Temperature Dependence of the

Responsivity of Indium-Doped

Silicon Infrared Detectors

R. D. Larrabee and W. R. Thurber

Electron Devices Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Washington, DC 20234

August 1 979

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79i786R

Prepared for The Defense Advanced Research Projects Agency

Arlington, VA 22209

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NBSIR 79-1 786R

• 1 TEMPERATURE DEPENDENCE OF THE

RESPONSIVITY OF INDIUM-DOPED

SILICON INFRARED DETECTORS

R D. Larrabee and W R. Thurber

Electron Devices Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Washington, DC 20234

August 1979

Prepared for The Defense Advanced Research Projects Agency

Arlington, VA 22209

US. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
)
TABLE OF CONTENTS

Page

Abstract 1

I . Int roduction 1

II. Measurement Techniques . ... 1

III. Results of Measurements 4

IV. Conclusions 8

V. Acknowledgment 8

VI. Reference 8

LIST OF FIGURES

1. Schematic illustration of the test detectors 2

2. Schematic diagram of the apparatus used to measure the responsivity of the test detectors as a function of bias and operating temperature 3

3. Peak responsivity os. temperature for 20-V bias (400 V/cm) for

three test detectors from a single float-zoned crystal of indiumdoped silicon 5

4. Peak responsivi ty Vs. temperature for 20-V bias (400 V/cm) for

two test detectors from a single CZ crystal of indium-doped

silicon 6

5. Peak responsivity vs. bias voltage for the best two float-zoned

and best CZ test detectors at the temperature of maximum response

with 20-V bias

i i i

7
)
Temperature Dependence of the Responsivity of Indium-Doped

Silicon Infrared Detectors

R. D. Larrabee and W. R. Thurber

Electron Devices Division

National Bureau of Standards

Washington, D.C. 20234

Abstract

Measurements were made to determine the degradation of responsivity of indium-doped extrinsic infrared detectors at temperatures above their normal operating range. The responsivity

was found to fall off very rapidly with temperature above the

optimum, and if operation at liquid nitrogen temperature is

desired, one must accept a degradation in responsivity of an order of magnitude or more.

Key Words: Indium-doped silicon; infrared detectors; responsivity.

I. INTRODUCTION

An infrared laser-scan technique for measuring the uniformity of

photoresponse over the surface of a wafer is being explored to determine

its suitability for screening out starting wafers of indium-doped silicon

which, if processed, would yield unacceptable detector arrays. Since

measurement time and cost are important considerations, the technique is

being studied at liquid nitrogen temperature instead of the lower operating temperature of indium-doped silicon infrared imaging arrays. The

main drawbacks of operation at higher temperatures are: 1) decreased

s ignal-to-noise ratio and 2) large dependence of the photoconductivity on temperature. The present study was undertaken to examine the temperature

dependence of the photoconductivity in state-of-the-art indium-doped silicon detectors in order to better appreciate the nature and magnitude of

the effects of higher than normal array operating temperature.

II. MEASUREMENT TECHNIQUES

A schematic illustration of the test detectors which were fabricated

from wafers of indium-doped silicon for the present measurement are shown

in figure l. The evaporated aluminum contacts were alloyed at 65U°C for

5 min, but otherwise the silicon was not subjected to high temperatures

during the detector fabrication. A schematic diagram of the apparatus

used in making the responsivity measurements is shown in figure 2. Photons from a 500-K blackbody radiation source pass through a mechanical

chopper and through a germanium-sapphire filter and then are incident on

the test detector which is located on the cold finger of a mechanical re- frigerator. A conventional measurement of the responsivity at the wavelength of peak response requires that the relative spectral response be

measured in addition to the blackbody responsivity at each temperature of

1
ALLOYED ALUMINUM CONTACTS EDGE POLISHED TO

2
o o LU C_3

3

Figure

2.

Schematic

diagram

of

the

apparatus

used

to

measure

the

responsivity

of

the

test

detectors

as

a

function

of

bias

and

operating

temperature.
interest. A conversion factor is then computed from the relative spectral response and used to convert the measured blackbody responsivity to

a peak wavelength responsivity. In the present measurement, the

sapphire component of the germanium-sapphire filter is used to determine

the long wavelength response characteristic of the system instead of

relying on the cut-off characteristic of the indium-doped silicon test

detectors to perform this function. Therefore, it was not necessary to measure the relative spectral response characteristic of each test detector at each temperature. The ratio of peak responsivity to filtered

5QG-K blackbody responsivity was computed using the measured spectral

transmission characteristic of the room temperature germanium-sapphire

filter and found to be 10.64. Responsivity measurements were made both

with and without this filter, and it was found that the values obtained

without the filter (using a nominal peak-to-blackbody response ratio of

2.32) averaged about 12 percent higher than the values obtained with the

filter. All of the responsivity data reported below were measured with

this filter in place. The germanium component of the germanium-sapphire

filter was included to absorb any short wavelength radiation from the

blackbody source (and ambient room light) that could otherwise excite

electron-hole pairs across the bandgap of the silicon test detectors and

possibly perturb the dynamics of the extrinsic photoconductivity mechanism.

The test detectors were biased with a constant voltage source through a small load resistor (i.e., a resistor with a resistance that is

small compared to the test detector resistance at all temperatures of

interest). The change in current due to chopping the incident radiation

from the blackbody source was measured as the change in voltage across

the load resistor and converted to a value of responsivity in amperes per

watt of incident radiation at an assumed peak wavelength of 6.4 pm.

III. RESULTS OF MEASUREMENTS

The results of responsivity measurements for test detectors fabricated from three wafers cut from a single float-zoned crystal of indiumdoped silicon are summarized in figure 3. Wafers B8 and B6 were subjected to nuclear transmutation doping [1] (NTD) techniques to precisely

compensate the background shallow acceptor impurities while wafer B3 was

not given this treatment. These data were observed at 20-V bias with a variety of load resistors, each selected to have a small resistance com- pared to the test detector resistance as the temperature varied over the

range indicated in figure 3. The NTD treatment has produced about an or- der of magnitude improvement in the responsivity at the optimum temperature, but this responsivity falls off very rapidly at temperatures above

this optimum.

The results of responsivity measurements for test detectors fabricated from two wafers cut from a single crystal of Czochralski (CZ)

indium-doped silicon are summarized in figure 4. These data were also

observed with 20-V bias and a variety of load resistors. The responsivity of the test detector from wafer A5 was comparable to that measured

4
RESPONSIVITY

(amperes/watt)

TEMPERATURE jkelvin)

Figure 3. Peak responsivity Vs. temperature for 20-V bias (400 V/cm)

for three test detectors from a single float-zoned crystal of indiumdoped silicon. Wafers B8 and B6 were compensated by NTD techniques,

while wafer B3 was not given this treatment.

5
RESPOWSiVUY

(amperes/watt)

Figure 4. Peak responsivity vs. temperature for 20-V bias (400 V/cm)

for two test detectors from a single CZ crystal of indium-doped silicon.

6
RESPONSIVITY

(amperes/watt]

APPLIED VOLTAGE [volts]

Figure 5. Peak responsivity vs. bias voltage for the best two floatzoned and best CZ test detectors at the temperature of maximum response

with 20-V bias. The test detector designation and temperature of measurement are indicated for each curve.

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NBS-114A IREV. 0-78 1

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 79-1786R

No. 3. Recipient's Accession No. "

4. TITLE AND SUBTITLE

Temperature Dependence of the Responsivity of Indium-Doped

Silicon Infrared Detectors

5. Publication Date August 1979

Code | £

7. AUTHOR(S) R. D. Larrabee and W. R. Thurber

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Pro|ect/Task/Wort< linrt No.

11. Contract/Grant No. ARPA Order No. 2397

Program Code 9Y10

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP) Defense Advanced Research Projects Agency

1400 Wilson Boulevard

Arlington, VA 22209

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

| ] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas factual summary of most sighi ficant information. If document includes a significant bibliography or

literature survey, mention it here.)

Measurements were made to determine the degradation of responsivity of indium-doped

extrinsic infrared detectors at temperatures above their normal operating range. The

responsivity was found to fall off very rapidly with temperature above the optimum,

and if operation at liquid nitrogen temperature is desired, one must accept a degradation in responsivity of an order of magnitude or more.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless . separated by semicolons)

Indium-doped silicon; infrared detectors; responsivity.

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