Photoluminescence Characterization of Hydrogenated Nanocrystalline/Amorphous Silicon

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1153-A02-01

Photoluminescence Characterization of Hydrogenated Nanocrystalline/Amorphous Silicon J.D. Fields1, P.C. Taylor2, J.G. Radziszewski2, D.A. Baker2, G. Yue3, B. Yan3 1

Materials Science Department, Colorado School of Mines, Golden, CO Physics Science Department, Colorado School of Mines, Golden, CO 3 United Solar Ovanic LLC, Troy, MI 2

ABSTRACT The photoluminescence in a nc-Si/a-Si:H mixture has been investigated at varying excitation intensities, and temperatures We have also observed changes in the luminescence spectra, which are induced by sequential annealing at temperatures below the a-Si:H crystallization temperature (~ 600°C). Two predominant luminescence peaks are observed at ~ 0.95 eV and ~ 1.30 eV, which are attributed to band tail-to-band tail transitions near the nc-Si grain boundaries and in the a-Si:H bulk, respectively. The 0.95 eV band saturates approaching 500 mW/cm2 excitation intensity. Annealing the nc-Si/a-Si:H mixture brings out a new low energy peak, centered at ~ 0.70 eV, and which we believe to be due to oxygen defects. INTRODUCTION In order to optimize hydrogenated nanocrystalline/amorphous silicon (nc-Si/a-Si:H) for photovoltaic applications we must better understand its electrical and optical properties, especially those at the nc-Si/a-Si:H interfacial regions. We present data from an exploratory round of photoluminescence experiments, which shed light on the nature of the electronic structure of nc-Si/a-Si:H. Investigating the luminescence behavior of a nc-Si/a-Si:H mixture under various excitation intensity and temperature conditions reveals several differences between the electronic structure of the nc-Si phase from that of a-Si:H. Radiative transitions signified by emission at ~ 1.3 eV occur when excited carriers recombine radiatively across band-tail states in a-Si:H [1]. In poly-Si and mixed phase silicon the large number of defects in grain boundary regions, namely variation in bond lengths and angles, causes local fluctuations in the electric potential. These effects form localized states in the band gap and give rise to band-tails [2]. Emission at 0.9 eV in poly-Si [2, 3] occurs by carrier recombination across these band-tail states, and a band observed at ~ 0.9 – 1.0 eV in µcSi/a-Si:H systems is believed to involve similar band-tail states [4]. The decay of PL with increasing temperature occurs due to the increased probability of non-radiative recombination when carriers have sufficient (thermal) energy to hop to nearby states [1]. Dangling bonds, interfaces, and defects provide pathways for excited carriers to lower their energy without radiative recombination. Both the luminescence decay with increasing temperature and the excitation intensity dependence of PL depend on local electronic structure. Band-tail luminescence in a-Si:H typically increases with temperature at very low temperatures, plateaus at ~ 50 K, and decreases rapidly above 80 K [1, 5]. Detection of nc-Si luminescence at room temperature and its unique luminescence decay with increasing temperature s

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