bluuebunny/crossref_metadata_2025 huggingface dataset:
Metal–n-6H–SiC surface barrier height—Experimental data and description in the traditional terms
Authors: Syrkin A. L. , Andreev A. N. , Lebedev A. A. , Rastegaeva M. G. , Chelnokov V. E.
Published: July 2002

This work compares our experimental data on surface barrier height in structures metal–n-6H–SiC for several metals (Al, Au, Mo, Cr) with other data available and with classical models of surface barrier formation. We observed decreasing of barrier height with increasing of donors concentration for the structures Au–n-6H–SiC, Mo–n-6H–SiC and Al–n-6H–SiC. We estimate the average surface energy level for (0001)-Si-faced n-6H–SiC, appearing in capacitance–voltage (C–V) measurements, to be about 0.3Eg at room temperature (Eg is the energy band gap) and corresponding surface states density to be about 1013 cm−2 eV−1, based both on our data and data of other authors.
Dynamics of long-pulse laser transfer of micrometer-sized metal patterns as followed by time-resolved measurements of reflectivity and transmittance
Authors: Kántor Z. , Szörényi T.
Published: July 2002

Laser-induced transfer of thin films is a simple single-step technique for surface patterning. In this paper the optimization principles and processes are outlined which led to successful application of the long-pulse laser transfer technique. The critical analysis of experiments on ns-pulse laser transfer of thin films of a variety of metals and the optimization study of the long-pulse laser transfer technique suggests that efficient deposition of high-quality patterns of micrometer dimensions can only be expected when using long laser pulses which not only produce ablation of the thin film pattern in solid phase but also maintain sufficient temperature during transfer and even on landing, to ensure film adherence. In order to identify and understand the different time-dependent processes determining the laser transfer, studies using optical and electron microscopy and static and time-resolved optical measurements were performed.
On the nucleation of an intermediate phase at an interface in the presence of a concentration gradient
Authors: Hoyt J. J. , Brush L. N.
Published: July 2002

The nucleation of an intermediate phase at the planar boundary of two other phases in a diffusion couple is considered. An extension of classical nucleation theory is presented which yields the shape of the critical intermediate phase nucleus and the critical work of formation. With the use of a simple example it is demonstrated that the work of formation decreases, and hence the nucleation rate increases, with increasing interdiffusion time.
Comparison of stepped-well and square-well multiple-quantum-well optical modulators
Authors: Woodward T. K. , Cunningham J. E. , Jan W. Y.
Published: July 2002

The responsivity and transmission performance of 60-period p-i-n multiple-quantum-well (MQW) optical modulators having single or double steps in the quantum well (QW) has been experimentally compared to a square-QW control sample. It has been confirmed that significantly increased shift of the exciton with applied electric field [the quantum confined Stark effect (QCSE)] is obtained in the stepped QWs as compared to a conventional square QW. However, no meaningful increase in optical modulation performance is found, due to broadening of the exciton transition in the stepped QWs. All MQWs contained nominal 45 Å Al0.3Ga0.7As barriers. Single-step QWs consisted of a 15 Å GaAs region with an 85 Å Al0.1Ga0.9As step. Double-step QWs consisted of a central Al0.15Ga0.85As plateau, with 20 Å GaAs regions on either side. The square QW consisted of 90 Å of GaAs. Excellent agreement between measured QCSE and tunneling resonance calculations was found. Our results indicate that stepped MQW devices are intrinsically more susceptible to growth induced degradation than square MQW modulators.
Microstructural changes in GeSbTe film during repetitious overwriting in phase-change optical recording
Authors: Nobukuni N. , Takashima M. , Ohno T. , Horie M.
Published: July 2002

Microstructural changes that occur in a GeSbTe film during repetitious overwriting in phase-change optical recording were investigated. The GeSbTe active layer was melted by a focused laser diode (LD) beam during each overwriting process over amorphous mark formation. The repetitious solidification and liquefaction process in such a short time as 50–200 ns resulted in microstructural changes in the active layer: a segregation, sink, and void formations. The sink was formed in the low-density active layer due to the shrinking of the volume during the resolidification process. Sink formation could be suppressed when a high-density active layer, having more than 80% of the bulk density, was used. Such a high-density GeSbTe film, however, resulted in a void formation of the size of 0.1 μm. The voids were thought to be nucleated by residual vacancies and Ar precipitation, since the active layer contained a high concentration of Ar impurities, due to the atomic peening effect. The subsequent void coalescence and migration processes across the beam scanning direction could result in the formation of thermally discontinuous grooves at the edges of the written marks. The voids could also migrate along the LD beam scanning direction, accompanied by a material flow of the active layer in the opposite direction. These phenomena were also found to depend on the material used to fabricate the protective layers which sandwiched the active layer. A TaOx protective layer enhanced the void migration across the track, resulting in the removal of voids from the center of the track. Use of the ZnS:SiO2 compound protective layer confined voids to the center of the track. The ZnS:SiO2 protective layer also promoted the formation of thermally discontinuous grooves at the edges of amorphous marks. The material flow along the track resulted in a thicker active layer at the start of the consecutive LD irradiation, and also in a high void density region at the final edge of the irradiation having a length on the order of 10 μm. This tendency was found for both the ZnS:SiO2 and TaOx sandwiching media.
Variation of magnetic properties as a function of Mo content in the RFe12−xMox series and their nitrides (R=Y, Nd, Gd)
Authors: Yang Jun , Dong Shengzhi , Mao Weihua , Xuan Ping , Liu Zunxiao , Sun Yunxi , Yang Yingchang , Ge Senlin
Published: July 2002

The structure and magnetic properties of RFe12−xMox series and their nitrides with R=Y,Gd,Nd and x=0.8,1.0,1.5,2.0,2.5 have been studied by x-ray-diffraction and magnetic measurements. It is found that with the increment of Mo content x, the lattice parameters increase, whereas the saturation magnetization, Curie temperature, and anisotropy field decreases in the RFe12−xMox series. Upon nitrogenation lattice parameters, saturation magnetization, and Curie temperature increase. Correspondingly, the easy c-axis magnetocrystalline anisotropy of the Fe sublattice decreases in YFe12−xMoxNy and GdFe12−xMoxNy but increases in NdFe12−xMoxNy although its variation with the Mo content is the same as in their original counterparts. NdFe12−xMoxNy compound in the low Mo content shows great potential for permanent magnet applications.
dc-bias stress of non-stochiometric amorphous silicon nitride thin film diodes
Authors: Nieuwesteeg K. J. B. M. , van der Put A. A. , Johnson M. T. , de Kort C. G. C.
Published: July 2002

Photon-emission experiments on silicon-rich hydrogenated amorphous silicon-nitride metal–semiconductor–metal diodes, have shown the existence of hot electrons under applied field strengths of approximately 106 V/cm. The effective temperatures and mean free path between collision for the electrons were estimated from the spectra. It is shown that, in general, asymmetrical changes in the electrical characteristics of the devices occur after prolonged dc stressing at high fields. Two drift mechanisms can be distinguished. The first is called ‘‘cathodic’’ drift and is driven by recombination between band-tail carriers in the semiconductor. The other is called ‘‘anodic’’ drift, and results from the effects of hot electrons at the anode. The spatial and time dependence of these drift mechanism is explained using a simple model.
Effect of pretreatment process parameters on diamond nucleation on unscratched silicon substrates coated with amorphous carbon films
Authors: Feng Z. , Komvopoulos K. , Bogy D. B. , Ager J. W. , Anders S. , Anders A. , Wang Z. , Brown I. G.
Published: July 2002

Diamond nucleation on unscratched silicon substrates was investigated using a conventional microwave plasma-enhanced chemical vapor deposition system. Silicon substrates were coated with thin films of amorphous carbon using a vacuum arc technique. The carbon-coated silicon substrates were pretreated with a methane-rich plasma at relatively low temperatures and were subsequently exposed to the diamond nucleation conditions. The significance of the pretreatment on the diamond nucleation density was examined by varying the methane concentration, chamber pressure, and exposure time. Scanning electron microscopy demonstrated that densely packed spherical nanoparticles on the pretreated surfaces played the role of diamond nucleation seeds. Raman spectroscopy analysis showed that the nucleation seeds consisted of nonhydrogenated carbon and that their structure was influenced by the pretreatment conditions. Transmission electron microscopy revealed that the nucleation seeds comprised disordered graphitic carbon and ultrafine diamond crystallites. Submicrometer films of good quality diamond possessing significantly higher nucleation densities (∼5×1010 cm−2) were grown from nanoparticles produced under optimum pretreatment conditions. The enhancement of the diamond nucleation density is mainly attributed to the formation of a large number of nanoparticles, which provided sufficient high-surface free-energy sites for diamond nucleation, in conjunction with their high etching resistance to atomic hydrogen stemming from the significant percentage of sp3 atomic carbon configurations, as evidenced by the presence of nanocrystalline diamond in the nanoparticle structure.
Step-flow epitaxial growth on two-domain surfaces
Authors: Desjardins P. , Greene J. E.
Published: July 2002

A general analytical model is presented for the simulation of step-flow epitaxial growth on two-domain surfaces composed of alternating type-A and B terraces. Separate terms are included for adatom attachment and crossing probabilities at ascending and descending steps on each of the two terrace types. The model is used to follow the evolution of terrace size distributions during deposition, focusing primarily on the case of single-terrace adatom migration. Positive attachment asymmetries Δa (i.e., a larger attachment probability at ascending steps) were found, as in the case for one-domain surfaces, to lead to a slow smoothing of size distribution fluctuations. However, even very small negative Δa values result in a rapid increase in fluctuation amplitudes with a tendency toward step bunching and the formation of double-height steps. The two terrace size distributions diverge essentially immediately upon initiating growth since each terrace is bordered by two terraces of the opposite type and only short-range migration is required to stabilize the average widths of the two distributions. Fractional surface coverages fA (B) of A (B) terraces increase at the expense of B (A) terraces when ΔaB (A)≳ΔaA (B). Steady-state average terrace widths are achieved rapidly, within a few monolayers; however, size-distribution standard deviations σ evolve toward steady state slowly (for ΔaA,B≳0) with σA (B)∝exp(−ΔaA (B)θ/λ2) where θ is the number of deposited monolayers and λ is the fluctuation width. Allowing multiterrace migration decreases, under some growth conditions, the rate at which terrace size distributions diverge and introduces oscillations in σA,B(θ). Simulation results are compared with available experimental data.
Zinc incorporation into InP grown by atmospheric pressure metalorganic vapor phase epitaxy
Authors: Logan R. A. , Chu S. N. G. , Geva M. , Ha N. T. , Thurmond C. D.
Published: July 2002

Zinc incorporation into InP grown by atmospheric pressure metalorganic vapor phase epitaxy has been studied systematically as a function of Zn source flow rate, substrate orientation, and growth temperature. Within the growth conditions for device quality layers, a Zn saturation level exists which varies with substrate orientation. The incorporation kinetics is analyzed using a surface adsorption-trapping model. We demonstrate that the Langmuir state of adsorption–desorption process is not established during growth due to the interruption of the surface processes by layer growth. The existence of a saturation level at a given growth condition indicates that Zn atoms incorporate at defect sites instead of the normal growth sites for In. Two parameters are used to characterize the properties of the surface defect, i.e., the capture cross section and the time of desorption for Zn atoms. The implication of these parameters and their substrate orientation and growth temperature dependencies are discussed.
Interpretation of double-crystal x-ray rocking curves in relaxed strained-layer structures
Authors: Lourenço M. A. , Dunstan D. J.
Published: July 2002

Double-crystal x-ray rocking curves from metamorphic epitaxial heterostructures are broadened by the presence of misfit dislocations and this vitiates the usual interpretation of rocking curves using simulation software. However, analysis of the moments of the rocking curve can be useful. Using single layers, we show that the first moment is equivalent to the peak splitting, so that the average strain and average composition of an inhomogeneous layer can be derived from its first moments. The usefulness of the method is demonstrated on complicated heterostructures including graded layers.
Studies on 0.96 and 0.84 eV photoluminescence emissions in GaAs epilayers grown on Si
Authors: Liang Jiachang , Jiang Junhua , Zhao Jialong , Gao Ying
Published: July 2002

Studies on the 0.96 and 0.84 eV photoluminescence (PL) emissions at various temperatures in GaAs epilayers grown on Si with [As]/[Ga]=20–50 by metalorganic chemical vapor deposition were made. In terms of an Arrhenius plot and configurational coordinate model, the thermal activation energy and Franck–Condon (FC) shift for the 0.96 eV emission band were obtained by measuring the variations in its PL intensity and full width at half-maximum with temperature, respectively. The dependence on PL intensity versus temperature of the 0.84 eV PL emission could not be fitted with an Arrhenius plot. Instead, it could be fitted with the formula used for amorphous semiconductors or localized states which allowed us to relate this emission with the presence of defects in the heteroepitaxial GaAs layers grown on Si investigated. Taking into account the FC and band-gap shifts, the energy relationships of the transitions from donor to acceptor, from conduction band to acceptor, and from donor to valence band were reformulated. In terms of these transition-energy relationships and experimental data, the 0.96 eV emission was explained as the recombination luminescence of the donor–acceptor pair, composed of an arsenic vacancy and a gallium vacancy, and the 0.84 eV emission as the transition from the localized As interstitial–Ga vacancy complex center to Ga vacancy.
Trap level characteristics of rare-earth luminescence centers in III–V semiconductors
Authors: Taguchi Akihito , Takahei Kenichiro
Published: July 2002

We have applied a multiphonon-assisted energy transfer model, which has been verified for the Yb-doped InP system, to Er-doped GaAs, Nd-doped GaP, and Nd-doped GaAs. By applying this model, the temperature dependence of the decay time of the 4f-shell luminescence can be calculated using two parameters. One parameter is the energy transfer probability between the rare-earth 4f shell and the semiconductor host, and the other is the energy which has to be compensated for in the energy transfer processes. The values of these two parameters were determined by fitting the calculated temperature dependence to the experimentally obtained results. The calculated temperature dependences fit with the experimental results well, showing that the energy transfer mechanism in these materials is similar to that in InP:Yb. The estimated values of energy which have to be compensated for enable us to estimate the energy level positions responsible for the rare-earth intra-4f-shell luminescence. The values of the transition matrix elements obtained by the fitting were rather large, suggesting that the energy transfer between the rare-earth 4f shell and the semiconductor host is efficient in spite of the fact that it is a phonon-assisted process.
On the threshold voltage and channel conductance of polycrystalline silicon thin-film transistors
Authors: Dimitriadis C. A. , Tassis D. H.
Published: July 2002

A model for the grain-boundary barrier height of undoped polycrystalline silicon thin-film transistors is developed based on a rodlike structure of the grains with a square cross section and a Gaussian energy distribution of the trapping states at the grain boundaries. An analytical expression for the threshold voltage is derived in terms of the distribution parameters of the grain-boundary trapping states, the grain size, and the gate oxide thickness. Comparison between the developed model and the experimental drain current versus gate voltage data has been made and excellent agreement was obtained. The key parameters affecting the threshold voltage and the channel conductance of the transistor were investigated by computer stimulation. The threshold voltage is mainly affected by the grain size and the gate oxide thickness. For the improvement of the channel conductance, besides the passivation of the grain-boundary trapping states, the increase of the grain size and mainly the scaling down of the gate oxide thickness are the key factors.
Thermally induced transformations at an ion implanted α–Fe2O3/α–Al2O3 interface: A Mössbauer spectroscopy study
Authors: Bhagwat Sunita , Joshi Sushama , Ogale S. B. , Marest G. , Benyagoub A. , Moncoffre N. , Thimon F.
Published: July 2002

Thermally induced chemical transformations at the interface between a 100 nm α–Fe2O3 and a polycrystalline α–Al2O3 substrate are studied and compared for the cases of the as-formed and ion bombarded interfaces. The thermal annealings are carried out under vacuum at 450 °C for different time durations and the transformations are examined by conversion electron Mössbauer spectroscopy. It is shown that in the sample prebombarded with 110 keV Ar+ ions, the formation of Fe3−xAlxO4, FeAl2O4, and FeO phases is favored at variance with a nonbombarded sample in which primarily a defective Fe3O4-y phase is produced with some amount of FeO. This difference is explained in terms of the annealing process of the oxygen vacancies produced during Ar implantation near the interfacial region.
Magnetic and structural characteristics of LaCo9+δSi4 and LaCo8.5+δSi4.5 alloys (δ=0–4)
Authors: Huang M. Q. , Wolf Jason , Wallace W. E.
Published: July 2002

LaCo9+δSi4 and LaCo8.5+δSi4.5 alloys with δ=0 to 4 have been synthesized and characterized. The stoichiometric alloys (δ=0) have low Tc and small moments, whereas the alloys with δ≠0 have larger Tc and magnetization.
Correlation between giant magnetoresistance and the antiferromagnetic volume fraction in superlattices
Authors: Takahashi Y. , Inomata K.
Published: July 2002

We investigated the giant magnetoresistance (GMR) of metallic magnetic superlattices and its correlation to the volume fraction of antiferromagnetically aligned magnetic layers. It is assumed that the superlattices are composed of a mixture of regions having, respectively, antiferromagnetic and ferromagnetic alignments. The calculated correlation curves for Co/Cu and FeNiCo/Cu superlattices show good agreement with the experimental results. The upper limits of the GMR in these superlattices are predicted in the case of a perfect antiferromagnetic alignment. It is shown that the observed nonlinear behavior of correlations is attributed to the film quality of superlattices. By assuming the existence of different regions of ferromagnetic but antiparallel alignment, it is understood that the observed results of no GMR for small volume fraction are due to an apparent phenomenon. The GMR is analyzed from the point of view of the preparation conditions of superlattices.
The influence of ambient magnetic environments on high-T
c superconducting quantum interference device gradiometers
Authors: Keene M. N. , Exon N. J. , Humphreys R. G. , Chew N. G.
Published: July 2002

The performance of high-temperature superconducting gradiometers have been evaluated in fields of up to 70 μT to determine their limitations in unshielded environments. The sensors incorporate first-order gradiometric superconducting quantum interference devices (SQUIDs) coupled to first-order gradiometric flux transformers. A gradiometer was operated successfully at all angles with respect to an ambient field with less than 20% variation in critical current and voltage/flux ratio. Magnetic hysteresis at 77 K was measured for field excursions up to 70 μT. The hysteresis factor, referred to the SQUID, averaged 10−3 Φ0/μT. Flux movement in the SQUID washers rather than the flux transformer was found to be the dominant cause of the hysteresis and the presence of the flux transformer reduced the hysteresis. This is interpreted as due to the coupling coil pinning the vortices against radial motion. The spectral noise density of the gradiometer was only weakly dependent on the orientation with respect to static fields of up to 70 μT except for narrow angular ranges where dominating random telegraph signal processes occurred. In contrast, field cooling the gradiometer above 15 μT was found to substantially increase the low-frequency noise. A gradiometer was successfully transported and operated out of doors with no shielding. Only a relatively minor degradation of performance was observed.
Optical gain and spontaneous emission in InGaAs/GaAs multiple quantum well laser diodes
Authors: Schönfelder Alexander , Ralston John D. , Czotscher Konrad , Weisser Stefan , Rosenzweig Josef , Larkins Eric C.
Published: July 2002

We present gain and spontaneous emission measurements in In0.35Ga0.65As/GaAs multiple quantum well lasers. First, the gain is extracted from measured amplified spontaneous emission through a laser facet. Second, unamplified spontaneous emission is detected through the transparent GaAs-substrate. Taking advantage of the fundamental relationship between gain and spontaneous emission, the separation of the electron and hole quasi-Fermi levels, ΔEF, at various bias currents below laser threshold is accurately determined. Theoretically, ΔEF is calculated utilizing a simple two-subband structure model. Fitting the theoretical predictions to the experimental data allows us to determine the threshold carrier density (nth=2.0×1018 cm−3), the in-plane hole effective mass (mv1=0.21m0), and the amount of band gap shrinkage at threshold (ΔEg,th=33 meV).
Temperature evolution of domains in potassium niobate single crystals
Authors: Lian Li , Chong T. C. , Kumagai H. , Hirano M. , Taijing Lu , Ng S. C.
Published: July 2002

The behavior of domain walls in KNbO3 single crystals with temperature variation from room temperature to 300 °C has been investigated in situ by a heating visualization system. It has been observed that domain walls show active behavior in a small temperature range from the phase transition temperature of 225 °C, the range being about 10 °C in the orthorhombic phase and 15 °C in the tetragonal phase. The 90° domain walls are generated randomly and extend themselves rapidly within the crystal in high density. The 60° domain walls, which are only observed in orthorhombic phase, do not appear randomly but are formed along the boundary of intersecting 90° domain walls. The results suggest that the 90° domain walls are most likely caused by microdefects within the crystal, and that dislocations at the junction of intersecting 90° domain walls supply nucleation sites for the 60° domain walls.
Structure and magnetic properties of Fe1−xNix/Cu Invar superlattices
Authors: Tang W. , Gerhards Ch. , Heise J. , Zabel H.
Published: July 2002

We have prepared by sputtering techniques a series of fcc [Fe1−xNix/Cu]×10 superlattices with sublayer thicknesses of 3 nm and with Ni concentrations x ranging from 0.26 to 0.54. The use of MgO single-crystal substrates and Cu sublayers in the superlattice growth ensures a well-defined fcc crystal structure in the Fe–Ni sublayers with a Ni concentration as low as 26 at. % and down to liquid helium temperatures. The magnetization of the Fe–Ni sublayers in the superlattices starts to deviate from the well-known Slater–Pauling curve at 40 at. % Ni, and continues to drop until the fcc–bcc transition is completed. A strong dependence of the magnetization on temperature was also observed for the Fe–Ni sublayers in the Invar range, consistent with the behavior of bulk Fe–Ni Invar alloys.
Characterization of epitaxial La0.7Ba0.3MnO3 structures using ferromagnetic resonance
Authors: Robson M. C. , Kwon C. , Kim K.-C. , Sharma R. P. , Venkatesan T. , Lofland S. E. , Bhagat S. M. , Ramesh R. , Domínguez M. , Tyagi S. D.
Published: July 2002

We have compared a single layer of La0.7Ba0.3MnO3 and a trilayer structure of SrTiO3/La0.7Ba0.3MnO3/SrTiO3, both grown epitaxially on a LaAlO3 substrate, using information obtained by ferromagnetic resonance (FMR). The trilayer samples have a more uniform magnetization and are not susceptible to environmental degradation. This may be due to the strain relief that the buffer SrTiO3 layer provides for the La0.7Ba0.3MnO3 layer. We have also studied the magnetic homogeneity of the trilayer structure as a function of the deposition temperature. The perpendicular FMR linewidth, Γ⊥, shows a clear window in the deposition temperature where the linewidth is <50 Oe. However, the parallel linewidth, Γ∥, is nearly ten times larger than Γ⊥ with only a weak dependence on the deposition temperature. This broadening of the parallel linewidth compared to the perpendicular linewidth can be explained by invoking a local unidirectional anisotropy in the plane of the film.
Optimal control of force microscope cantilevers. II. Magnetic coupling implementation
Authors: Bruland K. J. , Garbini J. L. , Dougherty W. M. , Sidles J. A.
Published: July 2002

We describe the implementation of optimal controllers for damping the motion of cantilevers used in magnetic resonance force microscopy. We demonstrate that optimal control is achievable and that torsional magnetic coupling provides an effective actuation method. Cantilever Brownian vibrational amplitude was reduced from 2 to 0.16 Å and resonant quality was reduced from 2000 to 5. Applied control fields were sufficiently small that they would not affect magnetic resonance phenomena.
Microstructure of diamond and β-SiC interlayer studied by synchrotron x-ray scattering
Authors: Je J. H. , Noh D. Y.
Published: July 2002

The microstructure of the diamond and the β-SiC interlayer grown on Si substrates was studied in synchrotron x-ray scattering experiments. In the process of growing diamond using microwave plasma chemical vapor deposition, a β-SiC interlayer was always formed epitaxially regardless to the orientation and the pretreatment of substrates. The crystalline axes of the β-SiC interlayer were parallel to the substrate crystalline axes. The pretreatment of the silicon substrates greatly enhanced the growth rate of the β-SiC. Meanwhile, the diamond particles were preferentially grown along both the 〈111〉 and the 〈001〉 directions on all the pretreated substrates. The diamond particles that were grown with the preferred growth direction matching to the substrate normal crystalline axis direction exhibited partial epitaxy, while others were grown nonepitaxially. The substrate pretreatment also enhanced the growth of diamond particles significantly.
Deposition-rate reduction through improper substrate-to-electrode attachment in very-high-frequency deposition of a-Si:H
Authors: Meiling H. , van Sark W. G. J. H. M. , Bezemer J. , van der Weg W. F.
Published: July 2002

We have tracked down one of the major causes for nonuniformities in film thickness in large-area deposition of hydrogenated amorphous silicon, a-Si:H. To simulate improper substrate-to-electrode attachment we deliberately introduced a gap behind the substrate. The rf-excitation-frequency dependence of the influence of this gap on the deposition rate is presented. We show that a local small gap behind the glass has a detrimental effect on the local deposition rate, and therefore on the uniformity of the films. For example, at a frequency of 60 MHz typically the reduction of the deposition rate amounts to 25% when a gap of 1 mm is present. To explain the observed effects the plasma-sheath dynamics are considered. The relations between the dc self-bias voltage, the amplitude of the applied rf voltage, and the deposition rate are determined experimentally. A theoretical model that explains the reduction of the deposition rate is presented. We conclude from the model that the ion density in the sheath is independent of the excitation frequency.
Pressure generation and measurement in the rapid vaporization of water on a pulsed-laser-heated surface
Authors: Park Hee K. , Kim Dongsik , Grigoropoulos Costas P. , Tam Andrew C.
Published: July 2002

The transient pressure generated by the interaction of short-pulsed laser light with the liquid–solid interface is studied quantitatively. A KrF excimer laser beam of tens of nanoseconds pulse duration irradiates water on a solid surface and induces rapid thermal expansion and explosive vaporization. The pressure pulses launched into water by such processes are detected experimentally by the photoacoustic probe beam deflection method and a broadband piezoelectric transducer. The peak intensities of the traveling pressure wave measured by these two methods are compared with the theoretical thermoelastic predictions. The measurements show that a compressional pressure wave packet is radiated from the water-solid interface with the peak intensity of the order of 1 MPa at laser fluences up to about 100 mJ/cm2. Simultaneous monitoring of the bubble growth kinetics by the optical specular reflectance probe has been performed. It is observed that the pressure generation is enhanced by the bubble expansion in the superheated water for laser fluences exceeding the bubble nucleation thresholds.
Electron paramagnetic resonance of Nb-doped BaTiO3 ceramics with positive temperature coefficient of resistivity
Authors: Jida Shin’suke , Miki Toshikatsu
Published: July 2002

Paramagnetic centers in Nb-doped BaTiO3 ceramics are measured at 77–500 K by electron paramagnetic resonance (EPR) for investigating the role of the centers on the well-known positive temperature coefficient of resistivity (PTCR) effect (PTCR at the Curie temperature). EPR detects four signals; an anisotropically broad singlet signal at g=2.005, a sextet signal due to Mn2+, a Cr3+ signal, and a Ti3+ signal. The former two signals arise in the rhombohedral and cubic phases, but disappear in the tetragonal and orthorhombic phases. The Cr3+ signal appears in all of the phases, while the Ti3+ signal is detected only at low temperatures. The singlet signal also arises in undoped, barium-deficient BaTiO3 ceramics, therefore the signal is attributable to barium-vacancy-associated centers rather than Nb4+ ions or Fe3+ ions proposed by several authors. In this article, we propose that the singlet signal is due to vacancy-pairs of VBa–F+ type, i.e., the vacancy pair of VBa–VO capturing one electron. The electrical resistivity data show a polaronic character of low-temperature conduction and a high resistivity jump around the Curie temperature. The low-temperature polaronic conduction is explained in terms of electron-hopping between Ti4+ and Ti3+ ions. The resistivity jump at the Curie temperature occurs along with the EPR intensity increase of the singlet signal, the Mn2+ signal and the Cr3+ signal. We conclude that the PTCR of Nb-doped BaTiO3 ceramics is strongly associated with the trap activation of the VBa–VO vacancy-pairs and manganese centers at the tetragonal-to-cubic transition.
Characterization of Stark components and peak emission cross sections of intermanifold and inter-Stark transitions of Nd3+ in Ba0.25Mg2.75Y2Ge3O12
Authors: Sardar D. K. , Stubblefield S. C.
Published: July 2002

The Stark components of the lower-lying manifolds of Nd3+ in Ba0.25Mg2.75Y2Ge3O12 have been characterized using the well-resolved emission spectra for the 4F3/2→4I9/2 and 4F3/2→4I11/2 transitions at 300 K. The emission cross sections of these intermanifold transitions as well as the inter-Stark transitions of R1→X5 at 940.4 nm and of R2→Y3 at 1061.8 nm have been determined. Finally, these results are compared with those of the standard laser host, Nd:YAG, and the potential of Nd:BAMGAR as a laser host is discussed.
Composition effects on the crystal structure of CuInSe2
Authors: Merino J. M. , Martín de Vidales J. L. , Mahanty S. , Díaz R. , Rueda F. , León M.
Published: July 2002

By x-ray powder diffraction and the Rietveld refinement method, the atomic positions in CuInSe2 were determined for compositions close to stoichiometry. The Se position, x(Se), was found to be correlated to the Cu content. According to a model proposed by Jaffe and Zunger [Phys. Rev. B 29, 1882 (1984)], changes in x(Se) induce a variation in the optical band gap, Eg. This could explain the spread in energy gap, Eg, found by many authors for this compound. The increase in the lattice parameter, a, correlated with the difference between the energy dispersive analysis of x rays and x-ray diffraction determined Cu contents, suggests the presence of a fraction of Cu atoms as interstitials.
Diffusion of gold into polycrystalline silicon investigated by means of the radiotracer 195Au
Authors: Poisson Ch. , Rolland A. , Bernardini J. , Stolwijk N. A.
Published: July 2002

Gold diffusion experiments were performed in three types of polycrystalline silicon over the temperature range 551–1265 °C by using the radiotracer 195Au and the serial sectioning technique. Depending on temperature, material structure, and diffusion time, different types of profiles were obtained, some of them being noninterpretable within the framework of classical solutions of Fick’s equations in the presence of grain boundaries. In contrast, all these experimental profiles were successfully analyzed by using a new diffusion model applicable to host material/impurity systems revealing strong segregation effects and a negligible diffusion flux along the extended defects. At temperatures higher than about 1100 °C, effective diffusion coefficients slightly lower than true lattice diffusion coefficients previously reported in the literature were measured. Decreasing the temperature, the effective Au diffusivity depends on the structure of the samples, which leads to different Arrhenius plots exhibiting a downward curvature. A gold segregation enthalpy of about 141 kJ mol−1 was estimated from the observed diffusion behavior.
Bulk solidification and recalescence phenomena in amorphous Ge films upon picosecond pulsed laser irradiation
Authors: Siegel J. , Solis J. , Afonso C. N. , García C.
Published: July 2002

Melting and rapid solidification are induced in amorphous Ge films upon irradiation with 10 ps laser pulses at 583 nm. The role of heat flow during the solidification process was investigated by comparing the behavior of films grown on substrates with different thermal properties. The melting and solidification kinetics are followed in real time by reflectivity measurements in the nanosecond time scale and the induced structural changes are analyzed by means of Raman spectroscopy in micro-Raman configuration. If the thermal diffusivity of the substrate is high enough, the film reamorphizes via bulk nucleation of the amorphous phase from the melt. When the thermal diffusivity of the substrate is reduced, the initial nucleation of the solid phase leads to an increase in the liquid temperature (recalescence) and in the melt duration, thus promoting the formation of the crystalline phase.
Dopant-selective etch stops in 6H and 3C SiC
Authors: Shor J. S. , Kurtz A. D. , Grimberg I. , Weiss B. Z. , Osgood R. M.
Published: July 2002

A novel photoelectrochemical etching process for 6H– and 3C–SiC is described. This method enables n-type material to be etched rapidly (up to 25 μm/min), while a buried p-type layer acts as an etch stop. Dissolution of SiC takes place through hole–catalyzed surface dissolution. The holes are supplied either from the bulk (e.g., p-SiC) or by UV photogeneration (in n- or p-SiC). The differing flatband potentials of n- and p-type SiC in HF solutions allow the selection of a potential range for which hole current injection occurs only in n-type materials, facilitating dopant-selective etching. This process can be utilized in controlled etching of deep features, as well as in precise patterning of multilayer films.
Vibration dynamics of tapered optical fiber probes
Authors: Yang Y. T. , Heh D. , Wei P. K. , Fann W. S. , Gray M. H. , Hsu J. W. P.
Published: July 2002

The motion of tapered fiber probes was studied both theoretically and experimentally. A continuum-mechanical model, including both the intrinsic and the external loss, is proposed to account for the vibration dynamics of the probe. The intrinsic loss was found to be the dominant damping factor experimentally. Analytical solutions based on a realistic probe geometry were obtained for the model in the presence of intrinsic loss. The results are compared with the measured overall motions of the tapered probe. The calculations agree well with the experimental results.
Effect of oxynitridation on charge trapping properties of ultrathin silicon dioxide films
Authors: Fukuda Hisashi , Murai Shoji , Endoh Toshiaki , Nomura Shigeru
Published: July 2002

The physical properties and charge trapping behavior of rapid thermal N2O-oxynitrided (RTON) and rapid thermal NH3-nitrided (RTN) ultrathin SiO2 films have been investigated. The results of secondary-ion-mass spectrometry and Fourier transform infrared reflection measurements indicate that although nitrogen atoms are incorporated into the RTON and RTN films, only the RTN film shows a large number of NH bonds in the bulk SiO2. Using an analytical model, the number of oxide charge traps, the capture cross section, and the charge trap generation rate for the RTON and RTN SiO2 films were determined. Under high-field stress, the RTON SiO2 film has a much smaller number of electron and hole traps and a lower electron trap generation rate, resulting in a larger charge-to-breakdown QBD value compared to that of pure SiO2 film. In contrast, a large number of electron traps which originate from NH and SiH bonds is present in the RTN film. The differences in the charge trapping phenomena and oxide breakdown characteristics are strongly related to the chemical bonding state in the bulk oxide.
The energy band alignment of Xc, Γc, and Γv points in (Al0.7Ga0.3)0.5In0.5P/AlxIn1−xP heterostructures
Authors: Ishitani Y. , Nomoto E. , Tanaka T. , Minagawa S.
Published: July 2002

Better carrier confinement in 0.6-μm-band laser diodes can be achieved by incorporating an AlInP layer into the (Al0.7Ga0.3)0.5In0.5P cladding layers. The effectiveness of this heterostructure, though, cannot be analyzed without detailed knowledge of the energy band alignment at the Xc, Γc, and Γv band extrema. We conducted photoluminescence and photoreflectance measurements at 12–100 K on (Al0.7Ga0.3)0.5In0.5P/AlxIn1−xP heterostructures (x=0.47–0.61) free from long-range ordering, and analyzed the results to obtain basic data on the alignment scheme. In these measurements we observed the Γc to Γv and the Xc to Γv transitions in bulk Al0.53In0.47P and (Al0.7Ga0.3)0.5In0.5P alloys, the AlxIn1−xP Xc to (Al0.7Ga0.3)0.5In0.5P Γv transition in (Al0.7Ga0.3)0.5In0.5P/AlxIn1−xP superlattices, and the Xc to Γv and to the Γc to Γv transitions in 20-nm-wide AlxIn1−xP layers in (AlyGa1−y)0.5In0.5P/AlxIn1−xP/(AlyGa1−y)0.5In0.5P double heterostructures (x=0.33–0.39, y=0.7–1.0). We found that the energy level of Xc in AlxIn1−xP decreased by 0.09 eV as x increased from 0.47 to 0.61, the Xc of AlxIn1−xP crossed the Γc at 0.340 (±0.008), and the Γv of AlxIn1−xP crossed the Γv of (Al0.7Ga0.3)0.5In0.5P at x=0.47(±0.01). The share of the band offset at Γc for x=0.53 was 75(±3)%.
Relationship between electrical transport and hole concentration in YBa2Cu3O7−x ultrathin films probed by electric fields
Authors: Auer R. , Schneider R.
Published: July 2002

The hole concentration in YBa2Cu3O7−x ultrathin films has been changed by electric fields using superconducting field effect transistors. The transistors consisted of YBa2Cu3O7−x/SrTiO3/Au trilayer systems which have been prepared in situ by pulsed laser deposition onto (100) SrTiO3 substrates and patterned by suitable metal masks during the deposition process. The hole concentration change has been determined by measuring the field-induced dielectric polarization of the SrTiO3 dielectric. The relative changes (modulations) of the resistance RDS, the superconducting transition temperature Tc, and the critical current density jc have been studied as a function of the relative change of the hole concentration n. The relationships were found to be linear with coefficients α=1, β=2.5, and γ=3 for the modulations of RDS, Tc, and jc, respectively, within the free carrier model. A quite small hole concentration of n=1×1021 cm−3 could be determined. In the discussion, the possible influence of weak links on the results has been considered.
Growth of ZnTe by metalorganic vapor phase epitaxy: Surface adsorption reactions, precursor stoichiometry effects, and optical studies
Authors: Lovergine N. , Longo M. , Prete P. , Gerardi C. , Calcagnile L. , Cingolani R. , Mancini A. M.
Published: July 2002

The metalorganic vapor phase epitaxy growth of ZnTe by di-isopropyl-telluride and di-methyl-zinc (Me2Zn) precursors is investigated by studying the epilayer growth rate as a function of both growth temperature and precursor transport rates. The ZnTe growth is a thermally activated process involving the heterogeneous pyrolysis of both Zn and Te alkyls onto the ZnTe surface. The growth rate dependence on growth conditions is explained in terms of surface adsorption-desorption reactions, assuming that the incorporation of Zn and Te atoms into ZnTe takes place through their selective adsorption on different surface lattice sites. There is also evidence that the occurrence of a competitive species for the surface adsorption of Zn atoms, which is identified as the CH3⋅ (methyl) radical, is produced by the pyrolysis of Me2Zn. Photoluminescence (PL) and absorption measurements performed on ZnTe allow to identify two new donor-acceptor pair (DAP) bands, originated from the recombination of a Ga donor with two acceptor centers, whose ionization energies are 56 meV for the higher energy band and around 140–150 meV for the lower energy one. Hall measurements show that the 56 meV acceptor is responsible of the p-type conductivity of the layers. The nature of the impurities originating such PL features is discussed with the support of secondary ion mass spectrometry. It is shown that Ga, Si, and C are dominant impurities in the layers, whereas Cu does not occur in our ZnTe. Unintentional C doping occurs in ZnTe as a consequence of the strong methyl and iso-propyl radical surface adsorption. We show that C is incorporated as an acceptor in ZnTe, originating the DAP bands observed in the PL spectra. Within this view, the 56 meV ionization energy acceptor is tentatively assigned to substitutional C atoms on Te lattice sites.
Silicide formation by concentration controlled phase selection
Authors: Pretorius R. , Mayer J. W.
Published: July 2002

It is proposed that direct formation of epitaxial CoSi2 and NiSi2 as the first phase, is due to the interlayer between the metal and silicon acting as a diffusion barrier, which decreases the metal concentration at the growth interface. Such concentration controlled phase selection (CCPS) is explained thermodynamically by utilizing the effective heat of formation (EHF) model. This approach is also used to explain silicide formation with metal alloys. Concentration controlled phase selection (CCPS) is not only applicable to silicide formation but should in general enable materials scientists to form phases with desirable properties, by controlling the concentrations of the reactants at the growth interface.
Giant magnetoimpedance in highly textured (110)[001] FeSi3%
Authors: Carara M. , Sommer R. L.
Published: July 2002

In this paper, results on the magnetoimpedance (MI) effect in sharply textured (110)[001] FeSi3% are reported. This material has high permeability and magnetocrystalline anisotropy and has a very low cost. The measurements were made for circular and rectangular samples at different angles (θ) between the [001] easy axis and the direction of the probe current. A very high MI (up to 360% at 100 kHz) has been obtained in the studied samples, which under adequate angles (θ), exhibit a single peak centered on zero dc field.
Surface magnetoelastic behavior of magnetic multilayers (invited)
Authors: Szymczak Henryk
Published: July 2002

Experimental and theoretical research on magnetoelastic properties of nanoscale magnetic multilayers is reviewed. It has been suggested that localization of magnetoelastic strains at the interface leads to the dependence of effective magnetostriction constants on the thickness of magnetic sublayers. Various experimental techniques can be used to measure magnetoelastic tensor components. It is shown that the most effective method to study magnetostriction of magnetic nanoscale films and multilayers is strain modulated ferromagnetic resonance. It is argued that this method can be used to separate the intrinsic surface effects from the interdiffusion induced effects. The origin of the surface magnetoelastic coupling is analyzed and qualitatively attributed to many different mechanisms. It is shown that the state of the interface in magnetic multilayers is an important parameter to which the magnetostriction seems to be sensitive.
Experimental study of magnetization reversal processes in nonsymmetric spin valve
Authors: Gornakov V. S. , Nikitenko V. I. , Bennett L. H. , Brown H. J. , Donahue M. J. , Egelhoff W. F. , McMichael R. D. , Shapiro A. J.
Published: July 2002

We have investigated a nonsymmetric bottom giant magnetoresistance spin valve with the structure Si/NiO/Co/Cu/Co/Ta, as well as single ferromagnetic Co layers on antiferromagnetic NiO, with or without a nonmagnetic Cu spacer. Magnetic hysteresis loops have been measured by SQUID magnetometry, and magnetic domain structures have been imaged using an advanced magneto-optical indicator film (MOIF) technique. The MOIF technique demonstrated that the first stage of magnetization reversal is characterized by nucleation of many microdomains. With increasing reversed field, the domain walls move over small distances (5–20 μm) until annihilation. The domain size was observed to increase with the thickness of the Co layer. When an alternating magnetic field was applied, the domain structure was dramatically changed.
Oblique sputtering of amorphous TbFeCo thin films on glass substrates and the effect of deposition angle on perpendicular magnetic anisotropy
Authors: Hsieh Yung-Chieh , Gadetsky Sergei , Suzuki Takao , Mansuripur M.
Published: July 2002

We measured the magnetic hysteresis loops for several obliquely deposited amorphous TbFeCo films. The experimental results show that the direction of the average magnetic anisotropy (i.e., the easy axis of magnetization) is no longer along the surface normal. With the help of computer simulations, we have quantified the effects of oblique deposition in terms of the deviations of local anisotropy directions from the surface normal. We also found that, with the increasing of the deposition angle, the compensation point shifts toward the Fe-rich side and the films become thinner.