Undoped GaAs Quantum Research Articles

High performance p++-AlGaAs/n++-InGaP tunnel junctions for ultra-high concentration photovoltaics.

A p++-AlGaAs: C/n++-InGaP: Te tunnel junction with a record peak tunneling current density of 5518 A/cm2 was developed. This was achieved by inserting a 6.6 Å undoped GaAs quantum well at the junction interface, and the numerical model demonstrated that trap-assisted tunneling contributes to the high peak tunneling current. Furthermore, we found that the p++-AlGaAs: C/n++-InGaP: Si + Te tunnel junctions have lower resistance and better stability than p++-AlGaAs: C/n++-InGaP: Te tunnel junctions in the operating temperature range of the multijunction solar cells, and the peak tunneling current density of the p++-AlGaAs: C/n++-InGaP: Si + Te tunnel junctions excess 3000 A/cm2 with a voltage drop of 7.5 mV at 10000 suns.

Stability of High-Density Two-Dimensional Excitons against a Mott Transition in High Magnetic Fields Probed by Coherent Terahertz Spectroscopy.

We have performed time-resolved terahertz absorption measurements on photoexcited electron-hole pairs in undoped GaAs quantum wells in magnetic fields. We probed both unbound- and bound-carrier responses via cyclotron resonance and intraexciton resonance, respectively. The stability of excitons, monitored as the pair density was systematically increased, was found to dramatically increase with increasing magnetic field. Specifically, the 1s-2p_{-} intraexciton transition at 9T persisted up to the highest density, whereas the 1s-2p feature at 0T was quickly replaced by a free-carrier Drude response. Interestingly, at 9T, the 1s-2p_{-} peak was replaced by free-hole cyclotron resonance at high temperatures, indicating that 2D magnetoexcitons do dissociate under thermal excitation, even though they are stable against a density-driven Mott transition.

Detection of the Anomalous Velocity with Subpicosecond Time Resolution in Semiconductor Nanostructures.

We report on the time-resolved detection of the anomalous velocity, constituting charge carriers moving perpendicular to an electric driving field, in undoped GaAs quantum wells. For this we optically excite the quantum wells with circularly polarized femtosecond laser pulses, thereby creating a state which breaks time-inversion symmetry. We then employ a quasi-single-cycle terahertz pulse as an electric driving field to induce the anomalous velocity. The electromagnetic radiation emitted from the anomalous velocity is studied with a subpicosecond time resolution and reveals intriguing results. We are able to distinguish between intrinsic (linked to the Berry curvature) and extrinsic (linked to scattering) contributions to the anomalous velocity both originating from the valence band and observe local energy space dependence of the anomalous velocity. Our results thus constitute a significant step towards noninvasive probing of the anomalous velocity locally in the full energy-momentum space and enable the investigation of many popular physical effects such as the anomalous Hall effect and spin Hall effect on ultrafast time scales.

Terahertz-Induced Optical Emission of Photoexcited Undoped GaAs Quantum Wells

Intense terahertz (THz) pulse induces photoluminescence (PL) flash from undoped high-quality GaAs/AlGaAs quantum wells under continuous wave laser excitation. The number of excitons increases 10,000-fold from that of the steady state under only laser excitation. The THz electric field dependence and the relaxation dynamics of the PL flash intensity suggest that the strong electric field of the THz pulse ionizes impurity states during the 1 ps period of the THz pulse and release carriers from a giant reservoir containing impurity states in the AlGaAs layers.

Kerr detection of acoustic spin transport in GaAs (110) quantum wells

Time-resolved Kerr reflectometry (TRKR) is used to investigate the long-range transport of spins by surface acoustic waves in undoped GaAs (110) quantum wells. TRKR measurements under an applied magnetic field demonstrate the coherent precession of the optically generated electron spin during acoustic transport over several micrometers and yield information about the relaxation processes for moving spins.

Effect of a transverse applied electric field on electron drift velocity in a GaAs quantum wire: A Monte Carlo simulation

A Monte Carlo simulation is run to study the electron transport in a thin undoped GaAs quantum wire under the influence of a transverse applied electric field. Phonon and surface-roughness scattering are included. Electron drift velocity is investigated as a function of roughness amplitude at a temperature of 77 or 300 K and a longitudinal electric field of 104 or 105 V/m. A transverse applied field is shown to provide a means of controlling drift velocity, affecting the scattering rate.

Reversal of Coherently Controlled Ultrafast Photocurrents by Band Mixing in Undoped GaAs Quantum Wells

It is demonstrated that valence-band mixing in GaAs quantum wells tremendously modifies electronic transport. A coherent control scheme in which ultrafast currents are optically injected into undoped GaAs quantum wells upon excitation with femtosecond laser pulses is employed. An oscillatory dependence of the injection current amplitude and direction on the excitation photon energy is observed. A microscopic theoretical analysis shows that this current reversal is caused by the coupling of the light- and heavy-hole bands and that the hole currents dominate the overall current response. These surprising consequences of band mixing illuminate fundamental physics as they are unique for experiments which are able to monitor electronic transport resulting from carriers with relatively large momenta.

Photoluminescence spectroscopy of the low-density two-dimensional electron system in electric fields

We performed photoluminescence (PL) measurements of the two-dimensional electron system (2DES) in an undoped GaAs quantum well (QW) while controlling the electron density and electric field independently. We compared the PL spectrum at the same electron density but at different electric fields. The PL linewidth changes with electric field at low electron density below 4×1010 cm−2, but does not change with electric field at higher electron density. The sample we used is composed of a metallic mesh front gate with a 500-nm pitch. The phenomena we report here mainly originate from the longitudinal electric field bias and are commonly observed in a different-mesh-size sample.

Spin Transport in (110) GaAs-Based Cavity Structures

The anisotropic spin dephasing of optically generated electrons in an undoped (110) GaAs quantum well inside a microcavity structure is investigated by means of spatially resolved photoluminescence experiments at a temperature of T=80 K. The dynamic type-II potential modulation induced by a surface acoustic wave (SAW) is used to transport electrons spatially separated from holes. Thus, the D’yakonov–Perel’ (DP) and the Bir–Aronov–Pikus spin dephasing mechanisms are suppressed, and electron spins can be transported over long distances of about 24 μm, which correspond to spin lifetimes of at least 8 ns. The spin vector can be rotated by an external in-plane magnetic field or by the effective in-plane field resulting from the structural inversion anisotropy induced by an intense SAW. This rotation generates an in-plane spin component that is subject to the DP spin dephasing mechanism. By means of Hanle effect measurements the lifetime of this in-plane spin component is found to be of 0.7 ns.

Luminescence flashes induced by microwave radiation in undoped GaAs quantum wells

Bright flashes of exciton emission are observed in photoexcited, undoped GaAs/AlGaAs quantum wells under pulsed microwave (36 GHz) irradiation. The flash intensity is in the range of 10--100 times the steady-state photoluminescence intensity with a decay time of $(1--10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{s}$ (depending on the applied microwave power and photoexcitation). These observations indicate a reservoir of long-lived carriers that is formed at low temperature due to localization of photogenerated electrons and holes at spatially separated shallow traps. Microwave-heated electrons activate the localized carriers into free states by means of avalanche impact ionization. This gives rise to rapid exciton formation with subsequent luminescence flash. A detailed model based on the coupled rate equations for free electrons, excitons and localized electrons (holes) is presented.

Spin transport and manipulation by mobile potential dots in GaAs quantum wells

Mobile quantum dots (denoted as dynamic quantum dots) formed by the interference of orthogonal beams of surface acoustic waves (SAWs) in an undoped GaAs quantum well coherently transport electron spins over distances exceeding 70 μ m . We determine the spin-splitting parameter for a GaAs quantum well to be γ = 17 ± 2 eV Å 3 as well as investigate the effects of the SAW strain field on the spin dynamics.

Photoluminescence of GaAs(110) quantum wells modulated by surface acoustic waves

We studied the low temperature (15K) photoluminescence properties of un-doped GaAs(110) quantum wells modulated by surface acoustic waves (SAW) by photoluminescence (PL) spectroscopy technique. Under the influence of the SAW, the decreasing of PL intensity and the splitting of the PL lines were observed. The magnitude of the splitting can be up to 10meV when the power applied to the transducers is 20dBm. Furthermore, the spin injection in GaAs(110) quantum wells generated by circular polarization light under the influence of SAW was also discussed.

Terahertz-optical mixing in undoped and doped GaAs quantum wells: From excitonic to electronic intersubband transitions

The mixing of near-infrared and terahertz (THz) beams has been observed in $n$-doped GaAs quantum wells (QWs) and is shown to be quite different from that in undoped QWs. The resonant behavior of the sidebands demonstrates that mixing in doped QWs is primarily sensitive to electronic intersubband transitions while that in undoped QWs is due to excitonic intersubband transitions. These results demonstrate that THz-optical mixing can be used to probe the collective dynamics of a driven electron gas.

Self-Similar Optical Absorption Spectra in High Magnetic Fields

Employing a non-perturbative multi-band approach within empirical tight-binding theory, we show that the magneto-absorption spectrum of a periodic two and three-dimensional electron system possesses a structure that overall reflects the fractal Hofstadter butterfly eigenvalue spectrum. For small magnetic fields, the standard magneto-optical selection results of envelope theory are recovered. We predict that the Hofstadter butterfly can be directly observed by interband magneto-absorption from an undoped GaAs quantum well beneath a periodic array of stressors.

Intensity correlated nonlinear photoluminescence spectra in undoped andp-doped GaAs and GaAs quantum wells

We study the line shapes and excitation density dependence of the intensity correlated nonlinear (NL) photoluminescence (PL) spectra due to excitons and free electron-hole pairs in molecular beam epitaxy grown undoped and p-doped GaAs and GaAs quantum wells. The laser excitation intensity (I) is varied by nearly three orders of magnitude. The NL PL spectra show negative peaks at energies corresponding to free and bound excitons as I increases. These peaks do not diminish, but continue to increase with I up to the highest value of I used.

Photoluminescence and radiative lifetime of trions in GaAs quantum wells

Electron-trion photoluminescence spectra were measured in undoped high-quality GaAs quantum wells. The obtained line shape depends on temperature and is asymmetric with a tail towards lower photon energies. For a detailed understanding, we have solved numerically the trion Schrodinger equation in the quantum well. Both electron and hole trions are considered. Good agreement is found for the trion binding energy and for the luminescence line shape at different temperatures. The radiative lifetime of thermalized trions is found to increase linearly with temperature. Analytical results are given for both types of trions taking into account the finite photon wave vector (light-cone effect).

Electron and Hole Trions in Wide GaAs Quantum Wells

We study the photoluminescence lineshape of electron and hole trions as function of temperature both experimentally and theoretically. Trion spectra were measured in undoped high-quality GaAs quantum wells. Depending on the excitation energy, an excess hole or electron density is present in the quantum well. The optical response near the band edge is described within density matrix theory for the linear absorption regime. The calculated temperature-dependent asymmetric trion lineshape and the trion binding energy, found from a numerical solution of the trion Schr�dinger equation, as well as its dependence on external electric fields are compared to experiments.

Trions in GaAs Quantum Wells: Photoluminescence Lineshape Analysis

Trion photoluminescence spectra were measured in undoped high-quality GaAs quantum wells. The lineshape is asymmetric with a temperature-dependent tail towards lower photon energies. We have solved numerically the trion Schrödinger equation in the quantum well and found good agreement for both the trion binding energy and the luminescence lineshape at different temperatures. To analyse the relative weight of trion and exciton emission, an equilibrium mass-action law for the respective densities is studied. Using calculated radiative lifetimes, the emission decay after pulsed excitation is found to deviate strongly from a single-exponential shape in case the trions are initially saturated.

Realization of nanopillars on GaAs quantum well structure grown by molecular beam epitaxy

A double barrier heterostructure grown on a n ++ GaAs substrate by Molecular Beam Epitaxy (MBE) was used for the fabrication of random dot pillar arrays of p–i–n quantum well nanoscale devices. The nanostructure was fabricated so as to get the vertical confinement of carriers within the undoped GaAs quantum well (QW) layer of 9 nm thick sandwiched between two Al 0.33Ga 0.67As of 11 nm thick barrier layers using the conventional lithographic and Reactive Ion Etching (RIE) techniques. The width of the pillars had the range of 10–50 nm and the height of the pillars varied from 185 to 250 nm by controlling the etching time from 90 to 180 s, respectively. These nanopillars were characterized by the SEM and the nanoscale structure of the pillars was studied by the Scanning Tunneling Microscope (STM) by applying a negative bias voltage of 3635 mV and the tunneling current of 2.61 nA. The light emission was detected from the nanopillars when applying a forward bias voltage of 1.3 V at 4 K and the emitted light was observed at 830 nm.

Mixing of terahertz and near-infrared radiation in quantum wells in strong magnetic fields

It was recently discovered that undoped GaAs quantum wells illuminated simultaneously by near-infrared (NIR) and THz radiation in strong magnetic fields emit sharp, strong sidebands [Kono et al., Phys. Rev. Lett. 79 (1997) 1758]. We developed a perturbative theory, based on a magnetoexciton model, for this phenomenon and obtained the optical susceptibility. The theoretical results semiquantitatively explain all the experimental results. Moreover, the theory has revealed how the sideband resonances can be related to intra-exciton transitions, indicating the usefulness of the sideband generation as a new tool of magnetoexciton spectroscopy.