Surface sensitive synchrotron x-ray photoelectron spectroscopy (XPS) and real-time in situ XPS were used to study the thermal stability of the hydroxyl termination and downward band bending on the polar surfaces of ZnO single crystals. On the O-polar face, the position of the Fermi level could be reversibly cycled between the conduction band and the band gap over an energetic distance of approximately 0.8 eV (similar to 1/4 of the band gap) by controlling the surface H coverage using simple ultrahigh vacuum (UHV) heat treatments up to 750 degrees C, dosing with H2O/H-2 and atmospheric exposure. A metallic to semiconductorlike transition in the electronic nature of the O-polar face was observed at an H coverage of approximately 0.9 monolayers. For H coverage less than this, semiconducting (depleted) O-polar surfaces were created that were reasonably stable in UHV conditions. In contrast, the downward band bending on the Zn-polar face was significantly more resilient, and depleted surfaces could not be prepared by heat treatment alone.preprintPeer reviewe

The stability of the ubiquitous hydroxyl termination and downward band bending on the m-plane () and a-plane () faces of ZnO single crystals was investigated using synchrotron and real-time x-ray photoelectron spectroscopy. On these non-polar surfaces, a strong correlation was found between the surface band bending and surface OH coverage, both of which could be modified via heat treatment in ultra high vacuum (UHV). On the m-plane () face, a threshold temperature of ~400 o C was observed, after which there was a sudden increase in OH desorption and upwards movement of the near-surface bands, leading to a metallic-to-semiconductor transition in the electronic nature of the surface, and a change from surface electron accumulation to depletion. This loss of surface metallicity is associated with the disruption of a stable monolayer of chemisorbed hydroxyl groups that form a closed hydrogen-bonded network, across the rows of Zn-O dimers, on the m-plane () face. The downward band bending and surface electron accumulation layers on both the m-plane () and a-plane () faces could be modified and eventually removed by simple UHV heat treatment, with important implications for the processing and electrical performance of ZnO nanostructures and catalytic ZnO nanopowders, which usually contain a high proportion of these non-polar surfaces.

We present a comprehensive comparison of electrical properties of differently fabricated high quality Schottky contacts on ZnO thin films grown by pulsed laser deposition. Thermally evaporated Pd/ZnO Schottky contacts exhibit ideality factors as low as 1.06 due to their high lateral homogeneity. The effective Richardson constant determined using these homogeneous contacts is (7.7±4.8)A cm−2 K−2 close to the theoretical value of 32 A cm−2 K−2. However, their rectification ratio is at most five orders of magnitude due to their comparably small barrier height (≈0.7eV). The largest effective barrier height (1.11 eV) and rectification ratio (7×1010) was obtained for reactively sputtered PdOx/ZnO Schottky contacts. Eclipse pulsed laser deposited IrOx/ZnO Schottky contacts were found to combine very good lateral homogeneity (n≈1.1), with a reasonably large barrier height (0.96 eV) and large rectification ratio (≈9 orders of magnitude). Our results for differently fabricated Schottky contacts suggest that the barrier formation is highly dependent on the presence of oxygen vacancies close to the interface and the different compensation mechanisms involved.