Characterization of Au-Mediated a-Si:H Crystallization by In Situ Electrical Measurements
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CHARACTERIZATION OF Au-MEDIATED a-Si:H CRYSTALLIZATION BY IN SITU ELECTRICAL MEASUREMENTS A. A. PASA*1, C. A. ACHETE#, W. LOSCH# and G. H. BAUER* 2 *Inst. f. Phys. Elektronik, Univ. Stuttgart, Pfaffenwaldring 47, D-7000 Stuttgart, FRG #LEMI/COPPE/Universidade Federal do Rio de Janeiro, C.P. 68505, 21945-970 Rio de Janeiro, RJ, Brazil ABSTRACT The Au-mediated crystallization of a-Si:H has been investigated by in situ electrical measurements and scanning electron microscopy. The a-Si:H structure was prepared by evaporation of Au thin films of different thicknesses (50 - 600 A) onto intrinsic glow discharge deposited a-Si:H layers (= 5000 A). A rapid increase in the electrical resistance was associated to the nucleation and growth of Si crystals inside the Au film. The growth process promotes the accumulation of Au at the Si grain boundaries forming a metallic network. This network controls the electric resistance at the end of the crystallization reaction. The kinetics of transformation has been studied by isothermal treatments over the 140 - 180 'C temperature range. An activation energy of 2.1 eV was measured. An analysis of resistance data obtained by constant heating rate treatments suggest the rupture of the network interconnections at T > 170 'C. SEM pictures showing the formation of Au clusters confirmed this suggestion. INTRODUCTION Metal-semiconductor interaction plays a major role in contact formation in semiconductor devices. In case of a-Si and metals as Al, Ag and Au, this interaction promotes the Si crystallization at low temperatures. The metal-induced crystallization is a solid-state reaction and the metal acts as a medium for the crystallization [1,2]. The Au-induced crystallization of a-Si can be initiated at 130 TC [3], a temperature that is well below the crystallization temperature of pure Si of about 600 'C [4]. The crystalline nuclei growth laterally inside the Au film forming large dendrites [5,6]. The Si penetration into the metal produces a rearrangement and the Au accumulates between the Si grains. In a previous paper [7] we studied the Au-induced crystallization of a-Si:H by Raman scattering. It was observed a dependence of the crystallized volume on the Au thin film thickness and on the heat treatment temperature. In this paper we will report the characterization of a-Si:H crystallization in presence of Au by in situ electrical measurements together with scanning electron microscopy (SEM). EXPERIMENTAL PROCEDURE The a-Si:H films (= 5000 A) were deposited on Coming glass (7059) substrates by conventional glow discharge decomposition of SiH 4 (T, = 250 'C). The a-Si:H samples were transferred into the Au evaporation system immediately after opening the glow IPermanent address: LEMI/COPPE/UFRJ, C.P. 68505, 21945-970 Rio de Janeiro, RI, Brazil. Present address: FakultAt Physik, Carl von Ossietzky Universitat, D-2900 Oldenburg, FRG.
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Mat. Res. Soc. Symp. Proc. Vol. 311. ©1993 Materials Research Society
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discharge reactor. The Au films were thermally evaporated; their nominal thickness
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