Comparative Study on the Interatomic Force Constants and Elastic Properties of Zinc-Blende AlN,AlP, and AlAs

JETP Letters - The lattice constants, interatomic force constants and elastic constants of zinc-blende AlN, AlP and AlAs are calculated by ab initio pseudopotential plane wave method. The...     

Theoretical investigation of the elastic,electronic, thermodynamic and optical properties of the zinc-blende structure AlN under high pressure

The elastic, electronic, thermodynamic and optical properties of the zinc-blende structure aluminum nitride (AlN) under high pressure have been investigated using first-principles calculations. The dependencies of the elastic constants, the bulk modulus, the shear modulus and energy gaps on the applied pressure are presented, and the results are in good agreement with comparable experimental and theoretical values. Also, the energy band structure and density of states under high pressure have been analysed. Furthermore, the optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are discussed for radiation up to 50 eV.     

闪锌矿GaN弹性性质、电子结构和光学性质外压力效应的理论研究

采用基于密度泛函理论(DFT)的第一性原理平面波赝势方法,结合广义梯度近似(GGA)下的RPBE和局域密度近似(LDA)的CA-PZ交换-关联泛函对闪锌矿结构的GaN在高压的性质进行了系统研究. 计算结果表明:弹性常数、体模量、杨氏模量和能隙都具有明显的外压力效应,计算结果与实验值和理论值很好的符合. 同时利用计算的能带结构和态密度系统分析了GaN的介电函数、折射率、反射率、吸收系数和能量损失函数等光学性质及其外压力效应. 分析结果为GaN的设计与应用提供了理论依据. 关键词： 第一性原理计算 电子结构 光学性质 闪锌矿GaN     

Elastic modulus,optical phonon modes and polaron properties in Al1−xBxN alloys

The results of first-principles plane-wave-based pseudopotential method calculations of the elastic modulus, optical phonon modes, and polaron properties of zinc-blende Al_1?xB_xN are presented. Good agreement is obtained with the exciting experimental data for AlN and BN compounds. Our results provide predictions for the remaining mixed crystals Al_1?xB_xN (0 < x < 1) for which no direct experimental or theoretical data are presently available. The compositional dependence of features such as elastic constants and their related mechanical parameters, phonon frequencies, dielectric constants, and polaron parameters was investigated. The study may be useful for the characteristic analysis of AlBN-based quantum well devices.     

The charge neutrality level and the fermi level pinning in A<Superscript>3</Superscript>N (BN,AlN, GaN,InN) nitrides

The electronic band structure and position of the charge neutrality level (CNL) in BN, AlN, GaN, and InN compounds with cubic and hexagonal lattices are calculated within the density functional theory (DFT-GGA). It is shown that the charge neutrality level is shifted from the middle of the BN and AlN forbidden band to the upper half of the GaN forbidden band and to the allowed energy region in the InN conduction band as the cation atomic weight increases. This determines semiinsulating properties of BN and AlN, n-type conductivity of GaN, and n ⁺-type conductivity of InN upon saturation of these materials by intrinsic lattice defects due to hard radiation. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 24–31, December, 2008.     

First-principles calculations for elastic properties of rutile TiO2 under pressure

This paper studies the equilibrium structure parameters and the dependences of the elastic properties on pressure for rutile TiO2 by using the Cambridge Serial Total Energy Package （CASTEP） program in the frame of density functional theory. The obtained equilibrium structure parameters, bulk modulus B0 and its pressure derivative B′0 are in good agreement with experiments and the theoretical results. The six independent elastic constants of rutile TiO2 under pressure are theoretically investigated for the first time. It is found that, as pressure increases, the elastic constants C11, C33, C66, C12 and C13 increase, The variation of elastic constant C44 is not obvious and the anisotropy will weaken.     

Lattice dynamics properties of zinc-blende and Nickel arsenide phases of AlP

Ab initio calculations, based on norm-conserving non-local pseudopotentials and the density functional theory (DFT), have been performed to investigate the behaviour under hydrostatic pressure of the structural, electronic, elastic and dynamical properties of AlP, in both zinc-blende and nickel arsenide phases. Our calculated structural and electronic properties are in good agreement with previous theoretical and experimental results. The phonon dispersion curves, the elastic constants, Born effective charge, etc., were calculated with the local density approximation and the density functional perturbation theory (DFPT). Our results in the pressure behaviour of the elastic and dynamical properties of both phases are in agreement with the experimental data when available, in other case they can be considered as predictions.     

掺杂GaN/AlN超晶格第一性原理计算研究

第一性原理计算方法在解释实验现象和预测新材料结构及其性质上有着重要作用. 因此, 通过基于密度泛函理论的第一性原理的方法, 本文系统地研究了Mg和Si掺杂闪锌矿和纤锌矿两种晶体结构的GaN/AlN超晶格体系中的能量稳定性以及电学性质. 结果表明: 在势阱层(GaN 层)中, 掺杂原子在体系中的掺杂形成能不随掺杂位置的变化而发生变化, 在势垒层(AlN层)中也是类似的情况, 这表明对于掺杂原子来说, 替代势垒层(或势阱层)中的任意阳离子都是等同的; 然而, 相比势阱层和势垒层的掺杂形成能却有很大的不同, 并且势阱层的掺杂形成能远低于势垒层的掺杂形成能, 即掺杂元素(Mg_Ga, Mg_Al, Si_Ga和Si_Al)在势阱区域的形成能更低, 这表明杂质原子更易掺杂于结构的势阱层中. 此外, 闪锌矿更低的形成能表明: 闪锌矿结构的超晶格体系比纤锌矿结构的超晶格体系更易于实现掺杂; 其中, 闪锌矿结构中, 负的形成能表明: 当Mg原子掺入闪锌矿结构的势阱层中会自发引起缺陷. 由此, 制备以闪锌矿结构超晶格体系为基底的p型半导体超晶格比制备n型半导体超晶格需要的能量更低并且更为容易制备. 对于纤锌矿体系来说, 制备p型和n型半导体的难易程度基本相同. 电子态密度对掺杂体系的稳定性和电学性质进一步分析发现, 掺杂均使得体系的带隙减小, 掺杂前后仍然为第一类半导体. 综上所述, 本文内容为当前实验中关于纤锌矿结构难以实现p型掺杂问题提供了一种新的技术思路, 即可通过调控相结构实现其p型掺杂.     

闪锌矿结构AlN、AlP、AlAs和AlSb电子结构和光学性质的第一性原理研究

本文采用基于密度泛函理论下的第一性原理平面波赝势从头算量子力学方法,对闪锌矿结构AlN、AlP、AlAs和AlSb的电子结构和光学性质进行了研究。分析比较了这些化合物的能带结构、态密度、介电函数及折射率等性质,总结Al与不同Ⅴ族元素形成化合物时的性质变化规律。结果表明,四种材料有着相似的能带结构,都是间接带隙宽禁带半导体,但是在导带底AlN的能态结构与其它三种材料明显不同。随着从AlN到AlSb的变化,光学性质曲线发生红移。     

Carrier transport and luminescence properties of n-type GaN

The surface morphology, electrical properties and optical properties of Si doped n-type GaN were investigated. The intentional SiH₄ doped GaN films were grown by metal organic chemical vapor deposition with the electron concentration varying from 3×10¹⁶ cm⁻³ to 5.4×10¹⁸ cm⁻³. The surface morphology shows that the roughness and dislocation pits increase as the mass flow rate of SiH₄ increases, which indicates that the quality of GaN degrades gradually. The activation energy of Si in GaN with different n concentrations varies from 12 to 22 meV, which may originate from the interactions of donor wave functions. The carrier transport mechanism with increasing temperature from 100 to 420 K was concluded as the complex effect of both impurity scattering and phonon scattering. The position of the near band edge emission peak was determined by both renormalization of the band gap and B-M effect. The intensity variations of the yellow luminescence could be explained by the change of Ga vacancy concentration caused by Si doping. Supported by the National Basic Research Program of China (Grant No. 2006CB6049), the National Hi-Tech Research and Development Program of China (Grant No. 2006AA03A142), the National Natural Science Foundation of China (Grant Nos. 60721063, 60731160628 and 60676057), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20050284004) and the Natural Science Foundation of Jiangsu Province of China (Grant No. BK2005210)     

Electronic properties and elastic constants of wurtzite, zinc-blende and rocksalt AlN

Electronic properties and elastic constants of AlN in the wurtzite, zinc-blende and rocksalt structures are investigated using an ab initio pseudopotential method based on the density-functional theory with both the local-density approximation and the generalized gradient approximation for the exchange-correlation functional. The numerically calculated results compare well with the existing experimental data. For elastic constants of rocksalt AlN our results are predictions.     

由热学性质获取氩晶体原子间各阶力常数

本文基于晶格动力学和量子力学微扰理论推导了氩晶体的热膨胀系数和比热与原子间相互作用的各阶力常数之间的关系公式,在此基础上根据热膨胀系数和比热的数据计算了氩晶体内的原子间相互作用的各阶力常数,并根据这些力常数绘制了原子间相互作用势能曲线,经比对发现该势能曲线与Morse势能曲线能较好吻合,这表明,本文提出的从热膨胀系数和比热获取各阶力常数的方法是正确的。     

由热学性质获取氩晶体原子间各阶力常数

本文基于晶格动力学和量子力学微扰理论推导了氩晶体的热膨胀系数和比热与原子间相互作用的各阶力常数之间的关系公式,在此基础上根据热膨胀系数和比热的数据计算了氩晶体内的原子间相互作用的各阶力常数,并根据这些力常数绘制了原子间相互作用势能曲线,经比对发现该势能曲线与Morse势能曲线能较好吻合,这表明,本文提出的从热膨胀系数和比热获取各阶力常数的方法是正确的。     

Structures and electronic properties of SimN8-m（0 〈 m 〈 8） clusters： a density functional theory study

The geometries, electronic structures and related properties of SimN8-m（0 〈 m 〈 8） clusters are studied using density functional theory （DFT） with hybrid functional B3LYP. The calculated results reveal several trends. For any stoichiometric clusters, the lowest energy isomers with an alteration of N and Si atoms are favourable in energy if the numbers of Si and N atoms are large enough to form ... Si N-Si-N... alternative chains. The bond lengths of single Si-N bonds are very close to the corresponding values of the bulk and other SiN clusters. The geometries for N-rich and Si4N4 clusters are planar structures, but three-dimensional structures are favourable in energy for Si-rich clusters. With the increase of m, the isotropic polarizability and average polarizability increase, the total binding energies generally decrease, the HOMO-LUMO gap and vertical ionization potential oscillate with increasing number of valence electrons, and their values with even valence electrons are larger than those with odd valence electrons. The atomic charges, IR and Raman properties are also reported.     

The pressure dependences of elastic and lattice dynamic properties of AlAs from ab initio calculations

Ab initio calculations,based on norm-conserving nonlocal pseudopotentials and density functional theory(DFT),are performed to investigate the structural,elastic,dielectric,and vibrational properties of aluminum arsenide(AlAs) with a zinc-blende(B3) structure and a nickel arsenide(B81) structure under hydrostatic pressure.Firstly,the path for the phase transition from B3 to B81 is confirmed by analyzing the energies of different structures,which is in good agreement with previous theoretical results.Secondly,we find that the elastic constants,bulk modulus,static dielectric constants,and the optical phonon frequencies vary in a nearly linear manner under hydrostatic pressure.What is more,the softening mode of the transversal acoustic mode at the X point supports the phase transition in AlAs.     

Analysis of the forces acting on the saltating particles in the coupled wind-sand-electricity fields

Based on the theoretical model describing the saltation of sand particles in the coupled wind-sand-electricity fields, the numerical simulations of the forces acting on saltating particles, such as the aerodynamic drag force, Magnus effect, Saffman force and electrostatic force, are analyzed in comparison to the gravity force of the particles in the steady windblown sand movement. Furthermore, the laws of the above forces vary with the friction velocity, the diameter of the sand particle, the initial angular velocity and the lift-off velocity are discussed. Supported by the National Natural Science Foundation of China (Grant Nos. 10772075 and 10772074), the Key Project of the National Natural Science Foundation of China (Grant No.10532040), and the Program for New Century Excellent Talents in University (Grant No. NCET-04-0979)     

Electronic,optical, and mechanical properties of BN,AlN, and InN with zinc-blende structure under pressure

In this work, the electronic, optical, and mechanical properties of BN, AlN, and InN under the action of pressure are calculated. For each of these compounds, the energy band structure, band gaps(E~L_g, E~Γ_g, E~X_g), refractive index(n),dielectric constants(ε_∞, ε_0), elastic constants(C_11, C_12, C_44), and relevant parameters such as bulk(B_u), shear(S_h), and Young's(Y_0) moduli are studied, and other important parameters such as bond-stretching(α), bond-bending(β) force constant, internal-strain parameter(ζ), effective charges(e~*_T, Z~*), anisotropy factor(I_s), Poisson's ratio(P_o), Cauchy ratio(C_a), the ductility index(μ_D), and linear compressibility(C0_) are also calculated. The effects of pressure on all studied properties are investigated. Our results are in good agreement with the available experimental and theoretical data for BN,AlN, and InN.     

Pressure dependences of elastic and lattice dynamic properties of AlAs from ab initio calculations

Ab initio calculations, based on norm-conserving nonlocal pseudopotentials and density functional theory (DFT), are performed to investigate the structural, elastic, dielectric, and vibrational properties of aluminum arsenide AlAs with zinc-blende (B3) structure and nickel arsenide (B8₁) structure under hydrostatic pressure. Firstly, the path for the phase transition from B3 to B8₁ is confirmed by analyzing the energies of different structures, which is in good agreement with previous theoretical results. Secondly, we find that the elastic constants, bulk modulus, static dielectric constants, and the optical phonon frequencies are varying in a nearly linear manner under hydrostatic pressure. What is more, the softening mode of transversal acoustic mode at X point supports the phase transition in AlAs.     

Improvement of the relative entropy based protein folding method

The “relative entropy” has been used as a minimization function to predict the tertiary structure of a protein backbone, and good results have been obtained. However, in our previous work, the ensemble average of the contact potential was estimated by an approximate calculation. In order to improve the theoretical integrity of the relative-entropy-based method, a new theoretical calculation method of the ensemble average of the contact potential was presented in this work, which is based on the thermodynamic perturbation theory. Tests of the improved algorithm were performed on twelve small proteins. The root mean square deviations of the predicted versus the native structures from Protein Data Bank range from 0.40 to 0.60 nm. Compared with the previous approximate values, the average prediction accuracy is improved by 0.04 nm. Contributed equally to this work Supported by the National Natural Science Foundation of China (Grant No. 30670497), the Beijing Natural Science Foundation (Grant No. 5072002), and the Specialized Research Foundation for the Doctoral Program of Higher Education (Grant No. 200800050003)     

Influence of pressure on the properties of GaN/AlN multi-quantum wells – Ab initio study

Pressure dependence of physical properties of GaN/AlN multi-quantum wells (MQWs) was investigated using ab intio calculations. The influence of pressure was divided into two main contributions: pressure affecting the properties of GaN and AlN bulk semiconductors and an influence on systems of polar quantum wells deposited on various substrates. An influence of hydrostatic, uniaxial, and tetragonal strain on the crystallographic structure, polarization (piezoelectricity), and the bandgap of the bulk systems is assessed using ab initio calculations. It was shown that when a partial relaxation of the structure is assumed, the tetragonal strain may explain an experimentally observed reduction of pressure coefficients for polar GaN/AlN MQWs. The MQWs were also simulated directly using density functional theory (DFT) calculations. A comparison of these two approaches confirmed that nonlinear effects induced by the tetragonal strain related to lattice mismatch between the substrates and the polar MQWs systems are responsible for a drastic decrease of the pressure coefficients of photoluminescence (PL) energy experimentally observed in polar GaN/AlGaN MQWs.