bluuebunny/crossref_metadata_2025 huggingface dataset:
‘‘Oriented film growth,’’ not ‘‘epitaxy’’ in HTSC film growth
Authors: Roy Rustum , Guo Ruyan , Bhalla A. S. , Cross L. E.
Published: July 2002

A survey of the literature reveals that the following materials have been used as the substrates for ‘‘epitaxial’’ deposition of YBCO high Tc superconducting (HTSC) thin films: SrTiO3, ZrO2 (or YSZ), MgO, LaAlO3, Si and Al2O3 (with or without buffer layers), SiO2, Pt, MgAl2O4, even noncrystalline substrates. An analysis of the structural relations between the substrate and HTSC phase is presented. It is obvious that the term ‘‘epitaxy’’ is misapplied in many of these cases. We analyze the evidence for actual crystal structural control from the substrate influencing the oriented growth of the desired phase and find it nonexistent in many cases, very weak in others, and persuasive in a few cases. For the superconductor quality what matters is the quality of the HTSC film, not what it is sitting on. This quality can be quantified in four steps, the first three describing the degree of orientation in none (random-orientation), one (growth direction), or two (in film plane) crystallographic directions. The fourth is the degree of continuity of the film or absence of grain boundaries (i.e., a 100% perfectly oriented set of separated islands with grain boundaries will clearly be considerably poorer than a continuous (single) phase). We propose that instead of the term epitaxial one should focus only on the degree of orientation and continuity of the ‘‘oriented film.’’ The phase immediately contiguous to the film (including many so-called buffer layers) should be designated substrate. Any phase below such a substrate is structurally and chemically insignificant (to the YBCO growth), although it may be necessary to obtain the oriented ‘‘buffer layer.’’ Such phases should be designated as carriers, not substrates.
Quantum size effects in ultrathin Cu(100) films
Authors: Mankey G. J. , Willis R. F. , Ortega J. E. , Himpsel F. J.
Published: July 2002

We report an angle resolved photoemission study of quantum size effects observed in the electronic structure of pseudomorphic Cu(100) films. The 3d band of a one-layer film exhibits an 80% reduction in its perpendicular dispersion and a 10% reduction in its parallel dispersion. The perpendicular dispersion of the 3d band evolves rapidly with increasing film thickness, since the 3d electrons are strongly localized on the ion cores. Quantum-well resonances are produced due to the discretization of the perpendicular wave vector in the s,p band. They produce oscillations in the Fermi level intensity as they disperse through it when the film thickness is increased. The observed oscillation period of approximately six layers matches that of oscillations in the magnetic exchange coupling between ferromagnetic Co films separated by Cu spacer layers. The connection of the quantum-well states to bulk electronic structure is determined by comparing oscillation amplitudes for two different photon energies.
Brass-rubber adhesive interphase investigated via depth profiling by using Auger electron spectroscopy
Authors: Hammer G. E. , Shemenski R. M. , Hunt J. D.
Published: July 2002

The adhesive interphase formed between brass and sulfur crosslinked rubber is known to be a complex layer of metal oxides, sulfides, and rubber. Hostile aging of this system produces changes in the structure, morphology, thickness, and mechanical properties of this layer. In this work it has been demonstrated that the overall thickness of the sulfide film can be used to characterize the degradation of the adhesive layer. The thickness of the sulfide film was measured from depth profiles of the brass-plated steel surface obtained by using Auger electron spectroscopy with ion sputter etching following cryogenic fracture of the bond. Aging under several conditions indicated an inverse relationship between adhesion and interfacial sulfide thickness, with magnitude dependent upon properties of the rubber compound formulation.
Damage to thin gate oxide during lightly doped drain spacer oxide etching
Authors: Gabriel Calvin T. , Weling Milind G.
Published: July 2002

Charge buildup damage to gate oxide during lightly doped drain (LDD) spacer oxide etch was studied by measuring gate leakage of fully processed transistors over thin oxide attached to area-intensive or edge-intensive polycide antenna structures over thick oxide. Wafers were etched at the LDD spacer oxide etch step for various overetch times ranging from −7% to +35%. It was found that gate oxide damage from charge buildup depended strongly on spacer overetch time. No charge buildup was detected before endpoint (defined as the time required to completely remove oxide from flat silicon surfaces—the polycide has a cap oxide layer on it, so the gate electrode is not exposed at endpoint), but significant charge buildup occurred through the polycide surface when etching was continued past endpoint. Gate leakage failures for transistors attached to large area-intensive antennas increased monotonically with overetch time, leading to 100% failure after 20%–30% overetch. This charge buildup damage began to occur even before the cap oxide was completely removed and the gate electrode was exposed to the plasma, suggesting that either the tetraethylorthosilicate oxide itself was not perfectly insulating or it became leaky after exposure to this high-energy oxide etch plasma with a dc bias of about −600 V.
Raman spectroscopic study of the formation of t-MoSi2 from Mo/Si multilayers
Authors: Cai M. , Allred D. D. , Reyes-Mena A.
Published: July 2002

We have used Raman spectroscopy, large- and small-angle x-ray diffraction spectroscopy of sputter-deposited, vacuum-annealed, soft x-ray Mo/Si thin-film multilayers to study the physics of silicide formation. Two sets of multilayer samples with d-spacing 8.4 and 2.0 nm have been studied. Annealing at temperatures above 800 °C causes a gradual formation of amorphous MoSi2 interfaces between the Si and Mo layers. The transition from amorphous to crystalline MoSi2 is abrupt. The experimental results indicate that nucleation is the dominant process for the early stage and crystallization is the dominant process after nucleation is well advanced. In the thicker multilayer, a portion of the silicon crystallizes during annealing and a strong Raman signal is observed. An advantage of Raman spectroscopy is that the Raman signal of the silicide is observed even before the presence of MoSi2 can be seen using x-ray diffraction. This study indicates that Raman spectroscopy is an effective technique for characterizing the formation of crystalline silicides.
Study of the optical constants determination of thin films: Dependence on theoretical assumptions
Authors: Bueno R. M. , Trigo J. F. , Martínez-Duart J. M. , Elizalde E. , Sanz J. M.
Published: July 2002

We present a critical study of the known reflectance (R) and transmittance (T) method to extract the optical constants of thin film–substrate systems. In order to study the effect of the usual assumptions (homogeneous layers, semi-infinite substrate, normal incidence angle for reflectance measurement, etc.) in the loss of solution during the numerical inversion process, we have developed a theoretical model that overcomes these approximations. Simulated film–substrate systems as well as RT measurements from a ZnSe film onto a CaF2 substrate have been used to show how accurate film thickness and optical constants can be obtained using a more complete optical model.
Characterization of aluminum based oxide layers formed by microwave plasma
Authors: Katz-Tsameret Zivit , Raveh Avi
Published: July 2002

Polycrystalline, commercially pure, aluminum (1100-Al) and an AlZnMgCu alloy (7075-Al) were plasma oxidized in a microwave 2.45 GHz glow discharge. The resulting surface oxide layers were characterized by Auger electron spectroscopy, by x-ray photoelectron spectroscopy, and by Fourier transform infrared spectroscopy. In 1100-Al the surface layer was γ-Al2O3 oxide, followed by an Al–O interdiffusion layer, while magnesium oxide was the dominant phase in Al-7075. Furthermore, the 200 nm layer of Mg–O acted as an oxygen diffusion barrier and reduced the Al–O interdiffusion layer thickness. The compositions and the structures of the oxide layers are presented and discussed further including the effect of microalloying on the layers’ properties, providing some rationale for the current results.
Evidence of specific and nonspecific adsorption of perchlorate (ClO4) on silver(110)
Authors: Krasnopoler A. , Stuve E. M.
Published: July 2002

The coadsorption of ClO4 with H2O on Ag(110) was studied as a model of anion adsorption at the solid–liquid interface. Thermal desorption spectroscopy, low energy electron diffraction, and high resolution electron energy loss spectroscopy were used to investigate the interactions between perchlorate, water and the metal surface. The vibrational spectrum of adsorbed ClO4 is an indicator of its adsorption geometry. For ClO4 adsorbed on the clean surface, losses at 640, 915, 1020, and 1220 cm−1 indicate that adsorption occurs through three oxygen atoms (tridentate) with a symmetry of C3v or lower. When coadsorbed with H2O, the vibrational spectrum resembles that of fully solvated, tetrahedral ClO4 (Td symmetry) with bands at 610, 880, and 1090 cm−1. The increase in symmetry indicates that direct interaction with the metal is lost and is considered evidence for nonspecifically adsorbed ClO4 in ultrahigh vacuum. The thermodynamic feasibility of this process is also discussed.
Recombination limited outgassing of stainless steel
Authors: Moore Boude C.
Published: July 2002

Those who need a better vacuum, either in existing systems or for new construction, should be interested in this discussion, since there is at present no method of analysis to predict the bake time and temperature required to achieve a specific low outgassing rate from stainless steel. It has been known for a generation that atomic hydrogen can be baked out of stainless steel to achieve an outgassing rate of 10−12 Torr ℓ/(s cm2). The time and temperature required are in reasonable agreement with measured diffusion constants and diffusion theory. However, the diffusion theory completely fails when predicting lower outgassing rates. The time and temperature required greatly exceed those predicted. It was observed two generations ago that at low pressures the permeation of hydrogen through metals is no longer a function of the square root of pressure, but approaches zero. This has been attributed to surface effects, specifically including the recombination limit; the atomic hydrogen outgassed to the surface must recombine into molecular hydrogen before it can escape into the vacuum. The recombination rate is a function of the square of the surface coverage by atomic hydrogen. The concentration at the surface cannot be zero as assumed in conventional diffusion theory, but must remain at finite levels consistent with the outgassing rate. In this article a finite-difference analysis is made of a reported 2 h bake at 950 °C; the diffusion of hydrogen to the surface is modified to include a recombination limit. The outgassing rates with time are found together with the concentration profiles in the steel. The recombination coefficient is adjusted to match the observed postbake outgassing rate. It is concluded that the addition of the recombination limit to diffusion theory may be a viable analysis method to predict the results of specific time and temperature bake patterns.
Reactivities of carbon- and nitrogen-modified Mo(110): A comparison of modification effects by surface and interstitial adatoms
Authors: Frühberger B. , Chen J. G. , Eng J. , Bent B. E.
Published: July 2002

The surface properties of carbon- and nitrogen-modified Mo(110) have been investigated using high-resolution electron energy loss spectroscopy, near-edge x-ray absorption fine structure, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and temperature programmed desorption. Exposure of the Mo(110) surface to olefin molecules at 600 K resulted primarily in surface carbon atoms; subsequent annealing to 1200 K produced interstitial/subsurface carbon atoms. Higher concentrations of carbon atoms in the interstitial/subsurface regions could be achieved by repeated dosing/annealing cycles. Exposure to either N2 or NH3 led to the deposition of surface nitrogen atoms. Subsequent annealing of such surfaces resulted in N2 desorption without any detectable production of interstitial/subsurface nitrogen atoms. Reactions with ethylene or cyclopentene as probing molecules demonstrated that the reactivities of carbon- and nitrogen-modified Mo(110) depend strongly on the location of the adatoms. For example, Mo(110) surfaces modified primarily by surface carbon and nitrogen atoms are inert toward the dissociation of either ethylene or cyclopentene. In contrast, carbon-modified Mo(110) with interstitial/subsurface carbon atoms remains active toward the decomposition of these two molecules, although the decomposition pathways are altered from those on the clean Mo(110) surface.
Adsorption controlled Si(1−x)Gex growth during chemical vapor deposition
Authors: Kühne H. , Fischer A. , Morgenstern Th. , Zaumseil P.
Published: July 2002

The present investigation concerns Si(1−x)Gex thin film growth that, at certain deposition conditions, is characterized by nonlinearity in the partial growth rate of germanium versus the increase of the germane (GeH4) content in the reaction gas mixture. A linear increase had previously been understood as caused by a reaction rate limited growth mechanism. The nonlinear increase of the partial growth rate of germanium versus the GeH4 partial pressure will be explained by means of an adsorption controlled growth mechanism based on competitive adsorption of both silane and germane. Transition from one to the other growth mechanism is possible as shown by the discussion of corresponding experimental growth results.
Mechanistic studies of the thermal decomposition of metal carbonyls on Ni(100) surfaces in connection with chemical vapor deposition processes
Authors: Xu Mingde , Zaera Francisco
Published: July 2002

The thermal decomposition of Fe(CO)5, Cr(CO)6, Mo(CO)6, and W(CO)6 on Ni(100) surfaces and under ultrahigh vacuum conditions was studied by using temperature programmed desorption and x-ray photoelectron spectroscopies. The initial adsorption of those metal carbonyls is mostly molecular at low temperatures, but complete decarbonylation to the naked metal takes place in all cases upon thermal activation. Experiments with coadsorbed isotopically labeled 13CO provided indirect evidence for a stepwise mechanism for Fe(CO)5 which may include the formation of tetra- and tricarbonyl intermediates on the surface. For Cr(CO)6, Mo(CO)6, and W(CO)6, on the other hand, complete decomposition occurs in a narrow range of temperature, and no intermediate could be isolated on the surface. The deposition of metal films via metal carbonyl activation was studied under steady state conditions as well. Continuous deposition was seen at substrate temperatures as low as 300 K, but the grown films were found to incorporate both carbon and oxygen under most conditions tested and to change their morphology depending on the substrate temperature during deposition.
A simulation model for thickness profile of the film deposited using planar circular type magnetron sputtering sources
Authors: Hong Seungbum , Kim Eunah , Bae Byeong-Soo , No Kwangsoo , Lim Sung-Chul , Woo Sang-Gyun , Koh Young-Bum
Published: July 2002

The thickness profile of the film deposited by planar circular-type magnetron is simulated considering the relationship between magnetic field profile and target erosion rate. The model is confirmed by the measurement of the film thickness profiles of the Cr films deposited in this study. It is found that the model is applicable to the magnetron sputtering process at low gas pressure. Furthermore, it can be used to predict the thickness profile of the films deposited by magnetron sputtering with various shapes of magnets.
LPCVD silicon-rich silicon nitride films for applications in micromechanics, studied with statistical experimental design*
Authors: Gardeniers J. G. E. , Tilmans H. A. C. , Visser C. C. G.
Published: July 2002

A systematic investigation of the influence of the process parameters temperature, pressure, total gas flow, and SiH2Cl2:NH3 gas flow ratio on the residual stress, the refractive index, and its nonuniformity across a wafer, the growth rate, the film thickness nonuniformity across a wafer, and the Si/N incorporation ratio of low pressure chemical vapor deposition SixNy films has been performed. As a tool for complete characterization of the property-deposition parameter relations, a full factorial experimental design was used to determine the dominant process parameters and their interactions. From this study it could be concluded that, in decreasing order of importance, the gas flow ratio of Si and N containing precursors, temperature, and pressure are the most relevant parameters determining the mechanical and optical properties of the films and the deposition rate and nonuniformity in film properties across a wafer. The established relations between properties and deposition parameters were fitted with physical–chemical models, including a film growth model based on a Freundlich adsorption isotherm. The optimal deposition conditions for films to be used in micromechanical devices will be discussed.
Etch-stop characteristics of Sc2O3 and HfO2 films for multilayer dielectric grating applications
Authors: Britten J. A. , Nguyen H. T. , Falabella S. F. , Shore B. W. , Perry M. D. , Raguin D. H.
Published: July 2002

High-efficiency, high laser-damage threshold reflection gratings can be made by etching a grating structure into the top layer of a multilayer high-reflectivity stack. Spatial efficiency and wave-front uniformity can be maximized by taking advantage of the etch-stop properties of the layer underneath the top layer to be etched. The etch-stop layer must have a high optical damage resistance, which places severe restrictions on available materials. The etch characteristics of HfO2 and Sc2O3, two materials commonly used in high laser-damage optical coatings, have been evaluated. The etch rate selectivities of e-beam evaporated SiO2/HfO2/Sc2O3 thin films in a reactive ion etching system optimized for SiO2 etching are approximately 100/10/1. Gratings etched to a HfO2 etch-stop layer suffered from deposition of fluorinated Hf compounds on the SiO2 grating sidewalls, but sidewalls were clean when Sc2O3 was used as the etch-stop layer.
Properties of ZrO2 films on sapphire prepared by electron cyclotron resonance oxygen-plasma-assisted deposition
Authors: Moulzolf S. C. , Yu Yan , Frankel D. J. , Lad R. J.
Published: July 2002

Ultrathin films of ZrO2 have been grown on r-cut sapphire [α-Al2O3(101̄2)] substrates in the temperature range 50 °C<T<650 °C via electron beam evaporation of Zr in the presence of an oxygen plasma generated by an electron cyclotron resonance source. Reflection high-energy electron diffraction observations during deposition above 475 °C at growth rates of 0.5 Å/s indicate that an epitaxial cubic ZrO2 phase grows without the addition of stabilizing dopants. The epitaxial orientation is (001)c-ZrO2∥(101̄2)r-sapphire with [100]c-ZrO2∥[12̄10]r-sapphire. Above a critical thickness of 40 nm, monoclinic phase ZrO2 nucleates and coexists with the cubic ZrO2 phase. At faster growth rates (above 2 Å/s), epitaxial single phase monoclinic ZrO2 is produced with the same epitaxial orientation as the cubic phase. For deposition below 250 °C, the ZrO2 films are amorphous. All of these films are stoichiometric ZrO2 as measured by x-ray photoelectron spectroscopy.
Role of zero-point effects in catalytic reactions involving hydrogen
Authors: Gross Axel , Scheffler Matthias
Published: July 2002

According to the Heisenberg uncertainty principle of quantum mechanics, particles which are localized in space by a bounding potential must have a finite distribution of momenta. This leads, even in the lowest possible energy state, to vibrations, and thus, to the so-called zero-point energy. For chemically bound hydrogen the zero-point energy can be quite substantial. For example, for a free H2 molecule it is 0.26 eV, a significant value in the realm of chemistry, where often an energy of the order of 0.1 eV/atom (or 2.3 kcal/mol) decides whether or not a chemical reaction takes place with an appreciable rate. Yet, in many theoretical studies the dynamics of chemical reactions involving hydrogen has been treated classically or quasiclassically, assuming that the quantum mechanical nature of H nuclei, i.e., the zero-point effects, will not strongly affect the relevant physical or chemical properties. In this article we show that this assumption is not justified. We will demonstrate that for very basic and fundamental catalytic reaction steps, namely the dissociative adsorption of molecular hydrogen at transition metal surfaces and its time reverse process, the associative desorption, zero-point effects cannot only quantitatively but even qualitatively affect the chemical processes and rates. Our calculations (treating electrons as well as H nuclei quantum mechanically) establish the importance of additional zero-point effects generated by the H2 surface interaction and how energy of the H–H stretch vibration is transferred into those and vice versa.
The defective nature of the TiO2(110) (1×2) surface
Authors: Patel R. , Guo Q. , Cocks I. , Williams E. M. , Roman E. , de Segovia J. L.
Published: July 2002

(1×1) and the (1×2) reconstructed TiO2(110) surfaces have been studied with photoelectron spectroscopy of the valence bands and of the Ti 3p core level using synchrotron radiation. There is a higher defect concentration within the sample upon transformation of (1×1) to (1×2). The defects with both surfaces are isolated and are not limited to the surface. Defects induced by exposure to low-energy electrons at room temperature are mainly located at the topmost surface layer as a result of electron stimulated desorption. Electron beam irradiation, while destroying long-range order, yields a significant overlap between defect states and promotes the formation of band-gap states tailing towards the valence band.
Simulation of sputter deposition at high pressures
Authors: Smy T. , Tan L. , Winterton S. S. , Dew S. K. , Brett M. J.
Published: July 2002

Sputtering, although often optimized by trial and error, increasingly needs sound modeling techniques because of the high capital investment. Usually the simulation problem is broken down into three stages: emission at the target, transport through the host gas, and deposition at the substrate. It turns out that these stages are necessarily linked and must be properly decoupled. Here we are mainly interested in the transport through the vapor phase, which may be simulated by our Monte Carlo model, SIMSPUD. However, at high operating pressures, significant deposition takes place onto the target. To modify our Monte Carlo routine, rather than emitting particles in proportion to the erosion profile of the target and waiting for some of these to completely escape, all particles which land on the target are re-emitted until they ultimately escape to the substrate or chamber walls. This modification makes the net emission rate equal to the target erosion rate. At sufficiently high pressures, the Monte Carlo routine becomes very slow. In fact, the problem then becomes a diffusion problem best handled by numerical solution of the Laplace equation. At any rate, it is shown that when SIMSPUD has been modified in this way, good agreement with experiment is obtained over a wide range of working pressures.
Experimental test of the propagation of a He pressure front in a long, cryogenically cooled tube
Authors: Wallén E.
Published: July 2002

A series of measurements on the propagation speed of a He pressure front in a cryogenically cooled 75.3 m long, 4.3 cm i.d. stainless steel tube at 1.9 K and 4.25 K and a theoretical model for the phenomenon of limited propagation speed of such a He pressure front are described. The measurements have shown that it took 20 h for a He leak of 5.7×10−5 Torr l/s to be detected at the other end of the 75.3 m tube at 1.9 K and if the leak was increased to 1.4×10−4 Torr l/s, still at 1.9 K, the time for the He to be detected at the distant end was reduced to 8.7 h. With the tube at 4.25 K, it took 5.0 h for a He leak of 7.0×10−5 Torr l/s to be detected at the other end. The He travels through the tube in the form of a pressure front with a steep leading pressure gradient. A model involving the adsorption isotherm for He on stainless steel which is able to predict the time of arrival of the He pressure front at the distant end within 17% is described.
Reflectance anisotropy of the Si(100)1×2-As surface: Discrete dipole calculation
Authors: Patterson C. H. , Herrendörfer D.
Published: July 2002

The reflectance anisotropy (RA) spectrum of the Si(100)1×2-As surface is calculated using the discrete dipole model. The semiconductor is treated as a slab of pointlike dipole-polarisable bonds coupled by dipolar electrostatic fields. The response of the polarisable bonds to light in normal incidence is expressed in terms of the normal modes of the polarisable bonds which are called dipole waves. These may be classified as surface-localised, surface resonance and bulk dipole waves. We give an analysis of the optical response of this surface in terms of dipole moments localised on bonds that would be too cumbersome to implement using a band structure. The discrete dipole RA spectrum is compared to the experimental spectrum for this surface and a density functional calculation of the spectrum. The discrete dipole spectrum contains the same features as experiment. The origin of the peaks in the RA spectrum is discussed in terms of excitation of dipole waves polarised parallel and perpendicular to the arsenic dimer axes and also in terms of excitation of dipole moments in particular bonds. Parameters for the calculation were obtained from ab initio calculations on clusters.
Chemical and spectroscopic surface science investigation of MoO3 and MoO3/Al2O3 ultrathin films
Authors: Street S. C. , Goodman D. W.
Published: July 2002

The nature of MoO3 and MoO3/Al2O3 ultrathin films supported on Mo(110) and their interactions with CH3OH have been investigated using surface science techniques. The ultrathin (<50 Å) films were characterized by Auger electron spectroscopy, low energy electron diffraction, and high resolution electron energy loss spectroscopy (HREELS) of their phonon vibrational modes and electronic transition regions. The activity toward oxidation by the surfaces was investigated using temperature programmed desorption of methanol. The vibrational features associated with the terminal Mo=O and the bridging Mo–O–Mo in tetrahedral molybdate have been identified by HREELS. The intensity of the Mo=O loss (∼1000 cm−1) of unsupported molybdate decreases after several runs of methanol adsorption/desorption. The M=O feature appears in the MoO3/ alumina system only after annealing to >800 K, apparently as a function of auto-oxidation. Simultaneous with the rise of the vibrational loss at 1000 cm−1 is a feature in the ELS spectrum which appears at ∼1.8 eV associated with d–d transitions in MoO2.
Experimental study on intermediate and gradient index dielectric thin films by a novel reactive sputtering method
Authors: Tang Q. , Ogura S. , Yamasaki M. , Kikuchi K.
Published: July 2002

Here we report a novel reactive sputtering method, called two-stage high rate reactive sputtering method, its principles, and apparatus setup, and the results of optical properties of thin films obtained by this method. Then two typical antireflection coatings, one with an intermediate and homogeneous layer and the other with a gradient layer, have been successfully produced by this method. Their spectrum performances and microstructures are also presented.
Physical characterization of hafnium oxide thin films and their application as gas sensing devices
Authors: Capone S. , Leo G. , Rella R. , Siciliano P. , Vasanelli L. , Alvisi M. , Mirenghi L. , Rizzo A.
Published: July 2002

Thin films of hafnium oxide have been prepared by using a dual ion beam sputtering system. A study of their physical properties is reported. In particular, structural and compositional characterization was performed by means of x-ray diffraction and x-ray photoelectron spectroscopy techniques, showing a mixture of amorphous and polycrystalline structure and a substoichiometric composition. The atomic force microscopy results have shown a crater-like morphology probably due to the deposition process. In addition the gas sensing characteristics were analyzed in the presence of carbon monoxide. The variations in the electrical resistance have shown the capability of the films to detect CO and then the possibility to use hafnium oxides as a new sensitive material in the field of gas sensors.
Optimization and characterization of remote plasma-enhanced chemical vapor deposition silicon nitride for the passivation of p-type crystalline silicon surfaces
Authors: Lauinger Thomas , Moschner Jens , Aberle Armin G. , Hezel Rudolf
Published: July 2002

In a recent letter [Lauinger et al., Appl. Phys. Lett. 68, 1232 (1996)] we have shown that record low effective surface recombination velocities Seff of 4 cm/s have been obtained at ISFH on low-resistivity (1 Ω cm) p-type crystalline silicon using microwave-excited remote plasma-enhanced chemical vapor deposition (RPECVD) of silicon nitride at low temperature (300–400 °C). As an important application, this technique allows a simple fabrication of rear-passivated high-efficiency silicon solar cells with monofacial or bifacial sensitivity. In this work, we present details of the required optimization of the PECVD parameters and a characterization of the resulting silicon nitride films. All deposition parameters are shown to strongly affect Seff as well as the stability of the films against the ultraviolet (UV) photons of terrestrial sunlight. A clear correlation between Seff and the film stoichiometry is observed, allowing a simple control and even a rough optimization of the surface passivation quality by measurements of the refractive index of the films. An optimum passivation and UV stability is obtained for silicon-rich silicon nitride films with a refractive index greater than 2.3.
Determination and application of the depth resolution function in sputter profiling with secondary ion mass spectroscopy and Auger electron spectroscopy
Authors: Hofmann Siegfried , Schubert Johannes
Published: July 2002

The depth resolution function describes the relation between the measured depth profile and the true in-depth distribution of the composition. In high resolution depth profiling, blurring of the profile often is markedly asymmetric and needs several parameters for its precise description. The depth resolution function can be experimentally determined with appropriate reference materials, e.g., samples containing delta layers or sharp interfaces between two media, and it can be theoretically predicted. Recently, a semi-empirical model was developed based on the most fundamental physical mechanisms of atomic mixing, surface roughness, and information depth. These three parameters precisely describe the resolution function that was tested using results obtained on several structures and with different methods, e.g., Auger electron spectroscopy profiles of GaAs/AlAs multilayer interfaces, and secondary ion mass spectroscopy profiles of B delta layers in Si and of SiO2/Ta2O5 multilayers. It is shown how the depth resolution function is applied to reconstruct in-depth distributions of the composition of the measured profiles. Furthermore, information about thin layer thickness, roughness development, and graded composition at interfaces are readily obtained.
Miniature quadrupole residual gas analyzer for process monitoring at milliTorr pressures
Authors: Holkeboer D. H. , Karandy T. L. , Currier F. C. , Frees L. C. , Ellefson R. E.
Published: July 2002

Monitoring of gas composition for processes in the mTorr range is not possible with a conventional residual gas analyzer (RGA) unless a pressure reduction pumping system is used. A miniature sensor, having dimensions compatible with the mean-free path of gas molecules can operate directly at mTorr pressures while being less expensive and more compact than a conventional apparatus. This article describes a quadrupole RGA which is scaled down by a factor of 7 to operate at pressures up to 10 mTorr. This sensor requires different construction methods and drive electronics than a traditional RGA. A unique ion source with high output has been developed. The small sensor has resolution characteristics very similar to standard size models but sensitivity is reduced in proportion to the ratio of cross-sectional area for each quadrupole r0. Response is linear to the mTorr range and a correction method for gas scattering, based on pressure measured by a total pressure gauge included in the ion source design, extends the linear response to 10 mTorr.
Reactive ion etching of piezoelectric materials in CF4/CHF3 plasmas
Authors: Leech Patrick W.
Published: July 2002

The reactive ion etching of substrates used in piezoelectric devices (quartz, fused silica, LiNbO3, LiTaO3, and sapphire) has been characterized in CHF3/CF4-based plasmas. For quartz and fused silica, a regime of ion-enhanced chemical etching similar to that established by Steinbruchel [Ch. Steinbruchel, J. Electrochem. Soc. 130, 648 (1983)] for CF4 was indicated over the range of compositions from CF4=1 to CHF3=1. In this regime, the etch rate was dependent on the square root of the rf bias voltage (V1/2). The etch rate of both the quartz and fused silica was at a maximum in a CF4 plasma and decreased continuously with an increase in the ratio of CHF3/CF4 gases in the mixture. In comparison, the etch rates of LiNbO3, LiTaO3, and sapphire were invariant with changes in the ratio of the CHF3/CF4 gases, the flow rate, and the chamber pressure. The constancy of etch rate in these substrates has been attributed to a predominance of etching by a physical process of sputtering.
Structural change and heteroepitaxy induced by rapid thermal annealing of CaF2 films on Si(111)
Authors: Mattoso N. , Mosca D. H. , Schreiner W. H. , Mazzaro I. , Teixeira S. R. , Macedo W. A. A. , Martins M. D.
Published: July 2002

In this article we show that heteroepitaxial CaF2 films can be induced on Si(111) with a rapid thermal anneal. The change from preferentially oriented polycrystals to a single crystal with type-B epitaxy is visible by different structural techniques. The x-ray photoelectron spectroscopy results indicate the presence of a reacted layer at the CaF2/Si interface with a pronounced increase of fluorine atoms at this interface. Transmission electron microscopy results show that big structural changes occur due to the thermal pulse.
Interfacial reaction effects in the growth of MgO on GaAs(001) by reactive molecular beam epitaxy
Authors: Robey S. W.
Published: July 2002

MgO has important potential uses as a buffer layer for growth of perovskite and related oxides on GaAs. Equilibrium thermochemical data indicate MgO will be stable against interfacial reaction with GaAs but, under molecular beam epitaxy growth conditions, kinetic limitations can increase the probability of an undesired Mg or O reaction with the substrate and influence epitaxial growth. In situ reflection high-energy electron diffraction and Auger spectroscopy were combined with ex situ atomic force microscopy to study the potential for nonequilibrium processes to affect MgO growth on GaAs (001) by reactive molecular beam epitaxy. Under oxygen poor conditions, Mg reacts with GaAs to form an epitaxial phase with a cubic structure and a lattice constant of 0.62±0.01 nm. The growth mode is highly three-dimensional, producing a rough surface morphology. This phase is unstable to subsequent oxidation, which leads to segregation of Mg to the surface to form a polycrystalline MgO overlayer. Three-dimensional growth of the reaction product is inhibited by oxygen pressures providing a Mg-to-O2 flux ratio on the order of 2, for growth temperatures at 700 K. However, the epitaxial quality of MgO is still degraded at these oxygen pressures suggesting that, close to stoichiometric MgO conditions, the Mg–GaAs reaction still influences the growth of epitaxial MgO.
Growth and characterization of radio-frequency magnetron sputtered lead zirconate titanate thin films deposited on 〈111〉 Pt electrodes*
Authors: Ea-Kim B. , Aubert P. , Ayguavives F. , Bisaro R. , Varniere F. , Olivier J. , Puech M. , Agius B.
Published: July 2002

The physical, chemical, and electrical quality of the bottom electrode plays a key role in the fabrication of ferroelectric capacitors. In this article, we have used x-ray diffraction and transmission electron microscopy to study the stability of the Pt/TiN/Ti/SiO2/Si stack before PZT film investigations. The Pt layer deposited by radio-frequency (rf) magnetron sputtering on such structure was strongly 〈111〉 oriented. The nucleation, growth, and orientation of lead zirconate titanate [Pb(Zr0.40Ti0.60)O3] thin film, performed by rf magnetron sputtering from ceramic target on 〈111〉 oriented platinum electrode and crystallized by rapid thermal annealing, have been investigated. The studies reveal that the PZT thin films deposited at 200 °C, with a substrate-target distance equal to 5.5 cm, using 1 Pa Ar pressure, and plasma power density of 1.7 W/cm2 allow good control of the film composition. The optimal annealing conditions have been then determined to achieve a pure perovskite structure (mainly 〈111〉 oriented). Ferroelectric hysteresis loop measurements indicated a remanent polarization of 22.0 μC/cm2 and coercive field of 50 kV/cm for the phase composition Zr/Ti=40/60.
Surface composition of BN, CN, and BCN thin films
Authors: Tempez A. , Badi N. , Bensaoula A. , Kulik J.
Published: July 2002

Boron nitride (BN), carbon nitride (CN), and boron carbon nitride (BCN) thin films were deposited on sapphire and silicon using ion beam and electron cyclotron resonance plasma assisted physical vapor deposition. In situ Auger electron spectroscopy was used to investigate the effect of different growth parameters and postgrowth processing on the thin film surface composition. The bulk composition was determined by electron energy loss spectroscopy and electron microprobe analysis. Both BN and CN films show thermal stability up to 900 and 700 °C, respectively. Low growth temperatures favor nitrogen incorporation in CN films and the optimum temperature for quasistoichiometric BN is between 450 and 600 °C, depending on the nitrogen sources.
Quantitative characterization of a highly effective atomic hydrogen doser
Authors: Eibl C. , Lackner G. , Winkler A.
Published: July 2002

An atomic hydrogen doser of the Bertel type was characterized in terms of the degree of dissociation and angular distribution of the effusing particles. In this doser hydrogen is dissociated in a tungsten tube which is heated by electron bombardment. Various experimental techniques were used to determine the degree of dissociation as function of temperature and gas flux. It is shown that simple equilibrium considerations cannot be applied to obtain the degree of dissociation accurately. Nevertheless, for sufficiently small gas flux and temperatures above 1850 K, the degree of dissociation approaches 100%. The angular distribution was determined by a gold foil on a goniometer as detector, which is sensitive to atomic hydrogen only. The experimental results were compared with Monte Carlo simulations. A strongly forward focused distribution is observed which allows efficient atomic hydrogen dosing. This doser was used to measure absolute initial sticking coefficients for atomic hydrogen on various single crystal metal surfaces: S0(H)=1.0 on Ni(111), 0.9 on Ni(110), 0.7 on Al(111), 0.6 on Al(100) and 0.5 on a polycrystalline gold foil, respectively.
Effect of hydrogen dilution on the remote plasma enhanced chemical vapor deposition of chlorinated SiO2 films
Authors: Alonso J. C. , Vazquez R. , Ortiz A. , Pankov V. , Andrade E.
Published: July 2002

Chlorinated silicon dioxide films have been prepared at low temperatures (200 °C) and high deposition rates (900–1700 Å/min) by remote plasma enhanced chemical vapor deposition using mixtures of SiCl4, O2, Ar, and H2 under various hydrogen flow rate conditions. It was found that films deposited without hydrogen grow with the highest deposition rate, however they exhibit poor properties such as high etch rate, low density, and low refractive index. These oxides are also chemically unstable and easily hydrolyzable upon exposure to ambient moisture. It is assumed that the low chlorine content (detected by Rutherford backscattering) in these samples is due to the hydration of the weak Si–Cl bonds existing in the as-deposited films, which generates desorption of HCl and forms the Si–OH bonds observed in the corresponding infrared spectra. The addition of hydrogen to the process reduces the deposition rate but improves the properties and stability of the films by reducing the amount of chlorine incorporated during growth. At hydrogen flow rates moderately higher than the SiCl4 flow, dense and stable chlorinated oxides with properties close to stoichiometric SiO2 are obtained. The composition results indicate that in order to obtain these types of oxides the chlorine concentration should be limited to a value around 2.6 at. %.
Structure of the interfacial region between polycarbonate and plasma-deposited SiN1.3 and SiO2 optical coatings studied by ellipsometry
Authors: Bergeron A. , Klemberg-Sapieha J. E. , Martinu L.
Published: July 2002

Fabrication of optical films on plastic substrates presents a considerable challenge due to the necessity of controlling adhesion and the optical properties of the interface. In the present work, the surface of polycarbonate (PC) substrates was pretreated in a microwave plasma in N2 and He gases, and amorphous hydrogenated silicon nitride (SiN1.3) and dioxide (SiO2) films were deposited in a dual-mode microwave/radiofrequency plasma. Using ex situ variable angle spectroscopic ellipsometry (VASE), spectrophotometry, and x-ray photoelectron spectroscopy, it was found that the plasma-treated PC contains a crosslinked surface layer which can extend to a depth of more than 50 nm, and which presents a refractive index increase of about 1%–2%. When PC is plasma treated and coated with SiN1.3 or SiO2, the interfacial region (interphase) is structured: it comprises the crosslinked layer, and a less dense transition region between the polymer and the film. The interphase may extend up to 100 nm. Some of the results suggest that the plasma pretreatment stabilizes the surface, leading to a lower thickness of the interphase. The accuracy of ex situ VASE analysis is discussed and the importance of the interface structure on the design and mechanical properties of optical systems on plastic substrates is emphasized.
Thermal and electron driven chemistry of CCl4 on oxidized Si(100)
Authors: Junker K. H. , White J. M.
Published: July 2002

Dosed on oxidized Si(100) at 100 K, carbon tetrachloride adsorbs and desorbs without dissociation. The monolayer desorbs at 135 K, 10 K lower than the multilayer. This unusual behavior is attributed to stronger interactions between condensed CCl4 molecules than between CCl4 and SiO2. Irradiation with either low-energy (⩽50 eV) electrons or Mg Kα x rays causes C–Cl cleavage. For 50 eV incident electrons, the decay of the CCl4 temperature programmed desorption peak area occurs with an effective cross section of (2.0±0.1)×10−16 cm2. The same cross section characterizes the ejection of CCl (CCl+) during electron irradiation. After low electron fluences, C2 and C3 molecules desorb reflecting both electron-induced C–Cl bond dissociation and C–C bond formation. At 2.5 eV incident electron energy, the cross section is still high—10−17 cm2. Electron activation is attributed to a combination of impact ionization and electron attachment mechanisms.
Vapor deposition modeling for an entrenched wafer geometry
Authors: Willett L. J. , Loyalka S. K. , Tompson R. V.
Published: July 2002

Empirical models for chemical vapor deposition of SiO2 from tetraethylorthosilicate (TEOS) and O3 have been proposed using one- and two-precursor models of the surface rate limited reaction kinetics. In the one-precursor model, considered here, the wafer surface is described by a Robin boundary condition, Φ=Φsat−α̃(∂Φ/∂ñ), where Φ is the concentration of the gas phase generated reactant precursor, Φsat is its saturation concentration, ñ is a normal vector pointing outward from the diffusion region (into the wafer substrate), and α̃ is a surface reaction kinetics parameter. The validity of the diffusion model approximation for transport in the continuum and near continuum regimes dictates that Φ be a solution to the Laplace equation in the interior of a diffusion region “closed up” to form a polygon. The horizontal top side of the polygon represents the macroscopic free flow boundary layer. The Robin (wafer) boundary forms the base of a rectangle (flat Robin boundary) or five sides of an octagon (entrenched Robin boundary). Closure of the geometrical boundary (diffusion region) results from the abstract construction of outer vertical polygonal sides [homogeneous Neumann boundaries where (∂Φ/∂ñ)=0]. The closed-up polygon forms a rectangle in the case of a flat wafer surface and an octagon in the case of a wafer surface with a trench. For the rectangle, it suffices to model adsorption along the base (flat wafer substrate) with a constant α̃. For the entrenched wafer surface, a phenomenological curvature dependence in α̃ has been previously proposed to arise due to the introduction of corners and edges via the trench. We propose a spatial dependence in α̃ which unfolds from the transformation properties of the Robin boundary condition when an infinite rectangle (flat wafer geometry) is conformally mapped into an octagon (entrenched wafer geometry). The boundary conditions for the transformed wafer surface are then used in a Green’s function boundary integral equation formulation of the problem. Numerical solutions, presented for the diffusion current, the surface reaction kinetics parameter, α̃, and the deposition concentration, Φ, prove to be consistent with a conformal (uniform) film profile.
Study of ohmic multilayer metal contacts to p-type ZnSe
Authors: Rinta-Möykky A. , Uusimaa P. , Suhonen S. , Valden M. , Salokatve A. , Pessa M. , Likonen J.
Published: July 2002

A novel approach for making ohmic contacts to p-type ZnSe has been introduced. This approach includes growth of p-ZnSe at low temperature by a variant of molecular beam epitaxy, then treating the surface with KOH solution, followed by deposition of a Te/Pd/Pt/Au metal layer and annealing at 200–250 °C for 5 min. As a result, a stable ohmic contact up to a current density of 2 kA cm−2 was obtained. Using this contact fabrication procedure, a ZnSe-based quantum-well laser was demonstrated in continuous wave mode of operation at room temperature. The formation of the ohmic contact is suggested to be due to the presence of oxygen on the ZnSe surface, the creation of TeO2 at the metal/ZnSe interface, and the diffusion of Pd into ZnSe.