Investigation of Porous Silicon Films Structure by Optical Methods
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INVESTIGATION OF POROUS SILICON FILMS STRUCTURE BY OPTICAL METHODS
P. BASMAJI, V. GRIVICKAS, G.I. SURDUTOVICH, R. VITLINA and V.S. BAGNATO Instituto de Ffsica e Qufmica de Sio Carlos - USP, SMo Carlos, SP, 13560-970, Brazil ABSTRACT The structure of aged porous silicon (PS) has been investigated using reflectivity, photoluminescence (PL), optical absorption and photoconductivity decay. An optical anisotropy in a perpendicular to surface direction is observed for PS samples below a critical porosity of 70%. The photostability of the PL spectra imply three different structures of PS, these should be attributed to quantum size structures, polymeric type structures and fluor-related structures. The absorption edge is deconvoluted into a band-to-band absorption with energy ranging between E. = 1.6 - 2 eV and an absorption shoulder at hv < E,. The high energy PL band around 2.3 eV resembles that of fluoroindate glass. The observed rapid lifetime at high injection level is attributed to excitonic Auger-type recombination. INTRODUCTION Electronic and structural properties of PS layers have recently received considerable attention, mainly since efficient visible photoluminescence (PL) has been observed in this material [1]. The origin of this visible luminescence is still a controversial question [1-3]. Because of this important consideration, we have looked further into the nature of the PS structure using optical methods. Our samples were prepared by an anodic etching of p-type Si with HF solutions in water at 10-100 mA/cm2 . Different porosity was obtained by using a variety of etching-time intervals. Scanning electron and atomic force microscopy studies of cross sections of PS layers have shown a two-layered structure. The upper layer consists of a coarse grained PS material, while a continuous uniform lower layer has finely textured structure [3]. The textured material dominates in samples of porosity _•70%, while the coarse material dominates in samples of higher porosity. As will be shown below, the two types of material exhibit a definite difference in PL and optical absorption after aging in air. These samples are labeled PS-A and PS-B respectively. Also, we used metal fluorides (RbF, CsF) in some etching solutions in order to modify the structure of highly porous layers. Such samples are labeled below as PS-C. All the results were obtained on samples aged between 1-4 months in air at room temperature. REFLECTIVITY The known "anisotropic morphology" model of PS as in the collection of c-Si columns (wires) situated basically normally to the film surface [1] implies an optical anisotropy as well. For a description of the PS samples we accepted the model of homogeneous uniaxial substrate with optical axis Z normal to the surface. We are putting forward a simple procedure of determination of the refractive index components (N. = N,, 5 N.) by the measurement of the Brewster angle 0, and the polarized light intensity reflectances R. and Rp at small and grazing angles. For Brewster angle we Mat. Res. Soc. Symp. Proc. Vol
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