Correlation between reflectivity and photoluminescent properties of porous silicon films

Porous silicon (PS) layers were formed on p-type, 〈1 0 0〉 oriented, 15 Ω cm resistivity Cz silicon wafers by electrochemical etching in an HF:C₂H₅OH (1:2 by volume) electrolyte at room temperature at a constant current density 20 mA/cm². The etching duration was varied to achieve PS layers of different morphologies and thicknesses. Both the photoluminescence (PL) and the total diffused reflectivity spectra of the PS layers were measured. It was found that for the PS layers grown for etching durations of less than 90 s the PL emission is insignificant and reflectivity is quite low. Such PS layers can be used as antireflection coatings (ARC) on solar cells. The PS layers formed for etching durations greater than 90 s show a significant PL emission in 500800 nm range with peak lying in 630660 nm wavelength range. When etching duration increases from 90 s to 8 min the PL intensity increases and the PL peak shows a blue shift. With further increase in etching duration the PL intensity decreases and PL peak shows a red shift. The reflectivity of the photoluminescent layers increases with etching duration showing a highest value for a sample grown for 8 min. Further increase in etching duration up to 20 min the reflectivity decreases and then increases. Striking observation is that both the PL emission intensity and reflectivity in the wavelength range of 550800 nm are maximum for the PS layer grown for the etching duration of 8 min.