bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport properties of nitrogen doped p-gallium selenide single crystals
Authors: Sánchez-Royo J. F. , Segura A. , Chevy A. , Roa L.
Published: July 2002

Nitrogen doped gallium selenide single crystals are studied through Hall effect and photoluminescence measurements in the temperature ranges from 150 to 700 K and from 30 to 45 K, respectively. The doping effect of nitrogen is established and room temperature resistivities as low as 20 Ω cm are measured. The temperature dependence of the hole concentration can be explained through a single acceptor-single donor model, the acceptor ionization energy being 210 meV, with a very low compensation rate. The high quality of nitrogen doped GaSe single crystals is confirmed by photoluminescence spectra exhibiting only exciton related peaks. Two phonon scattering mechanisms must be considered in order to give quantitative account of the temperature dependence of the hole mobility: scattering by 16.7 meV A′1 homopolar optical phonons with a hole-phonon coupling constant g2=0.115 and scattering by 31.5 meV LO polar phonon with a hole Fröhlich constant αh⊥=0.741.
Degradation and failure of MEH-PPV light-emitting diodes
Authors: Scott J. C. , Kaufman J. H. , Brock P. J. , DiPietro R. , Salem J. , Goitia J. A.
Published: July 2002

Light-emitting diodes made with poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene) (MEH- PPV) using indium-tin-oxide (ITO) as anode and Ca as cathode have been examined as they age during operation in a dry inert atmosphere. Two primary modes of degradation are identified. First, oxidation of the polymer leads to the formation of aromatic aldehyde, i.e., carbonyl which quenches the fluorescence. The concomitant chain scission results in reduced carrier mobility. ITO is identified as a likely source of oxygen. The second process involves the formation of localized electrical shorts which do not necessarily cause immediate complete failure because they can be isolated by self-induced melting of the surrounding cathode metal. We have not identified the origin of the shorts, but once they are initiated, thermal runaway appears to accelerate their development. The ultimate failure of many MEH-PPV devices occurs when the regions of damaged cathode start to coalesce.
Traps in undoped semi-insulating InP obtained by high temperature annealing
Authors: Marrakchi G. , Cherkaoui K. , Karoui A. , Hirt G. , Müller G.
Published: July 2002

The presence and evolution of traps in undoped semi-insulating (SI) InP obtained by high temperature annealing (900 °C for 90 h) in poor or rich phosphorus atmosphere has been studied by means of photoinduced current transient spectroscopy. Six traps named A1 to A6 having activation energies ranging from 0.2 to 0.6 eV have been detected in three samples submitted to the same annealing process. The samples differ in the Fe concentration of the starting material and the applied phosphorus pressure in the annealing process. A comparison of the corresponding photoinduced current transient spectroscopy spectra shows that among the observed traps, the 0.2 eV one can be related to a phosphorus deficiency, and the 0.3 eV and 0.4 eV traps could be due to an excess of phosphorus during annealing. Moreover, the trap corresponding to iron (0.6 eV) has been observed in all the studied samples.
Detailed analysis of room-temperature photoreflectance of strained InxGa1−xAs/AlyGa1−yAs undoped single quantum wells
Authors: Hosea T. J. C. , Lancefield D. , Garawal N. S.
Published: July 2002

We report a detailed analysis of room-temperature photoreflectance data of four InxGa1−xAs/AlyGa1−yAs undoped strained single quantum wells with x≊0.20 and y≊0, 0.05, 0.10, and 0.20. We have compared our results with theoretical calculations based on an effective mass formalism, where possible variations of well and barrier composition and thickness have been included. For all the samples studied we have identified both allowed and forbidden transitions in the spectra and obtain good fits assuming a conduction-band offset Qc of 60%±5%. This detailed analysis shows for the first time that room-temperature photoreflectance measurements alone can be used to determine the well composition and thickness together with the barrier composition and the barrier offset.
Mössbauer effect study of face-centered-cubic-like Fe on Cu(001)
Authors: Keune W. , Schatz A. , Ellerbrock R. D. , Fuest A. , Wilmers Katrin , Brand R. A.
Published: July 2002

Epitaxial face-centered-cubic (fcc)-like Fe films have been investigated in situ in ultrahigh vacuum by 57Fe conversion-electron Mössbauer spectroscopy (CEMS). A broad distribution of hyperfine magnetic fields, P(Bhf), with an extrapolated most-probable field of 32(2) T at magnetic saturation has been observed in ∼3 monolayers (ML) thick films grown at ∼90 or 300 K, and in ∼7 ML thick films grown at ∼90 K. Such films are in a ferromagnetic high-spin state. Their local structure is characterized by an electric quadrupole interaction, eQVZZ/4, of +0.114±0.05 mm/s from which we infer an anisotropically expanded fcc (fct-like) structure with c/a≊1.06. Approximately 5–7 ML thick films grown at 300 K are observed to be in a low-spin antiferromagnetic state below TN∼70 K, and are fcc (c/a=1) above TN. Site-selective CEM spectra taken on such films provide direct evidence for a magnetically ‘‘live’’ surface layer. The fcc→bcc transformation occurring in 35 ML thick films deposited at 300 K was observed to be incomplete.
The effect of rare-earth filling on the lattice thermal conductivity of skutterudites
Authors: Nolas G. S. , Slack G. A. , Morelli D. T. , Tritt T. M. , Ehrlich A. C.
Published: July 2002

Polycrystalline samples of Ir4LaGe3Sb9, Ir4NdGe3Sb9, and Ir4SmGe3Sb9 have been made by hot isostatic pressing of powders. The lattice thermal conductivity of these filled skutterudites is markedly smaller than that of IrSb3; thus, void filling shows promise as a method for improving the thermoelectric properties of these materials. We present the lattice thermal conductivity of these filled skutterudites in an effort to quantify the impact of void filling in this structure. It is believed that the atoms ‘‘rattle’’ in the voids of the structure and therefore interact with a broad spectrum of lattice phonons, reducing their mean free paths substantially below that in the ‘‘unfilled’’ skutterudites. An additional phonon scattering mechanism is caused by phonon-stimulated transitions between the low-lying energy levels of the 4f electron configurations in the case of Nd3+ and Sm3+. Magnetic susceptibility and Hall-effect measurements are also presented.
Structural and magnetic properties of face-centered-cubic Fe films grown on Co(100)
Authors: Escorcia-Aparicio Ernesto J. , Kawakami R. K. , Qiu Z. Q.
Published: July 2002

A rich variety of magnetic and structural properties have been found in fcc Fe films grown on Cu(100). In order to better comprehend the relation between the magnetic and structural properties of fcc Fe, we investigated fcc Fe films grown by molecular beam epitaxy on fcc Co(100). Structural characterization by low-energy electron diffraction and reflection high-energy electron diffraction indicate that the structural properties of fcc Fe films grown on Co(100) at room temperature are very similar to those of fcc Fe on Cu(100), exhibiting three distinct regions (fct, fcc, and bcc), with characteristic reconstructions at the boundaries. Magnetic measurements with in situ surface magneto-optic Kerr effect (SMOKE) reveal in-plane magnetization at room temperature for the three regions. Regions I and III are ferromagnetic, while region II has a small Kerr signal which is constant throughout the region. Cusps in the coercivity of the SMOKE loops are found to correspond to transitions between the three regions. Oxygen absorption experiments performed at room temperature revealed no change in the magnetization of region II, suggesting that the live layers responsible for the magnetic signal in this region are not at the surface.
Resonant microwave cavity response of amorphous ribbons
Authors: Medina A. N. , Knobel M. , Salem-Sugui S. , Gandra F. G.
Published: July 2002

In this study we report microwave experiments on the amorphous ribbon Fe4.6Co70.4Si15B10 using the usual setup for magnetic resonance experiments at 9.4 GHz to observe the cavity response. Several samples cut from the same tape were annealed, either in the presence or in the absence of a magnetic field. For each sample the magnetization and magnetoimpedance (MI) curves were obtained, with MI ratios between 1% and 40%. The magnetic resonance spectrum consists of the usual ferromagnetic line and an anomalous low field cavity response for H<100 Oe. The shape of this low field signal depends on the thermal treatment and on the orientation of the external field. We believe that this signal is in part a ferromagnetic peak but it also carries information of the skin-depth changes with a sweeping magnetic field.
Structure and magnetic properties of mechanical alloyed Nd–Fe–Ti compounds and their nitrides
Authors: Jin Zhi-qiang , Sun X. K. , Liu W. , Zhao X. G. , Xiao Q. F. , Sui Y. C. , Zhang Z. D. , Wang Zhigang
Published: July 2002

The Nd–Fe–Ti intermetallic compounds with a ThMn12 structure have been successfully prepared by mechanical alloying and then annealed. It has been found that the phase of the hexagonal TbCu7 structure is formed in Nd8Fe84Ti8 mechanical alloy powders annealed at temperatures ranging from 650 to 850 °C. With an increase in the annealing temperature Ta, the metastable TbCu7 structure transforms into a ThMn12 structure at Ta=900 °C. Consequently, the Curie temperature increases from 180 °C of TbCu7 structure at Ta=650 to 300 °C of ThMn12 structure at Ta=1050 °C. In the series of NdxFe92−xTi8 annealed at 960 °C for 30 min, it has been observed that the ThMn12 structure exists only over the range of compositions of 5≤x<11, and the structure of the type Th2Zn17 is formed for x≥11. All the compounds are nitrited at 400 °C for 15 h, and the Curie temperatures are raised from 400 to 450 °C. The Nd9Fe83Ti8 powder, annealed at 960 °C for 30 min, and nitrided at 400 °C for 15 h, has a coercivity iHc=2.3 kOe. The low coercivity perhaps mainly results from an excessive growth of α-Fe grains due to the high annealing temperature necessary for forming the hard magnetic phase Nd(Fe,Ti)12Nδ.
Optical and structural properties of spontaneously formed long-range compositional modulation in (111)A and (111)B AlGaAs
Authors: Chin Albert , Lin B. C. , Gu G. L. , Hsieh K. Y.
Published: July 2002

We have studied the spontaneously formed long-range Al-rich and Ga-rich AlxGa1−xAs/AlyGa1−yAs superlattices in both (111)A and (111)B AlGaAs during epitaxial growth. The spontaneously formed compositional modulation has been investigated by cross-sectional transmission electron microscopy in AlxGa1−xAs (x=0.3–0.4) grown on (111)A and (111)B GaAs substrates at 600–700 °C. In contrast, similar superstructures are not observed in layers simultaneously grown on (100) substrates. The observed structural compositional modulation is closely related to the large peak energy redshift and peak intensity enhancement in 15 K photoluminescence (PL) of samples grown on both (111)A and (111)B orientations to those on (100). The respective redshifts of PL peak energy for (111)A and (111)B Al0.27Ga0.73As to that of (100) are 137 and 45 meV, while the PL integrated intensity enhancement to (100) amounts to 200 and 2000 times, respectively. The effect of compositional modulation is reduced with the increasing growth temperatures as shown in the considerably less compositionally modulated (111)A and (111)B Al0.40Ga0.60As grown at 700 °C.
Positron lifetime measurements on neutron-irradiated InP crystals
Authors: Navarro F. J. , Damonte L. C. , Marí B. , Ferrero J. L.
Published: July 2002

Neutron-irradiated InP single crystals have been investigated by positron-lifetime measurements. The samples were irradiated with thermal neutrons at different fluences yielding concentrations for Sn-transmuted atoms between 2×1015 and 2×1018 cm−3. The lifetime spectra have been analyzed into one exponential decay component. The mean lifetimes show a monotonous increase with the irradiation dose from 246 to 282 ps. The increase in the lifetime has been associated to a defect containing an Indium vacancy. Thermal annealing at 550 °C reduces the lifetime until values closed to those obtained for the as-grown and conventionally doped InP crystals.
Sheath expansion into a large bore
Authors: Sheridan T. E.
Published: July 2002

Sheath expansion into a cylindrical bore is studied for radii larger than the ion-matrix overlap radius in the context of plasma-based ion implantation. The temporal dependence of the sheath width, sheath speed, and ion impact energy and the dependence of the maximum and average ion impact energy on the radius of the bore are computed. It is found that the sheath speed increases as the radii decreases, and that it is, in all cases, greater than the planar sheath speed. After the sheath has reached the axis, the potential drop across the bore decays rapidly. Consequently, the ion impact energy first increases and then decreases in time. Both the maximum and average impact energy increase with radius.
Kinetics and thermodynamics constraints in Pt gettering by P diffusion in Si
Authors: Coffa S. , Franco G. , Camalleri C. M. , Giraffa A.
Published: July 2002

We have explored the mechanisms underlying the gettering of Pt atoms dissolved in crystalline Si. By using Pt implantation at different fluences followed by a thermal process at 970 °C for 5 h we were able to prepare crystalline silicon wafers containing a uniform Pt concentration in the range 2×1012–2×1014 atoms/cm3. Subsequently, a heavily doped n-type region was produced on one side of the wafer by P diffusion at 900 °C. Following this deposition process we have studied the kinetics of Pt gettering to the P-doped region in the temperature range 700–970 °C and for annealing times ranging from 30 min to 48 h. Lifetime measurements by means of a contactless technique were used to detect the depletion of Pt in the bulk of the wafer due to the gettering process. The large range of initial Pt concentrations that we have explored allowed us to identify and separate the kinetics and thermodynamics constraints that determine the gettering efficiency and to propose a phenomenological model for the gettering of Pt. In particular, it has been found that the kinetics of the gettering process are driven by the dissolution of immobile substitutional Pt atoms into interstitial sites. This process is assisted by Si self-interstitials and characterized by an activation energy of 0.4 eV. Moreover, the equilibrium distribution of Pt is thermodynamically determined by a segregation coefficient of the Pt atoms between the gettering sites and the silicon matrix. This segregation coefficient, and hence the gettering efficiency, decrease when the temperature of the gettering process is increased and is described by an activation energy of 2.5 eV.
Frequency stability enhancement from carrier-envelope resonance in a surface acoustic wave delay line oscillator
Authors: Planat Michel , Lardet-Vieudrin Franck , Martin Gilles , Dos Santos Serge , Marianneau Gilles
Published: July 2002

We have built a new injection-locking set-up which allows the study of low frequency noise in an oscillator. The interaction between a driving 100 MHz signal and a surface acoustic wave delay line oscillator undergoing a nonlinear modulational instability is investigated. Driving amplitude is used to control the synchronization range and therefore the frequency stability of a newly discovered carrier-envelope resonance process. In the soliton-locked regime, fundamental and subharmonic steps attached to a Lorentzian shape frequency-amplitude characteristic are found. They are explained thanks to a driven nonlinear Schrödinger model and the joined synchronization map.
Transient picosecond/subpicosecond Raman scattering studies of nonequilibrium electron distributions and phonons in CdTe
Authors: Grann E. , Chen Y. , Tsen K. T. , Ferry D. K. , Almeida T. , Chen Y. P. , Faurie J. P. , Sivananthan S.
Published: July 2002

Nonequilibrium electron distributions and phonons in CdTe have been studied by transient picosecond/subpicosecond Raman spectroscopy at T=300 K. Our experimental results show that for photoexcited electron–hole density of n≂1018 cm−3, the electron distributions can be reasonably well described by Fermi–Dirac distribution functions with effective electron temperature substantially higher than the lattice temperature. From an ensemble Monte Carlo analysis of the nonequilibrium phonon population as a function of photoexcited electron–hole pair density, the LO phonon lifetime in CdTe has been deduced to be τ≂0.75±0.25 ps.
Electrical characterization of rapid thermal annealed radio frequency sputtered silicon oxide films
Authors: Choi W. K. , Choo C. K. , Lu Y. F.
Published: July 2002

An investigation of the effect of rapid thermal annealing (RTA) on the electrical properties of rf sputtered silicon oxide films was carried out. The films were prepared with the argon sputtering pressure varied from 2 to 10 mTorr. It was found that the insulating property of the films improved when deposited at lower sputtering pressure. The as-deposited film with the highest conductivity was selected for the RTA experiments. It was found that RTA at T≳900 °C or at longer times reduces the interface trapped charge (Dit) and the fixed charge (Qf) densities to 1.8×1012 eV−1 cm−2 and 1.5×1012 cm−2, respectively. We concluded that RTA at a longer period of time is more effective in improving the film quality than raising the annealing temperature. Postmetallization anneal reduces Dit further to 3.5×1011 eV−1 cm−2.
Theory of nonradiative decay dynamics in intensely pumped solid-state laser media via laser photothermal diagnostics
Authors: Mandelis Andreas , Vanniasinkam Joseph
Published: July 2002

A rigorous analytical treatment of nonradiative deexcitation and thermal-conduction transient evolution in solid-state laser media, resulting from intense optical pumping with rectangular pulses from a time-gated laser source, is presented. This situation arises in rate-window photothermal detection from laser rods with bulk and surface absorptions, the latter being due to polishing during the manufacturing of the rod. Numerical simulations of the theory show that the surface nonradiative (optical-to-thermal) energy conversion term is likely to dominate even at absorptances on the order of 1%–2%. Therefore, polishing optimization appears to be necessary in order to minimize laser losses at the surfaces when laser rods are active in a resonator cavity. The present theory also provides physical insights into the very different nature of the bulk- and surface-originating heat-conduction transients, as well as on the profile of the superposition photothermal wave form and its dependence on the optical, thermal, and metastable-state relaxation parameters. As a prelude for input to the photothermal theory, a treatment of the excited-state dynamics of a typical laser manifold pumped by an intense laser beam, away from or near saturation, is developed, and the luminescence and photothermal energy source profiles are calculated analytically.
Schottky-barrier fluctuations in Pb1−xSnxSe infrared sensors
Authors: Paglino C. , Fach A. , John J. , Müller P. , Zogg H. , Pescia D.
Published: July 2002

Epitaxial Pb1−xSnxSe layers have been grown on Si substrates, and Schottky-barrier infrared sensors were fabricated in the layers using Pb blocking contacts. The observed current–voltage characteristics (saturation currents j0 and ideality factors n) as a function of temperature are quantitatively explained with a fluctuation model of the barrier heights [J. H. Werner, W. Güttler, J. Appl. Phys. 69, 1991 (1522)]. The amount of the mean barrier fluctuation σ, which is typically 10–30 meV, depends on the layer quality and fabrication procedure. Higher σ causes higher j0 with increasing saturation values at low temperatures. In addition, the fluctuations cause high n(>2) values at low temperatures. Layers with improved structural quality (higher mobilities and lower threading dislocation densities) lead to lower barrier fluctuations and, therefore, to increased sensitivities.
Thermal properties of the nonlinear optical crystal zinc tris (thiourea) sulphate
Authors: Kerkoc P. , Venkataramanan V. , Lochran S. , Bailey R. T. , Cruickshank F. R. , Pugh D. , Sherwood J. N. , Moseley R. , Goeta A. E. , Lehmann C. W. , Howard J. A. K.
Published: July 2002

The heat capacity of crystalline zinc tris (thiourea) sulphate, has been measured in the range from 220 to 500 K by differential scanning calorimetry, and was found to obey the relationship Cp(T)=2.76×10−3 T+0.366 J g−1 K−1. Thermal expansion data have been measured in the range from 150 to 473 K. From these data, the principal thermal expansion coefficients were found to be α1=6.41×10−5 K−1, α2=4.52×10−5 K−1, and α3=−4.32×10−6 K−1. The thermal conductivity tensor of this orthorhombic crystal was calculated from values of the thermal diffusivity in the directions normal to the (100), (010), and (001) crystal planes by the laser flash method. The thermal conductivity coefficients at 295 K are k1=0.27 W m−1 K−1, k2=0.34 W m−1 K−1, and k3=0.54 W m−1 K−1.
Electrical properties of InAlAs/InGaAs modulation doped structure after SiN passivated annealing
Authors: Wakejima A. , Onda K. , Ando Y. , Fujihara A. , Mizuki E. , Nakayama T. , Miyamoto H. , Kuzuhara M.
Published: July 2002

The effect of thermal annealing on the InAlAs/InGaAs modulation-doped structure, passivated with a SiN film, was investigated. In contrast to the capless sample, the passivated modulation-doped sample exhibited negligible degradation in the two-dimensional electron gas concentration (NS) after annealing at 280 °C, indicating that SiN is an efficient passivation material to ensure good thermal stability for the InAlAs/InGaAs modulation-doped structure. In addition, partial recovery in NS was found by annealing the SiN-passivated sample that had experienced serious NS degradation. A series of secondary ion mass spectroscopy (SIMS) measurements and Hall effect measurements revealed that the recovery of NS is associated with the discharge of fluorine atoms from the Si-doped n-type InAlAs layer.
Substitutional phosphorus doping of diamond by ion implantation
Authors: Hofsäss H. , Dalmer M. , Restle M. , Ronning C.
Published: July 2002

We have studied the lattice sites of ion-implanted radioactive 33P in natural IIa diamond using the emission channeling technique. 33P ions were implanted at room temperature with 30 keV and a rather low dose of 1011 cm−2 and the implanted samples were annealed in vacuum to 1200 °C. From the channeling effects of the emitted β− particles measured for all principal axial directions we obtain a fraction of 70±10% of substitutional P, a vanishing fraction on tetrahedral interstitial sites and a 30% random fraction. Possible displacements of the P atoms from ideal substitutional sites must be below 0.2 Å. This demonstrates that P is a substitutional impurity in diamond and efficient substitutional P doping of diamond can be accomplished by a conventional implantation and annealing procedure if low implantation doses are chosen.
Distribution of As atoms in InP/InPAs (1 monolayer)/InP heterostructures measured by x-ray crystal truncation rod scattering
Authors: Tabuchi M. , Fujibayashi K. , Yamada N. , Takeda Y. , Kamei H.
Published: July 2002

The interfaces of InP/InPAs(1 monolayer)/InP samples grown by organometallic vapor-phase epitaxy were investigated by the x-ray crystal truncation rod scattering measurement. The distribution of As atoms around the InPAs heterolayer was clearly revealed in the atomic scale from the measurement. It was shown that the distribution of As atoms into the layer under the InPAs layer was very small and that distribution of As atoms in the InP cap layer was, on the other hand, noticeable and the amount of As atoms was almost the same as that contained in the InPAs heterolayer. These results suggest that the extension of As atoms in the InP layer occurs due to the absorbed As atoms on the InPAs surface or As atoms remaining in the gas phase. Thus, to realize the abrupt InP/InPAs interface, the source-gas change sequence should be controlled to be very abrupt.
Effect of growth rate on step structure and ordering in GaInP
Authors: Chun Y. S. , Lee S. H. , Ho I. H. , Stringfellow G. B.
Published: July 2002

CuPt ordering is widely observed in GaInP epitaxial layers grown by organometallic vapor phase epitaxy. The formation of this spontaneously ordered structure during epitaxial growth is intimately related to the atomic-scale physical processes occurring on the surface, specifically surface reconstruction and the attachment of atoms at steps. For growth on singular (001) GaAs substrates the surface structure, measured using atomic force microscopy, is seen to consist of small islands surrounded by either monolayer or bilayer steps. An increase in the growth rate from 0.25 to 2.0 μm/h with a constant tertiarybutylphosphine partial pressure at 670 °C has no effect on either the degree of order or the step structure. Only the step spacing is observed to change. It decreases systematically as the growth rate is increased, following an approximate 1/(growth rate)  1/2 dependence. As the growth rate increases, the time atoms have to rearrange before being frozen due to coverage by the next layer decreases. This leads directly to the dependence observed. These observations are consistent with previous observations which appear to show a close correlation between step structure (monolayer versus bilayer) and ordering, although the causative factor has not been determined.
Measurement of the effects of the localized field of a magnetic force microscope tip on a 180° domain wall
Authors: Foss Sheryl , Dahlberg E. Dan , Proksch Roger , Moskowitz Bruce M.
Published: July 2002

Opposite polarity magnetic force microscope (MFM) profiles of domain walls (DWs) in magnetite were measured with a commercial MFM tip magnetized in opposite directions perpendicular to the sample surface. The influence of the tip field on a DW resulted in an overall more attractive interaction. The difference between opposite polarity DW profiles provided a qualitative measurement of the reversible changes in DW structure due to the localized field of the MFM tip. The dependence of the measured alteration on tip-sample separation was fit with a power law at different positions across the DW. The rate of decay of the alteration with tip-sample separation, quantified by the exponent of the power law fit, varied across the DW and was much slower than expected from a simple model.
A theoretical analysis of magneto-optical Faraday effect of YIG films with random multilayer structures
Authors: Inoue Mitsuteru , Fujii Toshitaka
Published: July 2002

The magneto-optical (MO) Faraday effect of Bi-substituted yttrium iron garnet (Bi:YIG) films with random multilayer structures is analyzed using the random matrix approach, and the Faraday rotation angles (θF) are examined in comparison with those of periodic multilayer films. The films consist of Bi:YIG films and SiO2 dielectric films, which are stacked randomly (periodically) so as to form the random (periodic) multilayer structures. Similarly to the case of MO Kerr effect, a considerable enhancement in θF takes place when the film structure meets the nonreflection condition of light. In particular, θF of the film reaches a maximum of 1.4°/μm at λ=1.15 μm, with the fraction of Bi:YIG layers in the random multilayer film is 41%. This value is about seven times larger than that of a single Bi:YIG film (θF=0.2°/μm), and cannot be attained with periodic multilayer films.
Microwave permeability of ferromagnetic thin films with stripe domain structure
Authors: Acher O. , Boscher C. , Brulé B. , Perrin G. , Vukadinovic N. , Suran G. , Joisten H.
Published: July 2002

We report the observation of multiple permeability peaks for thin ferromagnetic films, in the 10 MHz to 6 GHz range. This behavior is correlated with the presence of perpendicular anisotropy and stripe domains. Because the perpendicular anisotropy is much smaller than the saturation magnetization of the layer, we propose an adaptation of the classical domain mode resonance model to the configuration with oblique magnetization in the stripes.
Step bunching mechanism in chemical vapor deposition of 6H– and 4H–SiC{0001}
Authors: Kimoto Tsunenobu , Itoh Akira , Matsunami Hiroyuki , Okano Tetsuyuki
Published: July 2002

Step bunching in chemical vapor deposition of 6H– and 4H–SiC on off-oriented {0001} faces is investigated with cross-sectional transmission electron microscopy. On an off-oriented (0001)Si face, three Si–C bilayer-height steps are the most dominant on 6H–SiC and four bilayer-height steps on 4H–SiC. In contrast, single bilayer-height steps show the highest probability on a (0001̄)C face for both 6H– and 4H–SiC epilayers grown with a C/Si ratio of 2.0. The increase of C/Si ratio up to 5.0 induces the formation of multiple-height steps even on a C face. The bunched step height corresponds to the unit cell or the half unit cell of SiC. The mechanism of step bunching is discussed with consideration of surface formation processes.
Ordering dependence of pyroelectricity in GaxIn1−xP
Authors: Weil R. , Chack A. , Levy M. , Salzman J. , Beserman R.
Published: July 2002

The pyroelectric coefficient of GaxIn1−xP thin films epitaxially grown on GaAs was measured on samples with x close to 0.5 and order parameter, η, varying between 0.02 and 0.34. It was found that the pyroelectric coefficient varies with the square of the order parameter in accordance with theory. This work also shows that the perfectly random alloy, η=0, is pyroelectric as well.
Magnetoelastic properties of terfenol composites (abstract)
Authors: Dennis K. W. , Govindaraju M. R. , Jiles D. C. , Linde M. , McCallum R. W.
Published: July 2002

Recent work was undertaken to identify the possibility of using composite Terfenol, consisting of Tb–Dy–Fe in a nonmagnetic, nonmetallic binder, for magnetoelastic sensor applications in which the material needs to be formed into complex shapes, and in which the high levels of magnetostriction obtained in Tb–Dy–Fe alone are unnecessary. Recent results reported by Sandlund et al.1 have indicated the possible use of such a material in high frequency applications. In this work we have studied the material for low frequency, or even dc applications, in which the material could be used to sense an applied torque. The results showed that these composite materials need magnetic fields of typically 800 kA/m to obtain the magnetostriction levels of 350 ppm reported elsewhere.2 Similar work by Peters3 needed fields of over 1 MA/m to obtain magnetostrictive strains beyond 100 ppm. Such field strengths are impractical for low power devices. From the present work, 38 μm Terfenol powder in an epoxy or polymer binder gave magnetostrictive strains of 120 ppm and above for field strengths of 100 kA/m, which is much more appropriate for small low power sensors. A benchmark sensitivity of 18 μ T/N m for measurement of torque levels over the range ± 10 Nm was set based on the response of nickel which has previously been used in magnetoelastic sensors. In the present composite material axial magnetic induction sensitivities of 32 μ T/Nm and tangential field sensitivities of 12 A/Nm2 (=15 μT/Nm) were obtained, which were 50% better than nickel.
Boundary roughness scattering in single and coupled quantum wires in a magnetic field
Authors: Vurgaftman I. , Meyer J. R.
Published: July 2002

We investigate magnetic-field-dependent velocity relaxation due to boundary-roughness scattering in quantum wires, using a two-dimensional (2D) finite-difference wave-packet propagation technique. We find that for low disorder correlation lengths and far away from density-of-states singularities associated with subband minima and saddle points, the Born approximation adequately reproduces the qualitative features of the more general 2D results. However, naive application of the Born approximation in regions where disorder-induced broadening or higher-order scattering become important can lead to substantial errors. In particular, the mobility enhancement associated with the opening of a quasigap in coupled quantum wire structures can be overestimated by as much as several orders of magnitude.
Profiling of electrical doping concentration in ferroelectrics
Authors: Chai Francis K. , Brews J. R. , Schrimpf R. D. , Birnie D. P.
Published: July 2002

In this paper, the extraction of doping profiles in ferroelectric thin-film capacitors using ferroelectric capacitance–voltage (CV) measurements is studied. For a ferroelectric field-dependent permittivity model, the doping profile relation to measured CV curves for ferroelectric thin-film capacitors is found to be analogous to the well-known result of metal–semiconductor Schottky junctions with an easily determined effective dielectric constant. Computer simulation shows the electrical doping concentration of ferroelectric thin-film capacitors can be profiled accurately with the proposed model. Limitations of the model are investigated.
Band structure and confined energy levels of the Si3N4/Si/GaAs system
Authors: Chen Z. , Mohammad S. N. , Park D.-G. , Diatezua D. M. , Morkoç H. , Chang Y. C.
Published: July 2002

The band structure of strained Si (4–10 ML) on (001) GaAs, band lineups of the strained Si/(001)GaAs heterojunction, and confined energy levels of the Si3N4/Si/GaAs quantum well have been calculated via a pseudopotential method. It has been found that in this technically important Si3N4/Si/(001)GaAs structure, strained Si has a very narrow band gap (0.34 eV) at the Δ⊥ point in the Brillouin zone. For the strained Si/(001)GaAs heterojunction, the conduction band offsets from the Δ⊥ for Si to the Γ valley for GaAs is 0.83 eV, and that from the Δ⊥ valley for Si to the X valley for GaAs is 1.21 eV. The valence band offset is 0.25 eV. The lowest confined energy level in the conduction band of the Si3N4/Si/GaAs quantum well ranging from 4 to 10 monolayers is found to be 0.22–0.28 eV above the conduction band edge of strained Si, or 0.57–0.61 eV below the conduction band edge of GaAs, while the first confined energy level in the valence band is barely above the valence band maximum of GaAs. The accumulation and inversion take place at these confined energy levels.
Structural and magnetic evolution of rapidly quenched Sm–Fe–Si–C alloys
Authors: Tang Wei , Jin Zhi-Qiang , Zhang Jian-Ron , Zhang Shi-Yuan , Du You-Wei
Published: July 2002

Sm 2 Fe 15.5 Si 1.5 C 1.5 samples with a rhombohedral Th2Zn17-type structure have been prepared by high frequency induction melting. The crystallization, crystal structure, and magnetic properties of melt-spun and annealed ribbons have been studied by means of x-ray diffraction, scanning electron microscopy, differential thermal analysis, and magnetic measurements. The results show that the as-spun ribbons consist mainly of an amorphous phase, and that they crystallize in two steps. Upon annealing at 660 °C the ribbons form a metastable phase of the TbCu7-type and an α-Fe phase. Upon further annealing above 750 °C the metastable phase transforms to a 2:17-type phase coexisting with α-Fe. A saturation magnetization of up to 108 emu/g with a coercivity of up to 3.5 kOe is obtained in the ribbon annealed at 660 °C for 30 min. However, the reduced remanence of the ribbon σr/σs is only 0.6 and its hysteresis loop exhibits two independent magnetic components. Additionally, the grain size of the ribbons obtained by annealing at or above 660 °C for 30 min is much larger than the single domain size. Thus, it appears that there is no significant exchange coupling between the hard and soft magnetic phases of the ribbons.
Reduction of photoscission of σ bonds in polysilanes by fullerene doping
Authors: Ninomiya Shinsuke , Ashihara Yoji , Nakayama Yoshikazu , Oka Kunio , West Robert
Published: July 2002

We report here an effective reduction of photoscission of σ bonds in polysilanes by C60 doping. The temporal variation of photoluminescence in polysilanes has been measured as a function of a doping amount of C60. The results indicate that the photoscission of σ bonds is related to a photogenerated electron-hole pair rather than a hole or an electron. The energy released when the electron-hole pair recombines nonradiatively is more likely required in addition to the thermal energy to scissor σ bonds. The C60 doping does not affect the activation energy of the photoscission process but decreases the photoscission cross-section. It is also demonstrated that the photoluminescence quenching by the C60 doping is observed for polysilanes not only with aromatic side groups but also with saturated hydrocarbon side groups.
Sensitivity analysis of transient measurements using the microwave cavity perturbation technique
Authors: Subramanian V. , Murthy V. R. K. , Sobhanadri J.
Published: July 2002

The application of microwave cavity perturbation technique for the study of transients in semiconductors is becoming popular due to its simplicity in measurement procedure and high sensitivity. This paper discusses the effects of quality factor, sample size, and coupling factor on the sensitivity of the measurement. Also, it deals with a measurement approach for the study of triplet state transitions and excited state studies in organic solvents. Finally, a comparison between the cavity perturbation technique and the currently used reflection technique is made.
Thermal conversion of an iron nitride–silicon nitride precursor into a ferromagnetic nanocomposite
Authors: Maya L. , Thompson J. R. , Song K. J. , Warmack R. J.
Published: July 2002

Iron nitride films, FeN, in a pure form and in the form of a nanocomposite in silicon nitride were prepared by reactive sputtering using iron or iron disilicide, respectively, as targets in a nitrogen plasma. Iron nitride decomposes into the elements by heating in vacuum to 800 °C. Intermediate phases such as Fe2N or Fe4N form at lower temperatures. The nanocomposites contain the iron phases as particles with an average size of ∼5 nm dispersed in the amorphous silicon nitride matrix. The magnetic properties of the nanocomposites were established. The precursor FeN–Si3N4 film is paramagnetic, while the Fe–Si3N4, obtained by heating in vacuum, is ferromagnetic and shows typical superparamagnetic behavior. These films are of interest as recording media with superior chemical and mechanical stability and may be encoded by localized heating.
Acousto-optic surface-plasmon resonance measurements of thin films on gold
Authors: Caruso Frank , Jory Michael J. , Bradberry Geoffrey W. , Sambles J. Roy , Furlong D. Neil
Published: July 2002

In this work a highly sensitive optical technique was used to study the adsorption of thin organic layers onto a gold film from water, in situ and in real-time. Optical excitation of a surface-plasmon resonance (SPR) was used to probe the gold/water interface. The use of an acousto-optic tunable filter then provides a differential technique for monitoring the SPR position in optical wavelength. This allows optical changes at the metal/liquid interface to be measured as the adsorption of thin organic layers occurs. Adsorption of a poly(ethylene glycol) monododecyl ether surfactant (C12E8) and a 30-mer DNA oligonucleotide with a mercaptohexyl group at the 5′-phosphate end (DNA-SH) onto gold from water were examined. Conventional, angle-dependent reflectivity measurements taken on the same system provided complementary SPR data, allowing the sensitivity of the two techniques to be compared.
Nitrogen doping of Te-based II–VI compounds during growth by molecular beam epitaxy
Authors: Baron T. , Saminadayar K. , Magnea N.
Published: July 2002

We present a detailed study of p-type doping of Te-based compounds (CdTe, ZnTe) and alloys (CdMgTe, ZnMgTe, and CdZnTe) during molecular beam epitaxy, using nitrogen atoms produced by a direct current glow plasma source. Characterization includes capacitance–voltage, Hall effect, low temperature reflectivity and luminescence, double crystal x-ray diffraction (XRD), nuclear reaction analysis and secondary ion mass spectroscopy. Doping introduces shallow hydrogenic acceptors NTe, whose ionization energy was determined. For ZnTe, doping as high as p≈1020 cm−3 can be obtained when ≈1.5×1020 cm−3 nitrogen atoms are incorporated in the layer. This doping level decreases considerably for CdZnTe and ZnMgTe alloys as the Zn content decreases. The highest concentration obtained for CdTe is p≈1018 cm−3. For a CdTe layer with a doping level p≈1017 cm−3 the hole mobility is μp=235 cm2/V s at 65 K. In ZnTe, incorporation of nitrogen atoms in NTe configurations induces a noticeable change of the lattice parameter, a change which was not observed in CdTe:N layers. The XRD pattern of ZnTe:N/ZnTe pseudosuperlattices allows measurement of the Zn–N bond distance dZn–N=2.16±0.05 Å. Study of the doping efficiency as a function of the growth conditions indicates that the compensation mechanism is related to the formation of nitrogen interstitial defects or complex defects involving metal vacancies, but no deep center has been detected in luminescence. A comparative study of various doped telluride materials indicates that presence of Zn atoms strongly enhances the solubility limit of NTe, hence the key role of Zn in the doping efficiency. Results are discussed on the basis of geometric and thermodynamic considerations. The first argument is that the Zn–Te distance is the closest fit to the metal–N bond distance, and this minimizes the elastic contribution to the energy of formation of the NTe acceptor. As a corollary, the doping efficiency decreases when the alloy lattice parameter increases. The second argument compares the formation enthalpies ΔHF of II3N2 nitride (II being Zn, Cd, or Mg) to that of the IITe compound.
Oxide–GaAs interfacial electronic properties characterized by modulation spectroscopy of photoreflectance
Authors: Hwang J. S. , Wang Y. C. , Chou W. Y. , Tyan S. L. , Hong M. , Mannaerts J. P. , Kwo J.
Published: July 2002

Built-in electric fields and interfacial state densities (Dit) in a series of oxide–GaAs heterostructures fabricated by in situ molecular beam epitaxy were studied using room temperature photoreflectance. The samples investigated were air-, Al2O3–Ga2Ox–, and Ga2O3(Gd2O3)–GaAs. We found that the built-in electric fields are 48, 44, and 38 kV/cm for air-, Al2O3-, and Ga2Ox–GaAs samples, respectively. For the Ga2O3(Gd2O3)–GaAs sample, the built-in electric field is negligibly small, indicating a very low interfacial state density. Estimated by the low field limit criterion, Dit is less than 1×1011 cm−2 eV−1. Our results on the Ga2O3(Gd2O3)–GaAs sample are consistent with the data obtained previously using capacitance–voltage measurements in quasistatic/high frequency modes and photoluminescence measurements.
Monte Carlo simulations of argon ion transport in the downstream region of electron cyclotron resonance plasmas
Authors: Zhong X. X. , Wu J. D. , Wu C. Z. , Li F. M.
Published: July 2002

A Monte Carlo method to simulate the plasma characteristics of an electron cyclotron resonance (ECR) microwave discharge system is presented. We use the method to predict the ion behavior in the downstream region of an ECR argon plasma. The pressure and space dependences of the plasma potential, the effects of gas pressure on the ion velocity and angle distribution and the evolution of distributions are examined and discussed in detail. Our results of the simulation show that the ion parallel velocity distribution appears to be bimodal and is strongly dependent on the gas pressure, whereas the ion perpendicular velocity distribution is mainly determined by the applied magnetic field, and the ion beam tends to be parallel to the magnetic field.
Conductance, magnetoresistance, and interlayer coupling in tunnel junctions modulated by nonmagnetic metallic interlayers
Authors: Zhang Wu-Shou , Li Bo-Zang , Zhang Xiangdong , Li Yun
Published: July 2002

Based on the two-band model and free-electron approximation, we study the magnetism and transport properties of tunnel junctions with nonmagnetic interlayers (NM) between the ferromagnetic electrodes and tunneling barrier. We find that properties of the junctions are intermediate between tunnel junctions and metallic magnetic multilayers. The mean conductance, tunnel magnetoresistance, and interlayer coupling are all the oscillatory functions of the thickness of NM. It suggests that weak antiferromagnetic coupling can be attained by controlling the thickness of NM. Our results have potential in designing spin-polarized tunneling devices with large field sensitivity.
Quantitative magnetic force microscopy on perpendicularly magnetized samples
Authors: Hug Hans J. , Stiefel B. , van Schendel P. J. A. , Moser A. , Hofer R. , Martin S. , Güntherodt H.-J. , Porthun Steffen , Abelmann Leon , Lodder J. C. , Bochi Gabriel , O’Handley R. C.
Published: July 2002

We present a transfer-function approach to calculate the force on a magnetic force microscope tip and the stray field due to a perpendicularly magnetized medium having an arbitrary magnetization pattern. Under certain conditions, it is possible to calculate the magnetization pattern from the measured force data. We apply this transfer function theory to quantitatively simulate magnetic force microscopy data acquired on a CoNi/Pt multilayer and on an epitaxially grown Cu/Ni/Cu/Si(001) magnetic thin film. The method described here serves as an excellent basis for (i) the definition of the condition for achieving maximum resolution in a specific experiment, (ii) the differences of force and force z-derivative imaging, (iii) the artificial distinction between domain and domain wall contrast, and finally (iv) the influence of various tip shapes on image content.
Quantitative analysis of the concentration of interstitial O2 molecules in SiO2 glass using luminescence and Raman spectrometry
Authors: Skuja L. , Güttler B. , Schiel D. , Silin A. R.
Published: July 2002

The Raman band of the stretching vibration of interstitial oxygen molecules in glassy SiO2 is found at 1548.5±1 cm−1. By comparison with the Raman spectrum of gaseous O2 (Q band at 1555.5 cm−1), the concentration of the interstitial O2 molecules in Suprasil W1 synthetic fused silica is estimated as (8±3.5)×1017 (molecules O2)/cm3. This value is further used to calibrate the intensity of the Nd-YAG laser-excited O2 a→X infrared photoluminescence band at 1272.2 nm against the intensity of the fundamental Raman bands of glassy SiO2. In this way, quantitative measurements of O2 concentrations in SiO2 down to at least ≈1016 molecules/cm3 are possible.
Recovery of ErCo2 Fermi level by substitution of Co by Ni and Fe
Authors: Garcia F. , dos Santos H. , Soares M. R. , Takeuchi A. Y. , da Cunha S. F.
Published: July 2002

Magnetic and electrical resistivity measurements performed in Er(Co1−xNix)2 have shown that the critical concentration for the first-order magnetic phase transition is near to xc=0.10. For higher Ni concentration the results suggest that Co is no longer magnetic. Experimental results presented here for Er(Co0.8Ni0.2xFe0.2(1−x))2 and Er(Co0.9Ni0.1xFe0.1(1−x))2 systems, show that when Co is replaced by Fe, the 3d band is filled up, whereas for Ni substitution that band is depleted. The saturation moment obtained from isothermal magnetization measurements indicates that Co recovers its moment and couples parallel to the Fe moment and antiparallel to the Er moment. Characteristics of spin fluctuation are also observed. These results show that with a suitable combination of Co, Ni, and Fe we can recover the original density of the states of the ErCo2 compound.