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A. RAMIREZ PORRAS,* O.RESTO,* S.Z. WEISZ,* Y. GOLDSTEIN,** A. MANY,** AND E. SAVIR** *Department of Physics, University of Puerto Rico, Rio Piedras, PR 00931 **Racah Institute of Physics, The Hlebrew University, Jerusalem 91904, Israel

ABSTRACT Pulse measurements on the porous-Si/electrolyte system are employed to determine the surface effective area and the surface-state density at various stages of the anodization process used to produce the porous material. Such measurements were combined with studies of the photoluminescence spectra. These spectra were found to shift progressively to the blue as a function of anodization time. The luminescence intensity increases initially with anodization time, reaches a maximum and then decreases with further anodization. The surface state density, on the other hand, increases with anodization time from an initial value of -2x 1012 cm- 2 for the virgin surface to -1013 cmn2 for the anodized surface. This value is attained already after -2 min anodization and upon further anodization remains fairly constant. In parallel, the effective surface area increases by a factor of 10 - 30. This behavior is markedly different from the one observed previously for n-type porous Si. INTRODUCTION Porous silicon,1 -4 (PS) obtained by electrochemical etching procedures applied to crystalline Si surfaces, exhibits high luminescence efficiencies in the visible range. It is quite clear now that the visible luminescence originates from the band-gap enlargement due to quantum confinement. 4 At the same time, the reasons for the high-efficiency luminescence are still somewhat under debate. 4 ,5 It was suggested that it is the amorphous 6 or microcrystalline 7 nature of the porous Si that is responsible for the phenomenon, or that the formation of silicon compounds such as siloxene (Si6 0 3 146 ) or species of Si-H, Si-O and Si-F bonds are involved in the luminescence. 8 One way of gaining further insight into the luminescence process is to carry out a variety of measurements on samples of different porosity. To that end we have employed combined studies of the luminescence spectrum, the surface-state density and the effective surface area of the porous surface. Such studies were carried out at different stages of the anodization process and thus for different morphologies of the porous surface. The luminescence spectra were measured by conventional methods. The surface state characteristics and the effective surface area were determined by pulse measurements 9 on the PS/electrolyte system. This system is particularly suitable since a capacitative contact to the terrain of the porous surface is best achieved by an electrolyte, and it was successfully used 10 to investigate n-type PS. There we found 10 a strong correlation between the surface-state density near the conduction band edge and the luminescence intensity. In this paper we present similar measurements on p-type porous Si and we compare the results with those obtained on n-type material. EXPERIMENTAL The starting material was high-grade p-