An Electrochemical Study of Porous Silicon
- PDF / 1,678,607 Bytes
- 12 Pages / 420.48 x 639 pts Page_size
- 38 Downloads / 212 Views
AN ELECTROCHEMICAL STUDY OF POROUS SILICON J.D. L'ECUYER* AND J.P.G. FARR** *Centre de Technologie Noranda, 240 boul. Hymus, Pointe-Claire, Canada. H9R IG5 **School of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT. ABSTRACT The I-V and impedance characteristics of p and n-type silicon electrodes in HF solutions have been determined. Three different I-V regimes are observed, one of which is associated with the on-set of localized dissolution. The formation of porous silicon takes place via a surface state mediated charge transfer mechanism. The position of the main recombination-generation center is estimated at 400 mV above the valence band edge. Localized dissolution is initiated at or close to active adsorption sites. It is then favoured because of geometrical field enhancement effects. Porous silicon has a surface chemistry that can be significant in luminescence. INTRODUCTION Oxidized porous silicon (FIPOS) has been considered for SOT structures in integrated circuit technology [1]. Porous silicon is obtained by anodic dissolution of silicon in hydrofluoric acid (HF) solutions. It consists of a fine network of interlinked pores, the number and size of which are determined by the doping level, the impurity type and the electrochemical parameters. Both the morphology and the surface chemistry of porous silicon are important in determining photoluminescence. The selectivity of the anodizing process and the rapid oxidation rate of porous silicon have been exploited for SOT purposes. Original applications of porous silicon for dielectric isolation were based on the anodization of p-type regions [1]. More recent work has made use of the selective anodization of heavily doped n-type regions [2-5]. A typical SOT structure then consists of a n/n+/n sandwich, where the n+ region is converted into SiO2 following anodization and oxidation. High quality SOI-CMOS transistors have recently been made on such material [6]. The optimization of FIPOS SOT structures rested largely on the control of the anodizing process. This could only be achieved through a better understanding of the electrochemistry involved in the formation of porous silicon. Electrochemical experiments were therefore carried out in order to determine the dissolution mechanism by which localized attack of the silicon electrode is initiated and maintained. The effects are also described of oxidising and ageing after the formation of porous silicon, on composition, lattice strain and reactivity. EXPERIMENTAL PROCEDURE A range of commercially available [100] p and n-type silicon wafers was examined (see Figure 1). The back face of the wafers was metallized so as to provide a good ohmic conmact. Porous silicon layers could be formed without illumination on all of these samples, except 0.1 f~cm n-type material, in the solutions investigated (40% HF and 1:1 mixtures of 40% HF and ethanol). The wafers were cleaved into approximately 1 cm 2 pieces and mounted on brass supports using a conductive epoxy. The silicon-brass assembly was coated
Data Loading...
