Strained Quantum Dots in Porous Silicon

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STRAINED QUANTUM DOTS IN POROUS SILICON XUE-SHU ZHAO, PETER D. PERSANS, JOHN SCHROEDER and YEUN-JUNG WU Physics Department and Center for Integrated Electronics Rensselaer Polytechnic Institute, Troy, NY. 12180

ABSTRACT On the basis of Raman, photoluminescence, and absorption studies of porous and nanoparticle silicon we propose that the strong luminescence in porous silicon results from strained silicon quantum dots. A silicon nanoparticle is a special Jahn-Teller system induced by extended electron states rather than localized state. Thus Raman scattering and photoluminescence in porous silicon are multi-phonon assisted free electronic transition processes. all observed anomalous properties of porous silicon can be clearly explained by using this strained quantum dot model.

INTRODUCTION In this paper, we report detailed pressure-dependent Raman scattering, photoluminescence(PL) studies on p+ and p- porous silicon and excitation intensity effects on optical properties of p+ small silicon particles. The experimental results demonstrate that the optical properties of porous silicon are the same as those of small silicon particles. We propose that the strong luminescence of porous silicon indeed comes from the strained silicon quantum dots residing in etched silicon. A strained silicon nanoparticle is a special Jahn-Teller system induced by sixfold degenerate electron levels, namely extended states rather than localized states. Thus the constraint of crystal momentum conservation in optical transitions is totally relaxed in small silicon particles. Raman scattering and the PL of small silicon particles are thus typical multi-phonon assisted electronic transition processes. All observed anomalous properties of porous silicon can be explained by a strained quantum dot model.

EXPERIMENTAL RESULTS AND DISCUSSION PorOus silicon p+ and p- samples were prepared by etching of p+ and p- single crystal silicon wafers ( 0.01 Q)/ cm 2 for p+, 5-10 fj / cm 2 for p- wafer) in solutions of 20 % HF / H 2 0. The anodization was done with current density of about 150 mA / cm 2 for two hours. Both samples were prepared under the same conditions. the brown powder was collected from the top layer of the porous silicon wafer and mixed into an uniform colloid in ethyl alcohol. The colloidal absorption spectra were used to estimate the energy gap between occupied and unoccupied states for both samples. The absorption behavior of the colloid sample made from p+ porous silicon is quite similar to that of p+ bulk silicon and can be well fitted with ot=A (hv-Eg) 2 . The energy gap thus is found 1.98 eV for p+ porous silicon. For the p- colloid sample, the absorption curve can be Mat. Res. Soc. Symp. Proc. Vol. 283. @1993 Materials Research Society

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fitted with indirect absorption relation a=A (hv-Eg) 3/2. The energy gap for p- porous silicon thus measured is about 2.05 eV. The same colloid samples were loaded in a diamond anvil cell to measure Raman scattering and PL spectra under high pressure at room temperature. The p+ silicon parti

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