GaAs AlGaAs Research Articles

Properties of spectral characteristics of quasi-stationary electron states in an open multi-cascade nanostructure

The exact expressions for scattering matrix and transmission coefficient in open multi-cascade nanostructure are obtained. The theory of spectral characteristics of electron quasi-stationary states is developed in two approaches. For the nanostructure with GaAs wells and AlGaAs barriers, it is established that resonance bands appear in the N-cascade structure. Each one is formed by the energies of N states. Only the scattering matrix allows to unambiguously determine the resonance energies and resonance widths of all electron states in the multi-cascade structure. The approach of transmission coefficient, due to the collapse of resonances, gives the incomplete information about the spectral characteristics.

Optimization of a GaAs/AlGaAs p-i-n heterojunction nanowire solar cell for improved optical and electrical properties

GaAs/AlGaAs based nanowires are promising candidates for photovoltaic applications due to their high absorption coefficient, low surface reflection, and efficient collection of photogenerated carriers. This study focuses on optimizing the performance of p-i-n GaAs/AlGaAs nanowire solar cell arrays having a radial junction using optoelectronic simulations. The research investigates the optimal doping for the GaAs core and AlGaAs shell, as well as the impact of shell thickness and junction positions on solar cell performance. Additionally, the study examines the effect of various surface effects, including the presence of surface traps, surface recombination velocities, and associated lifetime degradation. Our studies find that a high doping density for the shell and core region is crucial for achieving an appropriate band configuration and carrier extraction. It also highlights that having a larger doping density is more important than having a larger lifetime. Finally, the research examines the effect of different aluminum compositions on photogeneration inside the nanowire and shows that having a high aluminum composition can confine most photogeneration to inner GaAs regions, potentially allowing for thicker AlGaAs shells, which can efficiently prevent surface recombination.

Ideal refocusing of an optically active spin qubit under strong hyperfine interactions.

Combining highly coherent spin control with efficient light-matter coupling offers great opportunities for quantum communication and computing. Optically active semiconductor quantum dots have unparalleled photonic properties but also modest spin coherence limited by their resident nuclei. The nuclear inhomogeneity has thus far bound all dynamical decoupling measurements to a few microseconds. Here, we eliminate this inhomogeneity using lattice-matched GaAs-AlGaAs quantum dot devices and demonstrate dynamical decoupling of the electron spin qubit beyond 0.113(3) ms. Leveraging the 99.30(5)% visibility of our optical π-pulse gates, we use up to Nπ = 81 decoupling pulses and find a coherence time scaling of [Formula: see text]. This scaling manifests an ideal refocusing of strong interactions between the electron and the nuclear spin ensemble, free of extrinsic noise, which holds the promise of lifetime-limited spin coherence. Our findings demonstrate that the most punishing material science challenge for such quantum dot devices has a remedy and constitute the basis for highly coherent spin-photon interfaces.

Anomalous Hall Effect in GaAs–AlGaAs Quantum Wells Doped by Nonmagnetic Impurities near the Metal–Insulator Transition

Anomalous Hall Effect in GaAs–AlGaAs Quantum Wells Doped by Nonmagnetic Impurities near the Metal–Insulator Transition

UV–Vis–NIR broadband response of GaAs-based photocathode with multilayer graded-band cascade structure

To improve the lattice matching, quantum efficiency and response range of the transmission-mode GaAs-based photocathode , a novel photocathode with multilayer graded-band cascade emission layer is proposed. The one-dimensional steady-state continuity equations and finite-difference time-domain methods are utilized to obtain the quantum efficiency and optical absorption characteristics of the proposed photocathode, respectively. The results show that, the built-in electric fields generated by the varying-composition and varying-doping structure can assist the photoelectrons generated in the sublayer itself and the front sublayers to transport towards the emitting surface and escape to the vacuum. Compared with the conventional AlGaAs/InGaAs structure, the proposed structure possesses better lattice matching and enhanced quantum efficiency in the UV and NIR wavelength range. A three-fold increase of quantum efficiency can be realized at 400 nm and the theoretical quantum efficiency at 1064 nm can reach 2%. Besides, the integral absorptivity is increased by 6.8% in the visible wavelength range, and 12.9% in the NIR wavelength range. It is found that the increase of the thickness of AlGaAs sublayer and GaAs sublayer mainly reduces the quantum efficiency in the UV–Vis wavelength range. When the InGaAs sublayer is thin enough, the increase of thickness would enhance the quantum efficiency in the NIR wavelength range and deteriorate that in the UV–Vis wavelength range. This work can contribute to the performance improvement of UV–Vis–NIR broadband GaAs-based photocathode. • A transmission-mode photocathode with multilayer graded-band cascade structure is proposed. • The quantum efficiency model of the proposed multilayer structure is deduced. • The quantum efficiency and optical absorption characteristics are investigated. • The lattice matching, quantum efficiency and response range are improved.

Spin Glass and Isotropic Negative Magnetoresistance in GaAs–AlGaAs Quantum Wells with a Virtual Anderson Transition

Spin Glass and Isotropic Negative Magnetoresistance in GaAs–AlGaAs Quantum Wells with a Virtual Anderson Transition

Investigation of the Depth and Rate of Ion Etching of QWIP Structures

In this study we investigated the dependences of the rate of ion-beam etching of the upper contact layer (GaAs:Si), an active region consisting of a 50-fold alternation of barrier layers (AlxGa1–xAs) and quantum wells (GaAs:Si) and the lower contact layer (GaAs:Si) along the depth of QWIP structures based on GaAs–AlGaAs, fabricated by molecular beam epitaxy (MBE), in order to determine the effect of the composition of various layers on the etching rate and the ability to complete the etching process to the desired depth in time.

Superconductivity in AuNiGe Ohmic contacts to a GaAs-based high mobility two-dimensional electron gas

To cool a high mobility two-dimensional electron gas (2DEG) at a GaAs–AlGaAs heterojunction to milliKelvin temperatures, we have fabricated low resistance Ohmic contacts based on alloys of Au, Ni, and Ge. The Ohmic contacts have a typical contact resistance of RC≈0.8 Ω at 4.2 K, which drops to 0.2 Ω below 0.9 K. Scanning electron microscope images establish that the contacts have the same inhomogeneous microstructure that has been observed in previous studies. Measurements of the contact resistance RC, the four-terminal resistance along the top of a single contact, and the vertical resistance RV all show that there is a superconductor in the Ohmic contact, which can be turned completely normal with a magnetic field of 0.15 T. We briefly discuss how this superconductivity may be affecting the electrical transport measurements of 2DEGs, especially how it may hinder the cooling of electrons in a 2DEG below 0.1 K.

Deep-level defects in high-voltage AlGaAs p–i–n diodes and the effect of these defects on the temperature dependence of the minority carrier lifetime

The variation of the effective lifetime of minority carrier lifetime τeff with temperature has been studied in the temperature range 300–580 K in AlxGa1−xAs base regions of p+–p0–i–n0–n+ high-voltage GaAs–AlGaAs diodes grown by liquid-phase epitaxy. It was found by using deep level transient spectroscopy that the emission/capture of electrons and holes in AlxGa1−xAs p0–i–n0 epitaxial layers is governed by the DX− states of the DX center formed by Se/Te background impurities. The Arrhenius plots associated with the τeff were used to determine the activation energy of the hole capture to the DX− level to be Ec = 159 meV. It is shown that the thermal capture of holes to the DX− level determines the relaxation time of nonequilibrium carriers in the AlxGa1−xAs base layers as well as its temperature dependence.

Active region designs and mounting schemes of THz quantum-cascade laser

THz quantum cascade-lasers (QCLs) are the best sources for generating efficient THz radiations, which are fabricated by the semiconductor heterostructure. The layers of GaAs quantum wells and AlGaAs barriers are formed with different thicknesses to design the heterostructure. In the present paper, we review different designs of THz QCLs active regions which are followed by a comparison between epitaxy-up and down processing techniques of the fabricated wafers. The present review should be helpful for the scientific community working in the scientific area of the THz radiation field and, in particular, for the researchers interested to switch from the epitaxy-up processing of the wafers to epitaxy-down mounting to make possible an increase of the operating temperature of the THz QCLs in the continuous wave operation mode.

Quantum-Confinement-Enhanced Thermoelectric Properties in Modulation-Doped GaAs-AlGaAs Core-Shell Nanowires.

Nanowires (NWs) hold great potential in advanced thermoelectrics due to their reduced dimensions and low-dimensional electronic character. However, unfavorable links between electrical and thermal conductivity in state-of-the-art unpassivated NWs have, so far, prevented the full exploitation of their distinct advantages. A promising model system for a surface-passivated one-dimensional (1D)-quantum confined NW thermoelectric is developed that enables simultaneously the observation of enhanced thermopower via quantum oscillations in the thermoelectric transport and a strong reduction in thermal conductivity induced by the core-shell heterostructure. High-mobility modulation-doped GaAs/AlGaAs core-shell NWs with thin (sub-40 nm) GaAs NW core channel are employed, where the electrical and thermoelectric transport is characterized on the same exact 1D-channel. 1D-sub-band transport at low temperature is verified by a discrete stepwise increase in the conductance, which coincided with strong oscillations in the corresponding Seebeck voltage that decay with increasing sub-band number. Peak Seebeck coefficients as high as ≈65-85 µV K-1 are observed for the lowest sub-bands, resulting in equivalent thermopower of S2 σ ≈ 60 µW m-1 K-2 and S2 G ≈ 0.06 pW K-2 within a single sub-band. Remarkably, these core-shell NW heterostructures also exhibit thermal conductivities as low as ≈3 W m-1 K-1 , about one order of magnitude lower than state-of-the-art unpassivated GaAs NWs.

Strong Modulations of Optical Reflectance in Tapered Core-Shell Nanowires.

Random assemblies of vertically aligned core–shell GaAs–AlGaAs nanowires displayed an optical response dominated by strong oscillations of the reflected light as a function of the incident angle. In particular, angle-resolved specular reflectance measurements showed the occurrence of periodic modulations in the polarization-resolved spectra of reflected light for a surprisingly wide range of incident angles. Numerical simulations allowed for identifying the geometrical features of the core–shell nanowires leading to the observed oscillatory effects in terms of core and shell thickness as well as the tapering of the nanostructure. The present results indicate that randomly displaced ensembles of nanoscale heterostructures made of III–V semiconductors can operate as optical metamirrors, with potential for sensing applications.

Dual wavelength optical duobinary modulation using GaAs–AlGaAs microring resonator

In this article, optical duobinary modulation technique has been used for modulating the dual-wavelength optical signal generated by the microring resonator system. The optical signals are de-multiplexed to two different optical signals with center wavelength of 1540 and 1545 nm afterward, these are modulated and transmitted over an optical fiber communication system, where the high performance of the transmitted signals is obtained. The NRZ binary signals are used to modulate the optical signals. The high-quality factor and performance of the system has been investigated by applying an optimization through the system parameters, where significant improvement could be obtained and highest quality factor of 32.871 shows the successful transmission. The second wavelength of the dual-wavelength has undergone the same modulation technique and has experienced the optical transmission after the system optimization has been performed, where the quality factor for the received optical signal at 1545 nm center wavelength is 39.282. These results show the introduced modulation technique is suitable to apply for the optical signals generated by the microring resonators.

High-Efficiency AlGaAs/GaAs Photovoltaic Converters with Edge Input of Laser Light

High-efficiency photovoltaic converters (PVCs) have been developed and fabricated by liquidphase epitaxy in the AlGaAs–GaAs system with laser light (λ = 850 nm) introduced through the edge surface in parallel to the plane of the p–n junction of the device structure. To raise the efficiency of light “capture” by the p–n junction, an AlxGa1–xAs waveguide layer is formed, in which the content of aluminum gradually varies from x = 0.55 to 0.15 so that the refractive index gradient is created in this layer and light beams are diverted toward the p–n junction. When a PVC (having no antireflection coating) is exposed to 0.1- to 0.2-W laser light, an efficiency of 41.5% is obtained. Depositing an antireflection coating on the edge surface of a PVC raises its efficiency to 55%.

Connecting Composition-Driven Faceting with Facet-Driven Composition Modulation in GaAs-AlGaAs Core-Shell Nanowires.

Ternary III-V alloys of tunable bandgap are a foundation for engineering advanced optoelectronic devices based on quantum-confined structures including quantum wells, nanowires, and dots. In this context, core-shell nanowires provide useful geometric degrees of freedom in heterostructure design, but alloy segregation is frequently observed in epitaxial shells even in the absence of interface strain. High-resolution scanning transmission electron microscopy and laser-assisted atom probe tomography were used to investigate the driving forces of segregation in nonplanar GaAs-AlGaAs core-shell nanowires. Growth-temperature-dependent studies of Al-rich regions growing on radial {112} nanofacets suggest that facet-dependent bonding preferences drive the enrichment, rather than kinetically limited diffusion. Observations of the distinct interface faceting when pure AlAs is grown on GaAs confirm the preferential bonding of Al on {112} facets over {110} facets, explaining the decomposition behavior. Furthermore, three-dimensional composition profiles generated by atom probe tomography reveal the presence of Al-rich nanorings perpendicular to the growth direction; correlated electron microscopy shows that short zincblende insertions in a nanowire segment with predominantly wurtzite structure are enriched in Al, demonstrating that crystal phase engineering can be used to modulate composition. The findings suggest strategies to limit alloy decomposition and promote new geometries of quantum confined structures.

THz Stimulated Emission from Simple Superlattice in Positive Differential Conductivity Region

Narrow band emissions at 2.6–2.8 THz are observed out of liquid helium cooled 1 mm disk chips prepared of a wafer with the very low n type doped weak barrier GaAs–GaAlAs superlattice of 1000 periods. The emissions are at about 8.0–18.0 V pulsed voltage applied to the chips in region of the chips positive DC differential conductivity that guaranties absence of inhomogeneous electric field domains in the chips. The emission frequency bands are estimated with a cyclotron resonance filter; the measurements show that the band width is of about that of the THz quantum cascade laser. By using long voltage pulses the chip heating above 100 K is achieved without substantial change in emission power. We speculate that the emission is super luminescence (amplification) of whispering gallery modes in the chips as a result of inverted Wannier-Stark level transitions under bias. The results are the first world demonstration of THz stimulated emission in a simple superlattice within region of positive DC differential conductivity; they give strong impetus for development of THz and higher frequency sources based on such simple superlattices; the sources should well compete with the THz quantum cascade lasers in particular at elevated temperatures.

Temperature dependence of THz emission and junction electric field of GaAs–AlGaAs modulation-doped heterostructures with different i-AlGaAs spacer layer thicknesses

Photocarrier dynamics in GaAs/AlGaAs modulation-doped heterostructures (MDH) having i-AlGaAs spacer layers of different thicknesses were investigated using temperature-dependent terahertz time-domain spectroscopy (THz-TDS) and photoreflectance spectroscopy. In particular, results are discussed in the framework of the temperature dependence of the heterojunction electric field and photocarrier velocity for two i-AlGaAs spacer layer thickness values. The junction electric field, THz emission intensity and bandwidth of the MDH samples all decrease as temperature decreases. In contrast, the THz emission intensity and bandwidth of a reference bulk undoped GaAs does not significantly vary with temperature. These results imply that THz emission of MDH’s originates primarily from carrier drift due to the GaAs/AlGaAs junction electric field. A general decrease in the THz emission bandwidth of the MDH’s is attributed to a decrease in carrier velocity at lower temperatures, presumably due to the weaker electric field. Moreover, the MDH sample with thinner spacer layer exhibited a higher junction electric field. This work demonstrates the study of temperature-dependent photocarrier transport and junction electric field measurements. The results may provide useful insights in the design of MDH-based devices.

Outside Nominal Operation Analysis and Design Considerations of Inverse-Class-E Power Amplifier

In this paper, design and analysis using analytical expressions for the inverse class-E power amplifier (PA) operating at the outside nominal operation, i.e., class- $\text{E}_{\mathrm{n}}$ PA, is presented. This operation is defined as nonzero current switch (n-ZCS) and nonzero-derivative-current switch (n-ZDCS) conditions. The generalized design equations as a function of design specifications, load resistance, and a given dc-supply voltage are derived. Two degrees of the design freedom achieved thanks to n-ZCS and n-ZDCS that are utilized for the simultaneous satisfaction of design specifications, such as peak-switch-voltage and peak-switch-current along with a given load resistance. The output power capability is affected by n-ZCS and n-ZDCS conditions, which can be obtained in the desired value using the controlled adjustment of introduced deviation parameters. In order to prove the validity of the proposed design methodology, an inverse class-E PA operating in 2 GHz with output power of 23 dBm is implemented in 0.25- $\mu \text{m}$ AlGaAs–InGaAs pHEMT technology. There is a close agreement between the theoretical and simulation results and the measurements of the fabricated prototype PA, which proved the validity of the obtained design expressions.

Photo-Acoustic Spectroscopy Reveals Extrinsic Optical Chirality in GaAs-Based Nanowires Partially Covered with Gold

We report on the extrinsic chirality behavior of GaAs-based NWs asymmetrically hybridized with Au. The samples are fabricated by a recently developed, lithography-free self-organized GaAs growth, with the addition of AlGaAs shell and GaAs supershell. The angled Au flux is then used to cover three-out-of-six sidewalls with a thin layer of Au. Oblique incidence and proper sample orientation can lead to circular dichroism. We characterize this chiral behavior at $$ 532\,{\text{nm}} $$ and $$ 980\,{\text{nm}} $$ by means of photo-acoustic spectroscopy, which directly measures the difference in absorption for the circularly polarized light of the opposite headedness. For the first time to our knowledge, circular dichroism is observed in both the amplitude and the phase of the photo-acoustic signal. We strongly believe that such samples can be used for chiral applications, spanning from circularly polarized light emission, to the enantioselectivity applications.

Investigating effect of temperature on barrier height of PWB diodes

The effect of temperature on the barrier height of a gallium arsenide (GaAs)/aluminium GaAs (AlGaAs) potential-well barrier (PWB) diodes is investigated and reported in this Letter. It was found that more electrons diffuse from the doped AlGaAs(silicon) into the undoped layers with increasing temperature, this modifies the electric field along these layers and hence the barrier height. The barrier height increases with increasing temperature at an increasing rate of 0.093 – 0.36 meV / K in the temperature range of 200–380 K in a similar way to the planar-doped barrier (PDB) diode. The result of the computer simulations, however, shows that the PWB diode is less sensitive than the PDB diode due to the active nature of the charge in the well.