Porous Silicon Formation by Means of Metal Particle Enhanced Etching and Its Structure Control

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(1) We investigate the electroless displacement deposition of metal particles (Pt, Rh, Pd, Cu, Ag, and Au) onto n-Si wafers from a metal-salt solution containing HF. The particle density of metals varies widely, depending on the kind of metal. Deposited metals can be classified into two types of nucleation behavior. One consists of the platinum group elements, which display lower particle densities than elements of the other group and depend on the type of pretreatment of the n-Si wafer. The second group consists of the copper group elements, which display higher particle density than the first group and are independent of pretreatment. The size of deposited particles decreases from hundreds nm to ten nm as the particle density increases.(2) Metal-particle-enhanced HF etching of Si can produce micrometer-sized macroporous and nanometer-sized microporous Si by simple immersion of metal-particle-modified Si in a HF aqueous solution without a bias or a particular oxidizing agent. The etching rate of Si and the structure of produced porous Si are changed by the dissolved oxygen concentration of the HF solution and the photoillumination intensity on Si during etching. For n-Si, porous layer is only formed on the metal particle deposited area of the wafers. Both macro and microporous layers are formed on n-Si wafers modified with Pt, gold, or silver particles. Pd particles show a unique behavior in which the etching of n-Si proceeds at a high rate accompanying hydrogen evolution under dark conditions and the absence of dissolved oxygen. For Pt-particle-deposited p-Si, porous layers are formed by HF immersion. Under continuous photoillumination, porous Si is formed mainly on the non-photoirradiation side surface of p-Si, as opposed to the photoirradiation side of n-Si.