IEEE TRASSACTIOSS ON ELECTRON DEVICES, VOL. ED-16, NO. 10, OCTOBER 1969

Mercury C'admium Telluride as an Infrared Detector Material

ERNIE L. STELZER, JOSEPH L.

Abstract-The properties of the alloy semiconductor Hgt-,Cd,Te and its application as an infrared detector material are reviewed. The selection of this alloy system as an infrared detector material is dis- cussed. Bulk and epitaxial crystal growth techniques are described and representative electrical data presented which show the free carrier concentration to be in the 101e1016 cm+ range.

The flexibility of this ternary system is then discussed relative to the compositional dependence of the energy gap. Photoconductive response data are presented for detectors having response peaks of 4-20 microns over the temperature range 15"-300"K. The expression

E, = 1 .6 (~ - 0.134) f 0.1335(0.435 - X ) (- - T 100 1 )

40" < T < 240'K

0.15 < x < 0.35

is shown to represent the compositional and temperature dependence of the energy gap within the specified limits.

I. INTRODUCTION

EVELOPMENTS in solid state detector physics in the last fifteen years have provided a large array of detector materials having peak wave-

lengths in the visible and infrared portion of the electro- magnetic spectrum. Since a significant number of appli- cations for these detectors involve transmission over long distances, such as for mapping and tracking, an im- portant parameter is the atmospheric transmission at the peak response.

The critical nature of this filtering effect for the infra- red portion of the spectrum can be seen in Fig. 1. This figure illustrates the transmission of the atmosphere as a function of wavelengths of 4-20 microns. Most im- portant are the transmission windows a t 4.5-5, 8-14, and 16-20 microns. The largest window, 8-14 microns, will initially be the focal point of our discussion. For an intrinsic detector this corresponds to an energy gap range of 0.09-0.15 eV. A review of the numerous ele- mental and binary compound semiconductors indicate none of these materials has the proper energy gap for 8-14 micron response. Thus, the fabrication of a detec- tor in this range necessitates one of the following.

the Solid State Sensors Symposium, Minneapolis, Minn., September Manuscript received March 3, 1969. This paper was presented at

Avionics Laboratory under Contracts AF33(615)-3679 and F-33615- 12-13, 1968. This work was supported in part by the Air Force

68-C-1050 and the Army Night Vision Laboratory under Contract

The authors are with the Corporate Research Center, Honeywell, DAAB09-68-C-0073.

Inc., Hopkins, Minn.

SCHMIT, AND OBERT N. TUFTE

1) The use of a doped extrinsic semiconductor where the excitation is from an impurity level to either the valence or conduction band. This results in either a free hole and bound electron or bound hole and free electron.

2) The growth of semiconductor alloys having the proper energy gap for intrinsic excitation.

Extrinsic photoconductivity in doped germanium has been investigated extensively during the past ten years, and these investigations have led to the development of high performance detectors in this wavelength interval [l]. However, for certain applications, intrinsic detec- tors offer potential advantages over extrinsic detectors. These advantages include a higher operating tempera- ture and a much larger optical absorption coefficient so that the detector thickness can be drastically reduced in the intrinsic detectors. Therefore, the development of intrinsic photoconductive detectors has also been pur- sued during the past several years [l ]. The most suc- cessful intrinsic detector material that has been de- veloped for photoconductive detectors in the 8-14 pm range is an alloy of HgTe and CdTe which is customar- ily written as Hgl-,Cd,Te, where x is the mole fraction of CdTe in the alloy.

This paper presents a review of the properties of Hgl-,Cd,Te and the current status of detector research. This review includes the methods of crystal growth and material preparation, the electrical properties of the crystals, and the fabrication and properties of detectors. New experimental data are presented to show the com- position and temperature dependence of the energy gap. These results show the designability of Hgl-,Cd,Te for detector applications in which a specific wavelength re- sponse is required a t a specified temperature.

11. COMPOSITIONAL DEPENDENCE OF THE ENERGY GAP IN Hg,-,Cd,Te

In the evaluation of combinations of materials, the two major considerations are 1) attainability of the proper energy gap and 2) the miscibility of the materials in the required compositional range.

Fig. 2 shows the energy band structure of HgTe [2]- [4] and CdTe [SI, [6] at the center of the Brillouin zone. HgTe, although originally considered a small bandgap semiconductor, is actually a semimetal. Note that the re and bands are reversed with a pseudo energy gap of approximately 0.2 eV. CdTe is a direct gap material having an energy gap of approximtely 1.6 eV. These

STELZER et al.: MERCURY CADMIUM TELLURIDE AS IR DETECTOR MATERIAL

N 20

88 1

0

ENERGY GAP vs COMPOSITION CdTe

COMPOSITION

0.3-0.5% MOLE 4 CdTe .8

Fig. 2. The energy gap of the Hg,-,Cd,Te system as a function of composition showing energy band structure for the endpoints.

gaps are temperature dependent, as will be discussed later. Since HgTe and CdTe are completely miscible [7] , i t is possible to determine a composition that will have the proper energy gap for 8-14 micron excitation. Assuming a linear dependence [ 8 ] , [ 9 ] of the energy gap with composition, an alloy with 20 percent CdTe should have a peak response in the 8-14 micron range. The energy gap goes to zero a t x = 0.15 so that detectors hav- ing peak responses beyond 14 microns are also possible with this system.

111. CRYSTAL GROWTH A . Modi$ed Bridgman Method

Growth of Hg,-,Cd,Te from the melt is accomplished by use of a modified Bridgman [SI approach. The three elements Hg, Cd, and Te in stoichiometric amounts plus some excess Hg are loaded into a thick wall quartz capsule, evacuated and sealed off. The sealed capsule is heated in a furnace and rocked back and forth to insure mixing, after which i t is solidified from one end by sequentially cooling the three zones of the furnace. This solidification is done rather rapidly (several inches per hour) in order to reduce thermal segregation. This re- sults in a largely single crystal ingot containing a den-

INGOT

Z 2 I

X 0.23 0 . 2 2 0 . 2 0 /

0 2 4 6 8 I O 12 14 16 18 2 0 22 CM FROM TIP

rnp

I x 1

13rnrn

. -.

SLAB

Fig. 3. The compositional uniformity of a typical ingot and slice of Hg,,Cd,Te grown by the modified Bridgman method.

dritic substructure. A subsequent high temperature an- neal is used to remove the dendrites and a low tempera- ture anneal is used to adjust stoichiometry. A represen- tative cross-sectional and axial compositional profile is shown in Fig. 3. The compositional gradient from the top to the bottom of the ingot results from the solidifica- tion process. This, however, as indicated by the cross- sectional view, is minimal in the last half of the ingot. Thus, arrays of detectors having the same peak wave- length response can be fabricated from a single slice. The results presented in this paper are from detectors fabricated from material grown by the modified Bridg- man growth technique.

B . Epitaxy The preparation of Hg,-,Cd,Te by epitaxial growth

has also been investigated as a possible method of ob- taining compositional uniformity over large areas. We have restricted the epitaxial investigations in our labo- ratory to techniques which start with the compounds HgTe and CdTe rather than the individual elements. The most successful method has been an isothermal

882 IEEE TRANSACTIONS ON ELECTRON DEVICES, OCTOBER 1969

HOT@ POWDER SOURCE T * 600.C 5 mm SPACING 3 ATMS MERCURY PRESSURE

X

0.2 - I 86 HOURS I

-0 40 80 I20 160 200 240 260

THICKNESS (MICRON51

Fig. 4. The compositional variation in an isothermally grown epi-

for HgTe deposited on CdTe a t a temperature of 600°C with 3 taxial layer as a function of thickness. The growth conditions are

atm of mercury and spacing of 5 mm.

process [lo], [ll]. With this technique, a HgTe source wafer is separated from a CdTe substrate by a quartz spacer to form a structure similar to a parallel plate capacitor. These materials, along with a controlled amount of excess mercury, are placed in a quartz am- poule and the ampoule is evacuated and sealed. The ampoule is placed in a furnace and maintained a t 600°C under isothermal conditions. This results in the deposi- tion of HgTe on the CdTe substrate and the interdiffu- sion of these compounds forms Hgl-,Cd,Te. The inter- diffusion mechanism leads to a gradient in the composi- tion of the epitaxial layer of the form shown in Fig. 4. The composition of the layer shown in Fig. 4 was mea- sured with an electron beam microprobe and the growth conditions are given in the figure. The compositionai variation over the surface of the epitaxial layer is less than 0.005 mole fraction, which is the limit of the micro- probe. Photoconductive detectors have been fabricated from expitaxial layers and peak responses corresponding to the surface composition have been obtained [ll]. However, the Compositional profile within the epitaxial layer degrades the performance of the detectors, so that the material near the surface must be isolated from the remainder of the layer. Possible ways of achieving this are being explored.

IV. ELECTRICAL PROPERTIES The experimental verification of the feasibility of

growing compositionally uniform Hgl-,Cd,Te is only a necessary but not sufficient condition for the fabrication of photoconductive infrared detectors. The critical material parameters besides composition are 1) the free carrier concentration, 2 ) the carrier lifetime, and 3) the quantum efficiency. In this paper we will only explicitly discuss the free carrier concentration.

Since these detectors depend on an intrinsic excita- tion, the number of electrically active impurities must be minimized. The free carrier concentration of this material was evaluated by standard Hall effect tech- niques. Typical results showing the temperature depen- dence of the Hall coefficient are given in Fig. 5 . The

significance of these data is twofold. The first is the ex- ponential decrease in the Hall coefficient at the higher temperatures which indicates that intrinsic conduction is the dominant transport mechanism. The slope of this line gives the value of the energy gap. An extrinsic car- rier concentration of 1014cm-3 is determined from the flat low temperature portion of the data.

V. DETECTOR FABRICATION AND EVALUATION The final criteria for the evaluation of this material

are the fabrication of detectors and the determination of the response. A cross-sectional view of a typical de- tector can be seen in Fig. 6. The material is mounted with epoxy on a germanium substrate, lapped to the de- sired thickness (20-30 microns), and indium contacts evaporated on the ends of the samples. Standard photo- lithographic techniques can be used in the Iatter process.

Representative detectivity data for a detector having an 8 micron cutoff are shown in Fig. 7 . The HCT da ta are compared to detectors peaking in the same range fabricated from other material and to the theoretical limit attainable. While the absolute detectivity is com- parable, a very important difference is the operating temperature. The Hgl-,Cd,Te response data are a t 77"K, while the doped germanium [12]-[16] detectors are operated anywhere from 4.2'K to 60'K.

VI. DESIGNABILITY

As seen in Fig. 2 the energy gap varies as some func- tion of the composition. Therefore this alloy system pre- sents the possibility of fabricating detectors having dif- ferent peak responses. Detectors have been fabricated from material varying in composition (x) from 0.17 to 0.35 mole fraction of CdTe. The photoconductive re- sponse was measured as a function of temperature and wavelength. Representative relative response data are shown in Fig. 8 for detectors a t 85°K. The temperature dependence of the energy gap, as determined from the cutoff wavelength from similar response results, is shown in Fig. 9 by the data points for the indicated composi- tions. These results allowed us to formulate an empirical

STELZER et al.: MERCURY CADMIUM TELLURIDE AS IR DETECTOR MATERIAL 883

' - L I

Fig. 5. Typical temperature dependent Hall coefficient (RH) data. The right-hand scale is the conversion to carrier concentration ?a assuming ?a = l/eRH.

I N D I U M

E P O X P /

F RELATIVE RESPONSE v s WAVELENGTH T = 85'K

21.5 - 2 2

17.5-18

I

t I I 2 4 6 8 IO 20 30

WAVELENGTH (MICRONS)

Fig. 8. Representative spectral relative response curves for various compositions (percent). The curves are not normalized to each other.

10 microns / I \r Hgi- ,Cd,Te ~ ~ 2 0 - 3 0 m i c r o n s 1

,050" I G E R M A N I U M I

Fig. 6. A cross-sectional diagram of a Hgl,Cd,Te detector mounted on a substrate.

I os 3L 1.0 2.0 WAVELENGTH 4 6 (MICRONS) 5 IO 20 40

- > 2 0.18 a U :: 0.16 0 LT W 6 0. I 4

0.12

0.10

0.08

0.06 0 .. 17.0-17.5

0.04 0 4 0 80 120 160 2 0 0 2 4 0 2 8 0 320

TEMPERATURE ( * K )

Fig. 9. The experimental temperature and compositional depen- dence of the energy gap of Hgl-,Cd,Te. The solid lines represent solutions to the empirical expression

E, = 1 . 6 ( ~ - 0.134) + 0.1335(0.435 - x)[(T/100) - 11. Fig. 7. The spectral detectivity of a number of low temperature

infrared detectors. The doped germanium results are from [12]- [16]. Typical frequency of the noise measurement was 1000 Hz.

884 IEEE TRANSACTIONS OK ELECTRON DEVICES, OCTOBER 1969

expression for the temperature and compositional de- pendence of the energy gap. While expressions have been formulated that express this dependence, they as- sume that the compositional dependence of the energy gap is linear through the end points. However, in an attempt to formulate a more exact expression, we have taken a linear fit to the experimental data over the com- positional range of 0.17 < x < 0.33. From these data we have formulated the expression1

E, = 1 . 6 ( ~ - 0.134) f 0.1335(0.435 - X) - - 1 ( 1 t O )

for the compositional and temperature dependence of the energy gap. In Fig. 9 this expression is shown as the solid lines for various compositions. While this new ex- pression is not exact or general, i t does provide a rea- sonable fit to the experimental data for the range of interest.

VII. CONCLUSION In summary, we have first established the motivating

reasons for selecting intrinsic Hgl-,Cd,Te for an in- frared detector material. These mainly were 1) large optical absorption coefficient, 2) band structure, and 3) complete miscibility.

These potential advantages were verified by experi- mental results that indicated a detectivity approaching the theoretical limit. Also the operational temperature (77°K) was higher than any other detector peaking in this wavelength range. Finally, we have suggested that this alloy system has the potential to be designable for detectors peaking over a range of wavelengths and have

indicate the following expression to be accurate over the whole com- Additional data obtained since the presentation of this paper

positional range.

E, = 1 . 5 9 ~ - 0.25 + __ 0.157 0.327 T-- Tx + 0 . 3 2 7 ~ ~ 300 300

However, the two expressions are similar over the specified range. The details are given in [9].

provided data showing cutoff wavelengths ranging from 4-25 microns.

ACKNOWLEDGMENT

The authors wish to acknowledge the many contribu- tions made by their colleagues at the Honeywell Cor- porate Research Center and Radiation Center. Special note should be made of Dr. P. Kruse, who provided the impetus for the initiation of the investigation on this system, and Mrs. Bernice Johnson for fabricating the detectors.

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ments of Infrared Technology. New York: Wiley, 1962, pp. 420- 423.

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[3] R. Piotrzkowski, S. Porowski, Z. Dziuba, J. Ginter, W. Giriat, and L. Sosnowski, “Band structure of HgTe,” Phys. Status Solidi,

[4] T. C. Harman, W. H. Kleiner, A. J . Strauss, 9. B. Wright, J. G. vol. 8, p. K13.5, 1965.

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[S] D. Long and J. L. Schmit, Semimetals and Semiconductors. New

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[lo] G. Cohen Solal, Y. Marfaing, F. Bailly, and M. Rodot, Compt.

[11] 0. N. Tufte and E. L. Stelzer, J . A”Z. Phys., October 1969. Rend., vol. 261, p 931, 1965.

[12] “Interim report on infrared detectors, Syracuse University Re- search Institute, Syracuse N.Y., Phys. Rept. 103-6, September 1, 1958.

[13] “Interim report on infrared detectors,” Syracuse University Research Institute, Syracuse, N. Y. , Phys. Rept. 103-7, December 1, 1958.

[14] “Interim report on infrared detectors,” Syracuse University Research Institute, Syracuse, N. Y . , Phys. Rept. 104-2, May 1, 1960.

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1161 W. Beyen et al., “Cooled photoconductive infrared detectors,” J . Opt. SOC. Am., vol. 49, p. 686, 1959.