Role of Hydrogenated Surface in the Photoluminescence of Porous Silicon
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ROLE OF HYDROGENATED SURFACE IN THE PHOTOLUMINESCENCE OF POROUS SILICON MASAHIKO HIRAO, TSUYOSHI UDA and YOSHIMASA MURAYAMA Advanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-03, Japan ABSTRACT Optical properties of hydrogenated silicon clusters are investigated by density functional pseudopotential calculation. Transitions between the band-edge orbitals are allowed, in contrast to the indirect gap in bulk silicon. The energy gaps of hydrogenated silicon particles of 15 to 30 A•in diameter are estimated to be 2.0 to 1.5 eV. When the cluster is dehydrogenated, localized states related to dangling bonds appear in the mid-gap, which decrease the photoluminescence intensity. These results agree with much experimental evidence and suggest that the photoluminescence of porous silicon is attributable to hydrogenated silicon particles. INTRODUCTION The discovery of the photoluminescence (PL) of porous silicon (PS) [1] and ultrafine silicon particles [2] opens a new path toward silicon-based optoelectronic devices. Extensive experimental efforts have been made to understanrd the mechanism of PL. Observation by high-resolution transmission electron microscopy (TEM) [3] shows that the porous structure consists of quantum-size wires with widths measuring several tens of A. On tihe other hand, small silicon particles several tens of A in diameter have been observed by scanning electron microscopy (SEM) [4] and also by TEM [5, 6]. The crystal structure remains in microstructures [6, 7], although its lattice constants are slightly larger. The concentrations of Sill and Sill2 species, observed by Fourier-transform infrared (FTIR) spectroscopy, strongly relate to the PL intensity [5, 8]. Heat annealing in a vacuum decreases the PL intensity with decreasing Sill= species concentration [9]. Time-resolved luminescence measurements [5, 10] show long lifetimes on the order of microseconds, with the lifetime strongly dependent on the PL energy. For example, the lifetime of 2.2 eV PL is about one-tenth that of 1.5 eV PL. The origin of PL has been interpreted by a quantum confinement effect in microstructures, however, another proposal names siloxene derivatives as the origin of PL based on the similarity of IR and PL spectra [11, 12]. There are several theoretical analyses of silicon clusters or wires. The electronic structure of hydrogenated silicon clusters is investigated using the tight-binding method [13] and the linear combination of atomic orbital method [14]. They showed the size dependence of the energy gap and optical properties. The tight-binding electronic structure and optical properties of one dimensional silicon wires are calculated [15]. First-principal electronic structure of hydrogenated silicon wires with diameters around 10 A was recently calculated and adsorption spectra [16], PL lifetime [17] and excitonic effects [18] have been investigated. All studies showed that a simple effective mass approximation (EMA) is not sufficient for quantitative discussion of the PL mechanism.
METHOD First-principal ps
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