bluuebunny/crossref_metadata_2025 huggingface dataset:
Dark current mechanisms and conditions of background radiation limitation of n-doped AlGaAs/GaAs quantum-well infrared detectors
Authors: Andersson J. Y.
Published: July 2002

The most important mechanism of dark current in n-doped quantum-well infrared photodetectors (QWIPs) is due to interaction of electrons with longitudinal optical phonons. Theoretical expressions are derived for the carrier lifetime, and for generation currents originating from both photoexcitation as well as from thermal excitation in a single quantum well. Detector gain is discussed briefly. Calculated values of thermal generation currents and the ratio of photocurrent to thermal current are found to accord well with experimental data. Finally the conditions of background radiation limitation of QWIPs are investigated and the theory gives a temperature of background radiation limitation TBLIP=81 K for a 9 μm cutoff detector with a two-dimensional grating and optical cavity, for 300 K background temperature, optics f number=1 with 100% optical transmission, provided that a photocurrent to dark current ratio of 1 criterion is used.
Magnetic properties and giant magnetoresistance in melt-spun Co-Cu alloys
Authors: Yu R. H. , Zhang X. X. , Tejada J. , Knobel M. , Tiberto P. , Allia P.
Published: July 2002

Magnetic, structural, and transport properties of as-quenched and annealed Co10Cu90 samples have been investigated using x-ray diffraction and a SQUID magnetometer. The largest value of MR change was observed for the as-quenched sample annealed at 450 °C for 30 min. The magnetic and transport properties closely correlate with the microstructure, mainly with Co magnetic particle size and its distribution. For thermal annealing the as-quenched samples below 600 °C, the Co particle diameters increase from 4.0 to 6.0 nm with a magnetoresistance (MR) drop from 33.0% to 5.0% at 10 K. Comparison with the theory indicates that the interfacial electron spin-dependent scattering mechanism correlates with GMR for Co particle diameters up to about 6.0 nm.
Plasma properties determined with induction loop probes in a planar inductively coupled plasma source
Authors: Meyer J. A. , Mau R. , Wendt A. E.
Published: July 2002

Electromagnetic fields in a planar rf inductively coupled plasma source, of interest for materials processing, were measured using a two-loop inductive (B-dot) probe. The two loops were oriented to measure the time derivative of the axial and radial components of the magnetic field Ḃz and Ḃr, respectively, at various positions in the r–z plane of the cylindrically symmetric argon discharge. Maxwell’s equations were used with this data to calculate amplitudes of the rf azimuthal electric field Eφ and current density Jφ, as well as the complex permittivity ε of the plasma, from which the electron density ne was calculated. The electron densities calculated using this technique were found to compare favorably to the results of measurements made with Langmuir probes. Electron drift velocities calculated from Jφ and ne were found to be comparable to electron thermal velocities in the region of highest Eφ and thus may contribute to local enhancement of electron impact reactions, thereby affecting process chemistry and uniformity. The peak in the drift velocity moved radially outward as the pressure increased due changes in the radial plasma density profile. This technique is applicable to chemistries where Langmuir probes are not practical.
Ultrasonic imaging of the group velocity surface about the cubic axis in silicon
Authors: Kim Kwang Yul , Bretz Kathleen C. , Every Arthur G. , Sachse Wolfgang
Published: July 2002

This article reports measurements of the group velocity surface of silicon in the region of the 〈100〉 axis. Pulsed ultrasonic beams generated by a piezoelectric longitudinal mode transducer are transmitted through water and focused onto a small spot on the surface of a (001) oriented silicon single crystal disk. This gives rise to transient elastic waves which propagate in a wide range of directions through the specimen and they are detected by a small piezoelectric longitudinal mode transducer fixed at the center of the opposite face. The imaging of the group velocity surface about the 〈100〉 axis is accomplished by scanning the focused beam in various directions along lines that intersect at epicenter. The scanned images reveal complex foldings of the slow transverse sheet of the ray surface about the 〈100〉 axis, in general agreement with the predictions of ray acoustics. However, the observed image manifests pronounced wave motions well beyond the cuspidal edges predicted by ray acoustics. This phantomlike feature or eidolon can be explained on the basis of diffraction resulting from the finite wavelength of the ultrasonic waves.
Halogen ion electric field assisted diffusion in fluorite and polyvinyl chloride during electron irradiation
Authors: Jbara O. , Cazaux J. , Remond G. , Gilles C.
Published: July 2002

Fluorite CaF2 and polyvinyl chloride specimens coated with a grounded thin gold layer have been irradiated with 12 and 30 keV electrons, respectively. The change of the halogen x-ray signal intensity (F Kα and Cl Kα) has been measured as a function of time and for various incident doses. The experimental results are fully explained by a simple model for the anion migration driven by the electrostatic field. An Auger process is suggested for the initial escape of anions from their lattice site. To our knowledge, the migration of halogen ion is reported for the first time.
High-temperature magnetic properties of TbCu7-type SmCo-based films
Authors: Hegde H. , Qian X. R. , Ahn Jong-Guk , Cadieu F. J.
Published: July 2002

In applications where large remanent moments and energy products are required at elevated temperatures, SmCo-based permanent magnets provide the best properties. In addition to their high Curie points, these SmCo-based ordered phases of 1-5, 2-17, and the disordered 1-7 phase show large uniaxial anisotropies at elevated temperatures. We have now measured the high-temperature hysteresis properties of single-phase TbCu7-type SmCo-based permanent magnet films. Such single-phase samples have the advantage that optimal magnetic properties can be obtained from the as-sputtered films without any subsequent heat treatments. Normally bulk samples in this composition range form the two phase cellular structure whose magnetic properties are very sensitive to heat treatments. Single-phase TbCu7-type films of composition Sm13Co58Fe20Cu7Zr2 were sputter synthesized such that the crystallite c axes were oriented in the film plane. The net anisotropy field of the films remained larger than the maximum applied field of 18 kOe even at the highest measurement temperature of 460 °C. Thus all measurements were from minor hysteresis loops. The in-plane coercivity showed a monotonic decrease from 6.0 kOe at room temperature to 1.3 kOe at 460 °C. The in-plane remanent flux density decreased from 8.5 kG at 50 °C to 5.8 kG at 460 °C.
Magnetic force microscopy of single-domain single-crystal iron particles with uniaxial surface anisotropy
Authors: New R. M. H. , Pease R. F. W. , White R. L. , Osgood R. M. , Babcock K.
Published: July 2002

Lithographic patterning techniques have been used to fabricate arrays of submicron particles from a [110] single-crystal iron thin film with a strong uniaxial surface anisotropy and with an easy axis of magnetization lying in the [001] direction (in the plane of the film). Magnetic force microscopy images indicate that these islands are single domain over a wide range of island shapes and sizes. The uniaxial surface anisotropy is stronger than the shape anisotropy for the island geometries used, so the easy axes of the islands all lie roughly in the [001] direction, regardless of the island shape. Magnetic force images were also taken as both the magnetic tip and sample were subjected to a gradually increasing externally applied field. This technique allows us to monitor the magnetization reversal of individual islands and provides a direct measure of their switching fields.
Giant magnetoresistance in Co/Cu multilayers: Influence of Co thickness at the first antiferromagnetic maximum
Authors: Kubinski D. J. , Holloway H.
Published: July 2002

The dependence of the magnetoresistive properties of Co/Cu multilayers on the thickness of the Co layers is described for the case when the Cu layer thickness is 9 Å, corresponding to the first antiferromagnetic maximum. Both the width and the height of the peaks in the magnetoresistance/field curves decrease with increasing Co layer thickness in the range 5–50 Å. This leads to an optimum Co layer thickness of 30–40 Å that maximizes the sensitivity to changes in magnetic field. This result may have practical significance for applications that require the relative lack of hysteresis that is found at the first antiferromagnetic maximum.
Infrared reflectance of AlN–GaN short period superlattice films
Authors: MacMillan M. F. , Devaty R. P. , Choyke W. J. , Khan M. Asif , Kuznia J.
Published: July 2002

The room-temperature infrared reflectance of AlN–GaN short period superlattice films has been measured. These superlattice films were deposited by switched atomic layer metalorganic chemical vapor deposition onto GaN or AlN buffer layers deposited on basal plane sapphire substrates. The measured reflectance spectra are compared to calculated spectra using an effective medium theory to model the dielectric function of the superlattice. The optical properties of the individual materials making up the samples are modeled with Lorentz oscillators using only bulk input parameters. The effects of film and substrate anisotropy and off-normal incidence are included in the calculation. Using this modeling technique, it is possible to obtain thickness estimates for the superlattice film and the buffer layer. The complicated structures seen in the reststrahl region reflectance of these films are also analyzed by comparison to the calculated spectra.
Phosphorus-related donors in 6H-SiC generated by ion implantation
Authors: Troffer T. , Peppermüller C. , Pensl G. , Rottner K. , Schöner A.
Published: July 2002

Aluminum-doped 6H-SiC epilayers were implanted with phosphorus and subsequently annealed in a temperature range from 1400 to 1700 °C. The annealing behavior of implanted phosphorus atoms was studied by the Hall effect, admittance spectroscopy, and photoluminescence. Phosphorus acts as a shallow donor. Two ionization energies of (80±5) meV and (110±5) meV are determined, which are assigned to phosphorus atoms residing at hexagonal and cubic lattice sites, respectively. Assuming first-order kinetics, the annealing process results in an activation energy of the phosphorus donors of 2.5 eV. A set of four lines at a wavelength of about 420/421 nm is observed in the low temperature photoluminescence spectra; the intensity of these lines increases in parallel with the electrical activation of phosphorus donors by raising the annealing temperature. It is proposed that these lines are phosphorus-related.
Scaling laws for the reduction of threading dislocation densities in homogeneous buffer layers
Authors: Speck J. S. , Brewer M. A. , Beltz G. , Romanov A. E. , Pompe W.
Published: July 2002

In the heteroepitaxial growth of films with large misfit with the underlying substrate (linear mismatch strains in excess of 1%–2%) the generation of misfit dislocations and threading dislocations (TDs) is ubiquitous for thicknesses well in excess of the equilibrium critical thickness. Experimental data suggest that the TD density in relaxed homogeneous buffer layers can be divided into three regimes: (i) an entanglement region near the film/substrate interface corresponding to TD densities of ∼1010–1012 cm−2; (ii) a falloff in TD density that is inversely proportional to the film thickness h, applicable to densities in the range ∼107–109 cm−2; and (iii) saturation or weak decay of the TD density with further increase in film thickness. Typical saturation densities are on the order of ∼106–107 cm−2. In this article, we show that the TD reduction may be described in terms of effective lateral motion of TDs with increasing film thickness. An analytic model is developed that successfully predicts both the 1/h scaling behavior and the saturation of TD densities. Long-range fluctuations in the net Burgers vector content of the local TDs is a cause for saturation behavior. These models are supported by computer simulations.
Interaction of wide band gap single crystals with 248 nm excimer laser radiation. IV. Positive ion emission from MgO and NaNO3
Authors: Ermer D. R. , Shin J.-J. , Langford S. C. , Hipps K. W. , Dickinson J. T.
Published: July 2002

We report quadrupole mass-selected, time-of-flight measurements of Mg+ from polished, single crystal MgO and Na+ from cleaved, single crystal NaNO3 exposed to 248 nm (5 eV) laser radiation. A large fraction of the ions emitted from these materials have energies well above the energy of the incident photon. As the fluence is raised from low values, the ion intensities show thresholdlike behavior with a high-order fluence dependence (roughly sixth order). At still higher fluences, the fluence dependence of Mg+ from MgO decreases to roughly second order. We attribute these emissions to weakly bound ions adsorbed atop surface electron traps; when the underlying vacancy is photoionized, the adsorbed ion is electrostatistically ejected at high energy. We argue that several photons are required to ionize a surface electron trap beneath an adsorbed ion, accounting for the high-order fluence dependence and satisfying conservation of energy. (Several 5 eV photons are required to produce a 10 eV ion.) We show that a sequence of single-photon absorption events involving photoionization, charge transfer, and retrapping account for this unusual fluence dependence. These emission intensities are strong functions of surface treatments which increase defect densities, e.g., abrasion in the case of MgO and electron bombardment in the case of NaNO3.
A simple technique for the separation of bulk and surface recombination parameters in silicon
Authors: Gaubas E. , Vanhellemont J.
Published: July 2002

A simple method for the separation of bulk and surface recombination parameters, based on the simultaneous control of time and amplitude characteristics of carrier concentration decay, is presented. To enhance the precision of the parameter extraction procedure, the amplitude is determined using a wavelength resulting in near surface carrier excitation while the effective lifetime is measured for homogeneous bulk excitation. For the fast interpretation of experimental data, a technique using nomographs of amplitude–decay time correlated dependencies on modulation depth is proposed.
Low temperature epitaxy of Si and Si1−xGex by utrahigh vacuum-chemical molecular epitaxy
Authors: Huang G. W. , Chen L. P. , Chou C. T. , Chen K. M. , Tseng H. C. , Tasi W. C. , Chang C. Y.
Published: July 2002

Pure Si2H6 and GeH4 are used to grow Si and Si1−xGex epilayers at 550 °C by ultrahigh vacuum-chemical molecular epitaxy. 0.1% B2H6 and 100 ppm PH3 diluted in H2 are used as the p- and n-type dopant gases in Si/Si1−xGex epitaxy. The Ge mole fraction x and the growth rate of Si1−xGex epilayers show very strong dependence on the total source gas flow rate ([GeH4]+[Si2H6]) and the source gas ratio ([GeH4]/[GeH4]+[Si2H6]). The results can be explained by the relationships of the source fluxes, relative incorporation efficiency at activated surface sites, and hydrogen desorption under different growth conditions. The boron concentration of Si1−xGex increases with increasing GeH4 flow rate by keeping Si2H6 and B2H6 flow rates constant. It may be due to the increase of the surface sites which is caused by the increase of the hydrogen desorption rate when a higher Ge mole fraction epilayer is grown. The phosphorus concentrations of Si and Si1−xGex show different behavior with PH3 flux at higher PH3 flow rates while one increases linearly and the other becomes saturated, respectively. These results can be explained by a model based on the different levels of the effects of phosphorus blocking of surface-activated sites between Si and Si1−xGex epilayers. This effect can also be used to explain the fact that a smaller decrease in the growth rates of Si1−xGex epilayers occurs at a higher PH3 flow rate.
Analysis of the anomalous spectral response of InP solar cells induced by high fluence proton irradiation
Authors: Yamaguchi Masafumi , Takamoto Tatsuya , Ohmori Masamichi
Published: July 2002

Proton irradiation damage of InP n+-p space solar cells has been examined over a wide range of energy (0.015–10 MeV) and at high fluence (∼1013 cm−2). Although the usual red spectral response degradation of InP cells has been observed under lower fluence proton irradiation, radiation testing has revealed an anomalous spectral response in the blue part of the spectrum. We propose a model to explain this phenomenon by considering conductivity-type conversion from p-InP into n-InP in a defect-induced region, determined by the proton range, of the p-InP base layer due to high fluence proton irradiation. This is confirmed by measurements of conductivity-type conversion of p-InP crystals by high fluence (⩾1016 cm−2) 1 MeV electron irradiation.
Theory of intermodulation in a superconducting microstrip resonator
Authors: Dahm T. , Scalapino D. J.
Published: July 2002

The penetration depth and surface resistance of a superconductor depend upon the superfluid current density. This dependence gives rise to nonlinear mixing in a superconducting microstrip resonator. Here we discuss the problem of intermodulation in which two signals at ω1 and ω2, laying within the pass band of a microstrip cavity resonance, mix and generate a signal at 2ω1−ω2. An expression relating the power generated at 2ω1−ω2 to the power transmitted at ω1 and ω2 is given. We focus on the high-Tc superconductors where it is believed that the order parameter has dx2−y2 symmetry. We find for a resonator with a large unloaded Q that intermodulation arises dominantly from the reactive nonlinear inductance of the superconducting film.
Suppression of quantum well intermixing in GaAs/AlGaAs laser structures using phosphorus-doped SiO2 encapsulant layer
Authors: Cusumano P. , Ooi B. S. , Helmy A. Saher , Ayling S. G. , Bryce A. C. , Marsh J. H. , Voegele B. , Rose M. J.
Published: July 2002

A phosphorus-doped silica (SiO2:P) cap containing 5 wt% P has been demonstrated to inhibit the bandgap shifts of p-i-n and n-i-p GaAs/AlGaAs quantum well laser structures after rapid thermal processing. The intermixing suppression has been attributed to the fact that SiO2:P is more dense and void free compared with standard SiO2 together with a strain relaxation effect of the cap layer during annealing. Band gap shift differences as large as 100 meV have been observed from samples capped with SiO2 and with SiO2:P. The n-i-p structure showed a higher degree of intermixing compared to p-i-n structure. This behaviour has been attributed to the rise of Fermi level in the n doped structure, through which the formation energy of Ga vacancies is reduced compared to the p doped structure.
Adhesion assessment of silicon carbide, carbon, and carbon nitride ultrathin overcoats by nanoscratch techniques
Authors: Deng Hong , Scharf Thomas W. , Barnard John A.
Published: July 2002

This paper presents experimental results for adhesion assessment of 20 nm ultrathin SiC, amorphous carbon and carbon nitride films deposited on silicon (111) substrates by sputtering. In the experiment, the ultrathin overcoats were scratched by a face-forward Berkovich diamond indenter with a continuous depth sensing nanoscratch system. Experimental results indicate that the nanoscratch system has very good sensitivity for detecting ultrathin overcoat cracking, delamination, and brittle fracture caused by scratching. A well-defined critical load for each film was determined by the nanoscratch techniques. The highest critical load (800 μN) is found for the CN film and the lowest (500 μN) is for SiC. Scanning electron microscopy (SEM) investigations to the surface damage of the samples reveal that the SiC overcoat was damaged more severely than other films due to its brittle fracture. The adhesion properties of the three films tested in this work are best in carbon nitride, followed by amorphous carbon and then silicon carbide.
Quantum tunneling from a critical state in small antiferromagnetic particle
Authors: Barbara B. , Barbero G. , Dobrovitski V. V. , Zvezdin A. K.
Published: July 2002

The magnetic relaxation in small (of order of hundreds nanometers) antiferromagnetic particle is considered. To explain very small tunneling (activated) volume in the particle exhibiting a single-domain behavior, we propose to account for the role of defects. The relaxation process is considered as a sequence of small jumps in a potential created by defects. Kinetics of this process is considered.
Transport properties of (Sm0.7A0.3)MnO3 (A=Ca2+,Sr2+,Ba2+,Pb2+)
Authors: Thomas R. M. , Ranno L. , Coey J. M. D.
Published: July 2002

Polycrystalline samples of the mixed-valence perovskite-type manganites (Sm0.7A0.3)MnO3 (A=Ca2+,Sr2+,Ba2+,Pb2+) have been synthesized and their transport properties studied. The electrical conductivity of the series involves a variable-range hopping mechanism in the paramagnetic state, which extends well below TC for three of the samples. Only (Sm0.7Sr0.3)MnO3 shows a metal-insulator transition. There is a peak in the resistivity at a temperature Tp, just below the Curie point, TC=70 K with a transition from thermally activated type to metallic conductivity at low temperature with a large residual resistivity, ρ0=6×10−4 Ω m. By comparison with the (Ln0.7A0.3)MnO3 compounds, Ln=La, Pr, Nd the samarium compounds are more localized. The decrease of both Tp and TC with decreasing lanthanide radius and increasing mass depends on the nature of the substituted divalent cation, but the data cannot be scaled on a universal curve with respect to average ionic radius or tolerance factor.
High-performance Co/Pt multilayer magneto-optical disk using ultrathin seed layers
Authors: Hatwar T. K. , Tyan Y. S. , Brucker C. F.
Published: July 2002

We have prepared high-performance disks based on Co/Pt multilayers using ultrathin seed layers. Both crystalline (such as ZnO) and amorphous [such as indium-tin-oxide (ITO) and Si] materials were used as seed layers. We found that very thin layers of these materials enhance coercivity, squareness of the Kerr loop, and perpendicular anisotropy of the Co/Pt superlattices. These beneficial effects are obtained with a seed layer as thin as 0.2 nm. This is incontrast with the earlier presumptions that indicated much thicker layers, about 80 nm of crystalline material, were required to improve the texture of the Co/Pt multilayer for sufficient enhancement in the coercivity. Further, the data indicated that disks with amorphous seed layers generally have higher perpendicular anisotropy and lower recording noise as compared to those with crystalline ZnO. We have prepared disks using a thin amorphous ITO seed layer and a simple disk structure: grooved PC/ITO/12[Co/Pt]/lacquer overcoat. The performance of these disks is similar to that a commercial TbFeCo media. This media has potential for low cost manufacturing.
Nanoroughness effect on Cr growth mechanism
Authors: Chang Jack J. K. , Chen Qixu , Chen Ga-Lane , Sinclair Robert
Published: July 2002

A semiquantitative model is proposed to explain a nanoroughness effect on the Cr growth mechanism. It is shown that nanoroughness is another important factor that affects the Cr texture formation and Cr grain size besides temperature. By properly controlling the nanoroughness of the seed layer, a Cr(200) texture can be induced at low temperature and a small Cr grain size with uniform distribution can be obtained.
The effect of Mn substitution on the properties of NiCuZn ferrites
Authors: Nam J.-H. , Hur W.-G. , Oh J.-H.
Published: July 2002

The properties of Mn-substituted NiCuZn ferrites, (Ni0.2Cu0.2Zn0.6O)1.03(Fe2−xMnxO3)0.97 and (Ni0.2−xMnxCu0.2Zn0.6O)1.03(Fe2O3)0.97 sintered at a low temperature, were investigated in this study. When Mn was substituted for Fe ranging from 0 to 0.04 and for Ni ranging from 0 to 0.01, initial permeability and bulk density increased within these substitution ranges. At the same time, these two NiCuZn ferrites have the larger grain size. In these two compositions, resistivity was decreased with Mn content but coercivity tended to vary sensitively with increasing Mn content.
Emission characteristics and dynamics of C2 from laser produced graphite plasma
Authors: Harilal S. S. , Issac Riju C. , Bindhu C. V. , Nampoori V. P. N. , Vallabhan C. P. G.
Published: July 2002

The emission features of laser ablated graphite plume generated in a helium ambient atmosphere have been investigated with time and space resolved plasma diagnostic technique. Time resolved optical emission spectroscopy is employed to reveal the velocity distribution of different species ejected during ablation. At lower values of laser fluences only a slowly propagating component of C2 is seen. At high fluences emission from C2 shows a twin peak distribution in time. The formation of an emission peak with diminished time delay giving an energetic peak at higher laser fluences is attributed to many body recombination. It is also observed that these double peaks get modified into triple peak time of flight distribution at distances greater than 16 mm from the target. The occurrence of multiple peaks in the C2 emission is mainly due to the delays caused from the different formation mechanism of C2 species. The velocity distribution of the faster peak exhibits an oscillating character with distance from the target surface.
Correlation of domain processes and magnetoresistance changes as a function of field and number of bilayers in Co/Cu multilayers
Authors: Aitchison P. R. , Chapman J. N. , Jardine D. B. , Evetts J. E.
Published: July 2002

The understanding of the magnetization processes in Co/Cu multilayers showing substantial giant magnetoresistance (GMR) is incomplete. To gain further insight into the coupling and resultant switching characteristics, a series of Co/Cu multilayers with 3–15 bilayers has been grown by dc magnetron sputtering. Microstructural and micromagnetic studies along with MR measurements were made on the samples. Lorentz microscopy was used to directly observe the evolution of domain structures under the influence of an applied field. It appeared that dissimilar domain structures occurred in different layers. Magnetic structures were generally submicron in size although in samples with few bilayers well defined domain walls were also observed during the reversal process. Their occurrence coincided with abrupt steps in the GMR curves.
Room-temperature detection of mobile impurities in compound semiconductors by transient ion drift
Authors: Lyubomirsky Igor , Rabinal M. K. , Cahen David
Published: July 2002

We show that the transient ion drift (TID) method, which is based on recording junction capacitance under constant reverse bias [A. Zamouche, T. Heiser, and A. Mesli, Appl. Phys. Lett. 66, 631 (1995)], can be used not only for measurements of the diffusion coefficient of mobile impurities, but also to estimate the concentration of mobile species as part of the total dopant density. This is illustrated for CdTe, contaminated by Cu, and intentionally doped by Li or Ag and for CuInSe2. We show also that, with some restrictions, the TID method can be used if the mobile ions are major dopants. This is demonstrated using Schottky barriers on CdTe, and p–n junction devices in (Hg,Cd)Te, and CuInSe2. The values that we obtain for the diffusion coefficients (for Li, Ag, and Cu in CdTe and for Cu in CuInSe2) agree well with measured or extrapolated values, obtained by other methods, as reported in the literature. Furthermore, we could distinguish between diffusion and chemical reactions of dopants, as demonstrated for the case of Cu in CdTe and Ag-doped (Hg,Cd)Te. In the former case this allows us to separate copper-free from contaminated CdTe samples.
The transition from symmetric to asymmetric discharges in pulsed 13.56 MHz capacitively coupled plasmas
Authors: Booth J. P. , Cunge G. , Sadeghi N. , Boswell R. W.
Published: July 2002

The behavior of a rapidly pulsed radio-frequency capacitively coupled parallel plate reactor has been investigated using time-resolved voltage probe, microwave interferometer, and optical emission techniques. The reactor was operated with 50 mTorr of argon and 100 W rf power (measured at the generator) at 13.56 MHz supplied to the 100-mm-diam powered electrode, with pulse durations of 25 and 100 μs. For low repetition rates (50 Hz) the voltage envelope has a characteristic form which has been entitled the “Bird’s Head.” There is no plasma present at the beginning of the pulse, so that an initial breakdown phase occurs. This phase lasts about 600 ns, after which time the plasma density is sufficiently high for the Debye length to enter the gap between the electrodes and for sheaths to form on the electrodes. In asymmetric parallel plate reactors the blocking capacitor in the matching circuit charges such that the powered electrode acquires a continuous negative voltage offset (the so-called dc bias). In this system the charging time of the capacitor is longer than the rise time of the rf voltage. Consequently, for the first few μs of the pulse the discharge is symmetric (no dc bias) and confined between the rf and the adjacent earthed electrode. As the bias voltage increases the discharge fills more of the reactor and becomes asymmetric. The rate at which the blocking capacitor charges (due to net electron current from the plasma to the powered electrode) is controlled by the Bohm-criterion limited flux of ions to the earthed walls of the reactor, as shown by particle-in-cell simulations in H. B. Smith, C. Charles, and R. W. Boswell, J. Appl. Phys. 82, 561 (1997). At high repetition rates (20 kHz) the plasma density is hardly modulated, there is no breakdown or symmetric phase, and only the electron temperature and dc bias are modulated. The conditions which lead to a symmetric discharge phase are defined. A simple analytical model is developed to describe the temporal evolution of the plasma density and electron temperature. The model is in good qualitative agreement with the observations, and predicts an average electron energy of 10’s of eV during the first few μs of the symmetric discharge.
Pulsed KrF laser annealing of Ni/Si0.76Ge0.24 films
Authors: Luo Jian-Shing , Lin Wen-Tai , Chang C. Y. , Tsai W. C.
Published: July 2002

Pulsed KrF laser annealing can suppress the island structure formation and Ge segregation associated with the interfacial reactions of Ni/Si0.76Ge0.24. For the Ni/Si0.76Ge0.24 films annealed at an energy density of 0.1–0.3 J/cm2 nickel germanosilicide associated with the amorphous overlayer was formed, while at energy densities above 0.4 J/cm2 cellular structures of Ge-deficient Si1−xGex islands surrounded by Ni(Si1−xGex)2 due to the constitutional supercooling occurred. For the continuous Ni(Si1−xGex) films grown at 200 °C, subsequent laser annealing at a higher energy density of 0.6–1.0 J/cm2 caused transformation into homogeneous Ni(Si0.76Ge0.24)2 films without island structure and Ge deficiency which readily appeared on furnace annealing at temperatures above 400 °C. At energy densities above 1.6 J/cm2 the same cellular structures as described above were also noted.
Fowler–Nordheim current–stress resistance of Si oxynitride grown in helicon-wave excited nitrogen–argon plasma
Authors: Okamoto Yoshinaga , Ikoma Hideaki
Published: July 2002

Fowler–Nordheim (FN) tunneling electron injection was performed in Al/Si oxynitride/Si capacitors and compared with the results for the thermal Si oxides grown at 850 and 900 °C as well as the plasma oxide. The Si oxynitride was grown by direct oxynitridation of the Si substrate using helicon-wave excited nitrogen–argon mixed plasma around room temperature. The shift of the threshold voltage Vth [the inversion voltage of capacitance–voltage (C–V) curves] was much larger for the negative stress than for the positive stress, similar to the case of SiO2. The Vth shifts were much smaller for the plasma-grown Si oxynitride than for the thermal and plasma SiO2 when the oxynitride sample was annealed at moderate temperatures (300–500 °C) in nitrogen ambient. These findings can be explained on the basis of surface plasmon and avalanche breakdown models proposed as the explanation of hot-carrier injection instability in SiO2. When the Si oxynitride samples were annealed at high temperature (800 °C), anomalous C– curves were observed and the degradation was very great, especially for positive bias FN stress. These results are also tentatively explained.
Effect of the shape of the first barrier on quantum well infrared photodetector performance
Authors: Liu H. C. , Li L. , Buchanan M. , Wasilewski Z. R.
Published: July 2002

A systematic study of a set of GaAs/AlGaAs quantum well infrared photodetectors with various shapes of the first barrier (which acts as the device emitter or collector barrier depending on the applied bias polarity) is reported on. Three types of barriers are investigated: a triangular, a thick, and a high barrier. It is found that the effect of the detailed emitter or collector barrier shapes on the detector performance is negligible.
Temperature dependence of the photoluminescence spectra of single crystals of CuInTe2
Authors: Rincón C. , Wasim S. M. , Marı́n G. , Sánchez Pérez G. , Bacquet G.
Published: July 2002

The temperature dependence of the photoluminescence spectra of CuInTe2 has been studied between 10 and 120 K. From the analysis of the data, emissions due to free and bound excitons have been identified. The variation with T of the free-exciton energy line is compared with the Manoogian–Woolley model for such a transition. An estimated value of the Debye temperature and the average phonon frequency are in good agreement with those reported from specific heat and reflectivity measurements, respectively. Using the existing theoretical models, binding and dissociation energy of excitons bound to ionized acceptors have been estimated. From the temperature dependence of the emission intensity of the bound exciton the activation energy is found to be (14±3) meV. This agrees quite well with the acceptor level of 15 meV reported in this compound.
Pulse poling of side-chain and crosslinkable copolymers
Authors: Michelotti Francesco , Toussaere Eric
Published: July 2002

The growth of the macroscopic second-order nonlinear optical properties of side-chain and crosslinkable copolymer species is theoretically modeled. The model describes the rotational diffusion of nonlinear diazo-dye dipoles, in the presence of crosslinking sites randomly distributed in the copolymer matrix, under application of a periodic electric poling field. Solutions for the fundamental equations, describing the time-dependent orientational probability distribution for the crosslinked dipoles, have been obtained in the frame of two complementary approximations. Such distributions have been used in order to evaluate the temporal growth of the second-order nonlinear optical properties and their asymptotic value when saturation of the crosslinking process is attained. The two approximations are shown to give equivalent results, in the range of parameters where both are valid. The influence of the poling electric field frequency and of the volume density of crosslinking sites on the asymptotic second-order nonlinear optical properties is discussed. The results of the theoretical model are compared to the experimental ones, obtained for two copolymers species: A Disperse red 1 side-chain copolymer and a crosslinkable evolution of the latter, Red acid magly. In the experiments the r33 component of the electro-optic coefficient is measured by means of a modified electro-optic ellipsometry setup. The comparison shows a good agreement between experimental and numerical results. The model can be used in order to find the best values of the copolymer parameters, necessary in order to maximize the final second-order nonlinear optical properties.
Exciton-stimulated modulation of recombination in solar cells
Authors: Karazhanov S. Zh.
Published: July 2002

This work reports an investigation of the effect of excitons on carrier recombination and solar energy conversion processes. The probabilities of exciton-stimulated modulation of the occupancy of r centers in CdS as well as the binding coefficient of free electrons and holes into excitons are estimated. Carrier recombination and transport theories are presented. The theories are applied to Cu2S/CdS solar cells for AM1 illumination at a temperature of 300 K. It has been shown by numerical estimation that exciton-stimulated modulation of the occupancy of deep impurities does give a significant reduction of carrier recombination losses and efficiency improvements for Cu2S/CdS solar cells.
Determination of the sp3/sp2 ratio in tetrahedral amorphous carbon films by effective medium approximation
Authors: Chen Z. Y. , Yu Y. H. , Zhao J. P. , Wang X. , Liu X. H. , Shi T. S.
Published: July 2002

In this article, a method for the determination of the sp3/sp2 ratio for highly tetrahedral amorphous carbon (ta-C) films is presented. This method is an optical characterization according to the Bruggeman effective medium approximation based on simulation of the infrared reflection spectrum. The simulation reflectivity is in good agreement with the experimental spectrum, and the obtained results of sp3 content are in good agreement with electron energy loss spectroscopy data, suggesting that this is an effective method for obtaining the sp3/sp2 ratio of ta-C films.
Magnetoresistive Co/Cu multilayers: Hysteresis, polycrystallinity, and irreversible changes on magnetization
Authors: Holloway H. , Kubinski D. J.
Published: July 2002

Magnetoresistive metal multilayers are known to undergo an irreversible decrease in the maximum value of their field-dependent resistance after exposure to a magnetic field. An explanation for this effect in terms of the creation of antiphase domain boundaries is considered and rejected on the basis of experimental tests that point instead to a strong correlation of irreversible loss of resistance with magnetoresistive hysteresis. The main features of the phenomenon are reproduced by a model that treats polycrystalline multilayers as assemblies of grains with random orientations of magnetic easy axes. The key feature of the model is that for each individual grain the magnetic state corresponds to an energy minimum that may be only local, rather than global. Trapping in local energy minima accounts for both magnetoresistive hysteresis and incomplete antiferromagnetic order at the maximum resistance that is attained after cycling. The model explains experimental observations, such as the relatively small hysteresis and resistance loss at the first antiferromagnetic maximum. It also predicts a strong Co-thickness dependence of the resistance loss in Co/Cu multilayers at the second antiferromagnetic maximum that is verified with experimental data for Co/Cu multilayers.
Room temperature colossal microwave magnetoimpedance in micron-size powders of La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3—A novel magnetic tape
Authors: Srinivasu V. V. , Lofland S. E. , Bhagat S. M.
Published: July 2002

We report measurements of microwave loss in micron-size powders of the manganites La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3, as functions of temperature and magnetic field. At zero field, the loss increases rapidly as the temperature is lowered below a frequency-dependent temperature T*. The more exciting observation is that at, 295 K, the loss drops by nearly 80% in an applied field of 600 Oe. Using this effect, a magnetic tape was “manufactured.”
Photothermal effects in liquid crystals on dye-doped polymer films
Authors: Ono Hiroshi , Harato Yoshiro , Kawatsuki Nobuhiro
Published: July 2002

Photothermal effects in nematic liquid crystals (NLCs) on a dye-doped polymer film are investigated using a crystal rotation method combined with a pump-probe technique. We consider both the temperature dependence of the refractive indices of the NLCs and reorientation of the NLC molecules due to a temperature rise at the polymer surface. The photothermal birefringence change is proportional to the pump beam intensity. The photothermal effects strongly depend on the nematic-isotropic transition temperature (TNI) of the NLCs. The proportional constants increased with decreasing TNI of the NLCs and the values for E44 (TNI=100 °C), E7 (59 °C), and 5CB (35 °C) are estimated to be −1.2×10−4, −2.8×10−4, and −2.6×10−3 (/mW), respectively. In addition, the pretilt angles of a 5CB cell are changed by the irradiation of the pump beam.
Structural defects and microstrain in GaN induced by Mg ion implantation
Authors: Pong B. J. , Pan C. J. , Teng Y. C. , Chi G. C. , Li W.-H. , Lee K. C. , Lee Chih-Hao
Published: July 2002

The optical and structural characteristics of GaN films implanted with Mg and Be ions, grown by low-pressure metalorganic chemical vapor deposition were studied. The low temperature (20 K) photoluminescence (PL) spectra of annealed Mg implanted GaN show a 356 nm near band edge emission, a 378 nm donor-acceptor (D-A) transition with phonon replicas, and a 528 nm green band deep level emission. The origin of the 528 nm green band emission and the 378 nm D-A emission might be attributed, respectively, to the Mg implantation induced clustering defect and the vacancy defect in GaN film. Observations of in-plane and out-of-plane x-ray diffraction spectra for as-grown undoped, Mg implanted and rapid thermal annealed GaN suggest that ion implantation induced anisotropic strain may be responsible for the observed PL emission characteristics.
Plasma-assisted deposition and characterization of Ti-containing diamondlike carbon coatings
Authors: Meng W. J. , Curtis T. J. , Rehn L. E. , Baldo P. M.
Published: July 2002

We report on the synthesis of Ti-containing diamondlike hydrocarbon (Ti-DLC) coatings by plasma-assisted physical vapor deposition and the characterization of coating microstructure and mechanical properties. We find that Ti-DLC coatings with Ti compositions higher than 5.5 at. % are composites consisting of nanocrystalline TiC in an amorphous hydrocarbon matrix. The hardness and elastic modulus of Ti-DLC coatings are found to increase monotonically with increasing Ti composition, while the ratio of the hardness to the modulus remains approximately constant.
High remanence (Sm, Zr)Fe7Nx+α-Fe nanocomposite magnets through exchange coupling
Authors: Hidaka T. , Yamamoto T. , Nakamura H. , Fukuno A.
Published: July 2002

We have investigated the influence of α-Fe content on the magnetic properties of (Sm, Zr)Fe7Nx+α-Fe nanocomposite permanent magnet materials. As α-Fe content increases, the saturation magnetization and the remanence increase while the coercivity decreases monotonously. Also we confirmed the influence of microstructure on magnetic properties. Uniform and fine grain sizes of around 20 nm realized high remanence and high squareness of demagnetizing curve such as Br=9.9 kG, HcJ=9.1 kOe, Hk/HcJ=29.9%, and (BH)max=17.8 MGOe. These experimental results agree well with micromagnetic calculations by other researchers. Compaction-molded bonded magnet was prepared by magnetic powders with magnetic properties of Br=9.9 kG, HcJ=8.2 kOe, (BH)max=17.5 MGOe and 2 wt% of epoxy resin. Bonded magnet with a density of 5.91 g/cm3 showed high magnetic properties as follows: Br=7.6 kG, HcJ=8.4 kOe, and (BH)max=12.0 MGOe.
Novel high-Tc transistors with manganite oxides
Authors: Dong Z. W. , Pai S. P. , Ramesh R. , Venkatesan T. , Johnson Mark , Chen Z. Y. , Cavanaugh A. , Zhao Y. G. , Jiang X. L. , Sharma R. P. , Ogale S. , Greene R. L.
Published: July 2002

One viable approach for transistor-like high-Tc three terminal devices is the quasiparticle injection device (QPID). We have fabricated Au/YBa2Cu3O7/LaAlO3/Nd0.7Sr0.3MnO3 (NSMO) and Au/YBa2Cu3O7/LaAlO3/LaNiO3 (LNO) heterostructures on (100) LaAlO3 substrates by pulsed laser deposition for studies of quasiparticle injection effects in high-Tc superconducting thin films. The effect of the injection of spin-polarized quasiparticles from a ferromagnetic NSMO gate was compared to that of unpolarized quasiparticles from a nonmagnetic metallic LNO gate. A current gain greater than nine has been attained for spin-polarized QPIDs, which is an order of magnitude larger than the gain of spin unpolarized QPIDs. Such large effects could be useful in a variety of active high-Tc/colossal magnetoresistance heterostructure devices.
Colossal magnetoresistance in Gd1/2Sr1/2MnO3
Authors: Garcı́a-Landa B. , De Teresa J. M. , Ibarra M. R. , Ritter C. , Drost R. , Lees M. R.
Published: July 2002

Colossal magnetoresistance has been observed in polycrystalline Gd0.5Sr0.5MnO3 perovskite. Irreversibility and sharp anomalies in the magnetostriction, magnetization, and magnetoresistance isotherms take place at low temperatures (<90 K), which have been attributed to the establishment of a charge-ordered state (CO). For temperatures lower than ≈42 K, the CO state coexists with a cluster-glass state, as deduced from the magnetic and specific heat behavior and neutron scattering experiments. The linear-thermal-expansion measurements show an abrupt drop for T≈TCO when a sufficiently high magnetic field is applied. The appearance of this anomaly has been attributed to the partial delocalization of the carriers by the field.
Logarithmic approximation for the energy band in nonparabolic semiconductors
Authors: Li Biao , Gui Yongsheng , Ye Hongjuan , Chu Junhao , Krishnamurthy Srinivasan
Published: July 2002

We propose a semi-empirical logarithmic approximation to the conduction band of small gap semiconductors. This approximation enables us to obtain analytical expressions for the intrinsic absorption coefficient and free electron density, and these expressions are valid for a large temperature range. We show that the exponential variation of the absorption coefficient, as observed by Chu et al. [J. H. Chu et al., J. Appl. Phys. 71, 3955 (1991)] in Hg1−xCdxTe, can be fitted very well for any temperature using only a few parameters whereas the analytical expression for free electron density is useful in optical and transport simulations.
Magnetic properties and ordering in C-coated FexCo1−x alloy nanocrystals
Authors: Turgut Z. , Scott J. H. , Huang M. Q. , Majetich S. A. , McHenry M. E.
Published: July 2002

C-coated FexCo1−x (x=0.50, 0.45, 0.40, 0.35, 0.30, 0.25) nanoparticles were produced using a rf plasma torch. The only C source was acetylene used as a carrier gas. Structural determination by x-ray diffraction indicated a single disordered bcc α-FeCo phase along with graphitic C for all compositions. A Scherrer analysis of the peak widths revealed particles to have an average diameter of 50 nm. A broad log-normal size distribution was found from transmission electron microscopy observations. Magnetic hysteresis loops have been measured to temperatures exceeding 1050 K and revealed relatively high room temperature coercivities (200–400 Oe), with a strong compositional variation similar to that observed in bulk alloys. Larger coercivities are consistent with particles near the monodomain size for these alloys. The temperature dependence of the magnetization revealed the effects of atomic ordering. The variation of the saturation magnetization as a function of temperature showed a discontinuity near the bulk order–disorder (α→α′) transformation temperature, as well as loss of magnetization at the α→γ structural phase transition temperature. Other features of M(T) near 500–550 °C are consistent with prior observations of a “550 °C structural anomaly” which has been observed in bulk alloys with less than perfect order.