bluuebunny/crossref_metadata_2025 huggingface dataset:
The Chemistry of GaN Growth
Authors: Kuech T.F. , Gu Shulin , Wate Ramchandra , Zhang Ling , Sun Jingxi , Dumesic J.A. , Redwing J.M.
Published: April 2011

The development of new chemically based growth techniques has opened the range of possible GaN applications. This paper reviews some of the challenges in the chemically based growth of GaN and related materials. Ammonothermal-based growth, hydride vapor phase epitaxy and metal organic vapor phase epitaxy (MOVPE) are chemically complex systems wherein the underlying mechanisms of growth are not well understood at present. All these systems require substantial experimental and theoretical efforts to determine the nature and kinetics of GaN growth. In the case of metal organic vapor phase epitaxy, the application of computational techniques based on density functional theory have augmented the more conventional experimental approaches to determining the growth chemistry. These chemical reaction schemes, when combined with computational thermal-fluid models of the reactor environment, provide the opportunity to predict growth rates, uniformity and eve ntually materials properties.
Performance characteristics of cw InGaN multiple-quantum-well laser diodes
Authors: Kneissl Michael , Wong William S. , Walle Chris. G. Van de , Northrup John E. , Treat David W. , Teepe Mark , Miyashita Naoko , Kiesel Peter , Johnson Noble M.
Published: April 2011

The performance characteristics are reported for continuous-wave (cw) InGaN multiple-quantum-well laser diodes grown on epitaxially laterally overgrown GaN on sapphire substrates by metalorganic chemical vapor deposition. Room-temperature cw threshold currents as low as 41mA with operating voltages of 6.0V were obtained. The emission wavelength was near 400 nm with output powers greater than 20 mW per facet. Under cw conditions laser oscillation was observed up to 90°C. A significant reduction in thermal resistance was observed for laser diodes transferred from sapphire onto Cu substrates by excimer laser lift-off, resulting in increased cw output power of more than 100mW.
Non-Stationary Photoconductivity of GaN Nanocomposites In Artificial Opal Matrix
Authors: Niehus M. , Koynov S. , Schwarz R. , Feoktistov N.A. , Golubev V.G. , Kurdyukov D.A. , Pevtsov A.B.
Published: April 2011

It was recently proposed to use synthetic opals as a host matrix for obtaining 3D arrays of electronic nanodevices [1]. In the present work the opal matrices were infiltrated with GaN. We study electronic properties of opal-GaN, by means of transient photoconductivity (TPC) measurements using 5 ns laser pulses at wavelengths above (266 nm) and below (532 nm) the GaN bandgap (3.4 eV). A broad plateau is observed in the photocurrent decay covering several orders of magnitude. We compare the results with measurements in conventional GaN.
Plasma-Induced Effects on The Thermal Conductivity of Hydride Vapor Phase Epitaxy Grownn-GaN/Sapphire (0001)
Authors: Florescu D.I. , Pollak Fred H. , Lanfor William B. , Khan Farid , Adesida I. , Molnar R.J.
Published: April 2011

We have measured high spatial/depth resolution (2-3 [.proportional]m) thermal conductivity (κk) at 300K before and after plasma-induced effects on a series ofn-GaN/sapphire (0001) samples fabricated by hydride vapor phase epitaxy (HVPE) using a ThermoMicroscope'as scanning thermal microscope (SThM). The sample thicknesses were 50 ± 5 [.proportional]m and the carrier concentrations ~ 8 × 1016cm-3, as determined by Hall effect measurements. The thermal conductivity before treatment was found to be in the 1.70 – 1.75 W/cm-K range, similar to that previously reported for HVPE material with this carrier concentration and thickness [D. I. Florescuet al., J. Appl. Phys. 88, 3295 (2000)]. The samples were processed under constant Ar gas flow and pressure fora fixed period of time (5 min). The only variable processing parameter was the DC bias voltage (125 – 500 V). After the initial 125 V procedure κ exhibited a decrease linear in the DC voltage in the investigated range. At 125 V the thermal conductivity was only slightly less (κ ~ 1.65 W/cm-K) than the untreated case. κ had dropped to ~ 0.3 W/cm-K for the 500 V situation. The implications of these results for device applications in the area of high power opto-electronics and high power electronics will be discussed.
Effect of Externally-Imposed Radial Strain on the Piezoelectric Response of MOCVD-Grown Gallium Nitride
Authors: Himes Jennifer A. , Willis James R. , Gulino Daniel A.
Published: April 2011

The large piezoelectric constants of GaN suggest possible application of GaN-based materials in piezoelectric sensors, among other areas. GaN's wide band gap implies that these sensors will fare well over a broad temperature range and/or in a harsh environment.

In this work, films of gallium nitride approximately 0.75 micron thick and grown by MOCVD were subject to an externally-imposed radial stress condition. Deposition was performed in a commercial MOCVD reactor (CVD, Inc.) at 1323K using trimethylgallium and ammonia as the chemical precursors. The substrate was one-inch diameter silicon (111). After deposition, titanium dots were deposited in various locations, including the wafer center, by evaporation. Stress was applied to the film/substrate system using a modified micrometer head (Mitutoyo) mounted to an Ionic Systems Basic Stressgauge (model 30285). Stress levels were calculated based on the magnitude of the imposed deflection as read from the micrometer head display, and the piezoelectric response at any particular dot with respect to the center dot was measured by measuring the voltage difference using a digital multimeter (Keithley 175). The micrometer head impinged on the center dot and served as one electrical contact point.

Effective piezoelectric coefficients were measured as a function of imposed radial stress. Applied stresses in the range of 1 to 5 GPa resulted in effective piezoelectric coefficients ranging from –0.6 to –2.0 × 10-5 C/m2
A new mechanism in the growth process of GaN by HVPE
Authors: Trassoudaine A. , Aujol E. , Cadoret R. , Paskova T. , Monemar B.
Published: April 2011

Experimental results obtained in two different HVPE reactors are analyzed and compared to the theoretical curves, taking into account the surface kinetics, the mass transfer, and parasitic deposition on the glass walls before the substrate. Unexpected high growth rate values, up to 50νm/h, were measured in conditions of expected substrate etching by HCl. A systematic experimental study of this new phenomenon is presented together with a theoretical analysis. This analysis suggests a new mixed general mechanism of growth.
Vapor Phase Synthesis and Characterization of Gallium Nitride Powders
Authors: Hara Kazuhiko , Matsuo Yoshinori , Matsuno Yuuki
Published: April 2011

GaN crystalline powders have been synthesized by the reaction of a Ga vapor with an ammonia gas at the reaction temperature Tr = 900 - 1100°C in an atmospheric-pressure open-tube reactor. The size of GaN particles ranges from 0.2 to 2νm. It was found that the structural and luminescent properties depend strongly on Tr. The mean size of the GaN particles increased as Tr is raised. The GaN powders exhibited photoluminescence (PL) dominated by the band edge emissions. Thermal quenching is relatively significant for the powders synthesized at lower Tr. This is presumably due to enhanced non-radiative recombination at the surface because of their smaller particle size.
Interaction Between Basal Stacking Faults and Prismatic Inversion Domain Boundaries in GaN
Authors: Komninou Philomela , Kioseoglou Joseph , Sarigiannidou Eirini , Dimitrakopulos George P. , Kehagias Thomas , Georgakilas Alexandros , Nouet Gerard , Ruterana Pierre
Published: April 2011

The interaction of growth intrinsic stacking faults with inversion domain boundaries in GaN epitaxial layers is studied by high resolution electron microscopy. It is observed that stacking faults may mediate a structural transformation of inversion domain boundaries, from the low energy types, known as IDB boundaries, to the high energy ones, known as Holt-type boundaries. Such interactions may be attributed to the different growth rates of adjacent domains of inverse polarity.
Microstructure of GaN Films Grown by RF-Plasma Assisted Molecular Beam Epitaxy
Authors: Komninou Philomela , Kehagias Thomas , Kioseoglou Joseph , Sarigiannidou Eirini , Karakostas Theodoros , Nouet Gerard , Ruterana Pierre , Amimer Khalid , Mikroulis Spyros , Georgakilas Alexandros
Published: April 2011

The influence of the variation of the Ga/N flux ratio during deposition and of the different substrate nitridation temperatures on the microstructure of 2H-GaN films grown on (0001) sapphire, by RF plasma MBE, is investigated by conventional and high resolution Transmission Electron Microscopy (TEM-HREM). The different growth rates of the inverse polarity domains in Ga-rich and N-rich specimens result in film surfaces of different roughness, whereas the stacking fault (SF) content is significantly higher in samples grown under N-rich conditions. Low temperature nitridation of the substrate results in a low density of defects in GaN film. Cubic GaN “pockets”, near the substrate/GaN interface that are present in low temperature nitridated samples are not observed in high temperature nitridated samples.
Novel Cubic ZnxMg1−xO Epitaxial Heterostructures on Si (100) Substrates
Authors: Kvit A. , Narayan J. , Sharma A.K. , Jin C. , Muth J.F. , Teng C.W. , Holland O.W.
Published: April 2011

We have synthesized new cubic phase of ZnxMg1−xO alloy, which can be grown epitaxially on MgO (100) by lattice-matching epitaxy, and on Si (100) substrate by our domainmatching epitaxy for integration with silicon microelectronic devices. Cubic ZnxMg1−xO films on MgO (100) and Si (100) substrates were grown using a rotating target in a single chamber “insitu” pulsed-laser deposition system. Integration of ZnxMg1−xO films with silicon was accomplished via titanium nitride (TiN) buffer layers where four lattice constants of TiN match with three of the silicon during epitaxial growth via domain epitaxy. Rutherford backscattering/ion channeling techniques were used to determine chemical composition and crystalline quality of the films for x = 0.0-0.18. Detailed X-ray diffraction and transmission electron microscopy studies confirmed the epitaxial nature of ZnMgO/MgO (100) and ZnMgO/TiN/Si (100) heterostructures, and showed the formation of the Mg2−xZnxTiO4 spinel at the interface with TiN. Using optical transmission measurements, the band gap of cubic Zn0.18Mg0.82O film was estimated to be approximately 6.7 eV. The potential use of these alloys for optical devices in the ultraviolet range is discussed.
Low Resistance Optically Transparent Contacts to p–type GaN Using Oxidized Ni/Au and ITO for LED Application
Authors: Lin C. H. , Hibbard D. L. , Au A. , Lee H. P. , Dong Z. J. , Szalkowski F. J. , Chen J. , Chen C.
Published: April 2011

We report on a high transparency low resistance contact to p-GaN composed of a thin oxidized Ni/Au bilayer overcoated with indium tin oxide (NiO/Au/ITO). The NiO/Au/ITO layer shows a specific contact resistivity, c, of 1.8 × 10−3 Ωcm2 that is nearly ten times lower than conventional Ni/Au annealed under N2. Measurements on fully processed LEDs with a NiO/Au/ITO current spreading layer (CSL) show an operating voltage of around 4 V at 20 mA that is comparable to LEDs fabricated with a conventional Ni/Au CSL and a dramatic improvement over the previous ITO data. LED top surface light emission through the NiO/Au/ITO CSL is shown to be greater than that from LEDs with a conventional semi-transparent Ni/Au CSL. Taken together, these results demonstrate the feasibility of using NiO/Au/ITO as a CSL for high performance GaN LEDs.
Can Laterally Overgrown GaN Layers be free of Structural Defects?
Authors: Cherns D. , Liliental-Weber Z.
Published: April 2011

Transmission electron microscopy has been used to examine dislocations present in an epitaxial laterally overgrown (ELOG) sample of GaN grown on (0001)sapphire. Studies of both plan-view and cross-sectional samples revealed arrays of dislocations present in the (11-20) boundary between the seed and the wing (overgrown) material and at the meeting front between adjacent wings, as well as dislocations in the form of half-loops extending into the wing regions. Both the boundary and half-loop dislocations had 1/3<11-20> Burgers vectors which were either perpendicular (boundary dislocations) or at 30°s (half-loops) to the boundary plane. Large angle convergent beam electron diffraction was used to show that the boundary dislocations and halfloops correlated respectively with tilts and twists of the wing material about (11-20). A model is proposed whereby the half-loops are generated from threading dislocations by shear stresses acting along the stripe direction. The origin, and elimination, of these stresses is discussed.
Degradation of Luminescence from GaN During Electron Bombardment: Effects of Beam Voltage, Current and Scanned Area
Authors: Campo E. M. , Cargill G. S. , Ramer J. , Schurman M. , Ferguson I. T.
Published: April 2011

Near-band-edge cathodoluminescence emission for some nominally undoped GaN samples decreases with time during electron bombardment in a scanning electron microscope. The rate of decrease depends on the incident beam voltage and current and on the size of the area being scanned, which is determined by the magnification used for the scanning electron microscope. Faster intensity decreases occur with lower beam voltages (10kV versus 20kV), higher beam currents (600nA or 200nA versus 35nA) and smaller scanned areas (120K magnification versus 15K magnification). For larger scanned areas, the maximum luminescence intensity occurs 10's or 100's of seconds after the start of electron bombardment, and the intensity then decreases with further bombardment. In some cases the size of the degraded region is larger than the bombarded area. These observations suggest that the degradation is caused by electromigration of impurities or other charged defects within the GaN resulting from local charging by electron bombardment.
Optical Constants, Critical Points, and Phonon Modes of GaAsN Single Layers
Authors: Leibiger G. , Gottschalch V. , Kasik A. , Rheinländer B. , Šik J. , Schubert M.
Published: April 2011

Spectroscopic ellipsometry (SE) is employed to study the optical properties of GaAs1−yNy [0% ≤ y ≤ 3.7%] single layers for photon energies from 0.75 eV to 4.5 eV and for wavenumbers from 100 cm−1 to 600 cm−1. We provide parametric model functions for the dielectric function spectra of GaAsN in both photon energy ranges. The model functions for photon energies from 0.75 eV to 4.5 eV excellently match dielectric function data obtained from a numerical wavelength-by-wavelength inversion of the experimental data. Criticalpoint analysis of the ellipsometric data is performed in the spectral regions of the fundamental band gap and the critical points E1 and E1+δ1. The band-gap energy is red shifted whereas the E1 and E1+δ1 transition energies are blue shifted with increasing y. For y ≤ 1.65% the observed blue shift of the E1 energy is well explained by the sum of the effects of biaxial (001) strain and alloying. The GaAsN layers show two-mode behaviour in the infrared spectral range (100 cm−1 to 600 cm−1). We detect the transverse GaAs- and GaN- sublattice modes at wavenumbers of about 267 cm−1 and 470 cm−1, respectively. The polar strength of the GaN TO mode increases linearly with y. This effect can be used to monitor the nitrogen composition in GaAsN layers.
Raman Analysis of Single Crystalline Bulk Aluminum Nitride: Temperature Dependence of the Phonon Frequencies
Authors: Hayes Jonathan M. , Kuball Martin , Shi Ying , Edgar James H.
Published: April 2011

The frequencies of the E2(high), A1(LO), A1(TO), E1(TO) and E1(LO) phonons of singlecrystalline bulk AlN were measured using micro-Raman spectroscopy over a temperature range from 10K to 1275K. A modeling of the temperature dependence of the AlN phonon frequencies considering the thermal lattice expansion and two-phonon decay mechanisms gave results in good agreement with the experimental data. At temperatures in excess of ∼300K an approximate linear shift of the phonon frequencies with temperature was found. In this high temperature regime, we determined a frequency shift of the E2(high) phonon of (-2.22 ± 0.02) ×10−2cm−1/K, which is very similar to values reported for bulk GaN. This suggests that similar parameters will be suitable for AlxGa1−xN alloys, commonly used in high-power high-frequency electronic devices. The results provide the basis for non-invasive local temperature monitoring in highpower III-nitride devices using micro-Raman scattering techniques.
High Magnetic Field Studies of AlGaN/GaN Heterostructures Grown on Bulk GaN, SiC, and Sapphire Substrates
Authors: Knap W. , Borovitskaya E. , Shur M. S. , Gaska R. , Karczewski G. , Brandt B. , Maude D. , Frayssinet E. , Lorenzini P. , Grandjean N. , Massies J. , Yang J. W. , Hu X. , Simin G. , Khan M. Asif , Skierbiszewski C. , Prystawko P. , Grzegory I. , Porowski S.
Published: April 2011

We present the results of the high magnetic field studies of properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures grown over high-pressure bulk GaN, sapphire, and insulating SiC substrates. The experimental results include the low field Hall measurements, cyclotron resonance measurements, and cryogenic temperature Quantum Hall Effect studies as well as room-temperature characteristics of High Electron Mobility Transistors fabricated on all these substrates. The room temperature high field measurements allow us to clearly separate the contributions of a parasitic parallel conduction from 2DEG conduction in all investigated heterostructures.

The magnetotransport measurements are performed in the magnetic fields up to 30 Tesla for temperatures between 50mK-300K. This high magnetic field in combination with very high mobilities (over 60.000 cm2/Vs) in the sample on the bulk GaN substrates allow us to observe features related both to cyclotron resonance and spin splitting. The temperature dependence of this splitting determines the spin and cyclotron resonance energy gaps and, in combination with cyclotron resonance and tilted field experiments, allows us to determine the complete energy structure of 2DEG conduction band. We also present the first experimental results showing so called “the exchange enhancement” of the energy gaps between spin Landau levels.
Surface structure and polarization effects in GaN Thin Films as studied by Electric Force Microscopy
Authors: Jones K. M. , Visconti P. , Yun F. , Reshchikov M. A. , Baski A. A. , Morkoç H.
Published: April 2011

Stress induced piezoelectric fields in III-nitride semiconductors induce charge at the surface that is comparable to the free carrier concentration intended by doping. These fields, coupled with mixed Ga-polarity and N-polarity within the film, are detrimental to device performance due to an additional scattering caused by a reversal of the electric field normal to the surface. Consequently, investigation of the polarization effects is important and timely as nitride semiconductor devices are being contemplated for system applications. By imaging the surface potential by electric force microscopy (EFM), both a qualitative and a quantitative measure of polarization induced charge distribution across the surface could be obtained. In this vein, we undertook an EFM study in a series of samples. EFM images of as-grown and progressively etched samples not only led to the identification and imaging of inversion domains, but also showed the evolution of different polarities. The polarization of the surface was calculated and was found to agree reasonably well with the values found experimentally. The role of polarization was addressed by extraction of the second harmonic term from EFM signal.
Universal behavior of the pressure coefficient of the light absorption and emission in InGaN structures
Authors: Perlin P. , Suski T. , Wisniewski P. , Gorczyca I. , Lepkowski S. , Hansen M. , DenBaars S.P. , Damilano B. , Grandjean N. , Massie J.
Published: April 2011

We have studied an influence of pressure on the emission and absorption spectra measured from various types of InGaN structures such as epilayers, quantum wells and quantum dots. While the known pressure coefficients of GaN and InN bandgaps are in the range 40-25 meV/GPa, the experimental observation for the light emission shift with pressure for InGaN alloys is dramatically different. With the increasing In content and thus decreasing emission energy the observed pressure coefficients become very small eventually reaching zero or even slightly negative values! We have observed a much weaker trend for the decrease of the pressure coefficient for the absorption edges of InGaN. First principle calculations of InGaN band structure and its modification with a pressure are not able to explain the huge effect observed in the emission experiment but are in a good agreement with the results obtained in optical absorption measurements. We discuss here the possible mechanisms which can account for extremely low pressure coefficient of the light emission and the discrepancy between sensitivity light emission and absorption on applied pressure in InGaN alloys.
Optimization of Laser Energy Fluence in Pulsed Laser Deposition of ZnO on Al2O3(0001)
Authors: Yang W. , Vispute R. D. , Choopun S. , Sharma R. P. , Shen H. , Venkatesan T.
Published: April 2011

The effects of laser energy fluence on the growth of pulsed laser deposited ZnO thin films on c-plane sapphire substrates were systematically investigated by using x-ray diffraction, Rutherford backscattering spectrometry with ion channeling, and scanning electron microscopy techniques. Optical and electrical properties of the ZnO epilayers were characterized by using ultraviolet-visible transmission spectroscopy and Van der Pauw measurements, respectively. It was found that the laser fluence has strong effects on the crystalline, optical and electrical qualities of the ZnO films. At low laser fluence, ZnO film grows via 3D-island mode with low deposition rate, loss of Zn near the surface and particulates on top of the film. High laser fluence may also cause simultaneous multi-layer growth and the degradation of crystalline, electrical, and optical quality of the ZnO films. The optimal laser fluence window was found between 1.2J/cm2 and 2.5 J/cm2 for obtaining high quality ZnO films for optoelectronic applications. The dependence of laser fluence on the ZnO growth mode, surface morphology and electrical and optical properties is discussed.
Pattern Evolution of NiSi2 on Si Surface upon High Current Pulsed Ni Ion Implantation
Authors: Cheng X.Q. , Zhu H.N. , Liu B.X.
Published: April 2011

Fractal pattern evolution of NiSi2 grains on a Si surface was induced by high current pulsed Ni ion implantation into Si wafer using metal vapor vacuum arc ion source. The fractal dimension of the patterns was found to correlate with the temperature rise of the Si substrate caused by the implanting Ni ion beam. With increasing of the substrate temperature, the fractal dimensions were determined to increase from less than 1.64, to beyond the percolation threshold of 1.88, and eventually up to 2.0, corresponding to a uniform layer with fine NiSi2 grains. The growth kinetics of the observed surface fractals was also discussed in terms of a special launching mechanism of the pulsed Ni ion beam into the Si substrate.
TiNi/GaAs Thin Film Structures for Gate Metallizations
Authors: Zhang Chichang , Christou Aris
Published: April 2011

Shape memory alloy TiNi thin films on GaAs have been investigated. A series of TiNi compositions were electron beam deposited on GaAs initially as thin multilayers of Ti and Ni. The intermetallic phase of TiNi was formed by annealing and complete intermixing of the multilayers at 370oC. The intermetallic phases were investigated with X-ray diffraction techniques. The annealing kinetics and resistivity investigations were carried out in order to minimize the sheet resistance of the intermetallic phase. TiNi Schottky barriers on GaAs have been fabricated and their performance will be reported. Additional investigations on surface morphology using the energy dispersive spectroscopy as well as TEM investigations show the correlation between microstructure, intermetallic phase formation and sheet resistance.
Model for Dopant and Impurity Segregation During Vapor Phase Growth
Authors: Arnold Craig B. , Aziz Michael J.
Published: April 2011

We propose a new kinetic model for surface segregation during vapor phase growth that takes into account multiple mechanisms for segregation, including mechanisms for inter-layer exchange and surface diffusion. The resulting behavior of the segregation length shows temperature and velocity dependence, both of which have been observed in experiments. We compare our analytic model to experimental measurements for segregation of Phosphorus in Si(001), and we find an excellent agreement using realistic energies and pre-exponential factors for kinetic rate constants.
In Situ Study of Titanium Film Growth On Different Substrates
Authors: Oberhauser P. , Poppeller M. , Abermann R.
Published: April 2011

The chemical and microstructural properties of a surface have a strong influence on the growth mode and the morphology of a film evaporated onto this interface. Changes in the growth stress of thin titanium films, measured in situ by a cantilever beam technique, evaporated under UHV-conditions are used to monitor the chemical and microstructural properties of a substrate surface. The starting substrate film used in this study was a quasi single-crystalline TiO2-film (d=50 nm) prepared by reactive evaporation of titanium in an oxygen atmosphere and subsequent annealing (20 min, 400°C). The Ti-growth stress on this substrate is compressive up to monolayer coverage and tensile at higher film thickness, which is interpreted to indicate a strong interaction between TiO2 and the arriving Ti atoms at the interface during monolayer formation and strained (tensile) layer epitaxy at higher film thickness. In a second series of experiments the TiO2-film was covered with Al-overlayers of varying thickness. Due to oxygen interdiffusion from the TiO2-film an amorphous Al-oxide layer is formed at the interface eliminating the high degree of order of the substrate TiO2-film. On this amorphous substrate the stress vs. thickness curve of the Ti-film, in terms of our stress model, is interpreted to indicate island formation and growth of a polycrystalline Ti-film. At Al-layer thicknesses above about 3 nm the Al-interface becomes metallic. The structure of this Al-surface depends on the film thickness and substrate temperature during its deposition. During deposition of the first Ti-monolayer on metallic Al a large incremental tensile stress (up to 45 GPa) is measured. The magnitude of this tensile stress is closely related to the surface microstructure of the Al substrate. The surface roughness deduced from the tensile interface stress is compared with the surface roughness measured by AFM.

For comparison, analogous experiments were made with Al2O3/Al substrate bilayers. The results of these experiments qualitatively agree with those on the TiO2/Al-substrate. The general shape of the stress vs. thickness curve is comparable, however quantitative differences are interpreted to be due to differences in the structure and/or chemical composition of the substrate Al-film.
The Capillarity Influence On Shape Of Small Liquid Inclusions Enclosed In A Solid Under Non-Stationary Thermal Conditions
Authors: Gershanov Vladimir Yu. , Garmashov Sergey I. , Minyaev Andrey R. , Ivanov Nickita E. , Nosuleva Irina Yu.
Published: April 2011

An analytical model taking into account the influence of capillarity on the process of changing the cross-sectional shape of a cylindrical liquid inclusion enclosed in an anisotropic crystal under non-stationary thermal conditions is suggested. It is shown that the capillary effect confines the possibilities for controlling the inclusion shape under non-stationary thermal conditions. The capillarity influence becomes stronger with decreasing cross-sectional area and increasing interfacial energy. The results of calculations of the limit inclusion shape under different thermal conditions are presented and discussed.
Modelling surface growth in IBAD with rate equations
Authors: Sillanpáá Jussi K. , Koponen Ismo T. , Grønbech-Jensen Niels
Published: April 2011

We develop a rate-equation model for surface growth in ion beam assisted deposition. In terms of the rate coefficients the model describes how island detachment and breakup, adatom diffusion and interlayer transitions of adatoms affect the growth. We identify parameter values corresponding to different modes of growth and discuss our results and ways to improve the model.
Why is KPZ type surface roughening so hard to observe?
Authors: Drotar Jason T. , Zhao Y.-P. , Lu T.-M. , Wang G.-C.
Published: April 2011

The Kardar-Parisi-Zhang (KPZ) surface roughening model was proposed nearly fifteen years ago. Although there have been many theoretical studies, there are very few experimental examples of thin film evolution obeying the KPZ equation. We discuss the physical basis of the KPZ equation and suggest possible reasons for the departure from KPZ behavior that is usually observed in surface growth/etching processes. Particularly, we construct a non-local, KPZ-like growth model that takes into account the effect of surface re-emission and show that, for certain limits, our model reduces to the KPZ model. We also discuss various experimental results in the context of known roughening models, including our model.
In/Si(111): Self-Assembled One and Two-Dimensional Electron Gases
Authors: Rotenberg Eli , Yeom H. W. , Takeda S. , Matsuda I. , Horikoshi K. , Schaefer J. , Lee C. M. , Krenzer B. , Rocha M. , Kevan S. D. , Ohta T. , Nagao T. , Hasegawa S.
Published: April 2011

We present angle-resolved photoemission measurements for ultrathin In films on Si(111). Depending on the coverage, this system self-organizes into a metallic monolayer with either 4×1 or √7×√3 symmetry relative to the substrate. Electronically, they behave like ideal one- and two-dimensional electron gases (1DEG and 2DEG), respectively. The 4×1 system has atomic chains of In whose energy bands disperse only parallel to the chains, while for the √7×√3 system, the dominant reciprocal space features (in both diffraction and bandstructure) resemble a pseudo-square lattice with only weaker secondary features relating to the √7×√3 periodicity. In both materials the electrons show coupling to the structure. The 1DEG couples strongly to phonons of momentum 2kF, leading to an 8×“2” Peierls-like insulating ground state. The 2DEG appears to be partially stabilized by electron gap formation at the √7×√3 zone boundary.
Reaction Kinetics Effects on Reactive Wetting
Authors: González Marta , Araujo Mariela
Published: April 2011

Chemical reactions at the interface between two liquids and a solid can lead to very complex situations. For example, after the reaction the substrate may become less wettable allowing the formation of running droplets. We propose a model where droplets of reactive fluids of a given radius, are injected into a porous system. In their advance, the droplets react chemically with the solid surface, making it less wettable. The velocity of the droplet after the reaction is calculated for different reaction kinetics including adsorption and solute solubility, for situations where gravity effects are negligible. In all cases, a transition from a Brownian motion regime to a ballistic displacement is observed.
Tribological Behavior of Very Thin Confined Films
Authors: Müser M. H.
Published: April 2011

The tribological properties of two smooth surfaces in the presence of a thin confined film are investigated with a generic model for the interaction between two surfaces and with computer sim- ulations. It is shown that at large normal contact pressures, an ultra thin film automatically leads to static friction between two flat surfaces - even if the surfaces are incommensurate. Commen- surability is nevertheless the key quantity to understand the tribological behavior of the contact. Qualitative differences between commensurate and incommensurate contacts remain even in the presence of a thin film. The differences mainly concern the thermal diffusion of the contact and the transition between smooth sliding and stick-slip.
Observation of Laser Speckle Effects in an Elementary Chemical Reaction
Authors: Monson Eric , Kopelman Raoul
Published: April 2011

An experimental demonstration is shown for non-classical reaction kinetics in a homogeneous system with an elementary reaction, A+B→C. Sensitivity to the initial distribution of reactants is observed, along with a new reaction-kinetics regime which is a direct consequence of speckles in the laser beam. The long-time regime gives the first experimental demonstration of the asymptotic self-segregation (“Zeldovich”) effect, in spite of the non-random, speckled initial distribution of reactant B. Monte-Carlo simulation results are consistent with the experiments, and spatial analysis of these results correlates the excess of long-wavelength components in the initial reactant distribution with an anomalous slowing of the reaction progress.
Mechanical Measurements at a Submicrometric Scale: Viscoelastic Materials
Authors: Basire Charlotte , Fretigny Christian
Published: April 2011

Adhesive and tribological properties of the tip of an AFM on viscoelastic samples are studied. The kinetics of the indentation process is shown to be governed by bulk rather than by contact edge dissipations. It is shown that the transition from static to dynamic friction regimes takes place at a critical strain. During the static friction regime, the contact size remains nearly constant. Owing to this property, viscoelastic moduli is measured in this regime. Obtained results, characteristic of a micrometric domain are in a very good agreement with the same properties measured on centimetric samples using dynamical mechanical analysis. Finally, a stick-slip friction regime is observed in a range of velocities.
Adsorption in Ordered Porous Silicon: a Reconsideration of the Origin of the Hysteresis Phenomenon in the Light of new Experimental Observations
Authors: Coasne B. , Grosman A. , Dupont-Pavlovsky N. , Ortega C. , Simon M.
Published: April 2011

Porous silicon formed in p+ single crystal silicon is an interesting material for the study of the behavior of a fluid confined in mesoporous media, because it is a simple system of non interconnected straight pores, all perpendicular to the substrate and it can be well characterized by different methods. Moreover, the pores may be closed at one end when the porous layer is supported by the substrate or opened at both ends when the layer is removed from the substrate. Surprising results have been obtained. A hysteresis loop of type H2 (IUPAC classification), which is generally obtained for highly interconnected porous material, is also observed. Furthermore, a hysteresis of same type is also observed in pores closed at one end which is in contradiction with the Cohan model. The steep desorption process does not account for the large pore size distribution extracted from transmission electronic microscopy study. It is believed that, during the desorption process, the presence of an adsorbed layer on the external surface of the pores and/or the coupling between the pores via the silicon walls must be taken into account.
A Grand Canonical Monte-Carlo Study of Argon Adsorption/Condensation in Mesoporous Silica Glasses: Application to the Characterization of Porous Materials
Authors: Pellenq R.J.-M. , Rodts S. , Levitz P. E.
Published: April 2011

We have studied adsorption of argon in a mesoporous silica Controlled Porous Glass (CPG) means of Grand Canonical Monte-Carlo (GCMC) simulation. A numerical sample of the CPG adsorbent has been obtained by using an off-lattice reconstruction method recently introduced to reproduce topological and morphological properties of correlated disordered porous materials. The off-lattice functional of (100 m2/g)-Vycor is applied to a simulation box containing silicon and oxygen atoms of cubic cristoballite with an homothetic reduction of factor 2.5 so to obtain 30Å-CPG sample. A realistic surface chemistry is then obtained by saturating all oxygen dangling bonds with hydrogen. The Ar, Kr and Xe adsorption/desorption isotherms are calculated at different temperatures. At sufficiently low temperature, they exhibit a capillary condensation transition with a finite slope by contrast to that theoretically predicted for simple pore geometriessuch as slits and cylinders. In the low pressure and temperature domain, we have identified different adsorption scenarios, which can be interpreted on the basis of a Zisman-type of criterium for wetting. We demonstrate that the BET surface area is strongly related to this criterion. At higher pressure, we demonstrate that the pore size distribution obtained by using the standard BJH analysis applied to both adsorption and desorption data qualitatively reproduces the main features of the chord length distribution.
The Influence of Boundary Conditions and Surface Layer Thickness on Dielectric Relaxation of Liquid Crystals Confined in Cylindrical Pores
Authors: Nazario Z. , Sinha G. P. , Aliev F.M.
Published: April 2011

Dielectric spectroscopy was applied to investigate the dynamic properties of liquid crystal octylcyanobiphenyl (8CB) confined in 2000 Å cylindrical pores of Anopore membranes with homeotropic and axial (planar) boundary conditions on the pore walls. Homeotropic boundary conditions allow the investigation of the librational mode in 8CB by dielectric spectroscopy. We found that the dynamics of the librational mode is totally different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. The interpretation of the temperature dependence of relaxation times and of the dielectric strength of the librational mode needs the involvement of the temperature dependence of orientational order parameter. For samples with axial boundary conditions, layers of LCs with different thickness were obtained on the pore walls as a result of controlled impregnation of porous matrices with 8CB from solutions of different liquid crystal concentration. The process due to rotation of molecules around their short axis with single relaxation time observed for bulk 8CB is replaced by a process with a distribution of relaxation times in thin layers. This relaxation process broadens with decreasing layer thickness.
Studies of Temperature-Dependent Excimer-Monomer Conversion in Dendrimeric Antenna Supermolecules by Fluorescence Spectroscopy
Authors: Caoa Youfu , Moore Jeffrey S. , Kopelman Raoul
Published: April 2011

Phenylacetylene (PA) dendrimer labeled with perylene (See Fig 1) is discovered to exhibit temperature-dependent emission spectra in certain organic solvents over the temperature range of 20-65°C. The monomer signal is increasing rapidly when temperature increases, while the excimer signal decreases slowly. Models of excimer formation and weakly associated pairs (M+M) dissociation dynamics are included, and the equilibrium constants at different temperatures are calculated. This behavior suggests potential applications in fluorescence-based thermometry.
Isotopes in Neutron Diffraction – Detailed Structural Analysis at the Metal-Insulator Transition in SmNiO3
Authors: Weller Mark T. , Henry Paul F. , Wilson C.C.
Published: April 2011

The use of isotopically enriched materials on high intensity neutron powder diffractometers allows the derivation of much higher quality structural information than has hitherto been possible. The technique can be applied to materials such as ferroelectrics, superconductors and oxides exhibiting colossal magneto-resistance, where small structural changes associated with phase transitions need to be characterised. Three isotopically pure samples of samarium nickelate have been studied around the metal-insulator transition at 403 K. Simultaneous multi-histogram refinements permit extraction of very high quality structural information that shows smooth variations in bond lengths and angles with all nickel oxygen distances decreasing as the electrons localise.
Deposition and Characterization of Y3Al5O12 (YAG) Films and Powders by Plasma Spray Synthesis
Authors: Parukuttyammaa Sujatha D. , Margolis Joshua , Liu Haiming , Parise John B. , Grey Clare P. , Sampath Sanjay , Gouma Perina , Herman Herbert
Published: April 2011

YAG powders and coatings were developed for the first time by a novel precursor plasma spraying technique using the radio frequency (RF) induction plasma technique. The XRD of the as -sprayed coating confirms the presence of YAG, H-YAP or O-YAP or a mixture of the above depending on the spray conditions. 27Al MAS NMR of the YAG coating corroborates the x-ray results. TEM studies on the coatings confirm that the coating consists of nano-structured particles. The successful spraying of these complex oxide coatings proves that chemistry of phase formation can be controlled in the plasma, thus opening up new avenues in material synthesis.
Photocarrier capture properties of light-induced defects in a-Si:H
Authors: Stradins Paul , Shimizu Satoshi , Kondo Michio , Matsuda Akihisa
Published: April 2011

Light-induced degradation of photoconductive properties is investigated as a function of lightinduced Si dangling bond defect concentration and subgap absorption. This relation is studied for a variety of sample preparation techniques including a-Si:D, wide range of temperatures during light exposure and annealing, and sample degradation pre-history. Under a given exposure condition, light-induced defects have a fixed effective electron capture coefficient, while its value varies greatly with exposure conditions (temperature during exposure or stepwise anneal) and with sample microstructure. This explains commonly observed non-linear dependences and hysteresis between defect concentration and mobility-lifetime product. Creation efficiency of less-stable defects with large capture coefficients is not influenced by the presence of a large number of stable defects. Structural change accompanying the defect creation, if any, is strongly related to the defect creation and is likely to occur locally around the defects, modifying their capture coefficients. Capture can vary both because of the changes in intrinsic capture probabilities and due to changes in the net defect charge balance. The former may be increased by the presence of H or photocarrier trap (e.g. weak bond tail state) in a close vicinity of Si dangling bond. The latter may be caused by a wide distribution of defect states in the gap or by simultaneous creation of negatively charged floating bond states near VB edge.
Hydrogen Equilibration and Metastability in Amorphous Silicon
Authors: Branz Howard M. , Zhang S.B.
Published: April 2011

We review evidence for the vital role of H in metastability and defect equilibration in hydrogenated amorphous silicon. H pairs act both as an H reservoir that equilibrates with DB's and dopants and as the metastable H state of light-induced metastability. With ab initio pseudopotential theory, we calculate energies and configurations for two novel H pairing sites associated with small hydrogenated vacancies. We describe microscopic models of equilibration and metastability based on our calculations.
Model of Hydrogen-Mediated Metastable Changes in a Two-Domain Amorphous Silicon Network
Authors: Baugh Jonathan , Han Daxing
Published: April 2011

A phenomenological model for the light-induced metastability of a-Si:H is proposed in which a two-domain model of the amorphous network plays a central role. Boundaries between high and low density domains are associated with a significant fraction of the clustered Si-H in aSi:H. Weakly bonded hydrogen at these boundaries catalyzes metastable local configuration changes in the Si network due to non-radiative carrier recombination, leading qualitatively to both the gross structural changes and the increase in electronic defect density that are observed experimentally.
Hydrogen Distributions and Model Lineshapes of ESR Signals of Dangling Bonds in a-Si:H
Authors: Fedders P. A.
Published: April 2011

For many years it has been assumed widely that hydrogen is involved in some way in the formation of light induced defects. However recently some doubt has been cast on this because of experimental evidence that there is little H near light induced dangling bonds. In this paper we present a number of model calculations comparing ESR spectra of dangling bonds with and without correlations with H positions. The above models include different distributions of H and correlations or anti correlations of nearby H. In all cases the spectra are compared to those with no correlations or clustering. Our results coupled with published experimental data strongly suggest that dangling bonds are not correlated positively with the presence of nearby H and, in fact, it appears that light induced dangling bonds are negatively correlated with nearby H.
Electronic transport study of high deposition rate HWCVD a-Si:H by the microwavephotomixing technique
Authors: Sheng S.R. , Braunstein R. , Nelson B.P. , Xu Y.
Published: April 2011

The electronic transport properties of high deposition rate a-Si:H films prepared by HWCVD have been investigated in detail by employing the microwave photomixing technique. The high deposition rates (up to 1 µm/min.) were achieved by adding a second filament, increasing deposition pressure, silane flow rate, and decreasing filament-to-substrate distance. The effect of the deposition rate on the resultant film properties with respect to the substrate temperature, deposition pressure and silane flow rate was studied. It was found that the film transport properties do not change monotonically with increasing deposition rate. The photoconductivity peaks at ∼70-90 Å/s, where both the drift mobility and lifetime peak, consistent with the deposition rate dependence of the range and depth of the potential fluctuations. High quality, such as a photoconductivity-to-dark-conductivity ratio of ∼105 and nearly constant low charged defect density, can be maintained at deposition rates up to ∼150 Å/s, beyond which the film properties deteriorate rapidly as a result of an enhanced effect of the long-range potential fluctuations due to a considerable increase in the concentration of the charged defects. Our present results indicate that medium silane flow rate, low pressure, and higher substrate temperature are generally required to maintain high quality films at high deposition rates.