bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical characteristics of the hollow anode discharge
Authors: Miljević Vujo I.
Published: July 2002

A new type of discharge realized in a diode consisting of a concave cathode and a special type of a hollow anode is presented. The concave cathode focuses electrons toward the hollow anode increasing the density of high-energy electrons and the efficiency of excitation and ionization in the hollow anode. The distinctive arrangement of dark and luminous regions between the cathode and anode has been observed in the discharge.
Energy band alignment in GaAs:(Al,Ga)As heterostructures: The dependence on alloy composition
Authors: Batey J. , Wright S. L.
Published: July 2002

The band alignment in GaAs:(Al,Ga)As heterostructures has been investigated over the full range of alloy composition. The valence-band discontinuity ΔEv is determined by measuring the activation energy for thermionic emission of holes from p-GaAs over an undoped, square (Al,Ga)As barrier. The use of p-type structures to measure ΔEv circumvents a number of complications involved in the measurement of ΔEc. The parameters required for analysis are determined by different measurements on the same structures and the analysis is performed so that the activation energy, extrapolated to zero bias, yields ΔEv directly. It is found that ΔEv is a linear function of the aluminum mole fraction xAl: ΔEv ≂0.55xAl (eV) (0≤xAl≤1). The validity of these data is supported by measurements of ΔEc in the direct band-gap regime, where complementary values of ΔEv and ΔEc add up to the expected band-gap difference. This relationship provides a simple description of the full band alignment in this heterosystem and should prove valuable as a test of the various heterojunction lineup theories. Moreover, these measurements have a number of important consequences, particularly from the viewpoint of heterojunction devices.
Thermal stability of epitaxial Al/GaAs Schottky barriers prepared by molecular-beam epitaxy
Authors: Missous M. , Rhoderick E. H. , Singer K. E.
Published: July 2002

The effect of annealing on the electrical characteristics of epitaxial aluminum/gallium arsenide Schottky barriers prepared by molecular-beam epitaxy has been studied and compared with the annealing behavior of polycrystalline aluminum contacts deposited in a conventional evaporation system. It was found that the epitaxial contacts showed remarkably stable electrical characteristics, the forward characteristics remaining exponential over 6 decades of current after annealing for 1 h at 500 °C, with a slight degradation of n value from 1.01 to 1.02 and a very small recombination component. In the reverse direction, the characteristics improved on annealing, with a reduction in current from 9×10−10 to 6×10−11 A at a reverse bias of 1 V. In contrast, the polycrystalline contacts showed a pronounced recombination component after annealing, with an increase in n to between 1.05 and 1.09, and a significant degradation in reverse characteristics. Auger depth profiling showed that the epitaxial contacts exhibited a more abrupt interface than the polycrystalline ones in the as-deposited state. After anealing, the epitaxial contacts retained an abrupt interface but the polycrystalline ones became even less abrupt, with considerable in-diffusion of the Al and out-diffusion of the Ga and As. An explanation of these phenomena in terms of increased grain-boundary diffusion in the polycrystalline film is suggested. The technological importance of the results is emphasized.
Amorphous silicon–tellurium alloys
Authors: Shufflebotham P. K. , Card H. C. , Kao K. C. , Thanailakis A.
Published: July 2002

Amorphous silicon–tellurium alloy thin films were fabricated by coevaporation over the composition range of 0–82 at. % Te. The electronic and optical properties of these films were systematically investigated over this same range of composition. The optical gap of these films was found to decrease monotonically with increasing Te content. Conduction near room temperature was due to extended state conduction, while variable range hopping dominated below 250 K. The incorporation of Te in concentrations of less than 1 at. % was found to produce an increase in the density of localized states at the Fermi level and a decrease in the activation energy. This was attributed to the Te being incorporated as a substitutional, fourfold coordinated, double donor in a-Si. At approximately 60 at. % Te, a decrease in the density of localized states at the Fermi level, and an increase in the activation energy and photoresponse was indicated. This was attributed to the possible formation of a less defective a-Si:Te compound.
Current, carrier concentration, Fermi energy, and related properties of binary compound polar semiconductors with nonparabolic energy bands
Authors: Mohammed S. Noor , Abidi S. T. H.
Published: July 2002

Using Kane’s energy–wave-vector relation (k.P model), it is shown that the nonparabolic band structure of binary compound polar semiconductors (BCPSCs) is, in fact, a perturbed parabolic band structure. The formula for this ‘‘parabolic’’ band structure is used to derive current, carrier concentrations, Einstein equation, and others. The formula for the Einstein equation is found to be of the same form as that for wide-gap semiconductors. Simplified relations for nonparabolicity coefficients have been derived. It is shown that these coefficients compare well with similar coefficients derived earlier from different considerations. Numerical calculations of effective mass, density of states, Fermi energy, and electron-impurity interaction energy have been carried out for a number of representative compounds. Good correspondence of theoretical results with available experiments indicates that the present model is quite useful for describing properties of BCPSCs.
Method of solution of a three-temperature plasma model and its application to inertial confinement fusion target design studies
Authors: Tahir N. A. , Long K. A. , Laing E. W.
Published: July 2002

In this paper the physical and the mathematical model of a three-temperature plasma is presented. The numerical algorithm used to solve this problem is also described. A special technique has been used to treat very rapid electron-ion and electron-radiation energy equipartition rates in inertial fusion plasmas. This model has been incorporated in the one-dimensional computer code medusa-ka. (N. A. Tahir and K. A. Long, ‘‘medusa-ka: A one-dimensional computer code for inertial fusion target design,’’ Kernforschungszentrum Karlsruhe Report, KfK-3454, 1983). Application of this model to study the problem of ignition and burn propagation in reactor-size inertial fusion targets is also briefly discussed.
Superconducting gravity gradiometer for space and terrestrial applications
Authors: Moody M. V. , Chan H. A. , Paik H. J.
Published: July 2002

A three-axis superconducting gravity gradiometer with a potential sensitivity better than 10−3 Eötvös Hz−1/2 is currently under development for applications in space. Although such a high sensitivity may be needed for only a limited number of terrestrial applications, superconductivity offers many extraordinary effects which can be used to obtain a gravity gradiometer with other characteristics necessary for operation in a hostile moving-base environment. Utilizing a number of recently devised techniques which rely on certain properties of superconductors, we have produced a design for a sensitive yet rugged gravity gradiometer with a high degree of stability and a common-mode rejection ratio greater than 109. With a base line of 0.11 m, a sensitivity of 0.1 Eötvös Hz−1/2 is expected in an environment monitored to a level of 10−2 m s−2 Hz−1/2 for linear vibration and 7×10−6 rad s−1 Hz−1/2 for angular vibration. A conventional stabilized platform can be used at this level. The intrinsic noise level, which is two orders of magnitude lower, could be achieved by monitoring the attitude with a superconducting angular accelerometer which is under development. In addition, the new gradiometer design has the versatility of adapting the instrument to different gravity biases by adjusting stored dc currents.
Liquid lithium ion source: Nonlinear behavior of liquid surface in electric field
Authors: Pregenzer Arian L. , Marder Barry M.
Published: July 2002

Calculations have shown that electrohydrodynamic instabilities can produce surface distortions that may locally enhance the electric field to the extent that field evaporation of ions can occur from a liquid metal surface. Linear analysis of the electrohydrodynamic equations of motion has yielded the regions of instability as well as the wavelengths and growth times of the dominant unstable mode as a function of applied electric field. In the calculation presented here, the liquid surface is followed well into the nonlinear regime using a surface integral method. The results agree with the linear theory where appropriate, and add further support to the feasibility of a large-area liquid-lithium ion source for inertial confinement fusion experiments on a light ion-beam driver.
Characteristics of interelectrode flashover in air with the existence of a weakly ionized plasma channel induced by a KrF laser (248 nm)
Authors: Sasaki J. , Kubodera S. , Ozaki R. , Uchiyama T.
Published: July 2002

The irradiation of a KrF laser (248 nm) into the interelectrode space affects the electric flashover characteristics there. The results obtained show that negative oxygen ions dominate formation of the interelectrode arc in air. A laser beam is introduced parallel to the interelectrode axis and the equipment is arranged to prevent the occurrence of photoelectric effects on the electrode surfaces. The time separation between laser firing and application of the interelectrode voltage is varied from 10 ns to 10 ms. The interelectrode spacing is set in the range 2–12 cm. The arc formation time and the flashover probability are measured. The initial ion number density is estimated experimentally to be on the order of 1011 cm−3 throughout the channel.
Surface effect-induced fast Be diffusion in heavily doped GaAs grown by molecular-beam epitaxy
Authors: Pao Yi-Ching , Hierl Tom , Cooper Tom
Published: July 2002

van der Pauw measurements, C-V measurement, and secondary ion mass spectrometry were used to study the rapid diffusion of Be in heavily doped GaAs epilayers grown by MBE. The concentration dependence of the Be diffusion was measured. A discrepancy exists at high doping level between our experimental results and the Be interstitial-substitutional diffusion model previously proposed. The threshold of fast Be diffusion versus doping concentration was determined and a diffusion coefficient as high as 2×10−12 cm2/s was observed at 600 °C. A surface Fermi-level pinning effect model has been proposed in addition to the interstitial-substitutional model, which has successfully explained the onset of fast Be diffusion in MBE-grown GaAs at high doping level.
Audio frequency dielectric response of the liquid-crystal CE8 in chiral and racemic forms
Authors: Cava R. J. , Patel J. S. , Rietman E. A.
Published: July 2002

The real and imaginary parts of the dielectric function of racemic and chiral CE8 have been measured between 30 Hz and 13 MHz at temperatures between 51 and 162 °C. Low- (1 kHz) and high-(105 Hz) frequency dielectric constants reflect changes in structure of the liquid-crystal phases and differences in behavior of the two forms. The low-frequency relaxation in the ferroelectric SmC* phase, near 1500 Hz, was studied in detail as a function of temperature and dc bias. The mean relaxation time does not present an obvious systematic variation with temperature although the low-frequency dielectric constant and distribution of relaxation times do. The mean relaxation time in the SmC* phase shifts continuously to shorter times with the application of dc bias.
Retarded and enhanced dopant diffusion in silicon related to implantation-induced excess vacancies and interstitials
Authors: Servidori M. , Angelucci R. , Cembali F. , Negrini P. , Solmi S. , Zaumseil P. , Winter U.
Published: July 2002

The influence of the lattice defects induced by silicon-ion implantation on the B, P, As, and Sb diffusivities is investigated after annealing between 700 and 900 °C. The nature and depth position of the residual implantation defects in undoped samples is determined by the analysis of the rocking curves obtained by triple-crystal x-ray diffraction and transmission electron microscopy. In particular, besides the interstitial dislocation loops and clusters below the original amorphous-crystal interface, the epitaxial regrowth of the amorphized silicon leaves a vacancy-rich surface layer and a deeper region enriched in interstitials. These regions correspond to those where Monte Carlo simulations of defect production foresee excess point defects. Accordingly, as the dopant is located in correspondence with the vacancy or interstitial clusters, different behaviors of anomalous diffusion are observed. In the deep region where an interstitial excess is present, B and P show marked enhanced diffusion, while only a small enhancement is exhibited by As and Sb. On the contrary, retarded diffusivity for B and light enhancement for As and Sb are observed in the surface layer. These different trends are consistent with the different accepted contributions of vacancies and interstitials to the diffusion mechanisms of the investigated dopants.
Zinc diffusion in n-type indium phosphide
Authors: van Gurp G. J. , Boudewijn P. R. , Kempeners M. N. C. , Tjaden D. L. A.
Published: July 2002

Profiles of Zn in n-type InP〈100〉 wafers after ampoule diffusion were measured by secondary-ion mass spectrometry, Auger electron spectrometry, differential Hall-effect measurements, capacitance measurements, and scanning electron microscopy. The results can be explained by an interstitial-substitutional mechanism, in which Zn diffuses as a singly ionized interstitial and is incorporated in the In sublattice as an electrically active substitutional acceptor or as an electrically inactive complex. At Zn concentrations lower than the background donor concentration the profile is cut off, as interstitial diffusion breaks down. The acceptor solubility increases with background donor concentration. Activation energies for diffusion and solubility were found to be 1.40 and 1.0 eV, respectively.
Determination of crystallographic polarity of CdTe crystals with Auger electron spectroscopy
Authors: Lu Y.-C. , Stahle C. M. , Morimoto J. , Bube R. H. , Feigelson R. S.
Published: July 2002

The crystallographic polarity of CdTe crystals has been studied with Auger electron spectroscopy. The polarity of various surfaces could be determined by normalizing the low-energy Te (NOO) peak with the high-energy Te (MNN) peak. Upon comparing the normalized ratios from both {111} surfaces, the ratio from (111) Te surfaces was found to be about 1.3 times that from (111) Cd surfaces. The identified polarity was consistent with recent results of anomalous absorption of x rays, the convergent beam method of transmission electron microscopy, and chemical etching. Air-cleaved {110} surfaces of CdTe crystals and surfaces of pure Te crystals were studied for comparison as well.
Dynamics and factors controlling regularity of thermomagnetically written domains
Authors: Shieh Han-Ping D. , Kryder Mark H.
Published: July 2002

The reproducibility of thermomagnetically written domains in rare earth–transition metal thin films was studied and as a result films capable of supporting very regular domains were fabricated. To understand the origins of the domains’ size and shape variations, we first studied the nucleation and growth of domains by using a high-speed magneto-optical sampling camera having a 10-ns exposure time. It is observed that the domain is nucleated within 20 ns after the leading edge of the write pulse. The domain grows in size until 50–150 ns after termination of the write pulse, then shrinks, and finally reaches an equilibrium size. For very short write pulses, no shrinkage of the domain is observed after termination of the write pulse. It is found that increased laser power or longer laser pulse duration increases the final domain size, but not in proportion to total energy applied. The domain growth rate varies from film to film, indicating it is influenced by the intrinsic magnetic parameters. The reproducibility and regularity of the final stable domains were then studied as a function of writing and magnetic parameters. Although both influence the regularity of nucleated domains, the latter have been found to have more profound influences than the former. Films with compensation temperature above ambient temperature, larger perpendicular anisotropy constant, smaller magnitude of magnetization, and smaller domain aspect ratio nucleate domains with a higher degree of reproducibility. Higher coercivity films also support more regular domains. Photographs of domains nucleated with different parameters demonstrate these trends. The experimental observations can be qualitatively explained by analyzing the stiffness of the domain wall defining the domains. The wall stiffness is determined by the two competing forces derived from wall energy and demagnetizing energy. When wall energy dominates, the stiffness is large and domains tend to be circular and highly reproducible. When demagnetizing energy dominates, wall stiffness is low and domains become irregular and less reproducible in shape.
The role of temperature in sample-to-sample comparisons of the 1/f noise of metal films
Authors: Fleetwood D. M. , Beutler D. E. , Masden J. T. , Giordano N.
Published: July 2002

We have performed an extensive study of the 1/f noise of several different kinds of metal films fabricated in different ways. An examination of the temperature dependence of the 1/f noise of tin and indium shows that the previously reported substrate dependence of the noise magnitude generally is only present near room temperature. At other temperatures, especially lower temperatures, the substrate dependence is much smaller or negligible. The noise magnitudes of platinum and bismuth films, on the other hand, depend strikingly on the way that the sample is made, and can vary strongly from sample to sample as a function of temperature. Taken together, these results illustrate that, in contrast to common practice, the room-temperature magnitude of the 1/f noise of metal films is often not a useful parameter on which to base comparisons of the relative noise magnitudes of different films. The implications of these results for the interpretation of some previous studies are discussed, and recommendations are made about the interpretation of future studies of the 1/f noise of metal films.
Temperature dependence of the anchoring strength at a nematic liquid crystal-evaporated SiO interface
Authors: Yokoyama H. , Kobayashi S. , Kamei H.
Published: July 2002

The out-of-plane anchoring strength at the interface between 4-n-pentyl-4′-cyanobiphenyl (5CB) and an obliquely evaporated SiO was measured as a function of temperature by applying the ‘‘high-electric-field technique’’ recently developed by Yokoyama and van Sprang [J. Appl. Phys. 57, 4520 (1985)]. The orientational ‘‘extrapolation length’’de for the interface was found to remain virtually constant at about 30 nm up to nearly 1 K below the clearing temperature Tc=308.4 K. At temperatures closer to Tc, however, it was observed to show an apparently critical increase, which approximately follows de∝[(Tc−T)/Tc]−0.45. At 0.043 K below Tc, de=183±3 nm, yielding the anchoring energy of (1.13±0.03)×10−5 J/m2. The implications of the observed temperature dependence are first investigated by extending Gibbs’ surface thermodynamics to a nematic-wall interface, deriving a general relationship between the temperature variation of de and the surface excess entropy. In particular, it is concluded on thermodynamic grounds that the exponent of −0.45 can never be associated with a real critical behavior, but is indicative of the occurrence of crossover to a more singular or noncritical behavior. To draw specific connections between the temperature dependence of de and the orientational order near the interface,

we develop a simple statistical mechanical theory of the anchoring strength, based on a van der Waals picture of the nematic-wall interface. We show that the extrapolation length consists of two essentially distinct contributions: one is from the anisotropic interactions at the interface and the other is unconventionally from the interfacial inhomogeneity of the order parameter itself. For the most relevant cases, the van der Waals theory states that a reduction of the anchoring strength should always be accompanied by a depression of the surface order parameter. By comparing the theory with the experiment, the surface order parameters just below and above Tc are found to be 0.04 and 0.01, respectively, in agreement with the estimates from the contact angle and the pretransitional birefringence experiments. It is also argued, based on the present model, that the order parameter inhomogeneity is predominantly responsible for the quasicritical behavior of the anchoring strength.
High magnetization limits of spinel ferrite
Authors: Dionne Gerald F.
Published: July 2002

Modifications to a previously reported theoretical model of the thermomagnetization properties of LiZnTi spinel ferrite have made possible the prediction of upper limits to the magnetization of the spinel system. Extrapolation of the molecular-field coefficient relations to the limits of the undiluted spinel, i.e., the fictitious case of unrestricted Fe3+ occupancy, provides a basis for computing all combinations of dilutant site distributions. For this situation, the maximum room-temperature magnetization and Curie temperature approach 8000 G and 1050 K, respectively. Computations for the more realistic monovalent cation system Fe3+1−xA1+x [B1+0.5−xFe3+1.5+x]O4, where A and B could represent any combination of Li, Na, or possibly Cu, yield an optimum magnetization that could reach 6500 G with a Curie temperature of about 700 K. In the context of this theoretical model, the cation site distributions for high magnetization in the Mn and Ni spinel ferrite families are also discussed.
Influence of junctions on photoluminescence decay in thin-film devices
Authors: Ahrenkiel R. K.
Published: July 2002

The photoluminescence lifetime of III-V thin films is a major tool for the determination of minority-carrier lifetime. Prior work has dealt with the effects of surface recombination for various device geometries. Other work has shown the effect of self-absorption and photon recycling on the effective lifetime. We describe deep homojunctions thin-film devices. Model calculations show that the effective lifetime, as measured in the photoluminescence experiment, is always less than the bulk minority-carrier lifetime. In high-mobility materials, the effective photoluminescence lifetime may be dominated by diffusion and unrelated to recombination. An analysis of the data produces the minority-carrier mobility.
Photoacoustic measurements of the thermal conductivity of some bulk polymer samples
Authors: Madhusoodanan K. N. , Thomas Mini R. , Philip Jacob
Published: July 2002

Thermal parameters such as thermal diffusivity and thermal conductivity of the polymers nylon 66, cellulose acetate, polyacetal, Teflon, and polystyrene have been determined using the photoacoustic technique. Samples having thickness of the order of 25–50 μm have been specially prepared from bulk solid samples for measurements. The experimental method involves the determination of the characteristic frequency fc by measuring the variation of the amplitude and phase of the photoacoustic signal as a function of the chopping frequency for a monochromatic incident beam. Front surface illumination geometry has been used in the measurements. The measured thermal properties agree with the existing values in literature.
Modulated ion beam studies of product formation and ejection in ion-induced etching of GaAs by Cl2
Authors: Ameen M. S. , Mayer T. M.
Published: July 2002

We have measured products’ identity and kinetic energy for ion-induced etching of GaAs by chlorine at room temperature. Modulated ion beams of 1-keV Ne+ are used to etch the surface in the presence of steady-state flux of Cl2 with a neutral/ion flux ratio of 0–100. The major product species observed are GaCl3 and AsCl3, and substantial amounts of elemental Ga and As. Subchlorides of Ga are observed for neutral/ion ratio <10. Kinetic energies were measured by analysis of time-of-flight waveforms. Sputtered Ga and As atoms, in the absence of surface chlorination have most probable kinetic energy of approximately 5 eV, in accord with the expected sputtering mechanism. GaCl3 and AsCl3 product species have most probable kinetic energies of 0.3–0.5 eV, and Ga and As atoms emitted from a chlorinated surface have most probable energy of 1–2 eV. No evidence for slow kinetic processes with substantial surface residence times was observed. These observations are discussed in light of other reports of product formation and ejection in plasma and ion-beam-assisted etching. The results suggest a mechanism involving synthesis and ejection of products during the collision cascade following ion impact.
Studies on solution-grown thin films of ZnxCd1−xS
Authors: Padam G. K. , Malhotra G. L. , Rao S. U. M.
Published: July 2002

Structural, optical, and electrical properties of thin films of ZnxCd1−xS (0≤x≤1.0) prepared by the solution-growth technique are reported as a function of x. As-grown films are composed of a hexagonal phase in the range 0≤x≤0.8 and of cubic phase for x=1.0. The lattice parameters a and c vary with x following Vegard’s law. The band gaps for as-grown ZnxCd1−xS films are found to vary linearly with x from 2.40 eV for CdS to 3.6 eV for ZnS. The resistivity increases from 109 to 1012 Ω cm in the whole range 0≤x≤1.0. The effect of indium doping and H2 annealing on the above properties is also investigated.
Atomic force profiling by utilizing contact forces
Authors: Yang R. , Miller R. , Bryant P. J.
Published: July 2002

Atomic force microscopy (AFM) is a new technology currently being developed. Stylus profilometry (SP) was developed earlier. A procedure is reported here which combines features of both SP and AFM. In this approach a stylus scans a sample surface and responds to contact forces. Force values are maintained within the elastic range of the sample, and high resolution is achieved by means of a sensitive tunnel gap feedback circuit control. Initial tests of performance have been conducted over a range of applied loads with tungsten styluses on phlogopite mica samples to image lamellar steps with subnanometer resolution.
Preparation and properties of the dc reactively sputtered tungsten oxide films
Authors: Kaneko H. , Nagao F. , Miyake K.
Published: July 2002

Preparation and properties of the dc reactively sputtered tungsten oxide films have been investigated in relation to the oxygen concentration in sputtering atmosphere. The films with 1500–20 700 Å thickness were deposited on the glass substrates maintained at 200 °C by dc reactive sputtering from a metallic tungsten target under a constant operating pressure of 6.5×10−2 Torr in Ar-1%–30% O2 gas mixture. The films formed in an Ar-3%–20% O2 gas mixture are crystalline WO3, and have an electrical resistivity of 107–1011 Ω cm which is dependent on the oxygen concentration of the sputtering atmosphere. These films have a spectral transmittance above 80% in the visible and near-infrared regions, and optical band gap of the films ranges from 3.15 to 2.98 eV, depending on the oxygen concentration. Densities of the film deposited in Ar-3% O2 and in Ar-20% O2 gas mixtures are 5.85 g/cm3 and 6.65 g/cm3, respectively. Electrochemichromic properties of the transparent-crystalline WO3 films were studied using asymmetric cells, and were found to be dependent on the crystal orientation of the films. The films with the orientation of WO3 (020) and WO3 (021) formed in an Ar-3%–6% O2 gas mixture have a very good electrochromic property, and the cells composed of the crystalline WO3 films have a higher coloration rate than the cells composed of the vacuum-evaporated amorphous film. The films with the orientation of WO3 (001) and WO3 (021) formed in an Ar-8%–20% O2 gas mixture were found to have a poor electrochromic property.
Magnetic anisotropy in TbFe thin films prepared at oblique incidence
Authors: Shin Sung-Chul , Agarwala Ashok K.
Published: July 2002

Effects of the incident vapor-beam orientation on magnetic anisotropy of TbFe thin films were investigated. By coevaporation of Tb and Fe on tilted substrates, the angle of incident vapor beam was made to vary from 0° to 60° with respect to each substrate normal. The magnetic measurements were carried out using a vibrating sample magnetometer and a torque magnetometer in applied fields of 15 and 20 kOe, respectively. Assuming an intrinsic anisotropy with a single easy axis, the easy axis orientation and the intrinsic anisotropy energy were estimated. It was found that perpendicular magnetic anisotropy did not deteriorate until the angle of incidence reached 30°. However, the easy axis became tilted from the film normal for angles of incidence larger than 30°; finally, at an incident angle of 60°, it was in the film plane. A rapid increase in the shape anisotropy energy and a simultaneous decrease in the intrinsic anisotropy energy with the angle of incidence are believed to be causes for the loss of perpendicularity in the film.
Characterization of roughness and defects at an Si/SiO2 interface formed by lateral solid phase epitaxy using high-resolution electron microscopy
Authors: Kawarada Hiroshi , Ueno Tomo , Ohdomari Iwao , Kunii Yasuo
Published: July 2002

Using high-resolution transmission electron microscopy, we have characterized on an atomic scale the interfacial roughness and the interfacial defects at the Si/SiO2 interface obtained by lateral solid phase epitaxial growth of Si on SiO2. At the matrix/SiO2 interface, the protrusions of Si with 2–3 Si(200) lattice planes make the interface indented. These images can be explained by the pyramid-type or edge-type pseudo Si(100)/SiO2 interface models composed of small {111} facets. Defects, mainly microtwins, are localized only within 50 nm from the interface, and are adjacent to the SiO2. It indicates that microtwins are interface-related and are supposed to form from atomic scale indents of the original a-Si/SiO2 interface.
Lead fluoroborate glasses
Authors: Gressler Carole A. , Shelby James E.
Published: July 2002

Glasses containing up to 60-mol % PbF2 have been made in the binary system PbF2-B2O3. The glass transformation and crystallization temperatures, thermal expansion behavior, and electrical conductivity of these glasses have been measured. A corresponding series of lead borate glasses were also studied to provide a basis for comparison of the effects of fluorine and oxygen on the properties of these glasses. The lead fluoroborate glasses appear to be anion conductors, with conductivities approaching the highest ever reported for fluorine conducting glasses.
Thermal properties of semi-insulating GaAs dilutely alloyed with InAs
Authors: Ohmer M. C. , Mitchel W. C. , Graves George A. , Holmes D. E. , Kuwamoto H. , Yu P. W.
Published: July 2002

The thermal diffusivity, specific heat, and thermal conductivity of InxGa1−xAs dilute alloys for 0<x<0.013 were determined at 25 °C. For x=0.005, the diffusivity and conductivity are reduced to 60% and 50% of the values for GaAs, respectively. The implications of a reduction in thermal transport due to In alloying are discussed in terms of both crystal growth of dislocation-free GaAs and potential circuit reliability problems associated with lower heat dissipation.
Optical properties and structure of microcrystalline hydrogenated silicon prepared by radio-frequency magnetron sputtering
Authors: Logothetidis S. , Kiriakidis G.
Published: July 2002

We have studied the optical properties of hydrogenated microcrystalline silicon films prepared by rf magnetron sputtering. Spectroscopic ellipsometry (SE) has been used to measure the dielectric spectra in the 1.66–5.6-eV range. Films produced with substrate temperature higher than 200 °C and hydrogen partial pressure ratio equal to 0.5 exhibited a microcrystalline structure. Distinct differences were observed between such films as amorphous and microcrystalline films in their dielectric function spectra and other optical quantities. An energy shift and a broadening of the E1 and E2 transitions for the microcrystalline samples were calculated from the analysis of SE data. It was observed that the rf power and the substrate temperature were the primary factors controlling the microcrystallite formation. A strong density dependence of microcrystallinity was observed, for all the optical parameters investigated. The effect of the hydrogen concentration and the hydrogen-bonding configuration on the optical properties is also discussed.
Laser-induced fluorescence spectroscopy for the determination of the absolute density and spatial distribution of Si atoms in a SiH4-He-Ar glow discharge
Authors: Takubo Y. , Takasugi Y. , Yamamoto M.
Published: July 2002

We report on the absolute density and the spatial distribution of silicon atoms in a SiH4-He-Ar dc glow discharge measured by the laser-induced fluorescence method. The absolute sensitivity of the fluorescence-photon-counting system was calibrated by using the same system to measure Rayleigh scattering by Ar gas in the discharge chamber. The fluorescence quantum efficiency and the polarization degree of emitted light were measured to confirm the absence of excited-state quenching induced by collisions with the buffer-gas atoms. The measured spatial distribution profile of the ground-state silicon atoms is compared with that of the excited-state atoms.
Influence of solute doping on the high-temperature deformation behavior of GaAs
Authors: Guruswamy S. , Rai R. S. , Faber K. T. , Hirth J. P. , Clemans J. E , McGuigan S. , Thomas R. N. , Mitchel W.
Published: July 2002

The role of isovalent dopants in the high-temperature deformation of GaAs has been studied in the temperature range 500–1150 °C. Additions of In, Sb, and B increase the critical resolved shear stress for deformation at a given strain rate and result in lowering the dislocation density of as-grown liquid-encapsulated Czochralski GaAs crystals. Phosphorus, because of its minor influence on the lattice strain, shows little enhancement of the yield stress. These results are consistent with a solute hardening model, in which the solute atom surrounded tetrahedrally by four Ga or As atoms comprise the hardening cluster. Codoping with In and Si hardens GaAs, but codoping with Si is less effective than the isovalent solutes In, Sb, and B, and produces softening at high temperatures. The effect of solutes on both dislocation nucleation and multiplication are reviewed here.
A diffusive transport relaxation technique for studying laser-induced chemical vapor deposition reactions at high pressures
Authors: Kodas Toivo T. , Comita Paul B.
Published: July 2002

A diffusive transport relaxation technique has been developed to study the chemical processes occurring during laser-induced chemical vapor deposition. The technique is based on modulation of the laser light intensity which results in a modulation of the surface reaction rate. This perturbation of the reaction rate produces transients in the gas-phase reactant and product concentrations for the case of gas-phase transport-limited deposition. Product species can be differentiated from species formed from the reactant during ionization and fragmentation due to the phase-sensitive nature of the detection scheme. The system response is examined theoretically by describing the response of the surface reaction rate to the modulated laser light and then examining the response of the gas-phase reactant and product concentrations to the modulated surface reaction. The technique was used to identify the gaseous products formed during laser-induced deposition of gold using dimethyl gold hexafluoroacetylacetonate.
An interferometric calorimeter for thin-film thermal diffusivity measurements
Authors: Saenger K. L.
Published: July 2002

We demonstrate that the thermal diffusivity of thin-film samples bonded to transparent substrates can be measured from the phase lag of thermal waves propagating from the sample surface into the substrate. The thermal waves are detected from the substrate’s interferometrically modulated reflectance. This technique is used to determine the thermal diffusivity of 5-μm-thick polymer films.
Resonance radiation trapping effects in CuCl laser
Authors: Srigouri K. , Prasada Rao T. A.
Published: July 2002

The effects of resonance radiation trapping of 3248 and 3274 Å upon the corresponding laser fluorescent lines of 5106, 5700 and 5782 Å of copper have been investigated as a function of temperature in a CuCl laser with and without helium buffer gas. The experimental values of the laser starting temperature at 315 °C and the dissociation level at about 10% in CuCl at resonance radiation trapping threshold are found to be in good agreement with the reported values from calculations.
Band lineup for a GaInP/GaAs heterojunction measured by a high-gain N
p
n heterojunction bipolar transistor grown by metalorganic chemical vapor deposition
Authors: Kobayashi T. , Taira K. , Nakamura F. , Kawai H.
Published: July 2002

A GaInP(N)/GaAs( p) heterojunction bipolar transistor was fabricated by metalorganic chemical vapor deposition (MOCVD) for the first time. The common-emitter current gain exceeded 200 at a current density around 10 A/cm2 and the offset voltage was as small as 50 mV. Thermionic emission theory indicates that the conduction-band discontinuity (ΔEc) at GaInP/GaAs heterointerface is as small as 30 meV at room temperature and this value was more than 160 meV smaller than 0.19–0.22 eV obtained by the C-V profile method. The band-gap energy for MOCVD-grown GaInP was 60 meV smaller than the intrinsic band-gap energy (1.91 eV), but this value is too small to explain the difference between the present ΔEc value and the previously reported ΔEc value.
Spherical shell model of an asymmetric rf discharge
Authors: Lieberman M. A.
Published: July 2002

A spherical shell model is used to study ion transport and bias voltage formation in asymmetric, capacitive rf discharges, which have unequal areas A and glow-to-electrode voltages V at the powered (a) and grounded (b) electrodes. Ions are generated by thermal electron ionization and are lost by ambipolar diffusion in the glow. Resonant charge transfer with a constant cross section is assumed to dominate the ion transport. We obtain the density ratio scaling na/nb∝(Ab/Aa )7/24, where n is the density at the glow-sheath edge. Three electrode sheath models are considered: collisionless ions, collisional (constant mobility) ions, and a constant-ion cross-section collisional law. Using these and the continuity of the rf current flow, we obtain the scaling of the electrode voltage ratio with the electrode area ratio: Va/Vb∝(Ab/Aa )q. For typical rf materials processing discharges, the constant cross section law yields q≊2.21. The effects of secondary electron ionization and local ionization near the sheaths due to stochastic heating are shown to further reduce the value of q.
Domain wall energy in the media of magneto-optical recording
Authors: Mansuripur M.
Published: July 2002

A method of calculating domain wall energy density from experimental observations is proposed. The technique requires measurement of the expansion and collapse fields for circular domains of arbitrary size.
Critical-current measurements of high-T
c superconductors using a transformer technique
Authors: Meisner G. P. , Taylor C. A.
Published: July 2002

We report transport critical-current measurements of the YBa2Cu3Oy high-Tc superconductors using a contactless transformer technique. A critical-current density of 15.9±1.0 A/cm was observed at 77 K for samples fabricated by a standard preparation method for the high-Tc superconductors. The addition of a small amount of silver to samples of YBa2Cu3Oy has a dramatic degrading effect on the critical current. This is in sharp contrast to the Meissner-effect results, where silver addition has a minimal effect. No significant systematic frequency dependence of the critical-current determination was found between 10 and 5000 Hz. Visual observations of ‘‘blips’’ in oscilloscope traces and an extrapolation of signals to zero blip height gave consistent results and established an uncertainty of ∼3% for critical-current determination. The range of dimensions of the ring-shaped samples yielded an uncertainty of ∼3% for the cross-sectional area. The overall uncertainty for an absolute measurement of the critical-current density is consequently ∼6%. The uncertainty in relative measurements of the critical-current density is very much less.
High-sensitivity, interference-free, Stark-tuned CO2 laser photoacoustic sensing of urban ammonia
Authors: Sauren Hans , Bicanic Dane , Jalink Henk , Reuss Jörg
Published: July 2002

Low-concentration (few ppbv), interference-free, on-line photoacoustic detection of ambient ammonia (NH3) is reported by Stark tuning the Q(J=5, K=5, M=5) NH3 absorption line into resonance with the CO2 laser. Measurements were made over a range of total pressure between 600 and 50 mbar.
Exciton line broadening in ZnSxSe1−x epilayers grown on GaAs by molecular-beam epitaxy
Authors: Newbury P. R. , Shahzad K. , Petruzzello J. , Cammack D. A.
Published: July 2002

ZnSxSe1−x epilayers grown on GaAs substrates by molecular-beam epitaxy were examined for exciton line broadening in photoluminescence as functions of x(0.0≤x≤0.49) and temperature (6–300 K). The variation of linewidth with x is partly explained by alloy broadening effects and partly attributed to the density of defects near the epilayer surface, which in turn is a function of x and layer thickness. The increase in linewidth with temperature is modeled up to 200 K by taking into account scattering due to acoustic and optical phonons. The band gap is observed to be a quadratic function of x at both low (6 K) and high (300 K) temperatures.
Nondestructive characterization of nitrogen-implanted silicon-on-insulator structures by spectroscopic ellipsometry
Authors: Fried M. , Lohner T. , de Nijs J. M. M. , van Silfhout A. , Hanekamp L. J. , Laczik Z. , Khanh N. Q. , Gyulai J.
Published: July 2002

Silicon-on-insulator structures implanted by 200-keV nitrogen with a dose of 7.5×1017 atoms/cm2 were studied by spectroscopic ellipsometry (SE). The SE measurements were carried out in the 300–700-nm wavelength (4.13–1.78-eV photon energy) range. For the analysis of the SE data we used the conventional method of assuming appropriate optical models and fitting the model parameters (layer thicknesses and compositions) by linear regression. Calculated data were in good agreement with measurements when a seven-layer model, consisting of surface oxide layer, thick silicon layer, upper two interface layers, thick nitride layer, and lower two interface layers, was applied. Results obtained by SE were compared with those from Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy. In contrast with RBS measurements, we found that the sensitivity of our optical model combined with the fitting technique was good enough to resolve the silicon-rich transition layers at the upper and lower interfaces of the buried nitride layer.
Calculation of the lattice thermal conductivity in GaAs-InAs alloys and comparison with experiment
Authors: Szmulowicz Frank , Madarasz Frank L. , Klemens Paul G. , Diller Jeffrey
Published: July 2002

The room-temperature lattice thermal conductivity of GaAs-InAs alloys is calculated and compared to recently measured thermal conductivity of GaAs dilutely alloyed with InAs [M. C. Ohmer, W. C. Mitchel, G. A. Graves, D. E. Holmes, H. Kuwamoto, and P. W. Yu, J. Appl. Phys. 64, 2775 (1988)] and with older data at higher concentrations. Our calculation considers both the normal and Umklapp anharmonic interaction, as well as mass differences and size distortions for the scattering by point defects. The results indicate that it is the size-distortion scattering which leads to a very abrupt decrease in the thermal conductivity for dilute alloys. The theory of Klemens and Callaway used in the present work is in semiquantitative agreement with experiments, consistent with both the experimental and theoretical uncertainties. Unlike previous calculations, we were able to reproduce a very sharp decline in the thermal conductivity upon alloying for very dilute alloys.
Capacitance-voltage characteristics of amorphous silicon thin-film transistors
Authors: Shur Michael , Hack Michael , Shaw John G. , Martin R. A.
Published: July 2002

We present the results of experimental and theoretical studies of capacitance-voltage characteristics of amorphous silicon thin-film transistors. The small-signal capacitances are calculated as derivatives of channel charge with respect to terminal voltages. The induced charge is evaluated using the same charge-control model for an amorphous silicon thin-film transistor as we used to calculate the current-voltage characteristics of this device. The calculated dependences of capacitances on bias voltages are in good agreement with our experimental data. The capacitance-voltage model is suitable for incorporation into a circuit simulator and can be used for computer-aided design of a-Si integrated circuits.
Molecular-beam epitaxial growth and transport properties of InAs epilayers
Authors: Kalem Ş.
Published: July 2002

InAs epilayers have been grown on (100)-GaAs substrates by molecular-beam epitaxy using an As2 source, and their properties were investigated by the Hall effect and resistivity measurements. The transport properties were found to be critically dependent on the epilayer thickness d>3.0 μm, flux ratio of constituents, and substrate temperature. Particularly, a strained layer superlattice at the InAs/substrate interface was found important in reducing defect density. For the layers grown at optimum conditions, the Hall mobility reaches a certain saturation for about d>3.0 μm. The mobility peaks at around liquid-nitrogen temperature for thick samples and at higher temperatures for thin as well as etched layers, which we explain in terms of impurity scattering of carriers. In addition to ionized impurity and optical phonon scattering, other possible mechanisms limiting mobility are discussed in these compensated layers. No carrier freezeout was observed at temperatures down to 10 K. Instead, an apparent increase in carrier density due to impurity conduction was found. The existence of three spatial regions contributing to the conduction was confirmed from etching experiments. The results were compared with those of the InAs epilayers grown under similar conditions on Si substrates.
Development of a Li− ion source
Authors: Walther S. R. , Morse E. C. , Leung K. N.
Published: July 2002

Sources of Li− ions are needed for diagnostic neutral beams for fusion research. Previous efforts to generate Li− beams have focused on electron capture in a gas or production on a low work-function surface in a plasma. Volume production of Li− by dissociative attachment of optically pumped lithium molecules has also been studied. This paper presents experimental results for volume production of a Li− ion beam from a plasma discharge. These results show that a discharge source of Li− ions can provide suitable current densities for diagnostic beams. A theoretical model for the formation of Li− ions from Li2 molecules in the lithium discharge is developed. The model is in good agreement with the experimental results and shows favorable parameter scalings for further improvement of the Li− ion source.