Porous Si (111) Fabrication Using Electrochemical Anodization: Effects of Electrode Distance and Current Density

Abstract

Porous silicon (PSi) has developed for many applications such as gas and humidity sensors. Various methods are available to fabricate PSi, and electrochemical anodization is common due to low cost and easy use. Current density, etching/anodization time, type of etching solution, and electrode distance are the parameters determining resulting pores. The substrate used n-type silicon wafer with (111)-orientation and resistivity of 1.5-4.5 Ω.cm with a size of 1.5×1 cm2. The cleaning process of the samples employed the RCA cleaning procedure. Conductive contacts required for the electrochemical anodization were aluminum on the samples. The electrodes were the Si sample acting as anode and platinum (Pt) electrode as a cathode. The etching solution using a mixture of HF (40%) and ethanol (99%) with a 1:1 ratio. The electrode distance was 1.5, 2.0, and 2.5 cm. The current density for each electrode distance was 10, 30, and 50 mA/cm2 with an anodization time of 30 min. SEM and UV-Vis characterizations were applied to obtain surface morphology and reflectance, respectively. For all samples, the reflectance of PSi was lower than the reflectance of the original silicon surface (no pores). This condition indicates that the PSi is suitable as an anti-reflective layer in a solar cell. However, the PSi of reflectance curves has irregular shapes as a function of wavelength for different electrode distance and the current density. The SEM images confirmed that the pores formed on the silicon surface were inhomogeneous. The pore size decreased with the increase of the electrode distance while it increased as the increase of the current density. There was a correlation between pores size and reflectance at specific wavelength numbers.