Chapter 1 The Emergence of Hg1-xCdxTe as a Modern Infrared Sensitive Material

SEMICONDUCTORS A N D SEMIMETALS, VOL. 18

CHAPTER 1

The Emergence of Hg,-,Cd,Te Infrared Sensitive Material

as a Modern

Paul W . Kruse I. HISTORICAL OVERVIEW . . . . . . . . . . . 11. REVIEWOF THE ELECTRICAL, OPTICAL, A N D STRUCTURAL

PROPERTIES.

. . . . . . . . . . . . . . . Effective Muss Ratio . .

1. Energy-Band Siructure 2. Forbidden Energy Gap

. . . .

3 . Electron 4. Fermi Energy and Intrinsic Concentruiion 5 . Electron Mobility . . . . . . . 6 . Hole Mobiliiy . . . . . . . . 7. Optical Absorption Edge . . . . . 8. Phonon Frequencies . . . . . . 9. Lattice Constunt and Density . . . REFERENCES. . . . . . . . .

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1

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5 6 7

9 10 11 13 17 17 18 18

I. Historical Overview? The usual course of development of a new semiconductor material begins with university research and ends with industrial exploitation. Such has not been the case with Hg,-,Cd,Te. From the outset development has occurred largely within industry and at national laboratories. It is only recently that university research has taken place. The basis for this anomaly lies in the unique role which Hg,-,Cd,Te plays in infrared detection for military applications. Although radar technology came of age during World War 11, infrared technology was in its infancy, consisting principally of active infrared image converters and single element PbS cells (Cashman, 1946). Neither was capable of passive thermal imaging, an emerging military need. In the decade following the war, the lead salt family, including PbS, PbSe, and PbTe, was exploited (Cashman, 1959). In addition to their use in missile guidance, PbSe and PbTe, with absorption edges at 77 K near 5 p m , were potentially useful for 3-5 pm thermal imaging. During the latter part of the period it was determined that InSb, a member of the newly discovered 111-V compound 1- As viewed from the limited perspective of the author.

1 Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-7.52118-6

2

PAUL W. KRUSE

semiconductor family, held promise as a material useful for thermal imaging (Reike et ul., 1959). Dopants for Ge were also discovered which had excitation energies useful for thermal imaging; those of greatest interest for military applications were Ge: Au and Ge: Hg (Levinstein, 1959). As the 1950s ended, those materials under active investigation for military thermal imaging systems were Ge: Hg and InSb. The former had the virtue of responding throughout the 8-12 pm atmospheric window, but required cooling to around 30 K. The latter operated at 77 K but operated only in the 3-5 Frn interval. Thus, the need existed for a new material which would combine 8-12 pm response and 77 K operation. A search therefore began for such a material. One candidate was gray Sn, which was thought to be a narrow-gap (i.e., 0.1 eV) semiconductor. It turned out that gray tin was semimetallic and unstable at room temperature. HgSe, another candidate in the late 1950s, was believed to be a semiconductor, but studies showed that it too was semimetallic (Blue and Kruse, 1962b). HgTe, a third candidate, was thought to be a narrow-gap semiconductor, but it also turned out to be semimetallic (Harman et al., 1958). What was really needed was an “InSb-like” material whose properties were similar to InSb but whose energy gap was about half as large. It was realized that an intrinsic photoconductor was better than an extrinsic one in terms of reduced cooling requirements, and that a direct-gap material was superior to an indirect one in terms of free-carrier lifetime, and thus speed of response. The sought-for material should be radiative lifetime limited in order to minimize the cooling needed to attain the photon noise, or BLIP, limit (Kruse er a / . , 1962b). All of these characteristics were sought in the as-yet undiscovered material. In 1959, Lawson, Nielsen, Putley, and Young published a paper reporting that the alloy system Hg,-,Cd,Te exhibited semiconducting properties over much of the composition range (Lawson et a/., 1959). The forbidden energy gap was found to be dependent on the composition variable x, ranging from a wide-gap semiconductor for x = 1 to a semimetal at x = 0. This was widely recognized as an InSb-like material which appeared promising, and it was therefore selected for investigation. Studies began at laboratories in the U.S. (Kruse et af., 1962b; Harman et al., 19611, France (Bailly et al., 1963; Rodot and Henoc, 1963), Poland (Galazka, 1963), and the Soviet Union (Kolomiets and Mal’kova, 1963). Because of the potential military application, secrecy surrounded some of these efforts. Early investigations, from 1961- 1965, were concerned with determining a method for preparing crystals of Hg,-,Cd,Te having the proper x value to have an absorption edge at 12 pm at the temperature of opera-

1. Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

3

tion. This was the optimum; if the edge were too short or too long, then the BLIP-limited value of D*(300 K) would be too 1ow.t It was realized that the optimum spectral response was obtained by convoluting the atmospheric transmission spectrum with the thermal emission spectrum of the earth (Kruse et al., 1962b). The initial problem to be faced was one of determining accurately the energy gap as a function of composition and temperature. This was not at all easy to do. There was the difficulty of determining the composition itself. Determining the composition by measuring the density was sufficiently accurate for a large sample, but it assumed that the composition was uniform throughout the measured volume. The x value of very small volumes could be measured with an electron beam microprobe, but the accuracy was poor. Measurement calibration data were also insufficient. Crystal growth was a major problem, especially because of explosions. Because of the high vapor pressure of free Hg, open-tube methods were not employed. The initial approach was the Bridgman technique, in which the elements were sealed within a quartz ampoule that was heated above the liquidus temperature appropriate to the composition, then lowered through a temperature gradient (Woolley and Ray, 1960; Blair and Newnham, 1961). The quality of the thick-walled quartz from which ampoules were constructed was inconsistent, and explosions were frequent. A sidearm at a lower temperature to establish the Hg vapor pressure was sometimes employed (Harman, 1967). Because of the health hazard, it was necessary to seal the tubes and furnaces in steel liners, with proper venting to remove Hg vapor in case of an explosion. It was soon realized that constitutional supercooling gave rise to a dendritic growth pattern within the crystal, in which there was a microscopic web of high-x material within a low-x surrounding. The initial attempts to avoid this employed a rocking furnace to thoroughly mix the melt. The ampoule was then lowered through a very large temperature gradient at a very slow rate. Such attempts were only partially successful. It was then discovered that a high-temperature anneal (just below the solidus temperature) would remove the dendritic structure. Electrical defects that resulted from deviations from stoichiometry were another problem. It was discovered that they could be controlled by a low-temperature anneal. Optical and galvanomagnetic studies were underway from the very beginning. The optical studies were directed toward determining the energy gap by measuring the energy of the absorption edge. The early edges t D*(300 K) is the signal-to-noise ratio measured in a I-Hz bandwidth in response to 1 W of radiant power from a 300-K blackbody incident on a detector normalized to 1 cm2 sensitive area.

4

P A U L W . KRUSE

were soft, and often of unusual shape, because the relatively large samples employed in the spectrometers incorporated composition gradients (Kruse and Blue, 1963; Blue, 1964). Even so, such measurements revealed that at 77 K, the desired temperature of detector operation, the required x value for a 12-pm absorption edge was about 0.20 (Scott, 1969; Schmit and Stelzer, 1969). The galvanomagnetic studies were mostly measurements of the Hall coefficient and resistivity as functions of temperature for a given composition (Blue and Kruse, 1962b; Galazka, 1963); thus energy gap, carrier concentration, and mobility could be determined. The early studies concentrated on n-type samples. It was found that Hg in excess of that required for stoichiometry needed to be loaded into the ampoule. Too much excess resulted in droplets and voids in the crystal. Too little resulted in p-type samples. The electron density of n-type ~ . samples of x = 0.20 material was usually about 1-2 x loi5 ~ m - Very unusual shapes were found in some of the Hall curves, e.g., double crossovers, and Hall coefficients that showed a dip at low temperatures. Some of these anomalies were later determined to arise from an n-type surface on lightly doped p-type material (Scott and Hager, 1971). The preparation and evaluation of infrared detectors made from the crystals was carried out in parallel with these studies. Attention was directed toward photoconductivity in n-type samples. Here too, spurious effects showed up early. Composition and purity gradients in the individual detector elements gave rise to the bulk photovoltaic effect (Kruse, 1965). These spurious effects confused the interpretation of the spectral response and responsivity measurements of the early detectors. Despite all of these obstacles, progress was rapid. By 1965 Hg,-,Cd,Te photoconductive infrared detector technology had advanced sufficiently so that prototype detectors could be made for thermal imaging systems. Photoconductivity continued to be of major interest; lifetime studies were undertaken (Ayache and Marfaing, 1967). Airborne thermal mappers based upon single elements or small linear arrays of InSb and Ge: Hg had been developed earlier and placed into limited production in the U.S. New mappers were forthcoming based upon small arrays of Hgo.,,,Cdo.20,Te detectors. Yield of high performance detectors was a problem; even today there are yield problems. During the second half of the 1960s much interest was devoted to the preparation of epitaxial layers of Hg,-,Cd,Te. A close-spaced method utilizing evaporation of HgTe upon a CdTe substrate was reported (Cohen-Solal et d.,1965). Another employed a temperature gradient between source and substrate (Tufte and Stelzer, 1969). Thin films of Hg,-,Cd,Te were also prepared by sputtering (Kraus ef al., 1967). The 1970s were a period in which (Hg,Cd)Te technology made rapid ad-

1 . Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

5

vances. Photoconductive detector technology matured, especially that for Hg,,,,Cd,,,,,Te used in 8-12 p m airborne mappers and in 8-12 p m FLIRs (forward looking infrared systems) (Kinch and Borrello, 1975). Other compositions suitable for use in the 1-3, 3-5, and 15-30 p m range were also investigated (Reine and Broudy, 1977). Photovoltaic detector development also advanced rapidly, including those types suitable for optical heterodyne receivers for CO, laser communications systems operating at 10.6 pm. During the early 1970s, (Pb,Sn)Te detector technology was also rapidly advancing (Melngailis and Harman, 1970). However, interest in (Pb,Sn)Te waned for various reasons, including its high dielectric constant and relatively large thermal expansion coefficient, and it is no longer a viable competitor for military infrared systems. Today (Hg,Cd)Te ranks as one of the most thoroughly studied semiconductors (Long and Schmit, 1970; Harman and Melngailis, 1974; Dornhaus and Nimtz, 1976). From an investment point of view, it is the third most important semiconductor, outranked only by Si and GaAs. Mercury cadmium telluride detector linear arrays of 60- 180 elements are in mass production in the U.S. for the common modular FLIR. It is the preferred material for second generation FLIRs, in the form of a (Hg,Cd)Te photovoltaic detector array bonded to a Si CCD chip. Hg,-,Cd,Te PV detector/% CCD hybrid matrix arrays are also under investigation for terminal homing missile seekers. In other military applications, arrays having spectral responses as short as 2-3 p m for detecting missile plumes, and as long as 15-30 p m for detecting spaceborne objects are under development. Civilian applications of Hg,-,Cd,Te thermal imaging systems are also being explored, including thermography for early detection of breast cancer. During the 198Os, Hg,-,Cd,Te technology, including research development and production, will continue to expand. Infrared detector development and production will increase rapidly. New phenomena and applications will emerge; recent examples are CCD shift registers (Chapman et ul., 1978), injection lasers (Harman, 1979), and nonlinear optics (Kruse et al., 1979). This trend will clearly continue; by the end of the 1980s, Hg,-,Cd,Te will be established as one of the most useful of all semiconductors. 11. Review of the Electrical, Optical, and Structural Properties

There exists a wealth of published data concerning the properties of Hg,-,Cd,Te (Dornhaus and Nimtz, 1976). Because of compositional nonuniformities and measurement inaccuracies, some of the early data

6

PAUL W . K R U S E

have been superseded by better values. The following brief overview includes data which are believed accurately to represent the compositional dependence of selected parameters. 1. ENERGY-BAND STRUCTURE

The band structure near the r point for three different values of forbidden energy gap is illustrated in Fig. 1 (Overhof, 1971). The left-hand part illustrates the semimetallic behavior found in HgTe and Hg,-,Cd,Te for which x is less than -0.16 at 0 K. The re state, which is the conduction-band minimum in CdTe and other zinc-blende semiconductors, lies at a lower energy than the restate, which is the valence-band maximum in CdTe. Thus the energy gap Eo is negative in HgTe. The usual light-hole valence band becomes the conduction band and the conduction band becopes the light-hole valence band. Because of the k p interaction (Kane, 1966) the conduction band and light-hole valence bands are nonparabolic. The central part of Fig. 1 illustrates the region near the r point when the forbidden energy gap is slightly positive. Here the light-hole valence band is the normal Ts state and the conduction band, the normal re.The conduction and light-hole valence bands are nonparabolic and symmetric, with the free-electron and light-hole masses at the re and Tspoints very small and equal. The right-hand part of Fig. 1 illustrates the region near the r point when

1. Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

7

the energy gap is relatively wide. The band structure is very similar to that of InSb, with a small amount of conduction-band nonparabolicity, which decreases and ultimately vanishes as the energy gap widens with increasing x value.

2. FORBIDDEN ENERGY GAP The dependence of the forbidden energy gap, i.e., the Ta-Ts transition, upon composition at 0 and 300 K is illustrated in Fig. 2 (Long, 1968). The temperature coefficient of the energy gap for CdTe is negative, which is

I. 6 1.4

1.2

-2,

1.0

c

a 0.8 Q

W

0.6 lr

w

z W

0.4

0.2 0 -0.2 -0.4

I HgTe

I

I

1

X

I

CdTe

FIG.2. Energy gap as a function of composition in Hg,-,Cd,Te. 0, interband magnetoreflection at 77 K; x , interband magnetoreflection at 4 K; f , optical absorption at 300 K; A,A, photovoltaic studies at 77 and 300 K; V, photoluminescence at 12 K. [From Long (1968).]

8

PAUL W . KRUSE

0.50 0.4 5

0.40 0.35

0.30 0.25 m

w

0.20

0.15 0.10

0.05 0 T (K)

FIG. 3. Dependences of the energy gap E , and the long wavelength limit A,, of Hg,,Cd,Te as functions of temperature and composition. [From J . L. Schmit and E. L. Stelzer ( I 969).]

the usual case for most semiconductors, but is positive for compositions rich in HgTe. Figure 3 illustrates the compositional and temperature dependences in more detail for x values equal to or less than 0.40, the region of most interest (Schmit and Stelzer, 1969). The left-hand ordinate is the forbidden energy gap expressed in electron volts, whereas the right-hand one is the corresponding absorption edge wavelength or photodetector long-wavelength limit. The data illustrate that the composition Hgo.,esCdo.sosTe is the proper choice for an infrared detector operating at 77 K having an energy gap of 0.10 eV (long-wavelength limit of 12.4 pm). The analytic expression for the data illustrated is

E, (eV)

=

1 . 5 9 ~- 0.25

+ 5.233(10-4)T(1-2.08x) + 0 . 3 2 7 ~ ~ (1)

1. Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

9

where Eg is the energy gap in electron volts, x is the composition variable, and T is the absolute temperature.

3. ELECTRON EFFECTIVE MASSRATIO The dependence of the electron effective mass ratio upon composition is illustrated in Fig. 4 (Long, 1968). The values illustrated are at the conduction band edge. The ratio goes to zero at the semiconductorsemimetal transition. In the region of most interest, the right-hand half of the figure, the effective mass is small and directly proportional to the gap, as predicted by the Kane model (Kane, 1966). More detailed data concerning the effective mass is illustrated in Fig. 5 (Schmit, 1970). The values shown have been calculated based upon the I

I

0.025

0.020

mz -

0.015

m0

0.010

0.005

0

0 HgTe

0.I

X

0.2

0.3

FIG.4. Dependence of conduction band edge effective mass ratio upon composition at 0 K in Hg,-,Cd,Te. 0, interband magnetoreflection at 77 K; X, interband magnetoreflection at 4 K; cyclotron resonance at 4 and 77 K; A,oscillatory magnetoresistance at 4 K . [From D. Long (1968).]

+,

10

PAUL W . KRUSE X

0.07

0.70 w

2

0.65

0.06

0.60

0.05

0.55 0.50

0.04

0.45 0.40 0.35

0.03

0.30 0.28

0.26

0.24

0.02

0.22

0.20 0.IB

0.16

0.0I

0

0

50

100

150 200 TEMPERATURE ( K )

250

300

350

FIG.5 . Temperature dependence of the electron effective mass ratio of Hg,-,Cd,Te. [From J. L. Schmit (1970).]

Kane model and the measured dependence of energy gap upon composition illustrated in Fig. 3. The electron effective mass values illustrated in Fig. 5 are referred to by Schmit as the parabolic equivalent effective mass, i.e., the electron effective mass which would have to be employed in the standard expression for the intrinsic concentration in order to give the intrinsic concentration value predicted by the Kane model. 4. FERMI ENERGYA N D INTRINSIC CONCENTRATION

The temperature dependences of the reduced Fermi energy and intrinsic concentration with composition as an independent parameter are illustrated in Figs. 6 and 7 (Schmit, 1970). The calculations upon which the figures are based employ the measured dependence of the energy gap upon composition and temperature illustrated in Fig. 3 and expressed in Eq. (1). A nonparabolic conduction band was used. The valence band was approximated by a single parabolic band with hole effective mass equal to 0.55 m, (free-electron mass).

1. Hg,-,Cd,Te

AS A MODERN INFRARED SENSITIVE MATERIAL

11

+3

F

0

-15 TEMPERATURE ( K )

FIG.6. Temperature dependence of the intrinsic reduced Fermi energy (measured from the conduction-band edge) of Hg,-,Cd,Te. [From J. L. Schmit (1970).]

5 . ELECTRON MOBILITY

Figure 8 illustrates the dependence of the free-electron mobility upon composition at 4 K (Long and Schmit, 1970). The parameter ,uIis the Hall mobility, i.e., the Hall coefficient divided by the resistivity. The curves are theoretical, with account taken of the dependence of free-electron mass upon concentration (Fig. 4); the scattering is assumed to be by singly ionized impurity or defect centers of density equal to the extrinsic electric concentration. Measured data points are also shown in the figure. Extremely high values of mobility for high-purity samples are observed near the semiconductor-semimetal transition. The dependence of the free-electron mobility at 4.2 K upon composition and free-electron concentration is illustrated in Fig. 9 (Scott, 1971). At this temperature, the mobility is determined by scattering from ionized impurities or defect centers, as has been seen in the calculations of Fig. 8. The values illustrated were calculated from the known dependence of electron effective mass upon composition (Fig. 4). The dependence of the electron Hall mobility upon composition and temperature, determined experimentally for n-type samples in which the free-electron concentration was less than 2 x 1015 cmP3, is illustrated in

12

P A U L W . KRUSE X

10'

lo1'

*-

10'6

E v

z

50

w

1015

I-

z u

0

z a 0

1014

0 m

2 a

z

I-

10':

1012

10"

50

100

150

200

250

300

350

TEMPERATURE ( K )

FIG.7. Temperature dependence of the intrinsic carrier concentration in Hg,-,Cd,Te.

[From J . L. Schmit, Honeywell Corporate Technology Center, personal communication, based upon revised data from J . L. Schmit. (1970).]

Fig. 10 (Scott, 1972). Below about 30 K, the value for Hp0.80Cd0.20Te is about 3 x 105 cm2/V sec, which is extremely high for semiconductors. The room-temperature mobility of Hgo.aoCdo.20Te is about 1 X lo4 cm2/V sec. As the x value increases, the mobility decreases monotonically. For x 2 0.25, the mobility increases as the temperature increases between 4 and 20 K. Over this composition and temperature range, singly ionized donor impurity scattering dominates.

1. Hg,-,Cd,Te

5 3

AS A MODERN INFRARED SENSITIVE MATERIAL

13

r

2 I o6

7

5

Y

:

3

c

0

-

0 u) 0

2

>

5

" 10

5

-

7

4

I

FIG.8. Hall mobility of electrons as a function of composition in Hg,-,Cd,Te at approximately 4 K.[From D. Long and J. L. Schmit (1970).]

Figure 11 illustrates the dependence of free electron Hall mobility upon composition at 300 K for n-type Hg,-,Cd,Te samples in which the freeelectron concentration was less than 2 x lOI5 (Scott, 1972). The highest value, about 3.5 x lo4 cm2/V sec, is obtained near x = 0.08, i.e., near the semiconductor-semimetal transition at room temperature, where the electron effective mass has its minimum value. 6. HOLE MOBILITY Most of the mobility data are for n-type samples. Figure 12 illustrates some data on the hole mobility determined from Hall effect and resistivity measurements on p-type samples (Schmit and Scott, 1971; Scott et af., 1976). The hole mobility at room temperature ranges from

I

14

PAUL W . KRUSE

I

I

I l l

15

I0

I

I

I

l

l

16

i

I

I0

l

i I'

7

CONDUCTION-ELECTRON CONCENTRATION ( c m V 3 )

FIG.9. Electron mobility at 4.2 K as afunction of composition in Hg,-,Cd,Te. [From M. w. Scott (1971).]

40-80 cm2/V sec. The temperature dependence is relatively small. As indicated, the p-type samples are relatively impure compared to the n-type ones of Fies. 10 and 11.

'"F

J W

CdTe

HgTe

MOLE

FRACTION CdTe

FIG.1 1 . Electron mobility at 300 K as afunction of composition. [From M. W. Scott (1972).]

T (K) FIG. 10. Temperature dependence of the electron Hall mobility in Hg,-,Cd,Te as a function of composition. [From M. W. Scott (1972).]

lo3

c

I00 TEMPERATURE ( K )

10

FIG. 12. Temperature dependence of the hole Hall mobility in Hg,,Cd,Te as a function of composition. [From J . L. Schmit and M. W. Scott (1971); M. W. Scott, E. L. Stelzer, and R. J . Hager (1976). Numbers (1)-(6) identify the samples.]

X

:

0.21 0.23 0.25 0 0

0

0 0

9

o

x o x x o x o x o x

O

o

0 0 0

o

L

w

o o

0 0

o

0

oo 0

ooo

a

0

d x x x x

I

I

0 2

0

0

0

0 0

0

0

0 0

0

0

:

0

0

0

0

0

O

l

0 3

o

n

0

0 0

0

0

0 0

0 0

0

0

,"

0

0

0

0

o

O 0

0

01

0

0 0

0 0

x

0

1

0

x x x

0

00

0 0

:,"

z

0

0

0 0

0

0

I

0 4 ENERGY ( e V )

0

0 0

1-

0 5

0 6

0 7

FIG. 13. Optical absorption coefficient as a function of composition in Hg,-,Cd,Te.at room temperature. [From M. W. Scott (1969).]

1 . Hg,-,Cd,Te

I10

AS A MODERN INFRARED SENSITIVE MATERIAL

0

0.2

HgTe

04

06 X

0.0

17

I CdTe

FIG. 14. Longitudinal and transverse phonon frequencies in Hg,-,Cd,Te at 71 (+,O) and 300 K (@,a). [From R . Dornhaus and G. Nimtz (1976).]

7. OPTICALABSORPTION EDGE Figure 13 illustrates the dependence of the optical absorption coefficient upon photon energy for various compositions (Scott, 1969). The edges are steep, as expected for a direct-gap semiconductor. Early results (Blue, 1964) showing edges with a more shallow dependence upon energy probably were obtained from samples of nonuniform composition. 8. PHONONFREQUENCIES

.

Figure 14 depicts longitudinal and transverse phonon frequencies as functions of composition at 77 and 300 K (Dornhaus and Nimtz, 1976). Most of the data illustrated were originally published by Baars and Sorger (Baars and Sorger, 1972). The LO and TO frequencies were deduced by Kramers -Kronig analysis of reflectivity measurements.

18

PAUL W . KRUSE I

\ 5

t

i

I

1

I

- 6.0

I

-

,DENSITY

7.5

* ,i

-1.0

6.470

- 6.5 t9

0

l I-n

z

0 V

-

6.475

>

n

-

t J

0

6.465

ATTIC€ 6.460

0

CONSTANT

I

1

I

I

1

I

I

I

I

0I

02

03

0.4

0.5

06

0.7 0.7

0.8

09

6.0

5.5

1.0

X

FIG.15. Lattice constant and density of Hg,_,Cd,Te as a function of composition. [From D. Long and J. L . Schmit (1970).]

9. LATTICE CONSTANT AND DENSITY Figure 15 illustrates the dependence of lattice constant and density upon composition in Hg,-,Cd,Te (Long and Schmit, 1970; Woolley and Ray, 1960; Blair and Newnham, 1961). There is a small deviation from Vegard's law, i.e., the lattice constant is not quite linear with composition. ACKNOWLEDGEMENT Many colleagues at the Honeywell Corporate Technology Center and the Honeywell Electro-Optics Operation have worked for almost two decades in Hg,-,Cd,Te technology. Among them are Dr. Donald Long, Mr. Joseph L. Schmit, Dr. M. Walter Scott, Dr. Obert N . Tufte, Mr. Ernest L. Stelzer, Mr. Robert J. Hager, Dr. Marion Reine, Dr. Robert Broudy, and Mr. Robert Lancaster. Thanks to Darlene Rue for typing the manuscript.

REFERENCES Ayache. J. C., and Marfaing, Y. (1967). C . R. Acad. Sci. Paris B265,568. Baars, J., and Sorger, R. (1972). Solid Sfare Cornmutt. 10, 875. Bailly, F., Cohen-Salal, G., and Marfaing, Y.(1963). C. R . A m d . Sci. Paris 257, 103. Blair, J., and Newnham, R. (1961). "Metallurgy of Elemental and Compound Semiconductors," Vol. 12, p. 393. Wiley (Interscience), New York.

1 . Hg,-,Cd,Te

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