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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 



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ADA 0328 51 




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 

^ - uJ 



0^ c 



Final Report for Period 15 August 1974 - 15 December 1975 



DISTRIBUTION STATEMENT 
Approved for public release; distribution unlimited. 



ECOM 



US ARMY ELECTRONICS COMMAND FORT MONMOUTH, NEW JERSEY 0 7 7 03 



~ ; a t ,: * 






Disclaimers 



The findings in this report are not to be construed as an 
official Department of the Army position, unless so desig- 
nated by other authorised documents. 

The citation of trade names and names of manufacturers in 
this report is not to be construed as official Government 
indorsement or approval of commercial products or services 

referenced herein. 



Disposition 



Destroy this report when it is no longer needed. Do not 
return it to the originator. 



UNCLAS 



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E AND ADDRESS 

Honeywell Radiation Center 
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It. CONTROLLING OFFICE NAME AND ADDRESS / t ) I 

US Army Electronics Command ( / /I 

Attn: AMSEL-CT-L-C V— ->■ 

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US Army Electronics Command f ! ^ 

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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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