Group IV Nanocrystals for Silicon Photovoltaics
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Group IV Nanocrystals for Silicon Photovoltaics X. Liu1,2, S. Saini1,2, M. Vanhoutte3, J. Bakalis2, W. Yau2, A. Eshed3, L.C. Kimerling3, N. Pervez4, I. Kymissis4, and C.W. Wong4 1
Dept. of Physics, CUNY-Graduate Center, New York, NY, United States Dept. of Physics, CUNY-Queens College, Flushing, NY, United States 3 Dept. of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, United States 4 Depts. of Electrical Engineering & Mechanical Engineering, Columbia University, New York, NY, United States 2
ABSTRACT Silicon nanocrystals (nc-Si), have been shown to act as opto-electronic centers enabling light emission by carrier recombination, when precipitated in a silicon nitride (Si 3N4) host. In this work, nc-Si and Germanium nanocrystals (nc-Ge) are studied in sputtered films of Si3N4 and SiGeN for application as tandem cell layers in a Si solar cell. The samples are annealed in a nitrogen gas and forming gas ambient, from 500 ºC to 900 ºC, to investigate the influence of temperature on photoluminescence and photoconductivity.
INTRODUCTION Silicon nanocrystals (nc-Si) embedded in silicon nitride have been investigated by the photonics community as a Si-compatible opto-electronic medium[1, 2]. Sputtering studies of Si3N4 have found the deposited film to be N-deficient, resulting in Si-rich silicon nitride (SRN)[3, 4]. The limited solid solubility of Si in Si3N4 results in an annealing-induced phase separation and formation of nanometer-sized nc-Si. PL peak wavelengths, from annealed nc-Si embedded in SRN, have been reported in the visible and near-infrared (near-IR) spectrum [1, 4, 5]; differing concentrations of excess Si have been shown to shift the PL peak wavelength[1]. In comparison with Si-rich SiO2, the photoluminescence (PL) intensity of SRN is optimized at a lower temperature anneal (T=700 ºC) than that of Si-rich SiO2 (T=1150 ºC), indicating a larger driving force for nucleation of nc-Si in SRN[5]. While electrical conduction were not reported for Si-rich SiO2, a Space Charge Limited Conduction (SCLC) mechanism and electroluminescence have been demonstrated in SRN, embedded with nc-Si[1]. In the photovoltaics community, nc-Si embedded in SiO2 and Si3N4[5] have been investigated as a scintillator for downconversion of UV photons to visible, in order to reduce UV thermalization within Si solar cells. Germanium nanocrystals (nc-Ge) embedded in SiO2, have been proposed by the photonics community as an alternate candidate for Si-compatible light emission or memory storage devices[6]. Unlike the formation of nc-Si, which is a precipitation process from a supersaturated solid, nc-Ge are synthesized by reaction with Si or catalysis via H2 of oxidized Silicon Germanium (SiGe)[6]. It has been shown that synthesized nc-Ge are under enormous compressive stress, thereby influencing the density, distribution and size of the nanocrystals[7]. PL of nc-Ge has been reported in the near-IR, consistent with the predictions of the carrier confinement effect[6]. In the photovoltaics community,
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