ABSTRACT
For the first time, porous silicon (PSi) was discovered in 1950s by Uhlir (1956) and Turner (1958), while performing electropolishing experiments on silicon wafers using a HF-containing electrolyte. They found that under the appropriate conditions of applied current and solution composition, the silicon did not dissolve uniformly but instead fine holes were produced. PSi formation was then obtained by electrochemical dissolution of Si wafers in aqueous or ethanoic HF solutions (Smith and Collins 1992; Lehmann 1993; Halimaoui 1995, 1997). However, until the 1990s, PSi had always been considered a somewhat esoteric material, of interest to primarily advanced research and development centers and companies dealing with silicon electronics (Watanabe and Sakai 1971; Watanabe et al. 1975; Imai 1981). In particular, in 1971, Watanabe and Sakai (1971) demonstrated the first application of PSi in electronics, the so-called full isolation by the porous oxidized Si process (FIPOS), where the PSi layers were used for device isolation in integrated circuits. In the 1980s, the silicon-on-insulator (SOI) in integrated circuits technology (Takai and Itoh 1986), the silicon on sapphire technology (SOS), and silicidation of PSi (Ito et al. 1989) were introduced as well. The high surface area of porous Si was found to be useful as a model of the crystalline Si surface in spectroscopic studies (Dillon et al. 1990), and as a sensing layer in chemical sensors (Anderson et al. 1990). Only in the last decades after the papers of Canham (1991) and Lehmann and Gosele (1991), who demonstrated room temperature lightemitting properties of PSi, electrochemically produced microporous silicon (μPSi) became one of the most studied materials in the field of material science. Although it should be noted that Pickering et al. (1984) observed low temperature photoluminescence in 1984. The discovery of room-temperature photo-and electroluminescence boosted research because of the huge potential in silicon-based integrated optoelectronics applications. As it is known, devices based on a normal single crystalline Si substrate cannot emit light efficiently. The appearance of PSi-based light emitting devices (LEDs) gave the hope that porous Si will make possible the integration of passive optical devices like gratings, waveguides, and so on, on the same substrate with Si LEDs. Such integration would make true an old dream-to produce integrated optoelectronic circuits on cost-effective and well-investigated Si substrates. In the same years, Lehmann and Foll (1990) showed the possibility to etch deep and straight macropores with diameters in the micrometer region with a predefined lateral arrangement into n-type Si by electrochemical processing (see Table 1.1). Later, this approach became the basis for the development of photonic crystals. In the same time period, the unique features of the material-its large surface area within a small volume, its controllable pore sizes, its convenient surface chemistry, and its compatibility with conventional silicon microfabrication technologies-inspired research into applications far outside optoelectronics. Subsequently, pores have been found in most if not all single crystalline semiconductors (Foll et al. 2003a,b; Rittenhouse et al. 2003; Fang et al. 2006; Santinacci and Djenizian 2008; Shen et al. 2008).
