High-mobility Two-dimensional Electron Gas Research Articles

3D simulation of quantum-wire confining potential for a GaAs/AlGaAs 2DEG heterostructure

The aim of this work is the study of electrostatically confined single and coupled quantum wires realized within a high-mobility two-dimensional electron gas (2DEG) at a GaAs/AlGaAs heterointerface. The one-dimensional channels are formed by the potential created by suitably biased surface electrodes. The shape of the bottom of the conduction band energy at the heterojunction has been obtained by numerically solving the three-dimensional Poisson equation over the whole structure at 20 K. Special attention has been paid to the depletion condition for the 2DEG within the quantum wires. To this purpose, the 1D Schrödinger equation is solved along the growth direction consistently with the Poisson solution, to accurately calculate the local electron distribution at the heterojunction. Finally, a single-electron wavefunction is propagated within the structure by means of a two-dimensional time-dependent Schrödinger solver, and results are shown for a single-qubit gate able to split the wavepacket. This investigation is part of a feasibility study carried out to identify the experimental requirements for the realization of basic quantum-computing gates.

Electron scattering in AlGaN/GaN structures

We present data on mobility lifetime τt, quantum lifetime τq, and cyclotron resonance lifetime τCR, of a sequence of high-mobility two-dimensional electron gases in the AlGaN/GaN system, covering a density range of 1 to 4.5×1012 cm−2. We observe a large discrepancy between τq and τCR(τq∼τCR/6), and explain it as the result of density fluctuations of only a few percent. Therefore, only τCR—and not τq—is a reliable measure of the time between electron-scattering events in these specimens. The ratio τt/τCR increases with increasing density in this series of samples, but scattering over this density range remains predominantly in the large-angle scattering regime.

Negative parabolic magneto-resistance induced by electron–electron interaction in two-dimensional electron gas with diffusive transport

We experimentally study the parabolic negative magneto-resistance (NMR) in high density (N s >10 12 cm −2) and low mobility (μ<10 3 cm 2/ V s ) two-dimensional (2D) electron gas realized by δ-doping a narrow (5 nm) AlGaAs/GaAs/AlGaAs quantum well. We show that the parabolic NMR in this 2DEG is well described by the theory which connects the parabolic NMR to the existing quantum logarithmic correction to the conductivity of the 2D electron system in the case of the diffusive transport.

Laterally patterned high mobility two-dimensional electron gases obtained by overgrowth of focused ion beam implanted Al 1− xGa xAs

The use of focused ion beam implantation doping of an inverted GaAs/Al 1− x Ga x As heterostructure during a growth interruption allows for the lateral modulation of the heterostructure doping. Hence, laterally patterned two dimensional electron gases (2DEGs) are obtained with no further processing steps required. We have performed the direct writing of a 2DEG with a Hall-bar pattern, such that only the application of ohmic contacts was necessary and the sample surface remained unharmed otherwise. The 2DEG has an electron density of 3.6×10 11 cm −2 and an electron mobility of 4.8×10 5 cm 2/ V s , as determined by magnetotransport measurements. A conventional mesa-etched Hall-bar with almost identical electronic properties has also been studied. Different behaviour of the longitudinal as well as the transversal magnetoresistance for the two Hall-bars is observed and can be concluded to be due to a different confinement potential.

Photoreflectance studies of (Al)Ga- and N-face AlGaN/GaN heterostructures

Photoreflectance studies were carried out on Al x Ga 1− x N/GaN heterostructures confining high-mobility polarization-induced two-dimensional electron gases. By analyzing the Franz–Keldysh oscillations for samples with (Al)Ga- and N-face polarity, we obtained values for the surface electric field up to F=380 kV cm −1 at room temperature. Taking into account spontaneous and piezoelectric polarization, the density of charged surface states and the bare surface potential are estimated. The results unambiguously prove the presence of donor- and acceptor-like surface states for samples with (Al)Ga- and N-face polarity, respectively. A change of the electric field was observed upon the exposure of the surface to a polar liquid, demonstrating the applicability of these structures for chemical sensors.

Photoreflectance studies of (Al)Ga- and N-face AlGaN/GaN heterostructures

Photoreflectance studies were carried out on Al x Ga 1− x N/GaN heterostructures confining high-mobility polarization-induced two-dimensional electron gases. By analyzing the Franz–Keldysh oscillations for samples with (Al)Ga- and N-face polarity, we obtained values for the surface electric field up to F =380 kV cm −1 at room temperature. Taking into account spontaneous and piezoelectric polarization, the density of charged surface states and the bare surface potential are estimated. The results unambiguously prove the presence of donor- and acceptor-like surface states for samples with (Al)Ga- and N-face polarity, respectively. A change of the electric field was observed upon the exposure of the surface to a polar liquid, demonstrating the applicability of these structures for chemical sensors.

Radiation-induced magnetoresistance oscillations in a 2D electron gas

Remarkable recent experiments have shown that microwave radiation can induce dramatic changes in the DC transport properties of a high mobility two-dimensional electron gas. In particular, the diagonal resistivity is an oscillatory function of ω/ ω c, where ω is the microwave frequency and ω c is the cyclotron frequency. The amplitude of the oscillations increases with microwave intensity and eventually saturates as the minimum resistivity approaches zero. We describe a simple model, first proposed many years ago by Ryzhii and collaborators, to explain the effect, and present simplified but detailed and non-perturbative calculations of the non-equilibrium response of the electron gas. We also review some of the many different theoretical pictures that have been proposed to explain the physics and discuss open questions which remain.

High power GaN/AlGaN/GaN HEMTs operating at 2 to 25 GHz grown by plasma-assisted MBE

We report on the growth and power performance of GaN/AlGaN/GaN high electron mobility transistors (HEMTs) grown by plasma-assisted molecular beam epitaxy (MBE) on semi-insulating SiC substrates. We detail the MBE growth conditions that consistently produce high mobility two-dimensional electron gases (2DEGs) with room temperature mobility of ∼1400 cm2/Vs at a sheet density of 1.2 × 1013 cm−2. Transistors fabricated from these layers have demonstrated power densities in excess of 8 W/mm at 2 GHz, 6 W/mm at 7 GHz, and 3 W/mm at 25 GHz. All power data is achieved without the use of a SiN surface passivation layer. Central to the achievement of high power operation is the reduction of RF dispersion. Our growth studies have focused on the suppression of RF dispersion and maximizing RF output power. Pulsed I–V and gate lag measurements are used to quantify the amount of dispersion in different heterostructure designs. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical control of spins in semiconductors

Recent and ongoing optical experiments on mechanisms and methods for control and gating of spin relaxation in semiconductor quantum wells are reviewed. We discuss work on high-mobility two-dimensional electron gases in (001)-grown GaAs/AlGaAs wells which reveals two new aspects of D'yakonov, Perel' and Kachorovskii (DPK) spin dynamics, namely oscillatory spin evolution in a quasi-collision-free regime at low temperatures and strong deviation from the standard expectation that spin-relaxation rate will be proportional to electron mobility at higher temperatures. The latter may indicate that electron–electron scattering, neglected hitherto, is important for spin relaxation. Experiments on (011)-grown GaAs/AlGaAs quantum wells confirm that this orientation leads to extension of electron spin memory by as much as two orders of magnitude at room temperature due to suppression of the major contribution to DPK spin relaxation. Results for a sample with built-in electric field give a strong indication that, for this growth orientation, room temperature spin memory may be gated by external applied voltage.

Magnetization measurements of high-mobility two-dimensional electron gases

De Haas--van Alphen (dHvA) oscillations are observed for Landau levels (LLs) with filling factors between 4 and 52, at temperatures in the range 50 mK to 1 K, in experiments on high-mobility GaAs/(Al, Ga)As heterojunctions. The oscillations become sawtooth-shaped at low filling factors, and theoretical fits to the data, assuming the two-dimensional electron gas to be a non-interacting Fermi system, show the shape of LLs to be close to a \ensuremath{\delta} function. The small residual width (\ensuremath{\sim}0.4 meV or less) fits equally well to either a Gaussian or a Lorentzian density-of-states model. In almost all cases, a constant background density of states has to be included to obtain a satisfactory fit. Weak odd-filling-factor dHvA peaks are detected at high fields, from which a g-factor enhancement of 15 can be inferred. Comparison of the scattering time derived from the fits before and after illumination, with the momentum relaxation time derived from transport, reveals a counterintuitive behavior in the bulk-modulation-doped sample.

An electronic Mach-Zehnder interferometer.

Double-slit electron interferometers fabricated in high mobility two-dimensional electron gases are powerful tools for studying coherent wave-like phenomena in mesoscopic systems. However, they suffer from low visibility of the interference patterns due to the many channels present in each slit, and from poor sensitivity to small currents due to their open geometry. Moreover, these interferometers do not function in high magnetic fields--such as those required to enter the quantum Hall effect regime--as the field destroys the symmetry between left and right slits. Here we report the fabrication and operation of a single-channel, two-path electron interferometer that functions in a high magnetic field. This device is the first electronic analogue of the optical Mach-Zehnder interferometer, and opens the way to measuring interference of quasiparticles with fractional charges. On the basis of measurements of single edge state and closed geometry transport in the quantum Hall effect regime, we find that the interferometer is highly sensitive and exhibits very high visibility (62%). However, the interference pattern decays precipitously with increasing electron temperature or energy. Although the origin of this dephasing is unclear, we show, via shot-noise measurements, that it is not a decoherence process that results from inelastic scattering events.

Precession and motional slowing of spin evolution in a high mobility two-dimensional electron gas.

Optical spin-dynamic measurements in a high-mobility n-doped GaAs/AlGaAs quantum well show oscillatory evolution at 1.8 K consistent with a quasi-collision-free D'yakonov-Perel'-Kachorovskii regime. Above 5 K evolution becomes exponential as expected for collision-dominated spin dynamics. Momentum scattering times extracted from Hall mobility and Monte Carlo simulation of spin polarization agree at 1.8 K but diverge at higher temperatures, indicating the importance of electron-electron scattering and an intrinsic upper limit for the spin-relaxation rate.

Two-dimensional electron-gas actuation and transduction for GaAs nanoelectromechanical systems

We have fabricated doubly clamped beams from GaAs/AlGaAs quantum-well heterostructures containing a high-mobility two-dimensional electron gas (2DEG). Applying an rf drive to in-plane side gates excites the beam’s mechanical resonance through a dipole–dipole mechanism. Sensitive high-frequency displacement transduction is achieved by measuring the ac emf developed across the 2DEG in the presence of a constant dc sense current. The high mobility of the incorporated 2DEG provides low-noise, low-power, and high-gain electromechanical displacement sensing through combined piezoelectric and piezoresistive mechanisms.

Effects of exchange and electron correlation on conductance and nanomagnetism in ballistic semiconductor quantum point contacts

The spontaneous magnetization of a quantum point contact (QPC) formed between two large quantum dots by a lateral confinement of a high-mobility two-dimensional electron gas is studied for a realistic GaAs/AlxGa1-xAs heterostructure. The model of the device incorporates the contributions from a patterned gate, doping, surface states, and mirror charges. To explore the magnetic properties, the Kohn-Sham local spin-density formalism is used with exchange and correlation potentials that allows for local spin polarization. Exchange is the dominant mechanism behind local magnetization within the QPC, while the correlation part is less prominent. However, the correlation potential gives rise to an important correction in the QPC potential. Below the first conduction plateau we thus find a magnetized regime corresponding approximately to a single electron spin. Using an approximate separable saddle potential we compute the conductance and recover the so-called similar to0.7 (2e(2)/h) conduction anomaly plus an additional anomaly at similar to0.4 (2e(2)/h) below which the magnetization collapses.

Fiber-coupled antennas for ultrafast coherent terahertz spectroscopy in low temperatures and high magnetic fields

For the purposes of measuring the high-frequency complex conductivity of correlated-electron materials at low temperatures and high magnetic fields, a method is introduced for performing coherent time-domain terahertz spectroscopy directly in the cryogenic bore of existing dc and pulsed magnets. Miniature fiber-coupled THz emitters and receivers are constructed and are demonstrated to work down to 1.5 K and up to 17 T, for eventual use in higher-field magnets. Maintaining the submicron alignment between fiber and antenna during thermal cycling, obtaining ultrafast (&amp;lt;200 fs) optical gating pulses at the end of long optical fibers, and designing highly efficient devices that work well with low-power optical gating pulses constitute the major technical challenges of this project. Data on a YBCO superconducting thin film and a high mobility two-dimensional electron gas are shown.

Ballistic cooling in a wideband two-dimensional electron gas bolometric mixer

We show that bolometric mixers fabricated from a high mobility two-dimensional electron gas can reach a regime where electron energy relaxation is dominated by ballistic, rather than diffusive, outflow of excited electrons. This ballistic cooling mechanism establishes the maximum physical limit on mixer speed for a transit-time limited device. Intermediate frequency bandwidths of nearly 40 GHz have been obtained in devices with channel lengths &amp;gt;1 μm in ballistically cooled devices.

Even-odd transition in the Shubnikov–de Haas oscillations in a two-dimensional electron gas subjected to periodic magnetic and electric modulations

We investigate low-temperature magnetotransport of high-mobility two-dimensional electron gases subjected to one-dimensional periodic magnetic and electric modulations. Our previous quantum perturbation theory is extended to lower temperatures and the energy broadening due to impurity scattering is incorporated. Numerical calculations are made for situations where several Landau bands overlap. We find that the Shubnikov--de Haas (SdH) oscillations are dominated by collisional resistance. The amplitudes of the SdH oscillations are strongly modulated and the positions of the SdH minima switch between even and odd Landau-level filling factors, in the resistance both parallel and perpendicular to the one-dimensional modulation. This is a consequence of the internal structure (i.e., smeared out van Hove singularities) of overlapping Landau bands. Our theoretical results are in good agreement with recent experiments.

Influence of processing parameters on the transport properties of quantum point contacts fabricated with an atomic force microscope

We describe a reliable technique for fabricating ballistic quantum point contacts (QPCs) with large energy separation between one-dimensional subbands. The technique is based on lithography with an atomic force microscope and wet chemical etching of a GaAs/AlGaAs heterostructure. The high-mobility two-dimensional electron gas located 55 nm below the surface is laterally confined by 20 nm or 50 nm deep grooves with separations ranging between 65–105 nm or 100–185 nm, respectively. The conductance characteristics at T = 4.2 K exhibit clear quasi-plateaux at multiples of 2e2/h. Both the conductance threshold voltage and the plateau widths are directly related to the QPC geometry. The energy separation ΔE1,2 of the lowest subbands is determined from the conductance under nonzero dc drain voltage. Upon reducing the QPC width, ΔE1,2 varies from 6 meV to 15 meV.

Detection of spin-polarized electrons injected into a two-dimensional electron gas.

The spin-dependent mean-free path of electrons in a high-mobility InAs two-dimensional electron gas (2DEG) is measured. Ferromagnetic metal/insulator/2DEG junctions are fabricated on a common channel in a nonlocal geometry and used as spin injectors and detectors. For electrons in spin-orbit eigenstates at 4.5 K, lower bounds for the spin mean-free path and relaxation time are Lambda(S) > or = 4.6 microm and tau(s) > or = 3.8 ps, respectively. The temperature dependence is weak over the range 4.5<T<150 K.

Electrical properties of high-mobility two-dimensional electron gases in Si/SiGe modulation-doped heterostructures grown on silicon-on-insulator substrates

The growth of high-mobility modulation-doped Si/SiGe heterostructures on silicon-on-insulator (SOI) substrates is demonstrated. The structural and electrical properties of the samples are comparable with those of similar samples grown on standard Si substrates. Electron mobilities as high as 2800 cm 2 V −1 s −1 at room temperature and 8.2×10 4 cm 2 V −1 s −1 at 4 K are obtained.