bluuebunny/crossref_metadata_2025 huggingface dataset:
Scanned electrical probe characterization of carrier transport behavior in InAs nanowires
Authors: Zhou X. , Dayeh S. A. , Aplin D. , Wang D. , Yu E. T.
Published: August 2006

Conductive atomic force microscopy combined with detailed calculations of electronic subband structure has been used to analyze electron transport behavior across the transition between ballistic and drift/diffusive motion in InAs nanowires grown by metal organic chemical vapor deposition. Using a conducting tip in an atomic force microscope as a local, positionable electrical probe, InAs nanowire resistance as a function of electron transport distance within the nanowire has been measured. For distance of ∼200nm or less, this resistance is observed to be nearly independent of distance, while for larger distances the resistance increases linearly with distance. Analytical calculations indicate that a resistance only weakly dependent on distance should be observed for distances up to a few times the electron mean free path, and calculations of the mean free path using a variety of approaches yield values in the range of ∼50nm, very consistent with the observation of distance-independent resistance for transport distances up to ∼200nm.
Dependence of field emission properties of carbon nanotube films on their graphitization
Authors: Ting Jyh-Hua , Li Tsung-Lung , Hong Ying-Chi
Published: August 2006

The purpose of this study is to investigate the effect of graphitization of carbon nanotube (CNT) films on their field emission efficiencies. CNT films were prepared by microwave plasma-enhanced chemical vapor deposition (MPCVD) method. Transmission electron microscopy images reveal the center hollowness and multiwalled structure of a CNT. The tip-growth mechanism of the CNTs prepared by MPCVD is confirmed by the nickel particles enclosed at the tips of the CNTs. The intensity ratio of IG∕(ID+IG) under the Raman spectrum was defined to characterize the degree of graphitization of the CNTs. At any methane flow ratios [CH4∕(H2+CH4)], the CNT graphitization increases with the microwave power. Also, the CNT graphitization increases with the CH4 flow ratio and begins to drop at the flow ratio of 15%. It is mainly attributed to the relative concentration of carbon radicals in the hydrocarbon-based plasma. Enhancement of the graphitization of CNT emitter array leads to the decrease of the threshold field and the increase of the field-controlled current density consistently. This consistency suggests that higher concentration of sp2 bonding enhances conductivity. Therefore, the field emission efficiency of the CNTs increases with the CNT graphitization. At the microwave power of 1200W and the methane flow ratio of 15%, the graphitization intensity ratio is 0.637. The optimal threshold field and field-controlled current density at the field of 3V∕μm are 1.38V∕μm and 8.934mA∕cm2, respectively.
Growth and application of highly ordered array of vertical nanoposts
Authors: Kuo Teng-Fang , Xu Jimmy
Published: August 2006

In this article, we discuss a few recent advances in fabrication and application of highly ordered nanopost arrays. These arrays are vertically aligned and are uniform in diameter, height, and spacing. They can be made from a large variety of materials ranging from metals, semiconductors, and carbon nanotubes. The keys to achieving a high degree of uniformity and ordering are in the formation and the use of a highly ordered nanopore array as a growth template or mask. The uniformity and vertical orientation greatly facilitate the use of these arrays in biomolecular interfacing, field (optical and acoustic) sensing and modulation, as well as field emitters.
Silicon resonant microcantilevers for absolute pressure measurement
Authors: Bianco S. , Cocuzza M. , Ferrero S. , Giuri E. , Piacenza G. , Pirri C. F. , Ricci A. , Scaltrito L. , Bich D. , Merialdo A. , Schina P. , Correale R.
Published: August 2006

This work is focused on the developing of silicon resonant microcantilevers for the measurement of the absolute pressure. The microcantilevers have been fabricated with a two-mask bulk micromachining process. The variation in resonance response of microcantilevers was investigated as a function of pressure (10−1–105Pa), both in terms of resonance frequency and quality factor. A theoretical description of the resonating microstructure is given according to different molecular and viscous regimes. Also a brief discussion on the different quality factors contributions is presented. Theoretical and experimental data show a very satisfying agreement. The microstructure behavior demonstrates a certain sensitivity over a six decade range and the potential evolution of an absolute pressure sensor working in the same range.
Selective molecular beam epitaxy growth of size- and position-controlled GaN∕AlGaN nanowires on nonplanar (0001) substrates and its growth mechanism
Authors: Sato Taketomo , Oikawa Takeshi , Hasegawa Hideki , Hashizume Tamotsu
Published: August 2006

Fundamental growth properties were investigated for the size-controlled selective MBE growth of AlGaN∕GaN nanowires on the GaN (0001) prepatterned substrates both experimentally and theoretically. The lateral size of the present GaN nanowire was determined by two facet boundaries formed within AlGaN barrier layers. From the series of wire growth experiments, the growth selectivity and the measured angle of the facet boundary strongly depended on the Al composition and the initial crystalline facets of the mesa patterned templates. The experimental evolution of the cross-sectional structures was well reproduced by a computer simulation based on the phenomenological growth model where the slope angle dependence of lifetime of adatoms was taken into account. The lateral width of present nanowires could be kinetically controlled by the growth conditions and the supply thickness of AlGaN layers.
Liquid-phase sensors using open-gate AlGaN∕GaN high electron mobility transistor structure
Authors: Kokawa Takuya , Sato Taketomo , Hasegawa Hideki , Hashizume Tamotsu
Published: August 2006

Liquid-phase sensing characteristics of open-gate AlGaN∕GaN high electron mobility transistor (HEMT) structures were investigated in aqueous solutions and polar liquids. In de-ionized water, the open-gate HEMT clearly showed good drain I-V characteristics with current saturation and pinch-off behavior, very similar to I-V characteristics of typical Schottky-gate HEMTs. We observed a fine parallel shift in the transfer curves according to change in the pH value in a solution, indicating the corresponding potential change at the AlGaN surface. The sensitivity for the potential change was 57.5mV∕pH, very close to the theoretical value of 58.9mV∕pH at 24°C for the Nernstian response to H+ ions. In the low drain bias region, the drain current linearly decreased with the pH value. This also indicated a systematic potential change at the AlGaN surface due to pH change. The present open-gate device showed a fast response to the pH change and a stable operation at fixed pH values. A possible mechanism for the pH response of the AlGaN surface is discussed in terms of equilibrium reactions of hydroxyls at the AlGaN surface with H+ in a solution. It was also found that the device was quite sensitive to changes in the electrostatic boundary conditions of the open-gate area by exposure to polar liquids. The drain current linearly decreased with increasing normalized liquid dipole moment.
Characterization of crystalline rare-earth oxide high-K dielectrics grown by molecular beam epitaxy on silicon carbide
Authors: Fissel A. , Czernohorsky M. , Osten H. J.
Published: August 2006

We have investigated the growth and electrical properties of crystalline Gd2O3 directly grown on 6H-SiC(0001) substrates by molecular beam epitaxy. Initially, Gd2O3 islands with hexagonal structure are formed. Further growth resulted in the formation of flat layers in a mixture of cubic bixbyite in [111] orientation and monoclinic structure. The fabricated capacitors exhibited suitable dielectric properties at room temperature; such as a dielectric constant of ε=22, a leakage current of 10−8A∕cm2 at 1V and breakdown fields >4.3MV∕cm for layers with 14nm thickness. These properties make Gd2O3 interesting for high-K application on SiC.
Synthesis of polycrystalline ytterbium monoxide thin films by molecular beam deposition
Authors: Losego Mark D. , Maria Jon-Paul
Published: August 2006

The synthesis of ytterbium monoxide (YbO) by molecular beam deposition (MBD) is investigated for developing a lattice-matched oxide to gallium nitride. Because Yb2O3 is the thermodynamically stable oxide at atmospheric pressure, YbO has previously only been prepared by high-pressure bulk synthesis reactions. This article demonstrates that through low growth temperature and the kinetic control of molecular fluxes allowed by MBD, the polycrystalline monoxide phase can be stabilized in thin film form on silicon substrates. Once deposited, the YbO rocksalt structure is found to be stable at room temperature and atmospheric pressure. Elemental analysis indicates that films are primarily composed of Yb and O. Elemental concentrations of H, C, and N, which are known to form phases with Yb of similar structure and lattice parameter to YbO, are shown to be below the 1at.% level.
Postannealing effect on pseudobilayer HfO2∕HfSixOy∕Si gate oxides formed by an inductively coupled sputtering process
Authors: Choi Won Joon , Lee Eun Joung , Lee Jong Hyun , Yang Jung Yup , Do Young Ho , Hong Jin Pyo
Published: August 2006

Pseudobilayer HfO2∕HfSixOy gate dielectrics in metal-oxide-semiconductor devices were prepared using an inductively coupled rf plasma sputtering technique. This sputtering method was designed to improve the uniformity and efficiency of formation of high-quality gate dielectrics at room temperature. Crystallization of the gate dielectrics was easily controlled from amorphous to monoclinic by varying the external power from 0to60W at RT. The chemical bond states of the interfacial layers in the as-deposited and postannealed samples were analyzed with an x-ray photoelectron spectroscopy (XPS) system. The XPS results revealed that the interfacial layers of the as-deposited and annealed samples were hafnium silicide and hafnium silicate, respectively. Compared with the as-deposited sample, the pseudobilayer HfO2∕HfSixOy gate dielectric annealed at 750°C yielded excellent electrical characteristics due to the hafnium silicate interfacial layer. The dielectric constant and leakage current of the postannealed samples were about 15 at 100kHz and less than ∼10−6A∕cm2 at −1.5V, respectively.
Megasonic-assisted development of nanostructures
Authors: Küpper David , Küpper Daniel , Wahlbrink Thorsten , Bolten Jens , Lemme Max C. , Georgiev Yordan M. , Kurz Heinrich
Published: August 2006

The effect of high frequency (1MHz) acoustic agitation (megasonic agitation) on development of electron beam exposed poly(methylmethacrylate) (PMMA) nanostructures is investigated. Test patterns consisting of dense holes, isolated lines, and gratings with high aspect ratios have been used. Compared to conventional dip development, the sensitivity of the development process is increased and the homogeneity of nanopatterns is improved considerably. Furthermore, experiments towards ultimate aspect ratios and resolution of PMMA in the range of 2–3nm with megasonically assisted development have been carried out. The physical mechanisms for the observed enhanced development performance which is particularly attractive for nanostructuring are discussed.
Low interface states and high dielectric constant Y2O3 films on Si substrates
Authors: Alarcón-Flores G. , Aguilar-Frutis M. , Falcony C. , García-Hipolito M. , Araiza-Ibarra J. J. , Herrera-Suárez H. J.
Published: August 2006

Y 2 O 3 films were deposited on c-Si substrates at temperatures in the 400–550°C range, with no further thermal treatment given to these samples, using the spray pyrolysis technique. The spraying solution was yttrium acetilacetonate disolved N,N-dimethylformamide. In addition, a solution of H2O–NH4OH was sprayed in parallel during the deposition process to improve the optical, structural, and electrical properties of the deposited films. The growth of a SiO2 layer between the yttrium oxide and the Si substrate during this deposition process resulted in interface state density values as low as 1010eV−1cm−2. An effective refractive index value of 1.86, and deposition rates close to 1Å∕s were obtained. The Y2O3 films were polycrystalline with a crystalline cubic phase highly textured with the (400) direction normal to the Si surface. An effective dielectric constant up to 13, as well as a dielectric strength of the order of 0.2MV∕cm was obtained for ∼1000Å thick as-deposited films incorporated in a metal-oxide-semiconductor structure.
Reaction mechanism of fluorinated chemically amplified resists
Authors: Yamamoto Hiroki , Kozawa Takahiro , Okamoto Kazumasa , Saeki Akinori , Tagawa Seiichi , Ando Tomoyuki , Sato Mitsuru , Komano Hiroji
Published: August 2006

The halogenation of resist materials is a well-known strategy for the improvement in resist performance particularly in electron beam and x-ray resists. However, for chemically amplified resists, the halogenation of polymers requires particular caution because halogenated polymers may interfere with acid generation. In this work, acid generation in poly[4-hydroxystyrene-co-4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropyl)-styrene] films was investigated using steady-state spectroscopy and pulse radiolysis. Acid yield decreased with an increase in the ratio of hexafluoroalcohol units. It was found that the reactivity of polymers with low-energy electrons (∼thermal energy) correlates to the decrease in acid yield.
Oxidized GaN(0001) surfaces studied by scanning tunneling microscopy and spectroscopy and by first-principles theory
Authors: Dong Y. , Feenstra R. M. , Northrup J. E.
Published: August 2006

Oxidized Ga-polar GaN surfaces have been studied both experimentally and theoretically. For in situ oxidization at 550°C using molecular oxygen, Auger electron spectroscopy indicates a saturation oxygen coverage of 2.1±0.5 ML (monolayer). For these surfaces scanning tunneling microscopy reveals two surface phases, one with 33×33−R30° periodicity and the other with disordered two times periodicity. Scanning tunneling spectroscopy revealed a surface band gap with size close to that of GaN, indicating that any states of the oxide lie predominantly outside of the GaN gap. From first-principles total energy calculations of surface formation energies two models of energetically favorable surfaces structures are developed, with oxygen coverages of 1.25 and 2 ML, respectively. Both structures satisfy electron counting and have surface band gaps close in size to that of GaN, in agreement with experiment.
Radiative recombination during acoustically induced transport in GaAs quantum wells
Authors: Alsina F. , Stotz J. A. H. , Hey R. , Santos P. V.
Published: August 2006

We investigate the dynamics of radiative trapping centers during the ambipolar transport of electrons and holes induced by acoustic fields. The studies used spatially resolved photoluminescence spectroscopy to determine the nature of the trapping centers for different transport conditions. For low and moderate acoustic powers, the primary traps are electron capturing centers while hole trapping becomes important for high acoustic powers. A remarkable feature of these radiative trapping centers is that their capture cross section depends on the acoustic power. We propose a model for electron trapping based on the injection of carriers induced by the surface acoustic wave piezoelectric field into states at the interface between the GaAs quantum well and the (Al,Ga)As barriers.
Characterization of ZnO nanorod arrays fabricated on Si wafers using a low-temperature synthesis method
Authors: Tak Youngjo , Park Dongseok , Yong Kijung
Published: August 2006

ZnO nanorod arrays fabricated on ZnO buffer layers on Si wafers were grown using a low-temperature solution method and were characterized by various techniques. Buffer layers were prepared using metal organic chemical vapor deposition and a sputter-oxidation method. Aligned ZnO nanorods were deposited at 90°C on the substrates by a hydrothermal treatment using a zinc salt and aqueous ammonia solution. The ZnO nanorod arrays were characterized by scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy. The as-grown ZnO nanorod arrays exhibited broad deep-level emission centered at ∼564nm. The intensity of the deep-level emission decreased and band edge emission centered at 379nm appeared after air annealing. Samples annealed in hydrogen showed only band edge emission.
Calculation of critical dimensions for wurtzite and cubic zinc blende coaxial nanowire heterostructures
Authors: Raychaudhuri S. , Yu E. T.
Published: August 2006

We employ a methodology, based on established approaches for determining the critical thickness for strain relaxation in planar films, to determine critical dimensions for coherently strained coaxial nanowire heterostructures. The model is developed and executed for various specific core-shell heterostructures in [111] zinc blende and [0001] wurtzite geometries. These calculations reveal that critical dimensions in such heterostructures can be quantified by a unique critical core radius and a critical shell thickness, which is dependent on the core radius. It is anticipated that this work will serve as a guide to determine the feasibility of specific coherently strained nanowire heterostructure designs.
Thermal diffusion of Co into atomically flat ZnO(000-1) surfaces investigated by scanning probe microscopies and low energy electron diffraction
Authors: Dumont J. , Seldrum T. , Couet S. , Moisson C. , Turover D. , Sporken R.
Published: August 2006

In this study, we established a cleaning procedure to obtain atomically flat (000-1) surfaces from pressurized melt grown ZnO. Ex situ chemical cleaning removes fluid layer and trapped contaminants from the surface. This was followed by cycles of sputtering and annealing in ultrahigh vacuum. It is critical that the ion energy be high enough to produce small and highly mobile molecular fragments. On the other hand, annealing must be done at moderate temperatures to avoid segregation of potassium to the surface. Secondly, the interaction of Co thin films on atomically flat ZnO(000-1) has been investigated by low energy electron diffraction, scanning tunneling microscopy, and Auger electron spectroscopy. A high density of islands nucleates at the earliest stages of the growth and a subsequent Volmer-Weber growth of these islands is observed. Upon annealing at 550°C, an atomically flat surface (Zn,Co)O(000-1) is restored due to the diffusion of the Co into the semiconductor.
Self-consistent full band two-dimensional Monte Carlo two-dimensional Poisson device solver for modeling SiGe p-channel devices
Authors: Krishnan S. , Fischetti M. , Vasileska D.
Published: August 2006

Even now there are no device simulators that can model hole carrier transport in p-channel Si or SiGe metal-oxide-semiconductor field-effect transistors (MOSFETs), by properly addressing the issues of hole band-structure and quantum confinement effects. The purpose of this work is to remedy this situation by presenting an approach to modeling p-channel devices using a two-dimensional (2D) Monte Carlo transport kernel that is coupled self-consistently to a 2D Poisson equation solver and to a six band k∙p band-structure module. The need for full band solver for hole transport is especially true in the case of surface channel strained Si and buried channel strained SiGe p-MOSFETs investigated here. We have paid special attention on properly implementing and investigating the role of interface roughness on the operation of these device structures. We find that SiGe p-channel MOSFETs show performance improvement only for low gate and drain biases; i.e., when the device is operated in subthreshold and weak inversion regime and carrier spillover into the surface channel and surface roughness do not play significant role on the device operation.
Wavelength tuning of InAs∕InP quantum dots: Control of As∕P surface exchange reaction
Authors: Nötzel R. , Anantathanasarn S. , van Veldhoven P. J. , van Otten F. W. M. , Eijkemans T. J. , Trampert A. , Satpati B. , Wolter J. H.
Published: August 2006

Wavelength tuning of single and vertically stacked InAs quantum dot (QD) layers embedded in InGaAsP∕InP (100) grown by metal organic vapor-phase epitaxy is achieved by controlling the As∕P surface exchange reaction during InAs deposition. The As∕P exchange reaction is suppressed for decreased QD growth temperature and group V-III flow ratio, reducing the QD size and photoluminescence (PL) emission wavelength. The As∕P exchange reaction and QD PL wavelength are then reproducibly controlled by the thickness of an ultrathin (0–2 ML) GaAs interlayer underneath the QDs. Submonolayer GaAs coverages result in a shape transition from QDs to quantum dashes at low group V-III flow ratio. Temperature dependent PL measurements reveal excellent optical properties of the QDs up to room temperature with PL peak wavelengths in the technologically important 1.55μm region for telecom applications. Widely stacked QD layers are reproduced with identical PL emission to increase the active volume, while closely stacked QD layers reveal a systematic PL redshift and linewidth reduction due to vertical electronic coupling which is proven by the linear polarization of the cleaved-side PL changing from in plane to isotropic.
Device interference in GaAs quantum wire transistors and its suppression by surface passivation using Si interface control layer
Authors: Jia Rui , Hasegawa Hideki , Shiozaki Naoko , Kasai Seiya
Published: August 2006

In order to establish feasibility of high density integration of gate-controlled GaAs nanodevices, this article investigates device interference in GaAs-based quantum wire transistors (QWRTrs) by using a side-gating test structure and attempts to suppress the observed anomalously large side-gating with surface passivation using a silicon interface control layer (Si ICL). QWRTrs were formed on AlGaAs∕GaAs etched quantum wires (QWRs) and were controlled by nanometer sized Schottky wrap gates. A Schottky side gate was formed at a distance dsg from the QWR. When dsg was large, the QWRTr showed weak side gating which can be explained by the electrostatic side gating. However, when the side gates was placed close to the nanowire with dsg<500nm, anomalously large side gating started to take place which cannot be explained by the electrostatic side gating. On the basis of detailed measurements of side-gating behavior and side-gate leakage currents at various temperatures, the anomalous side gating was explained by a model in which occupation of deep traps at the back AlGaAs∕GaAs interface of the QWR is modulated by tunneling injection of electrons from the side-gate edge resulting from strong Fermi level pinning by surface states. Based on this model, attempts to reduce surface states by surface passivation were made. Formation of the Si ICL structure on a regrown thin GaAs cap layer completely removed the anomalous side-gating effect.
Molecular beam epitaxy of Sm2O3, Dy2O3, and Ho2O3 on Si (111)
Authors: Craft H. S. , Collazo R. , Sitar Z. , Maria J. P.
Published: August 2006

We report on the epitaxial deposition of Sm2O3, Dy2O3, and Ho2O3 films on Si (111) substrates as progress towards developing a predictive model for gate oxide selection. To date, films have been characterized by reflection high-energy electron diffraction, x-ray diffraction (XRD), and x-ray photoemission spectroscopy (XPS). In the case of Sm2O3, difficulty was encountered in achieving the growth of phase pure films; growth of the desired bixbyite oxide phase was accompanied by significant amounts of monoclinic oxide as detected by XRD. Dy2O3 and Ho2O3 films can be produced free of unwanted phases when deposited using a background pressure of 1×10−6torr O2∕O3 and a substrate temperature between 425 and 550°C. Dy2O3 films with an x-ray peak width of 0.6° in ω were obtained. XPS studies of the Dy2O3∕Si interface are in progress, and verify the phase purity of the films.
Large reduction of leakage currents in AlGaN Schottky diodes by a surface control process and its mechanism
Authors: Kotani Junji , Kaneko Masamitsu , Hasegawa Hideki , Hashizume Tamotsu
Published: August 2006

Leakage currents in AlGaN Schottky diodes were investigated systematically by using a rigorous computer simulation based on the thin surface barrier model taking account of unintentionally doped surface donors. The leakage currents in AlGaN Schottky diodes have stronger bias dependence and smaller temperature dependences as compared with those of GaN diodes. It was shown that these features were associated with shallow oxygen donors located near the AlGaN surface. Then, an attempt was made to remove oxygen and suppress leakage currents by a surface control process using an ultrathin Al layer and subsequent annealing. An in situ x-ray photoelectron spectroscopy analysis indicated the formation of Al2O3 layer during the surface control process, suggesting efficient gettering of oxygen from the surface. C-V analysis directly indicated the reduction of shallow donors by the surface control process. A remarkable reduction of reverse leakage currents of four to five orders of magnitude took place in large area AlGaN Schottky diodes after the application of the surface control process. This process also reduced leakage currents of the gate of the heterostructure field effect transistor device by more than one order of magnitude and increased temperature dependences of current.
Magnetic properties of epitaxial Co-doped anatase TiO2 thin films with excellent structural quality
Authors: Kaspar T. C. , Droubay T. , McCready D. E. , Nachimuthu P. , Heald S. M. , Wang C. M. , Lea A. S. , Shutthanandan V. , Chambers S. A. , Toney M. F.
Published: August 2006

The heteroepitaxy of Co-doped anatase TiO2 on LaAlO3(001) has been refined with the goal of determining the relationship between structural quality and magnetic ordering. By significantly reducing the deposition rate and substrate temperature, well-ordered Co:TiO2 films with high crystalline quality were obtained by oxygen-plasma-assisted molecular beam epitaxy, as characterized by x-ray diffraction. These films exhibit uniform Co doping, with no evidence of Co segregation or secondary phases throughout the film depth or on the surface. Despite the improvement in crystalline quality and Co distribution, the films exhibit negligible ferromagnetism, with saturation moments of only ∼0.1μB∕Co. This loss of ferromagnetism is in stark contrast to faster-grown Co:TiO2 films, where higher growth rate and substrate temperature typically result in lower crystalline quality, a highly nonuniform Co distribution, and average saturation moments of ∼1.2μB∕Co. The presence of ferromagnetism in faster-grown Co:TiO2 does not appear to arise from intrinsic point defects present in the bulk material, such as charge-compensating oxygen vacancies, but is instead attributed to the presence of extended structural defects.
Studies of plasma surface interactions during short time plasma etching of 193 and 248nm photoresist materials
Authors: Hua Xuefeng , Engelmann S. , Oehrlein G. S. , Jiang P. , Lazzeri P. , Iacob E. , Anderle M.
Published: August 2006

As the device dimensions scale to 100nm, the use of photoresist materials is suitable for lithographic patterning at 193nm. The molecular structure of 193nm photoresist materials is significantly different from that of 248nm photoresist materials [H. Ito, IBM J. Res. Deu. 45, 683 (2001), T. Kajita et al., Proc. SPIE 4345, 712 (2001)], which leads to a number of undesirable consequences, including pronounced surface and line edge roughness during plasma etching [H. Ito, IBM J. Res. Deu. 41, 69 (1997), [E. Reichmanis et al., J. Vac. Sci. Technol. B 15, 2528 (1997), [L. Ling et al., ibid. 22, 2594 (2004)]. In this article, we present an investigation of the mechanisms for the surface/line edge roughening of photoresist materials during plasma etching using C4F8∕90%Ar discharges. We emphasized in our study short exposure times (the first few seconds) of the photoresist materials and structures to the plasma, a time regime that has not been well studied. Rapid modifications were observed for both 193 and 248nm photoresists during short time exposure. During the first seconds of plasma exposure, photoresist material densification and hydrogen depletion are important processes. It is also found that rough surfaces develop within a few seconds of exposure to the C4F8∕90%Ar discharges. Plasma exposure leads to the formation of rough edges on the top of trench sidewalls in photoresist trench and line structures. During prolonged exposure to the plasma, the roughness is transferred to produce striations on the sidewalls. After an initial stage, the roughening rate remains constant for 193nm photoresist, whereas for 248nm photoresist the roughening rate is negligible. This difference is possibly related to the preferential removal of carbonyl groups for the 193nm photoresist material, which has been revealed by x-ray photoelectron spectroscopy and seconday ion mass spectroscopy.
Impact of line edge roughness on copper interconnects
Authors: Leunissen L. H. A. , Zhang W. , Wu W. , Brongersma S. H.
Published: August 2006

The dimensions of transistor gates are scaling to small dimensions and the relative variation of the edge, referred to as line edge roughness (LER), is addressed as one of the critical issues for front-end technology. These local variations deteriorate the device performance [Croon et al., Tech. Dig. - Int. Electron Devices Meet. 2002, 307; Croon et al., Proceedings of the ESSDERC, 2003 (unpublished), p. 22]. However, also for backend, interconnects wires are scaling down and the relative contribution of the LER is increasing. The impact of LER for interconnects has rarely been studied yet. In this article we address the electrical performance of the resulting copper interconnects with additional LER generated with e-beam lithography. The added LER will have spatial frequencies and amplitudes that are similar to the current resists used.
Fabrication of submicrometer patterned two-dimensional electron gases by overgrowth of focused ion beam doped AlxGa1−xAs
Authors: Reuter D. , Riedesel C. , Wieck A. D.
Published: August 2006

Laterally patterned two-dimensional electron gases have been fabricated by overgrowth of locally implantation doped Al0.33Ga0.67As barriers. The heterostructures were grown by molecular beam epitaxy and the local doping was performed by focused ion beam implantation. Submicrometer patterning capability was found when testing the insulation behavior of doping interruptions between two conducting rectangles. For a narrow conducting wire, an electronic width of approximately 350nm was realized. The experiments show that the thermal processing necessary to remove the implantation damage might hamper the lateral resolution and has to be optimized.
PCSI 2006: Old problems and new visions
Authors: Cooper Larry
Published: August 2006

A brief review of the origin of the Conference on the Physics and Chemistry of Semiconductor Interfaces (PCSI) establishes a motivation—the need for an insulating material for GaAs field-effect transistors—for the resulting focus of PCSI on surface and interface issues that impact semiconductor devices. Highlights of past meetings emphasize the contributions and value of PCSI in promoting scientific advances in solid-state electronics. Turning to the future, motivation will again be established by identifying the wide spectrum of device concepts that offer great potential as the basis for future electronic, magnetic, and photonic technologies that will provide high-speed and low-power capabilities far exceeding present-day capabilities. Successful demonstrations of these nanoelectronics devices will only be possible if surface and interface issues are addressed at the earliest stages. In the future, PCSI will be dominated by many problems where nanostructures are fabricated and heterogeneous materials characterized and understood on scales measured in a few nanometers.
III-nitride growth and characteristics on ferroelectric materials using plasma-assisted molecular beam epitaxy
Authors: Lee Kyoung-Keun , Namkoong Gon , Doolittle W. Alan , Losurdo Maria , Bruno Giovanni , Jundt Dieter H.
Published: August 2006

The characteristics and growth of III-nitride materials on ferroelectric lithium niobate with varying Li mole composition have been investigated to achieve a better device performance of AlGaN∕GaN heterojunction structures. III-nitride growth on lithium niobate has been performed after high temperature (1000°C) furnace thermal treatments in dry air environment resulting in atomically flat surfaces on lithium niobate (LN). However, while this furnace thermal treatment results in improved surface smoothness and III-nitride adhesion, it also causes repolarization, ferroelectric domain reversal from a +z spontaneous polarization to a −z spontaneous polarization in the surface of congruent LN (48.39mole% of Li2O). On the other hand, near-stoichiometric LN (49.9mole% of Li2O) did not develop repolarization during the identical thermal treatment. Furthermore, as determined in situ by spectroscopic ellipsometry, congruent LN (CLN) shows a bigger variation of the pseudorefractive index and pseudoextinction coefficient during annealing at various temperatures in the range of 100–800°C, indicating surface modification at those temperatures. On the other hand, near-stoichiometric LN (SLN) shows little change in these indexes implying that SLN provides better surface stability at elevated temperatures in the vacuum environment. This improved vacuum tolerance of SLN has allowed Al0.20Ga0.80N∕GaN heterostructures to be grown on CLN and SLN substrates, with two times higher channel mobility resulting on SLN substrates. Thus, SLN appears to be a viable substrate for molecular beam epitaxial growth of III-nitrides in various applications.
Energy conversion from catalytic reaction to hot electron current with metal-semiconductor Schottky nanodiodes
Authors: Park Jeong Young , Somorjai Gabor A.
Published: August 2006

Exothermic catalytic reactions induce electronic excitation at the metal surface, leading to the production of energetic hot electrons. We monitored the flow of hot electrons for over several hours using two types of metal-semiconductor Schottky diodes, Pt∕TiO2 or Pt∕GaN, during the platinum catalyzed oxidation of carbon monoxide. The thickness of Pt film used as the catalyst was 5nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. The chemicurrent was well correlated with the turnover rate of CO oxidation separately measured by gas chromatography, suggesting the possibility of application as chemical sensors with high sensitivity.
Complexities in modeling the metal to molecule interface
Authors: Speyer G. , Akis R. , Ferry D. K.
Published: August 2006

Determination of the conductance through a metal-molecule-metal structure presents a difficult computational problem. One normally divides the problem into two parts: computation of the band structure and then computation of the transport through the device. The choice of a basis set for the band structure calculation presents an interesting challenge, in that the results can significantly affect the resulting energy structure throughout the device. An important consequence of the band structure calculation is the surface dipole, which generates the work function, and is therefore an important aspect of the surface barrier between the “contact” and the molecule. If this is not determined accurately, which is quite often the case, one cannot expect to achieve good results for the transport calculation. Nevertheless, we can use the results to compare differences between different molecules without having good quantitative results for the conductance, and this is demonstrated here.
Comparison of ultrathin SiO2∕Si(100) and SiO2∕Si(111) interfaces from soft x-ray photoelectron spectroscopy
Authors: Ulrich M. D. , Rowe J. E. , Keister J. W. , Niimi H. , Fleming L. , Lucovsky G.
Published: August 2006

The limitations of soft x-ray photoelectron spectroscopy (SXPS) for determining structural information of the SiO2∕Si interface for device-grade ultrathin (∼6–22Å) films of SiO2 prepared from crystalline silicon by remote plasma assisted oxidation are explored. The main focus of this article is the limitation of data analysis and sensitivity to structural parameters. In particular, annealing data shows a significant decrease in the integrated density of suboxide bonding arrangements as determined from analysis of SXPS data. These decreases and changes are interpreted as evidence for reorganization of specific interface bonding arrangements due to the annealing process. Moreover, these results suggest that sample preparation and processing history are both critical for defining the nature of the SiO2∕Si interface, and therefore its electrical properties. Quantitative estimates of the interface state densities are derived from SXPS data revealing ∼2 monolayers (ML) of suboxide as prepared and ∼1.5 ML of suboxide after rapid thermal annealing at 900°C for both Si(100) and Si(111) substrates. Comparison of the individual suboxide bonding state densities indicate for both Si substrate crystallographic orientations that annealing causes a self-organization of the suboxide consistent with bond constraint theory.
Spin manipulation in a double quantum-dot–quantum-wire coupled system
Authors: Sasaki S. , Kang S. , Kitagawa K. , Yamaguchi M. , Miyashita S. , Maruyama T. , Tamura H. , Akazaki T. , Hirayama Y. , Takayanagi H.
Published: August 2006

We have studied spin correlation in a double quantum-dot–quantum-wire coupled device revealed in low-temperature transport characteristics. We demonstrate nonlocal control of the Kondo effect in one dot by manipulating the spin states of the other. The modulation of the local density of states in the wire region due to the Fano-Kondo antiresonance and the Ruderman-Kittel-Kasuya-Yoshida exchange interaction are the two possible mechanisms underlying the observed features. When the dot states are indirectly probed in the side-coupled geometry, double suppression of the wire conductance is observed due to the Fano-Kondo antiresonance involving both dots.
Optical characterization of process-dependent charging in hafnium oxide structures
Authors: Carriles R. , Kwon J. , An Y. Q. , Sun L. , Stanley S. K. , Ekerdt J. G. , Downer M. C. , Price J. , Boescke T. , Diebold A. C.
Published: August 2006

We present a comprehensive study of Si∕SiO2∕Hf(1−x)SixO2 structures using optical second-harmonic generation (SHG), supported by spectroscopic ellipsometry, photoelectron spectroscopy, and noncontact electrical measurements. Rotationally anisotropic and spectroscopic SHG depend strongly on dielectric composition and on postdeposition annealing in NH3. Spectroscopic SHG shows that these variations are linked to variations in charge trapped in the high-κ material, which influences SHG via electrostatic-field-induced SH (EFISH) generation. Supporting measurements corroborate this interpretation. Noncontact electrical measurements and x-ray photoelectron spectroscopy show annealing-dependent trends in interface charge and nitrogen incorporation, respectively, that mirror annealing-dependent trends in EFISH generation. The results show that SHG can potentially be used as an in situ, real-time monitor of internal electric fields attributable to composition- and annealing-dependent fixed charge in the oxide layers.
Cathodoluminescence spectra of surface-oxidized GaN crystallites
Authors: Honda Tohru , Akiyama Miwako , Baba Taichi , Watanabe Makiko
Published: August 2006

GaN crystallites covered with gallium oxides were fabricated and their cathodoluminescence (CL) spectra were observed. The intensity of CL spectra observed from the GaN crystallites with the oxides was higher than that in the case without the oxides. This is due to the reduction of surface recombination and carrier confinement. Electroluminescent devices operating in the UV spectral region were also fabricated using the GaN crystallites covered with gallium oxides.
Negative transconductance region in the double-gated silicon field emitter arrays
Authors: Chen Liang-Yu , Akinwande Akintunde I.
Published: August 2006

We report double-gated n-type silicon field emitter arrays with focus aperture diameter of 1.2μm, gate aperture diameter of 0.4μm, and tips that are 400nm below the gate aperture. Focus transfer characteristics of the devices have three distinct regions of operation: a saturation region (VF>15V) in which the anode current is almost independent of focus voltage, a rapidly increasing anode current region (5V⩽VF⩽15V), and a cut-off region (VF<5V) in which there is no anode current. The anode current slope with respect to the focus voltage in the saturation region depends on the relative position of the tip to the gate, and it is very small if the gate screens the tip from the focus. The focus voltage at which the anode current plateaus increased with the gate bias. We also report a negative transconductance region in the gate transfer characteristics for focus voltages in the rapidly increasing anode current region. The peak anode current and the gate voltage at which it occurs increased with the focus bias. For focus voltages above 15V, there was no negative transconductance region in the gate transfer characteristics. The negative transconductance region is explained by the initial spread in the axial velocity (energy) of electrons leaving the tips. The cut-off region is attributed to the formation of a potential barrier between the tip and the anode.
Process for 20nm T gate on Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs epilayer using two-step lithography and zigzag foot
Authors: Lee K. S. , Kim Y. S. , Lee K. T. , Jeong Y. H.
Published: August 2006

After metallization, a 20nm T gate with a straight foot is not mechanically stable because the support given by the foot is too weak. We have proposed a zigzag gate foot to enhance mechanical support and developed a process using two-step electron beam lithography and zigzag foot shape to fabricate 20nm T gates for high performance Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs modulation-doped field-effect transistors. Two-step lithography reduces electron forward scattering by defining the foot on a thin (40nm) bottom layer of polymethyl methacrylate at the second step, the T-gate head having been developed at the first step. Adopting a low temperature development technique for the second step reduces the detrimental effect of head exposure on foot definition. With this process, stand-alone 20nm zigzag T gates have been successfully fabricated on an Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs epitaxial wafer using a 20keV electron beam. With a higher-voltage electron beam, this process can be used to fabricate sub-20-nm T gates.
Gun lens theory for nonparaxial trajectories by canonical mapping transformation: Characterization of general skewed rays inside electron guns by electron gun focal length
Authors: Fujita Shin , Shimoyama Hiroshi
Published: August 2006

The canonical mapping transformation (CMT) theory has been generalized to nonmeridional (skewed) rays. It has been derived that with minor mathematical modification a CMT optical parameter, “electron gun focal length,” can be applied to skewed rays. The generalization makes it possible to characterize the behavior of nonparaxial electron trajectories inside the gun by a small number of optical parameters derived in the CMT theory. Important gun properties such as the crossover size and the angular current intensity are readily available from the electron gun focal length. The relevant CMT optical parameters can be estimated either by ray tracing a few representative cathode trajectories or by use of the conventional paraxial trajectory method whose applicability is usually limited to paraxial trajectories. Interpretation of the parameters by the CMT method extends their validity to nonparaxial cases. It has been found that using the rotating coordinate system in the conventional paraxial trajectory theory is equivalent to adopting the canonical direction (direction based on the canonical momentum as opposed to the kinetic momentum) in specifying the ray condition in the CMT. The “vector potential shift” must be taken into consideration in the beam profile evaluation at the crossover when nonzero magnetic field exists on the cathode surface. The beams from the off-axis points on the cathode miss the optical axis and do not form a conventional crossover with consequent reduction in the average brightness of the electron beam when the beam current is increased by accepting the electrons from increasingly off-axis emission points on the cathode.
First-principles calculations of dielectric constants for ultrathin SiO2 films
Authors: Wakui Sadakazu , Nakamura Jun , Natori Akiko
Published: August 2006

We studied the dielectric properties of ultrathin SiO2 films using two methods, internal field method and dipole moment method, based on first-principles calculations of the ground state in a finite electric field. We evaluated the optical and static dielectric constants, ε∞ and ε0, of H-terminated β-quartz (0001) films through calculations without and with the lattice relaxation in the electric field applied, respectively. The calculated values of ε∞ and ε0 with these two methods are in good agreement with each other, and they reproduce well the experimental values. Both the optical and static dielectric constants hardly depend on the film thickness, and the spatial variation of the local dielectric constant is also very small. These results indicate that both the surface effect and the quantum confinement effect are small for the ultrathin H-terminated β-quartz (0001) films.
Shot noise models for sequential processes and the role of lateral mixing
Authors: Neureuther A. R. , Pease R. F. W. , Yuan L. , Baghbani Parizi K. , Esfandyarpour H. , Poppe W. J. , Liddle J. A. , Anderson E. H.
Published: August 2006

Shot noise in electron-beam and extreme ultraviolet (EUV) exposure fundamentally limits the useful sensitivity of resists. Here the exposure, amplification, and deprotection of chemically amplified resists are treated as a sequence of statistical events to determine the effect of shot noise. Noise among processes is additive, and a sequential Poisson process is used to illustrate the “acid bottleneck” that occurs when less than one acid electron is generated per input electron. Lateral mixing, due to effects such as electron scattering and acid diffusion, is accounted for in a binomial sorting model that shows that the effect is simply a function of the total number of quanta reaching the voxel irrespective of their path. Electron exposure is estimated to generate on average one acid every 3.8, 21, and 36nm along the trajectories of 5, 50, and 100keV electrons respectively. Acid generation appears well correlated with energy deposition, at about 22eV/acid required for EUV and electron exposure or three times the deep ultraviolet level. The model was validated by observing the fraction of small (24 and 32nm across) contacts that cleared as a function of exposure dose at 100keV. Approximately 4500 electrons were required to clear, independent of contact size. However, the slope was indicative of a process with only 50 effective events possibly owing to resist surface effects. Circular dissolution smoothing and mechanical fracture are suggested as speculative sources for the near feature size correlation length of the line-edge roughness.