204 Journal of Crystal Growth 69 (1984) 204-206 North-Holland, Amsterdam

LETTER TO THE EDITORS

SEEDED G R O W T H OF LARGE SINGLE-GRAIN CdTe F R O M THE VAPOR P H A S E

H. KUWAMOTO

Rockwell International Science Center Thousand Oaks, California 91360, USA

Received 10 April 1984; manuscript received in final form 8 May 1984

A method for growing large single crystals of CdTe ( - 18 cm 3) is described. The crystals were grown routinely by a vapor deposition technique employing seed crystals. Epitaxial growth was observed on the (111), but not on the (111), surfaces.

Growth of large CdTe single crystals has been a particularly important subject because of its v-ray and infrared detector applications [1]. It is, how- ever, extremely difficult to grow large single crystals of CdTe. Large CdTe substrates required for certain applications are typically sawed from multigrain boules grown by the Bridgman tech- nique [2]. On the other hand, single crystal CdTe can be grown by vapor growth techniques, but the crystal size obtained is small. Yellin et al. [3] recently described a new vapor growth technique for CdTe, but crystal sizes were 23 mm in diameter and 5 mm in length (2 cm3), which were much smaller than those grown by the Bridgman method [2].

We report here the results of the seeded vapor growth study of CdTe to obtain large single crystals ( > 15 cm3).

The growth process employed is as follows. A Bridgman-grown (111} CdTe seed plate was opti- cally polished, etched by Br-methanol and placed in a quartz ampoule, as illustrated in fig. 1. The source material was polycrystalline CdTe, also grown by the Bridgman technique. The system was first evacuated and backfilled with Ar three times, and then left under Ar until the furnace reached operating temperature. Subsequent evacuation of the system down to 10-3-10 -5 atm initiated the CdTe growth on the seed. The temperature of the seed was set in the range 680 to 850°C with a 5-10°C difference between the seed and source temperatures. In all cases, single crystals were

grown epitaxially on ( l l l ) A seeds. Growth condi- tions were found not to be critical. At the conclu- sion of growth, the furnace was cooled slowly to room temperature with a cooling rate of 15°C/h to prevent cracking of the grown crystal.

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VAq ~UUM Fig. 1. Schematic of apparatus: (1) three-zone furnace, (2) growth ampoule, (3) source, (4) seed plate, (5a) joint, (5b) joint with leak slits, (6) hole, (7) heat pipe.

0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

H. Kuwamoto / Seeded growth of large single-grain CdTe from vapor phase 205

CdTe has cubic symmetry, which suggests iso- tropic growth characteristics, except for the (111) direction. When ( l l l ) A and ( l l l ) B surface seeds were loaded side by side into the growth ampoule, epitaxial growth was not observed on the B surface, but only on the A surface seed under the above growth conditions, probably reflecting the dif- ference in growth rates on these surfaces.

In conventional closed-tube vapor transport growth of CdTe, it was found that the single crystal seed often recrystallized into a multigrain crystal [4]. This can be ascribed to sublimation and recondensation of the seed, resulting in conversion to a polycrystalline seed in the closed (evacuated) ampoule before the furnace reaches the growth temperature. This is probably the same phenom- enon as the crystals often found grown on the source charge itself at undercritical supercooling [5]. In the present experiments, however, large CdTe crystals ( - 18 cm 3) have been successfully grown epitaxially on seeds because the growth apparatus used [6,7] (fig. 1) prevents sublimation of the seed. The presence of the Ar suppresses the evaporation of Cd and Te 2 from the surface of the CdTe seed because the slow diffusion of Cd and Te 2 in Ar results in local vapor saturation around the seed and the source. In confirmation, it was observed that in a 100 Torr argon atmosphere, vapor sublimed from the seed deposited not on the wall of the ampoule, but only on the sides of the seed. A typical example is shown in fig. 2, suggest- ing that the vapor is saturated around the CdTe.

As long as the ampoule is filled with Ar, CdTe sublimation is negligibly small under the above conditions. In the closed ampoule system, many factors must be adjusted to avoid seed evaporation and condensation such as heating rate, ampoule geometry, temperature gradient, distance between seed and source, etc.; otherwise, the initial single crystal seed recrystallizes into a multigrain seed during the heating process. In the present method, the seed remains single and epitaxial growth can begin on it after completely evacuating the Ar. As a result, an epitaxially grown crystal can be routinely synthesized by this method.

The importance of the presence of the Ar can- not be overstressed. Whenever the ampoule was heated under vacuum, polycrystals resulted. It

Fig. 2. Vapor condensation on the side of a seed.

should be noted that the evacuation rate of Ar after the furnace reaches the set growth temper- ature should be fast enough to prevent seed re- crystallization. When the Ar pressure was kept at 100 Torr for two days, no growth was observed, but the seed was thermally etched. The result is seen in fig. 3. Two seed plates were placed in an ampoule, one with the ( l l l ) A surface and the other with the ( l l l ) B surface. Thermal etching characteristics of the A and B surfaces are found to be quite different. It is seen in fig. 3 that the A surface has triangular steps surrounded by {110} sides, while the B surface is e t ched randomly, leaving triangular islands in holes. Inoue et al. [8] reported a somewhat different observation of the thermal etch pits. They observed pyramidal pits on the A surface and fiat-bottomed ones on the B surface. The reason for this discrepancy is not clear, but it is presumably caused by the different etching temperatures.

One of the crystals grown by this method is shown in fig. 4. Two features should be noted: (1) an epitaxial growth on a triangular shaped seed can be seen, which suggests no significant growth perpendicular to the growth direction, and (2) the

206 H. Kuwamoto / Seeded growth of large single -grain (.'ciTe from vapor phase

Fig. 3. Thermal etch pits: (a) A surface, (b) B surface.

growth surface is seen to be convex to the growth direction with a flat growth facet on it. Examina- tion of the microstructure of the crystals grown by this process detected the presence of thin voids which were oriented parallel to the growth direc- tion. These may be the result of too rapid growth, and further experiments will be required to solve this problem. The electronic properties of the material are currently being measured.

In summary, we believe that large CdTe single crystals can be grown readily on a seed by an experienced crystal grower using the present method.

This research was partially supported under Sandia Contract DE-AC04-76DP00789. The au- thor is grateful to Professor F. Rosenberger for stimulating discussions and to Dr. W.J. Gunning for critical reading of the manuscript.

References

Fig. 4. CdTe single crystal grown on a triangular seed sur- rounded by polycrystals.

[1] K. Zanio, in: Semiconductors and Semimetals, Vol. 13 (Academic Press, New York, 1978).

[2] L. Wood, E.R. Gertner, W.E. Tennant and L.O. Bubulac, Proc. SPIE 350 (1982) 30.

[3] N. Yellin, D. Eger and A. Shachna, J. Crystal Growth 60 (1982) 343.

[4] E.R. Gertner, private communication. [5] E. Kaldis, in: Crystal Growth Theory and Techniques, Vol.

1, Ed. C.H.L. Goodman (Plenum, London, 1974) p. 80. [6] E.L. Kern, C.J. Fischer, J.W. Sherohman, S. Sen and C.H.

Seeger, in: Abstracts 5th Intern. Conf. on Vapor Growth and Epitaxy (ICVGE-5), 1981, p. 180.

[7] F. Rosenberger, in: Abstracts 7th Conf. on Crystal Growth, Fallen Leaf Lake, CA, 1983.

[8] M. Inoue, I. Teramoto and S. Takayanagi, J. Appl. Phys. 33 (1962) 2578.