Magnetic Resonance in Hydrogenated Nanocrystalline Silicon Thin Films
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1066-A11-01
Magnetic Resonance in Hydrogenated Nanocrystalline Silicon Thin Films Tining Su1, Tong Ju2, Baojie Yan3, Jeffrey Yang3, Subhendu Guha3, and P. Craig Taylor1 1 Department of Physics, Colorado School of Mines, Golden, CO, 80401 2 Department of Physics, University of Utah, Salt Lake City, UT, 84112 3 United Solar Ovonic LLC, Troy, MI, 48326 ABSTRACT We have investigated the localized electronic states in mixed-phase hydrogenated nanocrystalline silicon thin films (nc-Si:H) with electron-spin-resonance (ESR). The dark ESR signal most likely arises from defects at the grain boundaries or within the crystallites. With illumination with photon energies ranging from 1.2 eV to 2.0 eV, there is no evidence of photoinduced carriers trapped in the bandtail states within the amorphous region. Dependence of the light-induced ESR (LESR) upon the exciting photon energy reveals that, at different excitation photon energies, different regions dominate the optical absorption. This behavior may have potential consequences for understanding the light-induced degradation in nc-Si:H. INTRODUCTION Hydrogenated nanocrystalline silicon thin films (nc-Si:H) may be the most promising materials for the next generation solar cells. Due to the strong optical absorption at lower energies, these films are ideal for collecting light with energies below the optical gap of hydrogenated amorphous silicon (a-Si:H) [1-3]. The nc-Si:H films for the best device performance typically have a mixed phase, with a volume fraction of crystallinity of about 50% [4,5]. As a consequence, the material is highly inhomogeneous. The optical absorption and consequently the dynamics of the photo-induced carriers are complicated and not well understood, especially for energies below the optical gap of a-Si:H and crystalline silicon. Previously, we reported that the mixed-phase nc-Si:H sample shows negligible lightinduced degradation as measured by electron-spin-resonance (ESR) [6]. The optical absorption of low energy light near E =1 eV is most likely due to localized states at the grain boundaries [6]. However, when the sample is illuminated with light of higher energies, the absorption can occur in all three regions, namely the crystalline region, the amorphous region, and the grain boundaries. As a result, the dynamics of the photo-induced carriers can be very complicated. The carriers can diffuse across the grain boundaries, they can also be trapped at the localized states at the grain boundaries, and probably also in the bandtail states in the amorphous region. Measurements of the optical coefficients alone do not provide information on the details of the absorption, nor do they reveal details of the carrier dynamics. On the other hand, ESR provides details of the localized states, such as those trapped in bandtails and grain boundaries. We report an ESR study of the localized states in nc-Si:H, with or without illumination, and the dependence of the production of paramagnetic states on the exciting photon energy.
EXPERIMENT All nc-Si:H samples were ma
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