Modulation-doped GaAs Quantum Wells Research Articles

Fractional Quantum Hall State at Filling Factor ν=1/4 in Ultra-High-Quality GaAs Two-Dimensional Hole Systems.

Single-component fractional quantum Hall states (FQHSs) at even-denominator filling factors may host non-Abelian quasiparticles that are considered to be building blocks of topological quantum computers. Such states, however, are rarely observed in the lowest-energy Landau level, namely at filling factors ν<1. Here, we report evidence for an even-denominator FQHS at ν=1/4 in ultra-high-quality two-dimensional hole systems confined to modulation-doped GaAs quantum wells. We observe a deep minimum in the longitudinal resistance at ν=1/4, superimposed on a highly insulating background, suggesting a close competition between the ν=1/4 FQHS and the magnetic-field-induced, pinned Wigner solid states. Our experimental observations are consistent with the very recent theoretical calculations that predict that substantial Landau level mixing, caused by the large hole effective mass, can induce composite fermion pairing and lead to a non-Abelian FQHS at ν=1/4. Our results demonstrate that Landau level mixing can provide a very potent means for tuning the interaction between composite fermions and creating new non-Abelian FQHSs.

Field control of anisotropic spin transport and spin helix dynamics in a modulation-doped GaAs quantum well

Electron spin transport and dynamics are investigated in a single, high-mobility, modulation-doped, GaAs quantum well using ultrafast two-color Kerr-rotation micro-spectroscopy, supported by qualitative kinetic theory simulations of spin diffusion and transport. Evolution of the spins is governed by the Dresselhaus bulk and Rashba structural inversion asymmetries, which manifest as an effective magnetic field that can be extracted directly from the experimental coherent spin precession. A spin precession length L-SOI is defined as one complete precession in the effective magnetic field. It is observed that application of (a) an out-of-plane electric field changes the spin decay time and L-SOI through the Rashba component of the spin-orbit coupling, (b) an in-plane magnetic field allows for extraction of the Dresselhaus and Rashba parameters, and (c) an in-plane electric field markedly modifies both the L-SOI and diffusion coefficient. While simulations reproduce the main features of the experiments, the latter results exceed the corresponding simulations and extend previous studies of drift-current-dependent spin-orbit interactions.

Bound-to-bound midinfrared intersubband absorption in carbon-doped GaAs∕AlGaAs quantum wells

Bound-to-bound intersubband absorption in the valence band of modulation-doped GaAs quantum wells with digitally alloyed AlGaAs barriers was studied in the midinfrared wavelength range. A high-purity solid carbon source was used for the p-type doping. Strong narrow absorption peaks due to heavy-to-heavy hole transitions are observed with out-of-plane polarized light, and weaker broader features with in-plane polarized light. The heavy-to-heavy hole transition energy spans the spectral range between 206 to 126 meV as the quantum well width is increased from 25 to 45 Å. The experimental results are found to be in agreement with calculations of a six-band k∙p model taking into account the full band structure of the digital alloy.

Optical imaging of trion diffusion and drift in GaAs quantum wells

The in-plane motion of negatively charged excitons ${(X}^{\ensuremath{-}})$ in modulation-doped GaAs quantum wells (QW's) was studied by means of spatially resolved photoluminescence spectroscopy. In the highest-quality QW's, resonantly excited ${X}^{\ensuremath{-}}$ were observed diffusing from the excitation region, characterized by a mobility as high as $6.5\ifmmode\times\else\texttimes\fi{}{10}^{4}{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1},$ independent of temperature and electron density. Under the application of an electric field in the plane of the QW, ${X}^{\ensuremath{-}}$ were found to drift in the direction opposite to the field, whereas neutral excitons (X) do not drift under similar conditions. The results demonstrate that ${X}^{\ensuremath{-}}$ can exist as a free quasiparticle in the best-quality samples. The simultaneous motion of nonresonantly excited X and ${X}^{\ensuremath{-}}$ was studied as a function of the electron density in the QW. The data reveal that the spatial distribution of X and ${X}^{\ensuremath{-}}$ is mainly determined by the motion of X that locally form ${X}^{\ensuremath{-}}.$

Many body effects and internal transitions of confined excitons in GaAs and CdTe quantum wells

Many body effects contribute significantly to the energy states of electron–hole pairs confined in quantum wells in the presence of excess electrons. We present results of optically detected resonance spectroscopy of the internal transitions of photo-excited electron–hole pairs in the presence of excess electrons for GaAs QWs and CdTe QWs. Compared to the case of isolated negatively charged excitons, excess electrons produce a large blue shift of the internal transitions in modulation-doped GaAs quantum wells (QWs) for filling factor <2, and similar effects are found in CdTe QWs. For filling factor >2 no internal transitions are observed. These measurements demonstrate the strong effects of electron–electron correlations on the internal transitions of charged excitons in these quasi-2D systems and the importance of magnetic translation invariance. In the presence of excess electrons, the observed internal transitions are those of a magnetoplasmon bound to a mobile valence band hole.

Onset of exciton absorption in modulation-doped GaAs quantum wells

We study the evolution of the absorption spectrum of a modulation-doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ semiconductor quantum well with decreasing the carrier density. We find that at some critical electron density there is a sharp change in the line shape and the transitions energies of the exciton peaks. We show that this critical density marks an abrupt transition from a simple excitonic behavior to a Fermi edge singularity.

Coupling of intersubband charge-density excitations to longitudinal-optical phonons in modulation-doped GaAs quantum wells

We have investigated the interaction between the longitudinal-optical (LO) phonon and the lowest intersubband charge-density excitation of the two-dimensional (2D) electron gas formed in a modulation-doped single quantum well structure. The intersubband spacing is continuously increased by applying dc voltages up to about 200 V between a top gate and a back contact. The frequency of the LO-phonon mode confined to the quantum well exhibits an electron-density-dependent redshift which is attributed to enhanced screening effects by the 2D electron gas. Inelastic light-scattering spectra reveal a clear anticrossing behavior of the energies of LO-phonon and charge-density excitation, for which a coupling constant of 1.5(3) meV is obtained.

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.

On the Spin-Flip Mechanisms of Electrons in Semiconductor Quantum Wells

We have studied the temperature dependence of the spin-flip processes of electrons in p-type modulation-doped GaAs quantum wells. We have found that at low temperature (T ≤ 30 K) the exchange interaction between electrons and holes is the main mechanism responsible for the spin-flip of the electrons, whereas at higher temperatures the so-called Dyakonov-Perel mechanism dominates. Furthermore, we have found that the recombination time increases considerably with increasing lattice temperature from a value of τr ≈︂ 200 ps at 5 K to τr ≈︂ 400 ps at 60 K.

Mid-Infrared Photodetector Using Self-Assembled InAs Quantum Dots Embedded in Modulation-Doped GaAs Quantum Wells

AbstractWe have designed and fabricated a quantum dot infrared photodetector which utilizes lateral transport of photoexcited carriers in the modulation-doped A1GaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. A peak responsivity was as high as 2.3 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels. Furthermore, we found that this device has high operation temperature and very high photoconductive gain.

Raman scattering study of the plasmon modes in bilayer systems

We report Raman scattering measurements of the acoustic and optic plasmons in electron bilayer systems consisting of two modulation-doped GaAs quantum wells. The measured dispersion relations are successfully fitted using the RPA, with self-consistent wave functions for the quantum well structures and the electron number densities in the wells as fitting parameters. Measurements of the temperature-dependent Landau damping of the acoustic plasmon are compared with the results of RPA calculations including a 0 K Hubbard correction; good agreement was obtained only for low temperatures ( T< T F/2), indicating the need for further theoretical work on local field corrections at higher temperatures.

Fermi-edge singularities in photoluminescence from modulation-doped GaAs quantum wells

We have performed a detailed study of low-temperature photoluminescence (PL) from a pair of modulation-doped GaAs/Al${}_{x}$Ga${}_{1\ensuremath{-}x}$As multiple-quantum-well samples finding weak Fermi-edge singularities (FES's) associated with both heavy-hole and light-hole subbands. In contrast to previous experiments it appears that the presence of the FES's is not due to hole localization or the proximity of a second electronic subband. The many body enhancement above the single particle background is found to be comparable to that in systems with localized holes. The enhancement of the PL spectrum is significantly stronger than that of the absorption spectra from the same samples. Comparison with previously published data suggests that this asymmetry may be a general property of two-dimensional electron systems.

New route to reduce ionized impurity scattering in modulation-doped GaAs quantum wells

We propose a new concept for the reduction of impurity scattering in remotely doped GaAs single quantum wells by using heavy-mass X-electrons in the short-period AlAs/GaAs superlattice barriers which smooth the potential fluctuations of the ionized Si dopants. Electron mobilities as high as 120 m 2 /V . s and electron densities of 1.6 x 10 16 m -2 are obtained in 10 nm GaAs single quantum wells in the one-subband occupation mode without any parallel conductance. To demonstrate the applicability of our concept magnetotransport and photoluminescence measurements together with self-consistent calculations are presented. Limitations of the effectiveness of the reduction of impurity scattering by impurity segregation and intermixing in the superlattice are discussed.

Negatively and positively charged excitons in GaAs/AlxGa1-xAs quantum wells.

We report the observation of the positively charged exciton and of the triplet state of the negatively charged exciton in modulation doped GaAs quantum wells. Applying a gate voltage at high magnetic fields we find that the photoluminescence line of the two-dimensional electron gas smoothly transforms into a negatively charged exciton and not into a neutral exciton. The Zeeman splitting of this line exhibits an abrupt change at the metal-insulator transition.

Virtual excitation of the Fermi-edge singularity in modulation-doped quantum wells.

The Fermi-edge singularity (FES) in modulation-doped GaAs quantum wells is a unique many-body feature that arises from strong Coulomb correlations within a dense sea of electrons. Using intense ultrashort excitation, we show experimentally and theoretically that the nonresonant nonlinearity (ac Stark shift) of the FES is dramatically different than that of the excitonic ac Stark shift of undoped-quantum-well excitons.

Collapse of the Hartree term of the Coulomb interaction in a very dilute 2D electron gas.

A novel many-body behavior of fundamental electron-electron interactions in very dilute 2D electron gases is revealed by inelastic light scattering in modulation-doped GaAs quantum wells. With increasing hydrostatic pressure the 2D electron density is continuously reduced down to values below 1\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. A quantitative analysis of the energies of the elementary excitations of the electron gas shows the collapse of the Hartree term of the Coulomb interaction at densities of about 2\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, which is attributed to a metal-non-metal transition of the electronic system. Exchange and correlation effects, on the contrary, remain significant even at very low carrier densities.

Terahertz electromagnetic transients as probes of a two-dimensional electron gas

We demonstrate the application of terahertz electromagnetic transients to the study of the dynamical conductivity of a two-dimensional, high mobility electron gas in modulation doped GaAs quantum wells. The transient response of such an electron system on a picosecond time scale occurs in a regime where the implications of electron-electron interaction effects need to be considered.

Coherent optical spectroscopy of electron scattering in a two-dimensional electron gas in high magnetic fields.

We use time resolved four wave mixing to study electron scattering in modulation doped GaAs quantum wells in high magnetic fields up to 8 T. We find a decrease in scattering rates relative to the zero field behavior. The four wave mixing signal shows a strong enhancement at the lowest Landau level which we attribute to the Fermi edge singularity.

Electronic Raman scattering on modulation doped GaAs quantum wells: Conduction band structure and collective effects

We report on a detailed analysis of electronic Raman scattering results in modulation doped single quantum wells. We illustrate thereby the great power of this technique to get a detailed characterization of the electron density distribution and the band structure in doped heterostructures. We report in particular on a recent observation, for first time directly by spectroscopy, of the spin splitting of the conduction band of GaAs due to the lack of inversion symmetry.

Femtosecond Coherent Spectroscopy of GaAs Quantum Wells

Recent results on femtosecond time-integrated and time-resolved four-wave-mixing experiments in intrinsic and modulation-doped GaAs quantum wells are reported. A variety of new phenomena ranging from giant excitonic resonance for intrinsic samples, Fermi liquid behavior in modulation-doped samples, and interesting temporal evolutions provide new insights into the physical process responsible for the coherent signals.