Cryst. Res. Technol. I 25 I 1990 1 8 I 865 - 869

Original Papers

NUTAN SHUKLA, G. C. TRIGUNAYAT

Department of Physics and Astrophysics, University of Delhi, Delhi, India

Growth of CdBr, Crystals by Zone-melting and Study of their Growth Features

A detailed surface microtopographic study has been carried out on the (OOO1) faces of melt-grown CdBr, single crystals. Mainly the growth of the crystals takes place by continuous solidification at the solid-liquid interface. Rarely, the growth occurs by two-dimensional nucleation and still more rarely by spiral mechanism. Most spirals are seen to be polygonized such that the polygonization extends right upto the centres of the spirals.

1. Introduction

During the last four decades several different X-ray diffraction studies have been carried out on polytypic cadmium bromide crystals, which have similar structure as the richly polytypic cadmium iodide. Mostly the crystals have been grown from solution and vapour. The solution-grown crystals have been found to exhibit growth spirals on their basal planes (MITCHELL). In the present work an extensive optical microscopic study of melt-grown CdBr, crystals has been carried out, revealing the existence of a wide variety of surface features. The optical observations have been confirmed as well as supplemented through scanning electron microscopy. The observed features have been suitably interpreted.

2. Experimental

The crystals have been grown by zone-melting technique described earlier by NIGLI et al. with a few modifications. Analar grade CdBr, x 4 H,O, supplied by M/s Sigma, was dehydrated by heating at 300 "C for 48 hours and used as the starting material. Although normally it is not possible to grow single crystals of CdBr, from melt on account of the high vapour pressure of the compound (7 times that of CdI,), the use of a subsidiary heater, argon gas and prior pelletisation of the material enabled a successful growth of the crystals by keeping the vapour pressure below the equilibrium dissociation value. The additional heating provided by the subsidiary heater further enabled to produce an almost planar solid-liquid interface. It also improved the visibility inside the growth chamber. The argon gas flowing through the system was allowed to escape into a beaker filled with hydrobromic acid instead of water, used by the earlier workers. The use of water renders the entry of a considerable amount of water vapour into the growth chamber on account of the strongly hygroscopic nature of CdBr,. The use of HBr effectively checks such a possibility.

Sixteen zone passes were given to obtain very pure crystal ingots. No coating of the quartz boat was required, as CdBr, did not react with the quartz boat.

Samples of nearly 10 x 4 mmz were prepared by cleaving the ingot. The sample preparation was carried out in a similar manner as described earlier for melt-grown CdI, crystals (SHUKLA, TRIGUNAYAT). For carrying out the SEM studies, the samples were sputtered with gold.

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866 SHUKLA, TRIGUNAYAT: Growth of CdBr, Crystals

Fig. 1. Succession of parallel zig-zag growth steps following the crystal symmetry. Some growth steps are curved. Bunching of the steps is also seen (arrow mark). Optical micrograph

Fig. 2. Zig-zag growth steps like those shown in Figure 1 at higher magnification. Scanning electron micrograph

3. Results and discussion

The most widely observed features have been the large number of zig-zag growth steps on the surface. Examples are shown in Figures 1 and 2. The steps are mutually inclined at angles of 120". As usual, the most favourable directions for the steps are expected to be the directions of close-packing. The CdBr, crystals have three equivalent close-packed directions, specified either along the set [1120], [1210], [2110] or [iT20], [T2iO], [2iTO], the directions are inclined at 120" to one another. Sometimes, curved steps are also seen on the surface (Fig. 1). They originate when the rate of growth is relatively fast. Figure 1 also illustrates 'bunching' of the growth steps, viz. coalescence of two more growth fronts into a single step. The bunching can result from the presence of impurities and imperfections and from the evolution of latent heat (ELBAUM, CHALMERS).

In the case of melt-growth, the thermal conditions lead to a stepped solid-liquid interface on the growing crystal face. Neither two-dimensional nucleation nor a screw dislocation step are necessary for continuous solidification, since the stepped interface is perpetuated by the forward movement of the supercooled zone (CHALMERS, MARTIUS). The addition of atoms to the growing solid takes place most readily on to the least close-packed growth fronts of the crystals. Consequently, all non-close-packed fronts copiously receive atoms until they have disappeared. Thus the surface only consists of close-packed growth fronts.

An example of crystal growth by two-dimensional nucleation, observed far less frequently, is presented in Figure 3, which shows a large number of hexagonal nuclei on the crystal surface. At places, some hexagons also coalesce together. The well-shaped hexagons can be correlated with Jackson's a-factor (JACKSON; JACKSON et al.), as follows. At a given temperature, the dimensions of the critical size nucleus depend on the supercooling, but not its shape. However, after the formation of a polygonal nucleus, an increase in temperature (which amounts to a lowering of the a-value) results in rounding off of the corners of the nucleus (LEAMY et al.). On the other hand, if the temperature decreases (i.e. a rises) or remains constant, the nucleus maintains its original shape during further growth.

The present observation of hexagonal growth (Fig. 3) implies that the temperature did not increase during crystal growth and, therefore, a was not lowered. Besides, the polygonal shape of the two-dimensional nuclei implies that the growth should have started at a relatively low temperature, i.e. at a relatively high value of a.

Cryst. Res. Technol. 25 (1990) 8 867

Fig. 3. Growth of a large number of hexagonal cry- stallites on the surface. Optical micrograph

Fig. 4. Triangular growth spirals with polygonized centres. Optical micrograph

The solution-grown CdBr, crystals are known to display growth spirals on their basal planes (MITCHELL). They are believed to form at low supersaturations. However, in our melt-grown crystals the spiral growth has been observed only in nine cases (out of nearly 350 crystals). An example is shown in Figure 4. By comparison with the solution-grown crystals, these spirals, ought to have formed at relatively low degrees of supercooling. They all have been found to be polygonized. In two cases, the centres have also been seen to be polygonized. The polygonized spirals correspond to high a-values during crystals growth and spirals with polygonized centres imply still higher a-values (SUNAGAWA, BENNEMA). The turns of the spirals have not been found to be equally spaced, which means that the supercooling should not have been uniform over the surface during growth.

Usually the spirals have been found to be triangular in shape (Fig. 4), which is consistent with the rhombohedra1 symmetry of the face (the melt-grown CdBr, crystals have all been found to be 6R polytype). However, two nine-sided spirals have also been observed. One example is shown in Figure 5. It has been suggested earlier, that spirals of more than six sides, viz. 8, 9, and 12 sides, are generated from a ledge initially exposed in a wrong orientation, viz., a different orientation than the one along a close-packed direction (JAIN, TRIGUNAYAT).

Fig. 5. Nine sides spiral (the central region is not clearly visible). Optical micrograph

Fig. 6. Circular spirals, A and B, with only a few turns. The latter is eccentric. Formation of eccentric loops is Seen at C. Optical micrograph

868 SHUKLA, TRIGUNAYAT: Growth of CdBr, Crystals

Fig. 7. Composite spiral steps due to more than one screw dislocations concentrated in the small region indicated by arrow mark. The growth steps are seen surmounting an obstacle at AB. Optical micrograph

Fig. 8. Scanning electron micrograph of ripple-like growth steps

Sometimes, circular spirals with a few turns have also been observed (Figs 6 and 7). The circular spirals correspond to lower U-values. Then the growth step is rough and the kink density is high, so that the spiral steps advance independently of crystallographic direction. In Figure 6, the eccentric spiral B is formed as follows (SUNAGAWA, BENNEMA). In real growth conditions, there exists a temperature gradient and hence a supercooling gradient. In the region of higher supercooling the spiral advances more rapidly, resulting in relatively widely separated steps, leading to the formation of an eccentric growth spiral. Similarly, if two screw dislocations of opposite hands be present close together, eccentric loops can form, as seen at C in Figure 6.

In Figure 7 the growth steps belong to a composite spiral originating from more than one screw dislocations concentrated in a small area marked by arrow. The region AB in Figure 7 shows the interaction of growth steps with obstacles, e.g. foreign macroparticles, on the surface. According to Cmmov, the impurities, adsorbed on the surface, not only change growth rates and habit of the growing crystals as a whole but also the nature of the layenvise growth fronts. When the surface has impuritiy particles which are strongly adsorbed and sufficiently large as compared with the lattice spacing, a growth step stops at the site of contact and begins to move around it.

Ripple-like features, also known as corrugations, have also manifested, although rarely, on crystal surfaces, as shown in Figure 8. Such growth may be resulting from interface instabilities linked to constitutional supercooling.

We are thankful to Dr. Krishan Lal, Dr. S. K. Sharma and Mr. S. U. M. Rao for providing facilities and rendering necessary help for SEM studies at the National Physical Laboratory, New Delhi.

References

CHALMERS, B., MARTIUS, U. M.: Phil. Mag. 43 (1952) 686 CHERNOV, A. A.: Modern Crystallography 111, Crystal Growth, Springer series in solid sciences 36

ELBAUM, C., CHALMERS, B.: Con. J. Phys. 33 (1955) 196 JACKSON, K. A,: in: Liquid MetalsandSolidification, Amer. SOC. Metals, Cleveland, Ohio 1958, p. 174 JACKSON, K. A., UHLMAN, Dr., HUNT, J. D.: J. Cryst. Growth 1 (1967)

(1980) 168

Cryst. Res. Technol. 25 (1990) 8 869

JAIN, R. K., TRIGUNAYAT, G. C.: J. Cryst Growth 2 (1968) 185 LEAMY, H. J., GILMER, G. H., JACKSON, K. A,: in: Surface Physics of Chrystalline Materials, (J. M. BLA-

MITCHELL, R. S. : Nature 182 (1958) 337 NIGLI, S., CHADHA, G. K.; TRIGUNAYAT, G. C., BAGAI, R. K.: J. Cryst. Growth 80 (1987) 378 SHUKLA, N., TRIGUNAYAT, G. C.: phys. stat. sol. (a) 103 (1987) 337 SUNAGAWA, I.; BENNEMA, P.: Preparation and Properties of Solid State Materials 7, Growth Mechanism

KELY, ed.) New York 1975

and Silicon Nitride (W. R. WILCOX, ed.) New York and Base1 1982

(Received, accepted January 6, 1990)

Author’s address: Nutan SHUKLA, G. C. TRIGUNAYAT Department of Physics and Astrophysics University of Delhi Delhi-110007, India