Localized States and Porous Silicon Luminescence

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detection. Use of attenuated total reflection geometry allows for in-situ investigation of wet porous silicon as well as dry porous silicon. During the infrared measurements, luminescence spectra are recorded simultaneously through a second lock-in detection. The results reported here have been obtained on porous silicon samples fabricated on (100)-oriented, float-zone purified, p-Si crystals (4x101 5 B atoms cm- 3 ), by anodization at 20 mA cm- 2 in 20% aqueous HF, in the dark. This provides 70%-porosity layers with a thickness of -1 g.m per minute of anodization. Oxidized porous silicon is obtained by anodization at 0.2 mA cm-1 in IM HCl + 0.5M CaCl 2 solution. Care is taken not to expose porous silicon to ambient atmosphere during the substitution of this electrolyte to the fluoride solution, in order to ensure the wetting of the porous layer by the electrolyte for the oxidation. The Ca 2+ ions in the electrolyte act as F- scavengers. During the anodic oxidation, the current is actually modulated between zero and its nominal value, and the associated changes in infrared transmission (electromodulated spectra) and in luminescence (electroluminescence spectra) are recorded. RESULTS AND DISCUSSION Typical photoluminescence spectra of wet, dry and oxidized porous silicon are shown in Fig. 1 as well as the corresponding photomodulated infrared spectra. It can be seen that wet porous silicon exhibits a weak yellow-green luminescence, whereas dry and oxidized porous silicon exhibit the well known intense red luminescence. The photomodulated infrared spectrum of wet porous silicon has a Drude-like absorption shape originating from the photocarriers; fine analysis of the data demonstrates the photocarrier capability to diffuse on a characteristic length of -1 gm [11]. On the opposite red-luminescent samples exhibit a broad photomodulated infrared absorption whose shape may be approximately described by a Gaussian curve centered between 1000 and 2500 cm- , depending upon the exact preparation conditions. This demonstrates that in wet porous silicon, photocreated carriers thermalize in extended states, whereas in dry or oxidized porous silicon, they thermalize in localized states. However, it should be noticed that the nice Gaussian shape of the photocarrier absorption observed in dry or oxidized porous silicon is obtained only in porous layers of sufficient homogeneity. For inhomogeneous porous layers, a mixture of Drude-like and Gaussian-like absorption is observed instead. As it is commonly done in many standard recipes used for porous silicon preparation, the homogeneity of the porous layer may be significantly improved by adding some ingredient (e.g., ethanol) in the HF solution used for porous silicon fabrication. We did not used such recipes, since such additives have not been proved to be completely inert in the luminescence process. For instance, we have observed that using a Triton X-100 tensio-active additive in the HF solution improves the layer homogeneity, but, even at ppm levels, efficiently quenches the lu

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