The Depth Dependence of Photoluminescence and Electrolytic Electroluminescence in Porous Silicon Films
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THE DEPTH DEPENDENCE OF PHOTOLUMINESCENCE AND ELECTROLYTIC ELECTROLUMINESCENCE IN POROUS SILICON FILMS M. I. J. BEALE, T. I. COX, L. T. CANHAM, AND D. BRUMHEAD Defence Research Agency, St Andrews Road, Malvem, Worcs, WR14 3PS, UK
ABSTRACT The depth dependence of the photoluminescence (PL) spectrum, both in terms of intensity and wavelength of the peak, is correlated with the electrochemical and chemical processes occurring during the formation of the porous silicon. The depth dependence of the intensity of the electroluminescence (EL) under both anodic bias and cathodic bias, in solutions containing 1M H 2S0 4 and 0.1M Na 2S20, is found to be identical with that of the PL. These observations strongly suggest that the silicon skeleton is highly conductive during electrolytic EL, in marked contrast to its high resistivity in air. Strong evidence is provided linking the electrolytic EL to the PL. We extend the quantum confinement based model for PL to the case of electrolytic EL. INTRODUCTION The potential application of porous silicon to the fabrication of electroluminescent devices which can be monolithically integrated with VLSI circuits is motivating considerable research worldwide. Despite much discussion, agreement has not yet been reached on the mechanisms giving rise to the efficient photoluminescence (PL) in porous silicon. The depth dependence of the PL has been studied by several workers, Noguchi et al [1] found that all the PL came from the topmost layer. Prokes et al [2] studied a 7014m thick film under very intense excitation. They found the PL intensity decreased away from the surface of the film but that the peak position was independent of depth. Tischler et al [3] conclude that the depth dependence of the PL is a function of the method of sample preparation and its history. In this work we show that it is possible to control the PL profile by varying the electrochemical and chemical processes occurring during the formation of the porous silicon. Electroluminescence (EL) has been observed from porous silicon in aqueous electrolytes under both anodic bias [4] and under cathodic bias in the presence of the persulphate ion [5,6]. The mec'hanisms for these processes are not well understood and in particular the paths of current flow have not been established. We have chosen to study the depth dependence of these processes in an attempt to elucidate the current pathways within the porous silicon, i.e. to determine if the silicon skeleton is rendered conductive during electrolytic EL. Based on the quantum confinement model for PL [7,8] and the cross-sectional results, we propose the mechanisms giving rise to the efficient electrolytic EL. EXPERIMENTAL TECHNIQUES Porous silicon films were prepared so as to have silicon skeleton dimensions which were either uniform (Sample 1) or non uniform (Samples 2 & 3) in depth, as deduced from gravimetric and film depth measurements. Depth dependence can be induced by chemical dissolution of the silicon skeleton either during or subsequent to the anodisation process. Th
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