bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanometer-scale hole formation on graphite using a scanning tunneling microscope
Authors: Albrecht T. R. , Dovek M. M. , Kirk M. D. , Lang C. A. , Quate C. F. , Smith D. P. E.
Published: July 2002

We present a reproducible technique for forming holes on a graphite surface with a scanning tunneling microscope. The holes have an average diameter of 40 Å (20 Å minimum) with an average resolvable spacing of 60 Å. Holes are produced by applying a short voltage pulse (3–8 V, 10–100 μs) across the tunneling gap which removes one or more layers of graphite in a small region directly below the tip. Arrays of hundreds of holes have been formed with yields as high as 99.6%. The writing process has a higher success rate in air or in the presence of water vapor. This suggests that the physical mechanism is a chemical process.
New excited-state absorption measurement technique and application to Pr3+ doped fluorozirconate glass
Authors: Quimby R. S. , Zheng B.
Published: July 2002

A new technique for analyzing excited state absorption (ESA) is introduced, in which the relative ESA spectrum is determined directly from fluorescence measurements. This represents, to our knowledge, the first application of the McCumber theory [D. E. McCumber, Phys. Rev. 136, A954 (1964)] to the calculation of excited state absorption spectra. The new technique is used to study ESA transitions from the 1G4 level in Pr3+ doped fluoride glass (ZBLAN). An ESA transition around 1380 nm is found, which shifts the peak of the gain curve for a 1.3 μm Pr3+ fiber amplifier about 10 nm to shorter wavelengths, and gives rise to negative gain for wavelengths longer than 1380 nm. Another ESA transition around 840 nm is found, which is an order of magnitude stronger than the 1 μm ground state absorption.
ac losses in powder-in-tube Bi2Ca2Sr2Cu3O10 tapes at power frequencies
Authors: Orehotsky J. , Reilly K. M. , Suenaga M. , Hikata T. , Ueyama M. , Sato K.
Published: July 2002

ac losses in Bi2Ca2Sr2Cu3O10 tapes, which were fabricated by the powder-in-Ag tube method, were measured with the applied field parallel to the tape face for frequencies 15–180 Hz and at 4.2 and 65–77 K. The losses were found to be significantly lower than those for sintered bars of YBa2Cu3O7 and the frequency dependence of the losses at 77 K was linear indicating that the losses were hysteretic in origin. However, at 4.2 K a significant contribution to the losses from the eddy current losses in the Ag sheath was apparent for frequencies greater than 30 Hz. Furthermore, the magnetic field dependence of the losses at 77 K followed the Bean model surprisingly well.
Optical properties and ionic conductivity of KTiOAsO4 crystals
Authors: Loiacono G. M. , Loiacono D. N. , Zola J. J. , Stolzenberger R. A. , McGee T. , Norwood R. G.
Published: July 2002

Crystals of KTiOAsO4 were grown by the high temperature solution method and their optical and ionic conductivity properties evaluated. The band edge and IR cutoff were 370 and 4850 nm, respectively. A domain structure prevented efficient frequency conversion in as-grown crystals. The ionic conductivity at 22 °C and 120 kHz was 1.5×10−8, 7.3×10−9, and 1.7×10−6 S/cm for the [100], [010], and [001], respectively. The alleged ferroelectric Curie temperature was 852 ±2 °C.
Direct observation of porous SiC formed by anodization in HF
Authors: Shor Joseph S. , Grimberg Ilana , Weiss Ben-Zion , Kurtz Anthony D.
Published: July 2002

A process for forming porous SiC from single-crystal SiC wafers has been demonstrated. Porous SiC can be fabricated by anodizing n-type 6H-SiC in HF under UV illumination. Transmission electron microscopy reveals pores of sizes 10–30 nm with interpore spacings ranging from ≊5 to 150 nm. This is the first reported direct observation of porous SiC formation.
Gigantic negative transconductance and mobility modulation in a double-quantum-well structure via gate-controlled resonant coupling
Authors: Ohno Y. , Tsuchiya M. , Sakaki H.
Published: July 2002

Transport of two-dimensional electrons in a novel double-quantum-well (DQW) field-effect transistor was systematically studied with emphasis on the effect of resonant interaction. By introducing ionized impurities appropriately into one of the QWs, wave-function-dependent scattering process was sensitively controlled by the gate voltage Vg. A prominent valley structure was observed in the channel conductance −Vg characteristics at resonance with the peak-to-valley ratio of 3 at 4.2 K. This nonlinear characteristic is caused by the deformation of electron wave functions in the DQW and is found to be well explained by the theoretical calculation. The DQW structure can be utilized for both negative transconductance and velocity modulation devices.
Growth of 2H-SiC on 6H-SiC by pulsed laser ablation
Authors: Stan M. A. , Patton M. O. , Warner J. D. , Yang J. W. , Yang J. W.
Published: July 2002

A 2H-SiC thin film has been grown on a 6H-SiC substrate by laser ablation using an excimer laser. The deposition of 2H-SiC film occurred in a high vacuum system (≊10−6 Torr) with the substrate temperature near 1200 °C. Plan-view and cross-sectional transmission electron microscopy (TEM) were used to measure the lattice parameters and to identify the polytype. Cross-sectional TEM images clearly show the symmetry of the film as c-axis oriented 2H-SiC containing columnar grains with an average diameter of 20 nm and a length of 100 nm.
Interfacial layer formation of the CdTe/InSb heterointerfaces grown by temperature gradient vapor transport deposition
Authors: Kim T. W. , Park H. L. , Lee J. Y. , Lee H. J.
Published: July 2002

CdTe epitaxial films were grown by a simple method of temperature gradient vapor transport deposition on p-InSb (111) orientation substrates in the growth temperature range between 200 and 300 °C. The stoichiometry of the CdTe/InSb heterostructure was observed by Auger electron spectroscopy, and Auger depth profiles demonstrated that the CdTe/InSb heterointerface was not abrupt. Transmission electron microscopy verified the formation of an interfacial layer in the CdTe/InSb interface and the formation of the stacking faults in the CdTe thin film. These results indicated that the films grown at approximately 270 °C contained a formation problem of an interfacial layer due to interdiffusion from the InSb prior to the growth of the CdTe, and that the interfacial layer might deteriorate the electrical property of the CdTe epitaxial layer.
Growth of epitaxial strontium barium niobate thin films by pulsed laser deposition
Authors: Schwyn Thöny S. , Youden K. E. , Harris J. S. , Hesselink L.
Published: July 2002

We report the growth of epitaxial SrxBa1−xNb2O6 (SBN) thin films by pulsed laser deposition. The films were grown on (100) MgO substrates with thicknesses in the range 200–400 nm. Rutherford backscattering analysis showed that the films have stoichiometric composition identical to the target material. X-ray diffraction 2θ scans indicate single crystalline layers with the (001) orientation perpendicular to the substrate plane. Phi scans on the (221) plane, however, reveal that the films have two in-plane orientations. The unit cell of SBN is rotated in the plane of the film by ±18.4° with respect to the MgO substrate unit cell. This rotation is explained using a model which takes into account both the lattice match and the electrostatic energy within the heteroepitaxial interface.
Angular selective transmittance through electrochromic tungsten oxide films made by oblique angle sputtering
Authors: Le Bellac D. , Azens A. , Granqvist C. G.
Published: July 2002

Tungsten oxide films were made by magnetron sputtering with a large angle between the substrate normal and the direction of the deposition flux. Scanning electron microscopy displayed a microstructure composed of inclined columns. Spectrophotometric measurements on films with different amounts of electrochemically intercalated Li+ ions showed pronounced angular selective transmittance together with electrochromism.
High-pressure combustor for the spectroscopic study of solid propellant combustion chemistry
Authors: Edwards T. , Weaver D. P. , Adams R. , Hulsizer S. , Campbell D. H.
Published: July 2002

An apparatus for spectroscopic studies of solid propellants burning at pressures up to 7 MPa (1000 psi) is described. The propellants are burned in a high-pressure combustor equipped with sapphire windows for optical access and a servomechanism that raises the propellant as it burns, allowing examination of a specific portion of the propellant flame for extended times. The results presented involve the collection of propellant flame emission spectra with a diode array detector over a fixed spectral region.
Scanning electron microscope electron detector system with replaceable-dynode multiplier and wide-band isolation amplifier
Authors: Sexton B. A. , Davey P. I.
Published: July 2002

A secondary electron detector has been developed for Scanning Auger Microscopy applications. Secondary electrons are directly detected in vacuum by a long-life replaceable-dynode multiplier and the current amplified by a 1-MHz wide-band isolation amplifier. The overall system is simpler than a conventional scintillator–photomultiplier type and gives acceptable video images for beam currents larger than 1 nA.
X-ray focusing using square channel-capillary arrays
Authors: Chapman H. N. , Nugent K. A. , Wilkins S. W.
Published: July 2002

A class of imaging, condensing, and collimating devices for x rays is investigated which is based on the use of an array of small channels of square cross section. The focusing and collimating effect arises from external reflection of near-grazing-incidence rays at the interior channel surfaces. Rays are redirected by being singly reflected from two orthogonal channel surfaces and are imaged from a source point to a square region with a side length MT+1 times that of the channel side length, where MT is the transverse magnification. The image and source locations are related by a thin-lens formula. The point spread function and the efficiency of these focusing devices are calculated. Two energy regimes with different channel reflectivity characteristics are examined in detail: the hard x-ray regime (E>8 keV) and the soft x-ray regime (E<200 eV). For these cases the efficiency of focusing x rays depends only on the channel aspect ratio and reflectivity parameters. A discussion is made of channel plates of other configurations, such as arrays of rectangular channels. The results are supported by Monte Carlo simulations.
A new-regime Wiley–McLaren time-of-flight mass spectrometer
Authors: Chandezon F. , Huber B. , Ristori C.
Published: July 2002

A modified version of the classic Wiley–McLaren time-of-flight mass spectrometer (TOFMS) is presented here. Owing to a second-order compensation of the initial position effect, a mass resolution m/δm higher than 2000 is obtained with large ion volumes, of the order of 0.2 cm3 for Na+55 (1265 a.m.u.). Sodium clusters Na+n with n up to 530 (≂12200 a.m.u.) are separated. The spectrometer can be used for the analysis of ionized clusters in a mass range from 1 to about 20000 a.m.u. as well as for the measurement of the kinetic energy of molecular fragments after a Coulomb explosion of a molecule. The performance of this spectrometer is demonstrated with sodium clusters and molecules.
Target diagnostic system for the national ignition facility (invited)
Authors: Leeper R. J. , Chandler G. A. , Cooper G. W. , Derzon M. S. , Fehl D. L. , Hebron D. E. , Moats A. R. , Noack D. D. , Porter J. L. , Ruggles L. E. , Ruiz C. L. , Torres J. A. , Cable M. D. , Bell P. M. , Clower C. A. , Hammel B. A. , Kalantar D. H. , Karpenko V. P. , Kauffman R. L. , Kilkenny J. D. , Lee F. D. , Lerche R. A. , MacGowan B. J. , Moran M. J. , Nelson M. B. , Olson W. , Orzechowski T. J. , Phillips T. W. , Ress D. , Tietbohl G. L. , Trebes J. E. , Bartlett R. J. , Berggren R. , Caldwell S. E. , Chrien R. E. , Failor B. H. , Fernandez J. C. , Hauer A. , Idzorek G. , Hockaday R. G. , Murphy T. J. , Oertel J. , Watt R. , Wilke M. , Bradley D. K. , Knauer J. , Petrasso R. D. , Li C. K.
Published: July 2002

A review of recent progress on the design of a diagnostic system proposed for ignition target experiments on the National Ignition Facility (NIF) will be presented. This diagnostic package contains an extensive suite of optical, x ray, gamma ray, and neutron diagnostics that enable measurements of the performance of both direct and indirect driven NIF targets. The philosophy used in designing all of the diagnostics in the set has emphasized redundant and independent measurement of fundamental physical quantities relevant to the operation of the NIF target. A unique feature of these diagnostics is that they are being designed to be capable of operating in the high radiation, electromagnetic pulse, and debris backgrounds expected on the NIF facility. The diagnostic system proposed can be categorized into three broad areas: laser characterization, hohlraum characterization, and capsule performance diagnostics. The operating principles of a representative instrument from each class of diagnostic employed in this package will be summarized and illustrated with data obtained in recent prototype diagnostic tests.
Lattice dilation by free electrons in heavily doped GaAs:Si
Authors: Leszczynski M. , Bak-Misiuk J. , Domagala J. , Muszalski J. , Kaniewska M. , Marczewski J.
Published: July 2002

Lattice constants of GaAs layers grown by molecular beam epitaxy were examined by using the high resolution x-ray diffractometer. For highly doped samples (up to 9×1018 cm−3 of free-electron concentration) we observed an increase of the lattice constant with respect to the undoped layers. Since substitutional silicon atoms decrease the lattice constant of GaAs, the results are explained by the influence of free-electrons via the deformation potential of the Γ minimum of the conduction band. The best fit to our diffractometric data was obtained for the band-gap deformation potential equal to −8.5 eV.
Excited states in self-organized InAs/GaAs quantum dots: Theory and experiment
Authors: Grundmann M. , Ledentsov N. N. , Stier O. , Bimberg D. , Ustinov V. M. , Kop’ev P. S. , Alferov Zh. I.
Published: July 2002

In photoluminescence spectra of nanometer-scale pyramidal-shaped InAs/GaAs quantum dots allowed optical transitions involving excited hole states are revealed in addition to the ground state transition. Detailed theoretical calculations of the electronic structure, including strain, piezoelectric and excitonic effects, agree with the experimental data and lead to unambiguous assignment of the transitions.
Raman shifts in Si nanocrystals
Authors: Zi Jian , Büscher H. , Falter C. , Ludwig W. , Zhang Kaiming , Xie Xide
Published: July 2002

Raman shifts of Si nanocrystals versus size were studied theoretically by a bond polarizability model. Zero-dimensional spheres and one-dimensional columns were considered. The relation between the Raman shift and the size for Si spheres and columns was established, from which the size of Si nanocrystals can be obtained for a given Raman shift or vice versa.
Effect of phase coexistence at morphotropic phase boundary on the properties of Pb(ZrxTi1−x)O3 ceramics
Authors: Mishra S. K. , Pandey Dhananjai , Singh Anirudh P.
Published: July 2002

The temperature variation of planar electromechanical coupling coefficient (kp) in Pb(ZrxTi1−x)O3(PZT) ceramics near the morphotropic phase boundary (MPB) compositions has been correlated with structural changes to show that the maximum electromechanical response at a given composition (x) is a function of temperature (T) and occurs in the tetragonal phase field just outside the two-phase MPB region in the (x,T) plane. This clearly disproves the hypothesis that the coexistence of rhombohedral and tetragonal phases at MPB is responsible for the enhancement of the electromechanical response. In fact, coexistence of the rhombohedral phase with the tetragonal phase is shown to lead to the lowering of the electromechanical response.
Highly efficient and bright organic electroluminescent devices with an aluminum cathode
Authors: Jabbour G. E. , Kawabe Y. , Shaheen S. E. , Wang J. F. , Morrell M. M. , Kippelen B. , Peyghambarian N.
Published: July 2002

The electron injection process, which limits the electroluminescent performance of organic devices, has been enhanced tremendously by inserting a layer of LiF with appropriate thickness between the cathode and a quinacridone doped organic layer. Devices with an Al/LiF cathode demonstrated a luminance in excess of 20 000 cd/m2 and an external quantum efficiency of 3%, which is comparable to devices with a Mg/LiF cathode. These devices show maximum luminance of 45 000 cd/m2 prior to failure in continuous bias operation. For the same LiF thickness, the operating voltage for devices with Al/LiF was lower than the corresponding operating voltage for devices with Mg/LiF or Mg alone. Tunneling theory is used to explain this enhancement.
Network morphology of polymer stabilized liquid crystals
Authors: Dierking I. , Kosbar L. L. , Afzali-Ardakani A. , Lowe A. C. , Held G. A.
Published: July 2002

Monomer solubility is identified as the primary factor determining network morphology in polymer stabilized cholesteric liquid crystal textures. Poorly soluble monomers form coarse structures composed of discrete, oblong grains, whereas soluble monomers yield smooth, continuous polymer networks. A crossover from smooth to grainy structure is observed as a function of monomer concentration. The grainy structure results from precipitation polymerization and the observed behavior is well described by the Flory–Huggins theory of polymer solubility.
Polymeric wavelength filters with polymer gratings
Authors: Oh Min-Cheol , Lee Myung-Hyun , Ahn Joo-Heon , Lee Hyung-Jong , Han Seon Gyu
Published: July 2002

Wavelength filters with Bragg gratings are demonstrated based on low-loss polymer waveguides and high-refractive-index polymer gratings. Two kinds of fluorinated polymers, perfluorocyclobutane and fluorinated poly(arylene ethers) are used for the low-loss waveguide operating around the 1.55 μm wavelength. The polymer grating is made of Resole, and it is successfully integrated in the polymer waveguides. In order to fabricate the Bragg gratings on the polymer waveguides, we use a phase mask and a Hg lamp in a mask aligner as the illuminating source instead of the laser. This method provides uniform gratings on a large area as well as the alignment capability. In the fabricated wavelength filters, the reflectivity at the Bragg wavelength is 30 dB, the 3 dB bandwidth is as narrow as 0.6 nm, and the insertion loss is 3.7 dB.
Observation of perpendicular magnetic anisotropy in Ni/Pt multilayers at room temperature
Authors: Shin Sung-Chul , Srinivas G. , Kim Young-Seok , Kim Mu-Gyeom
Published: July 2002

We report the experimental observation of room-temperature perpendicular magnetic anisotropy in Ni/Pt multilayers having 7–26 Å Ni and 2.3–4.6 Å Pt layer thicknesses, prepared by dc magnetron sputtering on glass substrates at 7 mTorr Ar sputtering pressure. Perpendicular magnetic anisotropy was found to be sensitively dependent on both Ni- and Pt-layer thicknesses, and a maximum anisotropy energy of 1.74×105 erg/cm3 was obtained for (16 Å Ni/2.3 Å Pt)30 multilayer. Interestingly, magnetization measurements revealed that about two atomic layers of Ni at the interface were magnetically dead in our samples.
Self-similarity of fluid flows
Authors: Wang Liqiu
Published: July 2002

Relations among flows at different scales are developed through examining flow response to multiplicative changes of spatial and temporal scales. The scaling invariance of physical definitions leads to the self-similarity of flows and the scaling invariance of fundamental laws governing the flows. Also developed is both the necessary and sufficient condition for a physical quantity to be self-similar, which forms a general function dependence of any self-similar quantity on position and time.
Coherent terahertz radiation detection: Direct comparison between free-space electro-optic sampling and antenna detection
Authors: Cai Y. , Brener I. , Lopata J. , Wynn J. , Pfeiffer L. , Stark J. B. , Wu Q. , Zhang X. C. , Federici J. F.
Published: July 2002

We compare the use of free-space electro-optic sampling (FSEOS) with photoconducting antennas to detect terahertz (THz) radiation in the range of 0.1–3 THz. For the same average THz power and low-frequency modulation, signal-to-noise ratio and sensitivity are better with antenna detection at frequencies smaller than 3 THz. When the modulation frequency is increased to more than 1 MHz in FSEOS, both detection schemes have comparable performance. Using a singular-electric-field THz emitter, we demonstrate the feasibility of a THz imaging system using real-time delay scanning in FSEOS and only 20 mW of laser power.
Stress-induced amorphization at moving crack tips in NiTi
Authors: Okamoto P. R. , Heuer J. K. , Lam N. Q. , Ohnuki S. , Matsukawa Y. , Tozawa K. , Stubbins J. F.
Published: July 2002

In situ fracture studies have been carried out on thin films of the NiTi intermetallic compound under plane stress, tensile loading conditions in the high-voltage electron microscope. Local stress-induced amorphization of regions directly in front of moving crack tips has been observed. The upper cutoff temperature, TC–Amax, for the stress-induced crystalline-to-amorphous transformation was found to be 600 K, identical to that for heavy ion-induced amorphization of NiTi and for ion-beam mixing-induced amorphization of Ni and Ti multilayer specimens. 600 K is also both the lower cutoff temperature, TA–Cmin, for radiation-induced crystallization of initially-unrelaxed amorphous NiTi and the lowest isothermal annealing temperature, TXmin, at which stress-induced amorphous NiTi crystallizes. Since TXmin should be TK, the ideal glass transition temperature, the discovery that TC–Amax=TA–Cmin=TXmin=TK implies that disorder-driven crystalline-to-amorphous transformations result in the formation of the ideal glass, i.e., the glassy state that has the same entropy as that of the defect-free crystal. As the glassy state with the lowest free energy, its formation can be understood as the most energetically-favored, kinetically-constrained response of crystalline alloys driven far from equilibrium.
Initial stages in the carbonization of (111)Si by solid-source molecular beam epitaxy
Authors: Cimalla V. , Stauden Th. , Ecke G. , Scharmann F. , Eichhorn G. , Pezoldt J. , Sloboshanin S. , Schaefer J. A.
Published: July 2002

Silicon carbide can be reproducibly grown on (111)Si below 600 °C by carbonization using an elemental solid carbon source in molecular beam epitaxy. The initial stages were observed by in situ reflection high-energy electron diffraction. Prior to silicon carbide growth, the continuous carbon flux lead to a transition from the (7×7) reconstruction of clean (111)Si to a carbon-induced (∛×∛)R30° structure. Above 660 °C, the silicon carbide growth starts directly on the silicon surface via three-dimensional nucleation. Below 660 °C, first a thin silicon–carbon alloy was formed by diffusion of carbon into the surface near the region with a concentration exceeding the bulk solubility in silicon.
Quantification of scanning capacitance microscopy imaging of the pn junction through electrical simulation
Authors: O’Malley M. L. , Timp G. L. , Moccio S. V. , Garno J. P. , Kleiman R. N.
Published: July 2002

Determining the cross-sectional doping profile of very small metal–oxide–semiconductor field effect transistors and specifically the direct measurement of their channel length is necessary for true channel engineering to be possible. Scanning capacitance microscopy (SCM) has generated unprecedented images of the cross-sectional doping profiles of very small transistors. The bias voltage dependence of these images has motivated us to investigate the SCM technique in greater detail. Using electrical simulations, we have focused on the pn junction to establish the qualitative and quantitative relationship between the bias voltage and the pn junction location. The ability to confidently interpret the images produced with SCM will allow us to improve simulation models, trouble-shoot process flow, and determine the effective channel length of semiconductor devices.
High voltage (450 V) GaN Schottky rectifiers
Authors: Bandić Z. Z. , Bridger P. M. , Piquette E. C. , McGill T. C. , Vaudo R. P. , Phanse V. M. , Redwing J. M.
Published: July 2002

We fabricated high standoff voltage (450 V) Schottky rectifiers on hydride vapor phase epitaxy grown GaN on sapphire substrate. Several Schottky device geometries were investigated, including lateral geometry with rectangular and circular contacts, mesa devices, and Schottky metal field plate overlapping a SiO2 layer. The best devices were characterized by an ON-state voltage of 4.2 V at a current density of 100 A/cm2 and a saturation current density of 10−5 A/cm2 at a reverse bias of 100 V. From the measured breakdown voltage we estimated the critical field for electric breakdown in GaN to be (2.2±0.7)×106 V/cm. This value for the critical field is a lower limit since most of the devices exhibited abrupt and premature breakdown associated with corner and edge effects.
Characteristic of interface effect in Cu–C60 granular films
Authors: Li Xiang , Tang Y. J. , Zhao H. W. , Zhan W. S. , Wang Haiqian , Hou J. G.
Published: July 2002

In this letter, Cu–C60 granular film is prepared with coevaporation method at room temperature. The conductance of the film is measured by in situ method, and its microstructure is characterized by transmission electron microscopy. The charge transfer from Cu to C60 is investigated with Raman spectroscopy. The results indicate that the sample has the uniformly granular microstructure. The interaction between C60 and Cu at the Cu–C60 interfaces, which significantly affects the orientational order–disorder phase transition of C60 and induces the phase transition of C60 in the temperature range from 219 to 248 K. The mechanism of the characteristic of such phase transition is discussed.
Surface electromagnetic modes in layered conductors in a magnetic field
Authors: Gvozdikov V. M.
Published: July 2002

A transfer-matrix approach is developed for studies of the collective electromagnetic modes in a semi-infinite layered conductor subjected to a quantizing external magnetic field perpendicular to the layers. The dispersion relations for the surface and bulk modes are derived. It is shown that the surface mode has a gap in the long-wavelength limit and exists only if the absolute value of the in-plane wave vector q exceeds the threshold value q*=−1/(a ln|Δ|). Depending on the sign of the parameter Δ=(ε−ε0)/(ε0+ε), the frequency of the surface mode ωs(q,Δ) goes either above (for Δ>0) or below (for Δ<0) the bulk-mode frequency ω(q,k)=ω(q,k+2π/a) for any value of k. At nonzero magnetic field H the bulk mode has a singular point q0(H) at which the bulk band twists in such a way that its top and bottom bounds swap. Small variations of q near this point change dramatically the shape of the dispersion function ω(q,k) in the variable k. The surface mode has no dispersion across the layers, since its amplitude decays exponentially into the bulk of the sample. Both bulk and surface modes have in the region qa≫1 a similar asymptotic behavior ω∝q1/2, but ωs(q,Δ) lies above or below ω(q,k), respectively, for Δ>0 and Δ<0 (a is the interlayer separation; ε0 and ε stand for the dielectric constants of the media outside the sample and between the layers; q and k are the components of the wave vector in the plane and perpendicular to the layers, respectively).
Quantum interference effects in delta layers of boron in silicon
Authors: Krasovitsky Vit. B. , Komnik Yu. F. , Myronov M. , Whall T. E.
Published: July 2002

The behavior of the conductance upon changes in temperature (in the interval 1.5–40 K) and magnetic field (up to 20 kOe) is investigated for a series of samples with a δ〈B〉 layer in Si, with hole concentrations in the conducting δ layer of 2.5×1013–2.2×1014 cm−2. It is shown that the temperature and field dependences obtained can be explained successfully as a manifestation of the weak localization effect and the interaction of mobile charge carriers (holes) in a two-dimensional electron system under conditions of strong spin–orbit interaction. An analysis of the behavior of the quantum corrections yields the temperature dependence of the phase relaxation time of the carriers, τφ=AT−1, with A≈(1.4±0.3)×10−12 K⋅s, where this temperature dependence is treated as a manifestation of hole–hole scattering processes, and the values of the interaction constants are also obtained (λT≈0.64–0.73).
Spectrum of rotational states of a diatomic impurity in an atomic 2D cryocrystal
Authors: Poltavskaya M. I. , Chishko K. A.
Published: July 2002

The spectrum of rotational states is calculated for a three-dimensional rotator in a three-parameter potential having the symmetry group S6 (sixfold mirror–rotational axis). This potential models the crystalline field produced at a diatomic homonuclear impurity molecule by a two-dimensional monatomic matrix (with coordination number z=6) on a substrate having the form of a close-packed crystalline plane. The main computational algorithm is the Ritz variational procedure with trial functions classified according to the symmetry of the ground and excited states of the rotator. The partition function is constructed and the impurity heat capacity is calculated for two-dimensional cryomatrices (of the Ar and Kr types) containing a subsystem of noninteracting impurities of different spin modifications (of the N214 or N215 type). It is shown that for different relationships among the parameters of the crystalline potential for the two types of impurities indicated the heat capacity exhibits characteristic low-temperature anomalies in the form of peaks whose height, width, and position on the temperature axis are determined by the parameters of the intermolecular interaction in the system. The effects predicted by the theory should be completely accessible to experimental observation.
Phonon relaxation in CdSSe semiconductor quantum dots studied by femtosecond time-resolved coherent anti-Stokes Raman scattering
Authors: Waltner P. , Materny A. , Kiefer W.
Published: July 2002

We have observed the relaxation dynamics of coherent longitudinal optical phonons in CdSSe quantum dots embedded in a glass matrix by femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS). The phase relaxation time is directly deduced from the exponential decay of the signal. Oscillations with frequency of the longitudinal optical (LO) phonon are superimposed on the CARS signal. We consider two possible mechanisms, which result in such oscillations. The excitation of a LO phonon wave packet of the fundamental and the overtone would produce quantum beats. The second possibility is displacive excitation of coherent phonons. This mechanism does not contribute to the CARS signal, but it could attenuate it periodically with the phonon frequency.
Vibrational relaxation in pyridine upon supersonic expansion
Authors: Maris Assimo , Favero Laura B. , Danieli Roberto , Favero Paolo G. , Caminati Walther
Published: July 2002

The rotational spectra of five vibrational states of pyridine have been assigned and measured by millimeter wave absorption spectroscopy in a supersonic expansion. The intensities of the lines of the vibrational satellites with respect to the ground state after the supersonic expansion depend on the kind of carrier gas, backing pressure, pyridine concentration, and symmetry of the rotational and vibrational states. Several rotational transitions of the vibrational satellites have also been measured in a conventional cell to complete the spectral assignment.
Transport properties of modulation-doped InAs-inserted-channel In0.75Al0.25As/In0.75Ga0.25As structures grown on GaAs substrates
Authors: Richter A. , Koch M. , Matsuyama T. , Heyn Ch. , Merkt U.
Published: July 2002

We report on gate voltage dependent electron transport in modulation-doped In0.75Al0.25As/In0.75Ga0.25As heterostructures with strained InAs-inserted-channels grown on GaAs substrates. At temperatures of T=4.2 K we achieve mobilities of up to μ=215 000 cm2(V s)−1 and electron densities of nS=1.2×1012 cm−2 for the highest measured gate voltage of Vg=20 V. The electron effective mass m*=0.036 me is determined by temperature dependent Shubnikov–de Haas measurements. The observation of an anisotropic mobility when the first excited subband becomes populated proves interface scattering to be the limiting mechanism for the electron mobility.
Velocity derivatives in the atmospheric surface layer at Reλ=104
Authors: Kholmyansky M. , Tsinober A. , Yorish S.
Published: July 2002

We report first results of an experiment in which explicit information is obtained on the field of velocity derivatives (all the nine components of the tensor ∂ui/∂xj) along with all the three components of velocity fluctuations at Reynolds number as high as Reλ∼104. This includes information on such basic processes as enstrophy and strain production, geometrical statistics, the role of concentrated vorticity and strain, and the reduction of nonlinearity.
Free electron density and mobility in high-quality 4H–SiC
Authors: Pernot J. , Contreras S. , Camassel J. , Robert J. L. , Zawadzki W. , Neyret E. , Di Cioccio L.
Published: July 2002

The free electron density and low-field electron mobility of 4H–SiC is examined in the temperature range 35–900 K. In good samples the electron density is constant in the temperature range 300–900 K, which offers interesting possibilities for high temperature sensor applications. On the best sample an experimental electron mobility of 12 400 cm2/V s at 50 K is found. A complete description of the temperature dependence of the electron density and mobility is given. We take into account the effects of the two inequivalent lattice sites as well as the valley–orbit splitting of the ground state at the hexagonal sites. The dependence of room-temperature mobility on electron concentration is established, described theoretically and compared with the results obtained by different authors.
Crystallization of amorphous germanium in an Al/a-Ge bilayer film deposited on a SiO2 substrate
Authors: Katsuki Futoshi , Hanafusa Kenji , Yonemura Mitsuharu , Koyama Toshiyuki , Doi Minoru
Published: July 2002

The crystallization of amorphous Ge(a-Ge) in an Al (134 nm) and a-Ge (108 nm) thin-film bilayer deposited on a SiO2 substrate has been examined by a cross section transmission electron microscope technique. When crystallization of a-Ge begins at 125 °C, amorphous AlGe (a-AlGe) alloy is formed in the Ge layer. Then, the a-AlGe alloy layer also appeared at the surface of the bilayer. After complete crystallization, those amorphous layers disappeared and the bilayer film has been converted to a polycrystalline film. We discussed the crystallization of a-Ge and proposed the mechanism of the diffusion of Ge atoms from the inner a-Ge layer through the outer Al layer to the topmost surface that involves the formation of the metastable a-AlGe alloy in the Ge layer, followed by the crystallization of this alloy by the pseudo-eutectic reaction, leading to the decomposition into an equilibrium Al and Ge crystal mixture and a-Ge. Then, Ge atoms is released to the Al layer for the compensation of the Al diffusion down into the Ge layer and again forms the a-AlGe alloy in the Al layer. The a-AlGe alloy in the Al layer is also crystallized by the pseudo-eutectic reaction. Consequently, decomposed a-Ge is ejected from the inside to the surface of the bilayer, resulting in the surface Ge segregation.
Quantitative thermal imaging by synchronous thermoreflectance with optimized illumination wavelengths
Authors: Tessier G. , Holé S. , Fournier D.
Published: July 2002

Using thermoreflectance microscopy with a camera, we have designed a system which delivers submicronic images of the alternative temperature variations in integrated circuits working in a modulated regime. A careful choice of the illumination wavelength permits us to highlight the heating in chosen parts of the sample and to optimize the thermoreflectance signal. We measure and explain the modifications of the photothermal response which are induced by the presence of a passivation layer. A calibration conducted on various materials with a thermocouple gives access to the absolute alternative temperature variations in integrated circuits working at frequencies between 0.1 Hz (quasipermanent regime) and 5 MHz.
Determination of the orbital lineup at reactive organic semiconductor interfaces using photoemission spectroscopy
Authors: Schlaf R. , Merritt C. D. , Picciolo L. C. , Kafafi Z. H.
Published: July 2002

We determined the orbital lineup of the tris (8-hydroxyquinolinato) gallium (Gaq3)/Mg interface using combined x-ray and ultraviolet photoemission spectroscopy (XPS and UPS) measurements. The Gaq3/Mg system is a prototypical model structure for organic electron/low work function electrode transporting materials interfaces found in organic light emitting diodes (OLED). A Gaq3 thin film was grown in 15 steps on a previously sputter-cleaned Mg substrate starting at a 1 Å nominal thickness up to a final thickness of 512 Å. Before, and in between the growth steps, the sample surface was characterized by XPS and UPS. The results indicate the formation of a reaction layer of about 12 Å thickness at the Mg interface, which resulted in a 0.96 V interface dipole potential. At Gaq3 coverages higher than 256 Å, a strong charging shift occurred in the overlayer related UPS-emission lines, which was identified by measuring the high binding energy cutoff (secondary edge) of both the XP and UP spectra. The several magnitudes different x-ray and ultraviolet source photon intensities allow pinpointing charging shifts with high sensitivity. Due to the low work function of the reacted interface layer, the Gaq3 electronic states are aligned at a binding energy below the substrate Fermi edge that exceeds the magnitude of the optical gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). This allowed the conclusion that the ground state exciton binding energy of Gaq3 needs to be larger than 0.43 eV. Based on these considerations, the lowest possible electron injection barrier matching the experimental data was estimated to be 0.15 eV.
Nuclear magnetic resonance study of diffusion and relaxation in hydrating white cement pastes of different water content
Authors: Nestle Nikolaus , Galvosas Petrik , Geier Oliver , Zimmermann Christian , Dakkouri Marwan , Kärger Jörg
Published: July 2002

While the nuclear spin relaxation time changes in hydrating cement materials have been widely studied by various groups during the last 20 years, data on the self-diffusion behavior of the pore water during hydration of a cement paste are much scarcer. Taking advantage of improved spectrometer hardware for pulsed field gradient diffusometry and a specialized pulse sequence which is designed to compensate the detrimental effects of inner magnetic field gradients in the sample we have studied the water self-diffusion behavior in pastes prepared from white cement at various water/cement ratios. For the same mixtures, studies of the transverse spin relaxation behavior were also conducted. A comparison of the results from both techniques shows that the diffusion coefficient starts to decrease only much later than the relaxation times for all pastes studied.
Nucleation site location and its influence on the microstructure of solid-phase crystallized SiGe films
Authors: Rodrı́guez A. , Rodrı́guez T. , Olivares J. , Sangrador J. , Martı́n P. , Martı́nez O. , Jiménez J. , Ballesteros C.
Published: July 2002

In this work, the solid-phase crystallization kinetics of amorphous SiGe films deposited by low-pressure chemical-vapor deposition on oxidized Si wafers has been studied by x-ray diffraction, Raman spectroscopy, and ultraviolet reflectance. The microstructure of the fully crystallized films has also been analyzed using these techniques in combination with transmission electron microscopy. The Ge fraction of the films (x) was in the 0–0.38 interval. The samples were crystallized at temperatures ranging from 525 to 600 °C. The crystallization monitored by all techniques was found to follow the Avrami model. Different crystallization behaviors are distinguished depending on the Ge content of the films and the crystallization temperature. The results are discussed in terms of the identification of the nucleation site location and the dimensionality of the grain growth, taking into account the probe depth of the different techniques and the values of the Avrami exponent derived from the crystallization curves. The preferred orientations of the grains, the grain morphology, the lateral grain size, the presence of inter- and intragrain defects, the surface roughness, and the overall crystallinity of the fully crystallized films have also been studied and related to the observations concerning the crystallization process.
Vertical bistable switching of spin vortex in a circular magnetic dot
Authors: Kikuchi N. , Okamoto S. , Kitakami O. , Shimada Y. , Kim S. G. , Otani Y. , Fukamichi K.
Published: July 2002

We have investigated switching behaviors of a spin-vortex core in circular magnetic dots to evaluate them as a new candidate for magnetic memory elements. The spin-vortex core with the lateral dimension of exchange length can take two possible states with spins around the dot center directing either up- or downward. These two spin configurations are very stable and their polarities can be switched by a large vertical field. The mean switching field reaches about 2.5 kOe for Ni80Fe20 dots with 1 μm in diameter and 80 nm in height. This result could open the way for developing a new bistable magnetic memory element utilizing the spin vortex.
Relationship between channel mobility and interface state density in SiC metal–oxide–semiconductor field-effect transistor
Authors: Harada Shinsuke , Kosugi Ryoji , Senzaki Junji , Cho Won-Ju , Fukuda Kenji , Arai Kazuo , Suzuki Seiji
Published: July 2002

Temperature dependence of threshold voltage in n-channel SiC metal–oxide–semiconductor field-effect transistors (MOSFETs) was studied. Linear relation was observed between the threshold voltage shift when the temperature varies from −150 to 150 °C and the number of the interface states present within the energy range of 0.2–0.4 eV from the conduction band edge energy Ec. This relationship revealed that the interface state profile near Ec in n-channel SiC MOSFETs can be represented by that in n-type SiC MOS capacitors. The relationship between the channel mobility and the interface state profile also suggested that the interface states within the energy range of 0.2–0.4 eV from Ec have little influence on the channel mobility.
Nonequilibrium electron heating in inter-subband terahertz lasers
Authors: Kinsler P. , Kelsall R. W. , Harrison P.
Published: July 2002

Inter-subband laser performance can be critically dependent on the nature of the electron distributions in each subband. In these first Monte Carlo device simulations of optically pumped inter-subband THz lasers, we can see that there are two main causes of electron heating: intersubband decay processes, and inter-subband energy transfer from the “hot” nonequilibrium tails of lower subbands. These processes mean that devices relying on low electron temperatures are disrupted by electron heating, to the extent that slightly populated subbands can have average energies far in excess of the that of either the lattice or other subbands. However, although these heating effects invalidate designs relying on low temperature electron distributions, we see that population inversion is still possible in the high-THz range at 77 K in both stepped and triple-well structures, and that our 11.7 THz triple-well structure even promises inversion at 300 K.
Formation and decay mechanisms of electron–hole pairs in amorphous SiO2
Authors: Uchino T. , Takahashi M. , Yoko T.
Published: July 2002

We present theoretical evidence for the creation of an electron–hole pair at an edge-sharing SiO4 site that is supposed to exist in a-SiO2 as an intrinsic structural defect. The present electron–hole pair consists of a nonbridging oxygen hole center and an E′ center, but these paramagnetic defects do not form a close pair but are separately located by over ∼4 Å. The subsequent decay mechanism along with the related radiolytic process is also discussed.
Soft x-ray photoemission studies of the HfO2/SiO2/Si system
Authors: Sayan S. , Garfunkel E. , Suzer S.
Published: July 2002

Soft x-ray photoelectron spectroscopy with synchrotron radiation was employed to study the valence-band offsets for the HfO2/SiO2/Si and HfO2/SiOxNy/Si systems. We obtained a valence-band offset difference of −1.05±0.1 eV between HfO2 (in HfO2/15 Å SiO2/Si) and SiO2 (in 15 Å SiO2/Si). There is no measurable difference between the HfO2 valence-band maximum positions of the HfO2/10 Å SiOxNy/Si and HfO2/15 Å SiO2/Si systems.
Planar Hall sensors for micro-Hall magnetometry
Authors: Rahm M. , Raabe J. , Pulwey R. , Biberger J. , Wegscheider W. , Weiss D. , Meier C.
Published: July 2002

In this work we present a new method to fabricate planar Hall sensors from GaAs–AlGaAs heterojunctions, which can be used to examine the local stray field at a specific section of a micron-sized magnet. Instead of mesa etching we implanted oxygen ions with an energy of 1.5 keV which deplete the two-dimensional electron gas underneath the exposed areas but leave the wafer flat. Planar double Hall cross devices were employed to investigate 30 nm thick electroplated Ni rings with outer and inner diameters ranging from 1.2 to 2 μm and from 0.3 to 1.6 μm, respectively. By comparing the signals from both Hall crosses of the sensor, we can distinguish between local stray field variations and changes of the global magnetization pattern. A hysteresis loop measured at a temperature of 110 K suggests that magnetization reversal occurs via a magnetic vortex structure.
Room-temperature single-electron charging in electrochemically synthesized semiconductor quantum dot and wire array
Authors: Kouklin N. , Menon L. , Bandyopadhyay S.
Published: July 2002

Cylindrical quantum dots of diameter ∼8 nm and height 3–10 nm, and wires of diameter 50 nm and height 500–1000 nm, were self-assembled by electrodepositing semiconductors in the nanometer-sized pores of anodic alumina films. Current–voltage characteristics of both wires and dots show Coulomb blockade at room temperature, while the wires also show a Coulomb staircase when exposed to infrared radiation. These results establish that electrochemical self-assembly is a viable technique for producing nanostructures that have potential uses in room-temperature single electronics.
Magnetic hardening in iron-neodymium-boron permanent magnets
Authors: Hadjipanayis G. C. , Lawless K. R. , Dickerson R. C.
Published: July 2002

The origin of magnetic hardening has been examined in Fe-Nd-B magnets by correlating the magnetic properties with the microstructure and magnetic domain structure observed with electron microscopy. Hysteresis loops and susceptibility studies show a magnetic anomaly at around 140 K. Microstructure studies reveal the presence of at least four phases with compositions Fe14R2B, Fe4NdB4, a high-Nd-content phase, and α-Fe. Initial magnetization studies, magnetic viscosity data, the temperature dependence of coercivity, and magnetic domain studies all indicate domain wall pinning at grain boundaries.