Mechanisms for crystallographic orientation in the crystallization of thin silicon films from the melt
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I. INTRODUCTION Over the last six years, the technique of zone-melting recrystallization (ZMR) has been investigated as a means of producing device-quality crystalline films on amorphous insulating substrates.1"3 It has also made possible the study of certain aspects of the solidification process itself. Direct observation of the solid-liquid interface in silicon4^6 combined with analysis of subboundary morphology7'8 and heat flow7 have led to models for the solid-liquid interface structure. Two regimes of ZMR growth have been identified.7 In the first regime, when the interface velocity is approximately >0.5 mm/s, a characteristically branched subboundary morphology is observed. The faceted solid-liquid interface structure is thought to be a result of ledge growth along {111} planes, with ledge nucleation occurring at reentrant corners of the interface. In a second growth regime, in which the interface velocity is b = {(f,l-4>2)/2,
s-Si
FIG. 6. Schematic of the reentrant corner of the faceted interface at a tilt boundary between two adjacent grains with (100) texture. Indicated are fi, and tf>2, the angles between the zone motion direction and the in-plane (100) orientations of grains 1 and 2, respectively, and h, the angle between the tilt boundary and the zone motion direction. The interface velocities of grains 1 and 2 normal to the faceted interface are u, and v2, respectively.
isotropy was a small effect compared to textural growth velocity anisotropy. Here we develop a model for this small but observable effect on growth that relates the growth velocity anisotropy to the faceted interface structure at the tilt boundary formed between adjacent (100)-textured grains. Consider a reentrant corner of the solid-liquid interface which occurs at the grain boundary between two adjacent grains, both with (100) texture, as shown in Fig. 6. The reentrant corners of the interface are assumed to be faceted, however the interface as whole need not be faceted. If we assume that the interfacial rearrangement frequency associated with growth is the same on both facets, then grain 2 has a velocity normal to the plane of its interface v2 oc ktX, and grain 1 has a velocity vx with relative in-plane misorientation, x — 2, shown for 0.5 /nm thick (100) Sifilmsscanned at 0.5 5 mm/s. The solid line indicates the predictions of the geometric growth model.
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where 0, and 2 are the angles by which the in-plane {100) directions of grain 1 and grain 2 deviate from the zone motion direction. Figure 7 is a plot of the occlusion angle (f>b as a function of in-plane orientations x and when grain 1 is more closely aligned with the direction of zone motion than is grain 2. Conversely, when grain 2 is well aligned and grain 1 is misaligned with the zone motion direction, the quantity {x — 2) is positive and (f)b is positive, indicating that grain 2 is occluding grain 1. The solid line indicates the angles predicted by the model, and the points are data from a (100)-textured, 0.5 /um thick Si film scann
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