Showing papers in "Journal of Applied Physics in 1998"

Hexagonal pore arrays with a 50-420 nm interpore distance formed by self-organization in anodic alumina

Abstract: Self-organized hexagonal pore arrays with a 50–420 nm interpore distance in anodic alumina have been obtained by anodizing aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays with distances as large as 420 nm were obtained under a constant anodic potential in phosphoric acid. By comparison of the ordered pore formation in the three types of electrolyte, it was found that the ordered pore arrays show a polycrystalline structure of a few micrometers in size. The interpore distance increases linearly with anodic potential, and the relationship obtained from disordered porous anodic alumina also fits for periodic pore arrangements. The best ordered periodic arrangements are observed when the volume expansion of the aluminum during oxidation is about 1.4 which is independent of the electrolyte. The formation mechanism of ordered arrays is consistent with a previously proposed mechanical stress model, i.e., the repulsive forces between neighboring pores at the metal/oxide interface promote the formation of hexagonally ordered pores during the oxidation process.

Density functional theory for calculation of elastic properties of orthorhombic crystals: Application to TiSi2

Abstract: A theoretical formalism to calculate the single crystal elastic constants for orthorhombic crystals from first principle calculations is described. This is applied for TiSi2 and we calculate the elastic constants using a full potential linear muffin-tin orbital method using the local density approximation (LDA) and generalized gradient approximation (GGA). The calculated values compare favorably with recent experimental results. An expression to calculate the bulk modulus along crystallographic axes of single crystals, using elastic constants, has been derived. From this the calculated linear bulk moduli are found to be in good agreement with the experiments. The shear modulus, Young’s modulus, and Poisson’s ratio for ideal polycrystalline TiSi2 are also calculated and compared with corresponding experimental values. The directional bulk modulus and the Young’s modulus for single crystal TiSi2 are estimated from the elastic constants obtained from LDA as well as GGA calculations and are compared with the ...

On the optical band gap of zinc oxide

Abstract: Three different values (3.1, 3.2, and 3.3 eV) have been reported for the optical band gap of zinc oxide single crystals at room temperature. By comparing the optical properties of ZnO crystals using a variety of optical techniques it is concluded that the room temperature band gap is 3.3 eV and that the other values are attributable to a valence band-donor transition at ∼3.15 eV that can dominate the optical absorption when the bulk of a single crystal is probed.

Frequency response of cantilever beams immersed in viscous fluids with applications to the atomic force microscope

Abstract: The vibrational characteristics of a cantilever beam are well known to strongly depend on the fluid in which the beam is immersed. In this paper, we present a detailed theoretical analysis of the frequency response of a cantilever beam, that is immersed in a viscous fluid and excited by an arbitrary driving force. Due to its practical importance in application to the atomic force microscope (AFM), we consider in detail the special case of a cantilever beam that is excited by a thermal driving force. This will incorporate the presentation of explicit analytical formulae and numerical results, which will be of value to the users and designers of AFM cantilever beams.

Plasma assisted molecular beam epitaxy of ZnO on c -plane sapphire: Growth and characterization

Abstract: ZnO single crystal thin films were grown on c-plane sapphire using oxygen microwave plasma assisted molecular beam epitaxy. Atomically flat oxygen-terminated substrate surfaces were obtained by pre-growth cleaning procedures involving an oxygen plasma treatment. A two dimensional nucleation during the initial growth which is followed by a morphology transition to three dimensional nucleation was observed by in situ reflection high energy electron diffraction. X-ray diffraction (XRD) and photoluminescence investigations suggest that the ZnO epilayer consists of a high quality layer on top of a transition layer containing a high density of defects in the interfacial region. A full width at half maximum (FWHM) of 0.005° is obtained for the ZnO(0002) diffraction peak in an XRD rocking curve, while a broad tail extending from the peak can also be observed. The photoluminescence spectra exhibit dominant bound exciton emission with a FWHM of 3 meV at low temperatures and free exciton emission combined with a ver...

Demagnetizing factors for rectangular ferromagnetic prisms

Abstract: An analytic expression is given for the magnetometric demagnetizing factors of the general rectangular prism, with special emphasis on the particular case of a square cross section. It is argued that this demagnetizing factor should be used in numerical computations that assume such prisms, when comparing the results with the theoretical study of ellipsoids.

Fabrication of green and orange photoluminescent, undoped ZnO films using spray pyrolysis

Abstract: Photoluminescent, undoped ZnO films have been fabricated using spray pyrolysis of zinc nitrate solution The luminescent films had a polycrystalline hexagonal wurtzite type structure with no preferred orientation Photoluminescence intensity was critically dependent on substrate temperature during spray pyrolysis and on post-annealing temperature Green, photoluminescent films possessed a porous structure while orange films possessed a close packed granular morphology Green luminescence appears to be due to oxygen vacancies in a layer just below the crystallite surface

Ellipsometric determination of optical constants for silicon and thermally grown silicon dioxide via a multi-sample, multi-wavelength, multi-angle investigation

Abstract: Optical constant spectra for silicon and thermally grown silicon dioxide have been simultaneously determined using variable angle of incidence spectroscopic ellipsometry from 0.75 to 6.5 eV. Spectroscopic ellipsometric data sets acquired at multiple angles of incidence from seven samples with oxide thicknesses from 2 to 350 nm were analyzed using a self-contained multi-sample technique to obtain Kramers–Kronig consistent optical constant spectra. The investigation used a systematic approach utilizing optical models of increasing complexity in order to investigate the need for fitting the thermal SiO2 optical constants and including an interface layer between the silicon and SiO2 in modeling the data. A detailed study was made of parameter correlation effects involving the optical constants used for the interface layer. The resulting thermal silicon dioxide optical constants were shown to be independent of the precise substrate model used, and were found to be approximately 0.4% higher in index than publis...

Experimental and theoretical study of a glow discharge at atmospheric pressure controlled by dielectric barrier

Abstract: The aim of this paper is to confirm the existence of atmospheric pressure dielectric controlled glow discharge and to describe its main behavior. Electrical measurements, short time exposure photographs, and numerical modeling were used to achieve this task. Experimental observations and numerical simulation are in good agreement. Therefore, the analysis of the calculated space and time variations of the electric field together with the ion and electron densities helps to explain the discharge mechanisms involved, showing the main role played by the electron as well as helium metastable density just before the discharge is turned on.

Tin doped indium oxide thin films: Electrical properties

Abstract: Tin doped indium oxide (ITO) films are highly transparent in the visible region, exhibiting high reflectance in the infrared region, and having nearly metallic conductivity. Owing to this unusual combination of electrical and optical properties, this material is widely applied in optoelectronic devices. The association of these properties in a single material explains the vast domain of its applicability and the diverse production methods which have emerged. Although the different properties of tin doped indium oxide in the film form are interdependent, this article mainly focuses on the electrical aspects. Detailed description of the conduction mechanism and the main parameters that control the conductivity is presented. On account of the large varieties and differences in the fabrication techniques, the electrical properties of ITO films are discussed and compared within each technique.

Minimization of ion micromotion in a Paul trap

Abstract: Micromotion of ions in Paul traps has several adverse effects, including alterations of atomic transition line shapes, significant second-order Doppler shifts in high-accuracy studies, and limited confinement time in the absence of cooling. The ac electric field that causes the micromotion may also induce significant Stark shifts in atomic transitions. We describe three methods of detecting micromotion. The first relies on the change of the average ion position as the trap potentials are changed. The second monitors the amplitude of the sidebands of a narrow atomic transition, caused by the first-order Doppler shift due to the micromotion. The last technique detects the Doppler shift induced modulation of the fluorescence rate of a broad atomic transition. We discuss the detection sensitivity of each method to Doppler and Stark shifts, and show experimental results using the last technique.

Model for strain and magnetization in magnetic shape-memory alloys

Abstract: The large magnetic-field-induced strains observed in martensitic phases based on Ni2MnGa and in other magnetic shape memory alloys are believed to arise from a process of twin-boundary motion rather than magnetostriction. The dependence of strain on magnetization, e(M), generally shows a large component that is linear (rather than quadratic) in M below saturation (quadratic dependence being typical of magnetostrictive strain). A simple phenomenological model for the magnetization process and field-induced strain by twin-boundary and phase-boundary motion is proposed for both the strong and weak anisotropy cases. The model is shown to account for the nearly linear dependence of strain on magnetization in the martensitic phases of these materials. It shows the field dependence of the magnetization and strain to be functions of an effective stiffness constant, C, the transformation strain, e0, and the magnetic anisotropy of the martensitic phase, Ku, through two reduced field parameters, he=MsH/Ce02 and ha=M...

Indium-tin oxide treatments for single- and double-layer polymeric light-emitting diodes: The relation between the anode physical, chemical, and morphological properties and the device performance

Abstract: We report combined studies of the influence of chemical and physical treatments on the properties of indium–tin oxide (ITO) thin films. The ITO films were also used as transparent anodes of polymeric light-emitting diodes (LEDs) incorporating poly(p-phenylene vinylene) (PPV) as the emitter material, with, or without, doped poly(3,4-ethylene dioxythiophene) (PEDOT) as a hole-injection/transport layer. Structures based on a soluble green derivative of PPV, poly(4,4′-diphenylene diphenylvinylene) were also tested. We studied chemical (aquaregia, degreasing, RCA protocol) and physical (oxygen and argon plasmas, Teflon, and paper rubbing) treatments and, in contrast to recently published work, we find that for Balzer Baltracon ITO, oxygen plasma and not aquaregia yields the highest efficiencies and luminances and the lowest drive voltages. For oxygen-plasma-treated anodes, the device efficiency clearly correlates with the value of the ITO surface work function, which in turn depends on the time of treatment. I...

Scattering of electrons at threading dislocations in GaN

Abstract: A model to explain the observed low transverse mobility in GaN by scattering of electrons at charged dislocation lines is proposed. Filled traps along threading dislocation lines act as Coulomb scattering centers. The statistics of trap occupancy at different doping levels are investigated. The theoretical transverse mobility from Coulomb scattering at charged traps is compared to experimental data. Due to the repulsive potential around the charged dislocation lines, electron transport parallel to the dislocations is unaffected by the scattering at charged dislocation lines.

Electroluminescence from heterostructures of poly(phenylene vinylene) and inorganic CdSe nanocrystals

Abstract: Electroluminescence (EL) and photoluminescence (PL) from heterostructure thin films made of organic poly (phenylene vinylene), PPV, and inorganic semiconductor CdSe nanocrystals are investigated. In these devices, the organic PPV structure is built next to an indium tin oxide anode, using the technique of molecular layer-by-layer sequential adsorption, and serves primarily as the hole transport layer. The inorganic layer, adjacent to an Al electrode, is made of spin cast CdSe nanocrystals, passivated with either organic groups or with a wider band gap semiconductor, e.g., ZnS in the present case. We find that the electroluminescence signal is almost exclusively generated within the inorganic layer, with a very weak contribution from the PPV layer at higher applied voltage. The performance of these heterostructure devices is influenced by the thickness of the dot layer. Lifetime tests reveal promising stability, with devices operating continuously over 50–100 h. Values of the external quantum efficiency, η...

Contact analysis of elastic-plastic fractal surfaces

Abstract: Rough surfaces are characterized by fractal geometry using a modified two-variable Weierstrass–Mandelbrot function. The developed algorithm yields three-dimensional fractal surface topographies representative of engineering rough surfaces. This surface model is incorporated into an elastic-plastic contact mechanics analysis of two approaching rough surfaces. Closed form solutions for the elastic and plastic components of the total normal force and real contact area are derived in terms of fractal parameters, material properties, and mean surface separation distance. The effects of surface topography parameters and material properties on the total deformation force are investigated by comparing results from two- and three-dimensional contact analyses and elastic and elastic-perfectly plastic material behaviors. For normal contact of elastic-perfectly plastic silica surfaces and range of surface interference examined, the interfacial force is predominantly elastic and the real contact area is approximately ...

Al-doped zinc oxide films deposited by simultaneous rf and dc excitation of a magnetron plasma: Relationships between plasma parameters and structural and electrical film properties

Abstract: A new technique of the simultaneous excitation of a magnetron sputtering discharge by rf and dc was used for the deposition of undoped ZnO- and Al-doped ZnO (ZnO:Al) films. By this technique, it was possible to change the ion-to-neutral ratio ji/jn on the substrates during the film growth by more than a factor of ten, which was revealed by plasma monitor and Langmuir probe measurements. While for a pure dc discharge the ions impinging onto a floating substrate have energies of about Ei≈17 eV, the rf discharge is characterized by Ar-ion energies of about 35 eV. Furthermore, the ion current density for the rf excitation is higher by a factor of about five, which is caused by the higher plasma density in front of the substrate. This leads to a much higher ion-to-neutral ratio ji/jn on the growing film in the case of the rf discharge, which strongly influences the structural and electrical properties of the ZnO(:Al) films. The rf-grown films exhibit about the three times lower specific resistances (ρ≈6×10−4 Ω...

Luminescence studies of localized gap states in colloidal ZnS nanocrystals

Abstract: ZnS nanoparticles were prepared by chemical precipitation of Zn2+ with sulfur ions in aqueous solution. The ultraviolet-excited samples reveal detailed structure in the luminescence spectra. A doublet pattern observed in the long wavelength region is attributed to the coexistence of the two crystalline forms in ZnS particles. The visible luminescent radiation at 590.1 nm is due to Mn impurities. The dominant emission band at short wavelengths exhibits a quadruple fine structure with peaks located at 416.1, 423.9, 430.1, and 437.8 nm which are identified with optical transitions arising from vacancy and interstitial sites for both Zn and S atoms.

Resolution and contrast in Kelvin probe force microscopy

Abstract: The combination of atomic force microscopy and Kelvin probe technology is a powerful tool to obtain high-resolution maps of the surface potential distribution on conducting and nonconducting samples. However, resolution and contrast transfer of this method have not been fully understood, so far. To obtain a better quantitative understanding, we introduce a model which correlates the measured potential with the actual surface potential distribution, and we compare numerical simulations of the three-dimensional tip–specimen model with experimental data from test structures. The observed potential is a locally weighted average over all potentials present on the sample surface. The model allows us to calculate these weighting factors and, furthermore, leads to the conclusion that good resolution in potential maps is obtained by long and slender but slightly blunt tips on cantilevers of minimal width and surface area.

Underlying physics of the thermochemical e model in describing low-field time-dependent dielectric breakdown in sio2 thin films

Abstract: The underlying physics behind the success of the thermochemical E model in describing time-dependent dielectric breakdown (TDDB) in SiO2 thin films is presented. Weak bonding states can be broken by thermal means due to the strong dipolar coupling of intrinsic defect states with the local electric field in the dielectric. This dipole-field coupling serves to lower the activation energy required for thermal bond-breakage and accelerates the dielectric degradation process. A temperature-independent field acceleration parameter γ and a field-independent activation energy ΔH can result when different types of disturbed bonding states are mixed during TDDB testing of SiO2 thin films. While γ for each defect type alone has the expected 1/T dependence and ΔH shows a linear decrease with electric field, a nearly temperature-independent γ and a field-independent ΔH can result when two or more types of disturbed bonding states are mixed. The good agreement between long-term TDDB data and the thermochemical model su...

Low frequency oscillations in a stationary plasma thruster

Abstract: Stationary plasma thrusters are ion thrusters whose properties make them especially suitable for satellite station keeping or orbit transfer. In these thrusters, a magnetic field transverse to the electron flow towards the anode increases the electron collision frequency and makes possible the generation of a plasma at relatively low gas flow and gas density. The decrease of the plasma conductivity due to the magnetic field induces a large electric field in the plasma which accelerates the quasicollisionless ions whose trajectories are not significantly affected by the magnetic field. The purpose of this article is to clarify, using results from a simple model, the electrical properties of these thrusters and the low frequency oscillation regime which has been observed experimentally. The model is based on the assumption of quasineutrality of the plasma column and on a 1D transient hybrid treatment of electron and ion transport in the device. Electrons are considered as a fluid and ions are described with a collisionless kinetic equation. This model provides reasonable estimates of the plasma properties and is able to give a clear picture of the low frequency oscillations, qualitatively close to the experimental observations.

Structure and magnetic properties of (Fe0.5Co0.5)88Zr7B4Cu1 nanocrystalline alloys

Abstract: The development of Fe73.5Si13.5B9Nb3Cu1 (FINEMET) by Yoshizawa et al. and Fe88Zr7B4Cu1 (NANOPERM) by Inoue et al. have shown that nanocrystalline microstructures can play an important role in the production of materials with outstanding soft magnetic properties. The FINEMET and NANOPERM materials rely on nanocrystalline α-Fe3Si and α-Fe, respectively, for their soft magnetic properties. The magnetic properties of a new class of nanocrystalline magnets are described herein. These alloys with a composition of (Fe,Co)–M–B–Cu (where M=Zr and Hf) are based on the α- and α′-FeCo phases, have been named HITPERM magnets, and offer large magnetic inductions to elevated temperatures. This report focuses on thermomagnetic properties, alternating current (ac) magnetic response, and unambiguous evidence of α′-FeCo as the nanocrystalline ferromagnetic phase, as supported by synchrotron x-ray diffraction. Synchrotron data have distinguished between the HITPERM alloy, with nanocrystallites having a B2 structure from the ...

A phenomenological model for systematization and prediction of doping limits in II–VI and I–III–VI2 compounds

Abstract: Semiconductors differ widely in their ability to be doped As their band gap increases, it is usually possible to dope them either n or p type, but not both This asymmetry is documented here, and explained phenomenologically in terms of the “doping pinning rule”

Comparison of two methods for describing the strain profiles in quantum dots

Abstract: The electronic structure of interfaces between lattice-mismatched semiconductors is sensitive to the strain. We compare two approaches for calculating such inhomogeneous strain—continuum elasticity [(CE), treated as a finite difference problem] and atomistic elasticity. While for small strain the two methods must agree, for the large strains that exist between lattice-mismatched III-V semiconductors (e.g., 7% for InAs/GaAs outside the linearity regime of CE) there are discrepancies. We compare the strain profile obtained by both approaches (including the approximation of the correct C2 symmetry by the C4 symmetry in the CE method) when applied to C2-symmetric InAs pyramidal dots capped by GaAs.

Probing ultrafast carrier and phonon dynamics in semiconductors

Abstract: Over the past 2 decades there has been tremendous advancements in the field of ultrafast carrier dynamics in semiconductors. The driving force behind this movement other than the basic fundamental interest is the direct application of semiconductor devices and the endless need for faster response and faster processing of information. To improve and develop microelectronics devices and address these needs, there must be a basic understanding of the various dynamical processes in the semiconductors which have to be studied in detail. Therefore, the excitation of semiconductors out of their equilibrium and the subsequent relaxation processes with various rates has become a key area of semiconductor research. With the development of lasers that can generate pulses as short as a few femtoseconds the excitation and subsequent probing of semiconductors on an ultrashort timescale have become routine. Processes such as carrier momentum randomization, carrier thermalization, and energy relaxation have been studied in detail using excite-and-probe novel techniques. This article reviews the status of ultrafast carrier and phonon dynamics in semiconductors. Experimental techniques such as excite-and-probe transmission, time-resolved up-conversion luminescence, and pump-probe Raman scattering along with some of the significant experimental findings from probing semiconductors are discussed. Finally, a selfconsistent theoretical model, which correlates the carrier and phonon dynamics in germanium on an ultrashort time scale, is described in detail.

Young’s modulus of single-walled carbon nanotubes

Abstract: We report in detail that unlike other materials, carbon nanotubes are so small that changes in structure can affect the Young’s modulus. The variation in modulus is attributed to differences in torsional strain, which is the dominant component of the total strain energy. Torsional strain, and correspondingly Young’s modulus, increases significantly with decreasing tube diameter and increases slightly with decreasing tube helicity.

Quantifying strain dependence in “colossal” magnetoresistance manganites

Abstract: Experimental, phenomenological, and theoretical analyses are given of the dependence on strain of the ferromagnetic Tc of the colossal magnetoresistance (CMR) rare earth manganese perovskites. It is found that Tc is extremely sensitive to biaxial strain; by implication other physical properties are also. The results indicate that biaxial strain is an important variable which must be considered in the design of devices based on thin films and provide evidence in favor of the relevance of the Jahn–Teller electron-phonon coupling to the CMR phenomenon.

Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids: Application to Tm3+ and Ho3+ ions in LiYF4

Abstract: The measurement of branching ratios, cross sections and radiative lifetimes for rare earth ions in solids is considered The methods are applied to Tm and Ho in YLF as a test case De-activation rates for electric dipole and magnetic dipole emission are calculated for many of the lower lying manifolds in Tm:YLF and Ho:YLF in the context of the Judd-Ofelt theory to determine radiative lifetimes Measured values for the branching ratios as well as the absorption and emission cross sections are also presented for many of the excited state manifolds From these measurements, a methodology is developed to extract measured values for the radiative lifetimes These results are compared with the Judd-Ofelt theory as a guide for consistency and for determining the accuracy of the Judd-Ofelt theory in predicting branching ratios and radiative lifetimes The parameters generated by the methods covered here have potential applications for more accurate modeling of Tm:Ho laser systems

Thermodynamics and kinetics of the Mg65Cu25Y10 bulk metallic glass forming liquid

Abstract: The thermodynamics and kinetics of the bulk metallic glass forming Mg65Cu25Y10 liquid were investigated using differential scanning calorimetry and three-point beam bending. The experiments lead to the determination of the thermodynamic functions as well as the viscosity of the supercooled liquid. The viscosity shows a temperature dependence, which is consistent with that of a strong glass similar to Zr–Ti–Cu–Ni–Be bulk metallic glasses or sodium silicate glasses. This contrasts with more fragile conventional metallic glass formers or pure metals. The relatively weak temperature dependence of the thermodynamic functions of the supercooled liquid is related to these sluggish kinetics in the supercooled liquid. Entropy, viscosity, and kinetic glass transition are compared in the frameworks of the fragility concept and the Adam–Gibbs theory. Strong liquid behavior retards the formation of crystals kinetically and thermodynamically.

Transparent, metallo-dielectric, one-dimensional, photonic band-gap structures

Abstract: We investigate numerically the properties of metallo-dielectric, one-dimensional, photonic band-gap structures. Our theory predicts that interference effects give rise to a new transparent metallic structure that permits the transmission of light over a tunable range of frequencies, for example, the ultraviolet, the visible, or the infrared wavelength range. The structure can be designed to block ultraviolet light, transmit in the visible range, and reflect all other electromagnetic waves of lower frequencies, from infrared to microwaves and beyond. The transparent metallic structure is composed of a stack of alternating layers of a metal and a dielectric material, such that the complex index of refraction alternates between a high and a low value. The structure remains transparent even if the total amount of metal is increased to hundreds of skin depths in net thickness.