bluuebunny/crossref_metadata_2025 huggingface dataset:
Indirect plasma deposition of silicon dioxide
Authors: Meiners L. G.
Published: July 2002

Silicon dioxide layers were formed by reacting silane with oxygen. The oxygen was ionized in an rf discharge chamber physically removed from the deposition zone. The excited oxygen species, which left the ionization chamber, reacted with silane regardless of the temperature, during deposition, to be chosen as an important parameter and at the same time avoided the adverse effects of placing the sample within an rf discharge. MOS capacitors were formed on silicon substrates and their electrical properties were evaluated by capacitance-voltage (C-V) measurements. An analysis of the quasistatic C-V data indicates a midgap density of surface states NSS = 2×1011 cm−2eV−1 for SiO2 layers deposited at 300 °C.
Surface science study of selective ethylene epoxidation catalyzed by the Ag(110) surface: Structural sensitivity
Authors: Campbell Charles T.
Published: July 2002

The selective oxidation of ethylene to ethylene epoxide (C2H4+1/2O2→C2H4O) over Ag is the simplest example of kinetically controlled, selective heterogeneous catalysis. We have studied the steady-state kinetics and selectivity of this reaction for the first time on a clean, well-characterized Ag(110) surface by using a special apparatus which allows rapid (∼20 s) transfer between a high-pressure catalytic microreactor and an ultrahigh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The effects of temperature and reactant pressures upon the rate and selectivity are virtually identical on Ag(110) and supported, high surface area Ag catalysts. The absolute specific rate (per Ag surface atom) is, however, some 100-fold higher for Ag(110) than for high surface area catalysts. This is related to the well-known structural sensitivity of this reaction. It is postulated that a small percentage of (110) planes (or [110]-like sites) are responsible for most of the catalytic activity of high surface area catalysts. The high activity of the (110) plane is attributed to its high sticking probability for dissociative oxygen adsorption, since the rate of ethylene epoxidation is shown in a related work [Ref. 1: C. T. Campbell and M. T. Paffett, Surf. Sci. (in press)] to be proportional to the coverage of atomically adsorbed oxygen at constant temperature and ethylene pressure.
Low-temperature Ge heteroepitaxy on GaAs(001)
Authors: Chambers S. A.
Published: July 2002

It is shown through the use of low-energy electron diffraction and high-energy x-ray photoelectron diffraction that very nearly layer-by-layer epitaxial growth of ultrathin Ge films can be achieved on GaAs(001). Superior overlayer uniformity has been attained by minimizing the surface defect density on the substrate and growing at a low enough substrate temperature to prevent high rates of surface diffusion and the resulting agglomeration of adatoms. When these two conditions are achieved, clustering is minimized during the initial stages of epitaxy. We have found that the first monolayer of Ge intermixes with the substrate to a limited extent when grown at 220 °C. Partial third-layer formation occurs as a result of the deposition of two monolayer equivalents (ML eq), and some fifth-layer formation occurs during the deposition of the fourth ML eq, but is limited in extent. These films exhibit less clustering but are also not as well ordered as those grown at 320 °C.
Beam-size measurements in focused ion beam systems
Authors: Harriott L. R.
Published: July 2002

Accurate measurement of the beam diameter for a finely focused ion beam is difficult using conventional methods which have been applied in electron beam systems. Generally, the beam is scanned across an abrupt edge while measuring beam current or secondary electron intensity. For ions, the sputtering of the sample degrades the shape of the edge and thus the accuracy of the measurement. Furthermore, implantation can change the secondary electron contrast during the measurement. In this paper, a new method is suggested that avoids these difficulties. The ion beam diameter is determined by measuring the ratio of ion flux in a single pixel wide feature to that of a feature which is much larger than the beam diameter. This ratio can be measured using a threshold process. For this paper, we have used sputtering of a thin film as the threshold process and secondary ion mass spectrometry as the end-point determinant.
Stress modification in sputtered zinc sulfide and zinc oxysulfide thin films
Authors: Orent Tom
Published: July 2002

The physical characteristics of zinc sulfide and zinc oxysulfide thin films deposited by rf magnetron sputtering have been studied. The stress and composition were functions of the deposition conditions. The stress could be independently controlled by two methods. Increasing the gas pressure did not alter the composition, but it did result in a change in the microstructure of the deposit that was accompanied by a reduction in the stress. Adding increasing amounts of oxygen to the sputter gas at constant pressure resulted in the formation of oxygen-containing compounds in the film that reduced the stress. Composition, chemical bonding, and structure were studied by a variety of techniques. The kinetics of oxygen incorporation were studied, and an atomic model was proposed.
Modeling of multicomponent reactive sputtering
Authors: Moradi M. , Nender C. , Berg S. , Blom H-O. , Belkind A. , Orban Z.
Published: July 2002

Computer process modeling has become an important tool in the development of new thin-film processes. Previously, we presented a model that successfully describes the complex behavior of the reactive sputtering of a single-element target. The model enables one to predict, e.g., the hysteresis effect and the composition of the deposited film. There exists, however, a demand for developing reliable reactive-sputtering processes for more complicated materials. This calls for an extended reactive-sputtering model including more than one target element. In this article, we will for the first time present an extension of our previous model now taking the effect of multicomponent targets into consideration. The transition from metallic- to compound-sputtering mode normally occurs at different levels for different single-element targets. The new extended reactive-sputtering model predicts the transition from metallic to compound mode during multicomponent reactive sputtering of thin films. The results indicate that under normal operating conditions, only one transition region exists irrespective of the number of target elements involved in the process. This is experimentally verified for the process of reactive cosputtering of Al+Ti in an Ar/O2 atmosphere. We will also point out the complex composition behavior of the reactive-cosputtering process as compared with reactive sputtering from an alloy target.
Etching of a-C:H films by an atomic oxygen beam
Authors: Bourdon E. B. D. , Raveh A. , Gujrathi S. C. , Martinu L.
Published: July 2002

Hydrogenated amorphous carbon (a-C:H) films are considered for real time monitoring of atomic oxygen in Low Earth Orbit, and during plasma etching processes. a-C:H layers were deposited onto quartz crystal microbalances (QCM) in a dual microwave/radio frequency (mw/rf) plasma in methane and methane/argon mixtures. The QCMs were exposed to a neutral atomic oxygen beam in a system simulating the space environment, using a flux of ∼1016 atoms/cm2 s at 2.5 eV average incident energy. The etch rate Er was determined from the mass loss by in situ measurement of frequency shift of the QCM oscillator. The Er values, ranging from 1 to 20 ng/cm2 s, were found to increase with the hydrogen concentration in the films, and to decrease with increasing film density. Systematically higher Er values were found for a-C:H with a polymerlike character and for very hard ‘‘diamondlike films’’ with bonded hydrogen, in contrast to films with predominantly unbonded hydrogen. For comparison, Er values for crystalline chemical vapor deposited diamond and for several commercial polymers have also been measured, and found to be ∼0.5 and ∼50 ng/cm2 s, respectively.
Three-dimensional simulation of surface evolution during growth and erosion
Authors: Katardjiev I. V. , Carter G. , Nobes M. J. , Berg S. , Blom H.-O.
Published: July 2002

A concise review of the basic ideas of the generalized kinematic model of surface evolution during growth and erosion is presented. Interpretation of the main results of this model as well as some specific rules and advice as to how this model is applied in practical simulations are also presented. It is concluded that three-dimensional (3D) computer simulation of surface evolution is only possible by the use of adequate, theoretically motivated numerical methods. As a demonstration, 3D topography simulations during broad and focused ion beam bombardment, chemical vapor deposition, and reactive ion etching using the 3D code dinese are also presented.
Micromachined array of electrostatic energy analyzers for charged particles
Authors: Stalder R. E. , Boumsellek S. , Van Zandt T. R. , Kenny T. W. , Hecht M. H. , Grunthaner F. E.
Published: July 2002

The design, fabrication techniques, and first test results of a new type of micromachined energy analyzer for charged particles are presented. The novel design is based on a Bessel Box straight-line geometry with focusing in one dimension. The problem of small acceptance area due to the drastically reduced overall dimensions is accounted for by the use of a parallel array of small analyzers simultaneously. We describe the fabrication of a proof-of-concept micromachined Bessel Box array by means of anisotropic wet etching of 〈110〉 silicon, and we present ray-tracing simulations for a simple geometry. Finally, we show some test results of Auger electrons and discuss possible applications of generic small charged particle analyzers.
Chemical sensors with catalytic metal gates
Authors: Lundström Ingemar , Petersson Lars-Gunnar
Published: July 2002

We first review the principles of gas sensitive field effect devices with catalytic metal gates. A discussion about recent experiments under ultrahigh vacuum conditions is given. More particularly, we consider the properties of the interface between the catalytic metal and the insulator it is evaporated on. A few more of the applied research areas related to these devices are also discussed and some remaining research problems are suggested.
Interactions and penetration at metal/self-assembled organic monolayer interfaces
Authors: Jung D. R. , Czanderna A. W. , Herdt G. C.
Published: July 2002

The purpose of research on metals (M) deposited onto self-assembled monolayers (SAMs) is to understand the interactions between the metal and eventually metal oxide overlayers on well-ordered organic surfaces. Applications of M/SAM and inorganic/SAM research results to the understanding of real inorganic/organic interfaces in vacuum and under environmental conditions can potentially play a key role in the development of advanced devices with stable interfacial properties. The results of M/SAM studies to date are reviewed, M/SAM combinations ranked according to reactivity and penetration, and specific examples of reactive interfaces (Cu/COOH, and Cr/COOH) and nonreactive interfaces with penetration (Ag/COOH) are used to illustrate the extremes.
Determination of electron temperatures in plasmas by multiple rare gas optical emission, and implications for advanced actinometry
Authors: Malyshev M. V. , Donnelly V. M.
Published: July 2002

A method is described for determining the electron temperature of a low pressure plasma of the type used in microelectronics materials processing. A small amount of an equal mixture of He, Ne, Ar, Kr, and Xe is added to the process gas (in this example Cl2) and the intensities of optical emission lines from the Paschen 2p levels of the rare gases are recorded. The observed emission intensities are compared with those computed from a model that includes electron impact excitation from the ground state, as well as two-step electron impact excitation through intermediate metastable levels. This latter route is shown to be the dominant one for nearly half of the levels. Using adjusted, published electron impact excitation cross sections and assuming a Maxwellian electron energy distribution, the electron temperature (Te), the only adjustable parameter, was determined from the best match between the observed and computed intensities. For a high density, helical resonator Cl2 plasma at 10 mTorr, Te=2.2±0.5 eV was determined from this method. This value is about 1 eV lower than the typical values reported for high density inductively coupled Cl2 plasmas at similar pressures. While the precision of the method is expected to be high, the accuracy of Te determined from optical emission would likely be improved with more accurate cross section data. Implications for actinometric determination of species concentrations in plasmas are also discussed.
Characterization of defect levels in chemically deposited CdS films in the cubic-to-hexagonal phase transition
Authors: Vigil O. , Riech I. , Garcia-Rocha M. , Zelaya-Angel O.
Published: July 2002

Spectral photoconductivity, photoconductive quenching, photoluminescence, and thermally stimulated current measurements, have been carried out in order to study the evolution of defect energy levels in CdS thin films, grown in cubic phase by chemical bath deposition, as a function of thermal annealing temperatures in Ar+S2 atmosphere. The results are influenced by a cubic-to-hexagonal phase transition. From those measurements, a number of trap levels and deep levels in the forbidden band are determined. The results can be explained in terms of the evolution of native and phase transition generated defects in the sample structure.
Characterization of heat-treated ITO/InP solar cells
Authors: Rosenwaks Y. , Li X. , Coutts T. J.
Published: July 2002

The effects of heat treatments on the properties of InP/ITO junctions were studied using time resolved photoluminescence measurements, surface chemical analysis, and photo I–V characteristics. It was found that the surface recombination velocity (SRV) of etched InP increased from 600 to ∼4×104 cm/s after a 350 °C heat treatment and to about 4×105 cm/sfollowing annealing at 500 °C. On the other hand annealings performed on Ar plasma treated samples increased the substrate’s SRV to similar values following 80 and 200 °C heat treatments, respectively. Surface chemical analysis showed that the increase in SRV is associated with phosphorus loss from the crystal surface which creates recombination centers at the interface. The effect of these recombination centers on the open circuit voltage of ITO/InP solar cells is demonstrated and discussed.
Spectroscopic ellipsometry investigation of nickel silicide formation by rapid thermal process
Authors: Hu Yaozhi , Tay Sing Pin
Published: July 2002

Titanium and cobalt silicides are used widely in microelectronics fabrication. There are limitations for both silicides. With TiSi2, a linewidth dependence of sheet resistance for lines narrower than 0.35 μm has become dramatic. The transformation from the high-resistivity C49 phase to the low-resistivity C54 TiSi2 phase is nucleation limited. With CoSi2 there is much less linewidth dependence of the sheet resistance, but more Si is consumed to form the silicide. With the use of NiSi, these problems can be avoided as reported so far in literature. In this article we shall report an investigation of rapid thermal silicidation of nickel on single crystalline silicon wafers in the annealing range of 150–1150 °C. It has been found that there are five zones in the dependence of sheet resistance on silicidation temperatures as follows: below 175, 175–350, 350–650, 650–900, and above 900 °C. In order to extensively study the phase sequence for Ni/Si reactions and the kinetics of nickel silicide formation corresponding to the sheet resistance zones, the spectroscopic ellipsometry (SE) was used to characterize the thin silicide films. The sensitivity and usefulness of SE characterization for studying the intermetallic growth processes and the application of an appropriate algorithm for extracting the information from experimental data is demonstrated. The results obtained by this nondestructive SE technique are compared with measurements done by the well-established but destructive techniques, Rutherford backscattering, scanning electron microscopy, and energy dispersive x-ray spectrometry. A physical picture of nickel silicidation as a function of temperature is also presented.
Challenges in electron cyclotron resonance plasma etching of LiNbO3 surface for fabrication of ridge optical waveguides
Authors: Mitsugi Naoki , Nagata Hirotoshi , Shima Kaori , Tamai Masumi
Published: July 2002

Ridge-shaped optical waveguides are promising structures to provide broadband characteristics for LiNbO3 (LN) based optoelectronic devices. In this regard, electron cyclotron resonance plasma etching with fluorocarbons have been commonly applied in the preparation of ridge waveguides, 3–5 μm in height, but certain fabrication problems need investigation and clarification. The etched LN surface was found to be covered with LiF leading to a weak adhesive strength for the overlayer, such as a SiO2 film, over the ridge waveguides. When CF4 etchant was used, a notch appeared along the foot of the ridge waveguides. The notch was a possible origin for the chipping of the waveguides. Such an undesirable notch was found to be prevented by the use of a CHF3 etchant. Here, these inevitable problems during the plasma etching of the LN were presented and discussed.
Synthesis and characterization of nickel sulfide catalysts
Authors: Olivas A. , Cruz-Reyes J. , Petranovskii V. , Avalos M. , Fuentes S.
Published: July 2002

Nickel sulfide catalysts obtained by homogeneous precipitation were prepared by varying the homogenization time and the sulfiding temperature. Structural characterization studies by transmission electron microscopy and x-ray diffraction showed the formation of various metastable crystalline phases at low temperatures of sulfiding (Ni3S2, Ni3S4, Ni7S6, Ni9S8, NiS2, NiS1.03, NiS), depending on the preparation conditions. NiS millerite appears to be the main phase for samples obtained after 4 h homogenization, changing to NiS1.03 phase after 36 h. Catalytic tests of these samples for cyclohexanone hydrogenation showed that the intrinsic activity is higher for the NiS1.03 phase.
Nanofabrication of model catalysts and simulations of their reaction kinetics
Authors: Johansson S. , Wong K. , Zhdanov V. P. , Kasemo B.
Published: July 2002

Nanofabrication of Pt particles on SiO2, with particle sizes and separations down to ∼20 nm, and more complex structures consisting of Pt particles deposited on top of CeO2 islands, have been made by electron beam lithography and lift-off techniques. Under catalytic reaction conditions (nonflammable mixture of H2+O2 in Ar at 1000 K, 1 atm) the originally deposited, disk-shaped, and polycrystalline particles, transform to three-dimensional crystalline particles. This restructuring is attributed to the surfactant role of oxygen (promoting Pt mobility), and the nonwetting of late transition metals on oxides. The exothermic H2+O2 reaction is a possible additional driving force. The catalytic kinetics on 3D crystalline particles with well-defined facet planes were explored by Monte Carlo simulations. The latter demonstrate that the catalytic kinetics on nm sized, crystalline particles may be absolutely unique and nonpredictable from single-crystal studies, primarily due to the diffusive mass transport between differently oriented facets on the particles.
Magnetization dynamics: A study of the ferromagnet/antiferromagnet interface and exchange biasing
Authors: Camley R. E. , McGrath B. V. , Astalos R. J. , Stamps R. L. , Kim Joo-Von , Wee Leonard
Published: July 2002

We use a method which employs a dynamic calculation of magnetization motion to find both the static configuration and the spin wave excitations in a ferromagnet/antiferromagnet layered structure. Our results for the static structure are similar to those found in Koon’s model; i.e., in zero applied field the ferromagnet points perpendicularly to the easy axis of the antiferromagnet, and the surface spins of the antiferromagnet are in a surface spin flop configuration. The calculated hysteresis curve for this structure shows a small exchange bias, in agreement with typical experimental results. We explore how this bias depends on the parameters of the antiferromagnet and on the nature of the interface coupling. We further examine models for antiferromagnets with both compensated and uncompensated surfaces. We show that the structure calculated by Koon is unstable at higher reversed magnetic fields but that this instability can be suppressed by an easy-plane anisotropy.
Oxygen doping effect on Ge–Sb–Te phase change optical disks
Authors: Ebina Atsushi , Hirasaka Masao , Nakatani Kenji
Published: July 2002

High performance phase change optical recording disk has been developed by using an oxygen-doped Ge–Sb–Te recording layer. The 5×105 overwrite cycles were achieved with this 6 at. % oxygen-doped phase change optical recording disk. The transmission electron microscope observation shows that the crystal grain size was enlarged and the “self-sharpening effect” disappeared with increasing of oxygen concentration. It is thought that the oxygen doping made it easy to grow the uniform large grain size without the self-sharpening effect around the amorphous mark and therefore, the overwrite cyclability of the phase change optical recording disk was improved.
Characterization of plasma deposited Ta2O5 films using grazing incidence x-ray scattering
Authors: Four S. , Devine R. A. B. , Brunel M.
Published: July 2002

Tantalum pentoxide thin films were prepared by plasma enhanced chemical vapor deposition, using a TaF5 source and a microwave excited H2/O2 plasma. The thickness and the density were determined by grazing incidence x-ray scattering, and confirmed by ellipsometry and Rutherford backscattering spectroscopy. Surface roughness measurements by atomic force microscopy and by x-ray scattering differ but suggest the average roughness (rms) is typically 0.28 nm for the amorphous phase and <0.83 nm for the crystallized form.
Ionized physical-vapor deposition using a hollow-cathode magnetron source for advanced metallization
Authors: Klawuhn E. , D’Couto G. C. , Ashtiani K. A. , Rymer P. , Biberger M. A. , Levy K. B.
Published: July 2002

The hollow-cathode magnetron (HCM) source is a new and promising technology for metal deposition that maintains the simplicity of the physical-vapor deposition technology and combines it with a very high-density diffused plasma (>1012 #/cm3) for efficient ionization of sputtered metals. The HCM employs a magnetron discharge confined in an inverted cup-shaped target. As such, it does not require any additional rf or microwave sources for generation of metal ions. In this article, the HCM theory of operation will be reviewed and results will be presented for the application of this source for deposition of Ti(N), Ta(N), and Cu films. In addition, the effects of rf bias on step coverage and film properties will be discussed. In particular, enhanced bottom coverage (20% for Cu, 40% for Ta, and 30% for Ti) in narrow, high-aspect-ratio vias (0.25 μm, 5:1 AR) was obtained without the application of rf bias to the wafer. Both TiN and TaN films had the same step coverage as Ti and Ta films, respectively. Rf bias was utilized to further increase the bottom coverage and the sidewall coverage of the films, thus extending the technology to higher aspect ratios. In addition, rf bias was used to modify film properties such as grain size, grain orientation, and film texture.
Variable temperature and ex situ spin-polarized low-energy electron microscope
Authors: Tober E. D. , Witte G. , Poppa H.
Published: July 2002

The spin-polarized low-energy electron microscope (SPLEEM) at NCEM has been utilized to examine the magnetic domain formation in thin films of Co grown on Au(111) single-crystal surfaces. A new, low-temperature liquid nitrogen cooling system has been added to this instrument, achieving sample temperatures over a wide range (115–2300 K) and enabling the first SPLEEM images to be obtained at below room temperature. Furthermore, the deposition of protective, nonmagnetic capping layers on top of the Co/Au(111) surface is observed to have little effect on the domain shape except for decreasing magnetic contrast due to the attenuation of the diffracted electrons from the Co layer. The magnetic domain contrast is nearly completely restored upon thinning the capping layers by ion bombardment enabling the examination of ex situ prepared samples.
Polysiloxane resist as a probe for energy deposited in electron beam exposed resists
Authors: Paraszczak J. , Kern D. , Hatzakis M. , Bucchignano J. , Arthur E. , Rosenfield M.
Published: July 2002

Polymeric resists based on the O–Si–O backbone, called polysiloxanes, have the extraordinary property that they are highly sensitive to various forms of energy, and at the same time they are highly resistant to etching in an oxygen plasma. Thus, thin layers of these materials on top of thick layers of carbon based polymers can be patterned and then used as masks for plasma etching of the bottom layer. The proximity effect parameters for electron beam exposure of such a system have been reported previously by Jones, Paraszczak, and Speth. In the present paper, the thin layer of polysiloxane is used for the energy which is deposited in the top layer of the resist system. The effect of electron beam potential upon the distribution of deposited energy in the resist film is determined experimentally, and compared with Monte Carlo simulations. The results are used to discuss the effect of electron beam potential on the dose contrast achievable in single and double layer resist systems.
On the design and characterization of a novel arsine cracking furnace utilizing catalytic decomposition of AsH3 to yield a purely monomeric source of arsenic for molecular beam epitaxial growth of GaAs
Authors: Kapitan L. W. , Litton C. W. , Clark G. C. , Colter P. C.
Published: July 2002

Arsine (AsH3) used as an arsenic source in molecular beam epitaxy instead of solid arsenic has been found to decompose catalytically instead of thermally under MBE growth conditions. Dehydrogenation of AsH3 in a spiral quartz cracking furnace containing an electrically heated catalyst has produced a purely monomeric beam of arsenic suitable for the growth of MBE GaAs and related III–V compounds. Experimental procedures and results of the AsH3 cracking are reported together with electrical and optical characterization of the first purely As1 grown MBE GaAs.
The influence of input power on the performance of rf sputtered ITO/InP solar cells
Authors: Coutts T. J. , Pearsall N. M. , Tarricone L.
Published: July 2002

This paper is concerned with the choice and influence of the deposition conditions of ITO thin films onto p-type single crystal InP substrates for use as heterojunction solar cells. Our analyses, based on capacitance–voltage, current–voltage, spectral response, surface photovoltage, and reflection electron diffraction, indicate that adverse effects of the deposition process on the surface can be reduced by proper choice of deposition schedule. The general conclusions are (1) The reverse saturation current is 2–3 orders of magnitude lower for cells fabricated by using very low power deposition of the ITO than for those using high power deposition. (2) Either abrupt or graded interfaces can be formed depending on the input power to the sputtering process. (3) The extent of the grading increases with increased rf power input and is believed to be due to the diffusion of species (possibly of tin).
Compound sputtering cathodes of refractory metal silicides and thin film produced
Authors: Hoo Hung-Lee , Avins Jonathan B.
Published: July 2002

Purity, density, and homogeneity results from manufacturing processes used to produce Ti, Mo, Ta, and W silicide sputtering cathodes have been evaluated in the light of sputtering characteristics. Resulting thin film properties of sheet resistivity, stoichiometry, and homogeneity were also determined. Metallographic studies of cathode materials are presented, along with sputtering parameters for planar silicide cathodes and film annealing conditions.
Laser-induced plasmas for primary ion deposition of epitaxial Ge and Si films
Authors: Lubben D. , Barnett S. A. , Suzuki K. , Gorbatkin S. , Greene J. E.
Published: July 2002

Epitaxial Ge and Si films have been grown by primary ion deposition from laser-induced plasmas. The plasmas were formed by focusing 15 ns, 107–108 W cm−2, pulses of 5 eV photons from a KrF excimer laser onto Ge or Si single crystal wafer targets. Time-of-flight, current-voltage, and film-thickness distribution measurements established that neutral atoms and ions were emitted from Ge targets with mean velocities of 1.0–1.6×106 cm s−1 (corresponding to average kinetic energies of 40 to 100 eV) in a distribution that was strongly peaked in the direction normal to the target surface. Macroscopic (∼1 μm diam) particles were also emitted. A shutter, synchronously triggered with the laser pulses through a delay circuit, was used as a velocity filter for removing from the beam particles with velocities up to 1.2×104 cm s−1. Epitaxial Ge films were grown on semi-insulating (100) GaAs substrates at temperatures between 300 and 450 °C and epitaxial Si was grown on (100) Si at 700 °C. Deposition rates were typically 0.5–1 μm h−1. Hall effect measurements carried out on the Ge/GeAs heterostructures showed that the Ge films, ∼1 μm thick, were p type with room temperature carrier concentrations of ∼1×1018 cm−3 and hole mobilities of ∼150 cm2/V s.
Single-step lift-off process using chlorobenzene soak on AZ4000 resists
Authors: Fathimulla A.
Published: July 2002

A single-step lift-off process using chlorobenzene soak on AZ4110 resist is described. The effects of the soak time and temperature on the thickness of the overhang is presented. This process provides thick overhang with vertical side walls, which are essential for the successful lift-off. Finally, the effect of deep UV exposure on the lift-off is discussed.
The effects of elastic relaxation on transmission electron microscopy studies of thinned composition-modulated materials
Authors: Treacy M. M. J. , Gibson J. M.
Published: July 2002

Composition modulations in defect-free crystalline materials are generally accompanied by a lattice distortion where unit cell dimensions are not characteristic of the local composition. The distortion originates from the constraint that the interatomic spacings perpendicular to the modulation direction be commensurate from one layer to the next. Thus there is an internal stress field set up in the material which attempts to suppress unit cell expansions and contractions in these perpendicular directions. A well-known example is the tetragonal distortion observed in bulk, cubic strained layer superlattices, and spinodally decomposed alloys. However, it is frequently overlooked that these stresses can be significantly relaxed near surfaces of the bulk material, particularly those surfaces lying perpendicular to the modulation direction. This relaxation can be a dominating effect in materials which have been thinned in cross section for study by TEM. Lattice plane bending near the specimen surfaces can cause strong diffraction contrasts, and the local lattice spacings are representative of neither the bulk, tetragonally distorted material, nor the unstressed material. In this paper we discuss how this important, and oft-neglected relaxation affects TEM bright-field and dark-field image contrasts, high resolution lattice images, and interpretation of selected area diffraction data. We also derive expressions, relevant for all sample thicknesses, describing the relaxation strain field in cubic anisotropic materials, thinned perpendicularly to the modulation direction. The results are discussed in the context of two classes of modulated materials, namely spinodally-decomposed InGaAsP alloys and GeSi/Si strained layer superlattices.
Electron energy distributions in oxygen microwave plasmas
Authors: Heidenreich J. E. , Paraszczak J. R. , Moisan M. , Sauve G.
Published: July 2002

Knowledge of the electron energy distribution is of fundamental importance in the understanding of chemical reactions in plasmas. Many of these reactions are initiated by electron impact, and their relative importance depends on the kinetic energy of the plasma electrons. The Langmuir probe is an important plasma diagnostic tool, which yields the density and potential of the plasma, as well as the electron temperature. A major shortcoming of this technique is the presumption of Maxwellian electron energy distributions. Determination of the electron energy distribution for a non-Maxwellian plasma requires a modified Langmuir probe technique developed by Druyvesteyn. We have measured distributions in microwave (915 MHz) excited O2 plasmas and observed little variation over the range of parameters studied. The measured distributions were fitted to a function of the form g(E)=a√E exp(−bEx) and a compliation of 18 data sets yielded a value of x=1.53±0.07, whereas Maxwellian distributions would have yielded x=1. The average electron energies from the measured distributions were in surprisingly good agreement with values from the Langmuir probe electron temperatures.
Comparison of etch rates of silicon nitride, silicon dioxide, and polycrystalline silicon upon O2 dilution of CF4 plasmas
Authors: Riley Paul E. , Hanson David A.
Published: July 2002

The etch rates of Si3N4, SiO2, and doped polycrystalline silicon with CF4 plasmas have been examined as a function of O2 dilution and radio frequency (rf) (13.56 MHz) power density at constant residence time (25 mTorr and 25 sccm total process gas flow rate) at 22±2 °C in a batch reactor. At fixed reactor pressure and process gas composition, the etch rates of these materials increase linearly, with different slopes, with increasing rf power density. Under the conditions studied, the etch rates of SiO2 are lower than those of the other two materials and are primarily a function of rf power for CF4 plasmas that are diluted with 0%–25% O2 by flow rate. In comparison, the etch rates of Si3N4 and doped polycrystalline silicon are functions of both rf power density and gas composition, although at each rf power level (200, 300, and 400 W) the Si3N4 etch rate reaches a plateau at an O2 dilution of ∼10%–15% by flow rate. Although the doped polycrystalline silicon etch rate exhibits similar behavior at 200 and 300 W, a steady increase in etch rate is noted at 400 W over the range of O2 dilution examined in this work. At low O2 dilutions, the Si3N4 etch rate is greater than the doped polycrystalline silicon etch rate. However, with increasing O2 dilution, the doped polycrystalline silicon etch rate exceeds the Si3N4 etch rate, with the intersections of these etch rate curves shifting to greater O2 dilution at higher rf power settings.
The role of ultrathin AlAs interlayers in determining the interface Fermi energy of the epitaxial NiAl/AlAs/n-GaAs(001) system
Authors: Chambers S. A. , Loebs V. A. , Doyle D. H.
Published: July 2002

We have investigated the compositional, structural, and band bending effects of growing ultrathin AlAs interlayers between epitaxial NiAl and n-GaAs(001) by molecular-beam epitaxy (MBE). Such an interface is of interest because of the possibility of increasing the already large Schottky barrier height at the NiAl/GaAs interface by inserting a lattice-matched interlayer of larger band gap. We have found that the Fermi energy in the GaAs band gap, as measured by x-ray photoemission spectroscopy (XPS), is only 0.35–0.40 eV below the conduction-band minimum when very high quality AlAs is grown on n-GaAs(001) by MBE. This result is presumably due to the creation of an interface of very high crystallographic quality and the related low density of interface states. We have also measured the valence band offset by XPS to be 0.40±0.07 eV, independent of band bending. Subsequent MBE growth of NiAl at 250 °C invariably increases band bending at the AlAs/GaAs interface, leaving the Fermi energy ∼0.8 eV below the conduction-band minimum on the GaAs side. This increase in band bending is accompanied by disruption of the AlAs/GaAs interface, which appears to be driven by diffusion of Ni atoms through the thin AlAs interlayer. Post-growth annealing at 570 °C increases the extent of disruption at the AlAs/GaAs interface and drives the Fermi level to ∼1.0 eV below the conduction-band minimum. Combining the valence-band offset and band bending measurements, the barrier height of the NiAl/AlAs/GaAs(001) interfacial system is estimated to be 1.15–1.35 eV, which is nearly a factor of 2 larger than values exhibited by conventional metal/GaAs interfaces.
Adaptive neural network algorithms for computing proximity effect corrections
Authors: Frye Robert C.
Published: July 2002

Computing self-consistent local dose corrections for images to offset proximity effects poses significant problems because the number of computations needed cannot be done in a reasonable amount of time. Recently, we have demonstrated the use of an adaptive neural network method to increase the speed of these computations by several orders of magnitude. We have now implemented these corrections in practical hardware, and have introduced improvements in the algorithm that further reduce the computational complexity and time. The challenges in computing proximity effect corrections are to find algorithms that work well for general feature shapes, and that can be efficiently implemented. This paper will discuss the iterative computation of optimal corrections for electron scattering, and the limits of image resolution that can be obtained. It will describe the neural network algorithm used to obtain equivalent results more efficiently, and the method of adaptively determining the network’s parameters. Finally, it will discuss several recent modifications that significantly improve the network’s performance.
Luminescence in scanning tunneling microscopy on III–V nanostructures
Authors: Alvarado S. F. , Renaud Ph. , Abraham D. L. , Schönenberger Ch. , Arent D. J. , Meier H. P.
Published: July 2002

Using electroluminescence associated with scanning tunneling microscope in AlxGa1−xAs heterostructures, we show that: (a) luminescence due to recombination can be induced within single quantum wells of dimensions down to a few nm and can also be used to image them, (b) the energy of bulk bands can be determined, and (c) transport parameters can be measured, e.g., the thermalization length and the diffusion length of minority electrons. This technique opens up new possibilities for the study and characterization of semiconductors and devices—including the possibility of studying surface states and single trapping centers associated with lattice defects, impurities, chemisorbed species, etc.
Scanning cathodoluminescence microscopy: A unique approach to atomic-scale characterization of heterointerfaces and imaging of semiconductor inhomogeneities
Authors: Christen J. , Grundmann M. , Bimberg D.
Published: July 2002

Luminescence experiments provide a powerful and nondestructive approach to the ex situ investigation of semiconductor heterointerfaces which might be buried up to several μm below the surface in a given complex sample structure. Combined with the ability of taking images simply by scanning the exciting focused electron beam across the area under investigation, lateral fluctuations of electronic properties like the variation of the fundamental band gap Eg(x,y) can be directly visualized by scanning cathodoluminescence (CL). The novel experimental approach, cathodoluminescence wavelength imaging (CLWI), which involves recording of a complete CL spectrum at every scanning position (x,y), yields direct 3D images of the atomic-scale morphology of quantum wells (QWs) as sensed by the QW exciton: similar to the tip of a scanning tunneling microscope, the exciton samples the local fluctuations of QW thickness Lz and transforms this structural information Lz(x,y) into a spectral one, the lateral variation of band gap Eg(x,y) and thus the CL emission wavelength λ(x,y). Topological maps of QW interfaces can thus be recorded at various positions and at various magnifications. The interface roughness can be investigated statistically at lateral resolution starting with the diameter of the QW exciton up to the mm regime. The same experimental principle for recording λ(x,y) and Eg(x,y) maps is successfully applied for the analysis of patterned structures. In the nonlattice-matched system GaAs on Si, the lateral strain variation causes Eg(x,y) fluctuations and can thus be directly imaged by CLWI. Metalorganic chemical vapor deposition grown GaAs layers on micropatterned Si(001) substrates show strongly inhomogeneous doping with Si impurities. By means of CLWI the strong increase of this Si incorporation in the vicinity of free {111} surfaces is measured and Si concentration maps are recorded across the complete sample pattern.
Structural damage induced by Ga+ focused ion beam implantation in (001) Si
Authors: Chu C. H. , Hsieh Y. F. , Harriott L. R. , Wade H. H.
Published: July 2002

Ga+ focused ion beam (FIB) implantation in (001) Si was conducted in our JEOL JIBL-104UHV FIB system. Ga+ ions with energy of 100 kV were implanted into silicon at doses from 1×1014 to 1×1016 /cm2. The current and current density of the Ga+ FIB are 20 pA and 1 A/cm2, respectively. We have made patterns consisting of single-pixel lines with various spacings and rectangular areas of different sizes. The critical dose to form a continuous amorphous region in rectangle-scanned samples is less than 5×1014 /cm2. The samples were annealed either in vacuum or a rapid thermal annealing furnace at 550–800 °C for 1/2 h and 60 s, respectively. Both cross-sectional and plan-view transmission electron microscopy were conducted to investigate the lattice disorder and residual defects in as-implanted and post-implantation annealed samples. After thermal annealing, dislocation lines and loops were observed along the line direction in line-scanned samples. In rectangle-scanned samples, dislocation lines and loops near the original amorphous/crystalline (a/c) interface, as well as edge defects along the edge of the rectangle were found.
Magnetron reactive ion etching of GaAs in SiCl4
Authors: Meyyappan M. , McLane G. F. , Lee H. S. , Eckart D. , Namaroff M. , Sasserath J.
Published: July 2002

Magnetron reactive ion etching of GaAs was investigated for the first time in a SiCl4 plasma as a function of process parameters such as pressure and power density. The etch rate of GaAs is found to be much faster than the figures reported for conventional (unmagnetized) reactive ion etching. Vertical sidewalls and smooth surface morphology were obtained under the reported experimental conditions. Schottky diodes were fabricated on etched samples to assess the extent of residual damage. The ideality factor and barrier height of the etched samples were found to be close to those of an unetched control sample. The results indicate that SiCl4 magnetron etching has promise for use in GaAs device fabrication.
Recipe to minimize Te precipitation in CdTe and (Cd,Zn)Te crystals
Authors: Vydyanath H. R. , Ellsworth J. , Kennedy J. J. , Dean B. , Johnson C. J. , Neugebauer G. T. , Sepich J. , Liao Pok-Kai
Published: July 2002

Using a quasichemical approach, the total native defect concentration and the minimum deviation in stoichiometry have been calculated in CdTe crystals as a function of the Cd pressure at various temperatures. With this knowledge, CdTe and (Cd,Zn)Te wafers have been subjected to postgrowth step annealing treatment under conditions such that the crystals are in equilibrium with a Cd or (Cd,Zn) vapor corresponding to the minimum in deviation from stoichiometry at each annealing temperature. The step annealed CdTe and (Cd,Zn)Te wafers have been examined under infrared microscopy and have shown significant reduction in the concentration of Te precipitates, whereas the unannealed wafers have had numerous Te precipitates distributed throughout the bulk. HgCdTe epitaxial films have been grown on the step annealed CdTe and (Cd,Zn)Te wafers as well as on unannealed wafers from the same boule. Examination of the cross sections of the epitaxial films indicates appearance of Te precipitates in films grown on unannealed substrates, whereas no Te precipitation was evident in films grown on the annealed substrates leading to the inference that the occurrence of Te precipitates in the (Hg,Cd)Te films is possibly related to the presence of Te precipitates in the substrates. Thermal migration of Te under a temperature gradient during step annealing is suggested as a possible mechanism in the elimination of larger size Te precipitates whereas the extremely fine precipitates (<1 μm) appear to need in-diffusion of metal vapor for their elimination.
Low-temperature cleaning processes for Si molecular beam epitaxy
Authors: Thompson P. E. , Twigg M. E. , Godbey D. J. , Hobart K. D. , Simons D. S.
Published: July 2002

Hydrogen-terminated surface cleaning techniques of silicon substrates were investigated by using x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM). Either a 4% HF dip or an HF-terminated abbreviated Shiraki clean was used as the cleaning technique. Shiraki-cleaned samples were grown as control samples. XPS was used to measure the C, O, and F remaining on the surface at various stages of the cleaning/growth process, including after a 1 h bake at 200 °C prior to growth. XPS did not detect a significant difference in the adsorbate concentrations between the baked and unbaked samples. From SIMS, the lowest impurity concentrations at the epitaxial/substrate interface were achieved with the abbreviated Shiraki clean, approximately at the same levels as obtained with the standard Shiraki clean, 1.3×1013, 5.4×1012, 1.6×1010, and 4.2×1011/cm2 for C, O, F, and N, respectively. This was achieved without the 850 °C anneal required to desorb the oxide on the Shiraki–cleaned samples. The epitaxial structure chosen for the TEM study was a 30 nm Si buffer grown at 650 °C followed by a 60 nm Si0.7Ge0.3 quantum well and a 250 nm Si layer, both grown at 500 °C. The structures were observed both as-grown and after a 850 °C, 1/2 h anneal used to study defect propagation. There were no detectable differences in the as-grown structures using plan-view TEM. However when the samples were annealed, the HF-dipped specimen developed inclusions in the top Si layer which were not observed in the other samples. In contrast, the prebaked HF-dip-cleaned sample did not have these defects when annealed.
Sub-40 nm resolution 1 keV scanning tunneling microscope field-emission microcolumn
Authors: Kratschmer E. , Kim H. S. , Thomson M. G. R. , Lee K. Y. , Rishton S. A. , Yu M. L. , Chang T. H. P.
Published: July 2002

Recent advances in microcolumn design and fabrication have led to a significant improvement in the resolution obtained with a new 3.5 mm long microcolumn. At an electron energy of 1 keV and 2 mm working distance, the beam diameter, measured by the signal rise-time while scanning over a sharp edge, is 40 nm and the resolution observed in scanning transmission electron microscope images is about 30 nm. These experimental results agree well with results of the electron-optical modeling. All lenses and apertures in the column are made from silicon membranes and assembled using a novel ultrahigh vacuum compatible, multilayer anodic bonding technique. The scanning tunneling microscope aligned field-emission source uses an oxygen processed, 50 nm radius, cold 〈111〉 W field-emission tip. Good emission stability for scanning electron microscope operation has been achieved with a few percent of root-mean-square current fluctuation over about 30 min. With periodic tip flashes, the system has been operated reliably over several weeks. A novel in situ tip alignment technique with respect to the extractor electrode has been developed. This technique also provides information about the tip emission characteristics during column operation.