THE UNIVERSITY OF MICHIGAN INDUSTRY PROGRAM OF THE COLEGE OF ENGINEERING A SINGLE-CRYSTAL PHOTOCONDUCTIVE TELLURIUM DETECTOR G. Suits P. Rice April, 1958 IP-286

ACKNOWLEDGEMENT Permission has been granted by the U. S. Army Liaison Office for the distribution of this Project MICHIGAN report 2144-240-T by the College of Engineering Industry Program. ii

ABSTRACT Single-crystal tellurium photoconducting detectors have been found to possess high sensitivities and short response times comparable to those of recent lead-salt-film detectors. The special properties of telluritum detectors make them appear particularly promising for the following applications: 1. Two-color infrared scanning systems, 2. Scanning systems using matched linear arrays of detectors, and 3. Infrared sensors which operate under conditions of high atmospheric attenuation. iii

PREFACE Project MICHIGAN is a research and development project in the field of combat surveillance which has been carried on by the Engineering Research Institute of The University of Michigan since 1953 under an Army contract supervised by the Signal Corps. The project is engaged in research and development in various fields of science and engineering to improve combat surveillance methods and equipment to meet the long-range operational requirements of the Army in the field. Major emphasis is placed on research and development in the areas of optics vision, infrared, acoustics, seismics, radiometry, radar, and in the fields of data-link, data-processing, data-display, and control and guidance systems for aerial platforms. In addition, the project develops new combat surveillance concepts and it evaluates them and existing systems through simulation and analysis. Project MICHIGAN is now carried on for the U.S. Army Signal Corps under Department of the Army Prime Contract Number DA-36*039 SC-52654, iv

TABLE OF CONTENTS Page ACKNOWLEDGEMENT ii ABSTRACT iii PREFACE iv LIST OF FIGURES vi INTRODUCTION 1 PROPERTIES AND PREPARATION OF TELLURIUM 2 PRELIMINARY DETECTOR DATA 6 Reproducibility 6 Low Atmospheric Attenuation 6 Two-Color Application 6 Noise Characteristics 7 Matched Linear Array Potentialities 7 Range of Available Detector Sizes 8 Variation of Sensitivity with Temperature 8 REFERENCES 9 v

LIST OF FIGURES Number Page 1 Effect of Aging on Resistance Ratio of Tellurium Crystals 3 2 Sensitivity Profile for Te No. 29 (Soldered Contacts) 5 3 Spectral Response of a Tellurium Photoconductive Detector (Cell Area 0.5 m2) 5 vi

I. INTRODUCTION The Infrared Laboratory, in an investigation begun in July 1957, has found that single-crystal tellurium photoconducting detectors possess high sensitivities and short response times comparable to those of recent lead-salt-film detectors, The tellurium detectors have special properties which make them appear particularly promising for use in two-color infrared scanning systems, scanning systems using matched linear arrays of detectors, and infrared sensors which.operate under conditions of high atmospheric attenuation. During the fabrication and testing of the detectors, the crystals were grown from the vapor phase of the element so as to form small hexagonal prisms. These crystals had lengths which varied from 1 mm to 4 mm and widths which varied from 0.1 mm to 1.0 mm. The semi-conductor properties of tellurium had been measured by many investigators since 1918. The most recent and comprehensive contributions were made at Purdue University (Ref. 1) and at the University of Pennsylvania (Ref. 2 and 3). This work, however, had not been directed toward the evaluation of tellurium as an infrared detector. Data of this type were obtained by Moss (Ref. 4) for cooled thin evaporated films of the element. The sensitivities which he observed were only moderate (NEP of 10-8 to 10 9 watts for a 20000C tungsten-filament spectrum at 85 cps, with a 1-cps band pass). His observed response times ranged from 300 to 700 microseconds. Our investigation revealed that the single-crystal tellurium detectors possess higher sensitivities and shorter response times than did the films described by Moss. The details of this investigations appear below. -1

II. PROPERTIES AND PREPARATION OF TELLURIUM Tellurium is an element in the sixth column of the periodic table. It has a hexagonal crystal structure, a melting point of 450oC, a metallic lustre, and a band gap of about 0.34 ev. When the crystals are grown from the vapor phase, the crystal habit varies. The crystals start to grow in the form of thin hexagonal prisms. Continued condensation changes this habit to a more bulky shape. A low-pressure hydrogen atmosphere is used to control the diffusion rate down the thermal gradient of the tube. The initial tellurium was 99.999 per cent pure, and was obtained from American Smelting and Refining Company (Lot No. P8009). The typical hydrogen pressure varies from 1 to 5 mm (Hg), and the melt temperature is about 500~C. Distillation times of about 20 hours are allowed. After the distillation, the tube is cracked open and the crystals of proper shape are harvested. Contacts can be applied in two ways: (1) by soldering with bismuth to the tellurium, followed by soldering indium or lead-tin to the bismuth, or (2) by welding a hot wire into the tellurium. Platinum wire is most easily welded, although bare copper or gold wire serves well. After the contacts have been applied, the crystals are allowed to age for a few days. The aging process is not fully understood at present; however, it is quite clear from experiments that the beginning of the aging process starts with the application of the contact iad not with the exposure of the crystal to the atmosphere. The sensitivity of the crystal is found to be indicated by the ratio of resistance of the crystal at room temperature to the maximum resistance as the temperature of the crystal is reduced toward 77~K. The resistance ratio Rmax/Rroom is plotted in Figure la as a function of the time after exposure to the -2

0. 24 6 81024 6 8 12 I.I ~Ia *~. -pl:=.. -: ww TIME FROM APPLICATION OF LEADS - HRS Lu lb XFIG. 1 EFFECT OF AG ON R E RO OF T M 0.1 2 4 6 8 1.0 2 4 6 8 10 2 TIME FROM EXPOSURE TO AIR- HRS 1a cc a: LU 0.1 2 4 6 8 1.0 2 4 6 8'10 2 TIME FROM APPLICATION OF LEADS - HRS 1b FIG. 1 EFFECT OF AGING ON RESISTANCE RATIO OF TELLURIUM CRYSTALS - 3 -

atmosphere7 where the contacts are applied at different times. The data are replotted in Figure lb with the time measured from the time of contact application. The results clearly show that the aging effect is started at the time of contact application. So far no distinction has been found between the quality and aging characteristics of soldered contacts in contradistinction to welded contacts. In addition, the sensitivity profiles of the crystals indicate that maximum sensitivity occurs near the contact. The contacts are not rectifying, and the sensitivity shows no dependence on the direction of the current. A typical crystal-sensitivity profile is shown in Figure 2. -4

z O L) LU LU.w 1002nd Contact 90 1 80 A 70 _Thermal Sink and First Contact 60 50 \ 40 30 2010 DISTANCE ALONG THE CRYSTALFIG. 2 SENSITIVITY PROFILE FOR Te No. 29 (Soldered Contacts) z O 0 0. I — LU z w -I w O Lz 0 1 2 3 4 5 6 WAVELENGTH IN MICRONS FIG. 3 SPECTRAL RESPONSE OF A TELLURIUM PHOTOCONDUCTIVE DETECTOR (Cell Area 0.5mm2) - 5 -

III, PRELIMINARY DETECTOR DATA A number of crystals of the element were given preliminary tests. The observed noise equivalent power (NEP), which is the infrared signal power incident on the detector that results in a signal-to-noise ratio if 1, is of the order of 10l watts. The test conditions were: 5000K blackbody, 85-cps chopping rate, 5-cps bandwidth, and liquid-nitrogen-copled cells. The absolute response as a function of wave length has a peak NEP of about lO1ll watts at 3.5 microns, with a drop of a factor of ten at 4.2 microns on the long-wavelength side (Fig. 3)A. Reproducibility It appears at present to be a simple matter to produce tellurium detectors with good sensitivity and with a relatively high degree of reproducibility. Out of three separate batches, the average noise equivalent input (NEP divided by the area of the sensitive element) was 2.4 x 10-8 (~ 50 per cent) watts/cm2. B. Low Atmospheric Attenuation The peak sensitivity is the 4-micron atmospheric window. Since this window is a relatively transparent one, the attenuation of a blackbody signal with increasing atmospheric path length should be less with the Te detector than with detectors whose long-wavelength limit falls between 4 and 8 microns, where atmospheric absorption is greater. Co Two-Color Application Since Te cells have a fairly sharp peak near 3.5 $, they serve as their own filters, and can be used directly as one element of a "two-color" infrared detection system. Detection in the second color might be provided by a lead selenide, lead telluride, indium antimonide, or thermal detector. -6

D. Noise Characteristics The dark resistance of the Te cells studied is ideally suited for direct connection to a transistor preamplifier. Presently available transLstors possess noise factors at 1 kc of roughly 6 db above the Johnson noise of room-temperature source resistance in the 500 to 2000 ohm range. A typical Te detector has a resistance which falls in or near this range and provides the transistor preamplifier with its optimum source resistance. The present cells are 1/f noise limited, and the bias current on the detectors can easily be adjusted to bring the detector noise above the preamplifier noise so that the system becomes detector-noise limited. All of the data described above were taken under these conditions, using transistor preamplifiers. E. Matched Linear Array Potentialities The crystal habit of Te grown from the vapor is a slender hexagonal prism which is ideally suited to the construction of very small detector elements or for the fabrication of linear arrays simply by applying multiple contacts to a single crystal. The use of linear arrays permits the utilization of lower mechanical scanning speeds while acquiring increased resolution (through the use of smaller detector elements) and increased sensitivity (through the use of narrower bandwidths). A danger in such a linear array is the possibility of "cross-talk" between adjacent cells. However, preliminary tests of cross-talk between two adjacent 1 x 1 mm detectors made by applying contacts to a 1 x 2 mm crystal indicated the cross-talk signal to be lower than the primary signal level by more than 20 db. A secondary concern is the matching of cell characteristics. This appears at present to offer little difficulty since the crystals appear to be fairly homogeneous. -7

F Range of Available Detector Sizes There is probably no fundamental limitation to the size of Te detectors; however, all detectors to date have been grown from the vapor phase, and the-fabrication of detectors larger than about 1 num2 has not been achieved without serious sacrifice of response time. Grinding and etching (to make the cell thin) seems to irreversibly affect the speed of response. The problem is thus to find a means of preparing large thin plates in which the speed of response is either not lost or can be recovered. G. Variation of Sensitivity with Temperature The sensitivity of the Te detectors in their present form rapidly rises as the temperature falls below about 1000Ko At 789K the sensitivity is still rising. Accordingly, if the temperature is not held accurately constant the fluctuations in sensitivity due to fluctuations in temperature can be a source of noise. -8

REFERENCES 1. Semiconductar Research - Fixl Report, July 1951 - July 1954, Purdue Research Foundation, Contra6t DA 36-039-SC-15339. 2. Photoconductrity and Recombination Processes in Telluriuum David Redfield, University of Pennsylvania, Technical Report No. 13, September 13, 1955o 3o Department of Physics Quarterly Report Nos. 11 and 12 on Semiconductor Research, University of Pennsylvania, July - December 19550 4.'Photoconductivity in the Elements, T. S. Moss, Academic Press, Inc, 1952.