bluuebunny/crossref_metadata_2025 huggingface dataset:
Room temperature YBaCuO microbolometers
Authors: Phong L. Ngo , Qiu S. N.
Published: July 2002

Details of the fabrication process and figures of merit of resistive YBaCuO microbolometers are reported. Thin films of YBaCuO were prepared on Si wafers under conditions that promote formation of the semiconducting phase at room temperatures. Temperature coefficient of resistance with values of up to 0.04 K−1 was uniformly achieved on 10 cm wide wafer. Bulk micromachining was used to create 60×60 μm2 bolometers on Si3N4 bridges with a thermal conductance of ∼7.6×10−7 W/K. The optical responsivity and detectivity of these bolometers were respectively ∼7×104 V/W and 3×109 cm Hz1/2/W at low frequencies. These figures are consistent with those derived from the thermal properties of the bridge and are better than figures of other classes of room temperature bolometer. Good agreement was found between the spectral response of the bolometer and the spectral absorptance of the Si3N4 bridge, confirming the role of the latter as the heat absorber in the device. Under normal operating conditions and assuming F/1 optics, the noise equivalent temperature difference of focal planes that make use of YBaCuO microbolometers was estimated to be less than 50 mK in the spectral range of 8–14 μm.
Exchange bias in Fe/Cr double superlattices
Authors: Jiang J. S. , Felcher G. P. , Inomata A. , Goyette R. , Nelson C. S. , Bader S. D.
Published: July 2002

Utilizing the oscillatory interlayer exchange coupling in Fe/Cr superlattices, we have constructed “double superlattice” structures where a ferromagnetic (F) and an antiferromagnetic (AF) Fe/Cr superlattice are coupled through a Cr spacer. The minor hysteresis loops in the magnetization are shifted from zero field, i.e., the F superlattice is exchange biased by the AF one. The double superlattices are sputter deposited with (211) epitaxy and possess uniaxial in-plane magnetic anisotropy. The magnitude of the bias field is satisfactorily described by the classic formula for collinear spin structures. The coherent structure and insensitivity to atomic-scale roughness makes it possible to determine the spin distribution by polarized neutron reflectivity, which confirms that the spin structure is collinear. The magnetic reversal behavior of the double superlattices suggests that a realistic model of exchange bias needs to address the process of nucleating local reverse domains.
Cryogenic etching of deep narrow trenches in silicon
Authors: Aachboun S. , Ranson P. , Hilbert C. , Boufnichel M.
Published: July 2002

Deep and narrow anisotropic etching of silicon structures has been investigated in a low-pressure high density plasma reactor working with a cryogenic chuck. We have previously demonstrated the feasibility of this technique on such structures. Improvement of etch rate and profiles has been studied and new results show 2 μm wide trenches etched to a depth of 50 μm at an average etch rate of 5 μm/min with highly anisotropic profiles and very high selectivity (>500:1) toward the SiO2 mask. An evaluation of a commercially available reactor from Alcatel has been carried out and similar results are obtained. A phosphosilicate glass mask has been used to study the effect on profiles. It is shown that undercut is reduced while bowing is independent of the mask material. Since surface temperature strongly affects the profiles, wafer deformations in our cryogenic chuck have been measured and temperature evolution across the wafer has been estimated. A significant temperature difference of 10 °C between the chuck and the wafer is expected for thin wafers (210 μm).
Synthesis of diamondlike carbon films with superlow friction and wear properties
Authors: Erdemir A. , Eryilmaz O. L. , Fenske G.
Published: July 2002

In this study, we introduce a new diamondlike carbon (DLC) film providing a friction coefficient of 0.001 and wear rates of 10−9–10−10 mm3/N m in inert-gas environments (e.g., dry nitrogen and argon). The film was grown on steel and sapphire substrates in a plasma enhanced chemical vapor deposition system that uses a hydrogen-rich plasma. Employing a combination of surface and structure analytical techniques, we explored the structural chemistry of the resultant DLC films and correlated these findings with the friction and wear mechanisms of the films. The results of tribological tests under a 10 N load (creating initial peak Hertz pressures of 1 and 2.2 GPa on steel and sapphire test pairs, respectively) and at 0.2 to 0.5 m/s sliding velocities indicated that a close correlation exists between the friction and wear coefficients of DLC films and the source gas chemistry. Specifically, films grown in source gases with higher hydrogen-to-carbon ratios had the lowest friction coefficients and the highest wear resistance. The lowest friction coefficient (0.001) was achieved with a film on sapphire substrates produced in a gas discharge plasma consisting of 25% methane and 75% hydrogen.
Novel proton exchange membrane thin-film fuel cell for microscale energy conversion
Authors: Morse Jeffrey D. , Jankowski Alan F. , Graff Robert T. , Hayes Jeffrey P.
Published: July 2002

Thin-film, proton exchange membrane fuel cells are developed using photolithographic patterning, physical vapor deposition, and spin-cast deposition techniques. In this study, micrometer-thick layers of nickel (Ni) and platinum (Pt) electrodes, as well as the proton conducting electrolyte layer of perfluoronated sulfonic acid, are synthesized. The anode layer is conductive to pass the electric current and provides mechanical support to the electrolyte and cathode layer that enables combination of the reactive gases. The morphology desired for both the anode and cathode layers facilitates generation of maximum current density from the fuel cell. For these purposes, the parameters of the deposition process and post-deposition patterning are optimized for continuous porosity across both electrode layers. The power output generated through current–voltage measurement is characterized at various temperatures in the range of 60–90 °C using dilute (4%) hydrogen fuel.
Observation of the deHaas–van Alphen effect in a two-dimensional electron system
Authors: Störmer H. L. , Haavasoja T. , Narayanamurti V. , Gossard A. C. , Wiegmann W.
Published: July 2002

We report the first observation of the oscillatory magnetic moment as a function of the magnetic field (deHaas–van Alphen effect) of a two-dimensional electron gas. The measurements were performed at 1.5 K on a modulation-doped GaAs–(AlGa)As heterostructure using a SQUID magnetometer. The strength of the signal was enhanced by stacking 4000 layers equivalent to an area of 240 cm2. The data are in qualitative agreement with theory but the amplitude of the oscillations is about a factor of 30 weaker than expected. We attribute this discrepancy to sample inhomogeneities. More accurate measurements should allow a direct determination of the electronic density of states.
Plasma etching of organic materials. I. Polyimide in O2–CF4
Authors: Egitto F. D. , Emmi F. , Horwath R. S. , Vukanovic V.
Published: July 2002

Gas phase and surface phenomena responsible for etching polyimide in O2–CF4 rf plasmas have been investigated. The dependence of the etch rate of polyimide on the relative concentrations of these two gases was compared with data from optical emission spectra, mass spectra, and x-ray photoelectron spectroscopy (XPS). The increase in O atom number density in the plasma with addition of CF4 to O2 certainly increases etch rates over those achieved in pure oxygen plasmas. However, etch rates do not track identically with atomic oxygen emission intensities. Etch rates are enhanced through the reaction of fluorine in the polyimide surface layer while fluorine present in excess inhibits etching through competition with oxygen atoms, resulting in the formation of CF2 type bonding at the surface. Removal of the resulting passivation is most efficiently accomplished by exposure of the polyimide to a plasma with higher oxygen concentration, probably due to the presence of ions. The position of the maximum in etch rate distribution with respect to CF4 concentration depends on an optimum ratio of O to F atoms in the plasma.
Upper bound of the beam energy broadening in acceleration region
Authors: Sasaki Tateaki
Published: July 2002

Using an expression of the local space-charge force derived in a previous paper, we analyze the broadening of axial energy distribution of beam particles in the acceleration region and calculate an upper bound of the energy broadening 〈‖ΔE‖〉UB. Assuming that the emitter is a sphere of radius R with potential V, we obtain the expression 〈‖ΔE‖〉UB ∼∝ m1/4 I1/2e V−1/4 T1/2eff α−1e R0, where m is the particle mass, Ie is the total emission current, V is the acceleration voltage, Teff is an effective temperature showing the kinetic energy of particles at the emitter surface, and αe is the beam semiangle at the emitter. Theory gives 〈‖ΔE‖〉UB ∼several eV for typical electron guns operating in space-charge unlimited conditions. The result is compared with experiments and a discussion is given of the problem we have to solve to calculate the magnitude of 〈‖ΔE‖〉.
Diboride diffusion barriers in silicon and GaAs technology
Authors: Shappirio J. R. , Finnegan J. J. , Lux R. A.
Published: July 2002

TiB2 and ZrB2 are members of the larger group of interstitial compounds which the transition metals form with boron, carbon, and nitrogen. Such materials are of significant interest for microelectronic application owing to a combination of low resistivity and outstanding diffusion barrier properties. We report here studies on the application of these diborides as diffusion barriers for both Si and GaAs based device technology. rf diode sputtered thin film ZrB2 has an annealed resistivity of 25 μΩ cm, rivaling that of its most widely employed silicide competitor, TiSi2 (18 μΩ cm). In addition, in experiments with bilevel structures consisting of first-level ZrB2 and second-level Al, no evidence of reaction has been observed in RBS spectra for samples heat treated in N2 at 625 °C for 2 h. These compounds thus appear to offer the prospect of serving the dual role of low resistivity contact metal as well as a self-diffusion barrier in multilevel contact structures. Recent work on the application of these materials for GaAs contact technology is even more significant. Alloyed Ohmic contacts to n-GaAs, a traditionally difficult technological problem, have been fabricated in which the Ohmic metallization (Ni/Ge/Au) in contact with the GaAs substrate is separated from the overlying thick Au upper contact by an intervening e-beam deposited TiB2 layer. The resultant structure shows improved electrical stability on aging at 350 °C to those of a more conventional nature prepared similarly, but employing a Ni film in place of the TiB2. This improved stability is attributed to the effectiveness of the diboride as a diffusion barrier to the in-migration of Au, thus preventing further modification of the Ohmic contact metallurgy obtained on alloying.
The InP(110)/Sb interface: Ohmic behavior at large Sb coverages
Authors: Esser N. , Münder H. , Hünermann M. , Pletschen W. , Richter W. , Zahn D. R. T.
Published: July 2002

The Schottky barrier at the InP(110)/Sb interface was determined by Raman scattering for Sb coverages up to 60 monolayers (ML). Band bending variations are found to occur until ∼20 ML. This can be correlated with the Sb structure; from ordered through amorphous to crystalline growth. For higher coverages of Sb the band bending decreases and finally flatband conditions are obtained. This fact is confirmed by I–V measurements at 150 and 400 ML.
New high current low energy ion source
Authors: Hara T. , Hamagaki M. , Sanda A. , Aoyagi Y. , Namba S.
Published: July 2002

We have developed a new type of ion source which is excited by a low energy and high current electron beam. The source has an extractable ion current density up to 0.62 A/cm2 and a low acceleration voltage less than 60 V. This ion source will open a new way for low energy ion processing such as damageless ion etching, ion beam crystal growth, and deposition.
A study of the effect of key processing variables on the lithographic performance of Microposit SAL601-ER7 resist
Authors: Blum Lauren , Perkins Marjorie E. , Liu Hua-Yu
Published: July 2002

Two statistically designed experiments were performed to study the effect of the key processing variables on the lithographic properties of Microposit SAL601-ER7 electron beam resist. A Taguchi L9, nine-run design with four variables at three levels was used for a screening experiment. The effect of postexposure bake (PEB) temperature, PEB time, and developer normality on the contrast and the sensitivity of the resist was determined. A lithographic test of the process recommended by this experiment suggested that development time should be included as a variable in the next study. A four-variable central composite design was performed in order to obtain a more complete understanding of the process. These experiments demonstrated that the resist performs well with a broad range of processing conditions. A postexposure bake temperature of 115 °C provides the most robust process. The developer concentration and development time can be modified to attain either shorter development times or lower sensitivities. Many of the effects of the process variables can be explained by the unique chemistry of the resist.
An x-ray stepper for synchrotron radiation lithography
Authors: Kouno Eiichi , Tanaka Yoshiharu , Iwata Joji , Tasaki Yoshiyuki , Kakimoto Emi , Okada Koichi , Suzuki Katsumi , Fujii Kiyoshi , Nomura Eiichi
Published: July 2002

A prototype synchrotron radiation (SR) stepper for quarter-micron devices has been developed and installed at the Photon Factory in the National Laboratory for High Energy Physics, Japan. The stepper features are, (i) exposure in an atmospheric environment, (ii) large exposure area (25-mm sq), and (iii) alignment error detection at all times, including during exposure. The stepper consists of an SR extracting chamber, precision mechanical stages, and an alignment error detection system. An SR beam in UHV goes through a beryllium window into an atmospheric environment, and covers the 25-mm sq exposure area by using an oscillating mirror. Patterns on a mask are imprinted onto a wafer. Mask and wafer are, respectively, held in place with vacuum chucks. Their subsequent positioning movements are driven in 6 degrees of freedom by piezoelectric actuators for fine alignment and gap setting. The alignment system, based on the previous Fresnel lens optical system, newly employs a differential mode linear Fresnel zone plate alignment method. As the optical system for this method is located at the outside of an SR beam, it can detect an alignment error between a mask and a wafer at all times, including during exposure. Patterns measuring 0.2 μm were completely successfully imprinted on a wafer. Until now, 0.03-μm (3σ) positioning accuracy and 0.2-μm (3σ) overlay accuracy were achieved.
Moiré method and zone plate pattern inaccuracies
Authors: Vladimirsky Y. , Koops H. W. P.
Published: July 2002

Moiré patterns obtained from superimposed zone plates with common types of pattern inaccuracies are analyzed. Tolerances for the distortions characterized as ellipticity, radial displacement, and nonconcentricity of the zones, are obtained from the corresponding equations for lenses. The formulas for moiré fringes resulting from superimposed zone plate patterns having these defects are derived. The feasibility of moiré pattern techniques to detect and measure inaccuracies of the zone plate is demonstrated. The sensitivity and the limitations of the method are discussed.
Thermal stability of polyimidesiloxane (SIM-2000)
Authors: Sun S. P. , Murarka S. P. , Lee C. J.
Published: July 2002

Polyimides are finding increased use in integrated circuits as a dielectric and protective layer. Its low dielectric constant, ease of application, and ability to planarize the surfaces, permit their incorporation into very large scale integrated and ultra-large scale integrated circuit processing. However, there is no single polyimide available which possesses high-temperature stability at temperature >300 °C. A newer class of polymers called polyimidesiloxane (SIM-2000), resulting from the modification of polyimides by special equilibrated silicone blocks, has been found superior to commercial polyimides especially with respect to their high-temperature stability. In this paper, we present the results of our investigation of the high-temperature stability of a few polyimidesiloxane materials spun on various substrates including Si, SiO2, and Al.
The advanced unified defect model for Schottky barrier formation
Authors: Spicer W. E. , Liliental-Weber Z. , Weber E. , Newman N. , Kendelewicz T. , Cao R. , McCants C. , Mahowald P. , Miyano K. , Lindau I.
Published: July 2002

The advanced unified defect model (AUDM) for GaAs proposed in this paper can be looked upon as a refinement of the unified defect model (UDM) proposed in 1979 to explain Fermi level pinning on 3–5 compounds due to metals or nonmetals. The refinement lies in identifying the defect producing pinning at 0.75 and 0.5 eV above the valence band maximum as the AsGa antisite. Since the AsGa antisite is a double donor, a minority compensating acceptor is necessary. This is tentatively identified as the GaAs antisite. The concentration of As excess or deficiency due to processing or reactions at interfaces is particularly emphasized in this model. A wide range of experimental data is discussed in terms of this model and found to be in agreement with it. This includes the original data on which the UDM was based as well as more recent data including Fermi level pinning on the free-GaAs(100) molecular-beam epitaxy surface, Schottky barrier height for thick (∼1000 Å) Ga films on GaAs, and the LaB6 Schottky barrier height on GaAs (including thermal annealing effects). Of particular importance is the ability of this model to explain the changes in Schottky barrier height for Al and Au on GaAs due to thermal annealing and to relate these changes to interfacial chemistry.
High-pressure oxidation of InP through an SiO2 overlayer
Authors: Geib K. M. , Chang R. R. , Lile D. L. , Wilmsen C. W. , Hryckowian G. , Zeto R. J.
Published: July 2002

The chemical composition and growth rate of native oxides grown under a 400-Å-thick SiO2 cap on InP as a result of high-pressure annealing in steam are presented. The thickness of the oxide was found to increase monotonically as the growth temperature was increased in 50 °C steps from 300 to 450 °C for a constant growth time of 20 h. The oxide thickness ranged from near zero to ∼200 Å for the four temperatures. X-ray photoelectron spectroscopy indicates that the oxide is primarily InPO4 but that some In(PO3 )3 is also present. Capacitance–voltage measurements show that the surface state density is reduced by the steam annealing.
Linewidth of excitonic emission and Stark effect in a ZnSe–ZnS strained-layer superlattice
Authors: Kawakami Yoichi , Taguchi Tsunemasa
Published: July 2002

Temperature dependence of sharp photoluminescence and absorption peaks, which originate from the n=1 free heavy-hole exciton, in a ZnSe–ZnS strained-layer superlattice (Lb=50 Å and Lw=25 Å) with a multiple quantum-well structure has been investigated. A dominant mechanism responsible for broadening in the excitonic linewidth below 85 K is an ionized donor impurity scattering of free excitons that occurs in addition to the acoustic phonon scattering. On the other hand, it is suggested that the linewidth of the excitonic absorption peaks at high temperature is attributed to the scattering of free excitons with LO phonons. Stark shifts and a concomitant quenching in intensity of the exciton line were observed under electric fields.
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u scanning microprobe reflection high-energy electron diffraction observation of GaAs surfaces during molecular-beam epitaxial growth
Authors: Isu Toshiro , Hata Masayuki , Watanabe Akiyoshi , Katayama Yoshifumi
Published: July 2002

A specially designed molecular-beam epitaxy (MBE) system with scanning microprobe reflection high-energy electron diffraction (RHEED) has been developed. The incident electron beam is designed to focus down to 10 nm in diameter and scan the surface of the substrate under MBE growth conditions. Using the RHEED intensity variation produced by the scanning of the incident beam, we obtain a microscopic image, i.e., which is the scanning reflection electron microscope (SREM) image, highly sensitive to the surface structure. The SREM images of GaAs(001) using the specular beam spot revealed granular features over the entire surfaces of MBE-grown GaAs layers which come from undulations of the surface. The undulation wavelength was typically 0.1–0.5 μm and developed along [1̄10] direction during MBE growth at 570 °C. At higher substrate temperatures, little development of the undulation was observed. As an overall feature of MBE growth, two-dimensional layer-by-layer growth was observed over the whole surface having undulations.
Growth and characterization of epitaxial bismuth films
Authors: Partin D. L. , Thrush C. M. , Heremans J. , Morelli D. T. , Olk C. H.
Published: July 2002

Epitaxial bismuth thin films have been grown by molecular-beam epitaxy for the first time. These films were grown on 〈111〉 oriented single-crystal barium fluoride. Films grown at low temperatures (<150 °C) had microscopically rough surface morphologies. At higher temperatures, no film would nucleate. This led us to use a two-step growth process in which films were nucleated at 100 °C followed by growth of a thicker film near the melting point of bismuth. Electron microscope observations show the films to be featureless and defect-free on the scale of 0.1 μm. The films grow with their trigonal axis parallel to the 〈111〉 axis of the substrate, and Laue backscattering pictures show they are epitaxial. Mobilities at room temperature are as high as 2.5 m2 V−1 s−1, and increase to over 10 at 20 K and 100 at liquid helium temperatures. These values are far superior to those of other bismuth films grown to date, and approach mobilities observed in single-crystal bismuth. Preliminary results of a study of n-type Te-doped bismuth are also reported.
Interesting aspects of reflection high-energy electron diffraction oscillations during growth of GaAs(100)
Authors: Jamison K. D. , Resh J. S. , Horton C. C. , Bensaoula A. , Ignatiev A.
Published: July 2002

The intensities of several reflection high-energy electron diffraction (RHEED) beams have been recorded during molecular-beam epitaxial growth of GaAs(100) using a novel video intensity measurement system that records multiple RHEED beam intensities simultaneously. The RHEED beam intensities were recorded at varying angles of incidence and crystal substrate azimuth angles. Strong oscillations in the intensities in specular and nonspecular beams with the same period but varying phases have been measured. As noted by other investigators, the phase relationship of the oscillations of the various beams has been found to vary with incident and azimuthal angle. The results are examined with regard to recent studies of the role of Kikuchi processes on the phase of the specular beam. In contrast to other reports, it is found that although the diffracted intensities in the vicinity of the elastically diffracted beams are influenced by inelastically scattered electrons from the Kikuchi lines, these effects can not account for all the phase behavior observed.
Double 15-nm-wide metal gates 10 nm apart and 70 nm thick on GaAs
Authors: Chou Stephen Y. , Fischer Paul B.
Published: July 2002

A method that can fabricate two narrow but thick metal lines separated by a small gap on a bulk semiconductor substrate is described. By opening a 40-nm-wide trench in a single layer of 70-nm-thick polymethylmethacrylate resist on GaAs using high resolution electron beam lithography, and by double shadow evaporations and a lift-off, two 15-nm-wide metal lines 10 nm apart and 70 nm thick were fabricated on a bulk GaAs substrate. The pitch size of the double metal lines is 25 nm. This is a factor of 2 smaller than the previous smallest pitch size on bulk semiconductors. It is found that the width and spacing of the two lines are uniform over tens of microns. It is also found that metals shadow evaporated on top of the resist can be removed successfully by a lift-off, even though they were connected to the metals in the resist trenches. These results suggest that using this method metal lines with even finer linewidth (<15 nm) can be achieved on a bulk semiconductor substrate.
Reflection high energy electron diffraction characteristic absences in GaAs(100) (2×4)–As: A tool for determining the surface stoichiometry
Authors: Farrell H. H. , Palmstro/m C. J.
Published: July 2002

In general, techniques that monitor the in situ growth of semiconductor materials with electron diffraction [such as reflection high energy electron diffraction (RHEED)] provide no quantitative information about surface stoichiometry. However, sophisticated materials problems, such as heteroepitaxial systems, require such information. Here we demonstrate that the use of characteristic absences in the fractional order diffraction features in RHEED patterns provide detailed quantitative information for the molecular beam epitaxial growth of materials such as GaAs. It is expected that this approach will lead to the improvement of growth for such interface-sensitive systems as resonant tunneling devices.
Fabrication of coupled quantum dot arrays with a 100–150 nm period
Authors: Lee K. Y. , Kern D. P. , Ismail K. , Haug R. J. , Smith T. P. , Masselink W. T. , Hong J. M.
Published: July 2002

This paper demonstrates a novel method for fabricating two-dimensional lateral surface superlattices. The technique involves etching a grid into a GaAs/GaAlAs modulation-doped heterostructure using a mask consisting of an array of 50-nm diam Ni/Au dots with a 100–150 nm period. Bias applied to a Schottky gate deposited over the structured surface varies the coupling strength between dots. The devices can be tuned to operate in the superlattice (weak and strong coupling) or the quantum dot (no coupling) regimes. Low-temperature characterization of the superlattices fabricated specifically for transport and capacitance measurements reveal novel magnetic field effects.
Vacuum lithography for three-dimensional fabrication using finely focused ion beams
Authors: Harriott L. R. , Temkin H. , Wang Y. L. , Hamm R. A. , Weiner J. S.
Published: July 2002

Three-dimensional nanostructure fabrication for optoelectronic and quantum-effect devices can benefit from the greatly improved surface layer quality and low contamination offered by all vacuum processing. Ultrathin epitaxial layers can be grown with excellent uniformity and interface quality with techniques such as molecular beam epitaxy (MBE) to produce two-dimensional confinement structures. What has been lacking is a lateral patterning method which is compatible with dimensions required for quantum confinement and the high vacuum crystal growth environment. Surface contamination and damage produced during processing become a much more serious problem with feature sizes in the range of 100 nm and below. The usual lithographic processes involve exposure to produce a chemical change in a polymer resist with photons or electrons resulting in a pattern but also introduce surface contamination incompatible with epitaxial overgrowth. A number of high resolution vacuum compatible patterning techniques are possible with finely focused ion beams due to their unique ability to deliver momentum as well as energy to a target. We have developed in situ patterning process for InP-based compounds in which a focused ion beam is used to pattern a very thin (20–30 Å) surface oxide layer. The mask pattern is then transferred into the substrate by dry etching of a Cl2 atmosphere at 200 °C with or without the assistance of a broad area Ar ion beam. Results of experiments exploiting the oxide masking for the pattern formation on InP will be presented, including in situ formation and removal of the mask, and regrowth performed on patterned layers.
Scanning tunneling microscopy on cyclodextrin inclusion complexes
Authors: Shigekawa Hidemi , Morozumi Tatsuya , Komiyama Makoto , Yoshimura Masamichi , Kawazu Akira , Saito Yoshio
Published: July 2002

Scanning tunneling microscopy on both cyclodextrins (CyDs) and their structural complex formed with a guest compound (1-adamantanemethanol) was performed on specimens obtained by deposition of CyD aqueous solutions onto highly oriented pyrolytic graphite. The CyD and guest compound image sizes were found to be satisfactorily consistent with molecular dimensions. In the absence of guest compounds, both α- and β-CyDs strongly interacted with each other, and formed regularly ordered structures. Guest compound addition resulted in a breakdown of the regular CyD structure, thus indicating that mutual CyD molecular interactions are suppressed by CyD-guest complex formations.
Effects of mirror surface roughness on exposure field uniformity in synchrotron x-ray lithography
Authors: Wells G. M. , Nachman R. , Welnak C. , Singh S. , Guo J. , Khan M. , Turner S. , Cerrina F. , Vladimirsky Y. , Maldonado J.
Published: July 2002

In this paper we discuss the surface roughness tolerances for mirrors used in synchrotron x-ray lithography beamlines. These requirements are different for lithography beamlines than they are for traditional beamlines, which are imaging systems. For imaging optical systems it is desirable to minimize the surface roughness of the mirror in order to reduce the intensity of the scattered light. Proximity print x-ray lithography requires nonimaging optical systems designed to deliver a uniform power distribution at the exposure field. In such applications, the scattering of x rays is less important, provided that the total flux is not too severely degraded. From this point of view it is important to notice that scattering removes power from the central, coherent, beam and redistributes it over a wider angle. The loss in reflectivity is only apparent because energy is conserved in the process. Furthermore, the presence of scattered x rays can be used to improve the uniformity of the power distribution. The requirement on the surface quality of mirrors used for scanning x-ray illumination is thus relaxed with respect to that of typical imaging systems, providing considerable cost savings and simplifying manufacturing. The effect of the scattered light on the uniformity of illumination is dependent on intensity and angular distribution of the scattering. Experimental results will be compared to the results predicted by calculations.
Quantification of the strain in fully relaxed Si/Ge heteroepitaxial films and superlattices via molecular dynamics
Authors: Srivastava D. , Taylor R. S. , Garrison B. J.
Published: July 2002

The fully relaxed Si/Ge heteroepitaxial films are simulated via molecular dynamics with Tersoff’s potential for Si–Si, Si–Ge, and Ge–Ge interactions. The lattice mismatch induced strain is computed as a function of the thickness of the film. The strain values of the bulk Si and Ge layers are found to be in excellent agreement with those obtained via ab initio local density functional calculations. In thick uncapped epitaxial films, we find that the surface reconstruction induced relaxations significantly alter the strain values in the top five layers. When the film is less than five layers thick, we show for the first time that these relaxations do not cross the Si/Ge interface but are fully contained within the epitaxial film region. For all of the geometries studied, it has been found that at the Si/Ge interface, the adjacent Ge layer moves towards the interface and the adjacent Si layer moves away from the interface. Though very small in magnitude, these additional relaxations significantly alter the generally assumed geometries of the Si2Ge2/Si{001}, Si4Ge4/Si{001}, Si3Ge1/Si{001}, and Si1Ge3/Si{001} superlattices.
Detection of surface plasmons by scanning tunneling microscopy
Authors: Möller R. , Albrecht U. , Boneberg J. , Koslowski B. , Leiderer P. , Dransfeld K.
Published: July 2002

The influence of surface plasmons excited in a polycrystalline silver film on the tunneling current of a scanning tunneling microscope (STM) has been analyzed. The plasmons cause an additional flow of electrons from the tungsten tip to the silver surface on the order of up to 50 pA. This process is independent of the polarity of the applied bias voltage, thereby excluding effects of thermal expansion. The different nature of the ordinary tunneling current and the surface plasmon induced current is clearly revealed by their different dependence on the gap distance. The local distribution of the intensity of the surface plasmon induced signal reveals structures on a nanometer scale. Some of them are correlated to the surface topography.
A model for Si molecular-beam epitaxy based on scanning tunneling microscopy observations and computer simulations
Authors: Elswijk H. B. , Hoeven A. J. , van Loenen E. J. , Dijkkamp D.
Published: July 2002

Scanning tunneling microscopy observations of homoepitaxial growth on vicinal Si(001) and of coarsening of Si islands are used to develop a model of growth based on a configuration-dependent activation energy to hopping. In the model four contributions to this energy barrier are proposed: the activation energy to surface diffusion Es, the dimer formation energy Ed and two directional interaction energies between neighboring atoms not forming a dimer, E∥ and E⊥. Monte Carlo simulations are employed to compare experimental results with the results of the tested model and to deduce the atomic interaction energies: Es=(1.15±0.1) eV, Ed=(0.45±0.10) eV, E∥=(0.20+0.2−0.14) eV, E⊥=(0.10+0.1−0.07) eV. Preliminary results of simulations, including anisotropic diffusion, are presented.
Passivation of GaAs surface recombination with organic thiols
Authors: Lunt Sharon R. , Santangelo Patrick G. , Lewis Nathan S.
Published: July 2002

Exposure of GaAs crystals to solutions of organic thiols resulted in substantial reductions in nonradiative GaAs surface recombination rates. This process yielded improvements in steady state photoluminescence signals that were comparable to those obtained after a Na2S⋅9H2O (aqueous) treatment. Use of a series of thiols indicated that the chemically important surface electrical trap levels behaved as a polarizable, electron deficient center. X-ray photoelectron spectroscopy indicated that the thiols did not remove excess As0 nor form detectable levels of As2S3-like phases, implying that neither of these factors is required for effective surface passivation chemistry.
Stability and interdiffusion of short-period Si/Ge strained layer superlattices
Authors: Friess E. , Schorer R. , Eberl K. , Abstreiter G.
Published: July 2002

Interdiffusion of Si/Ge short-period superlattices is studied in detail with Raman spectroscopy. Folded acoustic modes and alloy modes from the interface region are found to be very sensitive to intermixing and concentration profile. Annealing at elevated temperature first leads to indiffusion of Si into the Ge layers. The diffusion constant depends strongly on the Si content and consequently varies during the interdiffusion process. The experimental results are qualitatively understood on the basis of a simple atomistic diffusion model. In high quality samples with sharp interfaces the onset of intermixing is observed already at temperatures as low as 450 °C.
Direct writing of iridium lines with a focused ion beam
Authors: Hoffmann P. , van den Bergh H. , Flicstein J. , Assayag G. Ben , Gierak J. , Bresse J.-F.
Published: July 2002

Direct writing of metal features narrower than 100 nm is accomplished by scanning a focused ion beam over a substrate covered with a thin solid metalorganic film. A metalorganic cluster coordination compound [Ir4(CO)11Br][N(C2H5)4] is spun on an oxidized silicon wafer from acetone solution and the resulting layer is irradiated with a focused 20 keV Ga-ion beam. The partially decomposed irradiated film regions show a marked decrease of solubility in acetone. Developing by dissolving the nonirradiated parts of the surface hence leaves narrow lines of partially reacted metalorganic precursor. High temperature treatment of these lines, either in vacuum or in a reactive gas stream, completes the decomposition to quite pure iridium, as is confirmed by micro-Auger analysis. In these preliminary experiments we obtained lines with room temperature electrical resistivities as low as 400 μΩ cm, at direct writing speeds of 40 μm s−1 to 0.8 mm s−1, which correspond to ion doses of 3.1×1016 ions/cm2 to 1.6×1015 ions/cm2, respectively. The width of the metal lines decreases with decreasing ion dose in this range from 180 down to 90 nm. The adhesion to the substrate increases with increasing ion dose. The thickness of the resulting iridium lines depends mainly on the thickness of the metalorganic precursor layer. The maximum possible thickness of the [Ir4(CO)11Br][N(C2H5)4] layer that can be used depends on the effective penetration depth of the ions in this layer.
Photoluminescence and photoluminescence excitation spectroscopy of H-related defects in strained InxGa1−xAs/GaAs(100) quantum wells
Authors: Sobiesierski Z. , Woolf D. A. , Frova A. , Phillips R. T.
Published: July 2002

The incorporation of hydrogen into strained In0.09Ga0.91As/GaAs(100) quantum well structures has resulted in the observation of luminescence from both shallow and deep radiative states located within individual In0.09Ga0.91As layers. The shallow H-related states exhibit binding energies which vary from 22.0±0.5 meV to 15.0±0.5 meV, corresponding to well widths of 40 and 160 Å, respectively. Photoluminescence (PL) measurements are presented, which follow the variation in intensity of the individual PL components as a function of both temperature and excitation intensity. PL excitation (PLE) spectra are used to define the electronic states which lead to excitation of each of the shallow H-related PL transitions. These PLE spectra also contain extra features due to photon recycling. In addition, PL spectra, excited at 1.54 eV, show a splitting of the intrinsic e1–hh1 line shape, for each well, irrespective of whether or not the sample has been hydrogenated. The merits of the possible recombination mechanisms, which might result in this additional structure, are discussed in terms of the available data.
Oxygen magnetically enhanced reactive ion etching of silylated resist patterns
Authors: Dijkstra Han J.
Published: July 2002

One of the critical steps in top-surface imaging lithographic processes such as diffusion enhanced silylated resist DESIRE is the dry development, i.e., the anisotropic oxygen plasma etching of the nonsilylated resist selective with respect to silylated resist. In this article the etch characteristics of the DESIRE process in a magnetically enhanced reactive ion etching etcher are described. The application of a magnetic field is shown to increase both the etch rate and the etch selectivity. The dependency of etch rates, etch selectivity, linewidth and residue formation on magnetic field, radio-frequency (rf) power, oxygen pressure, and flow are reported. The expected linewidth variation with varying etch selectivity, due to sloped silicon profiles in the resist, is observed when the selectivity variation is due to varying rf power or oxygen flow. However, it is not observed when the selectivity variation is due to varying magnetic field. An explanation is given. The etch rate of silylated resist is shown to be dependent on the properties of the thin SiO2 top layer formed during oxygen etching, which is dependent both on etching and on silylation conditions.
Magnetotransport properties of two-dimensional channels in pseudomorphic and strain relaxed In0.17Ga0.83As heterojunctions
Authors: Fernández J. M. , Chen Jianhui , Wieder H. H.
Published: July 2002

The electrical and galvanomagnetic properties of the two-dimensional electron gas present in pseudomorphic In0.17Ga0.83As/Al0.35Ga0.65As and in strain-relaxed In0.17Ga0.83As/In0.15Al0.85As modulation δ-doped heterojunctions were measured as a function of temperature using conventional Hall and resistivity measurements and 1.6 K Shubnikov de Haas oscillatory magnetoresitance measurements, which revealed single subband occupancy. Typical mobilities of the pseudomorphic specimens are, μ(300 K)=9.3×103 cm2/V s and μ(1.6 K)=5.51×104 cm2/V s for electron densities, ns=1.07×1012 cm−2 and 9.78×1011 cm−2, respectively. The strain-relaxed structures have higher electron mobilities, μ(300 K)=104 cm2/V s and μ(1.6 K)=7.61×104 cm2/V s for ns=9.37×1011 cm−2 and 8.67×1011 cm−2. Although the total scattering time τq calculated from the magnetic field-dependent amplitudes of their Shubnikov de Haas oscillations, is half that of the pseudomorphic structures, their low temperature, classical relaxation time τc is greater by 30%.
Interfaces in GaAs/AlAs: Perfection and applications
Authors: Bode Michael H. , Ourmazd Abbas
Published: July 2002

This article presents a short review of our present understanding of the structure of semiconductor interfaces and the way they may be used to investigate solid state processes at the atomic level. Until recently it was customary to claim the fabrication of atomically perfect interfaces, based on the results of photoluminescence (PL) experiments. This was based on the splitting of PL lines into closely spaced satellites, thought to stem from the recombination of excitons within large, atomically perfect terraces. However, quantitative microscopic data, obtained by chemical mapping [A. Ourmazd, F. Baumann, M. Bode, and Y. Kim, Ultramicroscopy 34, 237 (1990)], clearly indicate the presence of significant atomic scale roughness [A. Ourmazd, D. W. Taylor, J. Cunningham, and C. W. Tu, Phys. Rev. Lett. 62, 933 (1989)]. This has been confirmed more recently by high resolution PL and photoluminescence excitation experiments [C. A. Warwick, W. Y. Jan, A. Ourmazd, and T. D. Harris, Appl. Phys. Lett. 56, 2666 (1990)], by Raman scattering [D. Gammon, B. V. Shannabrook, and D. S. Katzer, Phys. Rev. Lett. 67, 1547 (1991)] and by high resolution x-ray measurements [M. Lagally, presented at the MRS Fall Meeting in Boston, 1991, Ca5.2]. All these experiments reveal a more complex interfacial configuration, produced by the interplay of roughness at different length scales.

The quantitative microscopic techniques developed for the study of interfacial structure also makes possible a class of experiments, where the interface is used as a ‘‘photographic emulsion’’ layer, capable of recording the passage of defects. This allows one to investigate solid state processes at the atomic level. For example, the intermixing caused by the passage of an implanted ion through a GaAs/AlAs interface of a multilayer may be used to track the ion and investigate the microscopics of its interaction with the host material [M. Bode, A. Ourmazd, J. A. Rentschler, M. Hong, L. C. Feldman, and J. P. Mannaerts, Mater. Res. Soc. Symp. Proc. 1989 157, 197 (1990)]. From this type of experiments it is possible to deduce the microscopic damage signature produced by individual implanted Ga+ ions [M. Bode, A. Ourmazd, J. Cunningham, and M. Hong, Phys. Rev. Lett. 67, 843 (1991)]. Intriguingly, the damage is charged, and thus strongly influenced by internal electric fields in the solid. This may be exploited to microscopically steer ion implantation damage in solids.
Dislocation reduction in HgCdTe on GaAs and Si
Authors: Shin S. H. , Arias J. M. , Edwall D. D. , Zandian M. , Pasko J. G. , DeWames R. E.
Published: July 2002

Long-wavelength infrared molecular-beam epitaxial (MBE) HgCdTe films with dislocation densities as low as 2.3 × 105 cm−2 on (211) GaAs and Si substrates have been obtained by postgrowth thermal annealing and thermal cycle annealing processes (300–490 °C). Experiments show that metalorganic chemical vapor deposition (MOCVD) HgCdTe epilayers require a higher thermal annealing temperature than MBE material and the difference in dislocation reduction between MBE and MOCVD HgCdTe materials is caused by dislocation movement under high-temperature and thermal stress conditions. The CdTe buffer layer has been observed to play a significant role for the dislocation reduction in the HgCdTe epilayer grown on GaAs or Si alternative substrates. To study the role of dislocations on MBE HgCdTe/GaAs, systematic measurements of the minority carrier lifetime of MBE HgCdTe grown on both CdZnTe and GaAs substrates were carried out. A strong correlation between minority carrier lifetime and dislocation density is observed.
A kinetics study of the electron cyclotron resonance plasma oxidation of silicon
Authors: Joseph J. , Hu Y. Z. , Irene E. A.
Published: July 2002

The electron cyclotron resonance plasma oxidation of silicon was investigated using in situ static spectroscopic ellipsometry during process and dynamic real time ellipsometry at oxidation temperatures between 80 and 400 °C and at various applied bias’. Successful optical modeling of the ellipsometric data was accomplished using a two layer model, in which the top layer is a pure silicon dioxide film over an interface layer. The kinetics results are compatible with the Cabrera–Mott theory for the oxidation by charged species in the limit of low electric field. The effect of applied bias suggests that the oxidizing species is O−. The energy activation is 0.18 eV, substantially lower than the thermal oxidation value.
Scanning tunneling microscopy of GaInAs/InP multiquantum well (110) surfaces observed under an ultrahigh vacuum
Authors: Osaka Fukunobu , Kato Takashi
Published: July 2002

A scanning tunneling microscopy study of GaInAs/InP multiquantum well (MQW) (110) surfaces cleaved under an ultrahigh vacuum is described. Constant-current mode images show that the tip position is always higher on GaInAs well layers than on InP barrier layers. From measurements of samples having different doping concentrations it is found that the tip-height change reflects the potential shape of the MQW structure. A theoretical analysis, including the spreading resistance effect, for the bias voltage dependence of the tip-height difference between the two layers makes clear that this difference is mainly determined by the differences both in the state density and in the barrier height between the two layers, and that the spreading resistance effect cannot be neglected for a sample having a low doping concentration. The analysis also shows that the band offset of a GaInAs/InP heterointerface can be determined by comparing the theory with the experiment.
Annealing behavior of Au(Te)/n-GaAs contacts
Authors: Piotrowska A. , Kamińska E. , Lin X. W. , Liliental-Weber Z. , Washburn J. , Weber E. , Gierlotka S. , Adamczewska J. , Kwiatkowski S. , Turos A.
Published: July 2002

The microstructure and electrical properties of Au/Te/Au/n-GaAs contacts annealed in the temperature range 360–480 °C, either with or without an insulating capping layer, have been investigated by the combined use of Rutherford backscattering spectrometry, x-ray diffraction, transmission electron microscopy, and current–voltage characterization. In addition, a thin-film collector method was applied to measure evaporative losses of As and Te, in view of their high volatility during heat treatment. The results give evidence that the thermally activated contact reaction strongly depends on the application of the capping layer. Contacts annealed without capping layer become ohmic at 420 °C. In the uncapped system, the contact reaction is dominated by the Au–GaAs interaction, as indicated by the appearance of Au7Ga2 phase and large arsenic losses. Moreover, tellurium is shown to partly evaporate during the heat treatment. For the sealed contacts, the losses of both Te and As are limited. Suppression of As vaporization restrains the Au–GaAs reaction, while reduction of Te sublimation activates the Te–GaAs reaction. Ga2Te3 and As2Te3 are the main reaction products, apart from unreacted Au. These reactions, however, do not lead to the formation of ohmic contacts to n-GaAs. Our results do not support the heterojunction model of ohmic contact, but rather testify in favor of the doping model.