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We investigate ultrahigh frequency electrical properties of nanoscale n + − i − n + diodes made of polar semiconductors. The calculations show that the coupling between optical vibrations of the lattice and the ballistic electrons strongly modifies and enhances the time-of-flight effects giving rise to narrow resonances of the diode impedance in the reststrahlen frequency range. Particularly, negative dynamic resistance is induced in close proximity to the optical phonon frequency. The resonant effects in the dynamic resistance of nanoscale GaAs and InP diodes are studied in detail. The obtained magnitudes of the negative dynamic resistance effect indicate that the nanoscale diodes are capable of generating electromagnetic radiation in the far-infrared spectral range under electric pumping. [ABSTRACT FROM AUTHOR]

Current fabrication of nanomaterials is facing the following two challenges: high selectivity toward specific chemical compositions or morphologies and their scalable production. This usually requires new extreme fabrication conditions beyond the conventional approaches. Thermal plasma jets are flows of partially ionized gases where gas and electron temperatures reach their equilibrium state around 10 000 K, and thus provide high fluxes of energy and chemically active species including electrons and ions with their strong spatial gradients. Such extreme environments can trigger reactions that are not thermodynamically favorable or require high activation barriers, leading to the production of materials with exotic chemical compositions or structures. Since their first operation in 1960, thermal plasma jets were soon recognized as a unique and effective medium for material transformation such as melting, vaporization, and pyrolysis and recently have also found their important applications in nanomaterial fabrication. In this Perspective, we briefly review the latest progress in the thermal plasma jet-assisted fabrication of nanomaterials from nanoparticles to low-dimensional nanostructures. A special focus is made on the advantages of the thermal plasma jet technology in nanostructure fabrication, discussing plasma properties responsible for the nanomaterial growth with high throughput, high purity, anisotropy, desired compositions, or narrow size distributions. This Perspective closes with an outlook of challenges and opportunities for further advancement in this emerging field. [ABSTRACT FROM AUTHOR]

Material growth method and Mg doping effects on Er3+ photoluminescence in Er-implanted GaN are examined.

The Monte Carlo simulation of the electron transport in gallium nitride (GaN) reveals that the intervally electron transfer has a prominent role in GaN in high electric fields resulting in a strong inversion electron distribution greater than the thermal energy. The expression applies to several wide gap semiconductors such as the GaN and SiC at room temperatures.

Electro-migration of ionized/electrically active impurities in CdZnTe (CZT) was successfully demonstrated at elevated temperature with an electric field of 20 V/mm. Copper, which exists in positively charged states, electro-migrated at a speed of 15 μm/h in an electric field of 20 V/mm. A notable variation in impurity concentration along the growth direction with the segregation tendency of the impurities was observed in an electro-migrated CZT boule. Notably, both Ga and Fe, which exist in positively charged states, exhibited the opposite distribution to that of their segregation tendency in Cd(Zn)Te. A CZT detector fabricated from the middle portion of the electromigrated CZT boule showed an improved mobility-lifetime product of 0.91×10-2 cm2/V, compared with that of 1.4×10-3 cm2/V, observed in an as-grown (non-electro-migrated) CZT detector. The optimum radiation detector material would have minimum concentration of deep traps required for compensation. [ABSTRACT FROM AUTHOR]

Thermal emission spectra mediated by surface phonon polariton are examined by using a theoretical model that accounts for generation processes. Specifically, the acoustic phonon fusion mechanism is introduced to remedy theoretical deficiencies of the near thermal equilibrium treatments. The model clarifies the thermal excitation mechanism of surface phonon polaritons and the energy transfer path under non-zero energy flow. When applied to GaAs and SiC semi-infinite surfaces, the nonequilibrium model predicts that the temperature dependence of the quasi-monochromatic peak can exhibit distinctly different characteristics of either sharp increase or slow saturation depending on the materials, which is in direct contrast with the estimate made by the near-equilibrium model. The proposed theoretical tool can accurately analyze the nonequilibrium steady states, potentially paving a pathway to demonstrate stimulated interaction/emission of thermally excited surface phonon polaritons. [ABSTRACT FROM AUTHOR]

Domain wall motion during the poling of near-morphotropic Pb(Zr,Ti)O3 PZT was observed using Piezoresponse Force Microscopy (PFM). Poling was conducted on bulk polycrystalline PZT in a series of steps, interrupted by vertical PFM scans, which were used to identify the domain evolution processes. The mechanisms of evolution in complex domain patterns such as herringbone and checkerboard structures are revealed. Of interest, in the case of a herringbone pattern consisting of two sets of lamellae angled to each other, one set of lamellae expands and is observed to overwrite the other, transforming the herringbone structure into a single lamination. Also, lengthening without broadening, and simultaneous lengthening and broadening of lamellar domain bands in checkerboard structures are observed. The observations show that 180° and non-180° domain switching can occur simultaneously in complex domain patterns. Methods are developed for identifying the polarization directions of the individual domains in near-morphotropic PZT. The methods combine a knowledge of the compatible domain configurations with crystallographic data from electron backscatter diffraction and PFM data. The resulting map of polarization directions enables clear identification of the polarization switching mechanisms. [ABSTRACT FROM AUTHOR]

Micropatterned ordered arrays of cobalt (Co) anti-dots on a uniform thin nickel (Ni) underlayer, known as bilayered magnetic anti-dot lattices (BMALs), were fabricated using photolithography and a controlled wet-etching process. The magnetization reversals in a rhomboid BMAL under the application of a field along 0° (i.e., easy axis) and 90° (i.e., hard axis) were investigated using field-dependent magnetic-force microscopy, and the angular dependence of squareness (Mr/Ms) and coercivity (Hc) were studied using magneto-optical Kerr effects. Although the magnetic reversals were dominated by domain-wall motions or domain rotations, reflecting the easy and hard axis characteristics, various other complex reversal processes were also found that depends on the history of the field application. The angular dependence of Mr/Ms and Hc in a BMAL system was completely different from that in a single-layer, Co magnetic anti-dot lattice. Interestingly, a unidirectional and uniaxial component representing the overall magnetic anisotropy was revealed in the BMAL system. The details of these complicated magnetization behaviors were investigated and elaborated. [ABSTRACT FROM AUTHOR]

A theoretical model is developed for the intrinsic generation-recombination (G-R) noise in bipolar graphene that stems from stochastic processes of interband carrier transitions in the zero-gap single layer. The correlation functions are obtained for microscopic Langevin sources of fluctuations relevant to the considered G-R processes. The spectral density of the G-R fluctuations is of a Lorentz shape with the characteristic frequency fc=1/2πτr, where the recombination time τr is expressed through the G-R rates and the carrier density. Numerical estimates clearly demonstrate the potential dominance of G-R noise over the Nyquist and flicker noise in monolayer graphene. [ABSTRACT FROM AUTHOR]

We present experimental results on the multilayering effects of an electric ring resonator. The electromagnetic response of the electric ring resonator is measured via a scattering matrix using a vector network analyzer at the X-band frequency. Structures of the electric ring resonator with up to four layers were tested and analyzed using commercial software. We demonstrate that, in an electric ring resonator, the electric and magnetic dipole polarization effect gives rise to resonance frequency splitting when the cell is multilayered. [ABSTRACT FROM AUTHOR]

Using photoluminescence (PL) and current deep-level transient spectroscopy (I-DLTS), we investigated the electronic defects of indium-doped detector-grade CdMnTe:In (CMT:In) crystals grown by the vertical Bridgman method. We similarly analyzed CdZnTe:In (CZT:In) and undoped CdMnTe (CMT) crystals grown under the amount of same level of excess Te and/or indium doping level to detail the fundamental properties of the electronic defect structure more readily. Extended defects, existing in all the samples, were revealed by synchrotron white beam x-ray diffraction topography and scanning electron microscopy. The electronic structure of CMT is very similar to that of CZT, with shallow traps, A-centers, Cd vacancies, deep levels, and Te antisites. The 1.1-eV deep level, revealed by PL in earlier studies of CZT and CdTe, were attributed to dislocation-induced defects. In our I-DLTS measurements, the 1.1-eV traps showed different activation energies with applied bias voltage and an exponential dependence on the trap-filling time, which are typical characteristics of dislocation-induced defects. We propose a new defect-trap model for indium-doped CMT crystals. [ABSTRACT FROM AUTHOR]

CdZnTe crystals often exhibit nonuniformities due to the presence of Te inclusions and dislocations. High concentrations of such defects in these crystals generally entail severe charge-trapping, a major problem in ensuring the device's satisfactory performance. In this study, we employed a high-intensity, high-spatial-resolution synchrotron x-ray beam as the ideal tool to generate charges by focusing it over the large Te inclusions, and then observing the carrier's response at room- and at low-temperatures. A high spatial 5-μm resolution raster scan revealed the fine details of the presence of extended defects, like Te inclusions and dislocations in the CdZnTe crystals. A noticeable change was observed in the efficiency of electron charge collection at low temperature (1 °C), but it was hardly altered at room-temperature. [ABSTRACT FROM AUTHOR]

Oxide semiconductors are attractive materials for thin-film electronics and optoelectronics due to compatibility with synthesis on large-area, inexpensive glass and flexible plastic substrate. However, development of thin-film electronics has been hampered by the limited number of semiconducting oxides that are p-type. Here, we report on the properties of zinc-cobalt-oxide (Zn–Co–O) films, deposited at room temperature using pulsed laser deposition, that exhibit p-type conduction. Films are deposited at room temperature in a background of oxygen using a polycrystalline ZnCo2O4 ablation target. The p-type conduction is confirmed by positive Seebeck coefficient and positive Hall coefficient. Both electrical resistivity and carrier density are dependent on oxygen background pressure used during deposition. Zn–Co–O films deposited in 50 mTorr oxygen pressure appear to be amorphous based on x-ray diffraction, and show an electrical conductivity as high as 21 S cm-1. Distinct rectifying current-voltage characteristics are observed for junctions between Zn–Co–O and n-type InGaZnO films, exhibiting a threshold voltage of ∼2.5 V. P-type Zn–Co–O appears promising for thin-film electronic device technology. [ABSTRACT FROM AUTHOR]

A nonvolatile magnetic switch is proposed by utilizing the unique properties of graphene-ferromagnet composite structures. The basic mechanism relies on the role of graphene in mediating and modulating the effective exchange bias between adjacent magnetic layers, which can lead to electrically controlled rotation of magnetic bits. Readout of magnetization states is also achieved electrically through the magnetoresistance effect in the graphene channel. The proposed switch can be used to realize both the memory and programmable logic elements. Theoretical estimates illustrate the feasibility of the concept as well as its potential advantage of low power consumption (∼10-19 J) for the intrinsic switching operation. [ABSTRACT FROM AUTHOR]

We fabricated spin light emitting diodes using oxide tunneling barriers between ferromagnetic materials (Ni0.8Fe0.2/Co0.9Fe0.1) and semiconductors (GaAs) and investigated the temperature-dependent carrier lifetime and spin relaxation time of the active GaAs layer. We observed the circular polarization of the free exciton from the electroluminescence spectra due to the spin injection from the ferromagnetic material, whereas the circular polarization of the conduction band to acceptor transition was negligible. From the temperature-dependent carrier lifetime and spin relaxation time of the active GaAs layer, we found that the spin injection efficiency was larger than 25% between 20 and 180 K, where the magnetic field dependence of the spin lifetime was ignored. [ABSTRACT FROM AUTHOR]

The output power of InGaN multiple-quantum-well laser diodes (LDs) emitting at 405 nm wavelength is compared for several Al composition in the AlGaN n-cladding layer. The Al composition has been varied from 2% to 6% to study the effect of n-cladding refractive index on threshold current and slope efficiency of the LDs. As the Al composition in the AlGaN n-cladding layer increases, both threshold current and slope efficiency decrease. This behavior can be explained by the change in optical field distribution with refractive index of the AlGaN n-cladding layer. It is found that the Al composition of ¿4% would be advantageous for achieving more than 100 mW output power and high level of catastrophic optical damage. [ABSTRACT FROM AUTHOR]

By analyzing photoconductive decay curves, we compared the surface recombination velocities of semi-insulating CdMnTe:In crystals grown by the vertical Bridgman method with or without surface passivation. Sulfur passivation effectively prevents the formation of a conductive Te oxide layer on the CdMnTe surface and reduces the surface recombination velocities by about one third. We demonstrated, from IR observations of the distribution maps of Te precipitates, that their configuration affects the anomalous photoconductive decay curves and the gamma-ray spectrum in some areas of the CdMnTe crystal. Notably, not only the size but also the spatial configuration of the Te precipitates modulates the carrier-transport properties. [ABSTRACT FROM AUTHOR]

A memory concept based on the interfacial exchange energy between itinerant holes in a quantum dot and magnetic ions in an adjacent magnetic insulator is theoretically investigated. A model based on the free energy analysis demonstrates the existence of bistable states through the mechanism of bound collective magnetic polaron, whose formation and dissolution can be controlled electrically via a gate bias pulse. The parameter window suitable for bistability is discussed along with the conditions that support maximum nonvolatility. The analysis is extended to the influence of material choices as well as different designs. The calculation results clearly indicate the possibility of room temperature operation, given the availability of insulating ferromagnetic or antiferromagnetic materials whose Curie temperature is above room temperature. [ABSTRACT FROM AUTHOR]

We have studied the interaction of low-energy nitrogen ions (0.3–2 keV N2+) with GaAs (100) surfaces by photoemission spectroscopy (PES) around N 1s and Ga 3d core levels and near-edge x-ray absorption fine structure (NEXAFS) around the N K edge. At the lowest bombardment energy, nitrogen forms bonds with both Ga and As, while Ga-N bonds form preferentially at higher energies. Thermal annealing at temperatures above 350 °C promotes the formation of GaN on the surface, but it is insufficient to remove the disorder introduced by ion implantation. Nitrogen interstitials and antisites have been identified in NEXAFS spectra, while interstitial molecular nitrogen provides a clear signature in both PES and NEXAFS. [ABSTRACT FROM AUTHOR]

Interfacial engineering via nanoparticle substrate surface decoration has been extended to coated conductor templates. Preformed BaTiO3 and BaZrO3 nanoparticles were applied to substrate surfaces, prior to YBa2Cu3O7-δ (YBCO) deposition, by using a scalable and inexpensive technique of solution-based suspension. Compared to untreated reference samples, nanodecoration yields improved in-field critical current density (Jc) as well as strong correlated pinning along the c-axis direction of the YBCO film. Accordingly, a much reduced falloff of Jc with magnetic field strength was observed in all of the modified samples. In addition, scaling behavior of the normalized volume pinning force density (Fp) with respect to temperature provided insight as to the differences in flux pinning mechanisms dependent on the decoration technique. Finally, with these results our earlier proof-of-concept demonstrations on nanoparticle modified single crystal substrates were replicated on technological substrates, pointing to the versatility of this simple technique for realization of enhanced high temperature superconductor performance. [ABSTRACT FROM AUTHOR]

Using femtosecond differential transmission spectroscopy, we observed a “nondegenerate” biexciton, consisting of an electron-hole pair in the dot ground state and an electron-hole pair in the excited state, in InGaAs self-organized quantum dots. We resonantly pumped the ground state transition in the quantum dots and observed an induced resonance in the probe differential transmission spectrum near the first excited-state transition, which we attribute to the formation of a nondegenerate biexciton state. The binding energy of 15 meV does not change with excitation power, thus reflecting a genuine feature of few-particle states. Our theoretical model calculations show good agreement with these experimental results. When a prepulse is used to generate a population inversion in the quantum dots, we also observed the effects of nondegenerate biexcitonic correlations in differential transmission. [ABSTRACT FROM AUTHOR]

In order to investigate the mechanism behind bistable resistance switching in NiO thin films, we have done detailed x-ray photon spectroscopy (XPS) and x-ray diffraction Analysis (XRD) on NiO and Ti doped NiO samples fabricated under various conditions. We discovered that a high initial resistivity was required for samples to undergo bistable resistance switching, and the presence of metallic Ni content in these samples was determined by XPS. XRD data also showed that NiO grown with a relative (200) orientation was preferred over those grown with relative (111) orientation. [ABSTRACT FROM AUTHOR]

We report the fabrication of the artificial nanopatterns of Mn12-acetate films using e-beam lithography. Scanning electron micrographs and atomic force micrographs of the patterns reveal the minimum lateral size (∼50 nm), height, and surface morphology of the patterns. X-ray photoelectron spectroscopy data indicate the presence of Mn12-acetate in the patterned structure. The thin film material indicates magnetic properties consistent with Mn12-acetate, supporting the conclusion that the lithographic chemicals used in this study do not interfere with the core properties of Mn12-acetate. The successful fabrication of Mn12-acetate nanopatterns enables a range of possibilities for designed hybrid systems with three-dimensional positional control on the nanometer scale. [ABSTRACT FROM AUTHOR]

We have studied the magnetic and electronic properties of Eu1-xSrxCoO3 with 0/16≤x≤12/16. In the Sr doping range from 2/16 to 11/16, the ferromagnetic clusters coexist spatially with an antiferromagnetic matrix. The magnetization at low temperatures strongly depends on the Sr concentration x, exhibiting a maximum near half doping, which is described in term of the double-exchange mechanism. The semiconductor-metal transition at low temperatures occurs at x between 4/16 and 5/16, which indicates that the ferromagnetic clusters coalesce, leading to a percolative metallic conduction up to the composition of x=10/16. As compared with those of La1-xSrxCoO3 and Nd1-xSrxCoO3, Eu1-xSrxCoO3 shows a larger critical Sr concentration for semiconductor-metal transition and lower ferromagnetic transition temperatures, which is attributed to the enhanced global and local lattice distortion due to the smaller cation radius of the Eu3+ ions. [ABSTRACT FROM AUTHOR]

We have studied the magnetic and electronic properties of Eu1-xSrxCoO3 with 0/16≤x≤12/16. In the Sr doping range from 2/16 to 11/16, the ferromagnetic clusters coexist spatially with an antiferromagnetic matrix. The magnetization at low temperatures strongly depends on the Sr concentration x, exhibiting a maximum near half doping, which is described in term of the double-exchange mechanism. The semiconductor-metal transition at low temperatures occurs at x between 4/16 and 5/16, which indicates that the ferromagnetic clusters coalesce, leading to a percolative metallic conduction up to the composition of x=10/16. As compared with those of La1-xSrxCoO3 and Nd1-xSrxCoO3, Eu1-xSrxCoO3 shows a larger critical Sr concentration for semiconductor-metal transition and lower ferromagnetic transition temperatures, which is attributed to the enhanced global and local lattice distortion due to the smaller cation radius of the Eu3+ ions. [ABSTRACT FROM AUTHOR]

Imaging effects in metallic photonic crystals (PCs) are examined theoretically based on the finite difference time-domain method. The analysis shows that, in metallic PC-based systems, far-field images do form at the opposite side of the PC “lens” and more importantly, follow the rule of geometric optics with respect to the changes in the source position as a direct proof of negative refraction. However, the comparison of ideal left-handed media with a metallic PC suggests that the focusing effect in the PC based system is different from that of the ideal left-handed media in many aspects, due to the inhomogeneous nature of the PC. Particularly, strong dependence on the individual geometry as well as the frequency in the PC-based system renders the effective index sensitive to the variations and potentially limits its application as a superlens. [ABSTRACT FROM AUTHOR]

The effects of InGaAs/GaNAs barrier combinations were investigated in the metal organic vapor phase epitaxy grown GaInNAs quantum wells (QWs). Significant improvement was made in the optical performance of the 1.3 μm range with the proposed structures. Structural investigation linked with the optical properties reveals that a thin InGaAs layer, placed between GaInNAs QWs and GaNAs layers, plays a role in redistributing the nitrogen within the QWs while maintaining a sharp interface. The amount of diffused nitrogen is proportional to the strain residing in the InGaAs layer, which explains the tendency of blueshift with the increase in GaNAs thickness. [ABSTRACT FROM AUTHOR]

The feasibility of electrically programmable waveguiding in a photonic crystal (PC) is explored based on the metal-insulator transition of vanadium dioxide (VO2). Unlike the ordinary PCs, wave propagation in the desired structure may be switched on/off or redirected by applying an electrical bias on the selective electrodes by taking advantage of the electrically induced VO2 phase transition and subsequent modulation of dielectric properties. The characteristics of the two-dimensional VO2-based PCs with line defects are analyzed using the iterative plane wave and finite difference time domain methods. Particularly, the influence of the Drude relaxation on waveguiding is examined as the high rate typical for metallic VO2 can lead to the signal loss. An optimized structure is proposed to minimize the loss and simplify the fabrication. [ABSTRACT FROM AUTHOR]

A magnetization switching method for magnetic random access memory (MRAM), recently proposed by Savtchenko et al. [U.S. Patent No. 6,545,906 (2003)], is known to have an important advantage of a wide window for bit writing over the conventional method based on the asteroid curve, but it has a serious problem of high switching fields. In an effort to solve this problem, the effects of the thickness asymmetry and antiferromagnetic exchange coupling of the synthetic antiferromagnetic free-layer structure on the switching field have been investigated by micromagnetic computer simulation. At conditions relevant to high-density MRAM, magnetization switching in the direct write mode occurs at reasonably low values of word- and bit-line fields (below 100 Oe), combined with a substantially wide window for bit writing. A much wider window is observed in the toggle mode, but the required switching fields are too high, being over 150 Oe. [ABSTRACT FROM AUTHOR]

Magnetic tunnel transistors were prepared on Si(100) substrates by magnetron sputter deposition. By means of spin filtering through a single Co90Fe10 base layer, magnetocurrent ratios of 53%–55% and high transfer ratios of (1–2)×10-4 for emitter-base bias of 1.5–2 V were obtained at 77 K. The bias dependence of the collector current showed the square-law behavior. From the modified Bell-Kaiser model, attenuation lengths of majority and minority spins of hot electrons are expected as 40±5 and 16±1 Å in the single Co90Fe10 layer, respectively. The decrease of transfer ratio was observed with decreasing base thickness from 80 to 30 Å, which may be related to the extension of the (Co2Si and Fe) intermediate region formed at Co90Fe10/Si interface in the thinner base layer. [ABSTRACT FROM AUTHOR]

The conditions for microwave power generation with hot-electron transport are investigated in a submicron GaN diode when it operates in the limited space-charge accumulation (LSA) mode. Applying a transport model based on the local quasistatic approximation, the analysis shows that the nitride diodes can support the LSA mode of oscillation in the terahertz-frequency range. For a 100 nm n-GaN diode with a cross section of 500 μm2 and the electron density of 1×1017 cm-3, the generated microwave power is estimated to be as high as ≈0.6 W with the corresponding dc-to-rf conversion efficiency of ≈9% and the negative differential resistance of ≈-1.3 Ω; which thus provides an efficient mechanism to achieve very high-frequency microwave generation in the nitrides. [ABSTRACT FROM AUTHOR]

We study the Coulombic effects of electron-hole plasma on the ground and first excited energy levels and carrier wave functions in a GaN/AlGaN quantum well (QW) structure. The coupled Schrödinger equations for electron and hole states are solved self-consistently in the Hartree-Fock approximation along with the Poisson equation. As expected, the decreasing QW width and increasing plasma density diminish the charge separation in the QW induced by the internal field, leading to the relatively reduced contribution of the Hartree interaction to the energy-level shift. In contrast, the calculation also demonstrates that the contribution of many-body effects becomes more pronounced. The resulting competition between the many-body and Hartree contributions causes a nonmonotonous dependence of the electron and hole energies on the plasma density at a given field. These findings are applied to explore the potential bistable behavior in the QW electroabsorption under near-band-edge photoexcitation. [ABSTRACT FROM AUTHOR]

An internal-type linear inductive antenna, that is, a double-comb-type antenna, was developed for a large-area plasma source having the size of 1020 mm×830 mm, and high density plasmas on the order of 2.3×1011 cm-3 were obtained with 15 mTorr Ar at 5000 W of inductive power with good plasma stability. This is higher than that for the conventional serpentine-type antenna, possibly due to the low impedance, resulting in high efficiency of power transfer for the double-comb antenna type. In addition, due to the remarkable reduction of the antenna length, a plasma uniformity of less than 8% was obtained within the substrate area of 880 mm×660 mm at 5000 W without having a standing-wave effect. [ABSTRACT FROM AUTHOR]

The magnetotransport properties of ferromagnetic Ga1-xMnxAs epilayers with Mn mole fractions in the range of x≈2.2%–4.4% were investigated using Hall effect measurements. The temperature-dependent Hall carrier concentration for a metallic sample with x≈2.2% was analyzed assuming an activation energy from two acceptor levels. It was found that the two acceptor levels with activation energies of 129.4 and 31.6 meV at B=0 Oe decreased to 87.6 and 30.7 meV, respectively, at B=5 kOe. The decrease in acceptor activation energy from 129.6 to 87.6 meV was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for the increase in carrier concentration. From magnetic-field-dependent Hall resistance data, the Curie temperature was estimated to be TC=60 and 70 K for Ga1-xMnxAs samples with x≈2.2 and x≈4.4%, respectively. The magnetoresistance measurements confirmed that the anomalous Hall effect existed in these samples that showed metallic and insulating behavior, respectively. [ABSTRACT FROM AUTHOR]

Oxidation of an AlOx insulating barrier in a magnetic tunneling junction (MTJ) was carried out by a tilted-plasma oxidation method. It was found that the tilted-plasma oxidation induced a gradual change in the extent of oxidation of an insulating layer, which consequently led to a gradual change in the tunneling magnetoresistance (TMR) and specific junction resistance (RA) of the MTJ. We found a linear relation in the TMR versus RA curve with positive and negative slopes for less- and overoxidized junctions, respectively, and a parabolic relation for optimally oxidized junctions. The crossover in the TMR versus RA curves provides an effective and useful way to optimize (and monitor) the oxidation condition of a tunneling barrier in MTJs especially of a tunneling barrier less than 10 Å thick. The tunneling junctions were also investigated after thermal annealing at various temperatures. The observations after thermal annealing were found to be consistent with transmission electrons microscopy images and a scenario of the partial formation of an additional ultrathin tunneling barrier at the top surface of the bottom magnetic layer. [ABSTRACT FROM AUTHOR]

The voltage dependence of magnetoresistance (MR) and the low-frequency voltage fluctuation were studied in magnetic tunnel junctions with an AlN or an Al[sub 2]O[sub 3] barrier formed by plasma nitridation or oxidation. The junctions consist of free and pinned NiFe layers by a FeMn antiferromagnetic layer. For the Al[sub 2]O[sub 3] junction, the variation of half-reduction voltage of MR is about 50 mV from 401.3 mV in maximum to 351.4 mV in minimum depending upon the oxidation time. For the AlN junction, the less nitrided junction with a nitridation time of 120 s shows the higher-half-reduction voltage by about 100 mV than for optimally nitrided or oxidized ones. From the low-frequency voltage noise data, the less nitrided AlN junction shows larger 1/f noise magnitude compared with other junctions with an optimal condition, implying more defect states in the barrier due to unnitrided Al metals. In order to investigate the influence of nitrogen on ferromagnetic layer, another junction with an Al[sub 2]O[sub 3] barrier was made after the pinned NiFe layer was exposed to N[sub 2] plasma for 10 s. This junction reveals a lower MR and worse bias voltage dependence than any junctions studied here. The voltage dependence of MR, therefore, is likely influenced by the interface state rather than the defects in the barrier, although the two factors play an important role in determining MR. [ABSTRACT FROM AUTHOR]

The influence of structural disorder on the magnetic, transport, and optical properties of Ni[sub 0.50]Al[sub 0.50] alloy films has been investigated. A significantly disordered state was obtained by using vapor-quenching deposition onto substrates cooled by liquid nitrogen. The loss of translational invariance in the disordered state leads to a prominent increase of the magnetic moment of alloy below 50 K. This growth can be explained by the appearance of antistructure Ni atoms (or their clusters) in the disordered state of alloy and their ferromagnetic coupling below 50 K. The temperature dependences of resistivity for both ordered and disordered states of the Ni[sub 0.50]Al[sub 0.50] alloy films exhibit the resistivity minimum at 17-18 K, which has a nonmagnetic nature and can be related mainly to the quantum corrections to the electron-electron interactions in the presence of weak localization. It was shown that the resistivity of the ordered Ni[sub 0.50]Al[sub 0.50] alloy films in 50-300 K temperature range originates mainly from the electron-phonon scattering, while in the disordered state the contribution from the electron-phonon-vibrating impurity scattering becomes dominant. The structural disordering also leads to a noticeable change in the optical properties of alloy, especially in the infrared region. The observed temperature and structural dependences of the resistivity as well as the optical properties in the intraband region confirm the thesis on partial localization of the electronic states near the Fermi level. [ABSTRACT FROM AUTHOR]

We have analyzed amplification of transverse phonons confined in quantum well (QW) heterostructures through piezoelectric electron–phonon interaction with drifting electrons. It was found that this mechanism of interaction couples the low-dimensional electrons and the shear-horizontal (SH) confined phonons. We have studied the electrostatic potential accompanying the SH waves and found that efficient interaction can be achieved for the lowest antisymmetric SH phonon branch in a narrow band of phonon frequencies. For AlGaAs QWs the amplification coefficient was calculated to be on the order of 100 cm[sup -1] in the sub-THz phonon frequency range. These results suggest an electrical method for coherent phonon generation in the technologically well-developed AlGaAs QW heterostructures. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the ∼1540 nm [sup 4]I[sub 13/2]-[sup 4]I[sub 15/2] emissions of Er[sup 3+] in Er-implanted and annealed GaN. These studies revealed the existence of multiple Er[sup 3+] centers and associated PL spectra in Er-implanted GaN films grown by metalorganic chemical vapor deposition, hydride vapor phase epitaxy, and molecular beam epitaxy. The results demonstrate that the multiple Er[sup 3+]PL centers and below-gap defect-related absorption bands by which they are selectively excited are universal features of Er-implanted GaN grown by different techniques. It is suggested that implantation-induced defects common to all the GaN samples are responsible for the Er site distortions that give rise to the distinctive, selectively excited Er[sup 3+]PL spectra. The investigations of selectively excited Er[sup 3+]PL and PLE spectra have also been extended to Er-implanted samples of Mg-doped GaN grown by various techniques. In each of these samples, the so-called violet-pumped Er[sup 3+]PL band and its associated broad violet PLE band are significantly enhanced relative to the PL and PLE of the other selectively excited Er[sup 3+]PL centers. More importantly, the violet-pumped Er[sup 3+]PL spectrum dominates the above-gap excited Er[sup 3+]PL spectrum of Er-implanted Mg-doped GaN, whereas it was unobservable under above-gap excitation in Er-implanted undoped GaN. These results confirm the hypothesis that appropriate codopants can increase the efficiency of trap-mediated above-gap excitation of Er[sup 3+] emission in Er-implanted GaN. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Top synthetic spin valves with structure Ta/NiFe/CoFe/Cu/CoFe(P1)/Ru/CoFe(P2)/FeMn/Ta on Si (100) substrate with natural oxide were prepared by dc magnetron sputtering system. We have changed only the thickness of the free layer and the thickness difference (P1-P2) in the two ferromagnetic layers separated by Ru, and investigated the effect of magnetic film thickness on the interlayer coupling field in a spin valve with a synthetic antiferromagnet. As the free layer thickness decreased from 70 to 20 Å, the interlayer coupling field was increased due to the magnetostatic coupling (orange peel coupling). In the case of the thickness difference in the pinned layers, the interlayer coupling field agreed with the modified Néel model suggested in the top synthetic spin valve structures. However, in the case of t[sub P1]=t[sub P2], and t[sub P1]=t[sub P2]+5 Å, it was found that the interlayer coupling field could not be explained by the modified Néel ! model. The deviation of the modified Néel model at the dip zone could be due to the large canting of the pinned layers, which depend on applied field and different thickness in synthetic antiferromagnetic structure. The dependence of Cu thickness on the interlayer coupling field was investigated and 10 Oe of the interlayer coupling field was obtained when the Cu thickness is 32 Å. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The properties of nanoscale spheres and tubes are of recent interest due to the discovery of the fullerene molecule and the carbon nanotube. These carbon structures can be modeled as nanoscale spherical or cylindrical shells. In this article, these nanostructures are treated in the thin shell approximation with the elastic properties taken to be those of the graphene sheet. A quantization prescription is applied to the classical elastic modes to facilitate the first calculations of the quantum-mechanical normalizations of selected modes. These modes are shown to be amenable to the study of electron-phonon interactions. Indeed, electron-phonon interaction Hamiltonians are derived. Moreover, it is shown for such a tube of finite length that the electron-phonon interaction strength depends on the axial position. As a special case it is shown that the dispersion relation for the clamped tube depends on the length of the tube. In this article we consider both the vibrational frequencies and the mode quantization for both spherical shell and the nanotube using realistic material parameters. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The influence of the structural disorder on the transport properties of Co[sub 0.50]Ti[sub 0.50] alloy films in a temperature range of 4.2-300 K has been investigated without and with a magnetic field of 0.5 T. The absence of translational invariance in a disordered state leads to an increase in the resistivity and causes a change from the positive to negative temperature coefficient of resistance. This fact is explained by the partial localization of electronic states near the Fermi level. It was established that a partial structural disordering enhances the role of the electron-phonon-vibrating impurity scattering in the transport properties and also makes the spin-diffusive scattering rather noticeable. The appearance of a low-temperature resistivity minimum for the disordered Co[sub 0.50]Ti[sub 0.50] alloy film arises from the quantum corrections to the electron-electron interactions in the presence of weak localization. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The optical and magneto-optical properties of ordered Fe[sub 3]Pt have been investigated by spectroscopic ellipsometry and magneto-optical Kerr spectroscopy. The diagonal component of the optical conductivity tensor of the compound exhibits a broad absorption peak at about 2 eV, which is shifted by about 0.5 eV to lower energies from the corresponding one in pure bcc Fe. The Kerr angle spectrum of the compound disperses quite similarly in both spectral trend and magnitude to that of pure Fe below 3.5 eV but differently above it. The lower-energy shift of the 2-eV-absorption structure of the compound is interpreted as due to the shift of the minority-spin Fe-d states toward E[sub F] through the hybridization with Pt-d states. The Kerr effect of the compound is attributable to a large spin-orbit coupling in Pt as well as the well-hybridized spin-polarized bands. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In laser materials processing, localized evaporation caused by focused laser radiation forms a plume of mixed vapor and ambient gas above the material surface. The beam is refracted and absorbed as it traverses the plume, thus modifying its power density on the surface. In this work, plume-beam interaction is studied using an axisymmetric, high-temperature gas-dynamic model of a plume formed by vapor from an iron surface. The beam propagation in the plume is calculated from the paraxial wave equation including absorption and refraction. The simulation results quantify the effects of plasma plume properties on the beam radius and laser power density variations at the material surface. It is shown that absorption and refraction in the plume have significant impacts on the laser-material interaction. Absorption of the beam in the plume has much less direct effect on the power density at the material surface than refraction does. However, absorption is essential for the formation of the plume, without which there is no refraction. Helium gas is more efficient than argon for reducing the beam refraction and absorption effects. Laser energy reflected from the material surface has significant effects on the plume properties. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents the results of a comparative study of the solid-state reaction in a series of Ti/Ni multilayered films (MLF) with a bilayer period of 0.65-22.2 nanometer and constant Ti to Ni sublayer thickness ratio. Spectral and sublayer-thickness dependence of the magneto-optical properties of the Ti/Ni MLF.

The properties of zinc-cobalt-oxide (Zn-Co-O) films deposited at room temperature by using pulsed laser deposition that display p-type conduction are described. Distinct rectifying current-voltage characteristics are seen for junctions between Zn-Co-O and n-type InGaZnO films, which has displayed a threshold voltage, and P-type Zn-Co-O is promising for thin-film electronic device technology.

The artificial nanoparticle surface decoration technique was extended to the surfaces of technologically important biaxially textured metallic substrates to describe improvements in flux pinning properties of YBCO films subsequently grown by pulsed laser deposition (PLD). The findings offer insight into the effectiveness of interface engineering to induce flux pinning defects in the high temperature superconductor (HTS) matrix.

The preparation of the artificial nanopatterns of [Mn.sub.12]-acetate is carried out by using e-beam lithography. The nanopatterns could find application in design and development of hybrid systems with three-dimensional positional control on the nanometer scale.