Pore morphology of heavily doped p-type porous silicon

Porous silicon (PS) is a nanostructured material generated by electrochemically etching silicon in electrolytes containing hydrofluoric acid (HF) with many potential application areas such as optoelectronics and biosensing. PS retains the advantages of silicon technology while adding the ability of controlling optical properties. Fabry-Pérot interferometers, Fabry-Pérot filters or distributed Bragg reflectors are some of the 1D structures that have been fabricated in PS under different etching conditions, e.g. changing anodization current and concentration of HF.

Tuning pore diameter is essential for some applications in which substances must be flown through the pores, so that a size-based filtering of the molecules can be done. However, macropore (>50nm) formation on p-type silicon is still poorly known due to the strong dependence with resistivity [1]. Electrochemically etching heavily doped p-type silicon usually forms micropores (<10nm) but it has been found that bigger sizes can be achieved by adding a solvent to the electrolyte (aqueous or organic).

In this work we present the results of using dimethylformamide (DMF), dimethylsulfoxide (DMSO), potasium hydroxide (KOH) and sodium hydroxide (NaOH) for macropore formation in p-type silicon with resistivities between 0.001 and 9 Ω∙cm, achieving pore size range from 5 to 100nm.

[1] G.X. Zhang, Porous silicon: morphology and formation mechanisms, Modern Aspects of Electrochemistry, number 39, edited by C. Vayenas et al., Springer, New York, 2005.