Photoluminescence of i-GaxIn1 ? xAs/n-GaAs heterostructures containing a random InAs quantum dot array

i-Ga _x In_{1 − x} As/n-GaAs heterostructures containing InAs quantum dots have been grown by ion beam deposition and their photoluminescence has been studied. The photoluminescence spectrum of the heterostructures contains three characteristic peaks. The peak at 1.1 eV has a large width and is due to the array of InAs quantum dots of different sizes, randomly arranged on the surface of the i-Ga _x In_{1 − x} As layer. From the position of another peak (hν = 0.94 eV), we have evaluated the composition of the Ga _x In_{1 − x} As solid solution: x = 0.64. The third photoluminescence peak corresponds to the intrinsic absorption edge of the n-GaAs substrate. The key features of luminescence photoexcitation in the heterostructures are discussed. We show that the Ga_0.64In_0.36As quantum well may accumulate not only photogenerated electrons but also electrons that come from a thin interfacial n-GaAs layer through ballistic transport.     

The effect of hydrogenation on the photoconductivity of ion-doped gallium arsenide structures

It was found that the hydrogenation of ion-doped n ⁺-n-n _i -GaAs structures in atomic hydrogen increases the rate of conductivity relaxation in the doped layers, decreases the effect of bias voltage applied to the n ⁺-GaAs layer on the photoconductivity, and improves characteristics of the Schottky barrier transistors and the related integrated circuits. These phenomena are probably due to the formation of hydrogen complexes with electrically active centers in GaAs.     

Characteristics of surface states at the insulator-semiconductor interface of ZnS:Mn thin-film electroluminescent emitters

The energy distribution of the density of occupied surface states (N _ss) at the cathode insulatorphosphor interface in ZnS:Mn electroluminescent thin-film (ELTF) emitters has been modeled on the basis of experimental data. Changes in this distribution depending on the parameters of exciting voltage pulses have been studied. It is established that the energy distribution of N _ss shifts toward deeper levels upon a decrease in the frequency of the exciting signal and the resulting increase in the pause between the adjacent switch-on states. This behavior corresponds to a cascade relaxation mechanism of electrons trapped on the surface states. Maximum values of the N _ss (∼2.5 × 10¹³ cm⁻²) and the specific density of surface states per unit energy (2 × 10¹⁴–10¹⁵ cm⁻² eV⁻¹) are determined for the cathode insulator-phosphor interface from which electrons are tunneling. Positions of the equilibrium (∼1.25 eV below the conduction-band bottom) and the quasi-equilibrium (0.6–1.25 eV) Fermi levels during the ELTF emitter operation are estimated.     

On Jonckheere's k-Sample Test Against Ordered Alternatives

In this paper a k-sample non-parametric test for trend is considered. Given a sample of size n_i , i = 1, …, k respectively from each of k populations, the test rejects the hypothesis that the k populations are identical if S = Σ ^k _i=2 S_i ≥ S_i . Here Si is the Mann-Whitney statistic computed when each observation in the i-th sample is compared with the combined observations from the first (i – 1) populations. A recurrence formula is derived for computing the exact distribution of S. Tables of exact probabilities and critical values are given for nominal values of α = 0.5, 0.2, 0.1, 0.05, 0.025, 0.01, and 0.005 for k = 3 and all possible sample sizes from 2 to 8, and for equal sample sizes for values of n = 2(1)6, k = 4(1)6.     

Electrical characteristics of Au/n-GaAs Schottky barrier diodes with and without SiO2 insulator layer at room temperature

In this study, our main goal is fabricated with and without insulator layer Au/n-GaAs Schottky barrier diodes (SBDs) to explain whether or not the insulator layer is effective on some electric parameters such as Φ_B, n, N_ss, and R_s. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of metal-semiconductor (Au/n-GaAs) are investigated and compared with metal-insulator-semiconductor (Au/SiO₂/n-GaAs) Schottky diodes. From the room temperature I-V characteristics of these devices, the main electrical parameters such as, ideality factor (n) and zero bias barrier height (Φ_bo) values of 1.25 and 0.73 eV for Au/n-GaAs, and 1.51 and 0.75 eV for Au/SiO₂/n-GaAs, were obtained. The interface distribution profile (N_ss) as a function of (E_c − E_ss) was extracted from the forward-bias I-V measurements by taking into account the bias dependence of the effective barrier height (Φ_e) and series resistance (R_s) for the Schottky diodes. The N_ss values obtained taking into account the series resistance values are lower than those obtained without considering the series resistance. The diodes show non-ideal I-V behavior with ideality factor greater than unity. This behavior is attributed to the interfacial insulator layer and the interface states. The I-V characteristics confirmed that the distribution of N_ss, R_s, and interfacial insulator layer are important parameters that influence the electrical characteristics of metal-semiconductor and metal-insulator-semiconductor Schottky diodes.     

Microwave-stimulated relaxation of internal strains in GaAs-based device heterostructures

We have studied the effect of microwave radiation (frequency, 2.45 GHz; specific power density, 1.5 W/cm²) on the relaxation of internal mechanical strains in the (i)n-n ⁺-GaAs structures, (ii) Au-Ti-n-n ⁺-GaAs diode structures with Schottky barriers, and (iii) GaAs-based Schottky-barrier field-effect transistors (SFETs). It is shown that exposure of the samples to the microwave radiation for a few seconds leads to relaxation of the internal mechanical strains and improves the quality of the semiconductor surface layer structure. This results in improved parameters of the GaAs-based device structures of both (diode and SFET) types, as manifested by increased Schottky barrier height, reduced ideality factor and back current in the diode structures, and increased gain slope and initial drain current in the SFETs.     

Massless and massive particle-in-a-box states in single- and bi-layer graphene

Electron transport through short, phase-coherent metal-graphene-metal devices occurs via resonant transmission through particle-in-a-box-like states defined by the atomically-sharp metal leads. We study the spectrum of particle-in-a-box states for single- and bi-layer graphene, corresponding to massless and massive two-dimensional (2-D) fermions. The density of states D as a function of particle number n shows the expected relationships D(n) ∼ n ^1/2 for massless 2-D fermions (electrons in single-layer graphene) and D(n) ∼ constant for massive 2-D fermions (electrons in bi-layer graphene). The single parameters of the massless and massive dispersion relations are found, namely Fermi velocity υ _F = 1.1 × 10⁶ m/s and effective mass m* = 0.032 m _e, where m _e is the electron mass, in excellent agreement with theoretical expectations.      

Selective Recovery of Chromium from Precipitates Containing d Elements and Actinides: III. Effect of S2O82-

Reaction of Cr(III) hydroxides and mixed Fe(III)-Cr(III) and Ni(II)-Cr(III) hydroxides with persulfate ion in alkaline solutions was studied by spectrophotometry. The Cr(VI) yield (at oxidant deficiency) corresponds to the S₂O₈ ^{2 -} : Cr(VI) molar ratio close to 1.5. The initial reaction rate V ₀ is described by the kinetic equation -d[Cr(III)]/d = k[Cr(III)][S₂O₈ ^{2 -}][NaOH]. The activation energy is 53 kJ mol^{- 1} within the 41.5-95°C range. V ₀ is higher than the rate of thermal decomposition of persulfate ion, i.e., Cr(III) reacts directly with S₂O₈ ^{2 -}. Oxidation of NiCr₂O₄·nH₂O and mixed Fe(III)-Cr(III) hydroxides proceeds faster than oxidation of pure Cr(III) hydroxide. This is due to the catalytic effect of Fe(III) and Ni(II). Additions of Co(II) and Cu(II) also accelerate the process. Pu(IV) in alkali solution under the action of persulfate is converted into a more soluble oxidized species, which can be reduced back to Pu(IV) with appropriate reductants.     

Photo-control of Charge and Orbital States in Transition-Metal Compounds Probed by Photoemission Spectroscopy

We have studied the charge-orbital states in spinel- type CuIr₂S₄ and perovskite-type Pr_0.55(Ca_1−y Sr _y )_0.45MnO₃ (PCSMO, y=0.25, 0.40) using X-ray photoemission spectroscopy (XPS) combined with laser illumination. The XPS data of CuIr₂S₄ are dramatically changed by the insulator–metal transition (IMT). The line shape of CuIr₂S₄ in the insulating phase is consistent with the charge disproportionation of Ir³⁺:Ir⁴⁺=1:1. Interestingly, a visible light irradiation at low temperature gives a resistance reduction but does not give any spectral change. In case of PCSMO, the IMT is also clearly observed on the XPS data and the peculiar temperature dependence for y=0.25 suggests the coexisting of the ferromagnetic metallic phase and the charge-orbital ordered insulating phase. The spectra after visible light illumination show the enhancement of the density of states at the Fermi level, accompanied with the drop of the resistance.     

On the properties of a radiation-induced defect responsible for the 1.0-eV IR absorption band in gallium arsenide

The IR absorption spectra were measured at 77 K for Te-doped n-GaAs samples irradiated by electrons with an energy of E=3 MeV. It was found that the samples exhibit two absorption bands in the IR range, corresponding to 1.0 and 0.8 eV.     

Hydrogen passivation studies in Pd/n-GaAs devices

The effect of hydrogen on the current (I-V) and capacitance (C-V, TSCAP) has been studied for Pd/n-GaAs diodes. Hydrogenation has been found to improve ideality factor of the diode. ReverseC-V characteristics show that the number of shallow and deep donors is reduced on hydrogenation. The TSCAP measurement shows the presence of two donor states at ∼ 0.48 and 0.72 eV which is being partially passivated on hydrogenation. The likely origin of these states is discussed.     

Photonics of UO 2 2+ complexes with tryptophan: 2. Competition of electron phototransfer from higher excited electronic and vibration-rotation levels of tryptophan to UO 2 2+ with nonradiative relaxation

The possibility of competition of photochemical reaction of tryptophan (Trp) (intraspheric electron phototransfer from Trp to UO ₂ ²⁺ and formation of hydrated electron) in higher singlet S _n and vibrational S _n ^v states with nonradiative vibration-rotation (S _n ^v → S _n ), internal (S _n → S _n?1), and solvation (S _n → S _n ^r ) relaxation was considered. The influence of the excitation wavelength of Trp on the quantum yield of fluorescence, the Stern-Volmer constant of quenching of Trp fluorescence with uranyl ion, and the activation energy of thermal quenching of Trp fluorescence was studied. The monotonic change in the constant of quenching of Trp fluorescence with uranyl ion is due to the fact that intrashperic electron phototransfer from the vibronic S _n ^v states is faster than the vibration-rotation relaxation. The experimental dependence of the activation energy of thermal quenching of the Trp fluorescence on the excitation wavelength strongly suggests that the photo-induced electron transfer occurs mainly from the higher relaxed S _n ^r states with the stable solvation shell rather than from the Franck-Condon S _n levels of tryptophan.     

Sound Speeds and Excess Isentropic Compressibilities of Ternary Mixtures of Tetrahydropyran and Aromatic Hydrocarbons at 308.15 K

Speeds of sound, u _ijk , of tetrahydropyran (THP) (i) + benzene (j) + toluene or o- or p-xylene (k), and tetrahydropyran (i)+toluene (j) + o- or p-xylene (k) ternary mixtures have been measured over the entire mole fraction range at 308.15 K and atmospheric pressure. The speed-of-sound data have been used to calculate isentropic compressibilities, (k_S)_ijk{(\kappa_S)_{ijk}} , and excess isentropic compressibilities, (k_S^E)_ijk{(\kappa_S^{\rm E})_{ijk}} . The (k_S^E)_ijk{(\kappa_S^{\rm E})_{ijk}} values for the investigated mixtures are correlated with the Redlich–Kister equation to estimate ternary adjustable parameters and standard deviations. The Moelwyn–Huggins concept (Huggins, Polymer 12:357, 1971) of interaction between the surfaces of components of binary mixtures has been extended to predict excess isentropic compressibilities of ternary mixtures by employing the concept of connectivity parameters of the third degree of a molecule (which in turn depends on its topology). It has been observed that (k_S)_ijk{(\kappa_S)_{ijk}} values predicted by the Moelwyn–Huggins concept compare well with corresponding experimental values.     

Topology of gap nodes in superfluid3He: π4 Homotopy group for3He-B disclination

The topologically stable zeros in the energy spectrum of Fermi excitations in superfluid³He both in uniform phases and in textures are classified. This generalizes the classification of the defects of the order parameter in real coordinate space to the classification of zeros in the gap, which are the more general defects in coherent superfluid or superconducting states both in real space and momentumk space. The zeros are described by classes of mappings of the spherical surfacesS ⁿ , embracing the (6-n-1)-dimensional manifold of zeros in six-dimensional (k, r) space, into the space of the Bogolyubov-Nambu matrices, which describe the Fermi excitations. The examples of topologically nontrivial manifolds of zeros are discussed, including the closed line of zeros in five-dimensional space, which is described by the ₄ homotopy groups and exists in the core of the³He-B disclination. This object demonstrates the coupling between the real space topology of disclination and the extended space topology of zeros in the disclination core.     

An algorithm for spectral factorization using random search techniques

An approximation method for the energy spectrum of a stationary stochastic dynamical system is presented, which allows approximate functional rational factorization.This paper is in three parts. The first deals with a theoretical problem of approximation in Hardy Spaces, whose main result is the following:Let S⁽ⁱⁿ⁾, S be positive functions belonging to L¹(R_gw), such that log S⁽ⁿ⁾ and log S belong to L¹(R_ω, dω/1 + ω²).Let h⁽ⁿ⁾, h be the outer functions of the Hardy Space H²(Π₊) such that S^(g) = |h⁽ⁿ⁾|² and S = |h|² on iR.If S⁽ⁿ⁾ n^→∝ S in L¹(R_ω), and log S⁽ⁿ⁾ n^→∝ log S in L¹(R_ω, dω/1 + ω²), then: h⁽ⁿ⁾ n^→∝ h in H²(Π₊).The second part describes an effective algorithm, using random search methods, and gives an almost sure convergence result for it.The third part treats numerically two examples, permitting comparison of this algorithm with others (whenever there are…): the first example is a problem of approximation for a nonrational process (turbulence) that was considered in Ref. 22: the second example is a problem of model reduction (automatic) considered in Ref. 4.     

Influence of interfacial structural defects on the development of avalanche breakdown in heterojunctions

The reverse I–V characteristics of Cr/nn⁺-Si and Cr/nn⁺-GaAs Schottky barrier diodes and of heterodiodes produced by the vacuum epitaxy of germanium or Si_xGe_1?x onto nn⁺-silicon and nn⁺-GaAs were measured. The interfacial structural defects were studied by metallographic and transmission electron microscopy techniques. The effect of the density of structural defects at the interface on the I–V characteristics was investigated.     

Investigation of Charge Transfer Kinetics of Polyaniline Supercapacitor Electrodes by Scanning Electrochemical Microscopy

Scanning Electrochemical Microscopy (SECM) is introduced as a promising technique to probe localized interfacial kinetics at the interface of electrolyte/supercapacitor electrode based on polyaniline (PANI) by measuring approach curves from which heterogeneous charge transfer rate constants (k _eff) are extracted. The values correlate with the effectiveness of the electrode material for supercapacitor application. Specifically, measurements on PANI films of different thicknesses show that potential‐dependent rate constants are observed only for PANI films of up to 5 μm thickness. In addition to the thickness of PANI, k _eff is also found to be affected by the applied potential and surface morphology of PANI electrodes. These findings correlate with the macroscopic electrochemical performance of PANI electrodes which shows enhanced specific charge storage ability when their thickness is below 5 μm. Under these conditions, they deliver a specific capacitance of 486 F g⁻¹ and a rate capability of 89%. The observed correlation between microscopic kinetic data determined by SECM and macroscopic device characteristics provides rational guidelines for the optimization of the physical and structural properties of high performance supercapacitor electrodes.     

The effect of neutron irradiation on the microhardness of gallium arsenide

We have studied the effect of neutron irradiation on the microhardness of n-GaAs crystals. It is shown that the growth and saturation of microhardness with increasing radiation dose Φ, as previously reported in the literature, take place only in the dose range Φ∼10¹⁵−5×10¹⁶ cm⁻². As the neutron dose is increased further, the microhardness continues to grow due to the increasing role of the radiation-induced disordered regions in n-GaAs.     

Secruential Rank Tests I. Monie Carlo Studies of the Two-Sample Procedure

Monte Carlo studies of a sequential, two-sample, rank-sum test developed earlier by Wilcoxon, Rhodes and Bradley are reported. Ranking is accomplished within groups of observations in the sequential procedures and the theory is based on a model suggested by Lehmann. Design parameters are α and β, probabilities of Type I or Type II errors, m and n, the numbers of X- and Y-observations in a group of observations, and k ₁, the power in the Lehmann model wherein the alternative hypothesis states that the cdf of the Y-population is G(u) = F ^{k ₁} (U), F(u) being the cdf of the X-population. Studies are made for designs with (α = β = .05, m = n = 2(1)5, k _k = 1.5, 2.33, 4 and 9. Data appropriate to the Lehmann model are generated and studies are also made when G(u – μ _y ) = F(u).

Average sample numbers and powers of tests are estimated. The effects of truncation are examined. It is concluded that the Monte Carlo results substantiate the use of the usual Wald theory of sequential analysis and that the use of the sequential methods for data from normal populations differing only in locations is satisfactory for most practical applications.