Ni, Ir, Pd, Pt, and silver oxide Schottky contacts were fabricated on the Zn-polar face of hydrothermally grown, bulk ZnO. A relationship was found between the barrier height of the contact and the free energy of formation of its “metal” oxide. This is consistent with the dominating influence of oxygen vacancies $\left(V_{\mathrm{O}}\right)$ which tend to pin the ZnO Fermi level close to the $V_{\mathrm{O}}$ $(+2,0)$ defect level at approximately $0.7\mathrm{eV}$ below the conduction band minimum. Therefore, a key goal in the fabrication of high quality Schottky contacts should be the minimization of oxygen vacancies near the metal-ZnO interface.

A method of fabricating highly rectifying Schottky contacts on n-type ZnO using silver oxide has been developed and used to compare diode performance on hydrothermal and melt grown, bulk, single crystals. Silver oxide diodes on hydrothermal ZnO have lower ideality factors, lower reverse current voltage dependence, higher series resistance, and larger surface-polarity related differences in barrier height, compared to those on melt ZnO. These effects are explained by the large difference in resistivity between hydrothermal and melt ZnO. Barrier heights of 1.20eV were achieved on the Zn-polar face of hydrothermal ZnO which are the highest reported for n-type ZnO.

Planar Pd, Pt, Au, and Ag Schottky diodes with low ideality factors were fabricated on the Zn-polar (0001) and O-polar $\left(000\overline{1}\right)$ faces of bulk, single crystal ZnO wafers. The diodes were characterized by current-voltage and capacitance-voltage measurements. A polarity effect was observed for Pt and Pd diodes with higher quality barriers achieved on the O-polar face. No significant polarity effect was observed for Au or Ag diodes. The highest barriers were achieved with Ag as the Schottky metal with barrier heights varying between 0.77 and $1.02\mathrm{eV}$ . This is possibly due to varying degrees of oxidation of the Ag contacts.

This paper demonstrates the use of cost-effective solution-processed α-Ga ₂ O ₃ thin films (TFs) for electronic device applications. MESFETs based on AgO _x Schottky diode (SD) gates were fabricated on highly crystalline Sn-doped α-Ga ₂ O ₃ TFs, grown by mist chemical vapor deposition at atmospheric pressure and a substrate temperature of only 400°C. The rectification ratio and reverse breakdown voltage of typical SDs were 6 × 10 ⁶ and 19.6 V, respectively. The ON-OFF ratio of the corresponding transistors was 2 × 10 ⁷ . The MESFETs that could withstand drain voltages of up to 48 V were also realized.

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 ($\sim${}1/4 of the band gap) by controlling the surface H coverage using simple ultrahigh vacuum (UHV) heat treatments up to 750 \ifmmode^\circ\else\textdegree\fi{}C, dosing with HO/H 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.

Magnetotransport measurements and x-ray photoemission spectroscopy were used to investigate the surface conductivity of ZnO. Near-surface downward band bending, consistent with electron accumulation, was found on the polar and nonpolar faces of bulk ZnO single crystals. A significant polarity effect was observed in that the downward band bending was consistently stronger on the Zn-polar face and weaker on the O-polar face. The surface electron accumulation layer was found to significantly influence the electrical properties of high resistivity, hydrothermally grown bulk ZnO crystals at temperatures below 200 K, and is largely responsible for the anomalously low electron mobility reported for this material.