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Photoelectron study of the electronic and optical properties of porous silicon Van Buuren, Anthony W. H.


Various explanations have been proposed for the strong visible luminescence from porous silicon (PS), the most widely accepted being quantum confinement. In the quantum confinement model the bandgap of PS depends on the size of the microstructure. Double crystal x-ray diffraction on PS shows a narrow peak and a broad peak consistent with a two phase model for the structure of PS in which there is a macroporous backbone supporting a nanoporous surface layer. From the width of the diffuse peak we estimate the size of the silicon structures in the nanoporous region to range between 30-60A depending on the preparation conditions. Synchrotron radiation based techniques such as x-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES) allow direct measurement of the quantum shift in the conduction and valence band edges in porous silicon. High resolution measurements of the silicon L and K-edge absorption in porous silicon show that the absorption threshold is shifted to higher energy relative to bulk silicon, and the shift is dependent on how the porous silicon is prepared. The blue shift of the conduction band minimum in PS is understood qualitatively using a simple LCAO model. The x-ray absorption spectra also show an excitonic enhancement at the Si L edge in PS which increases with the quantum shift in the L-edge absorption. The enhanced excitonic absorption, is in agreement with the expectation that electron-hole interactions increase in silicon nanocrystals because of confinement effects. PES data show that the valence band of porous silicon is also shifted relative to the bands for bulk silicon. The quantum shift in the valence band is larger than the shift in the conduction band and is proportional to it with a proportionality constant of 2.0 that is independent of preparation conditions. An independent conformation of the relationship between the quantum shifts in the valence and conduction band edges is obtain from PS annealing experiments. The quantum shifts in the conduction and valence band edges of PS relative to bulk Si are found to decrease with progressively higher annealing temperatures, up to 550°C, at which point the band edge energies revert to the values for bulk Si. The ratio of the shift in the valence band edge to the shift in the conduction band edge remains approximately constant with annealing and equal to 1.9, in agreement with the ratio determined as a function of preparation conditions. This result suggests that the PS microstructure progressively becomes larger when heated between 400 and 500°C where the surface hydrogen evolves. The ratio of the valence band shift to conduction band shift is predicted to be 1.5 using an effective mass model for the quantum shifts. Measurements of the Si L-edge were used to probe the effects of different preparation procedures on the electronic structure of PS. When the porous silicon is made from n-type material with light exposure, the blue shift increases logarithmically with the anodizing current and anodization time. We explore the hypothesis that the etching reaction self-limits and that the quantum size effect is a key part of the self-limiting mechanism. Two models have been proposed to explain the light intensity and time dependence of the quantum shift in anodized n-type PS. The peak energy of the room temperature photoluminescence of PS is compared with the bandgap determined from the XAS and PES measurements for a series of PS sample prepared under different conditions. The photoluminescence bandgap is found to be smaller than the photoelectron spectroscopy bandgap, but exhibits the same trend with preparation conditions. The width of both the photoluminescence spectrum and the L- absorption edge increase with increasing blue shift, consistent with a distribution of quantum confinement energies. An alternative explanation for the visible PL in PS is emission from a surface siloxene (Si6H6O3) layer which is peaked near 550 nm. The structure of siloxene is known to consist of Si (111) layers terminated above and below by OH groups and H atoms. This is difficult to reconcile with photoelectron experiments which show that freshly prepared PS does not contain oxygen. Recently an oxygen-free form of siloxene (Si6H6) called layered polysilane has been synthesized. The x-ray absorption of the layered polysilane and PS are found to be remarkably similar. In particular, the K absorption edge of layered polysilane is shifted by 0.6 eV the same as that of the PS samples with the maximum conduction band shift. Conceivably PS could consist of (111) oriented layers of Si terminated with hydrogen with a chemical formula Si6nH6, where n is the number of layers and it depends on the preparation conditions. In this picture layered polysilane (n=l) is the limiting form of PS.

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