bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunability of bipolar conductivity in GaAs doping superlattices
Authors: Künzel H. , Fischer A. , Ploog K.
Published: July 2002

In GaAs doping superlattices the conductivity parallel to the constituent layers is tunable over wide ranges by injection or extraction of free carriers via selective electrodes. The transport simultaneously involves electrons in the n layers and holes in the p layers. The tunability of bipolar conductivity depending on temperature in the range 10 to 300 K and on the design parameters of the superlattice configuration is analyzed in detail. The carrier concentrations and mobilities as a function of bias Unp are determined independently from Hall effect measurements. The observed tunability of carrier concentration is in good agreement with calculations based on a simplified semiclassical approach. The carrier mobility is found to be strongly influenced by the carrier concentration and correspondingly by the effective thickness of the constituent superlattice layers. This behavior is analyzed in terms of the classical size effect. Only a minor influence of the temperature on the carrier mobility is detected. The temperature dependence of the carrier concentration is solely determined by the temperature dependence of the threshold voltage. The present detailed investigation is of importance for application of GaAs doping superlattices in bulk multijunction field effect transistors.
Lattice-matched single-crystalline dielectric films (BaxSr1−xF2) on InP(001) grown by molecular-beam epitaxy
Authors: Tu C. W. , Sheng T. T. , Macrander A. T. , Phillips J. M. , Guggenheim H. J.
Published: July 2002

We report lattice-matched single-crystalline dielectric films on a III–V compound semiconductor InP(001) substrate by molecular-beam epitaxial growth of BaxSr1−xF2. We used two methods to grow mixed fluoride films: two separate beams of BaF2 and SrF2 as well as a single beam of BaxSr1−xF2. In situ reflection high energy electron diffraction shows the parallel epitaxial relationship, BaxSr1−xF2(001)∥InP(001) and BaxSr1−xF2[110]∥InP[110]. The films were studied by double-crystal x-ray diffractometry for lattice parameters and Rutherford backscattering/channeling for compositions and crystallinity. Transmission electron microscopy shows that the films are single crystalline. Preliminary electrical results are also mentioned. Lattice-matched single-crystalline dielectric films can be important for epitaxial metal–insulator–semiconductor as well as semiconductor–insulator–semiconductor structures in a variety of device applications.
Enhanced luminescence from AlGaAs/GaAs single quantum well structures through improved interfaces
Authors: Masselink W. T. , Sun Y. L. , Fischer R. , Drummond T. J. , Chang Y. C. , Klein M. V. , Morkoç H.
Published: July 2002

In single GaAs quantum wells sandwiched between relatively thick AlxGa1−xAs, only weak photoluminescence which is dominated by extrinsic processes is observed. This is attributed to a degraded interface at the bottom AlGaAs/GaAs heterojunction. The situation worsens when the thickness of the bottom AlGaAs layer is increased, the GaAs thickness is reduced, or the Al mole fraction is increased. Inserting a graded three-period superlattice between the AlGaAs and GaAs quantum well at the bottom interface causes the intensity of the photoluminescence associated with exciton recombination to increase to more than 160 times that in the conventional quantum well. Furthermore, the impurity related photoluminescence from the improved quantum well is almost entirely absent; the photoluminescence is dominated by the recombination of the ground state n=1 heavy-hole free exciton. This dramatic improvement is indicative of the improvement of the interface resulting from the insertion of a thin superlattice.
Flow visualization in low pressure chambers using laser-induced biacetyl phosphorescence
Authors: Itoh Fumikazu , Kychakoff George , Hanson Ronald K.
Published: July 2002

Two optical probes, utilizing laser-induced biacetyl phosphorescence recorded by an image-intensified solid state camera, have been developed and used to characterize velocity fields in a low pressure chamber (similar to a plasma-enhanced CVD reactor). The method has been demonstrated at pressures as low as 0.5 Torr; measurements at lower pressures should be possible. Reactor flow velocity fields influence gas residence times and deposition rates. The method discussed in this paper can be used to investigate these effects.
Photoluminescence characterization of molecular beam epitaxy grown InxGa1−xAs(0.51<x<0.57)
Authors: Swaminathan V. , Stall R. A. , Macrander A. T. , Wunder R. J.
Published: July 2002

The results of a low temperature 5.5 K photoluminescence study on undoped n-type InxGa1−xAs(0.51<x<0.57) films grown on InP substrates by molecular beam epitaxy are presented. The photoluminescence spectrum near the band edge is dominated by a single peak of halfwidth 3–8 meV. This peak is attributed to a bound exciton transition for n<5×1015 cm−3 and to a donor-to-valence band transition for n>5×1015 cm−3. From the peak energy of this transition the band gap is determined as a function of x and compared with earlier studies. A weak emission band below the main peak, observed frequently in the films, is associated with a donor-to-acceptor transition and the acceptor is suggested to be carbon. The spatial variation in composition as determined by the variation in the photoluminescence bands is found to be Δx∼0.003 parallel to the growth direction and much less in the plane of the film. In this respect the molecular beam epitaxy grown films exhibit much less compositional variation compared to liquid phase epitaxy and vapor phase epitaxy films.
Molecular beam epitaxial growth and optical properties of Ga0.47In0.53As/Al0.48In0.52As on InP and Ga1−xInxAs/GaAs on GaAs quantum wells
Authors: Goldstein L. , Charasse M. N. , Jean-Louis A. M. , Leroux G. , Allovon M. , Marzin J. Y.
Published: July 2002

We have studied Ga0.47In0.53Al/Al0.48In0.52As quantum wells grown by molecular beam epitaxy (MBE). The smoothing effect of a Ga0.47In0.53As buffer layer results in narrow linewidth. Strained layer superlattices of Ga0.85In0.15As/GaAs exhibit excitonic transition at 77 and 300 K as seen by absorption measurement. A model is proposed to explain the different energy transitions.
Investigation of surface roughness of molecular beam epitaxy Ga1−xAlxAs layers and its consequences on GaAs/Ga1−xAlxAs heterostructures
Authors: Alexandre F. , Goldstein L. , Leroux G. , Joncour M. C. , Thibierge H. , Rao E. V. K.
Published: July 2002

We report a detailed investigation of the surface morphology of Ga1−xAlxAs layers grown by molecular beam epitaxy (MBE) as a function of the layer parameters and the growth conditions. Under some conditions Ga1−xAlxAs layers exhibit a surface roughness characterized by a degradation of the in situ electron diffraction pattern and a very broad photoluminescence peak. The amplitude of the roughness, as observed by optical microscopy and measured by alphastep profiler, increases with the layer thickness, the growth rate r, and goes through a maximum for a substrate temperature TS≂650 °C, a flux ratio R=JV/JIII≂6 and an Al composition x≂0.5. However, smooth layers can be obtained using growth conditions like high TS (≥700 °C), low flux ratio (R≤2.5), or low growth rate (<0.5 μm/h). No surface roughness was observed for GaAs and AlAs layers. Moreover, a high concentration of foreign atoms at the surface of the growing layer (Sn dopant for example) yields to a quite perfect surface morphology. This paper also contains data on the consequences of Ga1−xAlxAs surface morphology on the crystalline and optical quality as characterized by x-ray diffraction and 300 K photoluminescence, of single and multiquantum well structures grown on a Ga0.7Al0.3As buffer layer. This quality which decreases with increasing thickness of the Ga0.7Al0.3As buffer layer can be restored using optimized growth conditions or a tin doping flux. Some explanations of the mechanism responsible for the surface morphology of ternary layers are discussed.
Experimental results examining various models of Schottky barrier formation on GaAs
Authors: Spicer W. E. , Newman N. , Kendelewicz T. , Petro W. G. , Williams M. D. , McCants C. E. , Lindau I.
Published: July 2002

The mechanism of Schottky barrier formation on GaAs and other semiconductors seems to be getting more, rather than less, controversial. Therefore, it is important to examine new data as it becomes available. Four types of data will be mentioned. The emphasis will be on (1) the metal–GaAs chemical reaction products at the interface and their correlation (or lack thereof) with barrier heights and (2) the lack of evidence for discontinuity in Schottky barrier height as one goes from photoemission spectroscopy (PES) pinning results (thickness of order 1–1000 Å metal films). Also mentioned are (3) changes in barrier height for Al, Au, and Ag on diodes annealed up to 500 °C and (4) the effect on barrier height of growing ∼20 Å of native oxide on the GaAs before depositing a 1000-Å metal layer. These data will be presented for discussion and evaluated in terms of the various models of Schottky barrier formation now under discussion which appear most consistent with the unified defect model.
AuGa2 on GaSb(001): An epitaxial, thermodynamically stabilized metal/III–V compound semiconductor interface
Authors: Lince Jeffrey R. , Williams R. Stanley
Published: July 2002

A new type of metal/semiconductor junction has been developed. Crystalline thin films of the intermetallic compound AuGa2 have been grown by molecular beam epitaxy (MBE) on GaSb(001) substrates with the orientation (001)AuGa2∥(001)GaSb, [100]AuGa2∥[100]GaSb. The resulting films have been characterized by Auger electron spectroscopy (AES), low energy electron diffraction, electron energy loss spectroscopy, scanning electron microscopy, x-ray microprobe analysis, and Rutherford backscattering. The growth procedure is reported, and is discussed in terms of the Au–Ga binary phase diagram. This system is of special interest for contacting technology because the compounds terminate a pseudobinary cut through the Au–Ga–Sb bulk ternary phase diagram, which minimizes the amount of chemical interaction across the metal/semiconductor interface.
Thickness dependence of the reactions at the interface of Pd and Si〈111〉
Authors: Nemanich R. J. , Doland C. M.
Published: July 2002

The reactions at the interface of Si〈111〉 and Pd are studied as a function of the thickness of Pd coverage from 5 to 50 Å. Surface conditions are monitored with Auger spectroscopy and the structural interactions are probed by Raman spectroscopy. The results indicate that at 5 Å coverage, strong Si–Pd bonding is evident from the Auger, but the Raman spectra show no evidence of crystalline silicide formation. At 10 Å coverage a sharp peak is observed in the Raman spectrum which is attributed to the formation of crystalline silicide. The results indicate that the initial interactions can be modified after reactions proceed to greater thicknesses. The effect is attributed to critical cluster sizes necessary to form stable nuclei.
Tungsten patterning as a technique for selective area III–V MBE growth
Authors: Harbison J. P. , Derkits G. E.
Published: July 2002

A new scheme for selective area molecular beam epitaxial (MBE) growth is proposed which employs a tungsten masking layer deposited in situ within the MBE growth chamber. The complimentary etching characteristics of W and the III–V semiconductor in fluorine and chlorine based plasmas, allows for pattern definition in the tungsten mask layer without affecting the GaAs and a preetch of the GaAs exposed through the mask without disturbing the mask. This latter step was performed in an integrally connected plasma facility in the sample introduction vacuum chamber of the MBE system just prior to growth, thus miminizing surface contamination. Liftoff of the W mask and overlying polycrystalline GaAs is achieved using a fluorine based plasma etching step, leaving single-crystal free-standing selectively grown structures ∼1000 Å high and 1–5 μm wide with submicron step profiles. The GaAs surface, reexposed after removal of the W, is as smooth as that of the original starting wafer. The regrowth step can be preceeded by a plasma etch-back of the exposed GaAs to yield selectively regrown planar structures. Finally the process should eventually be able to be performed entirely in vacuum and is extendable to the broad range of other III–V compounds.
Core-level photoemission study of MBE-grown GaAs(111) and (100) surfaces
Authors: Katnani A. D. , Sang H. W. , Chiaradia P. , Bauer R. S.
Published: July 2002

MBE-grown GaAs(111) and (100) surfaces have been investigated using synchrotron radiation photoemission spectroscopy. A least-squares fit routine has been used to decompose the various contributions to the core-level line shape. For the As-terminated GaAs(111) surface, the As(3d) and Ga(3d) line shapes and intensities show that the surface coverage of As exceeds one monolayer and that only surface As atoms are involved in the reconstruction. Further, they indicate that the vacancy model developed for the Ga-terminated GaAs(111) surface cannot be extended to the As-terminated GaAs(111) surface. Distinct differences have been observed between primitive [4×6] and centered [c(4×4) and c(2×8)] GaAs(100) reconstructions. For the GaAs(100)c(4×4) surface, our results provide a better resolution of structure associated with 1/4 of a monolayer of an amorphous As overlayer from features arising from the order reconstructed layer. Our results suggest that the transfer of charge between Ga and As atoms occurs during the transformation of c(4×4) into c(2×8). Further, they show that the relative energy positions of the deconvoluted peaks are more characteristic of the different surface reconstructions. The heterojunction interface formation between As-terminated GaAs(111) and three surface reconstructions of GaAs(100) and Ge was studied. The observed differences among the various GaAs starting surfaces do not affect the band offset by more than the experimental uncertainty of ±0.05 eV.
Monte-Carlo simulations of MBE growth of III–V semiconductors: The growth kinetics, mechanism, and consequences for the dynamics of RHEED intensity
Authors: Ghaisas S. V. , Madhukar A.
Published: July 2002

Results of Monte Carlo simulations carried out to examine the nature of the growth mechanism in MBE of lattice matched III–V compounds are presented. The role of surface molecular reaction kinetics associated with the dissociative chemisorption of the group V molecular specie is explicitly investigated for the first time and shown to lead to a new configuration-dependent-reactive-incorporation (CDRI) growth process, quite distinct from the conventional notions of nucleation and continuous growth. For very slow dissociative reaction kinetics the CDRI growth process is shown to lead to a reaction-limited-incorporation (RLI) growth mechanism and the accompanying growth rate exhibits oscillatory behavior. For fast dissociative reaction kinetics, accompanied with a sufficiently fast surface interlayer migration kinetics of the group III atoms, the CDRI growth process gives rise to a configuration-limited-reactive-incorporation (CLRI) growth mechanism. The crystal growth rate once again exhibits oscillations. It is also shown that under conditions involving specially delicate balance between the various kinetic rates, growth can proceed without oscillations in the growth rate. An essentially layer-by-layer mode of material addition is realized in all the aforementioned cases. Consequences of the resulting growth front morphology for the time dependence of the RHEED specular beam intensity are investigated and shown to give rise to a damped oscillatory behavior, including in the absence of oscillations in the crystal growth rate.
Molecular beam epitaxy of GaAs using a mass-separated, low-energy As+ ion beam
Authors: Shimizu S. , Tsukakoshi O. , Komiya S. , Makita Y.
Published: July 2002

A new film growth technique of III-V compound semiconductors is under development where the sticking coefficient of group-V element can be increased to unity. In this technique, a group-III element is supplied to the substrate as a molecular beam, while a group-V element is supplied as a mass-separated, high-purity ion beam and implanted at low energy of about 100 eV. With this growth technique, epitaxial growth of GaAs on GaAs(100) is studied. Nondoped GaAs films are grown at the As+ ion energy of 100 and 200 eV. Single-crystal films with flat and smooth surface and free from oval defects are obtained at 220 °C or higher with the flux intensities of a Ga molecular beam and As+ ion beam at 2.0×1013 cm2 s and 2.0–4.4×1013 cm2 s, respectively. However, under Ga-rich conditions Ga droplets are observed on the grown films. The RHEED pattern shows c(2×8) structure in the former case. Photoluminescence spectra at 4 K of the films grown at 450 °C or higher show well-defined features due to bound exciton and donor-acceptor pairs at the As+ ion energy of 100 eV.
Quantum well diode lasers of lead-europium-selenide-telluride
Authors: Partin D. L. , Majkowski R. F. , Swets D. E.
Published: July 2002

The formation of a new semiconductor compound, lead-europium-selenide-telluride, was recently demonstrated using the molecular beam epitaxy (MBE) growth technique. This material may be grown lattice matched to PbTe substrates, and double heterojunction diode lasers were fabricated which covered the wavelength range 6.6 to 2.7 μm. Operation up to 147 K has been attained, which is the highest cw operating temperature ever observed in lead-chalcogenide semiconductors. Single quantum well diode lasers in a lead-chalcogenide materials system have now been fabricated for the first time. These devices were composed of PbTe quantum wells with Pb1−xEuxSeyTe1−y confinement layers. The quantum wells ranged in thickness from 300 to 2500 Å. Strong quantum effects are observed for the smaller well widths (Lz ≲1200 Å) because of the small carrier masses (me∼mh∼0.037m0). The shift in laser emission energy is in approximate agreement with that calculated from a finite square well potential if ΔEc∼ΔEv is assumed. The near equality of the electron and hole effective masses makes it difficult to accurately determine ΔEc and ΔEv. The threshold current is strongly reduced by quantum effects, apparently because of modifications of the density-of-states function. The laser emission from these devices has been characterized by near-field and far-field measurements. At high temperatures (T≳90 K), the emission of these mesa stripe lasers switches from gain guided to an index guided mode. Single-mode emission up to ∼1-mW power levels have been observed. These lasers have tuned from 6.45 μm (at 13 K) to 4.41 μm (at 174 K) under cw conditions, and to 4.01 μm (at 241 K) pulsed. It appears that the maximum operating temperature is limited by the quantum well barrier height, and a preliminary experiment with a higher barrier gave pulsed operation up to 260 K. These are the highest pulsed and cw operating temperatures ever reported for lead-chalcogenide diode lasers, and puts them within the operating range of thermoelectric coolers. The total cw tuning range of a single diode laser is as large as 740 cm−1, which is also much larger than previously observed. These properties greatly enhance the utility of these devices for spectroscopic and fiber-optic applications.
Photoconductivity of confined donors in Al0.3Ga0.7As/GaAs quantum wells
Authors: Mercy J. M. , Jarosik N. C. , McCombe B. D. , Ralston J. , Wicks G.
Published: July 2002

Capacitive coupling is used as a contactless method to study photoconductivity of confined donors in lightly doped AlGaAs/GaAs multi-quantum wells. The high sensitivity allows a clear observation of the 1s→2p+ intra-impurity transition line shape that can be compared with recent calculations. Evidence of a higher frequency line is shown, which is attributed to the 1s→3p+ transition.
Exposure of calcium fluoride resist with the scanning tunneling microscope
Authors: McCord M. A. , Pease R. F. W.
Published: July 2002

The scanning tunneling microscope (STM) can be used to generate very fine beams of very low energy electrons (below 100 eV). Under optimum conditions the beam diameter is proportional to beam voltage and hence, when using such a beam as an exposure tool it is desirable to employ the thinnest possible resists. The metal halides in general have several properties that make them attractive candidates for use in a scanning tunneling microscope. They are easily evaporated as thin films of thickness from 10 to 100 nm. It is important that an insulating resist film be thin so that the electrons can penetrate it and the tip does not crash into it. Thicker films require a higher accelerating voltage and this degrades resolution. The sensitivity of the metal halides is on the order of 1 C/cm2, which coincides with the dose range most readily obtainable in our instrument. Finally, the exposure voltage threshold of the metal halides is thought to be in the range of a few tens of volts. This is low enough to take advantage of the very fine low energy beams available from the STM, but high enough to exclude secondary electrons from contributing to the exposure process. The calcium fluoride was thermally evaporated onto a silicon substrate. When exposed to a beam of electrons, the fluorine dissociates and the calcium converts to calcium oxide, which can be dissolved in water. We have demonstrated the ability of the STM to expose lines in CaF2 with linewidths from tens to hundreds of nanometers and have shown the degradation of resolution as beam voltage increases.
High-resolution Fresnel zone plates for soft x rays
Authors: Vladimirsky Y. , Kern D. , Chang T. H. P. , Attwood D. , Ade H. , Kirz J. , McNulty I. , Rarback H. , Shu D.
Published: July 2002

Fresnel zone plates with minimum zone widths down to 50 nm were fabricated by electron-beam lithography, tested, and used in the scanning x-ray microscope at the Brookhaven National Synchrotron Light Source. The zone plates consist of gold rings on a silicon nitride window. Characterization of the zone plates included efficiency and resolution measurements in the first order focus. Spatial resolution down to 75 nm was measured. The best zone plates showed negligible astigmatism, and a first order efficiency in excess of 5% has been achieved. A new method for development monitoring during the fabrication procedure using an optical microscope with polarized light and crossed analyzer was introduced. For the inspection of the fabrication accuracy and, in particular, for ellipticity, a moiré pattern technique was employed. Using these zone plates as objective lenses, x-ray images of biological specimens were obtained with 70 nm resolution.
Resist development described by least action principle-line profile prediction
Authors: Barouch E. , Bradie B. D. , Babu S. V.
Published: July 2002

A new idea is introduced requiring that each development path will be the path of least resistance to developer penetration. Consequently, minimum dissolution time is required for the development of the final line profile. This idea manifests itself in a variational calculation of the path integral along each local development trajectory, from which the dissolution profile is obtained uniquely, as a solution of a nonlinear partial differential equation (PDE). The photoactive compound (PAC) concentration is obtained from the standard Dill’s equations for the exposure–bleaching process for both monotonic as well as standing waves. The procedure has been implemented and tested. It has been found to be very accurate and it eliminates the path crossings inherent in the predictions of the string algorithm. The arbitrary elimination of unfavorable points is avoided as well for all developing times.
Coherent point source electron beams
Authors: Fink Hans-Werner , Stocker Werner , Schmid Heinz
Published: July 2002

The concept of a point source for electrons in view of quantum mechanical experiments is described. The engineering of such atomic electron emitters is discussed and recent experiments in holography with low energy electrons are presented.
Band offset measurements for AlSb/ZnTe heterojunctions
Authors: Schwartz G. P. , Gualtieri G. J. , Feldman R. D. , Austin R. F. , Nuzzo R. G.
Published: July 2002

Core level photoemission measurements have been made for thin (40 Å) AlSb heterojunctions grown on ZnTe. The room temperature valence band offset we obtain is 0.35±0.11 eV and the heterojunction is type I (electrons and holes confined in AlSb). The Γ(ZnTe)–Δ(AlSb) conduction-band offset would then be 0.28 eV. The AlSb growth temperature was reduced to 325 °C in order to suppress interfacial reactions. Raman scattering indicates good crystalline quality for this growth temperature. Heterojunctions annealed in vacuo at 375 °C exhibit interfacial reactions which liberate elemental Te or possibly Al2Te3.
Microminiaturization of electron optical systems
Authors: Chang T. H. P. , Kern D. P. , Muray L. P.
Published: July 2002

The performance of miniaturized electron optical systems comprising a field emission microsource and a microlens for probe forming has been studied. A complete system measuring millimeters in length and diameter with performance exceeding that of a conventional system over a wide range of potentials (100 V–10 kV) and working distances (up to 10 mm) appears to be feasible. A scanning tunneling microscope aligned field emission microsource offers performance well suited for this application and a selective scaling approach has been developed to allow a wide range of potentials to be applied. Such miniaturized systems can be of significant importance to many areas of electron-beam applications.
Scanning attractive force microscope using photothermal vibration
Authors: Umeda N. , Ishizaki S. , Uwai H.
Published: July 2002

An attractive force microscope using photothermal vibration is presented. A cantilever is vibrated by the optical absorption of a laser beam whose intensity modulation frequency is at resonance frequency, and the vibration is detected by an optical deflection method. The characteristics of the photothermal vibration and the attractive force gradient versus tip-sample distance have been measured with a Ni foil cantilever. Topographic images of digital memory disks made from a polycarbonate and Mg–Al alloy were recorded.
Surface site specificity on the basal plane of graphite: 1.06 μm laser damage threshold and reactivity with oxygen between 350 and 2300 K
Authors: Balooch M. , Schildbach M. , Tench R. , Allen M. , Siekhaus W. J.
Published: July 2002

Scanning tunneling microscopy (STM) has been used to document changes in the nanometer-scale morphology of the basal plane of highly oriented pyrolytic graphite after in situ exposure to 7 ns, 1064 nm Nd:YAG, and to ms Nd:Glass laser pulses producing surface temperatures up to 2300 K, and after ex situ exposure to temperatures as low as 350 K in air. Laser damage produced by the ns pulses was visible by STM at fluences far below those that produce melting and effects visible by other imaging techniques. Damage appears first on step edges and consists of exfoliation of graphite layers and recession of steps through removal of mono- or multilayer patches. ms and long-term heating leads to reaction with oxygen, which proceeds by etching of both crystalline boundaries and step edges. In addition a high concentration of flat-bottom shallow pits (two monolayers deep) and a smaller number of conical pits (greater than 30 Å deep) can clearly be identified. The results give for the reaction at the grain boundaries an activation energy of 36 kcal/mole while for steps and shallow pits it is 53 kcal/mole, within the energy range found in global, not site-specific reaction studies. Site-specific reaction probabilities as low as 10−11 are measured at 350 K.
Low energy electron microscopy of nanometer scale phenomena
Authors: Bauer E. , Mundschau M. , Swiech W.
Published: July 2002

The physical principles of low energy electron microscopy (LEEM) are discussed. The application of this nonscanning imaging method to the study of surface phenomena on the 10 nm scale is illustrated by examples of the growth and desorption of metal films on metal and semiconductor substrates as well as by phase transitions in these films.
Experimental study of a focused ion beam probe size and comparison with theory
Authors: Orloff J. , Li J.-Z. , Sato M.
Published: July 2002

A study has been made to compare theoretical predictions of the performance of an optical column using a liquid metal ion source with experiment. Rather than estimating the beam size by calculating disks of confusion due to the chromatic and spherical aberrations, calculations of the current density profile were carried out using a new method which determines trajectories between object and image planes using the geometrical aberration theory. The trajectories are then used to find the current density which was then integrated to simulate the beam passing over a knife edge, so the rise distance could be estimated. It is shown that the focus conditions for optimum resolution of the beam may differ significantly from the conditions which optimize the rise distance of the beam. An experiment was then carried out to measure the actual rise distance for a variety of operating conditions of the actual ion gun. Good agreement was found between experiment and calculation.
Analysis of passivating oxide and surface contaminants on GaAs (100) by temperature-dependent and angle resolved x-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry
Authors: Schröder F. , Storm W. , Altebockwinkel M. , Wiedmann L. , Benninghoven A.
Published: July 2002

Surface contaminations on GaAs (100) wafers have been analyzed by means of temperature-dependent and angle resolved x-ray photoelectron spectroscopy (TOXPS, ARXPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). First, the dependence of the composition, chemical state, and diffusion behavior of the passivating thermal oxide on different annealing temperatures was investigated. The phases of this oxide can be described by a multilayer model and are consistent with the Ga–As–O equilibrium phase diagram. Second, the nature and amount of impurities at ambient and elevated temperatures was studied. No metal contaminants were found within the sensitivity of TOF-SIMS. The residual amount of carbon contamination at the desorption temperature of the oxide depends on the amount of photon irradiation at ambient temperature.
In situ characterization of InP surfaces after low-energy hydrogen ion cleaning
Authors: Gallet D. , Hollinger G. , Santinelli C. , Goldstein L.
Published: July 2002

Hydrogen ion cleaning procedures of InP(100) surfaces have been studied and the effect of hydrogen on carbon and oxygen contaminations were particularly investigated. The induced structural surface modifications were studied by x-ray photoelectron spectroscopy and in situ reflected high-energy electron diffraction just after ion bombardment and after annealing under an arsenic overpressure. Results show that cleaning with hydrogen ions leaves an indium-rich surface layer on the InP surface. After hydrogen ion bombardment, structural changes of the surface occur and their irreversibility after As stabilization depends on the hydrogen dose. If this dose does not exceed a critical value of about 3×1016 ions/cm−2, the structural properties of the InP surface can be restored after thermal annealing under arsenic overpressure. However, both unannealed and annealed surfaces show a strong pinning of the Fermi level, with the creation of defects 0.25 eV below the minimum conduction band, which indicates poor electronic properties always after hydrogen ion bombardment.
Self-limiting oxidation of Si nanowires
Authors: Liu H. I. , Biegelsen D. K. , Johnson N. M. , Ponce F. A. , Pease R. F. W.
Published: July 2002

Achieving tolerances of the order of 1 nm for sub-10 nm structures is both challenging and necessary for controlled experiments on such structures. Here the use of a self-limiting oxidation reaction to yield silicon (Si) wires of less than 10 nm diam with a tolerance of ±1 nm over 0.5 μm. The final self-limiting diameters were found to be controlled by oxidation temperature. For 30 nm initial Si column diameters, the asymptotic diameters were found to be 11 and 6 nm for dry oxidation at 800 and 850 °C, respectively. The mechanism of the self-limiting reaction is not yet fully understood but the tiny radius of curvature is obviously a factor. In addition, there appears to be an anomalous loss of Si; this may be due to sublimation of SiO.
Experimental and theoretical determinations of gate-to-emitter stray capacitances of field emitters
Authors: Busta H. H. , Pogemiller J. E. , Chan W. , Warren G.
Published: July 2002

By measuring the gate-to-substrate capacitance of an array of field emitters with and without emitters, it is possible to obtain the gate-to-emitter stray capacitance per tip. This experiment was performed on 30×30 and 100×100 monocrystalline silicon arrays with gate diameters varying from 1.5 to 7 μm and with different tip geometries. Typical values obtained are 0.1 fF/tip. The experimental values are compared with theoretical calculations using magic, a 2 and 1/2 dimensional particle-in-cell electromagnetics code. It is found that the stray capacitance depends strongly on the tip angle and other geometric parameters. Assuming that devices can be fabricated in which only the stray capacitance dominates, the expected cutoff frequency is gm/2πCstray=8 GHz, with gm=5 μS/tip for the best value of transconductance reported so far.
Reflection high energy electron diffraction observation of exchange reaction dynamics on InAs surfaces
Authors: Collins D. A. , Wang M. W. , Grant R. W. , McGill T. C.
Published: July 2002

We have used time-resolved reflection high energy electron diffraction (RHEED) measurements to study the dynamics of a surface, anion exchange reaction. In the experiment, InAs surfaces are exposed to Sbx fluxes and subsequent changes in the crystals’ RHEED patterns are examined. We find that when an InAs surface is initially exposed to an Sb flux the specular spot intensity first decreases, then recovers back toward its initial value. The shape of the intensity versus time curves is extremely reproducible if the absolute Sb flux and the Sb species are kept constant. The length of time required for the RHEED pattern to stabilize is much shorter for cracked Sb than for uncracked Sb. The RHEED dynamics are also faster if the total Sb flux increases. The behavior of the RHEED dynamics as a function of Sb flux and Sb species is consistent with the changes in the RHEED pattern being due to an Sb/As exchange reaction on the crystals’ surface. The RHEED data are compared to previously published x-ray photoelectron spectroscopy (XPS) data which studied exchange reactions on InAs surfaces exposed to Sb fluxes. The XPS study confirmed that the incident Sb did indeed exchange with As in the epilayer and estimated the exposure time needed to complete the Sb/As exchange reaction. The time scales for exchange associated with the RHEED and XPS data are in good agreement, which further indicates that the observed RHEED dynamics are due to the Sb/As exchange reaction. Preliminary results from exposing GaSb surfaces to As fluxes show similar RHEED and XPS behavior. This suggests that RHEED could be generally applicable to the study of surface exchange reaction dynamics.
Scanning tunneling microscopy/atomic force microscopy studies of bacteriophage T4 and its tail fibers
Authors: Ikai Atsushi , Imai Kensaku , Yoshimura Kousei , Tomitori Masahiko , Nishikawa Osamu , Kokawa Ryouhei , Kobayashi Masahito , Yamamoto Mitsuhiko
Published: July 2002

Atomic force microscopy and scanning tunneling microscopy were used to obtain molecular resolution images of biological structures such as proteins and viruses. Proteins of 10 to 20 nm scale could be reliably resolved with faintly recognizable subunit arrangements. Bacteriophage T4 was imaged with well-resolved head, tail, and tail fibers. When the tails were prepared from the headless mutants, the sheath proteins were stripped away from the tail, revealing the tube and the base plate in a T-shaped complex. Scanning tunneling microscopy was used on metal coated samples and provided a reliable height measurement for bacteriophage T4. Constant improvement in the instrumentation and sample preparation promises a bright future for the biomedical application of scanning probe microscopy.
Internal structure of C60 on Au(110) as observed by low-temperature scanning tunneling microscopy
Authors: Gaisch R. , Berndt R. , Schneider W.-D. , Gimzewski J. K. , Reihl B. , Schlittler R. R. , Tschudy M.
Published: July 2002

Scanning tunneling microscopy (STM) studies of C60 molecules on a Au(110) single crystal surface have been performed in ultrahigh vacuum and at low temperatures (5 and 50 K). Submonolayer, monolayer, and bilayer coverages of C60 have been sublimated onto reconstructed Au(110) 1×2 surfaces. STM images reveal ordered C60 overlayers which are commensurate with the atomic geometry of the substrate. Moreover, we have observed intramolecular patterns within single C60 molecules in unordered and ordered arrays of C60 in the first as well as in the second adsorption layer which may be associated with different orientations of the fullerenes on the highly corrugated gold substrate.
ZnSe nucleation on the GaAs(001):Se-(2×1) surface observed by scanning tunneling microscopy
Authors: Li D. , Pashley M. D.
Published: July 2002

We have used scanning tunneling microscopy (STM) to study the initial stages of the growth of ZnSe by molecular-beam epitaxy on the GaAs(001):Se-(2×1) surface. The GaAs(001):Se-(2×1) surface is formed by depositing Se onto the GaAs(001):Se-(2×4) surface and annealing at above 520 °C. It is a highly ordered (2×1) array of Se dimers, and so is structurally similar to the Se-terminated ZnSe(001)-(2×1) surface, and might be expected to provide a good starting surface for two-dimensional ZnSe growth. However, we find that ZnSe grows by the formation of three-dimensional islands on the GaAs(001):Se-(2×1) surface. Islands grow several layers high while much of the Se-terminated GaAs surface remains uncovered. We have compared our STM images to a simple statistical model of growth and conclude that the sticking coefficient of ZnSe on the GaAs(001):Se-(2×1) surface is about 1/5 of that on a continuous ZnSe film, whereas the sticking coefficient on top of the small ZnSe islands is close to that on a continuous ZnSe film. We can understand the low reactivity of the Se dimers on GaAs compared to Se dimers on ZnSe by considering electron counting. On the ZnSe surface, Se dimers are neutral, whereas on the GaAs surface Se dimers have a net positive charge which results in a lowering of the dimer energy, producing a relatively inert surface. We conclude that in order to grow structurally high quality ZnSe films on GaAs, which requires two-dimensional growth, formation of a Se-(2×1) structure on the GaAs(001) surface must be avoided.
Restricted motion of a GaAs surface Fermi level caused by excess As
Authors: Wada Yoshinori , Wada Kazumi
Published: July 2002

Atomic layer passivation (ALP) with atomically thin P-related compound semiconductor materials grown on (100) GaAs surfaces significantly enhances the photoluminescence intensity and also makes the electrical characteristics of GaAs metal–insulator–semiconductor (MIS) diodes (M/I/ALP/GaAs) nearly ideal. This study clarifies the change in GaAs interface characteristics caused by introducing As and subsequent passivation. Arsenic is introduced to GaAs surfaces by heating wafers in an AsH3 atmosphere before forming the passivation layer. Due to the As introduction, the enhancement of the PL intensity is suppressed to the magnitude of the intensity from conventional GaAs surfaces. The influence of As introduction on surface Fermi level pinning was also studied by fabricating MIS diodes. It is found that the range of capacitance becomes narrower than that observed in MIS diodes with ALP without an As layer. High-frequency analyses show that the surface Fermi level can move only within a limited energy range of 0.6 eV from the conduction band minimum of GaAs. It is shown that a reduction in the amount of As at the interface is necessary to unpin it in the ALP/GaAs system. The characteristics of As-induced interface states are discussed.
Calculation of electron field emission from diamond surfaces
Authors: Huang Z.-H. , Cutler P. H. , Miskovsky N. M. , Sullivan T. E.
Published: July 2002

The electron field emission from diamond surfaces is investigated theoretically using a model consisting of the projection of the energy-band surfaces in the 〈111〉, 〈110〉, and 〈100〉 emission directions. The effect of the negative electron affinity, the band bending, the image interaction, and surface states are examined in detail. It is found that the tunneling from the bulk conduction and valence bands is negligible in p-type diamond. While emission from surface states located about 1 eV below the conduction band has sufficient transmission probability to produce currents observed in experiments, there is no obvious transport mechanism in intrinsic, doped, or polycrystalline diamond to sustain a direct field emission current. We postulate two subbands in the intrinsic band gap, which may be generated by defects or impurities. With reasonable band parameters, the calculated j–F characteristics agree with experimental data.
Neural networks in plasma processing
Authors: Rietman Edward A.
Published: July 2002

Over the last few years neural networks have been studied for potential applications in plasma processing. The focus of this article will be on two neural network models for complementary metal–oxide–semiconductor production. The models were developed with strict statistical cross-validation and applied to developing a plasma gate etch controller and a plasma model of a contact etch process. For a gate etch controller, the process has been evaluated in a production environment and shown to improve the process variance and throughput. For a model of a contact etch process we demonstrate that the model is limited by the inherent noise in the training data and that the direct current bias and etch time are the key control factors that determine the product quality at the end of the etch.
Optical investigations of surface processes in GaP heteroepitaxy on silicon under pulsed chemical beam epitaxy conditions
Authors: Rossow U. , Dietz N. , Bachmann K. J. , Aspnes D. E.
Published: July 2002

Surface processes during the heteroepitaxy of GaP on Si under pulsed chemical beam epitaxy conditions were investigated simultaneously by the optical methods reflectance difference/anisotropy spectroscopy, p-polarized reflectance spectroscopy (PRS), and laser light scattering. Our studies were performed during both cyclic and interrupted growth, where the surface was exposed to individual pulses of the precursors triethylgallium (TEG) and tertiarybutylphosphine (TBP). The data show that the three optical probes provide different perspectives of growth. Several surface processes exhibit time constants of the order of 1 s. One such process is the clustering of Ga atoms, or less likely, of TEG fragments, that occurs with TEG exposure. The optical data also show that TBP dealkylation occurs essentially instantaneously upon arrival at the surface, and that TEG dealkylation is the rate-limiting step. The PRS data exhibit fine structure that shows that heteroepitaxial growth can be described by a four-phase model consisting of the substrate, a GaP layer, a surface reaction layer containing all adsorbed species not yet incorporated in the growing layer, and the ambient. By assuming that this surface layer is very thin we derive approximate equations that allow us to treat the PRS response quantitatively.
Atomic force microscope imaging of ribosome and chromosome
Authors: Li Min-Qian , Xu Lei , Ikai Atsushi
Published: July 2002

We present three-dimensional atomic force microscope (AFM) images of eukaryotic ribosome for what we believe to be the first time in air with a tapping-mode AFM. In AFM images, the typical structure of ribosome and its subunits is clearly shown. In addition, for the first time, to the best of our knowledge, an entire polytene chromosome of Drosophila has been imaged by AFM with scanning size 116×116 μm. The AFM image showed details of band pattern and puff regions. Compared with the optical image of the same specimen with staining, the AFM images may provide much higher spatial resolution with height information.
Butanethiol self-assembly on Au(001): The 1×4 Au missing row, c(2×8) molecular lattice
Authors: Poirier G. E.
Published: July 2002

We have characterized the molecular packing structure of butanethiol self-assembled monolayers (SAMs) on Au(001) using an ultrahigh vacuum scanning tunneling microscope. Butanethiol SAMs were formed on the clean surface using two different protocols: deposition by gas-phase transport and deposition from ethanol solution. The SAM growth proceeds by nucleation and growth of compact, anisotropic islands. The island growth results in relaxation of the compressed Au(001)(5×20) reconstruction with concomitant ejection of excess Au atoms onto the terraces. Although the reconstructed surface contains 0.26 monolayer excess Au atoms, 0.4 monolayer is ejected. The surface Au atoms arrange to form identical 1×4 missing row structures on both the original terrace level and the nascent island level. The thiols pack in a commensurate, distorted-hexagonal overlayer on this (1×4) missing row structure forming a c(2×8) molecular lattice.
Relationship between field emission characteristics and hydrogen content in diamondlike carbon deposited by the layer-by-layer technique using plasma enhanced chemical vapor deposition
Authors: Park Kyu Chang , Moon Jong Hyun , Chung Suk Jae , Oh Myung Hwan , Milne W. I. , Jang Jin
Published: July 2002

We deposited diamondlike carbon (DLC) films of various hydrogen contents by plasma enhanced chemical vapor deposition. The hydrogen content in the DLC film was controlled by a novel technique called a layer-by-layer deposition, in which the deposition of a thin layer of DLC and a CF4 plasma exposure on its surface were carried out alternatively. The DLC films with different hydrogen content could be obtained by varying the CF4 plasma exposure time. The emission current increases and turn-on field decreases with decreasing hydrogen content in the DLC film.
Interfaces of strained layer (GenSim)p superlattices studied by second-harmonic generation
Authors: Xiao Xudong , Zhang Chun , Fedotov A. B. , Chen Zhenghao , Loy M. M. T.
Published: July 2002

Second-harmonic (SH) generation has been applied to study the interfaces of short-period (GenSim)p strained layer superlattices. From the SH signal and its symmetry, we have concluded that (i) over an area of optical wavelength size, the superlattices were grown with nonuniform layer thickness in each period; (ii) along the growth direction, the atomic structure of Si-on-Ge and Ge-on-Si interfaces are not symmetric; and (iii) a structural change of the interfaces, possibly from the increase of misfit dislocation or other defect density, occurs when the thickness of the strained Ge layer in each period exceeds about six layers. The SH spectroscopic results showed defects contributed dominantly for (Ge7Si8)4, but substantial contribution from Si–Ge bonds for (Ge4Si4)3 and (Ge5Si5)3 still exists.