Single Barrier Heterostructure Research Articles

Experimental evidences of quantum confined 2D indirect excitons in single barrier GaAs/AlAs/GaAs heterostructure using photocapacitance at room temperature

We investigated excitonic absorptions in a GaAs/AlAs/GaAs single barrier heterostructure using both photocapacitance and photocurrent spectroscopies at room temperature. Photocapacitance spectra show well defined resonance peaks of indirect excitons formed around the Γ-AlAs barrier. Unlike DC-photocurrent spectra, frequency dependent photocapacitance spectra interestingly red shift, sharpen up, and then decrease with increasing tunneling at higher biases. Such dissimilarities clearly point out that different exciton dynamics govern these two spectral measurements. We also argue why such quantum confined dipoles of indirect excitons can have thermodynamically finite probabilities to survive even at room temperature. Finally, our observations demonstrate that the photocapacitance technique, which was seldom used to detect excitons in the past, is useful for selective detection and experimental tuning of relatively small numbers (∼1011/cm2) of photo-generated indirect excitons having large effective dipole moments in this type of quasi-two dimensional heterostructures.

Hidden peculiar magnetic anisotropy at the interface in a ferromagnetic perovskite-oxide heterostructure

Understanding and controlling the interfacial magnetic properties of ferromagnetic thin films are crucial for spintronic device applications. However, using conventional magnetometry, it is difficult to detect them separately from the bulk properties. Here, by utilizing tunneling anisotropic magnetoresistance in a single-barrier heterostructure composed of La0.6Sr0.4MnO3 (LSMO)/LaAlO3 (LAO)/Nb-doped SrTiO3 (001), we reveal the presence of a peculiar strong two-fold magnetic anisotropy (MA) along the [110]c direction at the LSMO/LAO interface, which is not observed in bulk LSMO. This MA shows unknown behavior that the easy magnetization axis rotates by 90° at an energy of 0.2 eV below the Fermi level in LSMO. We attribute this phenomenon to the transition between the eg and t2g bands at the LSMO interface. Our finding and approach to understanding the energy dependence of the MA demonstrate a new possibility of efficient control of the interfacial magnetic properties by controlling the band structures of oxide heterostructures.

Comparison of Electron Transmittance and Tunneling Current through a Trapezoidal Potential Barrier with Spin Polarization Consideration by using Analytical and Numerical Approaches

In this study, we report an analytical calculation of electron transmittance and polarized tunneling current in a single barrier heterostructure of a metal-GaSb-metal by considering the Dresselhaus spin orbit effect. Exponential function, WKB method and Airy function were used in calculating the electron transmittance and tunneling current. A Transfer Matrix Method, as a numerical method, was utilized as the benchmark to evaluate the analytical calculation. It was found that the transmittances calculated under exponential function and Airy function is the same as that calculated under TMM method at low electron energy. However, at high electron energy only the transmittance calculated under Airy function approach is the same as that calculated under TMM method. It was also shown that the transmittances both of spin-up and spin-down conditions increase as the electron energy increases for low energies. Furthermore, the tunneling current decreases with increasing the barrier width.

Enhanced current transport in GaN/AlN based single and double barrier heterostructures

Current transport through a unique structure design employing high quality GaN based heterostructure with sharp interfaces has been investigated. A novel approach of structure design has been adopted to enhance the current transport in GaN heterostructures with a specific number of barrier layers of AlN which includes Single Barrier Heterostructure (SBH) and Double Barrier Heterostructure (DBH) devices. The high band gap AlN can act as a barrier for current conduction between GaN layers and help in effectively enhancing the current transport in the device through tunneling phenomena. A highly enhanced current transport in comparison with No Barrier Heterostructure (NBH) has been observed in SBH and a further improvement is perceived in DBH. Moreover, the phenomenon of current conduction is explained through drift-diffusion model, in which current enhancement upon subsequent addition of high bandgap barrier layer has resulted in localized high electric field and thus charge carrier velocity overshoots. Further, this has also been explained via quantum model, by interference of transmitted and reflected electron wave at interfaces. UV photodetectors using such heterostructure designs with and without AlN barrier layers in metal-semiconductor-metal geometry have been fabricated. The UV photodetection device developed using DBH yields photoresponsivity 80 times higher as compared to NBH device under UV illumination (325nm). Employment of such structures will enable scaling up the production of highly efficient optoelectronic devices.

Spin injection via (110)-grown semiconductor barriers

We study the tunneling of conduction electrons through a (110)-oriented single-barrier heterostructure grown from III-V semiconductor compounds. It is shown that, due to low spatial symmetry of such a barrier, the tunneling current through the barrier leads to an electron spin polarization. The inverse effect, generation of a direct tunneling current by spin polarized electrons, is also predicted. We develop the microscopic theory of the effects and show that the spin polarization emerges due to the combined action of the Dresselhaus spin-orbit coupling within the barrier and the Rashba spin-orbit coupling at the barrier interfaces.

Bandgap and band offsets determination of semiconductor heterostructures using three-terminal ballistic carrier spectroscopy

Utilizing ambipolar tunnel emission of ballistic electrons and holes, we have developed a model-independent method to self-consistently measure bandgaps of semiconductors and band offsets at semiconductor heterojunctions. Lattice-matched GaAs/AlxGa1−xAs and GaAs/(AlxGa1−x)0.51In0.49P (100) single-barrier heterostructures are studied at 4.2 K. For the GaAs/AlGaAs interface, the measured Γ band offset ratio is 60.4:39.6 (±2%). For the heteroanion GaAs/AlGaInP (100) interface, this ratio varies with the Al composition and is distributed more in the valence band. The indirect-gap X band offsets observed at the GaAs/AlGaInP interface deviates from predictions by the transitivity rule.

Characterization of Single Barrier Microrefrigerators at Cryogenic Temperatures

The experimental characterization of single barrier heterostructure thermionic cooling devices at cryogenic temperatures is reported. The device studied was a cylindrical InGaAs microrefrigerator, in which the active layer was a 1 μm thick In0.527Al0.218Ga0.255As heterostructure barrier with n-type doping concentration of 6.68 × 1016 cm−3 and an In0.53Ga0.47As emitter/collector of 5 × 1018 cm−3 n-doping. A full field thermoreflectance imaging technique was used to measure the distribution of temperature change on the device’s top surface when different current excitation values were applied. By reversing the current direction, we studied the device’s behavior in both cooling and heating regimes. At an ambient temperature of 100 K, a maximum cooling of 0.6 K was measured. This value was approximately one-third of the measured maximum cooling value at room temperature (1.8 K). The paper describes the device’s structure and the first reported thermal imaging at cryogenic temperatures using the thermoreflectance technique.

Magnetic-field-induced Fermi-edge singularity in the tunneling current through an InAs self-assembled quantum dot

The results of the investigation of tunneling transport through a GaAs/(AlGa)As/GaAs single-barrier heterostructure containing InAs self-assembled quantum dots at low temperatures are reported. An anomalous increase in the tunneling current through the quantum dots has been observed in the presence of a magnetic field both parallel and perpendicular to the current. This increase is a manifestation of a Fermi-edge singularity appearing in the current due to the interaction of a tunneling electron with the electron gas in an emitter.

Monte Carlo simulation of electron transport in degenerate and inhomogeneous semiconductors

An algorithm is proposed to include Pauli exclusion principle in Monte Carlo simulations. This algorithm has significant advantages to implement in terms of simplicity, speed, and memory storage; therefore it is ideal for the three-dimensional device simulators. The authors show that even in moderately high applied fields, one can obtain the correct electronic distribution. They give the correct definition for electronic temperature and show that in high applied fields, the quasi-Fermi level and electronic temperature become valley dependent. The effect of including Pauli exclusion principle on the band profile, electronic temperature, and quasi-Fermi level for the inhomogeneous case of a single barrier heterostructure is illustrated.

Resonant tunneling effect and tunneling magnetoresistance in GaMnAs quantum‐well double‐barrier heterostructures

We have investigated the d2I /dV2-V characteristics and tunneling magnetoresistance (TMR) in the GaMnAs quantum-well (QW) double-barier heterostructures, and compared them with those of the GaMnAs-based single-barrier heterostructure. We obtained a relatively high TMR ratio of 126.5% in the single-barrier heterostructure at 2.6 K. The value of d2I /dV2 and the TMR ratio of this single-barrier heterostructure monotonically approached zero with increasing the bias voltage. On the other hand, in the double-barrier structures, oscillatory behaviors were seen in the negative bias region of d2I /dV2-V curves. With increasing the QW width, these peaks shifted to a smaller voltage and the period of the oscillation became short. This means that these oscillatory behaviors are induced by the resonant tunneling effect mediated by holes tunneling through the GaMnAs QW. Also, we observed that TMR is increased at these resonant peak bias voltages in the double-barrier heterostructures. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Resonant tunnelling of electrons in multi-step single-barrier heterostructures

We have studied the quantum transport of electrons in a three-step single-barrier AlGaAs heterostructure under electric field. Using the quantum transmitting boundary method and Tsu–Esaki approach, we have calculated the transmission coefficient and current–voltage characteristic. The difference of the effective mass among the three barriers is taken into account. Effects of the barrier width on transmission coefficient and peak-to-valley current ratios are examined. The largest peak-to-valley current ratio is obtained when the ratio of widths of the left, middle, and right barrier is fixed at 4:2:1. The calculated results may be helpful for designing devices based on three-step barrier heterostructures.

Magnetic-field-induced singularity in the tunneling current through an InAs quantum dot

The tunneling transport through a GaAs/(AlGa)As/GaAs single-barrier heterostructure with self-assembled InAs quantum dots is studied experimentally at low temperatures. An anomalous increase in the tunneling current through the quantum dots is observed in magnetic fields both parallel and perpendicular to the current. This result cannot be understood in the framework of the single-electron approximation. The proposed explanation of the phenomenon is based on the modified Matveev-Larkin theory, which predicts the appearance of a singularity in the tunneling current through the zero-dimensional state in a magnetic field because of the interaction between the tunneling electron and the spin-polarized three-dimensional electron gas in the emitter. The absence of spin splitting in the experimental resonance peaks is caused by the complete spin polarization of the emitter in relatively weak magnetic fields.

Observation of negative differential resistance in GaAlAs single-barrier heterostructure at room temperature

Observation of experimental negative differential resistance at room temperature due to electron tunneling in a multiple-step single-barrier GaAlAs heterostructure is reported. Theoretical investigations of a three-step single-barrier heterostructure were conducted using the transfer matrix method and the Tsu–Esaki approach to obtain the transmission coefficients and current–voltage characteristic, respectively. The system was designed based on these calculations, and grown with molecular beam epitaxy. The diode exhibited negative differential resistance at 300 K and had a peak-to-valley current ratio above unity, which was in agreement with the predicted values.

NONEQUILIBRIUM ELECTRONS AND PHONONS IN THIN FILM THERMIONIC COOLERS

The effect of hot carriers on the cooling performance of single barrier heterostructure thermionic coolers is studied theoretically. By studying nonequilibrium characteristics of electrons and phonons in the device, fundamental limitation in the cooler performance is analyzed. In particular, we investigated the effect of various boundary conditions at heterojunctions on the electron and phonon temperature distributions. These boundary conditions have a strong impact on the device operation. Thin film devices under high voltage or in high current density are examples of situations where electrons and phonons are not in equilibrium and a coupled transport equation should be solved for an accurate analysis. In a thermoelectric/thermionic device one measures the lattice temperature while cooling occurs in the electron gas. Although at low currents electrons and phonons have the same equilibrium temperature, by increasing the current they may have different temperatures, which can lead to a reduction in cooling power density. We will show that in materials with faster electron energy relaxation, that is, higher electron-phonon coupling, thermionic cooling performance is less affected by high current injection and that recently demonstrated SiGe thin film coolers are not limited by hot carrier effects.

Resonant tunneling via donorXstates in the AlAs barrier and binding energies of donors bound toXXYandXZvalleys

Magnetotransport in GaAs/AlAs/GaAs single-barrier heterostructures, incorporating unintentional donors in the barrier, is studied. Resonant tunneling is observed through the quasiconfined states in the AlAs layer which originate from the ${X}_{\mathrm{XY}}$ and ${X}_{Z}$ conduction-band minima and through two distinct states of the donors bound to the ${X}_{\mathrm{XY}}$ and ${X}_{Z}$ valleys. This allowes us to determine directly the binding energies of ${X}_{\mathrm{XY}}$- and ${X}_{Z}$-related donors at the center of a 5-nm AlAs barrier as ${E}_{B}{(X}_{\mathrm{XY}})\ensuremath{\approx}70 \mathrm{meV}$ and ${E}_{B}{(X}_{Z})\ensuremath{\approx}50 \mathrm{meV},$ respectively. Furthermore, we observe an additional oscillatory fine structure of the donor resonances which we attribute to a difference in the binding energies of donors located at different position in the AlAs layer.

Resonance tunneling and nonlinear current in heterobarriers with complex law of carrier dispersion

New effects in resonance electron tunneling in a GaAs/AlxGa1−xAs/GaAs single-barrier heterostructure are analyzed with applied electric bias taken into account. Γ-X mixing of electron states at the interfaces is responsible for the Fano resonance in the barrier transmission coefficient. Trajectories of the Fano resonances and their interplay with the Breit-Wigner resonances in an electric field are studied. The current-voltage characteristic of the heterobarrier is calculated. A knowledge of the differential conductance makes it possible to obtain the Fano resonance profile and determine its parameters.

Interface electronic states and boundary conditions for envelope functions

The envelope-function method with generalized boundary conditions is applied to the description of localized and resonant interface states. A complete set of phenomenological conditions which restrict the form of connection rules for envelope functions is derived using the Hermiticity and symmetry requirements. Empirical coefficients in the connection rules play role of material parameters which characterize an internal structure of every particular heterointerface. As an illustration we present the derivation of the most general connection rules for the one-band effective mass and 4-band Kane models. The conditions for the existence of Tamm-like localized interface states are established. It is shown that a nontrivial form of the connection rules can also result in the formation of resonant states. The most transparent manifestation of such states is the resonant tunneling through a single-barrier heterostructure.

Resonant tunnelling via states of the X-related donors located at different atomic layer in AlAs barrier

Magnetotransport in GaAs/AlAs/GaAs single-barrier heterostructures incorporating unintentional donors in the barrier is studied. Resonant tunnelling was observed both through the quasiconfined states in the AlAs layer which originated from the X XY and X Z conduction band minima and through two distinct states of the donors bound to the X XY and X Z valleys. Furthermore, we observed an additional oscillatory fine structure of the donor resonances which we attribute to the difference in binding energies of donors located at different position of the AlAs layer. Magnetic field behaviour of the fine structure shows that the binding energy of X-related donors depends on both magnetic field and donor position in the barrier.

N-type negative differential conductance in quasiballistic single-barrier heterostructures

We propose a mechanism for negative differential conductance in nanometric single barrier heterostructures when transport is controlled by tunneling processes and space charge effects. Theoretical expectations are confirmed by Monte Carlo simulations of a model GaAs–GaAlAs–GaAs semiconductor heterostructure.

Monte Carlo study of dc and ac vertical electron transport in a single-barrier heterostructure

Steady-state dc and large-signal ac vertical electron transport in a single barrier ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ structure is investigated by ensemble Monte Carlo simulations self-consistently coupled with the Poisson equation. The comparison of the calculated steady-state current-voltage characteristic with the experimental data indicates the strong influence of $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}\ensuremath{\Gamma}$ resonant tunneling on the dc current-voltage characteristic of the ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ structure. The Monte Carlo technique is applied for an evaluation of the capacitance-voltage characteristic of this structure. The calculated capacitance-voltage characteristic is compared with the corresponding experimental results. Frequency-dependent harmonic amplitudes are also evaluated from the calculated large-signal ac response.