bluuebunny/crossref_metadata_2025 huggingface dataset:
Low frequency polarization behavior of xBiScO3–yBiGaO3–(1−x−y)PbTiO3 piezocrystals
Authors: Zhang Shujun , Priya Shashank , Shrout Thomas R. , Randall Clive A.
Published: March 2003

Perovskite ferroelectric xBiScO3–yBiGaO3–(1−x−y)PbTiO3 (BS–BG–PT) single crystals were successfully grown using the high temperature solution method. The dielectric constant (K) of tetragonal crystals at room temperature was determined to be 500–600 with dielectric loss tangents less than 0.3% at a frequency of ∼1 kHz. The Curie temperature Tc was found to be around ∼420 °C, with longitudinal piezoelectric coefficient of ∼300 pC/N for 〈001〉 oriented tetragonal crystals. The remnant polarization Pr was 46 μC/cm2 with a coercive field Ec of 43 kV/cm at 1 Hz and dc field of 60 kV/cm. Based on dielectric and polarization measurements, it was found that the low frequency regime was dominated by a space charge conduction mechanism. A space charge model is developed which correlates the polarization with the dielectric relaxation. The relaxation time on the order of ∼2.2 ms was obtained in the BS–BG–PT single crystal using this model.
Effect of Si cap layer on parasitic channel operation in Si/SiGe metal–oxide–semiconductor structures
Authors: Sareen Alok , Wang Yun , Södervall Ulf , Lundgren Per , Bengtsson Stefan
Published: March 2003

We investigate the effects of silicon cap layer thinning on channel carrier confinement in silicon/strained silicon-germanium (Si/SiGe) metal–oxide–semiconductor (MOS) structures. The silicon cap thickness is shown to have a critical influence on the induced parasitic channel in the silicon cap, which lowers the transconductance value of the buried SiGe channel in hole carrier channel p metal–oxide–semiconductor field-effect transistor devices. This can have serious consequences on future implementation of SiGe ultrashort channel devices where gate induced parasitic channel and short channel effects can be pronounced. An exact methodology is devised for consumption of silicon cap layer using a modified Radio Corporation of America surface standard clean self-terminating chemical oxide and rapid thermal oxide growth. This provides (i) precise control over the silicon cap and thermal oxide thickness and (ii) a limit on the amount of thermal budget induced strain relaxation in the buried SiGe layer. The threshold voltages for inversion of carriers at the SiO2/Si and the Si/SiGe interfaces are extracted as functions of Si cap thickness. A transition from dual to single channel operation in MOS devices at room temperature on thinning the silicon cap layer is observed with capacitance–voltage measurements. Cross-section transmission electron microscopy and secondary ion mass spectroscopy techniques are used to calibrate and support the developed methodology for Si cap layer etch control and the effect of Ge species at the oxide–heterostructure interface. There is a high fixed charge density, which indicates Ge pileup at the SiO2/SiGe interface. Our analysis suggests that about 1 nm of silicon cap retention is necessary to minimize the gate induced parasitic channel and to decouple the oxide interface trap influence on the channel carriers in the SiGe.
Spectroscopic study of Ce- and Cr-doped LiSrAlF6 crystals
Authors: Kontos A. G. , Tsaknakis G. , Raptis Y. S. , Papayannis A. , Landulfo E. , Baldochi S. L. , Barbosa E. , Vieira N. D.
Published: March 2003

Undoped, as well as Ce3+ and Cr3+ doped LiSrAlF6 (LiSAF) crystals have been examined by Raman spectroscopy in order to identify the system’s molecular vibrations and the role of doping. Furthermore the electron spin resonance (ESR) spectroscopic technique has been used to study the distortions of the local crystalline environment of Ce. The analysis of the main ESR resonance shows that the Ce3+ ions substitute for Sr2+ ones in the structure and permit the description of the electronic eigenfunctions and energy splitting of its F25/2 ground state. We have tackled the distortion of the trigonal crystalline environment, induced by Na+ codopants acting as charge compensators to Ce3+, and we have assigned the ESR resonances to Na occupying sites at various distances from the Ce atoms. Our results indicate growth of high quality crystals.
Dielectric spectroscopy of blends of polyvinylalcohol and polypyrrole
Authors: de Oliveira H. P. , dos Santos M. V. B. , dos Santos C. G. , de Melo C. P.
Published: March 2003

Polymeric blends composed of conducting polypyrrole chains dispersed in a matrix of polyvinylalcohol containing ferric chloride represent a class of materials whose electrical behavior is intermediary between those of insulating and conducting polymers. To investigate the transport and polarization characteristics of these films we examine in this work their dielectric relaxation spectrum in the frequency domain. A relaxation in frequency identified in the resistance–reactance diagram is followed as a function of the relative concentration of the two polymers, as different types of dopants are used to promote the conducting behavior of polypyrrole. We also analyze the change in the relative dc and ac contributions to the total conductivity of the samples as the amount of incorporated polypyrrole is increased.
Particle-in-cell/Monte Carlo simulation of a capacitively coupled radio frequency Ar/CF4 discharge: Effect of gas composition
Authors: Georgieva V. , Bogaerts A. , Gijbels R.
Published: March 2003

A one-dimensional particle-in-cell/Monte Carlo model is developed to study a capacitively coupled radio frequency discharge in a gas mixture of argon and CF4. The simulation takes into account the following charged particles: electrons, two kinds of positive ions (Ar+, CF3+), and two kinds of negative ions (F−, CF3−). The model considers electron–Ar collisions, electron−CF4 collisions, various kinds of collisions of CF3+, F−, CF3−, or Ar+ with Ar or CF4, and positive–negative ion recombination. The probability for the positive–negative ion recombination is determined from a recombination rate constant. The ion–neutral elastic and reactive collisions are simulated by an ion–molecule collision model for endothermic reactions. The typical results of this model are electron and ion densities, fluxes and energy distributions, collision rates, and electric field and potential distributions. The simulation is performed for 0.1/0.9, 0.5/0.5, and 0.9/0.1 ratios of a Ar/CF4 mixture, as well as for pure Ar and pure CF4 discharges at a pressure of 200 mTorr. It is observed that at high CF4 concentration the discharge behaves as a typical electronegative discharge and that CF3+ is the major positive ion. At low CF4 concentration, keeping the other operating parameters the same, the double layer structure and the electron density maxima at the bulk–sheath interface, which are representative for an electronegative discharge, disappear and the Ar+ density exceeds the CF3+ density by more than 1 order of magnitude. The results show that the F− ions are the dominant negatively charged species for all Ar/CF4 ratios investigated.
Hydrogen-terminated defects in ion-implanted silicon probed by monoenergetic positron beams
Authors: Uedono Akira , Mori Toshiki , Morisawa Kunitomo , Murakami Kouichi , Ohdaira Toshiyuki , Suzuki Ryoichi , Mikado Tomohisa , Ishioka Kunie , Kitajima Masahiro , Hishita Shunichi , Haneda Hajime , Sakaguchi Isao
Published: March 2003

Hydrogen-terminated vacancies in Si+-implanted Si were studied by means of positron annihilation. After the Si+-ion implantation, hydrogen atoms were introduced into the damaged region using a hydrogen plasma [hydrogen-atom treatment (HAT)]. Monoenergetic positron beams were used to measure Doppler broadening spectra of the annihilation radiation and the lifetime spectra of positrons. It was found that the line shape parameter, S, corresponding to the annihilation of positrons trapped by vacancy-type defects, decreased after HAT. This was attributed to the trapping of positrons by H-decorated vacancy-type defects. Isochronal annealing experiments revealed a strong correlation between positron annihilation parameters and Raman intensities of Si–H, suggesting that hydrogen atoms are released from vacancy-type defects after annealing at 600 °C.
Origins of the elastic behavior of nanoparticle chain aggregates: Measurements using nanostructure manipulation device
Authors: Suh Yong J. , Friedlander Sheldon K.
Published: March 2003

Nanoscale studies were conducted on the dynamic behavior of individual nanoparticle chain aggregates (NCAs) and their networks. For this purpose, device was fabricated to apply tension to NCA under controlled conditions. The device is composed of a specimen support and a cartridge. The specimen support is a deformable alloy disk with a narrow slit across which the NCAs are deposited; the cartridge is used to connect the specimen support to a specimen elongation support holder. The aggregates were stretched using the specimen holder to widen or narrow the slit gap at speeds from 0.5 to 300 nm/s and the motion was observed with a transmission electron microscope. Most of the studies were made with carbon NCA (primary particle size between 11 and 16 nm) generated by laser ablation of a graphite target. The aggregates were deposited on the specimen support (disk) to form bridges across the slit. When tension was applied, the NCA chains remained attached at the slit edges; the chains stretched as kinks on the scale of a few particle diameters were straightened by rotation and/or grain boundary sliding at particle–particle interfaces. After the chain became taut, increasing tension produced little additional extension. Eventually, the chain broke, the tension relaxed, and the elastically strained portions along the NCA recovered. This led to fast contraction of the two broken ends. In one of the cases studied in detail, a small primary particle in the chain doubled in length before the chain broke at this site. This probably occurred because of the high tensile stress in the small particle. In separate experiments, a network of carbon NCA was produced by increased deposition around the slit of a specimen support. Chains in the network broke successively as the network stretched. Some of the chains broke midway and not at the junctures with each other. They contracted fast showing behavior similar to that of the individual aggregates. Possible applications to the behavior of nanocomposite materials composed of blends of NCAs and molecular polymers (e.g., rubber) are described.
Comment on “Pd growth and subsequent Schottky barrier formation on chemical vapor cleaned p-type GaN surfaces” [J. Appl. Phys. 91, 732 (2002)]
Authors: Bermudez V. M.
Published: March 2003

This Comment attempts to clarify issues relating to the determination of the energy separation between the valence band maximum and the Fermi level in surface-sensitive photoemission experiments on wurtzite GaN. Such considerations are relevant to the measurement of band bending at the surface of wide-band gap materials.
Characterization and modeling of the nitrogen passivation of interface traps in SiO2/4H–SiC
Authors: McDonald K. , Weller R. A. , Pantelides S. T. , Feldman L. C. , Chung G. Y. , Tin C. C. , Williams J. R.
Published: March 2003

The relationship between nitrogen content and interface trap density (Dit) in SiO2/4H–SiC near the conduction band has been quantitatively determined. Nitridation using NO significantly reduces Dit near the conduction band, but the effect saturates after ≈2.5×1014 cm−2 of nitrogen. These results are consistent with a model of the interface in which defects such as carbon clusters or silicon suboxide states produce traps with energies corresponding to the sizes of the defects. Nitrogen passivation results in the dissolution of the defects, which then lowers the energies of the traps in the band gap.
Defect evolution of low energy, amorphizing germanium implants in silicon
Authors: King A. C. , Gutierrez A. F. , Saavedra A. F. , Jones K. S. , Downey D. F.
Published: March 2003

The defect evolution upon annealing of low energy, amorphizing germanium implants into silicon was studied using plan-view transmission electron microscopy. Implants with energies of 5–30 keV at an amorphizing dose of 1×1015 Ge+ cm−2 were annealed at 750 °C from 10 s to 360 min. For the implant energies of 10 and 30 keV, the defects form clusters which evolve to {311} defects that subsequently dissolve or form stable dislocation loops. However, as implant energy drops to 5 keV, the interstitials evolve from clusters to small, unstable loops which dissolve within a small time window and do not form {311}’s. To determine the effect of the free surface as an interstitial recombination sink for 5 keV implants, the amorphous layer of a 10 keV implant was lapped to less than the thickness of a 5 keV amorphous layer and then annealed. We found that the defect dissolution observed for the 5 keV implant energy was dependent on the implant energy and not the proximity of the end-of-range damage to the surface. The activation energy of the observed rapid defect dissolution at 5 keV was calculated to be 1.0±0.1 eV.
Effects of oxygen contamination on diffusion length in p+−n GaInNAs solar cells
Authors: Balcioglu A. , Ahrenkiel R. K. , Friedman D. J.
Published: March 2003

We have studied deep level impurities in p+−n GaInNAs solar cells using secondary ion mass spectroscopy (SIMS), capacitance–voltage (C−V), and deep-level transient spectroscopy (DLTS). These films were grown by atmospheric and low-pressure metalorganic vapor phase epitaxy. The base layer is doped with silicon and the emitter layer is Zn doped. Two types of devices have been studied: devices grown with and without the addition of oxygen impurity. Using SIMS, the oxygen concentration was found to be about 2−3×1019 and 1×1017 cm−3, respectively. C−V measurements at temperatures below 190 K have revealed that carrier freeze out occurs in high oxygen samples, whereas we did not observe this phenomenon in low oxygen devices. In addition to observation of several trap levels in all samples, we observed two additional near midgap traps designated E3 (electron) at EC −0.59 eV and H3 (hole) at EV +0.59 eV only in high oxygen devices. We present evidence that these levels (E3 and H3) are associated with the oxygen defect and are an effective recombination center. We observed a logarithmic correlation between the concentration of the oxygen recombination center and the device quantum efficiency. From this correlation, the hole diffusion length is 0.06–0.15 and 0.12–0.31 μm with and without back reflection, respectively. We found that the hole diffusion length is strongly dependent on the concentration of the oxygen recombination center. We conclude that the oxygen recombination center is a lifetime-limiting defect and, therefore, controls the hole diffusion length.
Quantitative control and detection of heterovalent impurities in ZnO thin films grown by pulsed laser deposition
Authors: Sumiya M. , Fuke S. , Tsukazaki A. , Tamura K. , Ohtomo A. , Kawasaki M. , Koinuma H.
Published: March 2003

Impurities in ZnO specimens, including targets for pulsed laser deposition and thin films resulting from their use, were analyzed by secondary ion mass spectroscopy (SIMS). Negatively charged complex ions bound with oxygen (71Ga16O− and N14O-16) were found to be the most reliable species with which to evaluate the Ga and N content of ZnO films by clarifying possible mass interference effects in SIMS analysis. Calibrations were carried out to determine the Ga concentration (CGa) and the nitrogen concentration (CN) by normalizing the signal intensities for G71O-16 and N14O-16, respectively, to that for Z70O-16. Alternative ablation of pure single crystal and Ga-doped ceramic ZnO targets was found to be effective not only for achieving systematic control of the Ga concentration in ZnO:(Ga,N) films, but also for minimizing the contamination of undesired impurities from the sintered targets. The substrate temperature plays a decisive role in control of CN due to a thermally activated desorption process of N-related species during deposition. Systematic control of the CN/CGa ratio in a ZnO:(Ga,N) film was carried out on a ScAlMgO4 substrate by introducing a controlled temperature gradient on the substrate during deposition. A region with the correct concentration ratio of CN/CGa=2, where p-type conduction of the ZnO film was theoretically predicted, was included in the composition spread sample in which the CN/CGa ratio was continuously varied over a wide range.
Implant isolation of ZnO
Authors: Kucheyev S. O. , Jagadish C. , Williams J. S. , Deenapanray P. N. K. , Yano Mitsuaki , Koike Kazuto , Sasa Shigehiko , Inoue Masataka , Ogata Ken-ichi
Published: March 2003

We study ion-irradiation-induced electrical isolation in n-type single-crystal ZnO epilayers. Emphasis is given to improving the thermal stability of isolation and obtaining a better understanding of the isolation mechanism. Results show that an increase in the dose of 2 MeV O16 ions (up to ∼2 orders of magnitude above the threshold isolation dose) and irradiation temperature (up to 350 °C) has a relatively minor effect on the thermal stability of electrical isolation, which is limited to temperatures of ∼300–400 °C. An analysis of the temperature dependence of sheet resistance suggests that effective levels associated with irradiation-produced defects are rather shallow (<50 meV). For the case of implantation with keV Cr, Fe, or Ni ions, the evolution of sheet resistance with annealing temperature is consistent with defect-induced isolation, with a relatively minor effect of Cr, Fe, or Ni impurities on the thermal stability of isolation. Results also reveal a negligible ion-beam flux effect in the case of irradiation with 2 MeV O16 ions, supporting high diffusivity of ion-beam-generated defects during ion irradiation and a very fast stabilization of collision cascade processes in ZnO. Based on these results, the mechanism for electrical isolation in ZnO by ion bombardment is discussed.
Study of conduction mechanism and electroluminescence in CdSe/ZnS quantum dot composites
Authors: Hikmet R. A. M. , Talapin D. V. , Weller H.
Published: March 2003

Charge transport and electroluminesence properties of colloidally synthesized CdSe/ZnS core–shell nanocrystal quantum dots (QDs) were studied. Nanocrystals were prepared using the conventional technique of pyrolisis of organometallic reagents in a hot coordinating solvent medium. Thin film diodes were produced by depositing a layer of QDs on top of a layer of conducting polymer poly (3,4-ethylenedioxythiophene): polystyrenesulfonate followed by the deposition of a metal electrode. For all the metals used in this study only a small rectification could be observed upon reversing the bias. The efficiency of the light generation showed strong dependence on the work function and bias of the metal electrodes, indicating that light generation in these devices is a result of recombination of injected holes and electrons. Assuming that the charge transport is due mainly to electrons, the voltage dependence of the current in QD composites could be explained by space charge limited current (SCLC) in the presence of defects. At sufficiently high voltages the traps could be filled and a quadratic dependence of current on voltage, characteristic of trap free SCLC, could be observed. The mobility and the trap density were estimated to be μn=1.5×10−10 m2/V s and Nt=8×1023 m−3, respectively, and the characteristic trap depth was estimated to be around 0.15 eV.
Temporal and temperature variations of dc magnetic aftereffect measurements of Fe3O4 powders
Authors: Seyoum H. M. , Bennett L. H. , Della Torre E.
Published: March 2003

The temperature variation of the magnetic relaxation process obtained from dc magnetization measurements are investigated in fine Fe3O4 powder in the temperature range of 5 K<T<300 K. The maximum decay rate, S=d(M)/d[ln(t)], shows two distinct relaxation processes. The two processes display remarkably similar pronounced peaks 10, 35, 60, and 85 K below the Verwey transition (Tv) and different behavior above Tv. At the same time, high field irreversibility in M(T) curves in zero-field cooled and field cooled processes and a considerable increase in coercivity have also been observed below Tv. This is related to the freezing of disordered spins in reorganization of the domain structure.
Thermoelectric properties of highly grain-aligned and densified Co-based oxide ceramics
Authors: Zhou Yuqin , Matsubara Ichiro , Horii Shigeru , Takeuchi Tomonari , Funahashi Ryoji , Shikano Masahiro , Shimoyama Jun-ichi , Kishio Kohji , Shin Woosuck , Izu Noriya , Murayama Norimitsu
Published: March 2003

Highly grain-aligned Ca3Co4O9 and (Ca2.7Sr0.2La0.1)(Co3.9Cu0.1)O9 ceramics were prepared by the magnetic alignment technique, and then treated by a spark plasma sintering process to increase their bulk densities. Thermoelectric properties were investigated from room temperature to 700 °C in air. Grain alignment is effective in lowering the electrical resistivity and has no obvious influence on the Seebeck coefficient, thus resulting in enhancement of the thermoelectric power factor. Substitution of Sr, La and Cu does not appreciably change the electrical resistivity and Seebeck coefficient, but significantly reduces the thermal conductivity.
Effect of interstitial Ar, Ne, He, and O2 on the glass and phase transitions in solid C60
Authors: Gu Min , Tang Tong B.
Published: March 2003

C 60 pellets charged with He, Ne, Ar, or O2 under high pressure, have been studied with complex impedance and high-resolution C13 nuclear magnetic resonance spectroscopes. Ar and O2 were found to depress both the face-centered-cubic–simple cubic and the glass transitions. Storage at room temperature hardly affected these temperatures shifts in the first case, but the shifts decreased exponentially in the O2 case. The effective barriers for the glass transition are determined as 218±15 and 182±20 meV in Ar0.49C60 and (O2)0.5C60, respectively. Neither He nor Ne had a discernible influence on the temperatures and the barrier energy. These observations may be explained as follows. (1) The intercalation of Ar causes the C60 lattice to expand, and the resulting “free volume” increment suppresses Tg. (2) The reduction in effective barriers (by ∼70 meV) weakens the restorative force of the cage libration mode, which decreases Tc. (3) For (O2)0.5C60, an anharmonic coupling between the cage libration and the tumbling of O2 diatomic molecules further modified the potential surface and reduced the effective barriers, therefore inducing a larger temperature shift. (4) Its anharmonic tumbling motion makes O2 more susceptible to the “paddle wheel” action of rotating C60 so that it diffuses readily at room temperature.
Band gap and cathode- and photoluminescences from LiInO2 films
Authors: Kushida K. , Koba T. , Kuriyama K.
Published: March 2003

LiInO 2 is one of the candidate materials as a solid-state scintillator for solar neutrinos due to an inverse β−-decay of In115 to Sn115. LiInO2 films (space group I41/amd) having a tetragonal phase are grown by a sol–gel spin-coating method using Li and In acetates as raw materials. Six Raman active modes related to the space group are observed at 156, 250, 278, 382, 520, and 576 cm−1. The band gap of LiInO2 is found to be direct with a forbidden gap of 4.2 eV from an optical absorption spectrum. As a virtual experiment for testing a scintillation due to β−-ray self-emitted from In115 in LiInO2 by solar neutrinos, a cathode luminescence (CL) study is performed. A CL peak is observed in the wavelength range of 533–556 nm (2.3–2.2 eV) for various electron energies (5–20 kV) at room temperature. Possible origins of the yellow emission (∼2.2 eV) are oxygen vacancies introduced during the sol–gel synthesis and/or residual impurities included in the coating solution. A 2.3 eV photoluminescence is also observed at 20 K.
Effective medium approximation for the effective optical constants of a bilayer and a multilayer structure based on the characteristic matrix technique
Authors: El-Haija A. J. Abu
Published: March 2003

This article presents a derivation of the effective optical constants of a bilayer using the characteristic matrix technique. The derivation is simple, and in essence, adds an extra step to an approximate treatment of near-normal incidence spectroscopy, based on the characteristic matrix technique for stratified media. By this method, the effective optical constants of a bilayer consisting of two thin layers, each with a thickness much less than the wavelength of the incident radiation, are determined. This step leads to a modified procedure for calculating the optical properties of a layered stack that consists of thin bilayer units. The method, though still an effective medium approximation, is distinct from previous approximations. It introduces a more consistent parallelism to the fundamental theory of the characteristic matrix technique, and leads to a simplified analog of a multilayer viewed as one system of definite effective optical constants. Such a task has been difficult to achieve for structures with an arbitrary number of layers.
Numerical modeling of pyrolytic laser-induced chemical vapor deposition
Authors: Koutlas G. N. , Vlachos N. S.
Published: March 2003

Laser chemical vapor deposition (LCVD) is a technique to deposit thin films of oxidation, corrosion, and wear resistant as well as electronic, optoelectronic, and superconductor materials. In order to understand the underlying mechanisms of such a process we have developed a numerical model using computational fluid dynamics (CFD). The Navier–Stokes equations governing the flow, heat transfer, and chemical reactions of the gases are solved numerically while the temperature distribution in the substrate is determined by solving the corresponding heat conduction equation. The present CFD model provides an opportunity to assess the important parameters concerning the LCVD process, such as the gas flow field, temperature distribution, concentration of reactants/products, and deposition height. Indicative results are presented for the deposition of titanium carbide upon AISI 1060 carbon. These results provide understanding of the LCVD process and enable its optimization.
Emission characteristics of vapor transport equilibrated Er:LiNbO3 crystals
Authors: Zhang De-Long , Pun E. Y. B.
Published: March 2003

At room temperature, polarized visible and near infrared emission characteristics of 488 nm pumped vapor transport equilibration (VTE) treated Er:LiNbO3 crystals were investigated in comparison with the corresponding as-grown ones. The observed characteristics of the VTE crystal, whether it precipitates or not, include the retained polarization dependence, slight shift (less than 5 Å) of peak position, narrowing of some individual emission peaks, the appearance of additional emission peak near 1536.5 nm and the about 10% lengthening of 1.5 μm lifetime. In addition, in the case of lower Er doping level, the spectral shape are well reserved for the VTE crystals and no obvious changes in both visible and IR emission intensity. On the other hand, for those highly doped crystals, the VTE treatment not only results in a definite change of relative intensity between some emission peaks in IR region, but also causes the drop of both visible and IR emission intensity. Combining with the related absorption characteristic allow to conclude that the drop of the emission intensity results from the scattering of the nanocrystalline grains inside these crystals formed by VTE treatment.
Judd–Ofelt analysis of the Er3+ (4f11) absorption intensities in Er3+−doped garnets
Authors: Sardar Dhiraj K. , Bradley William M. , Perez John J. , Gruber John B. , Zandi Bahram , Hutchinson J. Andrew , Trussell C. Ward , Kokta Milan R.
Published: March 2003

Spectroscopic and laser properties of three different Er3+−doped garnet systems are characterized by employing the Judd–Ofelt (JO) analysis. The three garnet hosts are Y3Al5O12 (YAG), Y3Sc2Ga3O12 (YSGG), and Gd3Ga5O12 (GGG). The JO model has been applied to the room temperature absorption intensities of Er3+ (4f11) transitions to establish the so-called JO intensity parameters: Ω2, Ω4, and Ω6 in the three garnet hosts. The intensity parameters are used to determine the radiative decay rates (emission probabilities of transitions) and branching ratios of the Er3+ transitions from the excited state J manifolds to the lower-lying J′ manifolds. The predicted decay rates and branching ratios of these Er3+ transitions in YAG, YSGG, and GGG hosts are compared. From the radiative decay rates, the radiative lifetimes of the Er3+ excited states are determined in the three garnets and are also compared. We also report the spectroscopic quality factors, Ω4/Ω6, obtained for the three garnets. The quantum efficiencies of the I413/2→I415/2 Er3+ transition in YAG, YSGG, and GGG are determined to be ∼79%, 82%, and 85%, respectively.
Temperature dependence of electron spin resonance and electrical conductivity in P+-implanted C60 films and their derivatives
Authors: Fahim N. F. , Kojima N. , Yamaguchi M. , Ohshita Y. , Barsoum B. N. , Eid A. E.
Published: March 2003

The temperature dependence of electron spin resonance (ESR) has been measured to clarify the origin and nature of paramagnetic states responsible for the observed ESR signal in P+-implanted C60 films. Also, the temperature dependence of electrical conductivity was made and compared with ESR linewidths to detect the transport mechanism in these films. The ESR experiments were performed at 9.4 GHz in a wide temperature range from 3.7 to 300 K. The temperature dependence of the ESR signal intensity and spin susceptibility revealed that the unpaired spin follows the Curie law at T<20 K, while a clear deviation was observed at T>20 K. The Curie behavior of the spin susceptibility results from localized dangling bond electrons, while a deviation occurs due to delocalization and excitation of electrons into states as the temperature increases. The temperature dependence of linewidths indicates that the spin mobility occurs by hopping, taking into account that the electron–phonon interaction as spin relaxation mechanism at T>Tmin. The observed minimum in linewidths can be attributed to a decrease in the narrowing effects as the temperature decreases. Finally, the temperature dependence of the electrical conductivity and ESR resonance signal linewidth suggests that the transport mechanism occurs by hopping motion and the unpaired spins are partially localized.
Electronic subbands of Monte Carlo simulated doping profiles defined by a split gate potential during thermal treatment
Authors: Sundqvist P. A. , Narayan V. , Willander M.
Published: March 2003

We studied a model in which an external one-dimensional parabolic potential defined by split gates is used during heat treatment to calculate impurity profiles with an approximately Gaussian distribution in a semiconductor layer. Using a recently published model, the impurities are moved by a Monte Carlo procedure, to calculate equilibrium impurity profiles for different layer thicknesses and initial doping levels. The samples are cooled and the electronic subbands are then calculated self-consistently by coupling the Schrödinger equation with a charge neutral Poisson equation for temperatures between 40–300 K. The model includes temperature and doping concentration dependent impurity ionization rates. The polarity and strength of the split gate voltage may be altered to affect the subband energies and wave functions. When a parabolic potential with a negative constant was added, we found that it is possible to produce a charge density that consists of two peaks located near the quantum well walls. This effect is slightly washed out at room temperature. For a parabolic potential with a large and positive constant, the charge density becomes sharply localized at the middle of the quantum well. Throughout the calculations, we have used slightly nonsymmetric doping profiles.
Effects of As pressure and growth temperature on the growth of TlGaAs films by molecular-beam epitaxy
Authors: Kajikawa Y. , Kobayashi N. , Nishimoto N.
Published: March 2003

Growth of Tl0.05Ga0.95As on GaAs (001) substrates by molecular-beam epitaxy has been performed at substrate temperatures of 230, 210, and 190 °C under various As pressures. At a growth temperature of 230 °C, Tl droplets were observed on the surfaces after the growth under low As pressures, while no droplets were observed when As pressure was increased. On the other hand, when grown at 190 °C, no droplets were observed over the entire range of the As pressure examined. Except for a sample grown at 230 °C under the lowest As pressure, a clearly resolved peak appeared in addition to a substrate peak in each x-ray diffraction curve, indicating the existence of an epitaxial TlGaAs layer. However, the weak intensity of the diffracted peaks due to the TlGaAs layers suggested that single-crystalline growth is replaced by polycrystalline growth at some thickness, hepi. We estimated the epitaxial thickness, hepi, through fitting theoretical simulation to the experimental diffraction curves, and found that hepi increases with increasing As pressure and with decreasing growth temperature. The effects of As pressure and growth temperature on Tl droplet formation and on hepi are discussed in terms of a competition between Tl–As and Tl–Tl bond formation.
High-mobility organic thin-film transistors based on α,α′-didecyloligothiophenes
Authors: Halik Marcus , Klauk Hagen , Zschieschang Ute , Schmid Günter , Radlik Wolfgang , Ponomarenko Sergei , Kirchmeyer Stephan , Weber Werner
Published: March 2003

We have fabricated organic thin-film transistors and integrated circuits based on the small-molecule organic semiconductors α,α′-didecylquaterthiophene, α,α′-didecylquinquethiophene, and α,α′-didecylsexithiophene. The organic semiconductors were deposited by thermal evaporation, with solution-processed and cross linked poly-4-vinylphenol serving as the gate dielectric layer. We have found that bottom-contact devices based on these materials have better electrical performance than top-contact devices, presumably due to more efficient carrier injection from bottom contacts due to the presence of the relatively long alkyl chains substituted at the α- and ω-positions of the oligothiophene molecules. Bottom-contact transistors have carrier mobility as large as 0.5 cm2/V s and on/off current ratio as large as 105, and ring oscillators fabricated using bottom-contact transistors and α,α′-didecylsexithiophene as the organic active layer have signal propagation delay as low as 30 μs per stage.
Direct verification of the ion-neutral synergism during hydrocarbon film growth
Authors: Hopf C. , von Keudell A. , Jacob W.
Published: March 2003

Elementary ion-induced surface processes during plasma deposition of amorphous hydrogenated carbon films are studied in a particle-beam experiment employing sources for noble gas ions and CH3 radicals. Two processes govern film formation: (1) Ion-assisted film growth: Incident ions displace surface-bonded atoms and create dangling bonds which then serve as chemisorption sites for incident radicals, and (2) Ion-induced hydrogen release: Incident ions alter the film composition by preferential removal of bonded hydrogen in the subsurface of the growing film. It is shown that both elementary processes are in quantitative agreement with displacement yields as calculated by computer simulations.
Tunneling through InAs quantum dots in AlGaAs/GaAs double barrier resonant tunneling diodes with InGaAs quantum well emitters
Authors: Lin S. D. , Lee C. P.
Published: March 2003

Using InGaAs quantum well emitters, AlGaAs/GaAs double barrier resonant tunneling diodes with and without self-assembled InAs/GaAs quantum dots (QDs) have been studied extensively. Because the energy state of the emitter was lower than the level of the ground state within InAs QDs, the resonant tunneling was observed clearly near zero bias in all devices. From the results of bias-dependent photoluminescence and current–voltage characteristics, we obtain unambiguously the resonant tunneling through the InAs QDs, both controllably and reproducibly.
Directional luminescence from quantum dots in quantum well
Authors: Shen M. Y. , Asayama R. , Goto T. , Yao T.
Published: March 2003

Directional luminescence from coupled quantum states of individual ZnSe quantum dots (QDs) in ultrathin single quantum well structure, ZnS/(ZnSe)1/ZnS has been observed. The cause of the formation of the states is the Coulomb interaction between holes in the ZnSe QDs. Despite the random distribution of the sizes and positions of the QDs, the coherent length of the coupled quantum states reaches about 360 nm, which can be estimated directly through the emission angle dependence of the directional luminescence.
Electron energy distribution during high-field transport in AlN
Authors: Collazo R. , Schlesser R. , Roskowski A. , Miraglia P. , Davis R. F. , Sitar Z.
Published: March 2003

The energy distribution of electrons transported through intrinsic AlN heteroepitaxial films grown on SiC was directly measured as a function of applied field and AlN film thickness. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through films thicker than 95 nm at an applied field between 200 and 350 kV/cm occurred as steady-state hot electron transport following a Maxwellian energy distribution with a characteristic carrier temperature. At higher fields (470 kV/cm), intervalley scattering was evidenced by a multicomponent energy distribution featuring a second peak at the energy position of the first satellite valley. Velocity overshoot was observed in films thinner than 95 nm and at fields greater than 550 kV/cm. In this case, a symmetric energy distribution centered at an energy above the conduction band minimum was measured, indicating that the drift component of the electron velocity was on the order of the “thermal” component. A transient transport length of less than 80 nm was deduced from these observations.
Model of ferrite-cored probes for eddy current nondestructive evaluation
Authors: Theodoulidis Theodoros P.
Published: March 2003

A model of an axisymmetric probe coil with a cylindrical ferrite core in the presence of a layered conducting half space is developed. The boundary value problem is formulated in terms of the magnetic vector potential, which is expanded in a series of appropriate eigenfunctions. The corresponding eigenvalues are the real roots of a function with the radius and the magnetic permeability of the core as parameters. The unknown coefficients of the series are computed by solving a matrix system, which is formed by applying the usual interface conditions. Expressions are derived for the induced eddy current density and the impedance of the coil. Numerical results for the latter are in good agreement with experimental data.
Nonlinear alternating current response of colloidal suspension with an intrinsic dispersion
Authors: Huang J. P. , Gao L. , Yu K. W.
Published: March 2003

When a sinusoidal (ac) field is applied to a suspension containing nonlinear dielectric particles, the electrical response will generally consist of ac fields at frequencies of the higher-order harmonics. The situation is further complicated by an intrinsic dielectric dispersion, which often occurs due to the surface conductivity or inhomogeneous structure of the particles. We develop a perturbation method to investigate the effect of intrinsic dielectric dispersion on the harmonics of local field as well as induced dipole moment. The results showed, for weak intrinsic dispersion strength, the ratio of the third to first harmonics of the induced dipole moment decreases as the frequency increases, which is qualitatively in agreement with experiment. However, for a strong dispersion strength, the harmonics ratio increases as the frequency increases. Moreover, an increase in the intrinsic relaxation time may increase the strength of the harmonics.
Injection cascade lasers with graded gap barriers
Authors: Vasilyev Yu. B. , Suchalkin S. D. , Polkovnikov A. S. , Zegrya G. G.
Published: March 2003

We propose cascade structures based on type-II heterostructures with graded barriers separating electron and hole layers. The electron–hole separation is controlled by an external bias due to modification of the barrier shape. This principle can be used for fabrication of a class of devices. In particular, bipolar interband cascade lasers are considered in which an external bias changes the rate of electron–hole radiative recombination by several orders of magnitude. Theoretical calculations predict the feasibility of these lasers.
Pulsed laser deposition with a high average power free electron laser: Benefits of subpicosecond pulses with high repetition rate
Authors: Reilly Anne , Allmond Chris , Watson Shannon , Gammon Jason , Kim Jung Gi
Published: March 2003

We have conducted experiments exploring pulsed laser deposition of thin films using the high average power Thomas Jefferson National Accelerator Facility Free Electron Laser. The combination of parameters of this laser, including subpicosecond pulses, high average power, high repetition rate, and tunability, makes it a unique tool for the study of the effects of laser characteristics on thin-film quality. When compared to ablation and deposition with an ultrafast, high energy per pulse, low repetition rate laser (amplified Ti:sapphire), we find that the lower energy per pulse with high repetition rate of the free electron laser leads to very different plasma emission and produces films with high quality with the potential of very high deposition rates. This is demonstrated in the optical spectroscopy of plasma emission from Ti and the growth of Ni80Fe20 thin films.
Formation trends in quantum dot growth using metalorganic chemical vapor deposition
Authors: El-Emawy A. A. , Birudavolu S. , Wong P. S. , Jiang Y.-B. , Xu H. , Huang S. , Huffaker D. L.
Published: March 2003

We discuss the results of a growth matrix designed to produce high quantum dot (QD) density, defect-free QD ensembles, which emit at 1.3 μm using metalorganic chemical vapor deposition (MOCVD). In our study, we balance the nucleation rate and adatom surface migration to achieve high surface densities (1×1011 dots/cm2) and avoid QD coalescence or defects that commonly characterize MOCVD-grown QD ensembles designed for longer wavelength emission. Room-temperature photoluminescence (PL) spectra from corresponding surface QDs depend on QD size and density and show an emission wavelength up to 1600 nm. Ground-state PL from capped QDs is measured at 1.38 μm with a 40 meV linewidth. We demonstrate the ground-state 1.3 μm electroluminescence from a QD light-emitting diode structure grown on n-type GaAs.
Electron paramagnetic resonance characterization of Cr3+ impurities in a β-Ga2O3 single crystal
Authors: Yeom T. H. , Kim I. G. , Lee S. H. , Choh S. H. , Yu Y. M.
Published: March 2003

β- Ga 2 O 3 single crystals doped with the Cr3+ ion were grown in an O2 atmosphere using the floating zone method. Electron paramagnetic resonance (EPR) spectra of the Cr3+ ion were recorded with an X band EPR spectrometer at 20 °C. The rotation patterns of Cr3+ spectra in the crystallographic planes together with spin-Hamiltonian parameters showed unequivocally that the actual local site symmetry of the Cr3+ ion in β-Ga2O3 is not orthorhombic, as previously reported, but monoclinic. The spectroscopic splitting tensor g and the zero-field splitting parameters Bkq s were determined with an effective spin Hamiltonian. The Cr3+ ions replace Ga3+ ions in oxygen octahedra rather than in oxygen tetrahedra. The energy levels of the ground state of a Cr3+ ion embedded in the β-Ga2O3 crystal were calculated.
Wave structure in the contact line region during high speed droplet impact on a surface: Solution of the Riemann problem for the stiffened gas equation of state
Authors: Haller K. K. , Ventikos Y. , Poulikakos D.
Published: March 2003

Upon impact of a liquid droplet on a rigid substrate at sufficiently high velocity, a wave front adjacent to the impacted surface is created, separating the compressed area from the undisturbed bulk of the liquid. At a certain point in the evolution of the phenomenon, the free surface at the vicinity of the contact line yields and an intense lateral jetting phenomenon occurs. The often employed assumption that the wave front is composed of a single shock wave across its entire surface leads to the emergence of an anomaly, prior to the jetting eruption. This anomaly can be removed by the proposition of the existence of a multiple wave structure at the contact line region. In this article, the wave structure at the contact line region is explored analytically. The exact one-dimensional Riemann problem solution for the stiffened gas equation of state is presented, under the assumption of an isentropic flow in the smooth flow region (i.e., in the region with no contact discontinuities). For the case of impact velocity V=500 m/s and a water droplet with radius R=0.1 mm, we show that in addition to the single shock structure, solutions consisting of an expansion fan and shock wave are physically admissible. In this scenario, the pressure created at the contact line decreases compared to the pressure of the single shock structure. It is demonstrated that the liquid particle velocity in the intermediate region between the waves is lower than the velocity in the bulk of compressed liquid. An algorithm for estimating the intermediate region state, when the left and right states are known, is also presented.
Model relating process variables to film electrical properties for reactively sputtered tantalum oxide thin films
Authors: Jain Pushkar , Bhagwat Vinay , Rymaszewski Eugene J. , Lu Toh Ming , Berg Soren , Cale Timothy S.
Published: March 2003

An experimentally verified analytical model for pulsed dc reactive sputtering of tantalum oxide films is described. The influences of important process variables, like oxygen flow rate and sputtering ion current, on the oxygen partial pressure in the chamber, deposition rate, as well as film breakdown and leakage characteristics, are predicted using this model. The experimentally established existence of multiple oxygen partial pressures at a given oxygen flow rate (hysteresis loop) is theoretically explained using steady state analysis. The partial pressure of oxygen during deposition can be used to correlate and predict the critical oxygen flow rate required to achieve insulating dielectric films (with no metallic tantalum) for a specified sputtering ion current. The experimental results suggest that in order to ensure the electrical reliability of tantalum oxide films, deposition should be done at oxygen flow rates more than that required to go beyond the hysteresis region in partial pressure versus flow rate curve. In addition, the observed hystereses in breakdown field and leakage current density of tantalum oxide films closely follow the hystereses in the oxygen partial pressure curve with oxygen flow rate. The stabilities of different steady state operating points with respect to fluctuations in flow rate are discussed.
Geometries for high spatial resolution Hall probes
Authors: Guillou H. , Kent A. D. , Stupian G. W. , Leung M. S.
Published: March 2003

The Hall response function of symmetric and asymmetric planar Hall effect devices is investigated by scanning a magnetized tip above a sensor surface while simultaneously recording the topography and the Hall voltage. Hall sensor geometries are tailored using a focused ion beam, in standard symmetric and asymmetric geometries. With this technique we are able to reduce a single voltage probe to a narrow constriction 20 times smaller than the other device dimensions. We show that the response function is peaked above the constriction, in agreement with numerical simulations. The results suggest a way to pattern Hall sensors for enhanced spatial resolution.