Structural and optical characterization of polycrystalline CdSe<Subscript>x</Subscript>Te<Subscript>1-x</Subscript> thin films

Solid solutions of CdSe_xTe_1-x (0.7x1) were synthesized by vacuum fusion of stoichiometric amounts of CdSe and CdTe constituents in a silica tube. X-ray and electron microscope diffractometry techniques revealed that the CdSe_xTe_1-x thin films were polycrystalline with a hexagonal structure. The variation of lattice constants with composition was found to obey Vegards law. The compositional dependence of the optical constants, the refractive index n and the absorption index k, of the films was determined in the spectral range of 400–2000 nm. The dispersion of the refractive index of the films could be described using the Wemple–DiDomenico single oscillator model. Changes of the dispersion parameters were also studied as a function of the mole fraction x. A plot representing ²=f(h) showed that the CdSe_xTe_1-x thin films of different compositions have two direct transitions corresponding to the energy gaps E_g and E_g+. The variation in either E_g or E_g+ with x indicates that this system belongs to the amalgamation type. The variation follows a quadratic dependence and the bowing parameters were found to be 0.4 and 0.5 eV, respectively. PACS 78.20.-e; 81.15.-z     

Peculiar properties of nanogranular Ni<Subscript>x</Subscript>Si<Subscript>100-x</Subscript> magnetic films obtained by ultrashort pulsed laser deposition

The use of ultrashort laser pulses enables the deposition of films composed of mono-component nanoparticles exhibiting similar shape and size. Films made of nickel (Ni) and silicon (Si) nanoparticles have been produced and investigated in view of the expected interesting properties, resulting from the uniform distribution of metallic, magnetic Ni particles among semiconductive, non-magnetic Si particles. The morphology of the deposited nanoparticles and the related magnetic and magneto-transport characteristics of the films have been studied for different Ni contents, evidencing properties deriving from the peculiar deposition technique and in particular the important role of the free volume inclusions and the particles tendency to not coalesce. PACS 75.75.+a; 81.07.-b; 81.16.Mk; 73.50.Jt     

Magnetic and electrical investigations of Fe<Subscript>85-x</Subscript>Co<Subscript>x</Subscript>B<Subscript>15</Subscript> metallic glasses

The magnetic and electrical properties of metallic glasses with the general formula Fe_85-xCo_xB₁₅ were investigated over a large temperature range to study their concentration-dependent physical parameters. All of the samples investigated (x=17,21,30, and 40) were soft ferromagnets with coercive fields H_c1 Oe and high Curie temperatures slightly above 1200 K. The temperature-dependent magnetization behaved irregularly, and exhibited hysteresis during heating and subsequent cooling through the Curie temperature. The variation of the magnetization with temperature demonstrates that one or more phase transformations (crystallization) occurred in the course of the heating. The electrical resistivities exhibited positive temperature coefficients and minima at temperatures below 50 K. We did not observe a nonmonotonic variation of the magnetic and electrical properties with a monotonic change of the Fe_85-xCo_xB₁₅ composition that would correlate with the earlier proposed formation of strong nanoclusters in the vicinity of particular stoichiometrically close Fe:Co ratios. The good soft magnetic characteristics make the Fe_85-xCo_xB₁₅ metal glasses promising candidates for engineering materials in inductive applications. PACS 71.23.Cq; 75.75.+a     

Growth of polycrystalline LaNi<Subscript>1-x</Subscript>Co<Subscript>x</Subscript>O<Subscript>3</Subscript> (<Emphasis Type="BoldItalic">x</Emphasis> =0.3,0.5) thin films on Si(100) by pulsed laser deposition

Polycrystalline LaNi_1-xCo_xO₃ (x=0.5,0.3) thin films have been deposited on polished Si(100) substrates by pulsed laser deposition. The films are grown at 650 °C in ambient oxygen pressure of 0.4 mbar with an incident laser fluence of 1.5 J/cm² delivered by a KrF excimer laser. The lattice parameters of the as-grown films are slightly larger (0.05–0.4%) than those of the powders used to prepare the targets. The films exhibit weak texturing along the (012) direction. The low-temperature magnetic properties of the films, i.e. the coercive force, the remanence and the saturation magnetization, are enhanced compared to the powders. Furthermore, the x=0.3 film exhibits a low, almost temperature-independent resistivity above 200 K [(300 K)30 cm] and thus we propose it as a potential candidate material for electrode applications, e.g. in ferroelectric devices. PACS 68.55.-a; 73.61.-r; 81.15.Fg     

Electronic structure of paramagnetic In<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript> As nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In_1-xMn_xAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero k_z (k_z is the wave vector in the wire direction), and the spin-splitting bands cross at k_z = 0, whose k_z-positive part and negative part are symmetrical. A proper magnetic field makes the k_z-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero k_z. In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.     

Scaling and self-similarity in Pb<Subscript>1-x</Subscript>Fe<Subscript>x</Subscript>S nanoparticle films

This paper addresses the issues of scaling and self-similarity in typical nanoparticle films. The role played by microscopic processes contributing to growth on these issues is probed. While we perform this investigation for a specific system viz., Pb_1-xFe_xS nanoparticle films for clarity of the procedures, the analysis is general and can be applied to a variety of systems obtained using different deposition techniques.     

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     

Intensely emitting CdTe nanocrystals retained initial photoluminescence efficiency in sol–gel derived Si<Subscript>1-x</Subscript>Zr<Subscript>x</Subscript>O<Subscript>2</Subscript> glass

Emitting CdTe nanocrystals (NCs) were embedded in pure glass matrices (Si_1-xZr_xO₂, x≤0.15) using a controlled sol–gel method, where the pre-hydrolyzed condition, the molar ratio of Zr/Si, the gelation time, the pH, and the amount of alcohol were judiciously optimized considering the surface condition of the NCs and the mechanism of the glass formation. As a result, the prepared glass phosphor exhibited high photoluminescence efficiencies (40% for green and 60% for red when Zr/Si was 5–10%) by retaining their initial values as in CdTe colloidal solution. To our knowledge, these values are the highest among those ever obtained for any solid matrices containing NCs. Because of the existence of Zr, the prepared glasses exhibit much better resistance against the ambient atmosphere, heat-treatment, and boiling water compared with pure silica glass (x=0) or the glass prepared from our other methods using a silane coupling agent. Thus, the obtained glass is promising for applications such as optical devices. PACS 78.67.Bf; 78.55.Qr; 78.55.Et     

Non-critical phase-matched second harmonic generation in Gd<Subscript>x</Subscript>Y<Subscript>1-x</Subscript>COB

We have characterized non-critical phase-matching (NCPM) for both Type I and Type II second harmonic generation (SHG) in y-cut Gd_xY_1-xCOB using a nanosecond optical parametric oscillator (OPO). The variation of the NCPM wavelength with temperature was investigated for different values of the compositional parameter x. Efficient SHG of 1064 nm was achieved by choosing the suitable compositional parameter x=0.28 and by tuning the temperature of the crystal to 52 °C. Using a 25-mm-long Gd_0.28Y_0.72COB crystal, conversion efficiencies of 41 and 43% were obtained respectively from a mode-locked Nd:YAG and a Q-switched Nd:YAG laser. PACS 42.25.Lc; 42.65.Ky; 42.70.Mp; 42.79.Nv     

Photoacoustic spectra of Zn<Subscript>1-x</Subscript>Be<Subscript>x</Subscript>Te near the energy gap

The results of experimental studies of optical and structural properties in bulk crystals of Zn_1-xBe_xTe (x = 0.02, 0.06 and 0.12) were presented. The amplitude and phase photoacoustic (PA) spectra were measured and analyzed in dependence on the wavelength of the excitation optical beam, at different frequencies of modulation, using the PA microphone (PAmic) and PA piezoelectric (PApze) spectroscopy methods. The differences in PA spectra of as grown and annealed in zinc vapor samples were observed.     

Energy spectra of exciton states in disk-shaped GaAs-Ga<Subscript>1-x</Subscript>Al<Subscript>x</Subscript>As quantum dots under growth-direction magnetic fields

A theoretical study, within the effective-mass approximation, of the effects of applied magnetic fields on excitons in disk-shaped GaAs-Ga_1-xAl_xAs quantum dots is presented. Magnetic fields are applied in the growth direction of the semiconductor heterostructure. The parity of the excitonic envelope function related to the simultaneous exchange of z_e→-z_e and z_h→-z_h is a good quantum number and the wave function, both the odd and even parity, can be expanded as combination of products of the quantum well electron and hole function that preserves the parity with appropriate Gaussian functions. We have simultaneously obtained the energies of the excitonic ground and excited states and discuss the behavior of these energies as a function of the magnetic field.     

Electrical switching studies on As<Subscript>40</Subscript>Te<Subscript>60-x</Subscript>Se<Subscript>x</Subscript> and As<Subscript>35</Subscript>Te<Subscript>65-x</Subscript>Se<Subscript>x</Subscript> glasses

The I–V characteristics of bulk As₄₀Te_60-xSe_x and As₃₅Te_65-xSe_x glasses have been studied with a current sweep of 0–18 mA-0, over a wide range of compositions (4≤x≤22). All the glasses studied showed a threshold electrical switching behaviour. The number of switching cycles withstood by the samples has been found to depend on the ON-state current. It is seen that the switching voltages increase with increase in selenium content. Further, the switching voltages are found to be almost independent of the thickness of the sample (d), in the range 0.18–0.3 mm. Also, the switching voltages and the number of switching cycles withstood by the samples are found to decrease with temperature. Received: 6 November 2002 / Accepted: 8 November 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +91-80/360-0135, E-mail: sasokan@isu.iisc.ernet.in     

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     

First-principles investigation of electronic and optical properties in wurtzite Zn<Subscript>1-x</Subscript>Mg<Subscript>x</Subscript>O

A first-principles study has been performed to evaluate the electronic and optical properties of wurtzite Zn_1-xMg_xO. Substitutional doping is considered with Mg concentrations of x = 0, 0.0625, 0.125, 0.1875 and 0.25, respectively. Mg incorporation can induce band gap widening due to the decrease of Zn 4s states. The imaginary part of the dielectric function shows that the optical transition from band edge emission decreases slightly with increasing Mg contents. The optical band gap also increases from 3.2 to 3.7 eV with increasing Mg contents from 0.0625 to 0.25. The calculated results suggest that relatively high Mg concentration is necessary for effective band gap engineering of wurtzite Zn_1-xMg_xO.     

Rashba effect in Ga<Subscript>x</Subscript>In<Subscript>1-x</Subscript>As/InP quantum wire structures

An overview is given on the Rashba effect in Ga_xIn_1-xAs/InP quantum wires. First, the effect of Rashba spin–orbit coupling on the energy level spectrum of quantum wires with different shapes of the confining potential is theoretically investigated. The wave functions as well as the spin densities in the quantum wire are analyzed for different magnetic fields. It is found that, owing to the additional geometrical confinement, a modification of the characteristic beating pattern in the magnetoresistance can be expected. The theoretical findings are compared to measurements on two different types of wires: First, single wires and, second, sets of parallel wires. A characteristic beating pattern in the Shubnikov–de Haas oscillations is observed for wires with an effective width down to approximately 400 nm. The beating pattern is significantly better resolved for the samples with sets of parallel wires, owing to the effective suppression of conductance fluctuations. A comparison with theoretical simulations confirms that the strength of the Rashba effect is basically not affected by the geometrical confinement of the wires. However, for wires with a very small effective width the strong carrier confinement leads to a suppression of the characteristic beating pattern in the Shubnikov–de Haas oscillations. PACS 71.70.Ej; 73.63.-b; 71.70.Di     

Structural properties of zinc-blende Ga<Subscript>x</Subscript>In<Subscript>1-x</Subscript>N: ab initio calculations

We present a theoretical study of the structural properties, namely lattice constant, bulk modulus and its pressure derivative of zinc-blende Ga_xIn_1-xN. The calculations are performed using first-principles calculations in the framework of the density-functional-theory within the local density approximation under the virtual crystal approximation. The computed values are in good agreement with the available experimental data. The composition dependence of the studied quantities is examined. Besides, the deviation of the alloy lattice constant from Vegard's law is evaluated.     

Ab initio study on elastic and thermodynamical properties of Ti<Subscript>1-x</Subscript>Zr<Subscript>x</Subscript>C

Elastic and Thermodynamical properties of Ti_1-xZr_xC have been investigated using LAPW + lo within the density-functional theory with the generalized gradient approximation. We have studied the stability of the alloy Ti_1-xZr_xC as a function of Zr composition in rocksalt (B1) structure by calculating the elastic constants C₁₁, C₁₂ and C₄₄ using the tetragonal and trigonal distortions. Mechanical properties such as Poisson ratio, bulk, shear and Young’s modulii of Ti_1-xZr_xC are calculated. The Debye temperature and hardness are also computed for the first time to our knowledge for Ti_1-xZr_xC in various compositions.     

Synthesis and characterization of Zn<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript>S nanocrystalline films prepared on glass substrates

Nanocrystalline Zn_1-xMn_xS films (x=0.04, 0.08 and 0.12) were deposited on glass substrates at 400 K using a simple resistive thermal evaporation technique. All the deposited films were characterized by chemical, structural, morphological, optical and magnetic properties. Scanning electron microscopy and atomic force microscopy studies showed that all the films investigated were in nanocrystalline form with the grain size lying in the range 10–20 nm. All the films exhibited cubic structure and the lattice parameters increase linearly with composition. The absorption edge shifted from the higher-wavelength region to lower wavelengths with increase in Mn concentration. The magnetization increased sharply with increase of the Mn content up to x=0.08 and then decreased with further increase of the Mn content. Particularly, Zn_0.92Mn_0.08S concentration samples show a weak ferromagnetic nature, which might be the optimum concentration for optoelectronic and spintronic device applications. PACS 75.50.Pp; 78.66.Hf; 75.70.Ak; 75.75.+a     

Effect of external actions on the magnetic properties and corrosion resistance of Co<Subscript>70.5</Subscript>Fe<Subscript>0.5</Subscript>Cr<Subscript>4</Subscript>Si<Subscript>7</Subscript>B<Subscript>18</Subscript> amorphous alloy

Variations in the magnetic characteristics (specific saturation magnetization and coercive force) of Co–Fe–Cr–Si–B amorphous alloy (AA) are studied after high-pressure torsion (HPT) and heat treatment. The behavior of AA magnetic properties is analyzed with respect to structural transformations caused by external actions. The corrosion resistance of AA upon transitioning from an amorphous to a crystalline state is investigated. The established optimum annealing and HPT conditions yield a satisfactory combination of the magnetic properties and corrosion resistance of the investigated alloy.