bluuebunny/crossref_metadata_2025 huggingface dataset:
Cracking a Tough Nut with a big Computer
Authors: Abraham Farid F. , Schneider D. , Land B. , Lifka D. , Skovira J. , Gerner J. , Rosenkrantzt M.
Published: April 2011

We have simulated the dynamical failure of three-dimensional notched solids under tension using molecular dynamics and up to 100 million atoms. We discovered a dynamical brittle-to-ductile transition in the rapid cleavage of rare-gas solids when the crack velocity approaches one-third of the Rayleigh sound speed. At this transition, the crack tip has already begun to roughen on the atomic scale. This suggests that the brittle crack undergoes a dynamic instability which immediately leads to the initiation of plastic failure by the spontaneous emission and proliferation of dislocations and crack arrest.
Dynamics and Stability of Surfactant Coated thin Spreading Films
Authors: Matar Omar K. , Troian Sandra M.
Published: April 2011

Within lubrication theory, we investigate the hydrodynamic stability of a thin surfactant coated liquid film spreading strictly by Marangoni stresses. These stresses are generated along the air-liquid interface because of local variations in surfactant concentration. The evolution equations governing the unperturbed film thickness and surface surfactant concentration admit simple self-similar solutions for rectilinear geometry and global conservation of insoluble surfactant. A linear stability analysis of these self-similar flows within a quasi steady-state approximation (QSSA) yields an eigenvalue problem for a single third-order nonlinear differential equation. The analysis indicates that a thin film driven purely by Marangoni stresses is linearly stable to small perturbations of all wavenumbers. The insights gained from this calculation suggest a flow mechanism that can potentially destabilize the spreading process.
Structure of Polymer Chains Confined in Vycor
Authors: Lal Jyotsana , Sinha Sunil K. , Auvray Loic
Published: April 2011

We observe by Small Angle Neutron Scattering (SANS) the structure of polystyrene chains in semi-dilute solutions confined in model porous medium, Vycor. The size of the free polymer chains in solution is always larger than the pore diameter, 70 Å. The use of a suitable mixture of hydrogenated and deuterated solvents and polymers enables us to directly measure the form factor of one single chain among the others. The penetration of the chain in the porous media is almost complete for the concentration (Φ = 20%) and the range of molecular weights (35000 <M< 800000) used. The measured radius of gyration of confined chains is always smaller than the radius of gyration of free chains in the equivalent bulk solution.
Imaging and Resistivity Studies of Saturation Gradients in Two-Phase Flow in 3-D Porous Media
Authors: Kawamoto E. H. , Wong Po-Zen
Published: April 2011

We have carried out x-ray radiography and computed tomography (CT) to study two-phase flow in 3-D porous media. Air-brine displacement was imaged for drainage and imbibition experiments in a vertical column of glass beads. By correlating water saturation Sw with resistance R, we find that there is a threshold saturation S* ≈ 0.2, above which R(SW) ∼ Sw−2, in agreement with the empirical Archie relation. This holds true for both drainage and imbibition with littlehysteresis, provided that Sw remains above S*. Should Sw drop below S* during drainage, R(Sw) rises above the Archie prediction, exhibiting strong hysteresis upon reimbibition. This behavior suggests a transition in the connectivity of the water phase near S*, possibly due to percolation effects.
Simulations of Lubricants in Confined Geometries
Authors: Stevens Mark J. , Mondello Maurizio , Grest Gary S.
Published: April 2011

We examine the shear flow of hexadecane and squalane confined between plates with nm separation using molecular dynamics simulations. For both molecules substantial slip occurs at the walls and the density profile exhibits strong oscillations. In contrast to surface force apparatus measurements, our calculated effective viscosities are not much greater than bulk viscosities, but the simulations shear rates are much larger than the experimental ones. The actual viscosity calculated using the shear rate measured from the observed velocity profile is almost equal to the bulk viscosity within uncertainty.
Investigations of Ultra-Thin Single Layer A-Si:H Films
Authors: Koehler S. A.
Published: April 2011

Measurements are presented as direct evidence of tail states in ultra-thin a-Si:H single layer films. Including tail states in computer simulations completely removes the staircase structure in the differential optical spectra, previously associated with the quantum confinement of carriers.
Single Wafer Amorphous Silicon Process Evaluation
Authors: O'Meara David , Chang Chow Ling , Blumenthal Roc , Hegde Rama I. , Prabhu Lata , Kaushik Vidya
Published: April 2011

Single wafer amorphous silicon deposition was characterized through process modeling and film characterization for application in semiconductor production. DOE methodology was used to determine the main deposition parameters, and the responses were limited to device production requirement properties of surface roughness, deposition rate and degree of crystallinity of the as-deposited film. The data trends and models show that deposition temperature and silane flow are the main factors. Increasing either or both factor increases the deposition rate and the surface roughness. The surface morphology, evaluated by AFM, SEM and TEM, was found to be rougher at extreme growth conditions than the poly crystalline film formed after anneal. The as-deposited surface morphology was not a result of pre-anneal crystal formations as determined by TEM cross sections of samples before and after anneal. Lack of crystalinity is important for impurity diffusion considerations. Device application of the single wafer a-Si process will be a compromise between growth rate (and associated throughput) and surface roughness that can be tolerated.
Electronic and Optical Properties of Bulk GaN and GaN/AlGaN Quantum Well Structures
Authors: Suzuki M. , Uenoyama T.
Published: April 2011

Electronic structures and optical gains of bulk GaN and GaN/AlGaN quantum wells (QWs) are theoretically investigated for the wurtzite and the zincblende structures, using the k-p theory. It is found that the lower crystal symmetry, that is the wurtzite, is preferable for the lower threshold carrier density in the bulk. Although the QW structure leads to symmetry lowering only in the zincblende, we can not find a significant benefit of the zincblende QWs. As for the reduction of the threshold carrier density, biaxial strains are more effective in the zincblende. However, the threshold carrier density is still higher than in the wurtzite. It is proposed that the uniaxial strain in the c-plane of the wurtzite is more useful for reducing it.
Implantation Activation Annealing of Si-Implanted Gallium Nitride at Temperatures > 1100 °C
Authors: Zolper J. C. , Han J. , Biefeld R. M. , Van Deusen S. B. , Wampler W. R. , Pearton S. J. , Williams J. S. , Tan H. H. , Karlicek R. J. , Stall R. A.
Published: April 2011

The activation annealing of Si-implanted GaN is reported for temperatures from 1100 to 1400 °C. Although previous work has shown that Si-implanted GaN can be activated by a rapid thermal annealing at ∼1100 °C, it was also shown that significant damage remained in the crystal. Therefore, both AlN-encapsulated and uncapped Si-implanted GaN samples were annealed in a metal organic chemical vapor deposition system in a N2/NH3 ambient to further assess the annealing process. Electrical Hall characterization shows increases in carrier density and mobility for annealing up to 1300 °C before degrading at 1400 °C due to decomposition of the GaN epilayer. Rutherford backscattering spectra show that the high annealing temperatures reduce the implantation induced damage profile but do not completely restore the as-grown crystallinity.
Molecular Dynamics Simulation of Transport in Diamond and GaN: Role of Collective Excitations
Authors: Miskovsky N. M. , Lerner P. B. , Cutler P. H.
Published: April 2011

In this paper we consider the role of collective excitations in transport of charge carriers in the conduction band of diamond and GaN. The present molecular dynamics simulation uses a Monte Carlo algorithm to determine the time between collisions with scattering rates calculated quantum mechanically using the Fermi Golden Rule. For very thin films (L=0.01 μm) and for low fields (F<10 V/μm), the energy spectra of both diamond and GaN contain a series of peaks which are attributed to the absorption and emission of discrete plasmons. In diamond. the intervalley LA, LO and TO scattering becomes increasingly more important at higher fields while electron-plasmon interactions decrease. For thicker diamond films (L=0.1 μm), there is hot electron transport for F∼10 V/μm and quasi-ballistic transport for F-100 V/μm. In GaN with L=0.01 μm and fields F∼10 V/μm, there is a discrete series of peaks in the energy spectrum corresponding to excitations associated with polar optical scattering.
Integrated Vapor Phase Cleaning and Pure no Nitridation for Gate Stack Formation
Authors: Glowacki F. , Froeschle B. , Deutschmann L. , Sagnes I. , Laviale D. , Bensahel D. , Halimaoui A. , Martin F. , Bauer A. J.
Published: April 2011

The purpose of this publication is to give an insight into process development performed in two modules which belong to a cluster tool designed for the gate stack process sequence of cleaning, gate oxidation, and polysilicon chemical vapor deposition. For the first time, following the hardware and software MESC-based standards, two suppliers have integrated complementary modules to build a cluster tool. This equipment answers the demands of the 1C Manufacturers and follows the “best of breed” approach. Four single wafer rapid thermal process chambers, a Vapor Phase Cleaning (VPC) and a Rapid Thermal Oxidation/Nitridation (RTO/N) module from STEAG-AST Elektronik, a polysilicon and a nitride chemical vapor deposition module from ASM International are currently connected together to prove the feasibility of the single wafer processing gate stack cluster tool.
Surface Charging of Phosphors and its Effects on Cathodoluminescence at Low Electron Energies
Authors: Seager C. H. , Warren W. L. , Tallant D. R.
Published: April 2011

Measurements of the threshold for secondary electron emission and shifts of the carbon Auger line position have been used to deduce the surface potential of several common phosphors during irradiation by electrons in the 0.5 – 5.0 keV range. All of the insulating phosphors display similar behavior: the surface potential is within ±1 V of zero at low electron energies. However, above 2–3 kV it becomes increasingly negative, reaching hundreds of volts within 1 keV of the turn-on energy. The electron energy at which this charging begins decreases dramatically after Coulomb aging at 17 μA/cm2 for 30–60 min‥ Measurements using coincident electron beams at low and high electron energies to control the surface potential were made to investigate the dependence of the cathodoluminescence (CL) process on charging. Initially, the CL from the two beams is identical to the sum of the separate beam responses, but after Coulomb aging large deviations from this additivity are observed. These results indicate that charging has important, detrimental effects on CL efficiency after prolonged e-beam irradiation. Measurements of the electron energy dependence of the CL efficiency before and after Coulomb aging will also be presented, and the implications of these data on the physics of the low-voltage CL process will be discussed.
Deposition and Characterization of Eu:Y2O3 Red Phosphor Thin Films
Authors: Jones S. L. , Kumar D. , Singh Rajiv K. , Holloway P. H.
Published: April 2011

Thin film phosphors are very promising for the fabrication of flat panel field emission displays (FEDs). In the present paper we have reported the growth and characterization of Eu:Y2O3 phosphor thin films. The effect of surface roughness and crystallinity on the brightness of phosphor films have been studied. A post annealing treatment of the films have been found to result in the realization of Eu:Y2O3 films with 70% brightness compared to powder materials.
Effects of Interfacial States on Asymmetric Polarization Switchings of Epitaxial Bi4Ti3012 Thin Films
Authors: Park Bae-Ho , Hyun Sang-Jin , Mun S. H. , Noh T. W. , Lee J. , Lee See-Hyung , Jo W.
Published: April 2011

Epitaxial Bi4Ti3012 (BTO) films with Lao0.5Sr0.5Co03 (LSCO) or Pt bottom electrodes were grown on MgO(OOl) substrates by pulsed laser deposition. Surprisingly, a symmetric Pt/BTO/Pt capacitor showed a highly asymmetric polarization switching and an asymmetric Pt/BTO/LSCO capacitor revealed a nearly symmetric polarization switching. To understand these intriguing phenomena, Auger electron spectroscopy and x-ray photoemission spectroscopy depth-profiles were used. The evidences for interdiffusions at the bottom BTO/Pt interface were found. To get further understanding on the interfacial states, a capacitance-voltage (C-V) measurement was performed on the Pt/BTO/Pt capacitor. By fitting the C-V data with a back-to-back Schottky diode model, built-in voltages at the top and the bottom interfaces were determined to be 1.1 V and 3.2 V, respectively. From the obtained built-in voltages, an asymmetric band diagram for the Pt/BTO/Pt structure was suggested. Therefore, the imprint failure can be explained by existence of asymmetric interracial states.
Magnetic and Transport Properties of Micron Size Magnetic Participate Arrays Fabricated on a 2D Superconducting Nb Films and Sub-Micron Magnetic Wires
Authors: Otani Yoshichika , Fukamichi Kazuaki , Kitakarni Osamu , Shimada Yutaka , Pannetier Bemard , Nozieres Jean-Pierre , Matsuda Tsuyoshi , Tonomura Akira
Published: April 2011

Micron size particulate arrays of amorphous R-Co (R = Sm and Gd) alloys with a thickness of 200 nm were fabricated on top of a 20 nm thick Nb film by means of e-beam lithography and sputter deposition. The arrays consist of 2 × 2 μm2 square particles with a spatial periodicity of 4 μm along an edge of the square. Interesting oscillations due to the fluxoid quantization were observed both in the transition lines and the magnetization curves of the underlying Nb film. The superconducting state of the Nb film was found to be modulated by stray fields of the square magnetic particles.

Sub-micron size ferromagnetic Fe wires were also prepared in order to investigate the magnetoresistance effect associated with pinning and depinning processes of a magnetic domain wall confined in the wire. Low temperature transport, Lorentz microscopy and electron holography measurements demonstrated a clear change in resistivity associated with the depinning of a pinned single domain wall.
Mocvd of Cu on Teflon-AF and Alumina-Modified Teflon-AF
Authors: Martini D. , Sutcliffe R. , Kelber J.
Published: April 2011

XPS studies of Cu(I)hfac(COD)adsorbed on clean and alumina-modified Teflon-AF surfaces show that on the clean polymer surface, Cu(0) formation occurs between 300 K and 600 K in UHV. The corresponding reduction is hindered by the presence of an alumina adlayer. In addition, the Cu Auger kinetic energies indicate the formation of nanoparticles on the alumina surface at both 300 K and 600 K. Annealing from 300 K to 600 K results in a ∼4 eV shift of the Cu(2p) transitions to higher binding energies. These facts indicate that the Cu precursor reacts with the alumina surface and results in limited mobility and hindered Cu(I) to Cu(0) reduction on the modified polymer surface.
Cross-Sectional Tem Sample Preparation of Phase-Change Optical Disk by Ion Milling
Authors: Kouzaki T. , Yoshioka K. , Ohno E.
Published: April 2011

It is very difficult to prepare cross-sectional TEM samples for phase-change optical disks by conventional argon ion milling because of the difference of ion milling rates between multilayers and the polymer substrate. We have been successful in preparing samples of those optical disks by ion milling method with dissolution of the polymer substrate in advance. The cross-sectional structure was observed more clearly in this method rather than in ultramicrotome method.
A Novel Sample Design for Easier X-Tem Specimen Preparation of Surface Coated Hardened High Speed Steel
Authors: Østergaard Klaus F. , Mørch K. A.
Published: April 2011

A novel design of test sample for X-TEM specimen preparation is presented. The test sample is particurlarly suited for surface coatings on hardened steel (heat treated materials). The technique for cutting and preparation of a specimen from the test sample is given in some detail and a few hints about ion thinning and X-TEM analysis are included.
Grain Refinement in Undercooled Metals
Authors: Xiao J. Z. , Yang H. , Kui H. W.
Published: April 2011

Recently, it was demonstrated that grain refinement in metals can take place through two mechanisms, namely, dynamic nucleation and remelting of initially formed dendrites. In this study, it was found that Ni99.45B0.55 undergoes grain refinement, both by dynamic nucleation or by remelting, depending on the initial bulk undercooling just before crystallization. The nature of the grain refinement process is confirmed by microstructural analysis of the undercooled specimens.
A Possible Mechanism For Atomic Transport in Amorphous Metals
Authors: Van Ee Leon D. , Tthijsse Barend J. , Sietsma Jilt
Published: April 2011

The diffusion process at a temperature just below the glass-transition temperature is studied in a computer model of amorphous Ni81B19. The observed diffusion events cause structural changes that are highly localized and are correlated with low-frequency localized vibrational modes. The excitation of such a mode can result in a “jump”, which is identical to the diffusion event. The jump has a cooperative character involving some tens of atoms and can be of a reversible or of an irreversible nature.
Onion-Like Nanoscale Structures and Fullerene-Type Cages Formed by Electron Irradiation of Turbostratic Bx.C1-x (x<0.2)
Authors: Golberg D. , Bando Y. , Kurashima K. , Sasaki T.
Published: April 2011

Flakes of CVD grown BxC1-x, (x<0.2) films were exposed to intense electron irradiation (flux density up to ∼100 A/cm2) in a 300 kV high resolution electron microscope equipped with a field emission gun. The starting flakes revealed a turbostratic BxC1-x structure. The composition of the starting materials and irradiated products was determined by using electron energy loss spectroscopy (EELS). Depending on the electron dose applied, irradiation of the turbostratic material led to formation of soap-bubble-like irregularly-shaped objects (linear dimensions of ∼2–5 nm), onion- and semi-onion-like structures (d∼10nm), nested fullerenes (3–14 shells) and elementary fullerene-type cages (d∼0.7 nm). It is thought that these curled and closed nanostructures arise from a continuous bending of the hexagonal Bx C1-x sheets under electron irradiation. Finally, some possible structural models of BxC1-x fullerenes are considered.
In-Situ TEM Phase Formation in Cold Rolled Aluminum-Nickel Multilayers
Authors: Sieber H. , Perepezko J. H.
Published: April 2011

Multilayer samples of Nickel and Aluminum with an overall composition of Al-20Ni were prepared by cold rolling of elemental foils. The sample microstructures and phases were characterized by XRD, SEM and TEM/SAED, and the reactive phase formation was then examined by DSC measurements. XRD, SEM and TEM measurements show that the rolling procedure results in a decrease of the Al and Ni layer thicknesses (down to 100 nm in average) and a decrease of the grain size (down to less than 50 nm). No phase formation is observed during the cold rolling procedure. In isochronal DSC scans of the Al-Ni multilayers, the formation of the Al3Ni phase was found to be a two step reaction process due to 2-dimensional nucleation and lateral growth and a 3-dimensional phase thickening. While XRD measurements showed Al3Ni as the only phase that forms, more detailed TEM investigations of the samples after DSC treatment also showed a small amount of an amorphous Al-Ni phase, formed by a thermally activated solid state amorphization reaction (SSAR). In-situ TEM heating of the amorphous areas under the electron beam in the microscope yielded the crystallization of the amorphous phase to a B2 structure and a growth of the B2 grains up to 100 nm in size.
Multiwafer Movpe of III-Nitride Films for Led and Laser Applications
Authors: Beccard R. , Schoen O. , Schineller B. , Schmitz D. , Heuken M. , Juergensen H.
Published: April 2011

Process for mass production of GaN and its related alloys, InGaN and AlGaN, have been optimized to achieve high device yield and low cost of ownership. Here we present some of the latest results obtained from AIX 2000 HT Planetary Reactor® in a configuration of 7×2” which provides unique uniformity capabilities due to the two fold rotation of the substrates. GaiN single layers with background electron concentrations below 5·1016 cm-3 and intended doping levels up to 1018 cm-3 p-type and 1020 cm-3 n-type with state of the art homogeneities have been achieved. Thickness homogeneities have been shown to be better than 1% standard deviation on full 2” wafers, while composition uniformity of ternary material is determined by room temperature photoluminescence mappings. Low temperature photoluminescence and reflectance spectra of single layer GaN revealed free exciton transitions.
Low Temperature GaN Growth on Nitridated Sapphire Using Remote Plasma Enhanced Ultrahigh Vacuum Chemical Vapor Deposition
Authors: Kim Dong-Jun , Kim Kyoung-Kook , Paek Jong-Sik , Yi Min-Su , Noh Do-Young , Kim Hyo-Gun , Park Seong-Ju
Published: April 2011

GaN epitaxial thin films were grown on a nitridated sapphire at low temperature (550°C) using remote plasma enhanced ultrahigh vacuum chemical vapor deposition system and these films were investigated by Rutherford backscattering spectroscopy (RBS), X-ray diffraction(XRD) θ-rocking technique and the Ultraviolet-Visible-Nearinfrared (UV-VIS-NIR) absorption spectrum. The FWHM of the X-ray θ-rocking curve was about 0.4 degree using the (0002) reflection from the GaN layer with 5000Å thickness grown on the nitridated sapphire. An analysis of XRD and the UV-VIS-NIR absorption spectrum showed that the crystalline and optical qualities of GaN are dependent on the nitridation time of the sapphire even at low temperature when a plasma source is used for nitridation. This means that the density of protrusion, which is formed by a relaxation of the elastic energy caused by the lattice difference between the sapphire and AlxO1-xN, with the sapphire nitridation time plays a key role in the crystalline and optical properties of grown GaN films. The RBS channeling data and the FWHM value of the θ-rocking curve for GaNr(0002) also indicated that the truncated hexagonals are tilted towards each other. These results showed that the GaN epitaxial film can be successfully grown on nitridated sapphire by RPE-UHVCVD even at low temperature.
Fine Structure And Magnetooptics Of Excitonic Levels In Wurtzite GaN
Authors: Eckey L. , Hoffmann A. , Thurian P. , Broser I. , Meyer B. K. , Hiramatsu K.
Published: April 2011

We present a comprehensive study of the direct photoluminescence from excitonic states in GaN using polarization-dependent and magnetooptical measurements in fields up to 15 T. We measure and identify fine-structure splittings in the states n=l and n=2 of the A-exciton. The splitting between the energy levels of longitudinal and transverse states in the A-exciton is 0.96 meV. Emission from the spin triplet exciton in the state n=l is observed. We also identify emissions from three sublevels of the state n=2 of the A-exciton. From the analysis of our data we obtain the effective-mass parameters of electrons and holes, g-values and the Rydberg energy of the A-exciton.
Luminescence Properties of Si-Doped GaN and Evidence of Compensating Defects As the Origin of the Yellow Luminescence
Authors: Goepfert I. D. , Schubert E. F. , Redwing J. M.
Published: April 2011

We investigate the optical properties of n-type Gallium Nitride (GaN) with concentrations ranging from 5×1016 to 7×1018 cm−3. The near-band edge ultraviolet (UV) transition increases monotonically with the doping concentration. The photoluminescence linewidth of the near-bandgap optical transition increases from 47 to 78 meV as the doping concentration is increased. The broadening is modeled by taking into account potential fluctuations caused by the random distribution of donor impurities. Excellent agreement is found between experimental and theoretical results. We also investigate the origin of the yellow luminescence in GaN. At low excitation densities the experimental ratio of the UV-to-yellow photoluminescence does not change significantly as the doping concentration is increased by two orders of magnitude. Analysis of the luminescence in terms of a theoretical model indicates that the yellow luminescence is due to compensating impurities or defects.
Deep Trap Characterization In GaN Using Thermal And Optical Admittance Spectroscopy
Authors: Krtschil A. , Witte H. , Lisker M. , Christen J. , Birkle U. , Einfeldt S. , Hommel D. , Topf M. , Meyer B. K.
Published: April 2011

Deep defect levels and the optical as well as thermal transitions of carriers from the levels into the corresponding bands were analyzed using Thermal and Optical Admittance Spectroscopy. High resistivity GaN-layers grown by MBE and heterostructures consisting of n-type GaN-layers grown with Low Pressure Chemical Vapor Deposition on 6H-SiC substrates are investigated. In the MBE-grown GaN layers we determine deep electron traps with thermal activation energies of EA=(0.45±0.04)eV and EA=(0.65±0.03)eV. Furthermore, three different kinds of optical transitions were distinguished by Optical Admittance Spectroscopy: near band gap transitions including the transition between the valence band and a shallow donor 50meV below the conduction band, a peak at 2.1eV associated with the yellow photoluminescence band and various deep level-band transitions in the infrared region.

The high sensitivity of the TAS to interface defect states was used to investigate GaN/SiC heterostructures. We found an interface defect state at 70 … 90meV. Furthermore, one level was obtained originating from the epitaxial GaN-layer having an activation energy of 63±3meV. A defect distribution was identified in the p-type SiC-substrate with activation energies between 160meV and 180meV.
GaN And Related Materials For High Power Applications
Authors: Shur M. S.
Published: April 2011

Unique properties of GaN and related semiconductors make them superior for high-power applications. The maximum density of the two-dimensional electron gas at the GaN/AlGaN heterointerface or in GaN/AlGaN quantum well structures can reach 5×1013 cm−2, which is more than an order of magnitude higher than for traditional GaAs/AlGaAs heterostructures. The mobility-sheet carrier concentration product for these two dimensional systems might also exceed that for GaAs/AIGaAs heterostructures and can be further enhanced by doping the conducting channels and by using “piezoelectric” doping, which takes advantage of high piezoelectric constants of GaN and related materials. We estimate that current densities over 20 A/mm can be reached in GaN-based High Electron Mobility Transistors (HEMTs). These high current values can be combined with very high breakdown voltages in high-power HEMTs. These breakdown voltages are expected to reach several thousand volts. Recent Monte Carlo simulations point to strong ballistic and overshoot effects in GaN and related materials, which should be even more pronounced than in GaAs-based compounds but at much higher electric fields. This should allow us to achieve faster switching, minimizing the power dissipation during switching events. Selfheating, which is unavoidable in power devices, raises operating temperatures of power devices well above the ambient temperature. For GaN-based devices, the use of SiC substrates having high thermal conductivity is essential for ensuring an effective heat dissipation. Such an approach combines the best features of both GaN and SiC technologies; and GaN/SiC-based semiconductors and heterostructures should find numerous applications in power electronics.
Heteroepitaxy And High Density Nucleation Of Diamond On Mirror-Polished Silicon
Authors: Lin Zhangda , Sun Xiaosong , Yu Ge , Lee S. T.
Published: April 2011

By virtue of its excellent electronic properties, diamond may become an important material for high temperature, high speed, high power and high compact electronic devices. However, most of these devices, single crystal or epitaxial films are required. Thus, for obvious reason, large-area heteroepitaxial diamond films are desired. Silicon is the most important material severed as a substrate for diamond heteroepitaxy. However, diamond can hardly be grown on mirror-polished Si, with nucleation density being only about 104 cm−2. Although scratching Si substrate could enhance greatly the nucleation density, it destroyed seriously the periodic structure of the Si surface. Thus diamond film grown on such a substrate will be randomly oriented polycrystallites. In order to overcome this difficulty, several methods have been developed, including (1) bias enhanced nucleation; (2) electron emission enhanced nucleation; (3) nucleation enhanced by slight surface modification; and (4) very low pressure ( 0.1 torr ) nucleation. We will introduce the second, third, and fourth methods, which were developed in our laboratory recently. Special attention will be paid to the relation between heteroepitaxy and high density nucleation. The incubation time for diamond nucleation is a critical parameter for high density nucleation and heteroepitaxy.
High Frequency Capacitance Measurements On Metal-Insulator-Semiconductor Structures In Thermal Non-Equilibrium Condition
Authors: Sadeghi M. , Jauhiainen A. , Liss B. , Sveinbjörnsson E. Ö. , Engström O.
Published: April 2011

We simulate the charge carrier traffic between the energy bands and the interface states in structures like Al/SiO2/6H-SiC, Al/diamond/Si and Al/SIPOS/Si to explain their high frequency capacitance-voltage behavior. The structures have in common that traditional electrical measurement techniques performed at room temperature are strongly influenced by non-equilibrium carrier conditions at the insulator-semiconductor interface. This can result in large errors in the interface data extracted from such studies when thermal equilibrium conditions are assumed. In this work, high frequency capacitance-voltage data are compared to numerical simulations which include such thermal non-equilibrium conditions to enable more accurate estimates of interface state parameters in such structures.
Cl2-Based Dry Etching Of The AIGaInN System In Inductively Coupled Plasmas
Authors: Cho Hyun , Vartuli C. B. , Abernathy C. R. , Donovan S. M. , Pearton S. J. , Shul R. J. , Han J.
Published: April 2011

Cl2-based Inductively Coupled Plamas with low additional dc self-biases(−100V) produce convenient etch rates(500–1500Å.min−1) for GaN, AIN, InN, InAiN and InGaN. A systematic study of the effects of additive gas(Ar, N2, H2), discharge composition and ICP source power and chuck power on etch rate and surface morphology has been performed. The general trends are to go through a maximum in etch rate with percent C12 in the discharge for all three mixtures, and to have an increase(decrease) in etch rate with source power(pressure). Since the etching is strongly ion-assisted, anisotropic pattern transfer is readily achieved. Maximum etch selectivities of approximately 6 for InN over the other nitrides were obtained.
Design And Fabrication Of Nitride Based High Power Devices
Authors: Bandić Z. Z. , Piquette E.C. , Bridger P.M. , Kuech T.F. , Mcgill T. C.
Published: April 2011

We modeled the breakdown voltage, critical current density and maximum operating frequency of several GaN and GaN/AlN based high power and high temperature electronic devices. Important model parameters which influence device design and performance are minority carrier recombination lifetime and critical field for electric breakdown.

GaN Schottky devices have been fabricated in the planar geometry. Current-voltage measurements indicated the importance of the vertical geometry for achieving large breakdown voltages. The minority carrier (hole) recombination lifetimes have been measured by electron beam induced currents (EBIC). The measured hole lifetime of 7 ns and estimate for the critical field indicate the possibility of GaN/AlGaN thyristor switch devices operating at 5KV with current densities approximately equal to 200 A/cm2 and at frequencies above 2MHz.

The GaN structural and optical materials quality and processing requirement for etching is discussed.
Room Temperature Growth of Conducting ZnO Films
Authors: Witanachchi S. , Ying Y. , Miyawa A. M. , Mukherjee P.
Published: April 2011

Single and dual-laser ablation techniques have been used to grow conductive ZnO films at room temperature by ablating a Zn metal target in oxygen ambient. The emission spectroscopy of the material plumes shows a significant presence of oxygen ions and Zn ions in the dual-laser ablated plume. Furthermore, dual-laser ablated plumes expanded rapidly in the radial direction resulting in large-area uniform films. The electrical properties of the films deposited on glass substrates depend critically on the ambient oxygen pressure. Conductivities of the order of 103 (ω.cm)-1 have been obtained for films deposited at room temperature by the dual-laser ablation process.
Tailoring the Optical Properties of Si Nanocrystals In SiO2: Materials Issues And Nanocrystal Laser Perspectives
Authors: Brongersma M. L. , MIN K. S. , Boer E. , Tambo T. , Polman A. , Atwater H. A.
Published: April 2011

Si nanocrystals (diameter 2 – 5 nm) were formed by 35 keV Si+ implantation at a fluence of 6×1016 Si/cm2 into a 100 nm thick thermally grown SiO2 film on Si (100), followed by thermal annealing at 1100 °C for 10 min. The nanocrystals show a broad photoluminescence spectrum, peaking at 880 nm, attributed to the recombination of quantum confined excitons. Rutherford backscattering spectrometry and transmission electron microscopy show that annealing these samples in flowing O2 at 1000 °C for times up to 30 min. results in oxidation of the Si nanocrystals, first close to the SiO2 film surface and later at greater depths. Upon oxidation for 30 min. the photoluminescence peak wavelength blue-shifts by more than 200 nm. This blueshift is attributed to a quantum size effect in which a reduction of the average nanocrystal size leads to emission at shorter wavelengths. The fabrication of a laser based on SiO2 waveguides doped with Si nanocrystals seems possible, if the nanocrystal size distribution can be narrowed down into the percent range.
Comparative Study of Implanted and Sputtered Systems of Si Nanograins Embedded In SiO2
Authors: Charvet S. , Madelon R. , Rizk R. , Garrido B. , Pó;rez Rodríguez A. , López M. , González Varona O , Morante J. R.
Published: April 2011

Si nanograins embedded in silicon oxide matrix have been obtained by thermal annealing at 1100°C during one hour, either after implantation of Si+ into thermal SiO2 layers or after magnetron cosputtering of both Si and SiO2. The spectral distribution of the photoluminescence (PL) in both systems is similar and peaks in the red visible region (1.45–1.60 eV). The results obtained indicate that for high PL efficiency, the films have to be annealed at temperatures high enough to achieve phase separation. This was observed from the gradual shift of the infrared bands of the silica matrix toward those of thermal SiO2, together with the appearance of the crystalline silicon Raman vibrational mode in the spectra. The similar behavior of samples obtained by ion implantation and by sputtering suggests a more complex origin of the PL than quantum confinement.
Time-Resolved Photoluminescence from nm-Sized Silicon Crystallites In SiO2
Authors: Linnros J. , Galeckas A. , Pareaud A. , Lalic N. , Grivickas V. , Hultman L.
Published: April 2011

Time resolved photoluminescence (PL) decays have been measured for Si nanocrystals embedded in silicon dioxide. The nanocrystals were formed by Si implantation followed by thermal annealing at 800 – 1200 °C. The observed PL peaked in the wavelength range 640 – 850 nm and the PL decay exhibited a stretched exponential lineshape, characterized by a relatively large time constant. A nonlinear dose dependence of the PL yield and an observed redshifting for increasing doses and/or higher annealing temperatures is discussed in terms of a nucleation and growth mechanism for the nanocrystals. Finally, we argue that Auger recombination is effective at high excitation densities explaining a wavelength dependent saturation of the PL intensity.
Adhesion of Lipid Membranes Mediated by Electrostatic and Specific Interactions
Authors: Maier Christian W. , Behrisch Almuth , Kloboucek Annette , Merkel Rudolf
Published: April 2011

We used the micropipet aspiration technique for a study of biomembrane adhesion. Adhesion was caused by contact site A, a highly specific cell adhesion molecule, reconstituted in lipid vesicles of DOPC with 5 %(mol/mol) DOPE-PEG2000. We found adhesion and subsequent receptor aggregation in the contact zone. Additionally, electrostatic modulation of membrane adhesion was studied. Whereas addition of the negatively charged lipid SOPS to the lecithin (SOPC) host membrane suppressed adhesion due to electrostatic repulsion, a positively charged lipid (DOTAP) was surprisingly ineffective. This might be due to either phase separation of the mixture or DOTAP changing other membrane properties as bending stiffness and the Hamaker constant.
A Three-Dimensional Monte Carlo Model for Phosphorus Implants into (100) Single-Crystal Silicon
Authors: Son Myung-Sik , Hwang Ho-Jung
Published: April 2011

An alternative three-dimensional (3D) Monte Carlo (MC) dynamic simulation model for phosphorus implant into (100) single-crystal silicon has been developed which incorporates the effects of channeling and damage. This model calculates the trajectories of both implanted ions and recoiled silicons and concurrently and explicitly affects both ions and recoils due to the presence of accumulative damage. In addition, the model for room-temperature implant accounts for the self-annealing effect using our defined recombination probabilities for vacancies and interstitials saved on the unit volumes. Our model has been verified by the comparison with the previously published SIMS data over commonly used energy range between 10 and 180 keV, using our proposed empirical electronic energy loss model. The 3D formations of the amorphous region and the ultra-shallow junction around the implanted region could be predicted by using our model, TRICSI (TRansport Ions into Crystal-Silicon).
Atomie Force Microscope Study of Two-Dimensional Dopant Delineation by Selective Chemical Etching
Authors: Choi Kwang-Ki , Seong Tae-Yeon , Lee Seonghoon , Hwang Hyunsang , Sohn Yong Sun
Published: April 2011

Selective chemical etching and atomic force microscope (AFM) examination has been performed to delineate two-dimensional (2-D) dopants profiles of p/n-type well and junction areas. Selectivity strongly depended on the types of dopants and the ratio of etching solutions. Calibration showed that the carrier concentrations in both p/n-type regions could be delineated down to a level of ∼1×1017/cm3. The AFM-induced profiles were compared with the calculated data provided by the 2-D process simulators such as TRIM and SUPREM-IV.
Characterization of Interatomic Potentials by a Calculation of Defect Energy
Authors: Kogure Y. , Doyama M.
Published: April 2011

Potential functions used in molecular dynamics simulations for metals are characterized through a calculation of the third-order elastic constants, the Gruneisen parameters, and the molecular dynamics simulation of point defects. The Lennard-Jones potential and the embedded atom method potentials for noble metals (Cu, Ag, Au) are characterized by using a common program code.
A Tight-Binding Model for Optical Properties of Porous Silicon
Authors: Cruz M. , Beltran M. R. , Wang C. , Tagüeña-Martinez J.
Published: April 2011

Semi-empirical tight-binding techniques have been extensively used during the last six decades to study local and extended defects as well as aperiodic systems. In this work we propose a tight-binding model capable of describing optical properties of disordered porous materials in a novel way. Besides discussing the details of this approach, we apply it to study porous silicon (p-Si). For this purpose, we use an sp3s* basis set and supercells, where empty columns are digged in the [001] direction in crystalline silicon (c-Si). The disorder of the pores is considered through a random perturbative potential, which relaxes the wave vector selection rule, resulting in a significant enlargement of the optically active k-zone. The dielectric function and the light absorption spectra are calculated. The results are compared with experimental data showing a good agreement.
Tight-Binding Formalism for Ionic Fullerides and its Application to Alkali-C60 Polymers
Authors: Saito Susumu , Louie Steven G. , Cohen Marvin L.
Published: April 2011

We present a tight-binding formalism which can properly treat various ionic füllendes. In the Hamiltonian we include the intrafullerene Coulomb repulsion energy and the Madelung energy of the ionic lattice, both of which depend on the possible charge disproportion between fullerenes. This Hamiltonian requires a self-consistent treatment, but it is applicable to much larger systems than first-principles methods. Using this formalism we have studied the electronic structure of the one-dimensional A1C60 polymer. The present generalization of the tight-binding model is found to be important for ionic füllendes and a moderate-amplitude charge-density-wave state is found to be a possible stable state.
Effective Interatomic Interactions VIA The TB-LMTO Method
Authors: Drchal V. , Kudrnovský J. , Pasturel A. , Turek I. , Weinberger P. , Gonis A. , Turchi P. E. A.
Published: April 2011

The energetics of metallic alloys, their surfaces or interfaces, and magnetic multilayers is studied in terms of effective interatomic (or interlayer) interactions that are determined from ab initio electronic structure calculations using the TB-LMTO method combined with the coherent potential approximation and the method of surface Green functions. First the theoretical background (force theorem, Lloyd formula, generalized perturbation method for bulk and surfaces, vertex cancellation theorem, method of infinitesimal rotations) is discussed, and then the applications to the phase stability of bulk alloys, surface segregation in disordered alloys, magnetism-induced ordering in two- and three-dimensional systems, phase diagram of two-dimensional alloys, interlayer exchange coupling in metallic multilayers, and the construction of Heisenberg-like Hamiltonians for magnetic systems are presented.
Magnetotransport in Thin Films of Lan-nxCa1+nxMnnO3n+1 (n=2,3, and ∞)
Authors: Asano H. , Hayakawa J. , Matsui M.
Published: April 2011

With a use of the epitaxial a-axis thin films of perovskite series Lan-nxCa1+nxMnnO3n+1 (n=2,3, and ∞) with fixed carrier concentration (x=0.3), the transport properties of the series compounds have been examined to be associated with the difference in the number of the MnO2 layers. Results have indicated that a reduction in the number of layers results in systematic changes in the various features. These include an increase in resistivity, a decrease in resistivity peak temperature Tcρ corresponding to the metal-insulator transition, an enhancement of the maximum MR near Tcρ, and an increase in low temperature intrinsic MR. In order to explain the variation in these features with the number of MnO2 layers, it is necessary to take both anisotropie c-axis transfer interaction and two-dimensional spin fluctuation into account.
The Magentic Susceptibility in Ultrathin Films of Magnetic Materials
Authors: Ghatak Kamakhya P. , Bose P. K. , Majumder Gautam
Published: April 2011

In this paper we have studied the dia and paramagentic susceptibilities of the holes in ultrathin films of magnetic materials in the presence of a parallel magentic field on the basis of a newly derived dispersion law for such systems. The numerical computations are performed taking Hg1-xMnx Te and Cd1-xMnx Se as examples. Both the susceptibilities increses with decreasing doping and film thickness respectively. It is important to note that not only the paramagnetic-to-diamagnetic susceptibility ratio for the present case deviates from (1/3) in conventional semiconductors, but also that is a critical region, where quenching of the diamagnetic occurs. The theoretical analysis is in agreement with the experimental datas as given elsewhere.
Sub-200 Oe Giant Magnetoresistance in Manganite Tunnel Junctions
Authors: Xiao Gang , Gupta A. , Li X. W. , Gong G. Q. , Sun J. Z.
Published: April 2011

Metallic manganite oxides, La1-xDxMnO3 (D=Sr, Ca, etc.), display “colossal” magnetoresistance (CMR) near their magnetic phase transition temperatures (Tc) when subject to a Tesla-scale magnetic field. This phenomenal effect is the result of the strong interplay inherent in this class of materials among electronic structure, magnetic ordering, and lattice dynamics. Though fundamentally interesting, the CMR effect achieved only at large fields poses severe technological challenges to potential applications in magnetoelectronic devices, where low field sensitivity is crucial. Among the objectives of our research effort involving manganite materials is to reduce the field scale of MR by designing and fabricating tunnel junctions and other structures rich in magnetic domain walls. The junction electrodes were made of doped manganite epitaxial films, and the insulating barrier of SrTiO3. The interfacial expitaxy has been imaged by using high-resolution transmission electron microscopy (TEM). We have used self-aligned lithographic process to pattern the junctions to micron scale in size. Large MR values close to 250% at low fields of a few tens of Oe have been observed. The mechanism of the spin-dependent transport is due to the spin-polarized tunneling between the half-metallic electrodes, in which the spins of the conduction electrons are nearly fully polarized. We will present results of field and temperature dependence of MR in these structures and discuss the electronic structure of the manganite inferred from tunneling measurement. Results of large MR at low fields due to the grain-boundary effect will also be presented.
In-Situ X-Ray Characterization of LiMn2O4: A Comparison of Structural and Electrochemical Behavior
Authors: Rodriguez Mark A. , Ingersoll David , Doughty Daniel H.
Published: April 2011

LixMn2O4 materials are of considerable interest in battery research and development. The crystal structure of this material can significantly affect the electrochemical performance. The ability to monitor the changes of the crystal structure during use, that is during electrochemical cycling, would prove useful to verify these types of structural changes. We report in-situ XRD measurements of LiMn2O4 cathodes with the use of an electrochemical cell designed for in-situ X-ray analysis. Cells prepared using this cell design allow investigation of the changes in the LiMn2O4 structure during charge and discharge. We describe the variation in lattice parameters along the voltage plateaus and consider the structural changes in terms of the electrochemical results on each cell. Kinetic effects of LiMn2O4 phase changes are also addressed. Applications of the in-situ cell to other compounds such as LiCoO2 cathodes and carbon anodes are presented as well.