Effective method to extract optical bandgaps in Si nanowire arrays

Abstract

A simple method to extract the optical bandgap of Si nanowire (SiNW) arrays that utilizes the reflection spectra of freestanding SiNW arrays is presented in this Letter. At a fixed nanowire diameter, three different wire lengths reproducibly formed a cross point in their reflectance curve plots. The cross point wavelength corresponded to the optical bandgap, as verified by the classical Tauc's model. The optical bandgap of the SiNW arrays (112nm in average diameter) was measured to be similar to 1.19 eV, which is larger than the similar to 1.08 eV bandgap of bulk Si. Further decreasing the wire diameter to 68nm caused an increase of the bandgap to similar to 1.24 eV, which is closer to the optimal bandgap (similar to 1.40 eV) required to achieve the highest conversion efficiency in single-junction photovoltaic devices. Our method suggests that the multijunction tandem structure can be realized via control of the diameter of SiNW arrays. (C) 2011 Optical Society of America