GaAs Heterostructures Research Articles

Growth and characterization of GaAs1−x−ySbxNy/GaAs heterostructures for multijunction solar cell applications

The GaAsSbN dilute-nitride alloy can be grown lattice-matched to GaAs with a bandgap of 1 eV, making it an ideal candidate for use in multijunction solar cells. In this work, using molecular beam epitaxy in conjunction with a radio-frequency nitrogen plasma source, the authors focus first on the growth optimization of the GaAsSb and GaAsN alloys in order to calibrate the Sb and N compositions independently of each other. After the optimum growth conditions to maintain two-dimensional growth were identified, the growth of GaAsSbN films was demonstrated. Both a GaAsSb0.076N0.018/GaAs heterostructure (100 nm thick) and a GaAsSb0.073N0.015/GaAs quantum well (11 nm thick) were grown. X-ray diffraction analysis reveals quite high crystal quality with a small lattice mismatch of 0.13%–0.16%. Secondary ion mass spectrometry profiling revealed that nitrogen was unintentionally incorporated in the GaAs buffer layer during the plasma ignition and stabilization. Nevertheless, a low temperature photoluminescence peak energy of 1.06 eV was measured for the GaAsSbN heterostructure sample while the quantum well emitted photoluminescence at 1.09 eV, which demonstrates promise for realizing 1-eV solar cells.

High-resolution error detection in the capture process of a single-electron pump

The dynamic capture of electrons in a semiconductor quantum dot (QD) by raising a potential barrier is a crucial stage in metrological quantized charge pumping. In this work, we use a quantum point contact (QPC) charge sensor to study errors in the electron capture process of a QD formed in a GaAs heterostructure. Using a two-step measurement protocol to compensate for 1/f noise in the QPC current, and repeating the protocol more than 106 times, we are able to resolve errors with probabilities of order 10−6. For the studied sample, one-electron capture is affected by errors in ∼30 out of every million cycles, while two-electron capture was performed more than 106 times with only one error. For errors in one-electron capture, we detect both failure to capture an electron and capture of two electrons. Electron counting measurements are a valuable tool for investigating non-equilibrium charge capture dynamics, and necessary for validating the metrological accuracy of semiconductor electron pumps.

Efficient nitrogen incorporation in GaAs using novel metal organic As–N precursor di-tertiary-butyl-arsano-amine (DTBAA)

III/V semiconductors containing small amounts of nitrogen (N; dilute nitrides) are discussed in the context of different solar cell and laser applications. The efficiency of these devices is negatively affected by carbon (C) incorporation, which comes either from the direct C–N bond in the N precursor unsymmetrical 1,1-dimethylhydrazine (UDMHy) used conventionally or from the alkyl groups of the conventional precursors for gallium (Ga), indium and arsenic (As) containing carbon. This C is incorporated together with the N due to the strength of the C–N bond. A further important issue in dilute nitride growth is the very low N incorporation efficiency in the crystal from UDMHy, which can be as little as 1% of the N supplied in the gas phase. Therefore, new metal organic chemicals have to be synthesized and their growth characteristics and suitability for dilute nitride growth have to be explored. This work presents the chemical di-tertiary-butyl-arsano-amine (DTBAA), which was synthesized, purified and tested as an N precursor for metal organic vapor phase epitaxy (MOVPE). Computational investigations show β-hydrogen and isobutane elimination to be the main reaction channel in the gas phase with high reaction barriers and absence of small fragments containing C as products. The loss of N via N2, as in UDMHy, can be excluded for unimolecular reactions of DTBAA. The Ga(NAs)/GaAs heterostructures were grown by MOVPE as initial test material and a systematic N incorporation study is presented in this paper. It is shown that high quality Ga(NAs) can be grown using DTBAA. The N incorporation was confirmed by high resolution X-ray diffraction and photoluminescence studies. All samples grown exhibit as grown room temperature photoluminescence and smooth surface morphologies. Furthermore, DTBAA shows extremely high N incorporation efficiency, which makes this molecule a very promising candidate for further research into dilute nitride material growth.

Knight shift and nuclear spin relaxation inFe/n-GaAs heterostructures

We investigate the dynamically polarized nuclear-spin system in Fe/\emph{n}-GaAs heterostructures using the response of the electron-spin system to nuclear magnetic resonance (NMR) in lateral spin-valve devices. The hyperfine interaction is known to act more strongly on donor-bound electron states than on those in the conduction band. We provide a quantitative model of the temperature dependence of the occupation of donor sites. With this model we calculate the ratios of the hyperfine and quadrupolar nuclear relaxation rates of each isotope. For all temperatures measured, quadrupolar relaxation limits the spatial extent of nuclear spin-polarization to within a Bohr radius of the donor sites and is directly responsible for the isotope dependence of the measured NMR signal amplitude. The hyperfine interaction is also responsible for the $2\text{ kHz}$ Knight shift of the nuclear resonance frequency that is measured as a function of the electron spin accumulation. The Knight shift is shown to provide a measurement of the electron spin-polarization that agrees qualitatively with standard spin transport measurements.

Anisotropic spin relaxation inn-GaAs from strong inhomogeneous hyperfine fields produced by the dynamical polarization of nuclei

The hyperfine field from dynamically polarized nuclei in n-GaAs is very spatially inhomogeneous, as the nu- clear polarization process is most efficient near the randomly-distributed donors. Electrons with polarized spins traversing the bulk semiconductor will experience this inhomogeneous hyperfine field as an effective fluctuating spin precession rate, and thus the spin polarization of an electron ensemble will relax. A theory of spin relaxation based on the theory of random walks is applied to such an ensemble precessing in an oblique magnetic field, and the precise form of the (unequal) longitudinal and transverse spin relaxation analytically derived. To investigate this mechanism, electrical three-terminal Hanle measurements were performed on epitaxially grown Co$_2$MnSi/$n$-GaAs heterostructures fabricated into electrical spin injection devices. The proposed anisotropic spin relaxation mechanism is required to satisfactorily describe the Hanle lineshapes when the applied field is oriented at large oblique angles.

Energy scale of compositional disorder in Ga(AsBi)

We report on a study of compositional disorder in Ga(AsBi) structures. Temperature-dependent photoluminescence measurements on Ga(AsBi)/GaAs heterostructures with different Bi contents are performed. Experimental observations show an essentially non-monotonous dependence of the energy scale of disorder on the Bi content. Our theoretical analysis concludes that this peculiar behavior is a consequence of an essential bowing of the valence band edge as a function of Bi content and of a specific compositional dependence of the hole effective mass in Ga(AsBi) compounds.

Type I band alignment in GaAs81Sb19/GaAs core-shell nanowires

The composition and band gap of the shell that formed during the growth of axial GaAs/GaAs81Sb19/ GaAs heterostructure nanowires have been investigated by transmission electron microscopy combined with energy dispersion spectroscopy, scanning tunneling spectroscopy, and density functional theory calculations. On the GaAs81Sb19 intermediate segment, the shell is found to be free of Sb (pure GaAs shell) and transparent to the tunneling electrons, despite the (110) biaxial strain that affects its band gap. As a result, a direct measurement of the core band gap allows the quantitative determination of the band offset between the GaAs81Sb19 core and the GaAs shell and identifies it as a type I band alignment.

Magnetic-field-induced photocurrent in metal-dielectric-semiconductor heterostructures based on cobalt nanoparticles SiO2(Co)/GaAs

Magnetic-field influence on photocurrent in heterostructures of silicon dioxide films with cobalt nanoparticles SiO2(Co) grown on gallium arsenide GaAs substrate has been studied in the avalanche regime at room temperature. High values of magnetic-field-induced photocurrent were found in the vicinity and above the GaAs bandgap of ∼1.4eV. For photon energies E>1.4eV the photocurrent significantly increases, while the avalanche process is suppressed by the magnetic field, and the current flowing through the heterostructure decreases. The photocurrent is enhanced in the SiO2(Co 60at%)/GaAs heterostructure at the magnetic field H=1.65kOe by factor of about 10 for the photon energy E=1.5eV. This phenomenon is explained by a model based on electronic transitions in magnetic fields with the spin-dependent recombination process at deep impurity centers in the SiO2(Co)/GaAs interface region.

Two‐energy‐scale model for description of the thermal quenching of photoluminescence in disordered Ga(As,Bi)

AbstractThe thermal quenching of the photoluminescence (PL) intensity in Ga(As,Bi)/GaAs heterostructures is studied experimentally and theoretically. We observed an anomalous plateau in the PL thermal quenching at intermediate temperatures under low excitation conditions. Our theoretical analysis based on a well‐approved theoretical approach shows that this peculiar behavior of the temperature‐dependent PL intensity cannot be interpreted assuming a single‐scale monotonously energy‐dependent density of localized states (DOS). Experimental data clearly point at a non‐monotonous DOS with at least two energy scales. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excess noise and deep levels in GaAs detectors of nuclear particles and ionizing radiation

Experimental results on various electric noises in the Al/GaAs structures with Schottky barriers and the AlGaAs/i-GaAs heterostructures that are employed in GaAs detectors of high-energy nuclear particles and ionizing radiation are reviewed. Methods for the study of the parameters of deep levels (DLs) are described. The systematization of DLs that are formed in GaAs barrier structures and heterostructures in the presence of various impurities and defects of crystal lattice is performed. The results on the effect of DLs on the 1/f noise of the detectors and the data on radiation stability under the action of high-energy electrons and γ radiation are presented. Practical recommendations on the type and production technology of a detector with the minimum level of the 1/f noise are proposed. The method for the measurement of the 1/f noise of the detectors based on the Al/i-GaAs barrier structures is recommended for evaluation of the radiation stability and the breakdown voltage of high-voltage detectors.

Second Landau level fractional quantum Hall effects in the Corbino geometry

For certain measurements, the Corbino geometry has a distinct advantage over the Hall and van der Pauw geometries, in that it provides a direct probe of the bulk 2DEG without complications due to edge effects. This may be important in enabling detection of the non-Abelian entropy of the 5/2 fractional quantum Hall state via bulk thermodynamic measurements. We report the successful fabrication and measurement of a Corbino-geometry sample in an ultra-high mobility GaAs heterostructure, with a focus on transport in the second and higher Landau levels. In particular, we report activation energy gaps of fractional quantum Hall states, with all edge effects ruled out, and extrapolate the conductivity prefactor from the Arrhenius fits. Our results show that activated transport in the second Landau level remains poorly understood. The development of this Corbino device opens the possibility to study the bulk of the 5/2 state using techniques not possible in other geometries.

Hierarchy of modes in an interacting one-dimensional system.

Studying interacting fermions in one dimension at high energy, we find a hierarchy in the spectral weights of the excitations theoretically, and we observe evidence for second-level excitations experimentally. Diagonalizing a model of fermions (without spin), we show that levels of the hierarchy are separated by powers of R^{2}/L^{2}, where R is a length scale related to interactions and L is the system length. The first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalized single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power laws at the spectral edge. We measure momentum-resolved tunneling of electrons (fermions with spin) from or to a wire formed within a GaAs heterostructure, which shows parabolic dispersion of the first-level mode and well-resolved spin-charge separation at low energy with appreciable interaction strength. We find structure resembling the second-level excitations, which dies away quite rapidly at high momentum.

Effect of magnetic field enhancement of the photocurrent in ferromagnetic metal-dielectric heterostructures SiO2(Co)/GaAs

Heterostructures of silicon dioxide films containing cobalt nanoparticles SiO2(Co) grown on GaAs substrate exhibit at room temperature high values of magnetic field enhancement of photocurrent in the vicinity and above the GaAs bandgap of ∼1.4 eV. For photon energies E above the GaAs bandgap, the photocurrent significantly increases, while the avalanche process is suppressed by the magnetic field, and the current flowing through the heterostructure decreases. The photocurrent is enhanced in the SiO2(Co 60 at. %)/GaAs heterostructure at the magnetic field H = 1.65 kOe by a factor of about ten for the photon energy E = 1.5 eV. This phenomenon is explained by a model describing electronic transitions in magnetic fields with the spin-dependent recombination process at deep impurity centers in the GaAs interface region.

Even-denominator fractional quantum Hall physics in ZnO

The fractional quantum Hall effect, occurring for rational Landau-level filling factors, is commonly observed in GaAs heterostructures. Now, unusual even-denominator fractional quantum Hall states are reported for an oxide 2D electron system.

Characteristics enhancement of a GaAs based heterostructure field-effect transistor with an electrophoretic deposition (EPD) surface treated gate structure

A Pt/AlGaAs/InGaAs/GaAs heterostructure field-effect transistor (HFET), prepared by an electrophoretic deposition (EPD) approach on gate Schottky contact region, is fabricated and studied. The EPD-based Pt-gates with three different molar ratios (ω0) are examined by scanning electron microscopy (SEM) image. Good Pt-gate coverage with effective reduction of thermal-induced defects at Pt/AlGaAs interface is achieved through a low temperature EPD approach. Experimentally, for a gate dimension of 1μm×100μm, a lower gate current of 1.9×10−2mA/mm, a higher turn-on voltage of 0.85V, a higher maximum drain saturation current of 319.3mA/mm, and a higher maximum extrinsic transconductance of 146.8mS/mm are obtained for an EPD-based HFET at 300K. Moreover, comparable microwave characteristics of an EPD-based HFET are demonstrated at different temperature ambiences. Therefore, based on the improved DC performance and inherent benefits of low cost, simple apparatus, flexible deposition on varied substrates, and adjustable alloy grain size, the proposed EPD approach shows the promise to fabricate high-performance electronic devices.

Thermal quenching of photoluminescence in Ga(AsBi)

We report on a comparative experimental and theoretical study of the thermal quenching of the photoluminescence (PL) intensity in Ga(AsBi)/GaAs heterostructures. An anomalous plateau in the PL thermal quenching is observed at intermediate temperatures under relatively low excitation intensities. Theoretical analysis based on a well-approved approach shows that this peculiar behavior points at a non-monotonous density of states (DOS) in the disorder-induced band tails with at least two-energy-scales. While in previous studies carried out at relatively high excitation intensities a single-energy-scale was sufficient to fit the thermal quenching of the PL in Ga(AsBi), our study at lower excitation intensities proves that two-energy-scales of disorder contribute to the thermal quenching of the PL. Possible energy shapes of the DOS, which can fit experimental data, are revealed.

Tunneling current in Si-doped n type-GaAs heterostructures infrared emitter

In the present work, we measured the forward bias current-voltage (I–V) characteristics of Si-doped n type gallium arsenide (GaAs) heterostructures infrared emitter over a wide temperature range from 350 to 77 K. Results showed that the slopes of the exponential curve changed slowly with temperature. The analysis of the various tunneling mechanisms indicated that the tunneling current varied approximately as a function of ∼ exp(− αE g + βeV) where the parameters α and β varied indistinctively with temperature and voltage. The dependence of forward tunneling current on the temperature and bias can be explained by thermally induced band gap shrinkage and bias induced route change respectively. These results will be helpful for application of the optoelectronics device in both high and low temperature ambiences.

Recent experimental progress of fractional quantum Hall effect: 5/2 filling state and graphene

Abstract The phenomenon of fractional quantum Hall effect (FQHE) was first experimentally observed 33 years ago. FQHE involves strong Coulomb interactions and correlations among the electrons, which leads to quasiparticles with fractional elementary charge. Three decades later, the field of FQHE is still active with new discoveries and new technical developments. A significant portion of attention in FQHE has been dedicated to filling factor 5/2 state, for its unusual even denominator and possible application in topological quantum computation. Traditionally, FQHE has been observed in high-mobility GaAs heterostructure, but new materials such as graphene also open up a new area for FQHE. This review focuses on recent progress of FQHE at 5/2 state and FQHE in graphene.

Molecular Beam Epitaxy of Ultra-High-Quality AlGaAs/GaAs Heterostructures: Enabling Physics in Low-Dimensional Electronic Systems

Among very-low-disorder systems of condensed matter, the high-mobility two-dimensional electron gas (2DEG) confined in aluminum gallium arsenide (AlGaAs)–gallium arsenide (GaAs) heterostructures holds a privileged position as a platform for the discovery of new electronic states driven by strong Coulomb interactions. Molecular beam epitaxy (MBE), an ultra-high vacuum (UHV), thin-film deposition technique, produces the highest quality 2DEGs and has played a central role in a number of discoveries that have at their root the interplay of reduced dimensionality, strong electron-electron interactions, and disorder. This review attempts to describe the latest developments in heterostructure design, MBE technology, and the evolution of our understanding of disorder that result in improved material quality and facilitate discovery of new phenomena at ever finer energy scales.

Dependence of the ground-state transition energy versus optical pumping in GaAsSb/InGaAs/GaAs heterostructures

In this work, we report on the time-resolved photoluminescence studies of a double quantum well In0.2Ga0.8As/GaAs0.8Sb0.2/GaAs heterostructure which, in contrast to the GaAsSb/GaAs structures, is expected to provide effective confinement of electrons due to additional InGaAs layer. The studies at 4.2 K have revealed a complicated nonmonotonic dependence of the ground-state transition energy on the concentration of nonequilibrium charge carriers in the quantum well. The effect observed in this work is important in terms of creating sources of radiation, including stimulated emission, on the basis of InGaAs/GaAsSb/GaAs structures.