bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunable and adaptive bandpass filter using a nonlinear dielectric thin film of SrTiO3
Authors: Findikoglu A. T. , Jia Q. X. , Wu X. D. , Chen G. J. , Venkatesan T. , Reagor D. W.
Published: July 2002

We have prepared an electrically tunable and adaptive 3-pole half-wave bandpass coplanar waveguide filter incorporating a 1.2-μm-thick paraelectric SrTiO3 bottom layer and a 0.4-μm-thick superconducting YBa2Cu3O7−x top electrode layer on a LaAlO3 substrate. By applying a separate bias voltage on each pole and also on each coupling capacitance of the device at 4 and 76 K, the filter response is not only fine tuned to achieve symmetric and optimized filter characteristics with less than 2% bandwidth centered around 2.5 GHz, but also broadband tuned to shift the passband by more than 15%.
Direct integration of ferroelectric La–Sr–Co–O/Pb–Nb–Zr–Ti–O/La–Sr–Co–O capacitors on silicon with conducting barrier layers
Authors: Dhote A. M. , Madhukar S. , Wei W. , Venkatesan T. , Ramesh R. , Cotell C. M.
Published: July 2002

The growth of a polycrystalline La–Sr–Co–O/Pb–Nb–Zr–Ti–O/La–Sr–Co–O ferroelectric capacitor heterostructure is demonstrated on platinized conducting barrier layers of TiN/poly-Si/Si substrate for integration into high density nonvolatile memory (HDNVM). The concept of conducting bottom layers allows the realization of three-dimensional stacked capacitor-transistor geometry with a direct contact to the memory capacitor, occupying significantly less area on the chip required for HDNVM application. The growth of the ferroelectric heterostructure is achieved at a low temperature of 500–550 °C, compatible with existing Si based technology, without losing the structural and phase integrity of the ferroelectric stack and conducting bottom layers. The fatigue-free characteristics up to 1011 cycles at room temperature and 100 °C, imprint test and good retaining capability evaluated on these capacitors prove their suitability for HDNVM devices.
Strongly localized excitons in gallium nitride
Authors: Wetzel C. , Fischer S. , Krüger J. , Haller E. E. , Molnar R. J. , Moustakas T. D. , Mokhov E. N. , Baranov P. G.
Published: July 2002

We report on strong excitonic luminescence in wurtzite GaN at 3.309 and 3.365 eV (T=6 K). These lines lie well below the band gap and are found commonly in layers grown by different techniques and on different substrates. From detailed photoluminescence investigations we find small thermal activation energies and a very weak electron–phonon coupling. The photoluminescence behavior under hydrostatic pressure is indicative of strongly localized defects. These findings are similar to observations of excitons localized at extended defects such as dislocations in II–VI compounds.
Highly resistive CH-doped GaN grown by plasma-assisted metalorganic chemical vapor deposition
Authors: Sato Michio
Published: July 2002

Highly resistive (≳106 Ω cm) GaN thin films were grown on (0001) sapphire substrates by plasma-assisted metalorganic chemical vapor deposition using triethylgallium and N radicals. Considerable amounts of C and H were incorporated in the GaN films and the incorporated C was electrically inactive after annealing in N2 up to 900 °C, suggesting that thermally stable CH complex made the GaN films highly resistive. The C concentrations were reduced when the Ga and N sources were alternately supplied.
Photoluminescence in ultrafine zinc sulfide thin film
Authors: Chen Qianwang , Li Xiaoguang , Qian Yitai , Zhu Jingsheng , Zhou Guien , Zhang Weiping , Zhang Yuheng
Published: July 2002

Ultrafine (60 nm×10 nm) ZnS:Cu thin film with thickness 200 nm has been prepared by a hydrothermal technique at a relatively low temperature (140 °C). The optical properties of the Cu-doped ZnS thin film were found to be different from that of the bulk material and normal thin film with large grain size. Compared to the bulk material, the peak position of the blue band shifts to a higher energy (from 445 to 430 nm), and a possible new blue band (peak position at 415 nm) appears as the copper concentration in the ZnS thin film reaches 5×10−3 g/g, which is attributed to the equilibrium doping condition and the combination of the Cu+ ion with a dangling sulfur bond on the interface of fine grains, respectively.
Inhibited oxidation in low-temperature grown GaAs surface layers observed by photoelectron spectroscopy
Authors: Ng T.-B. , Janes D. B. , McInturff D. , Woodall J. M.
Published: July 2002

The surface oxidation characteristics of a GaAs layer structure consisting of a thin (10 nm) layer of low-temperature-grown GaAs (LTG:GaAs) on a heavily n-doped GaAs layer, both grown by molecular beam epitaxy, have been studied using x-ray photoelectron spectroscopy (XPS). Between the layer growth and XPS characterization, the unannealed LTG:GaAs sample and a control sample without the LTG:GaAs surface layer were exposed to the atmosphere. The rate of surface oxidation in the sample with a LTG:GaAs surface layer was significantly lower than the oxidation rate of the control sample. This direct observation of inhibited oxidation confirms the surface stability of comparable structures inferred from earlier electrical measurements. The inhibited surface oxidation rate is attributed to the bulk Fermi-level pinning and the low minority carrier lifetime in unannealed LTG:GaAs.
Optical gain of strained hexagonal and cubic GaN quantum-well lasers
Authors: Ahn Doyeol , Park Seoung-Hwan
Published: July 2002

In order to estimate and compare the would-be performances of both hexagonal and cubic GaN quantum-well lasers, the optical gains of strained-layer hexagonal and cubic GaN quantum wells are studied theoretically taking into account non-Markovian relaxation. The 6×6 multiband effective-mass Hamiltonians are used to calculate the band structures of both hexagonal—and cubic—quantum wells. As a numerical example, the optical gains of strained cubic and hexagonal quantum wells with various well widths are calculated. It is expected that the optical gains of the cubic-phase quantum well are larger in magnitudes than those of the hexagonal GaN quantum well over the wide range of carrier densities due to the heavier effective mass of the HH1 band of the latter at the zone center.
Optical transitions in InxGa1−xN alloys grown by metalorganic chemical vapor deposition
Authors: Shan W. , Little B. D. , Song J. J. , Feng Z. C. , Schurman M. , Stall R. A.
Published: July 2002

We present the results of optical studies of InxGa1−xN alloys (0<x<0.2) grown by metalorganic chemical vapor deposition on top of thick GaN epitaxial layers with sapphire as substrates. Photoluminescence (PL) and photoreflectance measurements were performed at various temperatures to determine the band gap and its variation as a function of temperature for samples with different indium concentrations. Carrier recombination dynamics in the alloy samples were studied using time-resolved luminescence spectroscopy. While the measured decay time for the alloy near-band-edge PL emissions was observed to be generally around a few hundred picoseconds at 10 K, it was found that the decay time decreased rapidly as the sample temperatures increased. This indicates a strong influence of temperature on the processes of trapping and recombination of excited carriers at impurities and defects in the InGaN alloys.
Small atom diffusion and breakdown of the Stokes–Einstein relation in the supercooled liquid state of the Zr46.7Ti8.3Cu7.5Ni10Be27.5 alloy
Authors: Geyer U. , Johnson W. L. , Schneider S. , Qiu Y. , Tombrello T. A. , Macht M.-P.
Published: July 2002

Be diffusivity data in the bulk metallic glass forming alloy Zr46.7Ti8.3Cu7.5Ni10Be27.5 are reported for temperatures between 530 and 710 K, extending 85 K into the supercooled liquid state of the alloy. At the glass transition temperature Tg, a change in temperature dependence of the data is observed, and above Tg the diffusivity increases more quickly with temperature than below. The data in the supercooled liquid can be described by a modified Arrhenius expression based on a diffusion mechanism suggested earlier. The comparison with viscosity data in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5 reveals a breakdown of the Stokes–Einstein relation, indicating a cooperative diffusion mechanism in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5.
Picosecond photoresponse of carriers in Si ion-implanted Si
Authors: Chin Albert , Lee K. Y. , Lin B. C. , Horng S.
Published: July 2002

Picosecond photoresponse of carriers in Si ion-implanted Si samples has been measured using femtosecond transient reflectivity measurement. A threshold peak implant dose of 1016 cm −2 is required to achieve picosecond carrier lifetime. At this dosage, carrier lifetimes of 0.9 and 1.4 ps are measured for the as-implanted and 400 °C annealed Si substrates, respectively. The increase in carrier lifetime upon annealing is attributed to the reduction in the concentration of trap and recombination centers. Sheet resistance also shows a strong dependence on the annealing temperature. An eightfold increase in sheet resistance is obtained for annealed samples, and a reduction in hopping conduction, manifested by the e−1/T temperature dependence, may be responsible for the increase in resistance. Further evidence of decreasing hopping conduction can be also observed from the more than two orders of magnitude in reduction of sheet resistance as the peak dosage decreases from 1016 to 1014 cm−2.
Gray-scale in polymer-stabilized antiferroelectric liquid crystal displays
Authors: Strauss J. , Kitzerow H.-S.
Published: July 2002

We have investigated an antiferroelectric liquid crystal which is stabilized by a polymer network. The latter is formed by in situ photopolymerization of a mesogenic diacrylate. The pure liquid crystalline monomer shows the well-known tristable electro-optic switching due to a field induced transition from the antiferroelectric to the ferroelectric state. The change of the optical transmittance due to this transition becomes more continuous if a polymer network is present. Thus, different gray shades can be easily achieved in polymer-stabilized antiferroelectric liquid crystals.
Spatially and spectrally resolved imaging of GaAs quantum-dot structures using near-field optical technique
Authors: Toda Yasunori , Kourogi Motonobu , Ohtsu Motoichi , Nagamune Yasushi , Arakawa Yasuhiko
Published: July 2002

We present experimental result on spatially and spectrally resolved imaging of GaAs quantum dot (QD) structures using a near-field optical microscope. Three photoluminescence (PL) peaks which originate from QD, quantum well (QW), and bulk regions were observed at liquid-He temperature. Carriers were observed to diffuse effectively from the AlGaAs barriers to the QD and QW regions. The region of the intension carrier capture manifests itself by emitting a sharp PL peak, and is estimated to be about 300 nm.
A model for visible photon emission from reverse-biased silicon p-n junctions
Authors: Obeidat Amjad T. , Kalayjian Zaven , Andreou Andreas G. , Khurgin Jacob B.
Published: July 2002

We report visible (380–650 nm) electroluminescence from reverse-biased silicon p-n junctions and from n- and p-type field-effect transistors designed for a standard chip-fabrication process. We measured the spectra of over 40 junctions and devices and found that they differed from previously reported silicon electroluminescence spectra. We use a hot carrier recombination model and account for Fabry-Perot effects to explain the observed electroluminescence spectrum. Our model’s prediction is in good agreement with the measured spectra.
Nanostructure fabrication by selective growth of molecular crystals on layered material substrates
Authors: Ueno Keiji , Sasaki Kentaro , Takeda Natsuko , Saiki Koichiro , Koma Atsushi
Published: July 2002

Nanostructures consisting of C60 molecules were fabricated on a GaSe/MoS2 heterostructure. A submonolayer film of a lamellar compound semiconductor GaSe was grown on a MoS2 substrate to form nanoscale holes or grooves surrounded by monolayer steps. Atomic force microscope (AFM) observation indicates that C60 molecules grow only in the bare MoS2 nanoregions at a substrate temperature of 180 °C. C60 molecules fill up those holes and grooves, and nanoscale C60 domains with polygonal shapes can be formed. This selective growth method can be combined with nanoscale patterning made by a scanning tunneling microscope or AFM to produce nanostructures of molecular crystals with designed shapes.
Low noise NbN lattice-cooled superconducting hot-electron bolometric mixers at submillimeter wavelengths
Authors: Kawamura Jonathan , Blundell Raymond , Tong Cheuk-yu Edward , Gol’tsman Gregory , Gershenzon Eugene , Voronov Boris , Cherednichenko Sergey
Published: July 2002

Lattice-cooled superconducting hot-electron bolometric mixers are used in a submillimeter-wave waveguide heterodyne receiver. The mixer elements are niobium nitride film with 3.5 nm thickness and ∼10 μm2 area. The local oscillator power for optimal performance is estimated to be 0.5 μW, and the instantaneous bandwidth is 2.2 GHz. At an intermediate frequency centered at 1.4 GHz with 200 MHz bandwidth, the double sideband receiver noise temperature is 410 K at 430 GHz. The receiver has been used to detect molecular line emission in a laboratory gas cell.
Electron transport properties through InAs self-assembled quantum dots in modulation doped structures
Authors: Horiguchi N. , Futatsugi T. , Nakata Y. , Yokoyama N.
Published: July 2002

We report electron transport properties through InAs self-assembled quantum dots in a modulation-doped structure with split gates. We observed drain current modulation with respect to gate voltage due to electron transport through the quantum level of InAs dots. The energy gaps estimated from the temperature dependence study of valley current and the voltage difference between the drain current peaks were consistent with each other and as large as 14 meV. The energy gaps can be explained by the charging energy of the InAs dots.
Enhanced harmonic emission from ionized clusters in intense laser pulses
Authors: Hu S. X. , Xu Z. Z.
Published: July 2002

We present a two-dimensional model to simulate the enhanced harmonic emission from ionized argon clusters irradiated by an ultrashort (∼25 fs) laser pulse at moderate intensity (∼1014 W/cm2). The low-order harmonics (below the 15th) are enhanced by nearly two orders of magnitude relative to those of monomer argon. Furthermore, the harmonic plateau is extended to a higher frequency. The clustering environment is responsible for the enhancement of harmonic emission.
In situ optical second-harmonic-generation monitoring of disilane adsorption and hydrogen desorption during epitaxial growth on Si(001)
Authors: Hu X. F. , Xu Z. , Lim D. , Downer M. C. , Parkinson P. S. , Gong B. , Hess G. , Ekerdt J. G.
Published: July 2002

The kinetics of disilane adsorption and hydrogen desorption during low-temperature, ultrahigh vacuum chemical vapor deposition on Si(001) is investigated in situ in real time by monitoring the instantaneous hydrogen coverage using optical second-harmonic generation. A simple two-site adsorption model and first-order desorption are used to establish a reactive sticking coefficient and to predict the Si(001) epitaxial growth rate. The reactive sticking coefficient is temperature independent between 740 and 920 K and equal to 0.04±0.01. Predicted growth rates are in agreement with published growth rates obtained from gas-source molecular-beam epitaxy of Si(001).
Fabrication and superfluorescence of rare-earth chelate-doped graded index polymer optical fibers
Authors: Kobayashi Takeyuki , Nakatsuka Shiro , Iwafuji Takami , Kuriki Ken , Imai Nana , Nakamoto Takashi , Claude Charles D. , Sasaki Keisuke , Koike Yasuhiro , Okamoto Yoshi
Published: July 2002

Rare-earth chelate-doped graded index (GI) polymer optical fibers (POF) are proposed and fabricated. The attenuation loss was measured to be 0.4 dB/m at 650 nm for a GI POF doped with 1 wt % of europium (Eu) chelate. Lifetime shortening and spectral narrowing verified the occurrence of superfluorescence at 614 nm in the Eu chelate-doped GI POF pumped with xenon flashlamps. The demonstration of superfluorescence shows that rare-earth chelate-doped GI POFs are appealing as optical amplifiers and superfluorescent sources in a variety of communication and sensor applications.
Lateral current-constriction in vertical devices using openings in buried lattices of metallic discs
Authors: Wernersson L.-E. , Carlsson N. , Gustafson B. , Litwin A. , Samuelson L.
Published: July 2002

Vertical channels in semi-insulating GaAs were realized by epitaxial overgrowth over lattices of W discs, including n×n vacant positions. The Schottky depletion around the metallic inclusions is responsible for the semi-insulating behavior, and, thus, conducting channels may be created in designed openings in the lattice. By measuring the current transport in structures with varying sizes of the channels, we demonstrate that the current is actually restricted to floating through the vacant positions. We further buried the metal discs, including openings, 60 nm above a resonant tunneling structure, and measured a negative differential resistance, with peak currents scaling with the opening area. These experiments demonstrate the effective combination of semiconductor heterostructures and embedded metal features for submicron, vertical injection into a heterostructure device.
Oligophenyl-based organic thin film transistors
Authors: Gundlach D. J. , Lin Y.-Y. , Jackson T. N. , Schlom D. G.
Published: July 2002

Organic thin film transistors (TFTs) have been fabricated using thermally evaporated films of the oligophenyls p-quaterphenyl (p-4P), p-quinquephenyl (p-5P), and p-sexiphenyl (p-6P). The field-effect mobility of these TFTs ranges from 10−2 cm2/V s for p-4P to 10−1 cm2/V s for p-6P with on/off current ratio from 105 to 106. These values are comparable to those achieved using the more widely studied organic semiconductors alpha-sexithienyl (α-6T) and pentacene. X-ray diffraction reveals a high degree of molecular ordering, believed to be important for obtaining high field-effect mobility in organic TFTs.
Hall magnetometer in the ballistic regime
Authors: Peeters F. M. , Li X. Q.
Published: July 2002

A Monte Carlo simulation of the ballistic motion of electrons in a mesoscopic Hall bar containing a local inhomogeneous magnetic field profile is presented. We find that in the regime of low magnetic fields, the Hall resistance is determined by the average magnetic field in the cross junction and is independent of the shape and position of this profile in the junction. Information on the size of the local magnetic field profile can also be obtained from the Hall measurement. The bend resistance, on the other hand, is much more precise on the exact details of the local magnetic field in the cross junction.
Charge release of lanthanum-doped lead zirconate titanate stannate antiferroelectric thin films
Authors: Xu Baomin , Moses Paul , Pai Neelesh G. , Cross L. Eric
Published: July 2002

The charge release speed and backward phase switching time of lanthanum-doped lead zirconate titanate stannate antiferroelectric thin films were investigated by directly measuring the switching current upon removal of the applied electric field. The backward switching time is about 6 ns. The maximum switching current density can reach 9400 A/cm2, and more than half of the stored charge can be released in 10 ns. These results show that the obtained antiferroelectric thin films are very promising for decoupling capacitor applications in high speed multichip modules.
Time-domain measurement of intersubband oscillations in a quantum well
Authors: Heyman James N. , Kersting Roland , Unterrainer Karl
Published: July 2002

We report time-domain measurements of electron intersubband oscillations in quantum wells. We use an interferometric technique to measure the change in the profile of a few-cycle THz pulse due to propagation through a modulation-doped Al0.3Ga0.7As/GaAs multiple quantum well structure (10×510 Å wells). From this data we obtain the absorption and the index of refraction due to electrons in the quantum well, and due to the GaAs substrate. Unlike existing studies of coherent charge oscillations of electrons and holes in heterostructures excited by ultrashort pulses of band-gap light, our all-THz measurements quantitatively determine the linear optical properties of the quantum well electrons.
Measurement of interface trap states in metal–ferroelectric–silicon heterostructures
Authors: Alexe Marin
Published: July 2002

Interface trap density distributions within Si for metal–bismuth titanate–silicon capacitors fabricated by chemical solution deposition were investigated. The interface trap density was measured by a conductance technique at room temperature and a value in the order of 1011–1012 eV−1 cm−2 was found depending on the ferroelectric crystallization temperature. An increase in the annealing temperature results in an increase in the interface trap density.
Negative electron affinity mechanism for diamond surfaces
Authors: Krainsky I. L. , Asnin V. M.
Published: July 2002

The energy distribution of the secondary electrons for chemical vacuum deposited diamond films with negative electron affinity (NEA) was investigated. It was found that while for completely hydrogenated diamond surfaces the negative electron affinity peak in the energy spectrum of the secondary electrons is present for any energy of the primary electrons, for partially hydrogenated diamond surfaces there is a critical energy above which the peak is present in the spectrum. This critical energy increases sharply when hydrogen coverage of the diamond surface diminishes. This effect was explained by the change of the NEA from the true type for the completely hydrogenated surface to the effective type for the partially hydrogenated surfaces.
Optical spectroscopy of self-assembled type II GaSb/GaAs quantum dot structures grown by molecular beam epitaxy
Authors: Hogg R. A. , Suzuki K. , Tachibana K. , Finger L. , Hirakawa K. , Arakawa Y.
Published: July 2002

We report an optical spectroscopic study of GaSb/GaAs quantum dots (QDs) formed by the Stranski–Krastanow growth mode using molecular beam epitaxy. We identify the QD luminescence by photoluminescence obtained at different excitation energies and densities. We show that, for these structures, not only the spectral position of peaks, but also their relative intensities are critically dependent upon the density of photogenerated carriers. Photoluminescence excitation (PLE) measurements confirm our assignment of the QD related peaks and a feature ∼25–27 meV higher in energy than the PLE detection energy is discussed in terms of phonon relaxation.
Vacancy defects in thin-film La0.5Sr0.5CoO3−δ observed by positron annihilation
Authors: Keeble D. J. , Krishnan A. , Friessnegg T. , Nielsen B. , Madhukar S. , Aggarwal S. , Ramesh R. , Poindexter E. H.
Published: July 2002

Vacancy-related defects in laser ablated thin films of the conducting metal–oxide La0.5Sr0.5CoO3−δ were detected using a variable-energy positron beam. The nonstoichiometry, δ, was altered by varying the oxygen partial pressure within the deposition chamber present during cooling. Conductivity measurements confirmed the change in oxygen content. Increased positron trapping at vacancy defects was observed with increasing nonstoichiometry. It is proposed that vacancy clusters were present in the film cooled in 10−5 Torr oxygen.
Observation of excitonic transitions in InSb quantum wells
Authors: Dai N. , Brown F. , Barsic P. , Khodaparast G. A. , Doezema R. E. , Johnson M. B. , Chung S. J. , Goldammer K. J. , Santos M. B.
Published: July 2002

We report the observation of interband exciton transitions in InSb/AlxIn1−xSb multi-quantum-well samples. The exciton peaks are identified with the use of a simple quantum well model. The strain present in the InSb wells alters the spectrum significantly from that for unstrained III–V materials and makes it possible to use the exciton spectrum in determining the band offset.
Room temperature spin relaxation in GaAs/AlGaAs multiple quantum wells
Authors: Britton R. S. , Grevatt T. , Malinowski A. , Harley R. T. , Perozzo P. , Cameron A. R. , Miller A.
Published: July 2002

We have explored the dependence of electron spin relaxation in undoped GaAs/AlGaAs quantum wells on well width (confinement energy) at 300 K. For wide wells, the relaxation rate tends to the intrinsic bulk value due to the D’yakonov–Perel (DP) mechanism with momentum scattering by phonons. In narrower wells, there is a strong dependence of relaxation rate on well width, as expected for the DP mechanism, but also considerable variation between samples from different sources, which we attribute to differences in sample interface morphology.
Selective formation of one- and two-dimensional arrayed InGaAs quantum dots using Ga2O3 thin film as a mask material
Authors: Hahn Cheol Koo , Park Young Ju , Kim Eun Kyu , Min Suk-Ki , Jung Suk Koo , Park Jung Ho
Published: July 2002

We report on the selective formation of InGaAs quantum dots (QDs) by molecular beam epitaxy. Nanoscale patterned Ga2O3 thin film deposited on the GaAs (100) substrate was employed as a mask material. Due to the enhanced migration effect of the group-III adatoms, such as Ga and In on Ga2O3 mask layer, the InGaAs QDs formed on the patterned substrate results in coalesced islands unlike those formed on the nonpatterned substrate. The estimation of the relative volume of the islands per unit area revealed that the desorption process as well as the migration of the Ga and In adatoms might occur on the Ga2O3 layer during the growth process, providing a good selective growth of self-assembled QDs.
Electrical detection of optically induced charge storage in self-assembled InAs quantum dots
Authors: Finley J. J. , Skalitz M. , Arzberger M. , Zrenner A. , Böhm G. , Abstreiter G.
Published: July 2002

Spectrally resolved photoresistance investigations of charge storage effects in self-assembled InAs quantum dots (QDs) are reported. Resonant optical excitation of the QDs produces a strong increase of the lateral resistance of a spatially separated electron channel (ΔR) which reflects the stored charge density. This photoresponse is persistent for many hours at 145 K and can be controllably reversed electrically. Pronounced oscillations observed in the spectral variation ΔR are shown to reflect the excitation spectrum of the QD ensemble showing resonances that arise from both direct and phonon-assisted absorption processes.
Lateral confinement in a resonant tunneling transistor with a buried metallic gate
Authors: Wernersson Lars-Erik , Suhara Michihiko , Carlsson Niclas , Furuya Kazuhito , Gustafson Boel , Litwin Andrej , Samuelson Lars , Seifert Werner
Published: July 2002

We have fabricated a resonant tunneling transistor by epitaxial overgrowth over a tungsten grating placed 30 nm above a GaAs/GaInP semiconductor, double barrier, resonant tunneling heterostructure. The Schottky depletion around the buried metal contacts controls the current to a vertical transistor channel. The lateral extension of this channel is defined by a square opening in the grating with a side length of 1.4 μm, which corresponds to a sub-μm electrical width. The transport properties at 20 K show a fine structure in the resonant tunneling characteristics, and it is affected by the gate bias. These effects are discussed in terms of lateral quantum confinement in the transistor channel defined.
Well-aligned carbon nitride nanotubes synthesized in anodic alumina by electron cyclotron resonance chemical vapor deposition
Authors: Sung S. L. , Tsai S. H. , Tseng C. H. , Chiang F. K. , Liu X. W. , Shih H. C.
Published: July 2002

Vertically aligned carbon nitride nanotubes with a uniform diameter of about 250 nm have been synthesized on a porous alumina membrane template (50–80 μm thick) in a microwave excited plasma of C2H2 and N2 using an electron cyclotron resonance chemical vapor deposition system. A negative dc bias voltage was applied to the substrate holder of graphite to promote the flow of ionic fluxes through the nanochannels of the alumina template. This allowed the physical, and subsequent chemical, absorption of species on the walls of the nanochannels that resulted in the formation of the carbon nitride nanotubes. The hollow structure and vertically aligned properties of the nanotubes have been clearly verified by field-emission scanning electron microscope images. The absorption band between 1250 and 1750 cm−1 in the Fourier transform infrared spectroscopy spectrum proves that nitrogen atoms have been incorporated into an amorphous network of carbon.
Field-enhanced emission rate and electronic properties of a defect introduced in n-GaN by 5.4 MeV He-ion irradiation
Authors: Goodman S. A. , Auret F. D. , Koschnick F. K. , Spaeth J.-M. , Beaumont B. , Gibart P.
Published: July 2002

A deep level defect ER3, introduced in n-GaN by high energy (5.4 MeV) He ions, was characterized by deep level transient spectroscopy (DLTS). This defect, 0.196±0.004 eV below the conduction band, with an apparent capture cross-section of 3.5±1×10−15 cm2, is introduced uniformly in the region profiled by DLTS at a rate of 3270±200 cm−1. The emission rate of this defect depends on the electric field strength in the space-charge region. This emission rate is modeled according to the Poole–Frenkel distortion of a square well with a radius of 20±2 Å or alternatively, a Gaussian well with a characteristic width of 6.0±1 Å. Hence, we conclude that ER3 is a point defect which has a field dependence not explained by the classical Poole–Frenkel enhancement.
Imaging and probing electronic properties of self-assembled InAs quantum dots by atomic force microscopy with conductive tip
Authors: Tanaka Ichiro , Kamiya I. , Sakaki H. , Qureshi N. , Allen S. J. , Petroff P. M.
Published: July 2002

Atomic force microscopy with a conductive probe has been used to study both the topography and the electronic properties of 10-nm-scale self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy on n-type GaAs. The current flowing through the conductive probe normal to the sample surface is measured for imaging local conductance, while the deflection of cantilever is optically detected for disclosing geometrical structure. The conductance on InAs QDs is found to be much larger than that on the wetting layer, allowing imaging of QDs through measurements of local current. We attribute this change in conductance to the local modification of surface band bending associated with surface states on InAs QD surface. Mechanisms of electron transport through QDs are discussed based on current–voltage characteristics measured on QDs of various sizes.
Room temperature growth of cubic boron nitride
Authors: Feldermann H. , Merk R. , Hofsäss H. , Ronning C. , Zheleva T.
Published: July 2002

Boron nitride thin films were deposited at room temperature with various ion energies by mass selected ion beam deposition on cubic boron nitride (c-BN) previously nucleated on Si (100) substrates at a higher temperature. Selective area diffraction, electron energy loss, and infrared spectroscopy results reveal continued growth of the cubic phase. The reported temperature threshold of about 150 °C for c-BN film formation is therefore unmistakably related to the initial nucleation of c-BN, whereas the growth of c-BN appears to be temperature independent. The latter is in accordance with predictions of the cylindrical thermal spike growth model recently proposed by our group.
Variation of nonlinearity parameter at low fundamental amplitudes
Authors: Barnard Daniel J.
Published: July 2002

Recent harmonic generation measurements of the nonlinearity parameter β in polycrystalline Cu–Al alloys have shown a transition to lower values at low fundamental amplitude levels. Values for β at high (>10 Å) fundamental levels are in the range predicted by single-crystal second- and third-order elastic constants while lower fundamental levels (<4 Å) produce values that are a fraction of those predicted. The effect is believed genuine and has been demonstrated recently in a commercial aluminum alloy by others. The source of the effect is unclear but initial results may require a reexamination of current methods for measurement of third-order elastic constants.
Static compression of Ti3SiC2 to 61 GPa
Authors: Onodera A. , Hirano H. , Yuasa T. , Gao N. F. , Miyamoto Y.
Published: July 2002

Synchrotron x-ray diffraction measurements have been made on hexagonal crystal Ti3SiC2 under pressures up to 61 GPa and at room temperature. No phase transition was observed within the pressure range studied. Both the a and c axes exhibited decreases with pressure, accompanied by a decrease in the c/a ratio to about 50 GPa, beyond which the ratio increased. The bulk modulus deduced from the volume-versus-pressure data was 206±6 GPa (with its pressure derivative 4.0±0.3), being close to that of TiC.
High contrast, ultrafast optically addressed ultraviolet light modulator based upon optical anisotropy in ZnO films grown on R-plane sapphire
Authors: Wraback M. , Shen H. , Liang S. , Gorla C. R. , Lu Y.
Published: July 2002

An optically addressed ultraviolet light modulator has been demonstrated which exploits the optical anisotropy in a ZnO film epitaxially grown on (011̄2) sapphire. This device achieves both high contrast and high speed by exploiting the anisotropic bleaching of the anisotropic absorption and concomitant ultrafast polarization rotation near the lowest exciton resonances produced by femtosecond ultraviolet pulses. The resultant modulation is characterized by a contrast ratio of 70:1, corresponding to a dynamic polarization rotation of 12°, and it decays to a quasiequilibrium value within 100 ps.
Low-temperature thermopower in nanostructured silver
Authors: Zhu X. F. , Zhang H. F. , Williams J. S. , Wang R. P. , Zhang L. D. , Wu X. J.
Published: July 2002

The variation of thermopower with temperature from 77 to 300 K has been measured in samples of nanostructured silver. It is observed that a dramatic change occurs in the thermopower of the nanostructured silver as grain size is reduced into the nanoscale region. In particular, with the grain size decreasing to the nanometer scale, the slope of the electron-diffusion component of thermopower as a function of temperature increases and the phonon-drag component of thermopower is shifted to lower temperatures. The changes in the thermopower correlate well with changes in the microhardness.
Nanoscale conduction modulation in Au/Pb(Zr, Ti)O3/SrRuO3 heterostructure
Authors: Yoshida Chikako , Yoshida Akira , Tamura Hirotaka
Published: July 2002

Nanoscale conduction modulation in an Au-tip/Pb(Zr, Ti)O3(PZT)/SrRuO3 heterostructure was observed using the Au-coated cantilever of scanning probe microscopy (SPM) as a mobile top electrode. In the ferroelectric layer of the heterostructure, a polarization-induced pattern 0.2 μm wide was written by applying a dc voltage and confirmed by the SPM piezoresponse method. The written pattern was read-out by detecting the change in conductance through the Au-tip/PZT/SrRuO3 heterostructure. Furthermore, the conduction modulations were explained by a model in which the Au-tip/PZT band bending is modified by an effective change injected into the PZT surface.
Evidence of energy coupling between Si nanocrystals and Er3+ in ion-implanted silica thin films
Authors: Chryssou C. E. , Kenyon A. J. , Iwayama T. S. , Pitt C. W. , Hole D. E.
Published: July 2002

Silica thin films containing Si nanocrystals and Er3+ were prepared by ion implantation. Excess Si concentrations ranged from 5% to 15%; Er3+ concentration for all samples was 0.5%. Samples exhibited photoluminescence at 742 nm (attributed to Si nanocrystals), 654 nm (defects due to Er3+ implantation), and at 1.53 μm (intra-4f transitions). Photoluminescence intensity at 1.53 μm increased ten times by incorporating Si nanocrystals. Strong, broad photoluminescence at 1.53 μm was observed for λPump away from Er3+ absorption peaks, implying energy transfer from Si nanocrystals. Erbium fluorescence lifetime decreased from 4 ms to 1 ms when excess Si increased from 5% to 15%, suggesting that at high Si content Er3+ ions are primarily situated inside Si nanocrystals.
Scanning capacitance microscopy of AlGaN/GaN heterostructure field-effect transistor epitaxial layer structures
Authors: Smith K. V. , Yu E. T. , Redwing J. M. , Boutros K. S.
Published: July 2002

Scanning capacitance microscopy is used to characterize local electronic properties in an AlxGa1−xN/GaN heterostructure field-effect transistor epitaxial layer structure. Lateral inhomogeneity in electronic properties is clearly observed, at length scales ranging from ∼0.1 to >2 μm, in images obtained at fixed bias voltages. Acquisition of a series of images over a wide range of bias voltages allows local electronic structure to be probed with nanoscale spatial resolution both laterally and in depth. Combined with theoretical analysis of charge and potential distributions in the epitaxial layer structure under applied bias, these studies suggest that the dominant factor contributing to the observed variations in electronic structure is local lateral variations in AlxGa1−xN layer thickness.
Laser-induced nano-oxidation on hydrogen-passivated Ge (100) surfaces under a scanning tunneling microscope tip
Authors: Lu Y. F. , Mai Z. H. , Qiu G. , Chim W. K.
Published: July 2002

Laser-induced nano-oxidation on hydrogen-passivated Ge (100) surfaces under a scanning tunneling microscope tip in air has been investigated. A 532 nm Nd:YAG pulsed laser with a pulse duration of 7 ns was used. A 2×2 oxide dot array with dot sizes between 20 and 30 nm and an oxide single line with a width less than 30 nm have been created using an electrochemical-etched tungsten tip under laser irradiation. The modified regions were characterized by atomic force microscope. The apparent depth of oxide layer as a function of laser intensity has been studied. The advantages and drawbacks of using a continuous wave laser and a pulsed laser will be discussed.
Microscopic theory of hot-carrier relaxation in semiconductor-based quantum-cascade lasers
Authors: Iotti Rita C. , Rossi Fausto
Published: July 2002

A microscopic analysis of basic nonequilibrium phenomena in unipolar quantum devices is presented. In particular, energy-relaxation processes governing the hot-carrier dynamics in the active region of GaAs-based quantum-cascade lasers are investigated by means of a generalized ensemble Monte Carlo simulation. Such analysis is essential in determining the validity range and limitations of purely macroscopic models with respect to basic device parameters, like injection current and temperature.
Optical measurement of interaction potentials between a single microparticle and an evanescent field
Authors: Wada Ken-ichiro , Sasaki Keiji , Masuhara Hiroshi
Published: July 2002

Radiation pressure exerted on a single microparticle within an evanescent field has been analyzed by a three-dimensional potential measurement system. Temporal fluctuation of the particle position due to the thermal Brownian motion was sequentially measured, and the potential energy was estimated by a thermodynamical analysis. A potential profile corresponding to the evanescent-field-induced radiation pressure could be obtained as an energy difference between two potentials measured with and without illumination of the evanescent field. The exponential dependence on the longitudinal position was clearly observed. The lateral radiation force on a 4.5 μm polystyrene latex particle in water was determined to be 90 fN at the laser intensity of 1.0 kW cm−2.
Characterization of electron trap states due to InAs quantum dots in GaAs
Authors: Walther C. , Bollmann J. , Kissel H. , Kirmse H. , Neumann W. , Masselink W. T.
Published: July 2002

InAs quantum dots grown in a GaAs matrix are investigated using capacitive transient spectroscopy and transmission electron microscopy (TEM). Two deep levels are detected which are energetically too deep to be the intrinsic electron levels of the quantum dots. Both TEM as well as the detailed non-exponential behavior of the electron capture indicate that the traps do not result from dislocations in the GaAs matrix. We propose that the measured trap levels are due to point defects in or near the quantum dots.
Higher-harmonics generation in tapping-mode atomic-force microscopy: Insights into the tip–sample interaction
Authors: Hillenbrand R. , Stark M. , Guckenberger R.
Published: July 2002

We present an experimental analysis of the nonlinear tip–sample interaction in tapping-mode atomic-force microscopy by exploiting anharmonic contributions of the cantilever motion. Two aspects of a concept aiming at a full reconstruction of the tip–sample interaction are demonstrated: higher flexural eigenmode vibrations excited by the impact of the oscillating tip on the sample are used to measure the tip–sample interaction time; by imaging at higher harmonics of the driving frequency material contrast is obtained.
In situ, real-time measurement of wing tilt during lateral epitaxial overgrowth of GaN
Authors: Fini P. , Munkholm A. , Thompson Carol , Stephenson G. B. , Eastman J. A. , Murty M. V. Ramana , Auciello O. , Zhao L. , DenBaars S. P. , Speck J. S.
Published: July 2002

By performing in situ, real-time x-ray diffraction measurements in the metalorganic chemical-vapor deposition environment, we have directly observed the emergence and evolution of wing tilt that occurs during the lateral overgrowth of GaN from stripes patterned in a SiO2 mask. This was done by repeatedly performing line scans through the 101̄3 peak in the direction perpendicular to the [101̄0]GaN stripe direction. The wing tilt developed as soon as the wings started forming, and increased slightly thereafter to reach a value of ∼1.19° after 3600 s of growth. Upon cooldown to room temperature, the tilt increased to ∼1.36°, indicating that thermally induced stresses during cooldown have only a small effect on wing tilt. However, changes in mask density, composition, and stress state during early lateral overgrowth must be considered as possible origins of wing tilt.
Achromatic linear polarization rotator using twisted nematic liquid crystals
Authors: Zhuang Zhizhong , Kim Young Jin , Patel J. S.
Published: July 2002

In this letter, we propose a simple configuration that is capable of rotating a linear polarization state of light by a certain angle for a wide range of wavelengths. The device consists of three liquid-crystal cells: two homogeneous cells and one twisted nematic (TN) cell. It is well known that a thick TN cell can rotate the linear polarization state of light by following the twisted structure. However, for a thin TN cell, achromatic polarization rotation is not possible. By the use of the Poincaré sphere model of the TN structure, we demonstrate that if one thin homogeneous cell is placed before and another one is placed after a thin TN structure, then a linear polarization state can be transformed close to the eigenmodes of TN. Therefore, this structure can be used to achieve the achromatic polarization rotation. This letter provides a detailed discussion of the theoretical analysis and a demonstration of the achromatic linear polarization rotator by the use of a 1.9 μm TN cell for the wavelength range 450–700 nm.
Enhancement of transverse trapping efficiency for a metallic particle using an obstructed laser beam
Authors: Gu Min , Morrish Dru , Ke Pu Chun
Published: July 2002

We report that the transverse trapping efficiency for a metallic particle can be enhanced by use of a laser beam obstructed by a circular opaque disk. In the case of gold particles, the enhancement factor for a p- or s-polarized trapping beam is at least 1.7 or 2.5, respectively. The dependence of the transverse trapping efficiency for gold particles (diameter=2 μm) on the size of the obstruction is measured and agrees with the theoretical prediction based on the ray-optics model.
Transmittance and resistivity of semicontinuous copper films prepared by pulsed-laser deposition
Authors: So S. K. , Fong H. H. , Yeung C. F. , Cheung N. H.
Published: July 2002

Thin copper films were grown on glass by pulsed-laser deposition. The simultaneous in situ monitoring of the electrical resistance and optical transmittance of the growing film yielded highly reproducible and consistent data about percolation onset and film conductivity, both being useful indicators of film quality. When prepared under favorable conditions, films as thin as 1.5 nm would percolate, and became fully continuous at 5 nm, with conductivity reaching 30% of that of bulk copper.
Z-scan determination of the third-order optical nonlinearity of gold:silica nanocomposites
Authors: Debrus S. , Lafait J. , May M. , Pinçon N. , Prot D. , Sella C. , Venturini J.
Published: July 2002

Third-order nonlinear optical properties of Au:SiO2 thin films were studied at the surface plasmon resonance wavelength by the z-scan technique using a nanosecond laser. Films were prepared by a multilayer deposition sputtering technique. They were composed of 2 nm mean diam gold particles, with a metal volume fraction of 20%. Z-scan measurements performed both with and without aperture showed a very large nonlinear absorption masking the nonlinear refraction. The nonlinear absorption coefficient β was found to be negative and equal to −1.1×10−2 cm/W. The different mechanisms contributing to this absorption are discussed and the large value of β is correlated to the duration of the laser pulses. Moreover, it is shown that a mean field theory is not appropriate to evaluate the effective susceptibility at high metal concentrations.
Properties of metastable hydrogen-related defects in n-type GaAs studied by isothermal deep-level transient spectroscopy
Authors: Tokuda Yutaka , Kamiya Kazuhiro , Okumura Tsugunori
Published: July 2002

Metastable hydrogen-related defects (M3/M4) in n-GaAs were studied in detail by using isothermal deep-level transient spectroscopy. In order to clarify the electric-field dependence of the electron-emission process, the double-correlation technique was applied to both M3 and M4 defects. It was clearly shown that with increasing electric field, the M4 level observed around 140 K split into two discrete components labeled M4(1) and M4(2). The result supports the recent finding that the M4 defect consists of two different configurations. On the other hand, the M3 defect gave a single peak in the double-correlation spectra around 230 K. The electric field dependence of the emission rates suggests that the M3 and M4(1) defects have a donor-like nature, while the M4(2) defect is acceptor-like. It was speculated that the metastable defect coupling with the M3 level is only M4(2) out of two components of the M4 defect.
Theory of solutions in the energetic representation. I. Formulation
Authors: Matubayasi Nobuyuki , Nakahara Masaru
Published: July 2002

The energetic representation of the molecular configuration in a dilute solution is introduced to express the solvent distribution around the solute over a one-dimensional coordinate specifying the solute–solvent interaction energy. In this representation, the correspondence is shown to be one-to-one between the set of solute–solvent interaction potentials and the set of solvent distribution functions around the solute. On the basis of the one-to-one correspondence, the Percus–Yevick and hypernetted-chain integral equations are formulated over the energetic coordinate through the method of functional expansion. It is then found that the Percus–Yevick, hypernetted-chain, and superposition approximations in the energetic representation determine the solvent distribution functions correctly to first-order with respect to the solute–solvent interaction potential and to the solvent density. The expressions for the chemical potential of the solute are also presented in closed form under these approximations and are shown to be exact to second-order in the solute–solvent interaction potential and in the solvent density.