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We present time-resolved photoluminescence investigations of InAs/GaAs structures containing quantum dots with the ground state at 1.43 eV. State fllling efiect and a Pauli blocking efiect were clearly observed. These efiects signiflcantly in∞uenced dynamics of excitation transfer from upper to lower state inside a dot leading to non-exponential dynamics. Numerical model based on nonlinear rate equations was proposed. The model described well the experimental data providing values of: lifetime of the ground state 0:53 § 0:03 ns, lifetime of excited state (when the ground state is full) 1:1 § 0:2 ns, and internal relaxation time (when the ground state is empty) 0:07 § 0:01 ns.

Coulomb blockade studies of lateral quantum dots and measurements of the quantised conductance of quantum point contacts, in high magnetic fields, reveal novel features which are hysteretic in magnetic-field sweep direction. These features are associated with long-lived eddy currents, induced in the 2D electron gas leads in these devices as the magnetic field sweeps and the 2DEG enters the quantum Hall effect state. Torsion-balance magnetometry measurements confirm the presence of these induced currents, and their influence on the nanostructures. The decay of the eddy currents, after the magnetic field sweep is stopped, exhibits two distinct regimes: a fast initial exponential decay followed by a much longer power-law decay in which the size of the eddy current falls typically to 60% of its original value in one day. The interpretation of these observations is that the Coulomb blockade and quantum point contact devices are influenced by the local Hall potential at the edges of the 2DEG leads. The Hall potential changes the local chemical potential of the leads, and hence the properties of the devices, in a manner which reverses when the field sweep direction is reversed.

GaInNAs(Sb)/GaNAs double quantum well (DQW) structures were grown on GaAs substrates using solid-source molecular beam epitaxy with N 2 /Ar gas mixtures in a radio frequency plasma cell. A novel method of in situ antimony mass spectrometry is introduced which permits flux monitoring in the presence of large arsenic background pressures. For a DQW sample grown without Sb, bright and narrow (38.1 meV) room temperature photoluminescence (PL) emission at 1509 nm was achieved after optimized rapid thermal annealing. In two samples grown with antimony fluxes of approximately 0.012 and 0.028 monolayers/s the PL intensity improved and very bright PL was observed at 1518 and 1551 nm with linewidths of 33.1 and 35.0 meV, respectively. The integrated PL intensities of each of these two samples was equivalent to the emission for a reference GaInNAs/GaAs DQW sample emitting closer to 1.3 μ m . More strikingly, the intensity of the Sb-free 1509 nm sample was only lower by a factor of 2. This suggests that the N 2 /Ar plasma approach has benefits for the material quality, as well as providing efficient flux control, yielding good material even without Sb. High-resolution X-ray diffraction and transmission electron microscopy measurements indicate excellent crystal quality for all samples. Secondary ion mass spectrometry reveals a dramatic tendency for Sb segregation during growth, resulting in very asymmetric incorporation with most of the Sb atoms located at the top interface.

We present transport properties of AlGaN/GaN heterostructures grown over high-pressure bulk GaN substrates. The experimental results include the conductivity tensor measurements in a magnetic field up to 23 T in a wide temperature range 2 K–300 K for Hall bar samples. The room temperature high field data allow us to clearly separate contributions of a parasitic parallel conduction from 2DEG conduction in all investigated heterostructures. The room temperature mobility limit for 2D electrons in GaN/AlGaN heterojunctions grown on defect free GaN bulk substrates is around 2400 cm2/Vs. The Quantum Hall Effect studies are performed in the magnetic fields up to 23 T and temperatures between 1.6 K and 15 K This high magnetic field in combination with very high mobility (over 60000 cm2/Vs) in the sample grown on the bulk GaN substrate allow us to determine the activation energy in cyclotron gap from longitudinal magnetoresistance. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-assembled InAs/GaAs quantum dot molecules have been studied spectroscopically in order to obtain a comprehensive picture of the exciton states confined in them. Photoluminescence studies show a characteristic splitting of the quantum dot shells, which clearly hints at a tunnel coupling of the dots. This coupling was confirmed by exciton fine structure studies, where distinct anticrossings are observed in its magnetic field dispersion. These avoided crossings would not occur if the two dots were decoupled. Four-wave mixing studies show a strong shortening of the exciton dephasing in the molecules as compared to quantum dots which appears to be problematic for application of excitons as the genuine bits in quantum information processing. Finally, we demonstrate that the molecule coupling can be controlled by applying an electric field along the molecule axis.

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.

For the majority of applications involving detection of weak signals or thermal imaging, the quantum well infrared photodetector (QWIP) is designed to have the highest possible detectivity and operating temperature. The device parameters, such as the doping density, are chosen accordingly. In a different direction, the intrinsic short carrier lifetime (~ 5 ps) makes QWIPs well suited for high speed and high frequency applications. In such cases, since lasers are normally used, a high dark current can be tolerated. The most important parameter is then the absorption efficiency. For system simplicity and potential wide use, room temperature or near room temperature (reachable by thermo-electric cooling) operations are desirable. This paper discusses the QWIP design for high absorption and elevated temperature operation, and present a systematic experimental study on a set of GaAs/AlGaAs QWIPs with different doping densities. High absorption (~ 100%) and up to room temperature operation are achieved in devices having high doping densities and 100 quantum wells.

We explore growth effects leading to size and compositional limitations in the production of self-assembled quantum dots (QD) emitting at long wavelengths. Molecular beam epitaxy grown QDs are studied as a function of arsenic pressure at a specific InAs coverage, and as a function of InAs coverage for three arsenic pressures. As a function of increasing the arsenic pressure used in QD growth, the photoluminescence (PL) of capped QDs is first redshifted at low arsenic pressures, and then blueshifted at high arsenic pressures. Microscopy of uncapped QDs shows that as the arsenic pressure increases, the QD density increases while the average QD width and height decrease monotonically; these trends are consistent with the shift in PL for the high arsenic pressure samples, but are inconsistent with the shift in PL for the low-pressure samples. This points to a modification of the QDs during capping. We discuss prior reports pertaining to arsenic pressure and capping effects, and in this context describe our observations of the effects of adjusting the arsenic pressure on the formation of QDs and the mechanism by which QDs may be modified during capping.

The exciton states in self-assembled InAs/GaAs quantum dot molecules have been studied by optical spectroscopy and the results have been compared to detailed theoretical calculations. We demonstrate that excitons in symmetric molecule structures form entangled quantum states with the degree of entanglement controlled by tunneling and Coulomb interaction.

Detailed Raman measurements were performed on the optical phonons in the disordered alloy Ga1−xAlxAs at room temperature. The results obtained are compared with earlier measurements and expressions are given for the Al concentration dependence of the AlAs-like and GaAs-like longitudinal and transverse optical modes. Copyright © 2002 John Wiley & Sons, Ltd.

We report planar integration of tapered terahertz (THz) frequency quantum cascade lasers (QCLs) with metasurface waveguides that are designed to be spoof surface plasmon (SSP) out-couplers by introducing periodically arranged SSP scatterers. The resulting surface-emitting THz beam profile is highly collimated with a divergence as narrow as ~4° × 10°, which indicates a good waveguiding property of the metasurface waveguide. In addition, the low background THz power implies a high coupling efficiency for the THz radiation from the laser cavity to the metasurface structure. Furthermore, since all the structures are in-plane, this scheme provides a promising platform where well-established surface plasmon/metasurface techniques can be employed to engineer the emitted beam of THz QCLs controllably and flexibly. More importantly, an integrated active THz photonic circuit for sensing and communication applications could be constructed by incorporating other optoelectronic devices such as Schottky diode THz mixers, and graphene modulators and photodetectors. Published version

We report our research on processing of AlGaAs/GaAs structures for THz quantum-cascade lasers (QCLs). We focus on the processes of fabrication of Ti/Au claddings for metal-metal waveguides and the wafer bonding with indium solder. We place special emphasis on the optimum technological conditions of these processes, leading to working devices. The wide range of technological conditions was studied, by use of test structures and analyses of their electrical, optical, chemical and mechanical properties, performed by electron microscopic techniques, energy dispersive X-ray spectrometry, secondary ion mass spectroscopy, atomic force microscopy, fourier-transform infra-red spectroscopy and circular transmission line method. On the basis of research a set of technological conditions was selected, and devices lasing at the maximum temperature 130K were fabricated from AlGaAs/GaAs structures grown by molecular beam epitaxy (MBE) technique. Their threshold-current densities were about 1.5kA/cm2. Additionally we report our initial stage research on fabrication of Cu-based claddings, that theoretically are more promising than the Au-based ones for fabrication of low-lossy waveguides for THz QCLs.

The effect of interface roughness scattering on performance of indirectly-pumped terahertz quantum cascade lasers is studied and a dual-barrier structure is proposed to improve its performance in terms of threshold current density and operating temperature., CLEO : Science and Innovations, 2014, June 8-13, 2014, San Jose, California

We present experimental and theoretical results demonstrating the readout of a single spin based quantum bit by current detection. The quantum bit has been built by isolating a single electron spin in a lateral dot with a tunable number of electrons. The spin state of the dot was read from the current amplitude resulting from the injection of spin polarized electrons. The current amplitude showed a systematic dependence on the spin state of the dot controlled by the number of electrons.

We present experimental results on quantum-well photodetectors for visible or near-infrared and middle- or far-infrared dual-band detection. We report on two types of devices based on (1) InGaAs/InP and (2) GaAs/AlGaAs quantum wells. In the first case, InGaAs/InP quantum-well infrared photodetectors (QWIPs) for both near and middle infrared spectra are shown. In the second case, large bandgap top contacts were used on standard GaAs/AlGaAs QWIPs so that visible light could reach the quantum-well region and be absorbed via interband transitions. Two large band gap top contacts were investigated, using a high Al fraction AlGaAs and a short period GaAs/AlAs superlattice. We evaluate and analyze the detector performance. We find that such devices are potentially useful for applications involving dual-band simultaneous detection and imaging.

Single and multiple layers of self-assembled InAs quantum dots (QDs) produced by the indium-flush technique have been studied by transmission electron microscopy (TEM) in an effort to develop techn ...

In this work, experimental results showing very slow (up to 104 s) dark current transients in n-type GaAs/Al0.27Ga0.73As quantum-well photodetectors (QWIPs) are reported. The transients with amplitudes of 0.1% to 65% of the steady-state current have been observed at 77 K. These effects are believed to be associated with initially ionized deep levels acting as traps reducing the positive charge in the structure. The time constant of the dark current (transient) decreases with increasing temperature with an experimentally determined activation energy ∼75 meV . A fitting of the capture cross section to σ(T)=σ 0 exp (E c /k B T) gives an estimate for the capture activation energy of Ec∼35 meV.

Quantum dot laser diodes with up to five well-defined electronic shells are fabricated using self-assembled quantum dots (QDs) grown by molecular beam epitaxy. At 77 K, we tune the lasers from the first to the fourth excited state of the QDs by varying the cavity length, this covers a wavelength range from 869 to 963 nm. At room temperature, we obtain lasing from the second to the fourth excited state covering the 938 to 984 nm wavelength range. For high injection currents, a large part of the QD ensemble contributes at once to the stimulated emission yielding a lasing emission linewidth having a full width at half maximum of 25 nm. By increasing the energy spacing between the QD energy level contributing to the lasing and the wetting layer energy levels, improved thresholds at higher temperatures are observed, leading to lasing below 100 A/cm2 at room temperature.Quantum dot laser diodes with up to five well-defined electronic shells are fabricated using self-assembled quantum dots (QDs) grown by molecular beam epitaxy. At 77 K, we tune the lasers from the first to the fourth excited state of the QDs by varying the cavity length, this covers a wavelength range from 869 to 963 nm. At room temperature, we obtain lasing from the second to the fourth excited state covering the 938 to 984 nm wavelength range. For high injection currents, a large part of the QD ensemble contributes at once to the stimulated emission yielding a lasing emission linewidth having a full width at half maximum of 25 nm. By increasing the energy spacing between the QD energy level contributing to the lasing and the wetting layer energy levels, improved thresholds at higher temperatures are observed, leading to lasing below 100 A/cm2 at room temperature.

The spin dynamics of free carriers is studied by time-resolved photoluminescence in n-type GaAs/AlGaAs modulation-doped multi-quantum wells. We study a series of samples with different two-dimensional electron densities. The photoluminescence spectra obtained at fixed time delay present energy splitting for the two different circularly polarized configurations. This energy splitting increases with the laser power and decreases as the intrinsic electron density increases. We interpret these results as a combination of two effects: the screening of the Coulomb interaction which decreases the exciton binding energy and the phase-space filling, which enhances the splitting between the two polarized exciton populations.

We investigate correlation effects in few-electron lateral quantum dots between filling factor ν=2 and 1. The correlations are the non-trivial spin depolarization events associated with the evolution of the total spin of the droplet with increasing magnetic field. We monitor the spin evolution by measurements of coulomb blockade (CB) peaks on lateral quantum dot devices containing from 5 to 20 electrons. Measurements are made as a function of the magnetic field, temperature, and source–drain bias. The temperature and high source–drain bias dependence of the CB peaks in the vicinity of the spin-flip are consistent with spin depolarization events due to electronic correlations.

We confirm the existence of a new regime for small few-electron open dots, in which the transport occurs through individual eigenstates of the corresponding closed dot. In particular, at low magnetic fields the characteristic features in the conductance are related to the underlying eigenspectrum shells. When the number of modes in the leads is reduced, a more detailed structure becomes discernible within the shells due to single eigenlevels. At higher fields Landau level condensation is evident as well as the crossing of levels collapsing to different Landau levels.

This paper considers the nature of anodic and thermal oxides on GaAs and AlxGa1−xAs, with x=0.2, 0.5 and 0.8. Anodic oxides were produced in NH4H2PO4 solution and thermal oxides formed in O2 at 500°C. The chemical composition of the oxides (⩽30 nm thick) has been evaluated using secondary ion mass spectrometry (SIMS), complemented by X-ray photoelectron spectroscopy (XPS) and other techniques. Anodic oxides on GaAs comprise both Ga2O3 and As oxides. For AlxGa1−xAs, Ga is enriched in the outer part of the film due to Ga-phosphate formation. The higher the Al content in the oxide, the less Ga-phosphate is present. Al is enriched at the inner interface, the amount increasing with increasing Al content in the substrate. These anodic films basically comprise a three-layer structure: Ga phosphate:Ga/As oxide:Ga/As/Al oxide. Thermal oxides on GaAs consist almost entirely of Ga2O3, with a small As enrichment at both the gas–oxide and the oxide–substrate interfaces. This is also the case for thermal films on Al0.8Ga0.2As, but now the main components of the film are Ga2O3 and Al2O3. Unlike anodic oxides, thermal oxides on both AlxGa1−xAs and GaAs are stable in the air and do not change perceptibly over a period of several months.

A new procedure for the growth of stacked self-assembled quantum dot layers is described. The main effect of the procedure is to convert the quantum dot population into a population of quantum disks of approximately equal height. Proposed model of the overgrowth process for highly strained 3D islands invokes mechanisms which may lead to a variety of dot shape modification and opens up new ways of influencing the dot population by deliberate control of the overgrowth process. Demonstrated very good performance of test devices indicate that the proposed procedure may have positive impact on the further development of quantum dot lasers.

Magnetospectroscopic studies of the InAs/GaAs wetting layer (WL) accompanying self-organized quantum dots (QDs) grown in Stranski–Krastanow mode are presented. Sharp emission lines from “natural” QDs formed in the WL are observed and their evolution in magnetic field is investigated. A quadruplet or a doublet splitting of the emission lines has been observed in magnetic field, which is attributed to a charged and a neutral exciton in a single dot. The metastable transformation of the spectrum from the former to the latter configuration has been observed and explained in terms of the charging of a single dot.

We have fabricated few electron lateral dots containing between 10 and 20 electrons. Single electron spectroscopic techniques have been used to investigate the regime between ν =2 and ν =1, i.e., the transition from a spin unpolarized quantum dot to a spin polarized quantum dot. We compare the experimental results to non-interacting, Hartree–Fock and exact diagonalization calculations. The comparison confirms that correlations are important in this regime. Spin-flips are found to be complex and rich phenomena involving a nontrivial spin polarization dependence on magnetic field and a resulting collapse of the Zeeman gap due to electronic correlations.

A systematic experimental and theoretical study has been performed on the conductance oscillations of a rectangular dot as a function of the dot width at different fixed magnetic fields (including zero field). We provide a semiclassical interpretation of the fluctuations at zero and very low magnetic fields $(\ensuremath{\lesssim}0.05$ T) in terms of specific families of periodic orbits, which occur in a classical rectangular billiard. At higher magnetic fields $(0.1\ensuremath{\lesssim}B\ensuremath{\lesssim}0.25 \mathrm{T})$ the fluctuations become more aperiodic with reduced amplitudes while the Fourier spectrum contains no significant peaks. With a further increase in the field the cyclotron orbit fits within the dot and now a single frequency dominates the fluctuations, which is consistent with the (semi)classical predictions in this field region. Finally, as the edge-state regime is reached, Aharonov-Bohm oscillations take over.

Magnetooptic Kerr effect in GaAs/Al0.312 Ga0.688 As multiple quantum wells was investigated in the integer quantum Hall regime. The measurements have been performed in magnetic fields up to 14.5 T, at the temperature of 1.8 K. Experimental data indicate the discontinuous behavior of the magnetooptic Kerr effect spectrum as a function of the filling factor. For odd filling factor values v = 3 and 5 we observe the large spin splitting. The effects cannot be explained in the one-particle model.

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.

The integer and fractional quantum Hall effects have been investigated using the acoustoelectric effect in both two-dimensional hole and electron systems fabricated by MBE on GaAs. Measurements have been made of the longitudinal and transverse acoustoelectric voltages as a function of magnetic field up to 16 T at a temperature of 340 mK. We report behaviour close to ν= 1 2 in the dispersion and acoustoelectric effect that cannot be explained on the basis of quasi-DC measurements. Finally, we find that a tensor relationship exists in the acoustoelectric effect similar to that reported recently in thermoelectric measurements.

Conductance fluctuations have been studied in a soft wall stadium and a Sinai billiard defined by electrostatic gates on a high mobility semiconductor heterojunction. These reproducible magnetoconductance fluctuations are found to be fractal confirming recent theoretical predictions of quantum signatures in classically mixed (regular and chaotic) systems. The fractal character of the fluctuations provides direct evidence for a hierarchical phase space structure at the boundary between regular and chaotic motion., Comment: 4 pages, 4 figures, data on Sinai geometry added to Fig.1, minor changes

In InAlAs/InGaAs double heterojunction bipolar transistors (DHBTS) with composite collectors, the reduction in current gain as the collector-base voltage is decreased (current blocking) is almost independent of temperature. This is in contrast to InP/InGaAs devices, where the blocking strongly increases as temperature is reduced. We attribute the temperature independence in InAlAs/InGaAs devices to electrons that are not thermalized as they cross the base. Our results have implications in the design of collectors for InAlAs/InGaAs DHBTs.

Far-infrared transmission measurements have been performed on a two-dimensional electron gas (2DEG) with a lateral modulation of its concentration induced using an electrostatic gating technique. A strong coupling of the plasmon with the cyclotron resonance harmonics (of index up to n=5), known as Bernstein modes, is observed when the photon energy is close to the plasmon energy of the modulated 2DEG. The behavior of these modes is studied as a function of magnetic field and modulation strength for λ=163 and 229 μm. A classical theory based on the Boltzmann kinetic equation [A. V. Chaplik and D. Heitmann, J. Phys. C 18, 3357 (1985)] gives a good account of the main experimental features.

Room temperature Raman measurements on a series of molecular beam epitaxy grown In0.53Ga0.47−xAlxAs epilayers lattice matched to InP substrates have been carried out for the composition range 0⩽x⩽0.08. This low Al composition range is important for tailoring the well gap energy in communication device fabrication. The Raman data show three-mode phonon behavior for films with Al concentrations as low as x=0.03. In addition to detecting the fundamental one-phonon scattering processes, overtone, and combinations of the GaAs-like and AlAs-like optic phonon modes are recorded. These are then used to determine the composition dependent phonon mode shifts. By comparing the separate estimates of the free carrier band-gap energy using low temperature absorption and photoluminescence measurements, unintentional doping of the epilayers by the substrate is revealed.

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 temperature dependence of capacitance and ac conductance for GaAs/AlGaAs multiquantum well (MQW) structures with 4, 8, 16, and 32 wells has been studied at different bias and frequency ranges. The dominant contribution to ac conductance depends on temperature and frequency, and changes from thermally assisted tunneling through the first excited state to thermionic emission with activation energies of 100±15 meV and 170±10 meV, respectively. In the temperature interval 80–100 K, conductance and capacitance have two distinct regions of frequency dispersion due to tunneling and thermionic emission respectively, giving rise to two relaxation times. Above 100 K, increased conductance makes electrical response of the MQW structure so fast that at frequencies up to 1 MHz relaxational properties are masked. A model based on a series circuit comprised of barrier resistance and capacitance in parallel shows results in good agreement with experimental data, indicating strong nonuniform conductance of MQW structures.

Nontrivial capacitance behavior, including a negative capacitance (NC) effect, observed in a variety of semiconductor devices, is discussed emphasizing the physical mechanism and the theoretical interpretation of experimental data. The correct interpretation of NC can be based on the analysis of the time-domain transient current in response to a small voltage step or impulse, involving a self-consistent treatment of all relevant physical effects (carrier transport, injection, recharging etc.). NC appears in the case of the non-monotonic or positive-valued behavior of the time-derivative of the transient current in response to a small voltage step. The time-domain transient current approach is illustrated by simulation results and experimental studies of quantum well infrared photodetectors (QWIPs). The NC effect in QWIPs has been predicted theoretically and confirmed experimentally. The huge NC phenomenon in QWIPs is due to the non-equilibrium transient injection from the emitter caused by the properties of the injection barrier and the inertia of the QW recharging., Comment: 9 pages, Latex, 11 ps figures; to be published in IEEE Trans. on Electron Devices

We have performed a detailed study of low-temperature photoluminescence ~PL! from a pair of modulationdoped GaAs/AlxGa 12xAs 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. @S0163-1829~97!00831-X# The Fermi-edge singularity ~FES! is a many-body enhancement of optical spectra that results from the response of a degenerate electron gas to the photocreation or annihilation of a positively charged hole. This phenomenon was first understood in simple metals where the absorption of an x-ray causes the excitation of an electron from a localized atomic core level to the Fermi level ( m) in the conduction band. 1,2 Fermi-edge enhancement of optical spectra from a twodimensional semiconductor system was first reported by Skolnick et al. 3 in photoluminescence ~PL! from

Although the Al x Ga (1 − x ) As alloy system has been extensively investigated, there are still considerable uncertainties in measuring the value of x . Here a new Al x Ga (1 − x ) As calibration structure, grown by molecular beam epitaxy, has been used to establish unambiguous alloy compositions. Such ‘standard’ Al x Ga (1 − x As layers were measured by high-resolution X-ray diffraction, photoluminescence, and Raman spectroscopy to determine the compositional variations of the measured physical parameters. The formulae provided give a reliable calibration base for other characterization tools. A detailed analysis of the X-ray results shows that Vegard's law does not hold for the variation of the Al x Ga (1 − x ) As lattice constant with x .

We investigate the transition to a Sinai geometry by introducing a circular pattern at the center of a square mesoscopic billiard defined in a high quality AlGaAs/GaAs crystal. The transition induces a novel quantum interference structure in the magnetoresistance with a characteristic field scale over an order of magnitude smaller than previously reported in mesoscopic billiards. A systematic comparison of fine and coarse structures, which differ by an order of magnitude in field scale, demonstrates the first observation of geometry-induced ``self-similarity'' in the magnetoresistance of a semiconductor system.

During the production of InxGa1-xAs quantum wells by the crystal growth technique of molecular beam epitaxy, the indium tends to accumulate in the surface layer and to float on the crystal growth surface. This effect delays incorporation of the indium into the quantum well and results in a graduated value of x across the well. Detailed indium concentration profile information is essential if accurate theoretical models are to be applied to describe these quantum wells. Therefore, to quantify this indium segregation we combine the complementary results of two experimental techniques: transmission electron microscopy (TEM) and double-crystal X-ray diffraction (DCXRD). The indium concentration well profiles were determined through an analysis of the contrast variation in (200) dark-field TEM images. Differences of less than 0.5% indium concentration were discernible. The DCXRD analysis provided an accurate value for the total amount of indium in the individual wells which was used as the scaling factor for the TEM concentration profile. To ensure that the observed contrast differences were due to variations in indium concentration and were not merely TEM sample preparation artefacts, high-quality cross-sectional TEM samples with parallel cleaved surfaces free of ion-milling damage were required. Accurate thickness values for the samples were also needed. A TEM sample preparation technique was developed to provide these requirements. Indium concentration profiles were taken from intensity line scans across dark-field TEM images of these samples. These profiles were interpreted by a simple kinematical theory (structure factor) calculation, and then by a more rigorous model using the dynamical theory of electron diffraction. The results of these two models were then checked for consistency with the results of the DCXRD measurement. The model profiles of indium segregation within the quantum wells, calculated using the structure factor approach for up to 35% indium content, matched very closely the profiles produced by the use of dynamical theory. Both sets of model profiles strongly supported the DCXRD results. These results demonstrate that a simple structure factor calculation in conjunction with a DCXRD measurement can allow an accurate measure of quantum well profiles and of the abruptness of InxGa1-xAs on GaAs and GaAs on InxGa1-xAs interfaces. This information is a powerful aid in the modelling and understanding of devices based on these structures.

Although the AlxGa1−xAs alloy system has been extensively investigated, there are still considerable uncertainties in measuring the value of x. Here a new AlxGa1−xAs calibration structure, grown by molecular beam epitaxy, has been used to establish unambiguous alloy compositions. Such “standard’’ AlxGa1−xAs layers were measured by high-resolution x-ray diffraction, photoluminescence, and Raman spectroscopy to determine the compositional variations of the measured physical parameters. The phenomenological equations derived from these measurements can now be used to establish the Al content of unknown alloys with confidence. In addition, the results show that Vegard’s law does not hold for the variation of the AlxGa1−xAs lattice constant with x. The small quadratic term has very important implications for a correct analysis of x-ray results.

This paper reports an experimental study of the effects of different metal claddings on the performance of terahertz quantum cascade lasers. The experimental results show that by using a metal cladding made of Ta/Cu/Au to replace that of Pd/Ge/Ti/Pt/Au, the maximum lasing temperature of the devices is increased from 132 to 172 K, and the threshold current density of the devices at 10 K can be reduced from 0.74 to 0.68 kA cm−2. The improvement of the device performance is attributed to lower optical losses associated with the metal cladding layers. The different effects of the metal contacts on device optical properties and electrical properties are also discussed.

We have performed magnetotransport experiments in the high-temperature regime (up to 50 K) of the integer quantum Hall effect for two-dimensional electron gases in semiconducting heterostructures. While the magnetic field dependence of the classical Hall law presents no anomaly at high temperatures, we find a breakdown of the Drude-Lorentz law for the longitudinal conductance beyond a crossover magnetic field B_c ~ 1 T, which turns out to be correlated with the onset of the integer quantum Hall effect at low temperatures. We show that the high magnetic field regime at B > B_c can be understood in terms of classical percolative transport in a smooth disordered potential. From the temperature dependence of the peak longitudinal conductance, we extract scaling exponents which are in good agreement with the theoretically expected values. We also prove that inelastic scattering on phonons is responsible for dissipation in a wide temperature range going from 1 to 50 K at high magnetic fields., 14 pages + 8 Figures

A rate equation model is presented to analyze the performance of terahertz quantum cascade lasers based on consecutive phonon-photon-phonon emissions. Devices lased up to 128.5, 138, and 144 K at 2.5, 3.2 and 2.6 THz, respectively. © Optical Society of America., CLEO: QELS_Fundamental Science, CLEO:QELS FS 2013, 9 June 2013 through 14 June 2013, San Jose, CA

We designed and demonstrated a terahertz quantum cascade laser based on indirect pump injection to the upper lasing state and phonon scattering extraction from the lower lasing state. By employing a rate equation formalism and a genetic algorithm, an optimized active region design with four-well GaAs/Al0.25Ga0.75As cascade module was obtained and epitaxially grown. A figure of merit which is defined as the ratio of modal gain versus injection current was maximized at 150 K. A fabricated device with a Au metal-metal waveguide and a top n(+) GaAs contact layer lased at 2.4 THz up to 128.5 K, while another one without the top n(+) GaAs lased up to 152.5 K (1.3 (h) over bar omega/k(B)). The experimental results have been analyzed with rate equation and nonequilibrium Green's function models. A high population inversion is achieved at high temperature using a small oscillator strength of 0.28, while its combination with the low injection coupling strength of 0.85 meV results in a low current. The carefully engineered wavefunctions enhance the quantum efficiency of the device and therefore improve the output optical power even with an unusually low injection coupling strength. (C) 2013 AIP Publishing LLC.

We experimentally investigated the effect of oscillator strength (radiative transition diagonality) on the performance of resonant phonon-based terahertz quantum cascade lasers that have been optimized using a simplified density matrix formalism. Our results show that the maximum lasing temperature (Tmax) is roughly independent of laser transition diagonality within the lasing frequency range of the devices under test (3.2‐3.7THz) when cavity loss is kept low. Furthermore, the threshold current can be lowered by employing more diagonal transition designs, which can effectively suppress parasitic leakage caused by intermediate resonance between the injection and the downstream extraction levels. Nevertheless, the current carrying capacity through the designed lasing channel in more diagonal designs may sacrifice even more, leading to electrical instability and, potentially, complete inhibition of the device’s lasing operation. We propose a hypothesis based on electric-field domain formation and competition/switching of different current-carrying channels to explain observed electrical instability in devices with lower oscillator strengths. The study indicates that not only should designers maximize Tmax during device optimization but also they should always consider the risk of electrical instability in device operation. V C 2013 American

High temperature operation of terahertz (THz) sources based on quantum cascade lasers (QCLs) is discussed. THz QCLs are compact, powerful sources but can only operate at cryogenic temperatures. State-of-the art THz QCLs are made with GaAs/AlGaAs heterostructures and use a single composition of AlGaAs for the barrier material. It was recently shown that multi-composition barriers in the band structure can result in gain > loss at temperature as high as ~240K. We demonstrate early experimental results that yield QCLs that operate up to 184K – similar to QCLs based on single composition barrier designs. An alternative method of producing room-temperature THz is based on intra-cavity difference-frequency generation (DFG) in mid-infrared (mid-IR) QCLs. Here we report devices with record conversion efficiency. THz DFG QCLs reported previously are highly inefficient since THz radiation produced more than ~100 μm away from the exit facet is fully absorbed due to high THz losses in the QCL waveguide. Our lasers use a non-collinear Cherenkov DFG scheme to extract THz radiation from the active region. Dual-color mid-IR quantum cascade lasers with integrated giant optical nonlinearity are grown on semi-insulating (S.I.) InP substrates. THz radiation is emitted at an angle into the substrate with respect to the mid-infrared pumps. Since S.I. InP is virtually lossless to THz radiation, this scheme allows for efficient extraction of THz radiation along the whole waveguide length. As a result, our sources demonstrate large mid-infrared-to-THz conversion efficiency. Devices tested at room-temperature produced 18μW peakpower and 75μW/W2 conversion efficiency.

Spin qubits based on interacting spins in double quantum dots have been demonstrated successfully. Readout of the qubit state involves a conversion of spin to charge information, which is universally achieved by taking advantage of a spin blockade phenomenon resulting from Pauli's exclusion principle. The archetypal spin blockade transport signature in double quantum dots takes the form of a rectified current. At present, more complex spin qubit circuits including triple quantum dots are being developed. Here we show, both experimentally and theoretically, that in a linear triple quantum dot circuit the spin blockade becomes bipolar with current strongly suppressed in both bias directions and also that a new quantum coherent mechanism becomes relevant. In this mechanism, charge is transferred non-intuitively via coherent states from one end of the linear triple dot circuit to the other, without involving the centre site. Our results have implications for future complex nanospintronic circuits., The authors thank P. Hawrylak and C.Y. Hsieh for discussions. M.B., R.S. and G.P. acknowledge financial support from the Spanish Ministry of Education (grant no. MAT2011-24331, MEC) and from the Marie Curie Initial Training Network (grant no. 234970, EU). M.B. and R.S. were supported by the Consejo Superior de Investigaciones Científicas through the JAE and JAE-Doc programmes, co-financed by the Fondo Social Europeo. G.G. acknowledges funding from the National Research Council Canada – Centre national de la recherche scientifique collaboration and Canadian Institute for Advanced Research. A.S.S. acknowledges funding from the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute for Advanced Research.

Spin qubits have been successfully realized in electrostatically defined, lateral few-electron quantum dot circuits. Qubit readout typically involves spin to charge information conversion, followed by a charge measurement made using a nearby biased quantum point contact. It is critical to understand the back-action disturbances resulting from such a measurement approach. Previous studies have indicated that quantum point contact detectors emit phonons which are then absorbed by nearby qubits. We report here the observation of a pronounced back-action effect in multiple dot circuits where the absorption of detector-generated phonons is strongly modified by a quantum interference effect, and show that the phenomenon is well described by a theory incorporating both the quantum point contact and coherent phonon absorption. Our combined experimental and theoretical results suggest strategies to suppress back-action during the qubit readout procedure., 25 pages, 8 figures