Role of subsurface defects in metal-ZnO(0001) Schottky barrier formation

The authors fabricated diodes of Au, Al, Ni, Pt, Pd, W, Ta, and Ir on single crystal ZnO(000 (1) over bar) surfaces from different vendors and measured their Schottky barriers, idealities, and reverse currents on as-received and remote oxygen (20% O-2/80%,He) plasma-treated surfaces. Using low temperature nanoscale depth-resolved cathodoluminescence spectroscopy (DRCLS) under the metal, the authors identified the presence of defect transitions at 2.1, 2.5, and 3.0 eV that change dramatically depending on the process steps and choice of metal. I-V measurements exhibited transitions from Ohmic to Schottky and lower idealities for Pt, Au, Ir, and Pd with plasma treatment. ZnO with low defect densities yield lower idealities and reverse currents. Deep level optical and transient spectroscopies correlated bulk and surface defects, showing deep levels at 2.54 and 0.53 eV, while DRCLS shows that these densities can increase by > 100 times at the surface. The magnitude of the metal's influence correlates directly to the relative defect concentrations near the surface and in the bulk. These results show that metals can induce defect states at the metal-semiconductor interface and impact device performance. (C) 2007 American Vacuum Society.