bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical characterization and evaluation of the conduction band offset for ZnCdSe∕ZnMgSe quantum wells grown on InP(001) by molecular-beam epitaxy
Authors: Sohel Mohammad , Zhou Xuecong , Lu Hong , Perez-Paz M. Noemi , Tamargo Maria , Muñoz Martin
Published: July 2005

Lattice matched ZnMgSe grown on InP is of considerable interest for its potential applications as a cladding layer due to the high band-gap energy (∼3.6eV) and for use in intersubband devices such as quantum cascade lasers. Several lattice matched Zn0.5Cd0.5Se∕Zn0.13Mg0.87Se quantum wells (QWs) were grown on InP (001) substrates. Emission ranging from the near UV to the visible spectral range was achieved by varying the thickness of the wells. The QW fundamental transition as function of the QW thickness was experimentally studied and modeled using an envelope calculation. The contactless electroreflectance measurements of a Zn0.5Cd0.5Se∕Zn0.13Mg0.87Se single QW yielded multiple transitions from the QW, allowing us to estimate the conduction band offset of this heterostructure to be as high as 1.12eV.
Optical properties of (GaSb)3n(AlSb)n(1⩽n⩽5) superlattices
Authors: Choi S. G. , Srivastava S. K. , Palmstrøm C. J. , Kim Y. D. , Cooper S. L. , Aspnes D. E.
Published: July 2005

We report room-temperature pseudodielectric function spectra ⟨ε(ω)⟩=⟨ε1(ω)⟩+i⟨ε2(ω)⟩ from 1.5 to 6.0 eV of a series of (GaSb)3n(AlSb)n(1⩽n⩽5) short-period superlattices (SLs) and the corresponding ternary alloy, Al0.3Ga0.7Sb, as determined by spectroscopic ellipsometry. An in situ chemical etching procedure was performed to minimize the effect of surface oxides and other overlayers. For small periods (n⩽2), the dielectric responses of the SLs are similar to those of the random alloy. For larger n, structures due to the E1, E1+Δ1, E0′, E2, and E1′ interband transitions can be seen, with the E1, E1+Δ1, and E0′ structures showing a clear SL-period dependence. Two additional features were also observed near the E1+Δ1 structure for n⩾4.
Investigation of nitrogen flow variation into a radio frequency plasma cell on plasma properties and GaInNAs grown by molecular beam epitaxy
Authors: Yuen Homan B. , Wistey Mark A. , Bank Seth R. , Bae Hopil , Harris James S.
Published: July 2005

The radio frequency (rf) plasma and its operating conditions play a large, if not dominant role, in the differences of characteristics of molecular beam epitaxy-grown dilute-nitride samples between various research groups. Optimization of the rf plasma itself has become more important due to recent discoveries that ions cause significant damage to the optical quality of the material. An investigation on the effects of different nitrogen gas flows into the rf plasma cell was conducted. GaInNAs quantum wells were grown with different flow rates and their optical quality was examined using photoluminescence. A Langmuir probe was used to qualitatively analyze the ion flux originating from the plasma. We find that lower gas flow rates lead to a “sparse” operating condition and to higher ion counts which degrade the optical quality of the material. Higher flow rates are necessary to minimize ion counts and obtain higher quality materials.
Growth of high optical quality InAs quantum dots in InAlGaAs∕InP double heterostructures
Authors: Zhang Z. H. , Cheng K. Y.
Published: July 2005

Two methods were studied to improve the optical quality of InAs quantum dot nanostructures grown in lattice-matched InAlGaAs∕InP double heterostructures of a center wavelength around 1.55μm. By either inserting InAlAs∕InGaAs superlattices between the InAlGaAs waveguide and the upper InAlAs cladding layer, or depositing a tensile-strained InGaAs strain-balance layer after the quantum dot formation, the optical quality of the quantum dot samples is greatly improved and a strong room-temperature photoluminescence is observed. The InGaAs strain-balance layer can be used to reduce the overall strain of the heterostructure, which makes it possible to stack a large number of quantum dot layers to improve the uniformity of the dot size distribution and increase the optical gain volume for high performance photonic device applications.
Depth profiling the electronic structures at HfO2∕Si interface grown by molecular beam epitaxy
Authors: Lay T. S. , Chang S. C. , Din G. J. , Yeh C. C. , Hung W. H. , Lee W. G. , Kwo J. , Hong M.
Published: July 2005

The depth profile of the HfO2∕Si interface grown by molecular beam epitaxy (MBE) has been investigated by high-resolution x-ray photoelectron spectroscopy using synchrotron radiation beam and low energy Ar+ sputtering. At the interfacial layer, the Hf 4f spectra show a Hf silicide state without a Hf silicate signal. The O 1s and Si 2p spectra show weakly two silicon oxidation states of SiO and SiO2. The data suggest an inward reaction of the Hf and O species into the Si substrate during the MBE growth. The valence band offset (ΔEV) of ∼3.5eV is obtained for the HfO2∕Si interface by measuring the valence-band edges of HfO2 and Si.
In situ monitoring of formation of InAs quantum dots and overgrowth by GaAs or AlAs
Authors: Yakimov Michael , Tokranov Vadim , Agnello Gabriel , van Eisden Jobert , Oktyabrsky Serge
Published: July 2005

With a goal of development of a high performance medium for optoelectronic devices, formation of InAs self-assembled quantum dots (QDs) in GaAs∕AlAs short period superlattice was studied using primarily in situ molecular beam epitaxy techniques including reflection high-energy electron diffraction and Auger electron spectroscopy. The processes of formation, decay of QDs and overgrowth of wetting InAs layers were analyzed with the emphasis on the role of AlAs or GaAs capping layers. InAs critical coverage for QD formation on GaAs and AlAs surfaces was measured as a function of temperature, and diffusion coefficients of In adatoms were estimated. QD decay was studied, activation energy for the process was found to be 0.8 and 2.3 eV for GaAs and AlAs surfaces, respectively, indicating that QD decay process is likely driven by In intermixing with the substrate for GaAs surface and by evaporation of In from the AlAs surface. Intermixing of InAs with the capping layers was studied at growth temperatures. Typical InAs redistribution thickness was estimated as 6 ML at 475 °C, same for AlAs and GaAs, with In segregation efficiency R=0.86 in both cases.
Growth and characterization of plasma-assisted molecular beam epitaxial-grown AlGaN/GaN heterostructures on free-standing hydride vapor phase epitaxy GaN substrates
Authors: Storm D. F. , Katzer D. S. , Mittereder J. A. , Binari S. C. , Shanabrook B. V. , Zhou Lin , Smith David J. , Xu X. , McVey D. , Vaudo R. P. , Brandes G. R.
Published: July 2005

We have grown AlGaN/GaN high electron mobility transistor (HEMT) structures by plasma-assisted molecular beam epitaxy on free-standing n-GaN substrates grown by hydride vapor phase epitaxy. Reflection high energy electron diffraction patterns of the as-loaded wafers exhibit narrow streaks which persist throughout the growth. Atomic force microscopy shows smooth AlGaN surfaces with root-mean-square roughness of 10 Å over a 20×20μm2 area. High resolution x-ray diffractometry indicates that the AlGaN peak is ∼20% narrower than for similar structures grown on SiC. Hall mobilities, electron sheet densities, and sheet resistances were measured on ten 60×60μm2 Hall test patterns defined photolithographically across the surface of the 10×10mm2 sample. Buffer leakage measurements demonstrate that a Be:GaN layer effectively isolates the channel from the conductive substrate. Average sheet resistances and sheet densities were 380Ω∕◻ and 0.94×1013cm−2, respectively. These HEMT structures exhibit room-temperature Hall mobilities in excess of 1900cm2∕Vs. In addition, devices on these structures exhibit excellent pinch-off, low gate leakage, and saturated drain current densities of almost 700 mA/mm. Further details regarding the structural and electrical properties will be described along with device testing.
Regrown-emitter InP heterojunction bisucpolar transistors
Authors: Kadow C. , Gossard A. C. , Rodwell M. J. W.
Published: July 2005

We present the molecular beam epidaxy (MBE) growth and the dc device performance of the first fully epitaxial regrown-emitter InP heterojunction bipolar transistors (HBTs). Here the emitter layers are regrown by MBE onto a patterned base–collector template. This process is aimed at reducing the emitter resistance, a key parameter for scaling high speed logic circuits. Emitter resistance is reduced because the emitter contact is formed to an extrinsic emitter larger than the base–emitter junction. Scanning electron microscope images demonstrate controlled, uniform etching to the intrinsic base layer, high-quality epitaxial MBE growth onto the patterned surface, and good sidewall coverage. HBTs with base-emitter junctions ranging from 48×48 to 2.5×2.5μm2 have maximum dc current gains between 5.0 and 7.5. Comparison to regular HBTs shows that the dc current gains are limited by excess base currents. Possible contributions to the excess base currents are discussed. Transmission line method measurements show that low emitter resistance and low base resistance are feasible in this regrown-emitter HBT technology.
Self-assembled quantum-dot molecules by molecular-beam epitaxy
Authors: Suraprapapich S. , Thainoi S. , Kanjanachuchai S. , Panyakeow S.
Published: July 2005

Self-assembled InAs quantum-dot (QD) molecules having high dot density and aligned dot set structure, which is defined by nanotemplates, were realized by thin capping and regrowth technique in a molecular-beam epitaxy process. Thin capping of GaAs on InAs QDs leads to the creation of nanoholes having a camel-like nanostructure due to anisotropic strain fields along the [11¯0] crystallographic direction and anisotropic surface diffusion accompanying the QD collapse. Regrowth of InAs QDs on the nanohole templates initially results in the formation of QDs with good size uniformity in the middle of features with the shape of propeller blades. This takes place at the regrowth thickness of 0.6 monolayer (ML). The strain at propellers’ edge starts to play its role, creating sets of quantum dots surrounding the initial and centered dots at the regrowth thickness of 1.2 ML. The elongated configuration of propellers’ blades defines the pattern of QD sets having five to six dots on each side. The dot density of the QD molecules is 3×1010cm−2, one order of magnitude higher than that of initial dot density (2×109cm−2).
Integrated field effect transistors for microelectromechanical systems applications, modeling, and results
Authors: Young Ralph W. , Draper Bruce L.
Published: July 2005

We have demonstrated a simple technique for building n-channel metal-oxide-semiconductor-field effect transistors (MOSFETs) and complex microelectromechanical systems (MEMS) simultaneously, instead of serially, allowing a more straightforward integration of complete systems. The fabrication sequence uses few additional process steps and only one additional masking layer compared to a MEMS-only technology, but uses processes outside the bounds of conventional complementary metal-oxide-semiconductor very large-scale integrated devices. The process flow forms the MOSFET gate electrode using the first level of mechanical polycrystalline silicon (polysilicon), while the MOSFET source and drain regions are formed by dopant diffusions into the substrate from subsequent levels of heavily doped polysilicon that are used for mechanical elements. We have observed that phosphorus-doped gate polysilicon shows negligible dopant diffusion through the gate oxide during high-temperature MEMS processing anneals, whereas arsenic-doped gate polysilicon shows significant diffusion through the oxide. We have adapted a commercial process and device simulator to model a wide range of device designs and process flows.
Technique for site-specific plan-view transmission electron microscopy of nanostructural electronic devices
Authors: Bassim N. D. , Twigg M. E.
Published: July 2005

This brief report demonstrates the feasibility of preparing site-specific plan-view samples of simple electronic devices and nanostructures for the transmission electron microscope (TEM). By combining the techniques of tripod angle lapping and single-sided ion milling in a different way, we are able to target specific devices for analysis in the TEM. This is essentially performed by back-angle lapping a patterned wafer that contains multiple features so that an edge is at or near a level of electron transparency. At this point, the sample is glued to a copper ring using conducting epoxy, and a piece of protective transparent material (preferably a piece of glass) is waxed to the device face in order to protect the device from sputtering and material redeposition on the opposite side. The device is then milled in a liquid-nitrogen-cooled ion mill in order to avoid specimen heating (which would melt the wax). By aligning the device of interest perpendicular to the lapping angle, the device may be examined optically as it is milled back to the feature of interest. This technique was used to examine the active region of a gold nanoparticle-based device to observe the alignment of these particles between two contact pads. This result is extremely promising because it allows the observation of materials on a lateral scale that are extremely sensitive to damage by sputtering. This sample preparation technique would be useful for researching various processing steps in the design of electronic devices, as well as patterned samples in general.
Enhancement of He-induced cavities in silicon by hydrogen plasma treatment
Authors: Liu C. L. , Ntsoenzok E. , Vengurlekar A. , Ashok S. , Alquier D. , Ruault M. O. , Dubois C.
Published: July 2005

The formation of subsurface nm-size cavities in Si from He implantation followed by thermal anneal involves a complex interaction of He with vacancy clusters. We have attempted to promote cavity formation with vacancy-type defects arising from a hydrogen plasma treatment that is interposed between the implantation (40keV and 160keV He) and anneal (800°C-1h) steps. Cross-sectional transmission electron microscopy (XTEM) results show enhanced growth of He-induced cavities due to hydrogen in the 160keV He implanted sample, while no significant change is seen in the cavity spectrum for 40keV. In conjunction with Secondary Ion Mass Spectroscopy (SIMS) data, the results are tentatively interpreted in terms of the evolution of defects and hydrogen during annealing, their interactions with the He-cavities, and proximity of the layers to the surface.
Nanostructured carbon-metal composite films
Authors: Narayan Roger J. , Scholvin Dirk
Published: July 2005

Diamondlike carbon–silver and diamondlike carbon–titanium nanocomposite films with thicknesses greater than one μm were deposited on Ti–6Al–4V alloy substrates using a multicomponent pulsed laser deposition process. Z-contrast scanning transmission electron microscopy (STEM-Z) and electron energy loss spectroscopy (EELS) were used to determine film microstructure and carbon bonding, respectively. Self-assembled silver nanodot arrays were observed in the diamondlike carbon–silver films, and alternating titanium carbide/diamondlike carbon nanolayers were observed in the diamondlike carbon–titanium films. Scratch testing demonstrated good adhesion of diamondlike carbon–metal nanocomposite films to the Ti–6Al–4V alloy. Nanoindentation testing of the diamondlike carbon-metal films demonstrated excellent hardness and modulus values, within the range of 35GPa and 350GPa, respectively. Diamondlike carbon-metal composite films have numerous potential applications, including use in machine tools, medical prostheses, and electronic devices.
Etching silicon-containing bilayer resists in ammonia-based plasmas
Authors: Panda Siddhartha , Wise Richard , Mahorowala Arpan , Balasubramanium Vaidya , Sugiyama Kenro
Published: July 2005

The need to print smaller feature sizes has led to the shift from 248nmto193nm lithography. The disadvantages of 193nm ArF single-layer resist materials, such as lower depth of focus and lower etch resistance, have made thin-film imaging (TFI) techniques an attractive alternative. In the bilayer (comprised of an image layer and an underlayer) approach, a form of TFI, O2-based plasma chemistries are used for the transfer etches whereby oxidation of silicon in the image layer provides the required etch resistance. However, use of oxygen results in profile and critical dimension (CD) control issues. While gas additives have helped minimize these problems, there are other accompanying disadvantages. In this work, the feasibility of a non-O2-containing NH3-based plasma etch chemistry was evaluated. Effects of additive gases, such as N2, H2, and C2H4, were investigated. Surface analysis of the resist showed that nitrogen from the gas phase was incorporated in the surface of the image layer during the etch, and the etch resistance increased with this nitrogen content. Gas phase analysis showed that the relative N2H species density in the system was correlated with the nitrogen incorporated in the image layer, and is thus believed to govern the etch resistance of the image layer. The underlayer etch behavior was correlated with that of the atomic hydrogen species density. The effect of the additives on the cross section profiles and line edge roughness were studied. The highest CD biases were obtained with H2 as the additive, while near-zero CD biases were attained with C2H4 as the additive. Also, with the C2H4 additive, minimum line edge roughness and smoothest profiles were obtained.
Growth and characterization of Si-doped self-assembled InAs quantum dots
Authors: Nah Jongbum
Published: July 2005

Si-doped self-assembled InAs quantum-dot samples were grown, and the influences of doping on the structural and optical properties of quantum dots were investigated. As Si doping concentrations increased, increased size, reduced areal density, and broadened size distributions of quantum dots were observed. Photoluminescence spectra of Si-doped quantum dots showed that the peaks of ground state transitions shifted with varied doping concentrations while, the peak positions of wetting layers were unchanged. Photoluminescence intensities of excited state transitions of Si-doped quantum dots and of wetting layers were reduced as Si concentration increased.
Electron emission from boron nitride films deposited on patterned GaAs substrates
Authors: Shima Hidekazu , Funakawa Shingo , Kimura Chiharu , Sugino Takashi
Published: July 2005

Electron emission from a patterned substrate is investigated in order to achieve the uniform current density. Square mesas of 10μm×10μm and 100μm×100μm sizes are formed on (100) GaAs surface by a photolithography technique and wet etching. Each square mesa is 3μm high and its sidewalls are vertical. The field enhancement factor of the patterned substrate is estimated to be around 240. Field emission characteristics are compared between boron nitride (BN) films deposited on flat and patterned substrates. The threshold electric field is estimated to be 17 and 10V∕μm for BN films on flat and patterned substrates, respectively. It is demonstrated that the introduction of patterned substrates is effective in not only reducing the threshold electric field but also improving the uniformity of the electron emission.
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InAs quantum dots grown with an As2 source using molecular-beam epitaxy
Authors: Sugaya Takeyoshi , Komori Kazuhiro , Yamauchi Shougo , Amano Takeru
Published: July 2005

We demonstrate the effects of using an As2 source to fabricate self-organized InAs∕GaAs quantum dot (QD) structures. QDs grown with As2 and As4 sources have narrow photoluminescence (PL) linewidths (22 and 20meV, respectively) and their respective emissions at room temperature are 1.30 and 1.29μm. QDs grown with an As2 source have a longer wavelength emission than those grown with an As4 source under all growth conditions. The density of QDs grown with an As4 source is larger and the dot size smaller than those of QDs grown with an As2 source. These results indicate that QDs grown with As2 are larger, resulting in a longer PL wavelength.
Much improved flat interfaces of InGaAs∕AlAsSb quantum well structures grown on (411)A InP substrates by molecular-beam epitaxy
Authors: Imura M. , Kurohara H. , Masui Y. , Asano T. , Kitada T. , Shimomura S. , Hiyamizu S.
Published: July 2005

In 0.53 Ga 0.47 As ∕ Al As 0.56 Sb 0.44 quantum well (QW) structures were grown on a (411)A oriented InP substrate by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra at 12K indicated that interface flatness of a 2.4nm thick In0.53Ga0.47As QWs on the (411)A InP substrate, which can be utilized for 1.55μm range all-optical-switching devices using intersubband transition (ISBT), is much superior to that of QWs simultaneously grown on a conventional (100) InP substrate over the whole range of the growth temperature (Ts=480–570°C). The best value of full width at half-maximum of a PL peak (12K) from the 2.4nm thick In0.53Ga0.47As∕AlAs0.56Sb0.44 QW grown on the (411)A InP substrate at 570°C was 36meV, which is much smaller than the best value previously reported (58meV) for similar (100) In0.53Ga0.47As∕AlAs0.56Sb0.44 QWs. This result indicates that MBE growth of In0.53Ga0.47As∕AlAs0.56Sb0.44 QWs on the (411)A InP substrate significantly improves their interface flatness and 1.55μm range In0.53Ga0.47As∕AlAs0.56Sb0.44 ISBT devices fabricated on the (411)A InP substrate are expected to provide much better performance.
Influence of AlN nucleation layer on the epitaxy of GaN/AlGaN high electron mobility transistor structure and wafer curvature
Authors: Torabi A. , Hoke W. E. , Mosca J. J. , Siddiqui J. J. , Hallock R. B. , Kennedy T. D.
Published: July 2005

Significant wafer curvature has been observed for AlGaN/GaN high electron mobility transistor (HEMT) structures grown on SiC substrates by rf plasma molecular-beam epitaxy. The curvature is caused by residual compressive strain in the films, due primarily to the lattice mismatch between substrate and epilayer. The wafers exhibit more bow when an AlN nucleation layer is used, than when GaN/AlGaN is grown directly on SiC. However, in test structures, AlN nucleation layers are found to impart tensile strain in the wafer that is small due to the AlN thickness. Using high resolution x-ray diffraction with reciprocal space maps, thin GaN films are found to relax more readily when grown directly on SiC substrates than on AlN buffer layers. The compressive strain in the thick GaN buffer layer grown on AlN bows the wafer and increases the substrate x-ray diffraction (XRD) linewidth. The GaN buffer, despite its thickness, does not relax fully but retains some residual strain.
Enhanced strain relaxation rate of InGaAs by adatom-assisted dislocation kink nucleation
Authors: Lynch C. , Chason E. , Beresford R.
Published: July 2005

In situ stress monitoring has revealed that the strain relaxation rate of InxGa1−xAs∕GaAs thin films is significantly enhanced (over 20 times faster at 500 °C) by the presence of a growth flux. We have explained this effect with a model in which the energy for dislocation single kink nucleation is lowered in the presence of a supersaturation of adatoms. In this paper, we present additional measurements showing that the growth rate dependence of this enhancement agrees well with our model. We also address alternate explanations for these observations.
Molecular-beam epitaxy growth of low-threshold cw GaInNAsSb lasers at 1.5μm
Authors: Bank Seth R. , Wistey Mark A. , Yuen Homan B. , Goddard Lynford L. , Bae Hopil , Harris James S.
Published: July 2005

The solid-source molecular-beam of low-threshold GaAs-based GaInNAsSb lasers is discussed. A general narrowing of the window was observed with increasing wavelength, due to the increased nitrogen required (⩾1%), and has historically made high-performance devices more difficult to achieve beyond ∼1.35μm. The introduction of and reduction in plasma-related damage from the rf nitrogen dramatically improved material quality and widened the window. We validate these observations with 1.5μm edge-emitting ridge-waveguide lasers with cw threshold as low as 440A∕cm2, peak CW output powers of 431mW, peak cw wallplug efficiencies >16%, and pulsed output powers >1W.
Implementing multiple band gaps using inductively coupled argon plasma enhanced quantum well intermixing
Authors: Nie D. , Mei T. , Djie H. S. , Chin M. K. , Tang X. H. , Wang Y. X.
Published: July 2005

The implementation of multiple band gaps on a single InP substrate with an InGaAs∕InGaAsP quantum well laser structure via the control of local defect concentrations in the process of inductively coupled argon (Ar) plasma enhanced quantum well intermixing is reported. With multistep plasma exposure, different levels of near-surface point defect concentrations are established in different areas, which lead to different extents of band gap modification in a single-step rapid thermal annealing (RTA). Three distinct band gaps with blueshifts of 84, 66, and 3nm with respect to that of the as-grown sample are achieved in a process involving two steps of Ar plasma exposure and a single step of annealing in nitrogen ambient at 600°C for 2min. As an indication of material quality preservation, no intensity reduction and linewidth broadening in the photoluminescence caused by the process has been observed. This work demonstrates a practical approach of multiple band gap modification for the InP-based photonic integrated circuit application.
Nitrogen-incorporated multiwalled carbon nanotubes grown by direct current plasma-enhanced chemical vapor deposition
Authors: Yang Ji Hoon , Kim Bub Jin , Kim Yun Hee , Lee Young Jin , Ha Byung Ho , Shin Yong Sook , Park Serng-Yerl , Kim Hyun Suk , Park Chong-Yun , Yang Chul Woong , Yoo Ji-Beom , Kwon Myeung Hoi , Ihm Kyuwook , Song Ha-Jin , Kang Tai-Hee , Shin Hyun-Joon , Park Young-Jun , Kim Jong-Min
Published: July 2005

The nitrogen-incorporated multiwalled carbon nanotubes (N-MWCNTs) were synthesized by dc plasma-enhanced chemical vapor deposition with a gas mixture of C2H2, NH3, and N2. Nitrogens in the N-MWCNTs were pyridinic nitrogen and graphitic nitrogen. With increase in the flow rate of N2 gas during the synthesis of MWCNTs, the pyridinic nitrogen increased much more than graphitic nitrogen. The near-edge x-ray absorption fine structure spectra revealed that the density of states such as π*, σ*, and π*+σ* bands of the N-MWCNTs decreased with increase of concentration of pyridinic nitrogen incorporated in the MWCNTs. The intensity ratio of the D band to the G band of Raman spectrum increased with the incorporation of nitrogen into MWCNTs.
Fabrication of two- and three-dimensional photonic crystals of titania with submicrometer resolution by deep x-ray lithography
Authors: Awazu Koichi , Wang Xiaomin , Fujimaki Makoto , Kuriyama Taketo , Sai Akihide , Ohki Yoshimichi , Imai Hiroaki
Published: July 2005

Two-dimensional photonic crystals of titanium dioxide are predicted to have many advantages over semiconductor photonic crystals, e.g., silicon and GaAs: in particular, low optical loss in the near infrared region used for optical communication, low thermal expansion, and a refractive index which is close to that of optical fibers. However, it is difficult to create micronanostructures in titanium dioxide, since semiconductor microfabrication techniques cannot be applied to titanium dioxide. As the first step, we calculated the photonic band gap of titanium dioxide rod slab on SiO2. Band gap percent against thickness of the rod slab was also examined. Finally, we confirmed the most suitable structure for two-dimensional (2D) photonic crystals. A deep x-ray lithography technique was employed to create a very deep and precise template. Liquid-phase deposition was then used to faithfully deposit a tightly packed layer of titanium oxide onto the template. Finally, the template was selectively removed to obtain a photonic nanostructure. We also calculated the photonic band gap for the 3D structure of TiO2. A template for the most appropriate structure was fabricated using the method proposed by Yablonovitch. By employing the same method, we successfully obtained the 3D structure of TiO2. The refractive index of the obtained TiO2 followed by heating at 700°C was determined as being 2.5, which is close to that of the anatase phase.
Structure and magnetic properties of Cr-doped GaN
Authors: Kim J. J. , Makino H. , Sakurai M. , Oh D. C. , Hanada T. , Cho M. W. , Yao T. , Emura S. , Kobayashi K.
Published: July 2005

Structure and magnetic properties of the Ga1−xCrxN (x=0.013, 0.063, and 0.101) have been investigated. The lattice constant of the c axis is systematically decreased with increasing Cr content. The local structure around a Cr atom maintains tetrahedral symmetry up to x=0.101 the same as the local symmetry of GaN by x-ray absorption fine structure analysis. The analysis on x=0.013 indicated that the second nearest neighbor around the absorbing Cr atoms consists of only 12Ga atoms. However, for x=0.063, the second nearest neighbor around the absorber consists of Ga and Cr atoms with an unexpectedly high ratio of about Ga:Cr=2:1. In CrK-edge x-ray absorption near edge structure, we observed the oxidation state of Cr ion increases with increasing Cr content. Ferromagnetic behavior was observed in all Ga1−xCrxN films. However, the paramagnetic component also coexists with the ferromagnetic component. Total effective magnetic moment per Cr atom decreased from 3.17μB∕atom for x=0.013 to 1.05 and 0.79μB∕atom for x=0.063 and 0.101, respectively. The decreased magnetic moments of the x=0.063 and 0.101 is possibly caused by antiferromagnetic interaction of Cr–N–Cr networks in the high Cr content samples.
Specular reflectance spectroscopy for substrate temperature determination in radio frequency-plasma molecular beam epitaxy of nitride semiconductors
Authors: Katzer D. S. , Storm D. F. , Binari S. C. , Holm R. T. , Mahon R. , Yang M. J. , Freitas J. A.
Published: July 2005

Specular reflectance spectroscopy was applied to several types of substrates commonly used for nitride molecular beam epitaxial growth. Calibration curves were generated from measurements of the observed shift in the reflectance point-of-inflection energy versus temperature for semi-insulating (SI) 4H-SiC, SI 6H-SiC, n+6H-SiC, and n−2H-GaN between room temperature and up to 900°C. We have applied the technique to the molecular beam epitaxy of AlN and GaN on SiC and discuss the advantages and limitations of the technique.
Direct deposition of aligned nanorod array onto cylindrical objects
Authors: Fan J.-G. , Zhao Y.-P.
Published: July 2005

A simple and robust method is developed for directly depositing aligned and well separated nanorod arrays onto cylindrical objects based on oblique angle deposition. By rotating the uniform or tapered cylindrical object such as optical fiber with a proper tilting angle, aligned nanorods from materials such as metals, semiconductors, and metal oxides have been successfully deposited. With a slight modification, multilayer film∕nanorod and nanorod∕nanorod structures have also been demonstrated. This method has the advantage of being able to coat aligned nanorods on cylindrical objects at low temperature, and it is particularly promising for preparing nanostructured fiber optical probes for sensor applications.
Chemisorption of C60 on the Si(001)-2×1 surface at room temperature
Authors: Cheng C.-P. , Pi T.-W. , Ouyang C.-P. , Wen J.-F.
Published: July 2005

A synchrotron-radiation photoemission study of C60 deposited on a clean Si(001)-2×1 surface at room temperature is presented. It is found that the C60 molecules are chemisorbed on the Si(001)-2×1 surface with a characteristic of the covalent bond. A Si–C60 hybridization causes charge transfer from the Si substrate to the C60 molecules. Consequently, induced surface components appear in the Si2p and C1s cores with opposite signs. To be specific for 1 monolayer coverage, two interface-induced Si2p components are clearly resolved at the core-level shifts of +0.38 and +0.97eV. For the C1s cores, in addition to a C60-related peak at 284.50eV, an induced peak shows up with a negative shift of −0.70eV, suggesting that the carbon atoms hybridized with silicon surface atoms gain charge.
Comparison of ZnO metal–oxide–semiconductor field effect transistor and metal–semiconductor field effect transistor structures grown on sapphire by pulsed laser deposition
Authors: Kao C. J. , Kwon Yong Wook , Heo Y. W. , Norton D. P. , Pearton S. J. , Ren F. , Chi G. C.
Published: July 2005

ZnO thin film field effect transistors with 1.5–20μm gate width were fabricated using either a metal gate [metal–semiconductor field effect transistor (MESFET)] or a metal–oxide–semiconductor (MOS) gate. In both cases we found that use of a thick (∼0.8–0.9μm) ZnO buffer was necessary on the sapphire or glass substrate prior to growing the active layers in order to reduce gate leakage current. Source/drain contacts of e-beam deposited Ti∕Al∕Pt∕Au showed specific contact resistances of 2.18×10−6Ωcm2 without annealing and the interdevice isolation currents were ∼10μA at 40V bias. The MOS structure with 50nm (Ce,Tb)MgAl11O19 gate dielectric showed a 1 order of magnitude lower gate leakage current than the MESFET, due to the relatively low barrier height of metals on n-type ZnO (0.6–0.8eV). Good drain–source current characteristics were obtained from MOS gate structures using P-doped ZnO channels, whereas the metal structures showed very poor modulation.
Molecular beam epitaxy growth of novel double-layer InAs quantum dot structures and their optical properties
Authors: Ohmori M. , Kawazu T. , Torii K. , Sakaki H.
Published: July 2005

We have designed and grown by molecular beam epitaxy (MBE) a novel sample with Schottky contact in which two sheets of InAs quantum dots (QDs) of different sizes are embedded at different positions of its depletion layer. Electronic states in these two respective QD layers are expected to be perturbed not only by the space-charge electric field but also by the carrier accumulation in QDs of both layers. We have investigated photoluminescence (PL) spectra of each dot layer under various bias conditions to clarify, in particular, how the local electric field is influenced by the accumulation of carriers in each dot layer, especially when two dot layers are closely spaced (25nm). The ground state transition energy of large dots shows an anomalous blue shift, which is as large as 11meV when a positive bias is applied. We show that the depopulation of charged dots is responsible for this anomaly.
LENGTH SCALE EFFECTS OF SMALL MODEL PROTEIN FOLDING
Authors: ZHANG LINSEN , ZHANG MING , CHEN CHANGJUN , XIAO YI
Published: July 2005

The purpose of this paper is to investigate the length scale effects of small protein folding by lattice modeling. Heat capacity is computed to analyze folding transition temperature of the model proteins of different lengths. The results show that there is an evident phase transition when the lengths of the model proteins are similar to those of foldons. Furthermore, it is found that the folding transition temperature is proportional to the sequence length.
MODELING EPIDEMIC BASED ON PENNA MODEL
Authors: HE MINGFENG , PAN QIUHUI , YU BINGLIN
Published: July 2005

A cellular automata model is developed aiming to study the epidemic on a lattice of two dimensions. The characteristics of individual including reproduction, death by inherited diseases and age are studied in the Penna model. The spreading of epidemic and the reproduction of individual are considered as cellular automata. We shall mainly discuss the influence on the number of epidemical patients by infection rate and death rate. The results show that the existence of epidemic mainly depends on the infection rate and the death rate but not the initial ratio of patients. There are two reasons for nonexistent epidemic. As long as the infection rate is less than 0.3, the epidemic cannot spread. On the other hand, if the infection rate and the death rate are both high, the individuals with epidemic die out and the epidemic cannot spread, either. The epidemic can exist all along when the combination of infection rate and death rate are in the mid area.
HYPERCHAOS IN THE FRACTIONAL-ORDER NONAUTONOMOUS CHEN'S SYSTEM AND ITS SYNCHRONIZATION
Authors: ZHANG HONGBIN , LI CHUNGUANG , CHEN GUANRONG , GAO XING
Published: July 2005

Recently, a new hyperchaos generator, obtained by controlling a three-dimensional autonomous chaotic system — Chen's system — with a periodic driving signal, has been found. In this letter, we formulate and study the hyperchaotic behaviors in the corresponding fractional-order hyperchaotic Chen's system. Through numerical simulations, we found that hyperchaos exists in the fractional-order hyperchaotic Chen's system with order less than 4. The lowest order we found to have hyperchaos in this system is 3.4. Finally, we study the synchronization problem of two fractional-order hyperchaotic Chen's systems.
THERMAL STABILITY OF BENZOROD ARRAYS: MOLECULAR-DYNAMICS SIMULATIONS
Authors: MALCIOGLU OSMAN BARIS , ERKOÇ ŞAKIR
Published: July 2005

A set of Benzorod arrays on a graphene substrate has been investigated by performing classical molecular-dynamics simulations. Benzorod is composed of aligned and dehydrogenated benzene rings that are stacked to form a rod-like structure. It has been found that the arrays considered are thermally stable up to elevated temperatures, with a dependence on length.
MOLECULAR TRANSPORT AND FLUIDICS IN CARBON NANOTUBE
Authors: WEI CHENYU , SRIVASTAVA DEEPAK
Published: July 2005

In this article we review recent developments in molecular transport and fluidics in carbon nanotube (CNT)-based nanochannels. Atomic molecular dynamics simulations and theoretical studies based on Fokker–Planck diffusion equation on the transport of large and long polymer molecules in CNTs are the focus of the article. Fast translocation and diffusion processes of large molecules in CNTs are reviewed and discussed, considering the effects of interfacial interactions and molecular conformations and structures at interface. The transport features for multiple molecules diffusing through CNTs are also discussed.
PROTEIN STRUCTURE AND FOLD PREDICTION USING TREE-AUGMENTED NAÏVE BAYESIAN CLASSIFIER
Authors: CHINNASAMY ARUNKUMAR , SUNG WING-KIN , MITTAL ANKUSH
Published: July 2005

Due to the large volume of protein sequence data, computational methods to determine the structure class and the fold class of a protein sequence have become essential. Several techniques based on sequence similarity, Neural Networks, Support Vector Machines (SVMs), etc. have been applied. Since most of these classifiers use binary classifiers for multi-classification, there may be Nc2 classifiers required. This paper presents a framework using the Tree-Augmented Bayesian Networks (TAN) which performs multi-classification based on the theory of learning Bayesian Networks and using improved feature vector representation of (Ding et al., 2001).4 In order to enhance TAN's performance, pre-processing of data is done by feature discretization and post-processing is done by using Mean Probability Voting (MPV) scheme. The advantage of using Bayesian approach over other learning methods is that the network structure is intuitive. In addition, one can read off the TAN structure probabilities to determine the significance of each feature (say, hydrophobicity) for each class, which helps to further understand the complexity in protein structure.

The experiments on the datasets used in three prominent recent works show that our approach is more accurate than other discriminative methods. The framework is implemented on the BAYESPROT web server and it is available at . More detailed results are also available on the above website.
DYNAMIC MODEL-BASED CLUSTERING FOR TIME–COURSE GENE EXPRESSION DATA
Authors: WU FANG-XIANG , ZHANG W. J. , KUSALIK ANTHONY J.
Published: July 2005

Microarray technology has produced a huge body of time-course gene expression data. Such gene expression data has proved useful in genomic disease diagnosis and genomic drug design. The challenge is how to uncover useful information in such data. Cluster analysis has played an important role in analyzing gene expression data. Many distance/correlation- and static model-based clustering techniques have been applied to time-course expression data. However, these techniques are unable to account for the dynamics of such data. It is the dynamics that characterize the data and that should be considered in cluster analysis so as to obtain high quality clustering. This paper proposes a dynamic model-based clustering method for time-course gene expression data. The proposed method regards a time-course gene expression dataset as a set of time series, generated by a number of stochastic processes. Each stochastic process defines a cluster and is described by an autoregressive model. A relocation-iteration algorithm is proposed to identity the model parameters and posterior probabilities are employed to assign each gene to an appropriate cluster. A bootstrapping method and an average adjusted Rand index (AARI) are employed to measure the quality of clustering. Computational experiments are performed on a synthetic and three real time-course gene expression datasets to investigate the proposed method. The results show that our method allows the better quality clustering than other clustering methods (e.g. k-means) for time-course gene expression data, and thus it is a useful and powerful tool for analyzing time-course gene expression data.
vALId: VALIDATION OF PROTEIN SEQUENCE QUALITY BASED ON MULTIPLE ALIGNMENT DATA
Authors: BIANCHETTI LAURENT , THOMPSON JULIE DAWN , LECOMPTE ODILE , PLEWNIAK FREDERIC , POCH OLIVIER
Published: July 2005

The validation of sequences is essential to perform accurate phylogeny and structure/function analysis. However among the thousands of protein sequences available in the public databases, most have been predicted in silico and have not systematically undergone a quality verification. It has recently become evident that they often contain sequence errors. To address the problem of automatic protein quality control, we have developed vALId, an interactive web interfaced software. Taking advantage of high quality multiple alignments of complete protein sequences (MACS), vALId first warns about the presence of suspicious insertions, deletions (indels) and divergent segments, and second, proposes corrections based on transcripts and genome contigs. In a first evaluation test, hundreds of indels and divergent segments were randomly generated in a manually refined MACS. The sensitivity (Sn) and specificity (Sp) of indel detection were excellent (0.96) while the mean Sn(0.49) and Sp(0.56) of divergent segment delineation depended on the percent identity between sequence neighbors. In a second test, 6195 sequences in 100 MACS corresponding to different functional and structural protein families were analyzed. 65% of the sequences were in silico predictions and 44% of eukaryote predicted proteins were partially incorrect with at least one suspicious indel or divergent segment.
NOISE-ROBUST SOFT CLUSTERING OF GENE EXPRESSION TIME-COURSE DATA
Authors: FUTSCHIK MATTHIAS E. , CARLISLE BRONWYN
Published: July 2005

Clustering is an important tool in microarray data analysis. This unsupervised learning technique is commonly used to reveal structures hidden in large gene expression data sets. The vast majority of clustering algorithms applied so far produce hard partitions of the data, i.e. each gene is assigned exactly to one cluster. Hard clustering is favourable if clusters are well separated. However, this is generally not the case for microarray time-course data, where gene clusters frequently overlap. Additionally, hard clustering algorithms are often highly sensitive to noise.

To overcome the limitations of hard clustering, we applied soft clustering which offers several advantages for researchers. First, it generates accessible internal cluster structures, i.e. it indicates how well corresponding clusters represent genes. This can be used for the more targeted search for regulatory elements. Second, the overall relation between clusters, and thus a global clustering structure, can be defined. Additionally, soft clustering is more noise robust and a priori pre-filtering of genes can be avoided. This prevents the exclusion of biologically relevant genes from the data analysis. Soft clustering was implemented here using the fuzzy c-means algorithm. Procedures to find optimal clustering parameters were developed. A software package for soft clustering has been developed based on the open-source statistical language R. The package called Mfuzz is freely available.
EVOLUTION OF THE NADR REGULON IN ENTEROBACTERIACEAE
Authors: GERASIMOVA ANNA V. , GELFAND MIKHAIL S.
Published: July 2005

The NAD biosynthetic pathway and NAD transformations in E. coli and S. typhi are well characterized. Using comparative genomics methods we describe the NadR regulon in other Enterobacteriaceae, identity new candidate regulon members and demonstrate that even a very simple regulon covering an essential methabolic pathway could be different in closely related genomes.
IMPROVING SUPERVISED LEARNING BY SAMPLE DECOMPOSITION
Authors: ROKACH LIOR , MAIMON ODED , ARAD OMRI
Published: July 2005

This paper introduces a new ensemble technique, cluster-based concurrent decomposition (CBCD) that induces an ensemble of classifiers by decomposing the training set into mutually exclusive sub-samples of equal-size. The CBCD algorithm first clusters the instance space by using the K-means clustering algorithm. Afterwards it produces disjoint sub-samples using the clusters in such a way that each sub-sample is comprised of tuples from all clusters and hence represents the entire dataset. An induction algorithm is applied in turn to each subset, followed by a voting mechanism that combines the classifier's predictions. The CBCD algorithm has two tuning parameters: the number of clusters and the number of subsets to create. Using a suitable meta-learning it is possible to tune these parameters properly. In the experimental study we conducted, the CBCD algorithm, using an embedded C4.5 algorithm, outperformed the bagging algorithm of the same computational complexity.
DIRECT ADAPTIVE FUZZY MOVING SLIDING MODE CONTROLLER DESIGN FOR ROBOTIC MANIPULATORS
Authors: MEDHAFFAR HANÈNE , DAMAK TARAK , DERBEL NABIL
Published: July 2005

This paper presents an adaptive moving sliding mode fuzzy control for robotic manipulators. The consequence parameters of the fuzzy control rules are adjusted online by applying the Lyapunov stability condition. Furthermore, parameters tuning is carried out in order to guarantee the sliding condition. Comparison between zero order and first order Sugeno model are presented. Moreover, a time-varying sliding surface is used with the aim of enhancing tracking-performance. Indeed, moving sliding surface improves the robustness during the reaching phase against uncertainties, parameter variations and extraneous disturbances. As an illustration, the trajectory control of a two degrees-of-freedom robotic manipulator is considered.
The Genetics and Evolution of Fluctuating Asymmetry
Authors: Leamy Larry J. , Klingenberg Christian Peter
Published: July 2005

▪ Abstract  Variation in the subtle differences between right and left sides of bilateral characters, or fluctuating asymmetry (FA), has long been considered to be primarily environmental in origin, and this has promoted its use as a measure of developmental instability (DI) in populations. There is little evidence for specific genes that govern FA per se. Numerous studies show that FA levels in various characters are influenced by dominance and especially epistatic interactions among genes. An epistatic genetic basis for FA may complicate its primary use in comparisons of DI levels in outbred or wild populations subjected or not subjected to various environmental stressors. Although the heritability of FA typically is very low or zero, epistasis can generate additive genetic variation for FA that may allow it to evolve especially in populations subjected to bottlenecks, hybridizations, or periods of rapid environmental changes caused by various stresses.