Correlated photoluminescence spectroscopy investigation of grain boundaries and diffusion processes in nanocrystalline a
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Correlated photoluminescence spectroscopy investigation of grain boundaries and diffusion processes in nanocrystalline and amorphous silicon (nc-Si:H) mixtures Jeremy D. Fields1, K. G. Kiriluk1, D. C. Bobela2, L. Gedvilas2, P. C. Taylor1 1. Department of Physics, Colorado School of Mines, Golden, CO, United States. 2. National Renewable Energy Laboratory, Golden, CO, United States. ABSTRACT Photoluminescence (PL) spectra obtained with correlated set of experiments investigating grain boundary characteristics and diffusion processes in nanocrystalline silicon alloys (nc-Si:H), provide insight regarding formation and passivation of electronic defects in these regions. Based upon current results and previous works we believe thermally driven processes induce a PL band centered at 0.7 eV upon thermal annealing, and most likely involve diffusion of hydrogen and oxygen near interfaces. A nc-Si:H sample set with varied crystal volume fraction, Xc, was subject to thermal annealing treatments at different temperatures – each exceeding the deposition temperature. Fourier-transform photoluminescence (FTPL) and Fourier-transform infrared absorption spectroscopy (FTIR), were employed to correlate the relative 0.7 eV defect band emergence with compositional changes indicative of Si–Hx and Si–O species, for each sample, at each temperature, respectively. Hydrogen effusion data provide additional perspective. We find the Xc to strongly affect susceptibility of nc-Si:H to oxygen related effects. The higher the Xc, the more readily oxygen penetrates the nc-Si:H network. We attribute this relationship to elevated diffusivity of oxygen in highly crystalline nc-Si:H materials, owing to their abundance of gain boundaries and interfaces, which serve as pathways for impurity migration. These findings corroborate the expectation that oxygen impurities and diffusion processes contribute to development of microstructural features giving rise to radiative recombination through deep defects in nc-Si:H. INTRODUCTION Photoluminescence (PL) spectra obtained with correlated set of experiments investigating grain boundary characteristics and diffusion processes in nanocrystalline silicon alloys (nc-Si:H), provide insight regarding formation and passivation of electronic defects in these regions. Based upon current results and previous works we believe thermally driven processes induce a PL band centered at 0.7 eV upon thermal annealing, and most likely involve diffusion of hydrogen and oxygen near interfaces. Reports from other authors attribute a 0.7 eV PL band in crystalline-Si (c-Si), polycrystalline-Si (poly-Si), and microcrystalline-Si (μc-Si:H) systems, emerging in response to thermal exposure, to oxygen related deep electronic defects [1-3]. Our previous investigations found the 0.7 eV PL band in nc-Si:H, labeled here a defect band, to originate from the nanocrystalline phase, and to involve defects of an analogous nature [4,5]. Observing correlations between oxygen contamination, thermal exposure, and defect band onset in all cases leads to
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