Symmetric Quantum Wells Research Articles

Binding energy of donors in symmetric triangular quantum wells

Hydrogen-like donor impurity states in symmetric triangular quantum wells are investigated by using a variational method. Both the effects of the variable effective mass of electrons and the spatially dependent dielectric constant are considered in the calculation. The numerical results whow that the binding energy depends on not only the effective mass and dielectric constant but also the spatial distribution of electron probability density. The binding energies of donor states get the maximums at the wellcenter. The results are also compared with those obtained in parabolic and square wells. It is seen that the triangular well support the highest binding energies for donor states.

Dependence of Optical Matrix Elements on the Boundary Conditions of the Continuum States in Quantum Wells

Unlike for the bound states, several different boundary conditions are used for the continuum states above the barrier in semiconductor quantum wells. We employed three boundary conditions, infinite potential barrier boundary condition, periodic boundary condition and scattering boundary condition, and calculated the local number of states, wavefunctions and optical matrix elements for the symmetric and asymmetric quantum wells. We discussed how these quantities are related in the three boundary conditions. We argue that the scattering boundary condition has several advantages over the other two cases. These results would be useful in understanding quantum well lasers and detectors involving continuum states.

Variational results for electron mobility in modulation-doped In0.53Ga0.47As/InP single symmetric quantum wells

In this paper, the low-field carrier mobility is investigated for quasi-2D electrons in a n-doped In0.53Ga0.47As/InP single symmetric quantum well. An accurate variational scheme is developed in view to determine the subband structure in this lattice-matched heterostructure. In this scheme, the Schrödinger–Poisson coupled equations are solved observing adequate matching conditions at the heterointerfaces, as well as exchange-correlation corrections to the Hartree potential. The results allowed us to compute the main scattering rates. Some interchanges in these scattering rates were found with respect to the limitation of electron mobility by varying the well and the spacer widths.

Electric field effect on the linear and nonlinear intersubband refractive index changes in asymmetrical semiparabolic and symmetrical parabolic quantum wells

By using the displacement harmonic variant method and the compact density matrix approach, the linear and nonlinear intersubband refractive index changes (RICs) in a semiparabolic quantum well (QW) with applied electric field have been investigated in detail. The simple analytical formulae for the linear and nonlinear RICs in the system were also deduced. The symmetrical parabolic QWs with applied electric fields were taken into account for comparison. Numerical calculations on typical GaAs QWs were performed. The dependence of the linear and nonlinear RICs on the incident optical intensity, the frequencies of the confined potential of the QWs and the strength of the applied electric field were discussed. Results reveal that the RICs in the semiparabolic quantum well system sensitively depend on these factors. The calculation also shows that the semiparabolic QW is a more ideal nonlinear optical system relative to the symmetric parabolic QW systems.

Subband structure and band bending in symmetric modulation-doped double quantum wells

We have calculated the subband structure and confinement potential of modulation-doped Ga Al x As-GaAs symmetric double quantum wells a function of the doping concentration. Electronic properties of this structure are determined by solving the Schrodinger and Poisson equations self-consistently. To understand the exchange correlation potential effects on the band bending and subband populations, this potential has been included in the calculation at 0 K and, at room temperature. We find that at low doping concentrations the effect of the exchange correlation potential is more pronounced on the subband populations.

Spectroscopy of Rashba spin splitting in InSb quantum wells

We use electron spin resonance in the far infrared to probe the Landau-level spin splitting in symmetric and asymmetric InSb quantum wells. The asymmetric wells exhibit a strong deviation in behavior from the symmetric wells at low magnetic fields with apparent $g$ factors far in excess of the bulk $g$ factor of InSb. These asymmetry-induced shifts in the spin resonance depend on Landau-level index as predicted by the Bychkov-Rashba model. We analyze the shifts to extract values for the Rashba parameter.

Theory of spin waves in diluted-magnetic-semiconductor quantum wells

We present a theory of collective spin excitations in diluted-magnetic-semiconductor quantum wells in which local magnetic moments are coupled via a quasi-two-dimensional gas of electrons or holes. In the case of a ferromagnetic state with partly spin-polarized electrons, we find that the Goldstone collective mode has anomalous $k^4$ dispersion and that for symmetric quantum wells odd parity modes do not disperse at all. We discuss the gap in the collective excitation spectrum which appears when spin-orbit interactions are included.

Spin splitting in symmetrical SiGe quantum wells

Spin splitting of conduction electron states has been analyzed for all possible point symmetries of SiGe quantum well structures. A particular attention is paid to removal of spin degeneracy caused by the rotoinversion asymmetry of a (001) heterointerface between two diamond-lattice materials. The asymmetry is shown to result in spin splitting of both Rashba and Dresselhaus types in symmetrical SiGe quantum wells. Consequences of the spin splitting on spin relaxation are discussed.

Collective excitations in double quantum wells with strong tunnel coupling

Spectra of collective and magnetic excitations in symmetric double quantum wells with strong tunnel coupling were studied by inelastic scattering. Gaps in the spectrum of collective and single-particle excitations associated with tunnel interwell splitting were measured. New excitation modes, namely, tunnel Bernstein modes, were detected in a magnetic field. Based on the dispersion and magnetic-field dependences of the energies of inelastic light scattering lines, a classification of excitations and magnetic excitations was given.

Nonmagnetic semiconductor spin transistor

We propose a spin transistor using only nonmagnetic materials that exploits the characteristics of bulk inversion asymmetry (BIA) in (110) symmetric quantum wells. We show that extremely large spin splittings due to BIA are possible in (110) InAs/GaSb/AlSb heterostructures, which together with the enhanced spin decay times in (110) quantum wells demonstrates the potential for exploitation of BIA effects in semiconductor spintronics devices. Spin injection and detection is achieved using spin-dependent resonant interband tunneling and spin transistor action is realized through control of the electron spin lifetime in an InAs lateral transport channel using an applied electric field (Rashba effect). This device may also be used as a spin valve, or a magnetic field sensor.

Role of the spin–orbit interaction in elastic scattering of electrons in quantum wells

We present a theoretical study of the spin-dependent electron scattering from screened impurities in III–V semiconductor quantum wells. The effective one band Hamiltonian and the Rashba spin-orbit interaction are used. We calculated the Mott scattering cross-section and the Sherman function for two-dimensional electrons spin-polarized parallel to the z-axis (direction of structure growth). We have found a large spin-dependent asymmetry in the elastic cross-section for electrons scattered from impurities in CdTe/InSb/CdTe symmetrical semiconductor quantum wells. The Sherman function amplitude for repulsive impurities in CdTe/InSb/CdTe quantum wells is predicted to be about 0.01

Fractional-dimensional polaron corrections in asymmetric GaAs-Ga 1− xAl xAs quantum wells

Polaron effects in asymmetric GaAs-Ga 1− x Al x As quantum wells (QWs) are investigated within the framework of the fractional-dimensional space approach and by using second-order perturbation theory. A well-width dependence of the polaron corrections with a dip and a peak is obtained for both symmetric and asymmetric QWs. The dip and the peak occur in the case of asymmetric QWs for larger well widths than in the case of symmetric QWs. An enhancement of the contrast between the dip and the peak of the polaron energy shift is found for the case of asymmetric QWs. These results show the convenience of using asymmetric QWs instead of symmetric ones in any experimental attempt of detecting the dip and the peak of the polaron energy shift.

Spin-selective positioning of wave functions in magnetically tunable symmetric triple quantum wells

Abstract We have experimentally demonstrated the wave functions distribution of the lowest multiplet of states in symmetric triple quantum wells (QWs) using magneto-absorption measurements. In this experiment, we utilized the giant Zeeman splitting of band edges in diluted magnetic semiconductors (DMSs) to map out the wave functions of states. We fabricated two triple QW systems consisting of Zn1−x−yCdxMnySe (DMS) and Zn1−xCdxSe (non-DMS) wells, separated by ZnSe (i.e., non-magnetic) barriers. Transitions involving the lowest multiplet of states (i.e., the ground state split by inter-well interactions) were clearly observed and well resolved at zero magnetic field. By analyzing the Zeeman shift shown by each transition, we are able to demonstrate the probability of finding an electron in a given well.

Random spin–orbit coupling and spin relaxation in symmetric quantum wells

The effective “spin–orbit” field acting upon spins of carriers in two-dimensional electronic structures (the Rashba field) arises due to the spatial asymmetry of the system. It will be shown in this letter that even in quantum wells with perfectly symmetric environment, a random Rashba field arises due to fluctuations of concentration of dopant ions. The magnitude of the field, its correlation function, and electron spin relaxation rate in this field are presented.

The Symmetry-Driven Transition in Magnetoluminescence at ν e = 2: The Rashba Method in Action

The magnetoluminescence spectra of symmetric quantum wells containing an electron gas show an abrupt transition from Landau level behavior (i.e. a linear shift of energy with field) to quadratic (exciton-like) behavior at the field at which the lowest Landau level is just filled, so that the electron filling factor νe = 2. Professor Rashba's insight was that this transition is a natural (although surprising, since the high field limit is by no means reached) consequence of the hidden symmetry which is known to hold in two-dimensional systems at high fields. This interpretation also accounts for the appearance ofmagnetoplasmon, “shake-up” and cyclotron satellites when νe > 2.

Terahertz/optical mixing in symmetric semiconductor quantum wells embedded in optical microcavities

A quantum well (QW) in the simultaneous presence of a terahertz field polarized in the growth direction and an incident optical field near an excitonic resonance results in substantial frequency mixing between the terahertz and optical fields. In particular, a response at new frequencies given by the input optical frequency plus or minus multiples of the terahertz frequency occurs-the terahertz sidebands. In a symmetric QW, the dominant contribution to terahertz-sideband formation is the high-frequency modulation of the overlap integral of the relevant conduction- and valence-subband envelope functions that determine the strength of the interband dipole moment. terahertz-sideband generation is shown to be strongly enhanced in a high quality-factor optical microcavity. Numerical values of the reflected intensity into the first terahertz sideband normalized with respect to the reflected intensity at the fundamental as large as /spl sim/10% are estimated. This suggests that terahertz-sideband generation in semiconductor microcavities is a promising option worthy of exploration for wavelength conversion for wavelength-division multiplexing applications.

Spectroscopic Signatures of the Alloy-Alloy Interface in InGaAs-GaAs(001) Stepped Quantum Wells: a Frequency- and Time-Resolved Study

Stepped quantum wells of InGaAs–GaAs(001) have been characterized by energy- and time-resolved (ultrafast) optical spectroscopies. The energy-resolved reflectivity spectra are in good agreement with our theoretical predictions while energy- and time-resolved photoluminescence measurements show a rather different behavior with respect to what we observe for symmetric quantum wells of similar structural characteristics and growth history. The data suggest that potential alloy fluctuations at the InGaAs–GaAs induce exciton localization.

Spontaneous quantum dot formation at In xGa 1− xAs ∣ In yGa 1− yAs interfaces

We have performed reflectivity and photoluminescence measurements on a set of In x Ga 1− x As ∣ In y Ga 1− y As ∣ GaAs(001) stepped/asymmetric quantum wells with an intentionally abrupt change of indium composition ( x=0.149, y=0.064). These measurements have been compared against extensive spectroscopic data gathered on a set of symmetric quantum wells with similar structural parameters. For both symmetric and asymmetric quantum well samples, reflectivity spectra agree well with theoretical calculations both in terms of peak energy positions and line shapes. The photoluminescence spectra from the asymmetric quantum well samples are consistent with the hypothesis of three-dimensional exciton confinement induced by local and random potential fluctuations at the In-alloy ∣ In-alloy interface. A brief examination of the possible formation mechanisms leading to the formation of such ‘natural quantum dots’ at this poorly studied interface will be presented.

Coupled two-layer plasmon modes induced in a single quantum well by in-plane magnetic fields

We present a complete investigation of the collective excitations in quantum wells subject to in-plane magnetic fields $\mathbf{B},$ based on the random phase approximation and self-consistently calculated ground states in the Hartree approximation. We find that, for a symmetric quantum well of width w, high in-plane $\mathbf{B}$ fields ${(l}_{B}\ensuremath{\ll}w)$ can, besides the usual optical intrasubband plasmons (OP's), give rise to an additional branch of undamped acoustic plasmons (AP's), which does not exist in a single well without $\mathbf{B}.$ In low $\mathbf{B}$ fields ${(l}_{B}\ensuremath{\gg}w),$ the magnetic-field broken symmetry of electron wave functions causes an anticrossing feature of the plasmon dispersions even in the symmetric well. In intermediate $\mathbf{B}$ fields ${(l}_{B}\ensuremath{\lesssim}w),$ an unusual anisotropy effect occurs: the number of the plasmon branches depends on the angle between the magnetic field and the in-plane plasmon wave vector $\mathbf{q}.$ For $\mathbf{B}\ensuremath{\Vert}\mathbf{q},$ there exist two branches of plasmons (OP's and AP's), whereas only one (OP) exists for $\mathbf{B}\ensuremath{\perp}\mathbf{q}.$

Absence of magneto-intersubband scattering inn-type HgTe quantum wells

Magneto-intersubband scattering (MIS), which is a general feature in III-V semiconductor heterostructures with two occupied subbands, is demonstrated to be absent in both asymmetric and symmetric type-III HgTe quantum wells (QW's). The analysis of the temperature dependence of Shubnikov--de Haas (SdH) oscillations and their fast Fourier transforms, together with the excellent agreement between the measured longitudinal resistivity and the calculated oscillations in the density of states in a magnetic field within the framework of an $8\ifmmode\times\else\texttimes\fi{}8$ $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model, completely rule out the MIS effect. The absence of the MIS effect in HgTe QW's is ascribed to a much more complex and irregular Landau level alignment which suppresses the intersubband scattering rate. A natural consequence of the absence of MIS is that the Rashba spin splitting can be properly identified from the SdH oscillations in a perpendicular magnetic field and at a constant temperature, in contrast to the recent findings for InAs QW's.