Stain etching of silicon provides a spontaneous, self-limiting chemical method to produce nanocrystalline silicon films ͑porous Si͒. Whereas the existence of etchants capable of producing porous Si has been known for some time, little has been known concerning how solution composition influences the efficacy of porous Si production and the properties of the resulting films. We demonstrate that the fluoride species may be derived from either HF or acidified NH 4 HF 2 . A range of oxidants may be used as long as their counterions do not lead to precipitation. However, a large positive electrochemical potential is not a sufficient condition for efficient porous Si production. Bubble production, which is deleterious to film homogeneity and long thought to be inherent to the process, can be avoided by the use of transition metal-containing oxidants. Properties of the film, such as morphology, growth rate, porosity, and the wavelength of the photoluminescence maximum, respond to the etchant composition. We observe a blue shift in photoluminescence, which correlates with an increasingly positive electrochemical potential ͑E 0 ͒ of the oxidant. It is argued that E 0 plays a role much like wavelength in photoelectrochemical etching and that smaller nanocrystals are produced with more positive values of E 0 .

Etching of silicon can be accomplished in a number of ways: in aqueous solutions of fluoride or KOH, or by simultaneous exposure to a gas such as SF6 or HCl and high power laser irradiation. Here we review how laser assisted etching, chemically enhanced laser ablation, and stain etching can be used to create pillars, pores, porosified pillars, and macropores. Pillar dimensions can range from 10μm tall with ∼200nm tips to >100μm high with 10μm tips. Ordered arrays can be produced and sharpening can lead to aspect ratio approaching 104. Porous silicon with a visibly photoluminescent nanocrystalline structure can be made by etching in fluoride solutions. Acidic fluoride+oxidant solutions (stain etchants) allow us to form porous silicon on substrates of arbitrary topology. Moreover, these etchants can be used to control the photoluminescence spectrum of the resulting film, presumably through control of the size distribution of the nanocrystalline structures that are responsible for photoluminescence.

We used a XeCl excimer laser (308 nm, ~3 J cm -2 ) and chemically enhanced laser ablation in the presence of SF 6 to create arrays of silicon pillars. Etching of these pillars with KOH leads to arrays of macropores whose morphology depends on surface crystallography. Several new stain etchants -containing some combination of HF, NH 4 HF 2 , HCl, HNO 3 , Fe(III), Mn(VII) and water -have been used to cover these structures with a layer of microporous silicon. Both pillars and macropores are black, i.e., exhibit low reflectivity across the visible spectrum. When porosified, pillars exhibit visible photoluminescence whose angular distribution differs from that of planar porous silicon with significantly more intensity at high angles. The peak photoluminescence wavelength depends on the stain etchant composition and exposure to air. The use of metal ions in place of HNO 3 in the stain etchant ameliorates the production of gas bubbles and leads to more uniform films.