bluuebunny/crossref_metadata_2025 huggingface dataset:
Internal photoemission in CoGa/GaAs Schottky barriers, possible injection of electrons into the L valley
Authors: Arghavani R. , Karunasiri R. P. G. , Kuo T. C. , Kim Y. K. , Wang K. L.
Published: July 2002

Internal photoemission in epitaxially grown films of CoGa/GaAs Schottky barriers by back illumination is used to study the injection of electrons into L and X valleys of the conduction bands of n-type GaAs. The general theory of Fowler predicts that the photocurrent per absorbed photon should vary as the square of the photon energy hν for [(hν−Φb/kT]≥0, where Φb is the difference between the Fermi level and Γ point in the GaAs conduction band. Unlike the case for Au/GaAs Schottky barriers, we observe two thresholds in the photocurrent measurements of uniform epitaxial CoGa/GaAs contacts. The second threshold formed at ≊0.3 eV higher than the first threshold (Γ point) is interpreted as the injection of carriers into the L valley of the GaAs from the metal side. A third threshold at ≊0.5 eV above the Γ point requires a photon energy greater than the energy gap of GaAs (1.4 eV) and hence cannot be observed in the back illumination mode.
Helium plasma enhanced chemical vapor deposited oxides and nitrides: Process mechanisms and applications in advanced device structures
Authors: Bright A. A.
Published: July 2002

New plasma-enhanced chemical vapor deposition (PECVD) dielectric processes enable deposition of films of much higher quality than previously possible by PECVD. The use of He-diluted PECVD processes (He-PECVD) for depositing near-thermal quality SiO2 and high-quality Si3N4 films is described. Excellent Si–SiO2 interfacial properties are achieved in this process by a dilute, low-rate, low-power plasma oxidation step prior to the deposition. The use of He-PECVD dielectrics in advanced bipolar and CMOS structures is described. In bipolar technology, high-performance SiGe heterostructures with extremely narrow base profiles have been fabricated using low processing temperatures in order to prevent diffusive broadening of the base dopant distribution and to avoid relaxation of the SiGe strained layer. When used in combination with a low-temperature emitter process, He-PECVD dielectrics make possible a completely low-temperature process, resulting in final base widths as low as 40 nm. In CMOS technology, successful fabrication of field-effect transistors (FET’s) with ultrathin (4.0–7.0 nm) He-PECVD gate oxides is described, with particular attention paid to producing a high quality (low Dit ) Si–SiO2 interface. Device characteristics with He-PECVD dielectric are comparable to those with thermal oxide, except that slightly lower transconductance is obtained in the He-PECVD oxide devices.
Nondestructive depth profile study of oxygen-exposed large-grain silver using angle-resolved Auger electron spectroscopy and ion scattering spectroscopy
Authors: Davidson Mark R. , Hoflund Gar B. , Outlaw R. A.
Published: July 2002

The adsorption of oxygen on polycrystalline Ag during exposure to 75 Torr of O2 for 1 h at temperatures ranging from 75 to 250 °C has been examined using ion scattering spectroscopy (ISS) and angle-resolved Auger electron spectroscopy (ARAES). Quantification of these data provide the profile of the O concentration in the near-surface region of the Ag as a function of exposure temperature. The O concentration in the outermost atomic layer increases from 0% according to forward-scattered ISS for the cleaned surface to a maximum value of about 50 at. % for the 250 °C exposure. A very small amount of subsurface O remains on the cleaned Ag surface after prolonged ion sputtering and annealing treatments. The total amount of subsurface O also increases monotonically with dosing temperature and reaches a uniform concentration level of 14 at. % over the region probed by Auger electron spectroscopy (AES). Exposures at higher temperatures (up to 600 °C) results in the same near-surface O concentration profile as the 250 °C exposure.
Scanning tunneling microscopy of DNA: The chemical modification of gold surfaces for immobilization of DNA
Authors: Bottomley L. A. , Haseltine J. N. , Allison D. P. , Warmack R. J. , Thundat T. , Sachleben R. A. , Brown G. M. , Woychik R. P. , Jacobson K. Bruce , Ferrell T. L.
Published: July 2002

We have investigated the coulostatic immobilization of DNA to a chemically modified gold surface. The chemical modification involves the covalent attachment of chemically polarizable groups by reaction of thiols and disulfides with clean gold surfaces. We have determined that chemically modified surfaces possessing pendant cationic groups bind DNA. In this paper we report the reliable and reproducible scanning tunneling microscopy (STM) imaging of DNA using a gold surface chemically modified with N,N,-dimethyl-2-mercaptoethylamine and present the first STM images of an entire genetically functional DNA molecule, the circular plasmid pBS+.
Growth of ferroelectric oxide thin films by excimer laser ablation
Authors: Krupanidhi S. B. , Maffei N. , Roy D. , Peng C. J.
Published: July 2002

The pulsed ultraviolet excimer laser ablation technique was employed to deposit a variety of ferroelectric oxide thin film compositions, including lead zirconate titanate, Pb(Zr,Ti)O3 (multiaxial), bismuth titanate, Bi4Ti3O12 (biaxial), and lead germanate, Pb5Ge3O11 (uniaxial), while maintaining the in situ stoichiometry of films close to that of targets. Growth process parameters such as deposition rates and the stoichiometry of the as-deposited films were examined and correlated with the ablation fluence, pressure, and substrate temperature. A threshold of ablation energy density has been identified for each type of ferroelectric composition. Evidence of intrinsic bombardment by energetic ablated species, during the thin film deposition, indicated an impact on the composition, structure, and the electrical properties as well. Electrical characterization of the ferroelectric films indicated a dielectric permittivity of 800–1000, a remanent polarization (Pr) of 32 μC/cm2, and a coercive field (Ec) of 100 kV/cm for polycrystalline PTZ films and corresponding properties of 150, 7 μC/cm2, and 20 kV/cm for in situ crystallized c-axis preferred oriented bismuth titanate thin films and a dielectric permittivity of 30, a Pr of 2.5 μC/cm2 and an Ec of 55 kV/cm for c-axis oriented lead germanate thin films. The polarization switching behavior in biaxial and uniaxial systems was strongly dependent on the orientation of the films while the multiaxial system was insensitive to film orientation.
Growth of epitaxial aluminum nitride and aluminum nitride/zirconium nitride superlattices on Si(111)
Authors: Meng W. J. , Heremans J.
Published: July 2002

Growth of epitaxial aluminum nitride (AlN) and crystalline superlattices of aluminum nitride/zirconium nitride (AlN/ZrN) on Si(111) by ultrahigh-vacuum dc magnetron reactive sputtering has been studied. Detailed structural characterizations were carried out with techniques of x-ray diffraction, transmission electron microscopy, and reflection high energy electron diffraction. Results on epitaxial orientation, defect configuration, and growth mode are discussed.
Au/Si interface: Experiments on substrate influence
Authors: Pasa A. A. , Paes H. , Losch W.
Published: July 2002

We have studied the influence of amorphous and crystalline substrate materials, such as a-Si and a-Si:H, as well as c-Si(111), (100), and hydrogen implanted c-Si(111) on the outdiffusion of Si through an evaporated Au thin film, and its subsequent oxidation in atmosphere. Using Auger electron spectroscopy depth profiling we observed that the SiO2 layer thickness d formed on top of the Au film strongly depends on the type of substrate material giving most enhanced Si outdiffusion from amorphous ones. The metastable phase AuxSi is detected on both interfaces between the Au/Si and the SiO2/Au layers. A temperature independent ratio, dH/d ≊ 2, is observed for the oxide layer thickness of substrates with and without hydrogen. This is a surprising result, which indicates the influence of nonthermodynamic effects, probably related to the surface structure of the semiconductor substrates and a strong influence of hydrogen. A model of the layer structure is developed, which permits the calculation of diffusion activation energies resulting in qHD ≂ qD ≂ 1.0 (eV) for the diffusing species on substrates with and without hydrogen, respectively.
Examination of the optimization of thin film transistor passivation with hydrogen electron cyclotron resonance plasmas
Authors: Ditizio R. A. , Fonash S. J. , Hseih B.-C.
Published: July 2002

Hydrogen plasmas generated by electron cyclotron resonance currently provide the most efficient plasma exposure technique available for passivating the grain boundaries of polycrystalline silicon. In this report, we show that careful optimization may be required when using this passivation approach on polycrystalline silicon gated, polycrystalline silicon thin film transistors fabricated using low temperature oxides. Optimization is found to be necessary for thin film transistors (TFTs) with polycrystalline Si gates in order to prevent the onset of high leakage currents which can develop when exposures are too long. The effects of exposure time, substrate temperature, microwave power level, pressure, and plasma dilution with an inert gas are examined to determine the conditions for optimal improvement in electrical performance. A model is also presented to explain this need for optimization of the electron cyclotron resonance hydrogen plasma passivation of poly-Si TFTs.
Calculated photocurrents and surface barrier heights
Authors: Aldao C. M. , Palermo A. , Weaver J. H.
Published: July 2002

This article solves the equations that dictate band flattening due to photo-induced electron–hole pair generation in the depletion region of a semiconductor. The calculations show the influence of the barrier height, the optical properties of the substrate, and the minority carrier properties. The calculations are compared to typical temperature-dependent photoemission results obtained after 10 Å of Ge were deposited onto p-GaAs(110). The results show significant band flattening when a lightly doped sample is brightly illuminated, even at room temperature.
Azimuthal and incident angle dependences in the second-harmonic generation from aluminum
Authors: Ying Z. C. , Wang J. , Andronica G. , Yao J.-Q. , Plummer E. W.
Published: July 2002

Optical second-harmonic generation (SHG) from clean Al(111) surface has been investigated using a Nd:YAG laser at 1064 nm. The second-harmonic signal exhibits both isotropic and anisotropic responses with respect to the rotation of the crystal azimuthal angle. The isotropic response can be largely accounted for by a free-electron contribution; the data agree with recent jellium-model calculations. On the other hand, the observation of anisotropic SHG suggests that the surface or bulk band structure of the material plays an important role in SHG and cannot be neglected in the description of SHG even for a simple-metal surface.
Electronic and chemical properties of Li–Au and Cs–Au films on Ru(001)
Authors: Rodriguez J. A. , Hrbek J. , Kuhn M. , Sham T. K.
Published: July 2002

The core and valence levels of Li–Au and Cs–Au films have been studied using synchrotron radiation. For Li–Au, the results of valence-band photoemission show no band gap in the region close to the Fermi level indicating that this compound has metallic properties. In contrast, the valence spectra of CsAu show the typical band gap expected for a highly ionic compound. The formation of alkali-gold bonds induces shifts in the core levels that are not controlled exclusively by changes in the charge of the metal centers. Alloys saturated with Cs or Li show binding-energy shifts of +1.3 or +0.7 eV, respectively, in the Au(4f) levels. LiAu and CsAu compounds are very reactive toward oxygen. The presence of oxygen shifts the Au(4f) levels of the alkali-gold alloys toward lower binding energy. The interaction of O2 with CsAu at 100 K produces O2−2 (the only oxygen species at low O2 exposures) and O−2. Annealing of the O2/CsAu film to 300 K leads to the disappearance of O−2 and the appearance of O2−. The adsorption of O2 on Li–Au alloys at 300 K produces the O−2 ion together with another oxygen species.
Reactive ion etching of SiGe alloys using fluorine-containing plasmas*
Authors: Zhang Ying , Oehrlein Gottlieb S. , de Frésart Edouard , Corbett James W.
Published: July 2002

The dry etching characteristics of strained Si0.85Ge0.15 thin films (≂800 nm thick), elemental Si and Ge in CF4 reactive ion etching (RIE) and electron cyclotron resonance (ECR) plasmas have been studied. The SiGe films were grown on Si substrates by ultrahigh vacuum/chemical vapor deposition. Etch rates were determined by in situ ellipsometry. For RIE, the dependence of the SiGe etch rate on CF4 pressure was studied, the pressure serving as a means to change the ion energy and the ion/neutral ratio of the plasma. In the ECR plasma separate rf biasing of the wafer was used to control the ion energy independent of the production of the plasma. For all conditions, the etch rates of SiGe alloy films are enhanced over the Si etch rates. X-ray photoelectron spectroscopy surface analysis shows that the SiGe surface etched in CF4 is Ge rich. The Ge enrichment increases with decreasing pressure in RIE and increasing rf bias in ECR etching. These results suggest that preferential sputtering is in part responsible for the change in the stoichiometry of the surface.
Photoion detection with a wide dynamic range using a quadrupole mass spectrometer for nonresonant multiphoton ionization of sputtered neutrals
Authors: Maruo T. , Higashi Y. , Tanaka T. , Homma Y.
Published: July 2002

A sputtered neutral mass spectrometer using nonresonant multiphoton ionization by an excimer laser has been developed. The composition of this instrument is similar to the dynamic secondary ion mass spectrometer. Depth profiling with a wide dynamic range can easily be performed by the combination of a quadrupole mass spectrometer and pulse-counting ion detection. The mass spectrum of photoionized neutrals of GaAs shows that the dynamic range of detection is more than six orders of magnitude and that the product of ionization efficiency and mass spectrometer transmission is about 2.1×10−3. The detection limit for Zn impurity in GaAs is under 1 ppm at a laser repetition rate of 50 Hz.
Copper deposition by electron cyclotron resonance plasma
Authors: Holber W. M. , Logan J. S. , Grabarz H. J. , Yeh J. T. C. , Caughman J. B. O. , Sugerman A. , Turene F. E.
Published: July 2002

An electron cyclotron resonance plasma reactor has been built in order to study the filling of high aspect-ratio features on semiconductor devices with metal. The reactor produces a plasma of copper which is nearly 100% ionized at the substrate, without the use of any buffer or carrier gas. The ion flux is dependent on both the feed rate of copper neutrals into the plasma region, and on the microwave power absorbed in the plasma. Solid filling of features having aspect ratios as high as 4.2 is demonstrated, and a simple model is derived to explain the fill characteristics.
New rotating target holder for laser ablation and ion beam sputter deposition of multicomponent and multilayered thin films
Authors: Auciello O. , Emerick J. , Duarte J. , Illingworth A.
Published: July 2002

A versatile ultrahigh vacuum rotating target holder has been designed and constructed for use in ion beam sputter and laser ablation deposition systems for producing multicomponent and/or multilayered thin films. The device features a capability for rotating the targets continuously during exposure to the beams to minimize the development of an undesirable surface topography that otherwise would occur if the targets were still during irradiation. In addition, each individual target holder can be tilted at a predetermined angle. The combined tilting and rotation produces a wobbling movement of the target surface that can be used advantageously to achieve a more uniform distribution of the materials ejected from the targets, which in turn can result in better composition and thickness uniformity of thin films over large area substrates. The device can be baked up to 150 °C depending on the choice of driving motor.
Surfactant-mediated epitaxy of Ge on Si(111): The role of kinetics and characterization of the Ge layers
Authors: Voigtländer B. , Zinner A.
Published: July 2002

Antimony as a surfactant in Ge epitaxy on Si(111) is known to suppress islanding at a growth temperature of 600 °C. We used scanning tunneling microscopy (STM) to study the growth at different temperatures. Only in the temperature range between 550 and 620 °C defect-free layer-by-layer growth is stabilized. At higher temperatures a transition to islanding (Stranski–Krastanov growth) occurs. At lower temperatures the crystal quality of the layer degrades. Together with model experiments of Si/Si(111) homoepitaxy these results show that three-dimensional islanding is suppressed by kinetically limited growth due to reduced diffusion length in surfactant-mediated growth. Several techniques were used to characterize the Ge layers: x-ray diffraction was used to determine the residual strain (0.1%), an upper limit for Sb doping of the Ge was determined by secondary ion mass spectroscopy to nSb<2×1018/cm3. The structural quality of the Ge films was excellent as analyzed by Raman spectroscopy. A STM analysis gives a lower limit for the density of dislocations threading to the surface (ndis<2×107/cm2).
Structural characterization of GaN and GaAsxN1−x grown by electron cyclotron resonance-metalorganic molecular beam epitaxy
Authors: Bharatan S. , Jones K. S. , Abernathy C. R. , Pearton S. J. , Ren F. , Wisk P. W. , Lothian J. R.
Published: July 2002

Electron cyclotron resonance-metalorganic molecular beam epitaxy has been used to deposit GaN and GaAsxN1−x layers on various substrates. This paper will report on the structural characterization of this material, as measured by x-ray diffraction and cross-sectional transmission electron microscopy. GaAsxN1−x layers grown on GaAs appear to be cubic while those grown on GaP are surprisingly hexagonal. The hexagonal phase is also observed under some growth conditions in material grown on GaAs, however, the cubic phase can be obtained by optimizing the parameters which affect the initial nucleation. Conditions such as pre-deposition annealing and growth temperature are critical in determining the phase and crystallinity of the resulting layers. Because of the reduced mismatch between GaN and GaP, the cubic phase of GaN can be more easily nucleated on GaP substrates than on GaAs wafers using similar growth conditions. However, the films under all growth conditions are fine-grained polycrystalline. The impact on film quality of various strain dissipation schemes, such as grading the group V species across the As (or P)–N interface, will also be discussed.
Be-doped GaAs layers grown at a high As/Ga ratio by molecular beam epitaxy
Authors: Zhang D. H. , Radhakrishnan K. , Yoon S. F. , Li H. M.
Published: July 2002

Beryllium doping levels of up to 2.25×1020 cm−3 in GaAs layers grown by molecular beam epitaxy (MBE) have been obtained at a high As/Ga flux ratio. The heavily doped p-type layers have mirrorlike surfaces and their root-mean-square roughness values do not significantly change as the hole concentration is increased. The mobility data show that the quality of the MBE layers is comparable to those prepared by liquid-phase epitaxy and vapor-phase epitaxy. Photoluminescence data confirm their high quality. In addition, the main emission line, attributed to transitions involving Be acceptors on Ga sites, shifts to a lower energy around 1.4628 eV as the hole concentration is increased to 2.25×1020 cm−3, due probably to degeneracy.
Improved quantification in secondary-ion mass spectrometry detecting MCs+ molecular ions
Authors: Gnaser Hubert
Published: July 2002

Molecular secondary ions composed of a sample atom M and a resputtered Cs+ projectile (MCs+) are successfully used for the quantification of secondary-ion mass spectrometry (SIMS) data. For a variety of different specimens it is demonstrated that the yields of these rather ubiquitous species exhibit little or no dependence on sample composition (matrix effect) even in the presence of electronegative elements and are thus well suited for quantitative SIMS evaluation. Specifically, for series of binary and ternary systems (a-Si:H, a-SiGe:H, a-SiC:H, and HCN) composition independent relative sensitivity factors could be established; thus, quantification by means of a single standard is feasible. Furthermore, from the correlation of the measured MCs+ secondary ion intensities relative sensitivity factors (and hence concentrations) can directly be derived, i.e., without any standard. The applicability of this quantification scheme is exemplified for a AlGaAs/GaAs multilayer structure and for the Ga focused ion beam implantation in InP. The results indicate that utilizing MCs+ molecular ions an accurate quantification of major components by SIMS is possible.
Ultraviolet photosulfidation of III–V compound semiconductors for electronic passivation
Authors: Zavadil Kevin R. , Ashby Carol I. H. , Howard Arnold J. , Hammons B. E.
Published: July 2002

A new vacuum-compatible passivation technique for III–V compound semiconductors has been developed. Sulfur passivation of GaAs(100) is produced by ultraviolet photolytic deposition of a sulfide species from vapor phase elemental sulfur. Photoluminescence studies of the photosulfided GaAs reveal a degree of passivation greater than or equal to that produced by conventional (NH4)2S solution treatments. X-ray photoelectron spectroscopy has shown that the sulfur resides on the surface as a single reduced sulfur species, either as sulfide or disulfide, indicating complete fragmentation of the S8 ring by UV light in proximity to the surface. The degree of photosulfidation depends strongly on surface preparation as demonstrated by the described surface oxide removal studies.
Local order measurement in SnGe alloys and monolayer Sn films on Si with reflection electron energy loss spectrometry
Authors: Wong Selmer S. , He Gang , Nikzad Shouleh , Ahn Channing C. , Atwater Harry A.
Published: July 2002

Measurements of local order are demonstrated in Sn-containing alloys and epitaxial monolayer thickness films by analysis of extended-edge energy loss fine structure (EXELFS) data obtained by reflection electron energy loss spectrometry (REELS). These measurements of short-range order provide a complement to the chemical information obtained with REELS and long-range order obtained using reflection high energy electron diffraction. The results suggest that EXELFS measurements are practical for samples mounted on the growth manipulator in a molecular beam epitaxy chamber. Advantages and limitations of reflection EXELFS are discussed.
Microfabricated metal and oxide fibers for biological applications
Authors: Gold J. , Nilsson B. , Kasemo B.
Published: July 2002

Particulates and fibers of ≤10 μm dimensions are important in many human–environment interactions. Two examples are asbestos fibers in the lung and particulate generation from orthopaedic implants. In this article, methods of producing fibers made from evaporated metal and oxide films using microfabrication techniques are described. Two different ‘‘lift-off’’ techniques are discussed in detail. Processing of the fibers allows unprecedented flexibility both in the choice of fiber material as well as control over fiber shape and dimensions. Monodispersed size distributions are obtainable. Fibers of silica, titanium, and gold have been produced and placed into suspensions ready for biological experiments.
Conventional and dynamic actinometry of discharges of hydrocarbon–oxygen–argon mixtures
Authors: Durrant Steven F. , Bica de Moraes Mário A.
Published: July 2002

Actinometric optical emission spectroscopy was used in the investigation of discharges of mixtures of C2H2, O2, and Ar to determine relative concentrations of the species CH, OH, H, CO, and O, which all increase with increasing proportions of oxygen in the plasma feed, Rox. The mean electron energy E and the mean electron density ρ were probed using Ar and He as actinometers. Additional measurements of the ratio of the intensities of the Hα to Hβ lines, I(Hα)/I(Hβ), showed that E was not strongly dependent on Rox. In contrast, ρ was found to fall with increasing Rox. A dynamic form of actinometry was also used to determine temporal trends in the relative concentrations of the above-mentioned species following cutting of either the O2 or C2H2 flows. On the basis of these data it is concluded that reactions involving oxygen play a major role in the production of the species CH, H, and CO. Plasma–polymer surface reactions contribute significantly to the production of the species H, CH, CO, and OH.
In situ diagnostic for etch uniformity
Authors: Buie M. J. , Pender J. T. , Soniker J. , Brake M. L. , Elta M.
Published: July 2002

Spatially resolved optical imaging experiments were performed in a parallel plate reactor known as the Gaseous Electronics Conference Reference Cell. They consisted of recording the optical emission discharges at various pressures and powers for 1024 points across the electrode at a height of 7 mm above the bottom powered electrode for the argon neutral line at 750.4 nm. Previously, our experiments [Pender et al., J. Appl. Phys. 74, 3590 (1993)] have shown that argon plasmas are nonuniform, specifically in the shape of annular rings with large intensities originating over the edges of the electrodes. In this work, we examined in situ the plasma uniformity as a single 3 inch silicon wafer with a simple test pattern was etched in a 30 sccm CF4 / 15 sccm Ar plasma environment. The results indicate that the plasma emissivity and etch depth are related. In the pressure range of 50 to 250 mTorr, the emissivity and the etch depth increase linearly with power. For a constant power, both emissivity and etch depth can be fitted to a quadratic polynomial with pressure as the dependent variable.
A scanning tunneling microscopy study of hydrogen adsorption on Si(112)
Authors: Baski A. A. , Whitman L. J.
Published: July 2002

The adsorption of atomic hydrogen on Si(112) has been investigated using scanning tunneling microscopy. The clean (112) surface is reconstructed into quasiperiodic nanofacets [each composed of a (111) and (337) terrace], and we find that atomic hydrogen does not remove this nanofacet structure at room temperature or 430 °C. This result demonstrates the surprising stability of the facet reconstruction.
Sensor for measuring the atomic fraction in highly dissociated hydrogen
Authors: Gardner W. L.
Published: July 2002

Atomic hydrogen is a very important constituent for a number of important processes. These include the removal of oxides from GaAs, the effective cleaning of oxides and hydrocarbons from Ge and Si surfaces, the chemical passivation and surface reconstruction of cleaned Si surfaces, the annealing of amorphous silicon, and the deposition of diamond films. Because the usual techniques for measuring the atomic fraction are either expensive and cumbersome to use or unsuitable for application to highly dissociated hydrogen, a specially designed sensor was developed. Sensor design is based on a diffusion tube with noncatalytic walls, having one end open to the atom source and a catalytic closure at the other end. The sensor is simple and inexpensive to fabricate, and determining atom density is straightforward. Sensor design also inhibits thermal runaway, which occurs when the atom density is high enough to impart enough recombination energy to the noncatalytic surface to substantially raise its temperature. While recombination coefficients for such surfaces are very low near room temperature, they increase nearly exponentially with temperature unless actively cooled. With the use of a straightforward calibration scheme to determine the variation in species fraction along the diffusion tube, the atomic fraction at the tube opening is determined. Design strategy, implementation considerations, and calibration method are presented. In addition, data obtained from an atomic hydrogen source are compared to relevant published data.
Study of surface reactions during plasma enhanced chemical vapor deposition of SiO2 from SiH4, O2, and Ar plasma
Authors: Han Sang M. , Aydil Eray S.
Published: July 2002

In situ attenuated total reflection Fourier transform infrared spectroscopy was employed in proposing possible surface reaction mechanisms during plasma enhanced chemical vapor deposition of SiO2 from a mixture of SiH4, O2, and Ar in a helical resonator plasma reactor. Infrared spectra taken during the oxide deposition revealed that the oxide surface is covered with OH when the deposition is conducted with a low SiH4 to O2 ratio (≪1). Both associated hydroxyls (SiOHassoc) and isolated hydroxyls (SiOHisol) were detected on the surface during growth. Two kinds of OH attached to Si exist on the surface: weakly bound OH that thermally desorb at 250 °C and strongly bound OH that desorb under ion bombardment. The thermal removal of weakly bound OH does not lead to SiO2 formation, but the ion-assisted removal of strongly bound OH by Ar+ ion bombardment results in oxide growth via the reaction 2SiOH(s)+Ar+→Si–O–Si(s)+H2O(g)+Ar+. Experiments undertaken to delineate the possible heterogeneous reaction paths between surface species and gas phase reactants showed that the molecular fragments of SiH4 (SiHx, x=1, 2, or 3) react with surface hydroxyl groups and produce surface hydrides such as HSiO3, H2SiO2, and H3SiO. The silane fragments also react with Si on the surface to give silicon hydrides whose Si constituent is backbonded to other Si atoms in addition to O [e.g., HSi(SiO2), H2Si(SiO), HSi(Si2O), H3SiSi, and H2SiSi2]. The latter reactions become more important as the OH on the surface is depleted, and the surface becomes silicon and silicon hydride rich. A subsequent exposure of the silicon- hydride-covered oxide surface to oxygen plasma creates SiOHassoc groups presumably by insertion of O into Si–H bonds of surface hydrides. The silicon hydrides are not observed during deposition using low SiH4 to O2 flow rate ratios (≪1): silane fragments, upon adsorbing onto the surface, are promptly oxidized by O atoms, leading to a SiOH-covered surface at steady state conditions. Bombardment of the surface by ions such as Ar+ and O+2 assists H2O desorption from SiOH forming Si–O–Si bonds.
Schiff base precursor compounds for the chemical beam epitaxy of oxide thin films. I. Deposition of CuO on MgO[001] using copper (II) bis(benzoylacetone)-ethylendiimine
Authors: Fritsch E. , Mächler E. , Arrouy F. , Berke H. , Povey I. , Willmott P. R. , Locquet J.-P.
Published: July 2002

A new precursor compound for the deposition of copper oxide thin films under molecular beam conditions, copper bis(benzoylacetone)-ethylenediimine, has been characterized by thermal analysis and in situ mass spectrometry. Its stability and decomposition behavior are reported as well as its use for the deposition of epitaxial copper oxide thin films on MgO.
Scanning probe anodization: Nanolithography using thin films of anodically oxidizable materials as resists
Authors: Sugimura Hiroyuki , Nakagiri Nobuyuki
Published: July 2002

We report scanning probe microscope (SPM) nanolithography that uses thin films patternable by scanning probe anodization as resists. An organosilane monolayer composed of trimethylsilyl [−Si(CH3)3] groups was prepared by chemical vapor deposition on a Si substrate. It was then patterned through the localized degradation of the monolayer as a result of anodic reactions induced beneath a SPM tip. The fabricated patterns on the resist were transferred into the substrate Si by area-selective chemical etching in an aqueous NH4F/H2O2 solution. Consequently, grooves narrower than 100 nm were successfully fabricated. We have also developed another resist system for SPM lithography that is compatible with a wide range of surfaces, including insulators. The resist consisted of a thin Ti film covered with a fluoroalkylsilane monolayer. The monolayer was degraded along a tip-scanning trace. This patterned resist was etched in a dilute HF solution. The chemical etching of the underlying Ti film proceeded selectively in the region where the monolayer had been degraded, and the pattern was transferred into the Ti resist film.
Temperature dependent compensation of Zn-dopant atoms by vacancies in III–V semiconductor surfaces
Authors: Ebert Ph. , Heinrich M. , Urban K. , Chao K.-J. , Smith A. R. , Shih C. K.
Published: July 2002

Scanning tunneling microscopy is used to study the compensation of Zn-dopant atoms by surface vacancies on InP and GaAs(110) surfaces as a function of the annealing time and temperature. An attractive interaction is observed between negatively charged Zn atoms and positively charged anion vacancies. Uncharged Zn-vacancy defect complexes are formed and their structure is analyzed. The concentration of the complexes increases with the vacancy concentration. Simultaneously the concentration of charged Zn decreases. The total observable Zn concentration is found to be the sum of the concentration of isolated charged Zn atoms and of the concentration of the defect complexes. This sum is constant at low temperatures, but decreases at 480 K. The observations are explained by the formation of subsurface defect complexes at higher temperatures and surface complexes at low temperatures.
Internal photoinjection and deep level luminescence at ZnSe/GaAs heterointerfaces
Authors: Yang X. , Brillson L. J. , Raisanen A. D. , Vanzetti L. , Bonanni A. , Franciosi A.
Published: July 2002

We have used photoluminescence spectroscopy to measure the relative intensity of the ZnSe and GaAs near band edge (NBE) emissions at ZnSe/GaAs interfaces fabricated by molecular beam epitaxy under different growth conditions. Using a HeCd laser to excite free electron-hole pairs near the buried heterointerface, we found systematic variations in the NBE relative intensity as a function of the Zn to Se beam pressure ratio used to grow the ZnSe epilayers. The results are in good agreement with the predictions of a simple expression for free carrier injection across the interface due to internal photoemission. In such a process, laser excitation results in free carriers which can move from GaAs to ZnSe over barriers due to the heterojunction band offsets and recombine across the band gap or via deep levels. The variation of such internal photoemission with different growth conditions supports the previously reported dependence of the ZnSe/GaAs band offset on the local interface composition.
Preparation and microstructural study of CeO2 thin films
Authors: Tian Chunyan , Du Yang , Chan Siu-Wai
Published: July 2002

Epitaxial and textured ceria films were prepared by conventional electron-beam evaporation on (001) LaAlO3, (11̄02) R-cut sapphire, (001) Au, and (001) Pd substrates using ceramic and metallic sources. Deposition temperature and deposition rate were varied to determine their effects on the formation of the CeO2 films. It was also found that the epitaxial formation of CeO2 films varied with different substrates and evaporation sources. Ceria films grew epitaxially on sapphire with a deposition rate less than 0.5 Å/s in the temperature range of 650–750 °C using either ceramic or metallic source. The epitaxial growth of CeO2 on LaAlO3 occurred at deposition rates as high as 1.0 Å/s at a deposition temperature as low as 600 °C using a metallic source, while on the (001) Au substrate, only a textured structure was observed between 580 and 700 °C and at deposition rates from 0.3 to 1.0 Å/s. On a (001) Pd substrate, the deposited CeO2 films showed a different microstructure corresponding to ceramic and metallic sources at deposition rates of 0.2–0.5 Å/s and from 700 to 750 °C. The epitaxial ceria films were achieved using a ceramic source, while the polycrystalline ceria films of multiple twinning were obtained using the metallic source. The epitaxial growth temperature of CeO2 on these substrates was empirically found to be around 700 °C (i.e., 0.35 Tm).
Nanometer scale patterning of C60 multilayers using molecular manipulation
Authors: Dunn A. W. , Moriarty P. , Beton P. H.
Published: July 2002

We show that the tip of a scanning tunneling microscope can be used to manipulate large groups of molecules and form lines in C60 multilayers that have widths of order 10 nm and lengths up to 1 μm. This modification is achieved by first moving the tip towards the surface and then sweeping it across a predetermined distance. This causes the second and higher layers of adsorbed C60 to accumulate in ordered islands that are several layers high and leaves exposed the first C60 layer, which is partially ordered in a hexagonal arrangement. Our results show that the first adsorbed C60 layer bonds strongly to the Si(111)–7×7 surface but that higher layers are bound much more weakly and can be routinely modified.
Controlled growth of WO3 films
Authors: LeGore L. J. , Greenwood O. D. , Paulus J. W. , Frankel D. J. , Lad R. J.
Published: July 2002

We have used reactive rf magnetron sputtering of a tungsten target in Ar/O2 mixtures and direct electron-beam evaporation of WO3 pellets with and without the presence of an electron cyclotron resonance (ECR) oxygen plasma to grow WO3 films on α-Al2O3(101̄2) single-crystal substrates (r-cut sapphire). The WO3 films exhibit a range of microstructures depending on deposition conditions. Using any of the deposition methods, the films are amorphous when grown at room temperature. Postdeposition annealing in O2 induces the formation of a random polycrystalline microstructure and an increased surface roughness. Growth of crystalline WO3 films can be achieved at deposition temperatures above 200 °C. During electron-beam evaporation of WO3 at 600 °C, reflection high-energy electron diffraction observations indicate that a tetragonal phase of WO3 grows epitaxially on r-cut sapphire with the (100) tetragonal plane coincident with the rectangular mesh of the r-cut sapphire substrate. Deposition of WO3 using plasma species generated by either a rf magnetron or ECR source promotes the formation of some nuclei of an orthorhombic and/or monoclinic phase coexisting with the tetragonal phase; the nearly identical lattice parameters make the orthorhombic and monoclinic phases difficult to distinguish. Compared to the polycrystalline films, the epitaxial films exhibit both a higher base-line conductivity and a larger conductivity change upon exposure to H2S gas.
Axially-resolved study of highly ionized physical vapor deposition
Authors: Dickson M. , Hopwood J.
Published: July 2002

The evolution of the ionization of sputtered aluminum is examined in an inductively coupled plasma. The experimental configuration consists of a conventional magnetron with an immersed, rf induction coil positioned between the target and the downstream substrate. Measurement of the number of aluminum atoms and ions as a function of distance from a magnetron target reveals a decreasing neutral density and a nearly constant ion density. The neutral metal density is well described by a simple diffusion model with a source term near the target while the metal ion density is approximately that of a diffusion-dominated, volume-generated plasma. These two spatial distributions result in a monotonically increasing ionization fraction versus distance from the target. Evidence suggests that the ionized physical vapor deposition method requires that sputtered species be nearly thermalized before ionization is probable.
Epitaxial growth of 3C–SiC on Si(100) by pulsed supersonic free jets of Si(CH3)4 and Si3H8
Authors: Ikoma Y. , Endo T. , Watanabe F. , Motooka T.
Published: July 2002

Heteroepitaxial growth of 3C–SiC on Si(100) by pulsed supersonic free jets of Si(CH3)4 and Si3H8 with various mixture ratios has been investigated. The heteroepitaxy is achieved at the substrate temperature of 900 °C without any carbonization process. The films grown by pure Si(CH3)4 contain inverse pyramidal pits surrounded by the {111} planes of Si, while {311} faceted pits are formed by mixing Si(CH3)4 with a small amount of Si3H8. When the Si3H8/Si(CH3)4 ratio further increases, pit formation is suppressed and instead Si islands are epitaxially grown from the pit region.
Effect of prepared GaAs surface on the sulfidation with (NH4)2Sx solution
Authors: Kang Min-Gu , Park Hyung-Ho
Published: July 2002

Surface properties of wet-cleaned or successively passivated GaAs with (NH4)2Sx solution were analyzed using x-ray photoelectron spectroscopy. All the treatments were carried out in a glove box under nitrogen controlled atmosphere. Every cleaning process with HCl or H3PO4 solution produced elemental As and the amount depends on the etching capability of acid to GaAs. Successive sulfidation treatment resulted in the formation of an As–S bond, and the observed quantity of the arsenic sulfide was closely related to the amount of elemental As. In particular, the As–S was hardly observed in the sulfidation treated GaAs surface after NH4OH treatment because elemental As had not been produced by the NH4OH treatment. The in situ annealed GaAs surface at 600 °C contained a large amount of elemental Ga and showed an effective formation of Ga–S after the sulfidation treatment. Therefore, it could be assured that the elemental forms of As and Ga bind with S through the sulfidation treatment using a (NH4)2Sx solution.
Reactive ion etching for mesa structuring in HgCdTe
Authors: Smith E. P. G. , Musca C. A. , Redfern D. A. , Dell J. M. , Faraone L.
Published: July 2002

Both wet chemical and dry plasma etching techniques have been investigated for mesa structuring in n- and p-type HgCdTe. Scanning electron microscopy (SEM) confirms the isotropic nature of a bromine-based wet chemical etching solution, and the anisotropic profile that results from reactive ion etching. Laser-beam-induced-current (LBIC) measurements reveal no significant modifications to the electrical properties for chemically etched HgCdTe material, but clearly demonstrate a p- to n-type conversion in p-type samples and n+ doping in n-type samples for reactive ion etching (RIE) (processing conditions: 400 mTorr, CH4/5H2, 0.4 W/cm2). LBIC measurements following low-temperature (200 °C) mercury annealing of RIE-processed samples indicate the full restoration of electrical properties to that of the initial as-grown wafers, thus preserving the beneficial aspects of RIE for anisotropic mesa structuring in HgCdTe.
Quantitative analysis of sputtered α- and α+β-brass surfaces by using Auger electron spectroscopy with principal component analysis-target factor analysis
Authors: Hammer G. E.
Published: July 2002

Previous quantitative analyses of ion sputtered brass alloy surfaces have indicated preferential sputtering of the zinc with a possible effect of crystalline structure in going from the pure face-centered-cubic α phase to the mixed α plus body-centered-cubic β phase. The application of principal component analysis-target factor analysis allows the use of the more surface sensitive MVV Auger transitions despite the complete overlap of the MVV peaks from copper and zinc. Quantitative analysis of these signals confirms the occurrence of preferential sputtering, including the phase dependence. Specifically, the zinc/copper concentration ratio from the surface vs the bulk shows a discontinuity as the bulk composition changes from the α to the α+β phase.
Analysis of alkali elements in insulators using a CAMECA IMS-6f
Authors: McKinley J. M. , Stevie F. A. , Granger C. N. , Renard D.
Published: July 2002

Secondary ion mass spectroscopic analysis of insulators is difficult because charging of the sample affects the trajectories of primary and secondary ions. Analysis of alkali elements in insulators is additionally complicated by the mobility of the species of interest under the influence of an electric field. Magnetic sector analyzers also present difficulties in providing charge neutralization because of the high electric field required to inject ions into the analyzer. Coating of the sample with a conductive film or grid has often been needed to provide a meaningful analysis. However, coating the sample adds a contaminant source that complicates alkali analysis, especially at the concentrations of interest to the semiconductor industry. In this article we provide, for a magnetic sector instrument, a method that requires no coating or sample preparation prior to analysis. If the electron penetration depth is matched to the thickness of the oxide layer, results show that SiO2 layers from 0.3 to 1.36 μm thick can be easily and repeatably analyzed for alkali elements using this approach.
Plasma-assisted reduction of carbon dioxide in the gas phase
Authors: Maya L.
Published: July 2002

The reduction of carbon dioxide by hydrogen, which constitutes the reverse water-gas shift reaction, is an active area of research because of its connection with the production of methanol and other fuels. Representative references are given, that have been reported in the catalysis literature where most of this research is described. In contrast with this, studies dealing with the plasma-assisted reduction of carbon dioxide, as a subset of the subject, are rather limited. A variety of products such as diamond, oxalic acid or fuel species have been obtained depending on the conditions. The present study was undertaken to explore the possibility of obtaining formic acid through the plasma-assisted reduction of carbon dioxide given the precedent that even a more complex molecule such as oxalic has been obtained. The production of formic acid was not anticipated to take place cleanly given the mechanistic complexity of such a process. The study was conducted nevertheless to seek an alternative to an electrochemical pathway to reduce carbon dioxide that has obvious shortcomings because of the requirement to dissolve the gas in a solvent, in addition to the limited concentration of reactant and products that might be obtained. Formic acid in the form of formate is a component of a cycle conceived to trap tritium from contaminated ground water that uses carbon dioxide from a selective oxidation step and hydrogen/tritium from the electrochemical reduction of the contaminated water. The electrochemical oxidation of formate is catalyzed by means of terpyridine bipyridine oxo ruthenium (IV), a complex that shows remarkable isotope effects so that tritiated formate is selectively enriched and may be separated by ion exchange.