Interface Optical Phonons Research Articles

Electron–interface-phonon interaction strength in Pöschl–Teller quantum wells is enhanced considerably compared to rectangular ones

We investigated the magneto-optical properties of the Pöschl–Teller GaAs/AlAs quantum well (QW) due to the electron–interface optical (IO) phonon interaction based on the optically-detected (OD) magneto–IO-phonon resonance (MIOPR) effect. The methods of the operator projection and the profile were used to calculate the MO-absorption power (MOAP) and measure the full width at half maximum (FWHM) of the OD-MIOPR peaks. The results are as follows: The well-width (LW), the temperature of the electron–phonon system (T), and the magnetic field (B) strongly affect the MOAP and the FWHM of the OD-MIOPR peaks for both the two emission and absorption cases of IO-phonons in both the materials of the well and the barrier. The FWHMs in both the materials of the well and the barrier for the absorption case of an IO-phonon are bigger than those for the emission case. The FWHM due to IO-phonon interaction in the GaAs material of the well is stronger than that in the AlAs material of the barrier for both the two IO-phonons emission and absorption cases. Especially, the result also showed that the FWHMs in the Pöschl–Teller QW are always bigger than those in the rectangular QW for the two emission and absorption cases of IO-phonons in both the GaAs and AlAs materials of the well and the barrier, respectively. Our specific results are expected to be promising data for potential applications in optoelectronic devices.

Quantum Phase Transitions of Interlayer Excitons in van der Waals Heterostructures

Quantum phases of interlayer excitons (IXs) in van der Waals heterostructures (vdWHs) play a critical role in determining the fundamental properties of these structures. However, quantum phase transitions between free state and self‐trapped state of IXs remain poorly understood. Herein, the physical pictures for quantum transitions of IXs stemming from the IX–interface optical phonon coupling are explored. The critical boundaries of the phase transitions among free, light, and heavy self‐trapped states are given and the dependence of boundaries on the structural parameters of vdWHs is discussed, implying the controllability of these IXs states in vdWHs. In order to distinguish different quantum phases further, a strategy that multiphonon Raman scattering mediated by these exciton states is proposed. It is found find that multiphonon overtones appear in Raman spectra for the self‐trapped states and not for free states, which clarifies the long‐standing puzzle that free exciton can induce multiphonon Raman processes. The results lay the significant theoretical foundation for understanding the quantum phases of IX and their modulation in vdWHs.

Ternary mixed crystal effects on optical phonons in wurtzite ZnO quantum wells with asymmetric MgZnO barriers

Optical phonons significantly relate to fundamental limits on dynamical processes in layered polar semiconductor devices. MgZnO layers commonly serve as barriers of ZnO quantum wells (QWs) to improve the electric and optical properties by quantum confinement while keeping the Mg fraction below 0.4 to avoid the crystal phase transformation from wurtzite to rocksalt. This work investigates ternary mixed crystal effects on optical phonons in wurtzite ZnO quantum well with asymmetric barriers of MgxZn1-xO and MgyZn1-yO over a full range of Mg mole fractions. Since MgZnO could be wurtzite (WZ) and rocksalt (RS) phases, the QWs can be denoted as WZ/WZ/WZ, WZ/WZ/RS, and RS/WZ/RS, respectively. Both asymmetries in the Mg fractions and crystal structures of the two barriers affect the optical phonons in QWs. RS/RS/RS quantum wells are also discussed for comparison. Since oscillatoring electrostatic potential of optical phonons is only permitted in the anisotropic wurtzite but forbidden in istropic rocksalt, more modes of confined optical phonons are found in a WZ/WZ/WZ QW, followed by WZ/WZ/RS one, and the least in the RS/WZ/RS one, while there is no confined optical phonons in RS/RS/RS quantum wells. Besides, the difference between the Mg fractions in two barriers leads to more modes of interface optical phonons due to the two unequal interfaces. The modes contained in the optical phonons spectra as well as their frequency ranges, dispersion relations, and electrostatic potentials, vary with Mg fraction in barriers. The strain on interfaces induced by the lattice mismatch leads to a slight blue shift of the dispersion relation as Mg fraction increases. Based on the results of this paper, the optical phonons related to electronic and optoelectronic properties in QWs could be further understood and adjusted.

Effects of asymmetric MgZnO barriers on polar optical phonon-limited electron mobility in wurtzite ZnO thin films

MgZnO barriers are commonly applied to passivate wurtzite ZnO films to enhance electron mobility, while the Mg mole fraction x is usually controlled below 0.4 to avoid phase separation. Few theoretical analyses have focused on electron mobility at large x since the phase separation leads to a complex scattering mechanism. This work investigates the effects of asymmetric MgZnO barriers on electron mobility, which is one source of complexity. Four asymmetric quantum wells simultaneously contribute to the electron mobility in proportions when the wurtzite and rock salt coexist in the mixed-phase MgZnO barriers with large Mg mole fractions. Besides, built-in electric fields also contribute to the asymmetry by tilting the bands. The polar optical phonon-limited electron mobility in asymmetric MgxZn1−xO/ZnO/Mg0.45Zn0.55O quantum wells is simulated between 176 and 333 cm2/V s as x ranges from 0.1 to 1. Our calculations show that confined optical phonons play a leading role in the quantum well with wurtzite barriers. Interface optical phonons are primary in the wells with rock salt barriers since most electrons are pushed close to the interface by the strong built-in electric field. The results indicate that wurtzite barriers are more favorable to achieving stable high mobility above 238 cm2/V s as the Mg mole fraction ranges from 0.14 to 0.33, which is commonly applied in practice.

Optical phonon limited electron mobility in ZnO nanowires wrapped by MgZnO shells

MgxZn1−xO shells are commonly used as a passivation barrier for improving electron mobility in ZnO nanowires by preventing electrons from charged surfaces. However, a high Mg mole fraction x instead makes lower electron mobility, which is usually attributed to the appearance of mixed-phase MgxZn1−xO as x increases. This work aims to find the optimal x for optical phonon limited electron mobility by considering the phase transformation in the MgZnO shell from wurtzite to rock salt, leading to a mixed-phase range of x. Our calculations show that the electron mobility μT can be effectively enhanced by keeping x below 0.057 when confined (CO1) optical phonons are only permitted for small wave vectors, and there is no interface (IF) optical phonon. Once x gets over 0.057, the propagating optical phonons are transformed into IF ones while CO1 phonons become permitted for all wave vectors resulting in a largely strengthened scattering effect and thus a drastic drop in the total electron mobility μT from 1215 to 310 cm2/V s. From then, μT begins to fall slowly as x increases even when the rock salt component in the shell appears to take the place of the wurtzite part, while the scattering from CO1 optical phonons remains primary. Furthermore, the enlarging core radius can weaken the electron–CO1 phonon interaction to enhance mobility.

Polaron mass of carriers in a thin film on ionic substrates

A new approach to establishing a strong electron--phonon interaction in heterostructures is proposed. A three-layer structure consisting of an ionic substrate, a semiconductor film, and a covering dielectric (with air or vacuum possibly serving as such a dielectric) is examined. Interface optical phonons emerge near the heterointerface. Their parameters are governed by the dielectric properties of the substrate. It is demonstrated that the effective mass of carriers in the film is altered in the presence of interface phonons. Depending on the substrate ionicity, the magnitude of this change may vary from several tens to hundreds of percent. It is shown that the conditions for strong electron--phonon interaction may be established in a large number of semiconductor films. Measurements of the effective mass of carriers in identical films positioned on different substrates should make it possible to identify a transition from a weak electron--phonon interaction to a strong one. Keywords: electron--phonon interaction, effective mass, interface phonons, polaron, thin films.

Comparison of the contribution of different types of interface-optical phonon modes to electron–phonon scattering in a quasi-two-dimensional system

Utilizing the operator projection technique, we theoretically study the contribution of different types of optical phonon modes to electron–phonon scattering in a quasi-two-dimensional system through calculating the expression for the magnetophonon resonance absorption power (MPR-AP) caused by scattering between the electrons with interface-optical phonons in parabolic quantum wells (PQWs). Electron interactions with different modes of interface-optical-phonons include the symmetric and antisymmetric modes in both the barrier and well layers together are considered in detail. The full width at half maximum (FWHM) of the resonant peaks due to different interface-optical-phonon modes was measured by using the profile method. Numerical results show that the strength of electron–interface-phonon interaction increases with confining frequency of parabolic confinement potential, temperature, and magnetic field under all antisymmetric- and symmetric-modes in both well and barrier layers. The electron-interaction with symmetric-interface-phonon has stronger strength compared to antisymmetric-interface-phonon in all well and barrier layers. The strongest-electron-scattering for symmetric phonon in well layer, meanwhile weakest that for antisymmetric phonon in barrier layer. At higher value of the photon frequency, the contributions of interface-optical-phonons is not significant. Our present calculations well accord with previous experimental and theoretical studies.

Interface optical phonons in double-channel AlGaN/GaN heterostructures: The ternary mixed crystal effect and size effect

Considering the anisotropy of wurtzite semiconductors, the interface optical phonons in double-channel AlGaN/GaN heterostructures are investigated by using a dielectric continuous model and transfer matrix method. Also, the ternary mixed crystal effect and size effect on the dispersion relations and electrostatic potentials of phonons are analyzed in detail. The results show that there are six branches of interface phonon modes in a double-channel heterostructure. For some values of Al composition, however, the phonon mode with the highest frequency may not exist, especially when the thicknesses of materials and the wave vectors of phonons are small. The ternary mixed crystal effect and size effect not only influence the values of phonon frequency and electrostatic potential, but also change the vibration mode of interface phonons. This suggests that the interface phonon vibrations can be controlled to reduce their adverse effects by changing the Al composition of AlGaN and the thickness of each layer in a double-channel heterostructure.

External electric and magnetic field effects on the polaron in a wurtzite nitride nanowire embedded in a nonpolar matrix

We examine the Frohlich polaron problem in wurtzite nitride cylindrical nanowire embedded in a nonpolar matrix within the framework of the Lee–Low–Pines variational approach. The effects of the external electric and magnetic fields and quantum-size confinement on polaron self-energy and effective mass due to electron interactions with both quasiconfined and interface optical phonons are investigated. The numerical results obtained for wurtzite GaN material are discussed in comparison with the zinc-blende structure cylindrical nanowires. The estimation of contributions of different phonon modes reveal that interface optical phonons are much more important in narrow wires. It is found that the electric field has a relatively pronounced influence on the polaron self-energy and effective mass, while the effect of the magnetic field is small. The obtained results will be of importance in further study of the phonon-assisted electro-optical properties in wurtzite semiconductor low-dimensional structures.

Cyclotron–interface phonon resonance line-width in asymmetric semiparabolic quantum wells

In this study, we investigated the cyclotron–interface phonon resonance line-width in GaAs/AlAs asymmetric semiparabolic quantum wells when electrons are scattered by symmetric (antisymmetric) interface optical phonons. The operator projection and profile methods were employed to calculate the expressions for the absorption power and the full width at half maximum (FWHM), respectively. We found that the electron–interface phonon interactions by the antisymmetric modes are weaker than those by the symmetric modes under all phonon emission and absorption cases. In addition, the FWHM is more dominant for interface phonon absorption compared with that for interface phonon emission. The interface optical phonon modes at the resonance peak with a lower-photon frequency are much more important for electron–interface optical phonon scattering than those at the resonance peak with a higher-photon frequency. Our results are consistent with previously reported experimental data.

Effect of spin–orbit coupling on the hot-electron energy relaxation in nanowires

The energy relaxation of hot electrons is proposed based on the spin–orbit (SO) interaction of both Rashba and Dresselhaus types with the effect of hot phonons. A continuum theory of optical phonons in nanowires taking into account the influence of confinement is used to study the hot-electron energy relaxation. The energy relaxation due to both confined (CO) and interface (IO) optical phonon emission on nanowire radius, electrical field strength, parameters of SO couplings and electron temperature is calculated. For considered values of the nanowire radius as well as other system parameters, scattering by IO phonons prevails over scattering by CO phonons. The presence of an electric field leads to the decrease of power loss in transitions between states with the same spin quantum numbers. With the increase of the electric field strength, the influence of the Dresselhaus SO interaction on the energy relaxation rate decreases. The effect of SO interaction does not change the previously obtained increasing dependence of power loss on electron temperature. The sensitivity of energy relaxation to the electric field also through the Rashba parameter allows controlling the rate of energy by electric field.

Effect of optical phonons scattering on electron mobility in asymmetric AlGaN/GaN quantum wells

Abstract Based on the dielectric continuum model, uniaxial model and force balance equation, the electron mobility in asymmetric wurtzite AlxGa1-xN/GaN/AlyGa1-yN quantum wells (QWs) is investigated theoretically in consideration of optical phonon scattering and varieties of Al compositions x and y. The dispersion relations and potentials of interface, confssined, half space and propagation optical phonon modes are given by the transfer matrix method including the effect of two-mode transverse optical phonons. Contrary to symmetric QWs, one branch of interface optical phonons vanishes and transforms into half space or confined optical one in certain intervals of Al component, whereas the confined optical phonons exist in a wider Al component range. The electron mobility in asymmetric AlGaN/GaN QWs can be modulated to be higher than that in symmetric ones. It is found that the total electron mobility decreases at first, then induces steps and finally decreases as x increases for certain y. The electron mobility also shows strong temperature-dependence. A significant modification on electron mobility appears for a narrower GaN well layer. Our results indicate that the asymmetric AlGaN/GaN QWs can be modulated to exhibit better transport properties than symmetric ones.

Contribution of remote interface polar phonons in the hole mobility of diamond

The role of remote interface polar phonon modes on the electronic transport properties of dimensionally confined diamond structures have been investigated in detail. By employing a dielectric continuum model, scattering rates, hole mobility and relaxation times in mesoscopic diamond-based devices are calculated. The performance has been analyzed by inserting polar over layers on diamond with wide-band gap materials such as AlN or w − BN. We have shown that a Fröhlich potential due to the interface optical phonon modes in the upper layer is created, that decays into diamond resulting in the existence of a remote polar phonon potential in the material. Hole scattering from remote polar phonons dominate on the order of two or more in magnitude resulting in substantial decrease in hole mobility in diamond. Thus, the present research will provide a milestone for understanding interface phonons that penetrate to the two-dimensional hole gas (2DHG) in confined diamond structures as compared to their bulk counterpart and can also be applied to other polar materials used for doping.

Electron–Phonon Interaction in Quantum Wells Based on Uniaxial Materials

The interaction of charged particles with interface optical phonons in quantum wells composed of uniaxially symmetric materials is studied theoretically. It is shown that the character of this interaction depends considerably on the degree of anisotropy of the phonon spectrum in the materials forming the quantum well and the barriers. In the case of strong anisotropy, the interaction turns out to be significantly weaker than it is in similar structures made of materials with cubic symmetry. In the case of weak phonon anisotropy, the interaction of charged particles with optical phonons can under certain conditions be described by a Frohlich-type coupling constant, and both weak and strong electron–phonon interaction can take place in different structures. The results obtained extend the possibilities of the optical diagnostics of quantum nanostructures.

Polaronic effects on the impurity-related linear and nonlinear optical properties in a nanowire with magnetic field

Abstract Using the Landau–Pekar variational method, the binding energies of a hydrogen-like impurity-bound magnetopolaron as well as related optical properties in a cylindrical nanowire are theoretically investigated. The influence of polar optical phonon confinement in a nanowire on the electron–phonon interaction is considered. The predictions of the impurity-related polaronic effects on the linear and nonlinear optical properties due to resonant transitions between hydrogen-like states are presented. A notable influence of the polaronic effect on optical characteristics and binding energies is observed. Particularly it is shown that the electron–phonon interaction leads to blue shifts of the extrema of all absorption coefficients and refractive index changes. The polaronic effect on the optical characteristics is noticeably manifested when the electron interacts mainly with either confined optical or interface optical phonons. It is found that, regardless of the magnetic field, the extrema of the refractive index change, without taking into account the electron–phonon interaction, are observed at the same photon energy, and taking into account this interaction, the extrema are somewhat distant from each other.

Электрон-фононное взаимодействие в квантовых ямах на основе одноосных материалов

AbstractThe interaction of charged particles with interface optical phonons in quantum wells composed of uniaxially symmetric materials is studied theoretically. It is shown that the character of this interaction depends considerably on the degree of anisotropy of the phonon spectrum in the materials forming the quantum well and the barriers. In the case of strong anisotropy, the interaction turns out to be significantly weaker than it is in similar structures made of materials with cubic symmetry. In the case of weak phonon anisotropy, the interaction of charged particles with optical phonons can under certain conditions be described by a Fröhlich-type coupling constant, and both weak and strong electron–phonon interaction can take place in different structures. The results obtained extend the possibilities of the optical diagnostics of quantum nanostructures.

Electron-interface-phonon interactions and its ternary effects on in ZnS/CdxZn1-xS core-shell heterostructures

Within the framework of dielectric continuum mode (DCM), random-element isodisplacement model (REIM) and standard quantization procedure, the electron-interface-phonon interactions and its ternary effects in ZnS/CdxZn1-xS core-shell heterostructures are investigated. The dispersion curves, the electron-phonon couplings and its ternary mixed crystal effects of interface optical (IO) phonons in ZnS/CdxZn1-xS core-shell heterostructures are calculated and discussed in detail. The results indicate that the IO phonons are more dispersive in low quantum number. With the increase of the cadmium element concentration, the IO phonon frequencies decrease linearly in high frequency range (HF-range), and decreases non-linearly in low frequency range (LF-range). The numerical results also indicate that the IO phonons with lower quantum number play a significant role in electron-phonon interactions. The electron-phonon coupling strengths increase linearly in HF-range, and increase non-linearly in LF-range with the increasing cadmium element concentration. The IO phonons in HF-range have prominent contributions to electron-phonon interactions.Within the framework of dielectric continuum mode (DCM), random-element isodisplacement model (REIM) and standard quantization procedure, the electron-interface-phonon interactions and its ternary effects in ZnS/CdxZn1-xS core-shell heterostructures are investigated. The dispersion curves, the electron-phonon couplings and its ternary mixed crystal effects of interface optical (IO) phonons in ZnS/CdxZn1-xS core-shell heterostructures are calculated and discussed in detail. The results indicate that the IO phonons are more dispersive in low quantum number. With the increase of the cadmium element concentration, the IO phonon frequencies decrease linearly in high frequency range (HF-range), and decreases non-linearly in low frequency range (LF-range). The numerical results also indicate that the IO phonons with lower quantum number play a significant role in electron-phonon interactions. The electron-phonon coupling strengths increase linearly in HF-range, and increase non-linearly in LF-range with the incr...

Interface optical phonons and its ternary mixed effects in ZnS/CdxZn1-xS quantum dots

Taking into account cadmium element concentrations, the interface optical (IO) phonons, electron-phonon couplings in ZnS/CdxZn1-xS quantum dots are investigated theoretically within the framework of macroscopical dielectric continuum model (MDCM) and modified random-element isodisplacement model (MREIM). The numerical results indicate that the cadmium element concentrations have important effects on the properties of IO phonons and electron-phonon interactions in a single ZnS/CdxZn1-xS quantum dot. The dependences between IO phonon frequencies and cadmium element concentrations are linearly decreases in high frequency range (HF-range), and are non-linearly decreases in low frequency range (LF-range). The numerical results also suggest that the dependent relations between the absolute value of electron-phonon coupling strengths and cadmium element concentration are linear relationship in high frequency range (HF-range), and are non-linear relationship in low frequency range (LF-range), especially in higher cadmium element concentration range.

Electron Scattering via Interface Optical Phonons with High Group Velocity in Wurtzite GaN-based Quantum Well Heterostructure

Here we present a detailed theoretical analysis of the interaction between electrons and optical phonons of interface and confined modes in a wurtzite AlN/GaN/AlN quantum well heterostructure based on the uniaxial dielectric continuum model. The formalism describing the interface and confined mode optical phonon dispersion relation, electron–phonon scattering rates, and average group velocity of emitted optical phonons are developed and numerically calculated. The dispersion relation of the interface phonons shows a convergence to the resonant phonon frequencies 577.8 and 832.3 cm−1 with a steep slope around the zone center indicating a large group velocity. At the onset of interface phonon emission, the average group velocity is small due to the large contribution of interface and confined mode phonons with close-to-zero group velocity, but eventually increases up to larger values than the bulk GaN acoustic phonon velocity along the wurtzite crystal c-axis (8 nm/ps). By adjusting the GaN thickness in the double heterostructure, the average group velocity can be engineered to become larger than the velocity of acoustic phonons at a specific electron energy. This suggests that the high group velocity interface mode optical phonons can be exploited to remove heat more effectively and reduce junction temperatures in GaN-based heterostructures.

Contribution of bulk and surface phonons to the properties of polaron in a Zn1−XCdXSe∕ZnSe heterojunction confined in a triangular potential

The contributions of bulk and surface phonons to the properties of a polaron in a Zn1 −XCdXSe∕ZnSe heterojunction confined in a triangular potential have been studied using the Lee-Low-Pines variational method and the Tokuda linear-combination operator method. We obtain expressions for the wave function of the system, the ground state energy, effective mass, the average number of phonons, the mobility and the lifetime of the polaron. Our numerical results show that the two Interface-optical (IO)-phonon modes and the longitudinal-optical (LO)-phonon mode contribute to the ground state energy, the average number of phonons, the mobility, the effective mass and the lifetime of the polaron. It is seen that both the ground state energy and the average number of phonons increase with an increase in the Cd composition X while the effective mass, mobility and lifetime of polaron decrease as the Cd composition X increases. We observe that there are more phonons in the bulk than at the surface. It is also noticed that the polaron disintegrates when the temperature is increased to certain values. Thus, at high temperatures, the electron-phonon interaction is destroyed. It is seen that even though the contribution of the LO phonon is more important; the contribution of the IO phonon to the polaron's properties is also quite significant.