Using the measured capacitance-voltage curves of Ni Schottky contacts with different areas on strained AlGaN∕GaN heterostructures and the current-voltage characteristics for the AlGaN∕GaN heterostructure field-effect transistors at low drain-source voltage, we found that the two-dimensional electron gas (2DEG) electron mobility increased as the Ni Schottky contact area increased. When the gate bias increased from negative to positive, the 2DEG electron mobility for the samples increased monotonically except for the sample with the largest Ni Schottky contact area. A new scattering mechanism is proposed, which is based on the polarization Coulomb field scattering related to the strain variation of the AlGaN barrier layer.

First-principles calculations on the mechanical properties and structural identifications of the fluorite-and pyrite-TiO 2 under high pressure have systematically been performed by the plane-wave basis pseudopotential method. Our calculated bulk modulus ͑272-324 GPa͒, shear modulus ͑97-128 GPa͒, elastic constant C 44 ͑40-73 GPa͒, and theoretical hardness ͑13 GPa͒ of the fluorite-TiO 2 show that it cannot be regarded as a potential candidate of superhard materials. Our results indicate that the differences of mechanical properties between the fluorite-and pyrite-TiO 2 are very small, which are at odds with the previous conclusions. In addition, we find that c-TiO 2 in the previous experiment is closer to the fluorite phase by analyzing the high-pressure behaviors of both cubic phases.

Ni Schottky contacts on AlGaN/GaN heterostructures have been fabricated, and one of the prepared samples has been annealed at 700 °C for half an hour. The barrier heights for the prepared samples were measured by internal photoemission. Based on the measured forward current-voltage (I-V) characteristics and using the two-diode model, the Ni Schottky barrier height at zero bias has been analyzed and calculated by self-consistently solving Schrodinger’s and Poisson’s equations, and the correlation expression between the barrier height at zero electric field and that at zero bias has been derived for Schottky contacts on AlGaN/GaN heterostructures. The calculated Schottky barrier heights corresponding to zero electric field for the prepared Ni Schottky contacts on AlGaN/GaN heterostructures agree well with the photocurrent measured results. Thus, the method for extraction of AlGaN/GaN heterostructure Schottky barrier heights from forward I-V characteristics is developed and determined.

The recently synthesized diamondlike BC 5 ͑d-BC 5 ͒ was shown to own a few interesting physical properties, which are yet to be confirmed due to the difficulty in the synthesis of adequately sized single crystals. Our density functional calculations not only confirm d-BC 5 to be an ultraincompressible and superhard material, but also reveal that it exhibits mechanical stability and metallic behavior. The higher energy barrier of d-BC 5 ͑0.057 eV/atom higher than diamond͒ implies that d-BC 5 is about 500 K more kinetically stable than diamond. Moreover, the trend that the mechanical properties drop with the increasing of boron content in diamondlike B-C compounds ͑d-BC x ͒ can be understood through analyzing the electronic structures. The combination of high hardness, strong stiffness, large activation barrier, and metallic feature allows this series of d-BC x promising applications as advanced abrasives and electronic devices at high temperature and high-pressure conditions.Diamond is widely accepted as the hardest material available in nature, but it is not used to cut iron and other ferrous metals due to the detrimental formation of iron carbide during high-speed machining. 1 New superhard materials are thus not only of great scientific interest, but also of practical importance. Years of synthetic and theoretical efforts 2-12 have been devoted to hunting for new diamondlike B-C-N-O phases with more thermal and chemical stability. Very recently, Solozhenko et al. 13 have synthesized d-BC 5 with the ultimate boron solubility in diamond. They reported that the d-BC 5 has high bulk modulus ͑335 GPa͒, extreme Vickers hardness ͑71 GPa͒, high-thermal stability ͑up to 1900 K͒ and conductive character. As we know, most of the superhard materials are insulators or semiconductors with covalent bonds. Hardness depends strongly on plastic deformation, which results in electron-pair bonds being broken and remade in a covalent crystal. Breaking an electron-pair bond means that two electrons become energetically excited from the valence band to the conduction band, so the hardness of a material is often related to its electronic structure, particularly to the band gap. 14 It therefore is a bit surprising that a conductive system is so hard. Moreover, because of the technical difficulties, no adequately sized single crystals may be obtained, and thus its interesting physical properties need to be confirmed theoretically. On the other hand, density functional calculations can provide accurate and reliable predictions of mechanical properties to validate these experimental values. In particular, theoretical understanding and interpretation of relative stability and conductive character is also of great importance. Therefore, the first-principles calculations that can provide further details and theoretical evidence about mechanical and electronic properties are highly demanded.In this paper, we investigate the mechanical and electronic properties of d-BC 5 using density functional calculations. At the same time, in order to insi...