xAs Superlattices Research Articles

First-principles study on the structural stability and electronic properties of GaAs/Ga1−xAlxAs superlattice nanowires

In this study, the first-principles method is used to study the structural stability and electronic properties of GaAs/Ga1−xAlxAs and AlxGa1−xAs/AlyGa1−yAs superlattice nanowires. Calculations show that GaAs/Ga1−xAlxAs superlattice nanowires with smaller Al component spacing have higher structural stability. The increase of Al composition will further weaken the stability of GaAs/Ga1−xAlxAs heterostructure nanowires. Compared with the pure GaAs nanowires, the work function of GaAs/Ga1−xAlxAs heterostructure nanowires is all reduced, indicating that the variable Al composition heterostructure can enhance the photoemission ability. In addition, heterostructures with different configurations exhibit different electronic properties. Moreover, a powerful built-in electric field is formed across the GaAs/AlxGa1−xAs superlattice interface, which will further promote the migration of photoelectrons from low Al composition layer to high Al composition layer. GaAs/Ga1−xAlxAs superlattice nanowires are promising to be the high-performance photocathode materials.

Electronic transport and band structures of GaAs/AlAs nanostructures superlattices for near-infrared detection

We report here the theoretical calculations of band structures E(d 1), E(k z , k p ) and effective mass along the growth axis and in the plane of GaAs/Al x Ga1−x As superlattices, in the envelope function formalism. The effect of valence band offset, well thickness and temperature on the band structures, has been also studied. Our results show that a transition from indirect to direct band gap in (GaAs) m /(AlAs)4 takes place between m = 5 and 6 monolayers at room temperature. Samples (GaAs)9/(AlAs)4 and GaAs(d 1 = 10 nm)/Al0.15Ga0.85As(d 2 = 15 nm) have a direct band gap of 1.747 eV at room temperature and 1.546 eV at T = 30 mK, respectively. Their corresponding cutoff wavelengths are located in the near infrared region. We have interpreted the photoluminescence measurements of Ledentsov et al. in GaAs(d 1 = 2.52 nm)/AlAs (d 1 = 1.16 nm) and the oscillations in the magnetoresistance observed by Kawamura et al. in GaAs/Al0.15Ga0.85As superlattice. In the later, the existence of discrete quantized levels along the growth direction z indicates extremely low interactions between adjacent wells leading to the use in parallel transport. The position of Fermi level predicts that this sample exhibits n-type conductivity. These results were compared and discussed with the available data in the literature and can be used as a guide for the design of infrared nanostructured detectors.

Structural parameters and their effects on the electronic transport properties in aperiodic superlattice profile

The aperiodic GaAs/AlxGa1−xAs superlattices (SL) with trimer disorder have been studied in this paper. The transfer-matrix technique and the exact Airy function formalism have been used to determine the miniband structure, the transmission coefficient, the resonance energy and resonant tunneling time (RTT). Although the disorder localizes the states on average, our numerical calculations show that the localization length of the states becomes more extended when the disorder is correlated (trimer case). We have also found that the RTT is of the order of several femtoseconds.

Effect of the aluminium concentration on the resonant tunnelling time and the laser wavelength of random trimer barrier AlxGa1−xAs superlattices

In this study, we investigated numerically the effect of aluminium concentration on the resonant tunnelling time (RTT) and the laser wavelength of random trimer barrier AlxGa1−xAs superlattices (RTBSL). Such systems consist of two different structures randomly distributed along the growth direction, with the additional constraint that the barriers of one kind appear in triply. An explicit formula is given for evaluating the transmission coefficient of superlattices (SL’s) with intentional correlated disorder. The method is based on Airy function formalism and the transfer-matrix technique. We discuss the impact of the aluminium concentration associated to the structural profile of the SL’s on the RTT and the laser wavelengths.

Mixed disorder in GaAs/AlxGa1−xAs superlattices and its effect on the range of wavelength infrared lasers

By introducing a mixed disorder on GaAs/AlxGa1−xAs superlattices, a wide range of infrared (15–248μm) was achieved. The studied systems consist of two different structures randomly distributed along the growth direction, with the additional constraint that the structure with height barrier V2 and thickness db2 appear only by doublet or triplet. The exact Airy – function formalism combined with a transfer matrix technique was used for evaluating the transmission probability and laser wavelength. The analysis of the proposed model shows that the far infrared wavelength is obtained when the aluminum mole fraction increase and mixed disorder is introduced by trimer.

Electronic transmission in non-linear potential profile of GaAs/AlxGa1−xAs biased quantum well structure

We study the effect of the nonlinearity on electrons transmission properties in a double barriers structure GaAs/AlxGa1−xAs superlattices. The nonlinearity is introduced as an effective potential in the Schrödinger equation and translates the electronic Colombian repulsion. We have used the transfer matrix formalism and the plane wave functions approximation to solve numerically the equation and calculate the electronic transmission coefficient. We have shown the occurrence of two allowed states within the same well instead of a single, translating the presence of two resonant states at two different energies. The first allowed state intensity strongly decreases with increasing the nonlinear parameter, whereas the second one called the degeneracy state increases. Both the two states evolve towards higher resonances energies.

Study of transmission properties in GaAs/AlxGa1−xAs superlattices generated by a specific sequences

In this work, we have numerically examined the transmission properties across Monomer Height Barrier Superlattices (MHBSLs), Dimer Height Barrier Superlattices (DHBSLs) and Trimer Height Barrier Superlattices (THBSLs) using the exact Airy function formalism and the transfer-matrix method. We have observed the formation of minigaps. We have demonstrated that the number of these minigaps is identical to the number the height barriers potential V2 “n” introduce in the ordered structure according to the sequence: Si=Si−1+(n+1) where: S1=2; i=1, 2, 3, 4… and n=monomer, dimer, trimer, quaterner…. Their widths present a linear variation with the applied bias and increases linearly with increasing of the difference between the heights of the potential barriers V1 and V2.

Electronic band structure of GaAs/AlxGa1−xAs superlattice in an intense laser field

We perform theoretical calculations for the band structure of semiconductor superlattice under intense high-frequency laser field. In the frame of the non-perturbative approach, the laser effects are included via laser-dressed potential. Results reveal that an intense laser field creates an additional geometric confinement on the electronic states. Numerical results show that when tuning the strength of the laser field significant changes come in the electronic energy levels and density of states.

Resonant tunneling in GaAs/Al xGa 1−xAs superlattices with aperiodic potential profiles

Using the exact Airy function formalism and the transfer-matrix technique, we have numerically investigated in this paper the effect of intentional correlations in spatial disorder on transmission properties of one-dimensional superlattices. Such systems consist of two different structures randomly distributed along the growth direction, with the additional constraint that barriers (wells) of one kind always appear in triply. It is shown that the intentional correlations in disorder and superlattices structural parameters are responsible to obtain resonant tunneling in aperiodic structure.

Analytical evidence of commuting resonance energy in correlated disorder trimer GaAs-AlxGa1–xAs superlattices

Using the exact Airy function formalism and the transfer-matrix technique, we have numerically investigated the effect of structural and topological correlated disorder, introduced by triplet, on the miniband structure of the GaAs/AlxGa1–xAs superlattices. These systems exhibit a miniband of extended states, around a critical energy, lying to the typical structure of the trimer cell. The states close to this resonant energy consist of weakly localized states, while in band tails i.e., for negligible transmission, the states are strongly localized. This is evidence of the suppression of localization in the RTB superlattices. The extended states due to the basic cell (triple barrier) remain for correlated structural and topological disorder. However, the commuting extended state exists only in the correlated structural disorder, where the periodicity of the system is preserved. Commuting resonance energy is theoretically demonstrated in this paper, and the obtained values are in good agreement with the existing numerical results.

Optimal control of AlAs oxidation via digital alloy heterostructure compositions

A thorough study of wet thermal oxidation in AlAs/AlxGa1−xAs superlattices is presented. The results shown here demonstrate that the final oxidation depth can be finely tuned via the composition and thickness of AlxGa1−xAs into the digital alloy. A complete model of oxidation in these structures is proposed, relying on diffusion through the AlAs layer, its oxidation and an additional effect due to the AlxGa1−xAs intermediate barriers. This barrier contribution is shown to further improve the control of the oxidation rate, and thereby fabrication of sophisticated AlOx/GaAs integrated optoelectronic devices.

The effect of quasi-periodicity on the resonant tunneling lifetimes of states in electricallybiased semiconductor superlattices

The tunneling lifetimes of quasi-resonant states for electrons invarious kinds of generalized Fibonacci and generalized Thue–MorseGaAs–AlxGa1−xAs superlattices have been evaluated numerically under variable dc bias conditions. All thequasi-periodic systems have been framed using the generalized block model. The variationof the lifetime at different quasi-resonant levels with respect to the external dc fieldundergoes a remarkable change due to the effect of quasi-periodicity. The occurrence of aminimum value of the average lifetime and its dependence on quasi-periodicity have beenanalyzed. It is shown that the low-order quasi-periodicity in the case of the generalizedFibonacci superlattice and the high-order quasi-periodicity for the generalized Thue–Morsesuperlattice hold promise for potential device applications. The impact of anincrease in the number of barriers on the tunneling lifetime has also been studiedexhaustively.

Photocurrent bistability in a GaAs∕AlxGa1−xAs superlattice under resonant-coupling conditions of Wannier-Stark localization states

We have investigated the effects of the resonant coupling between the Wannier-Stark localization states on photocurrent properties in a GaAs (6.8nm)∕Al0.1Ga0.9As (4.0nm) superlattice embedded in a p-i-n structure. The resonant-coupling profiles were confirmed with electroreflectance spectroscopy and analyzed by calculating the Wannier-Stark localization states as a function of electric field. It is found that the photocurrent-voltage characteristics exhibit peak structures, which results in negative differential resistance, owing to the changes of the optical-transition probabilities under the first- and second-nearest-neighbor resonant-coupling conditions between the first and second quantized electron states in the Wannier-Stark localization. Utilizing the negative differential resistance properties under the resonant-coupling conditions, we have demonstrated photocurrent-bistability operations of a self-electro-optic effect device.

Electronic band structure of GaAs/Al xGa 1−xAs superlattice in a periodic magnetic field

In this paper, a simple and exactly solvable model is used to investigate electronic band structure of GaAs/Al x Ga 1− x As superlattice in a periodic magnetic field. The behavior of electrons in three dimensions is studied in detail. We find that electrons not only form band structure along Z direction, but also form some permitted bands along y direction, while the energies along x direction are still continuous. Moreover, the characteristics of electronic band structure and its formative mechanism are discussed.

Electronic spectra of GaAs/Ga xAl 1− xAs superlattice with impurities arranged according to a Fibonacci sequence

We study the electronic density of states (DOS) of a GaAs/Ga x Al 1− x As superlattice with impurities arranged according to the Fibonacci sequence. Our theory uses a Green function method based on Dyson’s equation within the real-space renormalization-group approach in the framework of the tight-binding Hamiltonian for the Fibonacci quasiperiodic lattice. An exact decimation transformation has been done for the physical parameters characterizing the Fibonacci chain considering the scaling factor, under which the system is self-similar, equal to the golden mean τ=(1+ 5 )/2 . The electronic energy spectrum are then used to determine the electronic specific heat which, due to the localization of the electronic states, exhibits strong Schottky anomalies when the temperature goes to zero together with a log-periodic oscillation.

Unusual behaviour of the conductance in Gaussian superlattices

We study the conductance at finite temperature of pass-band GaAs-AlxGa1−xAs superlattices with Gaussian modulated Al mole fraction. Such structures present bands of almost unscattered electronic states and high peak-to-valley ratio in the j–V characteristic. We found a critical point, indicating the onset for the transition from the conducting to the nonconducting regime, by tuning the chemical potential in the vicinity of the band of unscattered states. Remarkably, the conductance of the Gaussian superlattice remains finite around the critical point even at zero temperature.

A Ground Miniband Width of a Superlattice with Rectangular Quantum Wells versus Constructive Parameters

A convenient formula for the ground miniband width as a function of constructive parameters of the superlattice is derived from the solution of the Kronig–Penney equation using the perturbation theory. The conditions of applicability of this formula are verified for the GaAs/AlxGa1–xAs superlattices used as infrared photodetectors.

Influence of the miniband width on the relaxation time of superlattice electrons scattered by impurity ions

Based on the Boltzmann equation, the influence of the miniband width on the relaxation time of nondegenerate electrons scattered by impurity ions in the GaAs/AlxGa1–xAs superlattice with doped quantum wells is numerically analyzed. The wave function being the eigenfunction of the ground state of the lower miniband of the superlattice is used to calculate the scattering probability. The dispersion of the longitudinal and transverse relaxation times versus the longitudinal wave vector is investigated.

Dependence of scattering of quasi-two-dimensional electrons by acoustic phonons on the parameters of a GaAs/AlxGa1−x As superlattice

The time of relaxation of quasi-two-dimensional electrons in a GaAs/AlxGa1−xAs superlattice in the case of scattering by acoustic phonons was calculated numerically in relation to the quantum-well width and the width and height of the potential barrier. The probability of scattering for electrons of the lower miniband was calculated using an approximate envelope wave function without consideration of the dispersion of the periodic component of the Bloch function in relation to the wave vector. A comparative analysis of the calculated relaxation times and the values obtained using the well-known approximate formula was performed.

Effect of Interface Roughness on the Dynamical Properties of Superlattices: A Monte Carlo Approach

The ensemble Monte Carlo (EMC) method has been used to study the vertical electron transport in GaAs/AlxGa1—xAs superlattices. The EMC method was applied in a slightly n-doped GaAs/AlxGa1—xAs superlattice (<1013/cm3) to obtain the transient drift velocity for Γ electrons. The following scattering mechanisms are included in the calculations: inelastic acoustic phonon, polar optical phonon, ionized impurity and interface roughness. The first three mechanisms are calculated using a three-dimensional approach, while the interface roughness is calculated using a two-dimensional approach. Intersubband scattering has not been taken into account because only one miniband in the superlattice was assumed. The goal is the study of the influence of interface roughness scattering on the Bloch oscillations and on the static response by the Monte Carlo approach.