Electron-optical Phonon Interaction Research Articles

Structure effects on electron-optical phonon interaction in GaAs/AlxGa1−xAs quantum wells

Based on the dielectric continuum model, we have studied the electron-optical phonon scattering rates in GaAs/AlxGa1−xAs quantum wells with different structure parameters. It was found that the scattering rate of the symmetric interface phonon mode has a stronger dependence on the Al composition in the barriers than that of the confined mode. The effective phonon energy emitted by hot electrons in GaAs/AlxGa1−xAs quantum wells with various Al compositions was estimated and the calculated value agrees with the experimental results qualitatively. For the dependence on the well width, scattering rates of the S+ mode drop considerably as the well width is increased. The dependence of the electron-optical phonon interaction on structure parameters can be clearly explained by the H and G factors defined in the article.

Emission current enhancement of MIM cathodes by optimizing the tunneling insulator thickness

The relationship between the thickness of the anodized Al/sub 2/O/sub 3/ tunneling insulator and the transfer ratio was investigated for metal-insulator-metal (MIM) cathodes to optimize the thickness in terms of a high transfer ratio and emission current. Combining ellipsometry, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), we determined the accurate thickness of an anodized Al film less than 20 nm-thick. With the knowledge of accurate thickness, we found that the transfer ratio increases as the insulator thickness increases from 5.2 nm to 10.6 nm, but saturates at 13.3 nm and decreases slightly at 20.1 nm. Optimizing the thickness of the insulator to 13.3 nm raised the transfer ratio of 0.1% for our previous work (Kusunoki and Suzuki, IEEE Trans. Electron Devices, vol. 47, pp. 1667-1672, 2000) to 0.7%. A high emission current of 14 mA/cm/sup 2/ was thus obtained. The existence of an optimal thickness for the anodized Al/sub 2/O/sub 3/ insulator was also clarified from a theoretical simulation. This is the result of a trade-off, as thickness increases, between the decreasing probability of cut-off at the surface workfunction barrier of the Ir-Pt-Au top electrode and the increased scattering of hot electrons inside the Al/sub 2/O/sub 3/ insulator and top electrode. The relationship is discussed on the basis of the absolute distribution of energy of the hot electrons, which we determined by simulating inelastic scattering driven by electron-optical phonon interaction in the Al/sub 2/O/sub 3/ insulator.

Investigation of electron–optical phonon interactions in moderate wide In xGa 1− xAs/GaAs strained quantum wells

In this presentation, we have calculated the electron–optical phonon scattering rate of GaAs/AlAs quantum wells and average electron energy loss rate as a function of well width of the GaAs/Al x Ga 1− x As quantum wells. We have also studied the Raman and hot electron–neutral acceptor luminescence in moderate wide In x Ga 1− x As/GaAs strained quantum wells (with 10 nm in well width and 30 nm in barrier width) to determine the dominant phonon mode emitted by the hot electrons in the wells at 15 K. The hot electron–neutral acceptor luminescence spectrum of the strained quantum well sample shows an oscillation period of about 22 meV which indicates that the hot electrons relaxed mostly through emissions of the InAs confined phonons.

Influence of Si doping on optical properties of wurtziteGaN

The band gap shift (BGS) of Si-doped wurtzite GaN for impurityconcentrations spanning the insulating to the metallic regimes has beeninvestigated at low temperature. The critical impurity concentration forthe metal-non-metal transition is estimated from the generalized Drudeapproach for the resistivity to be about 1.0×1018 cm-3.The calculations for the BGS were carried out within a framework of therandom phase approximation, taking into account the electron-electron,electron-optical phonon, and electron-ion interactions. In the wake ofvery recent photoluminescence measurements, we have shown and discussed thepossible transitions involved in the experimental results.

Raman and hot electron–neutral acceptor luminescence studies of electron–optical phonon interactions in GaAs/Al xGa 1− xAs quantum wells

Using two optical techniques, we have studied the hot electron–optical phonon interactions in GaAs/Al x Ga 1− x As multiple-quantum wells. Raman scattering measurements at 15 K are presented for the Al composition of x=0.3, 0.5, 0.7 and 1.0. The GaAs-like and AlAs-like phonon frequencies of the first-order modes are also measured as a function of Al composition. The optical phonon energies emitted by the photoexcited electrons in quantum wells are determined by using hot electron–neutral acceptor luminescence techniques. It is shown that the relaxation of hot electrons in the quantum wells is dominated by the GaAs LO phonon emission for small x, but by AlAs-like LO phonons for larger Al composition.

Effect of electron–phonon interaction on the impurity binding energy in a quantum wire

The effect of electron–optical phonon interaction on the hydrogenic impurity binding energy in a cylindrical quantum wire is studied. By using Landau and Pekar variational method, the Hamiltonian is separated into two parts which contain phonon variable and electron variable, respectively. A perturbative-variational technique is then employed to construct the trial wave function for the electron part. The effect of confined electron–optical phonon interaction on the binding energies of the ground state and an excited state are calculated as a function of wire radius. Both the electron-bulk optical phonon and electron-surface optical phonon coupling are considered. It is found that the energy corrections of the polaron effects on the impurity binding energies increase rapidly as the wire radius is shrunk, and the bulk-type optical phonon plays the dominant role for the polaron effects.

Optical phonons and electron–phonon interaction in quantum wells consisting of mixed crystals

The features of the optical phonons and the electron–optical-phonon interaction in quantum wells that involve layers of mixed crystal are studied theoretically based on the dielectric continuum approach. The interface optical (IO)-phonon energies and the electron–IO-phonon coupling strengths of Al 1−x In x As/ AlAs, InAs/ Al 1−x In x As , and Al 1− x In x As/Al 1− y Ga y As quantum wells are calculated numerically. It is found that the multi-mode effect of the mixed crystals changes greatly the property of the IO phonons in the quantum wells.

1/f NOISES OF HOMOPOLAR AND HETEROPOLAR SEMICONDUCTORS

The low-frequency noise spectral density with the 1/f spectrum for homopolar and heteropolar semi-conductors is theoretically obtained taking into account conduction electron–optical phonon interactions. The analytical expressions of the spectral density and Hooge's α H parameter are presented. The analytical temperature dependence of Hooge's parameter is compared with experimental data for n-Si and n-GaAs.

Calculation of spontaneous emission and gain spectra for quantum cascade lasers

In this paper, a quantum cascade laser has been treated as a three-level system, and the calculation of the spontaneous emission and gain spectra has been given. In the calculation, the conduction band nonparabolicity and the injection and exit of electrons have been considered. Results have shown that with increasing injection current, the spontaneous emission peak blue shifts, and the peak intensity increases near linearly with current. With increasing temperatures, the broadening of the spontaneous emission spectra has been attributed to the electron-optical phonon interactions. The peak gain of the stimulated emission has been shown to be determined mainly by the subband lifespans. We have pointed out that it is essential to obtain a long lifespan for the second excited state and short lifespan for the first excited state in order to obtain efficient population inversion and high peak gain for quantum cascade lasers.

Screening effects in the electron–optical-phonon interaction

We show that recently reported unusual hardening of optical phonons renormalized by the electron-phonon interaction is due to the neglect of screening effects. When the electron-ion interaction is properly screened optical phonons soften in three dimension. It is important that for short-wavelength optical phonons screening is static while for long-wavelength optical phonons screening is dynamic. In two-dimensional and one-dimensional cases due to crossing of the nonperturbed optical mode with gapless plasmons the spectrum of renormalized optical phonon-plasmon mode shows split momentum dependence.

The effect of electron-phonon interaction on the impurity binding energy in a quantum well

The effect of confined electron-optical phonon interaction on the hydrogenic impurity binding energy in a quantum well is studied. By using the Landau and Pekar variational method, the Hamiltonian is separated into two parts which contain phonon variables and electron variables. A perturbative-variational technique is then employed to construct the trial wave function for the electron part. The effects of electron-optical phonon interaction on the binding energies are calculated as functions of the well width. In the study of the polaron effects on the impurity binding energies in a quantum well, both confined bulk optical phonon and surface optical phonon effects are taken into account. It is found that the surface phonon effect prevails over the bulk phonon effect when the well width is small. The polaron effects on lower-lying excited states are also calculated. The line strengths of transitions from the ground state to lower-lying excited states are calculated as functions of the donor position and the well width.

Electron–optical-phonon interaction in quantum wells consisting of mixed crystals

A theory of electron--optical-phonon interactions in quantum wells consisting of mixed crystals is reported. According to whether the well or the barrier is made of mixed crystals, quantum wells consisting of mixed crystals are divided into three types. The expressions of phonon frequencies of all phonon modes and the electron--LO-phonon interaction Hamiltonian of all the types of the quantum wells are obtained based on the dielectric continuum approach. As applications of the present theory, the concentration dependences and the dispersion relations of the interface-optical-phonon frequencies in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As},$ ${\mathrm{Zn}}_{x}{\mathrm{Cd}}_{1\ensuremath{-}x}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e},$ and ${\mathrm{ZnS}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}/\mathrm{ZnS}$ quantum wells are calculated. The interaction energies of an electron in the lowest subband state with every optical phonon mode are also calculated. The features of the polaronic effect in quantum wells consisting of mixed crystals are discussed.

Band-gap shift in heavily dopedn-typeAl0.3Ga0.7Asalloys

The band-gap shift of heavily Si-doped Al0.3Ga0.7As alloys has been investigated theoretically and experimentally at low temperature. The calculations are carried out within a framework of the random phase approximation and the electron-electron, electron-optical phonon, and electron-ion interactions have been taken into consideration. The experimental data have been obtained with photoluminescence and photoluminescence excitation spectroscopy. Theoretical and experimental results fall closely together in a wide range of impurity concentration. @S0163-1829~99!02928-8#

Electron-optical phonon interaction and in-plane mobility in lead chalcogenide quantum-well structures

The in-plane electron mobility limited by scattering from optical phonons in lead chalcogenide (PbSnTe-type) quantum-well structures is studied theoretically. It is found that the mobility decreases drastically for quantum-well widths smaller than about 200 \AA{}. The scattering from the interface phonons is responsible for that. The nonparabolicity considerably reduces the mobility. Influence of the nonparabolicity on the mobility essentially depends upon the growth axis of the quantum-well structure. It is found that the nonparabolicity in quantum-well systems can be successfully represented by two following effects: (1) dependence of the in-plane electron effective mass upon the in-plane electron wave vector (subband nonparabolicity), and (2) dependence of the in-plane electron effective mass upon the subband number and quantum-well width (in-plane mass quantization). It is shown that, if the quantum-well width exceeds 250 \AA{}, influence of the nonparabolicity on the mobility is actually represented by the subband nonparabolicity alone, otherwise the in-plane mass quantization effect arises. The mobility of a many-subband system is found to be limited mainly by the intrasubband scattering. The mobility of electrons in a higher subband is smaller than that in the first one, which is due to the larger in-plane effective mass and stronger scattering rates in the higher subband. The mobility is essentially affected by electrons in a higher subband, while the gap between this subband and the ground one does not exceed ${k}_{B}T$. The electrophonon resonance effect is also considered.

Effect of electron–phonon interaction on surface states of polar crystals

A variational theory is proposed to study the problems of surface states of electrons in a semi-infinite polar crystal, by including the effects of electron–optical phonon interaction. The electronic surface-state energy is calculated. The numerical computations are performed for the surface-state energies and the average penetrating depths of the surface-state wave functions of the electrons in several I–VII, II–VI and III–V compound crystals. The results show that the electron–phonon interaction lowers the surface-state energy level and cannot be neglected. It is also found that the effect of electron–surface optical (SO) phonon interaction for surface-state levels is much bigger than the effect of electron–bulk longitudinal optical (LO) phonon interaction. It is indicated that the LO-phonon effect is unimportant.

Effects of interface phonon scattering in three-interface heterostructures

A detailed study of the electron–optical–phonon interaction in an asymmetric one-well/one-barrier heterostructure is given. Dispersion relations and phonon potential distributions for interface phonon modes are derived in the framework of the macroscopic dielectric continuum model. It is found that for intrawell relaxation processes the sum of the scattering rates by all interface polar-optical phonon modes is approximately independent of the width of the barrier layer. Consequently, a simplified Hamiltonian for electron–phonon interaction in a single quantum well can be used for scattering rate calculation in multiple heterointerface structures. The combined scattering rates by interface and confined phonon modes are compared with the results obtained in an idealized model using the bulklike phonon spectrum. The practical invalidity of the latter approximation is shown for electron kinetic energies comparable with the typical energy of optical phonons in the heterostructure.

Self-Consistent Approximation for Electron–Optical-Phonon Interaction in a Quantum Wire

Self-Consistent Approximation for Electron–Optical-Phonon Interaction in a Quantum Wire

Plasmon-LO-phonon coupling effects on the drag rate in double quantum-well systems

We study the Coulomb drag rate for electrons in a double-quantum-well structure taking into account the electron-optical phonon interactions. The full wave vector and frequency dependent random-phase approximation (RPA) at finite temperature is employed to describe the effective interlayer Coulomb interaction. The electron-electron and electron-optical phonon couplings are treated on an equal footing. The electron-phonon mediated interaction contribution is investigated for different layer separations and layer densities. We find that the drag rate at high temperatures (i.e., T ≥ 0.3 E F ) is dominated by the coupled plasmon-phonon of the system. Including the local-field effects in an approximate way we estimate the importance of intralayer correlations to be significant.

Coupled plasmon-phonon mode effects on the Coulomb drag in double-quantum-well systems

We study the Coulomb drag rate for electrons in a double-quantum-well structure taking into account the electron-optical phonon interactions. The full wave vector and frequency dependent random-phase approximation (RPA) at finite temperature is employed to describe the effective interlayer Coulomb interaction. The electron-electron and electron-optical phonon couplings are treated on an equal footing. The electron-phonon mediated interaction contribution is investigated for different layer separations and layer densities. We find that the drag rate at high temperatures (i.e., $T>~{0.2E}_{F})$ is dominated by the coupled plasmon-phonon modes of the system. The peak position of the drag rate is shifted to the low temperatures with a slight increase in magnitude, compared to the uncoupled system results in RPA. This behavior is in qualitative agreement with the recent measurements. Including the local-field effects in an approximate way we also estimate the contribution of intralayer correlations.

Electron–Interface–Phonon Interaction and Magnetopolaronic Impurity Transitions in Quantum Wells

The polaronic effect on the hydrogenic 1s–2p+ transition energy of a donor impurity located at the quantum well center in a double heterostructure is studied theoretically in detail. The electron–optical–phonon interaction Hamiltonian is derived on the basis of eigenmodes of lattice vibrations supported by the double heterostructure. Both the confined and interface phonon modes are included in the electron–phonon coupling. The transition energy is calculated as a function of the applied magnetic field for GaAs/Al 1-x Ga x As samples of well -widths d=125 Å, 210 Å and 450 Å by the second-order perturbation. Wide transition gaps are predicted around the two-level and three-level resonances for all three cases. It is found that the transition gap narrows with the increasing well-width but remains larger than the LO and TO phonon frequency difference for d=450 Å as is observed. We also perform the same calculation by assuming that the confined electron interacts with three-dimensional and two-dimensional phonon modes. The transition energy spectra from these calculations appear to be similar to those for a bulk sample, the spectrum splits at the resonance with the longitudinal optical phonon frequency only. From comparisons of our results with these calculations as well as with experiments, it is conclusively established that the wide gap of transition energy is solely due to the interface modes.