Changchun Chai

Novel silicon allotropes: Stability, mechanical, and electronic properties

One quasi-direct gap phase (Amm2) and three indirect gap phases (C2/m-16, C2/m-20, and I-4) of silicon allotropes are proposed. The detailed theoretical study on the structure, density of states, elastic properties, sound velocities, and Debye temperature of these four phases is carried out by using first principles calculations. The elastic constants of these four phases are calculated by strain-stress method. The elastic constants and the phonon calculations manifest all novel silicon allotropes in this paper are mechanically and dynamically stable at ambient condition. The B/G values indicate that these four phases of silicon are brittle materials at ambient pressure. The anisotropy properties show that C2/m-20 phase exhibits a larger anisotropy in its elastic modulus, shear elastic anisotropic factors, and several anisotropic indices than others. We have found that the Debye temperature of the four novel silicon allotropes gradually reduces in the order of C2/m-20 > Amm2 > C2/m-16 > I-4 at ambient pressure.

Two Novel C3N4 Phases: Structural, Mechanical and Electronic Properties

We systematically studied the physical properties of a novel superhard (t-C3N4) and a novel hard (m-C3N4) C3N4 allotrope. Detailed theoretical studies of the structural properties, elastic properties, density of states, and mechanical properties of these two C3N4 phases were carried out using first-principles calculations. The calculated elastic constants and the hardness revealed that t-C3N4 is ultra-incompressible and superhard, with a high bulk modulus of 375 GPa and a high hardness of 80 GPa. m-C3N4 and t-C3N4 both exhibit large anisotropy with respect to Poisson’s ratio, shear modulus, and Young’s modulus. Moreover, m-C3N4 is a quasi-direct-bandgap semiconductor, with a band gap of 4.522 eV, and t-C3N4 is also a quasi-direct-band-gap semiconductor, with a band gap of 4.210 eV, with the HSE06 functional.

Analysis of high power microwave induced degradation and damage effects in AlGaAs/InGaAs pHEMTs

Abstract The high power microwave (HPM) induced effects in AlGaAs/InGaAs pseudomorphic high electron mobility transistors (pHEMTs) are investigated by simulation and experiments. Simulated results suggest that the HPM damage mechanism is device burnout, which is caused by emerging current path and strong electric field intensity beneath the gate metal. Besides, analysis points out that the gate metal diffusion may be thermally activated, resulting in DC and RF performance degradation. Specifically, a positive shift will occur to the pinch-off voltage while accordingly the small-signal gain will decrease. The HPM injection experiments on the dual-stage pHEMT low noise amplifiers (LNAs) are carried out. Experimental results substantiate that deterioration happens both in the noise figure and in the small-signal gain, which is in agreement with the simulated results. Failure analysis indicates that the HPM induced failure of LNA is attributed to the failure of the first stage pHEMT. Finally, samples dissection analysis using the scanning electron microscopy (SEM) verifies the simulation analysis of the damage mechanism and the location susceptible to burnout. Meanwhile, the assumption of gate metal diffusion is validated by the observation of pits after removing the gate metal. The performance parameters deterioration can be utilized as the degradation (or damage) criteria, and the mechanisms analysis facilitates making reinforcing design.

Si96: A New Silicon Allotrope with Interesting Physical Properties

The structural mechanical properties and electronic properties of a new silicon allotrope Si96 are investigated at ambient pressure by using a first-principles calculation method with the ultrasoft pseudopotential scheme in the framework of generalized gradient approximation. The elastic constants and phonon calculations reveal that Si96 is mechanically and dynamically stable at ambient pressure. The conduction band minimum and valence band maximum of Si96 are at the R and G point, which indicates that Si96 is an indirect band gap semiconductor. The anisotropic calculations show that Si96 exhibits a smaller anisotropy than diamond Si in terms of Young’s modulus, the percentage of elastic anisotropy for bulk modulus and shear modulus, and the universal anisotropic index AU. Interestingly, most silicon allotropes exhibit brittle behavior, in contrast to the previously proposed ductile behavior. The void framework, low density, and nanotube structure make Si96 quite attractive for applications such as hydrogen storage and electronic devices that work at extreme conditions, and there are potential applications in Li-battery anode materials.

The Mechanical and Electronic Properties of Carbon-Rich Silicon Carbide

A systematic investigation of structural, mechanical, anisotropic, and electronic properties of SiC2 and SiC4 at ambient pressure using the density functional theory with generalized gradient approximation is reported in this work. Mechanical properties, i.e., the elastic constants and elastic modulus, have been successfully obtained. The anisotropy calculations show that SiC2 and SiC4 are both anisotropic materials. The features in the electronic band structures of SiC2 and SiC4 are analyzed in detail. The biggest difference between SiC2 and SiC4 lies in the universal elastic anisotropy index and band gap. SiC2 has a small universal elastic anisotropy index value of 0.07, while SiC2 has a much larger universal elastic anisotropy index value of 0.21, indicating its considerable anisotropy compared with SiC2. Electronic structures of SiC2 and SiC4 are calculated by using hybrid functional HSE06. The calculated results show that SiC2 is an indirect band gap semiconductor, while SiC4 is a quasi-direct band gap semiconductor.

Motion of current filaments in avalanching PIN diodes

The motion of current filaments in avalanching PIN diodes has been investigated in this paper by 2D transient numerical simulations. The simulation results show that the filament can move along the length of the PIN diode back and forth when the self-heating effect is considered. The voltage waveform varies periodically due to the motion of the filament. The filament motion is driven by the temperature gradient in the filament due to the negative temperature dependence of the impact ionization rates. Contrary to the traditional understanding that current filamentation is a potential cause of thermal destruction, it is shown in this paper that the thermally-driven motion of current filaments leads to the homogenization of temperature in the diode and is expected to have a positive influence on the failure threshold of the PIN diode.

A New Phase of GaN

The structural, mechanical, and electronic properties of the orthorhombic GaN (Pnma-GaN) are investigated at ambient pressure by using first-principles calculations method with the ultrasoft pseudopotential scheme. The elastic constants and phonon calculations reveal Pnma-GaN is mechanically and dynamically stable at ambient pressure. The calculated Young modulus of Pnma-GaN is 170 GPa, which is the three-fifths of wurtzite-GaN. Electronic structure study shows that Pnma-GaN is a direct semiconductor with band gap of 1.847 eV. The anisotropic calculation shows that wurtzite-GaN has a smaller elastic anisotropy than that of Pnma-GaN in Young’s modulus. In addition, when the composition of aluminum increases from 0 to 0.063 in the alloy, the band gap decreases initially and increases afterward for Pnma-Ga1−xAlxN, while, for wurtzite-Ga1−xAlxN, the band gap increases with the increasing composition x. Due to the structural porous feature, Pnma-GaN can also be expected to be a good hydrogen storage material.

First-principles Study of Structural, Elastic, Anisotropic, and Thermodynamic Properties of R3-B_2C

The structural, elastic, anisotropy, and thermodynamic properties of R3-B_2C were investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available theoretical results. The elastic constants, elastic modulus, and elastic anisotropies of R3-B_2C were also determined in the pressure range of 0-100 GPa. The calculated elastic modulus indicates that R3-B_2C is a potential superhard material. The calculated elastic anisotropic factors suggest that R3-B2_C is elastically anisotropic. A band structure study shows that R3-B_2C is a direct semiconductor with band gap of 0.170 eV. Moreover, we predict the thermodynamic properties and obtain the relationships among the thermal expansion, temperature, and pressure, as well as the variations of the isothermal bulk modulus, Debye temperature, Gruneisen parameter, and heat capacity.

Structural, Mechanical, Anisotropic, and Thermal Properties of AlAs in oC12 and hP6 Phases under Pressure

The structural, mechanical, anisotropic, and thermal properties of oC12-AlAs and hP6-AlAs under pressure have been investigated by employing first-principles calculations based on density functional theory. The elastic constants, bulk modulus, shear modulus, Young’s modulus, B/G ratio, and Poisson’s ratio for oC12-AlAs and hP6-AlAs have been systematically investigated. The results show that oC12-AlAs and hP6-AlAs are mechanically stable within the considered pressure. Through the study of lattice constants (a, b, and c) with pressure, we find that the incompressibility of oC12-AlAs and hP6-AlAs is the largest along the c-axis. At 0 GPa, the bulk modulus B of oC12-AlAs, hP6-AlAs, and diamond-AlAs are 76 GPa, 75 GPa, and 74 Gpa, respectively, indicating that oC12-AlAs and hP6-AlAs have a better capability of resistance to volume than diamond-AlAs. The pressure of transition from brittleness to ductility for oC12-AlAs and hP6-AlAs are 1.21 GPa and 2.11 GPa, respectively. The anisotropy of Young’s modulus shows that oC12-AlAs and hP6-AlAs have greater isotropy than diamond-AlAs. To obtain the thermodynamic properties of oC12-AlAs and hP6-AlAs, the sound velocities, Debye temperature, and minimum thermal conductivity at considered pressure were investigated systematically. At ambient pressure, oC12-AlAs (463 K) and hP6-AlAs (471 K) have a higher Debye temperature than diamond-AlAs (433 K). At T = 300 K, hP6-AlAs (0.822 W/cm·K−1) has the best thermal conductivity of the three phases, and oC12-AlAs (0.809 W/cm·K−1) is much close to diamond-AlAs (0.813 W/cm·K−1).

Elastic anisotropy and electronic properties of Si3N4 under pressures

First principles calculations are performed to systematically investigate the electronic structures, elastic, anisotropic and electronic properties of the monoclinic, tetragonal and orthorhombic structures of Si3N4 under pressure. Anisotropy studies show that three Si3N4 phases exhibit a large anisotropy. Furthermore, using the HSE06 hybrid functional, the monoclinic, tetragonal and orthorhombic phases are found to be wide band-gap semiconductors. The pressure induced band gap direct-indirect transition is found for monoclinic Si3N4. The elastic modulus, compressional and shear wave velocities as well as Debye temperatures as a function of pressure in three Si3N4 phases are also investigated in detail.