Volume 85, Issue 8, 15 April 1999
- classical phenomenology: electricity, magnetism, optics, acoustics, heat, mechanics (pacs 41-52)
- condensed matter: structure, mechanical, and thermal properties (pacs 61-68)
- condensed matter: electrical and magnetic properties (pacs 71-76)
- condensed matter: dielectric and optical properties (pacs 77-79)
- interdisciplinary physics (pacs 81-98)
- 43rd annual conference on magnetism and magnetic materials
- thin film media i
- cmr theory and layered manganites
- thermal viscosity and hysteresis modeling
- amorphous/nanocrystalline materials
- gmr: fundamentals i
- gmr and spin valves i
- new and magnetoelastic materials
- spin glasses and frustrated systems
- spin dynamics i
- gmr: fundamentals ii
- soft crystalline thin films
- instrumentation and measurement i
- magnetic imaging i
- surface and interface studies
- microwave materials
- shielding, levitation, propulsion
- modified hard magnets
- thin film media ii
- electronic and magnetic structure i
- magnetic memory and elements
- cmr-thin films i
- multilayer magnetism
- noncollinear spins
- microwave and mm wave devices
- hard magnets i
- power and control magnets
- exchange bias i
- thin films i
- recording heads, systems, and theory
- thin film media iii
- gmr and spin valves ii
- thin films ii
- spin dynamics ii
- magneto-optic materials
- amorphous/nanocrystalline alloys
- instrumentation and measurement ii
- magnetic imaging ii
- nanoparticles and domains
- numerical techniques and eddy currents
- microstructures and patterned nanostructures
- magnetic tunnel junctions i
- thin films iii
- recording systems
- superconductivity and mixed valence
- new magnetic materials
- cmr-bulk studies
- cmr-thin films ii
- gianto magneto-impedance
- magnetic sensors (not recording)
- particles and nanostructures i
- particles and nanostructures ii
- exchange bias ii
- particulate media
- cmr-manganites and other half-metallic ferromagnets
- hard-medium interface and tribology
- spin dynamics iii
- hard magnets ii
- bio/chemical magnetism
- new applications
- magnetic multilayers i
- magnetic multilayers ii
- anisotropic magnetoresistance and hall effect
- magnetic tunnel junctions ii
- standard problems in micromagnetics
- magnetoresistive magnetic random access memory
- recording heads
- multilayer coupling
- nanoscale and film hard magnets
- magnetic semiconductors and semimetals
- magnetic fluids
- magneto-optic media and materials
- crystalline magnetic films
- soft crystalline alloys
- superconductivity and mixed valence ii
- critical phenomena
- exchange bias iii
- thin film media iv
- patterned nanostructures
- computational micromagnetics
- electronics and magnetic structure ii
- magneto-elastic materials
Index of content:
- CLASSICAL PHENOMENOLOGY: ELECTRICITY, MAGNETISM, OPTICS, ACOUSTICS, HEAT, MECHANICS (PACS 41-52)
85(1999); http://dx.doi.org/10.1063/1.370294View Description Hide Description
The spontaneous emission factor β is an important parameter for the characterization of semiconductorlight emitting devices. In the analysis of superluminescent diodes, especially in the calculation of the optical intensity using rate equations, most authors have used the estimated value of β taken from laser diodes, despite the conceptual difference involved in each device. In this article, the spontaneous emission factor β for superluminescent diodes is discussed in detail, and a new method in calculating the average value of β is introduced. Based on this method, the values of β for gain-guided and index-guided structures are obtained.
A theoretical model for neutral velocity distributions at a planar target in plasma source ion implantation85(1999); http://dx.doi.org/10.1063/1.370295View Description Hide Description
A theoreticalmodel is developed to study the velocity distributions of neutrals striking a planar target in plasma sourceion implantation (PSII) for the case in which the pressure of the gas is large enough that the fast neutrals can be produced in the sheath by ion-neutral charge exchangecollisions. An analytic expression for the neutral velocity distribution at the target is provided. The theoretic results agree with Monte Carlo simulations.
production and loss mechanisms in fluorocarbon discharges: Fluorine-poor conditions and polymerization85(1999); http://dx.doi.org/10.1063/1.370296View Description Hide Description
The study of CF and radical production and loss mechanisms in capacitively-coupled 13.56 MHz plasmas has been extended to plasmaswith an Si substrate, and to plasmas, conditions where the atomic fluorine concentration is lower and where more polymerdeposition occurs on the reactor surfaces. Processes in the gas phase and at the reactor surfaces were investigated by time resolved axial concentration profiles obtained by laser induced fluorescence, combined with absolute calibration techniques. The results for CF were similar to those observed in the fluorine rich case, whereas the results for were strikingly different and more complex. This paper focuses on the radical, which, under these conditions is produced at all of the surfaces of the reactor, apparently via a long-lived surface precursor. The results can only be explained if large polymeric ions and/or neutrals are produced by polymerization in the gas phase. The gas-phase concentration is high, causing the otherwise slow gas-phase concatenation reactions to occur. These processes produce high-mass neutrals (and ions) which are the real polymer precursors. The radical therefore circulates in a closed cycle between the surface and the gas phase. The degree of polymerization is controlled by the fluorine atom concentration, which simultaneously controls the concentrations of of chain initiating species such as and of dangling bonds on the growing oligomers. This model appears to apply to fluorocarbon discharges in general, and agrees well with other results presented in the literature.
85(1999); http://dx.doi.org/10.1063/1.370297View Description Hide Description
The speed of conversion of infrared (IR) images by a planar semiconductorgas discharge system into the visible range has been investigated. Argon or nitrogen are used in the discharge gap having an electrode distance of 100 m. Using pulse radiation from an IR laser to excite the system, we have shown that the characteristic response time of the device with the cryogenic discharge in the gap can lie in the submicrosecond range. This characteristic of the system can be applied for a fast IR imaging at a rate higher than frame/s.
Ion energy distributions and sheath voltages in a radio-frequency-biased, inductively coupled, high-density plasma reactor85(1999); http://dx.doi.org/10.1063/1.370298View Description Hide Description
Ion energy distributions were measured at a grounded surface in an inductively coupled, high-density plasma reactor for pure argon, argon–helium, and argon–xenon discharges at 1.33 Pa (10 mTorr), as a function of radio-frequency (rf) bias amplitude, rf bias frequency, radial position, inductive source power, and ion mass. The ground sheath voltage which accelerates the ions was also determined using capacitive probe measurements and Langmuir probe data. Together, the measurements provide a complete characterization of ion dynamics in the sheath, allowing ion transit time effects to be distinguished from sheath impedance effects. Models are presented which describe both effects and explain why they are observed in the same range of rf bias frequency.
- CONDENSED MATTER: STRUCTURE, MECHANICAL, AND THERMAL PROPERTIES (PACS 61-68)
Reduction of density of subgrain boundaries and misfit dislocations in epitaxial (001) thin films: Effect on dielectric tunability85(1999); http://dx.doi.org/10.1063/1.370299View Description Hide Description
The effect of a buffer layer on the quality of rf magnetron sputteredepitaxial (001) thin films on a substrate has been investigated using high resolution transmission electron microscopy. Magnetron sputtered films generally exhibit a columnar subgrain morphology. By using a 75 nm thick buffer layer the subgrain boundary area was reduced considerably compared to single layer films. Subgrain widths around 130 nm were observed, which corresponded to an 80% reduction of the subgrain boundary area. The density of misfit dislocations was also reduced by 80%. By using a buffer layer, the dielectric tunability increased 30%, to a value of 1.43 (at V/μm, 2 MHz, 77 K). The influence of interfacial strain and misfit dislocations on the subgrain structure, and corresponding effect on the dielectric constant is elucidated.
85(1999); http://dx.doi.org/10.1063/1.370300View Description Hide Description
The use of a convergent beam to excite guided modes in a liquid crystal layer in order to determine the director profile is described. The technique has two main advantages over the conventional collimated beam procedure. First, the convergent beam guided mode (CBGM) technique uses a highly focused beam spot that simultaneously excites many guided modes and produces reflectivity data over a wide angle range. Such reflectivity can be captured in milliseconds with a charge coupled device array and can therefore potentially be used to study cell dynamics. Second, the CBGM technique removes the need to rotate the liquid crystal cell and consequently the focused beam remains completely stationary on the liquid crystal layer, allowing examination of single pixels. The work presented here is a demonstration of the CBGM technique via a study of the static director profile of a homogeneously aligned nematic (E7) liquid crystal layer.
85(1999); http://dx.doi.org/10.1063/1.370301View Description Hide Description
Photoexciteddefect clusters in the bulk of crystals are investigated using a microscopic fluorescence imaging system with 1 μm spatial resolution. The observed defect cluster concentration is approximately depending on the crystal growth method and sector of the crystal. The intensity of the emission clusters varies widely within the image field while a nearly uniformly distributed background is present. Spectroscopic measurements provided information on the emission characteristics of the observed defect population.
On the migration behavior of metal impurities in Si during secondary ion mass spectrometry profiling using low-energy oxygen ions85(1999); http://dx.doi.org/10.1063/1.370302View Description Hide Description
Secondary ion mass spectrometry(SIMS) was used to investigate the segregation of several metal impurities in Si under low-energy oxygen ion bombardment. Our results suggested that both the segregation of Ca, Cr, and Ta at the interface, and the antisegregation of Ti, Hf, and Zr into the oxide were thermodynamically driven. The migration behavior of Ca indicates that CaO, having a higher heat of formation than Si, was most probably formed under oxygen bombardment. Sharper in-depth profiles were obtained for Ti, Zr, and Hf (metals with lower heat of oxide formation than Si) by bombarding at angles of incidence for which a stoichiometric surface oxide is formed. The effect of impurity diffusivity is demonstrated through SIMSmeasurements at elevated temperatures (∼350–380 °C) for Cr, Zr, Ta, and Ti.
85(1999); http://dx.doi.org/10.1063/1.370303View Description Hide Description
Systematic studies of the postgrowth annealing of molecular beam epitaxial deposited SrS:Cu films are reported. In the as-grown SrS:Cu films, the grain size was small and the luminescence very weak. A step-annealing procedure in a atmosphere was developed and found to be a very efficient way to improve the crystallinity and luminescent properties without damage to the low temperature glass substrate and insulator layer of the device. A model is presented in terms of the free energy of formation and the reaction kinetics of Cu with It was proposed that the weak luminescence in the as-grown films was attributed to Cu atoms segregated at the grain boundaries in the SrS film. The oxidation of atomic Cu by and the diffusion of into the SrS lattice during annealing were responsible for the grain growth and the improved luminescent properties.
85(1999); http://dx.doi.org/10.1063/1.370304View Description Hide Description
The growth of GaS films on GaAs(001) surfaces by using thermal evaporation of layered-compound GaS has been investigated by Auger electron spectroscopy, low-energy electron-loss spectroscopy (LEELS), x-ray photoemission spectroscopy(XPS), and atomic force microscopy(AFM) as a function of deposition temperature. The LEELS spectrum of the filmsgrown at lower temperatures resembles that of a GaS single crystal, whereas that of the filmsgrown at reveals that GaAs surface was terminated by layer. XPS spectra suggest that after annealing at S atoms moved from As atoms to Ga atoms to form stable Ga–S bonds at the interface and As–S bonds are observed to be less stable. The band discontinuity at the GaS/GaAs(001) interface estimated by XPS showed the straddling-type I band alignment. Surface morphology of the films studied by AFM reveals the layer-by-layer initial growth of GaS.
85(1999); http://dx.doi.org/10.1063/1.370305View Description Hide Description
Laser interference crystallization of amorphous silicon (a-Si) thin films, a technique that combines pulsed lasercrystallization with holography, enables the fabrication of periodic arrays of polycrystallinesilicon (poly-Si) lines with lateral dimensions between 0.5 and 20 m. The lines consist of grains with well-defined grain boundary locations and lateral dimensions that are appreciably larger than the thickness of the initial a-Si:H film (up to 2 m for a 300 nm thick film). We investigated the dynamics of the crystallization process by two-dimensional finite element computer simulations of the heat transport and phase transitions during laser crystallization. The theoretical results were compared to: (i) measurements of the crystallization kinetics, determined by recording the transient changes of the reflectance during laser exposure, and to (ii) the structural properties of the crystallized films, determined by scanning force and transmission electron microscopy. The simulations indicate that the crystallization front responsible for the large grains propagates laterally from the edges of the molten silicon lines to their centers with a velocity of m/s. A substantial lateral growth only occurs for laser intensities large enough to melt the a-Si film around the center of the lines down to the substrate. Vertical crystallization, which is substantially slower (0.5 m/s), also participates in the solidification process. Using a transfer matrix approach, we converted the time-dependent phase and temperature distributions generated by the simulation program into values for the reflection and transmission of the film as a function of time during and after the laser exposure. A good agreement between the simulated and measured transient reflection was obtained both in the case of homogeneous crystallization as well as that of laser interference crystallization.
The optically active center and its activation process in Er-doped Si thin film produced by laser ablation85(1999); http://dx.doi.org/10.1063/1.370306View Description Hide Description
The local structure of erbium-doped silicon produced by the laser ablation technique is investigated by Er -edge x-rayabsorption fine structure analysis. The combined analysis of extended x-rayabsorption fine structure analysis and an x-rayabsorption near-edge structure simulation based on multiple-scattering theory reveals the most probable atomic coordination of the optically active center; Er bonded with six oxygen atoms has a symmetry. The optical activation process of this system is also discussed. The Si target with 10 wt% has two kinds of local structures, C-rare-earth grain and another Er phase incorporated in Si. The laser ablation homogenizes these phases, and deposits a new single-phase structure of the octahedron ( point group) on the substrates. In this phase, the optical transition probability is low due to the forbidden transition of Er in the crystal field originating from the higher-order symmetry of O. After annealing, degradation of the symmetry from to results in a crystal field suitable for inducing sufficient radiation transition.
85(1999); http://dx.doi.org/10.1063/1.370307View Description Hide Description
A spectroscopic ellipsometry technique is used to attempt a quantitative analysis of thin nanocompositefilms obtained by magnetron co-sputtering. The layers are first fabricated with varying values of deposition temperature and sputteredsilicon area, before being annealed at different temperatures. Using an effective medium model, the ellipsometry tool allowed the estimate of the thickness and the volume fraction of silicon agglomerates or grains, in addition to the optical parameters of the layer through a tentative determination of the dielectric function of the Si nanostructures. The Si content obtained by the ellipsometry approach agrees with those previously determined by direct measurements. Besides the obvious increase of incorporated Si with the sputtered area of this material, we noticed a maximum excess of Si for a deposition temperature in the range. In this respect, the peak position of the photoluminescence (PL) detected only in the annealed samples is found to be closely connected with the amount of excess Si, and in consequence, with the mean size of the thermally grown nanocrystallites. The systematic red shift of maximum PL with the rise of Si content, and implicitly with the increase of the particle size, regardless of the deposition conditions used for the fabrication of the original layer, is liable to allow conclusive argument as for the attribution of the PL in the 1.30–1.65 eV range to the quantum confinement effect. Besides the main contribution of the nanoparticle density, the quality of the interface, as examined by infrared absorption spectroscopy, appears determining for the PL efficiency, and is increasingly improved towards stoichiometric when the annealing temperature is increased up to 1100 °C.
85(1999); http://dx.doi.org/10.1063/1.370308View Description Hide Description
X-ray diffraction has been employed to investigate the strain relaxation of both components of a GaN/AlN bilayer on sapphire (0001) as a function of the GaN layer thickness. Below a critical thickness, GaN and AlN both relax with the same in-plane lattice constant, consistent with the energy minimum condition of elasticity theory for a bilayer. Above the critical thickness, however, the strain relaxations in the two layers were different. We can fit this strain relaxation behavior with a free standing bilayer model with an additional term describing the interaction of dislocations.
- CONDENSED MATTER: ELECTRICAL AND MAGNETIC PROPERTIES (PACS 71-76)
85(1999); http://dx.doi.org/10.1063/1.370309View Description Hide Description
A thorough detailed study of donor and acceptor properties in doped GaAs–(Ga,Al)As semiconductorsuperlattices is performed within the fractional-dimensional approach, in which the real anisotropic “impurity+semiconductor superlattice” system is modeled through an effective isotropic environment with a fractional dimension. In this scheme, the fractional-dimensional parameter is chosen via an analytical procedure and involves no ansatz, and no fittings either with experiment or with previous variational calculations. The present fractional-dimensional calculated results for the donor and acceptor energies in GaAs–(Ga,Al)As semiconductorsuperlattices are found in quite good agreement with previous variational calculations and available experimental measurements.
85(1999); http://dx.doi.org/10.1063/1.370310View Description Hide Description
A hydrodynamic (HD) transport approach based on a closed system of balance equations is developed from the maximum entropy principle. By considering a nonlinear expansion with respect to a local thermodynamic equilibrium, we determine an analytic expression for the distribution function as a function of macroscopic quantities such as density, velocity, energy, deviatoric stress, heat flux associated with charge carriers. From the determined distribution function and considering the collision interactions of carriers with phonons, all the constitutive functions appearing in the fluxes and collisional productions of the balance equations are explicitly calculated. The analytical closure so obtained is applied to the case of some submicron Si structures. Numerical HD calculations are found to compare well with those obtained by an ensemble Monte Carlo simulator thus validating the approach developed here.
85(1999); http://dx.doi.org/10.1063/1.370311View Description Hide Description
Expressions for the coefficient of performance and the cooling power of a thermionic refrigerator are derived taking into account the losses associated with thermal radiation and the need for a so-called passive conductor in the electrical circuit. Specific calculations are made for a thermionic device with a work function of 0.3 eV and it is shown that the performance is generally superior to that of a thermoelectricrefrigerator made from materials with a dimensionless figure of merit ZT equal to 4.
Photocurrent spectroscopy and study of subband parameters for heavy holes in nanoscale multiquantum well structures85(1999); http://dx.doi.org/10.1063/1.370312View Description Hide Description
Nanoscale multiquantum wells structures were studied by photocurrent spectroscopy. Photocurrent spectra showed clear steplike structures accentuated by exciton peaks. Many interband transitions were assigned from the spectral structure. As peaks of forbidden transitions, which appeared in large reverse bias voltages, were extrapolated to zero bias voltage on the photocurrent spectra, transition energies were estimated in a square potential well. New estimation methods of valence band parameters, heavy hole effective mass and valence band offset, were derived from a saturation of the heavy hole subband in the valence potential well, using the envelope function model in the effective mass approximation. The heavy hole effective mass in a direction normal to the quantum well plane and the valence band offset were and 0.22 eV.
Contribution of interface roughness to the infrared spectra of thermally grown silicon dioxide films85(1999); http://dx.doi.org/10.1063/1.370313View Description Hide Description
Interfacial oxide structures were studied by Infrared reflection absorptionspectroscopicanalysis. The longitudinal optical mode of the interfacial oxide films arose with lower frequencies and wider widths. We assumed and later verified that these changes resulted from the contribution of interfacial roughness. We therefore examined the contribution of interfacial roughness to the infrared spectra using the effective medium approximation model and via infrared spectra simulation. The experimental results explained by a model of that interface had an atomically roughness of within 1 nm and chemical structure changes effectively little fluctuation.