Carrier Transport in Ultra-Thin Nano/Polycrystalline Silicon Films and Nanowires

Carrier transport was investigated in two different types of ultra-thin silicon films, polycrystalline silicon (poly-Si) films with large grains > 20 nm in size and hydrogenated nanocrystalline silicon (nc-Si:H) films with grains 4 nm – 8 nm in size. It was found that there were local non-uniformities in grain boundary potential barriers in both types of films. Single-electron charging effects were observed in 30 nm × 30 nm nanowires fabricated in 30 nm-thick nc-Si:H films, where the electrons were confined in crystalline silicon grains encapsulated by amorphous silicon. In contrast, the poly-Si nanowires of similar dimensions showed thermionic emission over the grain boundary potential barriers formed by carrier trapping in grain boundary defects. INTRODUCTION Recently, there has been considerable interest in the low temperature growth of hydrogenated microcrystalline silicon (μc-Si:H) thin films for their potential applications in thin film solar cells and thin film transistors for flat panel displays [1,2]. Plasma-enhanced chemical vapor deposition (PECVD) is a novel technique that allows the film microstructure to be controlled by selecting appropriate deposition conditions [3]. For example, it is possible to prepare small grain nanocrystalline silicon (nc-Si:H) using a large H2/SiF4 gas mixing ratio. This controllability of grain size is promising for novel devices utilizing quantum confinement and single-electron charging effects. It is difficult to fabricate very thin (<50nm) nc-Si:H films with high crystallinity, large carrier mobility and large conductance using a low-temperature PECVD process. In addition, the transport properties of these heterogeneous materials are affected significantly by local microstructures. Thus carrier transport has been investigated in localized regions using an AFM/STM [4] and by fabricating wires ranging 0.1 μm – 4 μm in width [5]. In this paper, we report the fabrication of very thin nc-Si:H films and their characterization using 30-nm-wide nanowires to investigate the local carrier transport properties. We compare these results with those Mat. Res. Soc. Symp. Proc. Vol. 664 © 2001 Materials Research Society