Mechanical Properties and Electronic Structures of Cotunnite TiO2

Based on the first-principles plane-wave basis pseudopotential calculations, we investigate mechanical properties and electronic structures of the hardest known oxide, cotunnite TiO2. The calculated results show that cotunnite TiO2 has the highest bulk modulus （348 GPa） and hardness （32 GPa） among the high-pressure phases of TiO2, but its mechanical properties are not superior to those of c-BN. Moreover, the high hardness of cotunnite TiO2 can be understood from both the dense crystal structure （high valence electron density and short bond lengths） and the unusual mixtures of covalent and ionic bonding of Ti-O.     

欧姆接触金属退火对应力AlGaN/GaN异质结构中电子迁移率的影响

The influence of annealed ohmic contact metals on the electron mobility of a two dimensional electron gas （2DEG） is investigated on ungated AlGaN/GaN heterostructures and AlGaN/GaN heterostructure field effect transistors （AlGaN/GaN HFETs）. Current-voltage （I-V） characteristics for ungated AlGaN/GaN heterostructures and capacitance-voltage （C-V） characteristics for AlGaN/GaN HFETs are obtained, and the electron mobility for the ungated AlGaN/GaN heterostructure is calculated. It is found that the electron mobility of the 2DEG for the ungated AlGaN/GaN heterostructure is decreased by more than 50% compared with the electron mobility of Hall measurements. We propose that defects are introduced into the AlGaN barrier layer and the strain of the AlGaN barrier layer is changed during the annealing process of the source and drain, causing the decrease in the electron mobility.     

Ni肖特基接触厚度对Al0.3Ga0.7N/GaN应变异质结二维电子气密度的影响

厚度分别为50Å/50Å和600Å/2000Å的Ni/Au肖特基接触被沉积在Al0.3Ga0.7N/GaN应变异质结上。通过室温下测量的电容-电压曲线和电流-电压特性,我们计算得到厚度为600Å/2000Å的Ni/Au肖特基接触的二维电子气密度为9.131012 cm-2,而厚度为50Å/50Å 的Ni/Au肖特基接触的二维电子气密度为4.771012 cm-2。随着Ni/Au肖特基接触厚度由50Å/50Å增大到600Å/2000Å,正向20V时的饱和电流从60.88 mA增大到86.34 mA。通过薛定谔方程和泊松方程自洽计算,我们计算得到了两个样品的极化电荷密度,计算结果表明Ni/Au肖特基接触厚度厚的样品的极化较强。这样我们认为前面的实验结果是由于厚度为600Å/2000Å的肖特基接触增大了Al0.3Ga0.7N势垒层的张应力。     

Influence of Ni Schottky contact thickness on two-dimensional electron-gas sheet carrier concentration of strained Al_(0.3)Ga_(0.7)N/GaN heterostructures

Ni/Au Schottky contacts with thicknesses of either 50(?)/50(?) or 600(?)/2000(?) were deposited on strained Al_(0.3)Ga_(0.7)N/GaN heterostructures.Using the measured C-V curves and I-V characteristics at room temperature,the calculated density of the two-dimensional electron-gas(2DEG) of the 600(?)/2000(?) thick Ni/Au Schottky contact is about 9.13×10~(12) cm~(-2) and that of the 50(?)/50(?) thick Ni/Au Schottky contact is only about 4.77×10~(12) cm~(-2).The saturated current increases from 60.88 to 86.3...     

Determination of the relative permittivity of the AlGaN barrier layer in strained AlGaN/GaN heterostructures

Using the measured capacitance--voltage curves and the photocurrent spectrum obtained from the Ni Schottky contact on a strained Al_0.3Ga_0.7N/GaN heterostructure, the value of the relative permittivity of the AlGaN barrier layer was analysed and calculated by self-consistently solving Schr?dinger's and Poisson's equations. It is shown that the calculated values of the relative permittivity are different from those formerly reported, and reverse biasing the Ni Schottky contact has an influence on the value of the relative permittivity. As the reverse bias increases from 0 V to --3~V, the value of the relative permittivity decreases from 7.184 to 7.093.