Study of the Amorphous-to-Microcrystalline Transition during Silicon Film Growth at Increased Rates: Extensions of the E
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Study of the Amorphous-to-Microcrystalline Transition during Silicon Film Growth at Increased Rates: Extensions of the Evolutionary Phase Diagram Andre S. Ferlauto, Pablo I. Rovira, Randy J. Koval, Christopher R. Wronski, and Robert W. Collins Materials Research Laboratory and Center for Thin Film Devices, The Pennsylvania State University, University Park, PA 16802. ABSTRACT The microstructural and phase evolution of silicon films (Si:H) prepared by low temperature (200°C) rf plasma-enhanced chemical vapor deposition (PECVD) with hydrogen dilution of silane has been studied using real time spectroscopic ellipsometry (RTSE) and atomic force microscopy (AFM). Both RTSE and AFM support the concept of an evolutionary phase diagram that describes different regimes of bulk layer thickness and H2-dilution ratio R within which predominantly amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) films are obtained. In this study, the evolutionary phase diagram has been extended to identify four separate growth regimes: (i) a-Si:H with a smooth surface and a stable roughness layer thickness, (ii) a-Si:H with a rougher surface and an unstable roughness layer thickness, (iii) mixed-phase (a+µc)-Si:H, and (iv) fully-coalesced (single-phase) µc-Si:H. Based on this framework, the effect of increased rf power to achieve higher deposition rates in the rf PECVD process was investigated. INTRODUCTION It has been well established that moderate H2-dilution of the source gases used in plasmaenhanced chemical vapor deposition (PECVD) of amorphous silicon (a-Si:H) and its alloys at low temperatures (< 300°C) leads to improvements in the electronic quality of these materials [1]. The improved i-layer components prepared using this approach result in a-Si:H based solar cells with improved performance and higher resistance to light-induced degradation [2]. In 1994, it was pointed out that optimum a-Si1-xCx:H materials for the i-layers of wide gap solar cells are produced under conditions of maximum H2-dilution prior to the formation of the microcrystalline phase [3]. Later, a similar conclusion was reached with respect to a-Si:H i-layer materials for n-i-p solar cells [4]. More recently it was emphasized that, in order to optimize the deposition of a-Si:H i-layers near the amorphous-to-microcrystalline (a→µc) phase boundary, one must recognize that the location of the boundary and thus the phase of the film depends sensitively on the accumulated thickness and the substrate [5,6]. In Refs. 5 and 6, we applied real time spectroscopic ellipsometry (RTSE) techniques to analyze the microstructural evolution of Si:H films, determining the critical thickness at which detectable crystallinity develops as a function of the H2-dilution gas flow ratio, R=[H2]/[SiH4], for depositions on different substrates. This capability led to the concept of the evolutionary phase diagram for identifying the range of thickness and H2-dilution ratio where the amorphous and microcrystalline phases are predominant. The evolutionary phase diagram has proven us
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