bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of highly uniform silicon nanoparticles in high density silane plasmas
Authors: Shen Z. , Kim T. , Kortshagen U. , McMurry P. H. , Campbell S. A.
Published: August 2003

We have investigated silicon particles formed in an inductively coupled plasma (ICP) system and have measured their structural properties by electron microscopy. ICP silane plasmas are shown to be an interesting and versatile source of nanoparticles. Three regimes are mapped out: a regime of no observable particle growth at the lowest pressures, a regime of polydisperse and agglomerated particles at the highest pressures, and a regime yielding highly monodisperse particles at intermediate pressures. Our results suggest that an ICP allows this unique production of monodisperse nanoparticles at high concentrations. The particle size can be controlled from 15 to 200 nm and is determined by the plasma on time. For short plasma times, particle growth appears to be due primarily to coagulation, while in the second phase particle growth is attributed primarily to surface deposition. For H2 dilutions less than 92%, the growth rate is almost independent of H2 partial pressure. Transmission electron micrographs, however, indicate that the addition of hydrogen decreases the particle density. At higher dilution ratios, polycrystalline particles are obtained. Under all other conditions the particles are amorphous.
Effects of surface nitridation during nitrogen plasma ignition on optical quality of GaInAsN grown by solid source molecular beam epitaxy
Authors: Wang S. Z. , Yoon S. F. , Loke W. K.
Published: August 2003

This article reports the reflection high-energy electron diffraction (RHEED) results during the growth of GaInAsN by solid source molecular beam epitaxy using a plasma-assisted radio frequency nitrogen source. The results show that at sufficiently low nitrogen composition, the surface RHEED reconstruction is the same as that of GaAs. However, increase in nitrogen composition causes transformation of the RHEED pattern to (3×1) or (3×3) reconstruction, depending on the coverage of nitrogen radicals on the growth surface. This fundamental characteristic of the nitrogen-stabilized surface is accompanied by surface nitridation, which results in the formation of nonradiative recombination centers, hence deteriorating the GaInAsN quality. An overpressure of arsenic flux could, to some extent, minimize surface nitridation during nitrogen plasma ignition, and therefore provide an important means for quality improvement in GaInAsN. Keeping the GaInAsN quantum well away from the nitridated surface by employing a GaAs screen layer has been shown to result in improvement in the quantum well optical quality.
Annealing effects on the crystal structure of GaInNAs quantum wells with large In and N content grown by molecular beam epitaxy
Authors: Hierro A. , Ulloa J.-M. , Chauveau J.-M. , Trampert A. , Pinault M.-A. , Tournié E. , Guzmán A. , Sánchez-Rojas J. L. , Calleja E.
Published: August 2003

The impact of rapid thermal annealing on the optical emission of GaInNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy with high In and N content is shown to be highly dependent on the crystal structure of the QWs, as determined by transmission electron microscopy. Due to the presence of higher concentrations of nonradiative recombination centers, the annealing temperature required to obtain maximum photoluminescence emission is higher for the QW with strong structural modulation of the upper interface [at the onset of three-dimensional (3D) growth], intermediate for the two-dimensional (2D) grown QW with compositional fluctuations, and lower for the homogeneous 2D grown QW. Moreover, the transition from homogeneous 2D growth, to 2D growth with compositional fluctuations, and finally 3D growth, leads to progressively deeper carrier localization states below the conduction-band edge. Increasing annealing temperatures gradually shifts the localization states closer to the conduction-band edge, predominantly when compositional fluctuations are present. These results suggest a link between the formation of carrier localization centers and the presence of alloy fluctuations along the QW.
Efficiently explore the energy landscape of proteins in molecular dynamics simulations by amplifying collective motions
Authors: He Jianbin , Zhang Zhiyong , Shi Yunyu , Liu Haiyan
Published: August 2003

We describe a method for efficient sampling of the energy landscape of a protein in atomic molecular dynamics simulations. A simulation is divided into alternatively occurring relaxation phases and excitation phases. In the relaxation phase (conventional simulation), we use a frequently updated reference structure and deviations from this reference structure to mark whether the system has been trapped in a local minimum. In that case, the simulation enters the excitation phase, during which a few slow collective modes of the system are coupled to a higher temperature bath. After the system has escaped from the minimum (also judged by deviations from the reference structure) the simulation reenters the relaxation phase. The collective modes are obtained from a coarse-grained Gaussian elastic network model. The scheme, which we call ACM-AME (amplified collective motion-assisted minimum escaping), is compared with conventional simulations as well as an alternative scheme that elevates the temperature of all degrees of freedom during the excitation phase (amplified overall motion-assisted minimum escaping, or AOM-AME). Comparison is made using simulations on four peptides starting from non-native extended or all helical structures. In terms of sampling low energy conformations and continuously sampling new conformations throughout a simulation, the ACM-AME scheme demonstrates very good performance while the AOM-AME scheme shows little improvement upon conventional simulations. Limited success is achieved in producing structures close to the native structures of the peptides: for an S-peptide analog, the ACM-AME approach is able to reproduce its native helical structure, and starting from an all-helical structure of the villin headpiece subdomain (HP-36) in implicit solvent, two out of three 150 ns ACM-AME runs are able to sample structures with 3–4 Å backbone root-mean-square deviations from the nuclear magnetic resonance structure of the protein.
Role of ytterbium and ytterbium/cesium fluoride on the chemistry of poly(9,9-dioctylfluorene-co-benzothiadiazole) as investigated by photoemission spectroscopy
Authors: Fung M. K. , Tong S. W. , Lai S. L. , Bao S. N. , Lee C. S. , Wu W. W. , Inbasekaran M. , O’Brien J. J. , Liu S. Y. , Lee S. T.
Published: August 2003

Since ytterbium (Yb) possesses a low work function of 2.6 eV and Yb fluoride generally has a high negative free energy of formation, it is conceivable to use Yb, either directly or parasitically, with a metal fluoride, as a cathode in organic light-emitting diodes (OLEDs). In this work, the electronic structure and chemistry at the interface of Yb/poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) were investigated by ultraviolet and x-ray photoemission spectroscopy (UPS and XPS, respectively). We found that the deposition of Yb on top of F8BT foremost formed organometallic complexes with the sulfur but interacted partly with the nitrogen of F8BT, and eventually formed a Yb–C complex at higher Yb coverages. In the UPS spectra, Yb deposition increased the relative peak intensities corresponding to the σ-bonds originated from the aliphatic side chains, implying that some of the π-conjugated carbons in the polymer backbone may be destroyed. These results agree well with the disappearance of the π-to-π* transition as observed from the shake-up peaks of the carbon 1s core level in the XPS. The chemistry at the interfaces of Yb/CsF/Au and Yb/CsF/F8BT was also examined by XPS. In both cases, Cs was liberated from the CsF upon Yb deposition and the Yb reacted with the liberated F to form YbF3. In the Yb/CsF/F8BT system, the dissociated Cs did not exist in the metallic state, but reacted with the N atoms and carbon backbone in the F8BT. The low work function of Cs (2.2 eV) may further enhance the injection of electrons into the polymer layer. More importantly, the utilization of Yb/CsF as an electrode is polymer independent, and the CsF layer between the Yb and F8BT could retard Yb diffusion into the bulk polymer. The present results are supported by a calculation of the free energy needed to liberate the Cs in the metal/CsF system using a simple thermodynamic model. It is suggested that the use of Yb/CsF cathode in the polymer offers an advantage over most other metal fluorides because the current electrode has a higher affinity to liberate Cs due to the high negative heat of formation of YbF3, although Yb alone may not be a good electron injector in OLEDs.
Statistical model for prebreakdown current jumps and breakdown caused by single traps in magnetic tunnel junctions
Authors: Das J. , Degraeve R. , Groeseneken G. , Stein S. , Kohlstedt H. , Borghs G. , De Boeck J.
Published: August 2003

To obtain reliable magnetic tunnel junctions (MTJs) for sensor and memory applications, the quality of the Al2O3 tunnel barrier is extremely important. Here, we studied the reliability of MTJs with a 1.6 nm Al2O3 tunnel barrier formed by ultraviolet light assisted oxidation. In the stress measurements, prebreakdown current jumps and, finally, breakdown are observed. We show, by using statistics, that both the current jumps and the final breakdown can be attributed to single trap generation. Moreover, we can relate the current jump height to the trap location. In this way, we reveal the breakdown mechanism in MTJs and illustrate the importance of reliability studies.
Statistical thermodynamics of polymer solutions
Authors: Li Xiao Yi , Zhao De Lu
Published: August 2003

The previous theory of pure polymer fluid is extended to polymer solutions. The partition function and the complete thermodynamic description for polymer solutions on a lattice model are obtained. With neglecting of the influence of preferential interaction on configuration, the free energy of Flory–Huggins is recovered. The temperature-independent portion of effective interaction parameter χ is explained from the molecular origins, and the observed composition dependence of χ is provided on a molecular basis. The values calculated from the present theory match the experimental spinodals data. However, the mean field approximation has an important effect not only on spinodals and binodals, but also on the interaction parameter.
Dynamic properties of polyvinylmethylether near the glass transition
Authors: Casalini R. , Roland C. M.
Published: August 2003

Dielectric spectroscopy, encompassing 13 decades of frequency, was used to investigate local segmental relaxation in polyvinylmethylether (PVME). Measurements were obtained over a 110 degree range of temperatures, at pressures up to 725 MPa. At atmospheric pressure, time-temperature superpositioning is valid; however, application of pressure changes the shape of the dielectric spectrum. Similarly, the relaxation times and dc-conductivity have the same temperature dependence at ambient pressure, while a breakdown of the Debye–Stokes–Einstein relation is observed at elevated pressures. The pressure dependence of the relaxation times is weak, corresponding to an activation volume about equal in magnitude to the molar volume of the PVME repeat unit. The pressure coefficient of the glass transition temperature (Tg=247.5 K at ambient pressure) is small, 177 K/GPa. From the ratio of the isochronic and isobaric expansivities, =2.2, thermal energy is found to have a stronger effect on the relaxation times than does the volume, although the contribution from the latter is significant. A comparison was made of the relaxation properties of PVME to those of the structurally similar polyvinylacetate. Distinct, qualitative differences are noted at both ambient and elevated pressure.
Memory properties of a ferroelectric gate field-effect transistor with an adjoining metal–ferroelectric–metal assistance cell
Authors: Xiong Sibei , Sakai Shigeki , Ishii Kenichi , Migita Shinji , Sakamaki Kazuo , Ota Hiroyuki , Suzuki Eiichi
Published: August 2003

We prepared a ferroelectric gate field-effect transistor (FET) memory that consists of a normal metal–ferroelectric–metal-insulator–Si FET and an adjoining metal–ferroelectric–metal auxiliary cell. Due to this cell, the FET exhibited excellent nonvolatile memory properties of low-voltage operation and a long period of retention. After writing-voltage application of ±4 V, the drain current ratio between “ON” and “OFF” states was as large as six orders of magnitude and the memory window (difference in threshold voltage) was 3 V. Even 104 s after writing, the ON state drain current could be distinguished as being three orders different from the OFF state current. The ferroelectric FET with the auxiliary cell that demonstrated good nonvolatile properties has potential for application in high-density nonvolatile memories.
Hydrogen configurations, formation energies, and migration barriers in GaN
Authors: Wright A. F. , Seager C. H. , Myers S. M. , Koleske D. D. , Allerman A. A.
Published: August 2003

Hydrogen configurations, formation energies, and migration barriers in GaN have been obtained from density-functional theory (DFT) calculations employing the generalized-gradient approximation (GGA) for exchange and correlation. The H configurations and formation energies are similar to previous DFT results obtained using the local-density approximation (LDA) for exchange and correlation with the most notable differences being higher formation energies for H0 (0.2 eV) and H2 (0.1 eV) in the GGA calculations and a preference for H+ at a bond-centered site. GGA H migration barriers differ from LDA values with the largest differences being 0.4 eV higher GGA H+ migration barriers. Refined measurements of field-induced H+ drift in GaN p/n+ diodes have also been performed. Modeling of these results indicates that the sum of the H+ diffusion activation energy and the H+ binding energy to ionized Mg acceptors is 1.76 eV. The corresponding GGA result (1.6 eV) is in good agreement with the measured value.
Implantation of 60 keV copper negative ion into thin SiO2 films on Si: Thermal stability of Cu nanoparticles and recovery of radiation damage
Authors: Amekura Hiroshi , Kishimoto Naoki
Published: August 2003

Implantation of 60 keV copper negative-ion into 115 nm thick amorphous SiO2 films on a Si substrate (SiO2/Si) induces not only Cu nanoparticle formation in SiO2, but also radiation damage. Using ultraviolet, visible, and infrared reflection spectroscopy (0.031–6.52 eV), thermal recovery of the damage and stability of the nanoparticles were evaluated with isochronal annealing in Ar gas flow, from 100 to 1000 °C. The implantation induces Si–O–Si bond breaking in SiO2 layer, and amorphization of Si substrate, in addition to Cu nanoparticle formation in the SiO2 layer. A gradual recovery of the broken bonds in SiO2 was observed in a wide temperature range of 200–700 °C, which is characteristic in amorphous structures. Two-step recoveries at 600 and 900 °C are observed in recrystallization of Si substrate, indicating recoveries of pure and O-recoil rich regions in the Si substrate, respectively. Around 700 °C, intensity of the Cu surface plasmon resonance (SPR) peak drastically increases, suggesting coarsening of Cu nanoparticles. At 1000 °C, most of the Cu atoms escape from the SiO2 layer, probably due to thermally activated diffusion. Annealing temperature of 800–900 °C is adequate for a large SPR peak of Cu nanoparticles in SiO2 thin film on a crystalline Si substrate.
Investigation of carrier transport and carrier distribution in GaAs/(Al,Ga)As quantum-cascade structures
Authors: Ohtsuka T. , Schrottke L. , Hey R. , Kostial H. , Grahn H. T.
Published: August 2003

We have investigated the current density–electric field (j-F) characteristics and photoluminescence (PL) spectra of several GaAs/(Al,Ga)As quantum-cascade structures (QCS’s) up to the threshold field strength to obtain information on the carrier distribution and field inhomogeneities. Due to the complex structure of each single period of the cascaded device, the dark j-F characteristics of undoped QCS’s exhibit distinct current maxima, resulting in regions of negative differential conductivity, while a plateaulike feature appears under illumination. For doped QCS’s, a plateau with sawtoothlike structures appears, which are well known from doped, weakly coupled superlattices under electric-field-domain formation. However, in contrast to the splitting of the PL line that is observed for doped, weakly coupled superlattices, no splitting has been observed in the doped QCS’s. The j-F characteristic of an undoped QCS with thicker barriers than the original QCS exhibits a much more pronounced current maximum in the dark. Under illumination, the j-F characteristic of this structure shows a clear plateaulike feature, which contains additional fine structure indicating the existence of electric-field inhomogeneities within each period due to the separation of electrons and holes. The PL spectra of the undoped and doped QCS’s show emission from only the widest quantum well (QW) because the holes accumulate very quickly in this QW resulting from the strong coupling within the entire QCS. However, in the PL spectra of the undoped QCS with thicker barriers, PL lines from several quantum wells can be identified. The carrier distribution process in QCS’s is also discussed as a function of the applied electric field and compared for the different structures.
Rapid thermally annealed plasma deposited SiNx:H thin films: Application to metal–insulator–semiconductor structures with Si, In0.53Ga0.47As, and InP
Authors: Mártil I. , del Prado A. , San Andrés E. , González Dı́az G. , Martı́nez F. L.
Published: August 2003

We present in this article a comprehensive study of rapid thermal annealing (RTA) effects on the physical properties of SiNx:H thin films deposited by the electron cyclotron resonance plasma method. Films of different as-deposited compositions (defined in this article as the nitrogen to silicon ratio, x=N/Si) were analyzed: from Si-rich (x=0.97) to N-rich (x=1.6) films. The evolution of the composition, bonding configuration, and paramagnetic defects with the annealing temperature are explained by means of different network bond reactions that take place depending on the as-deposited film composition. All the analyzed films release hydrogen, while Si-rich and near-stoichiometric (x=1.43) ones also lose nitrogen upon annealing. These films were used to make Al/SiNx:H/semiconductor devices with Si, In0.53Ga0.47As, and InP. After RTA treatments, the electrical properties of the three different SiNx:H/semiconductor interfaces can be explained, noting the microstructural modifications that SiNx:H experiences upon annealing.
Alternating current conduction properties of thermally evaporated α-nickel phthalocyanine thin films: Effects of oxygen doping and thermal annealing
Authors: Anthopoulos Thomas D. , Shafai Torfeh S.
Published: August 2003

The ac conduction properties of thermally evaporated films of α-nickel phthalocyanine (α-NiPc) were studied in situ and ex situ employing symmetric gold ohmic electrodes in the frequency range of 20−106 Hz at various temperature regimes. ac conductivity was identified to be via a hopping-type mechanism in the lower temperature region and via a free-band conduction in the high temperature region. Upon exposure of the films to dry air, the low frequency ac conductivity was found to increase by 2 orders of magnitude, which was attributed to oxygen absorption within NiPc. The doping effect was partially reversed by thermal annealing of the films under high vacuum. Measurements on the dependence of capacitance and loss tangent (tanδ) on frequency were consistent and quantitatively explained by invoking an equivalent circuit model. Oxygen doping was found to increase the low frequency capacitance of NiPc. The phenomenon was understood in terms of reduction in the value of device internal resistance induced by oxygen absorption.
Structure of GaN quantum dots grown under “modified Stranski–Krastanow” conditions on AlN
Authors: Gogneau N. , Jalabert D. , Monroy E. , Shibata T. , Tanaka M. , Daudin B.
Published: August 2003

We propose a procedure to grow GaN quantum dots (QDs) on AlN by using the Ga surfactant effect in plasma-assisted molecular beam epitaxy. Self-formed GaN islands were spontaneously generated under vacuum, after evaporation of the Ga bilayer stabilizing the two-dimensional GaN layer grown under Ga-rich conditions. Island characteristics (size and density) are studied as a function of the nominal amount of GaN deposited. We demonstrate that the QD density can be controlled in the 3×1010 cm−2–2×1011 cm−2 range. It is shown that beyond a given amount of GaN nominally deposited, there is a coexistence between elastic and plastic relaxation, with GaN islands being formed on a partially relaxed two-dimensional GaN layer thicker than two monolayers.
Recombination dynamics of localized excitons in Al1−xInxN epitaxial films on GaN templates grown by metalorganic vapor phase epitaxy
Authors: Onuma T. , Chichibu SF. , Uchinuma Y. , Sota T. , Yamaguchi S. , Kamiyama S. , Amano H. , Akasaki I.
Published: August 2003

Recombination dynamics of excitons in nearly strain-free Al1−xInxN alloys on the GaN template were studied. Their band-gap energy showed a nonlinear dependence on the InN molar fraction x, and the bowing parameter was determined to be approximately −3.1 eV. Most of the alloys exhibited an extremely diffused band-edge, and consequently exhibited huge Stokes-type shifts up to 1–2 eV and full width at half maximum of the luminescence peaks up to 0.5 eV. The results suggested enhanced material inhomogeneity in AlInN compared to InGaN alloys. Since the time-resolved photoluminescence signal showed a pronounced stretched exponential decay, the spontaneous emission was assigned as being due to the radiative recombination of excitons localized in disordered quantum nanostructures. The integrated PL intensity at 300 K was as strong as 29% of that at low temperature, showing a potential use of AlInN alloys as infrared-to-UV light emitters.
Combining high resolution and tensorial analysis in Raman stress measurements of silicon
Authors: Bonera Emiliano , Fanciulli Marco , Batchelder David N.
Published: August 2003

We present the development of a Raman spectroscopy technique for the measurement of the tensorial nature of stress in silicon on a micrometric scale. After the detailed description of the theoretical bases of the experiment, we measure the stress tensor of a silicon surface close to a scratch. Then, we apply this method to discern which models are suitable for the description of the stress tensor in shallow trench isolations for microelectronics.
Modeling of photonic crystals using a real-valued transmission line matrix method
Authors: Romo G. , Smy T.
Published: August 2003

In this article, we present a stable real-valued transmission line matrix (TLM) method for simulating periodic photonic structures. Unique to this method is the circuit-type implementation of the periodicity (Bloch) condition at the sidewalls of the unit cell under simulation. Unlike previously published methods for implementing periodic boundary conditions in real-valued TLM networks, our circuit representation is shown to be passive since no energy is introduced from the boundaries of the simulation domain. The enforcement of Bloch boundary conditions at the sidewalls of the simulation domain, while preserving energy, results in an unconditionally stable TLM method free of high-frequency noise. Our approach is validated by simulating a two-dimensional array of dielectric materials, a structure usually referred to as a photonic crystal. The results are compared with those obtained by the plane wave expansion method and good agreement is found.
Hydrosilylation of crystalline silicon (111) and hydrogenated amorphous silicon surfaces: A comparative x-ray photoelectron spectroscopy study
Authors: Lehner A. , Steinhoff G. , Brandt M. S. , Eickhoff M. , Stutzmann M.
Published: August 2003

Alkene molecules were covalently bonded to hydrogen-terminated crystalline silicon (111) and hydrogenated amorphous silicon (a-Si:H) surfaces by thermally induced hydrosilylation. The resulting chemical surface structure was analyzed by x-ray photoelectron spectroscopy and compared to that of the corresponding silicon surfaces covered by a native oxide and terminated with hydrogen. Our results demonstrate successful hydrosilylation on both substrate materials. However, the presence of oxygen on the surface turns out to hinder the hydrosilylation reaction, as shown by the reduced concentration of hydrocarbons on the surface after prolonged exposure of the Si substrates prior to hydrosilylation. By monitoring both the O 1s and the Si 2p peaks, the oxidation kinetics of a-Si:H was found to be diffusion limited. Since stable hydrogen termination as a prerequisite of hydrosilylation can be achieved on a-Si:H surfaces with much less technological effort than on crystalline silicon surfaces, a-Si:H is a promising substrate for biofunctionalization procedures requiring less stringent process conditions.
First-principles calculations for nitrogen-containing single-walled carbon nanotubes
Authors: Zhao Mingwen , Xia Yueyuan , Lewis James P. , Zhang Ruiqin
Published: August 2003

We present calculations for possible configurations of nitrogen-containing single-walled carbon nanotubes and their electronic properties obtained with the ab initio tight-binding FIREBALL method. It is found that nitrogen atoms can be energetically incorporated into the carbon network in three forms: Substitution, substitution with formation of a vacancy structure, and chemical adsorption. The different forms exhibit different local densities of states near the Fermi levels, which might suggest a potential method to control the electronic properties of nitrogen-doped carbon nanotubes.
Room temperature luminescence from ZnSe1−xTex (x<1%) epilayers grown on (001) GaAs
Authors: Jacob A. P. , Zhao Q. X. , Willander M. , Yang C. S. , Chou W. C.
Published: August 2003

Photoluminescence experiments have been performed to systematically study the effect of thermal processing on ZnSe1−xTex (x<1%) epilayers. Our results show that, a ZnSeTe epilayer under proper post growth thermal annealing can emit light in the visible range of 5500–7000 Å at room temperature. Thus by systematically processing these samples, they could be used for II–VI laser diodes that can operate at room temperature. The results from hydrogen passivation study done on these samples are consistent with the previous reports that the broadband emission is related to an isoelectronic defect, i.e., excitons bound to the Te clusters.
Influence of nanostructure and nitrogen content on the optical and electrical properties of reactively sputtered FeSiAl(N) films
Authors: Lee S. J. , Snyder J. E. , Lo C. C. H. , Campos-Anderson K. M. , Anderegg J. W. , Jiles D. C.
Published: August 2003

In this study, the optical properties and dc resistivity of a series of FeSiAl(N) films reactively sputtered with different partial pressures of N were investigated. Spectroscopic ellipsometry was used to measure the real and imaginary parts of the complex dielectric functions. There is a distinct micro/nanostructural transition from single-phase columnar body-centered-cubic (bcc) grains for partial pressure (pp) of nitrogen in sputtering gas ⩽4% to a two-phase nanocomposite of equiaxed bcc nanograins in an amorphous matrix for films deposited with ⩾5% pp N. To assess the effect of surface oxidation on the optical properties, optical measurements were repeated on the 2 and 5% pp N films (representative of the two different types of films with different structures) after they were sputter etched in situ while performing depth profiling of the chemical composition using x-ray photoelectron spectroscopy. The low-nitrogen films (⩽4% pp N) showed a dielectric function typical of a metal whose charge carrier contribution can be described by a classical free electron Drude model. The nanostructured films (⩾5% pp N) showed a positive real part of the dielectric function ε1 and no evidence of free-carrier plasmon excitation. The optical conductivity decreased and the dc resistivity increased by about a factor of 2.5 as the film structure changed from a single phase columnar structure to the two-phase material that consisted of nanograins in an amorphous matrix.
Post-delay-time dependences of the formation of CdTe/ZnTe nanostructures and their activation energies
Authors: Oh S. H. , Choi J. C. , Park H. L. , Kim T. W.
Published: August 2003

Atomic force microscopy (AFM) and photoluminescence measurements were carried out to investigate both the formation of CdTe/ZnTe nanostructures and their electron activation energies for various delay times between the deposition of the CdTe layer and the introduction of nitrogen gas into the same chamber. The AFM images show that CdTe quantum dots (QDs) are formed. While the excitonic peak associated with the transition from the ground electronic subband to the ground heavy-hole band (E1-HH1) shifted to higher energy with increasing thickness of the CdTe thin film up to delay time of 10 s, it shifted to lower energy with an increasing thickness of the CdTe thin film for delay times from 20 to 60 s. The activation energy of the electrons confined in the 3.5 ML CdTe QDs grown with a delay time of 5 s was higher than it was for the electrons in CdTe nanostructures grown with delay times of 0 and 60 s. The present results can help to improve the understanding of how the formation of and the electron activation energy in CdTe/ZnTe nanostructures depend on the delay time.
Band gap engineering of SiCN film grown by pulsed laser deposition
Authors: Park Nae-Man , Kim Sang Hyeob , Sung Gun Yong
Published: August 2003

The band gap tuning of amorphous silicon carbon nitride thin (a-SiCN) film was demonstrated in the range of 2.3–3.0 eV by pulsed laser deposition using mixed targets. a-SiCN films were grown on silicon and quartz glass substrates at room temperature in a vacuum. Targets were fabricated by compacting a mixture of silicon carbide and silicon nitride powders. The stoichiometry of the film could be varied by the mixing ratio of the target. Ternary phase SiCN films were deposited at 30– 70 wt. % SiC in a target and their band gaps were controlled by appropriate adjustment of the carbon content. These findings indicate that the growth of a-SiCN films with a mixed target and their subsequent use as optoelectronic materials is a possibility.
Structural and optical properties of shape-engineered InAs quantum dots
Authors: Kim Jin Soo , Lee Jin Hong , Hong Sung Ui , Han Won Seok , Kwack Ho-Sang , Kim Jong Hee , Oh Dae Kon
Published: August 2003

Shape-engineered InAs quantum dots (QDs) were grown by using thin In0.15Ga0.85As and a monolayer (ML) of InAs with different periods on 3 ML InAs QDs and their structural and optical properties were investigated by transmission electron microscopy (TEM), photoluminescence (PL), and photoreflectance (PR) spectroscopy. Cross-sectional TEM images of the QD samples showed that the shape, particularly the height of the QD, could be effectively controlled without any significant degradation in QD quality, such as the generation of dislocations in the QD, thus changing the optical properties. PL and PR spectra indicated that all the layers required for the formation of shaped-engineered QD were grown without any degradation in QD properties. PL spectra of shape-engineered QDs that had been subjected to rapid thermal annealing showed an abnormal behavior compared to those of conventionally grown InAs QDs.
Thickness dependence of exchange bias and coercivity in a ferromagnetic layer coupled with an antiferromagnetic layer
Authors: Hu Jing-guo , Jin Guo-jun , Ma Yu-qiang
Published: August 2003

We present a theoretical investigation on the thickness dependence of exchange bias and coercivity in ferromagnetic/antiferromagnetic bilayer systems, in which the exchange coupling at the interface includes both bilinear (direct exchange) and biquadratic (spin-flop) terms. Two basic formulas for the exchange bias and the coercivity are derived analytically. From these two formulas, many interesting conclusions can be extracted, and a lot of experimental results can be illustrated. The results also show that, although, in general, the biquadratic term lowers the exchange bias, its role in the coercivity is duplex, i.e., it can be tuned to enhance or reduce the coercivity.
Design of high electron mobility devices with composite nitride channels
Authors: Singh Madhusudan , Singh Jasprit
Published: August 2003

Field effect transistors based on a GaN/AlGaN system have shown remarkable performance characteristics in a wide range of device applications. However, due to the large effective mass of GaN, the mobility in the channel is small. In this work, we consider a GaN/AlGaN structure with a thin InN channel of the order of a few monolayers. We find that mobility in the channel can improve considerably while breakdown characteristics are not expected to suffer. Mobilities of ≃2500 cm2 (V s)−1 are predicted along with high sheet charges for low interface disorder. Good agreement with experimental results is observed for higher degrees of disorder within the model. At higher electric fields, we find that most electron population transfers to higher valleys or other subbands that lie in AlGaN or GaN. We also compare the low-field mobility-charge product for this structure with the conventional AlGaN/GaN structure and find that the two values are similar.
Chemical sputtering of hydrocarbon films
Authors: Hopf C. , von Keudell A. , Jacob W.
Published: August 2003

Erosion of hydrocarbon films at room temperature due to argon ions and thermal atomic hydrogen is investigated in a particle-beam experiment. Physical sputtering by the ions is observed at energies ⩾200 eV and reaches a yield of 0.5 at an ion energy of 800 eV. The measured yields are in agreement with TRIM.SP computer simulations, and a threshold energy of ≃58 eV is derived for physical sputtering. Erosion by simultaneous fluxes of argon ions and thermal hydrogen atoms is observed at all energies investigated down to 20 eV and reaches a yield of about 3 at an ion energy of 800 eV and a hydrogen-atom-to-argon-ion-flux ratio of 400. It is proposed that the significant decrease of the threshold energy as well as the increase of the absolute yields is due to the process of chemical sputtering: Within a collision cascade caused by the incident ions, bonds are broken and instantaneously passivated by the abundant flux of atomic hydrogen. This leads to the formation of hydrocarbon molecules within the common range of ions and hydrogen atoms. Finally, the molecules diffuse to the surface and desorb. The threshold energy of chemical sputtering is on the order of typical carbon–carbon bond energies in organic compounds of several eV. Based on this mechanism a model for the energy dependence of the chemical sputtering yield is presented, which leads to good agreement with the data.
Dielectric relaxation and transition of porous silicon
Authors: Pan L. K. , Huang H. T. , Sun Chang Q.
Published: August 2003

Dielectric impedance measurements of porous silicon within the frequency range of 50 Hz–1.0 MHz and temperature range of 298–798 K revealed three semicircles in a Cole–Cole plot when the temperature is raised to 773 K; they are thought to correspond to contributions from the grain interior, grain boundary, and electrode/film interface, respectively. The enhancement in conductivity by heating follows an Arrhenius law with an activation energy transition from 0.07 to 0.79 eV at ∼565 K, which originates from band tail hopping that occurs around the Fermi edge. At a critical temperature, a high degree of dispersion in the real and imaginary parts of the permittivity also occurs at low frequencies. This dispersion behavior is interpreted as a combination of electron-lattice polarization associated to the band tail hopping and the crystal field weakening due to thermal expansion.
Determination of the diffusion length of charge carriers in nonstoichiometric sillenite-type crystals by the technique of nonsteady-state photocurrents
Authors: Vogt H. , Krätzig E.
Published: August 2003

Bi 12 GeO 20 and Bi12SiO20 single crystals have been grown from melts with different GeO2 and SiO2 contents, respectively, to investigate the influence of an intrinsic defect, the antisite defect (Bi on Ge or Si site), on the diffusion length and the mobility-lifetime product of excited charge carriers. Measurements of nonsteady-state photocurrents show that both quantities increase with decreasing concentration of the antisite defect.
Mg acceptor level in AlN probed by deep ultraviolet photoluminescence
Authors: Nam K. B. , Nakarmi M. L. , Li J. , Lin J. Y. , Jiang H. X.
Published: August 2003

Mg-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in Mg-doped AlN epilayers. From PL emission spectra and the temperature dependence of the PL emission intensity, a binding energy of 0.51 eV for Mg acceptor in AlN was determined. Together with previous experimental results, the Mg acceptor activation energy in AlxGa1−xN as a function of the Al content (x) was extrapolated for the entire AlN composition range. The average hole effective mass in AlN was also deduced to be about 2.7 m0 from the experimental value of the Mg binding energy together with the use of the effective mass theory.
Electron relaxation and transport dynamics in low-temperature-grown GaAs under 1 eV optical excitation
Authors: Sun Chi-Kuang , Chen Yen-Hung , Shi Jin-Wei , Chiu Yi-Jen , Gan Kian-Giap , Bowers John E.
Published: August 2003

Electron relaxation and transport dynamics in low-temperature-grown GaAs under 1 eV optical excitation was investigated by femtosecond transient transmission measurement and electro-optical sampling measurement in bulk samples and fabricated devices. An increase in the electron lifetime can be observed when the electron density is higher than 3×1017 cm−3. This effect is attributed to prolonged electron relaxation due to intervalley scattering of highly excited electrons and associated hot phonon effects. Our conclusion is further supported by bias-dependent studies where intervalley scattering was achieved using high electric fields.
Electric current-induced first-order effects on the insulator–metal transition and the colossal electroresistance in rare-earth manganates
Authors: Sudheendra L. , Rao C. N. R.
Published: August 2003

Passing electric currents through a single crystal of La0.77Ca0.23MnO3 causes a marked decrease in the temperature of the insulator–metal transition, TIM, the TIM decreasing with increasing current. The transition exhibits thermal hysteresis, the magnitude of which increases with an increase in applied current. Large negative electroresistance is observed close to room temperature while large positive electroresistance occurs at low temperatures. Similar results are observed with La0.9MnO3 as well.
Space-charge limited current in regioregular poly-3-hexyl-thiophene
Authors: Nikitenko V. R. , Heil H. , von Seggern H.
Published: August 2003

Temperature dependent current–voltage characteristics of an organic diode based on a thin film of regioregular poly-3-hexyl-thiophene (P3HT) are compared with results of a theoretical model assuming space-charge limited currents with a density of states (DOS), nonmonotonous in energy. This DOS was deduced from experiments utilizing the method of thermally stimulated currents. Both, experiments and theory result in an almost power-law dependence of j∝Vm, where the exponent m increases with decreasing temperature assuming a value of m=2 at room temperature. This effect can be accounted for by filling of deep traps at lower temperatures. Transport of charge carriers in P3HT seems to be limited by hopping in disordered regions rather than by the transport via extended states within crystalline grains.
Light emission from silicon-based porous anodic alumina formed in 0.5 M oxalic acid
Authors: Huang G. S. , Wu X. L. , Xie Y. , Shao X. F. , Wang S. H.
Published: August 2003

We have examined the photoluminescence (PL) spectra of silicon-based porous anodic alumina formed in 0.5 M oxalic acid. A broad PL band centered at 565 nm was observed. Spectral analyses reveal that the broad PL arises from optical transitions between the energy levels of F2+ center (a combined oxygen vacancy with one positive charge). In addition, two ultraviolet and one violet PL band, together with a blue band, were also obtained from this kind of Si-based porous anodic alumina. Based on annealing behavior of these PL bands, we suggest that the ultraviolet and violet PL bands are closely related to excess aluminum ions in the alumina membrane rather than oxygen vacancy defects reported previously. The blue emission originates in oxygen vacancies (F and F+ centers). This work provides a good understanding of the light-emitting properties of silicon-based porous anodic alumina templates.
Observation of ultrashort pulse propagation anisotropy in a semiconductor quantum nanostructure optical waveguide by cross-correlation frequency resolved optical gating spectroscopy
Authors: Tsurumachi Noriaki , Hikosaka Kazunori , Wang Xue-Lun , Ogura Mutsuo , Watanabe Naoki , Hattori Toshiaki
Published: August 2003

Femtosecond optical pulse propagation in a quantum well (QW) waveguide and a quantum wire (QWR) waveguide was investigated by cross-correlation frequency resolved optical gating (XFROG) spectroscopy. An optical pulse transmitted through the GaAs QW waveguide was found to stretch greatly from 140 fs to almost 1 ps due to nonlinear dispersion around the heavy hole exciton resonance at transverse electric polarization in a near resonant experiment. In contrast, only slight chirping of the pulse transmitted was observed either at transverse magnetic polarization or off resonance for both polarizations. In the GaAs QWR waveguide, the polarization anisotropy of a crescent shaped QWR could also be observed in terms of dispersion by XFROG spectroscopy in spite of the small absorption compared with that in the QW.
Size-dependent charge storage in amorphous silicon quantum dots embedded in silicon nitride
Authors: Park Nae-Man , Jeon Sang-Hun , Yang Hyun-Deok , Hwang Hyunsang , Park Seong-Ju , Choi Suk-Ho
Published: August 2003

Size-dependent charge storage was observed in metal–insulator–semiconductor structures containing amorphous Si quantum dots (a-Si QDs) grown by plasma-enhanced chemical vapor deposition. For a-Si QDs as large as 2 nm in diameter, one electron or one hole was stored in each a-Si QD. For small-sized a-Si QDs of 1.4 nm in diameter, however, the width of capacitance–voltage hysteresis was decreased, indicating that the charge density in the a-Si QDs was reduced. This can be attributed to the lowered tunneling barrier in the small-sized a-Si QDs resulting from a large quantum confinement effect. Long-term charge storage was observed in the fully charged a-Si QDs; this is attributed to a suppression of the discharge process by electrostatic repulsion among the charged dots.
YBa
2
Cu
3
O
7−δ
step-edge Josephson junctions fabricated on sapphire substrates
Authors: Ming Bin , Venkatesan T.
Published: August 2003

Sapphire is an ideal substrate for cryogenic device technologies. We report here the fabrication of YBa2Cu3O7−δ step-edge junctions on the R-plane (11̄02) sapphire substrates. With an “overhang” shadow mask produced by a photolithography technique, a steep step edge was created on the CeO2 buffer layer by Ar+ ion milling with optimized parameters for minimum ion-beam divergence. The step angle was determined to be in excess of 80° by atomic force microscopy. Josephson junctions fabricated from those step edges exhibited resistively shunted junction like current–voltage characteristics. IcRn values in the 200–500 μV range were measured at 77 K. Shapiro steps were observed under microwave irradiation. The magnetic field dependence of the junction Ic indicates uniform current distribution.
Optical absorption spectroscopy measurement of the gap shrinkage due to thallium incorporation in GaInTlAs alloys
Authors: Sibai A. , Olivares J. , Guillot G. , Bremond G. , Sánchez-Almazán F. , Gendry M. , Regreny P. , Hollinger G.
Published: August 2003

The optical properties of Ga(1−y)0.45In(1−y)0.55TlyAs epilayers grown at low temperature (230 °C) by solid-source molecular-beam epitaxy on InP substrates were characterized using optical absorption and photoluminescence techniques. Optical absorption measurements at room temperature show a gap shrinkage toward lower energies from 0.71 to 0.61 and 0.53 eV when the Tl content increases from 0% to 4%, and 8%, in good agreement with theoretical predictions. Low-temperature photoluminescence band-gap signals from GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing performed in order to improve the electronic quality of the layers. A band gap decrease as much as 41 meV for GaInTlAs with 1.9% Tl incorporation is measured by photoluminescence at 8 K.
Comparison of Cu electromigration lifetime in Cu interconnects coated with various caps
Authors: Hu C.-K. , Gignac L. , Liniger E. , Herbst B. , Rath D. L. , Chen S. T. , Kaldor S. , Simon A. , Tseng W.-T.
Published: August 2003

Electromigration in Cu Damascene lines with bamboo-like grain structures, either capped with Ta/TaN, SiNx, SiCxNyHz layers, or without any cap, was investigated. A thin Ta/TaN cap on top of the Cu line surface significantly improves electromigration lifetime when compared with lines without a cap and with lines capped with SiNx or SiCxNyHz. The activation energy for electromigration increased from 0.87 eV for lines without a cap to 1.0–1.1 eV for samples with SiNx or SiCxNyHz caps and to 1.4 eV for Ta/TaN capped samples.
High-power Er:Yb fiber laser with very high numerical aperture pump-cladding waveguide
Authors: Bouwmans G. , Percival R. M. , Wadsworth W. J. , Knight J. C. , Russell P. St. J.
Published: August 2003

One watt of laser power at 1535 nm has been generated from an Er:Yb codoped double-clad fiber structure, in which the inner cladding was suspended in air by a large number of thin silica webs. The acceptance numerical aperture (NA) of the inner cladding has been measured over a range of wavelengths, for lengths from 1 up to 100 m. For fiber lengths of ∼40 m, the NA at the pump diode wavelength was 0.88.
A critical examination of the P–h2 relationship in nanoindentation
Authors: Troyon M. , Martin M.
Published: August 2003

In this letter we derive a corrected analytical expression for the indentation load–depth relation during loading in an indentation experiment; P=Er(1/CEr/H+ε(m)/γπ/4H/Er)−2(h+hb)2. This expression takes into account the correction factor γ in the fundamental relations among contact stiffness, elastic modulus, and contact area, which is frequently forgotten in the literature, and the exact value of the geometric constant ε(m) of the indenter. A comparison with previously used expressions shows a much better agreement between hardness measurements from loading and unloading curve analyses.
InGaAs/GaAs three-dimensionally-ordered array of quantum dots
Authors: Mazur Yu. I. , Ma W. Q. , Wang X. , Wang Z. M. , Salamo G. J. , Xiao M. , Mishima T. D. , Johnson M. B.
Published: August 2003

We report on the first fabrication of (In,Ga)As/GaAs quantum dots with both vertical and lateral ordering forming a three-dimensional array. An investigation of the photoluminescence spectra from the ordered array of quantum dots, as a function of both temperature and optical excitation intensity, reveals both a lateral and vertical transfer of excitation.