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We report on transport characteristics of field effect two-dimensional electron gases (2DEG) in surface indium antimonide quantum wells. The topmost 5 nm of the 30 nm wide quantum well is doped and shown to promote the formation of reliable, low resistance Ohmic contacts to surface InSb 2DEGs. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus are observed to filling factor $\nu=1$ in magnetic fields of up to $B=18$ T. We show that the electron density is gate-tunable, reproducible, and stable from pinch-off to 4$\times 10^{11}$ cm$^{-2}$, and peak mobilities exceed 24,000 cm$^2$/Vs. Large Rashba spin-orbit coefficients up to 110 meV$\cdot$\r{A} are obtained through weak anti-localization measurements. An effective mass of 0.019$m_e$ is determined from temperature-dependent magnetoresistance measurements, and a g-factor of 41 at a density of 3.6$\times 10^{11}$ cm$^{-2}$ is obtained from coincidence measurements in tilted magnetic fields. By comparing two heterostructures with and without a delta-doped layer beneath the quantum well, we find that the carrier density is stable with time when doping in the ternary Al$_{0.1}$In$_{0.9}$Sb barrier is not present. Finally, the effect of modulation doping on structural asymmetry between the two heterostructures is characterized., Comment: Main text 7 pages 4 figures; Supplemental 17 pages 7 figures; added bandstructure simulations and MBE growth details, otherwise minor modifications to previous version

Terahertz quantum cascade lasers (THz QCLs) in metal-metal (MM) ridge waveguides have been fabricated and hetero bonded on aluminum nitride (AlN) substrate as heat sink submount. Compared with the conventional structure of THz QCLs in MM waveguides on gallium arsenide (GaAs) as receptor substrate, AlN performs superior heat dissipation properties for thermal management due to its much higher thermal conductivity. The light–current density–voltage (L-J-V) characterization shows comparable maximum operating temperature (Tmax) at 93-95 K for both THz QCLs bonded on AlN and GaAs under short pulse injection (250 ns). However, as the injected pulse duration increases for THz QCLs on GaAs, the light intensity drops quickly, eventually leading to lasing quenching when the pulse duration is above 30 µs at 80 K (heat sink temperature). On the other hand, THz QCL on AlN shows much stronger light intensity and slower decrease with the increase of the pulse duration; for example, the light intensity is 100 times higher for the THz QCL on AlN (pulse duration of 40 µs) than THz QCL on GaAs (pulse duration of 30 µs) at the same measurement conditions at 80 K. This study shows suspected joule heating plays a great role on THz QCLs operating from long duty cycle towards continuous-wave (CW) mode, indicating AlN substrate as a high thermal conductivity material produces superior thermal management for heat extraction and dissipation.

Parabolic quantum wells have unique properties that make them attractive for certain applications. However, growing them with molecular beam epitaxy is much more difficult than growing standard rectangular wells, as parabolic wells require a smooth, precise variation of the composition during growth. Typically, such composition variations have been approximated using the digital alloy technique, but there are limits on the quality of wells that can be produced this way. In our approach, we instead create a smooth parabolic potential in Al x Ga 1 - x As by varying the Al cell flux as a function of time. To compensate for thermal lag in the effusion cell, we develop a simple linear dynamical model, which can be inverted to find the temperature input required for a desired composition profile. With this approach, the composition in a 3 THz Al x Ga 1 - x As parabolic quantum well can be controlled to a root-mean-squared error of 0.4% Al. This approach can be easily generalized to other structures, and, importantly, can be used at typical growth rates ( 1.5 - 2.5 A / s ), which allows for the growth of parabolic well arrays.

Comprehensive studies on the surface morphological evolution of AlInSb metamorphic buffers and InSb QWs grown on top were conducted as a function of the GaAs (0 0 1) substrate offcut angles. We confirmed our earlier postulation that the vicinal surfaces defined by the hillock facets have the exact surface orientation needed to achieve large-area hillock-free surfaces. The related morphological transitions were discussed with a graphic illustration. The optimum substrate offcut for InSb towards [ 1 ¯ 1 0] direction was found to be around 0.5–0.6 ° with our growth conditions. On 2-inch GaAs (0 0 1) substrates with this offcut, a hillock-free and atomically smooth surface morphology was successfully achieved for modulation-doped InSb QWs.

We have realized quantized charge pumping using non-adiabatic single-electron pumps in dopant-free GaAs two-dimensional electron gases (2DEGs). The dopant-free III-V platform allows for ambipolar devices, such as p-i-n junctions, that could be combined with such pumps to form electrically-driven single photon sources. Our pumps operate at up to 0.95 GHz and achieve remarkable performance considering the relaxed experimental conditions: one-gate pumping in zero magnetic field and temperatures up to 5K, driven by a simple RF sine waveform. Fitting to a universal decay cascade model yields values for the figure of merit $\delta$ that compare favorably to reported modulation-doped GaAs pumps operating under similar conditions. The devices reported here are already suitable for optoelectronics applications, and with further improvement could offer a route to a current standard that does not require sub-Kelvin temperatures and high magnetic fields., Comment: 4 pages + references, 4 figures. Version 3: Updated Figure 2 with higher resolution data. Minor revisions in main text. Conclusions and arguments unchanged. Added Supplementary Material file. Version 2: Main results and analysis unchanged, minor revisions in presentation. Included new figure about fabrication details and expanded relevance of results to quantum optoelectronic applications

The paper presents the studies on technology of ~100 μm-emitting (~3 THz) Al0.15Ga0.85As/GaAs QCLs, the optimized scheme of fabrication of such lasers was elaborated. It was a result of the extensive work on decreasing waveguide losses while ensuring the proper performance of the electrical contacts and effective heat removal. The fabrication comprises Au-based claddings and Au-Au-mounting process. The yielded lasers operate up to the maximum temperature Tmax = 140 K, with threshold current density Jth ~ 1 kA/cm2 at 77 K.

This work presents a six-level scheme terahertz (THz) quantum cascade laser (QCL) design in which the resonant-phonon (RP) and the scattering-assisted (SA) injection/extraction are combined within a single Al0.15Ga0.85As/GaAs based structure. By utilizing extra excited states for hybrid extraction/injection channels, this design minimizes the appearance of an intermediate negative differential resistance (NDR) before the lasing threshold. The final negative differential resistance is observed up to 260K and a high characteristic temperature of 259 K is measured. These observations imply very effective suppression of pre-threshold electrical instability and thermally activated leakage current. In addition, the impact of critical design parameters of this scheme is investigated.

We present a two-junction photonic power converter operating under monochromatic illumination in the telecommunications O-band. The design, which is a direct extension of multi-junction solar cell technology, incorporates InAlGaAs absorbing layers lattice-matched to InP. Multi-junction detailed balance calculations are presented to benchmark device performance. A transparent tunnel diode with peak tunneling current near 1000 A/cm2 provides a series connection between the subcells. Current-voltage measurements under laser illumination at 1319 nm yield ideality factors near 1, with open-circuit voltages up to 0.38 V and 0.66 V for the one- and two-junction devices, respectively, under intensities of up to 0.33 W/cm2. Quantum efficiency measurements reveal that the two-junction device is current-matched near 1390 nm.

Time-resolved thermal imaging profile of mid-infrared quantum cascade laser was achieved by using 50 ns temporal resolution, observing temperature increase of 36.8% in the first 100 ns with subsequent decrease of 63.2% at Td=1.5 ps.

The optimization of metamorphic buffers for InSb/AlInSb QWs grown on GaAs (0 0 1) substrates is presented. With increasing surface offcut angle towards [ 1 1 ¯ 0 ] direction, the interaction of spiral growth around threading dislocations (TDs) with the offcut-induced atomic steps leads to a gradual change in the morphology of the AlSb buffer from one dominated by hillocks to that exhibiting near-parallel steps, and finally to a surface with increasing number of localized depressions. With the growth conditions used, the smoothest AlSb surface morphology was obtained for the offcut angles range of 0.8–1.3°. On substrates with 0° offcut, subsequent 3 repeats of Al 0.24 In 0.76 Sb/ Al 0.12 In 0.88 Sb interlayers reduces the TD density of AlSb buffer by a factor of 10, while 70 times reduction in the surface density of TD-related hillocks is observed. The remaining hillocks have rectangular footprint and small facet angles with respect to GaAs (0 0 1) surface: 0.4° towards [ 1 1 ¯ 0 ] direction and 0.7° towards [1 1 0] direction. Their triangular-shaped sidewalls with regularly spaced atomic steps show occasional extra step insertion sites, characteristic of TD outcrops. Many of the observed sidewalls are dislocation free and offer atomically smooth areas of up to 1 μm 2 , already suitable for high-quality InSb growth and subsequent top-down fabrication of InSb nanowires. It is proposed that the sidewalls of the remaining hillocks offer local vicinal surfaces with atomic step density optimal for suppression of TD-induced spiral growth, thus providing the important information on the exact substrate offcut needed to achieve large hillock-free and atomically smooth areas on AlInSb metamorphic buffers.

We demonstrate room temperature intersubband transitions in the THz range using continuously-graded Al x Ga 1-x As parabolic quantum wells. Below 100K, the linewidth of the transition reaches a record-low value of 5.7% of the central frequency.

We present modeled detailed balance efficiency of multijunction photonic power converters operating at 1310 nm at a laser intensity of 5.88× 105 W/m2, corresponding to our test laser power and cell size, in two scenarios: (1) with an absorbing substrate and (2) with a perfect specular back reflector. We show that, in the radiative limit, efficiency increases as a function of number of junctions in the case of an absorbing substrate. In this case, efficiency can reach as high as 69% with 15 junctions. In the case of a perfect specular back reflector, efficiency reaches more than 75% and is not sensitive to the number of junctions. This insensitivity allows freedom in future device design.

We show the high efficiency and voltage possible for a monochromatic multijunction photovoltaic device operating at the telecommunication wavelength 1310 nm. We use InAlGaAs lattice matched to InP and characterize our single-junction test structure, which we show has a bandgap of 0.864 eV. We use an extended detailed balance model to predict the efficiency and voltage of a series-connected multijunction device built from the same material, and show its advantages compared to a single-junction design. We show that a series-connected 20-junction device with In 0.532 Al 0.097 Ga 0.371 As can achieve a maximum power conversion efficiency over 69% in the radiative limit, at an input intensity of 5.6 × 105 W/m2. We also show that the device voltage at the maximum power point increases linearly with the number of junctions, with a slope larger than the single-junction voltage at maximum power point. Such behaviors of voltage and efficiency make a promising future for high-efficiency, high-voltage multijunction photonic power converters near 1310 nm.

Development of a photonic power converter for operation at 1310 nm would enable long distance optical power transfer over fiber. InAlGaAs lattice-matched to InP may be used as the absorber material in such a device. Material properties of InAlGaAs grown by molecular beam epitaxy (MBE) are studied by techniques including electro- and photoluminescence, spectroscopic ellipsometry, and quantum efficiency.

Fabrication of approx. 3 THz Al0.15Ga0.85As/GaAs QCLs grown by Molecular Beam Epitaxy equipped with Ta/Cu or Ti/Cu waveguide claddings will be presented. Our previous studies showed that copper layers as the waveguide claddings are most promising in THz QCLs technology. The theoretical predictions showed that lasers with Ti/Cu or Ta/Cu claddings (where Ti and Ta play the role of diffusion barriers and improve adhesion) show the smallest waveguide losses when compared with other metals. The main important issue of the presentation will be the wafer bonding of the QCL active region and GaAs receptor wafer. We will compare the results of ex-situ and in-situ bonding technology. The structures were tested by optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDXS). Our studies show that it was necessary to apply at least 5 nm-thick diffusion-barrier layers, as well as to keep all of the process temperatures below 400C in order to ensure the barrier tightness. The next important issue was control of composition of metallic claddings, in order to provide the control of the refractive index profiles of the claddings. The ridge structure lasers were fabricated with ridge width in the range 100 – 140 µm, formed by dry plasma etching in BCl3/Cl2/Ar mixture in ICP RIE system. The lasers operated with threshold current densities of approx. 1.2 kA/cm2 at 77 K and the Tmax = 130 K, when fed by 100-300 ns current pulses supplied with 0.3-1 kHz repetition frequencies. *This research is supported by The National Centre for Research and Development (bilateral cooperation, project no. 1/POLTUR-1/2016) and TUBITAK (Scientific and Technical Research Council of Turkey) project number 215E113.

The time-resolved (TR) temperature profile of actively biased mid-infrared quantum cascade lasers (MIR QCLs) was measured by using charge-coupled-device (CCD)-based thermoreflectance microscopy (TRM) with an ultrafast temporal resolution of 50 ns and a high spatial resolution of 390 nm. Based on the measured TR two-dimensional (2D) temperature profiles, the heat generation and dissipation dynamics within the lasers have been investigated. It is found that the active-region temperature increases quickly to a peak value (up to ∼100 °C above ambient room temperature) within 500 ns upon pulsed current injection of 6 A. The heat dissipation to the top and bottom cladding layers of the device is initially comparable, yet it evolves differently with time. Within 1–2 μs, the heat dissipation to the top cladding is substantially reduced and most of the heat is drained to the substrate through the bottom cladding layer. This constrained heat dissipation results in the elevated temperature in the active region, leading to thermal quenching of lasing operation, which is confirmed by experimental light–current–voltage measurement and theoretical thermal modeling. The TRM is an enabling tool for probing internal thermal dynamics of many active electronic and photonic devices, particularly for those needing special heat and thermal arrangement.

Scalable architectures characterized by quantum bits (qubits) with low error rates are essential to the development of a practical quantum computer. In the superconducting quantum computing implementation, understanding and minimizing materials losses is crucial to the improvement of qubit performance. A new material that has recently received particular attention is indium, a low-temperature superconductor that can be used to bond pairs of chips containing standard aluminum-based qubit circuitry. In this work, we characterize microwave loss in indium and aluminum/indium thin films on silicon substrates by measuring superconducting coplanar waveguide resonators and estimating the main loss parameters at powers down to the sub-photon regime and at temperatures between 10 and 450 mK. We compare films deposited by thermal evaporation, sputtering, and molecular beam epitaxy. We study the effects of heating in vacuum and ambient atmospheric pressure as well as the effects of pre-deposition wafer cleaning using hydrofluoric acid. The microwave measurements are supported by thin film metrology including secondary-ion mass spectrometry. For thermally evaporated and sputtered films, we find that two-level states (TLSs) are the dominating loss mechanism at low photon number and temperature. Thermally evaporated indium is determined to have a TLS loss tangent due to indium oxide of ~5x1e-05. The molecular beam epitaxial films show evidence of formation of a substantial indium-silicon eutectic layer, which leads to a drastic degradation in resonator performance., Comment: 10 pages main, 5 figures, 3 tables; 3 pages supplem., 3 figures

The grating coupled GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are integrated onto Si substrates using metal wafer bonding and epitaxial lift-off process. The 1 μm depth of hexagonal hole structure of grating was formed. The energy-dispersive x-ray spectroscopy results confirmed that the grating coupled QWIP is successfully mounted on an Si substrate. By evaluating the Raman spectra, PL, and surface roughness of bonded QWIP samples, the authors found that the grating does not induce any change in the optical or structural characteristics of actual QWIP layers. The dark current–voltage characteristics show a nearly identical dark current level between grating coupled QWIP and nongrating QWIP. The photocurrent spectrum shows that the peak photocurrent intensity of grating coupled QWIP is about 16 times higher than that of nongrating QWIP. This indicates that the grating effectively contributes to an increase in the light absorption of QWIP, showing large room for improvement of QWIP performance by further optimization of a grating structure.The grating coupled GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are integrated onto Si substrates using metal wafer bonding and epitaxial lift-off process. The 1 μm depth of hexagonal hole structure of grating was formed. The energy-dispersive x-ray spectroscopy results confirmed that the grating coupled QWIP is successfully mounted on an Si substrate. By evaluating the Raman spectra, PL, and surface roughness of bonded QWIP samples, the authors found that the grating does not induce any change in the optical or structural characteristics of actual QWIP layers. The dark current–voltage characteristics show a nearly identical dark current level between grating coupled QWIP and nongrating QWIP. The photocurrent spectrum shows that the peak photocurrent intensity of grating coupled QWIP is about 16 times higher than that of nongrating QWIP. This indicates that the grating effectively contributes to an increase in the light absorption of QWIP, showing large room for improvement of QWIP performanc...

The InAs/AlSb material system has proven to be an excellent choice for high-performance mid-infrared quantum cascade lasers. In this work, an unintentional displacement of Sb by residual As flux incident on the wafer was studied by direct monitoring of such flux during a simulated molecular beam epitaxy (MBE) growth sequence and dynamical simulations of high-resolution x-ray diffraction data collected on InAs/AlSb periodic structures grown under similar conditions. The results revealed that predominantly Al–As bonds detected at the InAs/AlSb interfaces, which were reported earlier, can be attributed to a residual bypass As flux on the wafer after closing the As shutter. Moreover, the experiments revealed that under typical growth conditions, AlSb binary barriers are converted into AlAsSb ternary layers with appreciable As content. The exact As content in the barriers is proportional to the effective As flux bypassing the closed shutter and thus depends on the particulars of the MBE system design and the exact As flux used for the growth of InAs wells.The InAs/AlSb material system has proven to be an excellent choice for high-performance mid-infrared quantum cascade lasers. In this work, an unintentional displacement of Sb by residual As flux incident on the wafer was studied by direct monitoring of such flux during a simulated molecular beam epitaxy (MBE) growth sequence and dynamical simulations of high-resolution x-ray diffraction data collected on InAs/AlSb periodic structures grown under similar conditions. The results revealed that predominantly Al–As bonds detected at the InAs/AlSb interfaces, which were reported earlier, can be attributed to a residual bypass As flux on the wafer after closing the As shutter. Moreover, the experiments revealed that under typical growth conditions, AlSb binary barriers are converted into AlAsSb ternary layers with appreciable As content. The exact As content in the barriers is proportional to the effective As flux bypassing the closed shutter and thus depends on the particulars of the MBE system design and the e...

InGaAs and InAlAs epilayers were grown on InP(111)B substrates by molecular beam epitaxy. Rather than focusing on a specific off-cut angle, the growths were done on rounded wafer edges, which expose a broad spectrum of vicinal surfaces with varying off-cut angle and off-cut azimuth. The epilayers were grown at several different growth conditions by varying the growth temperature, growth rate, and arsenic (As) overpressure. The epitaxial layers were characterized at the center and the edge of the wafers using Nomarski differential interference contrast microscopy and atomic force microscopy. It was shown that a minimum misorientation angle of ∼0.4° should be used in order to avoid pyramidal hillocks. At higher misorientations, 1.7°–3°, step bunching can lead to surface roughening.InGaAs and InAlAs epilayers were grown on InP(111)B substrates by molecular beam epitaxy. Rather than focusing on a specific off-cut angle, the growths were done on rounded wafer edges, which expose a broad spectrum of vicinal surfaces with varying off-cut angle and off-cut azimuth. The epilayers were grown at several different growth conditions by varying the growth temperature, growth rate, and arsenic (As) overpressure. The epitaxial layers were characterized at the center and the edge of the wafers using Nomarski differential interference contrast microscopy and atomic force microscopy. It was shown that a minimum misorientation angle of ∼0.4° should be used in order to avoid pyramidal hillocks. At higher misorientations, 1.7°–3°, step bunching can lead to surface roughening.

Statistical properties of neutral excitons, biexcitons and trions confined to natural quantum dots formed in the InAs/GaAs wetting layer are reported. The correlation of the trion binding energy and the biexciton binding energy was found. Magnetospectroscopic measurements of the excitons revealed also the correlation of excitonic effective g* factor of an exciton with the biexciton binding energy. The qualitative picture of the effect of quantum Confinement on the observed correlations is presented.

We present recent progress in the growth of nitride based laser diodes (LDs) made by Plasma Assisted Molecular Beam Epitaxy (PAMBE). In this work we demonstrate continuous wave (cw) LDs grown by PAMBE operating in the range 430–460 nm. The LDs were grown on c -plane bulk GaN substrates with threading dislocation densities (TDDs) ranging from 10 3 cm −2 to 10 7 cm −2 . The low TDDs allowed fabrication of cw LDs with the lifetime exceeding 2000 h at 10 mW of optical output power. The maximum output power for these LDs was 80 mW. We used AlGaN cladding-free design of LDs with InGaN waveguides. The key element to achieve lasing for wavelengths above 450 nm was substantial increase of the nitrogen flux available for growth in PAMBE. The increase of N flux is beneficial for growth of efficient InGaN QWs, which allowed demonstration of optically pumped lasing from single quantum well InGaN laser structures at 501 nm.

In this work we study the peculiar role of gallium and nitrogen atoms in the growth of InGaN by Plasma Assisted Molecular Beam Epitaxy (PAMBE). We investigate growth of InGaN layers on vicinal GaN (0001) substrates. Indium incorporation as a function of gallium and nitrogen fluxes was examined. We propose a microscopic model of InGaN growth by PAMBE postulating different indium adatom incorporation mechanisms on two nonequivalent atomic step edges of wurtzite crystal. The role of gallium and nitrogen fluxes during the growth of InGaN layers is discussed.

In this paper we describe our recent experiments on coherent manipulation of electron spin states formed in a highly tunable GaAs/AlGaAs triple quantum dot device. The coherent evolution of spin states is achieved by using fast pulses from an initialization point in the (201) charge configuration region of the stability diagram. We demonstrate the versatility of the triple dot system capable of tuning to different regimes controlled by the width of the (111) region and pulse parameters. In particular we observe Δ'1/2-Q3/2 (analogue of S-T+ in a double dot) and Δ'1/2-Δ1/2 exchange driven oscillations from both sides of the stability diagram involving all three spins. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Intra Landau level thermal activation, from localized states in the tail, to delocalized states above the mobility edge in the same Landau level, explains the $B_c(T)$ (half width of the dissipationless state) phase diagram for a number of different quantum Hall samples with widely ranging carrier density, mobility and disorder. Good agreement is achieved over $2-3$ orders of magnitude in temperature and magnetic field for a wide range of filling factors. The Landau level width is found to be independent of magnetic field. The mobility edge moves, in the case of changing Landau level overlap to maintain a sample dependent critical density of states at that energy. An analysis of filling factor $\nu=2/3$ shows that the composite Fermion Landau levels have exactly the same width as their electron counterparts. An important ingredient of the model is the Lorentzian broadening with long tails which provide localized states deep in the gap which are essential in order to reproduce the robust high temperature $B_c(T)$ phase observed in experiment., Comment: 15 pages, 12 figures

We utilize magnetic field dependencies to identify two hitherto unobserved quantum interference processes in a triple quantum dot circuit. The first observation involves the interplay of Landau-Zener-St\"uckelberg behavior from two separate anticrossings between two energy levels that anticross twice as a function of a detuning parameter. The second process involves quantum interference between all-exchange and hyperfine qubits activated in a three-spin system.

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Spin qubits involving individual spins in single quantum dots or coupled spins in double quantum dots have emerged as potential building blocks for quantum information processing applications. It has been suggested that triple quantum dots may provide additional tools and functionalities. These include the encoding of information to either obtain protection from decoherence or to permit all-electrical operation, efficient spin busing across a quantum circuit, and to enable quantum error correction utilizing the three-spin Greenberger-Horn-Zeilinger quantum state. Towards these goals we demonstrate for the first time coherent manipulation between two interacting three-spin states. We employ the Landau-Zener-St\"uckelberg approach for creating and manipulating coherent superpositions of quantum states. We confirm that we are able to maintain coherence when decreasing the exchange coupling of one spin with another while simultaneously increasing its coupling with the third. Such control of pairwise exchange is a requirement of most spin qubit architectures but has not been previously demonstrated., Comment: 12 pages, 13 figures, and 2 tables

We demonstrate InGaAs mid-infrared quantum well infrared photodetectors (MIR PV-QWIPs) that enable cost-effective mature GaAs-based detection and imaging technologies, with exceptional material uniformity, reproducibility, and yield, over a large area, with high spectral selectivity, innate polarization sensitivity, radiation hardness, high detectivity, and high speed operation at TEC temperatures without bias.

Increasing the operating temperature of infrared detectors is a prime importance for practical applications. The use of split-off band transitions has been proposed for high operating temperature infrared detectors. Initial results showed increasing the potential barrier for free carrier emission has led to increases in operating temperature from 150 K for a detector with an 8 μm threshold to room temperature for detector with a 4 μm threshold. However, these detectors showed a low responsivity due to the capture of carriers in each emitter. A proposal was made to use graded barriers with an offset between the barriers on the two sides of an emitter as a method of reducing the capture in the emitters. Two GaAs/AlGaAs samples with a single graded barrier (Al fraction x = 0.57 to 1 and 0.45 to 0.75, respectively) were used to test the effects. The sample with the lower barrier show responsivity increased by a factor of ∼10 or more compared to the higher graded barrier sample and detectors without the graded barrier. The higher graded barrier sample, space charge build up causes almost all potential drop across the first barrier, and hence reduces the response. Based on the modeling it is believed that this effect will be greatly reduced in detectors with multiple periods of graded barriers and emitters, allowing the full gain effects of the graded barriers to be realized., International Conference on Quantum Structure Infrared Photodetector, 2010

An analysis of dark current mechanisms has been performed on high-operating-temperature (up to 330 K) split-off (SO) band p+-GaAs/AlGaAs heterojunction infrared detectors (3-5 μm). In contrast to conventional 1-D current models due to carrier transport based on tunneling and/or thermionic emission mechanisms, a 2-D electrical model is used to explain nonuniformity degradation of zero-bias differential resistance (RoA) with temperatures as measured on SO detectors. The 2-D characteristic of carrier transport could have the limitation on high-temperature performances of detectors and, hence, needs optimizing. A theoretical model shows that this 2-D effect can be reduced by structural modifications such as using smaller mesa sizes, higher doping of the p+ -GaAs layer, and a higher potential barrier that prospectively provides better electrical uniformity for SO detectors working at high temperatures.

We study experimentally and numerically the polarization sensitivity of quantum well infrared photodetectors coupled to a diffraction grid. The polarization extinction ratio of such system is determined by two factors: polarization sensitivity of the diffraction grid and the intrinsic polarization sensitivity of the photodetector itself. The combined effect of these factors result in non-monotonic dependence of the polarization extinction ratio on the parameters of the diffraction grid. By varying the grid parameters, i.e., increasing the height and tuning the grid period, a maximum value for the polarization extinction ratio can be achieved. Both front side and back side illuminations of the photodetector are studied. The strongest polarization sensitivity is achieved under front side illumination.

The thermal dynamic behaviors of terahertz quantum cascade lasers are investigated using a spectrally matching terahertz quantum-well photodetector. The THz output power of a lasing device under a pulse excitation is found to decrease with time, which is attributed to the increase of active-region temperature. The measurements show that the duration of output lasing pulse ranges from a few microseconds up to 77 ?s, depending on device bias and heat-sink temperature. A theoretical analysis based on a two-dimensional heat diffusion model is presented. The model calculates lasing quenching time based on an ?average active-region temperature? criterion as well as a ?reserve of gain? criterion. The best fit for quenching time is found when the vertical thermal conductivity in the quantum well region is two orders of magnitude lower than that of substrate. The measured lasing quenching time is in very good agreement with theoretical modeling calculated using both criteria.

Quantum-wells and quantum dots and related semiconductor nanostructures have been widely investigated for infrared devices. Here we propose a new general approach to make use of polar optical phonons in quantum-wells for infrared (IR) and terahertz (THz) detection. As the first example, we show the coupling of phonon and intersubband transition leading to Fano resonance in photocurrent spectra. We investigate the phenomenon experimentally in specially designed GaAs/AlGaAs quantum-well infrared photodetectors. Finally, we discuss the future research and potentials.

A GaAs/AlGaAs heterojunction is used as a spin-split-off band IR detector operating at or around room temperature. This detector structure followed a similar layer architecture to the quantum well IR photo detectors (QWIP) and Heterojunction Interfacial Work function Internal Photoemission (HEIWIP) detectors. Compared to QWIPs, the emitter layer thickness is increased to avoid confinement. Unlike either the QWIPs or HEIWIPs, these detectors will have two energy gaps (barriers) to obtain the wavelength threshold which could be used to design detectors either for optimum operating temperature or optimum responsivity. The free carrier energy gap is determined by the Al fraction and the spin-split-off transition energy provides another handle on controlling the effective threshold of the detector. Unlike QWIPs, these will also detect normal incidence radiation. A preliminary detector showed a peak responsivity of 0.29 mA/W at 2.5 μm at room temperature.

We present a systematic study on a set of n-type GaAs-AlGaAs quantum-well infrared photodetectors (QWIPs) with varying Si doping density in the wells. It is revealed that the increase in doping density enhances proportionally the absorption efficiency and responsivity while increasing exponentially the dark current and hence the dark current noise. We experimentally confirm the theoretically predicted optimum conditions for background-limited infrared performance temperature and detector-noise-limited detectivity. It is suggested that, to achieve the optimal QWIP performance, the doping density in the wells should be determined according to application and the desired operating temperature. We point out that a simulation is highly recommended to achieve the best possible performance since the choice of doping may not be obvious. As shown here, an optimized doping for temperature is actually the worst for detectivity for the particular set of samples.

Magnetospectroscopic studies of individual excitonic states confined in potential fluctuations in the InAs/GaAs wetting layer (WL) are presented. A neutral exciton and a trion emission have been identified. They split in magnetic field in two components of orthogonal circular polarizations. The respective Zeeman splitting changes linearly with magnetic field up to 10 T. A significant scatter of the effective excitonic g *-factor is observed, reflecting the distribution of sizes and compositions of potential fluctuations in the WL. The distribution affects also diamagnetic shift of the excitonic emission. The observed properties of the excitons are consistent with a picture of shallow quantum dots formed in the WL due to In composition fluctuations. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work, we report on the growth of InGaN layers and InGaN/InGaN multi-quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PAMBE). We show that the incorporation of indium in InGaN layers can be controlled either by the ratio of Ga to N flux or the growth temperature. A method to increase the internal quantum efficiency of MQWs emitting green light at 500 nm by optimizing the growth temperature for the In content of each individual layer is proposed.

Plasma-assisted molecular beam epitaxy (PAMBE) has recently emerged as a viable tool for production of nitride blue-violet laser diodes operating at room temperature in continuous wave mode and high output powers [C. Skierbiszewski, P. Wisniewski, M. Siekacz, P. Perlin, A. Feduniewicz-Zmuda, G. Nowak, I. Grzegory, M. Leszczynski, S. Porowski, Appl. Phys. Lett. 88 (2006) 221108]. The present work reviews the current state of the art in this program as well as discusses its future directions. Two elements are given particular attention: (1) the epitaxial growth in metal-rich conditions, which enables effective lateral diffusion of N adatoms at low growth temperatures and (2) the role of threading dislocations in destabilizing the growth front. Low-temperature growth by PAMBE on dislocation-free GaN substrates is instrumental in achieving high performance of optoelectronic structures. The inherent to this process capability of sustaining two-dimensional step-flow growth mode (with straight and parallel atomic steps) at low growth temperatures opens up the way to the growth of strained multilayer structures with no compositional fluctuations and with flat interfaces.

Several designs of broadband quantum-well infrared photodetectors are considered. The performance characteristics are analyzed by modeling. Test devices designed to cover the 8–12 μm wavelength region are fabricated and characterized. The results show that a broad device can be realized and their performance is in accordance with expectation.

Readout of a spin qubit in a lateral gate-defined quantum-dot device typically involves a charge detector and a spin-to-charge conversion technique employing spin blockade. We investigate alternative mechanisms for spin-to-charge conversion involving metastable excited charge states made possible by an asymmetry in the tunneling rates to the leads. This technique is used to observe Landau-Zener-Stückelberg oscillations of the S-T+ qubit within the (1,0) ground state region of the charge stability diagram. The oscillations are π phase shifted relative to those detected using the standard technique and display a nonsinusoidal waveform due to the increased relaxation time from the metastable state.

The resistively detected nuclear magnetic resonance (RDNMR) performed on a two-dimensional electron gas is known to exhibit a peculiar 'dispersive' line shape at some filling factors, especially around $\nu =1$ . Here, we study in detail the inversion of the dispersive line shape as a function of the filling factor from $\nu =1$ to $2/3$ . The RDNMR spectra show a new characteristic W line shape in the longitudinal resistance, whereas dispersive lines detected in the Hall resistance remain unchanged. This W resonance, like the dispersive line, can be fitted correctly by a model of two independent response functions, which are the signatures of polarized and unpolarized electronic sub-systems.

Probabilities deduced from quantum information studies are usually based on averaging many identical experiments separated by an initialization step. Such initialization steps become experimentally more challenging to implement as the complexity of quantum circuits increases. To better understand the consequences of imperfect initialization on the deduced probabilities, we study the effect of not initializing the system between measurements. For this we utilize Landau-Zener-Stuckelberg oscillations in a double quantum dot circuit. Experimental results are successfully compared to theoretical simulations.

Scanning voltage microscopy results clearly show that the formation of electric field domains is responsible for the missing of lasing operation in a resonant-phonon based terahertz quantum cascade laser with a highly diagonal transition., CLEO: Science and Innovations, CLEO-SI 2015, 10 May 2015 through 15 May 2015

Generation of terahertz (THz), and sub-THz, coherent waves has been demonstrated at room temperature in GaAs-AlGaAs asymmetric quantum wells by mixing two CO2 laser beams. Two overlapping regimes were studied: the double resonance regime where three states are involved and the optical rectification which relies on two states only. The measured conversion efficiency of ~10-7 W-1 is in reasonable agreement with theoretical predictions. In addition, this technique of THz generation provides a model system to study the emission profile from an ensemble of radiating dipoles. For instance, backward emission and strong diffraction effects have been observed

We report on the measurements of the photoluminescence from the s-shell of a single InAs/GaAs quantum dot in magnetic fields up to 23 T. The observed multiline emission is attributed to different charge states of a single dot. Characteristic anticrossing of emission lines is explained in terms of hybridization of final states of a triply charged exciton (X−3).

Microwave induced resistance oscillations (MIROs) were studied experimentally over a very wide range of frequencies ranging from ~20 GHz up to ~4 THz, and from the quasi-classical regime to the quantum Hall effect regime. At low frequencies regular MIROs were observed, with a periodicity determined by the ratio of the microwave to cyclotron frequencies. For frequencies below 150 GHz the magnetic field dependence of MIROs waveform is well described by a simplified version of an existing theoretical model, where the damping is controlled by the width of the Landau levels. In the THz frequency range MIROs vanish and only pronounced resistance changes are observed at the cyclotron resonance. The evolution of MIROs with frequency are presented and discussed., Comment: 4 pages, presented at EP2DS, to be published in Physica E