First-principles calculations of $e$-ph interactions are becoming a pillar of electronic structure theory. However, the current approach is incomplete. The piezoelectric (PE) $e$-ph interaction, a long-range scattering mechanism due to acoustic phonons in non-centrosymmetric polar materials, is not accurately described at present. Current calculations include short-range $e$-ph interactions (obtained by interpolation) and the dipole-like Fröhlich long-range coupling in polar materials, but lack important quadrupole effects for acoustic modes and PE materials. Here we derive and compute the long-range $e$-ph interaction due to dynamical quadrupoles, and apply this framework to investigate $e$-ph interactions and the carrier mobility in the PE material wurtzite GaN. We show that the quadrupole contribution is essential to obtain accurate $e$-ph matrix elements for acoustic modes and to compute PE scattering. Our work resolves the outstanding problem of correctly computing $e$-ph interactions for acoustic modes from first principles, and enables studies of $e$-ph coupling and charge transport in PE materials.
The piezoelectric (PZ) polarization effect enhances the phonon group velocity in wurtzite nitrides. This enhancement influences the phonon relaxation rates. We calculate the modified phonon relaxation rates in binary wurtzite nitrides (GaN, AlN and InN) by considering process like umklapp process, point defect, dislocation, boundary and phonon-electron scattering. The result will be useful to study the effect of PZ polarization on thermal conductivity of binary wurtzite nitrides (GaN, AlN and InN).
New experimental results supported by theoretical analyses are proposed for aluminum silicon carbide (Al4SiC4). A state of the art implementation of the Density Functional Theory is used to analyze the experimental crystal structure, the Born charges, the elastic and piezoelectric properties. The Born charge tensor is correlated to the local bonding environment for each atom. The electronic band structure is computed including self-consistent many-body corrections. Al4SiC4 material properties are compared to other wide band gap Würtzite materials. From a comparison between an ellipsometry stud…
We have investigated the effect of piezoelectric (PZ) polarization property on group velocity of phonons in binary as well as in ternary wurtzite nitrides. It is found that with the presence of PZ polarization property, the phonon group velocity is modified. The change in phonon group velocity due to PZ polarization effect directly depends on piezoelectric tensor value. Using different piezoelectric tensor values recommended by different workers in the literature, percent change in group velocities of phonons has been estimated. The Debye temperatures and frequencies of binary nitrides GaN, Al…
We present results of a density functional theory study of MnO in the wurtzite structure. Our motivation is provided by recent experiments reporting ferromagnetism in Mn-doped wurtzite structure ZnO. We find that wurtzite MnO a) is not strongly energetically disfavored as compared with the ground state rocksalt MnO, b) shows strong magnetostructural coupling and c) has a piezoelectric response that is larger than that of ZnO. These predictions augur well for the creation of ferromagnetic piezoelectric semiconductor based on Mn-doped ZnO.
Ab initio quasi-harmonic thermoelasticity, piezoelectricity, and thermoelectricity of polar solids at finite temperature and pressure: Application to wurtzite ZnO
School of Physics and Technology, Xinjiang University（新疆大学物理科学与技术学院）；International School for Advanced Studies (SISSA)（国际高等研究院）；IOM - CNR（意大利国家研究委员会材料研究所）
Since AlN has emerged as an important piezoelectric material for a wide variety of applications, efforts have been made to increase its piezoelectric response via alloying with transition metals that can substitute for Al in the wurtzite lattice. Herein, we report density functional theory calculations of structure and properties of the Cr-AlN system for Cr concentrations ranging past the wurtzite-rocksalt transition point. By studying the different contributions to the longitudinal piezoelectric coefficient, we propose that the physical origin of the enhanced piezoelectricity in Cr$_x$Al$_{1-…
The origin of the anomalous, 400% increase of the piezoelectric coefficient in Sc$_x$Al$_{1-x}$N alloys is revealed. Quantum mechanical calculations show that the effect is intrinsic. It comes from a strong change in the response of the internal atomic coordinates to strain and pronounced softening of C$_{33}$ elastic constant. The underlying mechanism is the flattening of the energy landscape due to a competition between the parent wurtzite and the so far experimentally unknown hexagonal phases of the alloy. Our observation provides a route for the design of materials with high piezoelectric …
Accurate values for polarization discontinuities between pyroelectric materials are critical for understanding and designing the electronic properties of heterostructures. For wurtzite materials, the zincblende structure has been used in the literature as a reference to determine the effective spontaneous polarization constants. We show that, because the zincblende structure has a nonzero formal polarization, this method results in a spurious contribution to the spontaneous polarization differences between materials. In addition, we address the correct choice of "improper" versus "proper" piez…
We study the electroacoustic properties of aluminum scandium nitride crystals Al$_{1-x}$Sc$_x$N with the metastable wurtzite structure by means of first-principles calculations based on density functional theory. We extract the material property data relevant for electroacoustic device design, namely the full tensors of elastic and piezoelectric constants. Atomistic models were constructed and analyzed for a variety of Sc concentrations $0\le x\le50$ %. The functional dependence of the material properties on the scandium concentration was extracted by fitting the data obtained from an averagin…
A study of the mobility of n-doped wurtzite III-Nitrides is reported. We have determined the nonequilibrium thermodynamic state of the III-Nitrides systems, driven far away from equilibrium by a strong electric field, in the steady state, which follows after a very fast transient. The dependence of the mobility (which depends on the nonequilibrium thermodynamic state of the sample) on the electric field strength is derived, which decreases with the strength of electric field. We analyzed the contributions to the mobility arising out of the different channels of electron scattering, namely, the…
The free-carrier screening of macroscopic polarization fields in wurtzite GaN/InGaN quantum wells lasers is investigated via a self-consistent tight-binding approach. We show that the high carrier concentrations found experimentally in nitride laser structures effectively screen the built-in spontaneous and piezoelectric polarization fields, thus inducing a ``field-free'' band profile. Our results explain some heretofore puzzling experimental data on nitride lasers, such as the unusually high lasing excitation thresholds and emission blue-shifts for increasing excitation levels.
Ab initio calculations of the spontaneous polarization and piezoelectric properties of boron nitride nanotubes show that they are excellent piezoelectric systems with response values larger than those of piezoelectric polymers. The intrinsic chiral symmetry of the nanotubes induces an exact cancellation of the total spontaneous polarization in ideal, isolated nanotubes of arbitrary indices. Breaking of this symmetry by inter-tube interaction or elastic deformations induces spontaneous polarization comparable to those of wurtzite semiconductors.
The spontaneous and piezoelectric polarizations of III-nitrides considerably affect the operation of various III-nitride-based devices. We report an ab initio study of the spontaneous polarization (SP) and piezoelectric (PZ) constants of the III-nitride binary and ternary alloys with the hexagonal reference structure. These calculated polarization properties offer us a profound principle for polarization engineering of nitride semiconductor devices, based on which we propose a few heterojunctions which have nearly-zero polarization effect at the junctions that can potentially enhance optical a…
Tip-Based Proximity Ferroelectric Switching and Piezoelectric Response in Wurtzite Multilayers
Frantsevich Institute for Problems in Materials Science, National Academy of Sciences of Ukraine（乌克兰国家科学院弗拉特谢维奇材料科学问题研究所）；Institute of Physics, National Academy of Sciences of Ukraine（乌克兰国家科学院物理研究所）；Department of Materials Science and Engineering, University of Tennessee（田纳西大学材料科学与工程系）；Department of Materials Science and Engineering, Pennsylvania State University（宾夕法尼亚州立大学材料科学与工程系）
Recent advances in microelectromechanical systems often require multifunctional materials, which are designed so as to optimize more than one property. Using density functional theory calculations for alloyed nitride systems, we illustrate how co-alloying a piezoelectric material (AlN) with different nitrides helps tune both its piezoelectric and mechanical properties simultaneously. Wurtzite AlN-YN alloys display increased piezoelectric response with YN concentration, accompanied by mechanical softening along the crystallographic c direction. Both effects increase the electromechanical coupli…
First-principles full-potential linearized augmented plane wave (FLAPW) calculations are performed to clarify the role of the interface geometry on piezoelectric fields and on potential line-ups at the [0001]-wurtzite and [111]-zincblende GaN/Al junctions. The electric fields (polarity and magnitude) are found to be strongly affected by atomic relaxations in the interface region. A procedure is tested to evaluate the Schottky barrier in the presence of electric fields and used to show that their effect is quite small (a few tenths of an eV). These calculations assess the rectifying behaviour o…
Presently, the piezoelectric materials are finding tremendous applications in the micro-mechanical actuators, sensors and self-powered devices. In this context, the studies pertaining to piezoelectric properties of materials in the different size ranges are very important for the scientific community. The III-nitrides are exceptionally important, not only for optoelectronic but also for their piezoelectric applications. In the present study, we synthesized AlGaN via self catalytic vapor-solid mechanism by atmospheric pressure chemical vapor deposition technique on AlN base layer over intrinsic…
A number of diverse bulk properties of the zincblende and wurtzite III-V nitrides AlN, GaN, and InN, are predicted from first principles within density functional theory using the plane-wave ultrasoft pseudopotential method, within both the LDA (local density) and GGA (generalized gradient) approximations to the exchange-correlation functional. Besides structure and cohesion, we study formation enthalpies (a key ingredient in predicting defect solubilities and surface stability), spontaneous polarizations and piezoelectric constants (central parameters for nanostructure modeling), and elastic …
Lattice vibrations in materials induce perturbations on the electron dynamics in the form of long-range (dipole and quadrupole) and short-range (octopole and higher) potentials. The dipole Fröhlich term can be included in current first-principles electron-phonon ($e$-ph) calculations and is present only in polar materials. The quadrupole $e$-ph interaction is present in both polar and nonpolar materials, but currently it cannot be computed from first principles. Here we show an approach to compute the quadrupole $e$-ph interaction and include it in ab initio calculations of $e$-ph matrix eleme…
We review some experimental and theoretical aspects of coherent acoustic phonon generation in piezoelectric semiconductor multiple quantum wells. In order to model more advanced and complicated nano-acoustic devices, a macroscopic continuum theory for the generation and propagation of coherent acoustic phonons in piezoelectric semiconductor heterostructures is presented. The macroscopic approach is applicable in the coherent regime, and can be easily utilized to study coherent acoustic devices based on piezoelectric semiconductor heterosutructures. For each phonon mode, the corresponding coher…
Molecular Beam Epitaxy of Al$\mathrm{_{1-x}}$Sc$\mathrm{_{x}}$N Nanowires: Towards Group-III Nitride Piezoelectric Nanogenerators with Enhanced Response
First principles calculations of dynamical Born effective charges, quadrupoles and higher order terms from the charge response in large semiconducting and metallic systems
提出一种第一性原理电荷响应计算框架，通过纵向—横向响应关系高效获得 Born 有效电荷及高阶多极项，并用于大原胞半导体压电性和金属 TEM-EELS 分析。
Recent studies suggest that piezoelectric polarization can play an important role in determining the electronic and optical properties of nanoscale nitride heterostructures. Among a few models available, recent first-principles calculations performed by Prodhomme et al. provide a simple yet accurate description of linear and nonlinear piezoelectric coefficients in reduced dimensionality structures having wurtzite crystal symmetry. In this paper, first, within a fully atomistic VFF-sp3s* tight-binding framework, we employ the model proposed by Prodhomme et al. to evaluate the importance of nonl…
Group-III nitride alloys are currently used in various microwave communication applications because of the giant enhancement in electromechanical coupling after alloying with rocksalt nitrides such as YbN or ScN. Herein, the Yb-substitution induced enhancement for electromechanical coupling in wurtzite III-V nitrides is studied via theoretical calculations and experiments. The substitution induced mechanical softening and local strain can enhance electromechanical coupling. The mechanical softening induced by Yb substitution shows less dependence on the parent AlN or GaN, which is caused by th…
Acoustic phonons can be coherently generated by ultrafast displacive screening of potential gradients, often enhanced by the strong built-in piezoelectric fields, in wurtzite semiconductors. In such structures, transverse symmetry within the c plane hinders both the generation and detection of the transverse acoustic (TA) modes, and only longitudinal acoustic (LA) mode is generated. We show that even for c-GaN, the application of asymmetric potential distributions in the c plane can break the symmetry and selection rules, thus switching on the normally forbidden TA mode. This is in contrast to…
