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We report on investigations of the quantum Hall chiral edge states using a near-field scanning optical microscope that enables us to irradiate circularly polarized light from the probe tip with spatial resolution below the diffraction limit. We have found a clear evidence for the formation of spin-split incompressible strips near the edge of a two-dimensional electron system in high magnetic fields.

Superconductivity and optoelectronics have developed almost independently and had very rare interactions with each other in science and technologies. However recent interdisciplinary research opens up the potential for developing new optoelectronic devices. This review paper presents our recent theoretical and experimental demonstrations that superconductivity significantly modifies and accelerates photon generation processes. We have prepared superconducting light emitting diodes (LEDs) emitting at ∼1.6-µm optical-fiber communication band for the experimental demonstrations. This new-type LED operation is based on a unique physics related to Cooper-pair interband transition in a semiconductor, and further research leads to the solid-state simultaneously generated photon-pair sources for the potential application in quantum information and communication.

A real-space mapping of photovoltage near the edge of the Hall-bar of a GaAs/AlGaAs single heterojunction has been obtained using a dilution-refrigerator-based near-field scanning optical microscope in magnetic fields. The optical probe-sample surface distance dependence of photovoltage is investigated. We obtain photovoltage profile in the vicinity of the edge, which reflects the local chemical potential of the two-dimensional electron gas determined by the distribution of the compressible and incompressible strips.

Superconducting transport is measured between two superconducting electrodes at the n-type semiconductor side of a superconductor-based LED where a Josephson junction is formed. The characteristics of the Josephson junction are found to be modulated by applying voltage to the normal electrode at the p-type semiconductor side. The Josephson junction characteristics show an extraordinary sensitivity to the radiative recombination process, which we estimate as the recombination efficiency.

We measured the photoluminescence (PL) of a GaAs quantum well (QW) with a lateral potential modulation by the front-gate bias while changing both the electron density and electric field at low temperature. Near the onset of the two-dimensional electron gas, we observed an anomalous enhancement of PL intensity of the neutral exciton X 0 accompanied by a decrease in the intensity of the charged exciton X - . The sample was a 20-nm GaAs back-gate undoped QW with semi-transparent square-mesh surface gates. By comparing the results with those for a flat transparent surface gate sample we discuss the origin of this anomalous phenomenon. We present a possible explanation for this phenomenon, which is attributed to lateral drift motion of the X 0 in a laterally modulated exciton potential.

We have experimentally investigated the optical effect on the transport properties of superconductor-semiconductorsuperconductor (S-Sm-S) junctions composed of a two-dimensional electron gas in a GaAs/AlGaAs heterostructure and NbN superconducting electrodes. Illumination at λ 800 nm onto the whole junction area increases Andreev reflection (AR) probability at S-Sm interfaces. To explore this origin, we performed scanning photovoltage measurements by using an optical microscope. The obtained image plots of the photovoltage show that the illumination brings about the photovoltage at the S-Sm interfaces. This result implies that the illumination modulates the barrier height between S and Sm, which results in a modulation of AR probability.

Observation of drastically enhanced luminescence emitted from InAs quantum dots (QDs) which were in direct contact to superconducting Niobium (Nb) is reported. Although the PL intensity was essentially temperature independent without Nb stripe, drastic temperature dependence was observed with the presence of the Nb stripe. PL intensity was substantially increased below the Nb superconducting critical temperature (Tc). About 3-times enhancement was observed below Tc. This phenomenon is attributed to the Cooper pairs penetrated from the superconducting Nb stripe to adjacent InAs QDs by the proximity effect. This is the first experimental demonstration that the Cooper pairs enhance radiative recombination through QDs. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated the superconducting proximity effect in a sandwich structure of network-like carbon nanotubes coupled to NbN electrodes. The proximity effect gives rise to multiple Andreev reflection processes and enhanced magnetoconductance fluctuations that are similar to universal conductance fluctuation. Reentrant behavior caused by the proximity effect was observed. The proximity effect correction to the conductance disappears at low temperature and reaches a maximum value at about 8 K, which corresponds to Thouless energy. This reentrant behavior was also observed in the temperature dependance of fluctuation amplitude. These results are compared with theory.

We present measurements on the energy distribution profile of the ballistic hot carriers emitted from a quantum point contact. The hot carriers were injected to a quantum dot using magnetic focusing technique and analyzed by measuring differential conductance of the quantum dot. We found a thermally broadened energy distribution of the hot carriers near the bias voltage applied to the quantum point contact in an otherwise uniform distribution down to the equilibrium Fermi energy. We also measured an occupation ratio of available states by the injected hot carriers, and fully characterized their energy distribution profile. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study coherent transport and bound-state formation of Bogoliubov quasiparticles in a high-mobility In$_{0.75}%$Ga$_{0.25}$As two-dimensional electron gas (2DEG) coupled to a superconducting Nb electrode by means of a quantum point contact (QPC) as a tunable single-mode probe. Below the superconducting critical temperature of Nb, the QPC shows a single-channel conductance greater than the conductance quantum $2e^{2}/h$ at zero bias, which indicates the presence of Andreev-reflected quasiparticles, time-reversed states of the injected electron, returning back through the QPC. The marked sensitivity of the conductance enhancement to voltage bias and perpendicular magnetic field suggests a mechanism analogous to reflectionless tunneling--a hallmark of phase-coherent transport, with the boundary of the 2DEG cavity playing the role of scatters. When the QPC transmission is reduced to the tunneling regime, the differential conductance vs bias voltage probes the single-particle density of states in the proximity area. Measured conductance spectra show a double peak within the superconducting gap of Nb, demonstrating the formation of Andreev bound states in the 2DEG. Both of these results, obtained in the open and closed geometries, underpin the coherent nature of quasiparticles, i.e., phase-coherent Andreev reflection at the InGaAs/Nb interface and coherent propagation in the ballistic 2DEG., Comment: 6 pages, 5 figures

We investigated the superconducting proximity effect in carbon nanotubes (CNTs). Reentrant behavior caused by the proximity effect was observed in a sandwich structure of network-like CNTs coupled to NbN electrodes. The proximity effect gives rise to enhanced magnetoconductance fluctuations that are similar to universal conductance fluctuation. The proximity effect correction to the conductance and fluctuation amplitude disappears at low temperature and reaches a maximum value at about 8 K, which corresponds to Thouless energy. This reentrant behavior was also observed in the magnetic field dependance of fluctuation amplitude. We found that a correlation magnetic field of about 3 T induces the dephasing of the interference. This high magnetic field is due to the CNTs with small diameter. We discuss this proximity effect in these quasi-one-dimensional CNTs with random network structure in experiment and theory.

The electron–hole states in the fractional quantum Hall regime is investigated with a back-gated undoped quantum well by photoluminesccence in magnetic fields. The evolution of the photoluminescence spectra is discussed depending on the electron density. We find anomalies of the photoluminescence at the integer as well as the fractional filling factors.

The electric field dependence of the electron/hole wave function and the radiation energy of an exciton in a Be-δ-doped 80 nm quantum well (QW) is studied experimentally and compared it with variational calculation. The photoluminescence (PL) spectra show Stark shifts depending on the gate electric field and PL intensity of the exciton of the first excited state has a dip in the electric-field dependence which reflects the node of the electron wave function.

We report the magneto-optical measurement of GaAs quantum wire lattices of 0.7 micrometer period grown on GaAs(111)B substrates by using the selective-area MOVPE technique. We measure the photoluminescence (PL) spectra at 60 mK from the triangular and the Kagome lattice patterns as well as from the unpatterned single quantum well (SQW) as a reference. While the PL from acceptors is dominant in all the samples, the PL spectra have a different peak between the wire structures and the SQW. When we apply a perpendicular magnetic field to the samples, the PL intensities slightly decrease up to 100 mT and then increase at higher magnetic fields with periodic oscillations in the triangular and the Kagome lattices. This oscillation is possibly attributed to the interference effect of electrons in the lattice patterns threaded by the magnetic flux.

Targeting highly sensitive displacement and force sensors, we fabricated a piezoresistive cantilever that integrates a superconductor–semiconductor–superconductor (S–Sm–S) junction based on an InAs/AlGaSb heterostructure. The S–Sm–S junction is composed of a submicron Nb gap patterned on the InAs thin film, and a deflection of the cantilever is detected as a resistance change at the junction. We confirmed that the resistance change caused by induced strain (i.e., piezoresistance) has a strong dependence on bias current. When the maximum Josephson current ( I c ) is biased to the junction, the resistance change is significantly enhanced by more than a factor of 10 compared to that at the bias current above I c (the resistive state). The resulting maximum resistance change is 3.9 m Ω , which is three orders of magnitude larger than that obtained for our preliminary sample. This large piezoresistance at the S–Sm–S junction will lead to highly sensitive self-detective sensors.

We have fabricated network-like single-walled carbon nanotubes with superconducting NbN electrodes. The single-wall carbon nanotubes were synthesized on SiO2 substrates by the thermal chemical vapor deposition technique using Co catalyst. We obtained sufficiently good contact characteristics between the single-walled carbon nanotubes and the NbN electrodes by infrared annealing at heater temperature 700 °C for ~15 min in vacuum. We observed multiple Andreev reflection by measuring differential resistance as a function of applied voltage below 7 K. This multiple Andreev reflection is due to the proximity effect between NbN electrodes. The superconducting energy gap of NbN, 2Δ, is about 6 meV. The multiple Andreev reflection processes occur around the dips, i.e. ±2Δ/e and ±Δ/e. On the other hand, at above 8 K, the curves of different resistance are changed from dip to peak at zero bias voltage. This behavior is similar to reentrant behavior that has a maximum conductance corresponding to the correlation energy (Thouless energy) below superconducting critical temperature Tc. This reentrant behavior has been studied in normal metal or two-dimensional gas of semiconductor heterostructures coupled to superconductor.

We measured the photoluminescence (PL) spectra of a two-dimensional electron system induced in a Be-δ-doped GaAs/AlGaAs quantum well (QW) with a back gate. The electron density is controlled by means of the back-gate voltage. We estimated the electron density using the magneto-optical method and the PL linewidth, and also by undertaking transport measurements. We show that a uniform 2DES as large as 1 mm 2 is induced by the back-gate operation from 2.5 × 10 10 cm −2 . This experiment indicates that optical measurement with a back-gated QW is advantageous for studying the low-density 2DES.

We report on the fabrication of a ballistic SNS junction with a two‐dimensional electron gas (2DEG) in an InGaAs‐based heterostructure. We have observed the Josephson current as well as the subharmonic energy‐gap structures caused by the MAR. Moreover, we experimentally estimated the thermal noise by comparing measured IV characteristics with those obtained with an extension of the Ambegaokar and Halperin theory. As a consequence, we have observed giant thermal noise that is much larger than that expected with normal reservoirs. The experimentally obtained “giant” thermal noise can be qualitatively explained by the Martin‐Rodero theory that considers both the ballistic transport of the 2DEG and the thermal fluctuation in the coherent multiple Andreev reflection regime.

We present studies on the electric transport in a lateral GaAs/AlGaAs quantum dot defined by a patterned single connected metallic front-gate. This gate design allows to easily couple a large number of quantum dots and therefore holds high potential in the design of new materials with tailor-made band structures based on quantum dot superlattices of controlled shape. Clear Coulomb diamond structures and well pronounced tunneling peaks observed in experiment indicate that single-electron control has been achieved. However, the dependence on electron density in the heterostructure embedding the dot, which is controlled by an additional back-gate, reveals that transport characteristics are strongly influenced supposedly by potential fluctuations in the dot and lead regions.

We report on experimental studies of a quantum wire with side-coupled quantum dot defined by surface gate electrodes patterned on top of a GaAs/AlGaAs heterostructure. The dot is side-coupled to the wire by means of a split-gate electrode allowing for a control of coupling strength. Clear signatures of single-electron charging are observed in the low-temperature wire conductance within a broad range of tunnel couplings and wire widths. The oscillations in wire conductance associated with Coulomb peaks in the dot remain being visible for split-gate conductances exceeding G0=2e2/h and for mean wire conductances of up to 2.5G0. We explain this behavior as consequence of the two-dimensional sample geometry which can lead to asymmetric and effectively reduced coupling between the dot and the conducting modes in the wire and propose a modified sample layout that should be suitable to increase the coupling.

We fabricated NbN/multi-walled carbon nanotube (MWNT)/Al junctions using MWNTs standing in nano-porous alumina membranes. We observed a reduction in the resistance within the NbN superconducting energy gap voltage caused by Andreev reflection. Moreover, we found that the conductance scales in the form of power laws with respect to temperature and bias voltage, and the power-law exponent α becomes larger when NbN becomes superconductive. These experimental results can be qualitatively explained by the theory of superconductor-Tomonaga–Luttinger liquid hybrid systems.

We report the gate-voltage dependence of the electron g factor in InGaAs/InAlAs heterostructures with double InAs-inserted wells. The g factor has been determined from a series of Shubnikov–de Haas oscillations at various angles with respect to the layers under different gate voltages. The strain in InAs layers and the penetration of the electron wave function into InGaAs and InAlAs layers are the important factors of the reduction in the | g | factor. We have also observed the changes of the | g | factor with respect to the gate voltage, though this dependence is not clear in the simple calculation.

We report on electron g-factor in an InAs-inserted In0.53Ga0.47As/In0.52Al0.48As heterostructure. The gate voltage dependence of g-factor is obtained from the coincidence method. The obtained g-factor values are surprisingly smaller than the g-factor value of bulk InAs, and it is close to the bare g-factor value of In0.53Ga0.47As. The large change in g-factor is observed by applying the gate voltage. The obtained gate voltage dependence is not simply explained by the energy dependence of g-factor.

We present studies on the electronic transport in a quantum dot based on a back-gated undoped GaAs/AlGaAs heterostructure. A high-quality two-dimensional electron gas can be induced in the structure by applying a back-gate voltage. The quantum dot is well defined by means of a single metallic front-gate. We observe clear regions of Coulomb blockade and pronounced tunneling peaks in a broad range of source–drain and front-gate voltages. The formation of the quantum dot and energetic evolution of the associated tunneling peaks are studied as a function of the applied front-gate voltage. Excellent agreement to the calculated Coulomb diamond pattern using the ‘constant interaction model’ is achieved, if a front-gate voltage dependent change of the dot capacitance is taken into account.

We have successfully developed a circularly polarized near-field scanning optical microscope (NSOM) that enables us to irradiate circularly polarized light with spatial resolution below the diffraction limit. As a demonstration, we perform real-space mapping of the quantum Hall chiral edge states near the edge of a Hall-bar structure by injecting spin polarized electrons optically at low temperature. The obtained real-space mappings show that spin-polarized electrons are injected optically to the two-dimensional electron layer. Our general method to locally inject spins using a circularly polarized NSOM should be broadly applicable to characterize a variety of nanomaterials and nanostructures.

We propose the fabrication of a spin rectifying diode that utilizes the Rashba spin-orbit coupling, by the application of semiconducting triple barrier structures. This spin diode makes use of spin-dependent resonant tunneling levels that are formed in the triple barrier structures. We found that, for a certain emitter-collector bias voltage, it is possible to engineer the structure in such a way that a resonant level formed within the first quantum well matches that of the second quantum well only for a selected spin state, thus realizing an electronic spin rectifier. The calculated spin-filter efficiency of the transmitted current through the device, which is defined as |I↑-I↓|/(I↑+I↓), is found to be higher than 99.9%.

The values of the zero-field spin-splitting energy Δ0 in InAlAs/InGaAs/InAlAs heterostructures are investigated using anti-weak-localization analysis. The obtained values for Δ0 are compared with values that were theoretically predicted assuming Rashba spin–orbit coupling (denoted by ΔR). The good agreement between Δ0 and ΔR and their dependence on quantum well asymmetry suggest that our approach provides a useful tool for designing future spintronics devices using Rashba spin–orbit coupling.

We measure the magnetic flux dependence of the supercurrent in diffusive Nb/n-InAs/Nb mesoscopic SNS Josephson junctions with junction length ranging from 220 to 720 nm. In the longer junctions, the modulation periodicity of the maximum supercurrent by magnetic flux is a quantum flux, Φ 0 = h /2 e as usual. However, we find that the magnetic flux periodicity becomes twice or 2 Φ 0 = h / e when the junction lengths are shorter than 400 nm.

We have studied the differential resistance in a gated superconductor–semiconductor junction in the ballistic transport regime. The zero-bias resistance shows a minimum as a function of the gate voltage, while the junction normal resistance measured at a high bias voltage shows a monotonic increase. This is explained as follows: the conductance enhancement due to coherent Andreev reflection is controlled by the gate voltage which changes the transparency of the gate barrier. We have also fabricated a two-gated junction with a high transparency at the superconductor–semiconductor interface and measured its resistance as functions of two gate voltages. An increase in one gate voltage results in the resistance minimum as a function of the other gate voltage. The obtained results agree well with the so-called reflectionless tunneling theory.

We investigated a superconductor–semiconductor–superconductor junction formed by two superconducting NbN electrodes and a two-dimensional electron gas (2DEG) in an AlGaAs/GaAs heterostructure. We obtained a good ohmic contact between NbN/AuGeNi electrodes and 2DEG by annealing them at 450°C for 1 min in an N2 atmosphere. We observed a decrease in the resistance caused by Andreev reflection (AR) within the superconducting energy gap voltage in a zero magnetic field in this structure. We found that the peculiar features of the magnetoresistance in the transition region can be qualitatively explained by considering the existence of the AR in high magnetic fields.

We study a superconducting quantum point contact made of a narrow In$_{0.75}% $Ga$_{0.25}$As channel with Nb proximity electrodes. The narrow channel is formed in a gate-fitted constriction of InGaAs/InAlAs/InP heterostructure hosting a two-dimensional electron gas. When the channel opening is varied with the gate, the Josephson critical current exhibits a discretized variation that arises from the quantization of the transverse momentum in the channel. The quantization of Josephson critical current persists down to the single-channel regime, providing an unambiguous demonstration of a semiconductor--superconductor hybrid Josephson junction involving only a single ballistic channel., 6 pages, 4 figures

Two separate research topics are discussed. We discuss noise measurements of single electron transistors and the gain dependence of the noise. We will also describe the fabrication and preliminary measurements on several different superconductor/two-dimensional electron gas/superconductor structures.

Experimental and theoretical results of quantum transport in superconductor–semiconductor junctions are summarized. We will first show the experimental results of reentrant behavior of the conductance as well as of giant Andreev backscattering. Then Andreev reflection in the quantum Hall regime is described. The charging effect on the proximity correction is finally discussed.

We report studies of the in-plane magneto-transport properties of a 2DEG in an InAs inserted InGaAs/InAlAs heterostructure with a DC bias voltage. Temperature dependent zero-bias reduction of the longitudinal resistance was observed when high magnetic field is applied along the sample growth direction. We interpret the observed resistance reduction as the result of a Coulomb gap existing at the Fermi level of the 2DEG.

A gated inverted In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum well is studied via magneto transport. By analysing the gate-voltage-dependent beating pattern observed in the Shubnikov-de Haas oscillation, we determine the gate voltage (or electron concentration) dependence of the spin-orbit coupling parameter α. Our experimental data and their analysis show that the band nonparbolicity effect cannot be neglected. For electron concentrations above 2×10 12 cm −2 , it causes a reduction of α up to 25%. We report for the first time the α value for the second subband.

We report the fabrication of MgB2 nanowire based on a liftoff-like technique and its optical response. A Si/C bilayer mask is formed as the negative pattern of the nanowire by e-beam lithography and the standard liftoff process. A 10 nm-thick MgB2 thin film is deposited on the pattern, which produces MgB2 nanowire with a width of down to 200 nm. We do not need to liftoff the Si/C bilayer because the MgB2 is well separated at the edge of the Si/C bilayer due to its overhang structure. The optical response at 1.5 mum wavelength is measured at 4.2 K. For a 300 nm-wide nanowire, photoresponse signals with the repetition rate of 100 MHz are observed, and all signals disappear as the laser intensity decreases below 1.8 times 104 photon/pulse. On the other hand, the signals become intermittent as the laser intensity decreases for a 200 nm-wide nanowire. The signal disappears below 4times 102 photon/pulse. This shows that the nanowire works in the multi-photon detection regime.

A gated inverted ${\mathrm{In}}_{0.52}{\mathrm{Al}}_{0.48}{\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{0.53}{\mathrm{Ga}}_{0.47}{\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{0.52}{\mathrm{Al}}_{0.48}\mathrm{As}$ quantum well is studied via magnetotransport. By analyzing the gate-voltage-dependent beating pattern observed in the Shubnikov--de Haas oscillation, we determine the gate voltage (or electron concentration) dependence of the spin-orbit coupling parameter \ensuremath{\alpha}. Our experimental data and its analysis show that the band nonparabolicity effect cannot be neglected. For electron concentrations above $2\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2},$ it causes a reduction of \ensuremath{\alpha} up to 25%. We report the \ensuremath{\alpha} value for the second subband.

We report on the fabrication of InAs-inserted-channel InAlAs/InGaAs inverted HEMTs with NbN electrodes made by using a DC magnetron sputtering deposition and we describe the device characteristics we obtained. Excellent pinch-off characteristics were obtained even at /spl sim/10 K when NbN electrodes retain their superconductivity. For a 3-/spl mu/m-gate device, the maximum extrinsic transconductance at 10 K was 300 mS/mm, even at the very low drain voltage of 0.2 V. We found that the HEMTs with NbN electrodes, not only have superior characteristics at /spl sim/10 K that exceed the critical temperature of Nb, but are also able to combine with NbN Josephson junctions.

Measurements of superconducting and normal transport in a superconductor/semiconductor/superconductor junction with a long-split gate show that when the two-dimensional electron gas in the semiconductor is put into the pinched-off state by applying a gate voltage, the two superconducting electrodes couple through a long and narrow channel with a small number of modes. Multiple Andreev reflections the focusing of Andreev-reflected holes are observed in this situation, but the supercurrent decreases quickly as gate voltage is increased and it disappears when the channel is long and narrow. A sharp conductance peak due to the coherent motion of the electrons and holes in the narrow channel is also observed.

We report the fabrication of MgB 2 nanowire and its optical response. A 10-nm-thick MgB 2 thin film with the superconducting transition temperature T c = 21 K was synthesized by using molecular-beam epitaxy. The film was processed into a nanowire with a width of 300 nm and a length of 10 μm using e-beam lithography and Ar-ion milling. With a dc bias current applied close to the critical current ( I c ), the nanowire showed an electrical signal when illuminated by a laser pulse at the telecommunications wavelength.

To investigate the contribution of Cooper pairs to radiative recombination in a semiconductor, a junction fieldeffect transistor (FET) with a Nb/n-InGaAs/Nb junction was fabricated. Electroluminescence was observed from a 110nm-wide gap between the source and drain Nb electrodes in addition to the confirmation of the FET operation. Micro-photoluminescence observed from the gap in the Nb electrodes was drastically enhanced at the temperature lower than the superconducting critical value of ∼ 8K. This new finding is discussed based on the proximity effect at the super-normal junction. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated the transport of a two-dimensional electron gas (2DEG) in an In0.52Al0.48As/In0.7Ga0.3As heterostructure when exposed to light from infrared laser diodes (λ=0.78, 1.3 μm) by means of both Shubnikov–de Haas and Hall-effect measurements. We observed negative photoconductivity due to a reduction in the number of electrons in the 2DEG when they were illuminated by photons at λ=1.3 μm. We speculate that the negative photoconductivity originates from the diffusion and trapping of photo-induced hot electrons at deep impurity levels in the InAlAs barrier layer near the InAlAs/InGaAs interface.

We report on results of determination of the electron effective mass and the reduced mass as functions of the electron density ( n s ) at about 100 mK in perpendicular magnetic fields in the range between 1.5 r s 6 by magneto-photoluminescence measurements. We find that the obtained effective masses increase with decrease in n s at n s 1 × 10 11 cm - 2 .

We have studied low temperature transport characteristics of an independently contacted double quantum dot–quantum wire coupled system. Each quantum dot is connected to three leads; source, drain and wire. Fano resonances associated with one dot appear in the conductance of the other dot, which are gradually suppressed as the inter-dot tunnel coupling via the wire is reduced. When the dot-wire tunnel coupling is lost, conductance modulation similar to the Fano resonances reappear, which is ascribed to purely inter-dot electrostatic coupling effect.

Real-space mappings of a Hall photovoltage are performed by local laser excitation using optical microscope setup with a spatial resolution of about 2μm. The obtained images of broad and narrow strips presumably reflect the diffusion of the optically created electrons in the bulk and the edge states. The image near a current contact is considered to reflect the potential gradient near the contact.

We have investigated the device characteristics of InAs-inserted-channel In 0.52 Al 0.48 As/ In 0.53 Ga 0.47 As inverted high electron mobility transistors (HEMTs) with a novel ohmic structure. The ohmic contact between the ohmic electrodes and the two-dimensional electron gas (2DEG) formed in the InAs layer is obtained by direct contact with the ohmic electrodes-InAs, instead of an alloyed normal-metal and semiconductor as in a conventional inverted HEMT. The contact resistance of 0.11 Ωmm between the ohmic electrodes and the channel is smaller by a factor of 4, than that obtained using a conventional AuGeNi alloyed ohmic contact. For a 0.5 μm-gate device, a maximum extrinsic transconductance of 1.2 S/mm was obtained at 4.2 K, even at a very low drain voltage of 0.2 V. These results show that this ohmic contact formation allows us to obtain the improved HEMT characteristics.