Electrolytic coloration of O2-doped NaCl crystals

O₂²⁻-doped NaCl crystals are colored electrolytically by using a pointed cathode and a flat anode at various temperatures and voltages, which mainly benefit from appropriate coloration temperatures and voltages as well as anode structure of used electrolysis apparatus. Characteristic OH⁻, U, V₂^m, U_A, V₂, V₃, O²⁻-V_a⁺ complex, F, R₁, R₂ and M absorption bands are observed in absorption spectra of the colored crystals. Production and conversion of color centers in electrolytic coloration is explained. Current-time curves for electrolytic colorations and their relationships with electrolytic colorations were given.     

V color centers in electrolytically colored hydroxyl-doped sodium chloride crystals

Hydroxyl-doped sodium chloride crystals were successfully colored electrolytically by using pointed anode and flat cathode at various temperatures and under various electric field strengths. V₂ and V₃ color centers were produced in the colored crystals. Current-time curves for the electrolytic colorations were given, and activation energy for the V₂ and V₃ color center migration was determined. Production of the V₂ and V₃ color centers and formation of current zones for the electrolytic colorations of the hydroxyl-doped sodium chloride crystals are explained.     

Electrolytic coloration and spectral properties of OH-doped KBr polycrystals

OH⁻-doped KBr polycrystals were colored electrolytically by using a pointed cathode and a flat anode. Characteristic O⁻, OH⁻, U, Cu⁺ and absorption peaks were observed in resolved absorption spectrum of uncolored polycrystals. Herein the position of the absorption peak at room temperature was determined by using a Mollwo-Ivey plot. Characteristic V₂, V₃, Cu⁺, , I₂ and F spectral bands were observed in Kubelka-Munk functions of colored polycrystals. Current-time curve for electrolytic coloration of an OH⁻-doped KBr polycrystal and its relationship with electrolytic coloration process were given. Formation and conversion of color centers were explained.     

Electron correlation in two-dimensional systems: CHNC approach to finite-temperature and spin-polarization effects

Applying the classical-map hypernetted-chain method (CHNC) developed recently by Dharma-wardana and Perrot, we have studied the temperature and spin-polarization effects on electron correlation in the uniform quantum two-dimensional gas (2DEG) over a wide range of temperature T and spin-polarization ζ. The quantum fluid at the temperature T is mapped to a classical fluid at the temperature T_cf given by T_cf²=T²+T_q², where the quantum temperature T_q is determined by comparing the calculated correlation energy to that of Monte Carlo results for the fully spin-polarized quantum system at zero temperature. By the iterative solution of the modified HNC equation and the Ornstein-Zernike equation, we have obtained the pair distribution function (PDF) and correlation energy for the two-component classical 2DEG with a classical fluid temperature T_cf. The anti-parallel bridge function B₁₂(r) appearing in the modified HNC equation is determined by using the Monte Carlo correlation energy at T=0 or STLS (Singwi-Tosi-Land-Sjölander) result at T>0 and the numerical solution to the Percus-Yevick (PY) equation for the system of hard disks. By calculating the Pauli potential, the bridge function, PDFs, structure factors and correlation energy, we have shown that in some cases, the properties of the uniform quantum 2DEG depend remarkably on the temperature and spin-polarization.     

Electrical activation of boron-implanted silicon during rapid thermal annealing

The electrical activation of boron implanted in crystalline and preamorphized silicon has been investigated during rapid thermal annealing performed with halogen lamps. Samples implanted with B⁺ fluences ranging between 5×10¹⁴ and 1×10¹⁶cm⁻² and treated at temperatures between 900°C and 1100°C have been examined. When boron is implanted in crystalline Si, activation proceeds slowly atT<1000°C and cannot be completed in times typical of rapid thermal annealing (a few tens of seconds). The analysis of carrier profiles indicates that the time constant for activation is strongly affected by local damage and dopant concentration. If the total boron concentration exceeds equilibrium solubility, precipitation occurs concomitant to activation, even if the substitutional boron fraction is still lower than equilibrium solubility. ForT≧1000°C complete activation is obtained in times of about 10 s. In the case of preamorphized Si the activation occurs very quickly, during the recrystallization of the amorphous layer, for all the examined temperatures.     

Transmission properties of one-dimensional graded photonic crystals and enlargement of omnidirectional negligible transmission gap

We investigate numerically the transmission properties of one-dimensional (1D) graded photonic crystals (PCs) in detail. Under the condition of linearly graded structures, the gap broadening of three cases is calculated: the case when the thickness of low index layer is increased, the case when the thickness of high index layer is increased and the case when the thickness values of both layers are increased simultaneously. Then the gap broadening of another graded case with the thickness exponentially increased is presented. At last the enlargement of omnidirectional negligible transmission band gap is obtained.     

Effect of ambient hydrogen sulfide on the physical properties of vacuum evaporated thin films of zinc sulfide

Evaporated thin films of zinc sulfide (ZnS) have been deposited in a low ambient atmosphere of hydrogen sulfide (H₂S ∼10⁻⁴ Torr). The H₂S atmosphere was obtained by a controlled thermal decomposition of thiourea [CS(NH₂)₂] inside the vacuum chamber. It has been observed that at elevated substrates temperature of about 200 °C helps eject any sulfur atoms deposited due to thermal decomposition of ZnS during evaporation. The zinc ions promptly recombine with H₂S to give better stoichiometry of the deposited films. Optical spectroscopy, X-ray diffraction patterns and scanning electron micrographs depict the better crystallites and uniformity of films deposited by this technique. These deposited films were found to be more adherent to the substrates and are pinhole free, which is a very vital factor in device fabrication.     

Analytic and numeric Green’s functions for a two-dimensional electron gas in an orthogonal magnetic field

We have derived closed analytic expressions for the Green’s function of an electron in a two-dimensional electron gas threaded by a uniform perpendicular magnetic field, also in the presence of a uniform electric field and of a parabolic spatial confinement. A workable and powerful numerical procedure for the calculation of the Green’s functions for a large infinitely extended quantum wire is considered exploiting a lattice model for the wire, the tight-binding representation for the corresponding matrix Green’s function, and the Peierls phase factor in the Hamiltonian hopping matrix element to account for the magnetic field. The numerical evaluation of the Green’s function has been performed by means of the decimation-renormalization method, and quite satisfactorily compared with the analytic results worked out in this paper. As an example of the versatility of the numerical and analytic tools here presented, the peculiar semilocal character of the magnetic Green’s function is studied in detail because of its basic importance in determining magneto-transport properties in mesoscopic systems.     

Effect of sintering aid (CdCl2) on the optical and structural properties of CdZnS screen-printed film

Sulphides of zinc and cadmium have been utilized effectively in various opto-electronic devices. In the present work cadmium zinc sulphide (Cd_0.4Zn_0.6S) thin film has been deposited on ultra clean glass substrate by a simple inexpensive screen-printing method using cadmium sulphide, zinc sulphide, anhydrous cadmium chloride and ethylene glycol. Cadmium chloride has been used as sintering aid and ethylene glycol as a binder. Effect of sintering aid on the optical and structural properties of prepared cadmium zinc sulphide film has been investigated. The optical band gap (E_g) of the film has been studied by using reflection spectra in wavelength range 325–600 nm. It is found that reflection spectra suffer a drastic fall at two places, which is indicative of two band gaps of film viz. 2.38 eV and 2.9 eV corresponding to CdS and Cd_0.6Zn_0.4S, respectively. This is suggestive of the fact that cadmium zinc sulphide is a wide band gap semiconducting material. X-ray diffraction also confirms the formation of Cd_0.6Zn_0.4S composition.     

Electrolytic coloration of cuprous chloride crystals

Electrolytic coloration of cuprous chloride crystals is observed. Visible optical absorption spectra, coloration kinetics so as electric current kinetics are recorded at high temperature. EPR experiments are performed both at high temperature and with quenched samples. Two types of defects are found, one being metastable, the other stable, remaining after quenching. All experimental results do not agree with the classical theory of absorption of light by metal colloids. A model is proposed which qualitatively explains all experimental features of these phenomena.     

ZnSe sintered films: Growth and characterization

The II-VI compound semiconductor, ZnSe having wide band gap between 2.58 and 2.82 eV is a promising material for use in photovoltaic devices, blue light emitting diodes and laser diodes. Several methods have been used to prepare ZnSe thin films. We have deposited ZnSe films on ultra-clean glass substrate by sintering technique. The optical, structural and electrical properties of ZnSe thin films have been examined. The optical band gap of these films is studied using reflection spectra in wavelength range 325-600 nm and structure of these films is studied using XRD. The DC conductivity of the films was measured in vacuum by two-probe technique.Sintering is a very simple and viable method compared to other intensive methods. The results of the present investigation will be useful in characterizing the material ZnSe for its applications in photovoltaics.     

Studies of absorption spectra and the photovoltaic effect in LiNbO3:Mg:Fe crystals

The absorption spectra, photoconductivities and photovoltaic currents of LiNbO₃:Fe crystals with different Mg doping levels and Li/Nb ratios in the oxidized state have been investigated at room temperature. The Fe²⁺ ions in LiNbO₃:Mg:Fe with Mg content above a critical value are more easily oxidized than in crystals with Mg content below a critical value. The photoconductivity of LiNbO₃:Mg:Fe crystals with Mg content above a critical value is one order of magnitude higher than those with Mg content below a critical value, however, the photovoltaic current of the former is one order of magnitude lower than the latter. The differences are postulated to be due to different sites of Fe in these two classes of crystals.     

Determination of the copper diffusion coefficient in silicon from transient ion-drift

We use the transient ion drift in a depletion region of a Schottky barrier to determine ion diffusivities at moderate temperatures. The pulsed reverse bias leads to temperature dependent capacitance transients similar to deep level carrier emission transients. A simple theoretical model together with classical transient signal analysis provide the means to extract the ion diffusion constant. When applied to copper in silicon, diffusion data are obtained in a not yet investigated temperature range (280–400 K) which agree well with both low and high temperature diffusion data.     

Patterned aluminum nanowires produced by electron beam at the surfaces of AlF3 single crystals

Cubic- and rectangular-shape single crystals of α-AlF₃ in sizes of 5-50 μm have been synthesized by a solid-vapor phase process. Using the electron beam induced decomposition of AlF₃, a method is demonstrated for fabricating patterned aluminum nanowires in AlF₃ substrate in a scanning electron microscope. By controlling the accelerating voltage, the beam current and scanning time, it is possible to fabricate metallic nanowires of different sizes. The aluminum nanowires may act as nano-interconnects for nanoelectronics. This work demonstrates a potential technique for e-beam nanofabrication.     

A comparison of hydrogen incorporation and effusion in doped crystalline silicon,germanium, and gallium arsenide

The incorporation of deuterium into crystalline silicon, germanium, and gallium arsenide from a plasma source is investigated as a function of sample temperature during the plasma treatment. The total amount of incorporated deuterium and its bonding states are characterized by the thermal effusion (TE) technique for different dopants and doping levels. In all samples investigated, we find a strong influence of the passivation temperature on these quantities; however, there are large differences between different semiconductors and for different doping levels or dopants. The results are discussed in terms of plasma-induced defects, dopant-deuterium complexes, and surface effects.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Electrolytic coloration and spectral properties of OH- and Cu-codoped NaCl crystals

OH⁻- and Cu⁺-codoped NaCl crystals are colored electrolytically by using a pointed anode and a flat cathode at various temperatures and under various voltages. Absorption and fluorescence spectral properties of colored crystals are investigated. Absorption, excitation and fluorescence spectra reveal existence of V₂, V₂^m, V₃, OH⁻, U, U_A, O²⁻–V_a⁺, O₂⁺, Cu⁺, Cu⁰, Cu⁻ and OH⁻-perturbed Cu⁺ color centers in colored crystals. Formations of color centers are explained.     

A novel β→α phase transition of PbF2 at ambient condition driven by fluoride ions

We first qualitatively studied β→α irreversible phase transition of PbF₂ driven by fluoride ions and found that the speed of phase transition was strongly dependent on the concentration of fluoride ions in MeOH solution. These processes were traced by X-ray diffraction analysis. Possible mechanism of phase transition is discussed.     

Optical properties of Tm-doped GaSe single crystals

Optical properties of Tm-doped GaSe single crystals were investigated by measurements of optical absorption and photoluminescence. The single crystals were grown by the Bridgman technique. The X-ray diffraction analysis revealed that the single crystals were in the ε-type GaSe phase. The optical absorption spectra showed a sharp absorption peak at 582 nm near the band edge, which is due to direct free exciton. The temperature dependence of the energy of the exciton absorption peak was well fitted by the Varshni relation. In the photoluminescence spectrum at 10 K, we observed a very weak emission peak at 586 nm, a relatively strong emission peak centered at 613 nm, and several sharp and narrow emission peaks in the 790-840 nm region. The two emission peaks at 586 and 613 nm were associated with intrinsic emission lines due to direct free exciton and indirect bound exciton. The emission peaks in the 790-840 nm region, which were related to extrinsic emission, were assigned as due to the ³F₄→³H₆ transition of Tm³⁺ ions with a low symmetry of D₃ in the host lattice.     

Exact results for systems of electrons in the fractional quantum Hall regime

There has been a great deal of interest over the last two decades on the fractional quantum Hall effect, a novel quantum many-body liquid state of strongly correlated two-dimensional electronic systems in a strong perpendicular magnetic field. The most pronounced fractional quantum Hall states occur at odd denominator filling factors of the lowest Landau level and are described by the Laughlin wave function. It is well known that exact closed-form solutions for many-body wave functions, including the Laughlin wave function, are generally very rare and hard to obtain. In this work we present some exact results corresponding to small systems of electrons in the fractional quantum Hall regime at odd denominator filling factors. Use of Jacobi coordinates is the key tool that facilitates the exact calculation of various quantities. Expressions involving integrals over many variables are considerably simplified with the help of Jacobi coordinates allowing us to calculate exactly various quantities corresponding to systems with several electrons.     

Photoluminescence spectra of nitrogen implanted GaSe crystals

GaSe single crystals were N-implanted along c-axis with ion beams of 10¹⁴ and 10¹⁶ ions/cm² doses having energy values of 60 and 100 keV. The photoluminescence (PL) spectra of undoped and N-implanted GaSe crystals were measured at different temperatures. The PL intensity was observed to decrease with increasing implantation dose while the FWHM of the exciton peaks increased. In heavily doped crystals, due to the interaction with the radiation induced disorders, the wave vector selection rules are satisfied and an indirect exciton PL band is observed 36 meV below the direct exciton states.