Modulation-doped GaAs Quantum 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.

Polaronic interaction in a single modulation-doped GaAs quantum well with the Feynman-Hellwarth-Iddings-Platzman approximation

Absolute far-infrared magnetotransmission experiments have been performed in magnetic fields up to 33.5 T on a series of single GaAs quantum wells doped with different electron concentrations. The transmission spectra have been simulated with a multilayer dielectric model. The imaginary part of the optical response function, which reveals singular features related to the electron-phonon interactions, has been extracted. In addition to the expected polaronic effects due to the longitudinal-optical phonon of GaAs, additional interactions with interface phonons are observed. The main interaction is analyzed quantitatively with the Feynman-Hellwarth-Iddings-Platzman model, which is shown to predict correctly the concentration of carriers beyond which the Fr\"ohlich interaction is completely screened.

Dependences of the Transport Scattering Time and Quantum Lifetime on the Two-Dimensional Electron Gas Density in Modulation-Doped Single GaAs Quantum Wells with AlAs/GaAs Short-Period Superlattice Barriers

The dependences of the transport scattering time {\tau}t, quantum lifetime {\tau}q, and their ratio {\tau}t/{\tau}q on the density ne of the electron gas in modulation-doped single GaAs quantum wells with AlAs/GaAs short-period super-lattice barriers are investigated. The experimental dependences are explained in terms of electron scattering by remote ionized donors with an effective two-dimensional concentration n*R and background impurities with a three-dimensional concentration nB. An expression for n*R(ne) is obtained including the contribution of X-valley electrons localized in AlAs layers to the suppression of scattering by the random potential of remote donors. It is shown that the experimentally observed abrupt increase in {\tau}t and {\tau}q with an increase in ne above a certain critical value nec is related to a decrease in n*R. It is established that the drop in {\tau}t/{\tau}q observed for electron densities ne > nec occurs because scattering by the random potential of background impurities in this two-dimensional system with a decrease in n*R limits an increase in {\tau}t more considerably than an increase in {\tau}q.

Transport of a persistent spin helix drifting transverse to the spin texture

Time-resolved magneto-optic Kerr microscopy measures the effect of in-plane electric fields on the dynamics of a photoexcited spin distribution in a modulation-doped GaAs quantum well. The structure features nearly equal Dresselhaus and Rashba coefficients, such that there is negligible impact of spin-orbit coupling for electrons moving along the $[1\overline{1}0]$ or $[\overline{1}10]$ directions. Meanwhile, spin texture emerges for electrons moving in the $[110]$ or $[\overline{1}\overline{1}0]$ directions. The overall spin pattern resembles a persistent spin helix. An in-plane electric field, applied transverse to the spin texture (along the $[1\overline{1}0]$ or $[\overline{1}10]$ directions), introduces a drift of the spin packet and additional Larmor precessions, i.e., a marked decrease of the spatial periodicity of the spin pattern. The in-plane electric field also increases the temporal frequency of the evolving spin distribution, which is directly linked to the cubic Dresselhaus spin-orbit coupling term. Moreover, the in-plane field increases the diffusion coefficient by more than an order of magnitude. We attribute this effect to carrier heating and the separation of the photogenerated electron-hole dipole.

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.

Long-lived electron spins in a modulation doped (100) GaAs quantum well

We have measured T1 spin lifetimes of a 14 nm modulation-doped (100) GaAs quantum well using a time-resolved pump-probe Kerr rotation technique. The quantum well was selected by tuning the wavelength of the probe laser. T1 lifetimes in excess of 1 μs were measured at 1.5 K and 5.5 T, exceeding the typical T2* lifetimes that have been measured in GaAs and II-VI quantum wells by orders of magnitude. We observed effects from nuclear polarization, which were largely removable by simultaneous nuclear magnetic resonance, along with two distinct lifetimes under some conditions that likely result from probing two differently localized subsets of electrons.

Engineering artificial graphene in a two-dimensional electron gas

At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity, which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphenelike system and discuss an experimental realization in a modulation-doped GaAs quantum well. Ultrahigh-mobility electrons with linearly dispersing bands might open new venues for the studies of Dirac-fermion physics in semiconductors.

Limit to two-dimensional mobility in modulation-doped GaAs quantum structures: How to achieve a mobility of 100 million

Considering scattering by unintentional background charged impurities and by charged dopants in the modulation doping layer as well as by GaAs acoustic phonons, we theoretically consider the practical intrinsic (phonons) and extrinsic (background and dopants) limits to carrier mobility in modulation-doped AlGaAs-GaAs two-dimensional (2D) semiconductor structures. We find that reducing background impurity density to ${10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}3}$ along with a modulation doping separation of $1000\text{ }\text{\AA{}}$ or above will achieve a mobility of $100\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$ at a carrier density of $3\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}2}$ for $T=1\text{ }\text{K}$. At $T=4(10)\text{ }\text{K}$, however, the hard limit to the 2D mobility would be set by acoustic phonon scattering with the maximum intrinsic mobility being no higher than $22(5)\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$. Detailed numerical results are presented as a function of carrier density, modulation doping distance, and temperature to provide a quantitative guide to experimental efforts for achieving ultrahigh 2D mobilities.

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.

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.

Ultrafast dynamic control of spin and charge density oscillations in a GaAs quantum well.

We use subpicosecond laser pulses to generate and monitor in real time collective oscillations of electrons in a modulation-doped GaAs quantum well. The observed frequencies match those of intersubband spin- and charge-density excitations. Light couples to coherent density fluctuations through resonant stimulated Raman scattering. Because the spin- and charge-related modes obey different selection rules and resonant behavior, the amplitudes of the corresponding oscillations can be independently controlled by using shaped pulses of the proper polarization.

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.

Photoluminescence of Two-Dimensional Electron System in Modulation-Doped GaAs Quantum Well

We have carried out the photoluminescence measurements of two-dimensional electron system (2DES) in modulation-doped GaAs quantum well. It was found that the H band caused by radiative recombination between 2DES and holes shows a high energy tail below 4.2 K. We explain that the tail originates in the apparent breaking of the momentum conservation law, and that its reason is the electron scattering by neutral donors with the Bohr radius of ~10 nm. The distance between the adjacent donors, 80nm, is obtained from the coupling of the Landau levels in the different 2DES's. Excitonic effect is suggested by nonlinear dependence of the peak energy of the H band on magnetic fields and the lineshape analysis of the H band.

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.

Photoluminescence of modulation doped GaAs quantum wires on vicinal GaAs (110) substrates

Modulation-doped GaAs quantum wires are grown on the surfaces with giant steps which are naturally formed on vicinal GaAs (110) substrates by molecular beam epitaxy (MBE). It is found that photoluminescence (PL) peaks shift to lower energy with increasing doping level and to higher energy with increasing excitation intensity. These results indicate that the GaAs quantum wires are successfully modulation-doped.

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.