Metal-Assisted Chemical Etching of p‑type Si Nanopillars with Doping Level of 10²⁰ 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.