bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic energy-level structure of trivalent holmium in yttrium aluminum garnet
Authors: Gruber John B. , Seltzer Michael D. , Pugh Vincent J. , Richardson Frederick S.
Published: July 2002

The optical-absorption spectrum of Ho3+:Y3Al5O12 (Ho:YAG) has been analyzed between 220 and 2160 nm under variable temperature conditions between 4 and 60 K. Measurements between 220 and 470 nm were obtained from a Ho:YAG crystal that contained 15 at. % holmium. A crystal containing 0.5 at. % holmium was used to obtain the spectrum between 470 and 2160 nm. Within the temperature and total spectral range, more than 1500 transitions due to Ho3+(4f10) were observed and attributed to transitions between individual crystal-field (Stark) levels of the ground-state multiplet 5I8, and 49 different excited multiplet manifolds, 4f10(2S+1LJ), of trivalent holmium. From an analysis of these transitions it was possible to establish 419 of the 486 Stark levels predicted below 45 000 cm−1. This large number of experimentally characterized energy levels provided a suitable basis for a detailed analysis of the 4f10(Ho3+) electronic state structure in Ho:YAG. The analysis was carried out using a model Hamiltonian that assumes D2 site symmetry for the Ho3+ ions and incorporates all of the interactions known to have a significant influence on the 4fN electronic state structure of trivalent lanthanide ions in crystalline hosts. The radial-dependent parts of the interaction terms in the model Hamiltonian are represented in parametric form, and the resulting parameters are used as variables in fitting calculated-to-experimental energy-level data.

The parametric fits of energy-level data yield a rms deviation of ∼11 cm−1 between the calculated and observed locations of the 419 experimentally assigned levels (between 0 and 45 000 cm−1), and both the isotropic and anisotropic (crystal-field) interaction parameters of the model Hamiltonian are well characterized by these data fits. Among the 50 4f10[SL]J multiplet manifolds represented in the energy-level analysis, only three show major discrepancies between calculated and observed crystal-field splitting energies. The three problematic multiplet regions are: (5G6, 5F1), centered at 22 258 cm−1, 5G5, centered at ∼24 040 cm−1; and (5G3, 3L9), centered close to 28 950 cm−1. These same multiplet regions have posed problems in energy-level analyses of Ho3+(4f10) in other systems. The possible influence of correlation crystal-field (CCF) interactions on these multiplet manifolds is explored in the present study, but CCF effects appear to be small. Overall, the model Hamiltonian derived gives an excellent representation of 4f10(Ho3+) energy-level structure in Ho:YAG, and the eigenvectors of this Hamiltonian should provide a satisfactory basis for future calculations and modeling studies of Ho:YAG optical and magneto-optical properties.
Anomalous x-ray diffuse scattering of short-range order in Nd2−xCexCuO4−y
Authors: Kan X. B. , Renevier H. , Quintana J. , Peng J. L. , Cohen J. B.
Published: July 2002

Anomalous x-ray diffuse scattering is used to study the short-range order (SRO) and lattice distortions in the electron-type superconductor, Nd1.85Ce0.15CuO4−y. The intensity measurements were made at two energies below the Ce LIII absorption edge; the difference data greatly simplified the analysis to involve only atom pairs including Ce. The data sets were analyzed using a kinematic diffraction equation expanded up to the second order in atomic displacements. The fitted SRO parameters indicate the formation of Ce-Ce atom pairs along the 〈001〉 directions. For superconductivity to occur in this and other doped cuprate compounds, the size of the dopant free regions in the basal plane may have to exceed the superconducting coherence length.
Empirical dielectric function of amorphous materials for spectroscopic ellipsometry
Authors: Yamaguchi T. , Kaneko Y. , Jayatissa A. H. , Aoyama M. , Zotov A. V. , Lifshits V. G.
Published: July 2002

An empirical dielectric function (EDF) is proposed for optical characterization of amorphous materials. The EDF consists of the sum of damped harmonic oscillator terms whose square root amplitudes are distributed according to a hyperbolic function of photon energy connected to an exponential function. The usefulness of EDF is demonstrated by fitting it to the table values of dielectric constants for a-Si and a-Si3N4 and by applying it to the spectro-ellipsometric analysis of a-Si films deposited by vacuum evaporation and rf glow discharge methods.
The nature and distribution of nitrogen in silicon oxynitride grown on silicon in a nitric oxide ambient
Authors: Yao Ze-Qiang
Published: July 2002

Ultrathin dielectrics (<4 nm) were prepared by both growing and annealing in a nitric oxide (NO) ambient, using rapid thermal processing at different temperatures for various lengths of time. Compositions and residual nitrogen contents in various dielectrics were studied using x-ray photoelectron spectroscopy (XPS). The XPS depth profiles show that the nitrogen distribution of NO grown films is different from those that are NO-annealed (an initially grown pure oxides annealed in NO), N2O-grown and N2O-annealed (an initially grown oxide annealed in N2O) oxynitrides. The nitrogen distributes evenly throughout the dielectric with an atomic percentage of around 5.4 at. % for the NO-grown sample, while the nitrogen concentration is lower at the dielectric surface and piles up at the dielectrics/silicon interface with a peak of 5 at.% for the NO-annealed oxides. Deconvolution of Si 2p and N 1s XPS spectra has been used to study the bond structures. Both Si≡N and Si-N=H2 bonds have been found in the NO-grown and NO-annealed films, the later one is believed to be introduced by hydrogen impurities present in the NO gas or growth environment.
Electroluminescence from multilayer organic light-emitting diodes using poly(methylphenylsilane) as hole transporting material
Authors: Suzuki Hiroyuki , Meyer Harald , Hoshino Satoshi , Haarer Dietrich
Published: July 2002

We have investigated the optical and electrical properties of polysilane-based multilayer electroluminescent (EL) devices, utilizing poly(methylphenylsilane) (PMPS) as the hole transporting material, in order to elucidate the mechanism of EL emission in these devices. The EL devices which we fabricated have two or three functional organic layers. These layers are composed of a PMPS layer as well as a 3-(2′-Benzothiazolyl)-7-diethylaminocoumarin (Coumarin 6) doped polystyrene (PS) layer and/or a tris-(8-hydroxyquinoline) aluminum (Alq3) layer. An indium-tin-oxide-coated glass and an Al electrode were used as the hole and the electron injecting electrode, respectively. On the basis of a combined analysis of the basic characteristics of these devices, photoexcited fluorescence spectra and decay curves as well as the band diagram of these devices, we concluded that the recombination of charge carriers and the EL emission in the three-layer device occur both in the Coumarin 6:PS and the evaporated Alq3 layers.
Stationary modeling of two-dimensional states in resonant tunneling devices
Authors: Oriols X. , Suñé J. , Martín F. , Aymerich X.
Published: July 2002

One-side bound states are very important in vertical resonant tunneling devices which contain either lightly doped spacers or a small band-gap pseudomorphic layer adjacent to the barriers. By a proper choice of the boundary conditions, these states are modeled by stationary wave functions which contain the relevant information of the quasi-two-dimensional system under steady-state conditions. In particular, the wave functions allow the calculation of their contributions to the self-consistent charge density and the electrical current. In qualitative agreement with experimental results, it is demonstrated that the main resonant features of the current–voltage characteristic of these devices are due to resonant tunneling from an emitter two-dimensional electron gas. Finally, the proposed model is compared with a previous picture of other authors.
Sodium diffusion in glasses during electron irradiation
Authors: Jbara O. , Cazaux J. , Trebbia P.
Published: July 2002

Sodium migration in glasses during electron probe microanalysis is investigated. We observe the change in x-ray emission as a function of time and for various incident electron doses. A specific protocol is used in this study: independently drawn from a multivariate statistical analysis of the data and from an a priori simple model, an exponential decay of the Na signal is clearly established. This model for Na+ migration is only based on an electric field with a linear decrease behavior as a function of depth and it accounts for the main experimental results with no evidence of an incubation time prior to ion migration. The electron densities and the fraction of incident electrons remaining trapped into the glass are deduced from an estimate of ion mobility. The maximum electric field values at the coating/glass interface are also given.
A reduced moment-based model for precipitation kinetics and application to dopant activation in silicon
Authors: Clejan Iuval , Dunham Scott T.
Published: July 2002

A moment-based model for the time evolution of extended defect size distributions is introduced as a computationally efficient method for the quantitative modeling of precipitation processes. Because the model accounts for how the size distribution of aggregates changes with time, it is able to account for thermal history effects that are missed by simpler models. However, by considering only the lowest moments of the distribution rather than the full distribution itself, the model can be practically applied to nonhomogeneous systems for which it is impractical to include the full distribution at every point in space. The model is tested by implementing it in a process simulator and applying it to the simultaneous diffusion and activation/deactivation of dopants in silicon. Under the conditions compared, we find that the reduced model is nearly equivalent to a previously studied model [S. T. Dunham, J. Electrochem. Soc. 142, 2823 (1995)] maintains the full size distribution and is thus much more computationally intensive. Because the reduced model is derived directly from the more complete model using an energy-minimizing closure assumption, all parameters retain a physical interpretation and the model can be readily extended to a large range of systems.
A kinetic study of the C49 to C54 TiSi2 conversion using electrical resistivity measurements on single narrow lines
Authors: Saenger K. L. , Cabral C. , Clevenger L. A. , Roy R. A. , Wind S.
Published: July 2002

We describe a simple quasi-in situ resistivity technique and its application to the study of C49 to C54 TiSi2 conversion in narrow (0.1-1.0 μm) lines. This technique allowed comparison of both aggregate conversion versus time at temperature behavior and individual-line conversion versus time behavior for silicide lines of different linewidths. As linewidth decreased, the aggregate conversion versus time at temperature behavior slowed, and the conversion behaviors of individual lines having the same linewidth became more variable. Both of these observations are consistent with a nucleation-site-density controlled reaction under conditions of low nucleation site density. Correlations were also found between individual line behaviors and resistance to agglomeration; resistance to agglomeration (for 0.35–1.0 μm lines already in the C54 phase) was highest for lines which had ‘‘prompt’’ conversion behaviors (as measured by the sheet resistance drop during the first minute of the conversion anneal). Additional data concerning the sensitivity of the initial sheet resistances to formation anneal conditions and linewidth is also briefly discussed.
Values of capture cross sections of metastable defects in hydrogenated amorphous silicon
Authors: Bube Richard H. , Benatar Lisa E. , Bube Kenneth P.
Published: July 2002

Measurements of defect density, photoconductivity, and dark conductivity are used to obtain information about the values of the electron capture cross sections of charged and neutral metastable dangling-bond defects in high-quality, undoped, hydrogenated amorphous silicon at room temperature. Sixty measurements, obtained in the process of optical degradation experiments as a function of time at four different temperatures, have been analyzed using photoconductivity models corresponding to either one or two types of discrete-level, multivalent defects. A model with two types of defects is able to accurately describe both dark conductivity and photoconductivity results, and gives the following average values: an electron capture cross section of about 1×10−16 cm2 for neutral centers of both higher-lying (density not increased by light) and lower-lying (density increased by light) defects, of about 2×10−16 cm2 for positively charged higher-lying defects, and of about 20×10−16 cm2 for positively charged lower-lying defects.
Charge trapping in dry and wet oxides on N-type 6H–SiC studied by Fowler–Nordheim charge injection
Authors: Stein von Kamienski E. G. , Portheine F. , Stein J. , Gölz A. , Kurz H.
Published: July 2002

Metal-oxide-semiconductor capacitors fabricated by dry and wet oxidation at 1150 °C of n-type 6H–SiC exhibit a drastic negative charging during Fowler–Nordheim charge injection. This charging strongly depends on the fabrication conditions of the samples. The densities of interface states and oxide charges can be significantly reduced by a postoxidation anneal in Ar for as long as 60 min. A large part of the charge appears to be trapped at the interface. These charges are released from the traps by illumination at photon energies between 2 and 4 eV or by annealing below 300 °C. During charge injection interface states are created near the conduction band edge. Their density is strongly reduced by annealing at 150 °C. For the oxide charging we find capture cross sections in the range of 10−15–10−17 cm−2. In unannealed wet oxidized samples the traps exhibit properties similar to those of water-related traps in SiO2 on Si.
Magnetic properties of colossal magnetoresistive manganese oxides
Authors: Fontcuberta J. , Martínez B. , Seffar A. , Piñol S. , Roig A. , Molins E. , Obradors X. , Alonso J. , González-Calbet J. M.
Published: July 2002

The effects of isovalent substitution into the lanthanide sites in La1−xYxCa0.30MnO3−δ oxides is presented. We report on the electrical and magnetic properties of these materials for x=0–0.25. We have found that the ferromagnetic ordering temperature decreases and the cusp of the resistivity occurring at this temperature increases when the smaller lanthanide Y is introduced into the lattice. We will provide evidence that a key signature of the magnetic properties of these oxides is the presence of an unusual long tail in the magnetization versus temperature curves M(T), extending up to 3TM and which is very sensitive to the applied magnetic field and becomes increasingly important with the concentration of Y in the lattice. These features signal a strong polarizability of the spin clouds in these materials as our Mössbauer experiments will clearly demonstrate. Finally, we will show that the magnetoresistance of these compounds can be well-described in terms of a spin-disorder scattering mechanism.
Epitaxial Fe16N2 films grown on Si(001) by reactive sputtering
Authors: Brewer M. A. , Krishnan Kannan M. , Ortiz C.
Published: July 2002

We present a crystallographic template for the growth of the range of Fe-N phases on Si(001) by lattice matching on selected underlayers. Epitaxial films of pure α-Fe, γ′-Fe4N, and α′-Fe8N (N martensite) were grown individually by the optimization of reactive N2 sputtering parameters. The orientation relation of the Fe-N phases was Fe-N(001)∥Ag(001)∥Si(001) and Fe-N[100]∥Ag[110]∥Si[100]. Annealing the α′-Fe8N films resulted in the formation of α′-Fe8N/α″-Fe16N2 mixtures. In addition to the crystallographic and structural analysis, quantification of x-ray diffraction peak intensities confirmed that the α′/α″ mixtures contained as much as 46 vol % α″ (remaining α′). Vibrating sample magnetometry and SQUID magnetometry measurements of the α′ and α′(54%)/α″(46%) mixture, respectively, indicate enhanced magnetic moments for both the α′ and α″ phases with respect to pure Fe.
Influence of nitrogen profile on electrical characteristics of furnace- or rapid thermally nitrided silicon dioxide films
Authors: Bouvet D. , Clivaz P. A. , Dutoit M. , Coluzza C. , Almeida J. , Margaritondo G. , Pio F.
Published: July 2002

Thin silicon dioxide films nitrided in N2O by rapid thermal processing (RTP) or in a classical furnace were investigated by x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and electrical measurements on metal-oxide-semiconductor capacitors. Differences between the two nitridation processes were observed and explained. In lightly nitrided films, nitrogen occupies two configurations. Nitrogen is bound to three silicon atoms with at least one in the substrate or all three in the oxide. In RTP-nitrided films, both of these species are confined to within 1.5 nm of the Si/SiO2 interface. In furnace-nitrided films, the first species is also located close to the interface whereas the second one fills most of the regrown oxide thickness. In furnace-grown films, which are more heavily nitrided, a third structure due to Si2=N–O is observed throughout the layer. The electrical characteristics are well correlated with the amount of nitrogen at the interface that is bound to Si atoms in the substrate.
Cation distribution in NiZn-ferrite films determined using x-ray absorption fine structure
Authors: Harris V. G. , Koon N. C. , Williams C. M. , Zhang Q. , Abe M.
Published: July 2002

We have applied extended x-ray absorption fine structure (EXAFS) spectroscopy to study the cation distribution in a series of spin-sprayed NiZn-ferrite films, Ni0.15ZnyFe2.85−yO4 (y=0.16, 0.23, 0.40, 0.60). The Ni, Zn, and Fe EXAFS were collected from each sample and analyzed to Fourier transforms. Samples of Ni-ferrite, Zn-ferrite, and magnetite were similarly studied as empirical standards. These standards, together with EXAFS data generated from the theoretical EXAFS FEFF codes, allowed the correlation of features in the Fourier transforms with specific lattice sites in the spinel unit cell. We find that the Ni ions reside mostly on the octahedral (B) sites whereas the Zn ions are predominantly on the tetrahedral (A) sites. The Fe ions reside on both A and B sites in a ratio determined by the ratio of Zn/Fe. The addition of Zn displaces a larger fraction of Fe cations onto the B sites serving to increase the net magnetization. The fraction of A site Ni ions is measured to increase peaking at ≊25% for y=0.6. At higher Zn concentrations (y≥0.5) the lattice experiences local distortions around the Zn sites causing a decrease in the superexchange resulting in a decrease in the net magnetization.
Enhancement of magnetoresistance in Co(11̄00)/Cr(211) bilayered films on MgO(110)
Authors: Yao Y. D. , Liou Y. , Huang J. C. A. , Liao S. Y. , Klik I. , Yang W. T. , Chang C. P. , Lo C. K.
Published: July 2002

Epitaxial Co/Cr bilayered films have been successfully grown on the MgO(100) and MgO(110) substrates by molecular-beam epitaxy. According to the reflection high-energy electron-diffraction and x-ray-diffraction measurements the crystal structure of the film depends on orientation of the buffer and substrate. Epitaxial growth of biaxial Co(112̄0)/Cr(100) on MgO(100) substrate and of uniaxial Co(11̄00)/Cr(211) on MgO(110) substrate has been confirmed. The anisotropy magnetoresistance (AMR) is strongly influenced by the orientation of the Cr buffer. In Co(112̄0)/Cr(100) on MgO(100) AMR is isotropic for all in-plane fields. However, for Co(11̄00)/Cr(211) on MgO(110) we observed enhancement of AMR along the easy axis for temperatures below 150 K, while along the hard axis AMR has a local maximum at about 150 K. The easy axis data suggest that the longitudinal spin density wave of Cr and the crystal anisotropy of Co on Cr(211) plane dominate the enhancement of the AMR.
Studies of remanence in granular Ni–SiO2 films
Authors: Xu Y. , Zhao B. , Yan X.
Published: July 2002

We have studied microstructure and remanent magnetization of the granular Ni–SiO2 films. Both analysis of the transmission electron microscope images and that of the particle size distribution via remanence studies support the hypothesis that both superparamagnetic and ferromagnetic components coexists in the nearly percolating films.
Structure and magnetic properties of mechanically alloyed Nd(Fe,V)12Nx compounds
Authors: Yang Jun , Mao Ou , Altounian Z.
Published: July 2002

The structure and magnetic properties of Nd(Fe,M)12Nx (M=Ti,Mo,V) compounds prepared by mechanical alloying (MA) have been studied using x-ray diffraction and magnetic measurements. It is found that after annealing, single-phase Nd(Fe,V)12 compounds can be obtained over a large composition range, but can only be obtained in Nd(Fe,V)11 after MA. The crystallization of stoichiometric compounds at 750–950 °C always produces a small amount of α-Fe(V) in addition to the 1:12 phase. Below 750 °C, the crystallization is not complete and large amounts of α-Fe(V) are present. When nitriding at 450 °C, the α-Fe(V) phase disappears accompanied by the formation of an amorphous phase and a 1:7 nitride phase. The crystallized phase retains the 1:12 structure when nitriding at 480 °C. The highest coercivity of 8.8 kOe has been obtained for the NdFe10.5V1.5Nx compound. The difference between the coercivities of MA Nd(Fe,V)12Nx, Nd(Fe,Mo)12Nx, NdFe11TiNx, and Sm2Fe17Nx compounds is attributed to their different crystallization behavior and phase stability during nitrogenation.
Resonant Raman scattering study of InSb etched by reactive ion beam etching
Authors: Sendra J. R. , Armelles G. , Utzmeier T. , Anguita J. , Briones F.
Published: July 2002

A Raman study of InSb etched by reactive ion beam etching using a CH4/H2/N2 plasma generated by electron cyclotron resonance is presented. The evolution of the LO, 2LO phonon and phonon–plasmon coupled modes has been studied using resonant Raman spectra in different configurations. Results indicate an increase of the carrier density by a factor of about 60 and a decrease of the built-in potential due to the plasma process. The combination of both evolutions results in a prevalence of the electric field induced scattering upon the defect induced scattering mechanism.
Ion energy distributions in SF6 plasmas at a radio-frequency powered electrode
Authors: Becker F. , Rangelow I. W. , Kassing R.
Published: July 2002

An energy-resolving quadrupole mass spectrometer (E-QMS) was assembled underneath the powered electrode of a diode reactive ion etcher. The plasma ions reach the E-QMS through an orifice in the powered electrode with a diameter of 100 μm. The ion energy distributions (IEDs) of ionic species from SF6 plasmas in the pressure range of 0.1–1.5 Pa for dc bias potentials between 50 and 300 V and a rf of 13.56 MHz were investigated. The IEDs always show a saddle shaped peak at an energy corresponding to a total potential drop across the sheath given by USh=Udc+UP, where Udc is the dc bias potential and UP is the time averaged plasma potential. In the energy range from 0 eV to eUdc there are multiple peaks in the IEDs of SF+x (x=1..5), whereas the F+, F+2, and S+ IEDs show only a single peak in this range. These peaks are the result of ion generating collisions in the sheath. On pressure variation the IEDs do not change significantly. We also measure IEDs of negative ions. The IEDs of these ions show a broad distribution with an intensity maximum appearing at the half of UP and a width corresponding to max[UP(t)], where UP(t) is the time varying plasma potential. These correlations suggest that these ions originate from the plasma bulk.
A three-dimensional model for inductively coupled plasma etching reactors: Azimuthal symmetry, coil properties, and comparison to experiments
Authors: Kushner Mark J. , Collison Wenli Z. , Grapperhaus Michael J. , Holland John P. , Barnes Michael S.
Published: July 2002

Inductively coupled plasma (ICP) etching reactors are rapidly becoming the tool of choice for low gas pressure, high plasma density etching of semiconductor materials. Due to their symmetry of excitation, these devices tend to have quite uniform etch rates across the wafer. However, side to side and azimuthal variations in these rates have been observed, and have been attributed to various asymmetries in pumping, reactor structure and coil properties. In this article, a three-dimensional computer model for an ICP etching reactor is reported whose purposes is to investigate these asymmetries. The model system is an ICP reactor powered at 13.56 MHz having flat coils of nested annuli powering Ar/N2 and Cl2 plasmas over a 20-cm diam wafer. For demonstration purposes, asymmetries were built into the reactor geometry which include a wafer-load lock bay, wafer clamps, electrical feeds to the coil, and specifics of the coil design. Comparisons are made between computed and experimentally measured ion densities and poly-silicon etch rates in Cl2 plasmas. We find that the electrical transmission line properties of the coil have a large influence on the uniformity of plasma generation and ion fluxes to the wafer.
Photoluminescence and intersubband absorption spectroscopy of interdiffused Si/SiGe quantum wells
Authors: Boucaud P. , Wu L. , Guedj C. , Julien F. H. , Sajnes I. , Campidelli Y. , Garchery L.
Published: July 2002

The interdiffusion and thermal stability of narrow Si/SiGe multi-quantum wells is investigated by photoluminescence and intersubband spectroscopy. The photoluminescence exhibits a blueshift as a function of the temperature of annealing. The activation energy of the intermixing process and the interdiffusion coefficient are deduced from the photoluminescence shift versus temperature of the anneal. The intersubband absorption is measured by photoinduced infrared spectroscopy on the interdiffused samples for light polarized perpendicular (z polarization) or parallel (x polarization) to the layer plane. In z polarization, the absorption of annealed samples exhibits a redshift respective to the as-grown sample which is enhanced as more levels are confined in the well. The magnitude of this shift is in good agreement with simulations based on the data obtained by photoluminescence experiments. The redshift of the intersubband absorption in x polarization is lower than in z polarization due to the lower dependence of the spin-orbit level on the quantum well profile. Bound-to-continuum absorption is broadened in interdiffused quantum wells. This enhancement is explained by the local variation of the effective mass along the growth axis of the interdiffused well. At high temperature of annealing and beyond the onset of relaxation, we still observe intersubband absorption with a ratio between z and x polarization measured by photoinduced spectroscopy which is slightly decreased.
Two-step annealed polycrystalline silicon thin-film transistors
Authors: Choi Kwon-Young , Han Min-Koo
Published: July 2002

The performance of polysilicon thin-film transistors fabricated by two-step annealing, which consists of furnace annealing and the subsequent excimer laser annealing, is described. It was found that the average grain size of low-temperature furnace-annealed polysilicon films was several times larger than that of excimer-laser-annealed polysilicon films while the density of in-grain defect of low-temperature furnace-annealed films was much higher than that of excimer-laser-annealed film. However, the device characteristics of the low-temperature furnace-annealed polysilicon thin-film transistors were improved significantly by postannealing, such as high-temperature furnace annealing and excimer laser annealing, due to the effective elimination of in-grain defects. We found that the significantly improved temperature characteristics of drain current and hydrogen passivation rate of two-step annealed polysilicon thin-film transistors were also caused by the significantly reduced in-grain defects by postannealing. The density of trap states, which was extracted from the transfer curves of polysilicon thin-film transistors, was used to demonstrate the effects of modifying the deep and tail trap levels by two-step annealing.
Structural modification of ion-implanted and postannealed polycrystalline diamond studied by transmission electron microscopy
Authors: Jiang N. , Yagyu H. , Deguchi M. , Won J. H. , Mori Y. , Hatta A. , Kitabatake M. , Ito T. , Hirao T. , Sasaki T. , Hiraki A.
Published: July 2002

We have presented a convenient and effective method to ex situ study ion-implanted and postannealed polycrystalline diamond by transmission electron microscopy (TEM) without thinning the specimens. Chemical-vapor-deposited (CVD) diamond used for transmission electron microscopy study was directly deposited onto Mo TEM grids, and then implanted and postannealed. TEM images clearly reveal that there exists an ion-induced amorphous layer on the as-implanted CVD diamond surface, in which graphitelike structure is embedded. The amorphization processes depend on the irradiation conditions. Hydrogen plasma treatment was employed to anneal the as-implanted CVD diamond. High resolution electron microscopy images indicate that hydrogen plasma treatment can effectively remove the ion-induced surface amorphous layer without graphitizing the diamond. After treatment, high density ball-like diamond blisters appear on the surface, of which the average diameter is only about 2.5 nm, implying the critical size for the stable existence of CVD diamond crystallites may be on the order of a few atomic layers.
Microwave properties and strain-induced lattice defects of c-axis-oriented YBa2Cu3O7−δ thin films on silicon
Authors: Jaekel C. , Kyas G. , Roskos H. G. , Kurz H. , Kabius B. , Meertens D. , Prusseit W. , Utz B.
Published: July 2002

The microwave properties and the crystal structure of YBa2Cu3O7−δ thin films are examined as a function of film thickness. Already below the critical thickness for the formation of macroscopic fractures, we observe an increase of the residual surface resistance with thickness. Cross-sectional transmission electron microscopy reveals that this behavior is caused by structural changes of antiphase boundaries leading to normal-conducting regions between superconducting grains. The surface resistance is calculated within the weak-link picture.
Voltage offsets and imprint mechanism in SrBi2Ta2O9 thin films
Authors: Al-Shareef H. N. , Dimos D. , Warren W. L. , Tuttle B. A.
Published: July 2002

It is shown that voltage shifts in the hysteresis response of SrBi2Ta2O9 (SBT) thin-film capacitors can be induced using both thermal and optical methods. These voltage shifts are important since they can lead to imprint failure in ferroelectric memory devices. It is suggested that the voltage shifts in the hysteresis curve of SBT are caused by trapping of electronic charge carriers near the film/electrode interfaces, as has been previously reported for the Pb(Zr,Ti)O3 (PZT) system. In addition, a direct correlation is established between the magnitude and sign of remanent polarization (Pr) and the thermally induced voltage shifts (Vi), where Vi=αPr+β. It is also found that, unlike the PZT system, the thermally induced voltage shifts in SBT are smaller than those optically induced. One possible implication of this result is that the contribution of defect–dipole complexes to the voltage shifts in SBT is negligible. We suggest that the smaller contribution of defect-dipole complexes to the voltage shifts in SBT may be related to a smaller oxygen vacancy concentration in the perovskite sublattice of SBT as compared to that of PZT.
Quantitative simulation of a resonant tunneling diode
Authors: Bowen R. Chris , Klimeck Gerhard , Lake Roger K. , Frensley William R. , Moise Ted
Published: July 2002

Quantitative simulation of an InGaAs/InAlAs resonant tunneling diode is obtained by relaxing three of the most widely employed assumptions in the simulation of quantum devices. These are the single band effective mass model (parabolic bands), Thomas-Fermi charge screening, and the Esaki-Tsu 1D integral approximation for current density. The breakdown of each of these assumptions is examined by comparing to the full quantum mechanical calculations of self-consistent quantum charge in a multiband basis explicitly including the transverse momentum.
Gas collisions and pressure quenching of the photoluminescence of silicon nanopowder grown by plasma-enhanced chemical vapor deposition
Authors: Roura P. , Costa J. , Morante J. R. , Bertran E.
Published: July 2002

The quenching of the photoluminescence of Si nanopowder grown by plasma-enhanced chemical vapor deposition due to pressure was measured for various gases ( H2, O2, N2, He, Ne, Ar, and Kr) and at different temperatures. The characteristic pressure, P0, of the general dependence I(P)=I0 exp(−P/P0) is gas and temperature dependent. However, when the number of gas collisions is taken as the variable instead of pressure, then the quenching is the same within a gas family (mono- or diatomic) and it is temperature independent. So it is concluded that the effect depends on the number of gas collisions irrespective of the nature of the gas or its temperature.
Carrier photogeneration processes in layered organic systems incorporating bisazo pigment: Xerographic and electroabsorption study
Authors: Umeda Minoru , Yokoyama Masaaki
Published: July 2002

The role of the photoexcited state of an azo pigment as a reactant for photocarriers in a layered organic system was investigated using 2,6-anthraquinone bisazo (ABHN) or 1,4-distyrylbenzene bisazo (PDHN) pigment. Both bisazo pigments are highly sensitized by carrier transport molecules involved in the layered device. However, photocarrier generation in the bisazo pigments was quite different in the absence of sensitizing molecules; ABHN never generates, but PDHN generates to some degree by itself. The photocarriers are known to be generated via excitons of the azo pigments in the layered systems from the result of excitation-energy-independent quantum efficiencies. Electromodulated absorption spectra show that ABHN forms a Frenkel exciton and PDHN a charge-transfer-natured exciton. The different types of excitons explain the difference in carrier generation pathways of the optoelectronic systems based on these two azo pigments well.
MeV ion irradiation of Co/Cu multilayers
Authors: Cai M. , Veres T. , Roorda S. , Cochrane R. W. , Abdouche R. , Sutton M.
Published: July 2002

We examine the effect of MeV ion-beam irradiation on the giant magnetoresistance and related interlayer magnetic coupling in sputtered Co/Cu multilayers. At ion doses higher than 1013/cm2, the resistivity of the multilayers increases noticeably, well beyond that measured for pure copper or cobalt films. This increase in resistivity of the multilayers is tentatively ascribed to ion-beam-induced interface disorder. With increasing dose, the magnetic interlayer coupling passes systematically from a mainly antiferromagnetic (AF) coupling to a ferromagnetic one and, in parallel, the giant magnetoresistance (GMR) is progressively destroyed. A linear relationship between the GMR and the volume fraction of AF coupled regions is observed up to an ion dose of 2×1014/cm2.
Aspect-ratio-dependent charging in high-density plasmas
Authors: Hwang Gyeong S. , Giapis Konstantinos P.
Published: July 2002

The effect of aspect ratio (depth/width) on charge buildup in trenches during plasma etching of polysilicon-on-insulator structures is studied by Monte Carlo simulations. Increased electron shadowing at larger aspect ratios reduces the electron current to the trench bottom. To reach a new charging steady state, the bottom potential must increase, significantly perturbing the local ion dynamics in the trench: the deflected ions bombard the sidewall with larger energies resulting in severe notching. The results capture reported experimental trends and reveal why the increase in aspect ratio that follows the reduction in critical device dimensions will cause more problems unless the geometry is scaled to maintain a constant aspect ratio.
Lateral waves in magnetoelastic media: Analysis of magnetic field
Authors: Gourakishwar Singh O.
Published: July 2002

Reflection coefficients at the interface between two homogeneous, isotropic magnetoelastic media are obtained. An equation of caustic for lateral shear horizontal wave is also derived by using the method of steepest descent. These expressions are used in analyzing the relationship between magnetic field and reflection coefficient and could lead to discussions for using magnetic field in ultrasound imaging.
Spin-lattice relaxations of Be9 and Al27 in single crystalline alexandrite
Authors: Yeom Tae Ho , Hong Kwan Soo , Yu Insuk , Shin Hee Won , Choh Sung Ho
Published: July 2002

Nuclear spin-lattice relaxations of Be9 and Al27 in an alexandrite (BeAl2−xCrxO4, x=0.02) single crystal have been investigated in the temperature range 140 K–400 K. The paramagnetic impurity effect on the relaxation is negligible for both nuclei. The relaxation of the Be9 nuclear spins is found single exponential in time when the whole resonance lines are uniformly excited. When only the central resonance line is excited for Al27, the relaxation is well fitted with a linear combination of three exponential functions given by Simmons et al. [Phys. Rev. 127, 1168 (1962)]. Since the spin-lattice relaxation rate is proportional to the square of temperature for both nuclei, the dominant relaxation mechanism in this temperature range can be inferred as the Raman process.
Light-emitting diodes based on poly-p-phenylene-vinylene: I. Charge-carrier injection and transport
Authors: Karg S. , Meier M. , Riess W.
Published: July 2002

Detailed investigations of the device characteristics of poly-p-phenylene-vinylene (PPV) light-emitting diodes are reported. We analyze the influence of various hole- and electron-injecting electrodes on the current–voltage (I–V) characteristics and electroluminescence behavior. Our studies reveal that thermal conversion of the prepolymer on indium–tin–oxide (ITO) substrates leads—in contrast to conversion on Au and other high-work-function metals—to a p-type doping of PPV and, additionally, to the formation of an ohmic hole-injecting contact at the ITO/PPV interface. Hence, devices fabricated with low-work-function metals acting as the electron injecting contact (for example, Al and Ca) display Schottky behavior. These Schottky diodes are distinguished by a high rectification ratio ρr of about 106 and display electroluminescence at bias voltages as low as 1.5 V for ITO/PPV/Ca light-emitting diodes. The I–V characteristics can be quantitatively described within the modified Shockley equation, taking into account the voltage drop on a serial resistance, yielding ideality factors n ranging from 1.6 to 2.4 for different devices. At high current densities, space-charge-limited currents determine the device characteristics. The hole mobility μh is found to be of the order of 10−5 cm2/V s.
Microscopic analysis of electron noise in GaAs Schottky barrier diodes
Authors: González Tomás , Pardo Daniel , Reggiani Lino , Varani Luca
Published: July 2002

A microscopic analysis of current and voltage fluctuations in GaAs Schottky barrier diodes under forward-bias conditions in the absence of 1/f contributions is presented. Calculations are performed by coupling self-consistently an ensemble Monte Carlo simulator with a one-dimensional Poisson solver. By using current- and voltage-operation modes the microscopic origin and the spatial location of the noise sources, respectively, is provided. At different voltages the device exhibits different types of noise (shot, thermal, and excess), which are explained as a result of the coupling between fluctuations in carrier velocity and self-consistent field. The essential role of the field fluctuations to correctly determine the noise properties in these diodes is demonstrated. The results obtained for the equivalent noise temperature are found to reproduce the typical behavior of experimental measurements. An equivalent circuit is proposed to predict and explain the noise spectra of the device under thermionic emission-based operation.
Low temperature chemical vapor deposition growth of β-SiC on (100) Si using methylsilane and device characteristics
Authors: Liu C. W. , Sturm J. C.
Published: July 2002

The growth properties of β-SiC on (100) Si grown by rapid thermal chemical vapor deposition, using a single precursor (methylsilane) without an initial surface carbonization step, were investigated. An optimun growth temperature at 800 °C was found to grow single crystalline materials. A simple Al Schottky barrier fabricated on n-type SiC grown on Si substrates exhibited a “hard” reverse breakdown of 13 V with a positive temperature coefficient of 2×10−4 °C−1 up to 120 °C, indicating an avalanche mechanism. A Pt Schottky barrier fabricated on n-type SiC grown on tilted Si substrates to improve the surface morphology exhibited a breakdown voltage of 59 V, with a negative temperature coefficient. From the analysis of the electrical field distribution, the breakdown probably occurred at interface defects between SiC and Si, as suggested by Raman spectroscopy. To investigate minority transport behavior, SiC/Si heterojunction bipolar transistors (HBTs) were fabricated and compared to Si bipolar junction transistors. The collector currents of the SiC/Si HBTs were similar to those of Si control transistors, because both devices had the same base structures. Compared to Si control transistors, the base currents of SiC/Si HBTs increased. It seems that the interface defects between Si and SiC act as recombination centers to deplete back-injected holes, instead of being the barrier to stop hole currents, and thus to increase the base currents of SiC/Si HBTs.
Plasmon surface polariton coupling with dielectric gratings and the thermal decomposition of these dielectric gratings
Authors: Müller Klaus G. , Veith Michael , Mittler-Neher Silvia , Knoll Wolfgang
Published: July 2002

We demonstrate a simple method for plasmon surface polariton coupling. A dielectric layer sinusoidally modulated in height (dielectric grating) on top of a metal layer is used as the dispersive element for the momentum coupling between the incoming photon and the plasmon surface polariton. Hence, no modulation of the metal surface is necessary. This coupling method is used to study the thermal decomposition of dielectric gratings.
Detection of surface states in GaAs and InP by thermally stimulated exoelectron emission spectroscopy
Authors: Hullavarad Shiva S. , Bhoraskar S. V. , Bose D. N.
Published: July 2002

The energy distribution and relative densities of electronically active surface defects have been studied using thermally stimulated exoelectron emission (TSEE) spectroscopy. This novel and relatively simple technique has high sensitivity for detecting the surface states which are difficult to assess by other techniques. Here this technique is successfully used for detecting the pinned positions of the Fermi level in n-GaAs and n-InP which are, respectively, 0.91 and 0.43 eV below the conduction bands corresponding to 2Eg/3 and Eg/3 as expected. Antisite and oxygen related defects in these semiconductors are also identified at the surface. The relative TSEE peak intensities correlate very closely to the reported surface recombination velocities for these materials which are two to three orders of magnitude higher for GaAs. The effect of chromium on the surface states in these semiconductors, studied using semi-insulating GaAs, showed partial passivation of the surface defects in semi-insulating GaAs resulting in unpinning of the Fermi levels. Fe doped InP did not, however, show any sign of dopant induced deep levels.
Effect of an ultrathin Au interlayer at a Co/Cu(111) interface on the magnetic properties
Authors: Murayama Akihiro , Hyomi Kyoko , Eickmann James , Falco Charles M.
Published: July 2002

We have found from spin-wave Brillouin spectra that a half-monatomic thick Au interlayer at a Co/Cu(111) interface causes a significant improvement in magnetic uniformity in the Co ultrathin overlayer film deposited. The field-independent Brillouin spectrum width abruptly decreases with the insertion of the Au-interlayer, and an inhomogeneous transition at a critical field between out-of-plane and in-plane magnetizations also is suppressed. From the field dependence of the spin-wave energy, we find that the second-order uniaxial perpendicular anisotropy increases with the Au interlayer.
Luminescence from growth topographic features in GaN:Si films
Authors: Herrera Zaldivar M. , Fernández P. , Piqueras J.
Published: July 2002

Cathodoluminescence (CL) in the scanning electron microscope is used to investigate the nature of defects responsible for the luminescence associated with round and hexagonal-like topographic features of GaN:Si films. Round hillocks of the size of a few microns, which sometimes have a nanopipe related central hole, do not influence the luminescence emission of the film. Hillocks with sizes of several tens of microns show a marked CL contrast at the center and at the border. The origin of the observed contrast is attributed to a growth induced inhomogeneous distribution of point defects and impurities. Radiation with the electron beam of the scanning microscope causes a decrease of the CL intensity without spectral changes.
Shear strains in dry etched GaAs/AlAs wires studied by high resolution x-ray reciprocal space mapping
Authors: Darhuber A. A. , Bauer G. , Wang P. D. , Sotomayor Torres C. M.
Published: July 2002

We have fabricated GaAs/AlAs quantum wires and quantum dots by means of molecular beam epitaxy, electron beam lithography, and subsequent reactive ion etching using SiCl4 and O2. The nominal periods are 300 nm and 350 nm for both wire and dot samples. High resolution x-ray reciprocal space maps of the 350 nm samples exhibit not only satellites corresponding to a periodicity of 350 nm but also additional satellites corresponding to a period of three times 350 nm, whereas there are no such extra peaks in the maps of the 300 nm samples. These secondary satellites are shown to be associated with a discretization effect in electron beam writing. Moreover, we found, that the shear strain in the wires has a distinct influence on the intensities of these weak extra satellites. Hence, they provide a sensitive means for the assessment of shear strains in elastically relaxed quantum wires.
Energetic impact of small Ag clusters on graphite
Authors: Hall S. G. , Nielsen M. B. , Palmer R. E.
Published: July 2002

The impact of Ag clusters on graphite is studied with the scanning tunneling microscope. Agn− clusters (n=1, 3, 5, and 7) are projected at normal incidence to the surface at constant energy per cluster ion (1.5 keV/ion). We observe protrusions with diameters of ∼2.5 nm on the surface of graphite which we attribute to bumps in the graphite surface layer. We show evidence that the positions of the Ag atoms implanted during cluster bombardment remain highly localized.