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Conventionally, mechanical resonators exhibit evident degradation in quality factor and large frequency fluctuation at elevated temperatures above room temperature. Here, we show that the quality factor of up to 10 5 of a highly stressed GaN on Si nanomechanical resonators experiences little change as temperature increasing to 600 K and the temperature coefficient of the resonance frequency (TCF) is as low as several ppm/K, several times lower than those of the conventional GaN mechanical resonators. The high quality factor and low TCF at high temperatures are attributed to the high stress and the geometrical nonlinearity of dynamical strain in the GaN resonator, where the dissipation caused by the change of the material properties with the increasing temperature is compensated by the increased stiffness. This observation violates the universality of thermal energy dissipation in mechanical resonators. The results provide a universal strategy for engineering nanomechanical resonators with ultrahigh sensitivity and ultralow noise. [ABSTRACT FROM AUTHOR]

GaN films were grown on hydride vapor phase epitaxy (HVPE) AlN/SiC templates by metalorganic CVD (MOCVD) without annealing the reactor to eliminate the memory effect. A step-terrace structure and smooth surface were obtained for the GaN film, which had a thickness of ∼200 nm. Subsequently, AlGaN/GaN heterostructures for application in high electron mobility transistors (HEMTs) with thin GaN channels were fabricated without a C- or Fe-doped GaN buffer layer. The interface quality at the AlGaN/GaN heterostructure was good enough for a two-dimensional electron gas to exhibit Shubnikov–de Haas oscillation in a magnetic field at 1.8 K. The GaN HEMTs with a thin channel on the AlN/SiC templates exhibited both a pinch-off character and conventional properties. In view of both the shorter epitaxial growth time and higher thermal conduction, HVPE AlN/SiC templates are applicable to the fabrication of GaN HEMTs by MOCVD. [ABSTRACT FROM AUTHOR]

The control of carbon (C) impurity plays a crucial role in achieving high mobility GaN drift layers for GaN vertical power devices. We investigate the effects of NH3 partial pressure on the C incorporation in GaN and find out that the NH3 partial pressure is the key parameter controlling the C concentration. It is found that the C concentration is inversely proportional to the square of NH3 partial pressure under a fixed growth rate. Further physical analysis indicates that higher NH3 partial pressure results in a lower surface N vacancy (VN) concentration and, thus, the lower C incorporation efficiency. Under such a guidance, the C concentration in GaN can be reduced to as low as ∼1.7 × 1015/cm3 at a growth rate of 1 μm/h. As a consequence, a record high electron mobility of 1227 cm2/V·s at the room temperature is achieved for a GaN drift layer grown on Si substrates. Our strategy is effective in C control and paves the way for realizing high performance kV-class GaN vertical power devices on Si substrates. [ABSTRACT FROM AUTHOR]

The capacitance-voltage (C-V) hysteresis in the bidirectional measurements of the p-GaN metal-insulator-semiconductor (MIS) capacitor is suppressed by using a CaF2 dielectric layer and a post annealing treatment. The density of trapped charge states at the CaF2/p-GaN interface is dramatically reduced from 1.3 × 1013 cm2 to 1.1 × 1011/cm2 compared to that of the Al2O3/p-GaN interface with a large C-V hysteresis. It is observed that the disordered oxidized interfacial layer can be avoided by using the CaF2 dielectric. The downward band bending of p-GaN is decreased from 1.51 to 0.85 eV as a result of the low-density oxides-related trap states. Our work indicates that the CaF2 can be used as a promising dielectric layer for the p-GaN MIS structures. [ABSTRACT FROM AUTHOR]

Tailoring the electronic states of the dielectric oxide/diamond interface is critical to the development of next generation semiconductor devices like high-power high-frequency field-effect transistors. In this work, we investigate the electronic states of the TiO2/diamond 2 × 1–(100) interface by using first principles total energy calculations. Based on the calculation of the chemical potentials for the TiO2/diamond interface, it is observed that the hetero-interfaces with the C-OTi configuration or with two O vacancies are the most energetically favorable structures under the O-rich condition and under Ti-rich condition, respectively. The band structure and density of states of both TiO2/diamond and TiO2/H-diamond hetero-structures are calculated. It is revealed that there are considerable interface states at the interface of the anatase TiO2/diamond hetero-structure. By introducing H on the diamond surface, the interface states are significantly suppressed. A type-II alignment band structure is disclosed at the interface of the TiO2/diamond hetero-structure. The valence band offset increases from 0.6 to 1.7 eV when H is introduced at the TiO2/diamond interface. [ABSTRACT FROM AUTHOR]