High-Rate Growth of Stable a-Si:H
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ABSTRACT Correlation between the gas phase species in silane plasma measured by mass spectrometry and the properties of hydrogenated amorphous silicon (a-Si:H) films deposited by plasma enhanced chemical vapour deposition (PECVD) has been investigated. We have especially been interested in the higher-order silane related species in the plasma, whose contribution to the film growth is considered to be the cause of light-induced degradation in the film quality, especially at high growth rate. In this study, we varied excitation frequency, gas pressure and power density to vary the growth rates of a-Si:H films ranging from 2 A/s to 20 A/s. Molecular density ratio of trisilane, representative of higher silane related radicals, to monosilane has shown a clear correspondence to the fill factor after light soaking of Schottky cells fabricated on the resulting films. INTRODUCTION A-Si:H film is an important material in the solar cell industry. It is essential to achieve high growth rate and uniform deposition on large area substrates in order to reduce the production cost, but the film properties, initial and more importantly after light-induced degradation, are
known to deteriorate with increasing growth rate. The increase in the growth rate is conventionally obtained by increasing the silane partial pressure and the power; however, the amount of gas-phase higher-order silanes is known to increase with the silane pressure, while short lifetime radicals such as Si, Sill and Sill2 contribute to the film growth under a silane depletion condition [1]. These changes of film-growth precursors in the gas-phase and/or their subsequent interactions on the growth surface are considered to be the cause of the inferior film properties. It was reported that dihydride bonding configuration (Si-H 2) in the a-Si:H network structure is responsible for degradation [2]. The Si-H 2 fraction is known to increase with the increase in growth rate. According to the surface diffusion model, the film surface mostly covered with
hydrogen and precursors with long diffusion length are the ideal conditions for the growth of films with low dihydride configuration [3]. The causes of Si-H 2 are suggested to include the substrate temperature which affects the diffusion process of film precursors, positive ions in the plasma striking the film surface, and the presence of higher-order silane related radicals. These radicals can disturb the surface reaction of film precursors and can be incorporated in the film. As the dihydride modes can be suppressed by increasing the deposition temperature [4], we took notice of higher-order silanes, whose contribution is suggested to be relaxed by increasing the 105 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
temperature. There has been much work on gas-phase analysis using mass spectrometry in order to analyze the ionic and neutral species in the silane rf glow-discharge plasma [1, 5, 6, 7], but most work does not touch upon properties of films deposited from the relevant plasma, especially
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