Electron Gas In Quantum Wells Research Articles

INFLUENCE О F А STR О NG M А GNETIC FIELD О N FERMI ENERGY О SCILL А TI О NS IN TW О -DIMENSI О N А L SEMIC О NDUCT О R M А TERI А LS

This article shows that the Fermi levels of a nanoscale semiconductor in a quantizing magnetic field are quantized. A method is proposed for calculating the Fermi energy oscillations for a two-dimensional electron gas at different magnetic fields and temperatures. An analytical expression is obtained for calculating the Fermi-Dira c distribution function at high temperatures and weak magnetic fields. With the help of the propos ed formula, the experimental results in nanoscale semiconductor structures are investigated. Using fo rmula, Fermi energy oscillations are explained for two-dimensional electron gases in quantum wells (quantum wells, mainly GaAs/GaAlAs heterostructures) with a parabolic dispersion law. Keywоrds:quаntizing mа gnetic field, temperаture, Fermi energy, n аnоscаle semicоnductоrs,twо-dimensiоnаl structures, dispersiоn.

Linear intersubband optical absorption in the semiparabolic quantum wells based on AlN/AlGaN/AlN under a uniform electric field

The linear intersubband optical absorption in the polarization semiparabolic quantum wells (SPQWs) are investigated for typical AlN/AlxGa 1−x N/AlN. First, the one-dimensional Poisson and Schrödinger equations have been solved by the variational method within a finite potential barrier model and a bent band figured by all confinement sources (realistic model). Then, the intersubband optical absorption between the lowest two subbands has been theoretically studied under an uniform external electric field. Computed results including the effective confining potential profile, the wave function and the distribution of electron gas in quantum wells, intersubband optical absorption coefficient have been discussed. Our calculation shows that the positive interface polarization charges affect on the distribution of the two-dimensional electron gas (2DEG) in SPQWs so it has great influences on the total optical absorption coefficients significantly.

Strong confinement-induced engineering of the g factor and lifetime of conduction electron spins in Ge quantum wells.

Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the desirable but contrasting requirements of spin robustness against relaxation mechanisms and sizeable coupling between spin and orbital motion of the carriers. Here, we focus on Ge, which is a prominent candidate for shuttling spin quantum bits into the mainstream Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome these fundamental limitations by investigating a two-dimensional electron gas in quantum wells of pure Ge grown on Si. These epitaxial systems demonstrate exceptionally long spin lifetimes. In particular, by fine-tuning quantum confinement we demonstrate that the electron Landé g factor can be engineered in our CMOS-compatible architecture over a range previously inaccessible for Si spintronics.

Spin-dependent coherent transport in a double quantum dot system

We study spin-resolved resonant tunneling in a system of two quantum dots sandwiched between doped quantum wells. In the coherent (Dicke) regime, i.e., when quantum dot separation is smaller than the Fermi wavelength in a two-dimensional electron gas in quantum wells, application of an in-plane magnetic field leads to a pronounced spin-resolved structure of the conductance peak lineshape even for very small Zeeman splitting of the quantum dots' resonant levels. In the presence of electron-gas spin-orbit coupling, this spin-resolved structure gets washed out due to Fermi surface deformation in the momentum space. We also show that Aharonov-Bohm flux penetrating the area enclosed by electron tunneling pathways completely destroys conductance spin structure.

Droop Improvement in InGaN/GaN Light-Emitting Diodes by Polarization Doping of Quantum Wells and Electron Blocking Layer

-Polarization doping in quantum wells and electron blocking layer is proposed to improve the emission intensity and efficiency droop of InGaN light-emitting diodes (LEDs). Band diagrams and the internal quantum efficiencies of LEDs are theoretically studied by the ATLAS simulation program. Numerical results show that the internal quantum efficiency of polarization doped LED structures are improved by 25% as compared to un-doped structures which can be due to accumulation of high density two-dimensional electron gas in quantum wells.

Suris tetrons: possible spectroscopic evidence for four-particle optical excitations of a two-dimensional electron gas.

The excitations of a two-dimensional electron gas in quantum wells with intermediate carrier density (ne∼1011 cm-2), i.e., between the exciton-trion and the Fermi-sea range, are so far poorly understood. We report on an approach to bridge this gap by a magnetophotoluminescence study of modulation-doped (Cd,Mn)Te quantum well structures. Employing their enhanced spin splitting, we analyzed the characteristic magnetic-field behavior of the individual photoluminescence features. Based on these results and earlier findings by other authors, we present a new approach for understanding the optical transitions at intermediate densities in terms of four-particle excitations, the Suris tetrons, which were up to now only predicted theoretically. All characteristic photoluminescence features are attributed to emission from these quasiparticles when attaining different final states.

Alloy-disorder scattering of the interacting electron gas in quantum wells and heterostructures of Al x Ga1 − x As

The question whether alloy disorder is screened or unscreened is of fundamental importance. Therefore, we calculate the mobility of the interacting two-dimensional electron gas as realized in AlxGa1 − xAs quantum wells and heterostructures in the presence of alloy-disorder scattering. For the screening we use the randomphase approximation and we include many-body effects due to exchange and correlation. We propose to determine the alloy disorder potential VAD from mobility measurements. If we use VAD = 1.04 eV we can explain recent experimental results obtained for quantum wells and heterostructures with ultrahigh mobility. From the anomalous linear temperature dependence of the mobility measured in heterostructures, we conclude that the alloy disorder is screened. More experiments are needed to confirm the screening of the alloy disorder and we propose some measurements.

Electron spin dephasing in two-dimensional systems with anisotropic scattering

We develop a microscopic theory of spin relaxation of a two-dimensional electron gas in quantum wells with anisotropic electron scattering. Both precessional and collision-dominated regimes of spin dynamics are studied. It is shown that, in quantum wells with noncentrosymmetric scatterers, the in-plane and out-of-plane spin components are coupled: spin dephasing of carriers initially polarized along the quantum well normal leads to the emergence of an in-plane spin component even in the case of isotropic spin-orbit splitting. In the collision-dominated regime, the spin-relaxation-rate tensor is expressed in terms of the electric conductivity tensor. We also study the effect of an in-plane and out-of-plane external magnetic field on spin dephasing and show that the field dependence of electron spin can be very intricate.

Spin Hall effect due to intersubband-induced spin-orbit interaction in symmetric quantum wells

We investigate the intrinsic spin Hall effect in two-dimensional electron gases in quantum wells with two subbands, where a new intersubband-induced spin-orbit coupling is operative. The bulk spin Hall conductivity sigma(z)(xy) is calculated in the ballistic limit within the standard Kubo formalism in the presence of a magnetic field B and is found to remain finite in the B=0 limit, as long as only the lowest subband is occupied. Our calculated sigma(z)(xy) exhibits a nonmonotonic behavior and can change its sign as the Fermi energy (the carrier areal density n(2D)) is varied between the subband edges. We determine the magnitude of sigma(z)(xy) for realistic InSb quantum wells by performing a self-consistent calculation of the intersubband-induced spin-orbit coupling.

Magneto‐transport in single InGaAs quantum wells of different shapes

AbstractThe magneto‐transport measurements in InGaAs/InAlAs quantum wells (QW) obtained by MOCVD technology on InP substrates, and known as High Electron Mobility Transistors, were performed at low temperatures of 0.4 ‐ 15 K and high magnetic fields up to 10 T (with the magnetic field induction B perpendicular to the plane of the well). Three kinds of structures were studied. The Shubnikov‐de Haas oscillations demonstrating the occupancy of two subbands were observed. In order to determine the energies of Landau levels in the QW subbands, we have used the Zawadzki‐Pfeffer quazi‐two‐band model. Good agreement between calculated Landau levels and Fermi level from one hand, and positions of the Shubnikovde Haas oscillations peaks from the other, enable us to determine the parameters of 2D electron gas in QW: effective mass and carrier density.

Screening of exciton states by quasi-two-dimensional electron gas in quantum wells

Changes in the binding energy and oscillator strength of an exciton state due to screening by a quasi-two-dimensional electron gas were calculated self-consistently in a nonlinear approximation. It was shown that the collapse of the bound state proceeds at very small concentrations N s ≃5×109 cm−2, which is a consequence of taking into account the nonlinearity of the system response to the Coulomb perturbation.

Hydrostatic pressure dependence of negative-donor-ion singlet and singlet-like bound magnetoplasmon transitions in doped GaAs/AlGaAs quantum wells

Applied hydrostatic pressure modifies the Coulomb bound states of a quasi-two-dimensional electron gas in quantum wells by increasing the effective mass and by tuning the free electron density. Here, we explore these mechanisms by measuring the effects of pressure on the cyclotron resonance, the D 0 1 s → 2 p + transition, and the D −-singlet and singlet-like transitions in low-and high-density, modulation-doped GaAs quantum wells. For low doping density, detailed calculations employing a pressure-dependent electron mass agree well with the observed magnetic field and pressure dependencies. For high doping density and low fields, the blue-shift of the D −-singlet-like transition at fields below 8 T decreases with applied pressure as anticipated, due to loss of free electrons via the Γ–X crossover. However, near ∼7.5 T , this singlet-like transition exhibits an anomalous branching for pressures above 4 kbar , which indicates the presence of a resonant level and obscures the blue-shift at high fields.

Coulomb energy of a quasi-2D electron gas in a quantum well.

We calculate the Hartree, exchange and correlation energies of a quasi-2D electron gas in doped semiconductor quantum wells, using an expansion in Coulomb interaction. This expansion, also valid for the Hartree term in usual experimental situations, allows one to obtain the analytical well width dependence of the energies. We find that the finite width corrections to the exact 2D exchange-correlation energy are quite often as large as the Hartree contributions.

Intrinsic dephasing times of photoexcited electron-valence-hole pairs near the Fermi edge of a degenerate electron gas in quantum wells.

Various single-particle and collective-mode scattering processes that contribute to the intrinsic dephasing time of photoexcited electron--valence-hole pairs in degenerate, modulation-doped quantum wells are discussed. Detailed calculations show that sharp discontinuities in the energy dependence of the electron dephasing time can give information about short-range correlations associated with large wave vectors and high-frequency excitations of the coupled electron--LO-phonon system. However, to extract dephasing times from coherent optical excitations the effects of the complex valence-band structure, i.e., mixing of light and heavy holes in GaAs quantum wells, must be considered.

Optically induced intersubband absorption in the presence of a two-dimensional electron gas in quantum wells.

We identify long-lived photoexcited electrons in modulation-doped GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum wells by studying the effect of interband excitation on the electron intersubband absorption. The lifetime of these electrons is of the order of 1--10 \ensuremath{\mu}sec for doping levels corresponding to a two-dimensional electron density 1\ifmmode\times\else\texttimes\fi{}${10}^{10}$${\mathit{n}}_{\mathrm{\ensuremath{\square}}}$7\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. No photoinduced intersubband absorption is observed for ${\mathit{n}}_{\mathrm{\ensuremath{\square}}}$>7\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. For nominally undoped samples (${\mathit{n}}_{\mathrm{\ensuremath{\square}}}$1\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$), the photoinduced absorption is mainly due to short-lived free excitons. These results are consistent with the existence of localized holes that in lightly doped quantum wells greatly reduce the e-h recombination rate, leaving the electrons free for a long time.

Collective inter-subband excitations of electron gas in quantum wells in infrared absorption, Raman scattering and nonlinear optical processes

We discuss the role of collective inter-subband excitations of electron gases confined in quantum wells in infrared absorption, resonant Raman scattering and nonlinear optical processes. First, exchange as well as direct Coulomb interaction are invoked in a theoretical model to calculate the shift of the collective inter-subband modes from the single particle excitation energy. We then formulate the coupling of the collective inter-subband charge density mode to the electromagnetic field. The coupling mechanisms via exchange and direct Coulomb scattering for resonant Raman processes are discussed. An experiment is designed that can differentiate between the two mechanisms, based on their dependence on wavefunction overlap. Finally, we discuss nonlinear optical processes involving inter-subband transitions and collective excitations.

Exchange interactions in quantum well subbands

It is shown that exchange interactions in the two-dimensional electron gas in quantum wells could cause observable effects on subband energies and intersubband transition energies. In the case of doped quantum wells, the intrasubband exchange interaction can produce an energy shift which is substantially larger than the direct Coulomb energy shift. Theoretical estimates of such shifts are compared with experimental measurements of the infrared photoconductivity of multiple quantum well AlGaAs/GaAs structures with wells doped at about 1018 cm−3.

Piezoelectric and deformation potential acoustic scattering mobility of a two‐dimensional electron gas in quantum wells

AbstractThe piezoelectric‐scattering mobility in a semiconductor quantum well is evaluated incorporating the effect of the finite width of the structure. The mobility is found to be enhanced in wider channels, but the effect is smaller than that in the case of deformation‐potential acoustic scattering. The combined screened mobility due to deformation potential acoustic and piezoelectric scattering using the Fcrmi‐Dirac statistics is seen to vary inversely with the temperature over the range of about 20 to 60 K. The slope of this variation is greater for smaller electron concentration and wider channels.