bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of arsenic diffusion in Si1−xGex formed by epitaxy and Ge+ implantation
Authors: Mitchell M. J. , Ashburn P. , Bonar J. M. , Hemment P. L. F.
Published: April 2003

A comparison is made of arsenic diffusion in Si0.95Ge0.05 produced by epitaxy and ion beam synthesis using a 2×1016 cm−2 Ge+ implant into silicon. The arsenic diffusion depth at 1025 °C in the Si0.95 Ge0.05 epitaxy sample is enhanced by a factor of 1.26 compared with a similar Si control sample and by a factor of 1.30 in the ion beam synthesized sample. The arsenic diffusion in the Si0.95 Ge0.05 epitaxy sample is modeled by increasing the arsenic diffusion coefficient from the Si value of 1.92×10−15 to 5.15×10−15 cm2 s−1, and in the ion beam synthesized sample by using the same diffusion coefficient of 5.15×10−15 cm2 s−1 and increasing the “plus one” factor in the transient enhanced diffusion model from 0.01 to 1.5. Arsenic diffusion in a silicon sample implanted with 2×1015 cm−2 Si+ can be modeled using the same plus one factor of 1.5, thereby demonstrating the consistency of the modeling.
A single asperity study of Au/Au electrical contacts
Authors: Tringe J. W. , Uhlman T. A. , Oliver A. C. , Houston J. E.
Published: April 2003

Interfacial force microscopy (IFM) is used to measure the electrical contact properties of electroplated gold thin films of the type used in microelectromechanical system relays. Force and current levels consistent with those present in metal-metal contact switches are examined in an atmospheric-pressure, dry-nitrogen ambient at room temperature, and the nature of a nonmetallic contamination layer which limits contact resistance and lifetime is explicitly examined mechanically, electrically and chemically. The electrical and mechanical properties of the contamination layer on the gold substrate are observed by IFM both before and after being exposed to ozone for an extended period of time. The contamination film is characterized by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, and found to consist mostly of hydrocarbons; the film remains relatively stable in both composition and thickness following ozonation. However, some subtle chemical changes in the contamination layer induced by the ozonation process are found to profoundly affect the electrical properties of the gold-gold contact, reducing the resistance by more than 3 orders of magnitude and considerably reducing variability in the contact resistance between contact events. These results clearly demonstrate the critical role both positive and negative of the latent contamination present on the contact surfaces.
Two-dimensional growth of high-quality strontium titanate thin films on Si
Authors: Li H. , Hu X. , Wei Y. , Yu Z. , Zhang X. , Droopad R. , Demkov A. A. , Edwards J. , Moore K. , Ooms W. , Kulik J. , Fejes P.
Published: April 2003

Most semiconductor materials such as Si, Ge, and GaAs are subject to oxidation when exposed to oxidants. This results in difficulties in the heterointegration of epitaxial oxides on these semiconductors. Even though certain oxides may be thermodynamically stable when placed in contact with semiconductors, direct epitaxy of these oxides encounters kinetic difficulties due to the loss of epitaxy caused by the formation of an amorphous oxide at the interface. In this article, we address some important issues on the heteroepitaxy of oxides on semiconductors and show a stepped growth method that utilizes the kinetic characteristics of the growth process to suppress the oxidation of the substrate surface and thereby achieve oxide films with a high degree of crystallinity. The epitaxy of high-quality SrTiO3 (STO) thin films directly on Si was achieved. The chemical and structural properties of the STO/Si interface were evaluated in situ using reflection high-energy electron diffraction, x-ray photoelectron spectroscopy, and scanning tunneling microscopy, and ex situ using transmission electron microscopy and electron energy loss spectroscopy.
Polymer electrophosphorescent devices utilizing a ladder-type poly(para-phenylene) host
Authors: Yang X. H. , Neher D. , Scherf U. , Bagnich S. A. , Bässler H.
Published: April 2003

The properties of light-emitting diodes based on the electrophorescent platinum-porphyrin dye PtOEP blended into a ladder-type poly(para-phenylene) (LPPP) polymer host are presented. Due to the small difference between the highest occupied molecular orbital (HOMO) levels of the guest and the host, the operating voltage of single layer devices is almost independent of the dopant concentration in clear contrast to what has been observed in the case of wide-band gap host polymers. However, the efficiency and the color purity of these single-layer devices is quite poor, which can be attributed to the weak trapping of carriers on the phosphorescent dye. Incorporating an electron-transporting/hole-blocking layer greatly increases the efficiency of the devices but at the same time emission from the host becomes more significant. Adding a hole-transporting/electron-blocking layer further increases the efficiency of the devices. Pure red emission is, however, only obtained if the HOMO of the hole-transporting layer is well above that of the LPPP host. This effect is interpreted in terms of a direct injection of holes from the hole-transporting layer into the HOMO level of the dopant. Further, prompt fluorescence and phosphorescence studies indicate that energy transfer from LPPP singlet excitons contributes to the emission from the dopant, but that the efficiency of singlet exciton transfer is too small to explain the almost complete absence of host emission in the electroluminescence spectra of single- and three-layer devices.
Negative high-frequency differential conductivity in semiconductor superlattices
Authors: Romanov Yuriy A. , Mourokh Lev G. , Horing Norman J. M.
Published: April 2003

We examine the high-frequency differential conductivity response properties of semiconductor superlattices having various miniband dispersion laws. Our analysis shows that the anharmonicity of Bloch oscillations (beyond tight-binding approximation) leads to the occurrence of negative high-frequency differential conductivity at frequency multiples of the Bloch frequency. This effect can arise even in regions of positive static differential conductivity. The influence of strong electron scattering by optic phonons is analyzed. To achieve terahertz field amplification, we propose employing structures having minibands with effective electron mass that decreases as the electron energy increases.
Scattering of cylindrical thermal waves in fiber composites: In-plane thermal diffusivity
Authors: Salazar A. , Sánchez-Lavega A. , Celorrio R.
Published: April 2003

In this article, we present a general solution for the ac temperature field of an opaque material containing aligned subsurface cylinders produced by a modulated line illumination parallel to the cylinders. This work completes the case of plane illumination treated in a previous paper [J. M. Terrón, A. Salazar, and A. Sánchez-Lavega, J. Appl. Phys. 91, 1087 (2002)]. The model includes the multiple scattering effects suffered by the cylindrical thermal wave, generated by the line heat source, when interacting with the cylinders and with the sample surfaces. Numerical calculations illustrate the effect of buried cylinders on the sample surface temperature. This model is then used to calculate the in-plane effective thermal diffusivity of unidirectionally fiber-reinforced composites. We also calculate the decrease of this effective thermal diffusivity as a consequence of the presence of a thermal resistance between fibers and matrix. Experimental measurements using modulated photothermal techniques on calibrated samples confirm the validity of the results.
Raman studies of phosphorus incorporation in GaN1−xPx alloys
Authors: Chen D. J. , Shen B. , Zhang K. X. , Zhang R. , Shi Y. , Zheng Y. D. , Li Z. F. , Lu W.
Published: April 2003

Raman spectra for a series of high phosphorus compositional GaN1−xPx alloys, with phosphorus composition ratio up to 15%, grown by means of light-radiation heating, low-pressure metalorganic chemical vapor deposition have been investigated. The Raman spectra of GaN1−xPx alloys, recorded in backscattering geometry, exhibit four vibrational modes at 256, 314, 377, and 428 cm−1 compared with an undoped GaN sample. Those modes are assigned to the so-called quasilocal mode induced by P in GaN, disorder-activated scattering, and gap modes related to the Ga–P bond vibrations, respectively. The frequency of the A1(LO) mode is found to redshift, but the trend of the redshift weakens with larger x. This redshift is attributed to the effects of alloying and strain. Furthermore, the weakening trend of redshift with larger x illuminates that substitutional P atoms in the GaN1−xPx alloy are gradually getting saturated.
Exact solution for cooling of electronics using constructal theory
Authors: Ghodoossi Lotfollah , Eğrican Nilüfer
Published: April 2003

This article deals with the constructal-theory based solution for conductive cooling of electronics. The problem falls in the category of a more general “area to point” flow problem. Heat generated in a fixed area is to be discharged to a heat sink located on the border of the heat generating area through relatively high conductive link(s). This will maintain a limited temperature difference between the hot spot inside the heat generating area and the heat sink. Constructal-theory based solution in this article is supported by and matched with an analytical and exact heat transfer analysis of the physical problem. The solution procedure starts with heat transfer analysis and geometric optimization of the smallest heat generating area. The assembly of optimized smallest areas in a fixed but larger heat generating area by introducing a high conductive link and geometric optimization of the area leads to achieve the goal of conductive cooling of a larger area. Sequence of assembly of optimized areas in a relatively larger area and geometric optimization of this area is continued until the required area size to be cooled is obtained. The process of assembly and optimization steps leads to the formation of a tree-network of high conductive links inside the heat generating area. Along with geometric optimization of a heat generating area in each step, the tree-network of high conductive links is optimized with respect to high conductive material allocation in the heat generating area as well.
Competition between ferroelectric and semiconductor properties in Pb(Zr0.65Ti0.35)O3 thin films deposited by sol–gel
Authors: Boerasu I. , Pintilie L. , Pereira M. , Vasilevskiy M. I. , Gomes M. J. M.
Published: April 2003

Asymmetric metal–ferroelectric–metal (MFM) structures were manufactured by sol–gel deposition of a lead zirconate-titanate (PZT with Zr/Ti ratio 65/35) film on Pt-coated Si, with a Au top electrode. The average remnant polarization of 9 μC/cm2 and the coercive field of 39 kV/cm were obtained from the hysteresis loop measurements. A detailed analysis of the polarization–electric field (P–E), capacitance–voltage (C–V), and current–voltage (I–V) measurement results allowed us to estimate the near-electrode space-charge region thickness (roughly half of the film thickness at zero voltage), net doping concentration (around 1018 cm−3), built-in potential (in the 0.4–0.8 V range, depending on the injecting electrode), and dynamic dielectric constant (5.2). The current logarithm–voltage dependence for the field-enhanced Schottky emission obeys a “1/4” law. The spectral distribution of the short circuit current measured under continuous light illumination in the 290–800 nm range exhibits a cutoff wavelength at 370 nm and a maximum sensitivity at about 340 nm. The estimated band-gap energy of the PZT material is 3.35 eV. The MFM structure is discussed in terms of two back-to-back Schottky diodes with a ferroelectric material in between. It is concluded that the semiconductor properties of the films are not negligible and, in certain conditions, are dominating over the ferroelectric ones.
Microstructure evolution analysis in Co/Cu layers during the annealing process
Authors: Su Ming-Horng , Hwang Chi-Chuan , Chang Jee-Gong , Wang Shi-Hao
Published: April 2003

By means of molecular dynamics simulation, this article investigates the annealing process of a Co/Cu two-layer structure used for giant magnetoresistance applications. The many-body, tight-binding potential method is used to model the interatomic force which acts between the atoms, and the Langevin technique is incorporated into the motion equation such that the thermal control layer is maintained at a constant equilibrium temperature. The issues considered within this article include the annealing Cu surface roughness and Co/Cu interfacial roughness, the annealing morphology, the annealing microstructure, a comparison of Co and Cu migration abilities, and the extent of Co and Cu interdiffusion. The results of the present study indicate that the annealing temperature required to cause Co atom migration is greater than that which is required for Cu atoms. Consequently, once the annealing temperature exceeds a certain threshold value, a significant change in the Cu surface roughness will be observed before there is any obvious change in the Co/Cu interfacial roughness. It is also noted that the Cu film microstructure adopts a disordered state earlier in the annealing process when the annealing temperature is higher. Finally, it is determined that Co/Cu interdiffusion occurs at elevated annealing temperatures. The degree of interdiffusion becomes more pronounced as the annealing temperature increases, and in extreme cases, it is observed that Co atoms may diffuse to such an extent that they even appear on the Cu film surface.
Density and nitrogen content of ultrathin silicon oxide gate films grown using in situ pyrolytic-gas passivation
Authors: Yamada Hiroshi
Published: April 2003

The density and nitrogen content of 3.5–6.5-nm-thick silicon oxide films grown by ultradry oxidation using the recently proposed in situ passivation method that uses a little pyrolytic N2O gas were determined by a charged-particle activation analysis. It was confirmed that the density increases with decreasing humidity during oxidation as well as before and exhibits a remarkable increase only at a humidity of less than 1 ppb. The humidity dependence is almost the same as that found in our previous reports, where we obtained the electrical characteristics related to the condition of the oxide–silicon substrate interface, such as the potential barrier height energy and the time-dependent dielectric breakdown lifetime. Although the binding states of the N-related bonds are still unclear, it appears that a small microscopic structural change may be brought about near the oxide–silicon substrate interface.
Spatial filtering using media with indefinite permittivity and permeability tensors
Authors: Schurig D. , Smith D. R.
Published: April 2003

Bilayers of media for which the permittivity and permeability tensors are indefinite—that is, not all principle elements possess the same sign—can be used to construct low-, high-, and bandpass spatial filters. These filters possess sharp adjustable roll-offs, and can operate both below and above free-space cutoff to select specific spatial variation components or beam angles from a source or image.
Vibrational energy relaxation of aqueous azide ion confined in reverse micelles
Authors: Zhong Q. , Baronavski A. P. , Owrutsky J. C.
Published: April 2003

Vibrational energy relaxation (VER) times have been measured by ultrafast infrared spectroscopy near 2000 cm−1 for the antisymmetric stretching ν3 band of azide ion in water pools of nonionic reverse micelles (RM). The water pool radii were varied in the 1–3 nm range by adjusting the water-to-surfactant molar ratio, ω=[H2O]/[surfactant]. Compared to the value measured in this work for bulk water (0.81±0.06 ps), the VER decay times are about three times longer (2.5±0.2 ps) for the smallest RM studied (ω=1) and become shorter with increasing ω and RM size but do not reach the bulk value at the largest ω studied. Solvent shifts of the azide vibrational band in RMs have been previously reported [Langmuir 18, 7401 (2002)], and in a manner similar to the VER rates, tend toward the bulk water value with increasing ω. Studies of the VER dynamics of azide ion in RMs are used to investigate confinement effects on solvation and to explore the effects of continuously modifying the solute–solvent interaction by varying ω in the RMs.
Monte Carlo analysis of GaN-based Gunn oscillators for microwave power generation
Authors: Joshi R. P. , Sridhara V. , Shah P. , del Rosario R. D.
Published: April 2003

Monte Carlo studies of transferred electron oscillators based on bulk wurtzite GaN are presented. Two structures have been examined: (i) devices with the conventional single notch structure, and (ii) repetitive structures with serial segments to fashion a “multiple domain” device. Wurtzite material has been chosen because of the higher drift velocity and because analytical expressions for the band structure have recently become available. Performance parameters of interest such as the operating frequency, output power, and conversion efficiency are calculated. Variations due to changes in temperature, biasing voltage, and device length are also included. It is shown that multidomain Gunn diodes can lead to significant improvements in output power over conventional, single-transit structure, and so such multiple GaN diodes merit serious experimental study.
Study of the origin of luminescence in high indium composition InGaN/GaN quantum wells
Authors: Cheong M. G. , Liu C. , Choi H. W. , Lee B. K. , Suh E.-K. , Lee H. J.
Published: April 2003

We report the emission properties of various InGaN/GaN quantum wells. The photoluminescence of InxGa1−xN/GaN (x>20%) quantum wells under varying external bias voltages has been investigated. A redshift of the photoluminescence peak position and decrease of photoluminescence peak intensity with an increase in voltage were observed regardless of the direction of external bias applied. For the sample in which the quantum dot-like region is abundant in quantum well layers, the peak position and intensity were not influenced much by the externally applied bias voltage. The results show that the origin of strong emission from InGaN/GaN quantum wells can be attributed to exciton localization in the quantum dot-like region and they are consistent with those of high-resolution transmission electron microscopy.
Contactless measurement of bulk lifetime and surface recombination velocity in silicon wafers
Authors: Palais O. , Arcari A.
Published: April 2003

A method based on two phase shift measurements at two different modulation frequencies is proposed to determine simultaneously the actual bulk lifetime τb and the surface recombination velocity S in silicon wafers. Such a determination works, irrespectively, of the physical state of the surface or the passivation level, and is based on a microwave contactless technique, which allows mapping of τb and S with a spatial resolution of 50 μm.
Diffusion of deuterium (hydrogen) in previously hydrogenated (deuterated) III–V semiconductors
Authors: Theys Bertrand , Jomard François
Published: April 2003

Hydrogenated (deuterated) C-doped GaAs and Zn-doped InP layers have been exposed to a deuterium (hydrogen) plasma. Diffusion profiles have been measured by secondary ion mass spectroscopy and compared to those obtained after exposure of as-grown (without any previous plasma treatment) samples in identical plasma conditions. It is shown that a partial substitution of D (H) for H (D) occurs as a consequence of the breaking of the original H (D)–acceptor complexes. It is also demonstrated that, after the breaking, released H (D) species are trapped in the vicinity of the acceptor with which they were originally forming a complex.
Magnetic anisotropy and Mössbauer spectra in disordered lithium–zinc ferrites
Authors: Xu Z. C.
Published: April 2003

Single crystals of Zn doped lithium ferrite have been obtained by an improved flux method. The magnetic anisotropy and Mössbauer spectra of the system (Fe1−x3+Znx2+)[Fe(3+x)/23+Li(1−x)/2+]O4 with 0⩽x⩽0.5 have been investigated. According to the single-ion model, the magnetic anisotropy constant K1 depends mainly on the contribution of Fe3+ ions on tetrahedral sites and has always negative values (K1A≈−0.038 cm−1/ion; K1B≈0.0047 cm−1/ion for x=0.11–0.5) at room temperature. With increasing zinc content, the hyperfine magnetic field H on A sites is gradually decreased. The effect is presumably due to the spin canting and to the relaxation of the octahedral Fe3+ ions. The average reduction of the hyperfine magnetic field on B sites at 300 K is 22±2 kOe per Zn2+ nearest neighbor.
Theory of the structural phases of group 5B–6B metals and their transport properties
Authors: Nnolim Neme O. , Tyson Trevor A. , Axe Lisa
Published: April 2003

In order to predict the stable and metastable phases of the bcc metals in the block of the Periodic Table defined by groups 5B–6B and periods 4–6, as well as the structure dependence of their transport properties, we have performed full-potential linearized augmented plane wave computations of the total energies per unit cell as functions of the unit cell volume and the c/a ratio. In all cases, a metastable body centered tetragonal (bct) phase was predicted from the calculations. The energy barrier separating the calculated stable and metastable phases ranged from 0.09 eV/cell (vanadium) to 0.38 eV/cell (tungsten). The trends in resistivity as a function of structure and atomic number are discussed in terms of a model of electron transport in metals. Theoretical calculations of the electrical resistivity and other transport properties show that bct phases derived from the early group 5B elements are more conductive than the corresponding bcc phases, while bct phases formed from the early group 6B elements are less conductive than the corresponding bcc phases. Special attention is paid to the phases of tantalum where we show that the frequently observed β phase does not result from simple tetragonal distortions of bcc tantalum.
Gaussian random field models of aerogels
Authors: Quintanilla J. , Reidy R. F. , Gorman B. P. , Mueller D. W.
Published: April 2003

A model capable of predicting pore characteristics and rendering representative images of porous materials is described. A long-term goal is to discriminate between open and closed porosities. Aerogels are modeled by intersecting excursion sets of two independent Gaussian random fields. The parameters of these fields are obtained by matching small-angle neutron scattering data with the scattering function for the model. The chord-length probability density functions are then computed for the model, which contain partial clustering information for the aerogels. Visualizations of this model are performed and compared to electron microscopy images and gas adsorption pore size distributions.
Single photon emission from individual GaAs quantum dots
Authors: Hours J. , Varoutsis S. , Gallart M. , Bloch J. , Robert-Philip I. , Cavanna A. , Abram I. , Laruelle F. , Gérard J. M.
Published: April 2003

We report on photon correlation measurements on a single quantum dot formed at fluctuations of the interface of a GaAs/GaAlAs quantum well. We demonstrate that under pulsed nonresonant excitation, the quantum dot emits a single photon per pulse. This shows that after the photon emission, there is no refill of the quantum dot by the nearby two-dimensional reservoir of delocalized states. The possibility of delivering Fourier transform limited single photons makes this system a good candidate for exciton- and photon-based quantum information processing schemes.
Optimization of antiferroelectric liquid crystal devices at the degeneration point
Authors: Abdulhalim I.
Published: April 2003

Antiferroelectric liquid crystals (AFLCs) with 45° tilt angle have been shown recently [K. D’hav’e, Per Rudquist, S. T. Lagerwall, H. Pauwels, W. Drzewinski, and R. Dabrowski, Appl. Phys. Lett. 76, 3528 (2000)] to exhibit true dark state between crossed polarizers at normal incidence. We show that this situation is possible at any incidence or any tilt angle depending on the principal dielectric constants ε1–3. In particular, when the deviation of ε1 from the average (ε2+ε3)/2 is small, the medium behaves nearly as isotropic medium. Hence using this class of AFLC materials it will be possible to build full color, high-resolution displays with large viewing angular range. The optimum viewing angular cone will be obtained when the degeneracy point corresponds to 45° incidence angle and tilt angle anywhere from 0 to 45° depending on the values of the principal dielectric constants and, in particular, on the biaxiality. For the uniaxial case this angle is practically 41°–42° without dependence on the optical anisotropy.
Origin of dissipative losses in negative index of refraction materials
Authors: Greegor R. B. , Parazzoli C. G. , Li K. , Tanielian M. H.
Published: April 2003

Negative index of refraction materials have been postulated for many years but have only recently been realized in practice. In the microwave region these materials are constructed of rings and wires deposited on a dielectric substrate to form a unit cell. We have constructed, experimentally characterized, and simulated several of these structures operating in the 10–16 GHz range. The origin of the dissipative losses has been identified and effective schemes to reduce them devised and implemented. Numerical simulation and experimental verification shows that losses in negative index of refraction materials can be significantly reduced by minimizing the underlying losses of the constituent materials.
Influence of counter-ion-induced disorder in DNA conduction
Authors: Adessi Ch. , Anantram M. P.
Published: April 2003

Disorder along a DNA strand due to nonuniformity associated with the counter ion type and location, and in rise and twist are investigated using density functional theory. We then model the conductance through a poly(G) DNA strand by including the influence of disorder. We show that the conductance drops by a few orders of magnitude between typical lengths of 10 and 100 nm. Such a decrease occurs with on-site potential disorder that is larger than 100 meV.
Highly efficient molecular organic light-emitting diodes based on exciplex emission
Authors: Palilis L. C. , Mäkinen A. J. , Uchida M. , Kafafi Z. H.
Published: April 2003

Highly efficient exciplex emission is observed from molecular organic light-emitting diodes (MOLEDs) based on silole derivatives as emissive and electron transport materials, and a hole transporting amine derivative. A silole derivative, 2,5-di-(3-biphenyl)-1,1-dimethyl- 3,4-diphenylsilacyclopentadiene (PPSPP), which shows blue fluorescence (476 nm) with a high solid-state photoluminescence quantum yield of 85% was used as the emitter. Another silole derivative, 2,5-bis-(2′,2″-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene which exhibits high electron mobility, was used as the electron transport material. MOLEDs using these two siloles and N,N′-diphenyl-N,N′-(2-napthyl)-(1,1′-phenyl)-4,4′-diamine (NPB) as the hole transporter show electroluminescence (EL) emission centered at 495 nm. This red-shifted EL band relative to the blue fluorescence of PPSPP is assigned to a NPB:PPSPP exciplex. An operating voltage of 4.5 V was measured at 100 cd/m2 and an EL quantum efficiency of 3.4% was achieved at 100 A/m2.
Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers
Authors: Ku K. C. , Potashnik S. J. , Wang R. F. , Chun S. H. , Schiffer P. , Samarth N. , Seong M. J. , Mascarenhas A. , Johnston-Halperin E. , Myers R. C. , Gossard A. C. , Awschalom D. D.
Published: April 2003

We report Curie temperatures up to 150 K in annealed Ga1−xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of Ga1−xMnxAs epilayers may be important in determining their physical properties.
Fermi level on hydrogen terminated diamond surfaces
Authors: Rezek B. , Sauerer C. , Nebel C. E. , Stutzmann M. , Ristein J. , Ley L. , Snidero E. , Bergonzo P.
Published: April 2003

Atomic force microscopy and Kelvin probe experiments are applied to characterize hydrogen terminated patterns contacted with gold and aluminum on (100) diamond surfaces. On hydrogen terminated diamond the work function of 4.9 eV is detected, with an accuracy of about 0.1 eV. Taking into account the negative electron affinity of −1.3 eV and a band gap of 5.5 eV the Fermi energy is 0.7 eV deep in the valence band. Illumination of the sample results in a shift of the surface Fermi level by as much as 0.2 eV. This is attributed to a surface photovoltage effect.
Efficient red electroluminescence from organic devices using dye-doped rare earth complexes
Authors: Hong Z. R. , Lee C. S. , Lee S. T. , Li W. L. , Liu S. Y.
Published: April 2003

Using rare earth complexes with electron-transporting properties as host materials, and 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as a dopant, bilayer doped electroluminescent (EL) devices with efficient red light emission were fabricated. When a europium complex was adopted, the EL spectrum consisted of emissions from DCJTB and Eu3+ ions. At optimal dopant concentration, an EL efficiency of 5.7 cd/A at 0.04 mA/cm2 was observed. Although the EL efficiency decreased with an increase in current density, it remained higher than 2.0 cd/A with brightness of 347.0 cd/m2 at 5.7 V bias. DCJTB and Eu3+ ions collected the energy of singlet and triplet excitons, respectively, and then gave rise to pure red color emission, suggesting a promising way by which to utilize both singlet and triplet excited states in EL devices.
Temperature-dependent magnetic force microscopy investigation of epitaxial MnAs films on GaAs(001)
Authors: Plake T. , Hesjedal T. , Mohanty J. , Kästner M. , Däweritz L. , Ploog K. H.
Published: April 2003

We present variable-temperature magnetic force microscopy (VT-MFM) studies of epitaxially grown MnAs films on GaAs(001). Around a critical temperature of Tc=40 °C, the MnAs film undergoes a first order structural phase transition. Due to the strain involved, ferromagnetic α-MnAs and paramagnetic β-MnAs phases coexist as stripes along MnAs[0001]. The dimensions of the α-phase change from isolated dots at higher temperatures to well ordered stripes at lower temperatures. VT-MFM allows a close look at the evolution of domain patterns of MnAs micromagnets.
Fine control of L10 ordering and grain growth kinetics by C doping in FePt films
Authors: Ko Hyun Seok , Perumal A. , Shin Sung-Chul
Published: April 2003

We present the results on highly anisotropic oriented face-centered-tetragonal ordered equiatomic FePt thin films embedded in a C matrix prepared by cosputtering technique on MgO (100) substrates at 400 °C. We have found that doping of C in FePt alloy films is an effective and appropriate way to control the L10 ordering and growth of grain size down to 4 nm. Room-temperature magnetic measurements reveal that perpendicular magnetic anisotropy exists for these films with C concentration up to 25 vol %. These films are very promising and the results are of relevance for ultrahigh-density magnetic recording media.
Accelerated pre-oxidation method for healing progressive electrical short in organic light-emitting devices
Authors: Kim Youngkyoo , Choi Dongkwon , Lim Hyuntaek , Ha Chang-Sik
Published: April 2003

The accelerated pre-oxidation method (APOM) was applied to heal progressive electrical short (PES) in organic light-emitting devices. The APOM applied to the devices showed approximately 100 times lower leakage current and 25% improvement in device efficiency compared to a normal device without the APOM applied. The mean projected lifetime of the APOM devices was more than 2400 h at initial luminance of 100 cd/m2 and 90 °C, whereas the normal device turned off after 15 h of operation at the same temperature due to the PES phenomenon. The best stability at elevated temperature was achieved by using the APOM for the cathode layer.
Time-resolved magnetic domain imaging by x-ray photoemission electron microscopy
Authors: Vogel J. , Kuch W. , Bonfim M. , Camarero J. , Pennec Y. , Offi F. , Fukumoto K. , Kirschner J. , Fontaine A. , Pizzini S.
Published: April 2003

X-ray photoemission electron microscopy (X–PEEM) is a powerful imaging technique that can be used to perform element selective magnetic domain imaging on heterogeneous samples with different magnetic layers, like spin valves and tunnel junctions. We have performed nanosecond time-resolved X–PEEM measurements, on the permalloy layer of a Ni80Fe20 (5 nm)/Cu (10 nm)/Co (5 nm) trilayer deposited on Si(111). We used the pump-probe mode, synchronizing a magnetic pulse from a microcoil with the x-ray photon bunches delivered by the BESSY synchrotron in single bunch mode. Images could be acquired during and after the 20 ns long and 80 Oe high field pulses. The nucleation and subsequent growth of reversed domains in the permalloy could be observed, demonstrating the feasibility of element selective and time-resolved domain imaging using X–PEEM.
Temporal effect of low-temperature ferroelectric behaviors in Pb0.97La0.02(Zr0.60Sn0.30Ti0.10)O3 ceramics
Authors: Chan Wai-Hung , Chen Haydn , Colla Eugene V.
Published: April 2003

Kinetic behaviors of unusual low-temperature ferroelectric properties of Pb0.97La0.02(Zr0.60Sn0.30Ti0.10)O3 ceramic were studied. In a wide temperature range it reveals the antiferroelectric properties with pronounced polarization versus electrical field hysteresis double loops resulting from the field-induced antiferroelectric-to-ferroelectric switching. The reverse switching time is temperature dependent, and below 210 K this time overcomes the reasonable laboratory time scale. Kinetic data were analyzed to yield the activation energy. At as low as 100 K all experimental results suggest that the ground state of the system remains antiferroelectric, thus the time-dependent dielectric properties owe their origin to the kinetic effects.
Determination of radiation exposure history of common materials and computer hardware by using atomic (and magnetic force) microscopy
Authors: Sharma J. , Teter J. P. , Abbundi R. J. , Guardala N. A.
Published: April 2003

Defects produced by ionizing radiation are smaller than a micrometer and are unobservable in an optical microscope. An atomic force microscope was utilized to reveal their counts and structure in common materials like mica, silicon, organic solids, polymers, sugar, quartz, and calcite. A magnetic force microscope has shown the damage of radiation on computer hard disks. The present work shows that exposure to radioactive material leaves a permanent record, which can be read for dosimetric or forensic purposes by using atomic force microcopy on common objects or a magnetic force microscope on magnetic media.
Dielectric functions (1 to 5 eV) of wurtzite MgxZn1−xO (x⩽0.29) thin films
Authors: Schmidt R. , Rheinländer B. , Schubert M. , Spemann D. , Butz T. , Lenzner J. , Kaidashev E. M. , Lorenz M. , Rahm A. , Semmelhack H. C. , Grundmann M.
Published: April 2003

The optical dielectric functions for polarization perpendicular and parallel to the c-axis (optical axis) of pulsed-laser-deposition grown wurtzite MgxZn1−xO (0⩽x⩽0.29) thin films have been determined at room temperature using ellipsometry for photon energies from 1 to 5 eV. The dielectric functions reveal strong excitonic contributions for all Mg concentrations x. The band gap energies (E0A=3.369 eV for ZnO to 4.101 eV for x=0.29) show a remarkable blueshift. The exciton binding energy (61 meV for ZnO) decreases to approximately 50 meV for x≈0.17 and increases to approximately 58 meV for x=0.29. In contrast to ZnO, the MgxZn1−xO alloys are found uniaxial negative below the band gap energy, opposite to previously reported results.
Molecular beam epitaxy growth of GaAs1−xBix
Authors: Tixier S. , Adamcyk M. , Tiedje T. , Francoeur S. , Mascarenhas A. , Wei Peng , Schiettekatte F.
Published: April 2003

GaAs 1−x Bi x epilayers with bismuth concentrations up to x=3.1% were grown on GaAs by molecular beam epitaxy. The Bi content in the films was measured by Rutherford backscattering spectroscopy. X-ray diffraction shows that GaAsBi is pseudomorphically strained to GaAs but that some structural disorder is present in the thick films. The extrapolation of the lattice constant of GaAsBi to the hypothetical zincblende GaBi alloy gives 6.33±0.06 Å. Room-temperature photoluminescence of the GaAsBi epilayers is obtained and a significant redshift in the emission of GaAsBi of ∼84 meV per percent Bi is observed.
Peltier effect in normal metal–insulator–heavy fermion metal junctions
Authors: Goltsev A. V. , Rowe D. M. , Kuznetsov V. L. , Kuznetsova L. A. , Min Gao
Published: April 2003

A theoretical study has been undertaken of the Peltier effect in normal metal–insulator–heavy fermion metal junctions. The results indicate that, at temperatures below the Kondo temperature, such junctions can be used as electronic microrefrigerators to cool the normal metal electrode and are several times more efficient in cooling than the normal metal–heavy fermion metal junctions.
Organic molecular films on gold versus conducting polymer: Influence of injection barrier height and morphology on current–voltage characteristics
Authors: Koch N. , Elschner A. , Schwartz J. , Kahn A.
Published: April 2003

The current–voltage characteristics I(V) of model organic devices are studied under ultra-high-vacuum conditions. Active materials are N,N′-bis-(1-naphthyl)-N,N′-diphen-yl1-1,1-biphenyl1-4,4′-diamine (α-NPD) and pentacene, electrode materials are polycrystalline Au and the conductive polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). Despite a similar work function of electrode material surfaces (∼5 eV), hole injection from PEDOT/PSS is significantly more efficient than from Au, due to a smaller hole injection barrier. Hole injection characteristics from Au electrodes for devices made from α-NPD are independent of deposition sequence and substrate used. Pentacene devices exhibit serious asymmetries in that respect. These are caused by a strong dependence of morphology and preferred molecular orientation on the substrate for the crystalline material.
In situ preparation and interface characterization of TiO2/Cu2S heterointerface
Authors: Liu Guangming , Schulmeyer T. , Thissen A. , Klein A. , Jaegermann W.
Published: April 2003

The electronic structures and interface properties of the TiO2/Cu2S interface have been in situ studied after each growth step by x-ray and ultraviolet photoelectron spectroscopy. The p-doped Cu2S films (BEVBM=0.1 eV) were grown on the highly n-doped chemical vapor deposition prepared TiO2 (BEVBM=3.4 eV) films by thermal evaporation in a multistep growth procedure. The conduction band offset (0.7 eV), valence band offset (2.9 eV) and interface dipole (0.5 eV) were determined based on the quantitative examination of band bending occurring in the Cu2S films at higher coverage, leading to a staggered energy level configuration.
Reproducible fabrication of metallic silver nanostructures on a Si(111)-(7×7) surface by tip–material transfer of a scanning tunneling microscope
Authors: Fujita D. , Kumakura T.
Published: April 2003

A technique for reproducible fabrication of nanometer-scale silver dots, wires, characters, and figures on a Si(111) surface in ultrahigh vacuum using scanning tunneling microscopy and their electronic properties are presented. This method allows patterns to be formed at will by the computer-controlled application of voltage pulses between a silver-coated tip and the surface. Scanning tunneling spectroscopy shows unambiguous metallic behavior on the nanodots and depletion layer formation in the neighboring region. The dominant mechanism of nanodot formation can be attributed to the spontaneous formation of a point contact due to field-enhanced diffusion of silver atoms to the tip apex.
Zero-bias anomalies in electrochemically fabricated nanojunctions
Authors: Yu L. H. , Natelson D.
Published: April 2003

A streamlined technique for the electrochemical fabrication of metal nanojunctions (MNJs) between lithographically defined electrodes is presented. The first low-temperature transport measurements in such structures reveal suppression of the conductance near zero bias. The size of the zero-bias anomaly (ZBA) depends strongly on the fabrication electrochemistry and the dimensions of the resulting MNJ. We present evidence that the nonperturbative ZBA in atomic-scale junctions is due to a density of states suppression in the leads.
Supercontinuum generation in a highly birefringent microstructured fiber
Authors: Lehtonen M. , Genty G. , Ludvigsen H. , Kaivola M.
Published: April 2003

We present experimental results on supercontinuum generation in a highly birefringent microstructured fiber. We show that such a fiber offers clear advantages for continuum generation over weakly birefringent fibers. In particular, the polarization is preserved along the fiber for all the spectral components. Furthermore, the two eigenpolarizations exhibit different dispersion characteristics, which provide a convenient way of tuning the properties of the generated continuum. We investigate the impact of the pump wavelength and pulse duration on the continuum and use the results to generate an ultrabroadband continuum extending from 400 to 1750 nm.
Temperature-dependent low-frequency vibrational spectra of purine and adenine
Authors: Shen Y. C. , Upadhya P. C. , Linfield E. H. , Davies A. G.
Published: April 2003

Terahertz time-domain spectroscopy has been used to measure the vibrational spectra of polycrystalline purine and adenine over the temperature range 4–290 K. A number of well-resolved absorption peaks were observed in the frequency range 0.2–3.0 THz, which are interpreted as originating from intermolecular vibrational modes mediated by hydrogen bonds. We find that as the temperature is reduced, the observed absorption bands resolve into narrower peaks and some shift towards higher frequencies. We explain the temperature dependence of the spectra by the anharmonicity of the vibrational potentials and give an empirical expression to describe the frequency shift.
Mechanism map for a misfitting film on a viscous substrate
Authors: Sridhar N. , Cox B. N. , Srolovitz D. J.
Published: April 2003

Compressively stressed elastic films bonded to a viscous interlayer that separates the film from the substrate can experience stress relief by one of several mechanisms: buckling, in-plane expansion at free edges, or delamination from free edges. Here, results for edge relaxation and delamination are combined with prior work on buckling to provide a comprehensive map showing how the dominant stress relief and failure mechanisms depend on geometrical and material parameters. The mechanism map is constructed by comparing the characteristic relaxation times for each relaxation mechanism. The map provides the basis for simple engineering design rules to achieve a preferred failure mechanism.
Three-dimensional photonic crystals fabricated by visible light holographic lithography
Authors: Wang X. , Xu J. F. , Su H. M. , Zeng Z. H. , Chen Y. L. , Wang H. Z. , Pang Y. K. , Tam W. Y.
Published: April 2003

In this report, we present three-dimensional photonic crystals fabricated by a four-beam holographic lithography method using visible photoinduced polymerization. High-quality face-centered-cubic single crystals with a large range of polymeric matrix volume fraction were fabricated using optimal conditions obtained from computer simulations. Optical measurements of the crystals showing photonic band-gap-like behavior are presented for different polymeric matrix volume fractions.
Temperature-induced phase separation in chromium films
Authors: Saraf Laxmikant , Wang Chongmin , Engelhard Mark H. , Baer Donald R.
Published: April 2003

Vacuum evaporation of chromium on Si(100) results in an interfacial nanophase layer followed by a mixed grain region. In this work, the mixed grain region in chromium is nearly transformed into a single-phase region, which resulted in a maximum phase separation between the nano-δ-A15 phase and bulk α-bcc chromium. It is achieved by creating a temperature gradient across the chromium–silicon interface by using rapid evaporation at a growth rate of ∼3 μm/min at a source-to-substrate (SS) distance of 2 cm. Separately grown nanophase chromium films at SS distance of 32 cm show a predominant δ-A15 phase. Short-SS-distance rapid evaporation has the potential to produce selective phase separation by combining the temperature gradient with interfacial stress.
Decoration effects as origin of dislocation-related charges in gallium nitride layers investigated by scanning surface potential microscopy
Authors: Krtschil A. , Dadgar A. , Krost A.
Published: April 2003

The electrical charge state of threading dislocations in differently doped GaN is investigated by scanning surface potential microscopy in conjunction with tapping mode atomic force microscopy. The dislocations are found to be either negatively charged or neutral depending on the type of doping atoms in the layers, i.e., Mg acceptors, Si donors, or without intentional doping. The results are interpreted in terms of decoration of the dislocations with other defects resulting in a partial compensation of the core charge by the accumulated charges around.
High performance Co–Zr–B melt-spun ribbons
Authors: Saito Tetsuji
Published: April 2003

Co–Zr–B alloys have been produced by melt-spinning followed by heat treatments. The magnetic properties of the Co–Zr–B melt-spun ribbons depend on the annealing temperature. The maximum energy product of the optimally annealed Co–Zr–B melt-spun ribbons (BH)max=4.74 MGOe, is comparable to those of the hard ferrites. The hard magnetic phase of the Co–Zr–B system alloys is found to be the metastable Co5Zr phase.
Effect of overgrowth temperature on shape, strain, and composition of buried Ge islands deduced from x-ray diffraction
Authors: Stangl J. , Hesse A. , Holý V. , Zhong Z. , Bauer G. , Denker U. , Schmidt O. G.
Published: April 2003

We have investigated a series of samples containing SiGe islands capped at different growth temperatures. A layer of islands formed by deposition of 5 ML of pure Ge was capped with Si, deposited at temperatures of 460, 540, and 630 °C, respectively. The Ge composition profile and the shape of the buried islands are deduced from x-ray diffraction data. While for capping at high substrate temperatures a significant dilution of the Ge content and a flattening of the islands occur, capping at low temperatures maintains a high aspect ratio and a high Ge content of the islands. The maximum in-plane strain in the island remains as high as 0.005 for capping at low temperatures.
Nanoscale electron-beam-stimulated processing
Authors: Rack P. D. , Randolph S. , Deng Y. , Fowlkes J. , Choi Y. , Joy D. C.
Published: April 2003

Electron-beam-stimulated deposition and etching has been investigated as a clean, alternative method for nanoscale selective processing. Depositions using W(CO)6 and hydrocarbon sources have yielded efficient and selective electron-beam deposits. Primarily fluorine-based precursors have been used to etch a variety of materials. Initial results regarding the selective etching of silicon and silicon dioxide suggest that inelastic scattering of the primary electron beam with the gas occurs and is more severe at lower beam energies. The etch rate increases linearly with decreasing electron-beam energy, however, it is not clear if this is due to enhanced primary- or secondary-electron-stimulated processes. Feature sizes as small as 55 nm have been selectively processed.