Current-voltage measurements of Au contacts deposited on ex situ cleaned, n-type ZnO͑0001͒ ͓͑0001͔͒ surfaces showed reverse bias leakage current densities of ϳ0.01 ͑ϳ0.1͒ A / cm 2 at 4.6 ͑3.75͒ V reverse bias and ideality factors Ͼ2 ͑both surfaces͒ before sharp, permanent breakdown ͑soft breakdown͒. This behavior was due primarily to the presence of ͑1.6-2.0͒ ± 0.1 ͓͑0.7-2.6͒ ± 0.1͔ monolayers ͑ML͒ of hydroxide, which forms an electron accumulation layer and increases the surface conductivity. In situ remote plasma cleaning of the ͑0001͒ ͓͑0001͔͒ surfaces using a 20 vol % O 2 / 80 vol % He mixture for the optimized temperatures, times, and pressure of 550± 20°C ͑525± 20°C͒, 60 ͑30͒ min, and 0.050 Torr reduced the thickness of the hydroxide layer to ϳ0.4± 0.1 ML and completely eliminated all detectable hydrocarbon contamination. Subsequent cooling of both surfaces in the plasma ambient resulted in the chemisorption of oxygen and a change from 0.2 eV of downward band bending for samples cooled in vacuum to 0.3 eV of upward band bending indicative of the formation of a depletion layer of lower surface conductivity. Cooling in either ambient produced stoichiometric ZnO͕0001͖ surfaces having an ordered crystallography as well as a step-and-terrace microstructure on the ͑0001͒ surface; the ͑0001͒ surface was without distinctive features. Sequentially deposited, unpatterned Au films, and presumably the rectifying gold contacts, initially grew on both surfaces cooled in the plasma ambient via the formation of islands that subsequently coalesced, as indicated by calculations from x-ray photoelectron spectroscopy data and confirmed by transmission electron microscopy. Calculations from the current-voltage data of the best contacts revealed barrier heights on the ͑0001͒ ͓͑0001͔͒ surfaces of 0.71± 0.05 ͑0.60± 0.05͒ eV, a saturation current density of ͑4 ± 0.5͒ ϫ 10 −6 A / cm 2 ͑2.0± 0.5ϫ 10 −4 A / cm 2 ͒, a lower value of n = 1.17± 0.05 ͑1.03± 0.05͒, a significantly lower leakage current density of ϳ1.0ϫ 10 −4 A / cm 2 ͑ϳ91ϫ 10 −9 A / cm 2 ͒ at 8.5 ͑7.0͒ V reverse bias prior to sharp, permanent breakdown ͑soft breakdown͒. All measured barrier heights were lower than the predicted Schottky-Mott value of 1.0 eV, indicating that the interface structure and the associated interface states affect the Schottky barrier. However, the constancy in the full width at half maximum of the core levels for Zn 2p ͑1.9± 0.1 eV͒ and O 1s ͑1.5± 0.1 eV͒, before and after sequential in situ Au depositions, indicated an abrupt, unreacted Au/ ZnO͑0001͒ interface. Transmission electron microscopy confirmed the abruptness of an epitaxial interface. Annealing the contacts on the ͑0001͒ surface to 80± 5 and 150± 5°C resulted in decreases in the ideality factors to 1.12± 0.05 and 1.09± 0.05 and increases in saturation current density to 9.05 and 4.34 A / cm 2 , the barrier height to 0.82± 0.5 and 0.79± 0.5 eV, and in the leakage current densities to ϳ2 ϫ 10 −3 A / cm 2 at 6 V and ϳ20ϫ 10 −3 A / cm 2 at 7 V, respectively.

A layer containing an average of 1.0 monolayer ͑ML͒ of adventitious carbon and averages of 1.5 ML and 1.9 ML of hydroxide was determined to be present on the respective O-terminated (0001 ) and Zn-terminated ͑0001͒ surfaces of ZnO. A diffuse low-energy electron diffraction pattern was obtained from both surfaces. In situ cleaning procedures were developed and their efficacy evaluated in terms of the concentrations of residual hydrocarbons and hydroxide and the crystallography, microstructure, and electronic structure of these surfaces. Annealing ZnO(0001 ) in pure oxygen at 600-650°CϮ20°C reduced but did not eliminate all of the detectable hydrocarbon contamination. Annealing for 15 min in pure O 2 at 700°C and 0.100Ϯ0.001 Torr caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits (ϳ0.03 MLϭϳ0.3 at. %) of our x-ray photoelectron spectroscopy instrument. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO(0001 ) surface to a remote plasma having an optimized 20% O 2 /80% He mixture for the optimized time, temperature, and pressure of 30 min, 525°C, and 0.050 Torr, respectively, resulted in the desorption of all detectable hydrocarbon species. Approximately 0.4 ML of hydroxide remained. The plasma-cleaned surface possessed an ordered crystallography and a step-and-terrace microstructure and was stoichiometric with nearly flat electronic bands. A 0.5 eV change in band bending was attributed to the significant reduction in the thickness of an accumulation layer associated with the hydroxide. The hydroxide was more tightly bound to the ZnO͑0001͒ surface; this effect increased the optimal temperature and time of the plasma cleaning process for this surface to 550°C and 60 min, respectively, at 0.050 Torr. Similar changes were achieved in the structural, chemical, and electronic properties of this surface; however, the microstructure only increased slightly in roughness and was without distinctive features.

In this study, we have deposited Ti, Zr, and Hf oxides on ultrathin (∼0.5 nm) SiO2 buffer layers and have identified metastable states which give rise to large changes in their band alignments with respect to the Si substrate. This results in a potential across the interfacial SiO2 layer, significant band bending, and large shifts of the high-k valence band. The magnitude of the shift differs for the three materials and is dependant on both the SiO2 buffer layer thickness and annealing temperature. We propose a model where excess oxygen accumulates near the high-k-SiO2 interface providing electronic states, which are available to electrons that tunnel from the substrate.

The band alignment at the SiO 2-GaN interface is important for passivation of high voltage devices and for gate insulator applications. X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy have been used to observe the interface electronic states as SiO 2 was deposited on clean GaN͑0001͒ surfaces. The substrates, grown by metallorganic chemical vapor deposition, were n-(1ϫ10 17) and p-type (2ϫ10 18) GaN on 6H-SiC͑0001͒ with an AlN͑0001͒ buffer layer. The GaN surfaces were atomically cleaned via an 860°C anneal in an NH 3 atmosphere. For the clean surfaces, n-type GaN showed upward band bending of 0.3Ϯ0.1 eV, while p-type GaN showed downward band bending of 1.3Ϯ0.1 eV. The electron affinity for n-and p-type GaN was measured to be 2.9Ϯ0.1 and 3.2Ϯ0.1 eV, respectively. To avoid oxidizing the GaN, layers of Si were deposited on the clean GaN surface via ultrahigh vacuum e-beam deposition, and the Si was oxidized at 300°C by a remote O 2 plasma. The substrates were annealed at 650°C for densification of the SiO 2 films. Surface analysis techniques were performed after each step in the process, and yielded a valence band offset of 2.0Ϯ0.2 eV and a conduction band offset of 3.6Ϯ0.2 eV for the GaN-SiO 2 interface for both p-and n-type samples. Interface dipoles of 1.8 and 1.5 eV were deduced for the GaN-SiO 2 interface for the n-and p-type surfaces, respectively.

X-ray absorption spectroscopy (XAS) is used to study band edge electronic structure of high-transition metal (TM) and trivalent lanthanide rare earth (RE) oxide gate dielectrics. The lowest conduction band d-states in TiO 2 , ZrO 2 and HfO 2 are correlated with: 1) features in the O K 1 edge, and 2) transitions from occupied Ti 2p, Zr 3p and Hf 4p states to empty Ti 3d-, Zr 4d-, and Hf 5d-states, respectively. The relative energies of d-state features indicate that the respective optical bandgaps, E opt (or equivalently, E g), and conduction band offset energy with respect to Si, E B , scale monotonically with the d-state energies of the TM/RE atoms. The multiplicity of d-state features in the Ti L 2 3 spectrum of TiO 2 , and in the derivative of the O K 1 spectra for ZrO 2 and HfO 2 indicate a removal of d-state degeneracies that results from a static Jahn-Teller effect in these nanocrystalline thin film oxides. Similar removals of d-state degeneracies are demonstrated for complex TM/RE oxides including Zr and Hf titanates, and La, Gd and Dy scandates. Analysis of XAS and band edge spectra indicate an additional band edge state that is assigned Jahn-Teller distortions at internal grain boundaries. These band edges defect states are electronically active in photoconductivity (PC), internal photoemission (IPE), and act as bulk traps in metal oxide semiconductor (MOS) devices, contributing to asymmetries in tunneling and Frenkel-Poole transport that have important consequences for performance and reliability in advanced Si devices.