Coherent electron tunneling in triple coupled quantum wells

We present a numerical study of coherent electron tunneling in a triple coupled quantum well system. The evolution of the charge density is described in detail. In this heterostructure the electron will oscillate between the three wells, giving rise to a new type of oscillating luminescence signal. An experiment is proposed.     

Magnetocapacitance spectroscopy of multilayer electron systems in wide parabolic wells

We present a technique to detect quantum capacitance in the presence of strong density profile effects. We demonstrate our technique by performing capacitance-voltage subband spectroscopy on a system of coupled electron layers in a heterostructure which is the superposition of a wide parabolic well and multiple square wells. Events inducing Fermi level variations are detected as features in the derivative profile dC/dV_g as the electron gas width is reduced by an applied gate voltage. Quantum features are differentiated from density profile effects by considering differences in derivative profiles dC/dV_g for increasing magnetic field perpendicular to the electron layers. Results are compared with theory.     

Multicomponent Electron–Hole Liquid in Si/SiGe Quantum Wells

The density functional theory is used to calculate the energy of an electron–hole liquid in Si/Si_1–xGe_x/Si quantum wells. Three one-dimensional nonlinear Schrödinger equations for electrons and light and heavy holes are solved numerically. It is shown that, in shallow quantum wells (small x), both light and heavy holes exist in the electron–hole liquid. Upon an increase in the Ge content, a transition to a state with one type of holes occurs, with the equilibrium density of electron–hole pairs decreasing by more than a factor of 2.     

Quantized ν = 5/2 state in a two-subband quantum hall system

The evolution of the fractional quantum Hall state at filling 5/2 is studied in density tunable two-dimensional electron systems formed in wide wells in which it is possible to induce a transition from single- to two-subband occupancy. In 80 and 60 nm wells, the quantum Hall state at 5/2 filling of the lowest subband is observed even when the second subband is occupied. In a 50 nm well, the 5/2 state vanishes upon second subband population. We attribute this distinct behavior to the width dependence of the capacitive energy for intersubband charge transfer and of the overlap of the subband probability densities.     

Optical pumping of nuclear spin magnetization in GaAs/AlAs quantum wells of variable electron density

We report the use of time-resolved Faraday rotation to induce and probe the polarization of nuclear spins within a set of quantum wells with varying background electron density. The electron density was controlled over a broad range by making use of structures of mixed type-I/type-II GaAs/AlAs quantum wells that spatially separate photoexcited electron–hole pairs. We find that the optically detected nuclear magnetic field decreases quasi-monotonically with increasing electron density. The likely factors responsible for this behavior are increased electron spin-lattice relaxation, increased electron spin delocalization, and dilution of the electron spin polarization.     

Internal transitions of negatively charged magnetoexcitons and many body effects in a two-dimensional electron gas

Internal transitions of quasi-two-dimensional, negatively charged magnetoexcitons ( X-) and their evolution with excess electron density have been studied in GaAs/AlGaAs quantum wells. In the dilute electron limit, due to magnetic translational invariance, the optically detected resonance spectra are dominated by bound-to-continuum bands in contrast to the negatively charged donor system D-, which exhibits strictly bound-to-bound transitions. With increasing excess electron density Landau-level filling factors nu<2 the X--like transitions are blueshifted; they are absent for nu>2. The blueshifted transitions are explained in terms of a new type of collective excitation---magnetoplasmons bound to a mobile valence band hole.     

Magnetoresistance of a two-dimensional electron gas in a parallel magnetic field

The conductivity of a two-dimensional electron gas in a parallel magnetic field is calculated. We take into account the magnetic-field-induced spin-splitting, which changes the density of states, the Fermi momentum, and the screening behavior of the electron gas. For impurity scattering, we predict a positive magnetoresistance for low electron density and a negative magnetoresistance for high electron density. The theory is in qualitative agreement with recent experimental results found for Si inversion layers and Si quantum wells.     

Spin-orbit interaction in coupled quantum wells

We theoretically investigate the spin-orbit interaction in GaAs/AlxGa1 x As coupled quantum wells. We consider the contribution of the interface-related Rashba term as well as the linear and cubic Dresselhaus terms to the spin splitting. For the coupled quantum wells which bear an inherent structure inversion asymmetry, the same probability density distribution of electrons in the two step quantum wells results in a large spin splitting from the interface term. If the widths of the two step quantum wells are different, the electron probability density in the wider step quantum well is considerably higher than that in the narrower one, resulting in the decrease of the spin splitting from the interface term. The results also show that the spin splitting of the coupled quantum well is not significantly larger than that of a step quantum well.     

Temperature dependent symmetry to asymmetry transition in wide quantum wells

Quasi-two dimensional electron systems exhibit peculiar transport effects depending on their density profiles and temperature. A usual two dimensional electron system is assumed to have a δ like density distribution along the crystal growth direction. However, once the confining quantum well is sufficiently large, this situation is changed and the density can no longer be assumed as a δ function. In addition, it is known that the density profile is not a single peaked function, instead can present more than one maxima, depending on the well width. In this work, the electron density distributions in the growth direction considering a variety of wide quantum wells are investigated as a function of temperature. We show that the double peak in the density profile varies from symmetric (similar peak height) to asymmetric while changing the temperature for particular growth parameters. The alternation from symmetric to asymmetric density profiles is known to exhibit intriguing phase transitions and is decisive in defining the properties of the ground state wavefunction in the presence of an external magnetic field, i.e from insulating phases to even denominator fractional quantum Hall states. Here, by solving the temperature and material dependent Schrödinger and Poisson equations self-consistently, we found that such a phase transition may be elaborated by taking into account direct Coulomb interactions together with temperature.     

Magnetoelectric Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> and relativity theory

Based on the dielectric continuum phonon model, uniaxialmodel and force balance equation the mobility of two dimensional electrongas in wurtzite Al_xGa_1-xN/GaN/Al_xGa_1-xN quantum wells isdiscussed theoretically within the temperature range dominated by opticalphonons. The dependences of the electron mobility on temperature, Al molarfraction and electron sheet density are presented including hydrostaticpressure effect. The built-in electric field is also taken into account. Itis found that under normal pressure the main contribution to the mobility isfrom the scattering of interface optical phonons in narrow (for well widthd < 12 Å) and wide (for d > 117 Å and d > 65 Å for finitelythick barriers and infinitely thick ones, respectively) wells, whereas thatis from the scattering of confined optical phonons in a well with anintermediate width. It is shown that the electron mobility decreases withincreasing Al molar fraction and temperature, whereas increases obviouslywith increasing electron sheet density. The theoretical calculated electronmobility is 978 cm²/V?s which is higher than an available experimentaldata 875 cm²/V?s when x equals to 0.58 at room temperature. Theresults under hydrostatic pressure considering the modification of strainindicate that the mobility increases slightly as hydrostatic pressureincreases from 0 to 10 GPa.     

Photon-exchange energy transfer of an electron–hole plasma between quasi-two-dimensional semiconductor layers

Photon-mediated energy transfer is shown to play an important role for transfer of an electron–hole plasma between two quasi-two-dimensional quantum wells separated by a wide barrier. The magnitude and the dependence of the transfer rate of an electron–hole plasma on the temperature, the well-to-well distance, and the plasma density are compared with those of the standard Förster (i.e., dipolar) rate and also with the exciton transfer rate. The plasma transfer rate through the photon-exchange mechanism decays very slowly as a function of the well-to-well distance and is larger than the dipolar rate except for short distances. The transfer rate of plasmas saturates at high densities and decays rapidly with the temperature.     

Spontaneous spin polarization in doped semiconductor quantum wells

We calculate the critical density of the zero-temperature, first-order ferromagnetic phase transition in n-doped GaAs/AlGaAs quantum wells. We predict that this transition could be observed in narrow quantum wells at electron densities somewhat lower than the ones that have been considered experimentally thus far, and that there exists an upper limit for the well width beyond which there would be no transition as long as only one subband is populated. Our calculations are done within a screened Hartree-Fock approximation with a polarization-dependent effective mass, which is adjusted to match the critical density predicted by Monte Carlo calculations for the strictly two-dimensional electron gas.     

Remotely-doped graded potential well structures

We present a new method to obtain high-mobility three-dimensional electron gas systems. We have achieved control of carrier density and of carrier profile by growth of the first remotely-doped parabolic potential well structures. Computer-controlled molecular beam epitaxy is used to grow a layer of ultra-fine superlattices with a programmable composition gradient. This produces conduction-band potentials which, in the absence of doping, are equivalent to the potential profiles of fixed charge distributions. When conduction electrons are introduced into these graded wells through remote doping of the barrier regions, they distribute themselves in such a way as to produce a uniform chemical potential at thermal equilibrium. We illustrate through computer simulations employing Fermi statistics that electrons introduced into a wide parabolic potential well distribute themselves uniformly. More significantly, the carrier distribution in the well is remarkably insensitive to the dopant sheet charge in the barrier, the more so at lower temperatures. We have fabricated remotely-doped graded potential well structures of the proposed type by molecular beam epitaxy. These structures exhibit the above effects. Measured mobilities of such three-dimensional electron gases grown using the GaAs/Al_xGa_1−xAs system are higher than those of bulk-doped GaAs doped to give the same uniform electron concentration.     

Effect of ionized impurities on electron tunneling in GaAs/AlGaAs triple quantum wells

Photoluminescence and electroreflectance measurements in Si δ-doped GaAs/Al_0.35Ga_0.65-A triple quantum wells are performed in order to study the effect of ionized impurities on electron tunneling. It is theoretically shown that a thin quantum well with a δ-doping layer inserted between narrow and wide quantum wells of asymmetric double quantum wells enhances impurity scattering rate significantly. Photoluminescence decay time is found to decrease 30% at maximum compared with a sample without a δ-doping layer.     

The effect of exchange interaction on the energy spectrum of electrons in doped intentionally disordered superlattices

The electron density distribution is calculated for a doped superlattice with controlled vertical disorder caused by fluctuations of the layer thicknesses (quantum well widths) in the growth direction. At low temperatures, the exchange interaction leads to an increase in the scatter of quantum confinement levels and the formation of a soft gap in the electron density distribution over quantum wells of the superlattice.     

Staggered excitonic resonances of Stark-ladder transitions in an asymmetric double-well GaAs/AlAs superlattice

Excitonic effects on Stark-ladder transitions have been investigated experimentally and theoretically in a novel asymmetric double-well superlattice consisting of wide and narrow GaAs quantum wells separated by a constant AlAs barrier. In this superlattice strong electron resonance can occur under the applied electric field between the wide and narrow wells. It is found that due to existence of the two different heavy-hole localized states two types of excitonic resonances which are staggered in field are observed in the low-temperature photocurrent spectra. This field difference in the staggered exciton resonances is rigorously explained by variational calculations of the changes in the direct and indirect exciton binding energies with the field.     

Calculated effects of interface grading in GaAs----Ga1−xAlxAs quantum wells

Effects of interface grading on energy levels of electrons in GaAs---Ga_1−xAl_xAs quantum wells have been estimated using both a tight-binding formalism and an effective-mass Hamiltonian of the BenDaniel-Duke form. Graded interfaces a few atomic layers thich have only a small effect on energy levels in both schemes. Self-consistent calculations for electrons in a relatively wide (40 nm) quantum well show how the lowest levels change from those characteristic of the empty well to those characteristic of two weakly coupled heterojunctions as the electron density is increased.     

无限深势阱问题的可视化研究

基于计算软件对一些常见的无限深势阱进行了可视化研究,并设计了GUI界面实现对势阱的选择.在选定势阱后,设置好相应的势阱参数和量子数,便可依次绘制出低维势阱的波函数、概率密度函数和三维势阱中的电子云图像.文中分析了二维和三维势阱中能级的简并度问题,并重点讨论了不同势阱波函数和概率密度函数随量子数的变化规律.将势阱问题可视...     

Single-particle relaxation time of the two-dimensional electron gas in Si/SiGe: Many-body effects

The single-particle relaxation time of the two-dimensional electron gas in SiGe/Si/SiGe quantum wells is calculated. Many-body effects beyond the random-phase approximation become important at low electron density. For charged impurity scattering (remote doped), the importance of these many-body effects as functions of the electron density and spacer width is analyzed. Induced by many-body effects, a strong reduction of the single-particle relaxation time at low electron densities is predicted. The relation with the transport scattering time is described, multiple-scattering effects are commented, and the determination of many-body effects in existing samples is discussed.     

Combined exciton–electron excitation in quantum wells with a two-dimensional electron gas of low density

A combined exciton–cyclotron resonance is found in the photoluminescence excitation and reflectivity spectra of semiconductor quantum wells with an electron gas of low density. In external magnetic fields an incident photon creates an exciton in the ground state and simultaneously excites an electron between Landau levels. A theoretical model is developed and suggests the dominating contribution of the exchange exciton–electron interaction.