Interband transition studies of one-side modulation-doped AlxGa1−xAs/InyGa1−yAs/GaAs asymmetric step quantum wells

The results of S-dH measurements and the observation of quantum Hall plateaus at 1.5 K clearly demonstrated the existence of a 2DEG in modulation-doped Al_xGa_1−xAs/In_yGa_1−yAs/GaAs asymmetric step quantum wells. The PL data measured at several temperatures showed that the excitonic transition from the ground electronic subband to the ground heavy-hole band (E₀-HH₁) shifted to the lower energy side with increasing temperature. The value of the (E₀-HH₁) excitonic transition obtained from the PL measurements was in reasonable agreement with that determined from self-consistent calculations.     

Electronic and optical properties of (Al x Ga1−x )1−y Mn y As single crystal: a new candidate for integrated optical isolators and spintronics

We have explored the electronic and optical properties of cubic (Al _x Ga_1?x )_1?y Mn _y As system using the FP-LAPW method. The unit cell has 64 atoms, so that one manganese (Mn) atom is placed in the position of gallium site, which corresponds to 3.125 % doping concentration with x = 12.5 %. Our calculations, using local density approximation + U (Hubbard parameter) scheme, predict that the ferromagnetic state for AlGaMnAs, with a magnetic moment of about 4.014 μ_B per Mn dopant is more favorable. Despite its electronic properties being strongly affected by inducing small amounts of Mn substitutional atoms in the cationic sublattice of AlGaAs, (Al _x Ga_1?x )_1?y Mn _y As possesses optical properties strictly less than those of Al _x Ga_1?x As, especially its optical conductivity at the peak 1.256 eV. The results indicate that AlGaMnAs may be a good candidate for optoelectronics when exploited in optical fiber networks, and it can still be of great interest because of its promising potential when used for spintronics.     

Field effect on electron-hole recombination in Si/Si1−xGex/Si quantum wells having a W-like type II potential profile

A theoretical analysis, using a Schrödinger solver, is made to calculate the electric field-dependent interband transitions in a Si/Si_1−xGe_x/Si double QW strain-compensated in relaxed Si_1−yGe_y barriers. The conduction and the valence band present a W-like potential profile, resulting in a quasi-type I heterostructure. Three peculiar features are revealed as the electric field is increased: (i) two uncoupled e₁₁ and e₁₂ electron levels are generated, (ii) the e₁₁–hh₁ fundamental transition due to first silicon QW exhibits a red shift in emission energy while the e₁₂–hh₁ transition energy is bleu shifted, (iii) an improved wave function overlap for the e₁₁–h₁ fundamental transition, the latter property showing the advantage having two adjacent QWs in this W architecture.     

Electronic Structure and Magnetic Properties of Doped GaAs (Zinc Blende) with Double Impurities and Defects

Using ab-initio calculations based on Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA), we study theoretically the electronic and magnetic properties of a system based on GaAs material. These properties have been studied, with different point defects in GaAs, which are Gallium interstitials (Ga_i), Gallium antisites (Ga_As), Gallium vacancies (V_Ga), Arsenic interstitials (As_i) and Arsenic antisites (As_Ga), Arsenic vacancies (V_As), and in-doped or codoped system Ga_1?x Co _x As and Ga_1?y?z Co _y Fe _z As, respectively. This work presents detailed information about total and local density of states at different concentrations of these defects and doped elements; the stability of the ferromagnetic state compared with the spin-glass state has been discussed. The result of electronic properties shows that codoped GaAs material with Fe and Co become ferromagnetic, also doped GaAs with Co or Fe with a defect (hole in Ga) is stable in the ferromagnetic state. The Curie temperature is estimated from the total energy difference between the Disorder Local Moment (DLM) and the (FM) state by using a mapping on the Heisenberg model in mean field approximation.     

Potential barrier study at the InGaAs/InAs quantum dot interface in asymmetric multi quantum well structures

The photoluminescence and its temperature dependences have been investigated for the InAs quantum dots embedded in the asymmetric GaAs/In_xGa_1−xAs/In_0.15Ga_0.85As/GaAs quantum wells (dot-in-a-well, DWELL structures) as a function of the In content x (x = 0.10-0.25) in the capping In_xGa_1−xAs layer. The study of PL temperature dependences in the range of 80-120 K has revealed the potential barrier for electrons at the capping In_xGa_1−xAs/InAs QD interface. The value of mentioned barrier has varied in QD structures with the different In_xGa_1−xAs capping layer compositions and it was estimated in the studied asymmetric GaAs/In_xGa_1−xAs/In_0.15Ga_0.85As/GaAs DWELL structures. It is shown that the barrier for electrons equal to 48 and 24 meV appears in the InGaAs conduction band at the formation of InGaAs QW/InAs QD interface for the In compositions of x = 0.10 and 0.15, respectively. This barrier has not been detected in DWELLs with the In compositions x = 0.20 and 0.25. The energy gap offsets at the InAs/In_xGa_1−xAs interface in studied structures has been estimated and discussed as well.     

Optical nutation in GaAs/GaxA11−xAs quantum well structures

Abstract

The possibility of occurrence of the coherent optical transient effect known as optical nutation has been analytically established in the semiconductor quantum well (QW) structure, namely GaAs/Ga_xA1_1?xAs most extensively used in optical electronics. Ultra-short-pulse low-intensity band-to-band excitation of electrons to the 1s Wannier-Mott exciton state of the crystal has been considered to play an important role in the coherent radiation—QW interaction. Numerical estimations of the complex optical susceptibility and the transmitted intensity under the transient regime reveal ringing behaviour confirming the occurrence of optical nutation in III-V semiconducting QW structures.     

Band offsets of InXGa1−XN/GaN quantum wells reestimated

Recently In_XGa_1−XN/GaN heterostructures and quantum wells (QWs) have gained immense importance in the application of III-V nitride materials. Reported values of the ratios of conduction band offset to valence band offset for In_XGa_1−XN/GaN QW structures, ΔE_c:ΔE_v, vary widely from 38:62 to 83:17. While trying to explain the unusual shifts in the photoluminescence (PL) spectra, obtained from In_XGa_1−XN/GaN QW structures, it has been found that a band offset ratio, ΔE_c:ΔE_v = 55:45, explains all the experimental data precisely. In this paper detailed theories, procedures, results and discussions to establish the newly estimated band offsets will be presented.     

Effect of temperature and pressure on the electronic structure of GaxIn1−xAsyP1−y alloys lattice matched to GaAs substrate

Based on the empirical pseudo-potential within the virtual crystal approximation including the effective disorder potential, calculations of the electronic structure of the zinc blende quaternary Ga_xIn_1−xAs_yP_1−y alloys have been studied. The direct and indirect energy gaps of the considered alloys have been determined over the entire composition x and y in condition of being lattice matched to GaAs substrate. In addition, the effect of temperature and hydrostatic pressure of these energy gaps have been studied. Comparison of the calculated results with the experimental and published data showed good agreement.     

Properties of narrow-bandgap (0.3–0.48 eV) A3B5 solid solution epilayers grown by metal-organic chemical vapor deposition

Processes of epitaxial growth of narrow-bandgap (with bandgap value E _g ?? 0.3?0.48 eV) solid solutions GaInAsSb and InAsPSb on InAs substrates by metal-organic chemical vapor deposition at low pressure (76 Torr) are investigated. It is shown that, under chosen growth conditions, the InAsPSb epilayers have high crystalline quality, while the solid solutions Ga_1?x In _x As _y Sb_1?y and InAs _y P _z Sb_{1 ? y ? z} have compositions close to InAs (0.86 < x < 0.93, 0.62 < y < 0.9, 0.17 < z < 0.26) and manifest photoluminescence at room temperature.     

Effect of simultaneous chemical substitution of A and B sites on the electronic structure of BiFeO3 films grown on BaTiO3/SiO2/Si substrate

Electrical properties and electronic structure of Bi_1?xCa_xFe_1?yMn_yO_3?δ grown by pulsed-laser deposition on BaTiO₃/SiO₂/Si substrate were investigated. Results showed that Ca has drastic effect on symmetry of crystal and electrical poperties of BiFeO₃. On the other hand, Mn revealed to have more radical effect on optical properties and energy gap of the compound. XPS results represented that although Ca tend to decrease Fe valence state, Mn tends to stabilize it at 3+ (at least in this concentrations). UV–visible study yielded bandgap of 2.51–2.81 eV (at 300 K) for different concentrations of Ca and Mn. UV–visible spectra also revealed sub-bandgap defect transitions at 2.2 and 2.4 eV. Slave-particle approach has also been applied to elucidate nature of the metal–insulator transition.     

Dielectric characteristics of Ga doped TbMnO3

Low-frequency (0.1–200 kHz) dielectric properties of Tb_1?xGa_xMnO₃ and TbGa_yMn_1?yO₃ (x, y = 0.05, 0.1, 0.2, 0.3, 0.4) ceramic composites, which were synthesized by conventional solid-state reaction, were investigated in the temperature range from 77 to 350 K. Both dielectric constants and loss tangent (tan δ) increase with increasing temperature and decrease with increasing frequency, respectively. Interestingly, the dielectric constants of Tb_1?xGa_xMnO₃ are as large as that of the parent TbMnO₃, while the loss tangent reduces remarkably and less than 1 at high frequencies. These improvements demonstrate that Ga doped TbMnO₃ may have potential applications.     

Peculiarities of Silicon-Donor Ionization and Electron Scattering in Pseudomorphous AlGaAs/InGaAs/GaAs Quantum Wells with Heavy Unilateral Delta-Doping

The influence of the concentration of silicon donors on the electron-transport properties of pseudomorphous Al_0.25Ga_0.75As/In_0.2Ga_0.8As/GaAs quantum wells (QWs) in heterostructures with heavy unilateral δ-doping by Si atoms was studied in a broad temperature interval (2.1–300 K). High electron mobility (up to 35700 cm²/(V s)) at T = 4.2 K was observed at a 2D (sheet) electron density of 2 × 10¹² cm^–2 in the QW. A band mechanism limiting the ionization of donors at an increased level of doping is described. The nonmonotonic variation of electron mobility with increasing silicon concentration is explained. A growth in the mobility is related to increase in the Fermi momentum and screening, while the subsequent decay is caused by tunneling-induced degradation of the spacer layer with decreasing potential of the conduction band in the region of δ-Si layer. It is shown that the effect is not related to filling of the upper subband of dimensional quantization.     

On the effect of cobalt doping on thermoelastic martensitic transformations in ferromagnetic Heusler Ni50 − x Co x Mn29Ga21 magnetically controlled shape memory alloys

The phase composition, structure, magnetic and thermoelastic martensitic transformations, and physical properties (magnetic susceptibility, magnetization, electric resistivity) in multicomponent magnetic Ni_{50 ? x} Co _x Mn₂₉Ga₂₁ alloys (with a Co content of x = 0, 1, 2, 3, and 10 at %) exhibiting a magnetically controlled shape memory effect are studied. The critical temperatures of the magnetic transitions and thermoelastic martensitic transformations are determined. It is shown that the studied alloys in the austenitic state are substitutional solid solutions based on a quasi-ternary L2₁ superstructure (Ni, Co)₅₀Mn₂₉Ga₂₁.     

Interface characteristics of GaAsAlxGa1−x As superlattices grown by MOCVD

The structure and quality of the heterointerfaces of $\text{AlAs}\text{GaAs}$ semiconductor superlattices grown by the MOCVD technique have been examined on the atomic scale using high-resolution electron microscopy lattice imaging. The interface of either GaAs grown on AlAs (Al_xGa_1?xAs) or AlAs (Al_xGa_1?xAs) grown on GaAs is atomically smooth and without defects in both cases, and the interface quality is not degraded by increasing either the layer size or the Al composition.     

Light-emitting diodes based on Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>x</Subscript>As<Subscript>y</Subscript>Sb<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript> solid solutions grown from lead-containing melts

Light-emitting diodes (LEDs) with the maximum emission at λ=2.3 and 2.44 μm are obtained and characterized. The active region of LEDs is based on Ga_1?xIn_xAs_ySb_1?y solid solutions (x=0.21 and 0.25, respectively) grown by liquid phase epitaxy from lead-containing melts. The room-temperature external quantum yield of the 2.3 and 2.44 μm LEDs is 1.6 and 0.11%, respectively.     

Optical absorption edge of TlIn1 − x Ga x Se2 solid solutions

We have studied the optical absorption edge of TlIn_{1 ? x} Ga _x Se₂ (x = 1.0, 0.9, 0.7, 0.6, 0.5) solid solutions, identified the types of the observed optical transitions, and evaluated the band gap of the solid solutions and the energy of phonons involved in the transitions.     

Thermodynamic analysis of the metal-insulator transitions in V2O3 alloy systems

An overview is presented concerning the entropies (ΔS) and enthalpies (ΔH) associated with metal-insulator transitions in the (V_1?x M _x )₂O₃ M=Al, Cr, Ti systems, 0?x?0·12 and in non-stoichiometric V_{2(1 ?y} )O₃, 0?y?0·01. The recent literature and new data are analyzed with respect to gradiations in ΔH and ΔS withx ory, and their implications concerning the mechanism of the transitions. A revised phase diagram is presented.     

Optical and photoelectric properties of monoclinic Zn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>2</Subscript> crystals

The first data are presented on the band structure of monoclinic Zn_{1 ? x} Cd _x As₂ crystals. The fundamental absorption edge of Zn_{1 ? x} Cd _x As₂ for the E ‖ c polarization is shown to be dominated by an indirect allowed transition for absorption coefficients α^‖ < 6 cm^?1 and by a direct allowed transition for α^‖ > 6 cm^?1, both transitions involving excitonic levels. The absorption edge for E ⊥ c is due to a direct forbidden transition with the participation of excitonic levels. We have determined the band gap values for these transitions in the temperature range 80–300 K, the exciton binding energy, and the ionization energies of four deep acceptor levels produced in the band gap by structural defects.     

Growth by liquid phase epitaxy and evaluation of submicron InxGa1−xAsyP1−y layers for microwave field effect transistors

The growth of In-Ga-As-P layers 0.2 μm thick that are lattice matched to (100)-oriented InP substrates is described. A step cooling technique was employed to obtain uniform In_xGa_1?xAs_yP_1?y layers with band gaps of 0.9, 1.04, 1.15 and 1.2eV. Excellent surface morphology and good (±15%) thickness uniformity were achieved primarily by preventing substrate degradation by means of loss of phosphorus or by removing the degraded surface during the saturation of the melt. Van der Pauw and capacitance-voltage measurements were employed to characterize the epitaxial films. The field effect transistors fabricated from 1.15 and 1.2eV material exhibited a maximum available gain of about 7 dB at 10 GHz.     

Ferromagnetic and Charge-Ordered Phases in (Nd, Na)?CMn?CO Compounds

Polycrystalline Nd_1?x Na _x MnO₃ ( $x=0.05\mbox{--} 0.20$ ) compounds were prepared in single-phase form with a Pbnm space group. Paramagnetic to ferromagnetic transitions were observed up to a doping concentration of 15% with a maximum T _C of 113?K. The x=0.20 sample exhibits a charge order transition at 180?K, with a distinct behavior in magnetization versus field curve. The magnetization versus temperature plot of all these materials exhibits an anomaly at around 40?K, with a signature of spin-glass like behavior. The x=0.10 sample is found to exhibit a maximum spontaneous magnetization value of 3.1?? _B at 20?K. The magnetization data could be analyzed based on the Brillouin function model, by accounting for the ferromagnetic interaction. The effective spin contribution of 3d electrons for the double-exchange ferromagnetic interaction and the value of magnetic spin-canting angles were estimated. The measured magnetization is explained on the basis of competing magnetic interaction due to spin-canting. The charge order quenching and the first-order transition from the charge-ordered to ferromagnetic phase are observed for a threshold field of 3?T at 12?K. The electrical resistivity of the above samples exhibits insulating behavior.