Morphology of self-supporting porous silicon layers

Abstract

Porous silicon has been under investigation for many applications such as silicon surface micromachining [1], fabrication of thick and stress-free silicon nitride films [2], and formation of conducting tungsten films and silicon carbide layers [3]. The capability of growing silicon dioxide layers on porous silicon at low temperature and time parameters compared with those required for single-crystal silicon, has been utilized in semiconductor process technology and called fully isolated porous oxidized silicon (FIPOS [4]). The silicon dioxide layer that can be grown on porous silicon remains a part of the single-crystal silicon substrate, and has been applied in silicon-on-insulator (SOI) technology [5]. The self-supporting nature of porous silicon layers has suggested new applications in filters, molecular sieves, separators, catalyst supports and chemical sensors [6]. More recently [7] reports of the observation of visible photoluminescence from porous silicon have suggested the possibility of applying porous silicon in optoelectronic technologies. The reported light emission efficiencies are as high as 10% [8]. In view of the growing interest in porous silicon, it is important to study its material properties and morphology. The pore formation mechanism in n-type silicon as described by Gaspard et al. [9] suggests that the electrochemical etching reaction proceeds in reverse bias and the applied potential is dropped predominantly across the silicon wafer, and pore propagation is controlled by potential distribution in the wafer. The morphology of porous silicon depends on several factors such as the dopant concentration, dopant type, anodic or cathodic over potential and ambient light conditions [10-12]. The understanding of the mechanism by which porous silicon emits light in the visible region is not well established. Several mechanisms have been suggested which appear to explain the visible luminescence phenomenon. One of the mechanisms is the formation of "quantum wires" [7]. The predicted bandgap increase of silicon low-dimensional structures due to one-, twoand three-dimensional quantum confinement in the 1-5 nm range has been reported by Canham [8]. Transmission electron microscopic investigations reported by Beale et al. [13] have indeed shown that silicon nanometre-size porous structures can be formed by electrochemically etching silicon. Microcrystalline Si:H fabricated by reactive sputtering has also been