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TRANSPORT AND MICROSTRUCTURE OF MICROCRYSTALLINE SILICON ALLOYS G. LUCOVSKY, C. WANG, M.J. WILLIAMS, Y.L. CHEN AND D.M. MAHER Departments of Physics, and Materials Science and Engineering North Carolina State University, Raleigh, NC 27695-8202, USA ABSTRACT The microstructure and electrical properties of jic-Si and gc-Si,C prepared by remote plasmaenhanced chemical-vapor deposition, PECVD, are reviewed. The microstructure has been characterized by transmission electron microscopy, TEM, infrared, IR, absorption and Raman scattering. The electrical properties were characterized by temperature-dependent dark-conductivity measurements. These studies have explained significant quantitative differences between the carrier transport properties of gc-Si and jtc-Si,C alloys in terms of a band offset model for the interfacial potential steps between the amorphous and crystalline constituents of these material systems. INTRODUCTION There has been considerable interest in heavily doped jic-Si, and jtc-Si,C alloys with optical band-gaps >2.1 eV for use in photovoltaic and other thin-film devices [1]. The room temperature dark conductivities of doped gtc-Si and lgc-Si,C typically exceed those of a-Si:H and a-Si,C:H, respectively, by two to three orders of magnitude when they are prepared from source gas mixtures that include the same fractional content of p-type or n-type dopant gases. However, the highest dark conductivities for heavily doped tic-Si,C alloys with band-gaps of - 2.1 to 2.2 eV, are generally several orders of magnitude lower than those of doped pgc-Si, prepared from gas mixtures that include the same relative concentrations of the dopant-atom source gases. In addition, as both the C-atom, and dopant atom fractions are increased, the crystallite fraction decreases, and eventually goes to zero at very high doping densities, and/or high alloy atom ratios (see Table I). This limitation on crystallite formation is of great importance in device applications, and it has yet to be established whether it can be modified by using different deposition reaction pathways. The large differences in dark conductivities between amorphous and microcrystalline materials with the same chemical compositions are explained in terms of a band model that applies to diphasic microcrystalline materials in general, and to g.tc-Si and gic-Si,C alloys in particular. This model emphasizes the important role played by interfacial potential steps between the transport bands of the crystalline and amorphous components of the microcrystalline materials. These band offsets can limit the maximum values of the dark conductivity, especially when the bandgap of the crystalline phase is smaller than that of the amorphous phase, as is the case for the lgc-Si and g.tcSi,C alloys produced by remote PECVD in which the crystallites are Si. For example, the transport mechanism can be: i) by thermionic field-emission, or thermally-assisted tunneling between carriers in band states of the Si crystallites, and band-tail states of the amorphous constituent, as in