Contactless electroreflectance of GaNyAs1−y/GaAs multi quantum wells: The conduction band offset and electron effective mass issues

Interband transitions in GaN_yAs_1−y/GaAs multi quantum well (MQW) samples with y=0.012 and 0.023 have been studied by contactless electroreflectance spectroscopy (CER). Optical transitions related to absorption in the GaAs barriers and in the GaN_yAs_1−y/GaAs QWs have been observed and analyzed. The GaAs related transition exhibits clear Franz-Keldysh oscillations with the period corresponding to the built-in electric field of 14 and 17 kV/cm for samples with y=0.012 and 0.023, respectively. The portion of the CER spectrum related to absorption in the GaN_yAs_1−y/GaAs QW exhibits two clear resonances which are attributed to optical transitions between the ground and excited states confined in the QWs. The resonance attributed to the ground state transition is associated with absorption between the first light- and heavy-hole subbands and the first electron subband (11L and 11H) while the resonance attributed to the excited state transition is associated with absorption between the second heavy-hole subband and the second electron subband (22H). The energies of the 11H and 22H transitions have been matched with those obtained from theoretical calculations performed within the effective mass approximation. Thus, the GaN_yAs_1−y/GaAs QWs are type-I structures with a conduction band offset, Q_C, between 70 and 80%. Moreover, the incorporation of N atoms into GaAs is found to cause a significant increase in the electron effective mass. The determined values of electron effective mass for GaN_yAs_1−y/GaAs QW with y=0.012 and 0.023 are 0.105m₀ and 0.115m₀, respectively.     

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.     

Systematic theoretical investigations for contribution of lattice constraint to novel atomic arrangements in alloy semiconductor thin films

The atomic arrangements in zinc blende structured GaN_xAs_1−x thin films coherently grown on V-grooved substrates are theoretically investigated using empirical interatomic potentials and Monte Carlo simulation. The resultant atomic arrangements in GaN_xAs_1−x strongly depend on concentration x and substrate lattice parameter a_sub. Surface segregation of As or N is mainly found in GaN_xAs_1−x with large lattice mismatch to the substrate. On the other hand, the novel atomic arrangements such as layered segregation or ordered structure are found in GaN_xAs_1−x at the specific region such as (x, a_sub) = (0.5, 5.3), (0.3, 5.3), and (0.3, 5.1). This specific region corresponds to that with negative excess energy and with sufficient N and As atoms remaining in thin film layers even after their surface segregation. The formation of the novel atomic arrangements is discussed in terms of bond lengths in the surface layers. These results suggest that various novel atomic arrangements in alloy semiconductor thin films appear depending on x and a_sub which control degree of lattice constraint.     

Optical characterization of InGaAsN/GaAsN/GaAs quantum wells with InGaP cladding layers

Optical properties of InGaAsN/GaAs and InGaAsN/GaAsN/GaAs quantum well structures with InGaP cladding layers were studied by photoreflectance at various temperatures. The excitonic interband transitions of the InGaAsN/GaAsN/GaAs QW systems were observed in the spectral range above hν=E_g(InGaAsN). The confinement potential of the system with strain compensating GaAsN barriers became one step broader, thus more quantum states and larger optical transition rate were observed. A matrix transfer algorithm was used to calculate the subband energies numerically. Band gap energies, effective masses were adopted from the band anti-crossing model with band-offset values adjusted to obtain the subband energies to best fit the observed optical transition features. A spectral feature below and near the GaAs band gap energy from GaAs barriers is enhanced by the GaAs/InGaP interface space charge accumulation induced internal field.     

Hot electron energy and momentum relaxation in GaAs/AlAs and GaAs/Ga1-xAlxAs multiple quantum wells

Experimental results on high electric field longitudinal transport in GaAs/AlAs and GaAs/Ga_1-xAl_xAs multiple quantum wells (MQW) are presented and compared with the prediction of a dielectric continuum model. We draw from our experiments the following four conclusions.(i) In GaAs/Ga_1-xAl_xAs systems the dominant energy and momentum relaxation mechanism is through scattering with GaAs -modes.(ii) However, in GaAs/AlAs systems the AlAs interface mode is dominant in relaxing the energy and momentum of the quantum well electrons.(iii) The hot electron momentum relaxation as obtained from the high-field drift velocity experiments is strongly affected by the production of hot phonons as expected from a model involving a non-drifting hot phonon distribution.(iv) The importance of the AlAs interface mode in GaAs/Ga_1-xAl_xAs MQW is not the result of the intrinsic scattering rate but related to its shorter lifetime, compared to GaAs modes.     

Simple theoretical analysis of the Fowler-Nordheim field emission from microstructures and quantum wires of optoelectronic materials

We present a simplified theoretical formulation of the Fowler-Nordheim field emission (FNFE) under magnetic quantization and also in quantum wires of optoelectronic materials on the basis of a newly formulated electron dispersion law in the presence of strong electric field within the framework of k.p formalism taking InAs, InSb, GaAs, Hg_1−xCd_xTe and In_1−xGa_x As_yP_1−y lattice matched to InP as examples. The FNFE exhibits oscillations with inverse quantizing magnetic field and electron concentration due to SdH effect and increases with increasing electric field. For quantum wires the FNFE increases with increasing film thickness due to the existence van-Hove singularity and the magnitude of the quantum jumps are not of same height indicating the signature of the band structure of the material concerned. The appearance of the humps of the respective curves is due to the redistribution of the electrons among the quantized energy levels when the quantum numbers corresponding to the highest occupied level changes from one fixed value to the others. Although the field current varies in various manners with all the variables in all the limiting cases as evident from all the curves, the rates of variations are totally band-structure dependent. Under certain limiting conditions, all the results as derived in this paper get transformed in to well known Fowler-Nordheim formula.     

The magnetism for NN AFM ground state in Fe-based superconductor: Sr1−xKxFe2As2

The crystal structure, magnetism properties, and density of states for FeAs layered compound SrFe₂As₂ have been investigated by using the density functional theory (DFT) method. The magnetism under a checkerboard nearest neighbor anti-ferromagnetic (NN AFM) and ferromagnetic (FM) order ground-state have been analyzed with substitution for Sr with K ion in Sr_1−xK_xFe₂As₂. The results indicate that the distortion of FeAs tetrahedrons is sensitive to the electron doping concentration. The system magnetism was suppressed by K doping in NN-AFM ground state instead of FM. The density of states at Fermi level N(E_F) under NN AFM ground state would be regarded as a driving force for the increased T_c of Sr_1−xK_xFe₂As₂ system as observed experimentally. Our calculation reflects that NN AFM type spin fluctuation may still exist in the Sr_1−xK_xFe₂As₂ system and it may be an origin of strong spin fluctuation in this system besides the spin density wave (SDW) states.     

Ferromagnetic nature of (Ga, Cr)As epilayers revealed by magnetic circular dichroism

A systematic study of magnetic circular dichroism (MCD) was carried out in a wave length range 500-960 nm for (Ga_1−x, Cr_x)As epilayers with x=2.38% and 4.59% grown by the low temperature molecular beam epitaxy (LT-MBE) technique. Hysteresis characteristics showed up indeed in the magnetic field dependence of both MCD and magnetization measured by the superconductor quantum interference device (SQUID). The Curie temperature of the (Ga_1−x, Cr_x)As epilayer was determined to be about 12 K by the Arrott approach. The present result provides evidences that there is strong coupling of the Cr spins to the GaAs host band structure in (Ga_1−x, Cr_x)As samples. That affects the critical point of the semiconductor host, and makes the magnetization behavior in a plot of M∼B/T (magnetic field divided by temperature) substantially different from standard superparamagnetism.     

Magnetic resonance study of arsenic bonding sites in ternary chalcogenide glasses

⁷⁵As NQR and high-field NMR experiments have been performed on Ge_xAs_ySe_1−x−y glasses. Evolution of As bonding structure from arsenic sites with axially symmetric distribution of the electric field gradient (EFG) to highly asymmetric As surroundings has been revealed. Arsenic atoms form pyramidal structural units in Ge₂As₂Se₇ with no evidence of significant concentration of homopolar bonds. In Ge₂As₂Se₅ most of arsenic atoms form structural units with two As-As bonds per atom and asymmetric EFG distribution. Arsenic bondings become more complicated in Ge_0.33As_0.12Se_0.55 where all arsenic sites are highly distorted. The combination of NQR and NMR data provide valuable information on arsenic bonding dynamics in these glasses.     

Donor impurity states in coupled quantum well wires under hydrostatic pressure and applied electric field

In this work we study the binding energy of the ground state for a hydrogenic donor impurity in laterally coupled GaAs/Ga_1−xAl_xAs quantum well wires, considering the simultaneous effects of hydrostatic pressure and applied electric field. We have used a variational method and the effective mass and parabolic band approximations. The low dimensional structure consists of two quantum well wires with rectangular transverse section coupled by a central Ga_1−xAl_xAs barrier. Our results are reported for several sizes of the structure and we have taken into account variations of the impurity position along the growth direction of the heterostructure.     

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.     

Optical anisotropy of GaNAs grown on GaAs(0 0 1) substrate

In this paper GaN_xAs_1−x surfaces during growth are observed using reflectance difference or reflectance anisotropy spectroscopy (RDS or RAS). The epi-layer was grown by solid-source molecular beam epitaxy (MBE) system with a RF nitrogen prasma source. RD spectra showed broader structure and reduced amplitude compared to those of GaAs surfaces; GaAs(2 × 4)-like features were still observed with weak and blue-shifted peaks. In the low growth temperature region, an extra structure was also observed around 3.02 eV. We proposed that GaN_xAs_1−x surface can be classified into three types of the surface.     

Contactless electroreflectance spectroscopy of Ga(In)NAs/GaAs quantum well structures containing Sb atoms

Contactless electroreflectance (CER) spectroscopy has been applied to investigate the optical transitions in Ga(In)NAs/GaAs quantum well (QW) structures containing Sb atoms. The identification of the optical transitions has been carried out in accordance with theoretical calculations which have been performed within the framework of the effective mass approximation. Using this method, the bandgap discontinuity for GaN_0.027As_0.863Sb_0.11/GaAs, Ga_0.62In_0.38As_0.954N_0.026Sb_0.02/GaAs, and Ga_0.61In_0.39As_0.963N_0.017Sb_0.02/GaN_0.027As_0.973/GaAs QW structures has been determined. It has been found that the conduction-band offset is ∼50 and ∼80% for GaN_0.027As_0.863Sb_0.11/GaAs and Ga_0.62In_0.38As_0.954N_0.026Sb_0.02/GaAs QWs, respectively. It corresponds to 264 and 296 meV depth QW for electrons and heavy-holes in GaN_0.027As_0.863Sb_0.11/GaAs QW; and 520 and 146 meV depth QW for electrons and heavy-holes in Ga_0.62In_0.38As_0.954N_0.026Sb_0.02/GaAs QW. In the case of the Ga_0.61In_0.39As_0.963N_0.017Sb_0.02/GaN_0.027As_0.973/GaAs step-like QW structure it has been shown that the depth of electron and heavy-hole Ga_0.61In_0.39As_0.963N_0.017Sb_0.02/GaN_0.027As_0.973 QW is ∼144 and ∼127 meV, respectively.     

Optical second harmonic generation in asymmetric double triangular quantum wells

The second harmonic generation (SHG) in the asymmetric double triangular quantum wells (DTQWs) is investigated theoretically. The dependence of the SHG coefficient on the right-well width of the DTQWs is studied, and the influence of the applied electric field on SHG coefficient is also taken into account. The analytical expression of the SHG coefficient is analyzed by using the compact density-matrix approach and the iterative method. Finally, the numerical calculations are presented for the typical GaAs/Al_xGa_1−xAs asymmetric DTQWs. The results show that the calculated SHG coefficient in this coupled system can reach the magnitude of 10⁻⁵ m/V, 1–2 orders of magnitude higher than that in step quantum well, and that in double square quantum wells. Moreover, the SHG coefficient is not a monotonic function of the right-well width, but has complex relationship with it. The calculated results also reveal that an applied electric field has a great influence on the SHG coefficient. Applying an appropriate electric field to a DTQW with a wider right well can induce a sharper peak of the SHG coefficient due to the double-resonant enhancement.     

Influence of N cluster states on band dispersion in GaInNAs quantum wells

We use a modified band-anticrossing (BAC) model to investigate the band dispersion in a GaN_xAs_1-x/AlGaAs quantum well (QW) as a function of hydrostatic pressure. The band edge mass increases considerably more quickly with pressure than in the case of a GaAs/AlGaAs QW, and the subband separation also decreases significantly. We predict that the strong anticrossing interaction between the GaAs host conduction band and isolated N levels will inhibit tunnelling through the QW for a range of energy above the isolated N levels. The energy of N resonant states depends strongly on details of the local environment, giving a broader calculated distribution of N states in GaInNAs compared to GaNAs.     

Optical gain due to the Eu transition in the alloy of Ca1-xEuxGa2S4

Room temperature photoluminescence quantum efficiency of the alloy of Ca_1−xEu_xGa₂S₄ was measured as a function of x, and was found to be nearly unity under excitation at peak wavelength of excitation spectrum (510 nm) in the x range of 0.01≤x≤0.2. At larger x values, it tends to decrease, but still as high as 30% for stoichiometric compound EuGa₂S₄. Taking these backgrounds into account, pump-probe experiments were done with Ca_1−xEu_xGa₂S₄ for searching optical gain at x=0.2. The optical gain of nearly 30 cm⁻¹ was confirmed to exist, though the pumping induced transient absorption which seems to limit the higher gain was found.     

Local environment of nitrogen in GaNyAs1−y epilayers on GaAs (0 0 1) studied using X-ray absorption near edge spectroscopy

X-ray absorption near-edge spectroscopy (XANES) is used to study the N environment in bulk GaN and in GaN_yAs_1−y epilayers on GaAs (0 0 1), for y∼5%. Density-functional optimized structures were used to predict XANES via multiple-scattering theory. We obtain striking agreement for pure GaN. An alloy model with nitrogen pairs on Ga accurately predicts the threshold energy, the width of the XANES ‘white line’, and features above threshold, for the given X-ray polarization. The presence of large quantitities of N-pairs may point to a role for molecular N₂ in epitaxial growth kinetics.     

Effects of uniaxial pressure and annealing on the resistivity of Ba(Fe1−xCox)2As2

Single crystals of underdoped Ba(Fe_1−xCo_x)₂As₂ were detwinned by applying uniaxial pressure. The anisotropic in-plane resistivity was measured using the Montgomery method without releasing pressure. The resistivity along the a-axis shows metallic behavior down to 5 K, while the resistivity along the b-axis shows an insulator-like behavior in some temperature range. Annealing the sample radically reduces the residual resistivity for x=0, and at the same time the anisotropy becomes much smaller at low temperatures.     

Preparation, structure and oxide ion conductivity in Ce6−xYxMoO15−δ (x=0.1-1.4) solid solutions

The Ce_6−xY_xMoO_15−δ solid solution with fluorite-related structure have been characterized by differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), IR, Raman, scanning electric microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) methods. The electric conductivity of samples is investigated by Ac impedance spectroscopy. An essentially pure oxide-ion conductivity of the oxygen-deficiency was observed in pure argon, oxygen and air. The highest oxygen-ion conductivity was found in Ce_5.5Y_0.5MoO_15−δ ranging from 5.9×10⁻⁵ (S cm⁻¹) at 300 °C to 1.3×10⁻² (S cm⁻¹) at 650 °C, respectively. The oxide-ion conductivities remained stable over 80 h-long test at 800 °C. These properties suggested that significant oxide-ionic conductivity exists in these materials at moderately elevated temperatures.     

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

Compositional behavior of Urbach absorption edge is studied as well as the effect of compositional disordering on the parameters of exciton-phonon interaction, phase transition temperatures and electric conductivity in Cu₆P(S_1−xSe_x)₅Br_1−yI_y superionic solid solutions. The effect of different types of disordering on the optical absorption processes and specific features of compositional changes in the absorption edge spectra under S→Se and Br→I anion substitution in the mixed crystals are investigated. (x, T) phase diagrams for Cu₆P(S_1−xSe_x)₅X (X=I, Br) solid solutions are studied.