Microstructure control of silicon thin films through controlling charged nanoparticles generated during hot wire chemical vapor deposition

Abstract

Silicon thin film have been extensively used in electrical devices for thin film transistors (TFTs) and silicon thin film based solar cells. There are three kinds of silicon thin films from the viewpoint of microstructures: hydrogenated amorphous silicon (a-Si:H), hydrogenated microcrystalline silicon (µc-Si:H), and crystalline silicon thin film. The a-Si:H and µc-Si:H thin film generally fabricated by plasma enhanced chemical vapor deposition (PECVD) or hot wire CVD (HWCVD). Although low temperature (< 600 oC) direct deposition of crystalline film is urgently needed for flexible and high performance applications, high quality crystalline silicon film ordinarily prepared by post treatment of amorphous silicon films. This is the important issue. Until now, formation of nanostructures or films explained by classical crystal growth mechanism based on atomic or molecules growth. However, many puzzling phenomena and problems cannot be explained and solved by classical growth mechanism. Recently, many puzzling phenomena such as low temperature formation of nanocrystalline silicon came to the fore in PECVD or HWCVD. By the non-classical growth mechanism, namely, theory of charged nanoparticles (TCN), which have been extensively studied for 20 years, was revealed that the charged nanoparticles (CNPs) would be mainly involved in formation of nanostructure during not only thermal CVD but also HWCVD. Furthermore, it could explain many puzzling phenomena in CVD systems. In this study, based on the TCN and its phenomenological evidences, microstructure control of silicon thin films was studied. It was clearly shown that the silicon nanoparticles landed on a substrate and they formed the silicon film. In addition, the retrograde solubility of Si – H – Cl system was confirmed by capturing the silicon nanoparticles for few seconds. The crystalline silicon nanoparticles were remained at the condition of maximum HCl concentration. At this condition, polycrystalline silicon thin film was successfully deposited on a flexible polymer substrate at 200 oC substrate temperature. Furthermore, properties of silicon films increased by controlling the new processing parameter deposition delay time. Size of nanoparticles could be controlled by HCl concentration and deposition delay time. Moreover, small size CNPs which formed in the initial stage of reaction could be continuously supplied by pulsing the reaction. Based on the understanding of small size CNPs liquid like property in diffusion, microstructure of silicon films totally controlled from high to low crystallinity. Eventually, homo-epitaxial silicon films, which have 0.5 nm/sec growth rate and 50 nm thickness, could be deposited using small size CNPs at substrate temperature of 600 oC. These results indicate that diffusivity of nanoparticles could be enhanced by charge. This means that the charge enhances the atomic diffusion, which is a newly discovered physical phenomenon.