bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunneling microscopy of point defects on GaAs(110)
Authors: Lengel G. , Wilkins R. , Brown G. , Weimer M.
Published: July 2002

The structure of point defects situated at nominal arsenic or gallium sites on the (110) cleavage face of GaAs has been studied by scanning tunneling microscopy (STM). Alternate-bias imaging was used to simultaneously resolve the arsenic and gallium sublattices corresponding to filled and empty states, respectively, on both p- and n-type material. There is an interesting symmetry in the characteristic features associated with the most commonly observed defect of each type. Both types appear in the STM images as a missing surface atom, or, more precisely, a highly localized reduction in the corresponding filled- or empty-state density. For both types of defect, nearest neighbors within the same zigzag chain appear to be raised out of the surface, while second nearest neighbors show a slight depression. No large lateral displacements are detected in either the nearest or second nearest neighbor atoms. Point defects with this structure are observed exclusively at arsenic sites on p-type material, and exclusively at gallium sites on n-type material. The band bending accompanying the defects reveals that the arsenic defect on p-type samples is positively charged whereas the gallium defect on n-type samples is negatively charged. Migration of these defects in the [11̄0] direction along the zigzag chains, as well as in the [001] direction perpendicular to them, is occasionally observed in the STM scans.
Dose analysis of nitrogen plasma source ion implantation treatment of titanium alloys
Authors: Chen A. , Firmiss J. , Conrad J. R.
Published: July 2002

Dose analysis of plasma source ion implantation (PSII) treatment involves two issues: calculation of the delivered dose and measurement of the retained dose. This article discusses the delivered dose calculation method and the determination of constants used in this calculation, such as the secondary electron emission coefficient and average number of atoms per ion being implanted. Both can be measured experimentally at the University of Wisconsin PSII group. The retained dose is usually studied by Auger electron spectroscopy (AES). For nitrogen PSII treated titanium alloys, like all titanium nitrogen compounds, the chemical analysis by AES is complicated by two factors: (1) the Auger electron emission from nitrogen occurs at an energy that completely overlaps a transition from titanium; and (2) titanium oxide has a strong influence on the titanium transitions. In this study, a new technique that takes into account the influence of titanium oxide and solves the overlap problem has been developed to convert AES data to a concentration depth profile. A comparison of the concentration profile for nitrogen as observed in the AES data and calculated by the tamix code (computer simulation) shows good agreement.
Plasma source ion implantation of oxygen and nitrogen in aluminum*
Authors: Günzel R. , Wieser E. , Richter E. , Steffen J.
Published: July 2002

In a preliminary study we compare the surface properties of oxygen and nitrogen implanted in aluminum by the plasma source ion implantation (PSII) technique with beam line implantation of these elements. The acceleration voltage was 35 keV. Using PSII at these energies the areal density of the implanted species saturates at about 2×1017 cm−2 for nitrogen and 4×1017 cm−2 for oxygen. In the case of PSII with nitrogen, a relatively high concentration of oxygen is detected which is explained as due to the strong surface interaction with the oxygen contamination of the plasma. During PSII from a steady state oxygen plasma, the plasma surface interaction predominates the implantation. A surface layer of Al2O3 with a rough topology is formed. The surface hardness is lowered by this treatment. All other implanted samples show an increased dynamic hardness in a thin surface near layer of about 50 nm thickness in comparison to a nonimplanted sample. A maximum increase of the dynamic hardness by a factor 1.2 has been estimated for PSII with nitrogen and for PSII with oxygen from the plasma of the Townsend discharge.
Enhanced pitting corrosion resistance of 304L stainless steel by plasma ion implantation
Authors: Smith Preston P. , Buchanan R. A. , Roth J. Reece , Kamath Sanjay G.
Published: July 2002

High-nitrogen, austenitic stainless steels combine a variety of superior mechanical properties with an increased resistance to localized pitting/crevice corrosion in halide environments. However, nitrogen content in the bulk is restricted during normal melt/solidification processes due to limited solubility, which, if exceeded, leads to nitride formation and possible sensitization. Plasma ion implantation (PII) techniques may result in much higher nitrogen contents via nonequilibrium surface modification. PII of nitrogen into 304L stainless steel, and its subsequent effect on the pitting corrosion of this alloy, has been investigated. The implantation process at the University of Tennessee Microwave Plasma Facility results from a large-volume (206 ℓ  ), steady-state, uniform plasma, generated by up to 2 kW of 2.45 GHz microwave power. Samples were negative pulse biased to potentials of 10 and 20 kV, with doses ranging from 1014 to 1018 ions/cm2. Rutherford backscattering spectroscopy analyses of dose uniformity and depth profiling were also conducted. Potentiodynamic anodic polarization behaviors were determined in a deaerated 1.0 wt % NaCl solution. The highest pitting potentials, up to 1440 mV (standard hydrogen electrode) were measured at the lowest range of doses. These results indicate that nitrogen doses effective for tribological enhancement may be incompatible with those effective in resisting localized corrosion.
High-speed positive x-ray resist suitable for precise replication of sub-0.25-μm features
Authors: Ban Hiroshi , Nakamura Jiro , Deguchi Kimiyoshi , Tanaka Akinobu
Published: July 2002

A high-speed chemically amplified positive resist based on poly(p-hydroxystyrene) has been developed for x-ray lithography. This resist, CANI (meaning chemically amplified resist) by Nippon Telegraph and Telephone Corporation, has a sensitivity of 50–70 mJ/cm2 to soft x rays while maintaining sub-0.25-μm resolution and a rather large dose margin of 34%–40%. Another advantage of CANI is that the theoretical weight loss from the removal of t-butyl protecting groups by chemical amplification reactions is less than 4 wt %. This leads to little volume shrinkage at uv cure and reactive ion etching processes, and thus promises good controllability of pattern feature sizes between lithography and subsequent plasma etching processes.
Scanning tunneling microscopy studies of metal surfaces: Surface reactions, discrimination of chemically different elements, and surface alloying
Authors: Besenbacher F. , Lægsgaard E. , Pleth Nielsen L. , Ruan L. , Stensgaard I.
Published: July 2002

Recent novel results concerning the applicability of the scanning tunneling microscope for atom-resolved studies of chemical reactions, discrimination of chemically different elements, and alloying of nonmiscible metals at metal surfaces are discussed.
Reflectance modeling for in situ dry etch monitoring of bulk SiO2 and III–V multilayer structures
Authors: Hicks S. E. , Parkes W. , Wilkinson J. A. H. , Wilkinson C. D. W.
Published: July 2002

We investigate in situ dry etch monitoring via reflectometry. We present two approaches, the choice depending on the presence of layers (if any) deposited on the substrate. Firstly, we present a model of the normal incidence reflectance of a multilayer stack (of any number of layers) as a function of etch depth based on electrical transmission line theory. We have applied this model to the reactive ion etching of an InP/InGaAsP multilayer structure using CH4/H2/O2, and a GaAs/AlAs multilayer structure using SiCl4. In the case of the InP-based material, use of the model enabled us to distinguish individual 10.24 nm InGaAsP quantum wells. In the case of the GaAs-based structure, use of the model allowed us to determine the induction time for the removal of the native gallium oxide and to tell which AlAs quantum well had been reached during the etch, enabling cessation of the etch process just after the last well with no overetch. Agreement between the model and observed behavior of the reflectance was extremely good in both cases. Secondly, we have extended this approach to model the reflectance from a wafer with a suitable patterned dry etch mask and a laser beam covering both masked and etched areas, allowing the study of interference between the reflected beams from the two areas. We have compared the modeled and observed reflectance from a sample of NiCr masked bulk SiO2 and found agreement to be within 20 nm in a total etch depth of 1 μm.
Patterning a 50-nm period grating using soft x-ray spatial frequency multiplication
Authors: Wei Max , Attwood David T. , Gustafson T. K. , Anderson Erik H.
Published: July 2002

Soft x-ray spatial frequency multiplication is a technique that has the capability of reducing by a factor of 2 or more the finest period grating that can be written by other methods. We describe two geometries for this technique: a single-grating geometry that requires spatially coherent illumination, and a two-grating geometry that can be used with incoherent illumination. Starting with a parent grating of period p and area A, gratings of period p/2, p/4, p/6 over an area A/3, A/5, A/7, respectively, can be printed in the single-grating geometry. The two-grating geometry produces the same final grating periods for parent gratings of period p, but the final grating area is limited only by the sizes of the parent gratings. As an initial demonstration of this technique, we have used the single-grating geometry to pattern a 50-nm period grating in polymethylmethacrylate over an area of 30 μm×100 μm, starting with a 100-nm period parent grating with an area of 90 μm×100 μm. The x-ray source was a synchrotron undulator at λ=18 Å and required a 5-min exposure time. Recent developments in coherent radiation from x-ray undulator sources will give shorter exposure times and permit larger areas to be patterned in the single-grating geometry.
External field penetration effect on current-field characteristics of metal emitters
Authors: Il’chenko L. G. , Kryuchenko Yu. V.
Published: July 2002

Polarization and barrier potentials for the point charge at a metal–vacuum interface have been calculated in the Thomas–Fermi and random phase approximations taking into account spatial dispersion effects and quantum nature of conduction electrons. The effect of external field penetration into the metal on the barrier potential shape and emission current is demonstrated. Both isotropic (a spherical Fermi surface) and anisotropic (a flat Fermi surface) cases are considered. Comparison between these cases is made for equivalent conditions.
Investigation of an electron-beam microgun using a microtips array
Authors: Constancias C. , Herve D. , Accomo R. , Molva E.
Published: July 2002

A microtip field-emission electron microgun of a few cm3 has been developed for a novel type of semiconductor laser. The electron source is provided by an array of a field emissive microtips cathode each of 1.4 μm base diameter. The source current–voltage characteristic has been determined and its geometrical characteristics have been measured by collecting electrons on a phosphor screen in a region of constant electric field. We have observed that about 100 μA are typically emitted for gate-cathode voltage (VGC) of 80 V with a half-aperture angle of 28°. Beam focusing is then realized by two electrostatic lenses: a ‘‘thin hole lens’’ and a quadrupole lens. Using a cathode a 70×500 μm2 rectangular spot on a 10 kV biased anode has been obtained which yields a power density of about 3 kW/cm2. Less than 20% of the total electron beam (current) is lost in the electrostatic optics.
Far-infrared photoconductive magnetospectroscopy as a tool for studying shallow donor concentration profiles in wide GaAs/AlGaAs quantum wells
Authors: Chai Y. H. , Goodhue W. D. , Mueller E. R. , Waldman J.
Published: July 2002

The feasibility of using far-infrared photoconductive magnetospectroscopy to accurately determine the positions of shallow donors in multiple quantum well samples grown by molecular-beam epitaxy has been investigated and the technique successfully applied to silicon donors in quantum wells at doping levels below 2×1016 cm−3. Theoretical spectra calculated using a combined segregation and diffusion model for predicting donor position along with a theory of mapping donor 1s→2p+1 transition energies to magnetic fields are in good agreement with experimental spectra. A new value for the segregation decay rate, 3.1 nm/decade, is indicated for Si donors in wide GaAs/AlGaAs quantum wells grown at a substrate temperature of 680 °C. This new tool which is effective in the doping range of 1×1014 to 2×1016 cm−3 complements capacitance–voltage (C–V) techniques and secondary ion mass spectrometry. The technique is also applicable to a variety of heterostructures.
Growth of ZnSe epilayers on Fe(001) films by migration enhanced epitaxy and self-limiting epitaxy
Authors: Abad H. , Jonker B. T. , Cotell C. M. , Krebs J. J.
Published: July 2002

Low temperature growth of ZnSe epilayers on Fe (001) films using migration enhanced epitaxy (MEE) and self-limiting epitaxy (SLE) and the growth of Fe/ZnSe/Fe multilayers is reported. All samples were grown on GaAs (001) substrates. The samples were characterized in situ by reflection high energy electron diffraction and ex situ using magnetic and transmission electron microscopy measurements. Under Zn-rich conditions, MEE and SLE preserve the magnetic character of the incorporated Fe film. In Fe/ZnSe/Fe structures, the Fe films have significantly different coercive fields and resonant linewidths which may be related to the difference in defect densities between the two layers.
Atomic-scale modification on Si(111)7×7 surfaces
Authors: Ma Z. L. , Liu N. , Zhao W. B. , Gu Q. J. , Ge X. , Xue Z. Q. , Pang S. J.
Published: July 2002

In this paper the scanning tunneling microscope (STM) has been used to fabricate atomic-scale structures by removing atoms from the Si(111)7×7 surfaces at room temperature. Grooves a few nanometers in width are created. When the modification direction is along one of the base vector directions of the Si(111)7×7 surface (i.e., [1̄10], [1̄01], and [01̄1]), the grooves have atomically straight edges and lateral features as small as a 7×7 unit cell wide. The study of the modification process reflects that the atomic-scale structures are formed by extracting silicon atoms one by one from the sample surfaces. The dependence of critical current for creating a regular structure on Si(111)7×7 surface versus the bias voltage has been measured. Finally, based on the experimental data, the modification mechanism is designed.
Real time in situ monitoring of surfaces during glow discharge processing: NH3 and H2 plasma passivation of GaAs
Authors: Aydil Eray S. , Zhou Zhen H. , Gottscho Richard A. , Chabal Yves J.
Published: July 2002

Numerous device applications of GaAs are hampered by poor electronic properties of GaAs surfaces and interfaces. Room temperature NH3 and H2 downstream plasma passivation of native oxide contaminated GaAs surfaces is investigated using attenuated-total-reflection (ATR) Fourier-transform-infrared spectroscopy (FTIR) and photoluminescence (PL). Using ATR FTIR concentrations of –As–O, –As–H, H–OH, and C–H bonds are monitored in situ and in real time during exposure of the GaAs surface to H (D) atoms from a microwave discharge through NH3 (ND3) and H2 (D2). Photoluminescence intensity from the GaAs is monitored simultaneously with the FTIR spectra and used as a measure of surface state reduction. At room temperature, H atoms produced from the discharge remove –As–O and C–H contaminants but not Ga2O3. The appearance of As–H bonds and corresponding increase in PL are delayed significantly from plasma initiation, an increase in –O–H concentration, and removal of –As–O bonds. Because the As antisite defects are concentrated at the GaAs–oxide interface, the rates of As–H formation and PL enhancement may be limited by diffusion of H through the oxide and water layer which forms on the surface. We find that the concentration of physisorbed H2O on the GaAs surface increases throughout passivation. There are two sources of water detected on the surface: (i) reduction of As–O bonds and (ii) reaction of H with the quartz reactor walls. Ga2O3 grows on the surface via oxidation of GaAs by the physisorbed water. We surmise that higher H concentration in NH3 plasmas results in faster water accumulation on the surface and trapping of the As–H species subsurface. In H2 plasma, water accumulation on the surface is slower and trapping of As–H is not observed.
Vacuum plasma sprayed hydroxyapatite coatings on titanium alloy substrates: Surface characterization and observation of dissolution processes using atomic force microscopy
Authors: Campbell P. A. , Gledhill H. C. , Brown S. R. , Turner I. G.
Published: July 2002

Hydroxyapatite (HA), Ca5(PO4)3OH, is a biologically important material that occurs naturally in bone. Synthetic HA is thus a natural choice as a surface coating for materials that are used in implant surgery due to its inherent biocompatibility. HA coatings can be applied by several techniques, one of which is vacuum plasma spraying (VPS). This method is used commercially to apply HA to a titanium alloy substrate. A complete characterization of the initial, as-sprayed surface state was carried out using scanning electron microscopy (SEM), x-ray diffraction (XRD), and also atomic force microscopy (AFM). XRD measurements reveal the coating to be semicrystalline and SEM observations showed it to be irregular in appearance but with flatter regions interspersed. Large scale AFM scans (≳20 μm) of the as-sprayed coatings were difficult to obtain as the tip would frequently snag on higher outcrops, sometimes snapping the cantilever in the process. To prevent this from occurring, the scan range was reduced and images were only obtained within the flatter regions observed previously with SEM. From these data, the roughness values characterizing the flatter surfaces were estimated. Further, ex situ images of the coatings after they had been aged for periods of up to 28 days in a solution simulating the physiological environment within the body (buffered Ringer’s solution) were obtained. Changes in the topography were observed that may have a bearing on processes such as the cellular response of the surrounding tissue to the implanted material. The structural changes in the surface were again characterized by estimating the surface roughness values. Conclusions are drawn regarding the formation of the coating from plasma spraying and also the importance of the dissolution process in promoting bony ingrowth and rapid fixation of the implant.
Modeling of leakage mechanisms in sub-50 nm p+-n junctions
Authors: Jones Erin C. , Cheung Nathan W.
Published: July 2002

High leakage currents of ultrashallow junctions formed by B diffusion out of solid-phase epitaxially grown CoSi2 contacts grown from Co/Ti bilayers on Si are explained by the Shannon contact model. Depending on implant condition, the diodes behave either as p-n diodes or Schottky diodes with barrier height enhanced by the p-type diffusion. Diodes implanted with 11B+ at 7.5 keV and 1015 cm−2 dose show leakage current near 100 nA/cm2 at −5 V and behave like ideal p-n junctions after a 900 °C, 30 s postimplant anneal. Diodes implanted at 3.5 keV with 1015 cm−2 dose or at 7.5 keV with a 1014 cm−2 dose display higher leakage currents and other characteristics like Schottky diodes. Further analysis of the Shannon contact model shows that thermionic emission leakage current of Schottky-like diodes may limit p-n junction scaling: with p-type doping concentrations of 1018–1019 cm−3, the Shannon contact model predicts that a 20–30 nm p-type junction depth below metal contacts is necessary to keep leakage at acceptable levels. A capacitance–voltage method is suggested for finding the minimum junction depth of p-n junctions.
Vortex images in thin films of YBa2Cu3O7−x and Bi2Sr2Ca1Cu2O8+x obtained by low-temperature magnetic force microscopy
Authors: Yuan C. W. , Zheng Z. , de Lozanne A. L. , Tortonese M. , Rudman D. A. , Eckstein J. N.
Published: July 2002

We have imaged vortices in superconducting thin films with a low-temperature magnetic force microscope that utilizes microfabricated piezoresistive cantilevers with built-in tips. The films of YBa2Cu3O7−x and Bi2Sr2Ca1Cu2O8+x, are made by laser ablation and molecular beam epitaxy, respectively. The vortices usually appear as round features in the noncontact image with a diameter of about 1 μm or slightly less. In some cases the position of the vortices is correlated to defects observed in the topographic image of the same area. The vortices move sometimes, especially after taking a topographic (contact mode) scan.
The nanoscilloscope: Combined topography and AC field probing with a micromachined tip
Authors: van der Weide D. W. , Neuzil P.
Published: July 2002

We introduce a new concept for combined probing of topography and local AC fields, and report preliminary results of tip fabrication and scale-model measurements. The scanning force microscope (SFM) has been recently used to measure AC fields using the nonlinear response of the cantilever to voltage differences between the sample and scanning tip. By contrast, we extend the concept of near-field scanning optical microscopy (NSOM) down in frequency by employing a non-cutoff coaxial micromachined waveguide/tip, maintaining the topographic and tip–sample distance control capabilities of the SFM. Because this instrument will be used to measure both AC fields and topography with nanometer-level localization and better than nanosecond temporal resolution, we call it a nanoscilloscope.
Simulation of light propagation in optical linear and nonlinear resist layers by finite difference beam propagation and other methods
Authors: Erdmann A. , Henke W.
Published: July 2002

This article presents a new method for the calculation of light propagation in resist layers and compares it with scaled defocus and transfer matrix algorithms. Our method is not restricted to homogeneous resist and substrate layers. For the first time, a simulation of focus drilling is presented.
Imaging of organic molecular films using a scanning near-field optical microscope combined with an atomic force microscope
Authors: Yamada Hirofumi , Tokumoto Hiroshi , Akamine Shinya , Fukuzawa Kenji , Kuwano Hiroki
Published: July 2002

A scanning near-field optical microscope combined with an atomic force microscope is described. The instrument uses a microfabricated force-sensing cantilever made of single-crystal silicon with an integrated p-n diode at its end as a microphotodetector. The photosensitive cantilever efficiently detects the light scattered by the tip in the evanescent field generated by illuminating the sample by total internal reflection. Simultaneously the surface force is detected by monitoring the deflection of the cantilever. Both scanning near-field optical microscope and atomic force microscope images of a compact disk grating and a photosynthetic protein membrane were obtained. The membrane was deposited on the glass substrate using the Langmuir–Blodgett technique. The features in the scanning near-field optical microscope images are similar to those in the atomic force microscope. The possible crosstalk between the optical and the topography signal is discussed.
Molecular beam epitaxy growth kinetics for group III nitrides
Authors: Foxon C. T. , Cheng T. S. , Hooper S. E. , Jenkins L. C. , Orton J. W. , Lacklison D. E. , Novikov S. V. , Popova T. B. , Tret’yakov V. V.
Published: July 2002

We have used a modified molecular beam epitaxy method, in which the active species of nitrogen is derived from an Oxford applied research rf activated plasma source, to grow AlN, GaN, InN, Al(AsN) and Ga(AsN). In this paper we discuss, for the first time, the growth kinetics of both binary nitrides and ‘‘ternary alloys’’ in the light of both our observations and those of other groups working in this area and their relevance to the properties of films grown under various conditions.
Optimization of field emission arrays for inductive output amplifiers
Authors: Kodis M. A. , Jensen K. L. , Zaidman and E. G. , Goplen B. , Smithe D. N.
Published: July 2002

In an inductive output amplifier, an emission gated electron beam induces high-frequency fields in an output circuit via displacement currents, not convection current. Emission gated electron beams experience strong interactions when traversing a resonant or synchronous electromagnetic field, and this strong interaction is responsible for both the interesting nonlinear physics and the attractive efficiency and compactness of emission gated amplifiers. Field emission cathodes, due to their extremely low electron transit time and high transconductance, offer the opportunity to extend the advantages of emission gating into C and X bands. Design criteria for the joint optimization of the field emission arrays (FEAs) structure and the rf input and output circuits of inductive output amplifiers are presented. It is found that while output circuits yielding net efficiencies of 50% or greater are well within the state of the art, the gain is likely to be moderate (10–20 dB). With today’s FEA performance, a desirable operating regime is achievable, yielding a new class of compact, highly efficient, and moderate gain power booster amplifiers.
Imaging of ferromagnetic domains using photoelectrons: Photoelectron emission microscopy of neodymium-iron-boron (Nd2Fe14B)
Authors: Mundschau M. , Romanowicz J. , Wang J. Y. , Sun D. L. , Chen H. C.
Published: July 2002

Ferromagnetic domains of a single crystal of neodymium-iron-boron, Nd2Fe14B (one of the strongest permanent magnetic materials known) are imaged by focusing a beam of photoelectrons with electrostatic optics in a photoelectron emission microscope. Photoelectrons emitted from the surface are deflected laterally into two opposite directions by stray magnetic fields that exist above the domains. The photoelectron beam is partially split into two. Magnetic contrast is produced by blocking part of the beam and imaging with an edge of the beam. The magnetic contrast mechanism appears to be similar to the type I magnetic contrast mechanism known from scanning electron microscopy, in which stray magnetic fields above the ferromagnetic domains deflect secondary electrons either towards or away from the electron detector. Upon heating the sample above the Curie temperature, the ferromagnetic domains gradually disappear, as expected for a second order phase transition. They reappear upon cooling.
Study of the H2 remote plasma cleaning of InP substrate for epitaxial growth
Authors: Losurdo M. , Capezzuto P. , Bruno G.
Published: July 2002

The removal of the native oxide overlayers on the surfaces of III–V semiconductors, although widely investigated, requires further examination. Specifically, the main emphasis of research has been on the oxide layer characterization and, hence, on the development of cleaning procedures. This article attempts to study the H-atom plasma reduction of the native oxide on indium phosphide (InP) substrates, using a process parametric investigation and x-ray photoelectron spectroscopy (XPS) analysis. Oxide-free and stoichiometric InP surfaces are prepared by operating the plasma cleaning at the surface temperature of 270 °C, as a thermally activated process has been found. For the untreated substrate, the nature of the InP oxide changes, along the thickness, from indium phosphates (InPOx) to an indium oxide (In2O3) rich sublayer. High temperature plasma treatment is able to completely reduce the In2O3 oxide, unlike the wet-etching procedure. In addition, improved stability of the surface against reoxidation has been confirmed for plasma treated substrates.
Combined surface plasmon resonance and scanning force microscope instrument
Authors: Chen X. , Davies M. C. , Roberts C. J. , Shakesheff K. M. , Tendler S. J. B. , Williams P. M. , Davies J.
Published: July 2002

We have designed and constructed a combined surface plasmon resonance-scanning force microscope instrument capable of providing simultaneous in situ information on the dynamic behavior of the refractive index and spatial characteristics of surfaces. These data show considerable potential for revealing new insights into interfacial phenomena. In this article, we describe the instrument in detail and provide a brief example of the results obtained when studying the hydrolytic degradation of a bioerodable polymer film [poly(orthoester)] exposed to an acidic environment. Analyses of the plasmon resonance and force microscopy data reveals the kinetics of the polymer degradation process and the heterogeneities in polymer distribution and degradation, respectively.
Lithography with 157 nm lasers
Authors: Bloomstein T. M. , Horn M. W. , Rothschild M. , Kunz R. R. , Palmacci S. T. , Goodman R. B.
Published: July 2002

Projection photolithography at 157 nm was studied as a possible extension of current 248-nm and planned 193-nm technologies. At 157 nm, lasers are available with ∼8 W average power. Their line width is narrow enough as to enable the use of catadioptric, and maybe all-refractive optics similar to those used at 248 and 193 nm. The practicality of such designs is further enhanced by measurements of calcium fluoride, which show that its absorption is sufficiently small (∼0.004 cm−1) at 157 nm. Binary masks with chromium and chromeless phase shifting masks were fabricated on calcium fluoride as the transparent substrate. Robust photoresists at 157 nm still need to be developed, and they probably will be of the top surface imaging or bilayer type. Indeed, a silylation resist process was shown to have characteristics at 157 nm similar to those at 193 nm. The calcium fluoride based masks were integrated with the silylation process and a home-built, small-field, 0.5-numerical aperture stepper to provide projection printing with features as small as 80 nm. These initial results indicate that 157-nm lithography has the potential to become a manufacturing technology at dimensions well below 100 nm.
Microstructure, vibrational and electronic properties of GaN grown by molecular beam epitaxy on Al2O3(0001) and 6H-SiC(0001)
Authors: Freundt D. , Holz D. , Lüth H. , Romani M. , Rizzi A. , Gerthsen D.
Published: July 2002

GaN layers have been grown by molecular beam epitaxy with a rf plasma source on Al2O3(0001) and 6H-SiC(0001). The conductive n-SiC substrates were employed for the in situ characterization of the grown GaN layers by electron spectroscopies (HREELS and XPS). Transmission electron microscopy (TEM) in the conventional and high-resolution mode provides information regarding the structural properties. Plan-view TEM yields a threading defect density of 7×109 cm−2 in GaN/6H-SiC and 2×1010 cm−2 in GaN/Al2O3. Micro-Raman spectroscopy, by analysis of the coupled A1(LO)-phonon-plasmon mode, provides the free carrier concentrations to be n∼1–2×1017 cm−3 for GaN layers grown on both substrates. The in situ HREEL spectroscopy gives insight into the electronic and vibrational properties of the GaN surface. Deep levels at ∼900 meV above the valence band maximum induce a surface absorption structure which is reduced after heating at 600 °C and which might be assigned, according to the growth process and to the literature, to the presence of Ga vacancies in the as-grown layers, in particular in the region close to the surface. Heating the GaN/6H-SiC heterostructures at 900 °C causes complete decomposition of the GaN layer.
Photovoltaic study of nanocrystalline TiO2 film modified with dye molecules
Authors: Fang Jinghuai , Wu Jingwen , Lu Xiaomei , Gu Jianhua , Lu Zuhong
Published: July 2002

The nanocrystalline TiO2 film is modified simultaneously with porphyrin dye molecules and phthalocyanine dye molecules by chemical adsorption. Since porphyrin and phthalocyanine dye have different absorption character in the visible range, the TiO2 electrode sensitized by a combination of porphyrin and phthalocyanine dye molecules not only obtains a better spectral match to the solar spectrum, but also the photoresponse of the electrode is extended to cover most of the visible range. This kind of electrode exhibits a significant improvement of energy conversion efficiency.
Surface modification by the voltage pulse in a scanning tunneling microscope
Authors: Vladimirov G. G. , Drozdov A. V.
Published: July 2002

Experiments on the study of the modification mechanism of a metal surface by a voltage pulse in a scanning tunneling microscope are carried out. The received results shown that the value of a current flowing in a tip–sample circuit plays an important role during nanostructures formation. Measurements of a voltage drop in the tunneling gap at the surface modifications testify to the formation of a contact between a tip and sample. The received results are explained on the basis of the mechanism of thermal expansion of the tip apex region. The performed calculations of the emission characteristics for a case of a trapezoidal barrier testify to the proposed mechanism. They have shown that the change of emission current by a change of a potential difference is not great as for the field emission case. Moreover, the energy released by Nottingham effect essentially surpasses that released in the field emission case.
Low-stress sputtered chromium–nitride hardmasks for x-ray mask fabrication
Authors: Tsuboi Shinji , Kotsuji Setsu , Yoshihara Takuya , Suzuki Katsumi
Published: July 2002

We have developed low-stress chromium nitride (CrN) films as hardmasks for x-ray absorber etching. The stress in the CrN films is 3 MPa and its distribution (gradient) is less than 10 MPa in a 25×25 mm area. In addition, the CrN film is electrically conductive (1.4 Ω/□). We have fabricated 0.10 μm line-and-space patterns in 0.4-μm-thick tantalum germanide using a 75-nm-thick CrN hardmask. The results demonstrate that a sputtered CrN film is an excellent hardmask material for x-ray mask fabrication.
Scanning capacitance microscopy measurement of two-dimensional dopant profiles across junctions
Authors: Kopanski J. J. , Marchiando J. F. , Berning D. W. , Alvis R. , Smith H. E.
Published: July 2002

Cross-sectioned p+/p and p–n junction test structures were imaged with a scanning capacitance microscope (SCM). To maintain a constant difference capacitance, our SCM utilizes an electronic attenuator circuit with a dynamic range of 20 V to less than 1 mV. Dopant profiles are extracted from SCM images using a formalism, which rapidly determines the theoretical SCM response from a database of calculated C–V curves. A dopant profile from a p+/p junction determined via constant difference capacitance SCM is compared to a secondary ion mass spectroscopy profile from similar structures.
Direct comparison of two-dimensional dopant profiles by scanning capacitance microscopy with TSUPREM4 process simulation
Authors: McMurray J. S. , Kim J. , Williams C. C. , Slinkman J.
Published: July 2002

Quantitative two-dimensional (2D) dopant profiling of a gatelike structure is achieved by scanning capacitance microscopy (SCM) on a cross-sectioned polished silicon wafer. The gatelike structures consist of heavily implanted n+ regions separated by a lighter doped n region underneath a 0.56 μm gate. The SCM is operated in the constant change capacitance mode while scanning with a 37 nm radius tip. The 2D SCM data are converted to dopant density through a physical model of the SCM/silicon interaction. The model parameters are adjusted so that the SCM dopant profile far from the gate edge fits the vertical secondary ion mass spectrometry (SIMS) profile. A 15% error in average accuracy is found between SCM and SIMS profiles evaluated over the dopant range of 1020–1017 cm−3. The same model parameters are used for all points in converting the 2D SCM data, indicating that the accuracy of the full 2D result should be comparable to that of the vertical profile. A direct comparison of the 2D SCM and 2D TSUPREM4 results is made for the first time. The agreement is generally good, but there are some notable differences.
Electron emission characteristics of polycrystalline diamond films
Authors: Sugino Takashi , Kawasaki Seiji , Yokota Yuuko , Iwasaki Yukio , Shirafuji Junji
Published: July 2002

Electron emission characteristics have been investigated for polycrystalline diamond films subjected to H2 plasma treatment, O2 plasma treatment, and annealing. In the case of surface treatments of diamond film, the voltage commencing electron emission increases due to O2 plasma treatment or annealing and is recovered by the subsequent H2 plasma treatment. On the other hand, in the case when the rear face of the diamond film is treated with O2 plasma or annealed in diamond/Au samples, no recovery of the voltage occurs due to H2 plasma treatment after O2 plasma treatment or annealing. It is suggested that there exists another factor dominating the field emission characteristic other than field emission mechanisms such as electron injection at the diamond/metal contact and electron emission at the surface with negative electron affinity.
New strain-relieving microstructure in pure-Ge/Si short-period superlattices
Authors: Sunamura H. , Usami N. , Shiraki Y. , Fukatsu S.
Published: July 2002

High-temperature formation of highly strained pure-Ge/Si short-period superlattices (SPSs) and their photoluminescence (PL) properties are presented. We grew 99 period SPSs consisting of two-dimensional Ge layers separated by thin Si barriers. The evolution of undulations is newly observed for samples with small Si barrier thickness, which is attributed to a strain-related growth phenomenon, and similarities to the well-established island formation are discussed. Clear band-edge PL is observed in these high-temperature grown SPSs, and interesting PL properties associated with the undulation formation are presented.
Reflection high-energy electron diffraction studies of wurtzite GaN grown by molecular beam epitaxy
Authors: Hughes O. H. , Korakakis D. , Cheng T. S. , Blant A. V. , Jeffs N. J. , Foxon C. T.
Published: July 2002

We report a comprehensive reflection high-energy electron diffraction study of the surface structure of GaN as a function of substrate temperature and III–V ratio for growth using elemental gallium and active nitrogen derived from a rf plasma source. An emission spectroscopy analysis of the composition of the nitrogen plasma showed that the neutral atomic species dominated the growth process. The effect of substrate pretreatment is also discussed. It was found that good quality growth accompanied by a reconstructed surface are only obtainable during growth under a slightly Ga-rich regime and after pretreatment by nitridation and the growth of a low temperature buffer layer. Reconstruction mode diagrams are presented both for layers during growth and also for layers which have been cooled after growth. The implications of these plots are discussed in terms of surface vacancy densities.
Scanning tunneling microscope tip as a positionable contact: Probing a Josephson-junction array at subkelvin temperatures
Authors: Wildöer J. W. G. , van Oudenaarden A. , Harmans C. J. P. M. , van Kempen H.
Published: July 2002

We describe an experiment in which a scanning tunneling microscope (STM) is employed to selectively contact a lithographically fabricated structure at 350 mK. The STM enables us to probe the structure, a Josephson-junction array, at various positions. The experiment demonstrates that it is possible to combine the use of a scanning tunneling microscope with transport measurements on lithographically fabricated structures at temperatures below 1 K. We focus on the strategy to position the tip above the structure of interest, the influence of the STM on the electronic noise, and on the effective temperature of the sample.
Accuracy and efficiency in electron beam proximity effect correction
Authors: Wind S. J. , Gerber P. D. , Rothuizen H.
Published: July 2002

We present a suite of electron beam proximity effect correction algorithms which incorporate elements of various shape-to-shape techniques as well as field-based techniques. They are applied to a set of prototypical pattern data at simulated electron beam exposure conditions ranging from 25 to 100 keV on bulk substrates. The results are compared in terms of correction accuracy and computational efficiency. In addition, we show how these algorithms, when used in combination with one another, can be applied to hierarchical pattern design data such that the correction can be obtained with a high degree of accuracy without requiring unnesting of the hierarchy.
Can dry-etching systems be designed for low damage ab initio?
Authors: Deng L. G. , Rahman M. , Murad S. K. , Boyd A. , Wilkinson C. D. W.
Published: July 2002

Photoluminescence intensity measurements from GaAs/AlGaAs and InGaAs/InAlAs quantum well probe structures have been used to study dry-etch damage inflicted in low power reactive ion etching environments. Selective etching has been employed to accumulate damage in the materials under these relatively low damage conditions. The measured data are consistent with calculations for channeling effects of atomic ion species, using a microscopic ion channeling theory. The results indicate that atomic as opposed to molecular ion channeling may be the main mechanism for deep dry-etch damage in these environments, which suggests that gases can be selected as likely to cause low damage.
Measurement of nanomechanical resonant structures in single-crystal silicon
Authors: Carr D. W. , Sekaric L. , Craighead H. G.
Published: July 2002

We have used electron beam lithography to make very small (<30 nm linewidth) mechanical structures in single-crystal silicon. These structures can be driven capacitively by applying a voltage between the suspended portion and the underlying substrate. Optical interference techniques are used to detect and measure the motion of the structures with resonant frequencies above 40 MHz. We employed a design consisting of a square mesh with a 315 nm period, which results in a low mass (∼1×10−13 g) and large relative surface area (10−6 cm2). Also, by making suboptical-wavelength features, the optical properties can be altered, leading to an improved measurement sensitivity. We measured the oscillations at small amplitudes where the detected change in the optical reflection is proportional to the drive amplitude.
Scanning tunneling microscopy study of the adsorption of toluene on Si(001)
Authors: Borovsky Brian , Krueger Michael , Ganz Eric
Published: July 2002

Scanning tunneling microscopy is used to investigate the adsorption of toluene on the Si(001)-2×1 surface. At room temperature, adsorption occurs exclusively on top of dimer rows and results in several binding geometries. A metastable and weakly bound state is observed initially upon adsorption. This state converts to a more stable geometry. Conversions between the dominant adsorbed states are observed and can be induced by the scanning tunneling microscopy (STM) tip. These states closely resemble those of benzene on Si(001). Evidence that toluene interacts with more surface dimers than benzene is found in the bias dependence of the adsorbed states in STM images. Toluene decomposes upon annealing, leaving carbon on the surface.
Low temperature solid-source molecular-beam epitaxy growth of Al-free quantum well laser diodes using a GaP-decomposition source
Authors: Postigo P. A. , Lullo G. , Choy K. H. , Fonstad C. G.
Published: July 2002

Low threshold aluminum-free, InGaP/GaAs/InGaAs quantum well laser diodes have been grown at low temperature on GaAs using a GaP sublimation source for phosphorous and an atomic hydrogen cracker source for substrate surface preparation. The growth cycle was conducted entirely below 470 °C so that it is compatible with growth on GaAs very large scale integrated circuits, as in the epitaxy-on-electronics monolithic optoelectronic integration processes. The room temperature pulsed threshold current density of broad-area stripe contact laser diodes was consistently on the order of 0.38 kA/cm2. InGaP grown with the GaP cell is found by secondary ion mass spectroscopy to contain significant amounts of oxygen, particularly in Be- and Si-doped layers.
Application and calibration of a quartz needle sensor for high resolution scanning force microscopy
Authors: Clauss W. , Zhang J. , Bergeron D. J. , Johnson A. T.
Published: July 2002

We have investigated a force sensing technique for high-resolution scanning force microscopy which uses a piezoelectric quartz resonator oscillating perpendicular to the direction of the sample surface at a frequency of 1 MHz. The achievement of true atomic resolution on single-wall carbon nanotube surfaces illustrates the promises of this new technique which is especially well suited for ultrahigh vacuum or low-temperature conditions. The force sensitivity and the mechanical amplitude of the sensor oscillation were characterized semiquantitatively.
High-resolution imaging of single-stranded DNA on mica surface under ultrahigh vacuum conditions by noncontact atomic force microscopy
Authors: Matsumoto Takuya , Maeda Yasushi , Naitoh Yasuhisa , Kawai Tomoji
Published: July 2002

Dynamic mode atomic force microscopy with frequency-shift detection has been employed for imaging single-stranded DNA on mica surfaces under ultrahigh vacuum conditions. For the wet procedures used to prepare the samples, the strong adhesion force caused by the water layer at the surface is observed even in ultrahigh vacuum conditions. The DNA images can only be obtained with large frequency shifts, but provide sufficient resolution to see the coil or higher-order structures of the DNA molecules in detail. Contrast effects, which might include information on the detailed structure, are often observed depending upon the feedback conditions.
Process integration induced thermodesorption from SiO2/SiLK resin dielectric based interconnects
Authors: Baklanov M. R. , Muroyama M. , Judelewicz M. , Kondoh E. , Li H. , Waeterloos J. , Vanhaelemeersch S. , Maex K.
Published: July 2002

The thermodesorption from SiO2/SiLK resin dielectric assemblies at various stages of interconnect fabrication are studied by mass spectrometry. Species desorption from such an assembly is governed by the intrinsic material properties and the process step history, such as patterning chemistry and by environmental contamination. The thermodesorption of an as-cured SiLK resin film is compared to the desorption of species after different process steps. It is shown that as cured SiLK films contain very low amounts of moisture and volatile organic compounds. The plasma-enhanced chemical vapor deposition SiO2 hardmask is the main source of moisture in the SiO2/SiLK resin dielectric stack. Annealing at T⩾350 °C in both vacuum and nitrogen ambient conditions drastically reduces the species desorption. The origin and the kinetics of the desorbed species are described.