bluuebunny/crossref_metadata_2025 huggingface dataset:
Cryogenic liquid-jet target for debris-free laser-plasma soft x-ray generation
Authors: Berglund M. , Rymell L. , Hertz H. M. , Wilhein T.
Published: July 2002

A new target system based on a continuous cryogenic liquid jet for debris-free laser-plasma soft x-ray generation is described. The system is experimentally evaluated with liquid nitrogen as target material. With this target the photon flux is 4.5×1011 photons/(sr×pulse) from the λ=2.88 nm N VI line. Brightness and stability are also investigated for this table-top soft x-ray microscope source. The possibility to utilize other cryogenic liquids such as neon, argon, and xenon and, thus, making the system interesting for short-wavelength lithography applications, is also discussed.
A pressurized drop-tube furnace for coal reactivity studies
Authors: Ouyang Shan , Yeasmin Hasina , Mathews Joseph
Published: July 2002

The design and characterization of a pressurized drop-tube furnace for investigation of coal devolatilization, gasification, and combustion are presented. The furnace is designed for high-temperature, isothermal operation in a developing laminar flow regime. It can be operated at pressures up to 1600 kPa, and temperatures up to 1673 K, with variable reaction time, particle feeding rate, and with inert and various oxidizing atmospheres. Particle residence times can be varied between ∼0.02 and ∼10 s depending upon operating conditions and positions of injection and sampling probes. Observations ports are available for sample collections and for optical investigation of the reactions or temperature measurements. Characterization of gas temperature in the furnace shows that, although the gas temperature profile in the furnace is affected by the water-cooled injection probe, the furnace is able to achieve isothermal operation in a developing laminar flow regime. Results from a series of brown coal devolatilization tests demonstrated the suitability of the furnace for experiments in coal research.
Interface circuits for quartz-crystal-microbalance sensors
Authors: Eichelbaum Frank , Borngräber Ralf , Schröder Jens , Lucklum Ralf , Hauptmann Peter
Published: July 2002

The utilization of quartz-crystal-microbalance sensors in liquids yields new requirements to the applied interface circuits. In the present article, the fundamentals of the measuring principle and advantages and drawbacks of suitable interface circuits are discussed. Special requirements of oscillators as interface circuits are outlined. Possible solutions to those requirements are investigated and two recently developed oscillator circuits are presented.
On the initial growth of indium tin oxide on glass
Authors: Sun X. W. , Huang H. C. , Kwok H. S.
Published: July 2002

The initial growth mode of indium tin oxide on glass was studied. An interesting transition from a 3D island growth mode at low temperatures to 2D growth at higher temperature was observed. This transition from the Volmer-Weber mechanism to the Frank–van der Merwe mechanism occurs at about 150°C. It coincides with the transition from amorphous to polycrystalline growth of the thin films.
Z-propagating waveguide lasers in rare-earth-doped Ti:LiNbO3
Authors: Amin J. , Aust J. A. , Sanford N. A.
Published: July 2002

A means of reproducibly fabricating stable cw lasers in rare-earth-doped Ti:LiNbO3 has been demonstrated through judicious choice of waveguide orientation. Z-propagating waveguides have been fabricated in Nd- and Er-diffused Ti:LiNbO3 and room-temperature laser operation with greatly reduced photorefractive instability has been obtained. The reduced photorefractive damage susceptibility in this waveguide configuration has led to the realization of a 980 nm pumped laser in Er:Ti:LiNbO3, with a threshold of 10.5 mW of absorbed pump power and a slope efficiency of 8.5%.
Magnetocaloric effect in La0.67Ca0.33MnOδ and La0.60Y0.07Ca0.33MnOδ bulk materials
Authors: Zhang X. X. , Tejada J. , Xin Y. , Sun G. F. , Wong K. W. , Bohigas X.
Published: July 2002

The dependence of magnetization on the applied magnetic field and temperature was measured carefully near their Curie temperatures for two perovskite manganite samples: La0.67Ca0.33MnOδ and La0.60Y0.07Ca0.33MnOδ. It is suggested by the results that these materials can be utilized as both the thermal storage (passive regeneration) and as the working material (active regeneration) in an active magnetic regenerative refrigerator with very large temperature span, for their significant entropy change upon the application of a magnetic field and the easily tuned Curie temperatures.
Control of GaAs lateral growth rate by CBr4 during metalorganic chemical vapor deposition on patterned substrates
Authors: Kim Seong-Il , Kim Moo-Sung , Kim Yong , Son Chang Sik , Hwang Seong-Min , Min Byung-Don , Kim Eun Kyu , Min Suk-Ki
Published: July 2002

Lateral growth rate of GaAs is remarkably increased with supplying CBr4, which has been utilized as a p-type dopant source for carbon doped GaAs epilayers during metalorganic chemical vapor deposition growth. The lateral growth rate can be represented as a linear function of the CBr4 flow rate, while the GaAs vertical growth rate is relatively insensitive to the CBr4 flow rate. The maximum ratio of the lateral to vertical growth rate by CBr4 is about 29. With increasing the growth temperature, the lateral growth rate increases, but it decreases at a more elevated growth temperature than 700 °C. These results are also compared to the previous results obtained by CCl4. In all cases the lateral growth rate increments by CBr4 are larger than those by CCl4.
Theoretical evidence for efficient p-type doping of GaN using beryllium
Authors: Bernardini Fabio , Fiorentini Vincenzo , Bosin Andrea
Published: July 2002

Ab initio calculations predict that Be is a shallow acceptor in GaN. Its thermal ionization energy is 0.06 eV in wurtzite GaN; the level is valence resonant in the zincblende phase. Be incorporation is severely limited by the formation of Be3N2. It is shown, however, that co-incorporation with reactive species can enhance the solubility. H-assisted incorporation should lead to high doping levels in metal-organic-chemical-vapor deposition growth after post-growth annealing at about 850 K. Be-O co-incorporation produces high Be and O concentrations at molecular beam epitaxy growth temperatures.
Field-sweep rate dependence of magnetization and current–voltage characteristics in superconducting disks
Authors: Mawatari Y. , Sawa A. , Obara H. , Umeda M. , Yamasaki H.
Published: July 2002

We investigated distributions of electric field E and current density J in disk-shaped superconductors exposed to a perpendicular magnetic field Ba that is swept at a constant rate β=dBa/dt. The expressions of the dependence of the magnetization M on the field-sweep rate β were derived for arbitrary E–J characteristics taking into account the distributions of E and J accurately. We measured the dependence of M on β in a YBa2Cu3O7 disk, and estimated the E–J characteristics using the theoretical analysis. Furthermore, distribution of J(r) was estimated from the E(r) distribution and E–J characteristics.
Thermodynamics of diffusion under pressure and stress: Relation to point defect mechanisms
Authors: Aziz Michael J.
Published: July 2002

A thermodynamic formalism is developed for illuminating the predominant point defect mechanism of self- and impurity diffusion in silicon and is used to provide a rigorous basis for point defect-based interpretation of diffusion experiments in biaxially strained epitaxial layers in the Si–Ge system. A specific combination of the hydrostatic and biaxial stress dependences of the diffusivity is ±1 times the atomic volume, depending upon whether the predominant mechanism involves vacancies or interstitials. Experimental results for Sb diffusion in biaxially strained Si–Ge films and ab initio calculations of the activation volume for Sb diffusion by a vacancy mechanism are in quantitative agreement with no free parameters. Key parameters are identified that must be measured or calculated for a quantitative test of interstitial-based mechanisms.
Observation of room-temperature laser emission from type III InAs/GaSb multiple quantum well structures
Authors: Baranov A. N. , Bertru N. , Cuminal Y. , Boissier G. , Alibert C. , Joullié A.
Published: July 2002

Multiple quantum well InAs/GaSb laser heterostructures with type III (type II broken gap) band alignment in the active region have been grown by molecular beam epitaxy. Intense electroluminescence was observed at room temperature (RT) with peak emission wavelengths in the range 1.95–3.4 μm. RT lasing has been achieved at 1.98 and 2.32 μm for the structures with 6 and 12 Å thick InAs quantum wells, respectively.
Investigation of neutralized (NH4)2S solution passivation of GaAs (100) surfaces
Authors: Yuan Z. L. , Ding X. M. , Hu H. T. , Li Z. S. , Yang J. S. , Miao X. Y. , Chen X. Y. , Cao X. A. , Hou X. Y. , Lu E. D. , Xu S. H. , Xu P. S. , Zhang X. Y.
Published: July 2002

Synchrotron radiation photoelectron spectroscopy combined with scanning electron microscopy (SEM) and gravimetry has been used to study GaAs (100) surfaces treated with a neutralized (NH4)2S solution. Compared to the conventional basic (NH4)2S solution treatment, a thick Ga sulfide layer and strong Ga–S bond were formed on the GaAs surface after dipping GaAs wafers in a neutralized (NH4)2S solution. Gravimetric data show that the etching rate of GaAs in the neutralized (NH4)2S solution is about 15% slower than that in the conventional (NH4)2S solution. From SEM observation, fewer etching pits with smaller sizes were found on the neutralized (NH4)2S-treated GaAs surface.
Bis(dithienothiophene) organic field-effect transistors with a high ON/OFF ratio
Authors: Sirringhaus H. , Friend R. H. , Li X. C. , Moratti S. C. , Holmes A. B. , Feeder N.
Published: July 2002

A conjugated, fused-thiophene oligomer, bis(dithienothiophene) (BDT), has been synthesized and deposited by vacuum sublimation as the active layer in organic thin film transistors (TFTs). The TFTs show exceptionally high ON/OFF ratios up to 108 between accumulation and depletion with sharp turn-on characteristics comparable to that of amorphous silicon TFTs (subthreshold slope S=0.6 V/decade). Field-effect mobilities are 0.02–0.05 cm2/V s. The good performance is explained by the relatively high π-π* gap of the short-chain BDT molecule and the favorable coplanar π-π stacking in BDT, differing from the herringbone stacking in the oligothiophenes.
The physical origin of InAs quantum dots on GaAs(001)
Authors: Bottomley D. J.
Published: July 2002

We propose that large heteroepitaxial stress causes InAs to melt when deposited on GaAs(001) at approximately 770 K. This leads to mixing with the substrate in order to realize a local minimum in the Gibbs free energy of the liquid phase, producing an approximate liquid composition of In0.8Ga0.2As.The liquid phase facilitates mass transport, leading to quantum dot formation. Dot formation occurs after 2.0 monolayers of liquid material accumulate in order to minimize the surface tension but without reducing the net coordination of the liquid phase atoms. Ge quantum dots on Si(001) are also discussed, and the influence of viscosity effects is inferred.
Micromachined two-dimensional array piezoelectrically actuated transducers
Authors: Perçin Gökhan , Atalar Abdullah , Levent Degertekin F. , Khuri-Yakub Butrus T.
Published: July 2002

This letter presents micromachined two-dimensional array flextensional transducers that can be used to generate sound in air or water. Individual array elements consist of a thin piezoelectric ring and a thin, fully supported, circular membrane. We report on an optimum design for an individual array element based on finite element modeling. We manufacture the transducer in two-dimensional arrays using planar silicon micromachining and demonstrate ultrasound transmission in air at 2.85 MHz. Such an array could be combined with on-board driving and an addressing circuitry for different applications.
Scanning capacitance microscopy imaging of threading dislocations in GaN films grown on (0001) sapphire by metalorganic chemical vapor deposition
Authors: Hansen P. J. , Strausser Y. E. , Erickson A. N. , Tarsa E. J. , Kozodoy P. , Brazel E. G. , Ibbetson J. P. , Mishra U. , Narayanamurti V. , DenBaars S. P. , Speck J. S.
Published: July 2002

A combination of atomic force microscopy and scanning capacitance microscopy was used to investigate the relationship between the surface morphology and the near-surface electrical properties of GaN films grown on c-axis sapphire substrates by metalorganic chemical vapor deposition. Local regions surrounding the surface termination of threading dislocations displayed a reduced change in capacitance with applied voltage relative to regions that contained no dislocations. Capacitance–voltage characteristics obtained from these regions indicated the presence of negative charge in the vicinity of dislocations.
Optical shutter driven photochemically from anisotropic polymer network containing liquid crystalline and azobenzene molecules
Authors: Kurihara Seiji , Masumoto Kohji , Nonaka Takamasa
Published: July 2002

Polymer-dispersed liquid crystal (PDLC) with macroscopically uniaxial molecular alignment containing azobenzene molecules was produced. The PDLC was transparent because of this alignment. Upon photoirradiation to cause trans-cis photoisomerization of azobenzene molecules, the PDLC became translucent even at 30 °C. The PDLC system does not require the use of polarizers and electrodes to fabricate optical devices. The light scattering is ascribed to a biphasic morphology in the system produced transiently by means of the photochemical phase transition.
Determination of the thermal conductivity of diamond-like nanocomposite films using a scanning thermal microscope
Authors: Ruiz Facundo , Sun W. D. , Pollak Fred H. , Venkatraman Chandra
Published: July 2002

Amorphous “diamond-like/quartz-like” composites a-(C:H/Si:O) constitute a novel class of diamond-related materials with a number of unique bulk and surface properties. Using a thermal imaging microscope we have determined the thermal conductivity (κs) of a-(C:H/Si:O) films deposited on Si(001) substrates. We have developed a calibration procedure which makes it possible to use this method for the evaluation of not only topographical variations in κs but also absolute values.
Tunneling-enhanced recombination in Cu(In, Ga)Se2 heterojunction solar cells
Authors: Rau U.
Published: July 2002

This letter presents an analytical model for tunneling-enhanced recombination current in the space charge region of semiconductor junctions. We investigate current–voltage characteristics of different types of Cu(In, Ga)Se2-based heterojunction solar cells in a temperature range from 100 to 340 K. The temperature dependence of the saturation current and of the diode ideality factor of these devices are well described by the closed form expressions derived by the present approach.
Surface electromagnetic wave excitation on one-dimensional photonic band-gap arrays
Authors: Robertson W. M. , May M. S.
Published: July 2002

Experiments are described on the prism-coupled excitation of surface electromagnetic waves in one-dimensional photonic band-gap arrays. The low loss of photonic band-gap materials leads to narrow angular reflectivity resonances and high surface fields. These attributes, coupled with the ability to engineer the optical properties of photonic band-gap arrays, suggest these materials as powerful replacements for metal films in many applications that make use of surface-plasmon resonance.
Discrimination between nuclear recoils and electron recoils by simultaneous detection of phonons and scintillation light
Authors: Meunier P. , Bravin M. , Bruckmayer M. , Giordano S. , Loidl M. , Meier O. , Pröbst F. , Seidel W. , Sisti M. , Stodolsky L. , Uchaikin S. , Zerle L.
Published: July 2002

We have developed a detector, consisting of a cryogenic calorimeter with a scintillating crystal as an absorber, and a second calorimeter for the detection of scintillation light, both operated at 12 mK. Using a CaWO4 crystal with a mass of 6 g as the scintillating absorber, we have achieved a discrimination between nuclear recoils and electron recoils with a suppression factor of 99.7% at energies above 15 keV. This method will be applied for background rejection in the CRESST dark matter search (Cryogenic Rare Event Search with Superconducting Thermometers).
Electrochromic coloration efficiency of a-WO3−y thin films as a function of oxygen deficiency
Authors: Lee Se-Hee , Cheong Hyeonsik M. , Tracy C. Edwin , Mascarenhas Angelo , Czanderna A. W. , Deb Satyen K.
Published: July 2002

We report on how electrochromic coloration is affected by oxygen deficient stoichiometries in sputtered amorphous tungsten oxide (a-WO3−y) films. The electrochromic coloration efficiency increases with increasing oxygen deficiency in (a-WO3−y) films. No coloration is observed in nearly stoichiometric WO3 films. Raman spectroscopic studies reveal that the number of W5+ states generated with lithium insertion increases with the oxygen deficiency. Furthermore, there are no Raman peaks resulting from W5+ states in lithiated a-WO3−y films with near perfect stoichiometry, which is consistent with the absence of electrochromic coloration in those films. We conclude that the coloration efficiency of a-WO3−y films depends on the number of the W5+ states generated by lithium insertion and that the oxygen deficiency plays an important role in generating the W5+ states with lithium insertion.
High-quality diamond grown by chemical-vapor deposition: Improved collection efficiency in α-particle detection
Authors: Marinelli Marco , Milani E. , Paoletti A. , Tucciarone A. , Verona Rinati G. , Angelone M. , Pillon M.
Published: July 2002

Diamond films were grown on silicon by microwave chemical-vapor deposition using a CH4–H2 gas mixture. The crystalline quality of the films was assessed through their α-particle detection performance, a property highly sensitive to film quality, by using a 5.5 MeV Am241 source. A maximum collection efficiency η of 70%, 50% being the average value, was obtained in a 115-μm-thick sample after β-particle irradiation (“priming effect”). Unprimed efficiency η=50% maximum, 30% average, was also obtained on other samples. The dependence of the efficiency and the resolving power on the external electric field was studied as well. The results are interpreted by means of a Monte Carlo analysis of the α-particle detection process. It is concluded that, in the priming process, a saturation occurs of deep defects limiting the as-grown detector performance, and charge collection distance is only limited by grain boundaries located close to the substrate side. Therefore, there is indication that further improvement can be reasonably obtained by increasing film thickness.
Scaling of ferroelectric and piezoelectric properties in Pt/SrBi2Ta2O9/Pt thin films
Authors: Ganpule C. S. , Stanishevsky A. , Aggarwal S. , Melngailis J. , Williams E. , Ramesh R. , Joshi V. , Paz de Araujo Carlos
Published: July 2002

Scaling of the ferroelectric and piezoelectric properties in Pt/SrBi2Ta2O9/Pt thin films was studied. Focused ion beam milling was used to fabricate submicron devices (1×1, 0.5×0.5, 0.25×0.25, 0.09×0.09, and 0.07×0.07 μm2) and scanning force microscopy was used to examine their piezoelectric response. It was found that capacitors as small as 0.09×0.09 μm2 exhibit good piezoelectric/ferroelectric properties and that submicron (0.25×0.25 μm2) capacitors show resistance to bipolar fatigue with up to at least 109 cycles. The results were compared with similar capacitor structures milled in the Pb1.0(Nb0.04Zr0.28Ti0.68)O3 system where structures as small as 0.07×0.07 μm2 were analyzed.
Luminescence of Tb ions implanted into amorphous AlN thin films grown by sputtering
Authors: Jadwisienczak W. M. , Lozykowski H. J. , Perjeru F. , Chen H. , Kordesch M. , Brown I. G.
Published: July 2002

We report the observation of visible cathodoluminescence (CL) of Tb ions implanted into amorphous AlN films produced by sputtering. The implanted samples were subjected to thermal annealing treatment up to 1100 °C to optically activate the incorporated ions. The results show that up to 1000 °C annealing temperature the films remain amorphous and the Tb3+ emission intensity increases. The amorphous AlN:Tb films were characterized by x-ray diffraction, CL, and CL kinetics measurements. The sharp characteristic emission lines corresponding to intra-4fn-shell transitions are resolved in the spectral range from 350 to 750 nm, and observed over the temperature range from 7 to 330 K due to the transitions from D35 and D45 levels toward the FJ5 (J=2 to 6) multiplets. Finally, CL kinetics measurements have revealed that decay times of D35→7FJ and D45→7FJ transitions are in the range 0.94–0.77 and 0.49–1.61 ms at 300 K, respectively.
Two-photon absorption in GaN, GaInN, and GaAlN alloys
Authors: Krishnamurthy Srinivasan , Nashold Karen , Sher A.
Published: July 2002

We have carried out full band-structure-based studies of two-photon absorption coefficients in the GaN compound, and GaInN and GaAlN alloys. An accurate Hamiltonian and resulting wave functions and band structures are used in the calculations. The coefficients in GaN calculated without any adjustable parameters agree very well with recent experiments. We further show that the two-photon absorption coefficients can be increased or decreased by adding In or Al to the GaN compound. The spectral variation of the absorption coefficient can be explained in terms of the alloy-induced changes in the band gap and joint density of states.
Anisotropic strain in nitrided austenitic stainless steel
Authors: Mändl S. , Rauschenbach B.
Published: July 2002

Nitrogen treatment of austenitic stainless steels at temperatures around 300–400 °C leads to the formation of a hard and corrosion resistant phase called expanded austenite. The most puzzling feature of this phase is an anisotropic lattice expansion where the expansion along the (100) is about 20% larger than for the (111) direction. In this report an investigation of the lattice expansion of austenitic stainless steel after nitrogen plasma immersion ion implantation by x-ray diffraction is presented. Pole figure measurements as well as θ/2θ scans at different polar angles were performed. Comparing the data with three different structural models, fcc, tetragonal, and triclinic, it can be concluded that none of them yields good agreement with the data, so that the crystallographic characterization is still open. However, a model for the atomic origin of the observed lattice expansion based on the anisotropic Young’s modulus, which is found for every fcc metal, is presented, showing good agreement with the measured data as well as data taken from the literature.
Static and dynamic calibration of thin-film thermocouples by means of a laser modulation technique
Authors: Serio B. , Nika Ph. , Prenel J. P.
Published: July 2002

This article presents static and dynamic calibration methods of thin-film thermocouples (TFTCs) by means of a laser modulation technique. The static calibration is based on a combined analysis of theoretical and experimental results; knowing the experimental parameters (incident laser power, beam waist radius), the temperature of the exposed junction is theoretically calculated using a thermal model and the sensor electromotive force (emf) response is recorded. The ratio between the sensor emf and the corresponding temperature gives the TFTC sensitivity. The periodic method is used to determine both the frequency response and the 95% response time of the TFTC. Special test patterns with integrated Au/Pd thin-film thermocouples are developed and tested by these methods. Results of Au/Pd TFTC sensitivity are compared to the literature values and a good agreement is obtained.
Infrared spectra of cesium chloride aqueous solutions
Authors: Max Jean-Joseph , Chapados Camille
Published: July 2002

The aqueous solutions of CsCl were studied at room temperature by infrared (IR) spectroscopy in the entire solubility range, 0–1200 g/L, using attenuated total reflection (ATR) sampling. The influence of anomalous dispersion on the IR–ATR spectra was evaluated by calculating the imaginary refractive index, k(ν), of each sample. Factor analysis (FA) was used to determine the number and abundance of species in the solutions. FA applied to both k(ν) spectra and IR-ATR spectra produced two principal spectra with a similar abundance of species. This result indicates that, even at high salt concentration, the optical effects do not influence the chemical analysis of IR–ATR spectra. The spectral modifications related to the salt concentrations are mainly first order. Second order effects were observed, but being weak, were not investigated. The two principal spectra are related to the two species present in the solution: pure water and CsCl–solvated water. From the latter, 2.8±0.4 water molecules were calculated to be associated with each close-bound Cs+/Cl− ion pair. In the case of KCl and NaCl aqueous solutions, both of which showed the same number of species, the number of water molecules associated to an ion pair was 5.0±0.4. That the latter number is different from that of CsCl indicates that the interaction between water molecules and ion pairs is different when cation Na or K in the chloride salt is replaced by Cs.
Equilibrium state of hydrogen in gallium nitride: Theory and experiment
Authors: Myers S. M. , Wright A. F. , Petersen G. A. , Seager C. H. , Wampler W. R. , Crawford M. H. , Han J.
Published: July 2002

Formation energies and vibration frequencies for H in wurtzite GaN were calculated from density-functional theory and used to predict equilibrium state occupancies and solid solubilities at elevated temperatures for p-type, intrinsic, and n-type material. The solubility of deuterium (D) was measured in p-type, Mg-doped GaN at 600, 700, and 800 °C as a function of D2 pressure and compared with theory. Agreement was obtained by reducing the H formation energies 0.22 eV from ab initio theoretical values. The predicted stretch-mode frequency for H bound to the Mg acceptor lies 5% above an observed infrared absorption attributed to this complex. More limited solubility measurements were carried out for nominally undoped material rendered n-type by donors provisionally identified as O impurities, and results agree well with theory after the aforementioned adjustment of formation energies. It is concluded that currently recognized H states and physical processes can account for the equilibrium, elevated-temperature behavior of H examined in this work.
A study of the Au/Ni ohmic contact on p-GaN
Authors: Qiao D. , Yu L. S. , Lau S. S. , Lin J. Y. , Jiang H. X. , Haynes T. E.
Published: July 2002

The formation mechanism of the ohmic Au/Ni/p-GaN contact has been investigated. We found that it is essential to (i) deposit a structure of Au and Ni in the proper deposition sequence, and (ii) anneal the bilayer structure in an oxygen containing ambient. Our findings indicated that oxygen assists the layer-reversal reactions of the metallized layers to form a structure of NiO/Au/p-GaN. The presence of oxygen during annealing appears to increase the conductivity of the p-GaN. It is further suggested that Ni removes or reduces the surface contamination of the GaN sample before or during layer reversal. In the final contact structure, an Au layer, which has a large work function, is in contact with the p-GaN substrate. The presence of Au in the entire contacting layer improves the conductivity of the contact. An ohmic formation mechanism based on our experimental results is proposed and discussed in this work.
1/f
noise in pentacene organic thin film transistors
Authors: Necliudov P. V. , Rumyantsev S. L. , Shur M. S. , Gundlach D. J. , Jackson T. N.
Published: July 2002

We report on the flicker (1/f ) noise in pentacene organic thin film transistors (TFTs) of different designs. Our studies show that the TFT design affects the noise level and the noise dependence on the gate- and drain-source biases. The measured noise level was the lowest for the TFTs with a top source and drain contacts design. For these devices, the noise dependence at low drain current values resembled that for n-type crystalline Si metal–oxide–semiconductor field-effect transistors. The extracted Hooge parameter α, which allows comparing the noise level in different devices and materials, was 0.045 for the top-contact TFTs. This parameter value is several orders of magnitude lower than that for conducting polymers and only several times higher than that for hydrogenated amorphous Si (α-Si:H) TFTs. The bottom source and drain contacts TFTs had a much higher noise level with a noise dependence on the terminal voltages that differed from the noise voltage dependence for the top-contact TFTs. The Hooge parameter values were in the range of 5–20 for the bottom-contact TFTs. We estimated that the contact noise could be comparable to the channel noise for both top-contact and bottom-contact TFTs.
Radially expanding plasma parameters in a hot refractory anode vacuum arc
Authors: Beilis I. I. , Boxman R. L. , Goldsmith S. , Paperny V. L.
Published: July 2002

Electron temperature, plasma density, and plasma potential were measured with a Langmuir probe in the radially expanding plasma streaming from the interelectrode gap of a hot refractory anode vacuum arc. Plasma parameters were measured when the anodic plume was formed, during the first 20 s after arc ignition, at points located 3 to 18 cm from the electrode edge. In addition, the ion energy distribution was determined using a retarding field analyzer. As a function of radial distance, the electron temperature decreased from 1.2 to 0.6 eV, the plasma potential decreased from 3.7 to 1.7 V, and the plasma density decreased from 2.1013 to 1.8.1011 cm−3. The measured mean ion energy per unit charge state increased from 8 eV at an axial distance of 3 cm to 20 eV at 18 cm. The electron temperature and plasma potential decreased with arc duration by about 0.4 and 2 eV, respectively, near the electrode region. The relatively small magnitude of the ion energy observed near the gap may be caused by nonelastic resonance charge exchange scattering of the cathode jet ions on atoms that are reevaporated from the anode. The observed ion acceleration at larger distances from the gap is caused by the gradient in the electron pressure caused by the plasma expansion from the interelectrode gap into the ambient vacuum.
Low-dislocation-density GaN from a single growth on a textured substrate
Authors: Ashby Carol I. H. , Mitchell Christine C. , Han Jung , Missert Nancy A. , Provencio Paula P. , Follstaedt David M. , Peake Gregory M. , Griego Leonardo
Published: July 2002

The density of threading dislocations (TD) in GaN grown directly on flat sapphire substrates is typically greater than 109/cm2. The density of dislocations can be decreased by orders of magnitude using cantilever epitaxy, which employs pre-patterned sapphire substrates to provide reduced-dimension mesa regions for nucleation and etched trenches between them for suspended lateral growth of GaN or AlGaN. The substrate is pre-patterned with narrow lines and etched to a depth that permits coalescence of laterally growing III–N nucleated on the mesa surfaces before vertical growth fills the etched trench. Low-dislocation densities typical of epitaxial lateral overgrowth are obtained in the cantilever regions and the TD density is also reduced up to 1 μm from the edge of the support regions.
Empirical molecular dynamic study of SiC(0001) surface reconstructions and bonded interfaces
Authors: Koitzsch C. , Conrad D. , Scheerschmidt K. , Gösele U.
Published: July 2002

Empirical molecular dynamics simulations based on the Tersoff potential are carried out for SiC(0001) surfaces and bonded interfaces. It is demonstrated that such a classical interatomic potential is able to correctly describe SiC-4H (0001)3×3 and 3×3R30° surface reconstructions. The surprising accuracy of the empirical simulations compared to results of density functional methods as well as experiments is demonstrated not only by obtaining reasonable structural parameters, but also by the correct prediction of such intricate effects like buckling in the topmost carbon layer of the 3×3 surface and polymerization in the silicon wetting layer of the 3×3 reconstruction. Because of the established good applicability of the Tersoff potential the simulations are used to predict the formation of SiC interfaces to be generated by wafer bonding and so far experimentally unobserved. It is shown that the bond energy crucially depends on the local atomic structure at the interface. The resulting bond energies range from 0.56 up to 3.16 J/m2 depending on the initially prepared reconstruction and alignment of the surfaces.
Epitaxial growth of CrO2 thin films by chemical-vapor deposition from a Cr8O21 precursor
Authors: Ivanov P. G. , Watts S. M. , Lind D. M.
Published: July 2002

Presently, the best epitaxial thin films of CrO2 are made by chemical-vapor deposition (CVD) in a two-zone furnace with oxygen flow from a CrO3 precursor. The growth mode has previously been described as CrO3 vaporizing in the first zone, and thermally decomposing at higher temperature in the second zone onto a substrate. In the more recent works, the focus has been on the properties of the obtained layers rather than on deposition mechanisms. In the present experimental work, we attack the epitaxial growth of CrO2 by two completely different methods, namely, molecular-beam epitaxy (MBE) and CVD. We focus on the CVD process itself, and show the importance of an intermediate compound, Cr8O21, for the growth of CrO2 films. We show that it is not necessary to start the CVD from CrO3; instead, one can prepare Cr8O21 ex situ, and use it directly for the growth of high-quality CrO2 epitaxial layers, avoiding any contamination caused by the decomposition of CrO3 to Cr8O21. We discuss in parallel our failed attempts to deposit CrO2 from either CrO3 or Cr and oxygen plasma by MBE and our experiments with the CVD process, and conclude that CrO3 does not decompose directly to CrO2 and oxygen, as was expected. We propose a hypothesis that the role of Cr8O21 in the CVD process is to exude unstable molecules of CrO4, and that the reaction on the substrate is the decomposition CrO4→CrO2+O2.
Stimulated emission induced by exciton–exciton scattering in ZnO/ZnMgO multiquantum wells up to room temperature
Authors: Sun H. D. , Makino T. , Tuan N. T. , Segawa Y. , Tang Z. K. , Wong G. K. L. , Kawasaki M. , Ohtomo A. , Tamura K. , Koinuma H.
Published: July 2002

The mechanism of ultraviolet stimulated emission was investigated in ZnO/ZnMgO multiquantum wells. Stimulated emission induced by exciton–exciton scattering occurred throughout a range of temperatures from 5 K to room temperature. At temperatures higher than 160 K, stimulated emission due to electron-hole plasma recombination was also observed with a higher excitation threshold than that of exciton–exciton scattering. The exciton binding energies of multiquantum wells were larger than that of bulk ZnO and increased with a decrease in the well widths. This enhancement of exciton binding energy is due to the quantum-confinement effect and is favorable for the stability of exciton states.
Adaptive interferometer using self-induced electro-optic modulation
Authors: Kamshilin Alexei A. , Päiväsaari Kimmo , Klein Marvin , Pouet Bruno
Published: July 2002

We demonstrate the linear transformation of small phase excursions of a speckled wavefront into modulation of the transmitted intensity using the polarization self-modulation effect in photoconductive electro-optic crystals. The technique is more advantageous than the widely used photorefractive two-wave mixing technique.
Control and elimination of cracking of AlGaN using low-temperature AlGaN interlayers
Authors: Han J. , Waldrip K. E. , Lee S. R. , Figiel J. J. , Hearne S. J. , Petersen G. A. , Myers S. M.
Published: July 2002

We demonstrate that the insertion of low-temperature AlGaN interlayers is effective in reducing mismatch-induced tensile stress and suppressing the formation of cracks during growth of high-temperature AlGaN directly upon GaN epilayers. Stress evolution and relaxation is monitored using an in situ optical stress sensor. The combination of in situ and ex situ characterization techniques enables us to determine the degree of pseudomorphism in the interlayers. It is observed that the elastic tensile mismatch between AlGaN and GaN is mediated by the relaxation of interlayers; the use of interlayers offers tunability in the in-plane lattice parameters.
Electronically nonadiabatic trajectories: Continuous surface switching II
Authors: Hack Michael D. , Truhlar Donald G.
Published: July 2002

This paper presents several criteria that should be satisfied by any method such as the original continuous surface switching method that attempts to combine elements of the trajectory surface hopping method with elements of the self-consistent potential method for semiclassical electronically nonadiabatic molecular dynamics calculations. We present an improved, functionally simpler algorithm for the continuous surface switching method for nonadiabatic trajectory calculations. We show that this new algorithm satisfies nine criteria of reasonableness, whereas the original method satisfied only five of these; and we show that the accuracy of the new algorithm is somewhat better than the accuracy of the original method.
Laser-heated diamond anvil cell system for photochemical reaction measurements
Authors: Yusa H.
Published: July 2002

A laser heating system constructed for photochemical reaction within a diamond anvil cell features a tunable pulsed laser and an excimer laser ranging from ultraviolet to infrared in a continuous-wave Nd:yttrium–aluminum–garnet laser heating system. Pulse duration is measured during experiments using a high-speed photoreceiver and pulse modes are measured by a beam profiler. High-purity synthetic diamond with high ultraviolet transmittance are used to avoid damaging the anvils. Decomposition of silver iodide by an excimer laser irradiation is confirmed under high pressure. This laser technology will, combined with high-pressure experiments, open up avenues in photochemical reaction processes.
Progress toward long-pulse high-performance Advanced Tokamak discharges on the DIII-D tokamak
Authors: Wade M. R. , Luce T. C. , Politzer P. A. , Ferron J. R. , Allen S. L. , Austin M. E. , Baker D. R. , Bray B. , Brennen D. P. , Burrell K. H. , Casper T. A. , Chu M. S. , DeBoo J. C. , Doyle E. J. , Garofalo A. M. , Gohil P. , Gorelov I. A. , Greenfield C. M. , Groebner R. J. , Heidbrink W. W. , Hsieh C.-L. , Hyatt A. W. , Jayakumar R. , Kinsey J. E. , La Haye R. J. , Lao L. L. , Lasnier C. J. , Lazarus E. A. , Leonard A. W. , Lin-Liu Y. R. , Lohr J. , Mahdavi M. A. , Makowski M. A. , Murakami M. , Petty C. C. , Pinsker R. I. , Prater R. , Rettig C. L. , Rhodes T. L. , Rice B. W. , Strait E. J. , Taylor T. S. , Thomas D. M. , Turnbull A. D. , Watkins J. G. , West W. P. , Wong K.-L.
Published: July 2002

Significant progress has been made in obtaining high-performance discharges for many energy confinement times in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. Normalized performance (measured by the product of βNH89 and indicative of the proximity to both conventional β limits and energy confinement quality, respectively) ∼10 has been sustained for >5 τE with qmin>1.5. These edge localized modes (ELMing) H-mode discharges have β∼5%, which is limited by the onset of resistive wall modes slightly above the ideal no-wall n=1 limit, with approximately 75% of the current driven noninductively. The remaining Ohmic current is localized near the half-radius. The DIII-D electron cyclotron heating system is being upgraded to replace this inductively driven current with localized electron cyclotron current drive (ECCD). Density control, which is required for effective ECCD, has been successfully demonstrated in long-pulse high-performance ELMing H-mode discharges with βNH89∼7 for up to 6.3 s. In plasma shapes compatible with good density control in the present divertor configuration, the achieved βN is somewhat less than that in the high βNH89=10 discharges.
Quantum partition functions from classical distributions: Application to rare-gas clusters
Authors: Calvo F. , Doye J. P. K. , Wales D. J.
Published: July 2002

We investigate the thermodynamic behavior of quantum many-body systems using several methods based on classical calculations. These approaches are compared for the melting of Lennard-Jones (LJ) clusters, where path-integral Monte Carlo (PIMC) results are also available. First, we examine two quasiclassical approaches where the classical potential is replaced by effective potentials accounting for quantum corrections of low order in ℏ. Of the Wigner–Kirkwood and Feynman–Hibbs effective potentials, only the latter is found to be in quantitative agreement with quantum simulations. However, both potentials fail to describe even qualitatively the low-temperature regime, where quantum effects are strong. Our second approach is based on the harmonic superposition approximation, but with explicit quantum oscillators. In its basic form, this approach is in good qualitative agreement with PIMC results, and becomes more accurate at low temperatures. By including anharmonic corrections in the form of temperature-dependent frequency shifts, the agreement between the quantum superposition and the PIMC results becomes quantitative for the caloric curve of neon clusters. The superposition method is then applied to larger clusters to study the influence of quantum delocalization on the melting and premelting of LJ19, LJ31, LJ38, and LJ55. The quantum character strongly affects the thermodynamics via changes in the ground state structure due to increasing zero-point energies. Finally, we focus on the lowest temperature range, and we estimate the Debye temperatures of argon clusters and their size variation. A strong sensitivity to the cluster structure is found, especially when many surface atoms reorganize as in the anti-Mackay/Mackay transition. In the large size regime, the Debye temperature smoothly rises to its bulk limit, but still depends slightly on the growth sequence considered.
Micromagnetic modeling of the effects of stress on magnetic properties
Authors: Zhu B. , Lo C. C. H. , Lee S. J. , Jiles D. C.
Published: July 2002

A micromagnetic model has been developed for investigating the effect of stress on the magnetic properties of thin films. This effect has been implemented by including the magnetoelastic energy term into the Landau–Lifshitz–Gilbert equation. Magnetization curves of a nickel film were calculated under both tensile and compressive stresses of various magnitudes applied along the field direction. The modeling results show that coercivity increased with increasing compressive stress while remanence decreased with increasing tensile stress. The results are in agreement with the experimental data in the literature and can be interpreted in terms of the effects of the applied stress on the irreversible rotation of magnetic moments during magnetization reversal under an applied field.
Miniband formation in a quantum dot crystal
Authors: Lazarenkova Olga L. , Balandin Alexander A.
Published: July 2002

We analyze the carrier energy band structure in a three-dimensional regimented array of semiconductor quantum dots using an envelope function approximation. The coupling among quantum dots leads to a splitting of the quantized carrier energy levels of single dots and formation of three-dimensional minibands. By changing the size of quantum dots, interdot distances, barrier height, and regimentation, one can control the electronic band structure of this artificial quantum dot crystal. Results of simulations carried out for simple cubic and tetragonal quantum dot crystal show that the carrier density of states, effective mass tensor and other properties are different from those of bulk and quantum well superlattices. It has also been established that the properties of artificial crystal are more sensitive to the dot regimentation rather then to the dot shape. The proposed engineering of three-dimensional mini bands in quantum dot crystals allows one to fine-tune electronic and optical properties of such nanostructures.
Incorporation of beryllium on the clean and indium-terminated GaN(0001) surface
Authors: Northrup John E.
Published: July 2002

First-principles calculations of the energetics of Be incorporation in various sites on clean and indium-terminated GaN(0001) are presented. On the clean Ga-rich surface the Be atom prefers a surface substitutional Ga site over a near-surface interstitial site by about 1.0 eV, thus suggesting that interstitial concentrations could be suppressed near the surface during growth. Calculations for In-terminated surfaces suggest that it is possible to enhance incorporation of Be on the Ga site by performing the growth under conditions in which Be compensates compressive stress resulting from In incorporation.
Scanning Kelvin probe microscopy characterization of dislocations in III-nitrides grown by metalorganic chemical vapor deposition
Authors: Koley G. , Spencer M. G.
Published: July 2002

Scanning Kelvin probe microscopy has been used in conjunction with noncontact atomic force microscopy for characterizing dislocations in n-GaN and Al0.35Ga0.65N/GaN heterostructures. The surface potential variations around the dislocations present in the Al0.35Ga0.65N/GaN heterostructure have been observed to be 0.1–0.2 V with full width at half maximums (FWHMs) of 100–200 nm. On the other hand, n-GaN shows potential variations of 0.3–0.5 V having FWHMs of 20–50 nm. The dislocations (present in densities of ∼109 cm−2) have been found to be negatively charged for both n-GaN and Al0.35Ga0.65N/GaN heterostructure samples.
Dewetting and nanopattern formation of thin Pt films on SiO2 induced by ion beam irradiation
Authors: Hu Xiaoyuan , Cahill David G. , Averback Robert S.
Published: July 2002

Dewetting and nanopattern formation of 3–10 nm Pt thin films upon ion irradiation is studied using scanning electron microscopy (SEM). Lateral feature size and the fraction of exposed surface area are extracted from SEM images and analyzed as functions of ion dose. The dewetting phenomenon has little temperature dependence for 3 nm Pt films irradiated by 800 keV Kr+ at temperatures ranging from 80 to 823 K. At 893 K, the films dewet without irradiation, and no pattern formation is observed even after irradiation. The thickness of the Pt films, in the range 3–10 nm, influences the pattern formation, with the lateral feature size increasing approximately linearly with film thickness. The effect of different ion species and energies on the dewetting process is also investigated using 800 keV Kr+ and Ar+ irradiation and 19.5 keV He+, Ar+, Kr+, and Xe+ irradiation. The lateral feature size and exposed surface fraction scale with energy deposition density (J/cm2) for all conditions except 19.5 keV Xe+ irradiation.
Density and temperature dependence of the bulk viscosity of molecular liquids: Carbon dioxide and nitrogen
Authors: Rah Kyunil , Eu Byung Chan
Published: July 2002

A statistical mechanical formula is developed for the bulk viscosity of molecular liquids. It is expressed in terms of the self-diffusion coefficient of the liquid, intermolecular forces, and the site–site pair correlation functions. The density and temperature dependence of the bulk viscosity of carbon dioxide and nitrogen are calculated therewith and compared with experimental data wherever possible. In the case of liquid nitrogen for which experimental data are available the theoretical values of the bulk viscosity are well within the experimental error ranges in almost all cases. There are no experimental data to compare with the theoretical results for liquid carbon dioxide, but in the light of the comparison for nitrogen and the excellent shear viscosity results which were obtained in the same line of approach in the previous work the calculated bulk viscosity values of liquid carbon dioxide may be treated as theoretical predictions.