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 ($112\mathrm{nm}$ in average diameter) was measured to be $\sim 1.19\mathrm{eV}$, which is larger than the $\sim 1.08\mathrm{eV}$ bandgap of bulk Si. Further decreasing the wire diameter to $68\mathrm{nm}$ caused an increase of the bandgap to $\sim 1.24\mathrm{eV}$, which is closer to the optimal bandgap ($\sim 1.40\mathrm{eV}$) 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.

© 2011 Optical Society of America

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