Band-gap engineering with HfSxSe2-x -: art. no. 075205

The band gaps of HfS2 (1.96 eV) and HfSe2 (1.13 eV) cover the range of ideal band gaps for photovoltaic applications which is for a single-junction cell about 1.45 eV and for a two-junction cell about 1.1 eV and 1.8 eV. Additionally, in favor of applications the transition-metal dichalcogenides have a strong tendency to grow as thin films due to their quasi-two-dimensional character. Employing the chemical vapor transport method, single crystals of this ternary semiconductor were grown with nine different compositions. They were characterized in detail by low-energy electron diffraction, Laue diffraction, and energy dispersive x-ray analysis. In addition the absorption at room temperature was measured, revealing an absorption coefficient alpha(hnu) that is remarkably high for an indirect semiconductor. The determined band gaps of HfSxSe2-x show a nearly linear dependence on x, which is perfect for band-gap engineering. Furthermore, in the gap an exponential behavior of alpha(hnu) is observed, which could be interpreted as an Urbach tail.