Charge Carrier Absorption in Doped Microcrystalline Silicon Films
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ABSTRACT The infrared absorption in doped microcrystalline silicon thin films is analyzed by modelling the complex permittivity as the sum of the contributions resulting from interband transitions and from absorption by charge carriers. Their density and the drift mobility within grains is described by free carrier motion according to the Drude theory. However, for a good fit to experimental data a small trapping energy, reflecting the effect of grain boundaries, is included in the model. By comparing results obtained from the analysis of infrared data with conductivity and Hall measurements in films grown in a very high frequency (VHF) plasma and by hot-wire chemical vapor deposition (CVD), we show that infrared transmission measurements provide a simple access to transport parameters in these films. INTRODUCTION Microcrystalline silicon (pc-Si) is gaining considerable interest as a thin film semiconductor. Possible applications include thin film solar cells, where highly conductive doped layers may be combined with a thin amorphous absorber or alternatively with a microcrystalline absorber, if the thickness is raised to a few micrometers. For the growth of these films at relatively high rates from the gas phase and at temperatures between 200 and 400'C two methods are currently being investigated, plasma enhanced chemical vapor deposition (PECVD) in a capacitively coupled reactor using very high frequencies (VHF) [1] or hot-wire CVD [2]. Highly conductive doped microcrystalline silicon films, as envisaged for contact layers in solar cells, show significant absorption due to free charge carriers [3]. In crystalline silicon this phenomenon has been frequently employed for the material characterization, deducing the carrier density and mobility, if the effective mass of the charge carriers is known. This analysis has also been extended to doped poly- or microcrystalline thin films. In an early study [3], as well as in later work on phosphorous doped polycrystalline films prepared by CVD [4], the carrier density and mobility was evaluated using the Drude theory. Under the assumption of a wavelength independent refractive index n, the inverse of the absorption coefficient ox-1 is proportional to ?- 2 and this was shown to lead to satisfactory results for wavelengths smaller than 4 - 5 pm [3][4]. However, typical highly doped microcrystalline silicon films exhibit a broad minimum in the IR-transmission around X = 10 pm, clearly in contradiction with a•-1 _ -2. This observation has been tentatively attributed to grain size effects [5]. In order to overcome the simplification of assuming a constant refractive index, both transmittance and reflectance data may be analyzed thus yielding the wavelength dependence of both n and cx [6]. In this contribution we show that an evaluation of transmittance spectra is possible without additional reflectance measurements by modelling the dielectric function of the films and fitting the transmission predicted by the model to the experimental results. Transport 815 Mat. Res. Soc. Symp.
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