bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of radiation induced defects on the capacitance of silicon detectors
Authors: Darvas R. , Mudrik M. , Seidman A. , Zilberstein M. , Croitoru N.
Published: July 2002

The effect of neutron induced defects on Si detectors was studied both experimentally and theoretically. These defects form deep energy levels within the band gap. Four such levels were detected: 0.17, 0.24, 0.4, and 0.46 eV, using DLTS (deep level transient spectroscopy). The corresponding defects were identified and characterized. The capacitance was measured at different frequencies and temperature. It was shown that the capacitance of the irradiated detectors strongly depends upon temperature. A relation for this dependence (C-T) was found. Comparisons of DLTS with C-T results showed good agreement.
Nonlinear digital filtering of scanning-probe-microscopy images by morphological pseudoconvolutions
Authors: Weisman Andrew D. , Dougherty Edward R. , Mizes Howard A. , Miller R. J. Dwayne
Published: July 2002

A recently developed class of digital filters known as morphological pseudoconvolutions is applied to scanning tunneling microscopy (STM) images. These filters use a nonlinear branch of image processing known as morphology to improve the characteristics of both moving mean and moving median filters. They filter equally in both the x and y directions, so as not to introduce artifacts, and they have an adjustable parameter that allows the user to restore the observed image completely as the parameter tends to infinity. Very few assumptions are made concerning image and noise content; only the shape of typical data is taken into account. These filters are shown to outperform, both visually and in the mean-square-error sense, previously introduced Wiener filtering techniques. The filters are compared on typical STM-type images, using both modeled and actual data. The technique is general, and has been shown to perform very well on all types of STM and atomic force microscopy images.
Periodic grating-gate screening of plasmons in heterojunction structures
Authors: Ager C. D. , Wilkinson R. J. , Hughes H. P.
Published: July 2002

The far-infrared optical response of a structure consisting of a perfectly conducting grating gate on a modulation-doped GaAs/AlGaAs heterojunction has been calculated using the scattering matrix technique. The grating gate allows coupling between the incident radiation and the plasmon resonance of the two-dimensional electron gas (2DEG) in such a system, but also modifies the dispersion relation for the plasmon because of screening by the image charge distribution induced on the grating gate. Using the scattering matrix technique allows this screening effect to be modeled more accurately than has hitherto been possible, and our calculations show that the induced charge is concentrated at the edges of the grating fingers. The sinusoidal profile of the oscillating charge density in the 2DEG distorts so that the maximum amplitude of the oscillation is close to these edges, and the plasmon frequency is determined by the amount of distortion of the plasmon charge density profile and the additional screening gained through the distortion. Plasmon frequencies are calculated for a wide range of lamellar grating mark/space ratios, and are found to lie between the extremes predicted for plasmons in 2DEGs close to a vacuum interface and close to a continuous metal gate.
A Raman study of Au/Te/Au/GaAs (100) ohmic contacts
Authors: Münder H. , Andrzejak C. , Berger M. G. , Lüth H. , Borghs G. , Wuyts K. , Watté J. , Silverans R. E.
Published: July 2002

Ohmic contacts produced by high-energy pulsed laser beam alloying Au/Te/Au/n-GaAs are investigated by micro Raman spectroscopy. The results are compared to those from furnace annealed ohmic contacts. For the furnace as well as for the laser annealed ohmic contacts, no evidence for a doping of the contact region is found in the Raman spectra. The presence of a highly disordered GaAs surface layer is observed for both types of contacts. In addition, after furnace processing a Ga2Te3 layer is formed. These results are consistent with earlier Mössbauer studies. For the laser alloyed samples the results strengthen the role of a defective/disordered interface structure where conduction might occur by a resonant tunneling process involving localized gap states.
The dopant and compound forming behavior of As and Au impurities in Ga2Te3
Authors: Wuyts K. , Watté J. , Langouche G. , Silverans R. E. , Zégbé G. , Jumas J. C.
Published: July 2002

The dopant and compound forming behavior of As and Au impurities in Ga2Te3 is investigated by the combined application of Mössbauer spectroscopy, x-ray diffraction, optical transmission, and electrical measurements. Arsenic is shown to act as a n-type dopant in Ga2Te3, and a ternary semiconductor AuGa2Te3 is identified. The results allow for a concise description of the ohmic contact formation mechanism in alloyed Au/Te/Au/GaAs and related structures.
Surface trap creation in polyvinylidene fluoride and poly(vinylidene fluoride/trifluoroethylene) on peeling from a silicon substrate
Authors: Andry P. , Filion A. Y. , Perlman M. M.
Published: July 2002

Open-circuit dark-surface potential measurements on corona-charged solution cast films of polyvinylidene fluoride and a copolymer of vinylidene fluoride and trifluoroethylene show a slower decay after these films have been mechanically peeled from their silicon substrates. It is proposed that peeling produces a thin region of high trap density in the interface layer close to the peeled surface. A simple model of surface voltage decay is presented which accounts for near-surface trapping due to peeling as well as bulk trapping. Practical ranges for the interface layer thickness and trapping density are discussed.
Kinetic and structural study of the epitaxial realignment of polycrystalline Si films
Authors: Benyaïch F. , Priolo F. , Rimini E. , Spinella C. , Ward P.
Published: July 2002

The epitaxial realignment of undoped and As doped polysilicon films onto crystalline silicon substrates induced by high-temperature rapid-thermal annealing has been investigated. It is shown that the realignment mode and the kinetics of the process are intimately related to the microcrystalline structure of the layers under investigation, to the morphology of the native oxide film present at the interface, and to the presence of As atoms dispersed in the deposited layers. For layers having fine grain dimensions, compared to the film thickness, the realignment takes place via the motion of the crystal-polysilicon interface towards the surface. This is observed in undoped layers and in layers which have been subjected to a high-temperature anneal before As doping. The preimplant anneal disrupts the interfacial oxide film and reduces the thermal cycle needed to complete the realignment of the polysilicon layers. In layers which have not experienced any thermal treatment before As doping, it is seen that the grain size first increases to dimensions on the order of the film thickness, and the realignment transformation then proceeds by lateral growth of epitaxial columns in a manner similar to secondary grain growth. The kinetics of both realignment modes are thermally activated and the atomic limiting processes have been tentatively identified to be As diffusion in bulk Si for As doped layers and Si self diffusion for undoped films. The effect of the microcrystalline structure on the realignment kinetics is attributed to its relationship with the driving force governing the realignment transformation.
Temperature and substrate dependence of Ar sputtering of CoSi2 thin films
Authors: Hong Q. Z. , Harper J. M. E.
Published: July 2002

The temperature dependence of 300 eV argon ion sputtering of CoSi2 thin films has been investigated in the range 50–600 °C. Compared with the low-temperature values, the high-temperature etch rate of CoSi2 on Si decreases by a factor of 3, while the sputtering yield ratio of Si to Co increases by a factor of 3.5. This leads to effective motion of the silicide layer into the Si substrate at temperatures above 400 °C. During sputtering of CoSi2 on SiO2, the thickness of the silicide layer decreases almost linearly with bombardment time until all the silicide is removed. Similar behavior is observed in low-temperature sputtering of CoSi2 on Si. However, at elevated temperatures (400 °C<T≤600 °C), the etch rate of the silicide decreases significantly after an initial fast sputtering period. For CoSi2 on Si, experimental results show that while Si atoms are sputtered from the surface, Co atoms are released and diffuse to the silicide/Si interface where they react with the underlying Si substrate to form new CoSi2.
Spatially dependent effective mass and optical properties in finite parabolic quantum wells
Authors: Herling G. H. , Rustgi M. L.
Published: July 2002

The effects of the spatial dependence of the effective mass in GaAs/AlxGa1−xAs parabolic quantum wells on several of their optical properties have been calculated. The largest effects are on the conduction-band levels which are lowered with respect to those with constant effective mass. Effects on oscillator strengths, hole energy levels, and electron/hole overlaps are small.
Use of low-frequency capacitance in deep level transient spectroscopy measurements to reduce series resistance effects
Authors: Anand S. , Subramanian S. , Arora B. M.
Published: July 2002

In this article, we demonstrate the usefulness of a low-frequency capacitance deep level transient spectroscopy (LFDLTS) technique suitable for samples with a large built-in resistance in series with the depletion capacitance. As a case study, we investigate the effects of series resistance arising from the contact resistance and the bulk resistance of aluminum gallium arsenide epitaxial layers grown on semi-insulating gallium arsenide substrates. We show that there can be significant errors in the density and the activation energy of the traps (the so-called DX centers) determined by the conventional high-frequency (1 MHz) capacitance deep level transient spectroscopy measurements when the series resistance associated with the sample is high. On the other hand, the LFDLTS measurements are found to be free from these artifacts related to the series resistance effects.
Investigation of the oxidation kinetics of C54-TiSi2 on (001)Si by transmission electron microscopy
Authors: Huang G. J. , Chen L. J.
Published: July 2002

A transmission electron microscopy study of oxidation kinetics of C54-TiSi2 for both dry and wet oxidation has been carried out. Precautions were taken to determine the activation energies of oxidation in the temperature and time regime where the islanding of TiSi2 did not occur. For dry oxidation, activation energies for parabolic and linear growth were found to be 1.97 and 2.50 eV (with an error bar of ±0.1 eV), respectively. On the other hand, activation energies for parabolic and linear growth were found to be 1.88 and 2.10 eV (with an error bar of ±0.1 eV), respectively, for wet oxidation. The activation energy of parabolic rate constant is seen to be substantially different from those obtained previously. The difference is attributed to the occurrence and absence of islanding during oxidation in the previous and present study, correspondingly. The closeness of linear activation energy of TiSi2 oxidation with that of pure silicon is thought to be due to the fact that both are related to the breaking of the Si—Si bonds at the silicon surface. Orthorhombic TiO2 was observed to form on the surface of all samples wet oxidized at 880–1040 °C for 5–70 min. For dry oxidized samples, no TiO2 was detected in samples dry oxidized at 880–940 °C for 5–50 min and at 920 °C for up to 6 h. However, in samples dry oxidized at 920 °C for 10–15 h, TiO2 was found to form. For dry oxidation, it is thought that although the formation energy of TiO2 is considerably higher than that of SiO2, the nucleation barrier for forming TiO2 is higher. Only after prolonged oxidation was the nucleation barrier overcome and led to the formation of TiO2.
Ballistic pulse propagation in quantum wire waveguides: Toward localization and control of electron wave packets in space and time
Authors: Hayata K. , Tsuji Y. , Koshiba M.
Published: July 2002

A theoretical formulation of electron pulse propagation in quantum wire structures with mesoscopic scale cross sections is presented, assuming quantum ballistic transport of electron wave packets over a certain characteristic length. As typical mesoscopic structures for realizing coherent electron transmission, two traveling-wave configurations are considered: straight quantum wire waveguides and quantum wire bend structures (quantum whispering galleries). To estimate temporal features of the pulse during propagation, the walk off, the dispersion, and the pulse coherence lengths are defined as useful characteristic lengths. Numerical results are shown for ultrashort pulse propagation through rectangular wire waveguides. Effects due to an external electric field are discussed as well.
Evidence of silicon segregation as a function of arsenic overpressure in GaAs grown by molecular beam epitaxy
Authors: Fatt Yoon Soon
Published: July 2002

The effects of heavy Si doping in GaAs layers grown by molecular beam epitaxy as a function of varying arsenic to gallium (group V/III) flux ratio have been studied. By using a slow growth rate of 1500 Å/h, a high electron concentration of ≊5.5×1018 cm−3 was achieved, which decreases sharply following a decrease in the group V/III flux ratio. The low-temperature photoluminescence spectra also show a broad low-energy peak (≊1.284 eV) as the flux ratio decreases. Secondary ion mass spectroscopy measurements show that the effect of Si accumulation near the GaAs surface is increased at lower flux ratios.
Hydrogen termination of the NH4F-treated Si(111) surface studied by photoemission and surface infrared spectroscopy
Authors: Niwano M. , Takeda Y. , Kurita K. , Miyamoto N.
Published: July 2002

The Si(111) surface treated in a saturated solution of NH4F has been investigated using photoemission spectroscopy with synchrotron radiation and surface infrared spectroscopy (SIS) in the multiple internal reflection mode. Photoemission and SIS data clearly demonstrate that the NH4F-treated Si(111) surface is dominantly terminated with the monohydride Si (Si-H) oriented perpendicular to the surface and is free from silicon oxide. It is suggested that the absence of silicon oxide is closely related to the atomic flatness of this surface.
Electroluminescence efficiency increase due to recombination-enhanced annealing of recombination centers in GaAs
Authors: Uematsu Masashi , Yamada Kohji , Wada Kazumi
Published: July 2002

Electroluminescence (EL) efficiency increase due to recombination-enhanced annealing of recombination centers has been observed. The EL of the band-to-band emission from GaAs tunnel diodes has been investigated. The EL efficiency is found to increase as the diodes are degraded due to the recombination-enhanced process. The EL efficiency increase is attributed to the recombination-enhanced annealing of the recombination centers.
X-ray characterization of InAs laser structures grown by molecular beam epitaxy
Authors: Mazuelas A. , Meléndez J. , Dotor M. L. , Huertas P. , Garriga M. , Golmayo D. , Briones F.
Published: July 2002

An x-ray interference effect was used to characterize a set of strained layer laser structures containing N monolayers of InAs (N=1, 3, 5, 7) conveniently distributed in the quantum well active region. A sample containing 100 Å of Ga0.8In0.2As in the quantum well was also grown and characterized for comparison. Structural parameters such as thickness, chemical composition, and strain status of the different layers (cladding, waveguide, and quantum well layers) as well as the relaxation process and critical thickness due to increasing InAs content in the active region were studied. It was found that indium content was very close to the design values and that the whole structure is coherent with the substrate for 1 and 3 monolayers of InAs (and 100 Å of Ga0.8In0.2As) while the structure starts to relax by dislocation formation for 5 monolayers of InAs and is clearly relaxed for 7 monolayers of InAs. These x-ray results are in full agreement with transmission electron microscopy and characterization of the structures as laser devices.
Electrical and infrared investigation of erbium silicide
Authors: Unewisse M. H. , Storey J. W. V.
Published: July 2002

Schottky diodes fabricated from erbium silicide on n-type silicon have been studied to examine their suitability as infrared detectors. The devices were fabricated by thermal evaporation onto <100≳ silicon. The electrical barrier height of 0.28 eV measured by current-voltage and current-voltage-temperature techniques is consistent with that measured by previous workers. The photon barrier height is found to be close to the electrical barrier height. The quantum efficiency at 2.0 μm is found to be 0.52%, comparable to that of unenhanced PtSi, while the dark current is less than 10−7 A cm−2 at 77 K. We conclude that ErSi2 has considerable potential for use in infrared detector arrays.
Low-field diamagnetic response of hollow cylindrical samples of sintered high-T
c superconductors
Authors: Pace Sandro , De Luca Roberto
Published: July 2002

Magnetization curves of hollow cylindrical samples of sintered high-Tc superconductors are derived both numerically, by means of a Josephson junction array model, and analytically, by a critical-state model. In both cases the field dependence of the critical current density Jc is assumed to have the following analytic form: Jc(h,T)=Jc0(T) H02(T)/[H0(T)2+h2]. The two descriptions are seen to be qualitatively equivalent.
The effect of superdislocation on fracture: Dislocation-free zone in the front of a semi-infinite crack tip
Authors: Shiue Sham-Tsong , Lee Sanboh
Published: July 2002

The effect of superdislocation on mode II or mode III fracture has been investigated. The plastic zone between a semi-infinite crack tip and a superdislocation with a dislocation-free zone is simulated by a continuous dislocation distribution. The dislocation distribution functions in the crack and plastic zone are analytically derived. A close form of the number of dislocations in the plastic zone and stress field along the x axis are obtained. From the stress field, the stress intensity factor KIII at the crack tip and stress concentration at the superdislocation are available. When the distance between crack tip and superdislocation is larger than the plastic zone size, the stress intensity factor is proportional to the square root of plastic zone size KIII, but is inversely proportional to the distance between crack tip and superdislocation. On the other hand, when the distance between crack tip and superdislocation is almost the same as the plastic zone size, the stress intensity factor is inversely proportional to the square root of distance between crack tip and superdislocation. It is also found that the applied stress is determined by the superdislocation. Finally, our results can be reduced to several special cases.
Effects of high temperature irradiation on SmCo permanent magnets
Authors: Kähkönen O-P. , Kautto E. , Manninen M. , Talvitie M.
Published: July 2002

SmCo5 permanent magnets have been irradiated with 19.5 MeV protons at temperatures from about 300 to over 800 K. The results show that SmCo5 magnets are very sensitive to irradiation at elevated temperatures. The relative magnetic flux loss has been calculated with a model based on the local heating caused by the incoming particle. The theoretical result agrees well with the experimental data.
Reactions and orientation effects of the Fe/Pd/Cu/Si, Fe/Ag/Pd/Cu/Si, Fe/Au/Pd/Cu/Si, and Fe/Pt/Pd/Cu/Si structures using (100) and (111) Si
Authors: Chang Chin-An
Published: July 2002

(100)- and (110)-oriented Fe films, grown on Pd/Cu/Si, Ag/Pd/Cu/Si, Au/Pd/Cu/Si, and Pt/Pd/Cu/Si using (100) and (111) Si, respectively, have been heated to 600 °C, and the reactions studied. The (100) Fe films remain little changed both structurally and magnetically, after an anneal of 30 min between 300 and 400 °C in an ambient of N2-H2 (9:1). The saturation magnetization shows little change at 400 °C, while the coercivity increases at this temperature. Above 400 °C, the Fe films degrade depending on the structures grown. For the Fe/Pd/Cu/Si and Fe/Pt/Pd/Cu/Si structures, both the magnetic and structural characteristics of Fe remain partially intact at 600 °C. In both cases, the Pt and Pd layers have been consumed for the silicide formation above 400 °C. For the Fe/Ag/Pd/Cu/Si structure, Fe rapidly degrades at 500 °C, with Ag being little changed till 600 °C. For the Fe/Au/Pd/Cu/Si structure, the formation of the bulk Au-Si eutectic melt at 363 °C completely destroys the Fe layer above 400 °C. In comparison, a lower thermal stability is observed for the (110) Fe layers grown on (111) metal seeds using (111) Si, with reduced saturation magnetizations at 400 °C. A larger diffusion of Pd, Ag, Au, and Pt into the (110) Fe layers is also observed than those deposited on (100) Si. The mechanisms involved are discussed and are related to the different thermal stabilities observed.
Electric-field dependence of interband transitions in In0.53Ga0.47As/In0.52Al0.48As single quantum wells by room-temperature electrotransmittance
Authors: Dimoulas A. , Giapis K. P. , Leng J. , Halkias G. , Zekentes K. , Christou A.
Published: July 2002

Room-temperature electrotransmittance has been used in order to investigate the interband excitonic transitions in a 250-Å-thick In0.53Ga0.47As/In0.52Al0.48As single-quantum-well system as a function of an externally applied electric field. Parity forbidden transitions, involving conduction-band states with quantum numbers up to n=5, which become more pronounced at high electric fields were observed. The ground-state and the forbidden transitions showed a significant red shift due to the quantum confined Stark effect. A comparison with previously reported results on thinner InGaAs/InAlAs quantum wells indicated that the wide-well sample exhibits the largest shift, as expected from theory. Despite the appreciable Stark shift, the rather large, field-induced linewidth broadening and the relatively low electric field at which the ground-state exciton is ionized poses limitations on using this wide-quantum-well system for electro-optic applications.
A kinetic study on tungsten deposition from SiH4 and WF6
Authors: van der Jeugd C. A. , Janssen G. C. A. M. , Radelaar S.
Published: July 2002

The reaction kinetics of tungsten deposition from a SiH4/WF6 gas mixture was investigated for SiH4/WF6 feed-gas flow ratios lower than unity. By performing selective depositions, care was taken to ensure that the kinetic data was obtained in the surface-kinetics controlled regime. It was found that the reaction kinetics of the deposition process depends on the SiH4/WF6 ratio. For low SiH4/WF6 ratios (smaller than ≊0.7) the growth rate is slightly negative in [WF6] and almost quadratic in [SiH4]. The growth rate is almost independent of temperature between 200 and 350 °C. On the basis of our measurements it is argued that in this regime the surface is covered mainly with W, WFx, and F, and that the rate-limiting step is the dissociative adsorption of SiH4 on this surface. For higher SiH4/WF6 ratios (0.7≤SiH4/WF6≤1) the growth rate increases sharply as the SiH4/WF6 ratio increases. In this regime the growth rate cannot be expressed in the simple form: rate=A⋅[WF6]α⋅[SiH4]β since the growth rate is more sensitive to the SiH4/WF6 partial pressure ratio than to the absolute values of the SiH4 and WF6 partial pressure. For SiH4/WF6 ratios as high as 1, the growth rate is limited only by the flux of the species hitting the surface.
The combination of high-temperature sputtering with excimer laser planarization for submicrometer contact via filling
Authors: Yu C. , Sandhu G. S. , Doan T. T.
Published: July 2002

We have systematically investigated the use of high-temperature sputtering in conjunction with excimer laser planarization for planarizing Al alloy surfaces and filling submicrometer, high aspect ratio (∼1–3) contact vias. In general, the process window for the complete contact filling increased significantly with the increasing sputtering temperature, and decreased with shrinking contact/via geometry. Better as-deposited metal step coverage and profile obtained at higher sputtering temperatures were identified as the key parameters contributing to this significant improvement in the process window. The process window achieved by this combined process is significantly wider than that obtained by high-temperature sputtering alone. In addition, the poor surface morphology generally observed for the Al alloy films sputtered at elevated temperatures is significantly improved due to recrystallization during laser reflow.
Application of ‘‘critical compositional difference’’ concept to the growth of low dislocation density (<104/cm2) InxGa1−xAs (x≤0.5) on GaAs
Authors: Krishnamoorthy V. , Lin Y. W. , Park R. M.
Published: July 2002

Multilayer epitaxial structures consisting of InxGa1−xAs layers of various compositions were grown on GaAs substrates by the molecular beam epitaxy technique. Dislocation evolution and residual strain in these heterostructures were studied using cross-sectional transmission electron microscopy (XTEM) and high-resolution x-ray diffraction analyses, respectively. The multilayer heterostructures were designed such that the compositional difference between two adjacent InxGa1−xAs layers in the stack was less than a critical compositional difference of Δx=0.18, taking partial lattice-relaxation into account. XTEM studies of the stacked structures indicated dislocation evolution to be confined to the GaAs substrate and the InxGa1−xAs layers underlying the top InxGa1−xAs layer in the stack, the top InxGa1−xAs layer being essentially dislocation-free. This phenomenon is attributed to a monotonic increase in the yield strength of InxGa1−xAs at the appropriate growth temperatures with increasing values of x. Such behavior appears to persist up to an InxGa1−xAs composition of approximately x=0.5, whereupon a further increase in composition results in dislocation evolution in the top layer of the stack. It is postulated that the yield strength of InxGa1−xAs decreases with increasing values of x beyond x=0.5. Extremely low dislocation density InxGa1−xAs material was grown on GaAs using the stacked structure approach as evidenced by etch pit analysis. For example, dislocation densities of 1–2×103/cm2 and 5–6×103/cm2 were recorded from In0.35Ga0.65As and In0.48Ga0.52As top layers, respectively. Such InxGa1−xAs alloys would be potentially suitable for the fabrication of photonic devices operating at 1.3 μm (x=0.35) and 1.55 μm (x=0.48).
Wide electron gas with periodic density modulation
Authors: Sundaram M. , Wixforth A. , Hopkins P. F. , Gossard A. C.
Published: July 2002

We report the growth and characterization of a high-quality wide (∼2000 Å) three-dimensional electron gas (3DEG) with periodic density modulation (period ∼200 Å) in a modulation-doped wide parabolic potential well with a superimposed superlattice. Computer-controlled molecular beam epitaxy is used to synthesize the potential well as a graded AlxGa1−xAs digital alloy. The density-modulated 3DEG is compared to a uniform 3DEG of the same average density and width in a parabolic well without the superlattice. The Al mole fraction profiles for the two samples are measured in calibration runs immediately prior to actual growths. The density-modulated 3DEG has a low-temperature in-plane mobility in excess of 105 cm2/V s, compared to ∼2×105 cm2/V s for the uniform 3DEG. Capacitance-voltage measurements directly reveal the modulation of the density of the electron gas in the parabolic well with superimposed superlattice, and the absence of any density modulation for the gas in the bare parabolic well.
Nonresonant third order susceptibility measurements in a nematic liquid crystal-PCH-5
Authors: Philip John , Prasada Rao T. A.
Published: July 2002

We report for the first time the determination of the real part of the third order nonlinear susceptibility χ(3)(−ω;ω,0,0) above the nematic to isotropic phase transition temperature of a nematic liquid crystal 4 (trans-4′-4′-n-pentylcyclohexy)-benzonirtile (PCH-5) from electro-optic Kerr effect experiments. The value of χ(3) observed at 632.8 nm is found to be 6.2079×10−19 m2 V−2 close to the phase transition temperature at 55.1 °C.
Growth kinetics of silicon dioxide on silicon in an inductively coupled rf plasma at constant anodization currents
Authors: Choksi A. J. , Lal R. , Chandorkar A. N.
Published: July 2002

The kinetics of plasma oxidation of silicon at constant anodization current have been investigated. Oxidation was performed in an inductively coupled rf plasma anodization setup. The oxidation model is based on the assumption that the oxidant from the plasma is neutral atomic oxygen which captures an electron inside the oxide and forms a negative oxygen ion (O−). The O− ion then hops through interstitial sites in the oxide due to the high drift field during the anodization. A photon-assisted oxidation mechanism is assumed to occur at the SiO2/Si interface in which the energy to break Si—Si bonds at the interface is supplied by UV photons which are generated in the plasma. Based on these assumptions an analytical model was developed that successfully explains the dependence of oxidation rate on the gas pressure in the plasma, which was not explained by previous investigators. It has been shown that during constant current anodization, the space-charge effects inside the oxide are negligible. It was found that the oxidation rate shows a weak temperature dependence with a thermal activation energy of 0.04 eV. The data on oxide thickness versus the oxidation time were found to be in good agreement with the theory.
One-dimensional model of discharge pumped excimer lasers
Authors: Krause U. , Kleinschmidt J.
Published: July 2002

For many excimer laser applications for example, in medicine and semiconductor processing, the long-term stability of the output pulse energy and the laser-beam profile are very important. The spatial intensity distribution of an excimer laser with a Fabry–Perot cavity depends on many discharge conditions (e.g., the loading voltage, the total gas pressure, the concentrations of the gas components, the repetition rate, the electrode profiles, and the spatial distribution of the preionization intensity). A one-dimensional model of a discharge pumped excimer laser is described. It can be shown that the laser output power of a KrF laser depends on the spatial variations of the discharge parameters to a higher degree than does a XeCl laser. The double structure of the transverse intensity distribution for long-term KrF laser operation can be explained as a consequence of the depletion of F2 concentration within the gas mixture after many discharges.
Theory of atomic level sliding friction between ideal crystal interfaces
Authors: Sokoloff J. B.
Published: July 2002

Recent theoretical work on atomic level sliding friction is summarized. Some previous analytic results are verified by numerical simulations, and finite-size scaling arguments based on several time scales appropriate to finite crystals undergoing shear motion are used to interpret the results of the simulations and to give insight into the methods by which energy is dissipated in such a shearing process. One conclusion is that the existence of a lifetime for the lattice vibrations plays an important role in determining the velocity dependence of the force of friction, for any finite-size crystal. Finally the force of friction found in recent microbalance experiments between a solid rare-gas-element film on a metallic substrate is calculated by the present methods and compared to experiment. The main conclusion is that the observed friction is probably due to atomic level defects such as substitutional impurities. Larger-scale defects on the surface contribute a much smaller value.
Characterization of vacancy-type defects in Se-implanted GaAs by means of a slow positron beam
Authors: Fujii Satoshi , Shikata Shinichi , Wei Long , Tanigawa Shoichiro
Published: July 2002

Variable-energy (0–30 keV) positron beam studies have been carried out on 200 keV Se-implanted GaAs specimens before and after annealing for the electrical activation. From the measurements of Doppler broadened profiles of the positron annihilation as a function of the incident positron energy, it was found that vacancy clusters with high concentration were introduced especially in the annealed specimens after Se implantation. From the parallel measurement of electric characteristics, the higher activation efficiency was found to be obtained for the higher concentration of vacancy clusters. This fact implies that electrons supplied by the activation of Se also convert the charge state of As vacancies from positive to negative.
Triplet extinction coefficients of some organic compounds
Authors: Pavlopoulos Theodore G.
Published: July 2002

The triplet extinction coefficients εT of acridine, anthracene, benzanthracene, dibenzanthracene, chrysene, pyrene, and pyrene caroxaldehyde were measured at the visible spectral region at maximum triplet-triplet absorption. We used McClure’s method together with cw lasers as excitation sources to obtain the εT values. Because these triplet extinction coefficients were measured with improved accuracy, we suggest that these compounds be used as standards. We wanted to compare the accuracy of our values with the ones suggested as tentative standards. Further, we tested the validity of McClure’s method on anthracene by increasing the cw laser excitation intensities (power) Iex and recording triplet optical densities ODT. Under the experimental conditions we used, about 100 mW of cw laser power appears to be the upper limit.
Lattice disorder in LiNbO3 crystals induced by MeV Cu+ implantation
Authors: Shi Bo-Rong , Wang Ke-Ming , Wang Zhong-Lie , Liu Xiang-Dong , Xu Tian-Bing , Zhu Pei-Ran
Published: July 2002

The effects of 1 MeV Cu ion implantation in LiNbO3 crystals have been investigated using the Rutherford backscattering/channeling technique. The samples have been implanted with doses ranging from 5×1014 ions/cm2 to 2×1015 ions/cm2. The damage profiles have been deconvoluted from Rutherford backscattering spectra after considering the energy spreading due to difference of stopping power between nonchanneled and channeled particles. The energy dependence of the scattering cross section and stopping power have also been taken into account. The damage profiles have compared with TRIM’89 (Transport of Ions in Matter, version 1989) code calculation. For the lowest dose (5×1014 ions/cm2) implantation, the damage profile is in agreement with the defect profile calculated using the trim code. The damage peak is located shallower than the theoretical ion mean projected range, but the damage spread is larger than the ion spread. For higher dose implantations, a broadening of the damage profiles is observed, probably due to radiation enhanced defect migration during ion implantation. For doses higher than 2×1015 ions/cm2, an amorphous layer will be observed in the LiNbO3 crystal.
Quasi-epitaxial growth of diacetylene films by vacuum deposition
Authors: Kanetake T. , Tomioka Y. , Imazeki S. , Taniguchi Y.
Published: July 2002

The quasi-epitaxial growth of vacuum-deposited films of diacetylenes on various substrates, including uniaxially aligned polymer surfaces formed with rubbing treatment or Langmuir–Blodgett technique, is reported. The oriented polydiacetylene (PDA) films are readily obtained from these monomer films using photopolymerization. Highly oriented PDA films with excellent optical qualities have been obtained on rubbed polyethylene terephthalate surfaces. The typical dichroic ratio of these oriented PDA films is more than 100:1. This ratio is high enough to permit the application of these films to optical waveguide devices. The method for obtaining these highly oriented PDA films is described in detail. In addition, the structure, the morphology, and the optical anisotropy of these PDA films is examined by x-ray and electron diffraction, electron microscopy, and other optical means.
Potential barriers and current-voltage characteristics of p-doped graded AlAs-GaAs heterojunctions
Authors: Zeeb E. , Ebeling K. J.
Published: July 2002

Detailed calculations are presented of the potential shape of graded AlAs-GaAs heterojunctions and of the current-voltage characteristics across the heterobarriers using a variational wave-function model. Tunneling currents are taken into account and it is shown that common models based on thermionic emission are not applicable for low temperatures, high applied voltages, and heavily doped semiconductors. The resistivity of the junction is shown to be strongly temperature dependent. The recently reported low resistivities of graded-gap mirrors in vertical cavity laser diodes can be explained with a simple variational wave-function model.
Gd and Sm interfaces with Hg1−xCdxTe(110) and a general model of rare-earth/Hg1−xCdxTe(110) interface formation
Authors: Raisanen A. , Haugstad G. , Yu X. , Franciosi A.
Published: July 2002

Room-temperature synchrotron radiation photoemission studies of rare-earth/Hg1−xCdxTe(110) junctions were performed as a function of metal coverage for the rare-earth metals Sm, and Gd. These new results are compared to our previous results for Yb/Hg1−xCdxTe junctions. At low rare-earth metal coverages, we observe in all cases a rare-earth Te reaction which removes Hg and Cd from the interface region, and the three-dimensional island growth of a rare-earth telluride layer 4.0–4.5 monolayer thick. The most abrupt interface region is observed for Gd/Hg1−xCdxTe, in agreement with the more reactive character of this interface as compared to Yb/Hg1−xCdxTe and Sm/Hg1−xCdxTe. For higher rare-earth metal coverages, we observe the formation of a metallic rare-earth rich layer at the surface, which effectively traps Hg atoms diffusing across the interface through the formation of a rare-earth–Hg alloy.
Quantum conductance fluctuations in self-aligned, pinch-narrowed silicon metal-oxide-semiconductor field-effect transistors
Authors: Slimmer D. A. , Licini J. C. , Van Campen S. D. , Kwasnick R. F.
Published: July 2002

We have investigated the electrical behavior of pinch-narrowed n- and p-channel accumulation mode metal-oxide-semiconductor field-effect transistors. The devices were fabricated using a conventional twin well complementary metal-oxide-semiconductor process with gate widths smaller than 2 μm. The well known signature of mesoscopic behavior, nonmonotonic variation in the channel conductance versus gate voltage, is observed below 8 K in the p-channel devices. The amplitude of the fluctuations is consistent with the predictions of quantum conductance fluctuations.
Quantum well intersubband relaxation in resonant tunneling diodes and transistors
Authors: Broekaert Tom P. E. , Fonstad Clifton G.
Published: July 2002

The intersubband relaxation rate due to electron-electron interaction is calculated in analytical form and compared to the intersubband relaxation rates for polar optical phonon emission and for impurity scattering. It is found that for resonant tunneling diodes biased into the second resonance, the intersubband relaxation rate due to electron-electron scattering can become comparable to that due to phonon emission and can involve subpicosecond relaxation times. Taken together, these intersubband relaxation rates are sufficiently strong to significantly reduce the gain of a resonant tunneling transistor, i.e., a device in which an ohmic contact is made to the quantum well of a resonant tunneling diode. This leads us to propose a novel device structure similar to the resonant tunneling transistor, but where the subband for base potential modulation and the subband for hot charge transfer are located at different points in the Brillouin zone. This structure has the advantage that the intersubband relaxation rate is significantly reduced due to the increased momentum exchange on intersubband scattering. A possible material system that has all the needed band lineups to implement this device structure is the GaAs/AlAs/Ge material system.
The passivation of gallium arsenide surfaces with atomic sulfur
Authors: Gu G. Y. , Ogryzlo E. A. , Wong P. C. , Zhou M. Y. , Mitchell K. A. R.
Published: July 2002

A new technique for bonding sulfur atoms to gallium arsenide surfaces is described. In this technique the surface is exposed to gas-phase atomic sulfur at 60–70 °C. The resulting surfaces were characterized by angle-dependent x-ray photoelectron spectroscopy. The data show that, in contrast to passivation of GaAs by H2S at low temperatures, this technique yields a surface on which the sulfur is almost exclusively bonded only to gallium atoms.
Two dimensional infrared resolution of shear bands formed during low velocity impacts of NaCl crystals using fast infrared detectors
Authors: Woody D. L.
Published: July 2002

NaCl crystals were subjected to low velocity impacts provided by a drop weight impact machine. The shear bands were imaged with the use of arrays of fast responding infrared detectors. The detectors monitored the amount of energy given off as infrared photons by the impacted crystalline solid. The main emphasis of this study was the real time observance of the shear band formation during the impact. The signal was recorded over the duration of the impact, heating of the solid during its crushing, and the relaxation of the crystal. These crushing processes occurred over a 40–100 μs timescale. Rapid infrared photons corresponding to shear banding were recorded on a 30–90 ns timescale.
Electromagnetic scattering from a conducting cylinder covered with a multilayer dielectric
Authors: Lin Chisen , Yaqoob M. Tahir
Published: July 2002

We present here a new formulation for finding the scattered field from a conducting surface coated with a dielectric material, for a TM-incident field, by considering the equivalent conduction current on a conducting surface and the polarization current in the dielectric material. Two electric field integral equations are developed by applying the boundary conditions on a conducting surface and dielectric material. These equations are solved by the method of moments to find the conduction current and the polarization current. Since the scattered field originates in these two currents, the scattered field and the field-related parameters are readily obtained. Some illustrative examples are solved and compared with the exact results. The accuracy of the results shows the efficiency of our formulation.
Comparison of transport and magnetization critical currents in c-axis-oriented Y1Ba2Cu3O7−δ thin films
Authors: Blue C. T. , Blue C. A. , Boolchand P.
Published: July 2002

Transport, structural, and magnetic properties of c-axis-oriented Y1Ba2Cu3O7−δ thin films grown by off-axis in situ rf sputtering on MgO substrates have been investigated. By varying the substrate surface preparation prior to deposition, films could be grown with systematic improvements in their dc transport and structural properties. At the same time, these films display a systematic decrease in the irreversible magnetization ΔM(H) at all temperatures. Transport and inductive critical current densities in all cases displayed a temperature dependence that could be ascribed to flux creep and/or intragrain Josephson junctions. The irreversible magnetization for the films showed sample size dependence consistent with that of the Bean critical-state model. An anticorrelation between the transport and the magnetization critical current densities in the films was observed, which can be understood in terms of changes in the density of pinning sites as a function of substrate preparation.
Molecular beam epitaxy of Nd-doped CaF2 and CaSrF2 layers on Si and GaAs substrates
Authors: Bausá L. E. , Fontaine C. , Daran E. , Muñoz-Yagüe A.
Published: July 2002

The incorporation of Nd3+ in CaF2 layers grown on Si and GaAs substrates by molecular beam epitaxy is studied by photoluminescence spectroscopy. The results are in qualitative agreement with those obtained on CaF2:Nd homoepitaxial layers. A lower emission intensity (∼70%) at λ=1.0475 μm is attributed to residual stress and crystalline defects. Concentration quenching of photoluminescence appears at concentrations higher than 3.6 wt % Nd. The use of (Ca,Sr)F2 for lattice matching to GaAs leads to a significant inhomogeneous broadening of Nd3+ emissions due to disorder in the cationic sublattice.
Exciton binding energies and absorption in intermixed GaAs-AlGaAs quantum wells
Authors: Meney Alistair T.
Published: July 2002

The optical properties of excitons in layer-intermixed GaAs-AlGaAs quantum wells are studied theoretically. The electronic dispersion is obtained using the 6×6 Luttinger–Kohn Hamiltonian for the valence bands, and an accurate expression for the conduction band dispersion which includes the effects of nonparabolicity and warping to fourth order in k. The HH1-CB1 (1s) and LH1-CB1(1s) exciton binding energies are calculated as a function of diffusion time. The absorption for both TE and TM polarization is obtained at several wavelengths, and is seen to decrease significantly with increased intermixing. The decrease in absorption is larger for narrow wells, where the effects of intermixing are more pronounced for a given diffusion time.
A size estimation of ordered domains in Pb(Mg1/3Nb2/3)O3 ceramics
Authors: Wu Zhonglin , Gui Zhilun , Li Longtu , Zhang Xiaowen
Published: July 2002

The size of ordered domains in Pb(Mg1/3Nb2/3)O3 ceramics is estimated by minimizing the Helmholtz free energy of the system. All the differences between the ordered domains and the disordered matrix are taken into consideration. The charge imbalance plays an important role in the system. The theoretical estimation agrees satisfactorily with measured results.
Emitter composition and geometry related surface recombination current of AlGaAs/GaAs heterojunction bipolar transistors
Authors: Wu Chung-Cheng , Lee Si-Chen
Published: July 2002

The surface recombination current at the emitter-base heterojunction of AlGaAs/GaAs heterojunction bipolar transistors prepared either by liquid phase or molecular-beam epitaxy have been studied. An ideality factor close to 1.5 was observed for those transistors with an undoped AlxGa1−xAs (x=0,0.1,0.2) spacer layer being employed in between the emitter-base heterojunction. Evidence is provided indicating that the rate-limiting process for surface current transport is the electron injection through the surface channel near the emitter-spacer junction surface. It was also found that the emitter edge-thinning design will be most effective when the emitter-base junction is graded and the exposed base surface is a wide band-gap AlGaAs layer. By using compositional grading of the emitter-base junction and emitter edge-thinning design, high gain (4200) npn AlGaAs/GaAs heterojunction bipolar transistors prepared by molecular-beam epitaxy can be obtained.
Dependence of the x-ray photoacoustic signal intensity of copper on sample thickness and modulation frequency
Authors: Toyoda Taro , Masujima T. , Shiwaku H. , Ando M.
Published: July 2002

The hard x-ray photoacoustic signal intensities of copper have been measured at an x-ray absorption near-edge structure (XANES) region as functions of sample thickness (from 5 to 300 μm) and modulation frequency (from 5 to 50 Hz). It is shown that the photoacoustic signal intensities show maxima at the sample thickness close to 10 μm for each modulation frequency. The x-ray photoacoustic signal varies from f−1.0 ( f denotes modulation frequency) up to 20 μm sample thickness, whereas it varies from f−0.5 beyond a 20 μm sample thickness, which is not consistent with the Rosencwaig–Gersho theory for the photoacoustic effect in solids.