bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical absorption spectrum of hydrogenated amorphous silicon using photo-pyroelectric spectroscopy
Authors: Fan J. , Kakalios J.
Published: July 2002

The optical absorption coefficient α of doped and undoped hydrogenated amorphous silicon (a-Si:H) has been measured for photon energies from 2.2 to 1.0 eV using photo-pyroelectric spectroscopy (PPES). A simplified experimental setup and analysis for extracting α from the PPES data are described. In PPES the temperature rise induced in the a-Si:H thin film due to weakly absorbed light is detected via a pyroelectric polymer in thermal contact with the sample. This technique presently has a sensitivity of αd≳10−3, where d is the sample thickness.
Studies of electron-beam penetration and free-carrier generation in diamond films
Authors: Joshi R. P. , Schoenbach K. H. , Molina C. , Hofer W. W.
Published: July 2002

Experimental observations of the energy-dependent electron-beam penetration in type II-A natural diamond are reported. The experimental data are compared with results obtained from numerical Monte Carlo simulations, and the results are in very good agreement. The results also reveal that a threshold energy of about 125 keV is necessary for complete penetration for a 35 μm sample. It is found that over the 30–180 keV range, the energy dependence of the penetration depth and total path length exhibits a power-law relation. Monte Carlo simulations have also been performed to investigate the excess carrier-generation profiles within diamond for a set of incident e-beam energy distributions. The simulation results demonstrate the feasibility of tailoring the internal source function, and hence influencing the diffusion currents, the internal electric fields, and charge injection through the contacts.
Local investigation of recombination at grain boundaries in silicon by grain boundary-electron beam induced current
Authors: Palm J.
Published: July 2002

Grain boundary-electron beam induced current (GB-EBIC) is an alternative EBIC geometry without a Schottky contact or pn junction but with ohmic contacts on either side of a GB. The GB itself acts as a charge collecting plane. We developed this EBIC mode both experimentally and theoretically. We calculated the current profiles with the diffusion length and the recombination velocities of the GB and the surface as parameters by solving the three-dimensional diffusion problem. The influence of different diffusion lengths at both sides and the angle between GB and surface is discussed. By variation of the injection level and application of an external bias local current voltage characteristics of the GB are obtained from which change of the local barrier height with increasing injection level can be deduced. We can describe the measured recombination behavior by an electronic GB model with a double depletion layer and deep interface states in a detailed balance approach. We applied this EBIC mode to grain boundaries in solar cell silicon and observed significant variations of the local recombination behavior and the diffusion length along a single GB.
A parametric study of the phase formation of in situ BiSrCaCuO thin films by laser ablation
Authors: Lin Wen-Tai , Chen Yung-Fu , Kao Chih-Chang , Wu Kuo-Chung
Published: July 2002

The phase formation in BiSrCaCuO (BSCCO) films grown by in situ laser ablation has been studied as a function of excess concentrations of Ca and Cu in the target with oxidation gases such as O2 and N2O, gas pressures ranging from 0.02 to 10 Torr, and substrate temperatures ranging from 490 to 750 °C. A set of phase diagrams has been plotted illustrating O2 or N2O partial pressures vs substrate temperature, marking the stability fields and transitions among different phases of Bi2Sr2CuOx (2201), Bi2Sr2CaCu2Ox (2212), Bi2Sr2Ca2Cu3Ox (2223), and Bi2Sr2Ca3Cu4Ox (2234), amorphous products, and decomposition of the BSCCO films. In O2 and N2O, the first appearance of the 2212 phase is located at 510 °C/20 mTorr and 490 °C/100 mTorr, respectively. The BSCCO film decomposes at a high temperature that increases with increasing gas pressure. Higher gas pressures enhance the growth of the 2201 phase that may also be formed by cooling from partial film melting. Higher substrate temperature and gas pressure as well as the surplus of Ca and Cu in the target facilitate the growth of the 2223 and 2234 phases. The decomposition curve of the Ca and Cu enhanced BSCCO films is shown to be found at lower temperatures in comparison to the stability lines of the 2212 and 2223 phases; furthermore, its slope deviation occurs at 650–690 °C which is lower by as much as about 130 °C compared to that of stoichiometric superconductors. Nearly pure 2212, 2223, or 2234 films or a mixture of them can be grown within a temperature range of 575–690 °C at a pressure as high as 10 Torr, irrespective of the kind of oxidation gas.
Studies on the relation between the eigenvalue and enhancement of Kerr effect for magneto-optical materials
Authors: Fang Ruiyi , Peng Chubing , Ma Tingjun , Long Ping , Liu Jun , Nin Ying , Dai Daosheng
Published: July 2002

Based on the universal additivity law of the interference of light the following was achieved. (i) An analytic representation between the eigenvalue of the Kerr rotation angle θ0k and the enhanced θk for a double-layered film, θk=en1 θ0k, was obtained theoretically, where e is the enhancement factor and n1 is the refractive index of the dielectric medium. (ii) An analytic representation between θk and θ0k (θk=e013e123n2θ0k) of the trilayered film [bilayered transparent media and monolayered magneto-optical (MO) medium], was also obtained, where e013 and e123 are the enhancement factors: The calculated θk value is in good agreement with experimental results for e=1. In general, the theoretical calculation shows that θk is related to the thickness of the transparent media. Thus, if one wants to get an optimum value for θk, the thickness of the first transparent film layer (such as a glass substrate) must be selected carefully. It also shows that if the variation of the thickness of the glass substrate is 5 nm, θk could vary by 0.1° for MnBiCe films. (iii) The relation connecting θk with θ0k1 and θ0k2 for bilayered MO films has been theoretically studied, where θ0k1 and θ0k2 are the eigenvalues of the Kerr rotation angle of MO materials 1 and 2, respectively. Finally, θk for a Fe/Co film was calculated.
Interfacial solid-phase reactions, crystallographic structures, and electrical characteristics of Hf/(001)Si systems
Authors: Zaima S. , Wakai N. , Yamauchi T. , Yasuda Y.
Published: July 2002

Interfacial solid-phase reactions, crystallographic structures, and electrical properties of Hf/(001)Si systems have been investigated as a function of annealing temperature. For Hf/n+-Si systems, extremely low contact resistivities of 7.3×10−8 and 5.8×10−8 Ω cm2 have been achieved at annealing temperatures of 460 and 580 °C, respectively. A bilayer structure of an epitaxial Hf3Si2 and an amorphous layer is formed at the interface by annealing at 460 °C, and an epitaxially-grown Hf3Si2 layer covers the interfacial layer at 580 °C. The current flowing through the interface of Hf/n-Si is dominated by the thermionic emission for these annealing conditions. The Schottky barrier heights, whose values are 0.50 and 0.46 eV at 460 and 580 °C, respectively, are lower than those of HfSi and HfSi2. It has been found that the decrease in contact resistivity of the Hf/Si systems is mainly caused by the lowering of the Schottky barrier height and an improvement of interfacial defects by annealing.
2–10 nm scale plasma polymerized organic films
Authors: Senda K. , Vinogradov G. K. , Gorwadkar S. , Morita S.
Published: July 2002

Ultrathin organic films can be used in electronics for several applications. There are a few ways of preparation of such films and one is plasma polymerization. Traditionally, comparatively thick films, 100–1000 nm, are deposited by this method and studied. But, for electronic devices, 10 nm or thinner films are of great importance to any wet technique considered to be unsuitable for perfect layer formation. In this work we have deposited 2–10 nm scale ultrathin hydrocarbon films on silicon wafers. A capacitively coupled rf discharge was initiated in a short residence time box-type reactor in a uniform flow of an acetylene/argon mixture. The gas kinetic residence time in the plasma zone was limited to 5–70 ms to decrease synthesis of heavy gas phase products and to suppress particulate formation. Distribution of film thickness after the deposition was measured by an ellipsometer. The film surface topography was studied by an atomic force microscope that shows the films are smooth and pinhole free. Thus our preliminary results suggest that ultrathin uniform smooth organic films suitable for electronic applications can be deposited in a reproducible manner.
Influence of sintering temperature on electrical properties of ZnO varistors
Authors: Bai Shr-Nan , Tseng Tseung-Yuen
Published: July 2002

The electrical properties of the ZnO varistors are demonstrated in this present study to be affected by the sintering temperature. The variation of the non-ohmic behavior with sintering temperature is indicated from I-V and C-V measurements to be a result of the changes of the interface defect density at the grain boundaries and the donor concentration in the ZnO grains. A shallow Schottky barrier is formed as a result of a low interface defect density, which is caused by losing the liquid-phase sintered materials, such as Bi2O3, when the metal oxide additives along the grain boundaries are sintered at a high temperature. The dielectric characteristic of the ZnO varistors is also affected by the sintering temperature. From the dielectric loss analysis and the complex-plane analysis it is found that there are two intrinsic defects, V0. and Zni.., within the ZnO varistors. The natures of these defects as a function of sintering temperature are also mentioned.
Electron traps in InGaP grown by gas source molecular beam epitaxy
Authors: Kim H. S. , Hafich M. J. , Patrizi G. A. , Nanda A. , Vogt T. J. , Woods L. M. , Robinson G. Y.
Published: July 2002

Deep-level transient spectroscopy has been used to characterize n-type In0.48Ga0.52P grown by gas-source molecular beam epitaxy. Only one electron trap was detected in both unintentionally doped and Si-doped material, with the thermal emission energy barrier varying somewhat with measurement conditions. For a bias pulse duration of 10 ms, the emission barrier energy was 0.24±0.03 eV and the capture barrier energy was 0.06±0.02 eV. The trap concentration was less than 3×1014 cm−3 and was found to be independent of Si doping for concentrations up to 4×1018 cm−3 and to oxygen contamination in the range (0.5–1.5)×1018 cm−3.
Picosecond pulsed-field probes of magnetic systems (invited)
Authors: Freeman M. R.
Published: July 2002

Time-resolved methods using picosecond scale transient magnetic fields have been developed for investigations of ultrafast magnetic phenomena in a broad variety of systems. When combined with synchronous magneto-optic detection, the approach yields stroboscopic observations of magnetic dynamics with a time resolution of order 1 ps, and a corresponding spatial resolution down to the diffraction limit of optics. Results are presented from experiments on europium chalcogenide films at low temperatures. A continuum of behavior from paramagnetic relaxation to ferromagnetic resonance is observed. Electron-beam deposited EuS films are found to have surprisingly fast spin-lattice relaxation rates, of order (100 ps)−1, and the influence of spin-orbit interactions on the spin dynamics is studied through rare-earth alloying. Results are presented from experiments utilizing the magneto-optic sampling method for direct time-resolved screening measurements of normal and superconducting metal films.
Creation of nanometer-scale structures with the scanning tunneling microscope
Authors: Mascher Ch. , Damaschke B.
Published: July 2002

We used a scanning tunneling microscope to create nanoscale structures on Au surfaces by applying two kinds of voltage pulses. In the case of short rectangular pulses a sequence of identical pulses can lead to a number of different structures ranging from holes to hillocks. To separate the influence of the electric field and the current long triangular pulses have been applied under the condition of constant current. With this technique we are able to create either holes or hillocks, the material transport being from the positive to the negative electrode. We interpret our experiments as field evaporation of positive Au ions.
A fast Abel inversion algorithm
Authors: Gueron Shay , Deutsch Moshe
Published: July 2002

The inversion formula of Abel’s integral equation has numerous applications in many scientific fields. In practice, its input is experimentally obtained data that contains inevitable measurement noise. Avoiding error amplification is, therefore, a main concern of every algorithm. We present a new fast Abel inversion algorithm that, in contrast to other methods, requires a negligible number of transcendental operations. Storage requirements are reduced as well. The advantage of the algorithm is especially noticeable when multiple data sets need to be inverted. As demonstrated in the paper, the new algorithm attenuates the input errors.
Composition and temperature dependence of the refractive index in Hg1−xCdxTe
Authors: Liu Kun , Chu J. H. , Tang D. Y.
Published: July 2002

The intrinsic absorption spectra for very thin Hg1−xCdxTe bulk samples with compositions x ranging from 0.276 to 0.443 have been obtained at different temperatures. Based on the measured absorption curves and Kramers–Kronig relation, the refractive index dispersion has been calculated in the wavelength region near and below the band gap at temperatures from 4.2 to 300 K. For Hg1−xCdxTe samples with x from 0.276 to 0.443, the dispersion relation can be described by an empirical formula: n(λ,T)2=A+B/[1−(C/λ)2]+Dλ2, where the parameters A, B, C, and D are both temperature and composition dependent. In addition, a peak in the deduced dispersion spectra appears just at the wavelength corresponding to the band-gap energy Eg(x,T) of Hg1−xCdxTe.
Separation of charge transfer and surface recombination processes by simultaneous measurement of photocurrent and excess microwave conductivity profiles of Si(111) in NH4F
Authors: Lewerenz H. J. , Schlichthörl G.
Published: July 2002

Dynamic interface properties of n-type Si(111) in 0.2 M NH4F solution are used to demonstrate the possibility of separating charge transfer velocity kr and surface recombination velocity sr by simultaneous measurement of anodic photocurrent and light-induced excess microwave reflectivity. A judicious choice of experimental parameters allows one to follow the interfacial processes leading to passivation on an appropriate time scale. The n-Si(111) surface shows photocurrent doubling in the initial time regime followed by a region of quantum yield close to unity and finally passivation sets in. Relative changes in kr and sr can be deduced from a simple model consideration, based on calculating the integral of the excess carrier concentration at fixed electrode potential.
Thermally stable poled polyimides using heteroaromatic chromophores
Authors: Wong K. Y. , Jen A. K.-Y.
Published: July 2002

Guest–host poled polymers with large nonresonant electro-optic activity and high temperature stability were obtained using heteroaromatic chromophores and high temperature optical polyimides. An electro-optic coefficient r33 value of 10.8 pm/V measured at 1.52 μm was achieved. The electro-optic activity maintained more than 80% of its initial value after 600 h at 120 °C, and more than 60% after 700 h at 150 °C.
Time resolved Raman measurements in α-quartz shocked to 60 kbar
Authors: Gustavsen R. L. , Gupta Y. M.
Published: July 2002

An experimental method for obtaining time resolved Raman spectra in shock wave experiments is described. A key aspect of this method is the use of optical fibers to transmit light from a pulsed laser to the sample and to transmit the scattered light to the recording equipment. Using this method we have measured changes in the A1(466 cm−1) Raman line of α-quartz shocked along the z axis to 60 kbar. For the same density compression, the line shifts under shock loading are considerably larger than under hydrostatic loading. Because shifts of the A1(466 cm−1) line in quartz are correlated with changes in the Si—O—Si bond angle, which, in turn, is the primary compression mechanism in quartz, the present results indicate the role of nonhydrostatic stresses on compression at the molecular level.
1.5 μm room-temperature luminescence from Er-implanted oxygen-doped silicon epitaxial films grown by molecular beam epitaxy
Authors: Serna R. , Snoeks E. , van den Hoven G. N. , Polman A.
Published: July 2002

Oxygen-doped Si epitaxial films (OXSEF) grown by molecular beam epitaxy and subsequently implanted with Er show room-temperature luminescence around λ=1.54 μm. The 45-nm-thick films have an oxygen concentration of 10 at. % and were implanted with 7.8×1014 25 keV Er ions/cm2. The luminescence was optically excited with the 514 nm line of an Ar ion laser and is attributed to intra-4f transitions in Er3+. Thermal annealing at 700–800 °C is necessary to optimize the luminescence after implantation. Pure Si implanted and annealed under the same conditions does not show Er-related luminescence at room temperature. The emission from Er in OXSEF is attributed to the high concentration of oxygen in the films, which forms complexes with Er. The excitation of Er3+ is due to a photocarrier mediated mechanism.
Interfacial reactions in ultrahigh vacuum deposited Y-Si multilayer thin films
Authors: Lee T. L. , Chen L. J.
Published: July 2002

Interfacial reactions of ultrahigh vacuum deposited Y-Si multilayer thin films have been studied by both conventional and high-resolution transmission electron microscopy, Auger electron spectroscopy, and x-ray diffraction. An amorphous Y-Si intermixing layer with a composition approximately equal to YSi2 was found to form in multilayer films with a composition ratio of 1Y:2Si at room temperature. Homogenization in atomic composition in the amorphous phase proceeded in samples annealed at 250–350 °C. In samples annealed at 400 °C for 30 min, the amorphous layer was completely transformed to crystalline YSi2. The formation of crystalline Y5Si3 and YSi was detected in as-deposited samples with concentration ratios 1Y:1Si and 5Y:3Si as well as in samples prepared with excess Y. Y5Si3 was the only silicide phase present in 5Y:3Si films after 400 °C annealing. The results indicated that the phase formation and stability in Y-Si multilayers depend critically on the composition. Based on the prediction of a growth control model, it was concluded that the formation of amorphous layer at room temperature is controlled by nucleation.
Line shape of electromodulation in uniform electric field of δ-doped GaAs
Authors: Hsu T. M. , Chen Y. A. , Chang M. N. , Lu N. H. , Lee W. C.
Published: July 2002

The line shape of electromodulation in uniform electric field of δ-doped GaAs has been calculated and compared with the experiments. The calculations show that the Franz–Keldysh oscillations (FKO) beats observed in experiments are contributed form the interference of heavy-hole and light-hole transitions. The calculations also show that the linear equation of extrema is a good approximation to analyze the electric field from the FKO line shape, although the equation only considers the heavy-hole transition.
Point defects and thermoelectric properties of iron disilicide ceramics sintered with SiH4-plasma-processed micrograins
Authors: Miki T. , Matsui Y. , Teraoka Y. , Ebina Y. , Matsubara K. , Kishimoto K.
Published: July 2002

The point defects in β-iron disilicide (β-FeSi2) have been investigated by electron paramagnetic resonance (EPR) along with the thermoelectric properties. The samples used are FeSix (1.9≤x≤2.8) ceramics sintered with FeSix micrograins processed in SiH4-plasma and nonprocessed. EPR detects (1) several S=1/2 signals and (2) a multiplet signal. An S=1/2 signal with orthorhombic g factors (g1=2.061, g2=2.047, g3=2.024) is detected in all n-type specimens, and ascribed to unpaired electrons of donors associated with iron vacancies. The other S=1/2 signals are detected in both n-type and p-type specimens. The centers responsible for these signals are considered to exist in an oxidized intergrain region of the ceramics. The EPR intensity of the multiplet signal due to the closed-pair of a high spin Fe3+ ion (S=5/2) and an S=1/2 center is reduced by SiH4-plasma treatments. SiH4-plasma processing of FeSix micrograins prior to sintering changes the Seebeck coefficient sign and magnitude, depending upon the condition of plasma processing. This Seebeck change is explained in terms of the introduction of silicon- and/or iron-vacancies. SiH4-plasma processing increases the electrical conductivity of nearly stoichiometric specimens and reduces the EPR intensity of the multiplet signal. A possible model for the increased conductivity is discussed using an energy band model for the ceramic grains and grain boundaries. We consider that the change of the thermoelectric properties by SiH4-plasma processing is mainly due to the modification of defects in and near the ceramic intergrain region.
Interplay between exchange biasing and interlayer exchange coupling in Ni80Fe20/Cu/Ni80Fe20/Fe50Mn50 layered systems
Authors: Rijks Th. G. S. M. , Coehoorn R. , Daemen J. T. F. , de Jonge W. J. M.
Published: July 2002

Magnetoresistance curves have been measured for 80 Å Ni80Fe20/tCu Cu/60 Å Ni80Fe20/80 Å Fe50Mn50 layered systems (tCu=9–93 Å). After separating the contributions from anisotropic magnetoresistance and spin valve or ‘‘giant’’ magnetoresistance (GMR), the GMR curves were interpreted in terms of a minimum-energy model for the magnetization orientation of the Ni80Fe20 layers. The interlayer exchange coupling was found to be purely ferromagnetic, and monotonically decreasing with increasing tCu. The interplay of this coupling with the exchange biasing effect results in a critical interlayer thickness, tcritCu=15 Å, below which no perfect antiparallel alignment of the Ni80Fe20 magnetizations can be realized. The GMR, which would otherwise increase with decreasing Cu layer thickness, is therefore expected to reach a maximum at or slightly below tcritCu. Indeed, such a maximum was found (with ΔR/R=4.3%) at tCu=22 Å, a value well above tcritCu. This is attributed to the occurence of inhomogeneities in the interlayer exchange coupling.
Angle-resolved time-of-flight spectrometry of neutrals laser ablated from Nd1.85Ce0.15CuO4
Authors: van Ingen R. P.
Published: July 2002

Angle-resolved time-of-flight (ARTOF) distributions of O, Cu, and Nd atoms and of NdO molecules ablated from the alloy Nd1.85Ce0.15CuO4 were measured with a quadrupole mass spectrometer. From these distributions angle-resolved yield (ARY) distributions of the neutrals were obtained. The most probable kinetic energy was irrespective of the type of neutral approximately 2.3 eV. The experimental ARTOF spectra were fitted quite well with theoretical Maxwell–Boltzmann (FMB) distributions superimposed onto angle-dependent flow-velocity contributions. The temperatures and Mach numbers associated with these fits were about 1.20×103 K and 4.9, respectively, irrespective of the type of atom considered. The experimental ARY distributions were fitted with cosp θ distributions, where p≊6. This p value was much lower than expected from the FMB distributions. The results are discussed in terms of explosive surface evaporation followed by collisional modification in the ablation-induced plasmas of the nascent ARTOF and ARY distributions and in terms of explosive volume evaporation. Additional contributions to the experimental ARTOF and ARY distributions with respect to the fits are ascribed to additional thermal mechanisms.
Study of C49-TiSi2 and C54-TiSi2 formation on doped polycrystalline silicon using in situ resistance measurements during annealing
Authors: Clevenger L. A. , Mann R. W. , Roy R. A. , Saenger K. L. , Cabral C. , Piccirillo J.
Published: July 2002

In situ resistance versus temperature or time for reactions between 32 and 57.5 nm of titanium and undoped or doped polycrystalline silicon (boron, arsenic, or phosphorus, 7.9×1019–3.0×1020/cm3) has been measured and no clear correlation was found between the activation energy for the formation of the industrially important low-resistance C54-TiSi2 phase and its formation temperature. It is also demonstrated that with certain moderate doping levels typical of complementary metal-oxide-semiconductor manufacturing, boron or phosphorus-doped polycrystalline silicon can delay the formation of C54-TiSi2 more than arsenic-doped polycrystalline silicon. Finally, by using in situ resistance measurements, it is demonstrated that the ‘‘two-step’’ thermal annealing process similar to a salicide process requires less thermal annealing time at high temperatures to form C54-TiSi2 than a single ‘‘one-step’’ thermal anneal at the same temperature.
Variational approach to the calculation of the radii in the stagnant core model of shaped charge jet formation
Authors: Curtis J. P. , Kelly R. J. , Cowan K. G.
Published: July 2002

The formation of a jet and slug from a collapsing shaped charge liner can be viewed, by appropriate choice of reference frame, as the result of two fluid streams impinging upon each other. In this article we consider this formation process and develop further the concept of a stagnant core model. In this model a core region of material is supposed to be stationary at the junction where the liner material turns to form the jet and slug. In our two-dimensional treatment the boundaries of the core region and the free streamlines are assumed to be arcs of circles and the main problem is to determine the radii of these boundaries. However, unlike in previous work, a nonuniform flow field is assumed to exist in the circular flow region from the outset. The nonuniform flow field we derive needs to be matched with the (assumed) uniform flow in the impinging stream. To accomplish this a transition region in the impinging stream is postulated. Consideration of the mass and momentum balances in this region leads to further model equations. The first of these balances gives a relation between the radii of the free streamline and the stagnant core boundary. It is shown that there are no physically acceptable exact solutions to the model equations when the energy is minimized. However a very accurate approximate solution is shown to exist. This solution leads to an expression for the liner speed on the core boundary which is identical to the critical speed used in a recent study on the formation of incoherent jets. Physically sensible values of the free streamline radius are also shown to result from this approximate solution.
Charged particle densities and kinetics in a radio-frequency SF6 plasma
Authors: Kono A. , Endo M. , Ohata K. , Kishimoto S. , Goto T.
Published: July 2002

Electron, negative ion, and positive ion densities in a capacitively-coupled radio-frequency (rf, 13.56 MHz) SF6 plasma have been investigated as functions of the pressure (30–700 mTorr) and rf power. The decay of the charged particle densities in the afterglow has also been studied to obtain information about their kinetics. The electron density was determined by using a microwave cavity resonance technique. Negative ions were detected by measuring the density of photodetached electrons produced by pulsed laser irradiation of the plasma. The positive ion density was obtained from Langmuir probe measurements. At a rf power of 0.13 W/cm2 and at low pressures (≲100 mTorr) the positive ion density, which is virtually equal to the negative ion density, is found to be larger than the electron density by a factor of a few hundred; the ion/electron density ratio increases with increasing pressure to reach a value of a few thousand at high pressures. Wavelength-dependent photodetachment measurements indicate that the dominant contribution to the photodetachment signal at 266 nm comes from F−, but F− is a minor negative-ion species with respect to the density; that is, the photodetachment efficiency for the dominant negative ion species (supposed to be SF−5) is low. The recombination rate constant for F− (supposedly with SF+5) is suggested to be an order of magnitude larger than that for the dominant ion species, the latter being estimated to be slightly smaller than 10−7 cm3 s−1.
Magnetic properties of interstitially modified Nd3(Fe,Ti)29Xy compounds (X=H, C, and N)
Authors: Ryan D. H. , Cadogan J. M. , Margarian A. , Dunlop J. B.
Published: July 2002

The effects of light atom intercalation on the magnetic properties of the monoclinic compound Nd3(Fe,Ti)29 have been studied by Mössbauer spectroscopy and thermogravimetric analysis. Maximum contents of 4 nitrogen atoms and 6 hydrogen atoms per formula unit have been achieved, consistent with structural calculations. The associated lattice expansion ranges from 2% in the hydride to 6.5% in the nitride. Attempts to introduce carbon were unsuccessful as the material decomposed rapidly during the reaction. Both hydrogen and nitrogen additions lead to substantial increases in the magnetic ordering temperature, but only the nitrogen leads to an increase in the iron moment.
Relaxation and spin correlations in 119Sn-doped a-Fe90Sc10
Authors: Wiarda D. , Ryan D. H.
Published: July 2002

Combined 119Sn and 57Fe Mössbauer measurements have been made on a Sn-doped sample of a-Fe-Sc both around Tsg (117±2 K) to examine the onset of order, and at 12 K to investigate spin correlations. The different time scales probed by the Sn and Fe Mössbauer transitions allow us to confirm that cluster relaxation effects do not contribute to the ordering at Tsg, while the large transferred hyperfine field (Bhf) at the Sn sites, in an alloy known to exhibit isotropic spin freezing, is inconsistent with the simple view of the transferred field arising as a vector sum over the nearest-neighbor moments.
Scattering mechanisms and defects in InP epitaxially grown on (001) Si substrates
Authors: Hansen K. , Peiner E. , Tang G.-P. , Bartels A. , Schlachetzki A.
Published: July 2002

Carrier concentration and mobility of unintentionally doped InP layers, grown directly on Si using metal-organic vapor-phase epitaxy, have been studied. The formation of antiphase domains (APDs) was found to depend on annealing of the Si substrate in an AsH3 flow prior to epitaxial growth. Dislocation densities determined by the wet chemical delineation technique were (8±1)×107 cm−3, seemingly uncorrelated to APDs in the layers. In addition to a shallow donor and a compensating acceptor, a deep donor was observed affecting the temperature dependence of the free-electron concentration between 77 and 300 K. The electron mobility in this temperature range could be described in terms of the scattering mechanisms which are dominant in homoepitaxial InP, namely, scattering due to polar optical phonons, to ionized impurities, and to space charges. Electron scattering due to either of these mechanisms was strongly influenced by the occurrence of antiphase boundaries (APBs). The space-charge density as well as the degree of compensation of the epitaxial layers increases with the density of APBs. Degraded 300 K mobilities were obtained indicating the effect of local stress at the APB.
Magnetization reversal in systems of interacting magnetically hard particles
Authors: Wirth S.
Published: July 2002

A new procedure for investigating the switching behaviour of differently oriented particles in polycrystalline samples is introduced where the particle interaction is taken into account by the mean demagnetizing field. Based on specific remanence measurements the angular and field dependence of the probability of irreversible magnetization reversals (switching) is determined. This includes also the switching field distribution (SFD) of the material. The procedure is applied to a barium ferrite sample compacted from small particles. Due to the interaction the angular dependent switching fields are shifted to lower values compared to those predicted by the Stoner–Wohlfarth model and those measured for a strongly diluted sample, whereas the SFD is changed only slightly. This result as well as measurements of the initial susceptibility confirm single–domain particle behaviour of this material. Starting from our result for the switching probability it is shown that the angular dependence of the switching field can be deduced from angular dependent measurements of remanence coercivity only for samples with a very high degree of grain alignment. The determined probability is confirmed by comparing calculated and measured field dependence of the conventional remanence for the investigated material.
Evidence for band-to-band impact ionization in evaporated ZnS:Mn alternating-current thin-film electroluminescent devices
Authors: Ang W. M. , Pennathur S. , Pham L. , Wager J. F. , Goodnick S. M. , Douglas A. A.
Published: July 2002

Evidence is presented that the normal operation of evaporated ZnS:Mn alternating-current thin-film electroluminescent (ACTFEL) devices involves electron-hole pair generation by band-to-band impact ionization. Four observations are offered to support this assertion. These observations involve: (i) empirical field-clamping trends, (ii) experimental and simulated trends in charge transfer characteristics, (iii) experimental attempts to assess the interface distribution using a field-control circuit, and (iv) Monte Carlo simulation trends. Furthermore, the absence of overshoot in measured capacitance-voltage and internal charge-phosphor field curves indicates that a majority of the holes created by impact ionization are trapped at or near the phosphor/insulator interface. The multiplication factor (i.e., the total number of electrons transferred across the phosphor divided by the number of electrons injected from the phosphor/insulator cathode interface) is estimated, from device physics simulation of experimental trends, to be of the order 4–8 for evaporated ZnS:Mn ACTFEL devices operating under normal conditions.
Photoreflectance study of Si delta-doped low-temperature GaAs grown by molecular beam epitaxy
Authors: Cheng T. M. , Chang C. Y. , Hsu T. M. , Lee W. C. , Huang J. H.
Published: July 2002

Photoreflectance spectroscopy has been used to study the surface electric field of Si delta-doped GaAs grown by molecular beam epitaxy at a low substrate temperature (230 °C). Franz–Keldysh oscillations in the reflectance spectra are observed for samples annealed above 700 °C for 10 min. The deduced surface electric field increases with annealing temperatures and with a decrease in spacer thickness between surface and the delta-doped plane. The evolution of photoreflectance spectra can be explained by the activation of Si donors and Fermi level pinned at surface due to the redistribution of As precipitates.
Erbium in crystal silicon: Optical activation, excitation, and concentration limits
Authors: Polman A. , van den Hoven G. N. , Custer J. S. , Shin J. H. , Serna R. , Alkemade P. F. A.
Published: July 2002

The optical activation, excitation, and concentration limits of erbium in crystal Si are studied. Preamorphized surface layers of Czochralski-grown (Cz) Si(100), containing 1.7×1018 O/cm3, were implanted with 250 keV Er at fluences in the range 8×1011–8×1014 cm−2. After thermal solid-phase epitaxy of the Er-doped amorphous layers at 600 °C, Er is trapped in the crystal at concentrations ranging from 3×1016 to 7×1019 Er/cm3, as measured by secondary-ion-mass spectrometry. Photoluminescence spectra taken at 77 K show the characteristic Er3+ intra-4f luminescence at 1.54 μm. Photoluminescence excitation spectroscopy shows that Er is excited through a photocarrier-mediated process. Rapid thermal annealing at 1000 °C for 15 s increases the luminescence intensity, mainly due to an increase in minority-carrier lifetime, which enhances the excitation efficiency. Luminescent Er forms clusters with oxygen: the maximum Er concentration that can be optically activated is determined by the O content, and is (3±1)×1017 Er/cm3 in Cz-Si. The internal quantum efficiency for electrical excitation of Er in Cz-Si is larger than 3×10−6.
Trapping of photocarriers in Ga-doped Bi12GeO20 at 80 K
Authors: Bloom D. , McKeever S. W. S.
Published: July 2002

Data from measurements of optical absorption, photoconductivity, dark conductivity, thermally stimulated conductivity (TSC), and thermoluminescence (TL) on samples of undoped and Ga-doped, Czochralski-grown Bi12GeO20 single crystals are reported. The photoconductivity is n type, and the dark conductivity is p type. Undoped BGO exhibits a broad, band-edge absorption due to the optical excitation of electrons to the conduction band which gives the samples a yellow coloration. This absorption is reduced by the addition of Ga which acts as a compensating acceptor. When illuminated with light into this absorption band, but with photons of energy less than the band gap, photoexcitation of electrons occurs. These become trapped, inducing additional absorption and photoconductivity bands and TSC signals, but not TL. Excitation with photons of energy greater than the band gap induces both TSC and TL. Examination of the TSC and TL signals as a function of excitation wavelength allows the distinction between electron and hole trapping states for which trapping parameters have been determined. In addition, dark conductivity reveals three major hole states at energies of ∼Ev+1.41, ∼Ev+0.86, and ∼Ev+0.54 eV. These are believed to be empty donor states.
Evolution of photoluminescence of porous silicon under light exposure
Authors: Chang I. M. , Chuo G. S. , Chang D. C. , Chen Y. F.
Published: July 2002

Time evolution of the photoluminescence spectra of low current density electrochemically etched porous silicon are studied. When the sample is exposed to light illumination in atmosphere, the luminescence intensity decays initially. However, after a short period of degradation, it starts to enhance gradually. The profiles of the photoluminescence spectra including peak position and line width are very different for the decay and enhancement processes. When the sample is illuminated in vacuum, only a decay process is observed. Infrared spectrum shows that the hydrogen related bonding in the as-anodized sample is replaced by the oxygen related termination. A quantum confinement model together with the activity of nonradiative recombination centers is proposed to explain our observations. After oxidation under light exposure, the intensity of the photoluminescence is strong and stable. Thus, our study also provides a room temperature oxidation method for producing strong and stable luminescent porous silicon.
Development of highly conductive n-type μc-Si:H films at low power for device applications
Authors: Saha S. C. , Ray Swati
Published: July 2002

Highly conductive phosphorus-doped n-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by the usual (13.56 MHz) radio-frequency glow discharge of silane (SiH4), phosphine (PH3), and hydrogen (H2) in an ultrahigh-vacuum deposition system. The highest conductivity of the films obtained in this study is 100 S cm−1 after optimizing the hydrogen dilution ratio, chamber pressure, substrate temperature, and doping concentration of phosphorus. The formation of microcrystallinity in the material has been studied by transmission electron microscopy, x-ray-diffraction studies, and Raman spectroscopy. The volume fraction of microcrystallinity in these amorphous-microcrystalline mixed-phase materials has been estimated from Raman spectra. Sizes of the crystallites and volume fraction of microcrystallinity vary with hydrogen dilution, chamber pressure, and substrate temperature. The variations in the properties with deposition parameters have been explained in terms of the growth kinetics. The n-type μc-Si:H thin film, thus developed, has been applied in the first cell of a double-junction amorphous silicon solar cell. The prepared p-i-n–p-i-n stacked cell employing the n-type μc-Si:H film has exhibited appreciable improvement in open-circuit voltage, fill factor, and efficiency compared to the one with amorphous n layer in the inner n–p contact. Degradation of the cells prepared with and without μc-n layer has been studied.
Resonant cavity excitation system for radial array slab CO2 lasers
Authors: Bilida W. D. , Seguin H. J. J. , Capjack C. E.
Published: July 2002

A rf excitation system that can drive a gas laser constructed of multiple wide, but narrow-gap (slab), discharge volumes is presented. Resonant cavity techniques adopted from microwave applications are employed to provide uniform rf power division and impedance matching into each electrode channel. This approach when applied to a large radial array of gain channels can be used to create a very high-power laser system in an unusually compact package.
Modeling the optical dielectric function of II-VI compound CdTe
Authors: Kim Charles C. , Sivananthan S.
Published: July 2002

In previous papers, a new model was proposed for the optical dielectric function of zinc blende semiconductors and was applied successfully to the alloy series AlxGa1−xAs. It was found to be more generally valid than any previous model. In this paper, it is used to obtain an analytic expression for the dielectric function of the II-VI compound CdTe at room temperature. Unlike the previous application, additional polynomial terms in the real part of the optical dielectric function are not introduced, so that the Kramers-Kronig relationship between the real and imaginary parts of the dielectric function is satisfied exactly. As demonstrated before, the model covers not only the entire photon energy range of the given spectral data, but also is valid below and somewhat above the given spectral range. This is advantageous especially when spectral data are not provided in the range of interest, or when joining two separate data causes an artificial discontinuity. The model determines the optical dielectric function in the limit as the line broadening approaches zero, which is useful in calculating the optical dielectric function at all temperatures. For demonstration, the optical dielectric function at 70 K and 600 K is calculated and presented.
Recording long-term optical images of a Brownian particle in a Paul trap essentially free of thermally induced positional noise
Authors: Arnold S. , Li J. H. , Holler S. , Korn A. , Izmailov A. F.
Published: July 2002

We show experimentally that the positional uncertainty in long-term images of a microparticle in a Paul trap in air can be reduced to the optical limit, and below the pseudopotential limit. For this damped system, far below any Mathieu instability, the particle’s thermally induced positional noise is extremely sensitive to the phase of the driving field. Accumulating images strobed at the proper phase produces a long-term optical image which is essentially free of thermally induced positional noise. Although noise squeezing theory does not apply at such large dissipation, our results may be understood through recent theory of the Brownian parametric oscillator. Use of this theory coupled with our observations suggests that the extreme reduction in spatial variance observed by using our technique results from working in a low Reynolds number regime.
Microcavity effects on the spontaneous emission from InGaAs/GaAs quantum wells
Authors: Yang G. M. , MacDougal M. H. , Zhao H. , Dapkus P. D.
Published: July 2002

The spontaneous emission from an InGaAs/GaAs single-quantum well surrounded by AlAs/GaAs distributed Bragg reflectors (DBR) under the near-resonance condition between the exciton level and the confined optical mode is investigated. Under such conditions, on-axis spontaneous emission enhancement at the cavity resonant wavelength is clearly identified. The strength and character of the interaction of the exciton with the confined optical mode is determined by the dependence of photoluminescence spectra on the reflectivity of the DBR. Temperature dependence of the enhanced spontaneous emission shows the cavity resonant wavelength shifts at 0.85 Å/°C around room temperature. An increase of emission intensity at the cavity resonant wavelength with increasing temperature is also observed, which can be related to the increase of the interaction between excitonic emission and cavity mode.
Pattern dependence in selective epitaxial Si1−xGex growth using reduced-pressure chemical vapor deposition
Authors: Ito S. , Nakamura T. , Nishikawa S.
Published: July 2002

Pattern dependence of the growth rate and Ge content in selective epitaxial Si1−xGex growth using reduced-pressure chemical vapor deposition has been examined for the SiH2Cl2-GeH4-H2 system. Contrary to selective epitaxial Si growth where pattern dependence diminishes at low growth temperatures, the growth rate of selectively grown Si1−xGex layers depends on the ratio of the exposed Si area to the oxide-covered area on a wafer, at temperatures as low as 600 °C. The Ge content in the layers also depends on the ratio, and the dependence is greater at higher temperatures. Adding HCl to the gas phase decreases the pattern dependence of both growth rate and Ge content. Although, in terms of the growth rate, an optimum HCl flow rate for pattern-insensitive growth exists, the Ge content in the layers is always higher for the wafer with a smaller exposed area.
Electron energy states at the interface between semi-insulating polycrystalline silicon and crystalline silicon
Authors: Liss Bo , Engström Olof
Published: July 2002

A new model for the semi-insulating polycrystalline silicon (SIPOS)-silicon interface is proposed. The model explains the behaviour of the capacitance-voltage characteristics of a metal- SIPOS-silicon structure at low temperatures. A theoretical capacitance versus voltage (C–V) relationship has been calculated for low temperatures and fitted to measured data with very good agreement. The interface state density at the SIPOS–silicon interface has also been calculated using Terman’s method. We demonstrate that when determining the energy distributions of interface states between SIPOS and silicon by using C–V technique, thermal equilibrium between the interface and the silicon bulk has to be maintained by making the measurement at low temperature. Measurement at room temperature normally gives too low values of the interface state density, Dit. Dit values of approximately 1013 cm−2 eV−1 were found for the SIPOS-silicon interface with 25 at. % oxygen concentration.
Stoichiometry reversal in the growth of thin oxynitride films on Si(100) surfaces
Authors: Sutherland D. G. J. , Akatsu H. , Copel M. , Himpsel F. J. , Callcott T. A. , Carlisle J. A. , Ederer D. L. , Jia J. J. , Jimenez I. , Perera R. , Shuh D. K. , Terminello L. J. , Tong W. M.
Published: July 2002

Synchrotron-based O 1s and N 1s photoabsorption spectroscopy, O 1s, N 1s, Si 2p, and valence-band photoelectron spectroscopy (PES), and medium energy ion scattering (MEIS) have been used to determine the composition and thickness of oxynitride films grown in N2O on a Si(100) surface. Core-level photoabsorption spectroscopy is shown to be a very sensitive probe capable of measuring surface coverages lower than 0.1 monolayers of N (6.5×1013 N atoms/cm2). Film composition was monitored as a function of growth to demonstrate the stoichiometry reversal from primarily N-terminated surfaces in thin films to nearly pure SiO2 in films thicker than ∼20 Å. A sample with a 60 Å oxynitride film was depth profiled by etching in HF and was shown, via N 1s absorption spectroscopy, to have N segregation within 10 Å above the Si/SiO2 interface. Core-level PES and MEIS were used to study the growth mechanisms of oxynitrides on Si(100) and these data were used to create a schematic phase diagram showing three distinct regions of oxide formation. A critical N2O pressure was discovered at which oxide growth proceeds at over 1000 times its normal rate.