output level
must be specified.
Resolution - Measure of the optical quality of an image, regarding the
separation of close elements. Its units are: (1) optical
lines per Inch: (2) line-pairs per inch; (3) TV lines per
inch, and (4) elements or cells per inch. Units (3) and
(4) differ by a factor of two from units (1) and (2). e.g.:
a resolution of 50 optical lines (or simply optical
resolution of 50 lines) per inch, 50 line-pairs per inch,
100 TV lines per inch and 100 elements or cells per inch
are equivalent statements.
Spectral Distribution (of emission) - The power distribution of emitted
light as a function of the wavelength. Given generally in
relative units, with the maximum value as 100%.
Spectral Response (of photoresponsive elements) - Dependence of photo-
sensitivity (or photocurrent) on wavelength of illuminating
light. Measured generally by recording the current of the
photocell, when irradiating it with a constant power of
monochromatic light of changing wavelengths. If the dark
current Is not neglibible, its value Is substracted from
the measured currents. The spectral response curve is
given generally in relative units, the maximum of the curve
being set to 100%.
If the "slope of the current-light intensity curve is not
constant, the spectral response curve depends on the
intensity of the monochromatic illumination. To eliminate
this variable, another measuring method can be used
0
?
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 Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
NAVTRADE.VCEN 1400-1
peak radiant power gain (see under these headings).
In the case of the PC-EL Image Intensifier, where
the sizes of the input image and the output image
are the same, the luminous and brightness gains
are the same.
Gamma - Measure of the: image contrast of an intensifier panel.
It is equal to the slope of the trap fer har teristics
drawn on double logarithmic paper: ' --og (where
L Is the input light intensity, B is the output brightness).
Illumination - Density of the luminous flux on an illuminated surface.
Its units are the foot-candle (lumens per suare foot),
lux (lumens per m2) and phot (lumens per cm2) 1- ft-c =
10.764 lux = 1.0764 m phot (milliphot = 10-3 phot).
Image Intensifier or Image Amplifier - Device for increasing the bright-
ness of an optical image. Two varieties of such image
intensifiers are under development: (1) vacuum tube type,
using photoelectric emission, electron optics and cathodo-
luminescence; (2) solid state type, using PC. and EL
materials. The solid state type is much simpler, more
compact, and less expensive to fabricate than the vacuum
tube type, but has lower speed and lower picture quality
at the present state of the art.
Light Intensifier or Light Amplifier -.Device for increasing light
intensity. One element of an image intensifier.
Lumen (1m1 - Unit of luminous flux. It is equal to the flux through
a unit solid angle (steradian) from a uniform point source
of 1 candle.
Luminance - Light intensity per unit of projected area of source. Its
units are: candle/cm2, candle/ft etc.
Luminescence - Phenomenon of light emission caused by any other effect
than high temperature (thermal radiation), as electro-
luminescence (EL), photoluminescence (PL), bioluminescence,
cathodoluminescence etc.
Luminous Emittance - Luminous flux emitted per unit area. Its units
are: lumen/cm2 (Lambert), lumen/ft2 (foot-Lambert).
Luminous Gain - The ratio of the total output flux In luminous units
to the corresponding input flux in the same units. The
input spectrum, the output phosphor and the Input or
output flux level must be specified.
Luminous Flux - The total visible energy emitted by a source per unit
time. Its unit is the lumen.
89
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NAVTRADEVCEN 1440-1
0
?
0
preferably: the power of the monochromatic light Is
adjusted so that a constant photocell current Is obtained
when changing the wavelength. The reciprocal value of
this illuminating power as a function of wavelength will
give the spectral response curve.
Standard Briahtness Gain,- Same as brightness gain, If the Input
illumination source is a 2870 K color temperature tungsten
lamp.
Standard Lumiligug - Same as luminous gain, if the input flux Is a
2570 K color temperature tungsten lamp radiation.
(g-r-funition) - Where the absolute value of the slope
first derivative) is decreasing with increasing abscissa
independent variable). If the function is of the form
y ? c xn, it is sublinear when n < 1.
uoerlinear or Suoraline-r-Curve for function) - Where the absolute
value of the slope (first derivative) Is increasing with
increasing abscissa (independent variable). If the
function is described by the equation: y ? c xn, it is
superlinear when n ' 1.
J= - Unit of pressure, equal to 1 mm Hg.
Transfer Characteristic - Curve, representing the output brightness as
a function of the input light Intensity of the Image
intensifier. Drawn generally on double logarithmic paper.
91
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WAVTRADEVCEN 1440-1
Abbreviations and symbols used in,report:
A - constant of EL cell
A - Angstrom = 10-8 cm
AC - alternative current
oC - exponent in formulas
B - brightness in foot-Lamberts
Bo - constant of EL cells
co - sensitivity constant of PC cell
cd - dark current constant of PC cell
C - capacitance
Cl,C2 - capacitances of PC and EL cells respectively
CdS - cadmium sulfide
CdSe - cadmium selenide
d - width of PC cell
DC - direct current
e - charge of electron
EL - electroluminescent, electroluminescence
exp a - ea, where e = 2.7182, base of natural logarithm
E - dielectric constant
f - frequency in cps
fl - number of incident photons per second per unit area
F - number of incident photons per second per unit volume
ft-c - foot-candle
ft-L - foot-Lambert
g - conductance
G - gain
r - measure of contrast
h - Planck constant
1 - current
I - electrode distance
L - light intensity in foot-candles
A. - wavelength
m - exponent in formulas
M - constant of PC material
M - megohm - 106 ohm
? - micro- = 10-6 or micron - 10-6 m or mobility of carriers
n - exponent in formulas or density of carriers
N - number of carriers or photons
p - capacitance ratio
PC - photoconductive, photoconductor
RMS - root mean square
R - resistance
Rl - resistance of PC cell
4 - specific resistivity
s - susceptance
5' - conductivity
(T? - conductance of a square of any dimension
t - thickness of layer
t* - light abosrption thickness
l` - time constant or life time
Trr,- rise and decay time constants
92
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NAVTRADEVCEN 1440-1
trel - relaxation time: constant
V - Volts, voltage
Vo,Vl,V2 - Voltages (RMS) on image intensifier, on PC layer and on
EL layer respectively
W - Watts, power
xl,x2 - reactances of PC and EL cells respectively
X - luminous gain of image intensifier
Z - impedance
co - circular frequency (2 1. f)
.n. - ohm
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 Unclassified
Security Cla st or a ease 2005/05/02 : CIA-RDP78BO477OA002200060010-2
Cadmium Sulfide
Cadmium Selenide
Electroluminescence
Fortran
Image Intensifier
Photoconductivity
Sandwich Construction
Selenides
Solid State
Sulfides
Thin Films
Zinc Sulfide
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rr
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Services, Department of Commerce, for sale to the public, indi-
cate this fact and enter the price, if known.
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tory notes.
12. SPONSORING MILITARY ACTIVITY: Enter the name of
the departmental project office or laboratory sponsoring (pa)-
in for) the research and development. Include address.
43) ABSTRACT: Enter an abstract giving a brief arld factual
summary of the document indicative of the report, .even though
it may also appear elsewhere in the body of the technical re-
port. If additional space is required, a continuation sheet shall
be attached.
It is highly desirable that the abstract of classified reports
be unclassified. Each paragraph of the abstract shall end with
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formation in the paragraph, represented as (TS), (S), (C), or (U).
There is no limitation on the length of the abstract. How-
ever, the suggested length is from 150 to 225 words.
14. KEY WORDS: Key words are technically meaningful terms
or short phrases that characterize a report and may be used as
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Unclassified
Security Classification
?
0
?
Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
 Uncl> it @jd For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
AT
AT
DOCUMENT CONTROL DATA - R&D
(Security classification of title, body of abstract and' indexing annotation must be entered when the overall report is classified)
2a. REPORT SECURITY CLASSIFICATION
Unclassified _
2b. GROUP
3. REPORT TITLE
Solid State Image Intensifiers
4. DESCRIPTIVE NOTES (Type of report and inclusive dates)
Annual Report - Phase III June 26, 1963 to June 26, 1964
5. AUTHOR(S) (Lest name, first name, initial)
6. REPORT DATE
7a. TOTAL NO. OF PAGES
7b. NO. OF REFS
14
March 1965
99
NO.
9a. ORIGINATOR'S REPORT NUMBER(S)
b. PROJECT NO.
9b. OTHER REPORT NO(S) (Any other numbers that may be assigned
this report)
NAVTRADEVCEN 1440-1
d.
10. AVAIL ABILITY/LIMITATION NOTICES
No Limitation
11. SUPPLEMENTARY NOTES
12. SPONSORING MILITARY ACTIVITY
U. S.-Naval Training Device Center
Port Washington, N. Y.
13. ABSTRACT
Technical Report NAVTRADEVCEN 1440-1 describes the continuation of the work.
performed under contract N61339-562 and reported in technical report NAVTRADEVCEN
562-1. The objectives of the program were: (1) to improve the characteristics
of the low resolution (40 lines/inch) photoconductive-electroluminescent (PC-EL)
type image intensifier panel; (2.) to construct a high resolution (1000 lines/
inch) PC-EL type image intensifier; (3) to build an experimental model Compact
optical projection system, incorporating the image intensifier panel.
Panels with the best uniformity, least graininess and widest half-tone
range eproducing capability had maximum standard luminous gains (measured with
0
K tungsten light source) of 20 foot-Lambert per foot-candle and maximum
a 2870
output brightnesses of 30 foot-Lamberts at 200 volts and 2000 cps. Response
times between 50 and 500 mseconds were measured.
Work is being continued toward improving the speed, spectral response,
gain, and life characteristics of PC layers, developing the high resolution
image intensifier panel, and building a compact optical projection system with
the improved, image intensifier panels.
FORM
1 J AN 84 1473 Unclassified _
Security Classification
Approved For Release 2005/05/02 : CIA-RDP78BO477OA002200060010-2
 AT U. S. NAVAL TRAINING DEVICE CENTER
March 1965
Annual Rept. NAVTRADEVCEN 1440-1 UNC.
Approved For Release 2005/05/OSIA-RDP78B04770A002200060010-2
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The present Annual Report describes the contiii-
uation of the work performed under contract
'lnd reported in Final Report
562-1. The objectives of the pru