High-barrier rectifying contacts on undoped ZnO films with (NH ₄)₂S_x treatment owing to Fermi-level pinning

We report on the formation of high-barrier Ti and Ni contacts to (NH ₄)₂S_x-treated ZnO films grown by pulsed-laser deposition. The x-ray photoelectron spectroscopy (XPS) results show that the position of the Zn 3d core-level peak at the (NH₄)₂S _x-treated ZnO surface is the same as that at the Ti/(NH ₄)₂S_x-treated ZnO or Ni/(NH₄) ₂S_x-treated ZnO interfaces, suggesting the occurrence of Fermi-level (E_F) pinning and the formation of a barrier height of ∼2.7 eV. From the photoluminescence and XPS measurements, it is suggested that a high Zn-vacancy density might cause the ZnO E_F to be pinned close to the Zn-vacancy defect level at approximately 0.7 eV above the valence band maximum. In addition, the discrepancy in barrier-height values obtained from XPS and current-voltage measurements suggests the formation of S-Zn surface dipoles with S atoms on the surfaces.