A Model for Silicon Dendrite Growth During Laser/Plasma Deposition from a Silane Discharge
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A MODEL FOR SILICON DENDRITE GROWTH DURING LASER/PLASMA DEPOSITION FROM A SILANE DISCHARGE HAROLD M. ANDERSON AND PHILIP J. HARGIS, JR. * Department of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131 ** Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT A model for dendrite growths in polycrystalline Si films formed during laser/plasma deposition with a silane discharge and a pulsed KrF laser has been developed. The model assumes a thin (less than 10 nm) amorphous silicon (a-Si) film is deposited on a substrate prior to phase transformation due to laser heating. The observed dendritic structure of the overall polycrystalline Si films is attributed to Si crystals shooting from an excessively supercooled Si liquid bath. Supercooled liquid forms since the melting point for a-Si can be reached at relatively low KrF laser fluences. Latent heat evolved at the solid-liquid interface induces an interface temperature higher than that of the melt and the requisite negative temperature gradient for absolute bath supercooling. Since the formation of an undercooled liquid by fast melting a-Si is also an important first step in explosive crystal regrowth studies, these results may have important implications for crystal growth and transient annealing. A conical approximation model is used in this study to characterize the stability of the dendrite tip in terms of local temperature gradients, i.e., the degree of undercooling at the tip of the parabolic dendrite. The degree of undercooling and hence the tip radius appears to be significantly affected by small changes in the laser fluence. Stability criteria lead to a relationship between regrowth velocity, V, and the tip radius, R, of the 2 form VR - constant. The size and stability of the dendrite tip is determined from a balance between the destabilizing force due to thermal diffusion and the stabilizing capillary force. Based on the observed tip 2 radii formed at laser fluences from 0.13 to 0.25 J/cm , the model predicts regrowth velocities in a range between 2.0 and 20 m/s -- values consistent with transient annealing studies of a-Si.
INTRODUCTION Laser/plasma chemical processing has been shown [1,2] to provide new degrees of freedom for thin film process technology and the potential for fabricating materials with novel microstructure. Pulsed ultraviolet laser irradiation in combination with a silane glow discharge has been used to deposit morphologically unique silicon films on a variety of substrates, as shown in figure 1. Characterization studies [1,2] of these films have shown that, dependent on laser fluence, epitaxial growth occurs in regularly spaced columnar structures with a parabolic shaped tip, i.e., dendritic structures. Raman spectra of the films indicate that dendritic growth gives the film an overall polycrystalline quality, with the properties of individual columnar structures approaching that of bulk silicon. Previous analysis of the laser/plasma interactions leading to the formation of these films has established tha
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