Inorganic Materials, Vol. 36, No. 8, 2000, pp. 762-764. Translated from Neorganicheskie Materialv, Vol. 36, No. 8, 2000, pp. 920-923. Original Russian Text Copyright 0 2000 by Belyaev, Rubets, Kalinkin.

Elastic-Stress-Induced Phase Transformations in CdSxTel_x Films

A. P. Belyaev, V. P. Rubets, and I. P. Kalinkin St. Petersburg Technological Institute (Technical University),

Moskovskii pr. 26, St. Petersburg, 198013 Russia Received October 9, 1998; in final form, January 11, 1999

Abstract--Phase transformations in CdSxTel _ x layers deposited under highly nonequilibrium conditions were studied. The dark current and photocurrent were measured as a function of temperature both in the course of and after the phase transformations. The results of electrical measurements were contrasted with electron dif- fraction data. The observed temperature variation of conductivity during the thermally activated decomposition of the metastable solid solutions was shown to be consistent with the position of the Fermi level and electronic density of states.

INTRODUCTION

In an earlier study [1], we showed that the elastic stress developing at incoherent interfaces under highly nonequilibrium conditions may lead to soliton epitaxy, a fundamentally new mechanism of oriented film growth. In this work, we analyze the effect of volume elastic forces on the properties of solid solutions pre- pared under highly nonequilibrium conditions.

EXPERIMENTAL

We studied CdSxTel-x (0.4 < x < 0.6) layers tenths of a micron in thickness grown by vacuum evaporation onto muskovite substrates under highly nonequilibrium conditions [1]. The layers were polycrystalline both 10_ 3 before and after heat treatment. As shown by electron diffraction, the as-deposited layers were single-phase.

Electrical contacts to samples cut from the layers were made with silver paste. In electrical measure- ments under a vacuum of 10 -3 Pa, we used a V7-30 electrometer and a programmed temperature controller, 10-5 which maintained the temperature with a stability of +0.1 K or ensured constant-rate heating or cooling at 0.05-0.2 K/s. 6

Phase transformations were studied with and with- out an applied electric field. 10 -7

To assess the temperature effect on the characteris- tics of the electrical contacts, these were made both before and after annealing.

EXPERIMENTAL RESULTS

The main results of our experiments are displayed in Figs. 1--4.

Typical Arrhenius plots of dark conductivity are dis- played in Fig. 1. Curve 1 shows conductivity data for an as-deposited sample heated at 0.05 K/s. At low temper-

atures, we observe Arrhenius behavior with an activa- tion energy of about 0.5 eV. Starting at ---360 K, the slope becomes somewhat steeper and then decreases rapidly. At still higher temperatures, conductivity var- ies exponentially with an activation energy of 0.72 eV. During subsequent heating and cooling cycles, the exponential variation in conductivity persists through- out the temperature range studied (Fig. 1, curve 2).

Arrhenius behavior with the same activation energy was also observed in air.

10-9 1.8

T,K 500 400 300

I ! I

I

J 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

103/T, K -1

Fig. 1. Arrhenius plots of conductivity for CdSo.5Teo. 5 mea- sured (I, 3) in the course and (2, 4) after phase transforma- tions; heating at (1, 2) 0.05 and (3, 4) 0.2 K/s.

0020-1685/00/3608-0762525.00 �9 2000 MAIK "Nauka/Interperiodica"

ELASTIC-STRESS- INDUCED PHASE TRANSFORMATIONS 763

~, cm -1 10 o

10 4 -~.-~-~- 1 -e---e-4 2

10-3 , i 103 1.0 1.5 2.0 2.5 3.0

Energy, eV

Fig. 2. (1, 3) Spectral dependences of photocurrent and (2)absorption edge for CdSxTel_ x (3) before and (I, 2) after heat treatment.

10 -I 0

N 10- 2

0 Z

5 ~ 4 ~ 3 ~ ~ K 10- 3 I I I

.< 10 -4 . . t

~ 1 0 - 5

10-6 1.8

I

2.2 2.6 3.0 3.4 3.8

103/T, K -t

Fig. 3. Arrhenius plots of photocurrent for CdS0.sTeo. 5 heated at (1) 0.2 and (2) 0.05 K/s.

Curves 3 and 4 in Fig. 1 present conductivity data obtained at a heating rate of 0.2 K/s on another sample cut from the same film. These curves show no steep rise in conductivity.

Holding the samples at the highest temperature for a few hours had no effect on their conductivity.

As can be seen in Fig. 1, heat treatment reduced the conductivity of the films by three orders of magnitude. As shown by electron diffraction, these samples con- tained inclusions of a second phase, in accordance with earlier results obtained by optical absorption spectros- copy [2].

Samples with similar properties could be obtained by electron-beam heating in vacuum. Photoelectric measurements showed that the photocurrent increased with decreasing heating rate. For example, a portion of the sample heated at 0.05 K/s (Fig. 1, curves I, 2) showed, after holding at room temperature, a photocon- ductivity three orders of magnitude higher than that of another portion of the same sample heated by an elec- tron beam (illumination with a 90-W incandescent lamp). The photoconductivity of as-prepared samples was very low.

Typical photocurrent spectra are shown in Fig. 2. It can be seen that heating has little or no effect on the spectrum.

Figure 3 displays the Arrhenius plots of photocur- rent for samples heated at different rates. The shape of the curves is seen to be virtually independent of the heating rate. According to electron diffraction data, the samples heated at different rates were closely similar in phase composition and microstructure.

DISCUSSION

As shown by electron diffraction, the as-deposited films consisted of CdSxTe I -x solid solutions with x =

0.4-0.6. According to equilibrium phase-diagram data [3], these compositions fall in a two-phase region. There- fore, the as-deposited films were metastable owing to internal potential barriers. In II-VI films, such barriers are associated primarily with elastic stresses [4]. Heat treatment of such materials must induce decomposition of the metastable solid solution into CdTe- and CdS- rich solid solutions with boundary compositions in the equilibrium phase diagram.

The present results for the CdS-CdTe system are consistent with the above observations. Electron dif- fraction and optical examination of heat-treated sam- ples revealed phases with compositions falling in solid- solution ranges: Xl = 0.1-0.2 and x2 = 0.78-0.99. Sub-

~j

X

0 1.0

0.5

(E/Emax) 0"5 0.5 1.0

i i

+

1 2

) +

I

0.5 1.0

E/Emax

Fig. 4. Current-light curves for CdS0.sTe0.5; (I) heating rate of 0.05 K/s, (2) high illuminances (upper scale).

INORGANIC MATERIALS Vol. 36 No. 8 2000

764 BELYAEV et al.

sequent heat treatment had no effect on their electrical and optical properties. Thus, our samples underwent a transition from a metastable, single-phase state due to elastic stresses to the equilibrium, two-phase state.

The transition, driven by a reduction in free energy, can occur only via fluctuations, i.e., via nucleation of the new phase. This process lasts a certain period of time. Clearly, the conditions in the system during this period must have a significant effect on the kinetics of the phase transformation and the properties of the phases being formed. This accounts for the observed effect of heating rate on the electrical and photoelectric properties of the solid solutions, since the electrical properties are very structure-sensitive.

We suppose that the electrical properties of our films can be interpreted in terms of the electronic theory of disordered system [5]. The dark conductivity of the films is consistent with theoretical predictions [6],

6 = ~minexp[-(E c - EF)/kT], (1)

where t~min = 103 S/cm, and changes during heat treat- ment on account of changes in the electronic density of states, as follows from the observed spectral depen- dence of photocurrent and current-light data (Fig. 4). The insignificant effect of heat treatment on the photo- current spectrum indicates that the band gap remains virtually unchanged, and the shape of the spectrum tes- tifies to a monomolecular recombination mechanism, leading to a photocurrent,

iph = eFl.tfiGx, (2)

which is determined, through the lifetime of nonequi- librium carriers, by the defect concentration near the Fermi level [6]. Therefore, the difference in photore- sponse between the solid solutions after the phase transformation is associated with changes in the density of states near the Fermi level. Taking (1) into account, we suppose that curves 1 and 3 in Fig. 1 reflect the vari- ation in density of states during the thermally activated decomposition of the metastable solid solution.

In formulas (1) and (2), {~min is the minimal metallic conductivity; Ec is the mobility edge; E F is the Fermi energy; e is the electronic charge; F is the field strength; I.to is the drift mobility, weakly dependent on defect concentration and determining the temperature depen- dence of photocurrent (Fig. 3); and G is the generation rate of nonequilibrium carriers.

Note that the maximum in conductivity as a function of temperature was observed only at slow heating rates. We suppose that this is due to the slow response time of the electronics used in this work. At fast heating rates,

islands of the new phase are formed very rapidly, and the nucleation rate rises exponentially with tempera- ture, whereas the response time of our electronics was on the order of a few seconds.

CONCLUSION

Phase transformations in metastable CdSxTel_x solid solutions under elastic stress are accompanied by a sharp change in chemical potential, followed by grad- ual relaxation to the equilibrium value.

The microstructure and photoresponse of the phases resulting from the thermally activated transformation of the metastable solid solutions under elastic stress are determined by the heating rate.

The kinetics of dark conductivity during the ther- mally activated decomposition of the metastable solid solutions under elastic stress reflects the associated changes in the Fermi energy and electronic density of states.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundation for Basic Research, grant no. 99-03-32676.

REFERENCES

1. Belyaev, A.P., Rubets, V.P., and Kalinkin, I.P., Early Stages of Cadmium TeUuride Epitaxy on Hot and Cold Substrates, Neorg. Mater., 1998, vol. 34, no. 3, pp. 281-287 [Inorg. Mater. (Engl. Transl.), vol. 34, no. 3, pp. 214-219].

2. Belyaev, A.P., Rubets, V.P., and Kalinkin, I.P., Effect of Decomposition of CdTe-CdS Films on the Absorption Edge, Fiz. Tekh. Poluprovodn. (S.-Peterburg), 1997, vol. 31, no. 5, pp. 635--638.

3. Saraie, J., Kato, H., Yamada, N., et al., Preparation and Photoconductive Properties of Sintered Films of CdS- CdTe Mixed Crystals, Phys. Status Solidi A, 1997, vol. 39, no. 1, pp. 331-336.

4. Vityuk, V.Ya., Sanitarov, V.A., Kalinkin, I.P., and Zapleshko, N.N., Condensation Diagram for Thin CdSxTel-x Films, Neorg. Mater., 1982, vol. 18, no. 7, pp. 1126-1129.

5. Belyaev, A.P., Rubets, V.P., and Kalinkin, I.P., Conduc- tion Processes in Inhomogeneous CdSxTel_x, Thin Solid Films, 1988, vol. 158, pp. 25-36.

6. Mott, N. and Davis, E., Electronic Processes in Non- Crystalline Materials, Oxford: Oxford Univ. Press, 1979, 2nd ed. Translated under the title Elektronnye protsessy v nekristallicheskikh veshchestvakh, Moscow: Mir, 1982.

INORGANIC MATERIALS Vol. 36 No. 8 2000