HONEYWELL RADIATION CENTER LEXINGTON MASS F/G
10.6-MICRON <HG*CD)TE PHOTODIODE MODULE. (U)
OCT 76 T KOEHLER DAAB07-71-C-0236
ECOM-71-0236-F NL
F/G 17/5
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Research and Development Technical Report
ECOM-71-0236-F -
10.6 Micron (HgCd)Te Photodiode Module
T. Koehler
HONEYWELL RADIATION CENTER
2 Forbes Road
Lexington, MA 02173
October 1976
D D c
inmS)r?w) Qg~
Ip' wc i . 9 :o 1
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Final Report for Period 15 August 1974 - 15 December 1975
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ECOM
US ARMY ELECTRONICS COMMAND FORT MONMOUTH, NEW JERSEY 0 7 7 03
~ ; a t ,: *
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19. KEY WORDS (Contlnum on reweree aid* If neceeeary and Identify by block number)
Mercury Cadmium Telluride; Photodiode; 10.6 Micron Detector;
Heterodyne Detector
00o5
20. ABSTRACT (Continue on reveree aide It neceeeary and Identify by block nu^ber)^^ /
This report describes the development of a 1 6Any (Hg,pa)Te thermo-
electrically cooled photodiode module operating at 174 K. The objective
of the program was the demonstration of 170 K operation with quantum
efficiency of 20 p erce nt^., bandwidth of 50 MHz and a 1 mA reverse saturation
current in a^5 x 10 - ^ cm Brea device. Several approaches were assessed
in order to active ar heavil^-dpped junction that would minimize diffusion
current at 170 K. Two 10.6Mjum/*(Hg,Cd)Te Photodiode Modules, each consistin
DD 1473
EDITION OF I NOV SS It &JMOLETE „ ^£1 ^ UNCLASSIFIED
r SECURITY CLASSIFICATION OF THIS RAGE fWhan Data Bntmtmd)
SECURITY CLASSIFICATION OF THIS PA0e{RR«n Dtm han<
k of a n-on-p diffused (Hg,Cd)Te diode with an active area of 2 x 10 cm
mounted on a 6-stage thermoelectric cooler, wer^'cT&livered . the better
device had a minimum detectable power of 7 .TjlxClQ JJ W/Hz, 23 MHz band-
width, and 8.8 mA saturation current at 174 K. 'The power consumption
for the module was less than 20 watts.
security classification of this RAoer»h«< dm* Si»n4
PREFACE
j
n
This Final Report was prepared by Honeywell Radiation
Center, Lexington, Massachusetts, under Contract No. DAAB07-71-C-0236 ,
10.6 Micron (Hg,Cd)Te Photodiode Module. It covers the period from
15 August 1974 through 15 December 1975. The Contract Monitor has
been Mrs. Claire Burke at ECOM, Fort Monmouth, New Jersey.
The Project Engineer was Toivo Koehler. Materials Engi-
neers were R. Lancaster and B. Jindal. The devices were fabricated
by L. Gauthier and J. Faticanti. Device evaluation was performed
by R. Bechdolt and R. Healey, and theoretical computer modeling of
elevated temperature performance was done by S. Tobin.
The heterodyne measurements described in the Appendix
were made by Dr. Hans Mocker of Honeywell Systems and Research Center.
TABLE OF CONTENTS
PAGE
1 INTRODUCTION 1
1.1 PROGRAM OBJECTIVES 1
1.2 PROGRAM SUMMARY 1
2 THEORY AND TECHNICAL APPROACH 5
2.1 THEORY 5
2.2 APPROACH 6
2.2.1 Quench Technique 6
2.2.2 Ion Implantation of Gold in n-type (Hg,Cd)Te 9
2.2.3 Indium Diffusion in p-type 9
2.3 PERFORMANCE CONSIDERATIONS 9
2.4 THERMOELECTRIC COOLER SPECIFICATIONS 10
3 EXPERIMENTAL RESULTS 12
3.1 MATERIALS GROWTH AND JUNCTION FORMATION 12
3.1.1 Quench Technique 12
3.1.2 Impurity Doped p-type (Hg,Cd)Te 12
3.1.3 Indium Doped n-type Material and Gold Ion
Implantation 12
3.1.4 Reduced Area Indium Diffused Junction 13
3.2 MODULE PERFORMANCE TESTS 13
3.2.1 Modules 13
3.2.2 Detectors 18
3.3 DISCUSSION 23
3.3.1 Extended Model !.. 23
4 CONCLUSIONS AND RECOMMENDATIONS 28
5 REFERENCES ' 30
APPENDIX
HETERODYNE CHARACTERIZATION OF TWO TE- COOLED
(Hg , Cd)Te PHOTOMIXERS 31
DISTRIBUTION LIST 4 8
SECTION 1
INTRODUCTION
Heterodyne detection as a means for detecting weak signals
is useful in many systems applications, such as remote sensing, com-
munications, optical radar rangefinders, battlefield surveillance
and velocity and turbulence measurements . 2 gThe technique is not new;
minimum detectable power (MDP) of 7.x, 10 W/Hz has been reported
for copper doped germanium at 4.2 K' IHg,Cd)Te photodiodes have
achieved MDP of 8 x 10 W/Hz at 77°K^ ’ ' . The previous phase of
this program reported a (Hg,Cd)Te 10.6-ym photomixer on a nine-stage
thermoelectric cooler. Quantum efficiency or MDP has always been
near the theoretical limit, but progress has occurred by extending
this performance to higher operating temperatures where cooling
requirements could be simplified for the system.
This development program has addressed several critical
areas, such as quantum efficiency, leakage current at 170 U K and thermo-
electric cooler module miniaturization. The success of this develop-
ment will have a significant impact on the development of practical
CC >2 laser systems. Other critical components, such as miniature CO 2
waveguide lasers are already available.
1.1 PROGRAM OBJECTIVES
The objective of this program was the development of a
practical, fast response sensitive, reverse biased (Hg,Cd)Te photo-
diode detector for 10.6-ym radiation which utilizes a thermoelectric
(TE) cooler to reach an operating temperature in the 145 K to 190 K
range. ^Tt^photodiode detector with an active area greater than
5 x 10 cm was to have quantum efficiency greater than 20% at
10.6 ym, an electrical bandwidth greater than 50 MHz, and a maximum
1-mA reverse bias leakage current while operating at 170 K.
1.2 PROGRAM SUMMARY
Several approaches were investigated in achieving 10.6-ym
photodiodes with minimum saturation current at 170 K. The goal of each
approach was to maximize junction doping and thus, reduce diffusion
current at 170°K. The approaches are summarized as follows:
• Ingot or wafer^guenc^ed from higher temperatures to
produce 3 x 10 cm p-type material resulting from
high native defect concentration. Junctions are
formed by donor diffusion.
18 - 3
• Impurity doped ingot to form 3 x 10 cm" p-type.
Junctions to be formed by donor ion implantation.
1
• Form p + - n junctions by ion implantation on heavily
doped n-type material.
17 -3
• Reduce area of n-p junctions on 1 x 10 cm defect
dominated p-type material.
Tljie quench approach failed to produce concentrations higher
than 5 x 10 cm and was abandoned.
The imgurity doping approach produced copper doped ingots
with 3 x 10 1 cm p-type concentration. Indium implantation and
indium diffusion failed to compensate the acceptors and form n-p
junctions. Solubility limits of indium in (Hg,Cd)Te were considered
as a problem for this process.
Implantation of gold on heavily doped n-type (Hg,Cd)Te did
not produce junctions. Problems were associated with the rapid
diffusion of gold in (Hg,Cd)Te and the inability to compensate donors
after the implant damage removal anneal.
17 -3
The final devices were fabricated from the 1 x 10 cm
defect dominated p-type material with reduced junction area. As
summarized in Table 1.1, these devices had quantum efficiencies
greater tlji^n 20%. The minimum detectable power (MDP) of unit 1 was
7.7 x 10 W/Hz when operating in heterodyne mode at 170 K. This
unit had a saturation current of 8.8 mA. Our theoretical calculations
show that for these kind of diodes the saturation current cannot be
reduced by much for the following reason: the diffusion current density
at 170 K is ^united by the n-side Auger lifetime and cannot be lower
than 10 A/ cm . This is equivalent to a current of 5 mA in a 250 pm
diameter device. The consequence of a high dark current was high dc
power dissipation resulting in a thermal load on the thermoelectric
cooler which raised the operating temperature to 174°K. Also, the
local oscillator induced current did not dominate the noise of the
device .
An improvement in device design and fabrication can reduce
the saturation purrent which would result in lower MDP. This can be
achieved by reducing the detector area and by increasing the doping
level of the p-side. The ability to control the impurity leygls w^uld
enable the fabrication on n-p+ or n-pp+ devices (p+ > 1 x 1 (j cm )
with smaller dark current.
The bandwidth of 23 MHz was less than anticipated from RC
time constant calculations, and therefore, it was assumed to be
limited by minority carrier diffusion to the junction. This band-
width is equivalent to a response time of 6.91 x 10" seconds. At
170 K the n-side minority carrier lifetime is limited by intrinsic
Auger lifetime which is caljglate^ as 4.8 x 10~ 8 seconds. The p-side
radiative limit with 6 x 10 cm hole concentration is calculated
2
Table 1.1
RESULTS
Parameter
Wavelength
Operating Temperature
Quantum Efficiency
Responsivity
Electrical Bandwidth
Active Area
Capacitance
Dark Current
Minimum Detectable
Power (W/Hz)
R SHUNT
Goal
10.6 ym
170°K
20% min
1.6 A/ W min
50 MHz min
250 ym Diameter min
20 pF max
3 mA max
(1 mA desirable)
(None)
(None)
Achieved
Unit 1**
Unit 2***
10.6 ym
10.6 ym
174°K
174°K
21%
32%
1. 79 A/ W
2.72 A/ W
23 MHz
N/A
150 ym
150 ym
*
*
8 . 8 mA
10.5 mA
-19
7.7x10
4 . 0xl0 -17
800 R
600 n
i
Not measurable
•kk
Detector
FI
kkk
Detector
G1
-7 o -9
5.56 x 10 seconds. At 77 K a lifetime of 1 x 10 seconds is
measured in p-type and is limited by a Schockly-Read type recombi-
nation center. Radiative recombination rates have never been observed
in p-type (Hg,Cd)Te.
The observed bandwidth of 23 MHz is, therefore^ determined
by minority carrier diffusion in p-tyge (Hg,Cd)Te at 170°K. The
increase in lifetime from 77 K to 170 K can be explained by shifts
in Fermi level relative to the recombination center as a function of
temperature. Independent evidence supporting the increase of lifetime
is the increase of dif fusion^limited quantum efficiency as temperature
increases from 77°K to 170°K^ ' .
The following conclusions were made as a result of this
program:
• 170°K thermoelectrically-cooled 10.6-Mm (Hg,Cd)Te
photomixers are feasible.
• New performance and design limits were established
for those photomixers .
• The minimum diffusion current density in the present
devices is determined by the Auger lifetim^ of the n-side
of the junction at 170 K and it is 10 A/cm .
The approach of doping the p-side of the junction
to high concentrations in order to reduce diffusion
current density was valid although the technique of
quenching in defects to accomplish this did not
achieve concentrations above 5 x lQl? cm - ^ p-type.
2
The 10 A/ cm limit can be achieved either by doping
the p-side more heavily or increasing the electron
lifetime in p-type material. The latter approach
reduces the bandwidth, a result which was observed.
-19
A minimum detectable power (MDP) of 7.7 x 10 W/Hz
was demonstrated at 174 K in heterodyne operation.
Calculations indicate that MDP degrades by only a
factor of 3 with 5 mW total power dissipation if the
saturation current density is 10 mA instead of 1 mA.
This degradation can be compensated with an increase
in quantum efficiency. The impact of an increase in
current is on TE cooler power consumption and not
necessarily detector performance.
SECTION 2
THEORY AND TECHNICAL APPROACH
This section describes the basic device theory of (Hg,Cd)Te
photodiodes as it existed at the beginning of this grogram and how it
was utilized to design a 10.6-Mm photomixer for 170 K operation.
Several unique approaches were attempted to realize the design para-
meters .
2 . 1 THEORY
Several parameters are important for the achievement of
useful performance at 170 U K in a 10.6-pm (Hg,Cd)Te photomixer. These
are quantum efficiency, dark current, wavelength, and bandwidth.
Quantum efficiency has been demonstrated at 170°K by
Koehler, Burke and McNally (4- > 6) . Wavelength can be tuned for 10.6 pm
at 170 K by selecting (Hg,Cd)Te with composition of 0.194 mole frac-
tion of CdTe(^>6). Bandwidth at 77°K is RC limited or diffusion
limited by a 1 x 10“^ in second minority carrier lifetime in p-type
material. The most critical parameter, however, is dark current,
which would introduce excessive shot noise and require high local
oscillator power and which would introduce power dissipation levels
which could be impossible to overcome with a six-stage thermoelectric
coder.
At 170 K the dark current is diffusion limited and the
saturation value of this current can be expressed as :
J^ n i 2 (~Jr + — Jr
“ \ N. 1 T e N„ 1 T h
where n^ is the intrinsic carrier concentration at temperature T
and k is the Boltzmann constant. The first term in the parenthesis
is the contribution of electrons in the p-side of the junction and
the second term is due to holes in the n-side of the junction.
A simple model would be to assume that the mobility to
lifetime ratio remains constant as temperature increases to 170 K
and also that the ratio is independent of doping concentration.
Such a model predicts that we can achieve saturation current of
2 A/cm^ at 170°K by doping the p-side to 3 x 1018 cm“3. (4) The
values assumed for the parameters were as follows :
, , n 18 -3
3 x 10 cm
i m 16 “3
1 x 10 cm
in 5 2 „-l -1
10 cm V s
= 1.0 x 10 7 s
2 -1 -1
= 200 cm V s
= 5 x 10 s
.
Figure 2.1 shows saturation current density as a function
of carrier concentration.
A better model was developed during the program which
introduced temperature dependent mobilities based on lattice scatter-
ing and Auger lifetime for the n-side of the junction and radiative
recombination on the p-side. This model will be discussed in Section 3,
2 . 2 APPROACH
The basic approach was to achieve a heavily doped junction
in (Hg,Cd)Te having 0.194 mole fraction CdTe composition. Several
approaches were assessed in sequence:
• Quench Technique .
• Gold ion implantation in n-type (Hg.Cd)Te
• Tndium diffusion into impurity doped p-type (Hg.Cd)Te
• Standard technique in low concentration p-type with
reduced area
The two best devices from any technique would be candi-
dates for integration with TE coolers.
2.2.1 Quench Technique
Tne (Hg,Cd)Te annealed under mercury pressure has a unique
equilibrium p-type concentration at each anneal temperature. This
concentration is related to stoichiometric defects which behave as
acceptors. Figure 2.2 shows equilibrium acceptor concentration as a
function of anneal temperature. In principle each concentration
can be "frozen" in by quenching from the anneal. Therefore, to
achieve P of 3 x 10^° cm - 3 the wafer must be annealed at 600 C and
quenched. The junctions are fabricated by the standard planar
indium diffusion and mercury anneal process.
t •fcSKJ'Ws-V
6
Figure 2.2 EQUILIBRIUM DEFECT DETERMINED P-TYPE CONCENTRATION
8
2.2.2 Ion Implantation of Gold in n-type (Hg,Cd)Te
Gold is a known acceptor in (Hg,Cd)Te. This approach con-
sists of starting with 1.0 x cm-3 n-type (Hg,Cd)Te and implanting
gold through a photoresist implantation mask to form a heavily doped
p+ region. The implant is followed by a post-anneal; the time and
temperature must be determined experimentally. The dose and energy
can be calculated from LSS theory to give p+ of 3 x 10 1 cm"3.
2.2.3 Indium Diffusion in p-type
18 -3
The 3 x 10 cm p-type will be grown by doping an ingot
with copper. Junctions will be formed by indium diffusion or indium
implantation .
2.2.4 Standard Process with Reduced Area
The standard process consists of indium diffusion through
a ZnS diffusion mask in mercury vapor as described by D. L. Spears(8).
The objective here is to reduce diffusion current by reducing size
and also consider the effects of p-side minority carrier lifetime as
a function of carrier concentration, i.a., defect concentration.
2.3 PERFORMANCE CONSIDERATIONS
The minimum detectable power (MDP) in the heterodyne mode
of operation is a useful figure of merit for evaluating the 10.6-pm
TF. cooled photomixer. MDP pei unit bandwidth (B) is defined as:
This equation indicates that the theoretical performance
limit can be achieved by either reducing saturation current (In which
case we are ultimately amplifier noise limited. Amplifier noise is
not included as a factor for 170°K operation.), or by increasing the
local oscillator power. Because of the finite cooling capacity of
the TE cooler, this approach is also limited.
The total power dissipation (Pjj) in the photomixer consists
of optical absorption and dc power dissipation resulting from bias
(V r ) and current composed of a dark component and a local oscillator
induced component :
P
D
+ V
r
P L0>
( 3 )
If we assume a bias of 0.2 volt and quantum efficiency of
0.2 and 0.5, we can examine the effect of saturation current on MDP
for values of constant power dissipation. Figure 2.3 shows MDP for
3 mW, 5 mW, and 10 mW power dissipation levels for saturation currents
from 1 mA to 1C mA. Five milliwatt power dissipation is well within
the limit of a thermoelectric cooler. An increase in I from 1 mA
to 10 mA would degrade MDP by a factor of three. Therefore, useful
thermoelectrically cooled 10.6-ym photomixers can be made even if
the saturation current is not improved, but if quantum efficiency is
improved.
2.4 THERMOELECTRIC COOLER SPECIFICATIONS
The thermoelectric cooler was manufactured to the follow-
ing specifications :
Number of Stages
Height
Cold Surface
Vacuum Enclosure
Power
Current
Voltage g
Power (10 torr)
6
2.03 cm
0.42 cm x 0.42 cm
4.19 cm high x 6.35 cm dia
3.6 A
6.4 volts
19.5 watts
The units are manufactured by Marlow Industries and
include a power supply, heat exchanger and temperature monitoring
thermistors .
Focused
Figure
SECTION 3
EXPERIMENTAL RESULTS
This section describes (1) the results achieved with the
approaches utilized and (2) the performance of the two modules deli-
vered to ECOM.
3.1 MATERIALS GROWTH AND JUNCTION FORMATION
3.1.1 Quench Technique
This technique is described in Section 2.2.1. The highest
p-type carrier concentration achieved with this technique was
5 x. 10 17 cm“3. At temperatures above 450°C, the resultant concentra-
tion was independent of anneal temperature . Quenching was achieved
by immersing the ampoule in oil and breaking. It is assumed that
the time required to achieve equilibrium defect concentration is
less than the quench time above 500 C. Diodes were not fabricated
from this material since it did not differ significantly from as-grown
ingots .
3.1.2 Impurity Doped p-type (Hg,Cd)Te
18
Copper was used to dope an ingot of (Hg,Cd)Te to 3 x 10
cm _ 3. Hall measurements indicate that this method was successful.
Two techniques were used to fabricate junctions in this
material, both failed. Indium was evaporated on the surface and
diffused in a mercury atmosphere at 275 C. Indium was also ion
implanted ( 10 ^ cm"2 at 200 KeV) and annealed to remove damage. No
junctions were observed. Jt was speculated that the diffusion tem-
perature and post -anneal temperatures we re too low and the indium
concentration was determined by a solubility limit or the rapid
diffusion of indium in (Hg.Cd)Te diffused the indium so it did not
achieve sufficient concentration to overdope the 3 x 10^® cm-3 copper
concentration .
3.1.3 Indium Doped n-type Material and Gold Ion Implantation
This technique is described in Section 2.2.2. (Hg.Cd)Te
with donor concentration of 1 x 10^-^ cm- 3 was selected. The indium
donor had been introduced during crystal growth. A gold acceptor
was ion implanted and a range of post-anneal conditions were evaluated
with times ranging from 150 C to 275 C. No junctions were observed.
Gold implants in 5 x 10^-^ cm~3 material have produced junctions. It
was assumed that the post-anneal ceased gold diffusion to levels
where it could not overdope the 10^ cm donor concentration.
12
3.1.4
Reduced Area Indium Diffused Junction
The technique for fabricating these devices is described
in Section 2.2.4. Successful devices were achieved by this method.
Two typical process batches are described. Process 3201 used
2 x 10*7 cm-3 zone leveled p-type material and a 5 x 10"^ cm2 area
mask. Process 3233 used 6 x 10^® cm~3 quench annealed p-type
material and a 1.8 x 10 - ^ cm2 area mask. Table 3.1 describes two
devices from Process 3201. The reverse impedance to forward impedance
ratios are very high. The current densities are quite low and would
produce a device with 3.6-mA saturation current in a 150-ym diameter
device .
Table 3.2 summarizes some devices from Process 3233. The
current densities at 146 K were higher in Process 3233 than in Pro-
cess 3201. This may be associated with a lower p-side doping. Some
devices did not achieve a 10.6-ym cutoff wavelength. The quantum
efficiencies were good and increased with temperature. The optimum
quantum efficiency occurred at 30 mV at 77 K and at 200 mV at 146°K.
Below 200 mV, the device impedance was 5 ohms as determined by the
series resistance. Figure 3.1 shows the temperature dependence of
the saturation current density. These devices were selected from
Process 3233 for integration with thermoelectric coolers. Current
densities at 170 K were extrapolated from fixed measurement tempera-
tures of 77°K, 146°K and 193°K. At 170°K the devices had the follow-
ing properties .
Element
X SAT
J SAT
E2
9.5 mA
52 A/ cm
FI
6.75 mA
37 A/cm
G1
14 . 6 mA
80 A/ cm'
Q.E. (Peak)
0.55
0.427
0.67
Q.E. (10.6 pm)
0.27
0.21
0 32
Element E2 was rejected because of a chip on the edge of
the active area. Element FI was mounted in cooler S/N 1 and ele-
ment G1 was mounted in cooler S/N 2. Figure 3.2 shows a spectral
response curve of element 3233 FI.
3.2 MODULE PERFORMANCE TESTS
3.2.1 Modules
The elements mounted in a 0.44 cm x 0.44 cm flatpack were
cemented to the 6-stage thermoelectric cooler stock with GE 7031
varnish. The cooler and housing were prebaked at 105°C in vacuum.
13
Table 3.1
PROCESS 3201
Temperature
Parameter
Units
Element Al
77°K
R
series
o
8.3
R shunt
KQ
1.55
J SAT
A/ 2
A/ cm
0.2
146°K
R
series
O
ay
5.4
R shunt
0.8
J SAT
A / 2
A/cm
14.8
170°K
R
series
Q
4.3
R shunt
n
550
J SAT
A / 2
A/ cm
26
X
CO
ym
n 10.6
%
--
14
Element A2
10
3.9
0.16
6.25
1.2
9.6
4.8
600
18
10.84
14
f..
CM
e
o
ON CO
<u
CO ON
r-.
o
m
<r cm
O 00
o
cx
<t m
Ml" so
CO 1
CO vO
CO -o
Ml vO
in vo i
CO
• .
• •
• 1
• •
• •
• •
• • i
•
Q.E.
X
o o
o o
o
o o
o o
o o
o o
o
CO
1
cm
CO
o
vo
vO VO
•
CM
l-H
CM
r— 4
O CO
O CO vo
O to
O CO
i— 1 -3- VO
CO
CO
X
o E
u a
CO
o o
•nJ" r-4
Ml" r— ( O
f—4
r—4
>d- t-i o
a)
CO
r— 4
r-4 i— 4
»— 1 r-4
r— 1 i— 4 »— 1
r-l r-l
i— 1 r—4
r-4 r-4 rH
f-H
CO
00
x>
w
•
cfl
u
I —4
H
o
os
II
CL,
cd
a)
n
<
V-/
(X ✓"N
E X
vO O
id
vo
NvOO
vo
vo
MOO
(DO
Ml- r»
N nT N
mi-
N st N
H ^
r-4 r— <
r—4
r— 1
H r—4
r— 1
rH
rH rH
X
vo
OS
VO
Os
vO
Os
VO
Os
vO
OS
CM
O
u
c
<u
E
a>
CM
Q
w
CM
U
CM
[>4
u
CM
O
W
15
146 -- 0.65 49.3
With 10" torr vacuum, the cooler achieved 180°K; with 2 x 10 torr
vacuum, the cooler reached 174°K. Cooldown time to operating tempera-
ture was 3 minutes and 20 seconds. Approximately 19 watts are required
for operation. Figure 3.3 shows the 6-stage TE cooler. Figure 3.4
shows the assembled module with heat exchanger.
3.2.2 Detectors
Detectors were checked by measuring current density and
blackbody responsivity . Table 3.3 summarizes these results. Hetero-
dyne measurements were made at Honeywell Systems and Research Center
by Dr. Hans Mocker.
The Appendix contains a detailed report of his measurements .
The bandwidth on unit G1 was not measurable because of a low signal-
to-noise ratio. Blackbody signal measurements were also impossible
because of low signal-to-noise ratio.
Figure 3.5 shows the reverse current characteristic of
detector FI at 173 K during testing. Figure 3.6 shows the forward
and reverse current of detector G1 as a function of voltage.
Table 3.3
MODULE PERFORMANCE
Unit No. 1
Unit No. 2
Units
(Detector FI)
(Detector Gl)
Temperature
°K
173
174
J SAT
A/cm^
48.2
88.9
Ebb (4 kHz )
A/W
2.23
NM
r a
A/W
4.65
NM
peak
—
517c
NM
n 10 . 6 ym
—
267o
NM
Heterodyne
MDP
W/Hz
-19
7.7 x 10
4.0 x 10 -17
Bandwidth
MHz
23
NM
Figure 3.3 THERMOELECTRIC COOLER
wm
■
Vertical
5 mA / division
Horizontal
200 mV /division
Figure 3.6 FORWARD AND REVERSE CURRENT OF
DETECTOR G1 AS A FUNCTION OF
VOLTAGE
3.3
DISCUSSION
The simple model presented in Section 2 and used in the
initial phase of this program, assumed a constant mobility to life-
time ratio in the diffusion current equation. This assumption
focused our approach on producing a heavily doped junction for 170 K
operation. During the program we expanded the model to include
temperature dependence of mobilities and lifetimes and examine the
consequences on our approach. At the same time, we examined some
of the experimental findings in this extended theoretical framework.
The major experimental observations were that current density does
decrease with doping, quantum efficiency increases with temperature,
and bandwidth is diffusion limited by a time constant much longer
than one would expect for p-type material at 77 K which has a 1-ns
electron lifetime.
3.3.1 Extended Model
The only changes made in the model are: (1) electron
mobility, (2) hole mobility, and (3) lifetimes are functions of
temperature. The lifetime functions are based on a paper by Kinch. ^
The hole lifetime (x^) on the n-side is assumed Auger limited. Above
170 K, x^ is intrinsic Auger; i.e., independent of doping. The p-side
electron lifetime (t ) is assumed to be radiative. These lifetimes
are expressed as :
T h * T Ai < 2ni2 % 2 >
where
T.. = 7.6 x 10" 18 e 2 (1+M ) % (l+2y )
oo
x exp | [(l+2u )/(l+U )] Eg/kT j
23
r-
The value for the overlap integral F is selected as
0.5 to give the best agreement with the calculation by Buss in the
paper by Kinch, et al.
Other parameters are:
e =
cx>
12.5
*
m =
e
0.005
*
% =
0.55
n ± = (8.46-2.29 x + 0 . 00342T) (10 14 Eg' 75 )
x (T 1-5 exp (-Eg/ 2kT)
The minority carrier mobility is based on expression fit
to experimental data by S. Tobin. The minority carrier mobilities
are calculated by assuming a constant mobility ratio (p / p^)
of 150. The electron mobility in p-type (Hg,Cd)Te is expressed as:
p = 1.2 x 10 5 /T 0,4
e
The hole mobility in n-type (Hg,Cd)Te is expressed as:
7 2 3
p, = 3.3 x 10/ r ' J
n
Figure 3.7 shows the diffusion current density based on
this model as a function of donor concentration and acceptor concen-
tration at 170°K. The model indicates that if the p-side lifetime
is radiative, then the current density is determined by the n-side
diffusion. At concentrations less than 10 ^ cm*^, the hole lifetime
is intrinsic Auger and diffusion current can be lowered by doping
the n-side to higher concentrations. Above 10^-6 cm"3, the current
density cannot be improved by either p-side doping or n-side doping.
This effect is a consequence of hole Auger lifetime, becoming a
function of doping and cancelling any effects on diffusion current.
The current is still determined by the n-side diffusion . Q The value
of 10 A/cin represents a limit to current density at 170 K.
The n-side doping concentrations of less than 7.9 x 10^ cm
at 170°K, can be neglected because junctions will not occur below the
intrinsic concentration of 11-pm cutoff wavelength material. It can
be assumed that junctions that are observed have concentrations
higher than this.
24
170
Experimental Results, however, show detector FI has a cur-
rent density of 37 A/ cm and a 6.9-ns response time. This current
density and response are not possible with the n-side limited model.
One explanation would be that the p-side electron lifetime is not
radiative. Figure 3.8 shows calculations of the currenj^density at
170°K a function of for a concentration of 5 x 10 cin and
3 x 10 1 cm. The limits for various n-side concentrations are also
shown. If the detectors had radiative electron lifetime they would
be diffusion limited by the n-side.
16 - 3
Detector FI was fabricated in 6 x 10 cm p-type material
and therefore, fits this model very well. The figure suggests it
may be possible to achieve a wider bandwidth and improved current
density with a more lji^avil^ doped p-side. Detector batch 3201 was
fabricated in 2 x 10 cm p-t^pe material and had a current den-
sity in the range 18 to 26 A/cm at 170 K. Figure 3.8 predicts a 1-ns
response time or a 156 MHz bandwidth for these detectors.
Lifetimes of this magnitude are observed in 77°K (Hg,Cd)Te
photodiodes and are attributed to a Schockly-Read recombination cen-
ters in p-type material. The center has been hypothesized to be a
defect, and therefore, the decrease of lifetime would be consistent
with increased doping, i.e., defect concentration.
The model also predicts that the optically active region
in this case would be the p-side. A 1 x 10 lifetime produces a
4.7 -Mm diffusion length which is adequate for high quantum efficiency
if the junction is shallow and/or the n-layer is degenerate.
It should be noted that the bandwidth is not RC limited
in detector 3233F1. The p-side concentration was measured as
6 x lO 1 ^ cm2 . With 0.1 volt bias, 8.3 ohm series resistance and
2 x 10 cm area, the capacitance is 57.6 pF. With a 50 ohm ampli-
fier, this is equivalent to 47 MHz. The actual bandwidth i^prob^bly
higher because the n-side concentration is less than 6 x 10 cm
26
J n T J
170
SECTION 4
CONCLUSIONS AND RECOMMENDATIONS
I <
As a result of this program, the following conclusions
can be made about 10.6-um (Hg,Cd)Te photovoltaic photomixers
cooled to 170°K by thermoelectric coolers;
Operation^t 170 K was demonstrated with
7.7 x 10 W/Hz minimum detectable power and
23 MHz bandwidth.
The theoretical diffusion current density limit
with Auger hole lifetime on the n-side and
radiative electron lifetime on the p-side is
10 A/cm . In a 250-pm diameter device, this is
equivalent to 5 mA. Therefore, the design goal
of 1 mA is not realizable in this size. A 150-pm
diameter device would have a 2 mA diffusion cur-
rent which can be dominated by a local oscillator
current .
3. In the present devices, the radiative lifetime is
not observed in p-type material. A lifetime of
6.9 ns is observed and is assumed to be related
to defect related to the acceptor concentration.
The observe^ current densities of 20 A/cm
and 44 A/cm at 170 K are consistent with this doping
and lifetime. Improvements in present devices can
be made by increasing the p-type concentration. Spch
improvement would ultimately be limited to 10 A/cm
by the n-side diffusion current.
4. High diffusion current does not necessarily degrade
mixer performance, especially if the thermoelectric
cooler can handle 10 ~ watts of power dissipation.
18 -3
5. Techniques for achieving junctions with 3 x 10 cm
acceptor concentration were not demon|t|rate^. The
quench technique only achieved 5 x 10 ^gm goncen-
tration. Copper doping achieved 3 x 10 cm con-
centration, but conventional indium doping techniques
failed to produce an n-layer, i.e., a junction.
16 — 3
The devices delivered were fabricated in 6 x 10 cm
p-type material with 6 . 9 ns minority carrier lifetime at 170 K.
It is believed that this lifetime can be reduced to 1 ns in
17 -3
3 x 10 cm material. This type of device would permit greater
than 50 MHz bandwidth operation. It is recommended that devices of
this type should be demonstrated experimentally.
Th^ size goal of 250-ym diameter is not feasible because
of a 10 A/cin diffusion current limit at 170 K. Future devices
should be restricted to 150-ym diameter or less.
The program has devoted no effort to thermal stability.
Encapsulation techniques are available and should be applied to future
devices to permit bakeout of detector and thermoelectric cooler to
achieve long vacuum life for the module.
SECTION 5
REFERENCES
1. R. J. Keyes and T. M. Quist, Semi-conductors and Semi -metals ,
Vol. 5, 321, 1970.
2. M. C. Teich, "Coherent Detection in the Infrared," Semi-conductors
and Semi -metals , Vol. 5, Chapter 9, 1970.
3. B. J. Peyton, et al, IEEE J. Quantum Electronics, Vol. QE-8,
No. 2, February 1972.
4. T. Koehler and P. J. McNally, Optical Engineering, July/ Aug 1974.
5. M. A. Kinch, M. J. Brau, and A. Simmons, J. Applied Phys . ,
Vol. 44, No. 4, April 1973.
6. C. Burke and T. Koehler, sixth DOD Laser Conference, March 1974.
7. H. Mocker and T. Koehler, "Miniaturized Coherent Transmitter-
Receiver System," Proc . Electro-Optical Systems Conference, 593,
November 1975.
8. D. L. Spears, IRIS Detector Specialty Group Meeting, Washington,
DC, March 1973.
9. S. Tobin (Private Communication).
30
APPENDIX
HETERODYNE CHARACTERIZATION OF TWO TE-COOLED
(Hg ,Cd)Te PHOTOMIXERS
by
Dr. Hans Mocker
This Appendix summarizes the results of measurements on
two (2) thermoelectrically-cooled (Hg,Cd)Te Detector Modules, Type LK
170 Al., Serial Nos. Ul-Kl (referred to as Unit 1) and U2-K1 (referred
to as Unit 2) . Unit 1 contains detector FI and Unit 2 contains
detector G1 . These measurements were carried out at the Honeywell
Systems and Research Center.
Eight different types of measurements were made on Unit 1.
Unit 2 was only investigated for 1 to 3 due to its higher dark
current and lower NEP. The following measurements were made on
Detector Ul-Kl:
1. Current-voltage diagram (also for Unit Z) with local
oscillator power parameter.
2. Derived from 1: sensitivity to 1. o. power induced
current vs 1. o. power (also for Unit 2)
3. Determination of NEP (also for Unit 2)
A. Determination of S/N-ratio as a function of 1. o. power
for 5 different bias conditions.
5. Determination of current-voltage characteristics as a
function of 1. o. power (for 5 different bias condi-
tions) .
6. Determination of detector temperature as a function of
1. o. power for 5 bias conditions.
7. Noise investigations from motor.
8. Frequency response measurement (0-30 MHz).
I. DESCRIPTION OF THE EXPERIMENTAL SETUP
For the reported measurements the following experimental
setup was used (the schematic is shown in Figure 1). Two stable
Honeywell C02 lasers (Models 3000 and 7000) with a power output
of 3 watts and 8 watts, respectively, were used. Both lasers hetero-
dyned have a short-term stability of better than 10^ and a long-term
stability of better than 10?. The model 7000 was designated as local
oscillator (1. o.) and its power level coarse - attenuated by a
double - Brewster plate attenuator and fine attenuated by CaF2 -
flats. The 1. o. beam enters through a beamsplitter and is focused
by a germanium field lens on the (Hg,Cd)Te detector. The field lens
has a focal length of 1.5 inches. The model 3000 is designated to
generate variable signal powers. The laser beam can enter through a
series of optical CaF 2 - flat and an optical attenuation up to 100 dB
can be generated in this way. The beam is then reflected on a two-
dimensionally adjustable mirror and is then combined with the 1. o. -
beam at the germanium beamsplitter. Both beams thus fall spatially
coincident on the mercury cadmium telluride detector which is
mounted on a two-dimensional positioner.
Beam alignment is accomplished in the following way: the
1. o. beam is chopped by a low-frequency chopper and the two-dimensional
positioner of the detector adjusted to obtain maximum signal from the
detector. The detector can also be moved along the optic axis for a
one-time positioning at the focal point of the field lens. The signal
laser beam is then superimposed by adjusting the reflecting mirror M]^
with the signal laser beam being chopped. Fine adjustments are being
made once the beat signal of the two lasers has been obtained by
maximizing the S/N ratio on the spectrum analyzer.
The power level of the lasers that falls on the detector
can be measured by sliding into position a mirror M2 that reflects
the laser energy onto a pyroelectric radiometer (Laser Precision
MORK 3440). The transition under oscillation can be adjusted by
means of piezoelectric frequency control on each of the lasers and
monitored by a Wavelength Analyzer (Optical Engineering) after
insertion of a mirror M^.
Attenuation of the signal laser beam is accomplished by
insertion of pairs of calcium fluoride flats that are polished flat
to better than A/10 and parallel to less than 10 arc seconds. Indi-
vidual slabs are tilted by 7 degrees with respect to each other to
avoid a Fabry-Perot interferometer effect and not to create any
lateral beam offset. Each attenuation pair has an attenuation of
approximately 10 dB. An independent exact calibration with a CO 2
laser beam was made of each attenuator set.
The detector is evacuated by a 50- liter ion pump to a
vacuum of better than 10-6 Torr. A bias network as shown in Figure 2
was built to allow the setting of the dc voltage and current applied
to the detector. The temperature of the detector is monitored with
a thermistor and a digital ohmmeter. A calibration of the device
is shown in Table 1 .
The output of the detector is fed into an IF amplifier
with a center frequency of 10 MHz and a bandwidth of 2 MHz (type
RHG 1002). Both lasers are set off to a 10 MHz beat frequency by
means of piezoelectric frequency control. Due to their good long-
term stability, the lasers will hold the 10 MHz beat frequency
over many hours. The output of the IF amplifier is fed into a
33
Table 1
HOT SIDE
COL
D SIDE
TEMPERATURE
RESISTANCE
TEMPERATURE
RESISTANCE
(CELSIUS)
(K-OHMS)
(CELSIUS)
(K-OHMS)
0.00
3.820
-110.00
3641.757
1.00
3.670
-109.00
3305.013
2.00
3.526
-108.00
3002.936
3.00
3.388
-107.00
2731.621
4.00
3.257
-106.00
2487.640
5.00
3.132
-105.00
2267.975
6.00
3.013
-104.00
2069.970
7.00
2.899
-103.00
1891.284
8.00
2.790
-102.00
1729.848
9.00
2.686
-101.00
1583.834
10.00
2.586
-100.00
1451.624
11.00
2.491
-99.00
1331.784
12.00
2.400
-98.00
1223.041
13.00
2.313
-97.00
1124.265
14.00
2.229
-96.00
1034.450
15.00
2.150
-95.00
952.701
16.00
2.073
-94.00
878.220
17.00
2.000
-93.00
810.295
18,00
1.930
-92.00
748.288
19.00
1.863
-91.00
691.631
20.00
1.798
-90.00
639.814
21.00
1.736
-89.00
592.381
22.00
1.677
-88.00
548.921
23.00
1.620
-87.00
509.066
24.00
1.565
-86.00
472.486
25.00
1.513
-85.00
438.832
26.00
1.463
-84.00
407.987
27.00
1.414
-83.00
379.558
28.00
1.368
-82.00
353.377
29.00
1.323
-81.00
329.247
30.00
1.280
-80.00
306.990
31.00
1.239
-79.00
286.444
32.00
1.200
-78.00
267.463
33.00
1.161
-77. DO
249.914
34.00
1.125
-76.00
233.678
35.00
1.089
-75.00
218.645
36.00
1.056
-74.00
204.716
37.00
1.023
-73.00
191.800
38.00
0.991
-72.00
179.816
39.00
0.961
-71.00
168.689
40.00
0.932
-70.00
158.349
41.00
0.904
-69.00
148.736
42.00
0.877
-68.00
139.791
43.00
0.850
-67.00
131.464
44.00
0.825
-66.00
123.706
45.00
0.801
-65.00
116.474
46.00
0.777
-64.00
109.728
47.00
0.755
-63.00
103.431
48.00
0.733
-62.00
97.551
49.00
0.712
-61.00
92.055
50.00
0.692
-60.00
86.917
35
spectrum analyzer. Both a Hewlett-Packard analyzer (type 8555A)
for the frequency response measurements as well as Panoramic analyzer
(type SPA-3) have been used.
II. EXPERIMENTAL RESULTS
a) Current-Voltage Diagram .
The current-voltage diagram for Unit 1 (Detector FI) is shown
in Figure 3. The detector saturates with no local
oscillator power at about 7 mA. (See also Figure 4a)
The saturation curve has a fairly flat shoulder but at
a higher bias voltage the current starts increasing
again at a moderate slope. With increasing local
oscillator power the detector increases its saturation
level due to the local oscillator induced current. This
increase (see Figure 5 and 4b) is initially linear with
local oscillator power (up to 3 mW) and starts then to
saturate. The slope for detector FI is 0.8 mA/mW.
The current-voltage diagram for Unit 2 (Detector Gl) is
shown in Figure 6. This detector never reaches a flat
saturation plateau but only signs of onset of satura-
tion and this at substantially higher current levels
(12-13 mA) . The local oscillator induced current is
shown in Figure 7. It shows basically the same beha-
vior as for detector 1 with the exception of a higher
slope of 1.6 mA/mW and a reduced linear regime (<1.5 mW) .
From a comparison of both detectors it appears that the
relative local oscillator induced current is approximately
identical for both detectors.
b) S/N-Ratio Measurements .
S/N ratio measurements were made with the experimental
setup as described in Figure 1. For this purpose a
10 MHz offset frequency was chosen and the effective
bandwidth of the receiving system (Panoramic spectrum
analyzer) was 130 kHz; the noise figure of the IF-
amplifier was 4 dB. Under these conditions the minimum
detectable power level for detector FI was Ps* 1111 = 2 54
x 10 watts for detector Gl was Ps min = 1.3 x lO' 1 ^ watts.
Thus, the NEP for detector FI is NEP^ = 7.7 x 10"19 w/Hz
and for detector Gl NEP 2 = 4.0 x iq- 17 W/Hz.
36
8.1 irW
2.6 mW
0.29 n*J
0.09 i*.'
J 1 I I !__►
0.2 0.3 0.4 0.5 0.6 V Voltage
Figure 3 I-V DIAGRAM FOR DETECTOR FI IN UNIT 1
37
!
1
** t n*
(VERTICAL SCALE: 2 mA/division)
(HORIZONTAL SCALE: 0.5 Volts/division)
Figure 5 LOCAL OSCILLATOR INDUCED CURRENT VS LOCAL OSCILLATOR POWER
FOR DETECTOR FI
Figure 7 LOCAL OSCILLATOR INDUCED CURRENT VS LOCAL OSCILLATOR POWER
FOR DETECTOR G1
c) Local Oscillator Optimization Studies .
For this type of measurement the transmitter laser
was attenuated to approximately 10"8 watts and the
current and voltage values of the detector were
determined as the local oscillator level was
increased from 0 to 14 mW. The S/N ratio was
measured simultaneously on the spectrum analyzer.
The initial bias conditions for the detector are
shown in Table 2 :
Table 2
INITIAL BIAS CONDITIONS FOR DETECTOR Fl
Condition
1
2
3
4
5
Current (mA)
6.1
6.7
7.2
7.6
8.3
Voltage (V)
0.016
0.22
0.31
0.387
0.49
Figure 8 shows the change in detector character-
istics as a function of local oscillator power.
Figure 9 shows the corresponding plot of the S/N
ratio as a function of 1. o. power. One can see
that the S/N ratio is optimized for the bias con-
ditions 4 and 5 and that local oscillation power
levels between 0.5 and 1 mW are most suitable for
operation. The total dissipated power level for
a 1 mW 1. o. power and bias condition 4 and 5 is
as follows:
Condition 4: 2.1 mW bias +lmWl. o. = 3.1 mW total
Condition 5: 3 mW bias + 1 mW 1. o. = 4 mW total
Figure 10 shows the temperature of the (Hg,Cd)Te
detector as a function of the local oscillator power
for the 5 bias conditions as shown in Table 2. Thermal
heating of the element becomes noticeable at 1. o. power
levels of larger than 1 mW. The temperature of the
element increases for the bias conditions from 1
through 5 as expected. For the bias conditions 4
and 5 the temperature is reduced for 1. o. power
levels between 0.5 and 2 mW due to the stronger reduc-
tion in bias power. At higher 1. o. power levels the
temperature increases rapidly due to thermal heating.
Voltage
Figure 8 I-V DIAGRAM VERSUS LOCAL OSCILLATOR POWER FOR DETECTOR Fl
Figure 9 S/N RATIO AS A FUNCTION OF L.O. -POWER FOR VARIOUS BIAS CONDITIONS
FOR DETECTOR FI
Temperature
0-1 1.0 Local Oscillator Power 10
d) Frequency Response and Motor Noise Determination .
The frequency response of the (Hg,Cd)Te detector
was determined by generating a variable offset
frequency between 0 and 30 MHz. The S/N ratio
over this frequency interval was determined by
using the H.P. - spectrum analyzer as a receiver.
The "result of this measurement for detector FI
is shown in Figure 11. The results indicate that
the S/N measurement at 10 MHz did not see any
clipping since the response is virtually flat to
20 MHz. The 3-dB rolloff is at 23 MHz.
During measurements with a wideband, low noise
amplifier (5-500 MHz, noise figure 2 dB) it was
observed that the spectrum seen on the H.P.
analyzer showed a multiplicity of noise spikes
at discrete but in time slowly varying frequencies.
In order to determine their origin and order of
magnitude a beat signal of approximately 30 dB
was generated at a frequency of 10 MHz and sent
into a wideband amplifier and H.P. analyzer.
Besides the beat noise at 10 MHz (and a self-beat
of the laser at 5 MHz) a large number of noise beats
can be seen between 5 and 50 MHz. To confirm that
the noise is due to the immediate increase in
detector temperature. Thus, it appears that future
work should consider shielding the detector or
using a pneumatically driven turbine blade for cool-
ing purposes .
46
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