Large Red Shift of PL Peak Energy in High Growth Rate a-Si:H Prepared by Hot-Wire CVD
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Large Red Shift of PL Peak Energy in High Growth Rate a-Si:H Prepared by Hot-Wire CVD Daxing Han, Guozhen Yue, Jennifer Weinberg-Wolf and Jessica M. Owens Department of Physics & Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255 Yueqin Xu and Qi Wang National Renewable Energy Laboratory, Golden, CO 80401
ABSTRACT We characterized the electronic states and microstructure of high-growth-rate a-Si:H films by employing photoluminescence (PL) and Raman spectroscopies. The growth rate was from 50 to 115 Å/s compared to the standard rate of less than 10 Å/s. For the high-growth-rate a-Si:H films, we observed typical a-Si:H features in Raman but new features in PL. The new PL features are: a) the PL peak energy is as low as ~1.15 eV compared to the standard ~1.4 eV at 80 K; and b) the total intensity is more than one order of magnitude higher then the standard. We suggest that the nano-scale microstructure may be responsible for the anomalous PL features. INTRODUCTION a-Si:H made by hot-wire chemical vapor deposition (HW CVD) has attracted a great deal of attention because of the high deposition rate and some "magic" properties [1-3] as compared to the glow-discharge (GD) CVD materials. For device-quality HW a-Si:H, only 3-5 at.% H is needed, compared to 8-10 at.% in GD samples. A nuclear magnetic resonance (NMR) study suggests that most H atoms are highly clustered in a small volume fraction in HW films.[2] In the low hydrogen HW films, it was found that the internal friction is close to c-Si but much smaller than that in GD a-Si:H films.[3] Analysis of the first peak of the a-Si:H phase in the Xray diffraction (XRD) pattern demonstrates that its width, directly related to medium range order (MRO), is reduced by an increase of substrate temperature, Ts, in HW CVD growth. The width is narrower in the HW films deposited at Ts > 320 oC than that in conventional GD films.[4] These results, along with the high mass density, indicate that the HW a-Si:H network has a better overall structure order. On the other hand, for device-quality HW a-Si:H, the standard growth rate is 8-10 Å/s , compared to ~1 Å/s for GD samples.[1] More recently, scientists at the National Renewable Energy Laboratory (NREL) have made device-quality a-Si:H films at a very high growth rate by HW-CVD.[5] The growth rate can be as high as 50 to 160 Å/s compared to ~10 Å/s. The high growth rate was reached by adding a second filament, and decreasing the filament-to-substrate distance in the standard tubereactor.[1,5] These high growth rate a-Si:H films are interesting from both fundamental and industrial points of view. The preliminary studies of photo-response and defect density indicate that the same quality as the standard device-quality film has been achieved at such high growth rates.[5] Measurements of void density by small-angle X-ray scattering (SAXS) revealed an increase by well over an order of magnitude when going from one to two filaments.[5] To explore whether the material retains its structure and properties in su
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