Infrared Photoconductivity in Thin Silver Sulfide Films

91 Pages·2016·2.42 MB·English

by  Paul Joseph Aceto

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Open Access Master's Theses
1976
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Paul Joseph Aceto
University of Rhode Island
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INFRARED PHOTOCONDUCTIVITY IN
THIN SILVER SULFIDE FILMS
BY
PAUL JOSEPH ACETO
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
IN
ELECTRICAL ENGINEERING
UNIVERSITY OF RHODE ISLAND
1976
MASTER OF SCIENCE THESIS
OF
PAUL JOSEPH ACETO
Approved:
Thesis Committee
Dean of the Graduate School
UNIVERSITY OF RHODE ISLAND
1976
ABSTRACT
A study of thermally generated conductivity and photo
conductivity in thin layers of silver sulfide is presented.
Silver layers were vacuum deposited on sapphire sub-
strates and transformed into silver sulfide in a sulfur
atmosphere at 120° C. The dark current of single element
sensors was investigated as a function of temperature and
photocurrent was measured as a function of photon flux and
wavelength. The samples were photosensitive to radiation
in the spectral range 0.7 µm to 1 .8 µm. A maximum photo
response occured at a wavelength of 0.8 µm. The photo-
current in general increased with decreasing temperature.
The absorption edge for Ag s at 90 K was found to be at
2
.o
1.2 ~m making the calculated energy gap 1 eV. From dark
current measurements, recombination centers were found at
0.66 eV below the conduction band.
Vidicon type targets made from silver sulfide layers
were also investigated and showed an image fade-out effect
with a time constant in the order of seconds. Target re
solution was found to exceed 10 lp/mm and the targets
2
responded to incident photon flux levels of 30 µW/cm •
i
TABLE OF CONTENTS
ABSTRACT i
TABLE OF CONTENTS ii
LIST OF FIGURES iii
LIST OF SYMBOLS iv
ACKNOWLEDGMENT vi
INTRODUCTION 1
I. THEORETICAL 3
II. EXPERIMENTAL 13
III. RESULTS 36
IV. DISCUSSION 49
APPENDIX 60
BIBLIOGRAPHY 81
ii
LIST OF FIGURES
1. Elemental Sample Geometries 14
2. Transformation Furnace 16
3. Ag s after Transformation 18
2
4. Photomicrograph of an Island Target 20
5. Cryostat Interior 21
6. Circuit Configuration for Elemental Samples 24
7. I-V Phasor Diagrams 27
8. Vidicon Schematic 29
9. Silver Tracks in Thin Ag s Layers 34-35
2
10. Dark Current Versus Voltage 39
11 • Dark Current Versus Temperature 40
12. Photoresponse Versus Photon Flux 41
13. Spectral Response of Ag s 42
2
14. Photocurrent Versus Infrared Flood Time 44
15. Change in Photocurrent Versus Infrared Flood Time 46
1 6. Image using Infrared Irradiation 47
1 7. Comparison of Spectral Responses 54
1 8. Energy Band Diagram of Ag s 56
2
i_ii
LIST OF SYMBOLS
8 -1
c - Speed of light in a vacuum, 3 x 10 m-s
E - Energy, eV
- Energy of bottom of conduction band, eV
- Fermi level, eV
- Band gap, eV
- Energy difference from shallow traps to the
conduction band, eV
- Energy difference from recombination centers
to conduction band, eV
E - Energy of top of valence band, eV
v
e - Electronic charge, 1 .6 x 1 o-19 c
F - Photoelectron generation rate, cm-3-s -1
-2 -1
H Photon flux density, cm -s
- 34
h Planck's constant, 6.63 x 10- J-s
I - Current, A
- Photocurrent, A
2
- Current density, A-cm-
2
- Boltzmann's constant, 1 .38 x 10- 3 J-K-l
N - Effective density of states in the conduction
c
band, cm-3
N - Density of recombination centers, cm-3
0
- Density of shallow traps, cm-3
Effective density of states in the valence band, cm-3
n
Density of electrons in the conduction band, cm-3
iv
- Density of occupied recombination centers, cm-3
- Density of occupied shallow traps, cm-3
- Density of unoccupied recombination centers, cm-3
2
- Capture cross section for electrons, cm
T - Temperature, K
t - Time, s
1
v - Electronic drift velocity, m-s-
Tl - Per cent of photons absorbed
A - Wavelength, m
T - Electronic lifetime, s
p - Resistivity, 0-cm
v
ACKNOWLEDGMENT
I would like to thank Dr. S. Mardix for the guidance
he has given me during the course of this research and for
the many enlightening discussions which we have had. His
keen insight and good humor have helped me through many
periods of difficulty.
I would also like to thank Dr. H. Roehrig and Dr. S.
Nudelman for introducing me to this field.
In addition, I wish to thank the United States Army
Electroni cs Command, which provided part of the funds to
support this project under contract number DAAB0?- 69- C- 0420.
Finall y, I which to thank Mrs . Joan Lamoureux for
typing a beautiful manuscript.
vi
1
INTRODUCTION
The purpose of this thesis is to describe the results
of an investigation of infrared photoconductivity in thin
(in the order of 1 µm) silver sulfide films . Infrared
sensitive materials may be useful as both single element
IR detectors and scanned, two dimensional IR detectors .
Materials with a high resistivity are likely candidates
for investigation of photoconductivity. This is true be-
cause high resistivity materials have a low dark current,
hence low background. This low background allows for a
high signal to background ratio. In addition, conductivity
changes due to absorption of photons will not appreciably
affect a material which already has a high conductivity.
Materials with an absorption edge in the infrared are like-
ly to be photosensitive in the IR.
Silver sulfide (Ag S) was known as a photoconductor as
2
1
early as 1922 . There are two forms of Ag S: a-Ag S and
2 2
s
~-Ag2S. In the alpha form, Ag has a body centered cubic
2
2
arrangement of sulfur atoms and a relatively low electrical
resistivity (103 ohm-cm at room temperature)3. ~-Ag2s has
2
a monoclinic structure and a much higher resistivity (105
ohm-cm at room temperature)3. Some research has been per
formed on Ag S and it was found to have an absorption edge
2
in the IR4,5 . s· ~
ince has a higher resistivity and
~-Ag2S