bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic viscosity effects in the giant magnetoresistance of NiO/Permalloy/Cu/Permalloy exchange-biased films
Authors: Restorff J. B. , Wun-Fogle M. , Cheng S. F.
Published: July 2002

We have examined the effects of magnetic viscosity (magnetic aftereffect) on the giant magnetoresistance (GMR) of NiO/Permalloy(45 Å)/Cu(45 Å)/Permalloy(45 Å) exchange-biased spin-valve structures grown by direct current (dc) magnetron sputtering on Si substrates. One of the samples had a 6 Å Co layer on both sides of the Cu layer. GMR is a sensitive indicator of magnetic viscosity effects since any time dependence in the magnetization reversal of one of the layers is reflected directly in the GMR. Two different types of GMR measurements were made over the temperature range of −30 °C to 50 °C. The maximum time for the resistance to stabilize to 1 part in 104 ranged from about 70 to 200 s for both samples at the temperatures studied. The time dependency of the GMR was well described by C+S ln t, where S and C are constants; S is called the coefficient of magnetic viscosity. This equation characterizes the classic time dependent behavior of the magnetization. The maximum magnitude of S was found to decrease as the temperature was increased.
Inverted spin valves for magnetic heads and sensors
Authors: Lenssen K.-M. H. , De Veirman A. E. M. , Donkers J. J. T. M.
Published: July 2002

Inverted spin valves (Si substrate/Ta/X/Fe50Mn50/Ni80Fe20/Cu/Ni80Fe20/Ta with X=Cu or Ni80Fe20) have been investigated by transmission electron microscopy and magnetoresistance measurements. The newly proposed Ni80Fe20 buffer layer provides a significantly increased magnetoresistance ratio compared to the usual Cu buffer. This makes it possible to use inverted spin valves instead of conventional spin valves in magnetic heads and sensors, offering reduced corrosion sensitivity and a higher GMR ratio. First test structures for tape heads have been fabricated and evaluated.
Monitoring the microstructure of pearlitic steels by magnetoacoustic emission
Authors: Lo C. C. H. , Jakubovics J. P. , Scruby C. B.
Published: July 2002

Magnetoacoustic emission (MAE) and Barkhausen emission (BE) measurements have been made on hypereutectoid steel samples heat treated to produce completely pearlitic microstructures with various interlamellar spacings. Both the peak height ratio of the MAE profile and the amplitude of the BE profile were found to increase with increasing interlamellar spacing. These results can be explained in the context of the domain structure and the domain wall processes observed by Lorentz microscopy. The correlation of the spacing with the shape of the MAE and BE profiles shows that both techniques are useful for monitoring the pearlite interlamellar spacing.
Optical properties of α-irradiated and annealed Si-doped GaAs
Authors: Kunert H. W. , Brink D. J.
Published: July 2002

The influence of irradiation by α particles and subsequent isochronal annealing on n-GaAs doped with silicon, was investigated. Photoluminescence (PL) studies revealed the formation of an induced radiation center at 1.486 eV. In addition to the PL investigation we also employed capacitance–voltage (C–V) measurements and deep level transient spectroscopy (DLTS). The C–V measurements indicate a slight reduction in n-type carriers in treated samples and the DLTS spectra showed a number of commonly observed deep levels in irradiated samples (not annealed), which disappear after annealing. Based on the experimental evidence we propose that the 1.486 eV band is due to a donor-acceptor-pair transition. Alternative mechanisms are also discussed.
The effective dissipation in Nb/AlOx/Nb Josephson tunnel junctions by return current measurements
Authors: Cristiano R. , Frunzio L. , Nappi C. , Castellano M. G. , Torrioli G. , Cosmelli C.
Published: July 2002

Measurements of temperature dependence of the return current in high quality Nb/AlOx/Nb Josephson junctions are presented. From the experimental data, we obtain the effective resistance, i.e., the effective dissipation, for the retrapping process, according to the generalized resistively shunted junction model proposed by Chen, Fisher, and Leggett. We present a careful analysis, based on a comparison between the measured temperature dependencies of both the return and the quasiparticle tunneling current. We find that the junction subgap conductance, which includes the quasiparticle and the quasiparticle-pair interference terms, is responsible for the return process. The measurements have been performed on various samples, in a wide range of critical current densities from 50 to 2250 A/cm2, covering different damping regimes and spanning over the high and low temperature limits. Junctions with low critical current density show ideal dissipation which makes the return current scale with temperature according to the BCS exponential behavior without flattening out effects. This result may be relevant for the possible use of Nb/AlOx/Nb junctions in macroscopic quantum coherence experiments, which strongly require a very low dissipation.
Effect of the carbon acceptor concentration on the photoquenching and following enhancement of the photoacoustic signals of semi-insulating GaAs
Authors: Fukuyama A. , Morooka Y. , Akashi Y. , Yoshino K. , Maeda K. , Ikari T.
Published: July 2002

The spectral and the time dependent piezoelectric photoacoustic (PPA) measurements under the continuous light illumination were carried out at 85 K to investigate nonradiative recombination processes involving EL2 defect levels in carbon concentration controlled and not intentionally doped semi-insulating (SI) GaAs. The decrease of the PPA signal due to the photoquenching effect of EL2 is observed for a short period of illumination in the photon energy region from 1.0 to 1.3 eV. Since almost all of the carbon acceptors are compensated by deep donor EL2 in SI GaAs, electron occupancy of EL2 level can be controlled by changing the carbon acceptor concentration. It is found that the photoquenching becomes drastic with increasing the carbon concentration. After fully photoquenching, the PPA signal increases again through a local minimum by the continuous light illumination and finally exceeds the initial value before illumination until the saturation level is reached. The deep donor level EL6 and its metastable state are proposed. EL6 level donates electrons to compensate a part of carbon acceptors after EL20 to EL2* transition is accomplished. The nonradiative recombination through this level generates the PPA signal. Since the PPA measurement can detect lower concentration of EL6 than that of EL2, the higher sensitivity of the PPA measurements than the optical absorption measurements is pointed out. The usefulness of the PPA technique for studying the nonradiative transition through deep levels in semiconductors is also suggested.
Currents and electric fields induced in the human body when the arms are raised
Authors: King Ronold W. P.
Published: July 2002

The current induced in the human body when exposed to an incident electromagnetic field with the electric vector parallel to the length of the body and the magnetic field perpendicular to the chest was determined in an earlier study with the arms in close contact with the body. The analysis is now extended to the case when both arms are raised by an arbitrary angle. Formulas are derived for the current in the arms and the body when the latter is far from the earth, when it is barefoot on wet earth, and when it is standing on the earth with rubber-soled shoes. The current in the arms is shown to consist of two parts, that induced by the component of the incident electric field parallel to the arms and that generated in them by their connection to the body. The current in the body is significantly increased as the arms are raised.
In situ correlation between the optical and electrical properties of thin intrinsic and n-type microcrystalline silicon films
Authors: Hamma S. , Roca i Cabarrocas P.
Published: July 2002

Intrinsic and n-type microcrystalline silicon thin films were deposited on intrinsic hydrogenated amorphous silicon by the layer-by-layer technique. The growth of the samples has been analyzed in situ by kinetic ellipsometry, spectroscopic ellipsometry, and dark conductivity measurements. This in situ analysis has shown that the process of deposition can be divided into four phases: incubation, nucleation, growth, and steady state. Moreover we have found striking differences between the growth of undoped and n-type samples in both the kinetics of the formation of crystallites and the zone where the nucleation of crystallites takes place. According to our in situ conductivity measurements, the percolation threshold occurs for a crystalline volume fraction higher than 20% in both cases. Moreover, we can produce very thin (6 nm) and highly conductive (σd≈0.2 S cm−1) n-type microcrystalline silicon films on intrinsic amorphous silicon.
Fabrication of nanometer-sized magnetic structures using e-beam patterned deposition masks
Authors: Park Y. D. , Caballero J. A. , Cabbibo A. , Childress J. R. , Hudspeth H. D. , Schultz T. J. , Sharifi F.
Published: July 2002

We report on the development of a novel technique designed to study dimensional effects in sputter-deposited magnetic materials and multilayer nanostructures without the need for post-deposition patterning. Wires with widths ranging from 40 to 1000 nm have been deposited on Si wafer substrates through a deposition mask fabricated by e-beam lithography. A bilevel resist masking scheme allows smooth, near vertical sidewall profiles, and the resulting structures can be exposed using a simple lift-off process. This process was used to fabricate test structures of Fe, Cu, and Co which were characterized by scanning electron microscopy, atomic force microscopy, and ac resistivity measurements from 3 to 300 K. While the wire structures are geometrically well defined, transport measurements reveal high crystalline defect densities which must be eliminated to fabricate low-dimensional magnetoresistive structures.
Investigation into the magnetic properties of Zn1−xMnxTe thin films by the Faraday effect
Authors: Masterson H. J. , Lunney J. G. , Coey J. M. D.
Published: July 2002

An investigation of the concentration and temperature dependent Faraday rotation is performed in a series of Zn1−xMnxTe thin films produced by pulsed laser deposition. The films were deposited on (0001) sapphire substrates, and had Mn fractions in the range 0.08⩽x⩽0.84. An analytical expression for the peak rotation θFp near the band edge, was derived using a single oscillator model for the refractive index at the energy of the ground state free exciton, and it was shown that θFp is directly related to the sample magnetization in the high field limit of several Tesla. However, even at the moderate field strength of 0.4 T, used in the present work, the expression still approximately models the band edge Faraday rotation, and θFp can still be usefully employed as a probe of the magnetic properties of the films. By examining the variation in θFp with Mn concentration, it was seen that a roughly linear decrease occurred with increasing x, and this was assumed to be due to the formation of antiferromagnetic clusters of Mn2+ ions. Temperature dependent studies of (θFp)−1 for films with x=0.08 and x=0.55 revealed a linear Curie–Weiss behavior down to low temperatures, before exhibiting a characteristic downturn caused by uncoupled spins which contribute noticeably to the magnetic susceptibility in this temperature regime. Spin freezing was also observed from a cusp-like behavior in θFp, for films with x=0.55 and x=0.63. The temperature of the spin freezing agreed very well with its expected position based on the available magnetic phase diagram for Zn1−xMnxTe.
Characterization of amorphous hydrogenated carbon nitride films prepared by plasma-enhanced chemical vapor deposition using a helical resonator discharge
Authors: Kim Joo Han , Ahn Dong Hoon , Kim Yong Hwan , Baik Hong Koo
Published: July 2002

Amorphous hydrogenated carbon nitride thin films (a-CNx:H) have been prepared by plasma-enhanced chemical vapor deposition of N2 and CH4 gases using a helical resonator discharge. The structural and optical properties of the deposited a-CNx:H films have been systematically studied as a function of the substrate temperature and radio frequency (rf) substrate bias. The chemical structure and elemental composition of the a-CNx:H films were characterized by Fourier transform infrared spectroscopy (FT-IR), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The optical properties of the films were evaluated using transmission ultraviolet–visible spectroscopy. The morphology of the films was investigated by scanning electron microscopy and atomic force microscopy. The FT-IR and XPS studies demonstrate the presence of carbon–nitrogen bonds with hydrogenated components in the films. The film composition ratio N/C was found to vary from 0.127 to 0.213 depending on the deposition conditions. The Raman spectra, showing the G and D bands, indicate that the a-CNx:H films have a graphitic structure. It can be found that the optical band-gap Eg of a-CNx:H films is associated with graphitic clusters, while the decrease in Eg is correlated with an increase in the size and number of graphitic clusters. Combining the results of Raman and optical measurements, it can be concluded that a progressive graphitization of the films occurs with increasing the substrate temperature and rf substrate bias power, corresponding to bias voltage.
Laser detection of CH2 in CH4–H2 mixture dc discharges
Authors: Baravian G. , Sultan G. , Amorim J. , Hayaud C.
Published: July 2002

In CH4–H2 mixture dc discharges, the CH2 radicals are identified from hot H atom laser-induced fluorescence Doppler spectral profiles. We examine the possible contributions of hydrocarbon species present in the discharge, which could be dissociated by the laser photons and produce the fast H atom fragments responsible for these profiles. The dissociation energy, 4.2±0.2 eV, experimentally determined indicates that we detect mainly CH2 radicals. At a pressure of 2 Torr and a discharge current intensity of 10 mA, we show that the CH2 radical density increases almost linearly with the percentage—until 15%—of CH4 in the gas mixture, while the population density of the H atoms created in the plasma remains quasiconstant, except in the cathode sheath.
Optical properties of bulk AlGaAs
Authors: Zheng Jun , Lin Chih-Hsiang , Kuo C. H.
Published: July 2002

An analytical semiempirical model is presented for the dielectric function ε(E)=ε1(E)+iε2(E) of bulk AlGaAs. The imaginary part of the dielectric function ε2(E) is calculated based on a simplified model of band structure and the real part of the dielectric function ε1(E) is calculated from ε2(E) by directly integrating the Kramers–Kronig relation. The discrepancy between the calculated refractive index and experimental data for GaAs is less than 0.4% for photon energies from 1.35 to 1.8 eV.
Rayleigh–Taylor stability criteria for elastic-plastic solid plates and shells
Authors: Ruden E. L. , Bell D. E.
Published: July 2002

The Rayleigh–Taylor (R-T) instability theory is usually applied to the acceleration of one fluid by a lower density one, but also becomes applicable to a solid accelerated by a fluid at very high pressure. Approximate analytic R-T stability criteria are derived for both finite and infinitesimal perturbations of the driven surface of an incompressible solid plate of a given thickness, shear modulus, and von Mises yield stress uniformly accelerated by a massless fluid. The Prandtl-Reuss equations of elastic-plastic flow are assumed for the solid. A single degree of freedom, amplitude q, is assumed for the spatial dependence of the perturbation, which is approximated to be that of the semi-infinite half-plane ideal fluid linear R-T eigenfunction. The temporal dependence of q, however, is determined self-consistently from global energy balance, following a previously published model. The (significant) effect of the unperturbed solid’s stress tensor is included and related to the converging/diverging geometries of imploding/exploding cylindrical and spherical solid shells for which the model may be applied locally. Correlations with Phillips Laboratory’s quasispherical electromagetic implosions of solid shells are presented.
Optical properties of amorphous diamond prepared by a mass-separated ion beam: Correlation with the Raman spectra
Authors: Khriachtchev L. Yu. , Räsänen M. , Lappalainen R.
Published: July 2002

The optical properties of amorphous-diamond films prepared by a mass-separated ion beam are studied and compared with the shape of the corresponding Raman spectra. The Raman measurements with moderate spatial resolution indicate that the films possess the sp3 fraction distributed along the film surface, the thinner edges being less diamondlike than the thicker center. The optical parameters of the film material, such as the refractive index and absorption coefficient, follow the spatial changes of the sp3 fraction, and the amorphous-diamond center is characterized by a relatively high refractive index (n∼2.50) and quite low absorption (α∼0.2×104 cm−1 at 633 nm). Numerical correlation between the Raman spectra and optical parameters proposes a common origin for their spatial distribution, namely, impurities perturbing the three-dimensional diamondlike network and causing sp2-coordinated carbon areas.
Assessment of the theoretical basis of the Rule of Additivity for the nucleation incubation time during continuous cooling
Authors: Zhu Y. T. , Lowe T. C. , Asaro R. J.
Published: July 2002

The rule of additivity was first proposed by Scheil and Steinberg for predicting the incubation time for nucleation of solid phases during continuous-cooling phase transformations, and has since been widely used for both the nucleation incubation and the entire process of phase transformation. While having been successfully used to calculate the transformed volume fraction during continuous cooling in many steel alloy systems, there is experimental evidence that shows rule of additivity to be invalid for describing the incubation time for nucleation. Attempts to prove the validity of the rule of additivity for the incubation time have not met with much success, and much confusion still exists about its applicability to the incubation time. This article investigates the additivity of the consumption of the incubation time for nucleation during continuous cooling through an analysis based upon classical nucleation theory. It is rigorously demonstrated that the rule of additivity is invalid for the incubation time for nucleation. However, in practice, the relative error caused by using the rule of additivity could be very small in many cases due to the resolution limit of current experimental techniques. The present theory provides an explanation for the failure of the rule of additivity in predicting the incubation time for nucleation during continuous cooling.
Impact ionization in thin AlxGa1−xAs (x=0.15 and 0.30) p-i-n diodes
Authors: Plimmer S. A. , David J. P. R. , Rees G. J. , Grey R. , Herbert D. C. , Wight D. R. , Higgs A. W.
Published: July 2002

The electron and hole multiplication characteristics, Me and Mh, respectively, have been measured in two series of AlxGa1−xAs (x=0.15 and 0.30) p-i-n diodes where the i-region thicknesses, w, vary from 1.0 down to 0.025 μm. From these, the effective electron and hole ionization coefficients, α and β, respectively, have been determined and in the thicker structures agreement is found with data published previously in the literature. However, in the devices where w⩽0.1 μm, the dead space effect reduces multiplication below their bulk values at lower values of bias. As the bias is increased, α and β increase very rapidly suggesting that overshoot effects are compensating for the dead space.
Full-energy absorption of x-ray energies near the Xe L- and K-photoionization thresholds in xenon gas detectors: Simulation and experimental results
Authors: Dias T. H. V. T. , dos Santos J. M. F. , Rachinhas P. J. B. M. , Santos F. P. , Conde C. A. N. , Stauffer A. D.
Published: July 2002

Distributions of the number of primary electrons produced per incident mono-energetic x-rays in the 1- to 41-keV energy range, which includes the xenon L- and K-absorption edges, were simulated in xenon gas detectors with the Monte Carlo technique. These simulated full-energy absorption distributions are calculated as frequency plots of the number of primary electrons produced per incident x-ray photon. The simulation includes the absorption of x-rays and the de-excitation of the residual xenon ions, followed by the development of the primary electron cloud. The discontinuities observed in the Fano factor, w-value, energy linearity and energy resolution reflect the discontinuities of the Xe photoionization cross-section at the photoabsorption edges. The simulation results are compared with experimental values measured with a gas proportional scintillation counter, and with recent data from other authors. The discontinuities in energy linearity produce an ambiguity in determining the x-ray energy in certain narrow ranges containing the edges. However, our simulation results permit a detailed analysis of observations in these regions. At the K-edge, the discontinuities in the calculated Fano factor and energy resolution were found to depend on the extent to which the K-fluorescence produced by the xenon atoms is allowed to escape. A discussion of the asymmetry of the calculated full-energy absorption peaks is made in terms of the distinction between the different decay branches initiated by photoionization of the Xe atoms, and K-fluorescence escape is found to influence strongly the skewness of the calculated distributions.
Argon metastable densities in radio frequency Ar, Ar/O2 and Ar/CF4 electrical discharges
Authors: Rauf Shahid , Kushner Mark J.
Published: July 2002

The spatial distributions of excited states in radio frequency electrical gas discharges have been observed to be dynamic functions of gas mixture, pressure, and applied voltage. Recent measurements of two-dimensional profiles of excited states in the Gaseous Electronics Conference reference cell (GECRC) [McMillin and Zachariah, J. Appl. Phys. 77, 5538 (1995); 79, 77 (1996)] have shown that the spatial distribution of the Ar(4s) density varies considerably with operating conditions. The peak density of Ar(4s) systematically shifted in position, as well as changed in magnitude, with variations in pressure, applied voltage, and gas mixture. In this article, we present results from a two-dimensional computer simulation of Ar, Ar/O2, and Ar/CF4 discharges sustained in the GECRC with the intent of investigating the experimental trends. The simulations, performed with the Hybrid Plasma Equipment Model, agree well with experiments. They show that the shift in Ar(4s) densities is largely explained by the reduction in the electron mean free path, and local perturbations in the ambipolar electric field resulting from electrode structures. Additions of small amounts of O2 and CF4 decrease the Ar(4s) density due to quenching, and change its profile due to a transition to an electronegative plasma.
Theory of microwave propagation in dielectric/magnetic film multilayer structures
Authors: Camley R. E. , Mills D. L.
Published: July 2002

We explore the theory of microwave propagation in dielectric films, on which thin metallic ferromagnetic films have been deposited. Our aim is to study coupling between the microwave electromagnetic fields, and spin excitations in the ferromagnetic films. We present quantitative studies of attenuation provided by coupling to spin excitations, for various model structures including superlattices. We find strong attenuation of the microwaves, for frequencies near the ferromagnetic resonance frequency of Fe. Modest magnetic fields place this resonance above 20 GHz, and allow its frequency to be tuned. We note a transmission minimum occurs near the frequency γ(H0+4πMs), which is in the 70 GHz range for external magnetic fields H0 of a few kilograms. We explore the dependence of these phenomena on film thicknesses, and argue that such structures will move suitably for high frequency microwave devices.
Dispersion of birefringence in AgGaS2 and CuGaS2
Authors: Choi In-Hwan , Eom Sung-Hwan , Yu Peter Y.
Published: July 2002

The natural birefringence in two chalcopyrite semiconductors AgGaS2 and CuGaS2 have been measured in the visible at both room and liquid helium temperatures. This birefringence exhibits a sign reversal and considerable dispersion near the band gap in both samples. A phenomenological quasicubic model which was proposed to explain the dispersion of natural birefringence in wurtzite-type II-VI semiconductors is found to explain our experimental results quite satisfactorily provided exciton effect is included in fitting the data at low temperature.
Profile simulation of conformality of chemical vapor deposited copper in subquarter-micron trench and via structures
Authors: Burke Aaron , Braeckelmann Gregory , Manger Dirk , Eisenbraun Eric , Kaloyeros Alain E. , McVittie Jim P. , Han Joseph , Bang David , Loan James F. , Sullivan John J.
Published: July 2002

Copper profile evolution in ultralarge scale integration via and trench structures was investigated for thermal low pressure, low temperature, chemical vapor deposition (LPCVD) from CuI(tmvs)(hfac). The investigation examined copper profiles in specialized cantilever structures as a function of systematic changes in key processing conditions, namely, substrate temperature, precursor flux, and hydrogen reactant flow. Resulting experimental observations from cross section scanning electron microscopy were incorporated in a fast analytical simulator, using a two-dimensional adsorption/re-emission model, to simulate copper profile evolution. The deposition profiles were simulated using a single rate limiting precursor model. A comparison of simulation results and actual experimental profiles for thermal LPCVD copper showed that species re-emission within the via and trench structures play a critical role in achieving conformal step coverage and complete filling. In additional, precursor flux and substrate temperature were identified as the dominant parameters in the species re-emission process, with the probability for re-emission being inversely proportional to substrate temperature and directly proportional to precursor flux. The results of this study were employed in the development of an optimum LPCVD process window for complete copper filling of aggressive via and trench structures at growth rates above 2000 Å/min and as-deposited resistivity below 2.0 μΩ cm.
Consequences of asymmetric pumping in low pressure plasma processing reactors: A three-dimensional modeling study
Authors: Kushner Mark J.
Published: July 2002

Low pressure (<10s mTorr), high plasma density (>1011 cm−3) reactors are rapidly becoming the tool of choice for etching during microelectronics fabrication. Although gas injection and pumping are well characterized process parameters in higher pressure systems, the impact of the symmetry of gas injection and pumping on the uniformity of reactant fluxes to the substrate are less well known in low pressure systems. In this article, results from a three-dimensional plasma equipment model are used to investigate the consequences of asymmetric gas pumping on reactive fluxes in inductively coupled plasma reactors operating in Cl2. We find that for typical conditions (10 mTorr, 150 sccm, 400 W) the azimuthal symmetry of ion density and ion fluxes are little affected by, for example, side pumping. This results from the dominance of ambipolar electric fields which are largely determined by the uniformity of ion sources and shape of the chamber. Neutral reactant fluxes do suffer azimuthal asymmetries due to pumping, however the variations are typically <10%–15% over a pressure range of 5–20 mTorr. The magnitude of the side-to-side variations is largely determined by the relative rates of loss of reactant species to surfaces compared to pumping.
Study of Hall and effective mobilities in pseudomorphic Si1−xGex p-channel metal–oxide–semiconductor field-effect transistors at room temperature and 4.2 K
Authors: Lander R. J. P. , Emeleus C. J. , McGregor B. M. , Parker E. H. C. , Whall T. E. , Evans A. G. R. , Kennedy G. P.
Published: July 2002

A study of several Si0.8Ge0.2 p-channel heterostructures with self-aligned poly-Si metal–oxide–semiconductor gates were carried out. A novel fabrication process was developed which is compatible with the strained Si/SiGe system, and it has allowed Hall and resistivity measurements to be performed at room temperature and at 4.2 K. The structures were numerically modelled to calculate the charge distribution with temperature and with gate voltage and the results have shown good agreement with experiment. Hall measurements at 4.2 K have shown consistent SiGe channel Hall mobility enhancements of ×3 over the SiO2/Si channels in the same devices. Room temperature effective mobilities were measured for a buried Si0.8Ge0.2 p-channel metal–oxide–semiconductor field-effect transistor heterostructure using capacitance–voltage measurements to calculate the carrier density. Mobilities are consistently over 300 cm2/V s and the low temperature studies, together with measurements of comparable modulation doped heterostructures, and secondary-ion-mass spectroscopy depth profiles suggest that this mobility is at present limited by the quality and proximity of the SiO2/Si interface.
Improved efficiencies in light emitting diodes made with CdSe(CdS) core/shell type nanocrystals and a semiconducting polymer
Authors: Schlamp M. C. , Peng Xiaogang , Alivisatos A. P.
Published: July 2002

We report experiments on bilayer light emitting diodes made with organically capped CdSe(CdS) core/shell type semiconductor nanocrystals and an electroluminescent (EL) semiconducting polymer [poly(p-phenylenevinylene) or PPV]. The devices emit from red to green with external quantum efficiencies of up to 0.22% at brightnesses of 600 cd/m2 and current densities of 1 A/cm2. They have operating voltages as low as 4 V and lifetimes under constant current flow of hundreds of hours. Most of these numbers are significant improvements over similar devices made with CdSe nanocrystals. The devices show either nanocrystal-only EL or a combination of nanocrystal and PPV EL, depending on nanocrystal layer thickness. The nanocrystal EL is dependent on nanocrystal size. Some devices show a voltage dependent spectral output. The spectral output is consistent with a field dependent electron range in the nanocrystal layer limited by carrier trapping.
Spin-wave Brillouin study of ultra-thin Co/Au(111) films with perpendicular magnetic anisotropy
Authors: Murayama Akihiro , Hyomi Kyoko , Eickmann James , Falco Charles M.
Published: July 2002

We have studied ultra-thin Co/Au(111) films with perpendicular magnetic anisotropy by means of spin-wave Brillouin scattering. The ultra-thin Co films with thicknesses from 1 to 7 monolayer (ML) were grown by molecular beam epitaxy on 1 ML Au underlayers without intentional substrate heating. With 2 or more MLs of Co, spin-wave excitations are clearly observed. We find the magnetic field dependence of the spin-wave energy can be quantitatively explained by a calculation including out-of-plane magnetization due to uniaxial perpendicular magnetic anisotropy. The first-order perpendicular anisotropy constant systematically increases as the Co thickness decreases from 7 to 3 ML. The second-order perpendicular anisotropy constant is much smaller than that expected in Co(0001) bulk and also increases as the Co thickness decreases. The physical origin of the thickness dependence of the perpendicular anisotropy is discussed. We find the field dependence of the scattering intensity is different between Stokes and anti-Stokes processes. Finally, with 2 ML of Co, significant broadening of the spin-wave spectrum is observed, as well as the degradation of the perpendicular anisotropy. We believe this is the result of the effect of magnetic inhomogeneities due to the structural imperfections.
Tunneling induced charge generation in SiO2 thin films
Authors: Chen Chun , Wilson William L. , Smayling Michael
Published: July 2002

Charge buildup in thermally grown SiO2 thin films due to Fowler–Nordheim tunneling has been investigated. Both positive and negative oxide charge can be generated by the tunneling current. The stress fluence, voltage amplitude, and polarity dependence of the positive charge generation points to impact ionization occurring near the oxide substrate interface as the most likely source of this charge. The centroid location and density of the negative oxide charge are used to analyze the negative charge generation process at different stress voltage levels and temperature. A theoretical model is proposed and compared with the experimental data.
Limitations of the constant photocurrent method: A comprehensive experimental and modeling study
Authors: Schmidt J. A. , Rubinelli F. A.
Published: July 2002

We present experimental and modeling results for the subgap absorption coefficient of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) in order to explore the limitations of the constant photocurrent method (CPM). To properly model the subgap absorption coefficient, we have developed a simulation computer program including all the possible optical and thermal transitions between gap and extended states. Tail states are assumed to be either donor- or acceptorlike and midgap states are assumed to be amphoteric. The defect-pool model is also incorporated in our analysis. We have fitted several experimental dc-CPM spectra by using a standard set of parameters for the density of states. Our analysis demonstrates that in undoped samples the true absorption coefficient and the density of midgap states are underestimated by the CPM measurement. This is due to a nonconstant electron lifetime in the energy range between 0.8 and 1.2 eV. On the other hand, we show that in n-doped or p-doped samples, the CPM provides the correct answer. The energy range for the Fermi level where the CPM fails in a-Si:H is clearly established.
Tilted photoalignment of a nematic liquid crystal induced by a magnetic field
Authors: Andrienko Denis , Kurioz Yuri , Reznikov Yuri , Rosenblatt Charles , Petschek Rolfe , Lavrentovich Oleg , Subacius Darius
Published: July 2002

Nematic liquid crystal cells with polyvinyl cinnamate coated substrates were subjected to ultraviolet light. When this was done in the presence of an oblique magnetic field the photoalignment was found to be temporally and thermally robust, with a large pretilt angle and weak polar anchoring. Moreover, two easy axes with equal and opposite pretilt angle were obtained, such that a magnetic field could switch the director from one easy axis to the other.
Size effects of 0.8SrBi2Ta2O9–0.2Bi3TiNbO9 thin films
Authors: Zhu Jinsong , Zhang Xubai , Zhu Yongfei , Desu S. B.
Published: July 2002

The size effects of 0.8SrBi2Ta2O9–0.2Bi3TiNbO9 thin films, prepared by metalorganic deposition technique, were studied by determining how the ferroelectric properties vary with film thickness and grain size. It was found that the ferroelectric properties were determined by the grain size, and not by the thickness of the film in our studied thickness range of 80–500 nm. A 80 nm thick film showed good ferroelectric properties similar to the 500 nm thick film. The possible mechanisms for the size effects in SBT–BTN films are discussed.
Nitrogen-induced modifications in microstructure and wear durability of ultrathin amorphous-carbon films
Authors: Wei B. , Zhang B. , Johnson K. E.
Published: July 2002

A systematic experimental investigation was carried out to understand the effects of nitrogenation on the microstructure and wear durability of thin amorphous-carbon films. The films were fabricated with 0%, 10%, 15%, 20%, 30%, 40%, and 50% N2 in the sputter gas. Microstructure properties were characterized using Rutherford backscattering spectroscopy, electrical resistance, Raman spectroscopy, and x-ray photoelectron spectroscopy. Nanohardness and scratch wear resistance of the films were studied with an atomic force microscope equipped with a diamond tip. The head–disk interface tribological properties of the films were tested with industrial standard contact–start–stop wear instrumentation. The results indicate that the introduction of nitrogen into the carbon film increases the film lattice disorder and allows for the formation of carbon–nitrogen single, double, and triple bonds. The nitrogen atomic concentration in the film, electrical resistance, and the ratio of carbon–nitrogen bonds to carbon–carbon bonds increase with increasing N2 in the sputter gas. A significant addition in C≡N bond percentage is observed when the N2 exceeds 30% in the sputter gas. Both the nanohardness and scratch wear resistance of the carbon film can be significantly improved by incorporating an optimized nitrogen concentration in the film. In this study, the film processed with 30% N2 showed the highest nanohardness and wear resistance. The degraded nanowear resistance for the films processed with 40% and 50% N2 is attributed to the significant addition of C≡N bonds. Within a wide process range (15%–30% N2), the films exhibit excellent tribological performances at the head–disk interface. The wear mechanisms from the contact–start–stop wear tests are interpreted based on the understanding of film structures and film mechanical properties.
Energy level offset at organic semiconductor heterojunctions
Authors: Rajagopal A. , Wu C. I. , Kahn A.
Published: July 2002

We present an investigation via ultraviolet photoemission spectroscopy of the electronic structure of three organic-organic heterojunctions formed between the standard electron-transport emissive material tris(8-hydroxy-quinoline)aluminum (Alq3) and two hole-transport materials, i.e., 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA), and N,N′-diphenyl-N,N′-bis(l-naphthyl)-1-1′biphenyl-4,4″diamine (α-NPD). We measure directly the energy offsets between highest occupied molecular orbitals during the formation of the interfaces. We show that the relative positions of the highest occupied and lowest unoccupied molecular orbitals across the Alq3/PTCDA and Alq3/α-NPD interfaces are qualitatively different and explain, in part, the difference in the performance of electroluminescent devices based on these heterojunctions. We demonstrate the existence of charge transfer-induced dipoles which shift the molecular levels of one organic with respect to the other and invalidate the usual assumption of vacuum level alignment across organic heterojunctions. Finally, we show that the molecular level alignment is independent of the deposition sequence of the organic films and that transitivity applies to these organic “band offsets.”
Photoluminescence properties of MgxZn1−xSe single crystals
Authors: Park Sang-An , Song Ho-Jun , Kim Wha-Tek , Kim Hyung-Gon , Jin Moon-Seog , Kim Chang-Dae , Yoon Chang-Sun
Published: July 2002

Mg x Zn 1−x Se single crystals were grown by the closed tube sublimation method. The MgxZn1−xSe single crystals crystallized into zincblende and wurtzite structures in the composition ranges of x=0.0–0.1 and x=0.2–0.6, respectively. Blue and violet emissions with LO phonon replica and self-activated emissions in the MgxZn1−xSe single crystals were observed at 10 K.
Electroluminescence from mechanically damaged oxidized silicon
Authors: Yuan J. , Haneman D. , Andrienko I. , Li W.
Published: July 2002

We have found significant electroluminescence (EL), clearly visible in room light, to be produced from lightly oxidized Si wafers, which have been mechanically scratched, or indented with a diamond tip. The EL occurs in the visible and infrared ranges. Undamaged oxidized samples, where the oxide is chemically thinned to allow current from a top contact to pass, also show similar EL. However, damaged Si wafers that have only normal thin or no oxide, show negligible EL. A search for quantum-confined particles in indentation pits by scanning tunneling microscopy does not show any present. The results point strongly to the EL mechanism being related to the damage creating areas of optimum oxide thickness. The results are compared with those from spark-processed and laser-grooved silicon.
Fine iron particles formed in a sapphire crystal by the ion implantation technique
Authors: Kobayashi T. , Nakanishi A. , Fukumura K. , Langouche G.
Published: July 2002

The magnetic properties of iron atoms implanted into a sapphire crystal at a dose from 4×1016 to 1×1017 Fe/cm2 were investigated using conversion electron Mössbauer spectroscopy at low temperature. It was confirmed that the components in the Mössbauer spectra which have been assigned to the Fe4+ state so far by other researchers are caused by fine particles of metallic iron, and about 75% of the implanted iron atoms stabilize in the form of fine particles of α iron at high implantation dose. Growth of the fine particles was simulated with a computer, and the expected Mössbauer spectra were calculated considering the influence of the nearest neighbors and next-nearest neighbors on the magnetic hyperfine field of the iron atoms. The simulated spectra agrees well with the experimentally observed spectra on the whole.
Quantifying strain dependence in “colossal” magnetoresistance manganites
Authors: Millis A. J. , Darling T. , Migliori A.
Published: July 2002

Experimental, phenomenological, and theoretical analyses are given of the dependence on strain of the ferromagnetic Tc of the colossal magnetoresistance (CMR) rare earth manganese perovskites. It is found that Tc is extremely sensitive to biaxial strain; by implication other physical properties are also. The results indicate that biaxial strain is an important variable which must be considered in the design of devices based on thin films and provide evidence in favor of the relevance of the Jahn–Teller electron-phonon coupling to the CMR phenomenon.
On the optical band gap of zinc oxide
Authors: Srikant V. , Clarke D. R.
Published: July 2002

Three different values (3.1, 3.2, and 3.3 eV) have been reported for the optical band gap of zinc oxide single crystals at room temperature. By comparing the optical properties of ZnO crystals using a variety of optical techniques it is concluded that the room temperature band gap is 3.3 eV and that the other values are attributable to a valence band-donor transition at ∼3.15 eV that can dominate the optical absorption when the bulk of a single crystal is probed.
Raman and photoluminescence studies on intrinsic and Cr-doped ZnSe single crystals
Authors: Rai Brajesh K. , Katiyar R. S. , Chen K.-T. , Burger A.
Published: July 2002

Intrinsic and Cr-doped single crystals of ZnSe, grown by the seeded physical vapor transport technique, are studied using Raman and photoluminescence spectroscopic techniques. The coupling between the longitudinal–optical (LO) phonon and the hole plasmons, in Cr-doped ZnSe, results in a downward shift of the LO peak with increasing dopant concentration. The ratio of integrated intensities of LO and transverse–optical (TO) modes in ZnSe:Cr shows a systematic increase, with increasing temperature and decreasing excitation wavelength. This occurs due to the interaction of the LO phonon field with the surface electric field in the depletion layer as an enhancement of LO mode intensity. The localized Raman modes of CrZn–Se are seen in Cr-doped samples, which corroborate the photoluminescence findings of the presence of Cr2+ and Cr1+ deep levels. The LO and TO modes of intrinsic ZnSe show upward renormalization in their peak positions with decreasing excitation wavelength. The interaction of discrete phonons with the electronic continuum of the conduction band in ZnSe is attributed to the renormalization of Raman peaks. The large electron capture cross section of deep-level Cr2+ and Cr1+ impurities prevents the observation of band-to-band photoluminescence transitions at ∼2.67 eV in ZnSe:Cr; the charge transfer process Cr2+↔Cr1+ results in two emission bands at 2.25 and 1.40 eV.
Optically pumped hydrogen fluoride laser
Authors: Kessler William J. , Davis Steven J. , Miller Harold C. , Hager Gordon D.
Published: July 2002

We present results from a study of an optically pumped hydrogen fluoride laser. Rotation–vibration transitions within the (2,0) band near 1.3 μm are pumped, and both amplified spontaneous emission (ASE) and laser oscillation are observed on (2,1) band transitions near 2.7 μm. Longitudinal and transverse pumping schemes are discussed. We also have pumped the (3,0) band and observed ASE on both the (3,2) and (2,1) bands. This system serves as a prototype for other mid-infrared diatomic and polyatomic lasers based upon overtone pumping.
Direct experimental study of the magnetization reversal process in epitaxial and polycrystalline films with unidirectional anisotropy
Authors: Nikitenko V. I. , Gornakov V. S. , Dedukh L. M. , Kabanov Yu. P. , Khapikov A. F. , Shapiro A. J. , Shull R. D. , Chaiken A. , Michel R. P.
Published: July 2002

Direct observation of the magnetization reversal of epitaxial NiO/NiFe bilayers grown on (001) MgO and on polycrystalline Si substrates was performed by using the magneto-optical indicator film technique. It was shown that the unidirectional-axis magnetization reversal proceeds by domain nucleation and growth. A new phenomenon, an asymmetry in the activity of the domain nucleation centers, has been revealed. Remagnetization of the bilayer is shown to be governed by defect structures in the antiferromagnetic layer.
Anisotropy design in magnetic media: A micromagnetics study
Authors: Kaufman J. H. , Koehler T. , Moser A. , Weller D. , Jones B.
Published: July 2002

We have investigated the characteristics of model magnetic media using different strategies for the “anisotropy design.” We modeled media with intragrain anisotropy oriented random in the plane of the media, random in 3D, and aligned along the track direction, and for a large range of anisotropy field Hk and intergranular exchange J. We report a possible scaling function which collapses data for all three geometries onto a single universal curve. The rescaled data were then analyzed along with the signal-and-noise functions from the various model materials. For any geometry, we find a relation between exchange and anisotropy which maximizes the signal-to-(media) noise function.
90 MeV O16 ion irradiation effects on transport and magnetization in epitaxial thin films of La0.75Ca0.25MnO3
Authors: Bathe Ravi , Date S. K. , Shinde S. R. , Saraf L. V. , Ogale S. B. , Patil S. I. , Kumar Ravi , Arora S. K. , Mehta G. K.
Published: July 2002

The influence of 90 MeV O16 ion irradiation on the electric and magnetic properties of epitaxial thin films of La0.75Ca0.25MnO3 is studied. These films were deposited on LaAlO3(100) substrates by the pulsed excimer laser deposition technique. The thickness of each film was about 3000 Å. The high quality of the films was ensured by x-ray diffraction and transport measurements. The films were irradiated with 90 MeV oxygen ions at different doses (1011–1014 ions/cm2). It is observed that at a low dose of 1011 ions/cm2 the resistivity peak temperature (Tp) and the Curie temperature (Tc) increase, however as the dose is increased further, the Tp and Tc decrease. At the higher dose of 1014 ions/cm2, the film becomes insulating and does not show any peak in the resistivity–temperature curve down to 60 K. These results are discussed in terms of the effects of ion induced compositional inhomogeneity and strain on the film properties.