Barrier-Controlled Transport in Doped Microcrystalline Si
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Barrier-Controlled Transport in Doped Microcrystalline Si S. Brehme, P. Kanschat, W. Fuhs Hahn-Meitner-Institut Berlin, Abteilung Silizium-Photovoltaik, Kekuléstr. 5, D-12489 Berlin, Germany ABSTRACT Thin µc-Si films doped with P and B are grown on glass at temperatures between 300 °C and 400 °C by electron cyclotron resonance chemical vapor deposition (ECR-CVD). Hall mobilities in the films are found to be temperature-activated with activation energies being correlated to the doping concentrations. The in-grain mobility as determined by an electron spin resonance investigation is much higher than the Hall mobility the values of which are typically near 1 cm2/Vs at 300 K. The experimental results suggest that transport is dominated by potential barriers at the grain boundaries. An extended Seto model is used to probe the interface trap distribution. We obtained the best fitting results by assuming band-tail states decaying exponentially from the respective band edges. INTRODUCTION In recent years microcrystalline Si has found much attention because of its technological potential in device applications such as thin-film solar cells and transistors. An important prerequisite for further improvement of this material is the understanding of electronic properties such as carrier mobility and its correlation to other material parameters e.g. doping concentration. Barrier-limited transport is one of the more prevalent approaches to model theoretically the electronic properties of doped µc-Si films. Charged interface (IF) states situated in the distorted regions at the grain boundaries play a decisive role in this model. Unfortunately, there is no direct and simple method to probe the energetic distribution of these states. The existence of tail states near the band edges was theoretically described and experimentally proved [1,2]. On the other side there are studies of doped µc-Si films in which the authors explain their experimental results with discrete levels (δ distribution) [3, 4] or a distribution of IF states near midgap [5]. In this paper we study the electrical transport in highly doped µc-Si films experimentally and compare the results with numerical simulations. A temperature-activated behavior of the mobility is observed and an explanation is given assuming potential barriers at the grain boundaries. With numerical simulations based on an improved Seto model [3] we will demonstrate that the experimentally observed correlation between potential barriers and doping concentration can be understood if the presence of exponentially decaying tail states is assumed. EXPERIMENTAL DETAILS Thin films doped in-situ with P and B were grown on quartz and corning glass substrates at temperatures of 325 °C and 380 °C, respectively. The deposition was carried out in an ECRCVD system [6]. During the growth process the pressure was kept constant at a value between 0.9 and 10 mTorr and the microwave power was set to 1000 W. Only H2 was used as excitation
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gas. The gas doping ratios were varied in a wide range (PH3/SiH4:
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