The formation mechanism of gradient porous Si in a contactless electrochemical process
Recently, gradient porous silicon has been developed to meet the requirements of various applications due to its unique physical and chemical properties. In this paper, the formation mechanism and morphology of radially symmetric gradient porous silicon films fabricated using a contactless method and their dependence on different process parameters, such as HF concentration, solution pH, current density and wafer resistivity, have been investigated in detail. The design and geometry of the sample assembly allow decreasing current density radially inward on the silicon surface in contact with HF based etchant solution. In the presence of surfactants, an increase in the distribution range of porosity, pore diameter and depth was observed by increasing HF concentration or lowering pH of the etchant solution, as the formation of pores was considered to be limited by the etch rates of silicon dioxide. Gradient porous silicon was also found to be successfully formulated both at high (10 mA cm−2) and low (3 mA cm−2) current densities. Interestingly, the morphological gradient was not developed when dimethyl sulfoxide (instead of surfactants) was used in the etchant solution potentially due to limitations in the availability of oxidizing species at the silicon–etchant solution interface.