Real Time Spectroscopic Ellipsometry of Amorphous Silicon Grown at High Deposition Rates by Hot-Wire CVD
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Real Time Spectroscopic Ellipsometry of Amorphous Silicon Grown at High Deposition Rates by Hot-Wire CVD Brent P. Nelson and Dean H. Levi National Center for Photovolatics, National Renewable Energy Laboratory Golden, CO 80401, USA ABSTRACT We use real-time spectroscopic ellipsometry (RTSE) for in-situ characterization of the optical properties and surface roughness (Rs) of hydrogenated amorphous silicon (a-Si:H) grown by hot-wire chemical vapor deposition (HWCVD) with varying deposition rates (5 to 120 Å/s). Early time evolution of the Rs during growth is remarkably similar for all deposition rates. During the first few Ås of growth, there is a sharp increase in Rs as the a-Si:H nucleates in separate islands. This is followed by a reduction of Rs as these areas coalesce into a bulk film, which occurs at an average thickness of 100 Å. After coalescence the Rs rises to a stable value that is dependent upon growth conditions with a general tendency for the Rs to increase with growth rate. However, neither the Rs nor the material electronic properties are unique for a given deposition rate. Films grown under high silane flow and low pressure have a better photoresponse and a lower Rs than films grown at the same deposition rate but with low silane flow and high pressure. We observe a stronger correlation of film properties with Rs than with deposition rate; namely a monotonic decrease in photo-response, and increase in optical gap, with increasing Rs. INTRODUCTION Increasing the deposition rate of a-Si:H is important to increase product throughput as well as decrease initial capital investment of manufacturing facilities. We have successfully used hotwire chemical vapor deposition (HWCVD) to grow a-Si:H with photo-to-dark conductivity ratios over 105 [1], saturated defect densities below 5 × 1016 [2], and working solar cells with stabilized solar cell efficiencies over 4% [3] all at deposition rates up to 130 Å/s. We also know that while we maintain good material properties of these high growth rate materials, the films posses up to 3 vol.% voids between 18 and 58 Å in radius [4]. It is important to understand why a-Si:H can be grown at such high rates and yet maintain device quality electronic properties. Collins and coworkers performed keystone work in characterizing a-Si:H grown by plasma enhanced chemical vapor deposition (PECVD) using RTSE [5]. In analyzing their RTSE data, they observe three general stages of a-Si:H growth by modeling the evolution of the Rs. First islands of a-Si:H nucleate on the substrate increasing the Rs until these islands coalesce into a bulk film at which point Rs reaches a broad minimum. Eventually the surface roughness again increases. They correlate this second increase in surface roughness with a transition from stable a-Si:H growth to unstable growth and observe that the best materials are grown in the stable growth regime. We apply this modeling of the surface roughness evolution to observe the growth of a-Si:H by HWCVD at high deposition rates and never observe an extended region o
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