Bismuth Precipitation in "Monocrystalline" InBi ... Single crystal InBi, grown at the 50/50 at. pct composition by horizontal gradient freeze or zone refining methods ...
Bismuth Precipitation in "Monocrystalline" InBi M.A. FLETCHER, M. S. S. YOUNG, F. T. GEYLING, S. NAKAHARA, and J. M. PARSEY, Jr. Single crystal InBi, grown at the 50/50 at. pct composition by horizontal gradient freeze or zone refining methods, was found to contain Bi or Bi-rich precipitates ranging in size from 20 nm to several /xm. This secondary phase is accommodated in the surrounding InBi matrix and grows at appreciable rates at room temperature. This rapid growth is explainable by the 2/3TM rule, where the melting point of InBi, TM = 110.5 ~ Precipitation occurs even for very exact initial melt compositions. Qualitative modifications of the phase diagram in the region of 50 at. pct are proposed to explain these observations.
I.
INTRODUCTION
THE present investigation of InBi was motivated by the need for a suitable analog material to model the growth of bulk GaAs crystals under tractable experimental conditions. It was desired that the model material be a low melting temperature binary alloy so that the crystal growth process might be readily monitored with a variety of diagnostic tools. Although InBi apparently fulfills these requirements, it was found during this investigation to be unsatisfactory for the intended purpose due to second phase precipitation and compositional instabilities exhibited at room temperature. We investigated the nature of the precipitates in single crystal lnBi and suggest a probable mechanism for the precipitation process. II.
PREPARATION AND GROWTH
Starting materials were elemental In and Bi of 6N purity. The elements were combined and melted together under vacuum (approximately 10-3 torr) within one section of a three section quartz ampul. Initial mixing was performed while the melt (at 280 ~ to 300 ~ was contained in the first section. Transfer of the melt, through a small constriction, to the second section served to remove surface oxide layers which had formed during the charging and mixing procedures. Further oxide removal was accomplished by passing the melt through a second constriction and into the growth section of the ampul. Subsequently, the third section was sealed while under vacuum prior to final homogenization and crystal growth. InBi seed crystals ((111) longitudinal axis) were retained in a well at one end of the ampul to initiate growth of single crystals with a controlled orientation. A horizontal gradient freeze technique had been developed by which single crystals of InBi could be grown reproducibly from a homogenized melt. In the present experiments crystals were grown from stoichiometric melts, as well as In-rich (1.29 at. pct) and Bi-rich (0.4 and 1.77 at. pct) melts. The InBi crystals were nominally 13 cm long with a maximum diameter of 1.8 cm weighing approximately 100 grams.
M.A. FLETCHER is with the School of Medicine, Vanderbilt University. M. S.S. YOUNG, E T. GEYLING, S. NAKAHARA, and J.M. PARSEY, Jr. are Members of Technical Staff, AT&T Bell Laboratories, Murray Hill, NJ 07974. Manuscript submitted December 12, 1983. METALLURGICAL TRANSACTIONS A
Samples obtained from the various crystal growth experiments were analyzed by optical microscopy, TEM, SEM, X-ray fluorescence, atomic absorption, and differential scanning calorimetry (DSC). For all of the analyses, samples were obtained from the interior regions of the bulk single crystal material to avoid surface-related effects (i.e., oxides). DSC samples were 1 to 2 mm 3 in volume. The residual materials from the oxide removal processes were subjected to atomic absorption analysis to determine bulk compositions. III.
RESULTS
The remanent materials, entrapped within the first two sections of the ampul (representing between 0.028 and 0.53 wt pct of the InBi starting charge), were analyzed by atomic absorption methods. No imbalance in the In: Bi ratio was found in any of the remanent materials. This material was observed to be discolored after melting, having the bronze tint of bismuth oxide on the surface. Single crystals of InBi exhibited a smooth external morphology with no lineage on etched or relief-polished surfaces. Figure 1 illustrates a transverse section from a single crystal ingot. At low magnification (50x) the specimen transverse surface appeared homogeneous immediately following crystal growth and sample preparation (Figure 1). However, at high magnification (1500 x , oil immersion) the
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5 mm Fig. 1--Cross-section view of InBi ingot, free surface noted by arrow. VOLUME 15A, NOVEMBER 1984-- 1963
microstructure appeared to contain fine (~ 1 /xm) particles resolved as dark and light bands under polarized light illumination (see Figure 2). These particles were found to be distributed along preferred crystallographic directions in the bulk crystal matrix. As the single crystal samples grown from stoichiometric melts were aged at room temperature in air, the precipitates were found to increase in size, reaching about 3/.tin in dimension after - 3 6 0 days. Samples prepared from crystals grown from nonstoichiometric melts were clearly polyphasic to low magnification optical evaluation. Similar precipitate growth was found in the Bi-rich crystals. A detailed microstructural analysis of the InBi crystals was carried out by TEM. As expected, cleaved (001) specimens contained a second phase, the majority of which was aligned along the [llO] and Ill0] directions of the InBi matrix (see Figure 3). An electron diffraction analysis has shown that this second phase has the lattice parameter of hexagonal Bi. Lattice parameters of In, the In-Bi intermetallic phases, and In and Bi oxides were cross referenced without a successful match to the experimental data. This identification was further corroborated by SEM (backscatter) and X-ray fluorescence studies which showed
Fig. 3 - - T E M micrograph showing the microstructure of (001) InBi crystal. Arrows indicate two orthogonally-oriented Bi precipitates lying along the [110] and [1]0] directions of lnBi crystal, g = (200).
the second phase to be enriched in Bi with respect to In or pure Bi. In addition, using a computer-aided indexing method, ~ we could deduce one crystallographic relationship between InBi and Bi crystals. Computer-indexed patterns together with experimental data are shown in Figure 4. It is seen that a relationship (001)inBiH(211)~i and [ll0]Bi II[T02h, is present. In addition to this relationship, we found that Bi can also precipitate with other unidentified phase relationships. The absence of dislocations around the Bi precipitates and their oblong shape strongly suggests that these Bi precipitates have a semicoherent interface with the InBi matrix.
Fig. 2 - - Optical micrograph, taken by the phase contrast technique, showing a high density of small precipitates (black and white spots) in InBi crystal.
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Fig. 4--Electron diffraction pattern obtained from a (001) InBi crystal containing Bi precipitates (a), and the computer-simulated diffraction patterns of (001) InBi (b), and (211) Bi (c). 1964--VOLUME 15A, NOVEMBER 1984
METALLURGICAL TRANSACTIONS A
An interesting observation is that in aged TEM samples (90 days, room temperature, in air) the Bi-precipitates were found to increase significantly in size. The thin edges of the InBi foil were found to migrate toward thicker regions, leaving Bi precipitates which are apparently supported on a thin transparent amorphous indium oxide film (see Figure 6). The morphology of the Bi second phase is now clearly seen, and it appears to maintain the same shape and orientation as before aging and migration. The observation that the Bi precipitates did not migrate is consistent with the fact that Bi has a higher melting point than the InBi matrix. These results affirm the tendency for Bi to cluster and indicate the strong instability of InBi at room temperature. It should be emphasized that similar behavior of Bi precipitation was found in both the bulk specimens (interior regions) and thin film (TEM) samples. Accelerated aging (in vacuo, bulk material, elevated temperatures) and extended aging in air at room temperature (thin film samples) produced similar results, with different kinetics. Furthermore, freshly cleaved samples of bulk material subjected to TEM observation showed precipitates present at a very fine scale (
