Photoelectron spectroscopy, optical characterization, and density functional calculations of ${\mathrm{ZnO}}_{1-x}{\mathrm{S}}_x$ reveal that the valence-band (VB) offset $E_v(x)$ increases strongly for small S content, whereas the conduction-band edge $E_c(x)$ increases only weakly. This is explained as the formation of local ZnS-like bonds in the ZnO host, which mainly affects the VB edge and thereby narrows the energy gap: $E_g(x=0.28)\approx E_g(\mathrm{ZnO})-0.6\mathrm{eV}$. The low-energy absorption tail is a direct ${\Gamma }_v\to {\Gamma }_c$ transition from ZnS-like VB. The VB bowing can be utilized to enhance $p$-type ${\mathrm{N}}_{\mathrm{O}}$ doping with lower formation energy $\Delta H_f$ and shallower acceptor state in the ZnO-like alloys.

• Received 28 November 2005

DOI:https://doi.org/10.1103/PhysRevLett.97.146403