bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical effects of atomic hydrogen incorporation in GaAs-on-Si
Authors: Zavada J. M. , Pearton S. J. , Wilson R. G. , Wu C. S. , Stavola Michael , Ren F. , Lopata J. , Dautremont-Smith W. C. , Novak S. W.
Published: July 2002

We have introduced atomic hydrogen by two methods into GaAs layers epitaxially grown on Si substrates, namely, by exposure to a hydrogen plasma or by proton implantation. In both cases, when proper account is taken of shallow dopant passivation or compensation effects, there is a significant improvement in the reverse breakdown voltage of simple TiPtAu Schottky diodes. Proton implantation into undoped (n=3×1016 cm−3) GaAs-on-Si leads to an increase in this breakdown voltage from 20 to 30 V, whereas plasma hydrogenation improves the value from 2.5 to 6.5 V in n-type (2×1017 cm−3) GaAs-on-Si. Annealing above 550 °C removes the beneficial effects of the hydrogenation, coincident with extensive redistribution of the hydrogen. This leaves an annealing temperature window of about 50 °C in the H-implanted material, in comparison to 150 °C for the plasma-hydrogenated material. The hydrogen migrates out of the GaAs to both the surface and heterointerface, where it shows no further motion even at 700 °C. Trapping in the GaAs close to the heterointerface is shown to occur at stacking faults and microtwins, in addition to extended dislocations.
Temperature dependence of the band gaps of red HgI2
Authors: López-Cruz Elías
Published: July 2002

The photoconductive Moss–Hawkins [T. S. Moss and T. D. H. Hawkins, Phys. Rev. Lett. 1, 129 (1958)] technique was used in order to measure the temperature dependence of the band gaps of red HgI2 from 292 down to 10 K. In the case of the indirect gap, the theory of indirect transitions was successfully applied, obtaining a very good fitting with the phonon value reported in the literature. Both the indirect and direct gaps as a function of temperature show a Varshni-type behavior.
Characterization of Ga out-diffusion from GaAs into SiOxNy films during thermal annealing
Authors: Kuzuhara Masaaki , Nozaki Tadatoshi , Kamejima Taibun
Published: July 2002

The out-diffusion of Ga atoms during thermal annealing from a GaAs substrate into an SiOxNy encapsulating film has been studied using secondary ion mass spectrometry. The concentration of Ga atoms detected within the SiOxNy encapsulant annealed at 850 °C is found to increase with increasing the oxygen content of the encapsulant. The results are well correlated with the concentration change of the electron trap EL5 (Ec−ET =0.42 eV) evaluated from deep-level transient spectroscopy. We conclude that the controlled Ga out-diffusion by SiOxNy capped annealing causes enhanced electrical activation of Si implants and the generation of the EL5 trap during thermal annealing is ascribed to the Ga out-diffusion.
Demagnetizing field computation for thin films: Extension to the hexagonal lattice
Authors: Mansuripur M.
Published: July 2002

A previously published method of calculating the magnetic field pattern of thin films using fast Fourier transforms is extended to situations where the magnetization distribution is specified on a hexagonal lattice.
Dynamics of heterostructure hot-electron diodes
Authors: Arnold D. , Hess K. , Higman T. , Coleman J. J. , Iafrate G. J.
Published: July 2002

The transport properties of heterostructure hot-electron diodes are examined by Monte Carlo simulations. The transient analysis indicates that the limiting time constant for electronic switching from low to high conduction is of the order of the device transit time provided that tunneling- and thermionic-emission time constants associated with the formation of accumulation layers are of shorter duration; since the average electron velocities are of the order of 3×107 cm/s for typical device dimensions of 1000 Å, it follows that picosecond switching times should be possible for such device feature sizes. The effects of electron-electron interactions have also been considered; their influence on diode switching and on the switching of heterolayer devices in general are discussed.
Fast responding and highly multiplexible distorted helix ferroelectric liquid-crystal displays
Authors: Fünfschilling J. , Schadt M.
Published: July 2002

We present data on the electro-optics and the display performance of liquid-crystal devices based on the distorted helix ferroelectric (DHF) effect which demonstrate TV switching rates and low driving voltages. The DHF effect is based on S*c ferroelectric liquid crystals with very short pitch, which form in suitable cells a bookshelf arrangement of the smectic layers with a helical axis parallel to the plane of the display. The distortion of this helix by an applied electric field is responsible for the electro-optical effect: If the pitch is shorter than the wavelength, the distortion leads to a change of the effective refractive index. Unwinding of the helix is one of the limitations of these devices. We show that standard cell preparation techniques and driving schemes—including active matrix addressing—minimize helix unwinding and lead to highly multiplexible displays with short response times in the 10-μs region. Black–white contrast ratios >12:1 at driving voltages <2 V and grey scales are reported.
High-energy product Sm-Co-based sputtered films, crystal texturing, and magnetic properties
Authors: Cadieu F. J. , Hegde H. , Chen K.
Published: July 2002

Sm-Co-based sputtered films have been deposited that exhibit a remanence ratio for hysteresis loops measured out of the plane to in the plane of 0.04. These results have been obtained by thermalized sputtering this system in Ar, Ar+Xe mixtures, and in Xe. The use of Xe has allowed a more efficient thermalization of the massive Sm atoms so that a lower total pressure can be used than if Ar were used alone. The use of Ar-Xe mixtures as the sputtering gas has allowed high-energy product Sm2(Co, Fe, Cu, Zr)17 films with the TbCu7-type crystal structure to be obtained over a much wider range of substrate temperatures and sputtering pressures than has been possible for films sputtered only in Ar. Films sputtered in Ar will show a perpendicular to the in-plane remanence ratio of 0.36 vs 0.04 for Xe. The slope of the hysteresis loops was nearly linear from +18 to −4.5 kOe with values of −0.7%/kOe for the Xe films and a value of −1.2%/kOe for the Ar films. All of these films were directly crystallized onto heated substrates, which is necessary to control the crystal texturing.
Coercivity of magneto-optical media by spin dynamics
Authors: Suits J. C.
Published: July 2002

Spin dynamics computer simulations have been carried out to study the effect of pinning on domain-wall motion in TbFeCo-like media. These calculations were done on a 30×30×1 mesh, where the spin direction at each lattice site was calculated with the Landau–Lifshitz–Gilbert equation. The simulations were made in an IBM 3090 mainframe–personal computer environment where the result of the calculation is a movie that runs at three frames/second on an AT and shows graphically the domain-wall–defect interaction. The domain wall is caused to move in an external field toward a defect, and the maximum field that pins the domain wall was observed. The defects have finite length and zero magnetization, which correspond to voids or nonmagnetic second phase in the media. The simulation shows that small defects on the order of 100 Å in size can pin walls with pinning strength appropriate to the coercivity of magneto-optical media, i.e., local coercivities in the range 1–10 kOe. For sufficiently high fields a single wall may break up into two separate sections at the defect, and then join together beyond the defect to become a single wall again. For rectangular defects, the coercivity depends strongly and nearly linearly on defect length (parallel to the domain-wall surface) and only weakly on defect width for widths greater than about 50 Å (perpendicular to the wall surface).
Theoretical study of magnetic and magneto-optical properties of Fe-based transition-metal alloys
Authors: Ebert H. , Akai H.
Published: July 2002

Relativistic influences on the magnetic properties of transition-metal alloys manifest themselves for example by the occurrence of magnetocrystalline anisotropy, orbital contributions to the magnetic moment, and hyperfine fields. Among these effects, the magneto-optical Kerr effect is receiving much interest recently because of its application in the readout process of magneto-optical storage devices. We have studied the electronic structure of Fe-based transition metals by means of the spin-polarized version of the relativistic linear muffin-tin orbital method of band-structure calculation. By this way the mentioned consequences of relativistic effects, primarily the spin-orbit coupling, could be investigated without relying on the use of perturbational methods. Results for alloy systems, which were assumed to be of possible technical interest, are presented and discussed in comparison with experimental data as far as available.
Spin-dependent x-ray absorption in Co/Pt multilayers and Co50Pt50 alloy
Authors: Schütz G. , Wienke R. , Wilhelm W. , Zeper W. B. , Ebert H. , Spörl K.
Published: July 2002

The spin dependence of L2,3 absorption in 5d atoms oriented in a ferromagnetic matrix contains information on the spin density of the empty d-projected states of the absorbing atom. Spin-dependent absorption spectroscopy using circularly polarized synchrotron radiation was applied to study the polarization of the Pt atoms in the binary alloy Co50Pt50 and Pt/Co layered structures, which are promising candidates for magneto-optical recording. The spin-dependent absorption signals for vapor-deposited 250(4 Å Co+18 Å Pt) and 250(6 Å Co+18 Å Pt) multilayers indicate a ferromagnetic coupling on Pt and Co atoms with a significant Pt polarization. This is reduced on average by about 60% with respect to the Pt polarization in the Co50Pt50 alloy. The experimental results are discussed on the basis of spin-polarized band-structure calculations.
Phenomenological kinetic equation for Mn luminescence in ZnS films
Authors: Geoffroy A. , Bringuier E.
Published: July 2002

In this paper time-resolved measurements of Mn2+ luminescence in ZnS electroluminescent layers grown by atomic layer epitaxy are reported. The Mn concentration ranges from 0.03 to 1.4 mol %. The intensity of the luminescence versus time is cast in the form of a kinetic equation instead of an algebraic formula. This allows one to determine on a purely phenomenological basis the kinetic rates of mono- and bimolecular processes responsible for the relaxation. We show by a Laplace transform analysis technique that the long-time behavior is not dominated by the lifetime of the single Mn2+ ion in the host matrix. The influence of the electrical properties is evidenced on the second-order kinetic rate. The methods used provide a general framework for investigating complex decay processes.
Ultrathin Co/Pt and Co/Pd multilayered films as magneto-optical recording materials
Authors: Hashimoto S. , Ochiai Y. , Aso K.
Published: July 2002

Co/Pt and Co/Pd multilayered films were prepared by two-source dc magnetron sputtering. The multilayers with thin Co layers, one to three atoms thick, exhibited a perfect squareness of Kerr loop and an enhancement of Kerr rotation when the total film thickness became below several hundred angstroms. These ultrathin films have a high Kerr rotation of 0.2°–0.45° and strong perpendicular magnetic anisotropy even in the very thin-film thickness. The figure of merit of the ultrathin Co/Pt films is superior to that of TbFeCo system at 780 nm and becomes larger at shorter wavelength. The ultrathin multilayers possess very high corrosion resistance, and the layered structure is thermally stable up to 400 °C. Consequently, the ultrathin Co/Pt and Co/Pd multilayered films can be new magneto-optical recording materials, especially for higher density recording using a shorter wavelength laser.
Amorphic diamond films produced by a laser plasma source
Authors: Davanloo F. , Juengerman E. M. , Jander D. R. , Lee T. J. , Collins C. B.
Published: July 2002

Recently, attention has been focused upon laser plasma sources of thin-film diamond. These depend upon laser-ignited discharges in which intense pulsed currents flow through the small volume of carbon plasma ablated from graphite feedstock by a focused laser beam. The materials produced in this way generally resemble the hard amorphic films deposited by ion beams. This paper reports a detailed characterization of these films which we call amorphic diamond. The combination of an optical band gap of 1.0 eV with a grain size of 100–200 Å places this material far outside the range of possibilities available to the model of graphitic islands. A structure of very fine grained diamond would more readily explain the hardness of 13 GPa determined in the absence of any measurable fraction of hydrogen. Such amorphic diamond films have been grown uniformly on 100-cm2 areas at ambient room temperatures with no seeding or abrasion of the substrate.
The mechanism of epitaxial Si-Ge/Si heterostructure formation by wet oxidation of amorphous Si-Ge thin films
Authors: Prokes S. M. , Rai A. K.
Published: July 2002

Epitaxial Si-Ge/Si heterostructures have been formed from amorphous Si0.86Ge0.14, Si0.7Ge0.3, and Si0.56Ge0.44 films which were deposited at a vacuum of 10−7 Torr, followed by a wet oxidation process. The presence of an initial native oxide precluded solid phase epitaxy under standard annealing conditions, but epitaxy could be achieved by the use of wet oxidation. The samples were oxidized for various times at 900 °C and examined in reflected electron diffraction, ellipsometry, and cross-sectional and plan-view transmission electron diffraction. The formation of the epitaxial layer has been examined, and an epitaxial growth model is suggested.
Macroscopic quantum tunneling of the total magnetization in small ferromagnetic grains
Authors: Garg Anupam , Kim Gwang-Hee
Published: July 2002

The tunneling of the total magnetization of a single-domain spherical (∼100-Å-diam) particle is considered for various crystal symmetries, with special attention to the dissipative effect of phonons in the grain and any embedding matrix. Such dissipation is shown to have a spectral density J(ω)∼ω3 and to be very weak. Stray fields and interparticle interactions are also considered.
Effect of deformation on Pd2MnSn Heusler alloy studied with transmission electron microscopy, profile analysis of neutron powder diffraction pattern, and magnetization measurement
Authors: Kamiyama Takashi , Shinohara Takeshi , Tomiyoshi Shoichi , Minonishi Yasuhide , Yamamoto Hisao , Asano Hajime , Watanabe Noboru
Published: July 2002

The ferromagnetic Heusler alloy Pd2MnSn shows large reduction in magnetization by cold working, without any appreciable change in the Curie temperature. In order to clarify the relationship between the reduction and the defect structure in Pd2MnSn, transmission electron microscopic observation, high-resolution neutron powder diffraction, and magnetization studies have been performed. In the deformed Pd2MnSn sample, dislocations are distributed inhomogeneously and form a cell structure. In the cell walls, a large amount of antiphase domain boundaries exist. A method of Rietveld refinement has been proposed for analyzing the powder neutron diffraction patterns of the cold-worked sample, where all the contributions of defects (size, strain, and antiphase domain boundary) and the instrumental resolution to each reflection line are expressed in terms of Fourier coefficients, and inverse Fourier transforms of those product represent the calculated profile which is used for least-squares refinement. Analyzing the experimental data with the method proposed, it is concluded that the reduction in magnetization in the deformed Pd2MnSn is due to an increase in the number of small antiphase domains which couple antiferromagnetically to each other.
Magnetic-field-modulated written bits in TbFeCo thin films: Transmission electron microscopy Lorentz and scanning electron microscopy with polarization analysis studies
Authors: Aeschlimann M. , Scheinfein M. , Unguris J. , Greidanus F. J. A. M. , Klahn S.
Published: July 2002

Domains written thermomagnetically in TbFeCo thin films are studied with Lorentz transmission electron microscopy (TEM) and scanning electron microscopy with polarization analysis (SEMPA). Four different rare-earth/transition-metal compositions TbxFeyCo1−x−y are examined. The domain structures observed with both techniques are similar even though TEM Lorentz only detects the transverse component of the net magnetic field along the electron’s trajectory through the sample, while SEMPA detects the surface electron-spin polarization (magnetization). We find that the magnetic contrast in the SEMPA measurements is proportional to the magnetization of the transition-metal (TM) subnetwork which is antiferromagnetically coupled to the rare-earth (RE) subnetwork. This allows high-contrast SEMPA images to be acquired even when the system is magnetically compensated (Ms=‖MRE−MTM‖=0). The surface magnetization can be explained by assuming that the surface of the TbFeCo alloy consists of an outermost thin oxide layer followed by an Fe-rich subsurface layer. The importance of the demagnetizing field on the switching and domain nucleation process for thermomagnetically written bits is examined.
Iodine monofluoride discharge laser investigation
Authors: Harris D. G. , Blauer J. A. , Hurlock S. C.
Published: July 2002

The results of an experimental investigation to improve the performance of a discharge-pumped iodine monofluoride laser are reported. Lasing was observed at 478.7, 484.7, 490.7, and 496.5 nm. Electrical measurements of the discharge characteristics permitted the energy flow in the circuit to be followed and laser efficiencies to be calculated. Parametric studies of gas mixtures were carried out. By optimizing several parameters, single-pulse lasing energies greater than 50 mJ were obtained.
Deep levels in undoped bulk InP after rapid thermal annealing
Authors: Kim Eun Kyu , Cho Hoon Young , Yoon Ju Hoon , Min Suk-Ki , Jung Young Lae , Lee Wan Ho
Published: July 2002

Deep levels in rapid thermal annealed InP in metal-insulator-semiconductor (MIS) structures have been studied using deep level transient spectroscopy. Two different insulating layers used in forming MIS structures, a silicon nitride layer and an oxide layer, were fabricated by plasma enhanced chemical vapor deposition and concentrated nitric acid, respectively. In the samples annealed at temperatures between 700 and 900 °C for 10 s, two deep levels having apparent energy depths of 0.43 and 0.35 eV below the conduction band were newly generated. Then, it is considered that they are the defects related with phosphorus vacancy and its complex. Other deep levels observed between 0.55 and 0.79 eV below the conduction band were related with insulating layers. We show an evidence that they might be interface states in the junction of InP and insulator.
Mechanism of electrochromism for amorphous WO3 thin films
Authors: Hashimoto Satoshi , Matsuoka Hideki
Published: July 2002

The mechanism of electrochromism for an amorphous WO3 film has been studied. The film was prepared by using vacuum evaporation. X-ray phototelectron spectroscopy analysis has revealed that a state appears below the Fermi level after coloration in a LiClO4-propylene carbonate electrolyte and that the Fermi level increases in proportion to the amount of injected lithium. In addition, a decrease in the density of state of the conduction band has been observed in a colored film by using electron energy loss spectroscopy analysis in transmission electron microscopy. It has been concluded that the electrons injected occupy the conduction band after coloration and that electrochromism of amorphous WO3 film is due to an intraband transition between an electron injected in the conduction band and an empty state.
Predicted elastic constants and critical layer thicknesses for cubic phase AlN, GaN, and InN on β-SiC
Authors: Sherwin M. E. , Drummond T. J.
Published: July 2002

Elastic constants for zinc-blende AlN, GaN, and InN have been estimated from the elastic constants of the wurtzite phase. This has been accomplished by recognizing that the crystal structures of the wurtzite and zinc-blende phases are related by a simple rotation. This rotation was then applied to the elastic constants and a least-squares fit is used to match the results. Using the zinc-blende elastic constants the critical thickness of the nitrides on β-SiC substrates was calculated. The critical thickness of a single overlayer of AlN was calculated to be 14.1 nm, and for GaN the critical thickness was found to be 0.7 nm. In the elastic continuum model used there was no solution for the critical thickness of InN.
Formation of optical waveguides in KNbO3 by low dose MeV He+ implantation
Authors: Strohkendl F. P. , Günter P. , Buchal Ch. , Irmscher R.
Published: July 2002

MeV 4He+ implantation into single crystals of KNbO3 has been used to fabricate planar optical waveguides by generating a buried layer of lowered refractive index. We characterize the optical loss of these waveguides as a function of the ion dose D, and find a minimum loss of (1.0±0.2) cm−1 after implantation of 2 MeV ions at a dose of 5×1014 cm−2. This is by far the lowest dose ever for which efficient waveguiding has been reported. For good guides, the observed losses are mainly due to tunneling through the refractive index barrier with nontunneling contributions of less than 30%. The reduction Δnb of the refractive index nb in the region of the nuclear damage peak is proportional to the dose and we find Δnb/D=(7.8±1.6)×10−17 cm2.
Two spin-freezing behavior in concentrated amorphous ferrite oxides
Authors: Okuno S. N. , Hashimoto S. , Inomata K. , Morimoto S. , Ito A.
Published: July 2002

Amorphous CoFe2O4 with metal ions coordinated in a manner similar to the spinal crystal was prepared by the ion-beam sputter deposition method. The study of magnetic properties was carried out by dc- and ac-susceptibility measurements and Mössbauer spectroscopy. The strong spin-freezing behavior was observed in 80-Hz, 1-Oe ac susceptibility at two spin-freezing temperatures: 284 K (Tf1), which is considered to be caused by a random anisotropy of Co ions coordinated in distorted octahedral sites, and 86 K (Tf2), which might be caused by the frustration of antiferromagnetic interactions due to the amorphous structure. The dc susceptibility showed irreversibility below Tf1 which decreased with increasing applied field. However, a distinct change was not observed at Tf2. The Mössbauer spectrum revealed that some of the paramagnetic Fe ions were frozen below Tf1, with relaxation phenomena, and that the residual paramagnetic ions were frozen at Tf2. The time decay of the thermoremanent magnetization observed below Tf1 showed different behavior compared to that above and below Tf2.
Microwave dielectric loss studies on lithium-zinc ferrites
Authors: Raman R. , Murthy V. R. K. , Viswanathan B.
Published: July 2002

Microwave dielectric parameters are measured for lithium-zinc ferrites with chemical formula Li0.5−x/2Fe2.5−x/2ZnxO4 (x=0.0, 0.1, 0.3, 0.5, 0.6). All the samples except x=0.5, show an increase in the dielectric loss with temperature. A relaxation-type mechanism is observed for x=0.5. The results are compared with the low-frequency dielectric constant and loss.
Photoemission studies on metals using picosecond ultraviolet laser pulses
Authors: Srinivasan-Rao T. , Fischer J. , Tsang T.
Published: July 2002

We report the quantum efficiency of various metals irradiated by 266-nm, 4.66-eV laser pulses of 10-ps duration. The highest quantum efficiency obtained is 7.25×10−4 with samarium photocathodes. Current densities exceeding 66 and 21 kA/cm2 have been obtained from an area of 0.05 and 7 mm2, respectively. The maximum currents and current densities obtainable in these experiments are limited by the space charge. For surface fields exceeding 5×107 V/m on gold, the efficiency increases linearly with the field for the values investigated. Based on the quantum efficiency and optical damage threshold measurements, current densities exceeding 100 kA/cm2 seem feasible without damaging the photocathode.
Magnetic properties of Co/Pd multilayers determined by Brillouin light scattering and SQUID magnetometry
Authors: Harzer J. V. , Hillebrands B. , Stamps R. L. , Güntherodt G. , England C. D. , Falco Charles M.
Published: July 2002

Co/Pd multilayers with modulation wavelengths between 4 and 220 Å have been prepared by magnetically enhanced dc-triode sputtering on single-crystal sapphire substrates. Their saturation magnetization and volume and interface anisotropies have been investigated using Brillouin light scattering from collective spin waves and by SQUID magnetometry. The saturation magnetization of Co is found to be independent of the Co layer thickness and reduced by about 20% from the Co bulk value. From the comparison of the results of the two experimental methods, clear evidence for a Pd polarization is found and the polarization depth is estimated. Samples with Co thicknesses of 2 atomic layers and Pd thicknesses ≥5 atomic layers exhibit a perpendicular magnetization due to a large negative out-of-plane interface anisotropy. The properties of spin waves in Co/Pd multilayers with the direction of magnetization pointing out-of-plane are discussed with respect to an appropriate theoretical model.
High-sensitivity magnetoresistive transducers
Authors: Pant Bharat B. , Krahn Donald R.
Published: July 2002

Magnetoresistive transducers, combined with appropriate signal conditioning, can be used to detect magnetic-field changes from small currents, geomagnetic anomalies, and perturbations in the earth’s field from ferromagnetic objects. Two different designs of transducers are reported on−herringbone type, and barberpole biased type. Sensitivities as high as 3.5 mV/V/Oe have been obtained. The highest reported value seen in published literature is 1.75 mV/V/Oe (Philips Technical Publication 268), whereas values around 1.0 mV/V/Oe are typical. These high-sensitivity transducers are designed and fabricated based on previously published theoretical analysis [B. Pant, J. Appl. Phys. 67, 414 (1990)] of the response of thin magnetoresistive strips to external fields. This analysis indicates that to increase the sensitivity of these transducers one must reduce the ratio of strip thickness to its width and rotate the easy axis away from the long dimension of the strip wherever appropriate. The theoretical and experimental curves of bias field versus sensitivity for the herringbone design, and the transfer function for the barberpole design, are reported.
Two-point estimates of the spectral density function with finite separation and volume
Authors: Carlson A. W.
Published: July 2002

The spatial and temporal variation of a fluctuating quantity can often be described with sufficient accuracy as a sum of independent modes, whose powers are given by a function S(k) for each frequency. Many diagnostics are restricted to making measurements at two points, so that a determination of S(k) by spatial Fourier transformation is not possible. Nevertheless, in the limit of point measurements with infinitesimal separation, the mean wave number k̄ and the rms deviation σ about the mean can be exactly determined by repeated measurements of the phase difference between the tips. If the separation is finite, the method can be extended in such a way that the correct results are obtained for any separation as long as S(k) is Gaussian. For other spectral shapes, the error in the measurement of k̄ may be of the order of σ, and the fractional error in the measurement of σ may be substantial. A finite measurement volume reduces the sensitivity to short wavelength modes. Under certain assumptions, the measured values can be adjusted to yield better estimates of k̄ and σ.
Structural characterization of Co-Re superlattices
Authors: Melo L. V. , Trindade I. , From M. , Freitas P. P. , Teixeira N. , da Silva M. F. , Soares J. C.
Published: July 2002

Co-Re superlattices were prepared with nominal periodicities of 65–67 Å and varying bilayer composition. The structural characterization was made by x-ray diffraction and Rutherford backscattering spectrometry (RBS). First, second, and third order satellites are observed in the x-ray diffractogram at 2θ values and with intensities close to those predicted by simulation. This confirms the coherence of the superlattice. RBS measurements combined with RUMP simulations give information on interface sharpness and the absolute thicknesses of the Co and Re layers. Discrepancies between the experimental and simulated diffractograms are found for Co thicknesses below 18 Å.
Electrical characteristics and energy band offsets in n GaSb-p Ga0.83Al0.17Sb
Authors: Mebarki M.
Published: July 2002

p,n Ga1−xAlxSb/GaSb heterojunctions obtained by liquid phase epitaxy, were studied by capacitance-voltage characterization and spectral photoresponse. It is shown that the band offsets of this system are ΔEc=0.14±0.03 eV and ΔEv=0.07±0.03 eV, in agreement with other work. This preliminary study is used to propose a variation law of the electron affinity as a function of the composition x.
Free-electron laser with longitudinal wiggler in a waveguide partially filled with a relativistic electron beam
Authors: Mehdian Hassan , Hwang Un-Hak , Willett Joseph E.
Published: July 2002

Parametric excitation of a space-charge wave and a transfer magnetic mode in a cylindrical metallic waveguide partially filled with a relativistic electron beam is analyzed. The results of a numerical solution of the linear dispersion relation and determination of the dependence of amplitude gain, frequency enhancement, and the ratio of the frequencies of the excited waves on the ratio of beam radius to waveguide radius are presented.
Solutions to current crowding in circular vias for contact resistance measurements
Authors: Allen L. H. , Zhang M. Y. , Mayer J. W. , Colgan E. G. , Young R.
Published: July 2002

The effects of current crowding near circular contacts has been analyzed. We analyze a simple system of two parallel plates connected by a cylindrical plug. Under a given set of assumptions the problem can be reduced from three-dimensional to one-dimensional geometry. Given this assumption, analytic solutions are obtained for the current and voltage distributions within the plug. From these expressions the correct values for contact resistivity (Pc) are derived. Finally, the analytical expressions are compared with the results from two-dimensional numerical calculations.
Spectroscopic investigation of the electro-optic nonlinearity in poly(2,5-thienylene vinylene)
Authors: Gelsen O. M. , Bradley D. D. C. , Murata H. , Takada N. , Tsutsui T. , Saito S.
Published: July 2002

We report the results of a spectroscopic investigation of the quadratic electro-optic nonlinearity in poly(2,5-thienylene vinylene). Electroabsorption spectra closely resemble the second energy derivative of the unperturbed π-π* absorption and show oscillations associated with the strong vibronic features present therein. The dispersion of both the real and imaginary parts of χ(3) (−ω;0,0,ω) has been determined and we find that Re{χ(3)} has a peak value of 7.5×10−9 esu at 1.83 eV and Im{χ(3)} has a peak value of 6.2×10−9 esu at 1.78 eV. This nonlinearity is responsible for a photorefractive effect under resonant photoexcitation.
The angular dependence of the transmission of nontwisted liquid crystal displays
Authors: van Sprang H. A.
Published: July 2002

A model is presented which describes the properties of a sheet polarizer under oblique incident light. Experimental evidence indicates that the model describes the polarizer adequately when we take the index of the active (absorbing) layer as n=1.3. In the second part of the paper we present an analytical expression for the transmission of a nontwisted nematic sample (electrically controlled birefringence cell) under oblique incident light and as a function of externally applied electric fields. We obtain the expression by separate integration of phase and amplitude contributions to the final transmission. Combination of this model with the description of the sheet polarizer yields a satisfactory agreement with the experimental results. The speed of calculation using the above model is about 20 times that of a traditional fast 4×4 matrix calculation.
Self-consistent analysis in the presence of phase-randomizing processes for double-barrier structures
Authors: Alam Muhammad A. , Morrisey Richard A. , Khondker A. N.
Published: July 2002

We present a model, based on the nonequilibrium retarded Green’s function method of the quantum kinetic (Keldysh) theory, that describes carrier transport in three-dimensional quantum structures with translational invariance in the transverse direction. The transport equations include inelastic phase-breaking processes and describe the transport of both the coherent and incoherent electrons within the same framework with a set of first-order coupled linear differential equations. These equations can be solved without resorting to evaluating the Green’s function. The model accounts for local space charges in Poisson’s equation and is suitable for modeling the steady-state current-voltage characteristics of double-barrier structures. A realistic model for these devices should include the effects of inelastic processes and space charge simultaneously. However, as an illustration, we present numerical results for double-barrier devices by assuming that the electrons undergo elastic phase-breaking collisions only. Our simulation results show that the accumulated space charge is a function of phase-breaking collision and that the presence of dissipation within the contacts is partly responsible for the low observed peak-to-valley current ratio.
Growth of CdTe epitaxial films on p-InSb(111) by temperature gradient vapor transport deposition
Authors: Kim T. W. , Koo B. J. , Jung M. , Kim S. B. , Park H. L. , Lim H. , Lee J. I. , Kang K. N.
Published: July 2002

The growth of high quality CdTe epitaxial films on p-InSb(111) by a simple method of temperature gradient vapor transport deposition was carried out to investigate the possibility of the existence of a two-dimensional electron gas with high mobility at CdTe/InSb heterointerfaces. From the x-ray diffraction analysis, the grown layer was found to be a CdTe epitaxial film. Photoluminescence measurements at 15 K showed that a CdTe film grown on InSb(111) in the temperature range between 180 and 280 °C appeared to have an optimum crystal perfection at a substrate temperature of about 245 °C. These results also indicated that the CdTe films grown above 245 °C contained a significant problem due to interdiffusion from the InSb substrates during the growth.
Electron-transport mechanisms in metal Schottky barrier contacts to hydrogenated amorphous silicon
Authors: Heller D. E. , Dawson R. M. , Malone C. T. , Nag S. , Wronski C. R.
Published: July 2002

Detailed electrical studies have been carried out on nickel, palladium, and platinum Schottky barrier contacts to intrinsic a-Si:H films of 0.5, 1.0, and 3.0 μm thickness. The diode characteristics of the Schottky barrier structures are investigated using light and dark I-V measurements over a temperature range of 24–150 °C. The activation energies of the diode currents are compared to the electron barrier heights determined from internal photoemission (IPE) measurement. The bias dependence of the reverse diode currents is directly compared to the barrier lowering measured by IPE. The diode currents are explained in terms of a joint thermionic-emission/drift-diffusion model that includes changes in the barrier height with applied bias.
Efficient organic electroluminescent device using a single bipolar carrier transport layer
Authors: Littman Jon , Martic Peter
Published: July 2002

An electroluminescent device was developed in which a single bipolar transport layer is used. High efficiencies can be obtained from this device by judiciously selecting a dopant that enhances the radiant recombination probability of carrier pairs and spaces this recombination zone away from the cathode.
Interband transitions in InAsxP1−x/InP strained multiple quantum wells
Authors: Hwang S. J. , Shan W. , Song J. J. , Hou H. Q. , Tu C. W.
Published: July 2002

We report the optical study of the interband transitions in InAsxP1−x/InP strained-layer multiple quantum wells grown by gas-source molecular beam epitaxy. Low-temperature photoluminescence, photoluminescence excitation, and room temperature photomodulated transmission measurements were performed to investigate optical interband transitions. In addition to transitions associated with the heavy-hole and the light-hole bands, a transition involved with the spin-orbit split-off band was observed. We also observed spectral linewidth broadening due to compositional inhomogeneity and layer-thickness fluctuations from the sample using short-period superlattices as the well materials. Calculations based on the envelope-function approximation and phenomenological deformation potential theory, including both band nonparabolicity and strain-induced valence-band mixing, were compared with experimental data to identify the optical transitions between quantized states in the wells. We found good agreement between theory and experiment.
Low-temperature silicon homoepitaxy by rf sputtering
Authors: Miyazaki Takayuki , Adachi Sadao
Published: July 2002

We have shown that silicon homoepitaxial growth can be achieved by conventional rf diode sputtering at a low growth temperature (Ts∼400 °C). Spectroscopic ellipsometry and the electron channeling pattern are presented to show that the film deposited on a HF-treated substrate is a single-crystalline film while the film deposited without HF treatment is a polycrystalline film. Ar-discharge cleaning of the substrate is also found to greatly improve the surface morphology of the deposited films.
Calculation of stresses in InxGa1−xAs/InP strained multilayer heterostructures
Authors: Nakajima Kazuo
Published: July 2002

A theoretical model was proposed to calculate the stress in the multilayer heterostructures such as multiple quantum well and strained quantum well structures. The model was derived under consideration of the difference between crystalline parameters such as the lattice constant and thermal expansion coefficient of the composed crystal layers. In this model, each composed crystal layer is divided into many imaginary thin layers. The face force and strain balance was considered over all the imaginary thin layers with coherent interfaces. Using this model, the stress in the lattice-matched InP/In0.53Ga0.47As and strained InP/In0.82Ga0.18As multilayer heterostructures was calculated at 600 °C. In the multilayer, the compressive stress in the In0.53Ga0.47As and In0.82Ga0.18As layers is always larger than the tensile stress in the InP layers. The stress in the InxGa1−xAs layers decreases as the thickness of the InxGa1−xAs layers increases, and it increases by adding InP thick layers on the one side or both sides of the multilayer. The tangential and perpendicular lattice constants in the multilayer were calculated using this model. The perturbation of the InP lattice becomes smaller and that of the InGaAs lattice becomes larger by adding the thick InP layers. It is found from these results that the total stress at the InP/InGaAs heterointerface depends on only the lattice misfit, but the share of the total stress depends strongly on the structure.
Surface-plasmon-induced contrast in scanning tunneling microscopy
Authors: Rücker Markus , Knoll Wolfgang , Rabe Jürgen P.
Published: July 2002

The interaction of plasmon surface polaritons (PSPs) with the tunneling junction of a scanning tunneling microscope (STM) has been investigated by measuring the current response to a modulated plasmon excitation. From the dependence on modulation frequency, bias voltage, steady-state current, and metal film structure it is concluded that it originates mainly from PSP-induced heating and the associated expansion of the tunneling tip. The modulated current signal (or the equivalently modulated z-piezo voltage after the feedback loop) can be recorded while scanning the surface by STM. The resulting PSP maps reflect the polycrystalline metal film structure on the scale of a few nanometers convoluted to some extent with the tip shape and the PSP field distribution. The latter can be used to differentiate between different metal films on the micrometer scale.
Magnetic interaction between Co layers and Pd layers in Co/Pd multilayers
Authors: Miura Kaoru , Kimura Hitoshi , Imanaga Syunji , Hayafuji Yoshinori
Published: July 2002

The magnetic moment of Co/Pd multilayers where the Co layer is one layer thick has been calculated using the discrete variational-Xα cluster method. The moment of the multilayer per Co atom as a function of the number of Pd layers (nPd) shows a maximum at nPd=3. Moreover, it is found that the magnetic moment at the central Pd layer is induced antiferromagnetically for nPd=5. We conclude that the Ruderman–Kittel–Kasuya–Yosida interaction is dominant, rather than the d-d interaction between the central Pd layer and the Co layers, while the d-d interaction is dominant between the other Pd layers and the Co layers.
Interdiffusion in diluted magnetic PbTe/Pb1−xMnxTe quantum well structures
Authors: Krenn H. , Koppensteiner E. , Holzinger A. , Voiticek A. , Bauer G. , Clemens H.
Published: July 2002

An x-ray diffraction study of the growth of Pb1−xMnxTe/PbTe (x=0.022–0.037) multi-quantum well (MQW) structures of period lengths 205–950 Å is presented. In order to obtain a reliable Mn diffusion coefficient at the growth temperature T=270 °C, computed diffusion profiles are used as input for the calculation of diffractograms using kinematical diffraction theory. The satellites of the symmetric (222) Bragg reflections are compared to measurements. The analysis is based on a numerical solution of the diffusion equation which considers explicitly the effect that the boundary at the surface of the growing epitaxial layer moves during growth. Thus for the as-grown MQWs Mn interdiffusion is considered to occur already during the growth process. In addition, the Mn diffusion profiles of one sample which is isothermally annealed 4, 16, and 64 min at T=270 °C, are simulated using fixed boundary conditions. From comparison to both boundary conditions, the Mn diffusion coefficient of 8×10−17 cm2/s at T=270 °C is determined within an accuracy of a factor two. The results are well adapted to our special growth conditions, but the numerical model is applicable in generality. It uses only two essential input parameters: the diffusion coefficient and the growth rate.
Rapid thermal annealing characteristics of bulk AlInAs/InP and AlInAs/GaInAs/InP high electron mobility transistor structures with planar silicon doping
Authors: Kiziloḡlu Kürşad , Hashemi Majid M. , Yin Lie-Wei , Li Yuan Jing , Petroff Pierre M. , Mishra Umesh K. , Brown April S.
Published: July 2002

The effects of high temperature rapid thermal annealing processes on carrier concentration and mobility of bulk AlInAs and AlInAs/GaInAs high electron mobility transistor structures with planar Si doping are studied. At annealing temperatures of 700 °C and 800 °C, slight reduction in mobilities and carrier concentration are observed in samples annealed with a Si3N4 cap or GaAs pieces in close proximity. The reduction in mobility is thought to be due to enhanced diffusion of the donor Si atoms towards the two-dimensional electron gas channel. Preferential vacancy enhanced diffusion of Si atoms towards the surface is projected to be responsible for the loss in carrier concentration. At these annealing temperatures, the reduction in mobility in the samples annealed with SiO2 capping is more pronounced, and is as high as 80% at the measurement temperature of 15 K. This behavior is attributed to the outdiffusion of Ga and In atoms into the oxide thereby creating vacancies and resulting in interface mixing. Reduction in mobility and carrier concentration are much more substantial in the 900 °C anneals done with Si3N4 cap and GaAs pieces in close proximity. This indicates the destruction of the heterostructure integrity of the AlInAs/GaInAs interface. For the particular anneal with a SiO2 cap at this temperature, the carrier concentration increases above its reference value due to effective doping of the ternary material by the back-diffusing Si atoms from the SiO2 cap.
Interface localized states in coupled superlattices
Authors: Ihm G. , Noh S. K. , Falk M. L. , Lim K. Y.
Published: July 2002

A method of evaluating interface states for coupled superlattices has been developed. Explicit solutions for these states have been obtained within the framework of the Kronig–Penney model for the case of two semiconductor superlattices coupled by a tunnel barrier. The interface states are shown to be largely dependent on the coupling strength if the constituent superlattices have identical structure parameters, while insensitive to the tunnel barrier thickness if their structure parameters differ. In the latter case, electrons behave as if the system were composed of two decoupled semi-infinite superlattices.
Variation of the spin density at E
F with oxygen adsorption: Fe(110) vs O/Fe(110)
Authors: Wu Ruqian , Freeman A. J.
Published: July 2002

The spin densities at the Fermi energy for the clean and oxygen adsorbed Fe(110) surfaces calculated using the full potential linearized augmented plane-wave method are found to differ substantially (in sign) from the energy-integrated total spin density previously given in the literature. The energy-sliced spin density becomes negative in the Fe(110) surface-vacuum region despite the enhancement of the surface magnetic moment and is reversed in sign, to positive, by the addition of a p(1×1) oxygen overlayer. Both results have been observed in a recent experiment using spin-polarized metastable atom deexcitation spectroscopy.
Low-temperature photochromic response of gallium-doped bismuth germanium oxide
Authors: Hart D. W. , Hunt C. A. , Martin J. J.
Published: July 2002

Gallium is one of several dopants that electronically compensate the deep donor responsible for the yellow coloration observed in bismuth germanium oxide (BGO). A series of Ga-doped BGO crystals were grown using the Czochralski method to confirm the formation of a trapped hole center following exposure to near band edge light at low temperatures. The deep donor absorption shoulder disappeared for Ga concentrations in the 3%–4% (melt) range. Excitation at 10 K with 3.3 eV light produces photochromic absorption bands. For low gallium concentrations the photochromic bands were the same as the ones observed in undoped BGO. For concentrations of 4% or more the photoinduced absorption consisted of overlapping bands at 1.1, 1.55, and 2.45 eV. These bands grew uniformly with exposure time and annealed together in the 100–120 K temperature range. By comparison with earlier results on Al-doped BGO and bismuth silicon oxide (BSO), we believe that the 1.1 eV band in BGO:Ga and the 1.0 eV band in BGO:Al (and BSO:Al) are due to trapped hole centers ([GaO4]0 and [AlO4]0, respectively).