Al2O3 films, HfO2 films, and HfO2/Al2O3 stacked structures were deposited on n-type, Ga-face, GaN wafers using plasma-enhanced atomic layer deposition (PEALD). The wafers were first treated with a wet-chemical clean to remove organics and an in-situ combined H2/N2 plasma at 650 °C to remove residual carbon contamination, resulting in a clean, oxygen-terminated surface. This cleaning process produced slightly upward band bending of 0.1 eV. Additional 650 °C annealing after plasma cleaning increased the upward band bending by 0.2 eV. After the initial clean, high-k oxide films were deposited using oxygen PEALD at 140 °C. The valence band and conduction band offsets (VBOs and CBOs) of the Al2O3/GaN and HfO2/GaN structures were deduced from in-situ x-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The valence band offsets were determined to be 1.8 and 1.4 eV, while the deduced conduction band offsets were 1.3 and 1.0 eV, respectively. These values are compared with the theoretical calculations based on the electron affinity model and charge neutrality level model. Moreover, subsequent annealing had little effect on these offsets; however, the GaN band bending did change depending on the annealing and processing. An Al2O3 layer was investigated as an interfacial passivation layer (IPL), which, as results suggest, may lead to improved stability, performance, and reliability of HfO2/IPL/GaN structures. The VBOs were ∼0.1 and 1.3 eV, while the deduced CBOs were 0.6 and 1.1 eV for HfO2 with respect to Al2O3 and GaN, respectively.

This study reports a photoemission threshold of ∼1.5 eV from nitrogen-doped nanocrystalline diamond, which ranks among the lowest photo-threshold of any non-cesiated material. Diamond films on molybdenum substrates have been illuminated with light from 340 to 550 nm, and the electron emission spectrum has been recorded from ambient to ∼320 °C. The results display combined thermionic and photo-electron emission limited by the same low work function and indicate that the two emission processes are spatially separated. These results indicate the potential for a solar energy conversion structure that takes advantage of both photoemission and thermionic emission.

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The first part of this study examined oxide stability and cleaning of Ru surfaces. The surface reactions during H 2 plasma exposure of Ru polycrystalline films were studied using x-ray photoelectron spectroscopy (XPS). The $2 monolayer native Ru oxide was reduced after H-plasma processing. However, absorbed oxygen, presumably in the grain boundaries, remains after processing. A vacuum thermal anneal at 150 C substantially removes both surface oxide and absorbed oxygen which is attributed to a reduction by carbon contamination. The second part of the study examined the thermal stability of Cu on a Ru layer. The thermal stability or islanding of the Cu film on the Ru substrate was characterized by in situ XPS. After plasma cleaning of the Ru adhesion layer, the deposited Cu exhibited full coverage. In contrast, for Cu deposition on the Ru native oxide substrate, Cu islanding was detected and was described in terms of grain boundary grooving and surface and interface energies. The oxygen in the grain boundary has a negligible contribution to the surface energy and does not contribute to Cu islanding.

Vanadium oxide (VO2) is a narrow band gap material (Eg = 0.7 eV) with a thermally induced insulator-metal phase transition at ∼343 K and evidence of an electric field induced transition at T < 343 K. To explore the electronic properties of VO2, a sandwich structure was prepared with a 2 nm VO2 layer embedded between an oxidized Si(100) surface and a 2 nm hafnium oxide (HfO2) layer. The layer structure was confirmed with high resolution transmission electron microscopy. The electronic properties were characterized with x-ray and ultraviolet photoemission spectroscopy, and the band alignment was deduced on both n-type and p-type Si substrates. The valence band offset between VO2 and SiO2 is measured to be 4.0 eV. The valence band offset between HfO2 and VO2 is measured to be ∼3.4 eV. The band relation developed from these results demonstrates the potential for charge storage and switching for the embedded VO2 layer.