Solid Phase Epitaxial Regrowth of Microcrystalline Si Films on a (100) Si Substrate
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SOLID PHASE EP1TAXIAL REGROWTH OF MICROCRYSTALLINE Si FILMS ON A (100) Si SUBSTRATE
S. ROORDA, S. SAITO* and W. C. SINKE FOM-Institute for Atomic and Molecular Physics, Kruislaan 407 1098 SJ Amsterdam, the Netherlands.
ABSTRACT Microcrystalline Si, as produced by explosive crystallization of an amorphous Si layer on (100) Si, shows a two-stage annealing behaviour. Initially, solid phase epitaxial regrowth occurs very rapidly at temperatures at, or above 800 0C. After a few seconds, the regrowth rate slows down to the value typical for alignment of poly-Si. Solid phase epitaxial regrowth of microcrystalline Si is suggested to be strongly dependent on grain size and structure. INTRODUCTION In recent years, much research has been devoted to the mechanism of explosive crystallization of amorphous Si (a-Si)1- 3. When explosive crystallization occurs, a layer of a-Si is rapidly converted to very small crystallites via a liquid state which is undercooled with respect to the crystalline phase. Little attention has been paid to the properties of the material which results from explosive crystallization 4. After explosive crystallization of a layer of a-Si, the structure that can be observed is generally as follows. Near the surface a layer of crystallites with sizes of - 0.1 uAm is found. Under this large grained (lg) polycrystalline layer a microcrystalline (tLc) layer is found with much smaller grains. Transmission electron microscopy 2 and Raman spectroscopy5 results show that the average grain size in this layer ranges from 40 to 100 A. Finally, there is a layer of ac-Si which was not converted during explosive crystallization. This structure is shown schematically in Fig. 1. The relative thickness of the three layers depends on the circumstances during explosive crystallization, for example the energy density of the laser pulse that is used to trigger explosive crystallization. A laser energy density just above the threshold for explosive crystallization yields almost exclusively Wc-Si and a-Si and no large grained polycrystalline material. Now our attention is focused on the intermediate pc-Si layer. The grains in this layer are very small (only slightly larger than the stability limit for crystalline Si in the diamond structure, which is thought to be about 30 A[6]), and probably highly strained, as is the case for pc-Si prepared by the chemical transport method 7. In addition, pc-Si prepared by explosive crystallization is expected to be heavily defected, because crystallization occurred very rapidly from a highly undercooled melt. Consequently, some properties (such as grain growth and solid phase epitaxial regrowth rates) of gc-Si are expected to be different from those of lg poly-Si. In this experiment the solid phase epitaxial regrowth (SPE) of gc-Si on a (100) Si substrate is examined as a function of anneal time and temperature. By comparing the results with results reported in literature on alignment of lg poly-Si8 1- and on SPE of a-Si 1, insight can be gained concerning the nature of pc-Si. EXPERIMENTAL (
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