bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of defect distributions from admittance measurements and application to Cu(In,Ga)Se2 based heterojunctions
Authors: Walter T. , Herberholz R. , Müller C. , Schock H. W.
Published: July 2002

A method to deduce energy distributions of defects in the band gap of a semiconductor by measuring the complex admittance of a junction is proposed. It consists of calculating the derivative of the junction capacitance with respect to the angular frequency of the ac signal corrected by a factor taking into account the band bending and the drop of the ac signal over the space charge region of the junction. Numerical modeling demonstrates that defect distributions in energy can be reconstructed by this method with high accuracy. Defect distributions of polycrystalline Cu(In,Ga)Se2 thin films are determined by this method from temperature dependent admittance measurements on heterojunctions of Cu(In,Ga)Se2 with ZnO that are used as efficient thin film solar cells.
Preparation and characterization of thin, well-ordered aluminum oxynitride films on NiAl(001)
Authors: Bartolucci F. , Schmitz G. , Gassmann P. , Franchy R.
Published: July 2002

We deal with the formation of thin aluminum oxynitride (AlON) layers after adsorption of nitric oxide (or coadsorption of oxygen and ammonia) on NiAl(001) at 75 K and subsequent annealing at 1200 K. The adsorption of NO and formation of the AlON films are investigated by means of high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The AlON film shows a distinct (2×1) LEED pattern and the HREEL spectrum exhibits five loss peaks. An oxygen to nitrogen atomic ratio of ≂2 has been estimated from the AES analysis. The energy gap is determined to be Eg=6.6±0.2 eV. The structure of θ-AlON is derived from that of θ-Al2O3.
Exchange interaction effect on the dark current in n-type AlxGa1−xAs/GaAs multiple quantum wells infrared detectors
Authors: Huang Danhong , Manasreh M. O.
Published: July 2002

A many-body model based on the self-consistent screened Hartree–Fock approximation is used to study the electron-electron exchange interaction effect on the dark current in n-type AlxGa1−xAs/GaAs multiple quantum wells infrared detectors. This is accomplished by taking the difference between the dark current calculated from the single-particle model and that obtained from the many-body model. This difference is found to be independent of the electron mobility and the saturation velocity. The difference in the dark current was studied as a function of the bias voltage, doping concentration, and temperature. The results predict that the dark current obtained from the single-particle model is overestimated as much as an order of magnitude at low temperatures and high doping concentrations.
ac conduction in conducting poly pyrrole-poly vinyl methyl ether polymer composite materials
Authors: Saha S. K. , Mandal T. K. , Mandal B. M. , Chakravorty D.
Published: July 2002

Composite materials containing conducting polypyrrole and insulating poly (vinyl methyl ether) (PVME) have been synthesized by oxidative polymerization of pyrrole in ethanol using FeCl3 oxidant in the presence of PVME. The ac conductivity measurements have been carried out in the frequency range of 100 Hz to 10 MHz and in the temperature range of 110 to 350 K. The frequency dependent conductivity has been explained on the basis of a small polaron tunnelling mechanism.
Nitrogen plasma instabilities and the growth of silicon nitride by electron cyclotron resonance microwave plasma chemical vapor deposition
Authors: Pool F. S.
Published: July 2002

Nitrogen plasma instabilities have been identified through fluctuations in the ion current density and substrate floating potential. The behavior of the plasma instabilities was found to be confined to the pressure regime 0.9 mTorr<P<1.6 mTorr. The onset of instabilities in the nitrogen plasmas occurred following the transition from an underdense to overdense plasma, where an overdense plasma is defined for densities greater than the critical density nc=7.4×1010 cm−3. The instabilities are a consequence of the nonlinear dynamics present in electron cyclotron resonance (ECR) plasmas and indicative of a transition between plasma modes as the pressure increases from 0.9 to 1.6 mTorr. The plasma instabilities are suppressed with the introduction of silane for the deposition of silicon nitride, although the plasma still undergoes a transition from an underdense to overdense plasma at 1.0 mTorr. The transition pressure delineated regions of poor and optimum electrical properties of silicon nitride films deposited from a dilute nitrogen-silane (N2/SiH4=5) plasma. To evaluate growth conditions, the flux of energetic ions to deposited atoms was approximated by examination of the ratio of ion current density to deposition rate. This ratio was found to be well correlated to the electrical properties of ECR microwave plasma deposited silicon nitride films for pressures above the underdense to overdense transition at 1.0 mTorr.
Gas source molecular beam epitaxy growth of TlInP for new infrared optical devices
Authors: Yamamoto K. , Asahi H. , Fushida M. , Iwata K. , Gonda S.
Published: July 2002

Gas source molecular beam epitaxy growth of TlInP (thallium indium phosphide) on (100) InP substrate is described. TlInP is an important constituent material of newly proposed novel infrared optical device material, TlInGaP, which can be lattice-matched to InP. Reflection high-energy electron diffraction reveals the (2×2), (2×4), and (4×2) streak patterns corresponding to the P-excess, P-stabilized and group-III rich surfaces, respectively. However, facet pattern emerges in the low temperature (380 °C) growth. This pattern is consistent with the scanning electron microscopy image; hatched pattern along the [011] direction, which is caused by insufficient migration of group III adatoms on the surface. Double crystal x-ray diffraction rocking curves for TlP on InP and TlInP on InP samples clearly show the peak from TlP and TlInP as well as that from InP substrate. The peak position is changed with Tl/In flux ratio. Photoluminescence emission is observed for TlInP grown on InP.
Anomalous stochastic behavior of partial discharge on aluminum oxide surfaces
Authors: Van Brunt R. J. , von Glahn P. , Las T.
Published: July 2002

The stochastic properties of pulsating partial discharge (PD) generated by applying a low-frequency sinusoidal alternating voltage to a point electrode touching an aluminum oxide (Al2O3) surface in air have been investigated. The time dependence of such statistical characteristics as mean numbers of positive and negative PD pulses per half cycle and the amplitude and phase distributions of individual positive and negative PD pulses selected according to their order of occurrence in a cycle were extracted from records of the amplitudes and phases of all PD events that occurred while the voltage was applied for times up to 40 min. The discharge characteristics exhibit a dramatic sensitivity to the impurity content of Al2O3. In the case of high-purity (99.9%) Al2O3, the positive-PD pulses cease within 30 s after application of the voltage from which time the negative-PD pulses persist indefinitely in a relatively stationary pattern. The cessation of positive PD was not observed for Al2O3 samples of lower purity (96% or lower). A modified version of a previously developed Monte Carlo simulator of ac-generated PD that includes effects of transport and decay of surface charge between PD events was used to gain insight into the conditions that could give rise to the observed long-term behavior of PD for high purity Al2O3.
Observation of forward breakdown mechanism in high-pressure argon plasma produced by irradiation by an excimer laser
Authors: Tsuda Norio , Yamada Jun
Published: July 2002

When a XeCl excimer laser beam was focused in a high-pressure argon gas up to 150 atm, a hot and dense plasma was produced at the focal spot. The plasma developed not only backward but also forward, which differed from one produced by a visible laser. The radius of the forward plasma became smaller beyond the focal spot. There had been no explanation for the development mechanism of the forward plasma. A new model for the forward development mechanism called a forward breakdown wave was proposed. The forward plasma was calculated assuming that the laser beam transmitted through the plasma was further focused by the plasma. This model could predict the forward development behavior considerably well.
Phase evolution during ion-beam mixing of Ag–Cu
Authors: Wei L. C. , Averback R. S.
Published: July 2002

Ion-beam mixing of the immiscible Ag–Cu alloy system was investigated using 1.0 MeV Kr ion irradiation at temperatures ranging from 80 to 473 K. Mixing of binary (80 nm Cu/55 nm Ag) and multilayer [(8.5 nm Cu/12.2 nm Ag)×11] samples was characterized by backscattering spectrometry, electrical resistivity, and x-ray diffraction. Below room temperature, the Ag–Cu system is rendered completely miscible by Kr irradiation with the formation of a simple homogeneous phase. For irradiation at 473 K, the system again becomes immiscible, but with enhanced solubilities in the two terminal phases. At intermediate temperatures, two or three phases are formed, a nearly equiatomic phase, and one, and sometimes two, terminal phases. Irradiation at elevated temperatures of samples completely mixed at low temperature led to the decomposition of the near equiatomic phase; the new steady states were nearly the same as those obtained by direct irradiation of an as-deposited sample. Phase formation depended only weakly on the ion flux, although at the higher temperatures the decomposition reaction was favored by lower ion fluxes. The experimental results are explained using a diffusion model based on a competition between cascade mixing and thermally activated demixing.
Monitoring of aging properties of AlGaAs high-power laser arrays
Authors: Tomm J. W. , Bärwolff A. , Menzel U. , Voss M. , Puchert R. , Elsaesser Th. , Daiminger F. X. , Heinemann S. , Luft J.
Published: July 2002

Aging properties of AlGaAs high power laser arrays were monitored by photocurrent (PC) measurements. We demonstrate that this method allows a quantitative analysis of the aging process and provides insight into the microscopic degradation mechanisms. In graded index separate confinement heterostructures, the PC spectra in the spectral range of the laser transition display a substantial current decrease with aging. This is attributed to enhanced rates of surface recombination and defect generation in the graded gap region. In contrast, step index structures exhibit a much smaller change of PC. The PC results are complemented by measurements of low-frequency laser current noise and front facet temperature.
Imaging nanometer-thick patterned self-assembled monolayers via second-harmonic generation microscopy
Authors: Smilowitz L. , Jia Q. X. , Yang X. , Li D. Q. , McBranch D. , Buelow S. J. , Robinson J. M.
Published: July 2002

We have used the inherent surface sensitivity of second-harmonic generation to develop an instrument for nonlinear optical microscopy of surfaces and interfaces. This optical technique is ideal for imaging nanometer-thick, chromophoric self-assembled monolayers (SAMs), which have been patterned using photolithographic techniques. In this paper, we demonstrate the application of second-harmonic generation microscopy to patterned SAMs of the noncentrosymmetric molecule calixarene and discuss the resolution and sensitivity limits of the technique.
High shear rate viscosity measurements of perfluoropolyether lubricants for magnetic thin-film rigid disks
Authors: Hahm Chris D. , Bhushan Bharat
Published: July 2002

Lubricant rheology data relevant to the very high shear rates encountered in disk drive operation do not exits. In this study, the viscosity of eight perfluoropolyether lubricants is measured at shear rates up to 107 s−1 at ambient pressure. All eight lubricants display Newtonian behavior at all shear rates. No shear thinning was observed and the lubricants were found capable of sustaining extremely high tensile stresses, on the order of 1 MPa.
Electrical resistivity in sputtered Co90Fe10/Ag GMR multilayers
Authors: Jarratt J. D. , Klemmer T. J. , Barnard J. A.
Published: July 2002

An analysis of room temperature electrical transport properties in sputtered (Co90Fe10/Ag) giant magnetoresistive multilayers is presented. The Co90Fe10 and Ag thickness ranges are both well below bulk electron mean free paths. Layer thickness dependent resistivity is observed and compared to resistivity measurements of single layer Co90Fe10 and Ag films.
Noncollinear magnetic orders in Fe/Cr superlattices
Authors: Freyss M. , Stoeffler D. , Dreyssé H.
Published: July 2002

We calculate in a full self-consistent way the noncollinear distribution of magnetic moments in Fe5/Crn (n=1–6) superlattices by means of a d-band tight-binding model. Self-consistency is obtained on both magnitude and orientation of the moments: only the relative orientation Δφ between the central moments of two adjacent Fe layers is fixed, the other moments being free to orientate. We find that, when Δφ is varied from 0 to 180°, the total energy of the system behaves in accordance with the phenomenological proximity magnetism model proposed by Slonczewski only when the Cr thickness is not too small. For very thin Cr layers (n<2), the behavior is totally different.
Single crystal neutron diffraction study of the magnetic structure of TmNi2B2C
Authors: Sternlieb B. , Stassis C. , Goldman A. I. , Canfield P. , Shapiro S.
Published: July 2002

Neutron diffraction techniques have been used to study the magnetic structure of single crystals of the magnetic superconductor (Tc≅11 K) TmNi2B2C. We find that below approximately 1.5 K the magnetic moments order in an incommensurate spin wave with propagation vector qm=qm(a*+b*) [or qm=qm(a*−b*)] with qm=0.094±0.001. The spin wave is transverse with the moments aligned along the c axis, and the observation of relatively intense higher-order harmonics shows that the modulation is not purely sinusoidal but considerably squared. This incommensurate magnetic structure, which coexists with superconductivity below TN≅1.5 K, is quite different from those observed in the magnetic superconductors HoNi2B2C and ErNi2B2C. The origin of diffraction peaks observed in scans parallel to a* is briefly discussed.
High-density recording performance of media with CoCrPtTa/Cr/NiP on glass ceramic substrate
Authors: Lin Zhong-heng , Yen Edward T. , Chang Jack J. K. , Chen Qixu , Tang Ming-Ching , Chen Ga-Lane
Published: July 2002

The comparison of magnetic recording performance at high density between two kinds of CoCrPtTa/Cr/NiP thin films, one with Co(112̄0)//Cr(200) and another with Co(101̄1)//Cr(110) texture, on glass ceramic substrate is reported. The testing results have shown that the former films have lower transition noise, smaller partial erasure effect, and better offtrack capability in partial response maximum likelihood channel which also enhances the estimated areal density. The better performance of the films with Co(112̄0)//Cr(200) texture may be attributed to microstructure of Cr underlayer which results in bicrystal structure of magnetic layer and smaller Cr grain size.
Circular magnetic x-ray dichroism in the study of NiFe and AgNiFe thin films
Authors: Wiggins J. , Gago-Sandoval P. , Watson M. L. , van der Laan G. , Schille J. P.
Published: July 2002

Circular magnetic x-ray dichroism (CMXD) has been used as an element specific tool to investigate both AgNiFe heterogeneous alloy and NiFe thin films. The films were rf sputtered at a power of 100 W from a mosaic target in 8 μm of Ar. The absorption and CMXD spectra, recorded at the Ni and Fe L2,3 and L2,3 edges, were measured at station 1.1 of the UK Daresbury Laboratory Synchrotron Radiation Source. For the AgNiFe films, element specific hysteresis loops of both Fe and Ni within the Ag matrix were obtained for several samples in fields of up to 2.5 T. These results clearly showed that the Fe and Ni hysteresis loops superimpose, despite the fact that both the intrinsic anisotropy constants and magnetization of Ni and Fe are very different. This indicates that little or no segregation into elemental Ni or Fe particles occurs in these films. The result adds support to a previously developed phenomenological model of the giant magnetoresistance GMR of such systems, which presumes the existence of discrete NiFe particles. The ratio of the Ni and Fe orbital to spin magnetic moments was also determined for both the AgNiFe and NiFe thin films. The latter were produced by dc sputtering from Ni and Fe targets, and had compositions which varied from 100% Fe to 100% Ni. We find a reasonable correlation between the dependence of the observed m(spin)/m(orbit) ratios as a function of the films composition and previously published data for the gyromagnetic ratio of such films.
Giant magnetoresistance and interlayer coupling in Cu(111)/Ag67Co33 granular multilayers
Authors: Luo Yuansu , Moske Michael , Kaeufler Andrea , Lorenz Tilmann , Samwer Konrad
Published: July 2002

Cu/AgCo layered granular structures were prepared to study their field transport properties in comparison to their microstructure. The superlattices of the samples were characterized by x-ray diffraction. A combination of random and antiparallel alignments of magnetic particles is discovered expressing an oscillation of interlayer coupling similar to conventional multilayers. The weak antiferromagnetic coupling across thin Cu spacer layers between interfacial Co particles results in a notable enhancement of low-field sensitivity compared to granular single films. The oscillation period obtained here for Cu(111) interlayers is close to 3 atomic layers.
Infrared absorption spectra of manganese oxides La1−x−yRyCaxMnO3−δ
Authors: Kebin Li , Xijun Li , Kaigui Zhu , Jingsheng Zhu , Yuheng Zhang
Published: July 2002

Infrared transmission spectra were measured systematically in polycrystalline La1−xCaxMnO3−δ and several systems substituted by other rare earth (RE) ions such as Sm3+, Gd3+, Dy3+, etc., at La3+ sites or Fe ions at Mn sites but keeping the nominal Mn4+/Mn3+≈3/7. Two strong absorption peaks located around 600 and 350 cm−1, which are associated with internal phonon modes, i.e., stretching and bending modes of MnO6 octahedra, have been observed. Both of them are influenced by the concentrations of Ca and other RE ions substituting at the La and Mn sites. The details of the absorption peaks will be discussed on the basis of changes of the bond length of Mn-O and bond angle of Mn-O-Mn in MnO6 octahedra.
Photoemission study of the formation of intimate In–InGaAs(100) contacts at room and cryogenic temperatures
Authors: Cammack D. S. , McGregor S. M. , McChesney J. J. , Dharmadasa I. M. , Clark S. A. , Dunstan P. R. , Burgess S. R. , Wilks S. P. , Elliott M.
Published: July 2002

Previous current–voltage studies of In contacts deposited on atomically clean (intimate) In53Ga47As(100) have indicated the potential to “select” barrier heights in this materials system by cryogenic processing. Soft x-ray photoemission spectroscopy was used to determine the electronic and chemical nature of these interfaces, as a function of formation temperature. Metallization at room temperature results in a predominantly three-dimensional mode of growth, accompanied by the outdiffusion of As. Low temperature metallization appears to reduce clustering and inhibit As outdiffusion. It is proposed that the distribution of surface states and the fermi level pinning position are altered by the changes that occur in the geometry and bonding of the interface at low temperature.
Influence of the microstructure on the macroscopic elastic and optical properties of dried sonogels: A Brillouin spectroscopic study
Authors: Jiménez-Riobóo R. J. , García-Hernández M. , Prieto C. , Fuentes-Gallego J. J. , Blanco E. , Ramírez-del-Solar M.
Published: July 2002

The elastic and optical properties of organically modified silicates prepared by ultrasonics aided polycondensation of tetraethoxysilane and polidimethylsiloxane are studied by means of high resolution Brillouin spectroscopy. Nuclear magnetic resonance data evidence the microseparation of the organic and inorganic phases for the systems with high content of polymer formation. The elastic and optical properties are clearly influenced by the change in microstructure. We propose a three component mechanical model that qualitatively explains the observed variation of the elastic constant c11 versus molar fraction of dimethylsiloxane in these vitreous materials and develop a structural model that accounts for the observed dynamical behavior.
Influence of a surface film on conducting particles on the electrorheological response with alternating current fields
Authors: Wu C. W. , Conrad H.
Published: July 2002

A method was developed for the frequency dependence of the electrorheological (ER) response of suspensions of highly conducting particles with a low conducting surface film. At dc or low frequency ac field, the shear yield stress of the ER fluids considered is determined by the conductivity ratio σI/σf of the film to the host oil; at high frequency ac field it is determined by the permittivity ratio εI/εf. The critical frequency separating the conduction domain from the dielectric domain is proportional to σf/εf, the ratio of the conductivity to the permittivity of the host oil. To obtain a high shear yield stress at high frequency ac field, a high ratio of the permittivity of the surface film to the oil is desired and a reasonably thin surface film. Too thin a film increases the possibility of electrical breakdown in the film especially at a small ratio of εI/εf. An effective method for overcoming electrical breakdown in the surface film is to increase the permittivity of the film. Good agreement exists between the measured shear yield stress, current density, and the applied breakdown field for oxidized aluminum particles suspended in silicone oil and those predicted by the present model. Recommendations are given for the design of ER fluids with high yield strength and reasonable current density.
Magnetic and magneto-optical properties of MnPt3/Co multilayer
Authors: Kokuryu M. , Kato T. , Iwata S. , Tsunashima S.
Published: July 2002

Highly ordered MnPt3 alloy film was grown by molecular-beam epitaxy (MBE) at a temperature of 300 °C. The MnPt3 film exhibits a large Kerr rotation, which reaches a maximum value of −0.7° at 1.24 eV. MnPt3(2 nm)/Co(0.6 nm) multilayers were also grown by MBE method onto MgO(111) substrates at a temperature of 300 °C. It was verified by the observation of the reflection high-energy electron diffraction patterns that Co and MnPt3 layers in the film were grown epitaxially on MgO(111). The appearance of superlattice line due to Cu3Au structure in x-ray-diffraction patterns confirmed the formation of the ordered phase in the MnPt3 layer. The hysteresis loops for in-plane and film normal directions became close to each other, which implies that the perpendicular magnetic anisotropy was induced through the multilayered structure. The Kerr spectrum of the MnPt3/Co multilayer have a negative peak of −0.26° around 1.4 eV. This value is fairly smaller in magnitude compared to the one estimated by the effective reflectivity method using the optical and the magneto-optical constants for Co and MnPt3 alloy.
Formation of highly oriented diamond film on (100) silicon
Authors: Kim Yoon-Kee , Lee Jai-Young
Published: July 2002

Highly oriented diamond particles are deposited on mirror-polished (100) silicon substrates in a bell-jar-type microwave plasma deposition system using a three-step process consisting of carburization, bias-enhanced nucleation and growth. By adjusting the geometry of the substrate and substrate holder, very dense disk-shaped plasma is formed over the substrate when the bias voltage is below −200 V. This dense plasma is a prerequisite for the high orientation and is not obtained simply by increasing microwave power, but is obtained by introducing a graphite block between the substrate and the substrate holder. The plasma is concentrated over the substrate with a thickness of several millimeters. From the results of the optical emission spectra of the disk-shaped dense plasma, it is found that the concentrations of atomic hydrogen and hydrocarbon radicals are increased with negative bias voltage. It is also found that the highly oriented diamonds are deposited in the region, where the intensity ratios of carbonaceous species to atomic hydrogen are saturated. As a result, it is suggested that the highly oriented diamonds are obtained by a combination of the high dose of carbonaceous species and the increased hydrogen etching effects.
Electronic defect levels in relaxed, epitaxialp-type Si1−xGex layers produced by MeV proton irradiation
Authors: Monakhov E. V. , Nylandsted Larsen A. , Kringho/j P.
Published: July 2002

Proton-irradiation-induced electronic defects in relaxed, epitaxial p-type Si1−xGex layers grown by molecular-beam epitaxy have been investigated by deep level transient spectroscopy (DLTS) for 0⩽x⩽0.25. Three dominating lines in the DLTS spectra have been observed and correlated to the divacancy, interstitial carbon, and the interstitial-boron–substitutional-carbon pair. For all three levels the activation enthalpy relative to the valence band decreases with increasing Ge content. Annealing studies demonstrated that the annealing of the defect level, identified as interstitial carbon, is retarded with increasing Ge content, while the annealing temperatures of the two other defects are similar to those observed in silicon.
An improved analysis of admittance data for high resistivity materials by a nonlinear regularization method
Authors: Winterhalter J. , Ebling D. G. , Maier D. , Honerkamp J.
Published: July 2002

Admittance data of GaAs Schottky diodes are analyzed in order to calculate the activation energy and the cross section of deep levels. An improved model for high resistivity semiconductor material is introduced. In this model the admittance depends nonlinearly on the distribution of the relaxation times of the deep levels and on material parameters like the resistivity, the barrier height, and the band gap of the semiconductor. The reconstruction of a continuous distribution and of the parameters from experimental admittance data is an ill-posed inverse problem which is solved by a nonlinear regularization procedure. The method is used to analyze the deep levels and the semiconducting properties of a semi-insulating GaAs diode. Values for the resistivity, the barrier height, and the band gap could be confirmed and deep levels with activation energy in the range of 0.5–0.72 eV were determined to be electrical active.
The infrared vibrational absorption spectrum of the Si–X defect present in heavily Si doped GaAs
Authors: Ashwin M. J. , Newman R. C. , Muraki K.
Published: July 2002

Heavily silicon doped GaAs grown by molecular beam epitaxy using a single gallium isotope source (69Ga) has been studied by infrared absorption to reveal localized vibrational modes (LVMs) of Si complexes. The structure observed close to 367 cm−1 is the same as that present in normal GaAs:Si spectra and does not result from mixed Ga isotopes. The electron trap Si–X gives three LVMs at 368.4, 370.0, and 399.6 cm−1, typical of second neighbor donor-acceptor pairs, but inconsistent with a previous proposal that its structure is the planar defect VGa–SiAs–AsGa. It is now suggested that the defect is a perturbed SiGa–VGa center, involving a second Si atom or a second vacancy.
Carbon-related defects in carbon-doped GaAs by high-temperature annealing
Authors: Fushimi Hiroshi , Wada Kazumi
Published: July 2002

The behavior of defects in heavily carbon-doped GaAs induced by high-temperature annealing has systematically been studied. It is found that carbon-related defects decrease hole concentration and mobility, carbon concentration on the As sublattices and recombination lifetime, in terms of high-temperature annealing. We propose that extrinsic or intrinsic Frenkel-type defect formation would occur on the sublattices, resulting in formation of As vacancies as well as carbon interstitials. It is experimentally determined that one defect formation reaction consumes three holes, although theory has predicted four holes. It is suggested that the origin of scattering centers would not be equal to that of recombination centers. Finally, we like to point out that interstitial carbon (Ci) or As vacancies (VAs) play an important role of degradation devices with heavily carbon-doped layers.
Characterization of the electrical properties of semimetallic Bi films by electrical field effect
Authors: Butenko A. V. , Sandomirsky V. , Schlesinger Y. , Shvarts Dm. , Sokol V. A.
Published: July 2002

The electrical field effect (EFE) was used to investigate and to characterize the electrical properties of Bi films. The samples were prepared in a capacitor configuration with Al as the gate electrode, Al2O3 as the dielectric and the thin (∼1300 Å) thermal-evaporation deposited Bi film sample serving as the other electrode. The dependence of EFE on the electrical field (up to electrical displacements ∼107 V/cm or ≈1013[e]/cm2 “surface” charge) and on temperature (15–300 K), and also the temperature dependence of Hall constant, were determined. The experimental results are shown to be consistent with the EFE theory of a semimetal film, assuming that: (1) the film has an interface “dead layer” (∼600 Å) that does not contribute markedly to EFE due to its extremely low carrier mobilities. Only in the rest, “good” part of the film, the electroconductivity is modulated and leads to a measurable EFE; (2) the temperature behavior of EFE follows the temperature dependence of the electron and hole mobilities and its sign is determined by their relative magnitudes.
Image transformation in integrated quantum well infrared photodetector-light emitting diode
Authors: Ershov M. , Liu H. C. , Schmitt L. M.
Published: July 2002

We present an analysis of physical effects responsible for infrared image transformation in integrated quantum well infrared photodetector-light emitting diode (QWIP-LED). For a large-area device, the spatial smearing of the transformed image is determined by the lateral photocurrent spreading in the QWIP and the lateral diffusion of carriers injected into the LED. For devices with low QWIP photocurrent gain, the spatial resolution is limited by the carrier diffusion lengths in the QWIP and in the LED active region, which are much shorter than radiation wavelength, and hence, the transformed image is practically undistorted.
Effect of high magnetic fields on the noise temperature of a heterostructure field-effect transistor low-noise amplifier
Authors: Daw Edward , Bradley Richard F.
Published: July 2002

We present the results of measurements of the noise temperature of a heterostructure field-effect transistor (HFET) low-noise amplifier operating at 683 MHz in high static magnetic fields. These measurements indicate that the amplifier noise temperature increases from 4.2 K in zero field to 7.0 K at 3.6 T. The effect is highly dependent on the orientation of the amplifier with respect to the magnetic field. We propose an explanation of this effect in which the paths of the electrons in the HFET channel are altered due to the Lorentz force from the applied field, affecting the HFET transconductance. Calculations based on this model are shown to be consistent with our measurements.
Simulation of the formation of two-dimensional Coulomb liquids and solids in dusty plasmas
Authors: Hwang Helen H. , Kushner Mark J.
Published: July 2002

Dust particle transport in low-temperature plasmas has recently received considerable attention due to the desire to minimize contamination of wafers during plasma processing of microelectronics devices. Laser light scattering observations of dust particles near wafers in reactive-ion-etching (RIE) radio frequency (rf) discharges have revealed clouds which display collective behavior. These observations have motivated experimental studies of the Coulomb liquid and solid properties of these systems. In this paper, we present results from a two-dimensional model for dust particle transport in RIE rf discharges in which we include particle-particle Coulomb interactions. We predict the formation of Coulomb liquids and solids. These predictions are based both on values of Γ>2 (liquid) and Γ>170 (solid), where Γ is the ratio of electrostatic potential energy to thermal energy, and on crystal-like structure in the pair correlation function. We find that Coulomb liquids and solids composed of trapped dust particles in RIE discharges are preferentially formed with increasing gas pressure, decreasing particle size, and decreasing rf power. We also observe the ejection of particles from dust crystals which completely fill trapping sites, as well as lattice disordering followed by annealing and refreezing.
Magnetization distribution in thin films with perpendicular surface anisotropy
Authors: Bertram H. Neal , Paul D. I.
Published: July 2002

Magnetization patterns and remanence states of thin films with a surface anisotropy normal to the film plane are analyzed. The total film energy integral is expanded to second order to find both equilibrium and nucleation. It is shown that for a given film and applied field a critical surface anisotropy is required for the magnetizations to deviate from initial saturation. Analytic expressions are given for these critical conditions for both in-plane and normal applied field. Remanent state magnetization distributions are shown. For small thickness where the exchange energy keeps the magnetization approximately uniform throughout the film, the out-of-plane component increases monotonically with the magnitude of the surface anisotropy. For larger thicknesses, only a layer near the film surfaces rotates into the normal direction. In this case the nucleation condition for formation of these quasiwalls is independent of the film thickness. For large thicknesses the nucleation field varies quadratically with the surface anisotropy.
Effects of the defect structure on hydrogen transport in amorphous silicon
Authors: Acco S. , Beyer W. , van Faassen E. E. , van der Weg W. F.
Published: July 2002

Hydrogen evolution transients were measured for hydrogenated amorphous silicon prepared by Si implantation of crystalline silicon and subsequent hydrogen implantation. The evolution curves are found to be similar for different H concentrations but with entirely different atomic and nanoscale structures, as was evidenced by small-angle x-ray scattering and infrared absorption investigations [Phys. Rev. B 53, 4415 (1996)]. This behavior is explained by a hydrogen-diffusion controlled effusion with a limited density of sites in the amorphous material that can be occupied by hydrogen. The experimental effusion curves are modeled by using diffusion coefficients in the implanted layers that were determined by secondary-ion mass spectrometry. Diffusion through a highly disordered material of low H content is found to have an activation energy of 2.26 eV.
Switching from photochemical to photothermal mechanism in laser ablation of benzene solutions
Authors: Hatanaka Koji , Kawao Mitsushi , Tsuboi Yasuyuki , Fukumura Hiroshi , Masuhara Hiroshi
Published: July 2002

Nanosecond KrF excimer laser ablation of benzyl chloride, benzyl alcohol, toluene, ethylbenzene, and n-propylbenzene diluted in n-hexane, n-heptane, dichloromethane, and 1,2-dichloroethane was investigated by time-resolved photographic, photoacoustic, and absorbance measurements. Ablation threshold values, Fth, for high concentration solutions (α=250 cm−1, 0.6–1 M) were confirmed to be correlated to photochemical reactivity (β-bond cleavage) of the solute molecules, whereas no distinct relation between Fth and boiling point of solvents was observed. Time-resolved absorbance at the laser wavelength was almost constant during the excitation pulse, which means that the main light-absorbing molecules were fixed to the ground-state solutes. It is considered that this type of ablation is initiated by the photochemical fragmentation. On the contrary, Fth observed in relatively low concentration solutions (α=25 cm−1, 0.06–0.1 M) were about twice higher than those for the high concentration solutions, and had no direct correlation with the photochemical reactivity of the solute molecules. The time-resolved absorbance increased during the excitation pulse, and was ascribed to the fact that benzyl radicals produced by the photodissociation of solute molecules absorbed the excitation photons and converted them into heat through “a cyclic multiphotonic absorption process.” Furthermore, morphological aspects observed in nanosecond photography exhibited appreciable differences by varying the solute concentrations. These results clearly mean a concentration-dependent ablation mechanism; the ablation mechanism of the benzene derivative solutions switches from photochemical to photothermal as the solute concentration decreases.
The structural and luminescence properties of porous silicon
Authors: Cullis A. G. , Canham L. T. , Calcott P. D. J.
Published: July 2002

A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.
Erbium in Si: Estimation of energy transfer rate and trap depth from temperature dependence of intra-4f-shell luminescence
Authors: Taguchi Akihito , Takahei Kenichiro
Published: July 2002

We investigated the temperature dependence of erbium (Er) 4f-shell luminescence decay time and intensity in silicon based on a multiphonon-assisted energy transfer model, which has been verified for the Yb-doped InP system and has already been successfully applied to various rare-earth doped III–V semiconductors. The temperature dependence of the decay time of Er 4f-shell luminescence was calculated using two fitting parameters: the depth of a trap level related to the 4f-shell luminescence and the energy transfer probability between the Er 4f shell and the electronic state of the silicon host. For Si:Er codoped with oxygen and Si:Er codoped with nitrogen, the calculated temperature dependence was compared with the experimentally observed temperature dependence. A reasonably good fit was obtained between the calculated results and the experimental results, suggesting that the Er 4f-shell luminescence is caused by the multiphonon-assisted energy transfer. The estimated energy transfer probability for Si:Er,N is larger than that for Si:Er,O, suggesting a stronger interaction between the Er 4f shell and the Si host in Si:Er,N than in Si:Er,O.
Structural properties of GaN layers on Si(001) grown by plasma-assisted molecular beam epitaxy
Authors: Yang B. , Trampert A. , Brandt O. , Jenichen B. , Ploog K. H.
Published: July 2002

We report on the growth and microstructure of GaN films deposited on Si(001) substrates by plasma-assisted molecular beam epitaxy. GaN films grown directly on Si(001) are found to be phase mixtured, containing both cubic (β) and hexagonal (α) modifications. The origin of this phase mixture is identified to be due to the formation of amorphous SixNy at the GaN/Si interface during the nucleation stage. Therefore, a GaAs buffer layer is employed to prevent the formation of SixNy. GaN films grown on this GaAs/Si(001) structure are in fact predominantly cubic and exhibit the characteristic band-edge photoluminescence (PL) of β-GaN up to room temperature. However, the PL efficiency from these samples is low compared to that of β-GaN layers directly grown on GaAs(001). We explain the lower PL efficiency by the presence of additional structural defects, which are observed in the GaN/GaAs/Si(001) heterostructure by transmission electron microscopy.
Electron microscopy studies of high coercive melt-spun Sm2+δFe15Ga2C2 permanent magnets
Authors: van Lier J. , Zern A. , Labitzke H. , Thomas J. , Seeger M. , Kronmüller H.
Published: July 2002

The influence of over-stoichiometric Sm contents on the magnetic properties of melt-spun Ga stabilized Sm2Fe17 carbides has been investigated. For optimum heat treated Sm2+δFe15Ga2C2 ribbon flakes an increase of the room temperature coercivity could be observed from μ0HC=1.5 T for stoichiometric samples to μ0HC=2.2 T for δ=0.15. This effect is very interesting because of the improved temperature stability. At 500 K a maximum energy density of (BH)max=31.4 kJ/m3 for δ=0.09 can be observed. It is assumed that a Sm-rich nonmagnetic intergranular phase decouples the grains magnetically and is consequently the origin of the high coercivities of these samples. Electron microscopy studies of these melt-spun Sm2+δFe15Ga2C2 samples with different Sm contents have been performed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A broad grain size distribution (50–250 nm) is observed in the SEM, but there is no difference among samples with different Sm contents. TEM studies show no change in grain shape with variation of the δ value. High resolution TEM (HRTEM) studies indicate the existence of a very thin intergranular phase for samples with an excess of Sm which was proved using high resolution energy dispersive x-ray analysis (EDX) studies with a scanning TEM. These investigations show a significant increase of the Sm content at the grain boundaries for these highly coercive samples.
Fe
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Mössbauer study of the interstitial nitrogen atom effects of NdFe10.5V1.5Nx
Authors: Yang Jinbo , Mao Weihua , Yang Yingchang , Zhao Zhenjie , Li Fashen
Published: July 2002

Magnetic and Fe57 Mössbauer spectra measurements were carried out to study the interstitial nitrogen atom effects in the NdFe10.5V1.5Nx (0⩽x⩽2.1) compounds. It is found that saturation magnetization, magnetocrystalline anisotropy, hyperfine fields, and isomer shifts of NdFe10.5V1.5Nx are strongly enhanced upon nitrogenation, due to magneto-volume effects and chemical bonding effects of nitrogen atoms. On the basis of the variation of isomer shifts and hyperfine fields as a function of nitrogen concentration, and by using a set of phenomenological formulas, the contributions of magneto-volume effect and the chemical bonding effect to the hyperfine parameters are successfully separated. The pure magneto-volume effect and the chemical bonding effect of the nitrogen atoms will give a positive contribution to the average hyperfine fields and isomer shifts of the Fe atoms in the NdFe10.5V1.5Nx nitrides.
Spin wave scattering and intermode transitions induced by the magnetic vortex lattice in the ferrite–high-temperature superconductor film structure
Authors: Lutsev L. V. , Yakovlev S. V.
Published: July 2002

A model of dipole-exchange spin wave scattering induced by the periodic magnetic vortex lattice in the in-plane magnetized ferrite–high-temperature superconductor film structure is considered. The vortex magnetic field action leads to spin wave intermode transitions occurring in the ferrite film. It is found that an admixture of the higher mode in the crossing region of the surface spin wave mode with the higher mode (hybrid state) increases the direct intermode transition probability significantly. It is shown that mixing of modes and the formation of hybrid spin wave states depend on the magnetic inhomogeneity over the ferrite film thickness. The theoretical model is verified for a YIG–YBa2Cu3O7+δ structure.
The effect of boron and rare earth contents on the magnetic properties of La and Cr substituted α-Fe/R2Fe14B-type nanocomposites
Authors: Chang W. C. , Chiou D. Y. , Wu S. H. , Ma B. M. , Chen Q. , Bounds C. O.
Published: July 2002

The effect of phase transformations on the magnetic properties of rare earth lean (Nd0.95La0.05)9.5Fe82.5−xCr2B6+x (x=0 to 4.5) and (Nd0.95La0.05)7.5+yFe80.5−yCr2B10 (y=0 to 4) melt spun ribbons has been investigated. The phase mixture, after optimum thermal processing, was found to be strongly dependent upon the rare earth and boron contents. Two magnetic phases, namely α-Fe and R2Fe14B, were found in (Nd0.95La0.05)9.5Fe82.5−xCr2B6+x alloy ribbons with x ranging from 0 to 4.5. For a fixed rare earth content, increases in the boron concentration resulted in a higher volume fraction of the R2Fe14B phase, which led to an increase in the intrinsic coercive force from 7.1 kOe for x=0 to 12.6 kOe for x=4.2. A Br=9.6 kG, Hc=9.5 kOei, and (BH)max=15.5 MGOe have been obtained in the alloy ribbons with x=4.5. On the other hand, the increase in the total rare earth content, or y, was found to suppress the formation of the metastable Fe3B and/or R2Fe23B3 phases and to yield an α-Fe/R2Fe14B mixture for y>1. This increase in total rare earth content not only increases the volume fraction of both the R2Fe14B and α-Fe phases but also decreases the average grain size of these phases as evidenced by transmission electron microscopy analysis. This decrease in the average grain size may subsequently enhance the intrinsic coercivity and the remanence of the ribbons. A Br of 9.5 kG, a Hci of 13.2 kOe, and a (BH)max of 18 MGOe were achieved in (Nd0.95La0.05)11Fe77Cr2B10.
Effects of heteroepitaxial strain on Laves phases TbFe2 and DyFe2
Authors: Huth M. , Flynn C. P.
Published: July 2002

(110) oriented growth was observed for thin films of the highly magnetostrictive Laves phases TbFe2 and DyFe2 on Ta (110). On Nb (111), TbFe2 nucleates in the (111) orientation. Depending on the substrate temperature during growth, varying degrees of residual tensile strain were observed in the films. For TbFe2 grown on Ta (110) at 680 °C, x-ray diffraction revealed a splitting of the (220) Bragg reflection into a strain free component structurally coherent with the template, and a component with 0.5% biaxial tensile strain at room temperature. This strained component can be attributed to the difference in thermal expansion between the sapphire substrate and the TbFe2 film. Magnetic hysteresis measurements revealed that the axis of easy magnetization lies in the film plane for both (110) and (111) oriented samples.
Scanning magnetoresistance microscopy of La0.67Sr0.33MnO3 films
Authors: Petrov D. K. , Gupta A. , Kirtley J. R. , Krusin-Elbaum L. , Gill H. S.
Published: July 2002

We have developed a magnetic imaging scheme using the magnetoresistive spin valve head in a dc bias mode as a sensing element. By scanning the head in contact with the sample we obtain a submicron spatial resolution map of the normal component of the magnetic field in the temperature range 4.2–300 K. The writing element of the sensor can be used to alter the local magnetic structure in a controlled way. This technique was applied to image the magnetic domain structure down to 77 K in patterned thin films of La0.67Sr0.33MnO3, known for their colossal magnetoresistance. A reorientation of single or multiple domains in the films was accomplished by applying a local magnetic field with the writing element, while the effect on magnetotransport was monitored with the simultaneous measurement of current–voltage characteristics.