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*University of Western Ontario, London, Canada, N6A 5B7, [email protected] **C.S.R.F.,Synchrotron Radiation Centre, University of Wisconsin-Madison, Stoughton, WI. ABSTRACT Photoluminescence from porous silicon was examined using synchrotron radiation as an excitation source. The tunability of the excitation source permitted a wide range of excitation energies ranging from VUV to X-rays. This permitted site selective excitation where specific core levels (i.e. Si-K, O-K, Si-2p) were probed. In high porosity samples, luminescence bands of both surface and bulk origins were observed. All experiments exhibited a common luminescence maximum typically in the orange-red region of the visible spectrum. At certain specific excitation energies particularly in the VUV region additional peaks related to sites with oxygen character were also observed. The VUV excitation luminescence spectra of the porous silicon remarkably resembled that of oxygen deficient amorphous silicon dioxide glasses. INTRODUCTION Porous silicon has been the subject of intense study since room temperature optical luminescence was first reported [1]. The origin of the luminescence has received a great deal of scrutiny [2-7] and possible applications for porous silicon in opto-electronics have been examined [8]. Synchrotron radiation can be used as an excitation source for optical luminescence experiments to good effect. The synchrotron ring provides an intense collimated source of photons that are tunable over a wide energy range. This allows for the study of the luminescence at excitation energies near several relevant core level as well as valence binding energies such as: Si-K, Si-L, O-K, and valence. Further, by tuning the excitation energy, and using different detection schemes, the surface sensitivity of the experiment can also be

adjusted. A previous study [5] was performed at a bulk sensitive excitation energy. This study showed that in the bulk of the porous silicon layers, no species containing oxygen were involved in the luminescence. In recent studies we have used excitation energies that are more surface sensitive. These studies clearly indicate that in porous silicon samples that have been exposed to ambient atmospheric conditions, species containing oxygen are involved in luminescence from the surface of the porous silicon, albeit via a different luminescence channel. The implications of these results will be discussed. EXPERIMENT Porous silicon samples were prepared using a method described in detail elsewhere [9]. p+-type (Boron doped ) Si (100) wafers with a resistivity of 1-10 fl.cm were utilized. Current densities used ranged from 1 to 500 mA/cm2 with anodization times typically of 10-30 minutes. An electrolytic solution composed of 1:1 48% by weight HF: absolute ethanol was 547 Mat. Roe. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society

used. The resulting porous silicon films typically appear reddish-brown in colour and exhibited orange-red to green luminescence under UV excitation. All samples were stored in ambient