Calhoun: The NPS Institutional Archive
Theses and Dissertations
Thesis and Dissertation Collection
1973
Study of Pb [Subscript 1-x]Sn [Subscript x]Te and
Pb [Subscript 1-y]Sn [Subscript y]Se photodetectors.
Romsos, Arden Edwin.
Monterey, California. Naval Postgraduate School
http://hdl.handle.net/10945/16829
STUDY OF Pb.
Sn Te AND Pb.
Sn Se
1-x x
1-y y
PHOTODETECTORS
Arden Edwin Romsos
Monterey,
CaMorma 93940
u
I
L
\
otiierey, California
n^a^
* -z-
i
STUDY OF
Pb^ X3n XTe
and Pb.
Sn Se PHOTODETECTOIJS
y
-y
*>y
Arden Edwin Ronsos
Thesis Advisori
T. P. Tao
June 1973
Appslovnd
fan.
pubtic.
n.dLojOLi><i;
T157006
diAtnJJbutLon unfjjnit&d.
Sn Se Photcdetectors
Sn Te and Pb„
STUDY OF Pb,
1-x x
1-y y
ty
Arden Edwin ^Romsos
Captain, United States Marine Corps
B.Aero.E,, University of Minnesota, 19&5
Submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN ELECTRICAL ENGINEERING
from the
NAVAL POSTGRADUATE SCHOOL
/7)<r
Library
Mava! Postgraduate
Monterey, Calitorn.a
S
ABSTRACT
Two photodetection studies of the narrow gap IV-VI alloy semiconductors
were made.
In the first study, the photoconductive response of Pb Q qSn
and
Thin film samples were deposited on
Pb~ zroSn n opSe were investigated.
cleaved GaF^ and BaF ? substrates
..Se
"by
the one "boat evaporation method and
isothermally annealed to enhance their photoconductive sensitivities.
In agreement with
Photoresponse was obtained in Pb fi Q Sn n .Se samples.
previously reported results
"by
W. McBride and K. Holmquist of this
labo-
ratory,
(HI) oriented films were much less sensitive than (100) oriented
films.
No photoconductive response has been developed in Pb- /qSn- ~ ? Se.
In the second study, single hetero junction Pb.
cated by sequential depositions of p type Pb o^Sn.
n
Pb
Q
gSn
Q 2
Te were studied.
,
Sn Te diodes fabri-,
?
Te and n type
Their electrical properties were studied by
LCDR G. L. Smith in a companion thesis.
Operated at 90°K, 500°K black-
body responsivities up to 0.19 volt/watt have been obtained.
It is be-
lieved that this is the first time photovoltaic sensitivity has been found
in Pb 1
Sn Te hetero junctions ,
Noise measurements were also made.
It
was found that noise voltages in our photovoltaic diodes are much lower
than in photoconductive samples.
TABLE OF CONTENTS
I.
INTRODUCTION
II.
PHOTODETECTORS
A.
B.
9
H
13
THE PHOTOCONDUCTIVE DETECTOR
13
1.
Current Mechanisms
2.
Excess Carrier Lifetime
3.
Photoconductive Gain
...............
1^
.
1^
GENERAL THEORY OF PHOTOVOLTAIC DETECTORS
1.
Homo junction Photovoltaic Detectors
2,
Hetero junctions
.15
.
..........
19
C.
NOISE IN INFRARED PH0T0DETECT0R3
21
D.
FIGURES OF MERIT FOR INFRARED PHOTODETECTORS
22
E.
F.
.22
........
Parameters ........... ^
1.
Standard Radiation Source
2.
Detector Performance
3.
Spectral Response
..................
PRESENT DAY STATUS OF Pb-Sn CHALCOGENIDE PHOTODETECTORS
gSn. _Se Photoconductive Detectors
1.
Thin Film Pb
2,
Photovoltaic Detectors
Q
.
.
.
.
.
........
Sn Se ......
Pb.
1-y y
......
a.
Bulk Crystal
Pb.
Sn Te and
J
1-x x
b.
Thin Film Pb-Te Schottky Barrier Photovoltaic
Detectors
t
25
26
2^
2"
2^
27
27
SCOPE OF THIS STUDY
1.
Photoconductive Fb 4
Sn Se
i-y y
2,
Pb.
Sn Te Single Heterojunction Photovoltaic
l -x
x
Detectors, , .
.
.
.
.
...............
III.
13
27
on
SUMMARY OF PREPARATION, METALLURGICAL EVALUATION, AND ELECTRICAL
MEASUREMENTS OF Pb.
Sn Se THIN FILMS AND Pb
Sn Se SINGLE
j
*
J
7*.
HETEROJUNCTICNS
7. V
?
.
.
.
7
,
.
A,
B.
IV.
.............
1.
Sample Preparation
2.
Metallurgical Evaluation
3.
Electrical Measurements
4.
Isothermal Annealing
B.
.30
31
..........
.................
,
.31
.32
32
Sn Te SINGLE HETEROJUNCTIONS
1-x x
Pb.
...................
1.
Source Materials
2.
Material Preparation
3.
Diode Preparation
.32
.33
......
EXPERIMENTAL SETUP FOR MEASUREMENT OF BLACKBODY RESPONSE
A.
.
.
.
.33
.
.
.
3^
3^
DESCRIPTION OF EQUIPMENT
1
Vacuum Dewar
3^
2,
Bias Circuitry
37
3,
Radiation Source and Modulation Equipment
kt
Signal Measuring Equipment
........ 39
39
........
^2
MEASUREMENT PROCEDURE
1
Preparation
.......................
Photoconductive Detectors
a,
.....
.
.
^"2
.^2
............... ^2
Noise Measurements ................... ^3
2.
^3
Signal Measurements ............
3.
k.
Detector Performance Calculations ............ ^
STUDY OF PHOTOCONDUCTIVITY ............
.^5
Photovoltaic Detectors
b.
V.
30
3n Se THIN FILMS
i-y y
Pb.
A.
^5
RESPONSE MEASUREMENTS
1.
Pb
2'
F°0.68 Sn 0.32Se
Q
Sn
^5
^Se Samples
Samples
'
^
B.
VI.
VII.
50
NOISE MEASUREMENTS
53
STUDY OF PHOTOVOLTAIC RESPONSE
A.
I-V EVALUATION
53
B.
NOISE MEASUREMENTS
58
G.
PHOTORESPONSE MEASUREMENTS
58
63
CONCLUSIONS AND RECOMMENDATIONS
A.
B.
CONCLUSIONS
.
63
."3
1,
Pb<
Sn Se Photoconductors
2,
Pb i
Sn Te Single Hetero junction Photovoltaic Diodes .^3
RECOMMENDATIONS
.
Pb.
Sn Se
y
2.
Pb 1
Sn Te Single Hetero junction Photovoltaic Diodes
LIST OF REFERENCES
.
................
.^
.
65
6?
INITIAL DISTRIBUTION LIST
FORM DD 1^73
6^
63
1.
1-y
,
«
•
•
•
«
68
.
LIST OF FIGURES
1.
PHOTOSXCITATION MECHANISMS IN TERMS OF BAND DIAGRAM
12
2.
PHOTOSXCITATION MECHANISMS IN TERMS OF THE ABSORPTION
COEFFICIENT a Aim WAVELENGTH X
12
3.
CARRIER EXCITATION AT A P-N JUNCTION
16
k.
RECTIFICATION CHARACTERISTICS OF A PHOTOVOLTAIC DIODE UNDER
DARK (1) AND ILLUMINATED (2) CONDITIONS
16
5.
PHOTOVOLTAGE SENSITIVITY PROFILE HEAR A P-N JUNCTION
18
6.
ItETEROJUNCTION ENERGY BAND DIAGRAM
18
7.
GENERALIZED PHOTODETECTOR NOISE SPECTRUM
23
8.
TYPICAL SPECTRAL RESPONSE FOR AN INTRINSIC PHOTODETECTOR
9.
ENERGY GAP OF Vb, Sn Se AS A FUNCTION OF y, THE MOLE FRACTION
1-y y
OF SnSe AT 77 K
10.
...
.23
"
ENERGY GAP OF
P^_
.28
Sn Te AS A FUNCTION OF x, THE MOLE FRACTION
28
OF SnTe AT ?7°K
11.
BLOCK DIAGRAM OF THE BLACKBODY RESPOISE MEASUREMENT SYSTEM
12.
SECTIONED DRAWING OF THE DEWAR
13.
SCHEMATIC DIAGRAM OF DETECTOR BIAS CIRCUITRY
38
Ik.
SCHEMATIC DIAGRAM OF CHOPPER FREQUENCY REFERENCE GENERATION
CIRCUIT
40
15.
NOISE SPECTRUM FOR Pb
Se SAMPLES
51
16.
NOISE SPECTRUM FOR Pb
So SAMPLES
52
17.
RECTIFICATION CURVES FOR DIODES AT 97°K
18.
RECTIFICATION CURVES FOR DIODS C-3^ WITH DIFFERENT SCALING
19.
RECTIFICATION CURVES FOR DIODES AT 95°K
5^>
20.
RECTIFICATION CURVES FOR DIODE D-65 AT TWO TEMPERATURES
57
21
NOISE SPECTRUM FOR C SERIES DIODES
59
22.
NOISE SPECTRUM FOR D SERIES DIODES
.60
Q
Q 6Q
Sn
Sn
Q 1Q
32
.
.
.35
36
.
5^
.
.
.
55
LIST OP TABLES
I.
SUM14ARY OF METALLURGICAL AHD ELEGTHIGAL PROPERTIES OF
Pb
0.9
Sn
0.1
Se
'
'
•
Sn
II.
SUMMARY OF PHOTOCONDUCTIVS PROPERTIES OF Fb
III.
SUMMARY OF METALLURGICAL AND ELECTRICAL PROPERTIES OF
Se
Pt
Sn
0.68 0.32
IV.
W
'
Q 9
SUMMARY OF FIGURES OF MERIT OF Pb. Sn Te SINGLE
1-x x
HETEROJUNCTION PHOTOVOLTAIC DIODES
Q
jSe
...
M
k9
6l
ACKNOWLEDGEMENT
The author wishes to express his sincere appreciation to
Dr. T, F. Tao for his guidance, assistance, and continuous encourage-
ment during the pursuit of this study.
Also, thanks to the Office of
Naval Research and the Air Force Materials Laboratory for supporting
this research and to Mr, Ray Zahra for his help in preparing the samples,
I would also like to thank Jose Fernandez who began heterojunction research
at the Naval Postgraduate School and especially thanks to Gordon Smith
who was my partner in this research.
Finally, thanks to my wife, Janet,
for her patience and ability to translate my handwriting into a typewritten page.
8
I.
INTRODUCTION
Long-wavelength infrared (LWIR) devices, especially in the 8-1^ micron
region, have been very actively developed in the past decade.
Their appli-
cations include imaging, surveillance, remote sensing applications, and
use in C0 ? laser systems.
Narrow-gap IV -VI alloy semiconductor devices, namely Pb 1
Sn Te and
Sn Se have been getting most of the attention in the past two to three
Pb.
years (Refs. 1-6),
This is due to the fact that their energy gaps can be
made arbitrarily small by varying the alloy composition.
to be tailored to a specific wavelength range.
This allows them
They seem to have some
metallurgical advantages over another small energy gap ?J.loy system
Cd Te which has been extensively developed since the late 50' s,
Kg^
This thesis deals with photoconductivity
in Pb.
Sn Se and uhoto*
l-y y
voltaic response in Pb.
been done on Pb,
Sn Tc single hetero junctions.
Previous work has
Sn Se by RCA Princeton Research Laboratory, General
Electric Research Laboratory, Dr.
I.
Kasai and Dr. G.C. Wang, and by
K. Holmquist and V. McBride of Naval Postgraduate School (Ref.
7-8).
latter two reported quantitative photoconductivity results in Pb
in December 1972.
A recent study on single he tero junction
Pb.
Q
The
gSn_ .Se
Sn Te was
reported by J. Fernandez of Naval Postgraduate School (Ref. 9) also in
December 1972.
The purpose of this thesis was to further study the photoconductivity
in Pt
9
Sn
l
Se and to
lie
'
terniine if ?D
Sn n
n fo
->p Se
exhibited any photocon-
ductive response and to measure the photovoltaic response of single hetero junction Pb.
Sn To photovoltaic diodes.
This was a joint thesis project
conducted
"by
the author and LGDR G. L. SMITH, U.S.N.
SMITH concentrated
on device fabrication, I-V analysis, and C-V analysis while the author
of this thesis concentrated on the photoresponsa properties.
10
II.
PIIOTCDCTDJTQRS
The "basic principle of a photodetector is carrier generation, or
creation of electron -hole pairs, by incident light.
If the signal detec-
tion mechanism is "based upon measuring the change in conductivity due to
the added cairiers, the effect is known as the photoconductive effect.
If the carriers are generated at some point at which a potential barrier
exists, the charges will be separated, producing a voltage.
known as the photovoltaic effect.
This is
This addition of carriers is produced
in three different ways in semiconductors as shown in Figure
1
.
Figure 2
shows the three excitations in terms of the absorption coefficient a and
the wave-length k.
The first, intrinsic excitation, occurs when the photon energy of the
incident radiation is greater than the energy gap of the semiconductor.
The incident energy is absorbed by an electron in the valence band and the
electron is excited enough to pass into the conduction band.
This leaves
behind a hole in the valence band which creates an electron-hole pair
which is free to move and contribute to the electric conductivity.
The
second mechanism is the extrinsic transition or transition of carriers
from one energy band to an imperfection level in the energy gap.
This
produces free holes and bound electrons or free electrons and bound holes and
occurs when the incident energy is insufficient to excite a carrier across
the energy gap but large enough to excite the carriers to some impurity
level.
The third mechanism occurs when the incident energy is insuffi-
cient to excite carriers to any impurity level in the energy gap.
The
photon energy can then excite carriers in the conduction band which raises
the kinetic energy of electrons by the electric field,
11
n
o
E,
CARRIER
EXTRINSIC
INTRINSIC
I
FIGURE 1.
PHOTOEXCITATION MECHANISMS IN TERMS OF BAND DIAGRAM
o
M
%
o
o
o
H
EXTRINSIC
FREE
CARRIER
O
CO
WAVELENGTH
FIGURE
2.
PHOTOEXCITATION MECHANISMS IN TERMS OF THE ABSORPTION
COEFFICIENT a AND WAVELENGTH k
12
A.
THE PHOTOCONDUCTIVE DETECTOR
Current Mechanisms
1.
Considering only intrinsic excitations in a semiconductor the
change in free csjrrier concentration appears as a change in the conducThis is expressed in the following equations.
tivity of the material.
A<r
= e(Anii
+ Ap|^)
e
In the above equations a
ooo
.
n
,
and p
are the dark conductivity, free
electron, and free hole concentrations caused by thermal excitation, and
A<r,
u.
An, and Ap are the changes in these quantities due to incident light}
and
Li.
are the electron and hole drift mobilities and e is the elec-
tronic charge.
Incident electromagnetic radiation will be detected
either as a change in the voltage across the sample or as a change in
the current flowing through the sample.
Excess Carrier Lifetime
2,
At a given temperature, thermally excited carriers (n
always exist in a semiconductor.
and p
)
The excess carriers (An andAp), how-
ever, exist only when there is some form of external excitation.
gives these excess carriers a finite lifetime.
This
If the change in free
carriers with time or generation per unit volume is called f , then
d(An) =
dt
n =
or
Where
r
e
lifetime.
An
t
d( A p) _ A p _ „
dt
r,
„
h
e
fr e
p
* = f
t,
h
is the excess electron lifetime and
t,
h
is the excess hole
Since the phoconductivity depends on the density of free
carriers, it can be seen that excess carrier lifetime plays a very im-
portant part in photoconductive response.
13
3.
Photoconductive Gain
Photoconductive gain can
expressed as the ratio of excess
"be
carrier lifetime to the time required to move "between the electrodes.
The time between electrodes or transit time (T.) can he expressed in
terms of the distance between electrodes (L), the electron and hole mo-
bilities
(|i
and
|i,
)
and the applied voltage.
2
x
te
2
th
V
|!
(l,
V
Expressing the gain as
T
G -
T
e
h
,
+
rp
r
x
He
th
and combining the two equations gives
g =
and letting
o
b
Jk
+
'
x
e+
h>
^AG--4«b
Th )
Jj
This is an idealized picture and does not take into consideration the
material dielectric relaxation time
t
,
The maximum gain in a photo-
conductor is obtained when the transit time is equal to this dielectric
relaxation time.
b
G
=
max
Te
+
t,
h
x^
R
This again shows that photoconductive response is controlled by the excess carrier lifetimes and also by the material properties.
B.
GENERAL THEORY OF PHOTOVOLTAIC DETECTORS
A basic requirement of all photovoltaic detectors is the presence of
a junction.
In this study, homo junction and hetero junction photovoltaic
detectors are considered.
The basic sequence for both of these is the
14
.
production of excess carriers
"by
incident radiation, then separation of
the excess carriers by an internal electric field leading to a phot ovolt age
1,
Hoinqjunction Photovolta i c Detectors
The photovoltaic effect can he seen best by studying the energy
I" the absence of incident radiation on
band diagram shown in Figure 3.
the junction, the fermi levels (E^) of the n and p regions are aligned
and an internal electric field (s) exists at the potential barrier in the
direction of the arrow in Figure 3t
This internal electric field pro-
duces the same effect as the external bias on a photoconductor.
eliminates the need for biasing a photovoltaic detector,
This
A typical volt-
age-current characteristic of a photovoltaic detector is shown in Figure k
under dark and illuminated conditions.
When incident radiation falls on the semiconductor a motion of
electron minority carriers in the p-type portion across the barrier and
a motion of hole minority carriers in the n-type portion across the barrier
together constitute a current flow in the reverse direction of the barrier.
If the junction is short-circuited, the sum of the two currents is the
observed short-circuit current Isc, shown at point B on Figure 4,
If the
junction is open-circuited, a voltage is built up across the junction
just sufficient to counterbalance the current flow I,
This is the open-
circuit voltage V^p and is shown at point A in Figure h.
If radiation falls in a region remote from the junction, there
If, however, the radiation falls within
will be no photovoltaic effect,
one diffusion length of the junction, the carriers will be able to diffuse
to the junction and be separated, producing a photovoltaic effect.
The
sensitivity profile of photovoltage near the junction is shown in Figure 5«
The minority carrier diffusion lengths, L
15
and
L.
are defined as the
E
ELECTRON
p REGION
iS
n ESS ION
64iolb
*r
FIGURE 3.
FIGURE k.
•
CARRIER EXCITATION AT A p~n JUNCTION
RECTIFICATION CHARACTERISTICS OF A PHOTOVOLTAIC DIODE
UNDER DARK (l) AND ILLUMINATED (2) CONDITIONS
16
j
points where the relative photoresponse has decreased to a value of l/e
of the junction value,
The diffusion length is related to the mobility and lifetime in
the following manner
L
Where
T1
ep
and
TL
,
hn
ep
=
(
x
*
t
~
ep
D
_
2
Lnn = (x.hn
ep')
ep^
v
z
D.
hn')
carrier lifetimes. D
are the minority
*
ep
and
D,
hn
are
the minority carrier diffusion coefficients in p-type and n-type semiThe diffusion coefficients and mobilities are
conductors respectively.
related
"by
the following equation for non-degenerate semiconductors,
D-H|l
^
e
In detectors where the radiation is incident perpendicular to
the junction either the p or the n region must be very thin.
The sepa-
ration of excess carriers takes place in and near the junction so the
radiation must pass through the material to the junction for photovoltaic
response.
The depth of penetration of radiation is inversely proportion-
al to the absorption coefficient a.
10
k
cm
-1*
This means that for a typical a of
the depth of penetration is only approximately one micron.
The simplest relationship for a p-n junction is given by
I = I [exp(eV/kT) -1 ]
s
In this equation I
s
is the saturation current of the device.
This, how-
ever, does not agree with experimental results and must be modified as
follows,
I = I
s
[ exp (eV/ 1 kT) -1 ] +
G^V
- I
g(J
Here Gq„ is the conductance in shunt with the diode, 1^- is the current
induced by incident radiation and
1
is an empirical constant which nay
include several mechanisms causing the deviation from ideal rectification
17
r
c
>
Q
li:
n
DISPLACEMENT
FIGURE 5.
PKOTOVOLTAGE SENSITIVITY PROFILE NEAR A p-n JUNCTION
E
E
G2
^2
CI
%
FIGURE 6.
HETEROJUNCTION ENERGY BAND DIAGRAM
18
such as carriers created
phonon generation-recombination process
"by
inside and near the vicinity of the junction and tunneling.
For the open
ciricut case no current can flow and the open circuit voltage becomes
if
v
—
Isc ~ GsH v
JLisl iln r
L
oc "
t
+1
4.1
i
J
For the short circuit case there is no voltage across the junction so
1
2.
ho
Ketero junctions
Heterojunction photovoltaic detectors have a different energy
band diagram as shown in Figure 6.
One of the important differences is
This involves incident radiation with insuffi-
called the window effect.
cient energy to generate carriers in the wide bandgap material to pass
unattenuated to the narrow gap material where they are absorbed.
This
allows a high generation of carriers in and near the depletion region.
The current mechanisms in heterojunctions are not fully understood
at this time.
Several comprehensive studies have been made by Anderson
(Ref. 10) and Donnelly and Hilnes (Ref. 11).
current obeys the following relationships.
J = J
(e^/^kT
A e
JT = A
o
A = XqN
^V^
*
A
(
2
-£ )*
T
p
19
-1)
Anderson proposed that the
= current density
J
= saturation current
J
o
q
= electron charge
V
r applied voltage
V
- deviation from an ideal diode
k
r Boltzmanrf s constant
T
= temperature in
V np
= "barrier to hole current flow
X
= fraction of carriers with sufficient energy to cross the barrier
which actually cross the "barrier
= number of holes on the p side of the junction
N.
A
D
K
2
= diffusion coefficient of holes
p
= lifetime of holes
t
P
This is the model proposed for the heterojunction diodes investigated in
this research.
The above model is thoroughly investigated in Ref. 22.
20
C.
N0I33 IN INFRARED PHOTODETECTORS
Since noise is ultimately the limiting factor in any photodetector,
it must be given some consideration "before detector figures of merit can
"be
defined,
The four main contributors of noise in IR photodetectors are
Johnson
1.)
noise, 2) l/f noise, 3) Generation -recombination noise, and h) background
radiation noise,
Noise is thoroughly covered in (Refs, 1^-18).
Johnson noise is related to the free charge carriers moving about in
a crystal lattice.
Noise is caused when the carriers bump into each other
or into the crystal lattice atoms,
Johnson noise is independent of fre-
quency and current but varies with the absolute temperature (T), the resistance (r) and the noise equivalent bandwidth
V
=
OkTR Af
]
(
A f ) as shown by
2
where k is Boltzmann's constant.
As opposed to Johnson noise, l/f noise is dependent on frequency and
current through the detector.
The mechanisms which govern l/f noise are
not very well understood at the present time.
It is thought that in
photcconductors the major source of the noise is at the surface of the
material.
Significant noise of this type also is caused by non-ohraic
contacts.
In photodetectors, this noise is proportional to the density
of the surface states at the Fermi level.
Since the surface state density
at the Fermi level is temperature dependent, l/f noise is also dependent
on temperature and increases with decreasing temperature.
In many com-
mercial photodetectors l/f noise is negligible at frequencies above a
few hundred Hz.
In most photodetectors, the main source of noise at frequencies above
which i/f noise is troublesome is generation-recombination noise.
21
This
corresponds to the shot noise found in vacuum tubes.
It is due to valence
hand electrons being freed hy lattice vibrations which leaves them free to
The power spectrum of generation -re com-
combine during their lifetime.
bination is flat from low frequencies up to a frequency which is approximately the inverse of the free carrier lifetime.
Background radiation noise is always present and must be minimized to
insure that the signal-to-noise ratio is limited by the detector noise.
This noise is due to random fluctuations in the number of photons reaching the detector from warm objects in its surroundings and imposes an
upper limit on tho detectivity of the detector,
A typical noise figure which combines Johnson, l/f
,
recombination noise versus frequency is shown in Figure
D.
and generation7.
FIGURES OF MERIT FOR INFRARED PHOTODETECTORS
Infrared photodetectors are defined by certain figures of merit
describing the performance of the detector under specified operating
conditions.
Typical operating conditions are a 500 K blackbody radiation
source and an electronic noise bandwidth of 1Hz or 5Hz.
1.
Standard Radiation Source
Since the ideal blackbody can be very closely approximated by a
laboratory blackbody, it is chosen as the standard radiation source.
total power W(watts) incident on a detector of sensitive area A- (cm
The
)
p
a distance R (cm) from a blackbody radiator with an aperture A_ (cm
emissivity €^ and temperature
T^
DO
K) is given by the Stefan-Boltzmann
(
law
€
W _
" b
° T
BB
-if-
*D
.
'
*B
22
),
"
^
at
i/f NOISE
REQUENCY
FIGURE
7.
GENERALIZED PHOTODSTEJCTOR NOISE SPECTRUM
H
S
WAVELENGTH
FIGURE 8.
TYPICAL SPECTRAL RESPONSE FOR AN INTRINSIC PHOTODETSCTOR
23
Since a mechanical chopper is
Where a is the Stefan-Boltzmann constant.
placed between the source and the detector, the detector is exposed alter-
nately to radiation from the source and from the chopper
ature Tr,..(°K).
Made
at temper-
Peak-to-peak amplitude of the signal is then given by
Oil
(
T
" TCH
B3
*D
'
This value must be corrected by a value (a
sorption by optical windows and by (a
w)
which accounts for any ab-
which relates the RMS value of
)
the fundamental harmonic and the peak-to-peak amplitude of the signal,
This value (a
in Ref
.
7t
- 0.375) is calculated for the chopper used in this work
This gives the final form for the radiation signal on the
detector,
W
v
S
2i
. a
CH
°
€
a
^
B
VJ
^
T
BB " GH_L
(T
it
a t*
*B D 2
it
Det ector Performance Parameters
The first obvious figure of merit is the signal-to-noise ratio.
In order to keep the figures of merit standardized, the noise voltage is
always normalized to one Hz bandwidth.
found to be proportional to (Hz) 2
,
Since all noise voltages are
the noise voltage is normalized by
i
dividing by (Hz) 2
,
This gives
S
S/N =
i
=
V /(Hz) 2
N
^
V
NN
where V a is the signal voltage and V XTV is the normalized noise voltage,
Responsivity, R, is the next figure of merit and is the ratio of detector
signal to the input radiation signal.
VR = rp
W
S
volts/watt
Zk
It can be seen that responsivity
does not account for the detector noise which can lead to confusion, since
some detectors are very sensitive
are also very noisy.
"but
Noise equivalent power (NEP) takes the noise into account and is
defined as the radiation power required to give a signal to noise ratio
of unity,
W
mm
NEP
=
Watt
S
7-77—
V
"T
(Hz) 2
NN
Since a smaller NEP indicates a better detector, another figure
of merit was defined based on the inverted NEP and is called detectivity
The detectivity expresses the signal-to-noise voltage ratio obtain-
(D).
ed per watt of radiant power.
A value more commonly used instead of
detectivity is an area independent figure of merit (D*)»
common usage
i
D» -
D(y
D
V
i
2
=
^M
V
~-fW
g
JL
.
(Aj 2
(Aj 2
=
It nas "become
-^
WEI
1
cm(llz) 2 /w
to refer to (D*) as detectivity and whenever the term appears herein, it
will refer to (D*).
3.
Spectral Response
Since photodetectors are photon detectors instead of energy de-
tectors, it seems that the spectral response would be measured by radia-
tion from a constant photon source.
This, however, would require a
reference photodetector with a flat response over an extended infrared
wavelength region, which are not made.
The measurement is made with a
constant energy radiation at the detector
"by
monitoring the output from
the exit slit of a monochromator with a thermocouple throughout the
spectral range of measurement.
The response of the photodetector is then
normalized to a constant input powe-r by dividing it by the thermal de-
tector response at each corresponding wavelength interval.
25
A typical
.
spectral response curve for a photodetector is shown in Figure 8.
The
photoresponse is small at long wavelengths since the incident radiation
does not have enough energy for intrinsic or extrinsic excitation.
It
rises when the photon energy is near the energy gap in photoconductors
and homo junction photovoltaic detectors.
In heterojunctions the curve
is more complicated by having two different energy gaps and a not fully
understood conduction mechanism, but has a similar shape.
The drop at
shorter wavelengths is due to the increase of the absorption coefficient,
This causes all the incident energy to be absorbed very near the
surface
PRESENT DAY STATUS OF Pb-Sn GHALGOGENIDE PH0T0DETECT0R3
E.
1.
Thin Film Pb A n Sn n ,Se Photoconductive Detectors
U J
u «"
»
-
Photoconductivity in the two alloy systems, Pb.
Sn So has not been well developed.
Pb.
done on Pb.
j.
Sn Te than on Pb. Sn Se.
x
x —y
y
—x
in developing Pb,
Sn Te and
Considerably more work has been
Several attempts have been made
Sn Se photoconductive detectors at the RCA Princeton
Research Laboratory, however, no quantitative results indicating photo-
conductivity were reported.
by Dr.
I.
Kasai and Dr. C. G. Wang,
7
as high as low 10'
W.
Some work on Pb.
-
(cm-Hz 2 -W
Sn Se has also been done
In December, 1972, responsivities
-1
)
were reported by K. Holmquist and
McBride of the Naval Postgraduate School in Pb n Q Sn n .Se (Refs. 7-8).
2,
Photovoltaic Detectors
a.
Bulk Crystal Pb.
Sn Te and Pb.
Sn Se
1-x x
1-y y
Bulk crystal photovoltaic detectors have been developed in
both
?\_xS\?e
and Pb _ Sn Se by Melngailis and Harman (Ref. 3).
1
Most
of the work has been on Pb.
Sn Te due to their more favorable metallurgical
°
1-x x
26
.
properties.
For Pb
o^Sn
Q
responsivities as high as 190 Volt/Watt
.^Te
and detectivities as high as 10
10
r
cm-Hz"-W
-1
'
have been reported for diodes
with cutoff wavelengths in the 8-1^ micron atmospheric window.
b.
Thin Film Pb-Te Schottky Barrier Photovoltaic Detectors
The Scientific Research Staff of the Ford Motor Company
(Ref
19) have reported detectivities as high as 6X10
.
peak wavelengths of 5*5 microns on epitaxial Pb-Te.
11
'
—
cm-Hz 2 -W
-1
"
and
These values are
comparable to the best that have been reported for p-n junctions in bulk
Preliminary studies indicate that these detectors may also
crystals.
be made from Pb.
Sn Te
layers although there is yet no report of this
work
SGOPS OF THIS STUDY
F.
This study was concerned with the photoconductive response of
Pb
fi
Q
Sn
.Se and
n
Pb-.
£oSn„
» Se
?
and the photovoltaic response of a
Pb- o Sn- pr/Te n layer, Pb- o^Sn_
1.
^Te
p layer heterojunction diode.
Photoconductive Pb.
Sn Se
*-y y
The variation of the energy gap with variations in alloy compo-
sition for
Pb.,
Sn Se is shown in Figure 9t
This unique property allows
a detector made from this material to be tailored for operation at any
point in a given spectral region.
This thesis dealt with samples of two different compositions
y = 0.10, y = 0.32.
Figure 9«
1
The differences in the energy gap are shown in
As stated previously photoconductivity has been measured and
reported in Pb- gSn- .Se, however, no work has been attempted on the
photoconductive study of Pb
Q
ggSn
27
r,o 2e
*
The purpose of this thesis was
FIGURE 10.
ENERGY GAP OP Fb,
Sn Te AS A FUNCTION OF x, THE
MOLE FRACTION OF SnTe AT 77 K
FIGURE 9.
ENERGY GAP CF Fb*
SnSeAS A FUNCTION OF
Y
MOLE FRACTION OF SnSe'AT 77°K
23
y, THE
to further amplify on the reported photoconductivity in the y = 0,10
composition and to see if any photoconductivity could he found in the
y = 0.32 composition.
2,
Ph..
Sn Te Single Hetero junction Photovoltaic Detectors
Variations in alloy composition of Ph 1
Sn Te produce energy gap
Sn Se as shown in Figure 10.
changes similar to those in Phi
This
allows tailoring to a specific point in a given spectral region.
their small energy gaps,
"both Fh,
Sn Se and Ph„
Due to
Sn Te fit into the 8-14
micron atmospheric window.
In this thesis the alloy compositions used in the single hetero-
junctions were an n layer of x = 0.20 which gave an energy gap of 0.113
eV and a p layer of x = 0.14 which gave an energy gap of 0.160 eV at 85 K,
The second purpose of this thesis was to measure the photovoltaic and noise
properties of these single hetero junction
29
photodiodes.
III.
SUMMARY OF PREPARATION, METALLURGICAL EVA LUATION, AND
ELECTRICAL MEASUREMENTS OF Pb^ Sn Se THIN FILMS AND
Pb!
1-x
Fb,
A.
l-y
•
Sn Te SINGLE HETEROJUNCTIONS"'
x
Sn Se THIN FILMS
y
Sn Se studied in this research was in the form of thin films
The Ph.
l-y y
deposited on BaF~ and CaF ? substrates.
Metallurgical measurements were
made on the samples in order to determine their crystal structure, orientation, and thickness.
The samples were then isothermally annealed in
the presence of PbSn rich vapor to reduce their carrier concentrations.
Gold contacts were deposited and copper leads attached.
Hall measure-
ments were then made from 300 K to 90 K to determine carrier type and
concentration and Hall mobility.
1.
Sample Preparation
In this thesis y values of 0.10 and 0.32 wers used.
Stoichio-
metric proportions of Pb, Sn, and Se were weighed and melted at 1100 C
for Zk hours in a vacuum-sealed quartz ampoule.
composition (Pb,
annealing.
Sn
)>
Metal -rich alloy of the
Se was prepared in the same manner for use in
Substrates were made by cleaving half inch rods of BaF and
?
CaF ? along the (ill) plane.
These substrates were placed under vacuum
and a heating element was placed above them to control their temperature
during depositions.
Several small chunks of the Pb.
Sn Se source mate-
rial were placed in a graphite boat approximately six inches below the
substrate holder.
A tungsten heating coil was put around the boat and
the boat was covered with a molybdenum sheet with a small hole in it.
A
shutter was located between the holder and the boat to control the deposition.
Prior to the deposition, the vacuum chamber was evacuated to
30
approximately 10~
torr and the substrate was heated, to the desired
The boat was hea/ted to a temperature of
temperature (2?0 C to 325 C).
from 740°G to 820 G.
When the proper temperatures were reached, the
shutter was opened and the evaporated Pb, _ Sn Se condensed on the substrate forming a thin film.
The shutter was left open till the film
reached the desired thickness, then closed and the system was cooled to
room temperature.
2,
Metallurgical Evaluation
The metallurgical evaluation provided information on the thin
film thickness, crystal structure, orientation and composition.
The thickness was measured using the int erf erome trie method on
a Perkin-Elmer spectrophotometer.
Thickness was determined by measuring
the wavelength difference between peaks in a scan of the transmittance
versus wavelength.
Crystal structure was evaluated from Laue back-reflection photo-
graphs obtained on a Norelco Laue back-reflection x-ray unit.
Crystal
orientation and composition was determined from x-ray diffract ometer scans
done on a Norelco x-ray diffractometer using a copper target.
3.
Electrical Measurements
Carrier type, concentration and mobility were determined using
the standard Hall measurements.
The thin film sample was mounted on the
copper cold-finger, evacuated to 20 microns Hg or less and cooled to
approximately 90 K with liquid nitrogen.
The conductivity versus temper-
ature was determined during the cooling process by plotting the voltage
across a pair of contacts parallel to the current flow on the horizontal
input of an x-y recorder.
A thermocouple on the cold-finger was connected
to the vertical input of the recorder and a constant one milliampere
31
current was passed through the sample.
While the sample was gradually-
brought up to room temperature, the Hall voltage was measured across a
pair of contacts perpendicular to the plane
of.
the thin film.
A magnetic
field of 5000 Gauss was alternately turned on and off to produce the Hall
voltage.
The Hall voltage versus temperature was plotted on the recorder
and calculations of the Hall coefficient, conductivity, and carrier concentration, type, and mobility were made from the two plots.
Isothermal Annealing
^.
Isothermal annealing was used to reduce the carrier concentrations of the thin film samples after metallurgical evaluation.
The samples
were placed in quartz annealing ampoules along with a small amount of
metal -rich source material.
The ampoules were then evacuated, back-
filled with helium gas and sealed.
Annealing temperatures from 312 G to
were used from periods of time from three hours to four days for
5^-0 C
y = 0.10 composition material.
For y = 0.32 temperatures from 375 C to
550 G were used for periods of time from 2,5 hours to two days.
Sn Te SINGLE HETEROJUNCTIONS
1-x x
B.
Fb,
This is a brief summary of the preparation of single hetero junctions.
The work is covered in detail in Ref, 22.
Source Materials
!•
The p layer of the heterojunction was made from stoichi oris trie
Pb
Sn
n
Q Qfp G i
Q 2Q
).
ji,Te
-Te.
alloy and the n layer was made from metal rich (Pb. „_
Increasing the amount of PbSn changes the semiconductor
from p-type to n-type.
thesis.
Ref.
Values of 6 from 0.002 to 0.30 were used in this
Characteristics of materials with varying 6's are presented in
22.
32
2.
Material Preparation
Materials were deposited in two different ways.
The first method
was the same as used in the preceding section on Ph. J3n Se, except KC1
"X x
J.
was used as the substrate.
Since different composition materials are used
for the n layer and the p layer, the va.cuum chamber had to be opened while
the source materials were being changed.
This exposed the interface
between the two layers to the atmosphere for a considerable period of
time.
The second method of deposition eliminated this problem.
A multi-
source deposition unit with two deposition chambers was designed by Mr, Ray
Zahm,
This allowed the p layer source material to be loaded in one cham-
ber and the n layer source material in the other chamber.
After the
first layer was deposited, the substrate holder was rotated and placed
over the next deposition chamber and the second layer deposited,
This
allowed the entire deposition of the heterojunction to be carried out in
a 10
-6
torr vacuum.
layer of gold (5000A
3.
After deposition of the films was completed, a thin
)
was deposited on the top layer of the hetero junction,
Diode Preparation
When the substrate was removed after having gold deposited, it
was cleaved into squares approximately 1mm x 1mm,
These were then mounted
on a T0-5 header with the gold contacting the header.
used to form the bond.
Silver epoxy was
The substrate was then dissolved in water and one
mil copper wire was placed between the diode and header post using silver
epoxy.
33
IV.
A.
EXPERIMENTAL SETUP FOR MEASUREMENT OF BLAGKBODY RESPONSE
DESCRIPTION OF EQUIPMENT
The sane "basic setup was used for photoconductive and photovoltaic
response measurements.
The four main groups weret
1) the vacuum dewar,
2) the bias circuitry, 3) blackbody radiation source and radiation mod-
ulation equipment, and 4) the signal measurement equipment,
A block
diagram of the overall measurement system is shown in Figure 11.
It
Vacuum Dewar
All photoconductivity and photovoltaic measurements must be made
at cryogenic temperatures when narrow-gap semiconductors are used.
This
is because at room temperature, the energy gap is only several times kT
and the density of thermally-generated free carriers is so great that the
presence of photo -generated free carriers is virtually undetectable.
In
this research all samples were cooled by liquid nitrogen (boiling point
77 K) which required the use of a vacuum dewar.
The vacuum chamber pro-
vided an insulation between the liquid nitrogen and the atmosphere and
prevented frost from forming from water vapor in the air.
drawing of the dewar is shown in Figure 12.
A sectioned
A copper-constantan thermo-
couple was mounted on the copper cooling block for monitoring the tem-
perature of the sample.
When photovoltaic measurements were made, the
sample was mounted on a TO-5 header and this was fastened to the cooling
block.
The thermocouple was then positioned on the TO-5 header to get
an accurate temperature measurement.
Temperatures for both photoconduc-
tive and photovoltaic measurements reached as low as 93°K and held
constant at 100 K for extended periods of time.
3*
-tT^-
!
I
§
O
U
"=2
i
H
H
P-.
Pm
CO
°1
a
Q
OQ
OO
fcn
O
K
CQ Q
00
^ &i
S
<
->
ffi
o
00
Q
«a!
PL,
a
r
1
£
3
to
00
3B K
f3
g3 EH O
Pm H
00
^ W 00
><
fe O E 00
T
*3
<2
CQ
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00
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fid
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as
o
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11
P
Pm
35
VACUUM
<
FEED-THROUGHS"
MN^^
LIQUID
NITROGEN
RESERVOIR
:sr-r-v.\-K-i
VACUUM
CHAMBER
fe* xrcs^csrn:
.Ji.J
l^l=X72=Z=Z
SAMPLE
c£3;>
TZZZZZZZZZZZZZZZZZ^ZZZZZZA
VACUUM CONNECTION
FIGURE 12.
SECTIONED DRAWING OF THE DEWAR
36
COPPER
COOLING
BLOCK
2.
Bias Circuitry
Since a photovoltaic detector opera/tes at zero bias, this
circuitry was used only for the photoconduction measurements.
A schematic
The bias circuitry
diagram of the bias circuitry is shown in Figure 13.
was used to produce a low-noise current in the sample and to detect con-
ductance changes due to temperature changes in the sample as a voltage
signal.
l) the power source, 2)
The system had three main components!
the switch box, and 3) the junction box,
The power source was a standard
laboratory power supply (0-500V, 0-1 00mA) with an external low -pass filter
connected for additional filtering.
The resistance in the low-pass filter
was varied depending on the resistance of the sample
.
The switch box
following the power supply was used to make measurements of the current
through the sample or the voltage across it,
It could also be completely
eliminated from the circuit when measurements were being made,
The junc-
tion box contained the coupling capacitor and a load resistor.
The switch
box also enabled the load resistor to be switched out of the circuit for
detector resistance determination.
The load resistors were low noise,
wire wound resistors which were chosen to match the detector resistance.
This provided the maximum signal power transfer.
The internal circuitry
of the dewar consisted of a coaxial cable which provided the photocon-
ductor bias signal and also carried the photoconductive or photovoltaic
response signal to the lock-in amplifier.
For photovoltaic signals,
since no load resistor was required, the signal was fed directly through
a one hundred to one ratio transformer to the amplifier.
sistance samples (greater than
1
For high re-
k~ohm) the signal was fed through a
capacitor to the amplifier since the amplifier noise was very high for
these source resistances in the transformer mode.
37
33
3.
Radiation Source and Modulation Equipment
A Barnes model 11-101T-1 standard laboratory blackbody was used
as the source of radiation,
This source had a fourteen degree conical
cavity, a twenty degree field of view, an emissivity of 0.99i an aperture
of
O.fy
cm
2
,
The radiation was focused through a KRS-5 window on the
dewar which had a transmittance of 0.7 for wavelengths of 2-35 microns.
The temperature was maintained within one degree of 500 K with a Thermae
controller.
The temperature was measured using an iron-constantan thermo-
couple inserted through the aperture of the source.
The radiation was modulated at one kHz by a chopper blade nine
inches in diameter with fifty slots.
The blade was driven by an Electro-
Craft motor-generator and speed controller.
The chopper motor was mounted
directly above the blackbody on a metal stand.
The lock-in amplifier used to measure the signals from the sample
required a reference frequency input of precisely the same frequency as
the modulation.
This was produced by having the slotted chopper blade
modulate the radiation from a light -emitting diode which illuminated a
reverse biased silicon p-i-n diode.
This produced a signal sufficiently
strong and free of harmonics to synchronize the lock-in amplifier reference oscillator.
4,
The circuit used is shown in Figure 14.
Signal Measuring Equipment
Since the response of both the photovoltaic and photoconductive
detectors was from several nanovolts to microvolts and accompanied by a
large amount of noise, precision equipment had to be used for measurements,
A Princeton Applied Research Model 124 Lock-In Amplifier with a model 116
Preamp was used for measurement of the signal and the noise (Ref. 20).
The Preamp had a transformer and direct mode.
39
In the transformer mode
FIGURE
1U-
FREQUENCY
SCHEMATIC DIAGRAM OF CHOPPER
CIRCUIT
REFERENCE GENERATION
40
an Impedance matching transformer was placed "between the source and the
amplifier.
This was used when measuring low resistance samples and pro-
vided very low-noise amplification.
The direct mode, where the transformIn either mode the
er was by-passed, was used for high resistance samples.
entire amplifer contributed less than 3
<3b
of noise at
1
kHz.
At lower
frequencies used in noise measurement, the noise contribution went as
high as 6 db.
Following the preamplifier, the signal passed through a bandpass
amplifier of variable center frequency and bandwidth to filter out all
unwanted frequencies.
The signal was then passed into a synchronous de-
tector in which the signal was multiplied by the output of an oscillator
locked to the chopper reference frequency.
ba.nd of
This converted the selected
frequencies to an equivalent bandwidth centered at zero Hz by
the heterodyne process.
The output of the synchronous detector passed
through a low-pass filter with a variable time constant which eliminated
all unwanted frequency components.
The output of the lew-pass filter
was then read off an average reading meter on the lock-in amplifier.
The detector noise spectrum was also measured on the lock-in
amplifier.
This was done with the amplifier operating in the AGVM mode
in which the output of the bandpass amplifier went directly to a full-
wave rectifier and then to the low-pass filter and meter.
The "10^ ENBW"
setting of the Q selection dial was used which gave an equivalent noise
bandwidth that was ten percent of the center frequency.
Noise measure-
ments were made at frequencies of from 30 Hz to 10 kHz,
At low frequen-
cies it was necessary to use a high Q value to get rid of power line
frequency and its harmonics,
41
B.
MEASUREMENT PROCEDURE
Evaluation of detectivity and responsivity for both photoconductors
and photovoltaic detectors consisted of essentially the same procedure.
Noise evaluation was also the same except for the absence of a load re-
sistor for the photovoltaic detector.
These evaluations consist of prep-
aration, noise measurement, signal measurement, and calculations.
1.
Preparation
a,
Photoconductive Detectors
The BaF,- and CaF 9 substrates were attached to the copper
block of the dewar using silicone thermal grease which insured good heat
transfer.
The leads were then attached to the dewar using low-
temperature indium-alloy solder and the dewar was evacuated to approxi-
mately twenty microns Hg.
The blackbody was stabilized at 500°K by the
Thermae controller and the chopper was synchronized to 1kHz by means of
the Internal oscillator in the lock-in amplifier and an oscilloscope,
A
current was "then run through the sample which determined the room temp-
erature resistance.
The sample was then cooled to approximately 100 K
by pouring liquid nitrogen into the internal part of the dewar,
A current
was again passed through the sample determining the cold (100°K) resistance,
A low noise wire wound resistor of approximately the same resist-
ance as the detector was then soldered into the junction box.
b.
Photovoltaic Detectors
Since the thin film photovoltaic detector was mounted on a
TO-5 header, it had to be applied to the copper block in a different way,
A copper holder for the TO-5 header was made and fastened to the coldfinger with two copper screws.
The base of the header was then in con-
tact with the copper block which served as ground and only one lead was
42
Preparation was then the
cahle.
attached to the lead-in coaxial
sar,e
as
no load resistor was needed.
for the photoconductor except
Noise Measurements
2.
blaekdetector was shielded from the
For noise measurements the
AOVM mode and
lock-In amplifier was set to the
hody radiation and the
the
voltage was then read directly from
MO* ENBW" 4 position. The noise
amplifier meter for center
of from 30 Hz to 10 kHz.
fluencies
(less than
found that for low frequencies
WO
It was
Hz) the host values were
either side
at values which were 30 Hz on
obtained if readings were taken
normalized
These noise voltages were then
of 60 Hz and its harmonics.
pronoise using the noise figure contours
and corrected for amplifier
a correction
In the photoconductive detector
vided with the amplifier.
which
take care of the load, resistor
factor also had to he applied to
was
The following correction factor
resulted in a lower noise voltage.
The detector
used.
*N "
V
V
1
+
h
L
I Cl ° S io (NF/2 ° )]
= corrected noise voltage
N
V
= measured noise voltage
NM
= cold detector resistance
R
R
L
= load resistance
NF = amplifier noise figure
plotting V versus center frequency
N
noise spectrum was then obtained by
on log -log graph paper.
3.
Signal Measurements
the optimum bias current was
In the photoconductive detectors
which the maximum signal-to-noise
found by determining the current for
the
then exposed to radiation and
ratio was obtained. The detector was
*0
signal voltage was measured on the lock-in amplifier with the instrument
function set on normal and the Q selector set as low as possible.
The
synchronous phase selector was then adjusted for maximum signal and the
voltage was read off the amplifier meter.
As in the noise measurements,
the signal voltage had to be corrected for the load resistor,
vV
S
m v
d
SM V
+ JS);
R
L
V q r corrected signal voltage
Vq M= measured signal voltage
k.
Detector Performance Calculations
Using values specified for the equipment used, it was found that
the RKS value of the fundamental component of the modulated radiation
signal from the blackbody was
W
= 370 x 10
x A^ Watts
Then using values for signal and noise obtained the figures of merit were
evaluated.
1
tf.
R - V /W
s
s
,
NEP =
V*
S
y|-
,
D* - -3L-
V.
A.
STUDY OF PHOTOCONDUCTIVITY
RESPONSE MEASUREMENTS
1.
Pb
Se Samples
gSn1
The thickness, carrier concentrations, mobilities, low and high
temperature resistances, optimum bias current, signal voltage, normalized
noise voltage, signal-to-noise ratio, noise equivalent power, detectivity,
and responsivity are summarized in Tables I and II.
tures of two of the samples were known.
The ciystal struc-
Sample OB-6-1 was single-crystal
with (100) orientation and 0B-8-3 was polycrystalline with (lll)+(100)
orientation.
The Norelco x-ray diffractometer was not available so the
crystal orientation of the remaining Pb
Sn
n q
.Se and all the Pb~
/•„
Sn~ ooSe samples could not be obtained.
0,J<£
The results obtained in Ref
,
7 a*3 outlined below and compared
to the results obtained in this thesis.
It was found that there was a
good correlation between the photo-sensitivity and the type and orientation of the crystal structure of the sample.
Single -crystal (100)
crystals had good sensitivity while samples with some (111) crystal
orientation were lower in sensitivity.
Also single -crystal (100) samples
had low mobilities and increased in resistance from room temperature to
100 K.
Crystals with some (ill) crystal orientation had lower mobilities
and a decrease in resistance with decreasing temperature.
The data collected in this research correlates with that given
above, however, crystal orientation of only two samples was known.
Sample
0B-6-1 with (100) orientation increased from Zk ohms to 63 ohms and had
the highest detectivity measured.
^5
The crystal structure and orientation
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of the other sample known was polycrystalline (100)+(111).
The mobility
of this sample was approximately 30 times that of the (100) oriented
sample and its resistance decreased with decreasing temperature.
Its
detectivity was less than half that of the (100) oriented sample.
The
data obtained on the remaining two samples indicated that they had some
(111) crystal orientation.
very low.
Responsivities of all samples measured was
Assuming the orientation of the two unknown samples contained
some (ill) orientation it was observed that the responsivity varied .di-
rectly with the resistance of the sample.
This was probably due to the
fact that responsivity does not take into account the noise voltage
which is also proportional to the resistance.
The fact that both the
signal and noise in the (ill) samples were proportional to the resistance
kept the detectivities of the samples relatively close.
The reason for the higher detectivities in the (100) samples is
thought to be due to trapping centers increasing the lifetimes of the
majority carriers, thereby increasing the sensitivity.
The trapping cen-
ters arose due to (100) oriented crystal structure being grown on (ill)
oriented substrates which created crystal defects.
2.
Pfc>
£pSn_ ^pSe Samples
The thickness, carrier concentrations and mobilities, low and
high temperature resistances, and normalized noise voltage are listed in
Table III for four Pb
zroSn- „ Se
fi
?
samples.
The crystal structure was not
known for any of the samples due to the measuring equipment being inoperable.
The fact that all the resistances decreased with decreasing tem-
perature and the fact that all Hall mobilities were greater than 13^0
cm /v-sec seems to indicate that all the samples had some (111) crystal
orientation.
The resistances were all very low with cold resistances
43
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49
from 8 ohms to kZ ohms.
Pt>
n
The results of Ref. 7 showed photoresponse in
In this work
Sn_ ,Se for resistance values only as low as 130 ohms.
photoresponse was not obtained in R> n p/rSn.
^Se
samples.
The very low
resistance values seem to have been a major reason for this.
B.
NOISE MEASUREMENTS
Noise measurements were the same for both Pb. q^ n
Se samples.
<Se and Pb_ £pSn_
The results are presented in Figures 15 and 16.
~
Values of
normalized noise voltage was plotted versus frequency for the four sam-
Frequency values from 30 Hz to 10 kHz
ples of each alloy composition.
were used.
At 10 kHz the values approached the theoretical Johnson noise
for all the samples.
Below this frequency the noise voltage rose rapidly
in all but two samples.
In sample 6-1 the noise was constant at 73 nV/Rz 2
from 100 Hz down and in sample $-5 the noise did not rise as rapidly at
low frequencies as in the remaining samples.
majority of this noise was l/f noise.
It is thought that the
This is generally due to noise at
the surface of the material or non-ohmic contacts.
not as prominent in the
Pb..
Since this noise was
Sn Te heterojunctions which used the same
silver-epoxy to bond the wires, it seems the noise came mainly from the
semiconductor surface.
50
2
o
o—
fc
10"
10-
FREQUENCY (Hz)
FIGURE 15.
NOISE SPECTRUM FOR &>o.9(fi n 0.1Cp e SAMPLES
51
io
o
<5
5-4
o
5-2
°\
o
^o_ c
0^-
5-5
~-o—o-o
1-3
•o— o—
J—J
10 V
Kf
FREQUENCY (Hz)
FIGURE 16.
NOISE SPECTRUM FOR Pb
52
Q
^Sr^
32
Se SAMPLES
LJLIO,
I
10
VI.
STUDY OF PHOTOVOLTAIC p-jSPONSB
Heterojunction diodes were fabricated in series which were called
A, B, C, and D.
in Ref
22,
.
Detailed characteristics of these diodes are presented
Photoresponse was found in only the C and D series diodes.
Series G diodes were fabricated from J type source material with the n
layer on top.
Series D diodes were fabricated from I type source mate-
rial with the p layer on top.
mately two microns thick.
In both cases the top layer was approxi-
The study of photovoltaic response consisted
of I-V evaluation, noise measurements, and response measurements.
A.
I-V EVALUATION
The I-V rectification of the diodes was evaluated on a Tektronix
Model 576 curve tracer.
The diodes were dipped in liquid nitrogen to
determine if rectification was present.
The diodes which showed good
rectification were then mounted on the copper face of the dewar, and the
rectification was measured at approximately 95 K.
Figures 17-20 show the
results of these I-V measurements for the diodes which exhibited some
photoresponse.
Figure 18 shows the rectification curve for diode Q-Jk
on two different scales.
1
The bottom photo used a vertical scale of
microamp per division and a horizontal scale of 5 millivolts per divi-
sion.
ance R
This provided for an accurate measurement of the zero bias resist.
This allowed the figure of merit R A, which describes the quality
of a rectifying diode, to be measured.
Values of R A as high as 76 ohm-cm
were measured for diodes exhibiting photoresponse.
listed in Table IV.
These values are
Figure 20 shows the rectification curves for diode
D-65 at 95°K in the dewar and at 77°K while dipped in liquid nitrogen.
53
DIODE G-6-B
DIODE C-13
FIGURE 17.
RECTIFICATION CURVES FOR DIODES AT 97°K
5^
FIGURE 18.
RECTIFICATION CURVES FOR DIODE
C-3^ WITH DIFFERENT SCALING
55
DIODE D-8
DIODE D-91
FIGURE 19.
RECTIFICATION CURVES FOR DIODES AT 95 K
56
— — mmrnu _
:
fen
:
A
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—/
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95°K
77°K
FIGURE 20.
RECTIFICATION CURVES FOR DIODE D-65
AT TWO TEMPERATURES
57
There is considerable difference in the two curves, especially in the
reverse bias direction.
This may have been a contributing factor to
the poor photoresponse of the diodes.
The I-V characteristics of the
diodes are treated more thoroughly in Ref
B.
.
2Z
NOISE MEASUREMENTS
The same procedure was followed in making noise measurements on the
photovoltaic samples as on the photoconductive samples.
presented in Figures 21-22,
C-6-B and C-13.
The results are
The results are consistent except for samples
These two samples were measured the same day and had de-
creasing noise voltage at low frequencies.
It is thought that some error
was made in carrying out the noise measurements on these two samples
since they were the only ones exhibiting such behavior and such behavior
has not been reported in hetero junction diodes.
typical l/f noise spectrums.
The remaining diodes had
In contrast to the photoconductive samples
which approached Johnson noise at 10 kHz, the photovoltaic diodes approached Johnson noise at
1
kHz.
There was also considerably less increase in
the noise from high frequencies to low frequencies,
In the photoconduc-
tive detectors the noise Increased by a factor of approximately 100 from
10 kHz to 30 Hz, while in the photovoltaic diodes it increased by a factor
of approximately 10,
In photovoltaic detectors l/f noise is proportional
to the density of surface states at the Fermi level.
The noise levels in
the heterojunction diodes thus indicates a fairly low density of interface states.
Due to Johnson noise, the detectors with high resistances
showed the highest noise voltage.
C.
PHOTORESPONSE MEASUREMENTS
The various figures of merit for samples exhibiting some photoresponse
are presented In Table IV,
No photoresponse was observed in the G series
58
ID-*
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FREQUENCY (Hz)
FIGURE 21
.
NOISE SPECTRUM FOR C SERIES DIODES
59
-I
l__l
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10
io-V
10
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j3
D-65
12
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FIGURE 22.
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NOISE SPECTRUM FOR D SERIES DIODES
60
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diodes with a blackbody as the source of radiation.
values presented were obtained with a
of radiation.
1.50
The signal voltage
watt lightbulb as the source
It was not attempted to calculate figures of merit using
this large radiation source.
Responslvity and detectivity values for the
D series diodes were obtained with a 500 K blackbody as the radiation
source.
These values are far below those reported in Schottky barrier
diodes and in bulk crystal Pb.
Sn Te diodes.
It was observed in diode
D-65 that as the temperature rose, the signal voltage dropped rapidly.
At approximately 96 K the signal was 320 nV and at 103 K it had dropped
to 180 nV,
There was virtually no change in the noise at these two tem-
peratures which means a significant rise in the detectivity.
shows the rectification of this diode at 95 K and at 77 K,
Figure 20
There is con-
siderable difference in the reverse bias characteristics at the two temperatures.
There are indications that much better photoresponse could
be obtained at lower temperatures, however, not enough data was collected
Since much better photoresponse was ob-
in this thesis to confirm this.
tained in the D series diodes with the p layer exposed to incident radiation, it seems likely that the window effect helped the photoresponse.
62
VII.
CONCLUSIONS AND RECOMMENDATIONS
CONCLUSIONS
A.
1,
Sn Se Photoconductors
y
PTx,
1
-y
Although the amount of data gathered was limited, the photoconductive
response observed in Pb~ Q Sn_
Ref .
7.
..Se
was in agreement with that reported in
Single-crystal (100) oriented Pb_ qSn_ .Se material
.is
more sen-
sitive than material with a significant amount of (111) oriented structure.
The (100) oriented material also had much higher resistances than the ma-
terial with some (ill) orientation.
Pb
n
No photoresponse was observed in
£oSn_ ~ 3e samples tested and they all had very low resistances.
Noise
in both y ~ 0.10 and y = 0.32 compositions have considerable l/f noise
and were high, typically 100 times Johnson noise.
2,
Fo,
Sn Te Single Hetero junction Photovoltaic Diodes
Photovoltaic response has been developed in Pb.
hetero junction diodes.
It is believed that this is the first time photo-
voltaic response has been developed in Pb.
Sn Te hetero junctions .
noise spectrum has some l/f noise components up to
B.
Sn Te single
1
The
kHa,
RECOMMENDATIONS
1,
Pb,
i-y
Sn Se Photoconductors
y
Further work should be done on Pb Q £gSn
~gSe to see if higher
resistances and photoresponse can be developed.
The noise spectrum of
Pb 4 Sn Se semiconductors should be further investigated to determine if
1-y y
the noise comes from the surface or from non-ohmic contacts.
63
2.
Sn To Single Hetero junction Photovoltaic Diodes
Pb.
J-
•* J\.
.A.
Further investigation is needed to develop diodes which will
consistently yield photoresponse of higher values,
Blackbody response
at temperatures lower than 95 K should be investigated.
Spectral response
analysis of the photoresponse in the diodes should be made.
64
LIST OF REFERENCES
1.
Raytheon Infrared and Optical Research Laboratory, Special Microwave
Devices Operation Technical Report SM-228, Interim Report on Development of Lead-Tin Telluride Detectors by P.F, Debye, C.A. Kennedy,
and K.J. Linden, 7 April 1971.
,
2.
Rolls, V., Lee, R. and Eddington, R, J,, "Preparation and Properties
of Lead-Tin Telluride Phot odi odes," Solid State Electronics , v. 13,
p. 75-81, 1970.
3.
Melngallis, I. and Harman, T.G., "Single Crystal Le3£L-Tin Chalcogenides," S emiconductors and Semimetalst Infrared Detectors v. 5»
Academic Press, 1970.
,
4.
Strauss, A.J., "Metallurgical and Electronic Properties of
SnxTe and Pb.
Sn Se, and Other IV-VI Alloys,"
Transactions of
Pb,
J
1-x x
1-y y '
the Metallurgical Society of the AIMS v. 242, p. 3540365, March 1968,
'
,
5.
Calawa, A.R,, and others, "Crystal Growth, Annealing, and Diffusion
of Lead -Tin Chalcogenides," Transactions of the Metallurgical Society
"
of the AIMS , v. 242, p. 374-333, March lycSi
6.
Wang, C.C., Properties of Thin Film Pb.
.
Sn Te and Pb,
Sn Se
Infrared Photoconductors , Ph.D. Thesis, University of California,
Los Angeles, 1971.
7.
Holmquist, K.E., Thin-Film Pb
Q q
Sn
.Se Photoconductive Infrared
Photoconductivity Measurements , M.S. Thesis, U.S. Naval
Detectors
Postgraduate School, 1972.
t
8.
McBride, W.G., Thin-Film Pb
Q Q
Sn
.Se Photoconductive Infrared
Part I Metallurgical and Electrical Measurements , M.S.
Detectors.
Thesis, U.S. Naval Postgraduate School, 1972.
9.
Fernandez, J.M., Study of Ph.
Sn Te Diodes, M.S. Thesis, U.S. Naval
Postgraduate School, 1972.
10.
Anderson, R.L., "Experiments on Ge-GaAs Hetero junctions," SolidState Electronics , v. 5, p. 341-351, April 1962.
11.
Donnelly, J. P. and Milnes, A.G., "Current/Voltage Characteristics of
M
p-n Ge-Si and Ge-GaAs hetero junctions, Proceedings of IEEE v. 113,
p. 1468-1476, September 1966.
,
12.
Rose, A., Concepts in Photoconductivity and Allied Problems
1963.
13.
Bube, R.H., Photoconductivity of Solids
65
,
Wiley, i960.
,
Wiley,
lb,
Hudson, R.D., Infrared Systems Engineering , Wiley, 1969.
15.
Wolfe, W.L., ed., Handbook of Military Infrared Technology , U.S.
Government Printing Office, 1971.
16.
Kruse, D.W., McGlaughlin, L.D., and McQuistan, R.B., Elements of
Infrared Technology , Wiley, 1962.
17.
VanDerZeil, A., Fluctuation Phenomena in Semiconductors , Academic
Press Inc., 1959.
18.
Bennett, W.R., Electrical Noise , McGraw-Hill, i960.
19.
Logothetis, E.M., and others, "Infrared Detection "by Schottky Barriers
in Epitaxial PbTe," Applied Physics Letters v. 19, n. 9» 1 November
1971.
,
20.
Princeton Applied Research, Instruction Manual
Model 12^ 1971..
t
Lock -In Amplifier,
,
21.
Air Force Materials Laboratory Report AFML-TR-71-238 (unpublished),
Wright Patterson Air Force B&se, Narrow Gap Semiconductors "by T.F. Tao
and G.G. Wang, December, 1971.
U. S.
,
22.
Smith, G.L., Study of
PK
_Sn Te Single Heterojunction Diodes, M.S.
Thesis, U.S. Naval Postgraduate School, 1973.
66
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LCDR G. L. Smith
250? Valley Wood Rd.
Gautier, Mississippi
1
6.
S. R. Parker, Chairman
Department of Electrical Engineering
Naval Postgraduate School
Monterey, California
939^0
1
67
Security Classification
DOCUMENT CONTROL DATA
Security
c
las sil ication ol title,
-R&D
body ol abstract and indexing annotation mu.sl bt entered when the overall report
ting activity (CorponH »u;hor;
As, «
21.
Is
classified)
REPORT SECURITY CLASSIFICATION
Unclassified
Jl Postgraduate School
939^0
lerey, California
PI
R
2b.
CROUP
TITLE
T
Sn Se Phot odetec tors
Sn Te and Pb„
of Pb,
1-y y
1-x x
ly
J
iPTivE NOTES (Type
ol report and, inclusive dates)
r's Thesis? June 1973
<ORiSl (First name, middle
initial, .'««(
name)
Mn Edwin Romsos
>R T
DATE
OF PACES
7#.
TOTAL
9«.
ORIGINATOR'S REPORT NUMBER(S)
NO.
7b.
NO.
22
69
1973
TRACT OR GRANT
NO.
OF REFS
'ttJEC T NO.
vb.
OTHER REPORT
NO(S) (Any other number* that may be fealgned
this report)
RIBUTION STATEMENT
Approved for public release} distribution unlimited.
PLEMENTARY NOTES
12.
SPONSORING MILITARY ACTIVITY
Naval Postgraduate School
939^0
Monterey, California
TwcTphotodetection studies of the narrow gap IV -VI alloy semiconductors were
In the first study, the photoconductive response of Pb Q -Sn^Se and
Thin film samples were deposited on cleaved
, Q Sn„ 00 Se were investigated.
,oo
0,32
and BaF ? substrates by the one boat evaporation method and isothermally
Photoresponse was
jaled to enhance their photoconductive sensitivities.
In agreement with previously reported results
Sn ^Se samples.
ined in Pb
g
I.
McBride and K. *Holraquist of this laboratory, (111) oriented films were
less sensitive than (100) oriented films. No photoconductive response has
developed in Pb_ ggSn. r>^ e »
In the second study, single hetero junction
^ 0p8 6Sn
P^^S^Te
diodes fabricated by
Te were
0.2
tdied. Their electrical properties were studied by LCDR G. L. Smith in a
Operated at 90°K, 500°K blackbody responsivities up to 0.19 volt/
oipanion thesis.
It is believed that this is the first tine photovoltaic
a(t have been obtained.
^sitivity has been found in Pb« Sn Te hetero junctions. Noise measurements were
Aiential depositions of p type
Te and n t7pS Pb
.32
0.8
Sn
are much
It was found that noise voltages in our photovoltaic diodes
cler than in photoconductive samples.
made.
llo
F0 « M
NOV «ll*» / Sj
Y0101 -807-681
1^17^
1
II
t
1
(PAGE
1)
68
Security CU»sificatiort
A.-31408
ecurity Claetifiration
KEY WOKOS
BOLE
:
W
T
ROLE
WT
HOLE
WT
Letector3
nductor
junction
in Sellenide
in Telluride
ov
..1473
807-682
1
<
BACK)
69
Security Classification
A-
3
I
409
Thesis
R694 Romsos
1
I'*
S tUdy
7
L^7.r
'
C '**
f Pb
l- x SnTe and
Sn
Se
l. y
P hSto3etecy
tors.
pb
Pb
,°
1';17jI»
Thesis
R694 Romsos
Study of Pb.
Sn Te and
c."
X— X X
Pb.,
Sn Se photodecteci-y y
,
1
tors.
thesR694
bSmS**!?*'-** Subscript
3 2768 001 00297 5
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