bluuebunny/crossref_metadata_2025 huggingface dataset:
Negative ion formation in the scattering of CF3+ from graphite
Authors: Gleeson M. A. , Kropholler M. , Kleyn A. W.
Published: July 2002

The scattering of CF3+ from highly oriented pyrolytic graphite results in the formation of CF3−. Comparison is made with CF3+ scattering from a metal and from an insulator surface, which do not yield negative molecular ions. The outcome of the charge transfer processes that occur as CF3+ approaches a surface is critically dependent on the electronic properties of that surface. The ability to produce a stable negative ion close to a surface may be an important factor in the etching efficiency of the CF3 molecule.
Comparison of the effect of boron and nitrogen incorporation on the nucleation behavior and electron-field-emission properties of chemical-vapor-deposited diamond films
Authors: Lin I-Nan , Perng Kuoguang , Lee Lien-Hsin , Shih Chuan-Feng , Liu Kuo-Shung , Evans Gariant A. , Steeds John W.
Published: July 2002

Effect of boron (or nitrogen) doping on the nucleation behavior of chemical-vapor-deposited diamond films was investigated. It is observed that inclusion of B(OCH3)3 species in gases markedly enhances, whereas addition of (NH3)2CO species pronouncedly retards the nucleation of diamonds. Raman spectroscopic analyses reveal that the probable mechanism is the formation of boron–carbon clusters due to boron incorporation. While the boron (or nitrogen) species incorporated insignificantly modify the microstructure, these dopants markedly alter the electron-field-emission behavior of the diamond films. The field-emission properties are optimized for films grown with B(OCH3)3=2 sccm [or (NH3)2CO=6 sccm]. The nitrogen-doped films exhibit significantly superior electron-field-emission capacity to the boron-doped films, even though the latter possess much lower electrical resistivity [viz. (Je)n=1020 μA/cm2, (ρ)n=76 mΩ cm and (Je)b=360 μA/cm2, (ρ)b=2.1 mΩ cm].
Dynamic mechanical response of polymer networks
Authors: Edwards S. F. , Takano H. , Terentjev E. M.
Published: July 2002

The dynamic-mechanical response of flexible polymer networks is studied in the framework of the tube model, in the limit of small affine deformations, using the approach based on Rayleighian dissipation function. The dynamic complex modulus G*(ω) is calculated from the analysis of a network strand relaxation to the new equilibrium conformation around the distorted primitive path. Chain equilibration is achieved via a sliding motion of polymer segments along the tube, eliminating the inhomogeneity of the polymer density caused by the deformation. The characteristic relaxation time of this motion τe separates the low-frequency limit of the complex modulus from the high-frequency one, where the main role is played by chain entanglements, analogous to the rubber plateau in melts. The dependence of storage and loss moduli, G′(ω) and G″(ω), on crosslink and entanglement densities gives an interpolation between polymer melts and crosslinked networks. We discuss the experimental implications of the rather short relaxation time and the slow square-root variation of the moduli and the loss factor tan δ(ω) at higher frequencies.
Sources of resonance-related errors in capacitance versus voltage measurement systems
Authors: Polishchuk Igor , Brown George , Huff Howard
Published: July 2002

A frequency dependence of the capacitance of metal–oxide–semiconductor devices is often observed in wafer-level probe station measurements for frequencies exceeding 100 kHz. It is well established, however, that the true capacitance value in the SiO2 devices biased into accumulation should remain frequency-independent well into the gigahertz range. Consequently, the apparent frequency dependence of the capacitance versus voltage characteristic may be the result of a resonance present in the measurement setup. We present a quantitative analysis, which can be used to identify the sources of error, characterize a measurement system, and improve the precision of the collected data.
Bilayer quantum transistor
Authors: Hu Can-Ming , Heitmann Detlef
Published: July 2002

The ballistic transport properties of a bilayer structure are studied under the Landau–Büttiker formalism. An oscillatory conductance as a function of the front gate is found. The effect arises from the tunneling, which causes a phase difference between the symmetric and antisymmetric eigenstates. The gate electrode controls the effective tunneling gap and, therefore, the interference. The mechanism may be used to design a bilayer analogue of the spin field-effect transistor without ferromagnetic electrodes.
Femtosecond laser assisted scanning tunneling microscopy
Authors: Gerstner V. , Knoll A. , Pfeiffer W. , Thon A. , Gerber G.
Published: July 2002

The excitation of the tunneling junction of a scanning tunneling microscope using ultrashort laser pulses combined with detection of a tunneling current component which depends nonlinearly on the laser intensity allows, in principle, to simultaneously obtain ultimate spatial and temporal resolution. To achieve this goal, a laser system that produces ultrashort laser pulses is combined with an ultrahigh vacuum scanning tunneling microscope. The basic technical considerations are discussed and it is shown that atomic resolution can be achieved under pulsed laser excitation of the tunneling junction. The pulsed illumination gives rise to several contributions to the measured total current. Experimental evidence for signal contributions due to thermal expansion, transient surface potentials and multiphoton photoemission are presented.
Structural and dielectric properties of epitaxial Ba1−xSrxTiO3/Bi4Ti3O12/ZrO2 heterostructures grown on silicon
Authors: Canedy C. L. , Aggarwal S. , Li Hao , Venkatesan T. , Ramesh R. , Van Keuls F. W. , Romanofsky R. R. , Miranda F. A.
Published: July 2002

We report on the dielectric properties of an epitaxial heterostructure comprised of Ba1−xSrxTiO3, Bi4Ti3O12, and (ZrO2)0.91(Y2O3)0.09 grown on silicon substrates for potential use in microwave devices. Careful x-ray analysis indicates crystallographic alignment of all layers and transmission electron microscopy and Auger analysis reveals high quality epitaxy with minimal interdiffusion. The viability of using such heterostructures in actual microwave devices was assessed by incorporating the films in a coupled microstripline phase shifter design. The phase shifter devices, operating in the Ku band, had losses of less than 4 dB with a maximum phase shift of nearly 40° at 40 V. We compare this performance with a (Ba, Sr)TIO3/MgO phase shifter. These results presented represent significant progress towards integrating ferroelectric films with conventional silicon technology.
Investigation of interface and bulk fatigue scenarios in sol-gel derived Pb(Zr0.5Ti0.5)O3 films by asymmetric field driving
Authors: Li Biao , Koch F. , Meng X. J. , Cheng J. G. , Chu J. H.
Published: July 2002

A method to distinguish the bulk and interface scenarios of fatigue in sol-gel derived Pb(Zr0.5Ti0.5)O3 (PZT) thin films on Pt and LaNiO3 (LNO) substrates is proposed based on the asymmetric electric field driving of capacitors. The “hard” and “soft” failures of polarization, which are typical for sputtering and sol-gel deposited PZT, respectively, are realized in sol-gel derived Pt/PZT/Pt capacitors by changing the asymmetricity of driving pulses, indicating that the interface plays an important role in the polarization breakdown. For Pt/PZT/LNO capacitors, it is shown that the bulk pinning of domains appears to be more evident than the interface pinning. A model based on the electromigration and entrapment of charged defects subjected to the asymmetric field is also given, and the plausible approximation of space charge field versus charge density is discussed.
Reversible photoquenching of luminescence in conjugated polymers
Authors: Frolov S. V.
Published: July 2002

Photoluminescence in thin films of poly(p-phenylene vinylene) derivatives is studied using a two-beam excitation technique. It is demonstrated that excited state absorption leads to the reversible quenching of steady-state luminescence. Measurements of ultrafast exciton dynamics show that this quenching is due to exciton depletion by photogenerated metastable quenching centers.
Nonlinear optical studies of 3-methyl-4-methoxy-4′-nitrostilbene single-crystal films
Authors: Tan Shida , Bhowmik Achintya K. , Sodah Sunil , Thakur Mrinal
Published: July 2002

We report the second-order optical measurement of single-crystal thin-films of 3-methyl-4-methoxy-4′-nitrostilbene by transverse second-harmonic generation (SHG) technique. Phase mismatch was negligible in the measurements since the thickness of the films was less than the coherence length. Detailed polarization selective measurements were used to determine the magnitudes of the tensor elements of second-order susceptibility. The values of d coefficients at the fundamental wavelength of 1064 nm were measured to be d33=195±10 pm/V and d24=75±5 pm/V. Both type I and type II phase-matched propagation directions were identified in the crystal and the type II phase matching direction lies parallel to the film. The results show that these films are promising for applications such as phase-matched SHG in waveguides.
Effect of electric field on one-quantum photodecay of oxygen-deficient centers in germanosilicate fibers
Authors: Rybaltovskii A. O. , Zavorotny Yu. S. , Chernov P. V. , Bagratashvili V. N. , Tsypina S. I. , Kazansky P. G.
Published: July 2002

The effect of a strong electric field (up to 106 V/cm) on one-quantum UV photoexcitation and photodecomposition of germanium oxygen deficient centers (GODC) in twin-hole germanosilicate fibers with internal wire electrodes is reported. The fiber has been irradiated with the UV light of a deuterium lamp and triplet photoluminescence of GODC has been used to monitor the kinetics of its photodecay. Applying such an electric field did not affect the spectral characteristics of GODC but increased the rate of their one-quantum photodecomposition, while direct photoionization and charge separation did not take place. We have also shown that this effect is caused by the suppression of secondary photoinduced recombination processes of intermediates, rather than by acceleration of primary photodecomposition of GODC.
Spin-contamination of coupled-cluster wave functions
Authors: Krylov Anna I.
Published: July 2002

The propensity of approximate solutions of the electronic Schrödinger equation to break spin-symmetry is directly related to the quality of the approximate wave function, and thus can be used as a diagnostic tool. The quasi-variational nature of the (valence) optimized orbitals coupled-cluster doubles methods, (V)OO-CCD, enables a discussion of the stability of coupled-cluster wave functions in terms of both spin-contamination and a corresponding energy lowering relative to the pure spin solutions. The spin-contamination of (V)OO-CCD models has been studied for bond-breaking processes and diradicals. The main findings are: (i) The OO-CCD method is stable for a relatively large range of nuclear distortions and is capable of eliminating even very large spin-contamination of the unrestricted Hartree–Fock solution given that the molecular electronic configuration remains essentially single-reference. When a spin-contaminated solution arises, the energy splitting rapidly becomes large and 〈Ŝ2〉 approaches the Hartree–Fock value; (ii) The VOO-CCD method, which is designed to approximate a multi-reference model, remains stable over broader ranges; however, for pure diradicals it becomes unstable. In these cases, spin-contamination is also very large, but the energy lowering for the spin-unrestricted solutions is negligible; (iii) Higher order corrections described by perturbation theory lead to smaller energy splittings between restricted and unrestricted (V)OO-CCD energies. However, in case of spin-contaminated (V)OO-CCD solutions, these corrections may lead to unphysical shapes of the potential energy surfaces. Thus, in order to quantitatively characterize the quality of the wave functions, both spin-contamination and energy lowering due to the breaking of spin-symmetry must be considered.
Influence of the heterojunction on the field emission from tetrahedral amorphous carbon on Si
Authors: Rupesinghe N. L. , Chhowalla M. , Amaratunga G. A. J. , Weightman P. , Martin D. , Unsworth P. , Murray J.
Published: July 2002

In order to study the influence of the back barrier on the electron emission properties of tetrahedral amorphous carbon (ta-C), we have deposited identical films on p and n+ Si. The valence and conduction band offsets were measured for ta-C on p and n+ Si using in situ x-ray photoelectron spectroscopy and optical spectroscopy. From the band measurements it is shown that there is a substantial back barrier to emission. We show that for films having very similar properties, the electron emission can be influenced by the ta-C/Si heterojunction.
Short-cavity, edge-emitting lasers with high-reflectance, dielectric mirrors
Authors: Fitz J. L. , Horst S. C. , Hinkel D. S. , Turk H.
Published: July 2002

The fabrication and characterization of short-cavity, edge-emitting lasers operating at 861 nm is reported. These lasers were fabricated with high-reflectance, dielectric mirrors formed by plasma-enhanced, chemical-vapor deposition. Lasers with cavity lengths of 64 and 32 μm had continuous-wave, room-temperature threshold currents of 4.0 and 2.9 mA, respectively. To determine the reflectance of the deposited dielectric mirrors, a simple relationship was derived that used the change in the laser threshold current density resulting from mirror deposition.
Chaos in an additive-pulse mode-locked laser
Authors: Mozdy E. J. , Pollock C. R.
Published: July 2002

We present and analyze an experimental bifurcation diagram for the additive-pulse modelocked (APM) laser, reconfirming the existence of both quasiperiodicity and period-doubling bifurcations into chaos for this system. Large fiber coupling is required to observe period doublings into chaos, while quasiperiodicity occurs for small fiber coupling. The experimental chaotic time series are reconstructed in phase space using the method of delayed coordinates, then graphically compared with the expected orbits from APM simulations. This experimental bifurcation diagram for the APM laser demonstrates that the main contribution to APM instability is in fact deterministic.
Structure characteristics of methane-doped solid normal hydrogen
Authors: Galtsov N. N. , Prokhvatilov A. I. , Strzhemechny M. A.
Published: July 2002

Structure studies of the quantum crystal of solid normal hydrogen doped with methane and deuteromethane are carried out by powder x-ray diffraction within the temperature range 5 to 12 K. The concentration of the dopants in the gas mixtures used for sample preparation by quench condensing is varied from 0.5% to 5%. It is established that the equilibrium solubility of both methanes is substantially below the lower fraction indicated. The abnormally high apparent extra volumes per impurity may be ascribed to van der Waals complexes formed around isolated dopant particles.
Perturbation theory for surface-profile imaging with a capacitive probe
Authors: Garcı́a-Valenzuela A. , Bruce N. C. , Kouznetsov D.
Published: July 2002

We derive a perturbative series solution to the capacitance between two parallel electrodes with irregular profiles. The coefficients in the series are calculated using fast Fourier transform algorithms resulting in a very fast method. The applicability of the perturbative series solution is extended by introducing a spectral window function which can make the series converge in cases where the standard series does not converge. We show that the filtered perturbative solution is applicable to surface profiles with surprisingly large features. However, limitations on its applicability to surfaces with high spatial frequencies remain. Perturbation theory could be a powerful tool for simulating surface-profile images obtained by scanning a capacitive probe.
All selective metalorganic vapor phase epitaxy grown buried-heterostructure laser diodes with selectively oxidized AlInAs current-confinement layer
Authors: Ko Hyun-Chul , Nakamura Takahiro , Koui Tomoaki , Suzuki Naofumi , Shiba Kazuhiro , Kobayashi Kenichi
Published: July 2002

A laser diode structure, consisting of a 1.3 μm all selective metalorganic vapor phase epitaxy grown buried-heterostructure (ASM-BH) laser diode with a selectively oxidized-AlInAs current-confinement layer, is proposed and demonstrated. As the lateral oxidization is automatically stopped near the active layer by self-aligned AlInAs, a narrow current aperture with excellent controllability is realized. The fabricated laser diode shows low continuous-wave threshold current of 2.5 and 7.5 mA at 25 and 85 °C, respectively, for 250-μm-long devices with HR(75%)/HR(95%) coatings. The maximum operating temperature is higher than 145 °C.
Probability density functions of product concentration in a turbulent reacting coflow jet of NH3 and HCl
Authors: Jenkins T. P. , Kennedy I. M.
Published: July 2002

The distribution of product in a turbulent reacting coflow jet was investigated in which dilute HCl issued into a coflowing stream of dilute NH3. Mass concentrations of the product, NH4Cl aerosol, were inferred from Mie scattering measurements. Probability density functions (PDFs) of the product exhibited a preferred concentration, or mode, that remained constant across the jet while the flow was in an early stage of reaction, indicating that production occurred in large scale structures that extended across the jet. In contrast, in regions where the reaction had progressed to a significant degree, the mode marched to lower concentrations with radial distance from the centerline, such that the PDF resembled that of a passive scalar. In these regions, radial distributions of the mean and rms fluctuations of product concentration were similar to those of mixture fraction, measured using laser induced fluorescence of acetone. In regions where reaction was still in the early stages, product distributions differed markedly from mixture fraction distributions. The PDFs suggested that product was formed at the edges of large structures of rather uniform reactant concentration. A two-dimensional image of Mie-scattered laser light from the aerosol product supports this interpretation by revealing long, narrow regions of product.
Photoirradiation effect on polymer light-emitting device: Separation between recombination zone and photo-oxidized defects
Authors: Tada Kazuya , Onoda Mitsuyoshi
Published: July 2002

The photoirradiation effect on the characteristics of a polymer light-emitting diode with a semitransparent metal cathode is studied. The electroluminescence intensity as well as the photoluminescence quantum efficiency decreases due to photoirradiation in air, while the external quantum efficiency of electroluminescence does not change. This suggests that not the formation of quenching defects but the degraded carrier transport/injection nature of the polymer mainly affects the emission property of the device and that the recombination zone is separated from the photo-oxidized defects. Two models of the separation, the layer model and the scattering model, are proposed. The validity of the two models depends on the penetration depth of oxygen in the polymer.
Generation of atmospheric pressure plasma with a dual-chamber discharge
Authors: Park H. D. , Dhali S. K.
Published: July 2002

We discuss the experimental results of a method for generation of nonthermal plasma at atmospheric pressure. The device uses two chambers separated by a dielectric. A low-pressure chamber produces UV emission, which is coupled to the atmospheric pressure discharge. The electrical and optical characteristics of this device are discussed. Results show that the dual-chamber discharge is more uniform compared to conventional dielectric-barrier discharges.
Dominant role of surface states in photoexcited carrier dynamics in CdSe nanocrystalline films prepared by chemical deposition
Authors: Malý P. , Kudrna J. , Trojánek F. , Mikeš D. , Němec P. , Maciel A. C. , Ryan J. F.
Published: July 2002

We report on ultrafast carrier dynamics in nanocrystalline CdSe thin films prepared by chemical solution deposition. The photoluminescence and transient absorption dynamics have spectrally dependent decay faster at shorter wavelengths. The spectral behavior of the decay rates in samples with different nanocrystal sizes and the sensitivity of the dynamics on the surface modification indicate the dominant role of the surface states in the photoluminescence.
Characterization of excimer-laser-annealed polycrystalline silicon films grown by ultrahigh-vacuum chemical vapor deposition
Authors: Chen Ying-Chia , Sermon Wu YewChung , Tung I-Chung , Chao Chi-Wei , Feng Ming-Shiann , Chen Huang-Chung
Published: July 2002

Polycrystalline silicon (poly-Si) films grown by ultrahigh-vacuum chemical vapor deposition (UHVCVD) system and then annealed by excimer laser at room temperature have been investigated for the applications in polycrystalline silicon thin-film transistors (poly-Si TFTs). The results showed that the grain size of the laser-annealed poly-Si film decreased with laser energy density when a lower laser energy density below 157.7 mJ/cm2 was used. At about the threshold laser energy density (∼134.5 mJ/cm2), the finest grain structure could be obtained due to the partial melting in the top layer of the film. When the energy density of the excimer laser was larger than the threshold energy density, the large grain growth was initiated. The largest grain structure could be obtained at ∼184 mJ/cm2, while its surface roughness was better than that of the nonannealed UHVCVD poly-Si films. The surface roughening was suggested to arise from the specific melt-regrowth process but not the rapid release of hydrogen or capillary wave mechanism derived from laser-annealed amorphous silicon. By use of the laser-annealed UHVCVD poly-Si films as the active layer, the fabricated poly-Si TFT exhibited a field-effect mobility of 138 cm2/V s, a subthreshold swing of 0.8 V/dec, a threshold voltage of 3.5 V, and an on/off current ratio of ∼106.
The effect of As4 pressure on material qualities of AlGaAs/GaAs heterostructures grown on (111)B GaAs substrates
Authors: Yeo W. , Dimitrov R. , Schaff W. J. , Eastman L. F.
Published: July 2002

High-quality AlGaAs/GaAs heterostructures have been grown on on-axis (111)B GaAs substrates using molecular-beam epitaxy to study the effect of As4 pressure on the material qualities. It was found that the electrical and structural properties were strongly dependent on the As4 pressure. The quality of the heterostructure was investigated by Hall effect, atomic force microscopy, and high-resolution x-ray diffraction measurements. The electron Hall mobility dropped from 7200 and 90 000 cm2/V s to 930 and 1340 cm2/V s at room temperature and 77 K, respectively, as As4 pressure was increased from 4.9×10−6 to 1.1×10−7 Torr. The same trend was observed at the root-mean-square roughness of the AlGaAs/GaAs heterostructures measured by atomic force microscopy, and the intensity and full width at half-maximum of the x-ray diffraction curve.
Magnetic-history-dependent nanostructural and resistivity changes in Pr0.5Ca0.5Mn0.98Cr0.02O3
Authors: Hervieu M. , Mahendiran R. , Maignan A. , Martin C. , Raveau B.
Published: July 2002

We show that nanostructure and resistivity of Pr0.5Ca0.5Mn0.98Cr0.02O3 are sensitive to whether the sample is zero-field-cooled (ZFC) or field-cooled (FC) either in the “self-magnetic field (H≈2 T)” of the electron microscope or under the external magnetic field of 2 T. FC resistivity at H=2 T is lower than ZFC values below 140 K. The average value of the charge-orbital modulation vector (q=0.44) of the FC crystallites is lower than that of the ZFC crystallites (q=0.48) and the FC crystallites exhibit numerous defects like discommensuration, dislocations, and regions with loss of superstructures compared to the ZFC crystallites.
YBa
2
Cu
3
O
7
Josephson junctions on LaAlO3 bicrystals for terahertz-frequency applications
Authors: Kaestner A. , Volk M. , Ludwig F. , Schilling M. , Menzel J.
Published: July 2002

We prepared Josephson junctions made of YBa2Cu3O7 on symmetric 24° LaAlO3 bicrystal substrates. For better coupling of high-frequency radiation, we structured the contacts with logarithmic-periodic antennas made of YBa2Cu3O7. Characterization was done by measuring the voltage–current characteristics to determine the critical current IC, the normal-state resistance RN, the noise parameter Γ, and the Stewart–McCumber parameter βC as predicted by the resistively and capactively shunted junction model (RCSJ). The junctions showed excellent RCSJ-like behavior with ICRN products of up to 2.0 mV at 10 K and 220 μV at 77 K. To demonstrate the possibility to detect high-frequency radiation up to the THz regime, we used the Josephson junction as a detector for radiation from a far-infrared laser and we reconstructed the spectra by Hilbert spectroscopy.
Energy loss spectra of group III nitrides
Authors: Gavrilenko V. I. , Wu R. Q.
Published: July 2002

Electron energy loss spectra (EELS) of cubic and hexagonal BN, AlN, GaN, and InN have been calculated by using the first principles full potential linearized augmented plane wave method. Accurate calculations of linear optical functions are performed in a photon energy range up to 60 eV. The electron excitation energies related to the bulk plasmons are obtained for both reflection and transmission geometries. The predicted EELS data are discussed in comparison with available experimental results.
Hydrodynamic damping of tip oscillation in pulsed-force atomic force microscopy
Authors: Chen X. , Davies M. C. , Roberts C. J. , Tendler S. J. B. , Williams P. M.
Published: July 2002

Although proven a powerful technique for mapping adhesion and surface mechanical properties at high lateral resolution, pulsed-force (PF) atomic force microscopy (AFM) is problematic in liquid, due to heavy hydrodynamic damping of cantilever vibration. We present computer simulations using the simple harmonic oscillation model to explore the changes of deflection signal profile that occur from air to liquid environment. In agreement with experimental results, we find that oscillation phase lag plays a key role in the signal profile. When imaging in liquid, the deflection caused by liquid oscillation may exceed that caused by tip-sample contact repulsion and adhesion, which brings particular consideration for PF–AFM imaging in liquid.
Mode transition between growth and decomposition of oxides on Si(001): Kinetically determined critical coverage for oxidation
Authors: Suemitsu Maki , Enta Yoshiharu , Takegawa Youichi , Miyamoto Nobuo
Published: July 2002

Effects of preoxidation on the reaction kinetics of oxygen molecules at Si(001) surface have been investigated by real-time ultraviolet photoelectron spectroscopy. A mode transition from decomposition to growth of surface oxides was found to exist at a certain initial oxide coverage, which is kinetically, not energetically, determined. By considering a change of balance between Si adatom and oxygen-monomer fluxes at the perimeter of oxide clusters, this mode transition is quantitatively described as a bifurcation of an autocatalytic-reaction rate equation.
Mode behavior of photopumped AlGaAs–GaAs lasers confined by oxide-semiconductor distributed Bragg reflectors
Authors: Kellogg D. A. , Holonyak N. , Dupuis R. D.
Published: July 2002

Data are presented on photopumped AlGaAs–GaAs quantum-well (QW) heterostructure lasers with oxide-semiconductor distributed Bragg reflector (DBR) vertical confinement (and Fabry–Perot cleaved edges) that reduces the mode volume and influences the edge emission (EE) spectra. For narrow samples (≲20 μm), with the vertical DBRs “tuned” below the QW recombination transitions (E1 lowest, E2 higher), single mode EE operation occurs on E2 at a wavelength ∼77 meV above the lowest QW energy. For wide samples with an oxide-semiconductor vertical DBR located in a narrow band at the sample cleave edges, the DBR region acts as a spacial and spectral filter, thus increasing the possibility of single-mode EE operation.
Secondary electron emission from diamond: Physical modeling and application to scanning electron microscopy
Authors: Ascarelli P. , Cappelli E. , Pinzari F. , Rossi M. C. , Salvatori S. , Merli P. G. , Migliori A.
Published: July 2002

Secondary electron emission from diamond films is studied as a function of the primary electron beam energy and bulk material properties. A formulation of a simple model of the secondary electron emission coefficient, as a function of the primary electron beam energy, has been found to be helpful in defining physical criteria able to guide the optimization of the diamond film electron emission performance. The secondary electron mean escape depth deduced from the model is indeed related to the density of defects in the material and represents the main factor in determining the low energy secondary electron yield. These results are supported by Raman spectroscopy measurements, indicating a lower graphitic content and a higher crystalline quality of the diamond phase in films showing better secondary electron and photoemission yields. We demonstrate that a diamond film, acting as a stable and proportional electron multiplier, can be used as a converter of backscattered electrons into secondary electrons in scanning electron microscopy. It will be shown that the use of a diamond film converter is suitable to improve the signal to noise ratio of images providing an enhanced compositional contrast.
Design and evaluation of a low-cost, high-speed signal amplifier
Authors: Christian Noah P. , Alexander Andrew W. , Reilly James P.
Published: July 2002

A dc to 500 MHz amplifier design is described that has excellent performance. Surface mount technologies with present generation high-speed current-feedback operational amplifiers enabled the development of a low-cost circuit. The implemented design is robust and exhibits low noise.
Strong interface-induced changes on the numerical calculated Raman scattering in Si/3C–SiC superlattices
Authors: Bezerra E. F. , Freire V. N. , Souza Filho A. G. , Mendes Filho J. , Lemos V. , Ikoma Y. , Watanabe F. , Motooka T.
Published: July 2002

Raman spectra of (3C–SiC)8−δ/(3C–SiC0.5Si0.5)δ/(Si)8−δ/(3C–SiC0.5Si0.5)δ superlattices with interfacial transition regions of thickness δ varying from one to three monolayers are calculated. It is shown that severe frequency shifts (up to −86 cm−1) and the flattening of the folded optical phonons dispersion curves are due to the interfacial regions, strongly affecting the Raman spectrum in consequence. With increasing interface thickness, the Raman peaks are enhanced in the middle frequency range. These effects are mainly attributed to localization of atomic displacements at the Si/3C–SiC or the 3C–SiC/Si interfacial transition regions.
Effects of atomistic defects on coherent electron transmission in Si nanowires: Full band calculations
Authors: Ko Young-Jo , Shin Mincheol , Lee Seongjae , Park Kyoung Wan
Published: July 2002

The effects of atomistic imperfections on coherent electron transmission in Si[100] quantum wires a few nanometers wide are investigated using a tight-binding Green function approach. We find a significant suppression in the electron transmission by atomistic imperfections in these extremely narrow wires. Multiple conductance peaks or oscillations can be easily developed by the presence of only several vacancy defects, which can lead to a finite zero-conductance region around the subband edge. Several substitutional defects and surface dangling bonds generally result in decreased, oscillatory conductances with more significant effects found in narrower wires.
Quasilinear electro-optical response in a polymer-dispersed nematic liquid crystal
Authors: Nicoletta F. P. , Cupelli D. , De Filpo G. , Macchione M. , Chidichimo G.
Published: July 2002

Polymer-dispersed nematic liquid crystals are known to exhibit a quadratic electro-optic response. For practical applications, a linear dependence on the applied electric field is often required. In this letter, polymer-dispersed nematic liquid crystal films with a built-in dc electric field are shown to be good electro-optical devices with a linear response. The built-in electric field is obtained by energizing films with an external dc field. The electro-optical response dependence on the strength and frequency of the driving field is discussed.
Dynamics of optical nonlinearity of Ge nanocrystals in a silica matrix
Authors: Jie Y. X. , Xiong Y. N. , Wee A. T. S. , Huan C. H. A. , Ji W.
Published: July 2002

The optical nonlinearity and excited carrier lifetime in Ge nanocrystals (nc-Ge) embedded in a silica matrix have been investigated by means of single beam z scan and pump-probe techniques with laser pulse duration of 35 ps and 532 nm wavelength. The nc-Ge samples were prepared using magnetron cosputtering and postgrowth annealing at 800 °C. The nonlinear absorption coefficient α2 and refractive index n2 were found to range between 190 and 760 cm/GW, and 0.0026 and 0.0082 cm2/GW, respectively, and be proportional to the Ge concentration in the film. The confined excited carriers were found to depopulate with a lifetime of ∼70 ps. The nonlinearity in Ge nanocrystals is deduced to originate mainly from excited carrier absorption, with two-photon absorption providing a small contribution.
Solid-source molecular-beam epitaxy for monolithic integration of laser emitters and photodetectors on GaAs chips
Authors: Postigo P. A. , Fonstad C. G. , Choi S. , Goodhue W. D.
Published: July 2002

A solid-source molecular beam epitaxy has been used to grow Al-free InGaP/GaAs/InGaAs in-plane side emitting laser (IPSELs) devices on foundry available GaAs integrated circuits (IC) chips with integrated metal–semiconductor–metal photodetectors. The GaAs IC chips were cleaned at low temperature (470 °C) by monoatomic hydrogen prior to epitaxy. Br2 reactive ion etching was used after growth to make laser facets and parabolic mirrors for vertical emission. Using these techniques, we obtained laser emission from the integrated IPSEL devices, suggesting that these devices may be an alternative approach to that offered by vertical cavity surface emitting lasers in the fabrication of optoelectronic integrated devices.
Sulfur passivation of GaAs metal-semiconductor field-effect transistor
Authors: Dong Y. , Ding X. M. , Hou X. Y. , Li Y. , Li X. B.
Published: July 2002

A passivation technique consisting of a (NH4)2S dip followed by GaS deposition has been applied to a GaAs microwave-power metal–semiconductor field-effect transistor (MESFET). The breakdown characteristic of the MESFET is greatly improved upon the (NH4)2S treatment, and a stable passivation effect can be achieved by GaS film deposition. It is found that the FET current–voltage characteristics are closely related to variations in the pinning position of the GaAs surface Fermi level. With the surface passivated, a depletion layer can be properly formed and protected, which is of benefit to the control of the device parameters.
Characterization of photoresponse, photovoltage, and photonic gate response in a pseudomorphic p-channel modulation-doped field-effect transistor
Authors: Kim D. M.
Published: July 2002

Optoelectronic responses in a pseudomorphic p-channel modulation-doped field-effect transistor (p-MODFET) on GaAs have been characterized. Semiempirical models are provided and verified with experimental data. Significantly suppressed drain photoresponse and reduced gate photoresponse have been observed in p-MODFET due to the low hole mobility and high Schottky barrier on the InGaP layer. The drain photoresponse is predominantly modulated by the photoconductive effect of excess majority carriers while the photovoltage and photonic gate response are governed by the photovoltaic effect caused by excess minority carriers and energy barriers between the channel and dopant layers.
Photon avalanche in Cs2ZrCl6: Os4+
Authors: Wermuth Markus , Güdel Hans U.
Published: July 2002

The visible (VIS) upconversion luminescence of Cs2ZrCl6: x% Os4+ (x=0.1,1) upon excitation in the near infrared (NIR) is presented and analyzed. The crystals of Cs2ZrCl6: x% Os4+ (x=0.1,1) were synthesized by the Bridgman technique. VIS luminescence around 17 000 cm−1 is induced by NIR excitation between 10 500 and 14 500 cm−1. The three excited states Γ4 (3T1g), Γ5 (1T2g), and Γ1 (1A1g) with 15 K lifetimes of 20 ms, 8 μs, and 4 μs, respectively, in Cs2ZrCl6: 1% Os4+ participate in the upconversion. Excitation at the Γ4 (3T1g)→Γ1 (1A1g) energy shows the typical fingerprints of a photon avalanche. The dynamics of this excitation are calculated with a rate-equation model, and the observed concentration and temperature dependence of the processes are analyzed. The upconversion behavior of Cs2ZrCl6: x% Os4+ (x=0.1,1) is compared with the bromide analog Cs2ZrBr6: 0.2% Os4+.
Compressive strength of synthetic diamond grits containing metallic nanoparticles
Authors: Watson J. H. P. , Li Z. , Hyde A. M.
Published: July 2002

Synthetic diamonds made by high-temperature and high-pressure synthesis using an Fe–Co solvent/catalyst leave nanoparticles of the solvent/catalyst within the diamond matrix. These nanoparticles strongly affect the magnetic properties of the diamond. The magnetization versus field was compatible with the response of superparamagnetic particles. The mean size and separation of the inclusions were calculated. The inclusion size was found to be constant to within 10%, whereas the saturation magnetization varied by a factor of 100. A transmission electron microscope image of a single inclusion from a Fe–Co/diamond grit showed a dark, iron-rich core surrounded by a halo of material of intermediate contrast to the lighter diamond matrix. The size of the core is consistent with the magnetization measurements while the halo is of similar size to that determined by small-angle neutron scattering. The compressive strength increased linearly with the inclusion separation.
Phase separation curve of dilute He3–4He hcp solid solutions
Authors: Gan’shin A. N. , Grigor’ev V. N. , Maı̆danov V. A. , Omelaenko N. F. , Penzev A. A. , Rudavskiı̆ É. Ya. , Rybalko A. S.
Published: July 2002

A method is proposed for constructing the separation phase diagram of He3–4He solid solutions on the basis of precision measurement of the pressure jump due to phase separation at constant volume. The technique is implemented on high-quantity samples of the solid solutions, making it possible to obtain reliable and reproducible experimental data with no appreciable manifestation of hysteresis effects. The line of phase separation constructed from the experimental data is compared with the results of various theoretical approaches describing phase separation in solid solutions of helium isotopic mixtures. It is found that good agreement with experiment is observed only for the Edwards–Balibar model, which is an extension of the theory of regular solutions to take into account the differences of the crystal structures (hcp and bcc) of the phases existing in the system.
Coherent optical phonon generation by the electric current in quantum wells
Authors: Komirenko S. M. , Kim K. W. , Kochelap V. A. , Fedorov I. , Stroscio M. A.
Published: July 2002

This letter addresses the effect of generation of confined LO phonons by drifting electrons in quantum wells. We have derived a general formula for the phonon increment as a function of phonon wave vector, electron drift velocity, and structure parameters. Numerical estimates of the phonon increment and the phonon lifetimes have shown that AlAs/GaAs/AlAs and GaSb/InSb/GaSb quantum well structures can demonstrate the effect of coherent LO phonon generation by the electric current.
Terahertz-frequency electronic coupling in vertically coupled quantum dots
Authors: Boucaud P. , Williams J. B. , Gill K. S. , Sherwin M. S. , Schoenfeld W. V. , Petroff P. M.
Published: July 2002

We have studied terahertz absorption of samples containing two layers of self-aligned, self-assembled InAs quantum dots separated by a thin GaAs barrier. The vertically coupled dots were charged with electrons by applying a voltage bias between a metal gate and a doped layer beneath the dots. For a positive gate bias corresponding to flatband conditions, an absorption peak was observed near 10 meV (2.4 THz). The absorption is attributed to the inhomogeneously broadened transition between the quantum mechanically split levels (bonding and antibonding states) in the vertically coupled quantum dots.
Dual mode cross-slotted filters realized with superconducting films
Authors: Cassinese A. , Palomba F. , Pica G. , Andreone A. , Panariello G.
Published: July 2002

A planar superconducting single stage dual mode filter has been investigated and tested. The basic element of the device is a square patch resonator diagonally crossed by unequal slots to provide both a size reduction and a simple way to couple the two degenerate modes. The effect of further cuts, realized on the conductor surface, is also investigated with respect to the power handling capability and miniaturization requirements. Two different filter topologies operating in C band with 1% fractional bandwidth have been designed and fabricated using Nb and YBa2Cu3O7 films grown on 10×10×0.5 mm3 LaAlO3 (100) substrates. Device response is studied performing power and intermodulation measurements. Third order products start to appear above the noise floor (−120 dBm) at input power Pin≈−20 dBm, whereas thermomagnetic breakdown is observed above 20 dBm.
Second harmonic generation in microcrystallite films of ultrasmall Si nanoparticles
Authors: Nayfeh M. H. , Akcakir O. , Belomoin G. , Barry N. , Therrien J. , Gratton E.
Published: July 2002

We dispersed crystalline Si into a colloid of ultrasmall nano particles (∼1 nm), and reconstituted it into microcrystallites films on device-quality Si. The film is excited by near-infrared femtosecond two-photon process in the range 765–835 nm, with incident average power in the range 15–70 mW, focused to ∼1 μm. We have observed strong radiation at half the wavelength of the incident beam. The results are analyzed in terms of second-harmonic generation, a process that is not allowed in silicon due to the centrosymmetry. Ionic vibration of or/and excitonic self-trapping on novel radiative Si–Si dimer phase, found only in ultrasmall nanoparticles, are suggested as a basic mechanism for inducing anharmonicity that breaks the centrosymmetry.
Direct evidence for 8-interstitial-controlled nucleation of extended defects in c-Si
Authors: Schiettekatte F. , Roorda S. , Poirier R. , Fortin M. O. , Chazal S. , Héliou R.
Published: July 2002

The areal density of extended defects in P-implanted and annealed Si is observed to increase with ion dose to the power 8. A simple model based on Poisson statistics applied to point defects created during ion implantation shows that such a dependence corresponds to enhanced stability of interstitial clusters consisting of at least eight interstitial atoms, and it implies an interstitial “clustering” radius of 0.8 nm. The direct observation of “n=8” confirms the curious behavior observed earlier in transient-enhanced diffusion of B in Si, and provides a quantitative explanation of the threshold dose for the formation of extended defects in ion-implanted Si.
Fast accurate wavelength switching of an erbium-doped fiber laser with a Fabry–Perot semiconductor filter and fiber Bragg gratings
Authors: Li Shenping , Chiang K. S. , Gambling W. A.
Published: July 2002

We propose a simple passive method for achieving fast accurate wavelength switching in an erbium-doped fiber laser. The method relies on a laser cavity formed with a Fabry–Perot semiconductor filter (FPSF) and narrow-band fiber Bragg gratings (FBGs), where the FBGs set the wavelength references and the FPSF can be tuned by changing its injection current to select a particular wavelength from those set by the FBGs. Because of the nonlinearity in the FPSF, optical negative feedback can be built up in the laser to suppress relaxation oscillations caused by wavelength switching. With this method, a wavelength switching time of less than 25 μs has been achieved experimentally.
Experimental measurement of microwave-induced electron spin-flip time
Authors: Hu C. Y. , Tan P. H. , Ossau W. , Wojtowicz T. , Karczewski G. , Kossut J.
Published: July 2002

The electron spin resonance (ESR) is optically detected by monitoring the microwave-induced changes in the circular polarization of the neutral exciton (X) and the negatively charged exciton (X−) emission in CdTe quantum wells with low density of excess electrons. We find that the circular polarization of the X and X− emission is a mapping of the spin polarization of excess electrons. By analyzing the ESR-induced decrease in the circular polarization degree of the X emission, we deduce the microwave-induced electron spin-flip time >0.1 μs, which is much longer than the recombination time of X and X−. This demonstrates that the optically detected ESR in type I quantum wells with low density of excess electrons does not obey the prerequisite for the conventional optically detected magnetic resonance.
Monolithic integration of a quantum-well infrared photodetector array with a read-out circuit
Authors: Mandelik D. , Schniederman M. , Umansky V. , Bar-Joseph I.
Published: July 2002

We demonstrate the feasibility of monolithic integration of a quantum-well infrared detector and a read-out circuit on the same GaAs/AlGaAs crystal. Charge storage capability of 2×107 electrons in a 50×50 μm2 pixel is obtained. The operation of a 5×5 test array is reported, performing all the basic functions of a practical focal plane array.
Acceptor activation of Mg-doped GaN by microwave treatment
Authors: Chang Shoou-Jinn , Su Yan-Kuin , Tsai Tzong-Liang , Chang Chung-Ying , Chiang Chih-Lih , Chang Chih-Sung , Chen Tzer-Peng , Huang Kuo-Hsin
Published: July 2002

A microwave treatment method different from thermal annealing and low-energy electron beam irradiation was proposed to activate Mg dopants in p-type GaN epitaxial layer. From photoluminescence spectra and Hall effect measurements, it was shown that microwave treatment is a very effective way to activate the acceptors in Mg-doped p-type GaN layer. The activation of Mg dopant in p-type GaN layer may be explained as the breaking of magnesium–hydrogen bonding due to the microwave energy absorption.
Magnetic susceptibility and microstructure of hydrogenated amorphous silicon measured by nuclear magnetic resonance on a single thin film
Authors: Baugh Jonathan , Han Daxing , Kleinhammes Alfred , Wu Yue
Published: July 2002

A nuclear magnetic resonance technique for precisely measuring the bulk magnetic susceptibility of micron-thick hydrogenated amorphous silicon (a-Si:H) film is introduced. The large disorder-induced diamagnetic enhancement exhibited by a-Si:H is shown to provide a sensitive bulk measurement for detecting variations in structural order in a-Si:H films. Furthermore, this approach is shown to be effective in revealing the details of microstructure in a-Si:H, including the presence of microstructural anisotropy.
Apertured quantum dot microcavity light emitting diodes
Authors: Zou Z. , Chen H. , Deppe D. G.
Published: July 2002

Experimental data are presented showing efficiency enhancements due to size reductions of apertured quantum dot microcavity light emitting diodes. The aperture sizes are photolithographically controlled to generate four different aperture sizes that are nominally 4, 3, 2, and 1 μm diameters. Even at room temperature the efficiencies increase with decrease in aperture size.