Hong Piao

Investigation of the electronic properties of nitrogen vacancies in AlGaN

The effects of premetallization annealing on the electrical characteristics of GaN and AlxGa1−xN (x=0.25 and 0.5) Schottky diodes have been studied. Annealing above 800 °C in an Ar ambient led to a significant preferential N loss, and annealing at 1000 °C caused localized surface decomposition. The Pt/Au Schottky contacts on the annealed GaN became leakier, whereas those on the thermally damaged AlGaN exhibited more rectifying characteristics. Prolonged annealing produced more conductive AlGaN surfaces deficient in both N and Al, as revealed by x-ray photoelectron spectroscopy. These findings suggest that, as opposed to donorlike vacancies in GaN, N vacancies in AlGaN with an Al mole fraction ≥25% behave as deep-level states, compensating the near-surface region. Care must therefore be taken to prevent the loss of N during thermal processing of high-Al content AlGaN.

Multiscale Modeling and Analysis of the Nitridation Effect of SiC/SiO2 Interface

We explain the role of nitrogen in simultaneously increasing the inversion channel mobility and reducing the threshold voltage of SiC MOSFET. A variety of computational techniques have been used to compute the atomic scale configuration of a nitridated SiC/SiO2 interface, and the corresponding change in Fermi level, inversion channel mobility, and threshold voltage. X-ray photoelectron spectroscopy (XPS) has been used to investigate the SiC/SiO2 interface to determine the nitrogen concentrations and chemical bonding. We elucidate the physics behind improved channel mobility due to NO anneal and demonstrate that the trade-off between threshold voltage and inversion channel mobility can be correlated to the extent of nitridation.

Cubic gauche polymeric nitrogen under ambient conditions

The long-sought cubic gauche phase of polymeric nitrogen (cg-PN) with nitrogen-nitrogen single bonds has been synthesized together with a related phase by a radio-frequency plasma reaction under near-ambient conditions. Here, we report the synthesis of polymeric nitrogen using a mixture of nitrogen and argon flowing over bulk β-sodium azide or β-sodium azide dispersed on 100 nm long multiwall carbon nanotubes. The cg-PN phase is identified by Raman and attenuated total reflection-Fourier transform infrared spectroscopy, and powder X-ray diffraction. The synthesis of the cubic gauche allotrope of high energy density polymeric nitrogen under near-ambient conditions should therefore enable its optimized production and applications as a “green” energetic material and a potential catalyst for different chemical reactions.Polymeric phases of nitrogen are promising as environmentally-friendly, high energy-density materials, but are inherently unstable. Here, the authors report the synthesis and stabilization of polymeric nitrogen in its cubic gauche phase under near-ambient conditions, via plasma-enhanced chemical vapour deposition.

Silicon carbide oxidation in the presence of cesium: Modeling and analysis

In this work we have focused on investigating the interaction of cesium (Cs) atom/ion with the oxidant and carbon cluster defects at the SiC/SiO2 interface using atomistic scale computational techniques and experimental characterization methods. We observe that Cs behaves significantly different from sodium (Na) at the SiC/SiO2 interface. Our analyses indicate that Cs tends to form a strong bond with the incoming oxygen molecule, leading to the formation of Cs oxide and suboxides. Results suggest that Cs does not reduce the penetration barrier of the impinging oxidant (O2 molecule). Also, unlike Na, Cs is unable to increase the Fermi energy of SiC/SiO2 interface. Finally, lateral metal–oxide–semiconductor field-effect transistors (MOSFETs) were fabricated (using Cs) yielding mobilities less than 1 cm2/V s versus ∼100 cm2/V s fabricated using Na.

Analysis of Thin Phase-Shifter Films using Surface Analysis Techniques

Many types of phase-shifters have been developed for use in place of the TEM objective aperture [1]. The phase shifters act to increase phase contrast by providing high transfer of information over a very wide spatial-frequency range. Unfortunately, many of these devices fail shortly after being installed into the instrument due to charging in the electron beam, so we have been experimenting with surface deposition of novel thin-film metals. In some cases, it is essential that the electronscattering cross-section of the metal film be as small as possible, so the films must often be quite thin (less than 10 nm thick). Accurate analysis of such thin films is required to understand the composition of the layers, unexpected impurities both in the films and at the interfaces, the oxidation state of the layers, and the lateral uniformity of the layers.

High Spatial Resolution XPS and AES Applications in Understanding Microscopic Surface Phenomena

X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) are the most widely used techniques for quantitative surface analysis. The XPS is mainly dedicated to the identification of chemical composition/bonding on surfaces, while the high spatial resolution in AES has made it an established technique for determining the localized defects or other features distributed on surfaces. Although the spatial resolution in XPS has been much inferior to that in AES, the recent development of XPS instrumentation with near-micron spatial resolution has advanced the capability of elemental and chemical state imaging accompanied by small area analysis [1]. The goal of this presentation is to present how the combined uses of XPS and AES at enhanced spatial resolution have significantly improved the understanding of the chemistry of heterogeneous surfaces, particularly from the fields such as microelectronic materials/devices, thin films and coatings.