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We report experiments which demonstrate the effects of weak localization on the thermopower of a low-mobility 2D electron gas at a GaAs/AlxGa1?xAs heterojunction. The measurements rely on the weak-magnetic-field dependence of resistivity and the thermopower characteristic of weak localization. The effects are visible only at temperatures well below 1 K and are consistent with theoretical predictions that they occur only in the diffusion component, not in the phonon-drag component. We also find that phonon drag shows an unexpected field dependence at low temperatures. This does not appear to be related to localization and it is readily separated from the weak localization effects in diffusion.

The fluctuations of a square quantum dot have been studied as a function of the dot width. Measurements were made at magnetic fields ranging from the zero-field to edge-state regimes. The experimental results are compared with numerical quantum calculations and semiclassical periodic orbit theory.

We report far-infrared transmission measurements performed on a two-dimensional electron gas (2DEG) with a lateral modulation of its concentration induced using a split-gate technique. Under magnetic field, when the photon energy is close to the plasmon energy of the modulated 2DEG, a strong coupling of the plasmon with the cyclotron resonance harmonics (up to n = 5), known as Bernstein modes, is observed. At a fixed wavelength of , the behaviour of these modes is studied as a function of magnetic field while the modulation strength is varied in the quasi-two-dimensional regime by changing the gate voltage. Experimental data are compared with a classical theory based on the Boltzmann kinetic equation [A V Chaplik and D Heitmann 1985 J. Phys. C: Solid State Phys. 18 3357].

The quest for a universal quantum computer has renewed interest in the growth of superconducting materials on semiconductor substrates. High-quality superconducting thin films will make it possible to improve the coherence time of superconducting quantum bits (qubits), i.e., to extend the time a qubit can store the amplitude and phase of a quantum state. The electrical losses in superconducting qubits highly depend on the quality of the metal layers the qubits are made from. Here, we report on the epitaxy of single-crystal Al (011) layers on GaAs (001) substrates. Layers with 110 nm thickness were deposited by means of molecular beam epitaxy at low temperature and monitored by in situ reflection high-energy electron diffraction performed simultaneously at four azimuths. The single-crystal nature of the layers was confirmed by ex situ high-resolution x-ray diffraction. Differential interference contrast and atomic force microscopy analysis of the sample's surface revealed a featureless surface with root mean square roughness of 0.55 nm. A detailed in situ study allowed us to gain insight into the nucleation mechanisms of Al layers on GaAs, highlighting the importance of GaAs surface reconstruction in determining the final Al layer crystallographic orientation and quality. A highly uniform and stable GaAs (001)- reconstruction reproducibly led to a pure Al (011) phase, while an arsenic-rich GaAs (001)- reconstruction yielded polycrystalline films with an Al (111) dominant orientation. The near-atomic smoothness and single-crystal character of Al films on GaAs, in combination with the ability to trench GaAs substrates, could set a new standard for the fabrication of superconducting qubits.

We demonstrate the detection of mid-wavelength (3-5 mu m) infrared radiation by intersubband transitions in InxGa1-xAs/Al0.45Ga0.55As multiple quantum well structures grown on GaAs substrates. The peak detector response is shifted from 4.8 to 4.3 mu m by increasing the indium fraction from x=0.05 to x=0.20, while simultaneously decreasing the well width to keep the first excited eigenstate near the top of the wells. These detectors can be combined in detector stacks with conventional long-wavelength (8-12 mu m) infrared quantum well detectors for multi-band imaging applications.

Quantum well (QW) intermixing has been performed using low-energy broad-area ion implantation to increase the bandgap energy in a spatially selective manner. There is a maximum single dose beyond which further intermixing of the QWS is impeded by damage to the semiconductor surface. We demonstrate that this problem can be overcome by using a series of implants and rapid thermal anneals, with each rapid thermal anneal repairing the crystal surface. Using this technique we have observed shifts in optical bandgap for multiple implants greater than 2.5 times that observed for a single implant.

A systematic comparison of quantum well infrared photodetectors (QWIPS) made by different molecular beam epitaxy (MBE) systems is presented. QWIPS with identical layer specifications were requested from three different MBE facilities. Measurements of dark current, detector responsivities, infrared absorption and detectivities were carried out. Although good uniformity between detectors was obtained within the same MBE wafer, variations were observed between samples with the same specifications from different MBE systems. The results of this study illustrate the present accuracy of MBE technology.

The external efficiency of AlGaAs/GaAs photon-recycling light-emitting diodes operated at low temperature (80 K) and low current densities has been maximized by adjusting the beryllium doping concentration in the active region. A concentration of 8×1016 cm-3 was found close to the optimum: below this value the bimolecular recombination appears and above impurity-related transitions penalize the efficiency of photon-recycling effects.

The authors investigate the possibility of long-wavelength infrared (LWIR) photodetection using a single quantum well in an asymmetrical double-barrier resonant tunnelling structure. The goal is to produce very fast (>10 GHz) LWIR detectors. GaA-AlGaAs structures with both Si-doped and undoped quantum wells designed for the 8-12 mu m wavelength region have been studied. Electrons responsible for the absorption are dynamically stored in the well for the undoped case under a finite bias voltage. A circuit-limited temporal response risetime of about 800 ps was observed. The device physics can be understood using a simple resonant tunnelling model. To compare with the experiments quantitatively, self-consistent numerical calculations have been carried out.

Magnetic-field dispersion of the multiexcitons related to the p shell of a single quantum dot (QD) is analysed in this work. The reduced cyclotron effective mass of carriers is determined from the energy splitting between the p+-and p - related multiexcitonic emission lines. The reduced mass in the occupied QD was found to be larger than the mass related to the QD's single particle structure. The apparent increase of the reduced mass with increasing excitonic occupation of the dot is related to the mass renoralization due to electron-electron interactions within a multiexcitonic droplet. © Published under licence by IOP Publishing Ltd., 20th International Conference on the Application of High Magnetic Fields in Semiconductor Physics, HMF 2012, 22 July 2012 through 27 July 2012, Chamonix Mont Blanc

We demonstrate true-blue (450–460 nm) nitride-based laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy (PAMBE) on c-plane GaN substrates operating in continuous wave mode. The maximum optical power measured for these LDs is 80 mW, while the lifetime at the optical power of 10 mW is longer than 2000 h. We present a new LD design consisting of a separate confinement heterostructure with GaN claddings and thick In0.08Ga0.92N waveguides. We show that the key element responsible for the improvements in performance of the true-blue PAMBE LDs is the growth of InGaN quantum wells using a high nitrogen flux.

Room-temperature continuous wave lasing at 432nm with a threshold current of 7.6 kA/cm2 for nitride-based laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy is reported. The diodes were grown on c-plane GaN substrates with a threading dislocation density of 5 × 107 cm -2. We used a simplified laser structure design with GaN claddings where the optical modes were confined by the thick 120nm In 0:08Ga 0:92N waveguide. Our LDs show a high optical output power of 130mW, a differential gain of 0.5 W/A, and a lifetime of 50 h. © 2012 The Japan Society of Applied Physics.

We report an investigation of quantum Hall induced currents by simultaneous measurements of their magnetic moment and their effect on the conductance of a quantum point contact (QPC). Correlation of features in the noise of the induced currents, caused by the breakdown of the quantum Hall effect, for the two types of measurements provides conclusive proof of the common origin of the two effects. Common features in the magnetic moment and QPC resistance at Landau-level filling factors ν = 1,2 and 4 and their similar temperature and nonlinear sweep-rate dependences support this conclusion. In contrast, there is a distinct difference in the way the induced currents decay with time when the sweeping field halts at integer filling factor as detected by the two types of measurement. We attribute this difference to the fact that, while both effects are sensitive to the magnitude of the induced current, the QPC resistance is also sensitive to the proximity of the current to the QPC split gate, and we develop a model that describes semi-quantitatively the effects we observe. Although it is clearly demonstrated that induced currents affect the electrostatics of a QPC, the reverse effect, the QPC influencing the induced current, is not observed.

Our latest research results on GaAs–AlGaAs multiple quantum well spatial light modulators are presented. The thickness uniformity of the epitaxial layers across the 3-inch wafer grown by our molecular beam epitaxy is better than 0.1% and the variation of cavity resonance wavelength within the wafer is only 0.9 nm. A contrast ratio (CR) of 102 by varying bias voltage from 0 to 6.7 V is achieved after fine tuning the cavity by etching an adjust layer. Both theoretical and experimental results demonstrate that incorporating an adjust layer is an effective tuning method for obtaining high CR.

We have performed current–voltage (I–V) measurements on a terahertz quantum-well photodetector (QWP) at different temperatures and employed an emission-capture model to simulate the I–V curves. A temperature-dependent vertical electron drift mobility has been used to fit the curves from 7 K to 20 K. Photocurrents caused by 300 K background radiation have also been measured at different temperatures and a background-limited infrared performance (blip) temperature of 12 K for this terahertz detector has been determined. The current–temperature (I–T) curves derived from the measured dark I–V curves indicate that the thermionic emission process is the major mechanism for dark current in this terahertz detector.

The authors present a simulation and experimental study on the effect of phonon extraction level separation on the performance of GaAs-based three-well resonant-phonon terahertz quantum-cascade lasers (QCLs). The phonon extraction level separation is varied from 30 to 42 meV. Because of the efficient longitudinal-optical phonon scattering, the 36 meV QCL shows the largest gain, the best temperature performance and the highest output power. As for the lower (30 meV) or higher (42 meV) energy separation QCLs, the electron–longitudinal-optical phonon interaction still works by involving a transfer of in-plane momentum. The measured lasing characteristics are in qualitative agreement with simulation.

We present a theory and experiment demonstrating optical readout of charge and spin in a single InAs/GaAs self-assembled quantum dot. By applying a magnetic field we create the filling factor 2 quantum Hall singlet phase of the charged exciton. Increasing or decreasing the magnetic field leads to electronic spin-flip transitions and increasing spin polarization. The increasing total spin of electrons appears as a manifold of closely spaced emission lines, while spin flips appear as discontinuities of emission lines. The number of multiplets and discontinuities measures the number of carriers and their spin. We present a complete analysis of the emission spectrum of a single quantum dot with N=4 electrons and a single hole, calculated and measured in magnetic fields up to 23 Tesla., Comment: 9 pages, 3 figures, submitted to Europhysics Letters

Recently vertical quantum dots containing just a few electrons have been fabricated and studied. The long sought after artificial atom nature of these dots was confirmed in these dramatic experiments. In this paper, however, we describe the fabrication of a lateral few electron quantum dot defined by electrostatic gates over a high mobility 2DEG. We present some of the spectroscopic measurements we have performed on these dots and use them to confirm that the number of electrons in the dot lies between 10 and 20.

The magneto-optical spectrum of the shallow donors in n-GaAs is studied using a far-infrared laser at photon energies greater than the binding energy of the donors. The transitions from the (1s) ground state into the Landau levels are not observed but, instead, series of transitions from the ground state into donor states associated with the individual Landau levels. The strongest series corresponds to the (000)-(0N0) transitions and the second strongest series to (000-0N2) transitions where N is the index of the Landau level involved (and where the high-field notation is employed). The corresponding low-field notation is 1s-2p+, 1s-3d+2, 1s-4f+3, 1s-5g+4 etc. for the strongest series which can be followed up to the N=17 Landau level. Central cell structure due to the presence of several different donors can be observed on many of the lines associated with the N=1 and N=2 Landau levels. The 1s-3p+ transition can be used to determine chemical shifts accurately for the donor species present, as this line avoids the overabsorption effects which can distort the stronger 1s-2p+ line. A few weak lines remain unidentified.

Far-infrared photothermal ionisation spectroscopy is used to investigate the nature of the donors present in high-purity epitaxial films of GaAs grown by metal-organic chemical vapour deposition (MOCVD), metal-organic gas source molecular beam epitaxy (MOMBE) and solid source molecular beam epitaxy (MBE). Except for one lower mobility sample, the MOCVD samples showed the donor line X1 to be dominant with >50% relative intensity. The higher mobility samples showed the presence of two relatively closely spaced components to X1. Hydrostatic pressures up to 20 kbar were used to increase the chemical shifts and to vary the absorption strengths of the lines. By this means, and by the simultaneous observation of the same components on the 1s-2p and 1s-3p transitions, it was possible to show that these components were not artefacts of over-absorption but arose from two different donor species of very similar chemical shifts. Consequently, any identification of donor X1 as being solely due to silicon must be treated with caution. Germanium and sulphur accounted for the remaining donor contamination in the MOCVD material, with germanium usually being present at higher levels than sulphur. In contrast, earlier far-infrared and photoluminescence data with samples of different origin have shown germanium to be dominant in MOCVD material. MOMBE growth produced very similar proportions of X1, germanium and sulphur, indicating that the gas sources are responsible for the contamination. The MBE samples grown in reactors of different manufacture showed levels of sulphur donors comparable with the background carbon acceptors as well as traces of germanium, in addition to the silicon deliberately introduced. The results for the highest mobility MBE sample with deliberate silicon doping suggest that the lower energy component of the 'X1' doublet is due to silicon. The most likely candidate for the higher energy component is selenium. An even higher mobility MBE sample which was n-type without deliberate silicon doping showed the higher energy component attributed to selenium.

Far-infrared laser spectroscopy and Fourier spectroscopy are used in order to study the influence of hydrostatic pressure on shallow donor and cyclotron resonance spectra in high-purity epitaxial films of n-GaAs. Accurate pressure variations for the effective mass and dielectric constant are obtained at a temperature of 4.2K and pressures of up to 17.3kbar. These parameters are subsequently used to interpret the pressure variation of the chemical shifts of the 1s and 2s levels. It is shown that the dependence of these chemical shifts on magnetic field at fixed pressure can be described well by lowest-order perturbation theory. The ratio of the chemical shifts of the 1s level to the corresponding chemical shifts of the 2s level at zero magnetic field is found to be pressure independent and equal to 7.2+or-0.3 for all donors. A much stronger increase of chemical shifts than that predicted by lowest-order perturbation theory is observed on increasing pressure. In addition, a resonant interaction of the 1s level of the deepest shallow donor with a strongly localised level of the same impurity centre is observed on increasing pressure. A qualitative interpretation of all these phenomena is given.

High pressure equipment designed for low temperature magneto-optical experiments is described. The piston and cylinder type of pressure vessel was adapted to allow photocurrent experiments to be carried out at low temperatures and high magnetic fields. With a sapphire window the cell can be used in the far infrared region (3 mm-25 mu m wavelength) and in the near infrared at wavelengths shorter than 5.5 mu m. Hydrostatic pressures up to 1.8 GPa can be achieved at 4.2K. A suitable choice of window material allows the cell to be used in all spectral regions from far infrared to ultraviolet and over a temperature range from liquid helium to 500K. The use of a carbon bolometer allows transmission experiments to be performed.