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MATERIALS RESEARCH

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Surface and optical properties of porous silicon S. M. Prokes Naval Research Laboratory, Washington, D.C. 20375 (Received 13 July 1995; accepted 5 September 1995)

Although silicon is the material of choice in the semiconductor industry, it has one serious disadvantage: it is an extremely poor optoelectronic material. This is because it is an indirect gap semiconductor, in which radiative transition results in extremely weak light emission in the infrared part of the spectrum. Thus, the discovery of strong visible luminescence from a silicon-based material (porous silicon) has been quite surprising and has generated significant interest, both scientific and technological. This material differs from bulk silicon in one important way, in that it consists of interconnected silicon nanostructures with very large surface to volume ratios. Although the first mechanism proposed to explain this emission process involved carrier recombination within quantum size silicon particles, more recent work has shown that the surface chemistry appears to be the controlling factor in this light emission process. Thus, the aim of this work is to outline the data and arguments that have been presented to support the quantum confinement model, along with the shortcomings of such a model, and to examine more recent models in which the chemical and structural properties of the surface regions of the nanostructures have been incorporated.

I. INTRODUCTION 1

Porous silicon (PoSi) was first produced by Uhlir and Turner2 in their studies of electropolishing of silicon in dilute hydrofluoric (HF) solutions. They found that bulk silicon could be transformed into a material consisting of a network of pores if the anodization was performed in dilute HF in which the current densities were below those for electropolishing. Until recently, the major application of porous silicon was for Si-on-insulator (SOI) technology, where the active devices could be dielectrically isolated by the oxidation of the underlying porous silicon. The formation of this oxide layer was easy in porous silicon, since it contained a large amount of internal surfaces that were highly reactive.3 Recently, significant attention has been directed toward this material due to its visible photoluminescence (PL)4 properties. The first above gap PL in porous silicon was reported by Pickering et al.5 at low temperatures, but it was not until 1990 that Canham reported this type of PL at room temperature.4 Since silicon is an indirect gap semiconductor in which interband transitions need phonons,6 the transition probabilities in bulk silicon are 100 times smaller than in direct materials. This results in a quantum efficiency in the range of 1024 % with radiative recombination in bulk silicon producing light in the infrared (1.1 eV). Thus, the reports of visible light emission of high quantum efficiency (1–10%) from a silicon-based system (porous silicon) were very surprising, and have resulted in a large effort to determine the mechanism res