bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoluminescence of Excitons in n-Type In0.11Ga0.89N/In0.01Ga0.99N Multiple Quantum Wells
Authors: Monemar B. , Paskov P.P. , Bergman J.P. , Pozina G. , Kamiyama S. , Iwaya M. , Amano H. , Akasaki I.
Published: April 2011

In0.11Ga0.89N/In0.01Ga0.99N multiple quantum wells (MQWs) with heavily Si-doped barriers are shown to be very sensitive to a near surface depletion field. For a sample with 3 QWs, similar to what is often used in LEDs, only the QW most distant from the surface is observed in photoluminescence (PL). The appearance of a second lower energy PL peak below the ordinary QW exciton peak is a proof of a substantial band bending across the MQW structure. A similar effect seems to occur in pn-junctions having an MQW in the depletion region of a highly doped n-side. The strong depletion field is suggested to explain these results. The apparent absence of PL from the QWs closer to the surface (pn-junction) is tentatively ascribed to a loss of hole confinement in the strong depletion field.
Magneto-Spectroscopy of Two-Electron Transitions in Homoepitaxial GaN.
Authors: Wojdak M. , Baranowski J.M. , Wysmolck A. , Pakula K. , Stepnicwski R. , Potemski M. , Grzegory I. , Porowski S.
Published: April 2011

Two-electron transition occurs when the exciton bound to a neutral donor (DBE) recombines and leaves the donor in an excited state. The two-electron energy is therefore lower than that of the DBE peak by the difference in ground and excited state of the neutral donor. In a magnetic field the two-electron satellite splits into several components. These intra-donor excitations have been studied in homoepitaxial GaN up to magnetic fields reaching 23T. For Faraday (B‖c) configuration the two-electron transition splits mainly into 2s, 2p0, 2p+ and 2p- components. The total splitting between 2p+ and 2p- is equal to Landau energy. For Voigt (B???c) configuration in addition to transition to 2s, 2p0, 2p- and 2p+ there are additional lines which origin is discussed. It has been found that for two configurations of magnetic field the separation between 2p+ and 2p- is not exactly equal, what indicates the anisotropy of the electron effective mass. It has been found that m| = 0.205m0 and m??? = 0.225m0.
Measurement of the Effective Piezoelectric Constant of Nitride Thin Films and Heterostructures Using Scanning Force Microscopy
Authors: Rodriguez B. J. , Kim D-J. , Kingon A. I. , Nemanich R. J.
Published: April 2011

Piezoelectric properties of wurtzite AlN and GaN/AlN are investigated using scanning force microscopy (SFM). The magnitude of the effective longitudinal piezoelectric constant d33 of AlN and GaN/AlN thin films are measured and reported, and the d33 coefficients of these films are verified using an interferometric technique. Simultaneous imaging of the topography, and of the phase and magnitude of the piezoelectric strain is performed. Using a GaN film with patterned polarities, we demonstrate that polarity can be inferred from the phase image of the piezoelectric strain. We report d33=3±1 pm/V for AlN/SiC and 2 1 pm/V for GaN/AlN/SiC. Films grown by organo-metallic vapor phase epitaxy (OMVPE) on SiC, sputtered AlN films and films grown by molecular beam epitaxy (MBE) are characterized and compared.
Light pulse propagation in three-dimensional photonic crystals
Authors: Kitano H. , Minami F. , Sawada T. , Yamaguchi S. , Ohtaka K.
Published: April 2011

The phase characteristics of transmitted optical pulses in three-dimensional photonic crystals were investigated in the frequency and time domain by using the spectrally resolved cross-correlation technique. The temporal evolution of femtosecond pulses passing through polystyrene colloidal crystals exhibits a large phase distortion near the stop bands. The phase discontinuity around the band gap was observed in the frequency-domain. The phase of the transmitted pulses is found to change by π across the band gap. The dispersion curve estimated from the phase shift shows good correspondence with those calculated from a photonic band calculation. The group velocity significantly slows down near the stop bands. A large change of the group velocity dispersion is also observed near the band edges. These results are in good agreement with the band theory.
Polymer Dispersed Liquid Crystals as Mesoscale 2D and 3D Lattices
Authors: Escuti Michael J. , Crawford Gregory P.
Published: April 2011

We introduce and explore two- and three-dimensional lattices formed in Holographic-Polymer Dispersed Liquid Crystals (H-PDLC) materials, which exhibit an electrically controllable index modulation in multiple dimensions. As electro-optically active holograms, these materials exhibit fast dynamic switching phenomena (~100 microseconds), and are simple to fabricate. While many applications have been proposed for these materials, almost all are based on one-dimensional index modulations in various grating regimes. However, constraints in additional dimensions lead to a much greater sensitivity of the polymer morphology to monomer functionality, exposure irradiance, and grating pitch. In an effort to begin to understand this relationship, two-dimensional triangle lattices were created using two monomeric blends exposed over a range of powers. Final diffraction efficiency (Bragg regime), saturation voltage, and polymer morphology were examined from the resulting triangle lattices. Three- dimensional lattices are discussed and a six-beam holographic method is proposed. Photonic crystal applications are envisioned where the pseudo-bandgap can be electrically controlled.
Thermo-optical switches using coated microsphere resonators
Authors: Tapalian C. , Laine J.-P. , Lane P. A.
Published: April 2011

We report optical switching by a silica microsphere optical resonator coated by a conjugated polymer. Microspheres were fabricated by melting the tip of an optical fiber and coated by dipping in a 1 mg/ml toluene solution of poly(2,5-dioctyloxy-1,4-phenylenevinylene) (DOO-PPV). The resonator properties were characterized by evanescently coupling 1.55 µm light propagating along a stripline-pedestal anti-resonant reflecting optical waveguide into optical whispering gallery modes (WGMs). WGM linewidths less than 2 MHz were measured, corresponding to cavity Q > 108. WGM resonant frequency shifts as large as 3.2 GHz were observed when 405 nm pump light with a power density of ~100 mW/cm2 was incident on the microsphere. The time constant of the observed frequency shifts is approximately 0.165 seconds, leading us to attribute the frequency shift to thermo-optic effects. Such a system should be capable of thermo-optically switching at speeds on the order of 10 kHz.
Toward the Integration of Photonic Crystals with Optical Fiber
Authors: Suzuki Y. , Chen Lu , Kohnke Glenn E.
Published: April 2011

We have developed a novel silicon platform where light from optical fiber is coupled directly into and out of silicon-based photonic crystal structures with over 30dB suppression of transmission from 1400nm to 1700nm and defect energy levels tuned to within 2nm in the bandgap. Insertion losses as low as 3.5dB have been achieved. The optical spectra of our one-dimensional silicon-based photonic crystals can be quantitatively described by a simple model of light incident on a series of dielectric interfaces. The agreement between experiment and simulation and the low insertion losses are promising for the future integration of photonic crystals into optical communications.
Two Dimensional Photonic Crystal Modes and Resonances in Three-dimensional Structures
Authors: Fan Shanhui , Joannopoulos J. D.
Published: April 2011

We present three-dimensional analysis of two-dimensional guided resonances in photonic crystalslab structures. This analysis leads to a new understanding of the complex spectral properties of such systems. Specifically, we calculate the dispersion diagrams, the modal patterns, and transmission and reflection spectra of these resonances. From these calculations, a key observation emerges involving the presence of two temporal pathways for transmission and eflection processes. Using this insight, we introduce a general physical model that explains the essential features of complex spectral properties. Finally, we show that the quality factors of these resonances are strongly influenced by the symmetry of the modes, and the strength ofthe index modulation.
Residual Stress and Microstructure of Electroplated Cu Film on Different Barrier Layers
Authors: Volinsky Alex A. , Hauschildt Meike , Vella Joseph B. , Edwards N.V. , Gregory Rich , Gerberich William W.
Published: April 2011

Copper films of different thicknesses between 0.2 and 2 microns were electroplated on adhesion-promoting TiW and Ta barrier layers on <100> single crystal 6-inch silicon wafers. The residual stress was measured after each processing step using a wafer curvature technique employing Stoney's equation. Large gradients in the stress distributions were found across each wafer. Controlled Cu grain growth was achieved by annealing films at 350 C for 3 minutes in high vacuum. Annealing increased the average tensile residual stress by about 200 MPa for all the films, which is in agreement with stress-temperature cycling measurements.

After aging for 1 year wafer stress mapping showed that the stress gradients in the copper films were alleviated. No stress discrepancies between the copper on Ta and TiW barrier layers could be found. However, X-ray pole figure analysis showed broad and shifted (111) texture in films on a TiW underlayer, whereas the (111) texture in Cu films on Ta is sharp and centered.
X-Ray Diffraction Analysis and Modeling of Strain Induced Thermal Cycling in a Thin Aluminum (011) Bicrystal Film
Authors: Nowak D. E. , Thomas O. , Baker S. P. , Stach E. A. , Balzuweit K. , Dahmen U.
Published: April 2011

Heteroepitaxial films of aluminum bicrystals grown on silicon provide a model system in which to study plasticity in polycrystalline metal thin films. For the bicrystal films, dislocations are confined to move on two different slip plane orientations because of the orientation of the crystals on the substrate. In-situ transmission electron microscopy (TEM) observations during thermal cycling have shown two threshold temperatures for dislocation motion on cooling. A simple model uses the resolved shear stress on the possible slip planes to explain the TEM observations. Mechanisms responsible for the dislocation behavior are studied in-situ during thermal cycling between room temperature and 450°C with x-ray diffraction. The strains are determined using a sin2(Ψ) analysis at each temperature. Direct comparisons are made between the TEM observations, the model and x-ray diffraction results.
Plastic Relaxation in Thin Copper Films
Authors: Shu Jonathan B. , Baker Shefford P.
Published: April 2011

We have studied the isothermal relaxation behavior of 500 nm Cu films with SiNx passivation and barrier layers on Si substrates. Oxygen content in the Cu films was varied by deposition in various oxygen partial pressures in the range 10-10 to 10-4 Torr. The substrate curvature method was used to investigate the thermomechanical behavior of the Cu films. Isothermal relaxation experiments were performed in the temperature range 50-175°C. Comparison with constitutive creep deformation equations shows that the relaxation data in this temperature range are well described by power law behavior. Under certain isothermal conditions related to temperature, stress, and thermal history, anelastic recovery was observed– i.e. while in a tensile stress state and with the temperature held constant, the overall film stress was seen to increase over a relatively short time scale.
Controlled Stress Refractory Metallizations
Authors: Golecki Ilan , Eagan Margaret
Published: April 2011

Rhodium and iridium are highly electrically conductive refractory metals, which can be used as current-carrying thin-film metallizations. Their chemical inertness further enables their application at relatively high temperatures. However, due to the high elastic modulus of such metals, a residual tensile stress of 300 to 400 MPa is measured in evaporated thin films. We present novel results evidencing complete control over both the magnitude and the sign of the residual stress in such refractory thin films. The metallic layers are deposited by means of ion-beam-enhanced physical vapor deposition and both electrical resistivity and stress are controlled. Controlling the stress in this manner has enabled achieving thicker films and films with near-zero residual stress.
Effects of Pressure on the Vibrational, Structural and Electronic Properties of C60 Powder and Thin Films
Authors: Sharma S. C. , Ha B. , Rhee J. H. , Li Y. , Singh D. , Govinthasamy R.
Published: April 2011

We present results from a study of the vibrational, structural, and electronic properties of C60 powder and thin films. Raman spectroscopy and diamond anvil cell have been used to study pressure dependence of the Raman active modes of C60 powder. The material undergoes structural phase transition between 9 and 15 GPa. Some of the Raman modes soften, while others harden with increasing pressure. Thin films of C60 and La-doped C60 have also been studied by using Raman scattering, x-ray diffraction, x-ray photoelectron spectroscopy and uv photoemission spectroscopy. Whereas the powder and La-doped C60 films exhibit fcc crystalline structure, the C60 film appears disordered. Further, we observe a significant difference in the electronic valence bands of the doped and undoped films.
Nanoindentation Analysis of Viscoelastic Thin Films: Strain Rate and Adhesion Effects
Authors: Palacio Manuel Luis B. , Scherban Tracey , Sun Brad , Xu Jessica , Gerberich William W.
Published: April 2011

The mechanical properties of two viscoelastic thin films were measured, a polyimide and a low-k polymer dielectric. The static modulus and hardness were obtained from nanoindentation experiments using an elastic-plastic unloading model (J. Mater. Res., 13, 421 (1998)). Nanoindentation creep tests were also performed, where a value for the modulus was extracted by fitting the data to an equation based on the three-element standard linear solid model. Aside from being comparable to values reported in the literature, the moduli from creep experiments are at most 1.3 times lower than the static moduli. This decrement can be attributed to the differences in strain rate for the two methods and the effect of adhesive forces.
Stress Hysteresis and Mechanical Characterization of Plasma-Enhanced Chemical Vapor Deposited Dielectrics
Authors: Thurn Jeremy , Cook Robert F. , Kamarajugadda Mallika , Stearns Laura C.
Published: April 2011

Two plasma-enhanced chemical vapor deposited (PECVD) dielectric films pertinent to microelectronic-based applications were examined for thermo-mechanical stability. Both films–silicon nitride and silicon oxy-nitride–showed significant permanent non-equilibrium changes in film stress on thermal cycling and annealing. The linear relationship between stress and temperature changed after the films were annealed at 300°C, representing a structural change in the film resulting in a change in coefficient of thermal expansion and/or biaxial modulus. A double-substrate method was used to deduce both properties before and after the anneal of selected films and the results compared with the modulus deconvoluted from the load-displacement data from small-scale depth-sensing indentation experiments.
The Mechanical Properties of Electroplated Cu Thin Films Measured by means of the Bulge Test Technique
Authors: Xiang Yong , Chen Xi , Vlassak Joost J.
Published: April 2011

The mechanical properties of freestanding electroplated Cu films were determined by measuring the deflection of Si-framed, pressurized membranes. The films were deformed under plane-strain conditions. The pressure-deflection data are converted into stress-strain curves by means of simple analytical formulae. The microstructure of the Cu films was characterized using scanning electron microscopy and x-ray diffraction. The yield stress, Young's modulus, and residual stress were determined as a function of film thickness and microstructure. Both yield stress and Young's modulus increase with decreasing film thickness and correlate well with changes in the microstructure and texture of the films.
Effect of Microstructure on Microhardness of AlN Thin Films
Authors: Miyabe Shuichi , Aono Masami , Kitazawa Nobuaki , Watanabe Yoshihisa
Published: April 2011

Aluminum nitride (AlN) thin films with columnar and granular structures were prepared by ion-beam assisted deposition method by changing nitrogen ion beam energy, and the effects of the film microstructure and film thickness on their microhardness were studied by using a nano-indentation system with the maximum force of 3 mN. For the columnar structure film of 600 nm in thickness, the microhardness is found to be approximately 24 GPa when the normalized penetration depth to the film thickness is about 0.1. For the granular structure film of 700 nm in thickness, the microhardness is found to be approximately 14 GPa. These results reveal that the microhardness of the AlN films strongly depends on the film microstructure, which can be controlled by regulating the nitrogen ion beam energy.
Mechanics of Indium-Tin-Oxide Films on Polymer Substrates with Organic Buffer Layer
Authors: Park Sung Kyu , Han Jeong In , Kim Won Keun
Published: April 2011

This study was undertaken to derive the understandings of the electro- mechanical properties of ITO films on highly elongated polymer substrates which were subjected to thermally and mechanically stresses. The substrates are polycarbonate foils with 100, 180 and 200-mm thicknesses with gas barrier layer on their bottom surface. The metallic films used in the experiments are indium- tin-oxide (ITO) and tantalum (Ta) films. Strain- gauge measurements and numerical analyses using modified Stoney formula were carried out to determine the electro-mechanics in ITO films. The numerical analyses and empirical results show the thermally and externally induced mechanical stresses in ITO films on polymer substrates are responsible for the difference in thermal expansion between the ITO films and the substrate and for substrate material and thickness, respectively. Therefore, a ramped heating processing and a buffer layer were employed to improve the electro- mechanics, and then, the effects of the buffer layer were also quantified in terms of conductivity-strain variations.
Nanoindentation Induced Fracture in Hard Multilayer Thin Films
Authors: Karimi A. , Bethmont D. , Wang Y.
Published: April 2011

Depth sensing nanoindentation and nanoscratch testing were combined with atomic force microscopy (AFM) and electron microscopy observations to study mechanical properties and fracture behavior of a number of TixAl1-xNyC1-y hard thin films. Various failure modes were activated either by normal loading-unloading or by microscratching of the samples to provide an estimation of the fracture toughness and interfacial fracture energies. By changing chemical composition and deposition conditions various nanostructured thin films including monolitically grown single layer, nanocomposite, and multilayers were coated onto the tungsten carbide-cobalt substrates. All tested films exhibit elevated mechanical properties with high hardness (38 – 45 GPa) and modulus (500 – 570 GPa). Under sufficiently high load indentation the formation of corner Palmqvist type radial cracks were usually observed because of small modulus mismatch between coating and substrate, good adhesion, and in particular high toughness of both substrate and films in spite of great difference in their respective hardness. Various failure modes were activated and the sequences of fracture events were determined using stepwise or continuously increasing load scratch tests. Some other films were found to be more sensitive to tensile stress behind the indenter which generates regularly repeated microcracks on the scratch track. Other films in particular multilayers were appeared more susceptible to compressive stress ahead of the indenter leading to local delamination at the interface between layers and the formation of irregular microcracks under the contact zone.
Multiple Cracks of a Thin Film on a Ductile Substrate
Authors: Zhao Ming-Hao , Fu Ran , Zhang Tong-Yi
Published: April 2011

The present work proposes a Dugdale type model to theoretically predict the multiple cracks of a residually tensile stressed thin film on a ductile substrate. The results show that there exists a critical value, Rc, of the cracking resistance number, R. When R < R, the film > c cracks and the normalized crack spacing is determined by cracking resistance number and the stress ratio of the residual stress to the yield strength of the substrate.
Stress Development and Adhesion Behavior in Thin Ceramic Coatings Monitored by Positron Annihilation During Bending
Authors: Galindo R. Escobar , Veen A. van , Schut H. , Carvalho N.J.M. , Strondl C. , Hosson J. Th. M. de
Published: April 2011

Positron Beam Analysis was for the first time performed in combination with a four-point bending stage. Applications of this novel technique on PVD TiN (5 m thick), WC:H (2.5 m) and CrN (1.5 m m) layers on tool steel, low carbon steel and stainless steel substrates are discussed. It appeared from PBA results that for these systems the coatings did not fail by delamination during bending. SEM confirmed that there was no decohesion but instead, cracking of the coatings was observed. In order to weaken the interface and quantify adhesion properties we propose to induce blistering based on localised gas implantation.
The Role of Chemical Composition for the Ductility and Microstructure of Thin NiAl Films
Authors: Wellner Patrick , Kraft Oliver , Arzt Eduard
Published: April 2011

In this work, NiAl films deposited by magnetron co-sputtering onto silicon substrates with compositions between 45.0 and 51.5 at-% Al and a thickness of 1.2 μm were studied. The films exhibited grain sizes comparable to the film thickness. Furthermore, the films were found to have fiber textures with the strongest component being (110) for the very Ni-rich and (111) for the stoichiometric and Al-rich films. The residual stresses in the Ni-rich and stoichiometric films after annealing were found to be independent of composition, while the Al-rich films showed a significant decrease in film stress. This stress drop can be related to severe cracking in the Al-rich films, which has not been observed in the stoichiometric and Ni-rich films, indicating that the fracture toughness depends on composition. Based on a simple estimate, the fracture toughness of these Al-rich films was determined to be in the range of 1 to 2 MPa m-1/2, which is somewhat smaller than commonly reported bulk values for polycrystalline NiAl.
Atomistic Simulations of Deposition Processes of Epitaxial Layers
Authors: Shintani K. , Sugii H. , Nishimura M. , Nakajima T.
Published: April 2011

The deposition process of epitaxial layers is simulated by the method of molecular dynamics. The interaction forces between atoms are calculated using the Stillinger-Weber potential. Before the deposition of atoms, the substrate is equilibrated at a specified temperature. Atoms with identical initial velocities and with initial coordinates selected at random in the plane parallel to the growth surface arrive at the substrate to form an overlayer. During the deposition, the lower part of the substrate is held at an initially specified temperature by velocity scaling. How the growth morphology of an overlayer is influenced by the substrate temperature is investigated in connection with the behavior of deposited atoms.
STM analysis of copper thin films using hyperthermal copper ions
Authors: Pomeroy J.M. , Cooper B.H. , Brock J.D.
Published: April 2011

STM analysis of thin copper films grown on Cu(111) and Cu(100) at room temperature using hyperthermal ions reveals several morphological features not present in thermally grown films. Hyperthermal deposition of thin films has become a popular industrial technique due to observed decreases in film roughness and stress with increased grain sizes, but the link between incidence energy and these properties is poorly understood. Using a sophisticated molecular dynamics (MD) simulation, morphologies observed in experimentally grown films are correlated with the activation of hyperthermal atomistic mechanisms in the MD. Previous work has provided strong evidence for activation of adatom-vacancy pair production near 20 eV on Cu(111). Evidence will be presented here for the activation of sputter erosion near 40 eV. A proposal for quantifying the effect of energy on nucleation density by modifying the flux term in mean field nucleation theory will be presented. Other processes which directly contribute to smooth growth, such as atomic insertion near 10eV, have been proposed but are more difficult to correlate with the data due to their subtle morphological signatures.
GaAs Heteroepitaxy on Fluoride by Electron Beam Induced Surface Modification
Authors: Fritz Tobias , Haiml Markus , Schön Silke , Keller Ursula
Published: April 2011

GaAs saturable absorber materials are used in ultrafast nonlinear optics to obtain all-optical switching in optoelectronic devices. They are introduced to semiconductor saturable absorber mirrors (SESAMs) for the generation of ultrashort laser pulses. GaAs is grown on CaF2 by molecular beam epitaxy to fabricate devices, which provide a large high reflection bandwidth. The CaF2 surface was exposed to high- and low-energy electron irradiation before and during growth to increase the surface free energy for the subsequent GaAs overgrowth. A three-layer GaAs/ fluoride device was designed and fabricated to study the impact of the electron exposure on the growth mode and the surface roughness. The surface morphology of an optoelectronic device can cause nonsaturable losses, which degrade the device performance. Therefore, the effect of the electron exposure of the CaF2 layer on the surface roughness of the device was studied by atomic force microscopy. Measurements of the scattered light from the device surface allowed for a quantitative analysis of the nonsaturable losses attributed to the surface morphology of the device.
Stress origin and relaxation in epitaxial AlN thin films on SiC
Authors: Bathe Ravi , Vispute R.D. , Habersat Daniel , Sharma R. P. , Venkatesan T. , Scozzie C. J. , Ervin M. H. , Jones Ken
Published: April 2011

We have investigated the epitaxy, surfaces, interfaces, and defects in AlN thin films grown on SiC by pulsed laser deposition. The stress origin, evolution, and relaxation in these films is reported. The crystalline structure and surface morphology of the epitaxially grown AlN thin films on SiC (0001) substrates have been studied using x-ray diffraction (θ–2θ, ω, and Ψ scans) and atomic force microscopy, respectively. The defect analysis has been carried out by using Rutherford backscattering spectrometry and ion channeling technique. The films were grown at various substrate temperatures ranging from room temperature to 1100 °C. X-ray diffraction measurements show highly oriented AlN films when grown at temperatures of 750- 800 °C, and single crystals above 800 °C. The films grown in the temperature range of 950 °C to 1000 °C have been found to be highly strained, whereas the films grown above 1000 °C were found to be cracked along the crystallographic axes. The results of stress as a function of growth temperature, thermal mismatch, growth mode, and buffer layer thickness will be presented, and the implications of these results for wide band gap power electronics will be discussed.
Surface reconstruction and induced uniaxial magnetic fields on Ni films
Authors: Lukaszew R. A. , McNaughton B. , Stoica V. , Clarke R.
Published: April 2011

Metal ceramic interfaces are important in applications as diverse as magnetic storage media to support catalysts. Understanding the correlation between the films structure and its properties is very important for applications. For example the lattice mismatch between substrate and epilayer during heteroepitaxial growth induces strain as well as defects on the structure of the films that in turn affect the magnetic properties.

We have previously shown that it is possible to obtain epitaxial (001) and (111) Ni films grown on MgO substrates. [1] In particular we observed that the crystalline quality of the films improved considerably after 10 nm of film growth and annealing of the films indicated significant reduction of the surface roughness. In addition, after annealing (001) Ni films we observed nano-patterning of the surface through self-assembly of periodic “stripes”.

We will now present our analysis of the magnetic properties of these films, in particular the azimuthal dependence of the magnetization reversal, using MOKE, and we will attempt to correlate our findings with the surface characterization obtained with STM.
Morphologies of Self-Assembled Quantum Dots: A Variational Approach
Authors: Budiman R. Arief , Ruda Harry E.
Published: April 2011

We construct a 3D model for coherent island formation by (i) using a novel 3D strain tensor to account for bulk strains and (ii) representing adatom di.usion as an external field that perturbs an otherwise. at strained layer. Equilibrium shapes of coherent islands and wetting layer thickness are obtained. Coherently compressed layers are typically unstable, but become stable in tension. Comparisons with Si1-xGex/Si(001) and Si0.5Ge0.5/Si1-xGex(001) layers are discussed.
Growth and characterization of Ge nanostructures on Si(111)
Authors: Rosei F. , Motta N. , Sgarlata A. , Balzarotti A.
Published: April 2011

Scanning Probe Microscopy (SPM) in situ is used to study the evolution of Ge islands grown by Physical Vapor Deposition on Si(111) 7×7 reconstructed surfaces. Large 3D islands form on the Wetting Layer (WL), with average lateral dimension in the range 200 - 500 nm. The statistical distribution of the island shapes has been analyzed, showing that three types of shapes coexist under certain conditions: strained, partially relaxed and ripened (atoll-like) islands. We measured the contact angles of the island facets, and observed the depletion of the substrate around the ripened islands. These features are attributed to the misfit strain, which is partially relieved by interdiffusion of Si into the Ge layers.
Interface Engineering in Type-II CdSe/BeTe Quantum Dots
Authors: Ivanov Sergey V. , Shubina Tatiana V. , Sorokin Sergey V. , Toropov Alexey A. , Kyutt Reginald N. , Sitnikova Alla A. , Willander Magnus , Waag Andreas , Landwehr Gotfried
Published: April 2011

Stressor-controlled epitaxy has been proposed as an efficient method of CdSe quantum dot fabrication. The studies are performed on a type-II CdSe/BeTe system, where CdTe and BeSe inteface bonds play a role of intrinsic stressors. Predeposition of ~0.2 ML CdTe stressor ( Δ a/a= +15%), corresponding to a local maximum of RHEED specular spot intensity, appears to induce variation of stress field across the BeTe surface, caused by alternating regions with CdTe and BeSe bonds. It results in preferential nucleation of regularly arranged CdSe QDs at the BeSe sites with the following vertical chess-ordering in the CdSe/BeTe multilayers. The structures demonstrate bright up to RT PL in the 1.9-2.1 eV range and strong in-plane PL anisotropy related to non-equivalent bottom and top CdSe QD interfaces having estimated from x-ray diffraction total concentrations of CdTe and BeSe bonds of 0.3-0.4 and 0.6-0.7 ML, respectively.
Optical Characterization Of Self-Assembled Ge Dots On Silicon
Authors: Marabelli F. , Rastelli A. , Valsesia A. , Känel H.von
Published: April 2011

Self assembled quantum dots of Ge were obtained by magnetron sputter epitaxy of seven monolayers of Ge on a 33nm thick undoped Si buffer grown on top of a p-doped (100) Si substrate. The samples obtained in this manner were then capped with an increasing number of silicon layers in order to study the effect of Si deposition on the strain and the morphology of the dots. They were characterized “ex situ” by spectroscopic ellipsometry and Raman spectroscopy. The optical experiments revealed well defined differences between the capped and uncapped samples and among samples with different cap thicknesses.

By monitoring the energy and the splitting of the E0', E1 and E2 interband optical transitions of Ge and the Ge-Si vibrational mode, the optical measurements evidence strain effects as well as the formation of SiGe alloy, in agreement with the ``in situ'' STM measurements.
Stress relaxation during two-dimensional pseudomorphic epitaxial growth of metals
Authors: Andrieu Stéphane , Turban Pascal , Lapena Laurent , Muller Pierre
Published: April 2011

The analysis of RHEED diffraction peaks during two-dimensional (2D) pseudomorphic epitaxial growth leads to the well known RHEED oscillations but also to diffraction peak width and in-plane lattice spacing oscillations. These behaviors are evidenced in several metallic A/B systems in this paper. As in-plane lattice spacing oscillations are assumed to be due to elastic relaxation at the edge of the 2D Islands, we try to correlate the amplitude of the detected effect with the misfit. The width oscillations are assumed to be due to the scattering coming from islands. We actually show that the full width at half maximum (FWHM) at half coverage allows us to get a correct estimation of the nucleation density. We thus show experimentally that the in-plane lattice spacing oscillations amplitude depends on the nucleation density determined using FWHM measurements. Finally, a theoretical justification allows us to show that this amplitude also depends on the Young modulus ratio of both B and A layers.
Plume and Jetting Regimes in a Laser Based Forward Transfer Process as Observed by Time-Resolved Optical Microscopy
Authors: Young D. , Auyeung R. C. Y. , Piqué A. , Chrisey D. B. , Denham H. , Dlott Dana D.
Published: April 2011

Matrix-Assisted Pulsed Laser Evaporation Direct-Write was investigated by ultra high-speed optical microscopy. A layer of viscous fluid was irradiated with 355nm, 30 ns laser pulses in a laser-forward transfer configuration. The fluid response as a function of fluence was studied, and several distinct regimes of behavior were observed: plume, jetting and sub-threshold. However, the transition between plume and jetting regimes was not readily evident in a study of transfer pixel area vs. fluence, which may be explained by material-substrate interactions.
Direct Ink Jet Printing of Alumina Components
Authors: Ainsley C. , Reis N. , Derby B.
Published: April 2011

Alumina slurries containing up to 40% by volume ceramic particles suspended in alkane waxes have been developed with viscosity sufficiently low to allow direct ink-jet printing. Components fabricated by printing have been dewaxed, sintered and shrinkage and density measured. Small differences in shrinkage have been measured parallel and perpendicular to the droplet motion but these are within experimental scatter. The manufacturing process appears to generate bands of slightly larger grain size material at the interface between deposited layers.
Analysis of Drop-on-Demand Ink Jet Print Head for Rapid Prototyping
Authors: Shin Dong-Youn , Grassia Paul , Derby Brian
Published: April 2011

The rapid prototyping industry is growing dramatically because of its high potential to reduce product design and prototyping cycles. One of the recent technologies in this field is 3D printing using conventional ink jet technology. In order to maximize the capability of this process, it is required to understand the operating mechanism and drop formation process. The current work focuses on the mechanism of a piezoelectric cylindrical actuator and the hydrodynamic characteristics inside a print head in order to achieve more realistic boundary conditions for the numerical simulation of the drop formation process. Linearised Navier-Stokes equations for Newtonian fluid flow are solved analytically for the pump section with a constant radius and for the nozzle section with a tapering angle. Results from the developed solutions are input to Flow 3D and it is observed that analytical pressure histories show better agreement with numerical results than axial velocity histories. The presented analytic model can be used for fully further drop formation simulation as an upstream pressure boundary within an acceptable tolerance.
An ab initio Investigation on the Effects of Impurity in Aluminum Grain Boundary
Authors: Lu Guang-Hong , Tamura Tomoyuki , Kamiko Masao , Kohyama Masanori , Yamamoto Ryoichi
Published: April 2011

The electronic structure of AlS9 tilt grain boundary with segregated impurity atoms of Na, Ca, Si and S, respectively, has been investigated by an ab initio pseudopotential method. Na and Ca segregation causes the boundary to expand and the charge density to decrease significantly. There forms several weak bond regions. Si segregation increases the charge density between Si and the neighboring Al atom. There forms a stronger Al-Si bond that is a mixture of covalent and metallic character in the boundary. For S segregation, though there forms the stronger bond between Al and S atom, some Al-S bonds may become weaker than the former Al-Al bonds because of the charge density decrease. It is concluded that the mechanism of Na or Ca-promoted Al grain boundary embrittlement is one kind of ‘decohesion model’, that of Si is ‘bond mobility model’. It can't be decided the embrittlement mechanism by S segregation is classified into ‘bond mobility model’ or ‘decohesion model’.
Effect of Grain Boundaries and Indentation Load on the Electrical Properties of Nickel Base Super-alloys
Authors: Pinkos Kimberly , Laboy Celestina , Gerhardt Rosario A.
Published: April 2011

We have used impedance spectroscopy to evaluate the effect of the presence of grain boundaries and plastic deformation, as caused by hardness indentation, on the electrical response of several commercial nickel base super-alloys at room temperature. These alloys consist of a mostly nickel matrix that contains small precipitates of intermetallic phases such as Ni3Al or Ni3Ti(often referred to as gamma prime) as a reinforcing phase. Measurements were made as point contacts so that data could be tracked from point to point. Results indicate that the grain boundaries tend to have higher conductivities than the individual grains in most cases. It is speculated that this is due to the boundary regions having a different compositional profile than the center of the grains(as determined by gamma prime size, shape and distribution). Hardness indentation, on the other hand, had a more dramatic effect, by causing the magnitude of the imaginary impedance to change in size as well as position. Complementary microscopy results are included as supporting evidence for the effects discussed.
Phase Transformation Hysteresis in a Plutonium Alloy System: Modeling the Resistivity during the Transformation
Authors: Haslam Jeffery J. , Wall Mark A. , Johnson David L. , Mayhall David J. , Schwartz Adam J.
Published: April 2011

We have induced, measured, and modeled the β - α' martensitic transformation in a Pu-Ga alloy by a resistivity technique on a 2.8-mm diameter disk sample. Our measurements of the resistance by a 4-probe technique were consistent with the expected resistance obtained from a finite element analysis of the 4-point measurement of resistivity in our round disk configuration. Analysis by finite element methods of the postulated configuration of α' particles within model σ grains suggests that a considerable anisotropy in the resistivity may be obtained depending on the arrangement of the α' lens shaped particles within the grains. The resistivity of these grains departs from the series resistance model and can lead to significant errors in the predicted amount of the α' phase present in the microstructure. An underestimation of the amount of α' in the sample by 15%, or more, appears to be possible.
Effects of grain size distribution on Cole-Cole plots of polycrystalline spinels
Authors: Gutiérrez-Amador M. P. , Valenzuela R.
Published: April 2011

Spinel ferrites of composition Ni0.5Zn0.5Fe2O4 were prepared by coprecipitation. Different grain size and grain size distributions were obtained by various heat treatments. Their electrical properties were investigated by impedance spectroscopy in the temperature range 25-200°C and in the frequency range of 5Hz-13MHz. A decrease in resistivity was observed as grain size increased, which can be explained by an increase in the mean free path of electrons. Cole-Cole plots showed two well-resolved semicircles for samples with a narrow grain size distribution. As the width of the distribution increased, the semicircles exhibited a deformation and eventually became unresolved. These results are interpreted on the basis of a distribution of time-constant of the impedance response, associated with the grain size distribution.
Charge-Based Deep Level Transient Spectroscopy of Semiconducting and Insulating Materials
Authors: Polyakov V.I. , Rukovishnokov A.I. , Rossukanyi N.M. , Druz B.
Published: April 2011

The opportunity of the charge-based deep level transient spectroscopy (Q-DLTS) for study of the structures based on wide bandgap semiconducting and insulating materials such as diamond and Al2O3 was demonstrated. Using our isothermal Q-DLTS method with rate window (tm)scanning we obtained information about concentration, activation energy and capture cross-section of the native and extrinsic electrical active defects - trapping centers (TC) in slightly boron-doped polycrystalline diamond, diamond single-crystal and in the structures Al2O3 film on NiFe and Si substrates. In comparison with widely used capacitance-based deep level transient spectroscopy, Q-DLTS gives one possibility to investigate the structures in which a capacitance does not depend on the charge state of the surface and bulk traps.
Influence of Substrate Annealing Temperature upon Deep Levels in n-Type 4H SiC
Authors: Kordesch Martin E. , Perjeru Florentina , Woodin R. L.
Published: April 2011

The evolution of deep levels that depend upon annealing temperature is investigated for n-type 4H SiC-Ni Schottky barriers. Several samples, cut from the same wafer, have been left unheated or annealed at 400°C, 700°C and 900°C, in air. Deep level transient spectroscopy (DLTS) has been used to investigate deep levels in all four sets of samples. Electron traps with activation energies EC-ET = 0.19 to 0.5 eV are observed, as well as a hole trap in the sample annealed at 900°C at energy ET-EV = 0.14 eV.
Capacitance Spectroscopy of n-i-n and p-i-p GaAs Sandwich Structures with Nanoscale As Clusters in the i-Layers
Authors: Chaldyshev V.V. , Brunkov P.N. , Chernigovskii A. V. , Moskalenko A. , Bert N.A. , Konnikov S. G. , Preobrazhenskii V. V. , Putyato M. A. , Semyagin B. R.
Published: April 2011

Accumulation of electrons and holes in GaAs layers, that contained As clusters and were sandwiched between n- and p-type buffer GaAs layers, was revealed by capacitance-voltage measurements. As a result of majority-carrier accumulation, expansive depletion regions are formed in the adjoining buffer layers. Simulation of the capacitance-voltage characteristics, based on a numerical solution of the Poisson equation, shows that the accumulated charge density is ∼1 × 1012 cm−2, which is comparable with the concentration of As nanoclusters determined by transmission electron microscopy. The levels where the electrons or holes are accumulated lie close but above the GaAs midgap. A strong difference in the emission rates of the accumulated electrons and holes has been revealed.
Microwave Transient Photoconductivity Studies in Porous Semiconductors
Authors: Porteanu Horia-Eugen , Konstantinova Elisaveta , Kytin Vladimir , Loginenko Oleg , Timoshenko Victor , Dittrich Thomas , Koch Frederick , München T.U.
Published: April 2011

The dynamics of the photogenerated carriers in porous silicon and TiO2 anatase was studied at 35 GHz by measuring the change in time of the conductivity s and dielectric constanter. Localization of carriers leads to a positive change ofer, while quasifree carriers to a negative change. Size reduction in Si shortens the recombination time as long as the surface traps are not significant. Magnetic field investigations show opposite variation of conductivity in poroussilicon compared with TiO2.
Electrochemical Porosimetry
Authors: Song Hyun-Kon , Lee Kun-Hong
Published: April 2011

We have developed the electrochemical porosimetry analyzing microstructures of porous electrodes, which can give geometric information most meaningful in electrochemical systems. The methodology is based on the transmission line model with pore size distribution (TLM-PSD) that relates electrochemical impedance data with microstructural information. Pore length (lp), as well as pore size distribution, can be obtained by fitting the TLM-PSD to the experimental impedance data of a porous electrode. This geometric information was validated for the microporous, mesoporous and macroporous samples by comparing with the data obtained from conventional porosimetry. It was also shown that the electrochemical porosimetry could be used as a nondestructive probe to investigate the construction of electrochemical devices.
Microstructure and Damage of the Interlaminar Interface of Carbon Fiber Polymer-Matrix Composites, Monitored by Contact Electrical Resistivity Measurement
Authors: Wang Shoukai , Guerrero Victor H. , Kowalik Daniel P. , Chung D.D.L.
Published: April 2011

The microstructure and damage of the interlaminar interface of continuous carbon fiber polymer-matrix structural composites were monitored during dynamic changes in temperature, humidity and stress by contact electrical resistivity measurement. The stress was compressive, in the direction perpendicular to the interface. Temperature, humidity and stress had reversible effects on the resistivity, due to the effects on the interfacial microstructure, such as the extent of contact between fibers of adjacent laminae. These parameters had irreversible effects on the resistivity, due to relaxation and damage of the interface. The effects differed between composites with thermoplastic and thermoset matrices. The reversible effects allow the use of the contact resistivity as an indicator of temperature, humidity and stress.
A Percolation Equation for Modeling Experimental Results for Continuum Percolation Systems
Authors: McLachlan D.S. , Chiteme C. , Heiss W.D. , Wu Junjie
Published: April 2011

The standard percolation equations or power laws, for dc and ac conductivity (dielectric constant) are based on scaling ansatz, and predict the behaviour of the first and second order terms, above and below the percolation or critical volume fraction (øc), and in the crossoverregion. Recent experimental results on ac conductivity are presented, which show that these equations, with the exception of real σm above øc and the first order terms in the crossover region, are only valid in the limit σi/σc = 0, where for an ideal dielectric σi=ωε0εr.

A single analytical equation, which has the same parameters as the standard percolation equations, and which, for ac conductivity, reduces to the standard percolation power laws in the limit σi(ωε0εr)/σc = 0 for all but one case, is presented. The exception is the expression for real σm below øc, where the standard power law is always incorrect. The equation is then shown to quantitatively fit both first and second order dc and ac experimental data over the entire frequency and composition range. This phenomenological equation is also continuous, has the scaling properties required at a second order metal-insulator and fits scaled first order dc and ac experimental data. Unfortunately, the s and t exponents that are necessary to fit the data to the above analytical equation are usually not the simple dimensionally determined universal ones and depend on a number of factors.