bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

ABSTRACTThe 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

ABSTRACTWe 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

ABSTRACTWe 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

ABSTRACTTransmission 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

ABSTRACTNear-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

ABSTRACTSpectroscopic 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

ABSTRACTThe 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

ABSTRACTWe 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

ABSTRACTStress 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

ABSTRACTWe 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

AbstractThe 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

AbstractFractal 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

AbstractShape 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

AbstractWe 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

AbstractThe 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

AbstractAn 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

AbstractWe 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

AbstractThe 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

AbstractWe 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

AbstractChemical 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.