Microstructural and optical properties of strain compensated InxGa1−xAs/InyAl1−yAs multiple quantum wells

Transmission electron microscopy (TEM) and photocurrent (PC) measurements were carried out to investigate the microstructural and excitonic transitions in In_0.52Ga_0.48As/In_0.55Al_0.45As multiple quantum wells (MQWs). TEM images showed that high-quality 11-period strain-compensated In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs had high-quality heterointerfaces. Based on the TEM results, a possible crystal structure for the In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs is presented, and their strains are compensated. The results for the PC data at 300 K for several applied electric fields showed that several excitonic transitions shifted to longer wavelengths as the applied electric field increased. These results indicate that the strain-compensated In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs hold promise for electroabsorption modulator devices.     

Numerical optimization of In-mole fractions and layer thicknesses in AlxGa1−xN/AlN/GaN high electron mobility transistors with InGaN back barriers

The effects of the In-mole fraction (x) of an In_xGa_1−xN back barrier layer and the thicknesses of different layers in pseudomorphic Al_yGa_1−yN/AlN/GaN/In_xGa_1−xN/GaN heterostructures on band structures and carrier densities were investigated with the help of one-dimensional self-consistent solutions of non-linear Schrödinger-Poisson equations. Strain relaxation limits were also calculated for the investigated Al_yGa_1−yN barrier layer and In_xGa_1−xN back barriers. From an experimental point of view, two different optimized structures are suggested, and the possible effects on carrier density and mobility are discussed.     

Electronic parameters and electronic structures in modulation-doped highly strained InxGa1−xAs/InyAl1−yAs coupled double quantum wells

Electronic parameters of a two-dimensional electron gas (2DEG) in modulation-doped highly strained In_xGa_1−xAs/In_yAl_1−yAs coupled double quantum wells were investigated by performing Shubnikov-de Haas (S-dH), Van der Pauw Hall-effect, and cyclotron resonance measurements. The S-dH measurements and the fast Fourier transformation results for the S-dH at 1.5 K indicated the electron occupation of two subbands in the quantum well. The electron effective masses of the 2DEG were determined from the cyclotron resonance measurements, and satisfied qualitatively the nonparabolicity effects in the quantum wells. The electronic subband structures were calculated by using a self-consistent method.     

Experimental investigation of long-wavelength optical lattice vibrations in quaternary AlxInyGa1−x−yN alloys and comparison with results from the pseudo-unit cell model

Raman and Fourier transform infrared (FTIR) spectroscopies have been utilized to measure long-wavelength optical lattice vibrations of high-quality quaternary Al_xIn_yGa_1−x−yN thin films at room temperature. The Al_xIn_yGa_1−x−yN films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique with aluminum (Al) mole fraction x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. Pseudo unit cell (PUC) model was applied to investigate the phonons frequency, mode number, static dielectric constant, and high frequency dielectric constant of the Al_xIn_yGa_1−x−yN mixed crystals. The theoretical results were compared with the experimental results obtained from the quaternary samples by using Raman and FTIR spectroscopies. The experimental results indicated that the Al_xIn_yGa_1−x−yN alloy had two-mode behavior, which includes A₁(LO), E₁(TO), and E₂(H). Thus, these results are in agreement with the theoretical results of PUC model, which also revealed a two-mode behavior for the quaternary nitride. We also obtained new values of E₁(TO) and E₂(H) for the quaternary nitride samples that have not yet been reported in the literature.     

Interband transitions and electronic properties of InAs/GaAs quantum dots embedded in AlxGa1−xAs/GaAs modulation-doped heterostructures

Reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and double-crystal X-ray curves showed that high-quality InAs quantum dot (QD) arrays inserted into GaAs barriers were embedded in an Al_0.3Ga_0.7As/GaAs heterostructure. The temperature-dependent photoluminescence (PL) spectra of the InAs/GaAs QDs showed that the exciton peak corresponding interband transition from the ground electronic subband to the ground heavy-hole subband (E₁-HH₁) was dominantly observed and that the peak position and the full width at half maximum corresponding to the interband transitions of the PL spectrum were dependent on the temperature. The activation energy of the electrons confined in the InAs/GaAs QDs was 115 meV. The electronic subband energy and the energy wave function of the Al_0.3Ga_0.7As/GaAs heterostructures were calculated by using a self-consistent method. The electronic subband energies in the InAs/GaAs QDs were calculated by using a three-dimensional spatial plane wave method, and the value of the calculated (E₁-HH₁) transition in the InAs/GaAs QDs was in reasonable agreement with that obtained from the PL measurement.     

First principles investigation of optical properties of zinc-blende AlxGa1−xAs1−yNy materials

We have performed a first-principle Full Potential Linearized Augmented Plane Waves calculation within the local density approximation (LDA) to the zinc-blende Al_xGa_1−xAs_1−yN_y to predict its optical properties as a function of N and Al mole fractions. The accurate calculations of electronic properties such as band structures and optical properties like refractive index, reflectivity and absorption coefficient of Al_xGa_1−xAs and Al_xGa_1−xAs_1−yN_y with x≤0.375 and y up to 4% are presented. Al_xGa_1−xAs on GaAs have a lattice mismatch less than 0.16% and the lattice constant of Al_xGa_1−xAs has a derivation parameter of 0.0113±0.0024. The band gap energies are calculated by LDA and the band anticrossing model using a matrix element of C_MN=2.32 and a N level of E_N=(1.625+0.069x) eV. The results show that Al_xGa_1−xAs can be very useful as a barrier layer in separate confinement heterostructure lasers and indicate that the best choice of x and y Al_xGa_1−xAs_1−yN_y could be an alternative to Al_xGa_1−xAs when utilized as active layers in quantum well lasers and high-efficiency solar cell structures.     

Observation of new critical point in InxAl1−xAs alloy using spectroscopic ellipsometry

Using a spectroscopic ellipsometry, pseudodielectric functions 〈?〉 of In_xAl_1−xAs ternary alloy films (x = 0.43, 0.62, 0.75, and 1.00) from 0.74 to 6.48 eV were determined. Fast in-situ chemical etching to effectively remove surface overlayers using charge-coupled device detector and to avoid the reoxidation of the surface of films prior to the ellipsometric spectrum measurement was performed. At the high energy region, an additional critical point structure which is interpreted as the E′₁ transition from the band structure calculation of the linear augmented Slater-type orbital method was reported.     

Principal energy band gaps of the quaternary alloy AlxGa1−xSbyAs1−y

The correlated function expansion (CFE) interpolation procedure was presented to efficiently estimate principal energy band gaps and lattice constants of the quaternary alloy Al_xGa_1−xSb_yAs_1−y over the entire composition variable space. The lattice matching conditions between x and y for the alloy Al_xGa_1−xSb_yAs_1−y substrated to InAs and GaSb were obtained by optimizing the alloy lattice constant to that of the substrates. The corresponding principal band gaps (E(Γ), E(L), and E(X)) were also calculated along the lattice matching condition on each substrate (InAs and GaSb).     

Photoionization cross-section and binding energy of shallow donor impurities in Ga1−xInxNyAs1−y/GaAs quantum wires

We have investigated the effects of the nitrogen and indium concentrations on the photoionization cross-section and binding energy of shallow donor impurities in Ga_1−xIn_xN_yAs_1−y/GaAs quantum wires. The numerical calculations are performed in the effective mass approximation, using a variational method. We observe that incorporation of small amounts of nitrogen and indium leads to significant changes of the photoionization cross-section and binding energy.     

Study on the electronic structure and optical properties of different Al constituent Ga1−xAlxAs

Using quantum mechanics GASTEP software package based on the first principle density function theory, the electronic structure and optical properties of Ga_1−xAl_xAs at different Al constituent are calculated. Result shows that with the increase of Al constituent, the band gap of Ga_1−xAl_xAs increases and varies from direct band gap to indirect band gap; the absorption band edge and the absorption peak move to high-energy side; the static reflectivity decreases. With the increasing of the incident photon energy, Ga_1−xAl_xAs shows metal reflective properties in certain energy range. With the increasing of Al constituent, static dielectric constant decreases and the intersection of dielectric function and the x-axis move towards high-energy side; the peak of energy loss function move to low-energy side and the peak value reduces.     

Band offsets in In0.15Ga0.85As/ GaAs and In0.15Ga0.85As/Al0.15Ga0.85As studied by photoluminescence and cathodoluminescence

Photoluminescence and cathodoluminescence measurements of strained undoped In_0.15Ga_0.85As/GaAs and In_0.15Ga_0.85As/Al_0.15Ga_0.85As quantum well structures with emission lines attributed to the first electron–first heavy hole and first electron–first light hole excitonic transitions have been analysed theoretically within the eight-band effective mass approximation. For In_0.15Ga_0.85As/GaAs the results are consistent with either type I or type II alignment of the light hole band. In the case of In_0.15Ga_0.85As/Al_0.15Ga_0.85As our results indicate type II alignment for the light hole band and offset ratio ofQ = 0.83.     

Photoluminescence study in step-graded composition InxAl1−xAs/GaAs

We report on the lattice-mismatched growth of step-graded In_xAl_1−xAs buffer layers on GaAs (0 0 1) substrates by molecular beam epitay (MBE). The approach to growing highly lattice-mismatched epilayers is to interpose a buffer layer between the substrate and the active layer. Two samples G30 and G40 with active layer compositions, respectively, x = 0.46 and x = 0.41, are studied by photoluminescence (PL). At low temperature, the PL spectra show a large broadened band whose energy and intensity depend on the active layer composition. The step-graded layer compositions improved the crystalline quality of these structures and increase the active layer PL band intensity.     

Effect of hydrogen on modulation-doped AlGaAs/InGaAs/GaAs heterostructures: a photoluminescence study

The effect of hydrogen on donors and interface defects in silicon modulation doped Al_xGa_1−xAs/In_yGa_1−yAs/GaAs heterostructures has been investigated by photoluminescence (PL). Hydrogenation was carried out on two sets of samples, one set consists of high quality pseudomorphic heterostructures and another set having partially lattice relaxed structures prone to the defects. On exposure of high quality pseudomorphic structures to hydrogen plasma above 150 °C, a significant blue shift in the PL peak positions as well as bandwidth narrowing is observed. This indicates, the reduction in two-dimensional electron gas in the In_yGa_1−yAs quantum well due to hydrogen passivation of silicon donors in the Al_xGa_1−xAs supply layer. The reactivation of the donors is observed upon annealing the hydrogenated sample for 1 h at 250 °C under hydrogen ambient. Another interesting feature is a significant improvement in the PL of lattice-relaxed structures upon hydrogenation of the samples above 250 °C, which is attributed to the hydrogen passivation of interface defects due to the misfit dislocations.     

Structural characterization of InxGa1−xAs/Inp layers under different stresses

In_xGa_1−xAs layers on InP substrate can be subjected to compressive or tensile strain due to lattice parameter differences depending on the alloy composition. In order to examine in details the strain of InGaAs/InP epiatxial layers and its evolution after subjecting the layers to annealing at high pressure, X-ray synchrotron topography, high resolution X-ray diffraction and atomic force microscopy have been employed. The data show that the changes of structural properties of the InGaAs layers subjected to high temperature-high pressure treatment at 670 K-1.2 Gpa, strongly depend on initial strain state and defect structure. The annealing of samples under high pressure results in change of strain in tensile layers only. The behaviour of observed defects is discussed.     

A Hall coefficient investigation of ferromagnetic Ga1−xMnxAs layers on (100) GaAs substrates

Ferromagnetic Ga_1−xMn_xAs epilayers with Mn mole fraction in the range of x≈2.2-4.4% were grown on semi-insulating (100) GaAs substrates using the molecular beam epitaxy technique. The transport properties of these epilayers were investigated through Hall effect measurements. The measured hole concentration of Ga_1−xMn_xAs layers varied from 4.4×10¹⁹ to 3.4×10¹⁹ cm⁻³ in the range of x≈2.2-4.4% at room temperature. From temperature dependent resisitivity data, the sample with x≈4.4% shows typical behavior for insulator Ga_1−xMn_xAs and the samples with x≈2.2 and 3.7% show typical behavior for metallic Ga_1−xMn_xAs. The Hall coefficient for the samples with x≈2.2 and 4.4% was fitted assuming a magnetic susceptibility given by Curie-Weiss law in a paramagnetic region. This model provides good fits to the measured data up to and the Curie temperature T_c was estimated to be 65, 83 K and hole concentration p was estimated to be 5.1×10¹⁹, 4.6×10¹⁹ cm⁻³ for the samples with x≈2.2 and 4.4%, respectively, confirming the existence of an anomalous Hall effect for metallic and insulating samples.     

Magnetotransport, optical, and electronic subband properties in AlxGa1−xN/AlN/GaN heterostructures

The Shubnikov-de Haas (S-dH) results at 1.5 K for Al_xGa_1−xN/AlN/GaN heterostructures and the fast Fourier transformation data for the S-dH data indicated the occupation by a two-dimensional electron gas (2DEG) of one subband in the GaN active layer. Photoluminescence (PL) spectra showed a broad PL emission about 30 meV below the GaN exciton emission peak at 3.474 eV that could be attributed to recombination between the 2DEG occupying in the AlN/GaN heterointerface and photoexcited holes. A possible subband structure was calculated by a self-consistent method taking into account the spontaneous and piezoelectric polarizations, and one subband was occupied by 2DEG below the Fermi level, which was in reasonable agreement with the S-dH results. These results can help improve understanding of magnetotransport, optical, and electronic subband properties in Al_xGa_1−xAs/AlN/GaN heterostructures.     

Exciton bound to an ionized donor impurity in GaAs/Ga1−xAlxAs ellipsoidal finite-potential quantum dots under hydrostatic pressure

In the framework of perturbation theory, a variational method is used to study the ground state of a donor bound exciton in a weakly prolate GaAs/Ga_1−xAl_xAs ellipsoidal finite-potential quantum dot under hydrostatic pressure. The analytic expressions for the Hamiltonian of the system have been obtained and the binding energy of the bound exciton is calculated. The results show that the binding energy decreases as the symmetry of the dot shape reduces. The pressure and Al concentration have a considerable influence on the bound exciton. The binding energy increases monotonically as the pressure or Al concentration increases, and the influence of pressure or Al concentration is more pronounced for small quantum dot size.     

The role of strain in controlling the surface morphology of AlxGa1−xN following in situ treatment with SiH4 and NH3

Treatment of GaN with SiH₄ and NH₃ increases the size of surface pits associated with threading dislocations, allowing them to be easily imaged by atomic force microscopy. Here, we assess the effect of a similar treatment on Al_xGa_1−xN surfaces for x ≤ 0.4. For relaxed Al_xGa_1−xN epilayers, an increase in the observed size and density of threading dislocation pits is observed. However, if the Al_xGa_1−xN is under tensile strain, the treatment results in the appearance of nanometre-scale surface hillocks. These hillocks may prevent observation of the dislocation pits. The hillocks are found to consist of crystalline Al_xGa_1−xN, and hence are suggested to be formed by strain driven etching or transformation of the surface by SiH₄ and NH₃.     

Intersubband energies in Al1−yInyN/Ga1−xInxN heterostructures with lattice constant close to aGaN

We have studied the conduction band profile and the intersubband transition energy, E₁₂, of Al_1−yIn_yN/Ga_1−xIn_xN quantum well structures. We have considered how material parameters such as non-parabolicity and the uncertainty in the bowing parameter affect E₁₂ and the corresponding wavelength, λ₁₂. The calculations include strain and cover the transition range from telecommunication wavelengths (1.55 μm) to the mid-infrared (∼ 10 μm).     

The variation of electronic properties with the doping concentration of modulation-doped AlxGa1−xAs–GaAs double quantum wells

In this paper we have calculated the sub-band structure and the confinement potential of modulation-doped Ga_1−xAl_xAs–GaAs double quantum wells as a function of the doping concentration. The electronic properties of this structure were determined by self-consistent solutions of the Schrödinger and Poisson equations. To understand the effects of doping concentration on band bending, sub-band energies, and sub-band populations, the doping concentration on one right side of the structure is decreased while holding it constant on the left side. We found that at low doping concentrations on the right side, the effects of the doping concentration are more pronounced on band bending and sub-band populations.