Ternary Mixed Crystals Research Articles

Confined and propagating optical phonons in double-channel AlGaN/GaN heterostructures

Based on the dielectric continuous model, the confined and propagating optical phonon modes in AlGaN/GaN double-channel heterostructures are studied by using the transfer matrix method . Their dispersion relations and electrostatic potentials are discussed considering the ternary mixed crystal and size effects. The results show that both confined and propagating phonon modes can be classified into two groups: low- and high-frequency modes. Ternary mixed crystal effect only affects the low-frequency confined phonons , but has little influence on the high-frequency ones. The thicknesses of the AlGaN barriers have a significant impact on the confined phonons oscillating in both barriers and channels, but the thicknesses of the GaN channels only affect the confined phonons oscillating in channels. It is also found that the propagating modes can be ignored in the further researches because of the narrow frequency range allowing them to exist. These findings indicate that the scattering from the optical phonons in double-channel heterostructures can be modulated to improve the performance of related devices by adjusting the Al composition of AlGaN and the size of each layer. • Confined and propagating optical phonon modes were studied in AlGaN/GaN double-channel heterostructures. • The dispersion relation and electrostatic potential of phonon were analyzed in this structure. • The ternary mixed crystal effect and size effect were discussed in detail. • The anisotropy of wurtzite nitrides was included.

Influence of surface optical phonon on the electronic surface states in wurtzite group-III nitride ternary mixed crystals

An intermediate-coupling variational method is presented to investigate the surface electron states in wurtzite AxB1−xN (A, B=Al, Ga and In) ternary mixed crystals (TMCs). Corresponding effective Hamiltonian are derived by considering the surface-optical-phonon (SO-phonon) influence and anisotropic structural effect. The surface-state energies of electron, the coupling constants and the average penetrating depths of the electronic surface-state wave functions have been numerical computed as a function of the composition x and the surface potential V0 for the wurtzite AlxGa1−xN, AlxIn1−xN and InxGa1−xN, respectively. The results show that the surface-state levels of electron are reduced with the increasing of the composition x in wurtzite AxB1−xN. It is also found that the electron-surface-optical-phonon (e-SO-p) coupling lowers the surface-state energies of electron and the shifts of the electronic surface-state energy level in the wurtzite AlxGa1−xN and AlxIn1−xN increase with the increasing of the composition x. However, in the wurtzite InxGa1−xN, the case is contrary. The influence of the e-SO-p interaction on the surface electron states can not be neglected in wurtzite AxB1−xN.

Electronic mobility limited by optical phonons in symmetric MgxZn1-xO/ZnO quantum wells with mixed phases

In symmetric MgxZn1-xO/ZnO quantum wells (QWs), which are the basic structures of high electronic mobility transistors (HEMTs), the electron states and optical phonon modes are clarified with the dielectric continuum model, uniaxial model, and force balance equation. Then, the electronic mobility affected by optical phonons is obtained around room temperature by a weight model combined with Lei-Ting's force-balance equation, in consideration of mixed phases in MgxZn1-xO (0.37<x < 0.62). Our results show that the potentials of QW's barriers, sharply influenced by the built-in electric field, present a novel fluctuation in mixed phases region with increasing Mg composition x, contrasting to the results only influenced by ternary mixed crystals effect. Meanwhile, compared rocksalt (RS) phase barriers with wurtzite (WZ) ones in QWs, the confinement on both optical interface and confined phonons is stronger in the well and weaker in the barriers. Furthermore, the total electronic mobility in the QWs is mainly influenced by WZ and RS phases, in small (x < 0.37) and large (x > 0.62) Mg composition regions, respectively. In WZ phase, the mobility first reaches a minimum due to the strong polarizations, then rises to a maximum in RS phase. It indicates that the restriction of electronic mobility from different phases should be a primary consideration for the designation of HEMTs. Strong temperature and size dependences of the mobility are also revealed as well. Relatively thicker well width of QWs is more beneficial to increase electronic mobility.

Electron mobility influenced by optical phonons in AlGaN/GaN MISHEMTs with different gate dielectrics

The electron mobility influenced by optical phonons in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors with different gate dielectrics around room temperature is investigated theoretically. The electronic states are obtained by the finite difference method in consideration of built-in electric fields and the conduction band bending. The optical phonons are analyzed using the dielectric continuum model. Based on the theory of force balance equation, the electron mobility of two-dimensional electron gas is obtained for the structures with four different gate dielectrics of Al2O3, HfO2, SiO2 and Si3N4. Our results show that the electron mobility is the highest in HfO2 systems when Al composition in AlGaN is small, whereas the mobility is the highest in Al2O3 systems as Al composition increases to a certain value. The effects of the ternary mixed crystals, each layer’s size and the fixed charges on the sheet density and electron mobility are also discussed for different gate dielectric materials.

Effects of built-in electric field and donor impurity on linear and nonlinear optical properties of wurtzite InxGa1 – xN/GaN nanostructures**Project supported by the National Natural Science Foundation of China (Grant No. 11947414) and the Research Program of Science and Technology at University of Inner Mongolia Autonomous Region, China (Grant No. NJZZ19001).

The linear and nonlinear optical absorption coefficients (ACs) and refraction index changes (RICs) of 1s–1p, 1p–1d, and 1f–1d transitions are investigated in a wurtzite InxGa1 – xN/GaN core–shell quantum dot (CSQD) with donor impurity by using density matrix approach. The effects of built-in electric field (BEF), ternary mixed crystal (TMC), impurity, and CSQD size are studied in detail. The finite element method is used to calculate the ground and excited energy state energy and wave function. The results reveal that the BEF has a great influence on the linear, nonlinear, and total ACs and RICs. The presence of impurity leads the resonant peaks of the ACs and RICs to be blue-shifted for all transitions, especially for 1s–1p transition. It is also found that the resonant peaks of the ACs and RICs present a red shift with In-composition decreasing or core radius increasing. Moreover, the amplitudes of the ACs and RICs are strongly affected by the incident optical intensity. The absorption saturation is more sensitive without the impurity than with the impurity, and the appearance of absorption saturation requires a larger incident optical intensity when considering the BEF.

Optical vibration modes in multi-layer quantum dots of polar ternary mixed crystals

Within the framework of the dielectric continuum approach and the modified random-element isodisplacement model, the optical vibration modes as well as the corresponding electron-phonon interactions in spherical multi-layer quantum dots (QDs) consisting of ternary mixed crystals (TMCs) are investigated in detail. The dispersion relation and electron-phonon interaction Hamiltonian are obtained. As applications of the present theory, the numerical calculations for the ZnS/ZnxCd1-xS/ZnS and GaAs/AlxGa1-xAs/GaAs QDs are performed and discussed. Considering the “one-mode” and “two-mode” behaviors of the TMCs, the results show that there are 5 and 7 branches of interface/surface optical (IO/SO) phonon modes in the two systems, respectively. It is found that the effects of TMCs have a remarkable influence on the optical phonon frequencies and the electron-phonon interactions. The results also reveal that the presence of the cap layer introduces significant influence on the TMCs effects of optical phonon in multi-layer QDs. We hope that our theoretical results can stimulate further investigations of related photoelectric properties, as well as device applications in multi-layer QDs consisting of TMCs.

Interband optical absorption in wurtzite GaN/InxGa1−xN/GaN spherical quantum dots with built-in electric field

Based on the principle of density matrix and the finite element method, the interband optical absorption between the electron and hole has been investigated in a wurtzite [Formula: see text] spherical core–shell quantum dot (CSQD) including a strong built-in electric field (BEF). We have studied the effects of the size and the ternary mixed crystal on the optical absorption coefficients (ACs) and refraction index changes (RICs). The results indicate that the absorption peaks of ACs and RICs decrease rapidly, and show a redshift with the increase of the component [Formula: see text]. It is also found that the absorption peaks of ACs and RICs reduce obviously and depend on the core radius and the well width. When the core radius increases, the positions of the maximum ACs and RICs show a blueshift. At the same time, it presents a redshift when the well width increases. Particularly, the influence of the well width is much stronger than the core radius in the wurtzite [Formula: see text] spherical CSQDs. We hope that these results could provide guidance on both theoretical and experimental study related to the optical properties of spherical CSQDs.

Ab-initio study of the structural and optoelectronic properties of BaSe1-xSx alloys

Structural and optoelectronic properties of BaSe and BaS compounds and their ternary mixed crystals BaSe1-xSx for 0 ≤ x ≤ 1 which crystallize in NaCl-type structure (B1) have been studied with density functional theory (DFT) based full-potential linearized augmented plane wave (FP-LAPW) methodology. Structural properties of the binary compounds and their ternary alloys have been investigated using both generalized-gradient approximations (WC-GGA) developed by Wu-Cohen and (PBE-GGA) developed Perdew-Burke-Ernzerhof. For the electronic properties, in addition to WC-GGA, the recently developed modified Becke and Johnson (mBJ) potential was also used. The concentration dependences of lattice parameter, bulk modulus and band gap for the alloys exhibit nonlinearity. Optical properties of the alloys have been calculated in terms of their respective dielectric function, refractive index, and reflectivity.

Composition dependence of phonon and thermodynamic properties of the ternary AlGaN mixed crystal

Although ternary AlGaN has recently been studied, an accurate phonon spectrum of AlGaN mixed crystal is still scarcity. Based on the density functional theory (DFT), the atomic geometry, phonons and thermodynamic properties of AlxGa1-xN mixed crystal over the entire Al concentration range (0 ≦ x ≦ 1) are investigated. The properties are analyzed by using the generalized gradient approximation (GGA) and the local density approximation (LDA). The optimal doping position of each component is given explicitly. The accurate phonon spectrums of AlGaN mixed crystal under the entire concentration range are shown. The present phonon calculations of the stabled structures for AlxGa1-xN show that the vibration frequency increases to a higher frequency with the Al content increasing. The Debye temperature depends approximately linearly on the Al content at zero temperature and room temperature. Additionally, composition dependence of the high-frequency dielectric constants, Helmholtz free energy, zero point energy and specific heat are determined.

The structural and optical properties of ternary mixed crystals InxGa1−xAs with zinc-blende structure by first-principle calculations

The structural, elastic and optical properties of the ternary mixed crystals (TMCs) zinc-blende InxGa1-xAs for the In-composition varying from 0.0 to 1.0 by step of 0.125 are calculated by first-principle. We have adopted 16-atoms supercell structures of the InxGa1-xAs in this work, and the results for the equilibrium lattice constants, the elastic constants and the optical properties such as real and imaginary parts of the dielectric function, absorption coefficient, reflectivity, refractive index, extinction coefficient, energy loss function and real component of optical conductivity are presented and discussed in detail. The results show that the lattice constants vary with the composition almost linearly following the Vegard's law. Elastic properties such as bulk modulus B, shear modulus G and Young's modulus E decreases monotonically and non-linearly with increasing the alloy composition x. TMCs InxGa1-xAs are anisotropic and mechanically stable. And GaAs may be a less compressible alloy. The static dielectric constant, the static reflectivity and the static refractive index of optical parameters increases with increasing of indium composition. The incorporation of indium composition of absorption spectra leads to the redshifts phenomenon of optical absorption edge and a strong absorption is appeared from near visible to UV range. For x = 0.125, the energy loss function occurs a maximum value so-called plasmon frequency at 15.4eV and there is intense optical conductivity at lower photon energy range.

Influence of size and ternary mixed crystals on optical absorption of excitons in Zn[formula omitted]Mg[formula omitted]O/ZnO/Zn[formula omitted]Mg[formula omitted]O quantum wells

Optical absorption of exciton transitions in asymmetry Zn1−xMgxO/ZnO/Zn1−yMgyO quantum wells (QWs) is investigated and the effects of size and ternary mixed crystals (TMCs) are discussed in a wider TMC region. The different exciton states and interstate transition energies are obtained by the combination of a variational method and numerical calculations. Based on the density matrix approach and our weight model, the mixed phase of wurtzite and zinc blende in Zn1−xMgxO alloys within 0.37<x<0.62 is considered to discuss the optical absorption coefficients (OACs) in the presence of built-in electric fields and the Hartree potential. The results show that the peaks of OACs appear a merged phenomenon under size modulation. The change of OACs in wurtzite–wurtzite–wurtzite QWs is more obvious than that in zinc blende–wurtzite–zinc blende ones with increase of Mg component to demonstrate the TMCs effect. Our results are better agreement with the photoluminescence spectra of exciton transitions given by experiments.

Effects of ternary mixed crystals on interface/surface optical phonon in spherical core-shell quantum dots

Within the framework of the dielectric continuum approach and modified random-element-isodisplacement model, the optical vibration mode in a spherical core-shell quantum dot (CSQD) consisting of ternary mixed crystals (TMCs) are investigated. The dispersion relation and electron–phonon interaction Hamiltonian are derived. As a typical case, the numerical results for [Formula: see text] and [Formula: see text] CSQDs are obtained and discussed. Taking the one- and two-mode behaviors of TMCs into account, the effects of TMCs on interface/surface optical (IO/SO) phonon show that there are 3 and 5 branches of IO/SO phonon modes in [Formula: see text] and [Formula: see text] CSQDs for a given component of TMC, respectively. It is also found that the IO/SO phonon frequencies and electron–phonon interactions are strongly dependent on the component of TMCs and the size of CSQDs. We hope this work would be useful for the study of the phonon-related photoelectric properties in CSQDs consisting of TMCs.

Optical absorption via intersubband transition of electrons in GaAs/AlxGa1−xAs multi-quantum wells in an electric field * * Project supported by the National Natural Science Foundation of China (No. 61764012).

Based on the effective mass approximation, the Schrödinger equation and Poisson equation in GaAs/ AlxGa1−xAs multi-quantum wells (MQWs) are self-consistently solved to obtain the wave functions and energy levels of electrons in the conduction band for the ground first excited state by considering a lateral electric field (LEF). Then, the effects of size, ternary mixed crystal, doping concentration, and temperature on linear and nonlinear intersubband optical absorption coefficients (IOACs), and refractive index changes (RICs) due to the transition between ground states and the first excited states of electrons are discussed based on Fermi’s golden rule. The results show that, under a fixed LEF, with increase of Al composition and doping concentration, the IOACs produce a red shift. With increases of both widths of the wells and barriers IOACs appear as blue shifts and their amplitudes increase, but the barrier width change is much more important to affect nonlinear IOACs, whereas increasing the temperature results in a blue shift first and then a red shift of IOACs. When the other parameters are fixed but there is an increase in the LEF, IOACs occur with a blue shift, and the RICs have similar properties.

First-principle studies of optical properties of BexZn1-xO ternary mixed crystal

The optical properties of beryllium doped zinc oxide (BexZn1-xO) ternary mixed crystal are studied by the first-principles calculations within the framework of the density functional theory (DFT). It turned out that as the energy increases the real parεt ε1of the dielectric function of BexZn1-xO first reached a maximum peak and then gradually attenuated, the static dielectric constant of BexZn1-xO decreases as the Be component increases, the imaginary part ε2of the dielectric function represents light absorption, as the Be component increases, the three main peaks are constantly moving towards the high-energy tail. The refractive index and reflectivity of BexZn1-xO ternary mixed crystal are decreases with increasing Be concentrations, the blue shift of absorption spectrum of the system becomes more significant. Obtained smooth varying of the optical properties is related to the gradual decreasing of the lattice parameter of BexZn1-xO and increasing of the bandgap.

Electron mobility limited by optical phonons in wurtzite InGaN/GaN core-shell nanowires

Based on the force-balance and energy-balance equations, the optical phonon-limited electron mobility in InxGa1-xN/GaN core-shell nanowires (CSNWs) is discussed. It is found that the electrons tend to distribute in the core of the CSNWs due to the strong quantum confinement. Thus, the scattering from first kind of the quasi-confined optical (CO) phonons is more important than that from the interface (IF) and propagating (PR) optical phonons. Ternary mixed crystal and size effects on the electron mobility are also investigated. The results show that the PR phonons exist while the IF phonons disappear when the indium composition x &amp;lt; 0.047, and vice versa. Accordingly, the total electron mobility μ first increases and then decreases with indium composition x, and reaches a peak value of approximately 3700 cm2/(V·s) when x = 0.047. The results also show that the mobility μ increases as increasing the core radius of CSNWs due to the weakened interaction between the electrons and CO phonons. The total electron mobility limited by the optical phonons exhibits an obvious enhancement as decreasing temperature or increasing line electron density. Our theoretical results are expected to be helpful to develop electronic devices based on CSNWs.

Optical phonon scattering on electronic mobility in Al2O3/AlGaN/AlN/GaN heterostructures

Considering the built-in electric fields and the two-mode property of transverse optical phonons in AlGaN material, the electronic eigen-energies and wave functions are obtained by solving Schrödinger equation with the finite difference method. The dispersion relations and potentials of the optical phonons are given by the transfer matrix method. The mobility of the two dimensional electron gas influenced by the optical phonons in Al2O3/AlGaN/AlN/GaN heterostructures is investigated based on the theory of Lei-Ting force balance equation. It is found that the scattering from the half-space phonons is the main factor affecting the electronic mobility, and the influence of the other phonons can be ignored. The results show that the mobility decreases with increasing the thicknesses of Al2O3 and AlN layers, but there is no definite relationship between the mobility and the thickness of AlGaN barrier. The mobility is obviously reduced by increasing Al component in AlGaN crystal to show that the effect of ternary mixed crystals is important. It is also found that the mobility increases first and then decreases as the increment of the fixed charges, but decreases always with increasing temperature. The heterostructures constructed here can be good candidates as metal-oxide-semiconductor high-electron-mobility-transistors since they have higher electronic mobility due to the influence from interface phonons weakened by the AlN interlayer.

First-principles study on mechanical and elastic properties of BxAl1-xP alloys

Based on density functional theory calculations, systematic calculations of the structural properties, elastic anisotropy and mechanical properties of boron alloying aluminum phosphide (BxAl1-xP) ternary mixed crystal have been presented. The results of the lattice parameters, band gaps, elastic constants and elastic modulus accord with the experimental and others published data well. The band structure which is described by CASTEP method indicates they are direct gap semiconductors for the composition x = 0.25, 0.50 and 0.75. Beyond that, we studied the Debye temperatures together with the acoustic velocities for all the BxAl1-xP alloys using the obtained elastic modulus. Finally, we depicted the three dimensional surface constructions to explain the elastic anisotropy using several calculated different anisotropic indexes in our work.

Bazı Üçlü Karıştırılmış Hidrojen Bağlı Sıvı Kristallerin Termal özelliklerine 4-oktiloksi-4'-siyanobifenil (8OCB) mezojeninin etkisi

Bazı Üçlü Karıştırılmış Hidrojen Bağlı Sıvı Kristallerin Termal özelliklerine 4-oktiloksi-4'-siyanobifenil (8OCB) mezojeninin etkisi

Effects of two-mode transverse optical phonons in bulk wurtzite AlGaN on electronic mobility in AlGaN/GaN quantum wells

The two-mode property of bulk transverse optical (TO) phonons in ternary mixed crystals of wurtzite AlxGa1-xN has been investigated by introducing impurity modes in a modified random-element isodisplacement model. Based on the dielectric continuous model, the uniaxial model, and the Lei-Ting balance equation, the effects of the two-mode property on electrostatic potentials of interface optical and confined optical phonons in AlGaN/GaN quantum wells, as well as their influences on the electronic mobility (EM), are discussed by a component-dependent weight model. Our results indicate that the total EM decreases to a minimum at first and then increases slowly with x under the influences of the competitions from the eight branches of phonons. The further calculation shows that the total EM decreases with the increment of temperature in the range of 200 K &amp;lt; T &amp;lt; 400 K and reduction of well width d. As a comparison, the EM is calculated for an Al0.58Ga0.42N/GaN quantum well at room temperature, and our result is 1263.0 cm2/Vs, which is 1.44 times of the experiment value. Our result is expected since the difference between our theory and the experiment is mainly due to the neglect of interface-roughness and other secondary scattering mechanisms. Consequently, the two-mode property of bulk TO phonons in ternary mixed crystals does affect obviously on the electron transport in the quantum wells. And our component-dependent weight model could be extended to study the electric properties influenced by optical phonons in other related heterostructures.

Buffer layer influence on light absorption of electron intersubband transition in binary energy level systems of quantum wells

Due to the restriction of the ZnO buffer layer on the left barrier in a wurtzite asymmetric ZnO/MgxZn1-xO single quantum well (QWs) structure with finite barriers, the other factors such as the size of the well and right barrier, and Mg component, etc. will influence the critical value of the left barrier width to form a binary level energy system. By adopting a finite difference method to solve the Schrdinger equation, the eigenstates and eigenenergies of electrons in a two-dimensional electron gas are obtained, and the influences of buffer layer ZnO, size and ternary mixed crystal effects on the formation of binary energy level system in QW are discussed. In our computation, the influences of energy band bending, material doping and built-in electric fields on a realistic heterostructure potential are considered. Furthermore, based on the Fermi's golden rule, the optical absorption coefficient of electronic intersubband transition in QW and the influences of buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal effects are discussed. Our results indicate that the critical width of left barrier increases with the increases of the right barrier width and buffer layer thickness for a binary energy level system of ZnO/MgxZn1-xO single quantum well with a ZnO buffer layer on the left side. However, the critical width of left barrier decreases with the increase of well width and Mg component. Besides, the buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal also affect the light absorption induced by the electronic intersubband transitions. The increases of Mg component, the widths of right barrier and left barrier will increase the absorption peak and produce its blue-shift. Whereas, increasing well width will reduce the absorption peak and produce its red-shift. The above conclusions are expected to give theoretical guidance in improving the opto-electronic properties of materials and devices made of these heterostructures.