Luminescent properties of BexCd1−xSe thin films

We report photoluminescence (PL) study of Be_xCd_1−xSe epitaxial layers (x<0.21) grown by molecular beam epitaxy on InP substrates. Continuous wave PL spectra are taken within a 4.2-300 K temperature range. We observe an anomalous ‘s-shaped’ temperature dependence of emission energy and a severe decrease of emission intensity with the increase of temperature. We explain an ‘s-shaped’ temperature dependence of emission energy by exciton localization in the potential minima at low temperatures followed by thermal activation at higher temperatures. We attribute low emission intensity at high temperatures to exciton dissociation and electron/hole migration to non-radiative recombination centers.     

Spin transport properties in double-barrier systems with diluted magnetic semiconductor doped layers

We have studied spin-polarized currents in resonant tunneling diodes containing diluted magnetic semiconductor layers with different geometries. The spin polarization effects on the resonant current are studied within Green's function formalism following the diagrammatic technique for non-equilibrium processes as proposed by Keldysh, using the one-band tight-binding modeling. The magnetic impurities can be located inside the well or the barrier layers of a diode sample. The resonant I×V curves are analyzed as a function of the magnetic potential strength induced by the magnetic ions.     

Application of micro-Raman spectroscopy for the evaluation of doping profile in Zn δ-doped GaAs structures

Micro-Raman spectroscopy was used to characterize beveled Zn delta (δ)-doped GaAs structures. By adapting procedures previously developed for the study of Si δ-doped GaAs structures, Zn-doping profiles were obtained for a set of structures prepared with different doping levels. Values of the doping spike concentration and the full-width at half-maximum of the doping profile were compared with the values obtained by the electrochemical capacitance-voltage (EC-V) and secondary ion mass spectroscopy (SIMS) methods. The good correspondence between this Raman procedure and other well-known methods proves the validity of the technique for determining doping profiles in Zn δ-doped GaAs structures.     

Spin polarization in multilayer ferromagnetic semiconductor

We focus on transport of electron spins, which spin-polarized currents can be controlled and manipulated via the electron energy and momentum. We study in this paper the electronic properties of ferromagnetic phase of a multilayer ferromagnetic semiconductor in the mean-field and effective mass approximations, as a result of the magnetic interaction between holes and Mn ions, the magnetic layers acts as potential barriers for holes with spin-up, and as potential wells for the inverse spin polarization. As an example we calculate the dependence of hole density and the spin polarization in terms of the band offset v_w which describes the difference in electronegativity between the Mn and GaAs atoms. Our calculations are performed using a self-consistent procedure to solve Schrödinguer and Poisson equations taking into account the coulomb interaction between holes.     

High-k TixSi1−xO2 thin films prepared by co-sputtering method

We report on high-k Ti_xSi_1−xO₂ thin films prepared by RF magnetron co-sputtering using TiO₂ and SiO₂ targets at room temperature. The Ti_xSi_1−xO₂ thin films exhibited an amorphous structure with nanocrystalline grains of 3-30 nm having no interfacial layers. The XPS analyses indicate that stoichiometric TiO₂ phases in the Ti_xSi_1−xO₂ films increased due to stronger Ti-O bond with increasing TiO₂ RF powers. In addition, the electrical properties of the Ti_xSi_1−xO₂ films became better with increasing TiO₂ RF powers, from which the maximum value of the dielectric constant was estimated to be ∼30 for the samples with TiO₂ RF powers of 200 and 250 W. The transmittance of the Ti_xSi_1−xO₂ films was above 95% with optical bandgap energies of 4.1-4.2 eV. These results demonstrate a potential that the Ti_xSi_1−xO₂ thin films were applied to a high-k gate dielectric in transparent thin film transistors as well as metal-oxide-semiconductor field-effect transistors.     

Ondulating 2D waveguides. Dynamical and transport properties

We study the transport of non-interaction particles traveling in 2D waveguides, periodic in one direction. In particular, we study waveguides with smooth profiles yielding generic Hamiltonian chaos. We compute their diffusion σ²(t) and the time evolution of particle density. We find normal and super-diffusive behavior depending on the underlying dynamics. We compare our results with those of the Lorentz channel, the serpent billiard, and polygonal billiards.     

Parameter-dependent third-order nonlinear susceptibility of parabolic InGaN/GaN quantum dots

The electron states confined in wurtzite In_xGa_1−xN/GaN-strained quantum dots (QDs) have been investigated in the effective-mass approximation by solving the Schrödinger equation, in which parabolic confined potential and strong built-in electric field effect due to the piezoelectricity and spontaneous polarization have been taken into account. The third-order nonlinear susceptibility of the QDs in various directions (both parallel to z direction and vertical to z direction) have been calculated, and the magnitude reaches 10⁻¹⁴ m²/V². It has been shown from the results that the order of the built-in electric field in the strained QD is of MV/cm. Furthermore, the results of how the third-order nonlinear susceptibility depend on the radius R of QDs, the height L of QDs, the In content x of QDs and the relaxation rate Γ₁₀ have been given.     

Energy levels of a quantum ring in a lateral electric field

The electronic states of a semiconductor quantum ring (QR) under an applied lateral electric field are theoretically investigated and compared with those of a quantum disk of the same size. The eigenstates and eigenvalues of the Hamiltonian are obtained from a direct matrix diagonalization scheme. Numerical calculations are performed for a hard-wall confinement potential and the electronic states are obtained as a function of the electric field and the ratio r₂/r₁, where r₂ (r₁) is the outer (inner) radius of the ring. The effects of decreasing symmetry and mixing on the energy levels and wave functions due to the applied electric field are also studied. The direct optical absorption are reported as a function of the electric field.     

A study of the rectifying behaviour of aniline green-based Schottky diode

An Al/aniline green (AG)/Ga₂Te₃ device was fabricated and the current-voltage (I-V), capacitance-voltage (C-V) and capacitance-frequency (C-f) characteristics of the device have been investigated at room temperature. The values of the ideality factor, series resistance and barrier height obtained from Cheung and Norde methods were compared, and it was seen that there was an agreement with each other. It was seen that the forward bias current-voltage characteristics at sufficiently large voltages has shown the effect of the series resistance. In addition, it was seen from the C-f characteristics that the values of capacitance have been decreased towards to the high frequencies. The higher values of capacitance at low frequencies were attributed to the excess capacitance resulting from the interface states in equilibrium with the Ga₂Te₃ that can follow the ac signal.     

Phase separation, effects of biaxial strain, and ordered phase formations in cubic nitride alloys

The thermodynamics as well as the energetics and the structural properties of cubic group-III nitrides alloys have been investigated by combining first-principles total energy calculations and cluster expansion methods. In particular results are shown for the ternary In_xGa_1−xN and the quaternary Al_xGa_yIn_1−x−yN alloys. Phase separation is predicted to occur at growth temperatures, for both fully relaxed alloys. A remarkable influence of an external biaxial strain on the phase separation, with the formation of ordered phase structures has been found for the InGaN alloy. These findings are used to clarify the origin of the light emission process in InGaN-based optoelectronic devices. Results are shown for the composition dependence of the lattice constant and of the energy gap in quaternary Al_xGa_yIn_1−x−yN alloys.     

Comparative study of frame-compatible stereo 3D services and a novel method for spatial interleaving using HEVC

There are a number of ways to realize frame-compatible stereo 3D services, which includes the spatial and temporal interleaving based methods. We first present the detailed analysis of these methods with respect to compression efficiency, backward compatibility and the ability to re-use existing infrastructure. Simulations have been set up to benchmark the Rate–Distortion (R–D) performance of these options with the state-of-the-art Multiview Video Coding (MVC) and results show that temporal interleaving based method performs better when compared to spatial interleaving in both H.264/AVC as well as in HEVC compression schemes, with MVC still outperforms both the methods. In the case of the spatial interleaving method using HEVC, we propose to encapsulate the two views in different tiles with filters turned off across the tile boundary. Simulation results show that the proposed method, when compared to “no-tiles” case, improves decoder performance by approximately 50% with no reduction in R–D performance.     

The electrical characteristics of Sn/methyl-red/p-type Si/Al contacts

The junction characteristics of the organic compound methyl-red film (2-[4-(dimethylamino)phenylazo]benzoic acid) on a p-type Si substrate have been studied. The current-voltage characteristics of the device have rectifying behavior with a potential barrier formed at the interface. The barrier height and ideality factor values of 0.73 eV and 3.22 for the structure have been obtained from the forward bias current-voltage (I-V) characteristics. The interface state energy distribution and their relaxation time have ranged from 1.68 × 10¹² cm⁻² eV⁻¹ and 1.68 × 10⁻³ s in (0.73-E_v) eV to 1.80 × 10¹² cm⁻² eV⁻¹ and 5.29 × 10⁻⁵ s in (0.43-E_v) eV, respectively, from the forward bias capacitance-frequency and conductance-frequency characteristics. Furthermore, the relaxation time of the interface states shows an exponential rise with bias from (0.43-E_v) eV towards (0.73-E_v) eV.     

Tight-binding study of the optical properties of GaN/AlN polar and nonpolar quantum wells

The electronic structure of wurtzite semiconductor superlattices (SLs) and quantum wells (QWs) is calculated by using the empirical tight-binding method. The basis used consists of four orbitals per atom (sp³ model), and the calculations include the spin-orbit coupling as well as the strain and electric polarization effects. We focus our study on GaN/AlN QWs wells grown both in polar (C) and nonpolar (A) directions. The band structure, wave functions and optical absorption spectrum are obtained and compared for both cases.     

Contact potential of p-Al0.5Ga0.5As/n-GaAs structures

The built-in potential of Al_0.5Ga_0.5As (10¹⁸ cm^?3) gates on n-type (10¹⁷ cm^?3) GaAs channel layers in the case of heterojunction normally-off FETs has been measured via C-V and forward I-V methods. A built-in potential of 1.39 eV from C-V measurements and 1.36 eV from forward I-V characteristics which compare well with the theory (1.4 eV) have been deduced. The ideality coefficient is found to be 1.56.     

Tailoring the electronic properties of zigzag graphene nanoribbons via sp2/sp3 edge functionalization with H/F

Functionalizing the edges of graphene nanoribbons (GNR) plays a vital role to alter their electronic properties. In present work, the effect of sp²∕sp³ edge functionalization of zigzag GNR (ZGNR) via H/F is investigated to reveal their structural stability and spin dependent electronic properties. Depending upon the way of functionalization, nanoribbons under considerations are categorized in three groups viz. Group A: one edge is sp² functionalized while other edge is sp³ functionalized, Group B: both the edges are sp³ functionalized and Group C: both edges are sp³ functionalized, however, number of H and F atoms are different. All the group A and group C structures settled in magnetic ground state while group B nanoribbons are all nonmagnetic except F₂-ZGNR-H₂ which prefers ferromagnetic state. Further, it is noticed that F functionalization enhances the stability and is equivalent to p-type doping of nanoribbon along with affecting the electronic properties significantly. Interestingly, for selective structures, magnetic ground state has been obtained and the observed magnetic stabilization is found to be greater than room temperature thermal excitations which ensures the potential of considered ZGNR for practical applications. Present findings provide a viable way for tailoring the spintronic properties of ZGNR and absolute shifting of Fermi level which can play a crucial role for designing future nano-devices.     

Impurities in elliptical quantum corrals

Elliptical quantum corrals present interesting properties due to the combination of confinement and focalizing properties. We show here that two magnetic impurities located at the foci of the systems experience an enhanced interaction, as compared to the one they would have in an open surface. These impurities interact via a superexchange AF interaction J with the surface electrons in the ellipse. For small J they are locked in a singlet state, which weakens for larger values of this parameter. When J is much larger than the hopping parameter of the electrons in the ellipse, both spins decorrelate and form a singlet with the nearest electron, thus presenting a confined RKKY-Kondo transition. We can also interpret this behavior via the entanglement or von Neumann entropy between the localized impurities and the itinerant electrons of the ellipse: for small J the entropy is nearly zero while for large J it is maximum.     

The role of the interface insulator layer and interface states on the current-transport mechanism of Schottky diodes in wide temperature range

In order to interpret in detail the experimentally observed current-voltage-temperature (I-V-T) and capacitance-voltage-temperature (C-V-T) results of Al/p-Si metal-semiconductor Schottky barrier diodes (SBDs) we have been examined the samples in the temperature range of 150-375 K. In the calculation method, to confirm the relationship between the I-V-T and C-V-T results, we have reported a modification which includes the ideality factor, n, and tunnelling parameter δχ^1/2 in the forward bias current characteristics. In the intermediate bias voltage region (0.1 < V < 0.6 V), the semi-logarithmic plots of the forward I-V-T curves were found to be linear. From the reverse saturation currents I₀ obtained by extrapolating the linear region of curves to zero applied voltage, the values of zero bias barrier heights ?_B0 were calculated at each temperature. The values of ideality factor calculated from the slope of each curves were plotted as a function of temperature. The values of n are 3.41-1.40 indicating that the Al/p-Si diode does obey the thermionic field emission (TFE) mechanism rather than the other transport mechanism, particularly at low temperature. The high value of ideality factors is attributed to high density of interface states in the SBDs. The temperature dependence energy density distribution profile of interface state was obtained from the forward bias I-V-T measurements by taking into account the bias dependence of the effective barrier height and ideality factor. The interface states density N_ss decreasing with increasing temperature was interpreted by the result of atomic restructuring and reordering at the metal-semiconductor interface. After the modification was made to the forward current expression, we obtained a good agreement between the values of barrier height obtained from both methods over a wide temperature.     

Dielectric mismatch effect on coupled impurity states in a freestanding nanowire

We studied the coupled impurity states in a freestanding semiconductor nanowire (NW), within the effective mass approximation and including the effect of the dielectric mismatch, by using finite element method. Bonding and anti-bonding states are found and their energies converge with increasing distance d_i between the two impurities. The dependence of the binding energy on the wire radius R and the distance d_i between the two impurities is investigated, and we compare it with the result of a freestanding NW that contains a single impurity.     

Optical properties of supercrystals

We present a systematic study of the dependence of the optical properties on the geometry of the quantum dots, which form a quantum dots chain. We obtain electronic structure of 1D supercrystals by using a tight-binding method based on the s and p orbitals and wave functions of the quantum dots for the on-site parameters. For the hopping parameter we choose the extended Hückel method. We find a drastic dependence on this parameter especially on the electronic band structure. We also relate the tight-binding matrix elements with the momentum matrix elements taking the k-space gradient of the Hamiltonian for calculating the dielectric spectra of supercrystals made of cylindrical, spherical, and conical quantum dots. We compare the optical behavior of the chains in the three geometries and report anomalies for the conical quantum dots chain.     

Comparative studies on electronic transport due to the reduced dimensionality at the heterojunctions of GaAs/A1xGa(1−x)As and GaxIn(1−x)As/InP systems at low temperatures

Comparative studies on ac/dc mobility due to the reduced dimensionality of spatially confined low dimensional systems, at the heterojunctions of GaAs/A1_xGa_(1−x)As and Ga_xIn_(1−x)As/InP forming quasi-two dimensional (Q2D) and quasi-one dimensional (Q1D) systems have been made. The effect of various low temperature nonphonon scattering mechanisms such as ionized impurity, alloy disorder scattering and surface roughness scattering mechanisms; and phonon scattering mechanisms such as acoustic phonon via deformation potential and piezoelectric scattering mechanisms on the systems has been studied. It is found that the surface roughness scattering mechanism dominates in Q2D system whereas acoustic phonon scattering mechanism dominates in Q1D system due to which the nature and magnitude of the temperature dependent dc/ac mobility curves shows significant variation. Whereas, it is observed that the confinement does not change the nature of the frequency dependent real and imaginary parts of ac mobility curves. However, the mobility is found to be enhanced with effective mass and also due to the confinement, i.e. the mobility for Q1D system is higher than that for Q2D system.