Growth and Characterization of A1GaN/A1N/GaN HEMT Structures with a Compositionally Step-Graded A1GaN Barrier Layer

A new A1GaN/A1N/GaN high electron mobility transistor （HEMT） structure using a compositionally step-graded A1GaN barrier layer is grown on sapphire by metalorganic chemical vapour deposition （MOCVD）. The structure demonstrates significant enhancement of two-dimensional electron gas （2DEG） mobility and smooth surface morphology compared with the conventional HEMT structure with high A1 composition A1GaN barrier. The high 2DEG mobility of 1806 cm2/Vs at room temperature and low rms surface roughness of 0.220 nm for a scan area of 5μm×5 μm are attributed to the improvement of interracial and crystal quality by employing the stepgraded barrier to accommodate the large lattice mismatch stress. The 2DEG sheet density is independent of the measurement temperature, showing the excellent 2DEG confinement of the step-graded structure. A low average sheet resistance of 314.5Ω/square, with a good resistance uniformity of 0.68%, is also obtained across the 50 mm epilayer wafer. HEMT devices are successfully fabricated using this material structure, which exhibits a maximum extrinsic transconductance of 218 mS/ram and a maximum drain current density of 800 mA/mm.     

GaN MOS-HEMT Using Ultra-Thin A1203 Dielectric Grown by Atomic Layer Deposition

We report a GaN metal-oxide-semiconductor high electron mobility transistor （MOS-HEMT） with atomic layer deposited （ALD） Al2O3 gate dielectric. Based on the previous work [Appl. Phys. Lett. 86 （2005） 063501] of Ye et al. by decreasing the thickness of the gate oxide to 3.5nm and optimizing the device fabrication process, the device with maximum transconductance of 150mS/mm is produced and discussed in comparison with the result of lOOmS/mm of Ye et al. The corresponding drain current density in the 0.8-μm-gate-length MOS-HEMT is 800mA/mm at the gate bias of 3.0 V. The gate leakage is two orders of magnitude lower than that of the conventional A1GaN/GaN HEMT. The excellent characteristics of this novel MOS-HEMT device structure with ALD Al2O3 gate dielectric are presented.     

Hyperbolic Bending of Vortex Lines with Finite Number and Length in Rotating Trapped Bose-Einstein Condensates

The minimal energy configurations of hyperbolic bending vortex lines in the rotating trapped Bose-Einstein condensates are investigated by using a variational ansatz and numerical simulation. The theoretical calculation of the energy of the vortex lines as a function of the rotation frequency gives self-consistently vortex number, curvature and configuration. The numerical results show that bending is more stable than straight vortex line along the z-axis, and the vortex configuration in the xy-plane has a little expansion by increasing z.     

Coherent Structures in Transition of a Flat-Plate Boundary Layer at Ma = 0.7

Direct numerical simulation （DNS） of a spatially evolving fiat-plate boundary layer transition process at free stream Mach number 0. 7 is performed. Tollmien-Schlichting （T-S） waves are added on the inlet boundary as the disturbances before transition. Typical coherent structures in the transition process are investigated based on the second invariant of velocity gradient tensor. The instantaneous shear stress and the mean velocity profile in the transition region are studied. In our view, the fact that the peak value of shear stress in the stress concentration area increases and exceeds a threshold value during the later stage of the trallsition process plays an important role in the laminar breakdown process.     

Phase Transition and EOS of Marmatite （Zn0.76Fe0.23S） up to 623K and 17 GPa

In situ energy dispersive x-ray diffraction for natural marmatite （Zn0.76Fe0.23S） is performed up to 17. 7 GPa and 623 K. It is fit, ted by the Birch-Murnaghan equation of state （EOS） that Ko and α0 for marmatite are 85（3）GPa and 0.79（16）＊10^-4 K^-1, respectively. Fe^2＋ isomorphic replacing to Zn^2＋ in natural crystal is responsible for high bulk modulus and thermal expansivity of marmatite. Temperature derivative of bulk modulus （OK/OT）p for marmatite is fitted to be -0.044（23） GPaK^-1. The unambiguous B3-B1 phase boundaries for marmatite are determined to be Pupper（GPa）= 15.50 - 0.016T（℃） and Plower （GPa）=9.94-0.012T（℃） at 300-623K.     

Ab Initio Comparative Study of Zincblende and Wurtzite ZnO

By employing the first-principles pseudopotential plane-wave method, the physical properties of zincblende ZnO are investigated in comparison with those of the common wurtzite structure. Zincblende ZnO is predicted to be a direct gap semiconductor. Compared to the wurtzite structure, the zincblende ZnO is characterized by smaller bandgap and pressure coefficient, larger electron effective mass, increasing static dielectric constants and more covalent bonding. Furthermore, the optical properties including dielectric function and energy loss function of zincblende ZnO were obtained and analysed with some features. These aspects reveal promising applications of zincblende ZnO in optoelectronic devices.     

Hydrogen Passivation Effect on Enhanced Luminescence from Nanocrystalline Si/SiO2 Multilayers

Nanocrystalline Si/SiO2 multilayers are prepared by thermally annealing amorphous Si/SiO2 stacked structures. The photoluminescence intensity is obviously enhanced after hydrogen passivation at various temperatures. It is suggested that the hydrogen trapping and detrapping processes at different temperatures strongly influence the passivation effect. Direct experimental evidence is given by electron spin resonance spectra that hydrogen effectively reduces the nonradiative defect states existing in the Si nanocrystas/SiO2 system which enhances the radiative recombination probability. The luminescence characteristic shows its stability after hydrogen passivation even after aging eight months.     

抽取信号的χ2分布拟合方法

BESⅡ上测量ψ(2S)辐射衰变时, 鉴于本底道的污染非常严重, 提出了一种抽取信号事例数的χ²拟合方法. 大量的蒙特卡罗模拟表明, 信号道与各个本底道的χ²分布形状各不相同. 这些特点提供了从数据的χ²分布中抽出信号道事例数的χ²拟合方法. 从蒙特卡罗模拟的结果来看, χ²拟合方法的误差小于2%.     

柔性ITO衬底电化学沉积制备Cu/Cu2O薄膜

研究了使用电化学沉积法于碱性条件下在柔性ITO衬底上制备Cu/Cu₂O薄膜的方法。循环伏安曲线表明Cu₂O与Cu的阴极峰分别位于-500 mV(vs Ag/AgCl)和-800 mV(vs Ag/AgCl)附近。利用循环伏安法考察了生长温度和电解液pH值等对Cu₂O与Cu阴极峰电位的影响,阴极峰随生长温度的升高以及pH值的降低而略向阳极移动,沉积电流也随之相应增大。与弱酸性条件相比,上述两个阴极峰随pH值升高而移动的程度明显减小,这可能与碱性条件下C₃H₆O电离程度增大以及C₃H₆O根作为配体的过量程度有关。通过X射线衍射光谱和扫描电子显微镜的表征证实,在所研究的生长温度区间和pH值内可利用电化学沉积法在柔性ITO衬底上制备Cu/Cu₂O纳米混晶薄膜。在相同的生长温度和pH条件下,电化学沉积电位对样品表面形貌和晶体性质具有较大影响。     

为2007年亚奥赛物理实验命题的体会

笔者被聘为2007年亚奥赛物理实验命题,经过半年多的努力终于将科研成果——激光测温合理地转换为一个奥赛实验试题——干涉法测量玻璃热膨胀系数和折射率温度系数,本文主要介绍了命题过程及体会.