Study of electron-phonon interaction in quantum wells using optically excited nonequilibrium population of phonons

It is well known that nonequilibrium populations of phonons can be generated in bulk semiconductors by the relaxation of photoexcited hot electrons. In this paper we demonstrate that nonequilibrium phonons can also be generated in quantum wells by picosecond lasers and then probed by time-resolved Raman scattering. Using this method we have studied the interaction between hot electrons and interface and confined LO phonons in GaAs/AlAs quantum wells for well widths varying between 2 to 6 nm. The results are compared quantitatively with three macroscopic models of electron-LO phonon interaction. Only the model proposed by Huang and Zhu agrees both qualitatively and quantitatively with experimental results.     

Photoluminescence and exciton resonances over the scattered light in multiphonon spectra of resonant scattering in the CdTe/ZnTe superlattices with narrow quantum wells

Photoluminescence and Raman scattering spectra in CdTe/ZnTe heterostructures and superlattices with narrow quantum wells (4.8–9.2 Å) in a temperature range of 5–300 K have been measured. The temperature dependences of the intensity of exciton luminescence in isolated quantum wells have been studied, and the thermal activation energies associated with the effective barriers for electrons and holes have been determined. In CdTe/ZnTe heterostructures, the binding energies of an exciton with a heavy hole have been determined as functions of the quantum well width. The multiphonon Raman spectra that exhibit distinctive features, such as the weak intensity of nLO phonon lines of ZnTe (n < 8), the absence of their dependence on the number n (n > 2), and the multiple participation in scattering of acoustic LA phonons with large wave vector, have been investigated. The results have been explained based on the concept of the relaxation of hot excitons over the exciton band.     

Formation of excitons from free electron–hole pairs due to acoustic phonon interactions in quantum wells

A study of the process of exciton formation due to acoustic phonon interaction in quantum wells (QWs) is presented. Considering that excitons are formed from photoexcited free electron–hole pairs, we have derived the rate of such formation as a function of density and temperature of charge carriers and wavevectorK_|| of the center-of-mass motion of exciton, and finally applied our theory to GaAs/AlGaAs QWs. We have found that the formation of an exciton due to acoustic phonon emission is more efficient at relatively large values ofK_|| (hot excitons) whereas that due to longitudinal optical (LO) phonon emission is more efficient at relatively small values of K_||.     

Electron-phonon scattering in asymmetric semiconductor quantum-well structures

We calculate scattering rates of intrasubband and intersubband electronic transitions in asymmetric single quantum wells (QW's) and step QW's due to interface phonons, confined bulk-like LO phonons, and half-space LO phonons. The relative importance of the different phonon modes is analyzed. The results show that the electron-phonon scattering rates have intimate relation to the QW parameters.     

Terahertz acoustic-phonon signal generated by heated electrons in AlGaAs/GaAs-based quantum wires

In this paper, we explore theoretically the possibility of applying AlGaAs/GaAs-based quantum wire systems as a terahertz (THz) ultrasonic generator. For structures such as Al_xGa_1-xAs/GaAs-based low-dimensional semiconductor systems and semiconductor nanostructures, electrons are confined within the nanometer distance scale so that energies (e.g. electronic subband energy, electron kinetic energy, Fermi energy, etc.) are in the meV scale, which consequently results in the acoustic-phonons generated by heated electrons from these novel systems to be around the THz frequency range. Our theoretical results indicate that: (i) Al_xGa_-xAs/GaAs-based quantum wires are suitable for generating THz acoustic-phonon signals; (ii) both longitudinal and transverse acoustic-phonon modes contribute to the detected phonon signals; (iii) the THz ultrasound wave can be generated through both intra- and inter-subband scattering processes; and (iv) the strong dependence of the acoustic-phonon emission from a quantum wire on phonon frequency and phonon emission angle can be observed.     

Size dependent effective band gap,optical absorption coefficients and refractive index changes in a wide ZnS/Zn1−xMgxS strained quantum well

Exciton binding energy of a confined heavy hole exciton is investigated in a Zn_1−xMg_xS/ZnS/Zn_1−xMg_xS single strained quantum well with the inclusion of size dependent dielectric function for various Mg content. The effects of interaction between the exciton and the longitudinal optical phonon are brought out. The effect of exciton is described by the effective potential between the electron and hole. The interband emission energy as a function of well width is calculated for various Mg concentration with and without the inclusion of dielectric confinement. Non-linear optical properties are carried out using the compact density matrix approach. The dependence of nonlinear optical processes on the well width is investigated for different Mg concentration. The linear, third order non-linear optical absorption coefficients values and the refractive index changes of the exciton are calculated for different concentration of magnesium content. The results show that the exciton binding energy is found to exceed LO phonon energy of ZnS for x>0.2 and the incorporation of magnesium ions and the effect of phonon have great influence on the optical properties of ZnS/Zn_1−xMg_xS quantum wells.     

Calculation of the dark current in a quantum well infrared photodetector

Dark currents in a biased quantum well fabricated using Al_0.27Ga_0.73As/GaAs heterojunctions are calculated at two different temperatures including thermionic field emission currents arising from the electron scattering with phonons and plasmons. In the electron–phonon scattering process several modes due to heterojunctions such as the confined, half-space and interface longitudinal optic phonons are taken into account. It is found that the confined phonon scattering process results in maximum currents compared to those obtained in the half-space and interface scattering modes. However, the magnitude of the currents that resulted from the electron–plasmon scattering process is found to be higher than that found from the electron scattering with confined phonons. Comparison of the calculated dark currents with experiments shows that the thermionic emission currents due to phonon and plasmon assisted processes are essential to get better agreement with experiments than the previously employed bulk phonon scattering process.     

Cyclotron resonance study of conduction electrons in n-type indium antimonide under a strong magnetic field—II: Hot electrons

A system of hot electrons in the n-InSb under the application of a strong magnetic field has been studied by far IR cyclotron resonance. A three band model and an energy independent scattering time were assumed in analyzing the line shape variation with electric field applied either parallel or perpendicular to the magnetic field. Two kinds of electron temperature, inter- and intra-subband, were introduced to describe the electron distribution in energy space. The electron distribution function was found to deviate from an essentially Maxwellian form in the manner predicted by Yamada and Kurosawa. A remarkable difference exists between the two geometries: E∥H and E⊥U. A brief survey of cyclotron emission, and the reverse process of hot electron cyclotron absorption, is summarized at the end as an addendum.     

Two-dimensional electronic transport in In0.53Ga0.47As quantum Wells

Transport properties of the electrons itinerant two dimensionality in a square quantum well of In_0.53Ga_0.47As are studied in the framework of Fermi-Dirac statistics including the relevant scattering mechanisms. An iterative solution of the Boltzmann equation shows that the ohmic mobility is controlled by LO phonon scattering at room temperature, but below 130 K alloy scattering is predominant. The calculated mobilities with a suitable value of the alloy scattering potential agree with the experimental results over a range of lattice temperature. For lattice temperatures below 25 K where the carrier energy loss is governed by the deformation potential acoustic scattering, the warm electron coefficient is found to be negative. Its magnitude decreases with increasing lattice temperature and is greater for larger channel widths. Values of the small-signal AC mobility of hot electrons at a lattice temperature of 4.2 K are obtained for different sheet carrier densities and channel widths. Cut-off frequencies around 100 GHz are indicated.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

(CdSe)1(ZnSe)3/ZnSe短周期超晶格多量子阱的共振Ramam谱

在(CdSe)₁(ZnSe)₃/ZnSe短周期超晶格多量子阱中,根据一维线性链模型计算的结果与实验结果的比较表明,我们在不同的共振条件下分别观察到了来自多量子阱的阱中和垒中ZnSe限制纵光学声子模的Raman散射。与GaAs/AlAs量子阱的偏振选择定则不同,在共振条件下,我们在两种偏振配置下都观察到了阱中ZnSe限制模LO₁,并认为这种不同可能来源于样品特殊的电子子带结构和光学声子行为。 关键词：     

Hot electron energy and momentum relaxation in GaAs/AlAs and GaAs/Ga1-xAlxAs multiple quantum wells

Experimental results on high electric field longitudinal transport in GaAs/AlAs and GaAs/Ga_1-xAl_xAs multiple quantum wells (MQW) are presented and compared with the prediction of a dielectric continuum model. We draw from our experiments the following four conclusions.(i) In GaAs/Ga_1-xAl_xAs systems the dominant energy and momentum relaxation mechanism is through scattering with GaAs -modes.(ii) However, in GaAs/AlAs systems the AlAs interface mode is dominant in relaxing the energy and momentum of the quantum well electrons.(iii) The hot electron momentum relaxation as obtained from the high-field drift velocity experiments is strongly affected by the production of hot phonons as expected from a model involving a non-drifting hot phonon distribution.(iv) The importance of the AlAs interface mode in GaAs/Ga_1-xAl_xAs MQW is not the result of the intrinsic scattering rate but related to its shorter lifetime, compared to GaAs modes.     

Raman studies of InAs/In0.53Ga0.47As single quantum wells grown on InP substrate by M.B.E.

We analyzed using the Raman technique a series of single quantum wells of InAs/In _0.53Ga_0.47As at different thicknesses of InAs layer grown on a (100) InP substrate by MBE. These high lattice mismatch systems are particularly interesting for potential applications in the mid-IR wavelength range. The well thickness was between 6 and 12 monolayers. The In _0.53 Ga_0.47As grown on an InAs layer is subject to a tensile biaxial strain and the InAs to a compressive one. In the Raman spectra we observed an intense narrow line corresponding to the LO phonon of the InAs layer between a GaAs-like LO mode and a smaller InAs-like LO phonon typical of In_0.53Ga_0.47As. With the increase of the well thickness the experimental energy shift of the LO phonon of the InAs layer decreases, indicating a smaller strain, whereas the GaAs-like LO phonon of the alloy remains constant and the intensity ratio of these two modes becames smaller. The dominant and sharp features of the InAs LO and GaAs-like LO characterize the good quality of our structures. With the increase of the InAs layer thickness we also observed the appearance and the intensity rise of a weak peak around the frequency of the InAs TO mode. This peak could be associated with the TO mode that is forbidden in our scattering geometry. We believe that this is indicative of a slight deterioration of the structural perfection of the sample with the increase of the well thickness. To our knowledge, this is the first study of vibrational properties of InAs/In _0.53 Ga_0.47As single quantum wells grown on InP substrates.     

Luminescence from hot electrons relaxing by LO phonon emission in p-GaAs and GaAs doping superlattices

Laser excited hot electrons in GaAs relax by LO phonon emission within a few hundred femtoseconds, leading to a series of peaks in the distribution of hot electrons in the conduction band, which we observe in luminescence. We find that the luminescence peaks shift according to the acceptor binding energy for C?, Ge?, Zn?, and Be-p-doped GaAs layers grown by MBE and LPE. Thus we prove that recombination is between hot electrons and neutral acceptors. The series of peaks due to electrons from the heavy hole band agree well with $\text{k}\text{.}\text{p}$ band structure, while peaks due to those from light holes are about 15 meV lower than expected from the band structure. We show that the discrepancy is not due to heating or surface fields. The peak separation in the luminescence ladder is about 6% larger than the LO energy suggesting emission of renormalised LO phonons. We find thermalisation by LO emission also in GaAs nipi doping superlattices. In nipi crystals the emission is shifted to higher energies (by 12 meV for light and by 6 meV for heavy holes) due to a change in band structure caused by the space charge fields.     

Relaxation processes and mobility of electrons in a semiconductor quantum well with the modified Pöschl-teller confining potential

Relaxation processes and mobility of electrons in a semiconductor quantum well are studied. The modified Pöschl-Teller potential is used as a confining potential. Scattering rates due to impurity ions, acoustic and piezoacoustic phonons are calculated taking into account the screening of scattering potentials by charge carriers. It is shown that when degenerate electrons are scattered by acoustic phonons, the dependence of scattering rate on electron wave number ν_ac(k) is almost linear. At small k, the acoustic phonon piezoelectric scattering rate of degenerate electrons increases with k, and then it decreases slightly when k > 8 × 10⁷ m⁻¹. The ionized impurity scattering rate of degenerate electrons does not depend on temperature, is directly proportional to the electron density, and decreases with increasing k. Dependences of electron mobility on surface ion density and temperature are studied. It is shown that in the case of non-degenerate or slightly degenerate electron gas, a maximum appears in the temperature dependence of the mobility, and the screening effect is negligible. The screening significantly increases the mobility of electrons in the case of high degeneration. Obtained results are applied to GaAs-based quantum wells.     

Pressure effect on the electron mobility in AlAs/GaAs quantum wells

By taking the influence of optical phonon modes into account, this paper adopts the dielectric continuum phonon model and force balance equation to investigate the electronic mobility parallel to the interfaces for AlAs/GaAs semiconductor quantum wells (QWs) under hydrostatic pressure. The scattering from confined phonon modes, interface phonon modes and half-space phonon modes are analysed and the dominant scattering mechanisms in wide and narrow QWs are presented. The temperature dependence of the electronic mobility is also studied in the temperature range of optical phonon scattering being available. It is shown that the electronic mobility reduces obviously as pressure increases from 0 to 4GPa, the confined longitudinal optical (LO) phonon modes play an important role in wide QWs, whereas the interface optical phonon modes are dominant in narrow QWs, the half-space LO phonon modes hardly influence the electronic mobility expect for very narrow QWs.     

Magnetopolaron states involving confined phonons in a semiconductor quantum well

Energy splitting ΔE _res in double magnetopolaron energy spectrum in rectangular quantum wells as functions of the well width d have been calculated. We have considered in the capacity of interaction leading to resonant coupling between electrons and phonons the interaction with confined phonons and (for comparison) with bulk LO phonons. We have obtained the conditions when the interaction with bulk phonons yields correct results. Calculations for AlAs/GaAs/AlAs and AlSb/InSb/AlSb structures have been performed. Alongside the parameter ΔE _res for a polaron, whose resonant magnetic field is determined by the condition Ω=ω _L1, where Ω is the cyclotron frequency and ω _L1 is the LO phonon frequency in the quantum well (A-polaron), we have calculated ΔE _res for D-(Ω=2ω _L1) and F-polarons (Ω=3ω _L1), which is a factor of $\sqrt 2 $ and $\sqrt 3 $ , respectively, smaller than ΔE _res for the A-polaron. Since the splitting ΔE _res for the A-polaron is very large (up to 0.2?ω _L1), it is more convenient to study in experiments D-and F-polarons since their resonant magnetic fields are lower. We have predicted existence of “weak” magnetopolarons, in which the splitting is proportional to a higher power of Frölich’s coupling constant α than α ^1/2.     

Intersubband relaxation of hot carriers in quantum well systems

We use an ensemble Monte Carlo simulation of coupled electrons, holes and nonequilibrium polar optical phonons in multiple quantum well systems to model the intersubband relaxation of hot carriers measured in ultra-fast optical experiments. We have investigated the effect of various models of confined photon modes on the energy relaxation and intersubband transition rate in single quantum well and coupled well systems. In particular, the symmetry of the atomic displacement with respect to the quantum well has a marked effect on the relative intersubband versus intrasubband scattering rates, depending on whether one considers electrostatic boundary conditions(slab modes) or mechanical boundary conditions(guided modes). In single quantum wells systems, the overall intersubband relaxation time is not found to be strongly dependent on the confined mode model used due to competing effects of hot phonons and the relative intrasubband scattering rates. For coupled well systems, the relaxation rate is much more dependent on the exact nature of the phonon amplitude. Large effects are found associated with localized AlAs interface modes which dominate the intersubband relaxation time.     

Γ to Xzelectron transfer times in type-II GaAs/AlAs superlattices due to emission of confined and interface phonons

We calculate the Γ →  X_zelectron transfer times due to the emission of confined and interface LO phonons in type-II GaAs–AlAs and AlGaAs–AlAs superlattices. A dielectric continuum model is employed to describe the electron–phonon interaction, and the electron envelope wavefunctions are obtained from a Kronig–Penney model. The calculated transfer times are in good agreement with available experimental results. We have used two different sets of AlAs X-valley effective masses obtained from different experiments and we show that the transfer times calculated with the heavier masses are in closer agreement with the measured data.     

Phonon deformation potentials of CdTe

Abstract

Single and multiple quantum wells (QWs) of CdTe/Cd_1?xZn_xTe, of different composition (x), grown on a Cd_1?xZn_xTe (001) substrate are studied by Raman spectroscopy at low temperature, under resonance conditions. Two of the three phonon deformation potentials (PDPs) of the LO phonon of CdTe are calculated using the observed phonon frequency shifts due to a lattice-misfit bisotropic strain.     

Modifying electron-phonon scattering rates in semiconductor quantum wells with thin AlAs layers

We have studied theoretically the electron-phonon scattering rates in GaAs/AlAs quantum wells which have additional thin AlAs layers in them using the dielectric continuum approach for the phonons. The confined and interface phonon modes and the intersubband electron phonon scattering rates of these structures have been calculated. The system with an additional AlAs layer is found to have intersubband electron scattering rates which are increased modestly as compared to those for the corresponding quantum well. These results show that scattering rates in general are expected to depend only weakly on the effects of system structure on the optical phonon spectra.