bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of electron-beam induced microporosity on masking properties of polymethylmethacrylate in wet etching of nanometer structures
Authors: Maximov I. , Bogdanov A. L. , Montelius L.
Published: July 2002

The masking properties of nanometer scale polymethylmethacrylate (PMMA) features used for definition of sub-100-nm-wide bridges in InP/Ga0.25In0.75As two-dimensional electron gas heterostructures by wet etching were investigated. Bridges untreated after the development of PMMA showed very poor masking ability due to a substantial exposure by backward scattered electrons from the surrounding exposed areas. However, if the resist was post-baked after development at a temperature higher than the glass transition temperature, Tg, the masking properties were restored and wet etched nanobridges of a sufficient quality were obtained. The post-development bake temperature and time were optimized to provide enough resist “packing” and yet not to cause resist flowing resulting in unacceptable alteration of the pattern geometry.
Electrical properties of ion beam sputtered and ion assisted SiO2, SiOXNY, and SiNX films on silicon
Authors: Lambrinos M. F. , Valizadeh R. , Colligon J. S.
Published: July 2002

To develop methods for the formation of metal-insulator-semiconductor (MIS) devices at low temperature, thin SiO2, SiOXNY, and SiNX films were reactively sputtered onto nonheated silicon substrates using different sputtering and ion assist parameters. SiO2 was prepared by bombarding Si with Ar+ ions in Ar/O2 ambient whereas SiOXNY and SiNX films were prepared by bombarding Si3N4 with N2+ ions in N2/O2 and Ar/N2 ambients, respectively. In addition, 300 eV N2+ ion assistance was used during preparation of SiNX films to enhance film nitriding. The 100–300 eV Ar+ ion assistance was also used during preparation of SiO2 to investigate the electrical damage effects caused by energetic ion bombardment during sputtering of the insulator film. The bulk and interfacial electrical properties of these films, which were assessed from current–voltage and high-frequency capacitance–voltage (C–V) measurements on MIS structures, indicate that the Si native oxide and type of anneal play a key role in determining the final film-interface quality. SiOXNY sputtered films exhibit poorer C–V electrical characteristics with increasing nitrogen content resulting from the creation of positive fixed charge in the film and fast interfacial states. This damage can be suppressed using a native oxide but this increases the instability of the interface. Ion assistance of SiO2 films increased the level of electrical damage to the film in the bulk and in the vicinity of the interface. The majority of this damage manifests itself as fixed charge and trapping states in the film rather than fast interfacial states and leads to lower breakdown voltages and higher dielectric constants. Drift behavior of these films suggest that, with a native oxide in place, the former type of ion bombardment induced damage can be eliminated by a forming gas anneal although some deterioration of the electrical bulk properties remains.
Nanometer-scale gap control for low voltage and high current operation of field emission array
Authors: Lee Hyung-Il , Park Soon-Soo , Park Dong-Il , Hahm Sung-Ho , Lee Jong-Hyun , Lee Jung-Hee
Published: July 2002

A new technique for nanometer-scale gap formation was proposed and investigated for the possibility in vacuum microelectronic device application. When the temperature is down at the end of oxidation, the compressive stress induced at the initially connected slim polysilicon patterns forces to be separated into two parts, yielding wedge-shaped cathode and anode tips for a field emission array with extremely small interelectrode distance. The oxidation time and the polysilicon thickness are the key parameters to control the interelectrode gap. The fabricated field emission devices show very high emission current with low turn-on voltage.
Two-dimensional electron gas mobility as a function of virtual substrate quality in strained Si/SiGe heterojunctions
Authors: Churchill A. C. , Robbins D. J. , Wallis D. J. , Griffin N. , Paul D. J. , Pidduck A. J. , Leong W. Y. , Williams G. M.
Published: July 2002

The electron mobilities of two-dimensional electron gases in tensile strained Si grown on relaxed cubic SiGe alloys on Si (001) substrates are reported. The effects of using high and low temperature growth for the relaxed buffer layers, in an ultrahigh vacuum compatible chemical vapor deposition system using SiH4 and GeH4 gases, were investigated. We have measured electron mobilities of up to 2.6×105 cm2 V−1 s−1 for 4.5×1011 cm−2 carrier densities at 1.5 K; there is a strong correlation between surface morphology and underlying misfit dislocation volume densities which is reflected in the electron mobility. The highest mobility was achieved with high growth temperatures and high growth rates for the relaxed layers, while lower temperatures and growth rates produced samples with lower mobilities. We present transmission electron microscopy images, together with optical micrographs of the sample surfaces to demonstrate that substrate growth technology plays an important part in device performance and manufacturing compatibility.
Fabrication of strained InAs island ensembles on nonplanar patterned GaAs(001) substrates
Authors: Konkar A. , Heitz R. , Ramachandran T. R. , Chen P. , Madhukar A.
Published: July 2002

We report on the behavior of InAs deposited on nonplanar GaAs(001) substrates patterned with ≲0.5 μm wide stripe mesas oriented along the [11̄0] and 〈100〉 directions and with square mesas with a lateral size of ≳0.5 μm oriented along the 〈100〉 directions. Interfacet migration of In from the sidewalls to the mesa top leads to an enhanced InAs island density on the stripe as well as square mesa tops compared to that on the planar unpatterned region. Using such interfacet migration and InAs deposition amount less than needed for island formation on planar GaAs(001), we demonstrate complete selectivity in the positioning of InAs islands on the [11̄0] oriented stripe mesas of widths ≲100 nm, with islands forming exclusively on the mesa tops. These islands arrange in mesa-width-dependent parallel chains. They show photoluminescence (PL) comparable to that from the islands on the planar substrates. The polarization dependence of the PL suggests the presence of anisotropy in strain fields and potential elongation of islands in the [11̄0] direction. The significance of the stripe mesa edge orientation to the island formation is revealed by vastly different island densities and InAs morphology on the [11̄0] oriented versus 〈100〉 oriented stripe mesas. These differences reflect difference in migration from the different sidefacets that surround the (001) mesa top.
Estimation of the activation energy for Ar/Cl2 plasma etching of InP via holes using electron cyclotron resonance
Authors: Sabin E. W.
Published: July 2002

The temperature dependence of the via hole etch rate is reported for argon/chlorine plasma etching of indium phosphide in an electron cyclotron resonance etcher. The indium phosphide via hole etch rate was first established for several wafer chuck temperatures. Then the temperature of the wafer was estimated by measuring the heating effect of the plasma and the cooling effect of the helium backside cooling. The wafer temperature was then substituted for the wafer chuck temperature. The etch rate as a function of wafer temperature was found to exist in two regimes. The first regime is where the vapor pressure of the etch by product gas (indium chloride) is below the etcher chamber pressure. In this first regime the activation energy was calculated to be 0.45±0.05 eV. The second regime is where the vapor pressure of the etch by product gas (indium chloride) is above the etcher chamber pressure. In the second regime the activation energy was calculated to be 0.06±0.03 eV.
Scanning tunneling microscopy and x-ray photoelectron spectroscopy studies of atomic level structure and Fermi level pinning on GaAs(110) surfaces grown by molecular beam epitaxy
Authors: Ishikawa Yasuhiko , Tsurumi Naohiro , Fukui Takashi , Hasegawa Hideki
Published: July 2002

The relationship between atomic level structure and Fermi level pinning on n-type GaAs(110) surface grown by molecular beam epitaxy (MBE) is studied in situ by scanning tunneling microscopy (STM)/scanning tunnel spectroscopy (STS) and x-ray photoelectron spectroscopy (XPS). Fermi level by XPS on our MBE-grown n-GaAs(110) surfaces is found to lie at around EC−0.5 eV, indicating the presence of band bending. Among a few characteristic features observed by STM in addition to the basic (1×1) relaxation structure, the dominant holelike structure in the empty state image is correlated with band bending. It shows a gradual decrease of the apparent height over the several atomic distances toward the center in the empty state STM image and gradual increase of the apparent height in the filled state image, indicating the presence of negative charge. It accompanies random site deviations of surrounding atoms. The density of the holelike feature agrees roughly with the required number density of surface pinning centers to cause the observed band bending. This holelike structure originates most likely from a Ga vacancy. However, in STS spectroscopy measurements, a U-shaped continuous gap state distribution extending from the band edges, rather than a peaked discrete deep acceptor state distribution, is observed with a local downward shift of the surface Fermi level. A picture is presented where generation of Ga vacancy introduces local bond disorder and acts like a pinning center having a gap state continuum in accordance with a disorder-induced gap state model.
Anisotropic microstructure development during the reaction of Mg with GaAs
Authors: Robey S. W.
Published: July 2002

Reaction of Mg with the GaAs(001) surface occurs at substrate temperatures above 550 K and leads to three-dimensional (3D) growth of an epitaxial, cubic reaction phase with a lattice constant of 0.62±0.02 nm, 9% larger than GaAs. The resulting anisotropic, 3D heteroepitaxial microstructure was studied using in situ electron diffraction and Auger spectroscopy and ex situ atomic force microscopy (AFM). Three-dimensional structures develop that are elongated along the GaAs[110] direction with aspect ratios (length to width) up to 20. These structures appear to be composed of isotropic islands from which elongated, tapered ridges extend in one direction along [110]. Analysis of AFM images and size distributions suggest a critical size for the initiation of ridge growth. We speculate that these structures develop in response to misfit strain in the reaction phase.
Study of reaction and electrical properties at Ti/SiGe/Si(100) contacts for ultralarge scale integrated applications
Authors: Zaima Shigeaki , Yasuda Yukio
Published: July 2002

Solid-phase reaction, Schottky barrier heights and contact resistivities at the interface of Ti/Si0.8Ge0.2/Si(100) systems have been investigated. At annealing temperatures below 300 °C, Ti atoms preferentially react with Si atoms. Ge atoms start to react with Ti above 400 °C and C54–Ti(Si1−yGey)2 with a fraction of y=0.12 is formed by annealing at 650 °C. The annealing behavior of Schottky barrier heights suggests that the Ge composition of SiGe layers at the interface is Si-rich, which is consistent with the results on the interfacial reaction. For both n- and p-SiGe, the Schottky barrier heights lower than those of Ti/Si(100) are obtained at 650 °C, which is considered to be related to the reaction product such as C54–Ti(Si1−yGey)2. The contact resistivities smaller than those expected from the Schottky barrier heights are obtained for p+-SiGe at 580 °C. The sheet resistance of Ti/Si0.8Ge0.2/Si(100) and Ti/Ge(100) systems decreases at annealing temperatures above 550 °C and the values are as small as that of C54–TiSi2.
Surface chemical changes on field emitter arrays due to device aging
Authors: Wei Yi , Chalamala Babu R. , Smith Bruce G. , Penn Cecil W.
Published: July 2002

Emitter life and emission stability of field emission vacuum microelectronic devices are strongly influenced by the morphology and surface chemistry of the emission surfaces. Vacuum microelectronic devices like field emission displays (FEDs) are vacuum devices, which have continuous interaction between the emission surfaces and the residual gases in the vacuum envelope. Any significant variation in the shape, morphology, or chemical composition of the emitter surface will result in emission current changes. In this article, we report measurements of the surface chemical modifications of active FED cathodes due to their interaction with the residual gas environment within the vacuum envelope. The changes in emitter tip height is measured using high resolution scanning electron microscopy (SEM). Surface chemical changes are determined by micro-Auger electron spectroscopy and x-ray photoelectron spectroscopy (XPS). Observations with SEM did not show any measurable changes in the tip height and shape. The micro-Auger and XPS measurements indicate the accumulation of surface oxides on tips that have undergone aging.
Controlled conjugation of nanoparticles with single stranded DNA
Authors: Maeda Y. , Nakamura T. , Uchimura K. , Matsumoto T. , Tabata H. , Kawai T.
Published: July 2002

The controlled conjugation of Au nanoparticles using single-stranded DNA has been achieved by Watson–Crick base pairing interaction and a biotin-streptavidin reaction. Observation by atomic force microscopy have confirmed the one-to-one binding of Au nanoparticles to plasmid DNA. The results suggest that this method could be utilized to achieve nanoscale arrangements in a wide range of applications.
Comparative study of electron emission characteristics of silicon tip arrays with and without amorphous diamond coating
Authors: She J. C. , Huq S. E. , Chen J. , Deng S. Z. , Xu N. S.
Published: July 2002

Details are given of experiments carried out to compare the field electron emission characteristics of single crystal silicon tip arrays with and without amorphous diamond (a-D) coating. The coatings on the tip arrays were prepared using the filtered vacuum arc plasma deposition technique. In addition, the optical and electrical properties of the a-D coatings were measured. An anode probe technique was employed to measure the current–voltage characteristics and turn-on fields of the tip-array emitters before and after coating. It is found that the coating enhances the emission. The physical reasons for these effects are discussed.
Nitridation of the GaAs(001) surface: Thermal behavior of the (3×3) reconstruction and its evolution
Authors: Lu J. , Haworth L. , Hill P. , Westwood D. I. , Macdonald J. E.
Published: July 2002

The nitridation of the GaAs(001) surface using a radio frequency atomic nitrogen plasma source in a molecular beam epitaxy growth chamber has been studied. The resulting nitrogen induced GaAs(001) (3×3) reconstruction was investigated by in situ reflection high energy electron diffraction and x-ray photoemission spectroscopy (XPS). It was found that this reconstruction is only obtained in the temperature range 400–580 °C with a very low dose of atomic nitrogen. The nitrogen coverage corresponding to the (3×3) reconstruction was determined by quantitative XPS to be 0.30±0.09 ML. Below 400 °C an As–N species of disordered structure was found on the GaAs(001) surface. Subsequent annealing at about 500 °C produced the (3×3) reconstruction. Above 580 °C, nitridation lead to direct formation of β-GaN islands. In addition, the (3×3) reconstruction was found to be unstable to both exposure to atomic hydrogen and annealing. The N desorption activation energy of the (3×3) was estimated to be 2.75±0.55 eV. A surface phase diagram of the (3×3) has thus been deduced.
Nanoelectronic device applications of a chemically stable GaAs structure
Authors: Janes D. B. , Kolagunta V. R. , Batistuta M. , Walsh B. L. , Andres R. P. , Liu Jia , Dicke J. , Lauterbach J. , Pletcher T. , Chen E. H. , Melloch M. R. , Peckham E. L. , Ueng H. J. , Woodall J. M. , Lee Takhee , Reifenberger R. , Kubiak C. P. , Kasibhatla B.
Published: July 2002

We report on nanoelectronic device applications of a nonalloyed contact structure which utilizes a surface layer of low-temperature grown GaAs as a chemically stable surface. In contrast to typical ex situ ohmic contacts formed on n-type semiconductors such as GaAs, this approach can provide uniform contact interfaces which are essentially planar injectors, making them suitable as contacts to shallow devices with overall dimensions below 50 nm. Characterization of the native layers and surfaces coated with self-assembled monolayers of organic molecules provides a picture of the chemical and electronic stability of the layer structures. We have recently developed controlled nanostructures which incorporate metallic nanoclusters, a conjugated organic interface layer, and the chemically stable semiconductor surface layers. These studies indicate that stable nanocontacts (4 nm×4 nm) can be realized with specific contact resistances less than 1×10−6 Ω cm2 and maximum current densities (1×106 A/cm2) comparable to those observed in high quality large area contacts. The ability to form stable, low resistance interfaces between metallic nanoclusters and semiconductor device layers using ex situ processing allows chemical self-assembly techniques to be utilized to form interesting nanoscale semiconductor devices. This article will describe the surface and nanocontact characterization results, and will discuss device applications and novel techniques for patterning close-packed arrays of nanocontacts and for imaging the resulting structures.
Suppression of boron transport out of p+ polycrystalline silicon at polycrystalline silicon dielectric interfaces
Authors: Wu Y. , Niimi H. , Yang H. , Lucovsky G. , Fair R. B.
Published: July 2002

The transport of B atoms out of p+ polycrystalline silicon (poly-Si) gate electrodes through SiO2 gate oxides to the Si–SiO2 interface during dopant activation anneals degrades performance and reliability of hole-conducting (p-channel) field effect transistors. This article studies the suppression of B atom transport by using remote plasma processing to form ultrathin Si3N4 and silicon oxynitride diffusion barrier layers between p+ poly-Si gate electrodes and SiO2 gate dielectrics. Suppression of B atom transport has been monitored through electrical measurements, demonstrating that ∼0.8 nm of Si3N4, equivalent to a N areal density of ∼4.5×1015 atoms cm−2, is sufficient to effectively suppress B out diffusion during aggressive anneals of ∼1 min at 1000 °C. The suppression and transport mechanisms in nitrides, oxides, and oxynitrides have been studied by varying the N atom areal density by alloying. Quantum chemistry calculations suggest that B transport occurs through the formation of donor-acceptor pair bonds between B+ ions and nonbonding electron pairs on oxygen atoms with the transport process requiring a connected O atom percolation pathway. Donor-acceptor pair bonds with B+ ions are also formed with N atoms in nitrides and oxynitride alloys, but with a binding energy more than 1.5 eV higher than B+ ion O-atom bonds so that nitrides and oxynitride alloys effectively block B diffusion through the formation of a deep trapping site.
Fast, smooth, and anisotropic etching of SiC using SF6/Ar
Authors: So Myeong S. , Lim Seung-Gu , Jackson Thomas N.
Published: July 2002

Using magnetically enhanced reactive ion etching and an etch chemistry containing sulfur hexafluoride and argon we have dry etched silicon carbide (SiC) with etch rates as large as 1900 Å/min and obtained smooth etched surfaces and good etch anisotropy. Such etch processes are useful for a variety of SiC devices, including vertical UMOSFETs. Etch chemistry was found to greatly affect etch rate and final surface roughness. Etching in mixtures of sulfur hexafluoride (SF6) and oxygen, for example, resulted in relatively rough etched surfaces (typically 25 Å rms) with a maximum etch rate near 800 Å/min. Etching in SF6/Ar gave smoother surfaces and larger etch rates, most likely because physical sputtering by Ar+ ions helps to remove nonvolatile or low-volatility fluorocarbon and carbon-rich etch products. For example, using equal SF6 and Ar flow rates of 40 sccm, a process pressure of 10 mTorr, and a rf power density near 2.5 W/cm2, etch rates near 1000 Å/min are obtained. Atomic force microscopy indicates that the surface roughness of SiC etched under these conditions actually decreased slightly (from 6.1 Å to 4.4 Å rms). Scanning electron microscopy indicates good etch anisotropy with slight trenching at feature edges. Increasing the rf power density to 3.5 W/cm2 increases the etch rate to 1900 Å/min with only slightly increased surface roughness. The combination of large etch rate, smooth etched surfaces, and good etch anisotropy make magnetically enhanced reactive ion etching in SF6/Ar attractive for SiC device fabrication.
Directly sputtered stress-compensated carbon protective layer for silicon stencil masks
Authors: Hudek P. , Hrkút P. , Držik M. , Kostič I. , Belov M. , Torres J. , Wasson J. , Wolfe J. C. , Degen A. , Rangelow I. W. , Voigt J. , Butschke J. , Letzkus F. , Springer R. , Ehrmann A. , Kaesmaier R. , Kragler K. , Mathuni J. , Haugeneder E. , Löschner H.
Published: July 2002

Silicon stencil masks for ion beam projection lithography have a protective layer stopping the ions and thus preventing a change in the Si membrane stress. This is needed to maintain extremely tight pattern placement specifications even when they are irradiated with high exposure doses. The fabrication of carbon protective layers by indirect sputter coating which are suitable for helium ion beam exposure has already been reported. This article describes a method of forming very low stress carbon protective layers based on direct radio frequency sputter coating with nitrogen added to the argon sputter gas and in situ thermal treatment using commercially available equipment. The carbon layers thus produced are stable in conventional environments. The article deals also with the physical characterization of carbon layers and the protection performances of these coatings under helium ion beam exposure using accelerated lifetime testing.
Progress toward a 30 nm silicon metal–oxide–semiconductor gate technology
Authors: Tennant D. M. , Timp G. L. , Ocola L. E. , Green M. , Sorsch T. , Kornblit A. , Klemens F. , Kleiman R. , Kim Y. , Timp W.
Published: July 2002

We report on progress toward scaling both N-metal–oxide–semiconductor (MOS) and P-metal–oxide–semiconductor MOS transistors to a gate length of 30 nm. We describe lithography and pattern transfer results that are suitable to meet this goal. Scanning capacitance microscopy is used to determine the effective channel lengths and source drain junction depths on cross-sectioned devices to optimize the fabrication process. We present interim electrical results obtained for high performance, down to Lg=57 nm, N-MOS and P-MOS transistors made using this process. We have also used a device simulation program to predict subthreshold current for N-MOS transistors with gate lengths from 40 to 26 nm. The simulation provides insights into the effects of critical dimension control and edge roughness on leakage current, and has implications for extending large scale integration of MOS technology beyond 50 nm.
Vertical cavity surface emitting lasers incorporating structured mirrors patterned by electron-beam lithography
Authors: Berseth C.-A. , Dwir B. , Utke I. , Pier H. , Rudra A. , Iakovlev V. P. , Kapon E. , Moser M.
Published: July 2002

We demonstrate the operation of bottom-emitting vertical cavity surface-emitting lasers (VCSELs) with linear mode polarization which is controlled in an arbitrary orientation by the use of patterned metallic mirrors on top of the VCSEL surface. The top mirror is made in the shape of a 200 nm pitch grating, composed of alternating high reflectivity (Au) and low reflectivity (Cr) metal lines, whose orientation determines the polarization of the laser mode. The gratings were fabricated by high-resolution (<50 nm) electron-beam lithography and lift-off technique, and were aligned with the other parts of the VCSEL structure (top electrode, ion-implanted zone) fabricated by conventional photolithography. Various types of mirror shapes and sizes were fabricated, including square and circular grating envelopes, as well as circular mirrors with an average (radial) Gaussian reflectivity.
Novel method for silicon quantum wire transistor fabrication
Authors: Kedzierski Jakub , Bokor Jeffrey , Anderson Erik
Published: July 2002

Local stress-limited oxidation was used to fabricate silicon quantum wire transistors with a channel diameter of 5 nm. The oxidation of source and drain regions was prevented with a silicon nitride diffusion barrier. A novel wraparound gate was used to improve the gate control of the potential in the channel. The electrical properties of these devices were investigated at room temperature. Ideal subthreshold behavior, with the subthreshold swing equal to 60.3 mV/dec, was observed.
Characteristics of a free-standing superconducting nanobridge with an integrated heater fabricated using a self-aligned technique
Authors: Lee S.-B. , Hasko D. G. , Ahmed H.
Published: July 2002

We report on a novel method of controlling the supercurrent flow in a superconducting weaklink. We have fabricated a free-standing superconducting nanobridge with an integrated NiCr heater. This NiCr layer acts as a self-aligned mask for reactive ion etching and also as the normal metal heating element of the nanobridge. The heater is used to control the critical current of the superconducting weaklink part of the nanobridge; the free-standing nature of the nanobridge reduces the heater power required. We discuss the dependence of the current–voltage characteristics of the superconducting weaklink on heater current. This device has advantages over existing controllable weaklink devices.
Fabrication of submicrometer regular arrays of pillars and helices
Authors: Malac M. , Egerton R. F. , Brett M. J. , Dick B.
Published: July 2002

We use a glancing-angle deposition technique to produce regular lattices of submicrometer pillars and helices with a two-dimensional lattice constant below 1 μm and structure heights of 0.5–10 μm. Possible applications of such structures include photonic crystals and magnetic-storage media.
Cl
2
reactive ion etching for gate recessing of AlGaN/GaN field-effect transistors
Authors: Chen Ching-Hui , Keller Stacia , Haberer Elaine D. , Zhang Lidong , DenBaars Steven P. , Hu Evelyn L. , Mishra Umesh K. , Wu Yifeng
Published: July 2002

An effective gate recess etch process has been applied to AlGaN/GaN modulation-doped field-effect transistors (MODFETs), utilizing low power Cl2 reactive ion etching. In comparison to GaAs-based materials, GaN shows a greater robustness to ion damage under ion bombardment at very low ion energies (<70 V). It suggests that a viable gate recess etch process is possible. Recessed gate AlGaN/GaN MODFETs with gate to drain breakdown higher than −80 V have been demonstrated with an optimized lower power Cl2 reactive ion etching.
Charging and discharging of electron beam resist films
Authors: Bai M. , Pease R. F. W. , Tanasa C. , McCord M. A. , Pickard D. S. , Meisburger D.
Published: July 2002

Pattern placement imprecision due to charging of the workpiece is believed to be a significant contribution to the total positional error in electron beam lithography. In an earlier work, Liu et al. [J. Vac. Sci. Technol. B 13, 1979 (1995)] reported that the surface potential of exposed resist could be negative or positive according to the resist thickness and the electron energy. In that work the authors were constrained to use a flood beam. In this study, we report a new independent approach using a Kelvin probe electrometer to measure the surface potential after exposure by a focused beam. There is a qualitative agreement with the earlier work in that the surface potential tends to be less positive at lower electron energies and for thicker resists. We observed positive surface potentials at 10 and 20 keV beam irradiation. This positive charging is much more evident in polybutene sulfone than in UV5.
New three dimensional simulator for low energy (∼1 keV) electron beam systems
Authors: Lee Yongjae , Lee Woojin , Chun Kukjin , Kim Hoseob
Published: July 2002

In this article, we describe the models and the results of a new three dimensional (3D) lithography simulation programs for low energy (∼1 keV) electron beam systems. Monte Carlo simulation was performed to obtain the energy intensity distribution in e-beam resists, and the models we have used were tabulated Mott data for elastic scattering, Moller and Vriens cross sections for inelastic scattering, and modified Bethe equation plus discrete energy loss for energy loss. The energy intensity in poly(methyl) methacrylate was calculated with the exposure simulation program with various pattern shapes. In the development simulation program, the 2D or 3D resist profile could be implemented. The ray tracing model and the Neureuther equation were used for the development simulation. The simulated developed depths as a function of energy were compared with experimental results developed by Rishton and Schock. The maximum deviation from the experimental results was 12.4 nm (6%) at 2500 eV, and all the data were within error range. The optimum condition was obtained and the positive and negative resist profiles for 50 nm line and space pattern were realized with our simulation program.
Depth scale distortions in shallow implant secondary ion mass spectrometry profiles
Authors: Schueler B. W. , Reich D. F.
Published: July 2002

Secondary ion mass spectrometry analysis of ultrashallow boron implants is typically performed using sub-keV O2+ primary ion beams, either at normal or oblique (with O2 leak) incidence bombardment. This article investigates the distortion of depth profiles which may be due to sputter rate changes near the surface or primary ion beam induced mixing/roughening under the primary ion bombardment conditions mentioned above.
New aspects of nanopotentiometry for complementary metal–oxide–semiconductor transistors
Authors: Trenkler T. , Stephenson R. , Jansen P. , Vandervorst W. , Hellemans L.
Published: July 2002

Nanopotentiometry is a scanning probe microscopy (SPM) technique providing insight in the actual working behavior of semiconductor devices under operation. In nanopotentiometry, a conductive SPM tip is used as a voltage probe in order to measure the distribution of the electrical potential on the cross section of an operating device. The information thus provided is complementary to carrier profiling and is a method for the calibration of device simulations. The suitability of alternative SPM techniques for studying ultrashallow devices under operation is examined. Measurements have been carried out in deep submicron complementary metal–oxide–semiconductor devices. The impact of the changes in the doping profile on the potential distributions has been explored using simulations and experimental results. Due to further improvements in sample preparation and measurement methodology, the response of the devices to variable bias conditions could be studied in more detail. The formation of the conductive channel underneath the gate was investigated and compared to device simulations.
Ion implantation damage model for B, BF2, As, P, and Si in silicon
Authors: Son Myung-Sik , Hwang Ho-Jung
Published: July 2002

In this article we describe a newly proposed and consistent damage model in Monte Carlo simulation for the accurate prediction of a three-dimensional as-implanted impurity profile and point defect profile induced by ion implantation in (100) crystal silicon. An empirical electronic energy loss model for B, BF2, As, P, and Si self-implants over a wide energy range has been proposed for silicon-based semiconductor device technology and development. Our model shows very good agreement with secondary ion mass spectrometry data over a wide energy range. For damage accumulation, we have considered the self-annealing effects by introducing our proposed nonlinear recombination probability function of each point defect for computational efficiency. For the damage profiles, we compared the published Rutherford backscattering spectrometry (RBS)/channeling data with our results of phosphorus implants. Our damage model shows very reasonable agreements with the RBS/channeling experiments for phosphorus implants.
Aligned carbon nanotube films for cold cathode applications
Authors: Obraztsov A. N. , Pavlovsky I. , Volkov A. P. , Obraztsova Elena D. , Chuvilin A. L. , Kuznetsov V. L.
Published: July 2002

Thin film material of oriented multiwall carbon nanotubes was obtained by noncatalytical chemical vapor deposition in a glow-discharge plasma. The film phase composition, surface morphology, and structural features were studied by Raman and electron microscopy techniques. Low-voltage electron field emission of thin film nanotube material was obtained and examined in diode configuration. The I–V curves in Fowler–Nordheim coordinates were linear and the corresponding threshold average field was about 1.5 V/μm. The emission current density was up to 50 mA/cm2 at the field of 5 V/μm. The emission site density reached 107 cm−2 at the same value of electric field.
Field emission characteristics of boron nitride films
Authors: Sugino Takashi , Etou Yoshihiro , Tagawa Shigeru , Gamo Mikka Nishitani , Ando Toshihiro
Published: July 2002

Boron nitride (BN) films are deposited onto Si substrates by plasma-assisted chemical vapor deposition. It is found that a variation in the surface roughness occurs with increasing deposition time. Ultraviolet photoelectron spectroscopy analysis reveals that negative electron affinity (NEA) appears on the surface of BN films treated with H2 plasma and that NEA is maintained even after O2 plasma treatment. NEA is lost by annealing at 1100 °C. Field emission characteristics of BN films with various surface roughnesses suggest electron emission due to Fowler–Nordheim tunneling from the energy state in the band gap. A turn-on electric field as low as 8 V/μm is obtained for the H2 plasma treated BN film.
Resolution in scanning near-field cathodoluminescence microscopy
Authors: Pastré D. , Bubendorff J. L. , Troyon M.
Published: July 2002

The lateral resolution in scanning near-field cathodoluminescence microscopy, in the case of bulk materials, is theoretically and experimentally studied. Although the theoretical resolution of near-field optical collection systems is determined by the probe size and thus should not be dependent on the energy dissipation volume in the material, it is theoretically shown that the contribution of the far-field signal caused by the radiative centers situated far from the probe can alter the resolution by adding a background noise to the near-field signal. In order to see the role of the energy dissipation volume, the electron beam accelerating voltage is varied and its influence on lateral resolution is studied in the case of a material that does not present a large energy transfer range, a fluorite doped with 0.3% europium. Cathodoluminescence images of this one confirm that the resolution is improved by working at low accelerating voltage to limit the energy dissipation volume, hence the contribution of the far-field radiative centers. On the other hand, for materials having a large energy transfer range (several micrometers) and therefore for which the far-field contribution can be strongly disturbing, the question concerning the efficiency of the near-field collection systems arises. Our experimental results on indented MgO crystal, which presents a large energy transfer range, demonstrates that near-field collection, despite the large contribution of the radiative centers situated far from the tip, gives still a better resolution than far-field collection.
Using statistical experimental design to investigate the role of the initial growth conditions on GaN epitaxial films grown by molecular beam epitaxy
Authors: Lee Kyeong K. , Doolittle William A. , Brown April S. , May Gary S. , Stock Stuart R.
Published: July 2002

The nucleation and buffer growth of GaN on (0001) sapphire by molecular beam epitaxy are investigated using the design of experiments approach. Six factors are simultaneously varied: time and temperature for nitridation, buffer growth temperature, Ga cell temperature, growth time, and nitrogen plasma power during buffer growth. In situ reflection high-energy electron diffraction is utilized to monitor these steps. The quality of the epitaxial layers obtained is examined by means of electron mobility and atomic force microscopy. It is shown that the buffer layer growth rate has the greatest influence on improving the electrical properties of the subsequent GaN epitaxial layer. Depending on the growth conditions, the Hall mobility of the GaN epitaxial layer varies from 24 to 238 cm2/V s. Changes in surface morphology are correlated with improvements in electron mobility. We also discuss interaction effects between the factors. A trend extracted from a least-squares model reveals that 300 K Hall mobility is greatly improved at high growth rate and low nitrogen plasma power during buffer growth.
Optical properties of ZnMgSeTe quaternary alloys grown on ZnTe substrates by molecular-beam epitaxy
Authors: Chang J. H. , Wang H. M. , Cho M. W. , Makino H. , Hanada H. , Yao T. , Shim K. , Rabitz H.
Published: July 2002

Zn 1−x Mg x Se y Te 1−y quaternary alloys are grown on ZnTe(001) substrates for the first time. The photoluminescence properties of ZnMgSeTe are characterized by strong excitonic luminescence intensity with a narrow linewidth that indicate the high crystal quality and the composition homogeneity. It is deduced that the composition homogeneity of the alloy is originated from the stable sticking coefficient of group-VI species and the exclusion of impurity diffusion from the substrate. Type-I band lineup in the ZnTe/ZnMgSeTe single quantum well is estimated from the narrow and strong exciton luminescence line of the quantum well, and the band offset in this heterostructure is estimated based on the photoluminesence, photoluminescence excitation, and reflectance results as 20δEc:80δEv.
Depth-resolved detection and process dependence of traps at ultrathin plasma-oxidized and deposited SiO2/Si interfaces
Authors: Brillson L. J. , Young A. P. , White B. D. , Schäfer J. , Niimi H. , Lee Y. M. , Lucovsky G.
Published: July 2002

Low-energy electron-excited nanoluminescence spectroscopy reveals depth-resolved optical emission associated with traps near the interface between ultrathin SiO2 deposited by plasma-enhanced chemical vapor deposition on plasma-oxidized crystalline Si. These near-interface states exhibit a strong dependence on local chemical bonding changes introduced by thermal/gas processing, layer-specific nitridation, or depth-dependent radiation exposure. The depth-dependent results provide a means to test chemical and structural bond models used to develop advanced dielectric-semiconductor junctions.
Spins in the Vortices of a High-Temperature Superconductor
Authors: Lake B. , Aeppli G. , Clausen K. N. , McMorrow D. F. , Lefmann K. , Hussey N. E. , Mangkorntong N. , Nohara M. , Takagi H. , Mason T. E. , Schröder A.
Published: July 2002

Neutron scattering is used to characterize the magnetism of the vortices for the optimally doped high-temperature superconductor La
2-x
Sr
x
CuO
4
(
x
= 0.163) in an applied magnetic field. As temperature is reduced, low-frequency spin fluctuations first disappear with the loss of vortex mobility, but then reappear. We find that the vortex state can be regarded as an inhomogeneous mixture of a superconducting spin fluid and a material containing a nearly ordered antiferromagnet. These experiments show that as for many other properties of cuprate superconductors, the important underlying microscopic forces are magnetic.
Crystal Structure of an Initiation Factor Bound to the 30
S
Ribosomal Subunit
Authors: Carter Andrew P. , Clemons William M. , Brodersen Ditlev E. , Morgan-Warren Robert J. , Hartsch Thomas , Wimberly Brian T. , Ramakrishnan V.
Published: July 2002

Initiation of translation at the correct position on messenger RNA is essential for accurate protein synthesis. In prokaryotes, this process requires three initiation factors: IF1, IF2, and IF3. Here we report the crystal structure of a complex of IF1 and the 30
S
ribosomal subunit. Binding of IF1 occludes the ribosomal A site and flips out the functionally important bases A1492 and A1493 from helix 44 of 16
S
RNA, burying them in pockets in IF1. The binding of IF1 causes long-range changes in the conformation of H44 and leads to movement of the domains of 30
S
with respect to each other. The structure explains how localized changes at the ribosomal A site lead to global alterations in the conformation of the 30
S
subunit.
Variability in the El Niño-Southern Oscillation Through a Glacial-Interglacial Cycle
Authors: Tudhope Alexander W. , Chilcott Colin P. , McCulloch Malcolm T. , Cook Edward R. , Chappell John , Ellam Robert M. , Lea David W. , Lough Janice M. , Shimmield Graham B.
Published: July 2002

The El Niño–Southern Oscillation (ENSO) is the most potent source of interannual climate variability. Uncertainty surrounding the impact of greenhouse warming on ENSO strength and frequency has stimulated efforts to develop a better understanding of the sensitivity of ENSO to climate change. Here we use annually banded corals from Papua New Guinea to show that ENSO has existed for the past 130,000 years, operating even during “glacial” times of substantially reduced regional and global temperature and changed solar forcing. However, we also find that during the 20th century ENSO has been strong compared with ENSO of previous cool (glacial) and warm (interglacial) times. The observed pattern of change in amplitude may be due to the combined effects of ENSO dampening during cool glacial conditions and ENSO forcing by precessional orbital variations.
Sonic Hedgehog Control of Size and Shape in Midbrain Pattern Formation
Authors: Agarwala Seema , Sanders Timothy A. , Ragsdale Clifton W.
Published: July 2002

Little is known about how patterns of cell types are organized to form brain structures of appropriate size and shape. To study this process, we employed in vivo electroporation during midbrain development to create ectopic sources of Sonic Hedgehog, a signaling molecule previously shown to specify different neuronal cell types in a concentration-dependent manner in vitro. We provide direct evidence that a Sonic Hedgehog source can control pattern at a distance in brain development and demonstrate that the size, shape, and orientation of the cell populations produced depend on the geometry of the morphogen source. Thus, a single regulatory molecule can coordinate tissue size and shape with cell-type identity in brain development.
Microbial Genes in the Human Genome: Lateral Transfer or Gene Loss?
Authors: Salzberg Steven L. , White Owen , Peterson Jeremy , Eisen Jonathan A.
Published: July 2002

The human genome was analyzed for evidence that genes had been laterally transferred into the genome from prokaryotic organisms. Protein sequence comparisons of the proteomes of human, fruit fly, nematode worm, yeast, mustard weed, eukaryotic parasites, and all completed prokaryote genomes were performed, and all genes shared between human and each of the other groups of organisms were collected. About 40 genes were found to be exclusively shared by humans and bacteria and are candidate examples of horizontal transfer from bacteria to vertebrates. Gene loss combined with sample size effects and evolutionary rate variation provide an alternative, more biologically plausible explanation.
Haplotype Diversity and Linkage Disequilibrium at Human
G6PD
: Recent Origin of Alleles That Confer Malarial Resistance
Authors: Tishkoff Sarah A. , Varkonyi Robert , Cahinhinan Nelie , Abbes Salem , Argyropoulos George , Destro-Bisol Giovanni , Drousiotou Anthi , Dangerfield Bruce , Lefranc Gerard , Loiselet Jacques , Piro Anna , Stoneking Mark , Tagarelli Antonio , Tagarelli Giuseppe , Touma Elias H. , Williams Scott M. , Clark Andrew G.
Published: July 2002

The frequencies of low-activity alleles of glucose-6-phosphate dehydrogenase in humans are highly correlated with the prevalence of malaria. These “deficiency” alleles are thought to provide reduced risk from infection by the
Plasmodium
parasite and are maintained at high frequency despite the hemopathologies that they cause. Haplotype analysis of “A−” and ”Med“ mutations at this locus indicates that they have evolved independently and have increased in frequency at a rate that is too rapid to be explained by random genetic drift. Statistical modeling indicates that the A− allele arose within the past 3840 to 11,760 years and the Med allele arose within the past 1600 to 6640 years. These results support the hypothesis that malaria has had a major impact on humans only since the introduction of agriculture within the past 10,000 years and provide a striking example of the signature of selection on the human genome.
Zipf Distribution of U.S. Firm Sizes
Authors: Axtell Robert L.
Published: July 2002

Analyses of firm sizes have historically used data that included limited samples of small firms, data typically described by lognormal distributions. Using data on the entire population of tax-paying firms in the United States, I show here that the Zipf distribution characterizes firm sizes: the probability a firm is larger than size
s
is inversely proportional to
s
. These results hold for data from multiple years and for various definitions of firm size.
Localization of Long-Term Memory Within the
Drosophila
Mushroom Body
Authors: Pascual Alberto , Préat Thomas
Published: July 2002

The mushroom bodies, substructures of the
Drosophila
brain, are involved in olfactory learning and short-term memory, but their role in long-term memory is unknown. Here we show that the
alpha-lobes-absent
(
ala
) mutant lacks either the two vertical lobes of the mushroom body or two of the three median lobes which contain branches of vertical lobe neurons. This unique phenotype allows analysis of mushroom body function. Long-term memory required the presence of the vertical lobes but not the median lobes. Short-term memory was normal in flies without either vertical lobes or the two median lobes studied.
The Closest Living Relatives of Land Plants
Authors: Karol Kenneth G. , McCourt Richard M. , Cimino Matthew T. , Delwiche Charles F.
Published: July 2002

The embryophytes (land plants) have long been thought to be related to the green algal group Charophyta, though the nature of this relationship and the origin of the land plants have remained unresolved. A four-gene phylogenetic analysis was conducted to investigate these relationships. This analysis supports the hypothesis that the land plants are placed phylogenetically within the Charophyta, identifies the Charales (stoneworts) as the closest living relatives of plants, and shows the Coleochaetales as sister to this Charales/land plant assemblage. The results also support the unicellular flagellate
Mesostigma
as the earliest branch of the charophyte lineage. These findings provide insight into the nature of the ancestor of plants, and have broad implications for understanding the transition from aquatic green algae to terrestrial plants.
The Vibrational Spectrum of H
2
O
3
Authors: Engdahl A. , Nelander B.
Published: July 2002

This report presents positive infrared spectroscopic identification of H
2
O
3
, a higher oxide of hydrogen of importance for the understanding of the chain formation ability of atomic oxygen and a possible intermediate in hydrogen oxygen radical chemistry. All fundamental vibrations of H
2
O
3
, isolated in an argon matrix, have been observed. In addition, several bands of HDO
3
, D
2
O
3
, and H
2
16
O
2
18
O have been measured. One particular mode, the antisymetric O–O stretch at 776 cm
−1
, should be observable even in the presence of high water concentrations.