Band gap in Hg1-x ZnxTe solid solutions

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Band gap in Hg1-x ZnxTe solid solutions

B. Toulouse, R. Granger, S. Rolland, R. Triboulet

To cite this version:

B. Toulouse, R. Granger, S. Rolland, R. Triboulet. Band gap in Hg1-x ZnxTe solid solutions. Journal

de Physique, 1987, 48 (2), pp.247-251. �10.1051/jphys:01987004802024700�. �jpa-00210436�

Band gap in Hg1-x ZnxTe solid solutions

B. Toulouse, ^R. Granger, S. Rolland and R. Triboulet (*)

Laboratoire de Physique ^des Solides, ^{U.A. 786 au} C.N.R.S., I.N.S.A., F-35043 Rennes Cedex, ^France (*) Laboratoire de Physique ^des Solides, C.N.R.S., ¹ place ^{A. Briand,} F-92195 Meudon Cedex, ^France (Requ ^{le 15} juillet 1986, accept6 le 20 octobre 1986)

Résumé.

Les variations de la largeur de bande interdite des solutions solides Hg1-xZnxTe ^{sont données, pour la} première fois, ^en fonction de la température ^et pour ^toute la gamme de composition. Une formule empirique

traduisant ces résultats est présentée.

Abstract.

First experimental ^{results on the} band gap variations with temperature ^and composition ^of Hg1-xZnxTe solid solutions are given for the whole composition range. An empirical expression describing ^the

results is given.

Classification Physics ^Abstracts

78.20D

1. Introduction.

Solid solutions Hgi _xCdxTe (M.C.T.) have become the most widely used material for infrared detection in the middle infrared [1]. The main drawback of these semiconductors is the poor lattice stability ^{due to the}

weak Hg-Te bond which is weakened when Cd is

incorporated [2]. Alloying HgTe ^{with an other com-} pound ^{of lower bond} length ^should improve ^its stability [3] and ZnTe has been suggested ^as the second II-VI

binary [4].

The study ^of Hg1 - xZnx Te ^{solid solutions (MZT) has} recently ^started. Solid solutions of low x have been elaborated by liquid phase epitaxy [5], ^{molecular beam} epitaxy [6], isothermal vapor phase epitaxy [7] ^{and the} travelling heated method (T.H.M.) for bulk materials

[8]. ^{The first results reveal a} higher ^hardness [8] ^{and a}

lower interdiffusion coefficient [7] in MZT than in MCT. It is important ^{to know the band} parameters ^of these new materials which appear promising ^{for the}

elaboration of optoelectronic ^{devices in the infrared.}

We report first results on the optical band gap

EG ) ⁱⁿ Hg1 - xZnx Te solid solutions and give ^an empirical expression ^{to describe} EG ^{as a function of} composition (x) ^and temperature (T).

2. Material growth.

There is little thermodynamical ^{data for the MZT} system ^{but the} calculated liquid-solid phase diagram [9]

JOURNAL DE

PHYSIQUE.-T. 48,

2, FEVRIER

reveals a great difficulty ^{to obtain} good bulk materials

by direct solidification from the melt (see discussion in

[8]). Liquid phase epitaxy ^and travelling heated method

(THM) growths ^{allow us to} avoid this difficulty ; ^the

latter method has been chosen to obtain the large crystals necessary for _many studies. Tellurium is used

as a solvent.

The growth ^{of MZT} alloys has been achieved with the same THM method as for MCT [10]. ^{The source}

material is set up with two cylindrical segments ^{of ZnTe} and HgTe ^{whose section area ratio is x. This} cylinder ^is placed ^{in a} silica tube with the smallest backlash

possible ^after mechanical and chemical polishing. ^The growth conditions have been conjecturally extrapolated

from those of MCT alloys. ^{Table I} gives typical growth

conditions.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01987004802024700

248

The most important ^{condition to obtain a} homogene-

ous material is a flat interface between the molten zone

and the crystal during ^the growth. Ingots ^{with ten}

different compositions have thus been obtained.

3. Composition and lattice parameter.

The ingots ^{have been cut in slices} then thinned to about 50 _um and polished ^for optical measurements. About 15 checks of composition have been performed by microprobe analysis [11] ^{on each thinned} sample. ^An example ^of composition variations of a 2 x 2 mm2 sample ^is presented ⁱⁿ figure 1 ; ^a discussion about the

possible origin ^{of the} composition fluctuations is given

in [8]. Since, ^{it has} been verified that the liquid ^solid

interface is concave. Samples showing ^{a too} high composition ^variation ( Sx

0.06 ) ^{have been} rejec-

ted.

Fig. ^1.

^{Check of} composition variation (lW x ) .

The lattice parameter ^value (a) is deduced from

Debye Scherrer X ray diffraction patterns ^{obtained on} powder ^{from the} grinding ^{of the} samples (cf. Fig. 2).

Results are relatively scattered due to the composition inhomogeneity ^{of the} samples. Vegard’s law appears ^to be relatively ^better obeyed ^{in MZT} than in MCT [12].

4. Optical absorption coefficient.

Optical transmission has been measured on a

CODERG spectrophotometer ^{between 80 K and 300 K}

on as grown materials for x

0.2. Samples ^{with x}

^0.2

Fig. ^2.

^Lattice parameter ^versus composition.1: ^Values given ⁱⁿ [4].

were annealed at 260 °C under saturated _mercury pressure during ^{100 hours.}

Figure ^{3 shows} the variations of the optical absorp-

tion coefficient ( a ) ^versus photon energy h v ^at 3 temperatures ^{and for a} sample ^{with x}

^0.71.

Absorption coefficient fits very well the usual

( ahv ) 2 = C ( h v - EG ) law for direct optical ^transi-

tions as seen on figure ^{4. The} optical gap EGl ^is

obtained by extrapolating (ahv) 2 ^{to zero.}

Fig. ^3.

Example ^of optical absorption coefficient versus

photon energy for three temperatures.

Fig. ^4.

EOt evaluation.

In MCT the gap has ^{also been} empirically ^{defined as}

the photon energy where ^a reaches 500 cm- 1 [13]. ^The corresponding energy, ^called EG2, ^is designed by ^an

arrow in figure ^{3 and} reported in table II with EG1 ^for

MZT.

Actually ^the samples ^{do not have a} high composition homogeneity (cf. Fig. 1) ^and composition ^values ( (x) ) appearing in tables and figures ^{are an} average of all the measured values found in the illuminated area

of the sample during transmission measurements. The maximum deviation of composition ^{8x found on the}

tested area ( 2 x 2 mm2 ) ^{is given in table II where all}

the results have been gathered.

5. Band gap variations in MZT.

For each composition, EG ^follows very well a linear variation with temperature ^{T as} illustrated in figure ⁵

for three compositions.

EG (x) variations at 80 K and 292 K are plotted ⁱⁿ figure ^6. Experimental ^{values at} 80 K have been fitted with a third-order polynomial represented by ^{a solid}

line in figure ^{6 :}

..

dEG

The temperature coefficient

d; (x) ^{values are given}

Fig. ^5.

Energy gap variations with temperature ^{for three} compositions.

in table II and reported ⁱⁿ figure ^7. dEa/dT goes ^to

zero for a composition ^{near x}

^0.4 corresponding ^to EG = ^{0.75 eV.} The gap decreases with ^{T on} the ZnTe side.

A linear fit of dEG/dT variations with x has been made which appears ^{as a dashed} line in figure 7 ; ^for this fit the value corresponding ^to HgTe has been taken off because of its too low value. There is no evaluation of the band _{gap for} x 0.14. In order to reach an

Table II.

Experimental results and calculated values.

250

Fig. ^6.

Energy gap variations with composition ^{at 80 K and}

292 K. x et + : Values given ⁱⁿ [4].

empirical expression ^of EG ^{which must go to the} HgTe value, ^the experimental results of dEG/dT ^{have been.}

fitted with the expression

X>

Fig. ^7.

Band gap temperature coefficient dEG/dT ^varia-

tions with composition. ^{+ : Values} given ⁱⁿ [4].

The fit gives ao = 5.30 x 10-4, al

4.05 X 10- 4,

a2

6.83 x 10- 4 and corresponds ^to the solid line drawn in figure ^{7. The} expression ^{is therefore} conjec-

tural for x 0.14.

From the empirical expressions ^of EG ^and dEo/dT ^it

is then possible ^{to deduce a first} expression ^of the gap variations of Hgl _ xZnxTe ^{as a function of x and T:}

The calculated variations of EG ^{at T = 292 K are} reported ^{as a solide line in} figure ^6.

All the results are summarized in table II. This allows comparison between both experimental ^determi-

nations Eol ^and EG2, ^{and with} EGC calculated from the above formula.

One could note an oscillatory ^{behaviour of the} experimental points ⁱⁿ figures ^{2 and 7 which has not}

been taken into account in this study. ^In fact, ^{it would} be very interesting ^to point ^{out an} eventual ordered structure in the metallic sublattice as suggested ⁱⁿ [14]

for MCT. This ordering should appear ^at zinc to mercury ratio of 1:3, 2:2 and 3:1. Although ^{no definite} proof ^{has been} given ^{in MCT, the} strongest ^Zn-Te bond could enhance this phenomenon. Unfortunately

the sample inhomogeneity composition ^{does not} pre-

sently ^{allow us to ascertain such an} ordering.

6. Conclusion.

Hg1 - xZnx Te ^appears henceforth as an alternative material for optoelectronic applications in the infrared.

The first properties pointed ^{out let us} expect ^an

equivalent ^{or even better behaviour in devices. Particu-} larly ^the temperature coefficient _goes to zero for

x = 0.4 corresponding ^{to the near infrared} (1.6 um).

Acknowledgments.

The authors greatly acknowledge C. Pobla for lattice parameter measurements and microprobe analysis ^and

J. C. Chabreyron ^for sample preparation.

References

[1] ^{DORNHAUS, R. and} NIMTZ, G. in Narrow Gap Semiconductors, Vol. 98, Springer ^{Tracts in}

Modem Physics (Springer, Berlin), 1983, p. 119.

[2] SPICER, ^W. E., SIBERMAN, ^J. A., LINDAU, I., CHEN,

A. B., ARIEL SHER and WILSON, ^J. A., ^{J. Vac.}

Sci. Technol. A3 (1983) ^1735.

[3] EHRENREICH, ^{H. and} HIRTH, ^J. P., Appl. Phys. ^Lett.

46 (1985) ^668.

[4] ^ARIEL SHER, EGER, ^{D. and} ZEMEL, A., Appl. Phys.

Lett. 46 (1985) ^59.

[5] ^ARIEL SHER, EGER, D., ZEMEL, A., FELDSTEIN, H.,

RAIZMAN, A., ^J. Cryst. ^{Growth 72} (1985) ^108.

[6] SIVANATHAN, S., CHU, X., BOUKERCHE, ^{M. and} FAURIE, ^J. P., Appl. Phys. ^{Lett. 47} (1985) ^1291.

[7] POBLA, C., GRANGER, R., ROLLAND, S., TRIBOULET, R., presented ^at the 8th Int. Conf.

on Cryst. Growth, ^York July ^{1986, to be} pub-

lished in J. Cryst. ^Growth.

[8] TRIBOULET, R., LASBLEY, A., TOULOUSE, ^{B. and} GRANGER, R., presented ^at the 8th Int. Conf.

on Cryst. Growth, ^York July 1986, ^{to be} pub-

lished in J. Cryst. ^Growth.

[9] LAUGIER, A., Revue Phys. Appl. ⁸ (1973) ^259.

[10] TRIBOULET, R., ^NGUYEN DUY, ^{J. and} DURAND, A., J. Vac. Sci. Technol. A3 (1985) ^95.

[11] Microprobe analysis performed ^at microprobe ^fa- cility of C.N.R.S. in Bellevue and west micro-

probe facility, IFREMER Brest.

[12] KRUSE, P. W., Mercury ^Cadmium Telluride,

Semicond. Semimet. 18 (1981) ^18.

[13] HANSEN, ^G. L., SCHMIT, J. L. and CASSELMAN,

T. N., J. Appl. Phys. ⁵³ (1982) ^7099.

[14] BALAGUROVA, E. A. and KHABAROV, E. N., J.

Sov. Phys. ⁷ (1976) ^133.