Showing papers in "Modern Physics Letters B in 2001"

PROPERTIES OF ZnO:AL THIN FILMS, OBTAINED BY THE SOL-GEL METHOD

Abstract: Al-doped ZnO thin films have been prepared by the sol-gel technique, on slide-glass substrates. Al(NO3)3·9H2O was used as aluminum source and the doping range was 0.001-20 at %. The effect of the Al concentration in solution on the microstructure of the films is presented. The transmittance spectra, as well as the sheet resistance measurements, show the presence of electrically-active aluminum in the films.

FORMATION OF CONDUCTIVE CdO THIN FILMS ON PHOTOCONDUCTIVE CdS THIN FILMS FOR WINDOW LAYER APPLICATIONS IN SOLAR CELLS

Abstract: We report the formation of a thin layer of CdO on chemically deposited CdS thin films during air anneal at 370°C to 500°C for 5 min to 120 min. During a 5 min anneal, the sheet resistance of the CdS thin films drops from about 1013 □/9 to 3.5 k□/□(370°C) and 470 □/□(500°C). X-ray diffraction studies showed that this is associated with the formation of a thin layer of CdO layer, which occurs at temperatures above 370°C. The CdS, which remains under the conductive CdO top layer, is photosensitive - with photo-to-dark current ratio of 103 - 104. Essentially the air annealing converts the highly resistive and highly photosensitive intrinsic (i) CdS thin film into a (i)CdS-(n+)CdO layer. The technique offers prospects to convert the top part of a chemically deposited CdS thin film window layer of high photosensitivity, deposited on an absorber layer, to a conductive layer. This is of interest in thin film solar cell technology.

ABOUT KINKS IN AlGaAs LASERS LIGHT-CURRENT CHARACTERISTIC

Abstract: The origin of the kinks in semiconductor lasers light-current characteristic (L(l)) is theoretically alld experimentally analyzed. The devices are straight separate confinement heterostructure lasers. We have developed a model to calculate L(l) which considers two thermal loss mechanisms: the leakage current and Auger recombination. It is shown that the kinks in the light-current characteristic appear at temperatures at which the considered mechanisms crossover.

RBS ANALYSIS OF MBE GROWN SiGe/(001)Si HETEROSTRUCTURES WITH THIN HIGH Ge CONTENT SiGe CHANNELS FOR HMOS TRANSISTORS

Abstract: The growth of Si1-xGex quantum wells with high Ge composition (x>0.5) on Si(001) substrates by MBE is of great interest both for HMOS device applications and fundamental research. One of the possibilities to obtain a high Ge content Si1-xGex channel for mobile carriers, while retaining its strain, is to grow a relaxed Si1-yGey buffer layer on an underlying Si substrate. Such a buffer is termed a virtual substrate (VS), which is a constituent of SiGe metamorphic heterostructures. The effect of annealing on the structure of the Si(001)/VS/Si0.7Ge0.3/Si0.2Ge0.8/Si0.7Ge0.3 heterostructures was studied by grazing angle of incidence RBS. The thickness of the Si0.2Ge0.8 channel is inhomogeneous, which makes the analysis of the data by traditional means very hard. We have developed a model whereby the influence of the thickness inhomogeneity of each layer in the apparent energy resolution as a function of depth can be calculated. Automatic fits to the data were performed, and the roughness parameters, that is, the standard deviation of the thickness inhomogeneity of the relevant layers, were obtained, together with the thickness and stoichiometry of each layer. The results are compared with TEM and high resolution XRD experiments.

Optical doping of nitrides by ion implantation

Abstract: A series of rare earth elements (RE) were implanted in GaN epilayers to study the lattice site location and optical activity. Rutherford backscattering spectrometry in the channeling mode (RBS/C) was used to follow the damage behavior in the Ga sublattice and the site location of the RE. For all the implanted elements (Ce, Pr, Dy, Er, and Lu) the results indicate the complete substitutionality on Ga sites after rapid thermal annealing at 1000°C for 2 min. The only exception occurs for Eu, which occupies a Ga displaced site. Annealing at 1200°C in nitrogen atmosphere at a pressure of 1GPa is necessary to achieve the complete recovery of the damage in the samples. After annealing the recombination processes of the implanted samples were studied by above and below band gap excitation. For Er implanted samples besides the 1.54 μm emission green and red emissions are also observed. Red emissions from 5D0 → 7F2 and 3P0 → 3F2 transitions were found in Eu and Pr implanted samples even at room temperature.

WIGNER LATTICE ORDER, COLLECTIVE MODE, AND SUPERCONDUCTIVITY IN THE La1.985Sr0.015CuO4+δ SYSTEM

Abstract: We have studied far-infrared charge dynamics of Sr- and O- co-doped La1.985Sr0.015CuO4+δ with δ = 0.024 (p = 0.063 per Cu) and δ = 0.032(p = 0.07). We found that two-dimensional Wigner lattice order is the ground state of cuprates away from half-filling. We found that the presence of 2D Wigner lattice and the pinned Goldstone mode is essential for cuprate physics and superconductivity. We propose that all high Tc physics are based on the existence of these peculiar 2D electron lattices.

Energy levels in a spherical quantum dot with parabolic confinement under applied electric fields

Abstract: Using a variational procedure, we have studied the electric field effect on the electronic states in a spherical GaAs–Ga1-xAlxAs quantum dot with parabolic confinement in the case of both finite and infinite barrier heights. For single-particle states of electrons and heavy holes a relative weak Stark shift at weak fields and a transition to a stronger shift at higher fields is found. Compared with a spherical quantum dot with a rectangular confinement potential, the parabolic quantum dot presents a greater restriction to the carriers. As a result, the confinement energy is larger, while the electric field effects on the energy levels are weaker. We conclude that a spherical quantum dot with a parabolic confinement may be significant for practical application.

CuxSbySz THIN FILMS PRODUCED BY ANNEALING CHEMICALLY DEPOSITED Sb2S3-CuS THIN FILMS

Abstract: The possibility of generating ternary compounds through annealing thin film stacks of binary composition has been demonstrated before. In this work we report a method to produce large area coating of ternary compounds through a reaction in solid state between thin films of Sb2S3 and CuS. Thin films of Sb2S3-CuS were deposited on glass substrates in the sequence of Sb2S3 followed by CuS (on Sb2S3) using chemical bath deposition method. The multilayer stack, thus produced, of approximately 0.5 μm in thickness, where annealed under nitrogen and argon atmospheres at different temperatures to produce films of ternary composition, CuxSbySz. An optical band gap of ~1.5 eV was observed in these films, suggesting that the thin films of ternary composition formed in this way are suitable for use as absorber materials in photovoltaic devices. The results on the analyses of structural, electrical and optical properties of films formed with different combinations of thickness in the multilayers will be discussed in the paper.

Controlled not gate based on a two-layer system of the fractional quantum hall effect

Abstract: After a coordinate transformation, the wavefunction of the ground state of the fractional Quantum Hall Eect is found to be the congregation of free independent particles immune to thermal phonon disturbance. Based on the pseudo-spin states appearing in the two layer system of fractional Quantum Hall Eect, we propose a new controlled NOT gate with very large decoherence time. Recently, quantum computing has been attracting the attention of many researchers. Many quantum computing devices have been proposed. 1 The main difculty in practical operation of most of those devices is the existence of strong decoherence which causes error in the computing procedure. The eect appears as the decrease in the nondiagonal terms of the density matrix of the initial pure state through the interaction between the state and environment, which results in the decrease in interference. In this paper, we point out that (i) the wavefunction representing the quantum state of the fractional Quantum Hall Eect (FQHE) 2;3 is found, after appropriate coordinate transformation, to be the congregation of independent \free particles" with no particle{particle interaction, (ii) the decoherence time D of the \free particle" state is expected eectively to be innity due to ignorable thermal phonon disturbance, and (iii) the macroscopic quantum nature with particle-number deniteness (N t 0) of FQHE has excellent adaptability to the qubit device using particle number change in comparison with the superconductor device where the macroscopic quantum nature has phase deniteness ( t 0). These properties reveal the possibility that FQHE provides ideal quantum computing devices having particle quantum states with D!1.

EPR OF Mn2+ AND Fe3+ IONS DOPED IN BISMUTH–BORATE GLASSES

Abstract: Experimental EPR results of 99.5%[x B2O3(1-x) Bi2O3]0.5%MO glass samples (MO = MnO, Fe2O3 and 0.07 ≤ x ≤ 0.8) systems are presented. The resonance absorptions are centered at g ≈ 4.3 and g ≈ 2.0. For manganese-doped samples both lines show hyperfine structure for certain values of Bi/B ratio. In addition to these lines a well defined shoulder at g ≈ 9.8 is recorded from the samples doped with iron. The changes in matrix composition, corresponding to different values of x, induce changes in the surroundings of the resonance centers. Structural data obtained from the EPR measurements indicate various sites for Mn2+ and Fe3+ ions in environments characterized by different crystalline field intensities. The EPR data also provides evidence that the redox effect of bismuth–borate matrices on the doped paramagnetic ions is a function of Bi/B ratio.

The decoherence criterion

Abstract: The decoherence mechanism signals the limits beyond which the system dynamics approaches the classical behavior. We show that in some cases decoherence may also signal the limits beyond which the system dynamics has to be described by quantum field theory, rather than by quantum mechanics.

DEPENDENCE OF SECOND HARMONIC GENERATION ON SIZE OF NANOCRYSTALLITES OF ZnO

Abstract: We report second harmonic generation (SHG) from ZnO thin films deposited by the pulsed laser deposition technique at ambient oxygen pressures ranging from 10 mTorr to 1 Torr on glass substrate at room temperature (RT). The dependence of SHG on grain size and thickness of the film is discussed.

Morphological Stability of a Nanowire During Growth Process

Abstract: In this letter, we analyze the morphological stability of a nanowire during the vapor–solid (VS) side growth process at the same time as it grows along its axial direction by the vapor–liquid–solid (VLS) mechanism. The longitudinal perturbations due to the fluctuations of the growth conditions are introduced to describe the morphological variance of a nanowire along its fibre direction. It is shown that the side growth can induce the morphological instability for a nanowire with side perturbations if the perturbation's wavelengths are larger than a nanowire's perimeter.

A+B-->0 and A+A-->0 Chemical Reactions Under Enhanced Diffusion Conditions Through Thompson's Method

Abstract: In this work we use Thompson's method to study both A+B→0 and A+A→0 reactions under anomalous diffusion regimes. We consider a general action encompassing the two kinds of reactions, by including a non-brownian (γ ) and for the reaction rates ( ).

Microscopic Theory of the Quantum Hall Effect

Abstract: The phonon exchange between the electron and the elementary magnetic flux (fluxon) induces an attractive transition in the degenerate Landau states. This attraction bounds an electron–fluxon complex. The center-of-mass of the complex moves as a boson with a linear dispersion relation (∊ = cp). The 2D system of free massless bosons undergoes a Bose–Einstein condensation at kBTc = 1.954ℏcn1/2, where n is the boson density. For GaAs/AlGaAs, Tc ~ 1 K at the principal Landau-level occupation ratio ν = 1, where the electron number equals the fluxon number. Below Tc, there is an energy gap, which stabilizes the Hall resistivity plateau. The plateau value (j/N)(h/e2) at the fractional occupation ratio ν = N/j, for odd j, indicates that the composite boson containing an electron and j fluxons carries the fractional charge (magnitude) e/j due to the magnetic confinement.

Thompson's method applied to diffusion limited chemical reaction a+b→0, with different initial concentrations of the two species

Abstract: In this work we use Thompson's renormalization group method to treat diffusion limited chemical reactions A+B→0 (inert product), with unequal initial concentrations of the two species, by considering ρA(0)≪ρB(0). For d≤2, we obtain stretched exponential decaying of the specie A, which is in agreement with rigorous result of Bramson and Lebowitz. For d>2, we obtain simple exponential decaying of the specie A.

Dynamic critical exponents of the ising model with multispin interactions

Abstract: We revisit the short-time dynamics of 2D Ising model with three spin interactions in one direction and estimate the critical exponents z, θ, β and ν. Taking properly into account the symmetry of the Hamiltonian, we obtain results completely different from those obtained by Wang et al.10 For the dynamic exponent z our result coincides with that of the 4-state Potts model in two dimensions. In addition, results for the static exponents ν and β agree with previous estimates obtained from finite size scaling combined with conformal invariance. Finally, for the new dynamic exponent θ we find a negative and close to zero value, a result also expected for the 4-state Potts model according to Okano et al.

OPTICAL AND ELECTRICAL PROPERTIES OF THALLIUM SULPHIDE AND TlxMySz (M=Cu,Bi,Sb) thin films

Abstract: Tl2S thin films (0.05μm-1μm) were deposited from chemical bath containing thallium nitrate and thiourea. These films possess an indirect optical band gap of about 1 eV. Multi-layer films: Tl2S-CuS, Tl2S-Bi2S3 and Tl2S-Sb2S3 films were prepared, which when annealed at 300°C led to the formation of CuTlS2 (tetragonal, Eg.dir = 1.25 eV), Bi2Tl4S5 (orthorhombic, Eg.dir = 2.05 eV), TlSbS2 (triclinic, Eg.dir = 1.92 eV), as confirmed by XRD. Optical band gap of 1.4 eV - 2 eV suggests possible application as absorber materials in solar cell.

Ion-beam control of thin-film microstructural columnar angle

Abstract: The microstructural columnar angle of titanium oxide is shown to be rotated towards the vapor direction during oblique, ion-assisted deposition. The effect was found to increase with increasing ion energy, but experiments with the ion beam directed along and perpendicular to the tangent rule direction did not reveal dependence on the angle of incidence of the ions. The technique may allow access to deposition of biaxial materials in a regime where the columns run nearly parallel to the substrate.

Computer Simulation Study of a Nematogenic Lattice-Gas Model Based on the Nehring-Saupe Interaction Potential

Abstract: Over 25 years ago, Nehring and Saupe proposed an anisotropic nematogenic lattice model, whose restriction to nearest neighbours has the form Here the three-component vectors xj∈Z3 define centre-of-mass coordinates of the particles, and uk are three-component unit vectors defining their orientations; ∊ is a positive quantity setting energy and temperature scales (i.e. T*=kBT/∊); this model is seen to be the anisotropic counterpart to the generic Lebwohl–Lasher lattice model. It has often been used for approximate calculations of elastic properties, and has recently been studied by simulation [Hashim and Romano, Int. J. Mod. Phys.B13, 3879 (1999)]. We study here its lattice-gas extension, whose Hamiltonian is defined by where νk=0,1 denote occupation numbers, ∑{j

DEPOSITION OF SMALL Cu AND Au CLUSTERS ON Cu(001) SURFACE: STRUCTURE CHARACTERISTIC

Abstract: Deposition of small Cu and Au clusters on a Cu(001) surface was studied by molecular dynamics simulation using Finnis-Sinclair tight-binding potential. The impact energy varied from 0.1 to 6.0 eV/atom. The adlayer structure was found to be dependent on the specific substrate and projectile materials. The structure rearrangement of supported clusters on the surface was observed for both Cu and Au clusters. The Cu adlayer has a good fcc structure as that of substrate. However, the nanostructure was found for Au adlayer on a Cu(001) surface. Our results are consistent with other reports that the nanostructure is mainly caused by the lattice-parameter mismatch. Besides that, the cluster size and the impact energy are the key parameters that can affect the morphology of deposited films.

Role of quantum entanglement due to a magnetic impurity on current magnification effect in mesoscopic open rings

Abstract: We study the current magnification effect in presence of exchange scattering of electron from a magnetic impurity placed in one arm of an open mesoscopic ring. The exchange interaction causes entanglement of electron spin and impurity spin. Earlier studies have shown that such an entanglement causes reduction or loss of interference in the Aharonov–Bohm oscillations leading to decoherence. We find however, that this entanglement, in contradiction to the naive expectation of a reduction in current magnification leads to enhancement as well as suppression of the effect. We also observe additional novel features like new resonances and current reversals.

FORMATION OF THE BAND GAP OF CdS THIN FILMS GROWTH BY DIFFERENT TECHNIQUES

Abstract: We studied the formation of the band gap on CdS thin films deposited on glass substrates by chemical bath deposition (CBD) and close spaced sublimation (CSS) techniques. In order to obtain the first stages of films growth, we used small deposition times for both methods. Films growth by CBD were prepared by two modalities on the bath agitation: magnetic agitation with a magnetic stirrer, and a new proposal based on ultrasonic vibration. Morphological, structural, and optical results were obtained on films. Values of the band gap energy obtained by the absorption method, are agree with the values reported in the literature, but squared absorption coefficient vs. energy curves, showed differences between them. An explanation on this behavior with the optical window effects, the surface roughness, the structural results, the films thickness, and the growth technique is given. CBD technique with ultrasonic vibration produces films with more cleaner surfaces and better optical properties.

EFFECT OF ION-ENERGY ON THE PROPERTIES OF AMORPHOUS GaN FILMS PRODUCED BY ION-ASSISTED DEPOSITION

Abstract: Amorphous GaN films were deposited on various substrates viz. Si(100), quartz, glass, Al, stainless steel and glassy carbon by thermal evaporation of gallium in the presence of energetic nitrogen ions from a Kaufman source. The films were deposited at room temperature and 5 × 10-4 mbar nitrogen partial pressure. The effect of a low energy nitrogen ion beam during the synthesis of films was investigated for energies 40 eV and 90 eV. The N:Ga atomic ratio, bonding state, microstructure, surface morphology, and electrical properties of the deposited a-GaN films were studied by different characterisation techniques. The films are found to be X-ray amorphous in nature, which is confirmed by Raman spectroscopy. Rutherford Backscattering Spectroscopy (RBS) and Nuclear Reaction Analysis (NRA) indicate the N:Ga atomic ratio in the films. The 400-750 eV energy range is thought to be optimal for the production of single-phase amorphous GaN. The effect of ion-energy on optical, Raman, and electrical conductivity measurements of the films is also presented.

Optimization of heat treatments of african green sapphires

Abstract: Under suitable heating conditions designed for a particular sapphire it is possible to change the color in order to increase its value. It was found that heating African green sapphires in an oxygen atmosphere could improve their quality and thus increase their prices. The color of green sapphires is due to the presence of trace amounts of Fe3+, Ni3+, Fe2+ and Ti4+ ions. It is known that Fe3+ yields pale yellow and Ni3+ a golden color while the charge transfer mechanism between Fe2+ and Ti4+ gives sapphire its blue color. It is obvious that the green color is due to a high ratio of Fe2+ to Ti4+. By using an X-ray diffractometer, the maximum decrease of the c/a ratio of the hexagonal structure is found to occur after heat treatment at 1200°C. These results are also confirmed by the shift of emission spectra toward shorter wavelengths. Furthermore, the number of Fe2+ ions converted to Fe3+ was detected by electron spin resonance spectrometry and found to increase with temperature. The sapphire with trace Ti4+ ions, appeared slightly bluish green after heating at 1500°C. The optimal temperature is therefore at 1300°C, where we can achieve yellowish green sapphires instead.

PHYSICAL CHARACTERIZATION OF PURE AND SUBSTITUTED TETRAGONAL LaBaCaCu3O7-δ SUPERCONDUCTOR

Abstract: The high Tc superconductor, forming the focus of the present review, presents an interesting situation where, despite its oxygen content close to 7.0, the system prefers to crystallize in tetragonal structure, as distinct from exhaustively studied orthorhombic RE:123 (REBa2Cu3O7, RE = rare earth) compounds. Its optimum Tc is decided by the cation intermixing at various sites, unlike that in case of normal RE:123, where all cationic sites are fully occupied and fixed, leaving the anionic sites free for oxygen filling. In this, we review the rich crystal chemistry of this compound and discuss it in conjunction with its unusual superconducting properties. The article is divided in five parts: (1) first we discuss variously reported interesting finer structural details and superconductivity of pristine LaBaCaCu3O7-δ (La:1113) superconductor. The La, Ba and Ca sites are found to be intermixed with respect to their nominal sites of the standard RE:123 structure. (2) We examine the structural aspects and superconductivity of La1-xRExBaCaCu3O7 (RE = Nd, Dy, Sm and Pr). Unlike RE:123 and 124 compounds, the RE:1113 compounds form only with light rare earths such as La, Pr and Nd. Pr:1113 compound is found to be an insulator with Pr moments ordering magnetically at around 8 K, unlike at 17 K for Pr:123. (3) Studies of Pr substitution reveal that relative Tc depression due to Pr in the (La,Nd)1-xPrxBaCaCu3O7 system is less in comparison to that found for La or Nd site Pr substituted (La,Nd)1-xPrxBa2Cu3O7. (4) The results of superconductivity and structural details are reviewed for LaBaCaCu3-xMxO7 (M = Fe, Co, Ni, Ga and Zn) and finally (5) we summarize our conclusions. The results in terms of Tc depression due to these impurities in the parent La:1113 system are seen as similar to those observed for RE:124 (REBa2Cu4O8) and Nd:214 (Nd2-xCexCuO4), but are different from RE:123 and La:214. The results of structural details, superconductivity and magnetic ordering presented in the four parts for La1-xRExBaCaCu3-yMyO7 (RE = Nd, Dy, Sm, Pr with 1.0 ≥ x ≥ 0.0, and M = Fe, Co, Ni, Ga and Zn with 0.24 ≥ y ≥ 0.0 were obtained from X-ray and neutron diffraction and using AC and DC magnetic susceptibility, electrical resitivity and heat capacity data as a function of temperature. This review is mainly a consolidated work of the authors, including their collaboration from various laboratories and the reported data of other workers, all being duly acknowledged in the text.

Conductive copper sulfide thin films on polyimide foils for optical and optoelectronic applications

Abstract: Copper sulfide thin films of 75 nm and 100 nm thickness were coated on Kapton foils (PI) of 25 nm thickness by floating them on a chemical bath. The foils were annealed at 150°C-400°C in N2 converting the coating from CuS to Cu1.8S. The sheet resistance of the annealed coatings (100 nm) is 10-50 ohms/square which is almost unaltered after immersion in dilute HCl for 30-120 min. The infrared reflectance predicted for the coatings is 67%-77% at a wavelength 2.5 μm, which is nearly what is experimentally observed. The coated PI has a transmittance (25-35%) peak located around 550-600 nm. These thermally stable conductive coatings on PI foils might be used as conductive substrates for optoelectronic device structures.

HALL CONDUCTIVITY FLUCTUATIONS IN EPITAXIAL YBa2Cu3O7-δ THIN FILMS

Abstract: We present systematic measurements of the longitudinal and Hall resistivities in YBa2Cu3O7-δ epitaxial thin films. Above the critical temperature, Tc, the longitudinal resistivity, ρxx(T), increases linearly with temperature, whereas the Hall resistivity, ρxy(T), varies inversely proportional to T. Consequently, the cotangent of the Hall angle shows quadratic temperature dependence. The fluctuation contributions to both the longitudinal and Hall conductivities in the normal phase are studied. In the case of the longitudinal conductivity, 2D and 3D Gaussian regimes are observed. In addition, a genuine 3D-XY critical regime is discerned closely above Tc in experiments performed at low applied fields. The value of the corresponding critical exponent, λC=0.33, indicates that the dynamic universality class for the superconducting transition in the analyzed system is given by the Model E of Hohenberg and Halperin. The analysis of the Hall conductivity fluctuations above Tc reveals the occurrence of 2D and 3D Gaussian regimes which are consistent with those identified in the longitudinal conductivity results. Below Tc the Hall resistivity shows a sign reversal before attaining the zero resistance state.

A NEW APPROACH TO IDEAL AlGaAs MQW SOLAR CELLS

Abstract: A theoretical model has been developed which shows that the insertion of multi-quantum wells into the depletion region of a p-i(MQW)-n AlxGa1-xAs solar cell can significantly enhance the conversion efficiencies. Open-circuit voltages, short-circuit current densities, I-V curves and conversion efficiencies have been calculated as functions of the well and barrier band gaps, width and depth of the wells, number of wells in the intrinsic region and the recombination rate in the interfaces. Particular emphasis is placed on calculation of absorption of the AlxGa1-xAs quantum wells. These results are matched with p-i-n solar cells which are identical in all respects except that they do not have quantum wells. We demonstrated that for determined values of the studied parameters the conversion efficiencies of the quantum well solar cell is higher to corresponding cell without quantum wells.

DEBYE TEMPERATURE OF MgB2

Abstract: In this study we determined the Debye temperature of MgB2 powder by using the Rietveld refinement method based on the X-ray diffraction (XRD) data of this compound. MgB2 crystallizes with the AlB2 structure, a space group of P6/mmm (191), and the lattice constants are a=b=3.0840 A, c=3.5222 A. The temperature factors of the atom Mg and B are 0.38 and 0.29 respectively, and the Debye temperature of MgB2 is calculated to be 884 K at room temperature by Debye approximation.