Orientation, twinning and orientation-dependent reflectance in InSb-GaSb alloys

Mat. Res. Bull. Vol. 13, pp. 1175-1180, 1978. P e r g a m o n P r e s s , Inc. P r i n t e d in the United States.

ORIENTATION, TWINNING AND ORIENTATION-DEPENDENT REFLECTANCE IN InSb-GaSb ALLOYS Robert A. Lefever Ampex Computer Products Division, E1 Segundo, California, USA William R. Wilcox Clarkson College of Technology, Potsdam, New York, USA and Kalluri R. Sarma Motorola, Phoenix, Arizona, USA (Received September 14, 1978; Refereed)

ABSTRACT InSb-GaSb solid solution ingots prepared by directional solidification aboard Skylab and on earth consist primarily of twins with (iii) twin planes parallel to the direction of solidification and growth directions that lie along <211>, , or <321> crystallographic directions. The optical bireflectance exhibited by polished samples when observed by reflected plane-polarized white light is symmetrical with respect to twin boundaries, regardless of twin orientation relative to solidification direction, and displays a two-fold rotational symmetry rather than the four-fold symmetry normally associated with uniaxial strain birefringence. InSb-GaSb alloys were directionally solidified in horizontal ampoules, vertical ampoules, and in Skylab 3 and 4 (1-3). Starting compositions were 10%, 30% and 50% InSb. The 8 mm diameter feed material was prepared by rapid cooling to produce a dendritic structure. In the experiments the ingots were partially melted back in a gradient furnace, the furnace conditions were held for 16 hrs, and then the temperature was programmed down to cause directional solidification. The resulting composition profiles are described elsewhere (1,2,4). Both straight and curved grain boundaries were observed in longitudinal and cross-sectional 1175

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slices. In analogy with known behavior with other materials it was assumed that the straight boundaries separated (iii) twins while the curved boundaries separated randomly-oriented grains (1-3). Many fewer straight boundaries and slightly fewer curved boundaries were found in the Skylab-processed ingots than in the earth-processed ingots (3). Here we report additional studies which necessitate some modifications in our previous conclusions.

Orientation Preliminary x-ray back reflection data indicated that despite the fact that the material was polycrystalline the majority of the ingots exhibited a high degree of overall crystallographic uniformity, with the direction of solidification within about 8 to 15 degrees of a direction, suggesting a preferred growth direction (i). Further study revealed that for 8 ~f the 12 ingots examined, the solidification direction was 18±2- from in a particular direction, namely <211>. For three of the remaining samples the direction was and for the fourth it was approximately <321>. These directions all lie in the (iii) plane (twin plane), indicating that the high degree of orientation involved twinning and some condition that favored alignment of the twin plane parallel to the direction of solidification. NO consistent dependence was noted for orientation on gravity or composition. Twin Boundaries Careful X-ray back reflection examination of regions on the two sides of both curved and straight boundaries invariably revealed that a twin relationship existed. At higher magnifications in the optical microscope many of the apparently-curved boundaries were found to consist of short, straight segments. Finally, even those boundaries that appeared to be curved at high optical m a g n i f i c a t i o n were found to consist of short straight segments by electron transmission studies. Electron diffraction patterns demonstrated that the crystallographic relationship across the boundaries was one of twinning, i.e. <110> twinning on a (iii) twin plane. It is well known that compounds with the zinc-blende structure exhibit twinning, with a geometry corresponding to 1800 rotation about the (iii) twin plane, and it is apparent from the data that our ingots were composed primarily of such twins. The fact that the boundaries were segmented along various (iii) planes, giving the appearance of curvature on a macroscopic scale, suggests energy minimization by alignment perpendicular to the solid-liquid interface during solidification, in a manner commonly observed for grain boundaries. Since the energies of twin boundaries are much smaller than those of grain boundaries, the driving force for alignment is correspondingly smaller and the effect would not be expected to be observed as frequently. The alignment of the (iii) twin plane parallel to the solidification direction was originally thought to involve a self-

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perpetuating re-entrant angle mechanism of the type proposed for germanium by Hamilton and Seidensticker (5). However, while such a m e c h a n i s m m i g h t have played a role in propagation of our twins, it appears likely that the source of the twins was the dendrites in the original "cast" ingots. The highly dendritic nature of this material and the twins in many of the dendrites were readily apparent in the optical microscope at low magnification. When solidification from an interface with such a structure is initiated, propagation of the twins can be expected to occur, with only those most favorably oriented (i.e., with the (Iii) twin plane parallel to the direction of solidification) propagating throughout the length of the resolidified ingot. The higher twin content in earth-processed ingots (3) came about because new twins were frequently generated during solidification on earth, but very seldom during space processing. A possible m e c h a n i s m for this difference is the generation of twins as the result of strain at the interface associated with the attachment of foreign particles to the solid/liquid interface. Particles with densities greater than that of the melt would be expected to settle on the surface of the solid/liquid interface in the vertical configuration, be swept past the interface by convection in the horizontal configuration, and remain in suspension in the Skylab samples. Thus, the generation of twins at the interface would be expected to be greatest in the vertical solidification experiments and least in the Skylab samples, as found. If, as expected, the orientation of the newly generated twins is random with respect to the solidification direction the majority would grow out of the ingot, accounting for the rapid reduction in the number of twins with ingot length in the vertical and horizontal samples. Since those twins oriented at large angles to the solidification direction are not favorably oriented with respect to the curvature of the liquid/solid interface, they are not influenced thereby (see previous section) and thus propagate with straight boundaries.

Optical

Properties

When examined under the microscope with reflected white light, polished sections revealed no details associated with the crystallographic orientation. Except for occasional voids and microcracks they were essentially featureless. However, when examined with plane-polarized white light, they exhibited marked variations in reflectivity that were a function of twin structure and the orientation of twins relative to the direction of polarization. For all cases discussed below, the polarizer and analyzer were exactly crossed. Observations on one of the highest contrast samples illustrates the magnitude of the effect. For the direction of polarization parallel to a straight twin boundary, the reflected light was orange on one side of the boundary and blue-green on the other. However, if the sample was rotated only slightly relative to the direction of polarization the color disappeared leaving

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shades

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of gray.

It was first thought that this might be a strain birefringence effect associated with the growth process. However the symmetry of the effect was related solely to twin boundary orientation, regardless of twin orientation relative to solidification direction. The material solidified near the melt-back interface contained twins with boundaries at large angles to the solidification direction, but the bireflectance remained related only to the direction of the twin boundary. Likewise, in transverse sections there was no radially-oriented effect but one that was associated with twin boundary orientation only. A second observation also suggests that this is not a birefringence effect. As illustrated by Fig. i, the variation in intensity of reflected light exhibited two-fold symmetry with respect to the plane of polarization.

Fig.

0

0

0

\'/

7/

,,/× \ ' ,

,i / \,, e

180

1

Relative surface reflectivity, I/Io, as a function of analyzer angle (8) relative to polarizer with twin boundary parallel to direction of polarization. A and B are adjacent twins. Dashed curve is result expected for twin B if the effect were the result of optical birefringence.

The solid curves represent the behavior of a pair of adjacent twins. At zero degrees, that is with the twin boundary parallel to the plane of polarization, one twin showed m a x i m u m reflectivity while the second showed minimum reflectivity, and m a x i m u m contrast existed between the twins. With rotation of the sample, the reflectivity on one twin increased while that of the other decreased until, at 450 , adjacent twins were indistinguishable from one another. At 900 the contrast was again a maximum, but the individual twins reversed in relative intensity. At 1800 the original condition was established. Birefringence has been reported for isotropic semiconductors, including Si, Ge, GaAs and ZnSe, as a result of a wave vector dependence of the dielectric constant (6). However, birefringence varies as the sin22e and thus exhibits fourfold symmetry. A sin22e curve is plotted as a dotted line in the figure. There is additional evidence that indicates that birefringence was not responsible for the o r i e n t a t i o n - d e p e n d e n t reflectance in our samples. Birefringence results in rotation of the plane of polarization. If a b i r e f r i n g e n t crystal is placed in an extinction orientation when polarizer and analyzer are at

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90 o to one another and rotated slightly relative to the plane of polarization, the analyzer can be rotated away from the 90 ° position to re-establish the condition of extinction. This was not true in the present case. Minimum reflection always occurred when polarizer and analyzer were positioned at 90 ° to one another, regardless of alignment relative to twin orientation. The degree of contrast between twins in the m a x i m u m contrast orientation decreased as the average indium content of the sample increased - that is, it was greatest for ingots solidified from melts containing i0 mole % InSb and least for ingots solidified from melts containing 50 mole % InSb. Furthermore, for a given melt composition, vertically solidified and space processed ingots showed a smaller effect than horizontally solidified ingots. These observations are compatible with one another if we assume that the degree of contrast is related directly to concentration of indium in the solid. In the horizontally solidified ingots, convective stirring occurred in the melt and the indium concentration increased slowly with distance from the interface(l, 2,4). In the space processed ingots (with no gravitationallydriven convection) and in vertically solidified ingots (with convection suppressed by an unfavorable temperature gradient) the indium concentration was uniformly higher. Thus, vertically solidified and space processed ingots were similar to one another, and when grown from melts of the same composition were less bireflectant, due to higher indium content, than horizontallysolidified material. As shown in Table l, the later-solidified regions of the ingots prepared from melts of high indium content did not exhibit contrast between twins.

Regions

TABLE i. and Intensities of Bireflectance Earth

Composition

Horizontal

Processed Vertical

Processed

In0.1Ga0.9Sb

Entire Ingot (strong)

Entire Ingot (medium)

30 mm* (medium)

In0o3Ga0.7 Sb

Entire Ingot (medium)

i0 mm* (weak)

i0 mm* (weak)

In0.5Ga0.5Sb

20 mm* (weak)

3 mm* (very weak)

This loss of contrast occurred closer to the initial remelt interface as the indium content in the melt increased, and occurred sooner in the vertically-solidified and space-processed *Distance is length of ingot (from initial meltback that exhibited significant bireflectance.

interface)

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ingots for a given melt composition because the high content in the solid occurred sooner in these cases.

indium

Thus it appears clear from the data that this was not a strain birefringence effect but rather an o r i e n t a t i o n - d e p e n d e n t property of the alloy surface. We may have observed the effect of carrier generation by polarized light in which the number of carriers generated, and thus the reflectivity, depends on the direction of polarization relative to the crystallographic orientation of the surface. The fact that the magnitude of the optical effect increased with increasing indium concentration is compatible with a model of optical excitation of carriers since the energy gap (7) of indium antimonide (0.2355 ev) is considerably less than that of gallium antimonide (0.8128 ev) and both are capable of excitation by white light. Since the Fermi surfaces are anisotropic, the effect of plane polarized light on carrier generation would be expected to be orientation dependent. A_ccknowled@ement This work was NAS8-29847.

supported

by NASA contracts

NAS8-28305

and

References i.

.

W.R. Wilcox, J.F. Yee, M.C. Lin, K.R. Sarma and S. Sen, Skylab Science Experiments 38, Science and Technology (1975) p. 27. J.F. Yee, S. Sen, K. Sarma, M.C. Lin and W.R. Wilcox, Proceedings of the Third Space Processing Symposium. Skylab Results I (1974) p. 301.

3.

J.F. Yee, M.C. Lin, K. Sarma and W.R. Growth 3_O0, 185 (1975).

4.

R.A. Lefever, W.R. Bull., in press.

.

D.R. Hamilton (1960).

Wilcox,

and R.G.

and M.L.

K.R.

Sarma and C.E.

Seidensticker,

Cohen,

Wilcox,

Phys.

Rev.

J. Appl.

J. Cryst.

Chang,

Phys.

B 2, 1821

Mat.

31,

6.

J.P. Walter

(1970).

7.

M. Nuenberger, III-V Semiconductor Compounds, Handbook of Electronic Materials, Vol. 2, Plenum, New York, 1971, pp. 39, 83.

Res

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