bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of ferromagnetic semiconductor devices for spintronics
Authors: Lebedeva N. , Kuivalainen P.
Published: June 2003

We develop physical models for magnetic semiconductor devices, where a part of the device structure consists of a ferromagnetic semiconductor layer. First we calculate the effect of the exchange interaction between the charge carrier spins and the spins of the localized magnetic electrons on the electronic states, recombination processes, and charge transport in ferromagnetic semiconductors such as (Ga,Mn)As. Taking into account, e.g., the splitting of the conduction and valence bands due to the exchange interaction, we model the electrical characteristics of the basic magnetic semiconductor devices such as Schottky diodes consisting of a nonmagnetic metal/ferromagnetic semiconductor interface, pn diodes consisting of a ferromagnetic/nonmagnetic junction and bipolar transistors having a ferromagnetic emitter. The models predict that at temperatures close to the Curie temperature TC the electrical properties of the magnetic semiconductor devices become strongly dependent on the average spin polarization of the magnetic atoms. A feature in the models is that many device parameters such as diffusion lengths or potential barriers become spin dependent in magnetic semiconductor devices. In a ferromagnetic Schottky diode the sensitivity of the device current I to the external magnetic field may be as large as (∂I/∂B)I−1≈1/T at temperatures close to TC. In a ferromagnetic pn diode both the ideal and recombination currents become magnetic field dependent. In a ferromagnetic bipolar transistor the current gain shows the same sensitivity to the spin polarization as the dc current in the ferromagnetic pn diodes. According to our model calculations optimal structures showing the largest magnetization dependence of the electrical characteristics in III–V ferromagnetic semiconductor devices would be those where the magnetic side of the junction is of n type.
Spectroscopic characterization of 1450 nm semiconductor pump laser structures for Raman amplifiers
Authors: Constant S. B. , Tomić S. , Lock D. , Sale T. E. , Sweeney S. J. , Hosea T. J. C.
Published: June 2003

In order to characterize various different epilayer designs for semiconductor Raman amplifier pump lasers, combined electromodulated reflectance (ER) and photoluminescence (PL) studies were performed on wafer samples of InP / InGaAsP / InGaAsP edge-emitter laser structures in the infrared spectral region. Information about the quantum well (QW) transitions is obtained primarily from the ER, with additional corroboration provided by the PL. The ER spectra are fitted with a line shape model to obtain the ground-state and higher-order QW transition energies, which are found to agree well with theoretically calculated values. The ER spectra also provide the waveguide core and barrier compositions and built-in electric fields in the laser structures. The information provided by ER studies on the prefabrication wafers is found to corroborate well with diagnostic spontaneous emission measurements performed on actual laser devices fabricated from the same wafer batches.
Transferred hyperfine interaction for Rb87 and Cs133 in AMnCl3 (A=Rb and Cs) single crystals
Authors: Lim Ae Ran , Jung Jae Kap , Zax David B.
Published: June 2003

The transferred hyperfine interactions at the A nuclei in AMnCl3 (A=Rb and Cs) crystals were measured on both sides of their corresponding Néel temperatures, TN. The changes in the curves of the transferred hyperfine field, Hhf, near 94.6 and 67 K for the Rb and the Cs nucleis, respectively, were associated with paramagnetic-to-antiferromagnetic transitions in these crystals. The Hhf due to transfer of the spin density from the Mn2+ ion to the A ion was obtained in the paramagnetic and the antiferromagnetic regions. The change in the Hhf for Rb and Cs nuclei may be due to differences in the contributions of the Mn2+ ions. Also, the temperature dependences of Hhf for Rb and Cs nuclei in RbMnCl3 and CsMnCl3 change abruptly in the antiferromagnetic phase, and the abrupt increase in the Hhf is large enough to allow transfer of the spin density from the Mn2+ ion to the A ion for TN>T. This result is consistent with the increases in the spin-lattice relaxation times, which were observed in previous research, for both crystals.
Nanostructure and infrared photoluminescence of nanocrystalline Ge formed by reduction of Si0.75Ge0.25O2/Si0.75Ge0.25 using various H2 pressures
Authors: Taraschi Gianni , Saini Sajan , Fan Wendy W. , Kimerling Lionel C. , Fitzgerald Eugene A.
Published: June 2003

Nanocrystalline Ge in SiO2 was synthesized by the reduction of Si0.75Ge0.25O2 with H2, at various annealing temperatures (ranging from 700 to 900 °C), with various H2 partial pressures (100% N2, 6% H2:94% N2, and 100% H2), and for a range of times. Cross-sectional transmission electron microscopy (XTEM) reveals a strong dependence of nanocrystal distribution and structure on annealing temperature and H2 partial pressure. The oxide/semiconductor interface served as a seed layer for the growth of large Ge crystallites, whereas nanocrystals formed in the bulk of the oxide for high H2 partial pressures or low temperatures. The mechanisms involved in nanocrystal formation are discussed in the context of XTEM and Raman spectroscopy results. Select nanocrystalline Ge samples were characterized optically with infrared photoluminescence (PL) measurements at 4 K, using a 488 nm Ar laser excitation. Nanocrystalline Ge samples with small nanocrystals (<15 nm) exhibited a broad infrared PL background ranging from about 1.0–1.7 μm. The broad background is attributed to quantum confinement in the Ge nanocrystals, consistent with a wide distribution of nanocrystal sizes and hence a broad range of confinement energies.
Torsion-dependent magnetoimpedance in FeCuNbSiB nanocrystalline wires with vanishing or transverse anisotropy
Authors: Li Y. F. , Vázquez M. , Chen D. X.
Published: June 2003

Two pieces of Fe73.5 Cu1 Nb3 Si13.5 B9 amorphous wires were submitted to thermal treatment without (S1) or under (S2) applied tensile stress, so inducing nanocrystalline structure characterized by two different types of magnetic anisotropy. Sample S1 exhibits longitudinal domain structure with small anisotropy constant, while a transverse domain structure with large transverse anisotropy is induced by stress annealing for sample S2. The specific magnetoimpedance response of each nanocrystalline wire has been studied as a function of the applied torsion, ξ. Both samples show the giant magnetoimpedance (GMI) effect, with GMI ratios of 190% and 145% at ξ=0 for wires S1 and S2, respectively. Applied torsion modifies the form as well as the amplitude of magnetoimpedance, which is discussed in terms of the competition between the magnetoelastic anisotropy induced by the processing and the helical anisotropy induced by the torsion. The torsion impedance has been modeled for the case of sample S1 under large enough helical anisotropy. The observed giant torsion-impedance effect of 320% and 235% for samples S1 and S2, respectively, is proposed to be employed in magnetoelastic torsion sensors. The use of the GMI effect is further emphasized to be a powerful tool to explore the magnetic structure and the magnetic anisotropy.
Silicon nanocrystals and defects produced by silicon and silicon-and-gold implantation in silica
Authors: Barthou C. , Duong P. H. , Oliver A. , Cheang-Wong J. C. , Rodrı́guez-Fernández L. , Crespo-Sosa A. , Itoh T. , Lavallard P.
Published: June 2003

High-purity silica samples were implanted at room temperature with 2-MeV Si ions or sequentially with 2-MeV Si and 10-MeV Au ions. Three photoluminescence bands associated with the presence of defects are identified in the as-implanted samples. After a heat treatment at 1100 °C, a line appears at 750 nm in Si-implanted samples. This line is attributed to the emission from localized states in Si nanocrystals. At this temperature, the defects are only partially annealed. In the case of the samples implanted sequentially with Si and Au ions, the Si nanocrystal emission band appears, even without heat treatment.
Annealing-induced Ge/Si(100) island evolution
Authors: Zhang Yangting , Drucker Jeff
Published: June 2003

Ge/Si(100) islands were found to coarsen during in situ annealing at growth temperature. Islands were grown by molecular-beam epitaxy of pure Ge and annealed at substrate temperatures of T=450, 550, 600, and 650 °C, with Ge coverages of 6.5, 8.0, and 9.5 monolayers. Three coarsening mechanisms operate in this temperature range: wetting-layer consumption, conventional Ostwald ripening, and Si interdiffusion. For samples grown and annealed at T=450 °C, consumption of a metastably thick wetting layer causes rapid initial coarsening. Slower coarsening at longer annealing times occurs by conventional Ostwald ripening. Coarsening of samples grown and annealed at T=550 °C occurs via a combination of Si interdiffusion and conventional Ostwald ripening. For samples grown and annealed at T⩾600 °C, Ostwald ripening of SiGe alloy clusters appears to be the dominant coarsening mechanism.
Characterization of the metal–insulator interface of field-effect chemical sensors
Authors: Åbom A. Elisabeth , Haasch Richard T. , Hellgren Niklas , Finnegan Nancy , Hultman Lars , Eriksson Mats
Published: June 2003

The metal–insulator interface of hydrogen-sensitive metal–insulator–semiconductor capacitors, with SiO2 as the insulator and Pt as the metal contact, has been studied. The thin Pt films were prepared in ultrahigh vacuum by electron beam evaporation and dc magnetron sputtering. Deposition parameters were substrate temperature and sputtering pressure. The hydrogen responses of the differently prepared devices were measured in a semi-inert ambient (and used as a measure of the concentration of available adsorption sites for hydrogen at the interface). A large variation of responses was found for differently prepared sensors, and the magnitude of the response was found to increase for decreasing bonding strength between the Pt film and the SiO2 substrate, as determined by scratch adhesion measurements. The bonding strength was controlled via the energetics of the Pt deposition flux. The largest interfacial roughness, from cavities between noncoalesced metal grains, and the poorest adhesion, was obtained by a reduced surface diffusion during growth and incomplete coalescence of the metal grains on the oxide surface as studied by transmission electron microscopy and atomic force microscopy. From x-ray photoelectron spectroscopy studies it was concluded that no chemical bonds were formed between Pt and SiO2. It is inferred that the hydrogen active in the sensor response is adsorbed on the oxide side of the interface in a spillover process. The difference in hydrogen response between differently prepared devices can be explained by a difference in concentration of available adsorption sites giving rise to a detectable dipole moment, on the oxide due to a blocking by Pt atoms in contact with the oxide. Thus, the concentration of Pt atoms in contact with the oxide affects both the hydrogen response and the metal–oxide adhesion.
Two-dimensional structures of ferroelectric domain inversion in LiNbO3 by direct electron beam lithography
Authors: He J. , Tang S. H. , Qin Y. Q. , Dong P. , Zhang H. Z. , Kang C. H. , Sun W. X. , Shen Z. X.
Published: June 2003

We report on the fabrication of domain-reversed structures in LiNbO3 by means of direct electron beam lithography at room temperature without any static bias. The LiNbO3 crystals were chemically etched after exposure of the electron beam, and then the patterns of domain inversion were characterized by atomic force microscopy (AFM). In our experiment, an interesting phenomenon occurred when the electron beam wrote a one-dimensional grating on the negative c face: a two-dimensional (2D) dotted array was observed on the positive c face, which is significant for its potential to produce 2D and three-dimensional photonic crystals. Furthermore, we also obtained 2D ferroelectric domain inversion in the whole LiNbO3 crystal by writing the 2D square pattern on the negative c face. Such a structure may be utilized to fabricate 2D nonlinear photonic crystal. AFM demonstrates that a 2D domain-reversed structure has been achieved not only on the negative c face of the crystal, but also across the whole thickness of the crystal.
The CN free radical in acetonitrile discharges
Authors: Li Peng , Fan Wai Yip
Published: June 2003

Infrared diode laser absorption, Fourier-transform infrared (FTIR) and optical emission spectroscopies have been applied to the study of the CN radical and other transient species in acetonitrile/argon plasmas. A dc discharge was used to produce the CN radical in which both the infrared rovibrational spectrum around 2000 cm−1 and the B−X electronic emission spectrum around 388 nm were detected. The concentration of CN was determined to be in the region of 1011–1012 cm−3, aided by vibrational intensity calculations performed at UB3LYP/6−311+G** level of theory in Gaussian 98. An acetonitrile plasma was also generated in a static cell and by using FTIR spectroscopy; the main product observed under such conditions was HCN. Based on the experimental data, it appears that the major reaction pathways for CN in the plasma involve electron impact dissociation and excitation processes and its reaction with HCN. In order to provide a better overall understanding of the gas phase reaction pathways in the plasma, emission studies of excited NH, C2 and CH generated in the same plasma were performed as well.
Electrodeposition of Ni/SiC contacts
Authors: Oskam Gerko , Patel Parimal J. , Long John G. , Searson Peter C.
Published: June 2003

We report on the fabrication and characterization of n-type 4H–SiC/Ni contacts formed by electrodeposition from nickel sulfate solutions. We show that the nickel ion states overlap the conduction band of n-type 4H–SiC so that electrodeposition occurs by electron injection from the conduction band into the nickel ion acceptor states. Nickel deposition on n-type 4H–SiC is characterized by nucleation followed by diffusion-limited growth of three-dimensional clusters. As-deposited n-type 4H–SiC/Ni contacts are rectifying with ideality factors of 1.5–2. After annealing, the contacts are Ohmic due to the formation of Ni2Si.
Effects of thermal annealing on the radiation produced electron paramagnetic resonance spectra of bovine and equine tooth enamel: Fossil and modern
Authors: Weeks Robert A. , Bogard James S. , Elam J. Michael , Weinand Daniel C. , Kramer Andrew
Published: June 2003

The concentration of stable radiation-induced paramagnetic states in fossil teeth can be used as a measure of sample age. Temperature excursions >100 °C, however, can cause the paramagnetic state clock to differ from the actual postmortem time. We have heated irradiated enamel from both fossilized bovid and modern equine (MEQ) teeth for 30 min in 50 °C increments from 100 to 300 °C, measuring the electron paramagnetic resonance (EPR) spectrum after each anneal, to investigate such effects. Samples were irradiated again after the last anneal, with doses of 300–1200 Gy from Co60 photons, and measured. Two unirradiated MEQ samples were also annealed for 30 min at 300 °C, one in an evacuated EPR tube and the other in a tube open to the atmosphere, and subsequently irradiated. The data showed that hyperfine components attributed to the alanine radical were not detected in the irradiated MEQ sample until after the anneals. The spectrum of the MEQ sample heated in air and then irradiated was similar to that of the heat treated fossil sample. We conclude that the hyperfine components are due to sample heating to temperatures/times >100 °C/30 min and that similarities between fossil and MEQ spectra after the 300 °C/30 min MEQ anneal are also due to sample heating. We conclude that the presence of the hyperfine components in spectra of fossil tooth enamel indicate that such thermal events occurred either at the time of death, or during the postmortem history.
Investigation on intergrain exchange coupling of nanocrystalline permanent magnets by Henkel plot
Authors: Zhang Hong-wei , Rong Chuan-bing , Du Xiao-bo , Zhang Jian , Zhang Shao-ying , Shen Bao-gen
Published: June 2003

In a real magnet, the relation between isothermal remanence Jr(H) and dc demagnetization remanence Jd(H) is expressed as δm(H)=[Jd(H)−Jr(∞)+2Jr(H)]/J(∞). It is believed that nonzero δm is due to the interactions between particles in the magnet. Using Pr2Fe14B as a sample, the relation is examined by the micromagnetic finite element method. The positive value of δm is primarily caused by intergrain exchange coupling. The decrease of intergrain exchange coupling results in the drop of the maximum value of δm. However, the variation of anisotropy in grain boundaries produces no change in the maximum value of δm. A Henkel plot is suggested to be effective for checking intergrain exchange coupling in magnets.
Simultaneous imaging of ionic conductivity and morphology of a microfluidic system
Authors: Ionescu-Zanetti Cristian , Cheung Karen , Lal Ratneshwar , Lee Luke P.
Published: June 2003

We present a method for nanoscale simultaneous measurements of the conductivity and morphology of microfluidic systems. While device morphology is imaged by atomic force microscopy (AFM), the AFM tip is used as an electrode probe to measure the conductivity through a buffer in the fluidic channels to a reference electrode. Connectivity to a reference electrode can be probed simultaneously at a large number of test points along micro- and nanofluidic channels without the requirement of external fluidic connections. Since the placement of microelectrodes is essential to a number of microfluidic applications, this technique allows for the AFM tip to be used as a rapid prototyping tool.
Infrared and electrical properties of amorphous sputtered (LaxAl1−x)2O3 films
Authors: Devine R. A. B.
Published: June 2003

Amorphous (LaxAl1−x)2O3 (0.61⩽x⩽0.73) films have been deposited by sputtering in a partially reactive atmosphere. The average dielectric constant of the as-deposited films was 13.4 and 12.5 following annealing at 700 °C for 60 min in N2; both values were much lower than the single crystal values ∼24 and 28 for LaAlO3 and La2O3, respectively. Leakage current densities were ∼10−8 A cm−2 for an applied field of 1 MV cm−1 for film thicknesses ∼75 nm. Fourier transform infrared spectroscopy reveals transverse optic mode peaks at 723 and 400 cm−1 and corresponding longitudinal optic modes at 821 and 509 cm−1. The density of the amorphous phase is estimated to be ∼0.9 times the crystalline density.
Band-tail characteristics in polysilicon
Authors: Sun J. H. , Shen W. Z. , Meng F. Y.
Published: June 2003

The temperature- and growth-dependent Urbach band-tail characteristics in polysilicon wafers have been investigated both experimentally and theoretically for the improvement of solar cells. The band-tail parameter in polysilicon has been obtained by temperature-dependent transmission spectra, with the aid of a detailed calculation of the transmission profile. The band-tail parameter is in the range of 30–90 meV in polysilicon. A band-tail model based on the calculation of density of occupied states and the carrier–phonon interaction has been employed to analyze the temperature-dependent band-tail characteristics. It is found that the yielded structural information can be well explained by Raman measurements. We have correlated various aspects of the observed structural properties, such as the band-tail parameter, trap concentration, grain–surface charge density, and correlation length, with the conversion efficiency of the polysilicon solar cells.
Investigation of quantum effects on the Bloch electron velocity around closely spaced bands at high electric fields
Authors: Martinez A. , Lindefelt U. , Nilsson H.-E.
Published: June 2003

In conventional Monte Carlo simulations of electron and hole transport in semiconductors, the possibility of electric-field induced interband transitions between scattering events is not taken into account. In this article we study the effects of interband transitions on the electron (or hole) velocity and on the distance traveled by the charge carrier. The resulting quantum velocity, defined as the expectation value of the velocity operator for a state which is a superposition of Bloch states, contains an average group velocity and a quantum interference term. We calculate these terms for two cases, namely, for a simple parameterized two-band k⋅p model and for hole motion in wurtzite GaN, and compare with the one-band group velocity used in the conventional Monte Carlo method. The results show that interband transitions around closely spaced energy bands, which may arise in crystals with relatively large unit cells, like wurtzite GaN and hexagonal and rhombohedral SiC polytypes, may have a major effect on the particle velocity and particle trajectory, and therefore constitute a major influence on, for instance, electrical breakdown.
Spin-dependent recombination electron paramagnetic resonance spectroscopy of defects in irradiated silicon detectors
Authors: Eremin V. , Poloskin D. S. , Verbitskaya E. , Vlasenko M. P. , Vlasenko L. S. , Laiho R. , Niinikoski T. O.
Published: June 2003

Spin-dependent recombination (SDR) electron paramagnetic resonance (EPR) spectroscopy is applied for investigation of paramagnetic recombination centers in irradiated silicon p−n junction detectors (diodes) formed on float-zone (FZ) silicon wafers. The main radiation defects, associated with SRD-EPR spectra arising from excited triplet states, are assigned to complexes of two substitutional carbon atoms and one interstitial silicon atom (CS+SiI+CS) and to oxygen + vacancy (O+V) complexes (A-centers). In spite of the low concentration of oxygen in FZ silicon the A-centers are found to play an important role in the recombination process in the diodes. At temperatures T<100 K the SDR-EPR spectra are well observable by measurements of the microwave conductivity or by detecting a dc forward current IF below a forward-blocking voltage UFBL. At UFBL the IF has a steep jump followed by a decrease of the voltage over the diode and a negative resistance region with oscillations of the current. The SDR-EPR spectrum arising from the (CS+SiI+CS) complexes was found to decrease strongly in the forward biased diodes. This effect can be attributed to coexistence of two different configurations of the carbon related defect in the bulk n-type region of the diodes.
Optical studies of strained type II GaAs0.7Sb0.3/GaAs multiple quantum wells
Authors: Chen T. T. , Chen C. H. , Cheng W. Z. , Su W. S. , Ya M. H. , Chen Y. F. , Liu P. W. , Lin H. H.
Published: June 2003

We report a detailed investigation on the optical transitions of strained type II GaAs0.7Sb0.3/GaAs (100) multiple quantum wells. For the theoretical calculations, both of the elastic deformational potential of intrinsic compressive biaxial strain, and quantum confinement effects are included. The asymmetric photoluminescence spectra reveal the features of excited state transition and quantum confinement Stark effect at high and low temperatures, respectively. The asymmetry features have also been investigated and confirmed by low-temperature photoluminescence experiments under different excitation power. From polarized photoluminescence excitation and photoconductivity spectra, both of the type I and type II optical transitions can also be clearly identified.
Plasma radiation during laser-induced breakdown of dielectrics
Authors: Nitikant  , Sharma A. K.
Published: June 2003

A model of radiation emission during the laser breakdown of dielectrics by ultrafast pulses is developed. Initially the laser causes electron-hole pair production via the tunneling of valence electrons to the conduction band. These free carriers are heated by the laser and acquire enough kinetic energy, on the time scale of a few collisions, to knock down valence electrons, leading to avalanche production of electron-hole pairs, via collisions. As the free electron concentration acquires large values, the energy transfer from electrons to the lattice becomes brisk, causing melting and eventually plasma formation of the material. The bremsstrahlung radiation from the plasma scales linearly with the laser fluence.
Optical characterization of polymer dispersed liquid crystals for holographic recording
Authors: Lucchetta D. E. , Criante L. , Simoni F.
Published: June 2003

In this study we present an extended investigation made on holographic polymer dispersed liquid crystals. A spectroscopic study of the photosensitive mixtures during the polymerization and phase separation process was performed, and an optical characterization of the recorded phase gratings was carried out using two alternative methods. The behavior of the diffraction efficiency of the transmission phase gratings versus time, angle of incidence, and wavelength was experimentally determined, and the results were successfully analyzed using the available theories for anisotropic phase gratings. Finally, the electro-optical properties concerning the beam deflection and response times were measured.
Model of a He–Xe low-pressure dc positive column plasma
Authors: Gortchakov S. , Lange H. , Uhrlandt D.
Published: June 2003

The positive column plasma of dc glow discharges at low pressure in a mixture of helium and 2% xenon is studied. Such glow discharges are favored candidates for the design of mercury-free light sources. A self-consistent model of the column plasma is presented based on strict radially resolved treatment of the nonlocal non-equilibrium kinetics of the electron component, the space-charge potential and the densities of ions and excited atom states. A detailed reaction kinetic scheme of the low-lying xenon excited states has been developed and is included in the model. The model is validated by a comparison with measurements of the axial electric field and the densities of the lowest metastable and resonant xenon levels. The latter was obtained by tunable diode laser absorption and probe diagnostics. The initial results of the model show a pronounced radial structure of the rare-gas column plasma and nonlocal properties of the electron power budget caused by space-charge confinement. The impact of the uncertainty of atomic data used in the model on the quantitative results is investigated in detail. A large impact of the cross sections of electron-impact excitation of the xenon ground state and the excitation and ionization of the xenon excited states, which are known with insufficient reliability only, has been observed.
Elliptical air hole waveguides in slab photonic crystals
Authors: Sigalas M. M. , Chow E.
Published: June 2003

Waveguides in slab photonic crystals are analyzed. The waveguides are created by replacing a line of circular air holes with elliptical air holes. The spectral width of the waveguiding region is calculated to be as high as 20% of the midband frequency. Three-dimensional plane wave expansion and finite difference time domain results are presented and are in agreement between them.
Quantitative correlation between the local coercivity variation and magnetization reversal dynamics in Co/Pd multilayer thin films
Authors: Jang Hyuk-Jae , Choe Sug-Bong , Shin Sung-Chul
Published: June 2003

We report the existence of a quantitative correlation between magnetization reversal dynamics and spatial variation of the local coercivity, ΔHC, in Co/Pd multilayer thin films. The ΔHC was directly probed by measuring hysteresis loops on spatially resolved local regions by means of a magneto-optical microscope magnetometer and magnetization reversal dynamics was characterized by analyzing the wall-motion speed V and the nucleation rate R. We found a linear relationship between log(V/R) and log(ΔHC), where a small variation of the local coercivity results in a large V/R showing wall-motion dominant reversal behavior. A Monte Carlo simulation considering magnetic nonuniformity well predicts the observed experimental relationship.
Magnetism and giant magnetoresistance of HeFe6Ge6-type Y0.7Ce0.3Mn6Sn6 compound
Authors: Yao Jin-lei , Zhang Shao-ying , Yan Mi , Yang De-ren , Wang Ru-wu , Zhang Li-gang , Shen Bao-gen
Published: June 2003

Magnetic transitions and magnetoresistance (MR) effect of the HfFe6Ge6-type Y0.7Ce0.3Mn6Sn6 compound have been investigated in the temperature range of 5–370 K. The sample undergoes paramagnetic, ferrimagnetic, and antiferromagnetic ordering with cooling. In the antiferromagnetic state, a moderate magnetic field (⩽11 kOe) is sufficient to switch Y0.7Ce0.3Mn6Sn6 from the antiferromagnetic to ferrimagnetic state, and this results in the giant magnetoresistance effect of −36.5∼−6% from 5 K to 285 K at 50 kOe. It is found that the MR (MR=[ρ(H)−ρ(0)]/ρ(0)×100%) behavior of the sample can be expressed as MR=−A(M/MS)2 in the field range from antiferromagnetic to ferrimagnetic configuration, and the power-law magnetic-field dependence of the MR effect MR∝−Hq is shown in the ferrimagnetic state. The values of the exponent q are 0.35, 0.46, and 0.65 at 5 K, 75 K, and 285 K, respectively.
Fluorination of copper phthalocyanines: Electronic structure and interface properties
Authors: Peisert H. , Knupfer M. , Schwieger T. , Fuentes G. G. , Olligs D. , Fink J. , Schmidt Th.
Published: June 2003

We compare the electronic structure of differently fluorinated copper phthalocyanines (CuPC, CuPCF4, and CuPCF16) using x-ray photoemission spectroscopy and valence-band ultraviolet photoemission spectroscopy. Whereas the ionization potential (IP) is increased by more than 1 eV as a function of the degree of fluorination, further electronic properties such as the optical gap or the composition of the highest occupied molecular orbital and lowest unoccupied molecular orbital remain nearly unchanged. This fact renders these compounds an ideal tool for the investigation of the influence of the IP on the interface properties. At the interface to gold, besides interface dipoles we observe both downward and upward band bending. These phenomena depend clearly on the IP of the phthalocyanines.
Nanoscale ferromagnetic rings fabricated by electron-beam lithography
Authors: Heyderman L. J. , David C. , Kläui M. , Vaz C. A. F. , Bland J. A. C.
Published: June 2003

We have fabricated nanoscale ferromagnetic rings using electron-beam lithography with a lift-off process for pattern transfer. The resist thickness and electron-beam dose were tailored to produce 10 nm thick rings with outer diameters down to 90 nm. Arrays of rings were produced for magneto-optical Kerr effect measurements which show that down to the smallest diameters, reproducible magnetic switching into the flux-closure vortex state is maintained.
Thickness and growth temperature dependence of structure and magnetism in FePt thin films
Authors: Toney Michael F. , Lee Wen-Yaung , Hedstrom Jonathan A. , Kellock Andrew
Published: June 2003

We describe structural and magnetic measurements of polycrystalline, L10 chemically ordered Fe(55–60)Pt(45–40) films as a function of film thickness (from 3 to 13 nm) and growth temperature (270–370 °C). With increasing film thickness, the coercivity increases from about 1 kOe up to 11 kOe (growth at 400 °C), while for increasing growth temperature, the coercivity grows from 0.2 to 6 kOe for 4.3 nm thick films and 1.6 to 10 kOe for 8.5 nm thick films. There is a strong, nearly linear correlation between coercivity and the extent of L10 chemical order. In all the films there is a mixture of L10 and chemically disordered, fcc phases. The grain size in the L10 phase increases with both film thickness and growth temperature (increasing chemical order), while in the fcc phase the grain size remains nearly constant and is smaller than in the L10 phase. The films all contain twins and stacking faults. The relationship between the coercivity and the film structure is discussed and we give a possible mechanism for the lack of chemical order in the very thin films (lack of nucleation sites for the L10 phase).
Simple model of polarization offset of graded ferroelectric structures
Authors: Pintilie L. , Boerasu I. , Gomes M. J. M.
Published: June 2003

The abnormal polarization offset observed in case of graded ferroelectric structures is explained assuming the presence of a nonreversible part of polarization due to the imposed polarization gradient. It is shown that an upper limit for the offset should exist, fixed by the remnant polarization of the component materials considered as independent layers. This is valid for the structures in which polarization increases or decreases in steps from one layer to the other, but should also be valid for the structures with continuous gradient. However, large values of the charge offset could be achieved in the last case. The electric field dependence of the polarization offset is predicted, together with the possibility of obtaining large nonconventional pyroelectric coefficients. The simulation developed in the case of a bilayer ferroelectric structure and using data from the lead–lanthanum–zirconate–titanate system describes well the observed features of the graded ferroelectric structures.
Effect of (101̄0) crystal orientation on many-body optical gain of wurtzite InGaN/GaN quantum well
Authors: Park Seoung-Hwan
Published: June 2003

The optical properties of (101̄0)-oriented wurtzite (WZ) InGaN/GaN quantum well (QW) lasers are investigated using the multiband effective-mass theory and the non-Markovian gain model with many-body effects. These results are also compared with those of (0001)-oriented WZ InGaN/GaN QW with piezoelectric and spontaneous polarizations taken into account. The optical gain of the (101̄0)-oriented QW is largely enhanced over that of the (0001)-oriented QW. This can be explained by the fact that the optical matrix elements near the band edge (k=0) of the (101̄0)-oriented InGaN/GaN QW significantly increase compared to that of the (0001)-oriented structure, in addition to the reduction of the effective mass. The threshold current density Jth of the (101̄0)-oriented QW is expected to be about half that of the (0001)-oriented QW structure.
Giant magnetoelectric effect of a hybrid of magnetostrictive and piezoelectric composites
Authors: Wan J. G. , Liu J.-M. , Chand H. L. W. , Choy C. L. , Wang G. H. , Nan C. W.
Published: June 2003

A magnetoelectric (ME) hybrid structure is constructed by an efficient coupling between magnetostrictive Terfenol-D (Tb0.27-0.30Dy0.73-0.70Fe1.90-1.95)/epoxy and piezoelectric lead–zirconate–titanate [Pb(Zr0.52Ti0.48)O3]/epoxy composites. Significant ME effect produced by the piezoelectric k31 coupling with the longitudinal vibration of the magnetostrictive component over a wide range of frequency is observed. It is revealed that the ME effect can be enhanced by applying an optimized magnetic bias. A magnetoelectric voltage coefficient as high as 8700 mV/cm Oe is recorded at the resonance frequency of 59.2 kHz for the structure with an optimized magnetic bias of 0.7 kOe. Our measurement confirms that the eddy current loss remains negligibly small at an operating frequency as high as ∼200 kHz, predicting very promising applications of the present ME structure.
Admittance of a slot diode with a two-dimensional electron channel
Authors: Ryzhii V. , Satou A. , Shur M. S.
Published: June 2003

We calculate the frequency-dependent admittance of a diode with a two-dimensional electron channel in a slot between strip-like contacts. Hydrodynamic electron transport equations coupled with a two-dimensional Poisson equation for the self-consistent electric potential are used. Using the calculated expression for the admittance, we analyze the effect of the planar contacts and external capacitance on the plasma oscillations in the system under consideration. The obtained results are useful for the interpretation of experimental observation of plasma effects in high-electron mobility transistors and optimization of terahertz devices based on these transistors.
Analysis of dislocation scattering on electron mobility in GaN high electron mobility transistors
Authors: Joshi R. P. , Sridhara V. , Jogai B. , Shah P. , del Rosario R. D.
Published: June 2003

A Monte Carlo study of two-dimensional electron gas mobility in wurtzite GaN high electron mobility transistors is presented to include scattering due to edge dislocations strains. For self-consistency, numerical solution of Schrödinger, Poisson, and charge balance equations are used for the eigenfunctions. Electron mobility predictions around 1.71×103 cm2/V s, are in close agreement with the reported data. At the highest dislocation density of 1010 cm−2, mobility reductions of 16.8% and 8.6% are predicted for 77 and 300 K, respectively. Electron mobility behavior as a function of the channel density at different temperatures is also discussed.
Effects of pulsed anodic oxide on the intermixing in InGaAs/GaAs and InGaAs/AlGaAs quantum wells
Authors: Yuan Shu , Liu C. Y. , Zhao F. , Chan M. C. Y. , Tsui W. K. , Dao L. V. , Liu X. Q.
Published: June 2003

Intermixing in InGaAs/GaAs and InGaAs/AlGaAs quantum well structures after rapid thermal annealing with and without an anodic oxide cap on the surface was studied by low temperature (8 K) photoluminescence (PL). The PL peak energy was shifted towards higher photon energies (blueshift) in both types of samples, especially at annealing temperatures above 880 °C. The anodic oxide cap has been demonstrated to inhibit the band-gap blueshift of the quantum well structures. Secondary ion mass spectroscopy data indicated that Ga vacancies were injected from the anodic oxide cap into the epitaxial layers. These vacancies enhanced interdiffusion between group III atoms, and partially relaxed the strain in the structure, resulting in the effect of the suppression of the blueshift.
Growth of Si nanocrystals on alumina and integration in memory devices
Authors: Baron T. , Fernandes A. , Damlencourt J. F. , De Salvo B. , Martin F. , Mazen F. , Haukka S.
Published: June 2003

We present a detailed study of the growth of Si quantum dots (Si QDs) by low pressure chemical vapor deposition on alumina dielectric deposited by atomic layer deposition. The Si QDs density is very high, 1012 cm−2, for a mean diameter between 5 and 10 nm. Al2O3/Si QD stacks have been integrated in memory devices as granular floating gate. The devices demonstrate good charge storage and data retention characteristics.
Mechanism for linear and nonlinear optical effects in crystals of the Sr2Be2B2O7 family
Authors: Lin Zheshuai , Wang Zhizhong , Chen Chuangtian , Chen Shyong K. , Lee Ming-Hsien
Published: June 2003

Electronic structure calculations of Sr2Be2B2O7 (SBBO) family crystals including Sr2Be2B2O7, BaAl2B2O7, and K2Al2B2O7 have been performed based on a plane-wave pseudopotential method. Moreover, the linear optical coefficients and the static second-harmonic generation (SHG) coefficients of BaAl2B2O7 and K2Al2B2O7 are calculated at the independent-particle level. The calculated refractive indices and SHG coefficients are in good agreement with experimental values. In addition, a real-space atom-cutting method is adopted to analyze the respective contributions of the cations and anionic group to optical response. The results show that in these crystals, when the radii of the cations increase, their contributions to the SHG effect become slightly more pronounced, however, the contributions to the SHG coefficients from the (BO3)3− and (AlO4)5− anionic groups are dominant and comparable.
Realization of one-chip-multiple-wavelength laser diodes with II–VI/III–V compound semiconductors
Authors: Song J. S. , Cho M. W. , Oh D. C. , Makino H. , Hanada T. , Yao T. , Zhang B. P. , Segawa Y. , Chang J. H. , Song H. S. , Cho I. S. , Kim H. W. , Jung J. J.
Published: June 2003

A laser diode which includes II–VI device structure directly grown on III–V device structure is proposed. This idea makes possible one-chip-multiple-wavelength operation in the visible wavelength region by the vertical integration of individual light emitters, which is demonstrated by a ZnCdSe/ZnSe/ZnMgBeSe quantum-well structure for a blue-green light emitter grown by molecular beam epitaxy directly on a metalorganic chemical vapor deposition grown InGaP/InGaAlP device wafer for a red light emitting device. The feasibility of the II–VI/III–V complex light emitter is demonstrated by optical-pumping experiments. Optically pumped lasing at 504 and 664 nm is achieved from II–VI and III–VI laser structures on one chip simultaneously at room temperature with a threshold power of 115 and 84 kW/cm2, respectively. The present results clearly show the feasibility of epitaxial integration of II–VI and III–V laser structures, which will stimulate the investigation of multifunctional optical devices.
Light-induced absorption in photorefractive BaTiO3 crystals
Authors: Banyal Ravinder Kumar , Prasad B. Raghavendra
Published: June 2003

We present the experimental results on the measurement of temporal and steady-state light-induced absorption change in undoped and rhodium-doped barium titanate (Rh:BaTiO3) crystals at different wavelengths and intensities. Theoretical calculations based on a two-center charge transport model agree well with the experimental results which supplement the earlier studies carried out using photorefractive BaTiO3 crystals. We also report a wavelength-specific light-induced effect that modifies the absorption dynamics in a peculiar manner. This can be attributed to light-induced thermal effects. However, the possibility of additional photorefractive centers becoming active cannot be ruled out.
Structural magnetic properties and spin-glass behavior in La0.9Te0.1MnO3
Authors: Tan G. T. , Duan P. , Dai S. Y. , Zhou Y. L. , Lu H. B. , Chen Z. H.
Published: June 2003

We report the structural, magnetic, and electrical transport properties of pervoskite oxide La0.9Te0.1MnO3. This material, with space group R-3C, shows spin-glasslike features at low temperature and has a good colossal magnetoresistance behavior. The magnetoresistance ratio MR=[ρ(0)−ρ(H)]/ρ(0) is about 51% at 200 K in a field of 4 T. X-ray photoemission spectroscopy measurements suggest that Te ions are in the Te4+ state, while the Mn ions may be in the Mn2+ and Mn3+ valence state.
Molecular oxygen and moisture as traps in poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]: locations and detrapping by chain relaxation
Authors: Tseng Hao-En , Peng Kang-Yung , Chen Show-An
Published: June 2003

We investigate locations of molecular oxygen and moisture (O2/H2O) serving as traps in poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) by measurements of field-induced thermally stimulated current, photoexcitation thermally stimulated current (PE-TSC), and dynamic mechanical analysis. Two broad peaks of PE-TSC are found at 212 and 325 K in the samples with O2/H2O-rich MEH-PPV, which are in reasonable agreement with those of the side chain and main chain relaxation, respectively. These traps can be attributed to the presence of O2/H2O (most likely molecular oxygen) in the side chain region and the amorphous main chain region, and the detrapping process is induced by chain motions under elevated temperatures.
Elastic properties of hot-isostatically-pressed magnesium diboride
Authors: Nesterenko V. F. , Gu Y.
Published: June 2003

Magnesium diboride was hot-isostatically pressed using three qualitatively different cycles: dense material cooled under pressure (DMCUP), “standard” cycle with pressure and temperature simultaneously reduced, and isothermal pressure release. Elastic properties of dense MgB2 were measured at normal conditions using resonant ultrasound spectroscopy method. The highest values of elastic moduli correspond to the sample processed using DMCUP cycle. The data for fully dense samples are in satisfactory agreement with theoretical predictions based on quantum mechanics calculations. The effect of lower density on elastic constants is consistent with a theoretical approach based on elasticity theory taking into account effect of porosity.
Quantitative strain analysis in AlGaAs-based devices
Authors: Tomm Jens W. , Gerhardt Axel , Müller Roland , Biermann Mark L. , Holland Joseph P. , Lorenzen Dirk , Kaulfersch Eberhard
Published: June 2003

We present a strategy for quantitative spectroscopic analysis of packaging-induced strain using both finite element analysis and band-structure calculations. This approach holds for a wide class of AlGaAs-based, and related, devices, among them high-power “cm-bars.” The influence on the results of particular device structure properties, such as intrinsic strain and quantum-well geometry, is analyzed. We compare theoretical results based on a unaxial stress model with photocurrent data obtained from an externally strained cm-bar, and obtain better agreement than from alternative strain models. The general approach is also applicable to the analysis of all data that refer to changes of the electronic band structure, such as absorption and photoluminescence.
Polymer laser fabricated by a simple micromolding process
Authors: Lawrence Justin R. , Turnbull Graham A. , Samuel Ifor D. W.
Published: June 2003

We report polymer distributed feedback lasers fabricated using solvent-assisted microcontact molding. The poly[2-methoxy-5-(3,7-dimethyloctyloxy) paraphenylenevinylene] film is patterned by placing it in conformal contact with an elastomeric mould inked with a suitable solvent. When the resulting microstructured film is pumped with the 532 nm pulsed output of a microchip laser, we observe lasing above a threshold pump energy of 225 nJ. Above threshold the emission narrows to a linewidth of less than 0.6 nm at a wavelength of 638 nm. This micromolding technique may find application to a wide range of wavelength-scale microstructured organic photonic devices.
Pentacene organic transistors and ring oscillators on glass and on flexible polymeric substrates
Authors: Klauk Hagen , Halik Marcus , Zschieschang Ute , Eder Florian , Schmid Günter , Dehm Christine
Published: June 2003

We have fabricated organic thin film transistors, inverters, and ring oscillators on glass and on flexible polyethylene naphthalate, using the small-molecule hydrocarbon pentacene as the semiconductor and solution-processed polyvinylphenol as the gate dielectric. Depending on the choice of substrate, the transistors have a carrier mobility between 0.3 and 0.7 cm2/V s, an on/off current ratio between 105 and 106, and a subthreshold swing between 0.9 and 1.6 V/decade. To account for the positive switch-on voltage of the transistors, circuits were designed to operate with integrated level shifting. Depending on the type of substrate, ring oscillators have a signal propagation delay as low as 15 μs per stage.
Analysis of the transport process providing spin injection through an Fe/AlGaAs Schottky barrier
Authors: Hanbicki A. T. , van ’t Erve O. M. J. , Magno R. , Kioseoglou G. , Li C. H. , Jonker B. T. , Itskos G. , Mallory R. , Yasar M. , Petrou A.
Published: June 2003

Electron-spin polarizations of 32% are obtained in a GaAs quantum well via electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An analysis of the transport data using the Rowell criteria demonstrates that single-step tunneling is the dominant transport mechanism. The current–voltage data show a clear zero-bias anomaly and phonon signatures corresponding to the GaAs-like and AlAs-like LO phonon modes of the AlGaAs barrier, providing further evidence for tunneling. These results provide experimental confirmation of several theoretical analyses, indicating that tunneling enables significant spin injection from a metal into a semiconductor.