GaAs Conduction Band Research Articles

Resonant tunneling injection of electrons through double stacked GaAs/InAs quantum dots with nanohole electrode

Resonant tunneling diodes containing closely double-stacked InAs quantum dots (QDs) were grown on GaAs substrates by MBE. After growing a thin GaAs capping layer on the double-stacked InAs QDs, nanoholes were selectively formed just above the larger second QDs by thermal annealing. The Au thin film was deposited directly on top surface of the larger second QDs through the nanoholes. The second QDs contacted with Au film served as conducting dots, which can locally inject electrons into the underlying first QDs. In current versus voltage (I–V) measurements, (dI/dV) peaks were clearly observed in the forward bias voltage region. It was due to the tunneling current through a non-doped GaAs thin layer between double-stacked QDs and n-GaAs conduction band. The (dI/dV) peaks shifted toward the lower forward voltage region with increasing temperature. It was explained by the temperature dependence of the electron energy distribution in the GaAs conduction band.

Effect of the nonparabolicity on the resonant lifetimes and resonant energies of symmetric GaAs/AlxGa1-xAs double barrier nanostructures

We investigate the effect of nonparabolicity of the GaAs conduction band on the resonant lifetime and the associated lowest resonant energy for electrons tunneling through undoped symmetric GaAs/AlxGa1-xAs double barrier nanostructures. The effective mass theory is considered by employing the spatial and energy dependence of the effective masses for both well and barrier materials of the structure. It is found that, both resonant lifetime and the corresponding resonant energy are mainly affected by nonparabolicity, mass mismatch, and the aluminum content that controls the barrier height. The results of the present work showed a strong dependence of the resonant lifetime and the resonant energy on the barrier thickness and well width. Pronounced discrepancies are obtained between the results of the present work and other works which neglected the effects of the nonparabolicity and the mass mismatch. A good agreement is obtained in comparing the results of the present work with the experiment data.

Photoconduction of nanoparticle arrays pumped with near 1550 nm pulse lasers

When erbium (Er) concentration is greater than ∼7 × 1017/cm3, ErAs nanoparticles with various sizes are formed spontaneously in GaAs epitaxial layer during MBE growth. This letter presents experimental studies on transient photoconduction of ErAs nanoparticle arrays when pumped by laser pulses with wavelengths near 1550 nm. The experimental data suggests the photoconduction may be contributed by two mechanisms. One is extrinsic photoconductivity, where bound electrons from levels of nanoparticles pinned near the middle of GaAs forbidden gap transfer to the conduction band of GaAs (i.e. bound to conduction band). The other is displacement-current conduction traced to photon-assisted polarization of nanoparticles with two quantized energy levels. This investigation explains why these nanoparticle-embedded materials have a higher responsivity to pulsed lasers than continuous wavelength (CW) lasers.

Effects of dissipative electron tunneling manifested in the photocurrent of a GaAs p-i-n photodiode with a double InAs quantum dot layer

We report on the results of experimental studies of the photocurrent (PC) of photodiodes based on GaAs p-i-n structures with InAs/GaAs(001) double asymmetric quantum dot (DAQD) arrays obtained by self-assembling in the process of low-pressure metal-organic vapor phase epitaxy (LP-MOVPE). Three peaks were observed in the dependence of the PC on the reverse bias, measured under photoexcitation with a photon energy equal to the energy of the interband ground state transition in larger InAs QDs. These peaks were attributed to photoexcitation of electrons from the ground hole states in larger QDs into the ground electron states followed by resonant dissipative (with absorption or emission of optical phonons) and conservative tunneling into the GaAs conduction band via the ground electron states in smaller QDs. The PC dependence on the bias voltage agrees qualitatively with the theoretical field dependence of the probability of 1D dissipative tunneling between the QDs.

Effects of stacking periodicity on the electronic and optical properties of GaAs/AlAs superlattice: a first-principles study

The effects of stacking periodicity on the electronic and optical properties of GaAs/AlAs superlattice have been explored by density functional theory calculations. Among the (GaAs)m/(AlAs)m, (GaAs)1/(AlAs)m and (GaAs)m/(AlAs)1 (m = 1 to 5) superlattices, the band gaps of (GaAs)m/(AlAs)1 superlattices decrease significantly as the layer of GaAs increases, and the cut-off wavelengths are found to locate in the near infrared region. For (GaAs)m/(AlAs)1 SLs, the conduction bands shift toward Fermi level, resulting in the smaller band gap, while conduction bands of (GaAs)1/(AlAs)n SLs slightly shift to higher energy, which lead to comparable band gaps. The layer number of GaAs shows negligible effects on the reflectivity spectra of superlattice structures, while the absorption coefficient shows a red-shift with the increasing layer of GaAs, which is beneficial for the application of GaAs/AlAs superlattice in the field of near infrared detector. These results demonstrate that controlling the number of GaAs layers is a good method to engineer the optoelectronic properties of GaAs/AlAs superlattice.

Spectral Change of E− Band Emission in a GaAs:N δ-Doped Superlattice Due to Below-Gap Excitation and Its Discrimination from Thermal Activation

In this study, we examined the E− band luminescence of a GaAs:N δ-doped superlattice (SL) grown by metal organic vapor phase epitaxy with 0.15% nitrogen (N) using two-wavelength excited photoluminescence. It was observed that the photoluminescence (PL) intensity of the low-energy peak (P2) at 1.38 eV of the E− band was quenched larger compared to the high-energy peak (P1) at 1.41 eV. This was due to the superposition of below-gap excitation (BGE) light of energy 0.95 eV over the above-gap excitation light of energy 1.45 eV on the SL structure at a fixed temperature of 12 K. On the other hand, at higher temperatures, the PL intensity of the high-energy peak P1 was quenched higher compared to the low-energy peak P2 without any addition of the BGE light. We interpreted the experimental results by considering the carrier recombination (CR) model and concluded that the observed PL spectral and intensity change of the E− band emission due to BGE does not result from the thermal activation, but from the optical excitation among the E− band, conduction band, and CR levels in GaAs:N δ-doped structure. We concluded that to achieve high-efficiency intermediate band-type solar cells, it is essential to understand the CR mechanism through CR levels by determining their origin and eliminating them from the material.

Terahertz Driven Amplification of Coherent Optical Phonons in GaAs Coupled to a Metasurface.

We demonstrate amplification of longitudinal optical (LO) phonons by polar-optical interaction with an electron plasma in a GaAs structure coupled to a metallic metasurface using two-color two-dimensional spectroscopy. In a novel scheme, the metamaterial resonator enhances broadband terahertz fields, which generate coherent LO phonons and drive free electrons in the conduction band of GaAs. The time evolution of the LO phonon amplitude is monitored with midinfrared pulses via the LO-phonon-induced Kerr nonlinearity of the sample, showing an amplification of the LO phonon amplitude by up to a factor of 10, in agreement with a theoretical estimate.

Photoluminescence characterization of nonradiative recombination centers in MOVPE grown GaAs:N δ-doped superlattice structure

Nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structure grown by metal organic vapor phase epitaxy (MOVPE) are studied by two-wavelength-excited photoluminescence (PL) method. The PL intensity quenching due to the superposition of below-gap excitation (BGE) light at energies of 0.75, 0.80, 0.92, and 0.95 eV and an above-gap excitation (AGE) light either for the GaAs conduction band (1.69 eV) or E− band excitation (1.45 eV), respectively indicates the existence of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. The AGE density and BGE density dependence of the PL intensity have been studied for both excitation schemes. Recombination model based on two levels has been proposed to interpret the experimental results and energy distribution of NRR centers inside the GaAs:N δ-doped SLs and GaAs layers. The defect-related parameters are estimated systematically by solving the rate equations and fitting the results with the experimental data that give a phenomenological insight into the sample.

Optical properties of quasi-type-II structure in GaAs/GaAsSb/GaAs coaxial single quantum-well nanowires

The GaAsSb-based quantum well plays a very important role in optoelectronic devices due to its excellent wavelength tunability. When the dimension reduces, the quantum confinement effect will take place and the quantum well in nanowires will show many interesting characteristics. GaAsSb-based quantum-well nanowires are of contemporary interest. However, the properties of the quasi-type-II structure in a single quantum well nanowire have been rarely investigated. Here, we grow GaAs/GaAs0.92Sb0.08/GaAs coaxial single quantum-well nanowires and discussed their power-dependent and temperature-dependent photoluminescence. We find that due to the small band offset of conduction bands, both type-I like and type-II like emission exist in our nanowires. When electrons obtain enough thermal energy through collisions or surrounding environment, they will overcome the barrier and diffuse to the GaAs conduction band, which contributes to the type-II like recombination. These results show the optical property of the quasi-type-II quantum well in nanowires, which can pave the way toward future nanoscale quantum well devices.

G -factor of electrons in gate-defined quantum dots in a strong in-plane magnetic field

We analyze orbital effects of an in-plane magnetic field on the spin structure of states of a gated quantum dot based in a two-dimensional electron gas. Starting with a $k \cdot p$ Hamiltonian, we perturbatively calculate these effects for the conduction band of GaAs, up to the third power of the magnetic field. We quantify several corrections to the g-tensor and reveal their relative importance. We find that for typical parameters, the Rashba spin-orbit term and the isotropic term, $H_{43} \propto {\bf P}^2 {\bf B} \cdot \boldsymbol{\sigma}$, give the largest contributions in magnitude. The in-plane anisotropy of the g-factor is, on the other hand, dominated by the Dresselhaus spin-orbit term. At zero magnetic field, the total correction to the g-factor is typically 5-10% of its bulk value. In strong in-plane magnetic fields, the corrections are modified appreciably.

Two Topics of Optical Excitation Dynamics, Newly Unveiled by the Time- and Momentum-Resolved Photo-Electron Emission from the Conduction Band of GaAs: A Theoretical Review

We review the recent two topics of optical excitation and relaxation dynamics, newly unveiled by the time- and momentum-resolved photo-electron emission from the conduction band of GaAs. One is the real-time collective relaxation dynamics, resulting in the Fermi degeneracy formation in the Γ valley. We show that it takes almost infinite time to realize the exact Fermi degeneracy, due to a restricted selection rule for the intravalley transition of the photo-excited electrons. The other is the spontaneous and instantaneous intervalley transition from the Γ valley to the L one. By considering the electron-phonon coupling before the photo-excitation, such spontaneous intervalley transition is realized within the framework of the Franck–Condon principle of the photo-excitation.

Nonradiative recombination centers in GaAs:N δ-doped superlattice revealed by two-wavelength-excited photoluminescence

We use two-wavelength-excited photoluminescence (PL) to investigate nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structures grown by molecular beam epitaxy. The change in photoluminescence (PL) intensity due to the superposition of below-gap excitation at energies of 0.75, 0.80, 0.92, and 0.95 eV and above-gap excitation at energies of 1.69 or 1.45 eV into the GaAs conduction band and the E− band implies the presence of NRR centers inside the GaAs:N δ-doped SL and/or GaAs layers. The change in PL intensity as a function of the photon number density of below-gap excitation is examined for both bands, which enables us to determine the distribution of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. We propose recombination models to explain the experimental results. Defect-related parameters that give a qualitative insight into the samples are investigated systematically by fitting the rate equations to the experimental data.

Attosecond optical-field-enhanced carrier injection into the GaAs conduction band

Resolving the fundamental carrier dynamics induced in solids by strong electric fields is essential for future applications, ranging from nanoscale transistors1,2 to high-speed electro-optical switches3. How fast and at what rate can electrons be injected into the conduction band of a solid? Here, we investigate the sub-femtosecond response of GaAs induced by resonant intense near-infrared laser pulses using attosecond transient absorption spectroscopy. In particular, we unravel the distinct role of intra- versus interband transitions. Surprisingly, we found that despite the resonant driving laser, the optical response during the light–matter interaction is dominated by intraband motion. Furthermore, we observed that the coupling between the two mechanisms results in a significant enhancement of the carrier injection from the valence into the conduction band. This is especially unexpected as the intraband mechanism itself can accelerate carriers only within the same band. This physical phenomenon could be used to control ultrafast carrier excitation and boost injection rates in electronic switches in the petahertz regime. Significant enhancement of carrier injection into the conduction band is observed for GaAs subjected to intense resonant near-infrared laser pumping. Attosecond-resolved investigation reveals the interplay between the intra- and interband transitions.

Temperature effects in contacts between a metal and a semiconductor nanowire near the degenerate doping

We have investigated the nonlinear conductance in diffusion-doped Si:GaAs nanowires contacted by patterned metal films in a wide range of temperatures T. The wire resistance RW and the zero bias resistance RC, dominated by the contacts, exhibit very different responses to temperature changes. While RW shows almost no dependence on T, RC varies by several orders of magnitude as the devices are cooled from room temperature to T = 5 K. We develop a model that employs a sharp donor level very low in the GaAs conduction band and show that our observations are consistent with the model predictions. We then demonstrate that such measurements can be used to estimate carrier properties in nanostructured semiconductors and obtain an estimate for ND, the doping density in our samples. We also discuss the effects of surface states and dielectric confinement on carrier density in semiconductor nanowires.

Energy relaxation mechanism of hot-electron ensembles in GaAs: Theoretical and experimental study of its temperature dependence

We have recently demonstrated that the fast momentum relaxation due to electron-phonon scattering of hot electrons excited into the conduction band of GaAs leads to the formation of hot-electron ensembles spread over the Brillouin zone. In the present work, we study the energy relaxation of hot-electron ensembles in GaAs, by means of ab initio calculations and time-, energy-, and momentum-resolved spectroscopy. We theoretically show that when the temperature decreases, the energy relaxation time ascribed to electron-phonon interaction becomes faster than at ambient temperature and prove that this is indeed the case in the experimental results.

Bias-dependent electrical spin generation in Fe3Si/GaAs : Consistent behavior in nonlocal and local spin valves and in the three-terminal configuration

We study spin generation in lateral spin-valve structures consisting of ferromagnetic ${\mathrm{Fe}}_{3}\mathrm{Si}$ contacts on $n$-type GaAs transport channels. Different film-growth conditions are found to enable the comparison of ferromagnetic contacts with similar spin polarizations but different electrical characteristics. Strongly rectifying contacts are found to hinder the observation of spin-injection signals due to a strong bias dependence of the spin-generation efficiency. For contacts fabricated under optimized growth conditions, efficient spin generation enables the operation of nonlocal and local spin valves as well as the detection of spin signals in the three-terminal configuration. In all configurations, the observed signals exhibit a consistent bias-dependent spin generation and detection behavior which is explained by spin accumulation in the GaAs conduction band without the involvement of localized states.

Impurity-induced modulation of terahertz waves in optically excited GaAs

The effect of the photoinduced absorption of terahertz (THz) radiation in a semi-insulating GaAs crystal is studied by pulsed THz transmission spectroscopy. We found that a broad-band modulation of THz radiation may be induced by a low-power optical excitation in the spectral range of the impurity absorption band in GaAs. The measured modulation factor achieves 80%. The amplitude and frequency characteristics of the resulting THz modulator are critically dependent on the carrier density and relaxation dynamics in the conduction band of GaAs. In semi-insulating GaAs crystals, the carrier density created by the impurity excitation is controlled by the rate of their relaxation to the impurity centers. The relaxation rate and, consequently, the frequency characteristics of the modulator can be optimized by an appropriate choice of the impurities and their concentrations. The modulation parameters can be also controlled by the crystal temperature and by the power and photon energy of the optical excitation. These experiments pave the way to the low-power fast optically-controlled THz modulation, imaging, and beam steering.

Formation of hot-electron ensembles quasiequilibrated in momentum space by ultrafast momentum scattering of highly excited hot electrons photoinjected into the Γ valley of GaAs

We study ultrafast scattering dynamics of hot electrons photoinjected with high excess energies in the \ensuremath{\Gamma} valley of the conduction band of GaAs, using time- and angle-resolved photoemission spectroscopy and ab initio calculations. At ultrafast rates of the order of 10 fs, the packets in the \ensuremath{\Gamma} valley are transformed into hot-electron ensembles (HEEs) quasiequilibrated in momentum space but not in energy space. The energy relaxation of the HEEs takes place as a whole on a longer time scale with rates dependent only on the excess energy, irrespective of the momenta of hot electrons. Both momentum scattering and energy relaxation are ruled by the electron-phonon interaction.

Intervalley scattering of electrons by short-wave phonons in (GaAs)8(AlAs)8(001) superlattice

Intervalley transitions induced by short-wavelength phonons in the conduction band of a superlattice (GaAs)8(AlAs)8(001) are investigated on the basis of the pseudopotential method and in the phenomenological model of interatomic forces. The main attention in the study centers around the transitions associated with the vibrations confined inside the layers. It is shown that the deformation potentials for the majority of intervalley transitions in a superlattice exceed the potentials of corresponding transitions in the binary crystals because of the localization of atomic displacements and wavefunctions of electrons inside the same layer of the superlattice.The bottom of the conduction band in (GaAs)8(AlAs)8(001) superlattice corresponds to the states Γ1(1), Z3, M1, M4 originating from sphalerite's X¯ valleys localized in AlAs layers. Transitions between them are the most intense ones and they are caused by optical vibrations of Al atoms. “Semi-interface” vibrations being mainly localized in the one side of the GaAs layer are involved in the Γ1(2)−X1, Γ1(2)−R1 and X1−Z1 transitions which are analogs of Γ¯−L¯ transitions in binaries. The transitions Γ1(2)−M1 and Γ1(2)−M4 are governed by smooth parts of wave-functions and pseudopotentials. As a consequence their intensities are comparable with those of Γ¯−X¯ sphalerite transitions in spite that these states are localized in the different layers of the superlattice.

Instantaneous Intervalley Transition Just at the Franck–Condon State in the Conduction Band of GaAs

We propose two possible quantum microscopic mechanisms for intervalley transitions just at the Franck-Condon state in GaAs. The first is the simultaneously combined coulombic elastic transitions of two electrons,from the originally photo-excited $\Gamma$ valley to the mutually opposite L and -L valleys in the Brillouin zone. The second is the "elastic" scattering of an electron from the $\Gamma$ valley to the L one by the frozen phonon, which is not newly created in the excited state, but already exists, being inherited from the starting ground state, according to the Franck-Condon principle of the photo-excitation. Surprisingly, we can theoretically show that such the elastic electron-phonon scattering gives extremely fast time constant of the order of a few tens femtoseconds, as well as the above elastic electron-electron scattering does.