Electrons In GaAs Research Articles

Charge dynamics in the 2D/3D semiconductor heterostructure WSe2/GaAs

Understanding the relaxation and recombination processes of excited states in two-dimensional (2D)/three-dimensional (3D) semiconductor heterojunctions is essential for developing efficient optical and (opto)electronic devices, which integrate van der Waals 2D materials with more conventional 3D ones. In this work, we unveil the carrier dynamics and charge transfer in a monolayer of WSe2 on a GaAs substrate. We use time-resolved differential reflectivity to study the charge relaxation processes involved in the junction and how they change when compared to an electrically decoupled heterostructure, WSe2/hBN/GaAs. We observe that the monolayer in direct contact with the GaAs substrate presents longer optically excited carrier lifetimes (3.5 ns) when compared with the hBN-isolated region (1 ns), consistent with a strong reduction of radiative decay and a fast charge transfer of a single polarity. Through low-temperature measurements, we find evidence of a type-II band alignment for this heterostructure with an exciton dissociation that accumulates electrons in GaAs and holes in WSe2. The type-II band alignment and fast photoexcited carrier dissociation shown here indicate that WSe2/GaAs is a promising junction for advanced photovoltaic and other optoelectronic devices, making use of the best properties of van der Waals (2D) and conventional (3D) semiconductors.

Lateral semiconductor magnonics: an array of GaAs stripes atop the YIG layer

In this work, we demonstrate the numerical and experimental research of the spin-wave transport in a structure composed of a gallium arsenide (GaAs) stripe lattice interfaced to an yttrium iron garnet layer. We show that this structure can be considered as an array of an infinite number of laterally coupled ferrite-semiconductor waveguides. We show that the surface wave properties for colinear propagation along the semiconductor stripes are similar to the waves in magnetic films with partial metallization. In addition, the properties of these surface waves depend on the electron concentration of the GaAs and thus may be tuned. With regard to the wave propagation at a certain angle to the GaAs stripe lattice, the Bragg resonance forms and the corresponding band gap depend on the angle between the wave to the stripes and on the GaAs electron density. The Brillouin light scattering technique was used to experimentally observe the spin-wave beam transformation, and microwave measurements support the numerical data and reveal the mechanism of the dip formation and widening of the frequency range in the spin-wave transmission. The proposed structure could be used as a reconfigurable metasurface and magnonic beam separation unit.

Asynchronous propagation of atomic force and excited electronic charge in GaAs under proton irradiation

The studies for the interaction of energetic particles with matter have greatly contributed to the exploration of material properties under irradiation conditions, such as nuclear safety, medical physics and aerospace applications. In this work, we theoretically simulate the non-adiabatic process for GaAs upon proton irradiation using time-dependent density functional theory, and find that the radial propagation of force on atoms and the excitation of electron in GaAs are non-synchronous process. We calculated the electronic stopping power on proton with the velocity of 0.1–0.6 a.u., agreement with the previous empirical results. After further analyzing the force on atoms and the population of excited electrons, we find that under proton irradiation, the electrons around the host atoms at different distances from the proton trajectories are excited almost simultaneously, especially those regions with relatively high charge density. However, the distant atoms have a significant hysteresis in force, which occurs after the surrounding electrons are excited. In addition, hysteresis in force and electron excitation behavior at different positions are closely related to the velocity of proton. This non-synchronous propagation reveals the microscopic dynamic mechanism of energy deposition into the target material under ion irradiation.

Transport and noise of hot electrons in GaAs using a semianalytical model of two-phonon polar optical phonon scattering

Recent ab-initio studies of electron transport in GaAs have reported that electron-phonon (e-ph) interactions beyond the lowest order play a fundamental role in charge transport and noise phenomena. Inclusion of the next-leading-order process in which an electron scatters with two phonons was found to yield good agreement for the high-field drift velocity, but the characteristic non-monotonic trend of the power spectral density of current fluctuations (PSD) with electric field was not predicted. The high computational cost of the ab-initio approach necessitated various approximations to the two-phonon scattering term, which were suggested as possible origins of the discrepancy. Here, we report a semi-analytical transport model of two-phonon electron scattering via the Fr\"ohlich mechanism, allowing a number of the approximations in the ab-initio treatment to be lifted while retaining the accuracy to within a few percent. We compare the calculated and experimental transport and noise properties as well as scattering rates measured by photoluminescence experiments. We find quantitative agreement within 15% for the drift velocity and 25% for the $\Gamma$ valley scattering rates, and agreement with the $\Gamma-L$ intervalley scattering rates within a factor of two. Considering these results and prior studies of current noise in GaAs, we conclude that the most probable origin of the non-monotonic PSD trend versus electric field is the formation of space charge domains rather than intervalley scattering as has been assumed.

Complex exciton dynamics with elevated temperature in a GaAsSb/GaAs quantum well heterostructure

Exciton dynamics in a GaAsSb/GaAs quantum well (QW) heterostructure were investigated via both steady state and transient photoluminescence. The measurements at 10 K demonstrated the coexistence of localized excitons (LEs) and free excitons (FEs), while a blue-shift resulting from increased excitation intensity indicated their spatially indirect transition (IT) characteristics due to the type-II band alignment. With increasing temperature from 10 K, the LEs and FEs redistribute, with the LEs becoming less intense at relatively higher temperature. With increasing temperature to above 80 K, electrons in GaAs are able to overcome the small band offset to enter inside GaAsSb and recombine with holes; thus, a spatially direct transition (DT) appeared. Hence, we are able to reveal complex carrier recombination dynamics for the GaAsSb/GaAs QW heterostructure, in which the “S” shape behavior is generated not only by the carrier localization but also by the transformation from IT to DT with elevated temperature.

Hydrodynamics, viscous electron fluid, and Wiedeman-Franz law in two-dimensional semiconductors

Considering theoretically the transition between hydrodynamic and ballistic regimes in 2D semiconductors, we show that electrons in high-mobility 2D GaAs are by far the best system for the direct observation of collective hydrodynamic effects even in bulk transport properties independent of complicated transport features in narrow constrictions and small systems where Gurzhi phenomena are typically studied experimentally. We predict a strong hydrodynamics-induced generic violation of the Wiedeman-Franz law in bulk 2D GaAs systems for mobilities as modest as ${10}^{6}{\mathrm{cm}}^{2}/\text{Vs}$ and densities $1--5\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{cm}}^{\ensuremath{-}2}$ in the temperature range of $T=1--40\phantom{\rule{0.16em}{0ex}}\text{K}$.

The lock-on effect and collapsing bipolar Gunn domains in high-voltage GaAs avalanche p-n junction diode

We present experimental evidence and physics-based simulations of the lock-on effect in high-voltage GaAs avalanche diodes. The avalanche triggering is initiated by steep voltage ramp applied to the diode and in-series 50 Ω load. After subnanosecond avalanche switching the reversely biased GaAs diode remains in the conducting state for the whole duration of the applied pulse (dozens of nanoseconds). There is no indication of the p-n junction recovery that is commonly expected to develop on the nanosecond scale due to the drift extraction of non-equilibrium carriers. The diode voltage keeps a constant value of ∼70 V much lower than the stationary breakdown voltage of 400 V. Numerical simulations reveal that the conducting state is supported by impact ionization in narrow high-field collapsing Gunn domains as well as in quasi-stationary cathode and anode ionizing domains. Collapsing Gunn domains spontaneously appear in the dense electron-hole plasma due to the negative differential mobility of electrons in GaAs. The effect resembles the lock-on effect of GaAs bulk photoconductive switches but is observed in reversely biased p-n junction diode switched by a non-optical method.

Unified approach to cyclotron and plasmon resonances in a periodic two-dimensional GaAs electron gas hosting the Hofstadter butterfly

We present theoretical calculations for the cyclotron resonance and various magnetoplasmon modes of a Coulomb interacting two-dimensional GaAs electron gas (2DEG) modulated as a lateral superlattice of quantum dots subjected to an external perpendicular constant magnetic field. We use a real-time excitation approach based on the Liouville-von Neumann equation for the density operator, that can go beyond linear response delivering information of all longitudinal and transverse collective modes of interest to the same order. We perform an extensive analysis of the coexisting collective modes due to the lateral confinement and the magnetic field for a different number of electrons in each dot. In the limit of vanishing dot modulation of the 2DEG we find signs of the structure of the Hofstadter butterfly in the excitation spectra.

Density-dependent two-dimensional optimal mobility in ultra-high-quality semiconductor quantum wells

We calculate using the Boltzmann transport theory the density-dependent mobility of two-dimensional (2D) electrons in GaAs, SiGe, and AlAs quantum wells as well as of 2D holes in GaAs quantum wells. The goal is to precisely understand the recently reported breakthrough in achieving a record 2D mobility for electrons confined in a GaAs quantum well. Comparing our theory with the experimentally reported electron mobility in GaAs quantum wells, we conclude that the mobility is limited by unintentional background random charged impurities at an unprecedented low concentration of $\ensuremath{\sim}{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. We find that this same low level of background disorder should lead to 2D GaAs hole and 2D AlAs electron mobilities of $\ensuremath{\sim}{10}^{7}$ and $\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\text{V}\phantom{\rule{0.16em}{0ex}}\text{s}$, respectively, which are much higher theoretical limits than the currently achieved experimental values in these systems. We therefore conclude that the current GaAs hole and AlAs electron systems are much dirtier than the state-of-the-art 2D GaAs electron systems. We present theoretical results for 2D mobility as a function of density, effective mass, quantum-well width, and valley degeneracy, comparing with experimental data.

The influence of saturation of electron drift velocity on photorefractive effect in GaAs/AlGaAs quantum wells structures

The structures of the semi-insulating quantum wells based on GaAs/AlGaAs material composition are photorefractive materials of high sensitivity and short time of response. An important factor affecting the course of the photorefractive phenomenon in these materials is the nonlinear relationship between the velocity of electrons and the intensity of the electric field. The article compares two most frequently used models of nonlinear transport of electrons in GaAs: a classic two-valley model and an empirical model accounting for the saturation of the dependence of electron velocity on electric field intensity. It has been shown how the selection of the model influences the predicted distribution of space-charge field during the photorefractive two-wave mixing, which allows evaluating the correctness of the use of these models.

One-Dimensional Poisson Calculation for Electrically Controlled Band Bending in GaAs/AlGaAs Heterostructure.

Here, we describe the band-bending situation for introducing electrons in an undoped GaAs and AlGaAs quantum well. Our calculation has shown that an externally applied electric field can modulate two-dimensional electron gas (2DEG) without standard modulation doping. The topic of electrically modulated 2DEG has only background impurities, no intentional dopants, so scattering or dephasing by background potential fluctuations should be much reduced. Using our calculation, it is straightforward to confine carriers (in the range of 1010~1011 cm-2), when the external electric field is more than threshold voltage, 4 V to the surface metal gate.

Experimental conditions for the observation of electron-hole superfluidity in GaAs heterostructures

The experimental parameter ranges needed to generate superfluidity in optical and drag experiments in GaAs double quantum wells are determined, using a formalism that includes self-consistent screening of the Coulomb pairing interaction in the presence of the superfluid. The very different electron and hole masses in GaAs make this a particularly interesting system for superfluidity, with exotic superfluid phases predicted in the BCS-BEC crossover regime. We find that the density and temperature ranges for superfluidity cover the range for which optical experiments have observed indications of superfluidity, but that existing drag experiments lie outside the superfluid range. However we also show that for samples with low mobility with no macroscopically connected superfluidity, if the superfluidity survived in randomly distributed localized pockets, standard quantum capacitance measurements could detect these pockets.

First-principles hyperfine tensors for electrons and holes in GaAs and silicon

Understanding (and controlling) hyperfine interactions in semiconductor nanostructures is important for fundamental studies of material properties as well as for quantum information processing with electron, hole, and nuclear-spin states. Through a combination of first-principles density-functional theory (DFT) and $\mathbf{k}\cdot\mathbf{p}$ theory, we have calculated hyperfine tensors for electrons and holes in GaAs and crystalline silicon. Accounting for relativistic effects near the nuclear core, we find contact hyperfine interactions for electrons in GaAs that are consistent with Knight-shift measurements performed on GaAs quantum wells and are roughly consistent with prior estimates extrapolated from measurements on InSb. We find that a combination of DFT and $\mathbf{k}\cdot\mathbf{p}$ theory is necessary to accurately determine the contact hyperfine interaction for electrons at a conduction-band minimum in silicon that is consistent with bulk Knight-shift measurements. For hole spins in GaAs, the overall magnitude of the hyperfine couplings we find from DFT is consistent with previous theory based on free-atom properties, and with heavy-hole Overhauser shifts measured in GaAs (and InGaAs) quantum dots. In addition, we theoretically predict that the heavy-hole hyperfine coupling to the As nuclear spins is stronger and almost purely Ising-like, while the (weaker) coupling to the Ga nuclear spins has significant non-Ising corrections. In the case of hole spins in silicon, we find (surprisingly) that the strength of the hyperfine interaction in the valence band is comparable to that in the conduction band and that the hyperfine tensors are highly anisotropic (Ising-like) in the heavy-hole subspace. These results suggest that the hyperfine coupling cannot be ruled out as a limiting mechanism for coherence ($T_2^{\ast}$) times recently measured for heavy holes in silicon quantum dots.

Numerical Study of Carrier Transport in n+/n/n+GaAs/AlGaAs Heterostructure at High Current Densities

Carrier transport across isotype GaAs/Al0.4Ga0.6As graded heterojunction has been numerically studied at high injection levels in the framework of the energy balance model. It is shown that, under reverse bias, impact ionization in the domain formed at the heterojunction leads to accumulation of electron-hole plasma in GaAs, but does not cause a breakdown. The latter occurs when the domain reaches the N/N+ interface in AlGaAs and impact ionization is developed throughout the AlGaAs layer. By contrast, under forwarding bias, the initial domain at the heterojunction rapidly disappears with increasing voltage, and in its place, a peak of an electric field of the opposite sign arises since the carrier mobilities of GaAs and AlGaAs are different. Impact ionization in the peak gives rise to a new domain at the n+/n interface in GaAs, which leads to breakdown and the appearance of negative differential resistance. It is shown that a peak of electron temperature is formed near the heterojunction for both current directions. However, the reasons why it appears are different. Under reverse bias, the electrons are heated by the electric field in the domain, while under forwarding bias, it is caused by the energy gradient of the conduction-band edge in the graded-composition layer of the heterojunction. The surprising thing is that, under reverse bias, electrons in GaAs remain cold throughout the whole region extending to the junction at all currents, which results in the extremely great and physically unreasonable values of the velocity overshoot.

Theoretical prediction and experimental demonstration of inter-valley scattering induced Inverse Spin Hall effect for hot electrons in GaAs

Realization of the Inverse Spin Hall Effect (ISHE) with hot electrons in direct bandgap semiconductors is an intriguing puzzle. Here, we report the influence of steady state carrier accumulation in the satellite L valley on the establishment of ISHE in the GaAs epilayer. Steady state carrier accumulation is calculated by analytically solving the rate equations where the whole conduction band is divided into four distinct regions based on the energy and momentum. Electron-hole generation, energy and spin relaxation, and various recombination mechanisms are considered in the theoretical framework where carrier accumulation in the L valley of GaAs is driven by intervalley scattering. This is then followed by a set of experiments to measure the photoinduced ISHE at excitation energy (Eex) of 1.65, 1.94, and 2.33 eV, where significant differences are theoretically predicted. The measured values of ISHE signal are thereafter compared with the numerically calculated ones, which establish the validity of the proposed formalism. Further, the physical origin of ISHE signal is investigated for different regimes that lead to the observation of intervalley scattering induced ISHE at Eex = 2.33 eV.

Photoconductivity, carrier lifetime and mobility evaluation of GaAs films on Si (100) using optical pump terahertz probe measurements

The carrier lifetimes and electron mobility values were estimated for 2 μm thick GaAs films grown on Si (100) substrates by means of optical pump terahertz probe (OPTP) technique. The GaAs/Si films measured were epitaxial grown at different substrate temperatures (TS = 520 °C or TS = 630 °C). From x-ray diffraction measurements and Raman spectroscopy, the GaAs/Si films were shown to experience minimum strain at room temperature, and crystal misorientation in the (111) or (110) direction. With no measureable photoluminescence at room temperature, carrier lifetimes were measured via OPTP and found to be ∼20 and ∼35 ps for a fluence of ∼4 μJ cm−2, which is in the same order of magnitude as a reference bulk GaAs grown on SI–GaAs (TS = 630 °C) having a lifetime of ∼70 ps. From OPTP photoconductivity measurements, the estimated GaAs/Si films’ electron mobility are ∼2900 cm2 V−1 s−1 (TS = 520 °C) and ∼3500 cm2 V−1 s−1 (TS = 630 °C) at a pump-probe delay time of Δt = 50 ps, in which the bulk GaAs electron mobility is ∼5200 cm2 V−1 s−1.

GaAs Spectrometer for Planetary Electron Spectroscopy

AbstractWork toward producing a radiation‐hard and high temperature tolerant direct detection electron spectrometer is reported. The motivation is to develop a low‐mass, low‐volume, low‐power, multimission capable instrument for future space science missions. The resultant prototype electron spectrometer employed a GaAs p+‐i‐n+ mesa photodiode (10 μm i layer thickness; 200 μm diameter) and a custom‐made charge‐sensitive preamplifier. The GaAs detector was initially electrically characterized as a function of temperature. The detector‐preamplifier assembly was then investigated for its utility in electron spectroscopy across the temperature range 100 to 20 °C using a laboratory 63Ni radioisotope β− particle source (end point energy = 66 keV). Monte Carlo simulations using the computer program CASINO were conducted and showed that the spectrometer had a quantum detection efficiency which increased with increasing electron energy up to 70 keV; a quantum detection efficiency of 73% was calculated. The accumulated 63Ni β− particle spectra together with CASINO simulations of the detected spectra showed that the GaAs based spectrometer could be used for counting electrons and measuring the energy deposited per electron in the detector's active region (i layer). The development of a GaAs electron spectrometer of this type may find use in future space missions to environments of intense radiation (such as at the surface of Europa for investigation of electron‐driven radiolysis of ice) and high temperature (such as at Mercury, and comets passing close to the Sun).

Zero-field electron Larmor precession in NiFe/SiN/GaAs nanohybrid structure at room temperature

We investigate the spin dynamics of a NiFe/SiN/GaAs nanohybrid structure, in which the volume of NiFe is significantly reduced by ion milling. The photogenerated GaAs electron spins induce the electron Larmor precession with a long decay component (LDC) with a time constant τLDC of 550 ps under zero field at room temperature, indicating the presence of an effective magnetic field. The LDC reflects the relaxation of NiFe magnetization, whose time constant is modulated by the inner product of NiFe magnetization and GaAs electron spins in the external field. These results indicate the existence of an exchange interaction between the NiFe magnetization and GaAs electron spins.

Interpretation of thermal conductance of the ν=5/2 edge

Recent experiments [Banerjee et al, arXiv:1710.00492] have measured thermal conductance of the nu=5/2 edge in a GaAs electron gas and found it to be quantized as K \approx 5/2 (in appropriate dimensionless units). This result is unexpected, as prior numerical work predicts that the nu=5/2 state should be the Anti-Pfaffian phase of matter, which should have quantized K=3/2. The purpose of this paper is to propose a possible solution to this conflict: if the Majorana edge mode of the Anti-Pfaffian does not thermally equilibrate with the other edge modes, then K=5/2 is expected. I briefly discuss a possible reason for this nonequilibration, and what should be examined further to determine if this is the case.

Engineering the spin polarization of one-dimensional electrons

We present results of magneto-focusing on the controlled monitoring of spin polarization within a one-dimensional (1D) channel, and its subsequent effect on modulating the spin–orbit interaction (SOI) in a 2D GaAs electron gas. We demonstrate that electrons within a 1D channel can be partially spin polarized as the effective length of the 1D channel is varied in agreement with the theoretical prediction. Such polarized 1D electrons when injected into a 2D region result in a split in the odd-focusing peaks, whereas the even peaks remain unaffected (single peak). On the other hand, the unpolarized electrons do not affect the focusing spectrum and the odd and even peaks remain as single peaks, respectively. The split in odd-focusing peaks is evidence of direct measurement of spin polarization within a 1D channel, where each sub-peak represents the population of a particular spin state. Confirmation of the spin splitting is determined by a selective modulation of the focusing peaks due to the Zeeman energy in the presence of an in-plane magnetic field. We suggest that the SOI in the 2D regime is enhanced by a stream of polarized 1D electrons. The spatial control of spin states of injected 1D electrons and the possibility of tuning the SOI may open up a new regime of spin-engineering with application in future quantum information schemes.