Metal-Assisted Chemical Etching of p-type Si Nanopillars with Doping Level of 10^20 cm^–3

Silicon nanopillars (SiNPs) are quasi-1D Si nanostructures extending normally to the substrate. As Si nanowires, SiNPs display large surface-to-volume ratios and a remarkable reduction of their thermal conductivity by almost a factor one-hundred compared to bulk Si. SiNPs have found applications in energy-related fields, including thermoelectrics and photovoltaics, and in chemical sensing and biosensing. Among the available preparation techniques, Metal-Assisted Chemical Etching (MACE) has been extensively used to prepare high density SiNP forests, as the method is facile and scalable. However, while MACE lets easily obtain low-to-medium doped SiNPs, p-type SiNPs with doping level ≥ 1020 cm-3, essential for energetic applications, could never be obtained. In this paper we provide evidence that this is related to the competition between metal-assisted and noncatalyzed etching. Thus, guided by a model of the silicon-solution and silicon-metal-solution interfaces, we could develop a strategy that enabled for the first time the preparation by MACE of p-type SiNPs with doping levels as large as 1020 cm-3, using Au as the catalyst and Na2S2O8 as the oxidizing agent. The possibility of preparing heavily doped SiNPs largely extends MACE application to critical technological fields spanning from thermoelectrics and photovoltaics to batteries and sensing.