The Transition Between Amorphous Regrowth and Explosive Crystallization
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THE TRANSITION BETWEEN AMORPHOUS REGROWTH AND EXPLOSIVE CRYSTALLIZATION J.J.P.Bruines, R.P.M.van Hal, B.H.Koek, M.P.A.Viegers, and H.M.J.Boots, Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands. ABSTRACT The transition between amorphous regrowth and explosive crystallization of a 220nm thick amorphous Si layer on a crystalline Si substrate has been studied using time-resolved reflectivity, transmission electron microscopy, and Rutherford backscattering spectroscopy. Upon irradiation by 7.5ns FWHM pulses from a frequency-doubled Nd:YAG laser, interferences in the reflectivity indicate growth of amorphous Si from the surface. The observation of a narrow Cu peak, buried below the surface, points towards solidification from both the rear and the front. Transmission electron microscopy studies revealed the occurrence of small patches of polycrystalline Si. The relative amount of this polycrystalline Si is increased by longer laser pulses, higher substrate temperatures, and thicker amorphous Si layers. The results are discussed in terms of the temperature distribution and the time available for the nucleation of polycrystalline Si at the liquid-solid interface. INTRODUCTION The melting and solidification of amorphous silicon (a-Si) upon pulsedlaser irradiation has shown some intriguing phenomena [1-22]. These phenomena are related to differences in phase change kinetics [23-25], melt temperature, and latent heat between amorphous and crystalline Si. Especially the melt temperature reduction of approximately 200K with respect to that of crystalline Si (c-Si) and the decrease in the latent heat at the solid-liquid transition to -70% of the c-Si value play an important role [9,26,27]. Because of these last two features explosive crystallization (XCR) may occur. In this paper we speak of XCR whenever the release of excess latent heat from the spontaneous crystallization of the undercooled liquid Si (1-Si) leads to the melting of additional a-Si. XCR of a-Si, both normal to the surface and laterally, has been the subject of extensive research [1-17]. Transmission electron microscopy (TEM) images after pulsed-laser irradiation of a-Si layers with energy-densities below the threshold for epitaxial growth revealed the existence of two microcrystalline regions: large grain polycrystalline Si (LG p-Si) at the surface with fine grain (FG) p-Si material underneath [1-8]. The thickness of the LG p-Si layer corresponds approximately with the melt depth calculated without XCR. The total thickness of p-Si (LG+FG) can surpass the LG thickness by as much as an order of magnitude [3]. Measurements of the transient conductivity and the time-resolved reflectivity (TRR) by Thompson et al. [9] prompted them to propose the following model for XCR. The absorbed laser energy forms a primary melt at the surface. This liquid starts to solidify into LG p-Si from the primary melt depth towards the surface and the latent heat released by this solidification melts a thin layer of the underlying a-Si. Latent heat is again released during
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