Thermally activated negative photoconductivity below 6 K in <Emphasis Type="Italic">p</Emphasis>-GaAs/Al<Subscript>0.5</Subscript>Ga<Subscript>0.5</Subscript>As heterostructures and the effect of uniaxial compression

Thermally activated negative photoconductivity is observed in p-GaAs/Al_0.5Ga_0.5As:Be heterostructures under illumination with red light at temperatures below 6 K. As the temperature decreases, the concentration and mobility of 2D holes in the quantum well drop sharply, particularly under uniaxial compression. The phenomenon is quantitatively described under the assumption that a layer of deep donor-like traps with a low thermal activation barrier E_B = 3.0 ± 0.5 meV exists at a distance of about 7 nm from the heterojunction and that this barrier does not change with strain. Presumably, the traps may be the p-type dopant Be atoms diffusing from the active layer and occupying interstitial positions.     

Modification of the energy spectrum and magnetic breakdown in a system of 2D holes at the GaAs/Al<Subscript>0.5</Subscript> Ga<Subscript>0.5</Subscript>As heterojunction upon uniaxial compression

Uniaxial compression of the p-type GaAs/Al_0.5Ga_0.5 As heterostructures induces magnetic breakdown between the spin-split ground-state subbands of two-dimensional heavy holes. This phenomenon serves as direct experimental evidence of a strong qualitative modification of the energy spectrum of these structures upon uniaxial deformation. This modification has been revealed by numerical calculations, and, according to it, the subband spin splitting decreases upon compression, while the contours of the hole Fermi surface in both subbands touch one another in the compression direction in a pressure range of 2.5 kbar.     

High temperature dependence of the density of two-dimensional electron gas in Al<Subscript>0.18</Subscript>Ga<Subscript>0.82</Subscript>N/GaN heterostructures

Temperature dependence of the density of two-dimensional electron gas (2DEG) in Al_0.18Ga_0.82N/GaN heterostructures has been investigated by means of high temperature Hall measurements ranging from room temperature to 500 °C. It is found that the 2DEG density decreases with increasing temperature in the range from room temperature to 250 °C, and then increases with the temperature above 250 °C. It is thought that the decrease of the 2DEG density from room temperature to 250 °C is caused by the reduction of the conduction band offset at high temperatures. The increase of measured 2DEG density at higher temperatures is attributed to the background electron concentration in the GaN layer. Theoretical calculation of the 2DEG density in Al_0.18Ga_0.82N/GaN heterostructures at various temperatures is consistent with the experimental results using the multilayer Hall effect model. PACS 73.40.Kp; 73.61.Ey     

Photoluminescence and persistent photoconductivity of Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>N/GaN heterostructures

We have investigated the optical properties of Al_xGa_1-xN/GaN heterostructures (x=0.08, 0.15, 0.33) grown by metal organic chemical vapor deposition on sapphire using photoluminescence (PL) and persistent photoconductivity (PPC) measurements. For the Al_xGa_1-xN/GaN heterostructures (HS) containing high Al composition, we observed an anomalous temperature-dependent photoluminescence and persistent photoconductivity effects. These results show a strong dependence of the physical properties of Al_xGa_1-xN/GaN HS on the Al content and layer thickness. The anomalous temperature-dependent PL is usually attributed to the presence of carrier localization states. These phenomena are explained based on the alloy compositional fluctuations in the Al_xGa_1-xN/GaN HS. From the PPC measurements, the photocurrent (PC) quenching was observed for Al_xGa_1-xN/GaN HS and it is explained by the metastable states formed in the underlying GaN layer. Also, the mechanisms behind the PC quenching and PPC phenomena are explained in detail. PACS 72.20.Jv; 72.40.+w; 78.55.Cr     

Polarization effects in quantum-well In<Subscript>28</Subscript>Ga<Subscript>72</Subscript>As/GaAs heterolasers

The effect of externally introduced variable strains on the polarization properties of quantum-well In₂₈Ga₇₂As/GaAs laser radiation at room temperature is studied experimentally and theoretically. An analysis of the polarization effects at various values of the excess of the working current over the threshold is performed. Data on the energy for the splitting of the levels of light and heavy holes in the quantum well of the structure under consideration are obtained. It is experimentally proven that the effectiveness of the action of a variable strain on the polarization twist substantially increases with increasing quantum well width.     

Contactless gating,surface charging and illumination effects in a buried Al<Subscript>0.24</Subscript>Ga<Subscript>0.76</Subscript>As/GaAs quantum well structure

The conductivity of an Al_0.24Ga_0.76As/GaAs quantum well was studied as a function of the surface charge generated by electron bombardment of the sample in the absence of an externally applied surface electric field. Under a suitable rate of electron irradiation, it was possible to completely shut off the conductive channel, implying a surface density . Light illumination quenches the increase of the resistivity, apparently due to photoemission from the metastable surface states. Upon turning off the electron bombardment the surface charge on adsorbed layers of xenon and water at 8 K decays in room temperature darkness with a lifetime τ= 0.30 ±0.02 s. The average charging efficiency, is μ₀ ≃0.001. Surface charging is shown to be an effective method for contactless gating of field effect devices.     

Effect of interdiffusion of In and Al atoms on excitonic states in In<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>As/Al<Subscript>y</Subscript>Ga<Subscript>1−y</Subscript>As quantum dots

The effect of interdiffusion of aluminum and indium atoms on the exciton emission energy and binding energy in In_xGa_1?xAs/Al_yGa_1?yAs quantum dots is studied. It is shown that the emission energy increases monotonically with increasing diffusion length, while the binding energy has a maximum.     

Electron mobility for a model Al<Subscript>x</Subscript> Ga<Subscript>1-x</Subscript> As/GaAs heterojunction under pressure

A variational method and a memory function approach are adopted to investigate the electron mobility parallel to the interface for a model Al_xGa_1-xAs/GaAs heterojunction and its pressure effect by considering optical phonon modes (including both of the bulk longitudinal optical (LO) in the channel side and interface optical (IO) phonons). The influence of a realistic interface heterojunction potential with a finite barrier and conduction band bending are taken into account. The properties of electron mobility versus Al concentration, electronic density and pressure are given and discussed, respectively. The results show that the electron mobility increases with Al concentration and electronic density, whereas decreases with pressure from 0 to 40 kbar obviously. The Al concentration dependent and the electron density dependent contributions to the electron mobility from the scattering of IO phonons under pressure becomes more obvious. The variation of electron mobility with the Al concentration and electron density are dominated by the properties of IO and LO phonons, respectively. The effect of IO phonon modes can not be neglected especially for higher pressure and electronic density.     

Solar-blind MSM-photodetectors based on Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>N heterostructures

Solar-blind MSM photodetectors based on the AlGaN heterostructures have been fabricated and investigated. The influence of material properties on device parameters is discussed. Effect of different buffer layers on the detector performances has been examined. Detectors exhibit low dark currents and high sensitivity within the range of 250–290 nm. Effect of optical excitation energy on GaN-based MSM-detector performance is analyzed and discussed. At high excitation level the detector speed of response is limited by the field screening caused by the space-charge of the holes. The impulse response of GaN-based MSM-detector is compared favorably with GaAs MSM-device.     

Sacrificial etching of Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>As for III–V MEMS surface micromachining

A study of Al_xGa_1-xAs as a sacrificial film for surface micromachining is presented. Al_xGa_1-xAs etch rate and selectivity are measured over a range of aluminum mole fractions and HF etchant concentrations during the release of structural features up to 500 μm in width. The etch process is found to be diffusion limited, with an inverse power law relationship between etch depth and etch rate. Excellent selectivity greater than 10⁵ is achieved between sacrificial AlAs and structural GaAs, even for long etches up to 250 μm in length. Compared with previous studies of Al_xGa_1-xAs etching for epitaxial liftoff processing, measured etch rates for surface micromachining are approximately an order of magnitude lower, primarily due to the longer effective etch lengths required. However, unlike epitaxial liftoff, Al_xGa_1-xAs surface micromachining is compatible with higher HF concentrations which can provide comparable overall etch rates, with important implications for AlGaAs MEMS fabrication. PACS 81.05.Ea; 85.85.+j     

On the correlation between supercooling,superheating and kinetic arrest in a magnetic glass Pr<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>Mn<Subscript>0.975</Subscript>Al<Subscript>0.025</Subscript>O<Subscript>3</Subscript>

We report a quantitative investigation of the magnetic field-temperature phase diagram by taking into account a simple phenomenological model arising out of the interplay of kinetic arrest and thermodynamic transitions in a magnetic glass Pr_0.5Ca_0.5Mn_0.975Al_0.025O₃, through magnetization measurements. Such studies are necessary as kinetic arrest plays an important role in the formation of “magnetic glasses”, which has been observed in systems undergoing first order magnetic phase transitions. It has been shown that disorder in a system results in the formation kinetic arrest (H _K ,T _K ) band, like supercooling (H ^*,T ^*) and superheating (H ^**,T ^**) band. Quantitative proofs are given to show that (H _K ,T _K ) band is anticorrelated with (H ^*,T ^*) and (H ^**,T ^**) bands, while the later two are correlated among themselves. Analysis of time dependence of magnetization at different temperatures is carried out to establish the fact that the kinetic arrested state is different from the supercooled state.     

Influence of Al-concentration on the current density in GaAs/Al<Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>Ga<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>As generalized Thue-Morse superlattices

The influence of Al concentration on the current density in GaAs-Al _c Ga_{1- c} As heterostructure is studied for two different classes of the Generalized Thue-Morse superlattice (GTS): (i) width-barrier GTS and (ii) height-barrier GTS. The occurrence of resonances in the transmission is found to be highly dependent on the Al concentration as well as on the degree of quasi-periodicity of the system. Interesting features are noted for the current density profile with the increase in the Al content for the width-barrier case. The use of the height-barrier GTS with small Al content could be suggested to achieve the negative differential conducting regimes at comparatively low applied bias. The effects of the applied field as well as of the quasi-periodicity on the carrier localization are also studied.     

Theoretical study on electronic and optical properties of Zn-doped In<Subscript>0.25</Subscript>Ga<Subscript>0.75</Subscript>As photocathodes

First-principles calculations for intrinsic and Zn-doped In_0.25Ga_0.75As are performed based on density functional theory to study the influence of Zn doping on electronic and optical properties. The band structure, density of state, Mulliken population, dielectric function, complex refractive index, absorption coefficient and reflectivity of In_0.25Ga_0.75As are calculated. Results show that the Fermi levels of two Zn-doped models enter into the valence bands and Zn atom is more easily to replace In atom than Ga atom. The lattice constant of In_0.2344Ga_0.75Zn_0.0156As reduced after optimization, while that of In_0.25Ga_0.7344Zn_0.0156As increased to the contrary. The Mulliken bond population shows that the doping Zn atoms can enlarge the strength of In–As and Ga–As polar covalent bonds. Furthermore, the calculated absorption coefficient and reflectivity are used to characterize the performance of photoemission, indicates that the photoresponses of Zn-doped models are better than that of the intrinsic.     

Comparison between Si/SiO<Subscript>2</Subscript> and InP/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> based MOSFETs

The discrete breathers in graphane in thermodynamic equilibrium in the temperature range 50–600 K are studied by molecular dynamics simulation. A discrete breather is a hydrogen atom vibrating along the normal to a sheet of graphane at a high amplitude. As was found earlier, the lifetime of a discrete breather at zero temperature corresponds to several tens of thousands of vibrations. The effect of temperature on the decay time of discrete breathers and the probability of their detachment from a sheet of graphane are studied in this work. It is shown that closely spaced breathers can exchange energy with each other at zero temperature. The data obtained suggest that thermally activated discrete breathers can be involved in the dehydrogenation of graphane, which is important for hydrogen energetics.     

Diamagnetic susceptibility of hydrogenic donor impurity in a V-groove GaAs/Ga<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As quantum wire

In this work, the diamagnetic susceptibility and the bindingenergy of a hydrogenic donor impurity both in the parabolic andnon-parabolic conduction band models have been calculated withinthe effective mass approximation for a V-grooveGaAs/Ga_{1- x} Al _x As quantum wire. According to the resultsobtained from the present work reveals that (i) the value ofdiamagnetic susceptibility due to the non-parabolicity effect ishigher than that of parabolicity effect; (ii) the values ofdiamagnetic susceptibility and binding energy due to thenon-parabolicity effect is not appreciable at low Al molefractions; (iii) the diamagnetic susceptibility approaches to thebulk value both in L \(\rightarrow\) 0 or L \(\rightarrow\) ∞; (iv)the effect of non-parabolocity is not appreciable in the bindingenergy and energy dependent effective mass, for energies lowerthan 50 MeV.     

Gas sensor based on nanosize In<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> film

Nanosize films of In₂O₃:Ga₂O₃ (96:4 weight %) have been deposited on a glassceramic substrate by the method of rf magnetron sputtering. The surfaces of fabricated films were studied with use of a scanning electron microscope; sizes of grains were determined and the thicknesses of films were measured. In order to prepare a gas-sensitive structure, a thin catalytic palladium layer and ohmic comb contacts were deposited on the In₂O₃:Ga₂O₃ film surface by the method of ion-plasma sputtering. The sensitivity of sensors based on the glassceramic/In₂O₃:Ga₂O₃ (96:4 weight %)/Pd structure to different concentrations of propane and butane gas mixture, as well as to methane was investigated at temperatures of working substance from 250 to 300°C.     

Magnetic properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.47</Subscript>Al<Subscript>0.03</Subscript>O<Subscript>2</Subscript> as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.47Al_0.03O₂ was synthesized by wet chemical method and characterized by X-ray diffraction and analysis of magnetic measurements. The powders adopted the α-NaFeO₂ structure. This substitution of Al for Mn promotes the formation of Li(Ni_0.47²⁺Ni_0.03³⁺Mn_0.47⁴⁺Al_0.03³⁺)O₂ structures and induces an increase in the average oxidation state of Ni, thereby leading to the shrinkage of the lattice unit cell. The concentration of antisite defects in which Ni²⁺ occupies the (3a) Li lattice sites in the Wyckoff notation has been estimated from the ferromagnetic Ni²⁺(3a)–Mn⁴⁺(3b) pairing observed below 140 K. The substitution of 3% Al for Mn reduces the amount of antisite defects from 7% to 6.4–6.5%. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie–Weiss law agrees well with the combination of Ni²⁺ (S = 1), Ni³⁺ (S = 1/2) and Mn⁴⁺ (S = 3/2) spin-only values. Delithiation has been made by the use of K₂S₂O₈. According to this process, known to be softer than the electrochemical one, the nickel ions in the (3b) sites are converted into Ni⁴⁺ in the high spin configuration, while Ni²⁺(3a)–Mn⁴⁺(3b) ferromagnetic pairs remain, as the Li⁺(3b) ions linked to the Ni²⁺(3a) ions in the antisite defects are not removed. The results show that the antisite defect is surrounded by Mn⁴⁺ ions, implying the nonuniform distribution of the cations in agreement with previous NMR and neutron experiments.     

Low-temperature structural anomalies in Pr<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript>

The magnetic and crystal structures of the Pr_0.5Sr_0.5CoO₃ metallic ferromagnet have been studied by the neutron diffraction technique. It is demonstrated that below 150 K, the compound is mesoscopically separated into two crystalline phases with different spatial symmetries and with different directions of the magnetic anisotropy. The phase separation exists down to 1.5 K, and at temperatures below 90 K, the low-symmetry phase occupies about 80% of the sample volume. The main structural difference between the phases is the configuration of oxygen atoms around praseodymium and, to a certain extent, around cobalt. The ferromagnetic structure with the magnetic moment lying in the basal plane of the structure (μ_Co ≈ 1.7 μ _B at 1.5 K) arises at 234 K, whereas the component directed along the long axis of the unit cell appears at 130 K. The formation of the new structural phase and change in the orientation of the magnetic moment give rise to the anomalies of the physical and magnetic characteristics of this compound observed earlier at temperatures about 120 K.     

Electronic and structural properties of zincblende Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>N

Numerical calculations based on first-principles are applied to study the electronic and structural properties of zincblende Al_xGa_1-xN. The results show that the lattice constant has a very small deviation from the linear Vegard’s law. The direct and indirect bowing parameters of 0.295±0.034 eV and -0.125±0.060 eV are obtained, respectively, and there is a direct-indirect crossover near x=0.692. Besides, the bulk moduli and their pressure derivatives are monotonically increased with an increase of the aluminum composition x. The deviation parameter of the bulk modulus of -5.32±1.60 GPa is obtained. PACS 71.15.-m; 71.15.Nc; 71.55.Eq; 71.20.Nr; 42.70.Qs     

Geometries and electronic structures of Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript>, Ga<Subscript>3</Subscript>Se<Subscript>2</Subscript> and their anions. Theoretical insights

We hereby report a theoretical study on the equilibrium geometries, electronic structures and harmonic vibrational frequencies of Ga₂Se₃, Ga₃Se₂ and their anions. The ground and low-lying excited states of Ga₂Se₃⁻, Ga₂Se₃, Ga₃Se₂⁻ and Ga₃Se₂ are studied at the B3LYP and/or MP2 and CCSD(T) levels in conjunction with 6-311+G(d) and 6-311+G(2df) one particle basis sets. Ga₂Se₂⁻ adopts the C_2v kite geometry while Ga₂Se₃ has a ‘V’ geometry. Ga₃Se₂⁻ has a three-dimensional ‘D_3h ’ geometry and Ga₃Se₂ prefers the three-dimensional ‘C_2v ’ structure. Electron detachment energies from the ground electronic states of the anions to several neutral states are reported and discussed. At CCSD(T)//MP2 level, the adiabatic electron affinity (AEA) of Ga₂Se₃ is calculated to be 3.23 eV when using the 6-311+G(2df) basis set and that of Ga₃Se₂ is 2.77 eV with the 6-311+G(d) basis set. The findings of this research are analyzed and compared with gallium oxide and sulfide analogues.