bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cracking and Delamination of Heteroepitaxial Barium Hexaferrite Films
Authors: Oliver S.A. , Kozulin I. , Mcgruer N.E. , Yoon S.D. , Vittoria C. 
Published: April 2011

ABSTRACTExperimental results are shown for the structural and stress behaviors of c-axis oriented barium hexaferrite films on c-plane sapphire (Al2O3) substrates as a function of oxygen growth pressure and film thickness. It is shown that films deposited at 20 mTorr crack and delaminate at thicknesses above 15 µm, but that evidence for these large stresses is not apparent in either substrate curvature measurements or distortion of the c-axis lattice constant for thinner films (< 2 µm). In contrast, films (< 4 µm) deposited at 300 mTorr show large substrate curvatures that relax with increasing film thickness, in tandem with the formation of first large outgrowths, and then polycrystalline grains as the original c-axis film texture is randomized. Correlations between these observations are made to explain the complex evolution of properties in these films.
Solution Synthesis of Epitaxial Rare-Earth Oxide Thin Films on Roll-Textured Nickel
Authors: Morrell Jonathan S. , Xue Ziling B. , Paranthaman Mariappan , Chirayil Thomas G. , Vallet Catherine E. , Beach David B. , Specht Eliot D. 
Published: April 2011

ABSTRACTUsing solution chemistry, epitaxial films of rare-earth oxides of the general formula RE2O3 (where RE = Sm to Lu) were prepared on cubic-textured nickel tapes. Solutions of metal methoxyethoxides or metal acetate/methoxyethoxides in 2-methoxyethanol were used to coat rolltextured nickel tapes using either spin-coating or dip-coating. Coated tapes were subsequently heated in a reducing atmosphere at temperatures between 950 °C and 1160 °C for varying lengths of time. Film quality was determined using X-ray diffraction and electron and surface probe microscopy. Films were found to be oriented both in and out-of-plane of the substrate, free of pinholes and gross defects, and of sufficient quality for use as substrates for high Tc, superconductors.
Modeling of Metal(100) Homoepitaxial Film Growth at Very Low Temperatures
Authors: Caspersen K.J. , Stoldt C.R. , Thiel P.A. , Evans J.W. 
Published: April 2011

ABSTRACTWe model the growth of Ag films deposited on Ag(100) below 140K. Our recent Variable-Temperature Scanning Tunneling Microscopy (VTSTM) studies reveal “smooth growth” from 120-140K, consistent with earlier diffraction studies. However, we also find rougher growth for lower temperatures. This unexpected behavior is modeled by describing the deposition dynamics using a “restricted downward funneling” model, wherein deposited atoms get caught on the sides of steep nanoprotrusions (which are prevalent below 120K), rather than always funneling down to lower four-fold hollow adsorption sites. At OK, where no thermal diffusion processes are operative, this leads to the formation of overhangs and internal defects (or voids). Above 40K, low barrier interlayer diffusion processes become operative, producing the observed smooth growth by 120K. We also discuss how the apparent film morphology mapped out by the STM tip “smears” features of the actual film morphology (which are small at low temperature), and also can lead to underestimation of the roughness.
Heteroepitaxy: The Missing Link Between Surface Chemistry and Dry Corrosion
Authors: Yang Judith C. , Bharadwaj Mridula Dixit , Tropia Lori 
Published: April 2011

ABSTRACTWe have investigated the initial stages of Cu (001) oxidation in dry and moist oxidizing conditions using in situ ultra-high vacuum (UIHV) transmission electron microscopy (TEM). To investigate the role of moisture in the solid state reactions in Cu oxidation, we have examined the oxidation of Cu (001) with water vapor. Our observation indicate that water vapor causes reduction of Cu2O and retards the oxidation rate if both oxygen gas and water vapor are used simultaneously which contradicts the thermochemical data. We are also modeling the nucleation to coalescence of the oxide scale using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation and have noted a qualitative agreement.
Potential of GaN Gunn Devices for High Power Generation Above 200 GHz
Authors: Kamoua Ridha , Zhu Yiming , Corcoran Yunji 
Published: April 2011

ABSTRACTThis paper investigates the potential of Gallium Nitride (GaN) Gunn diodes for generating high power radiation at millimeter and sub-millimeter wave frequencies. Simulations were carried out based on an ensemble Monte Carlo computer model. This model accounts for thermal effects, series resistance, and device circuit interaction through the harmonic balance technique. The accuracy of the model has been validated in the case of InP Gunn devices at frequencies above 100 GHz[1]. Initially uniform and linearly graded doping profiles in the active region were considered. It is found that, similar to devices based on InP, the graded profile resulted in a much improved performance in terms of power, efficiency, and operating temperature. In particular, a GaN Gunn structure consisting of a 1 μm thick active region with a graded doping profile increasing from 6 × 1015 cm−3 at the cathode terminal to 4.6×1016cm−3 at the anode terminal yielded promising results. The DC bias voltage was estimated from the calculated velocity-electric field data to be about 25 V. With this bias, it was found that oscillations in the fundamental mode could be obtained over the frequency range from 220 GHz to 330 GHz subject to a load resistance of 1 Ohm and a maximum operating temperature of 800 K. The maximum output power was 30 mW at 290 GHz with a corresponding conversion efficiency of 0.25 %. Considerable improvement was obtained from a device with a heterojunction at the cathode. For a 1 μm device, an optimum power level close to 150 mW was predicted at 215 GHz with an efficiency of 2 %. This estimated power level is about an order of magnitude higher than what can be achieved from InP Gunn oscillators at the same frequency.
GaN pnp Bipolar Junction Transistors Operated to 250°C
Authors: Zhang A.P. , Dang G. , Ren F. , Han J. , Monier C. , Baca A.G. , Cao X.A. , Cho H. , Abernathy C.R. , Pearton S.J. 
Published: April 2011

ABSTRACTWe report on the dc performance of the first GaN pnp bipolar junction transistor. The structure was grown by MOCVD on c-plane sapphire substrates and mesas formed by low damage Inductively Coupled Plasma etching with a Cl2/Ar chemistry. The dc characteristics were measured up to VBC of 65 V in the common base mode and at temperatures up to 250°C. Under all conditions, IC ∼ IE indicated higher emitter injection efficiency. The offset voltage was ≤ 2 V and the devices were operated up to power densities of 13.9 kW·cm−2.
SiC epitaxial growth on porous SiC substrates
Authors: Melnychuck G. , Mynbaeva M. , Rendakova S. , Dmitriev V. , Saddow S. E. 
Published: April 2011

ABSTRACTWe report on the growth and crystal quality of CVD epitaxial layers grown on porous SiC (PSC) substrates. A layer of porous SiC was fabricated by surface anodization of commercial 4H and 6H-SiC (0001)Si face off-axis wafers. The 4H and 6H-SiC epilayers were grown on porous SiC (PSC) substrates using atmospheric pressure CVD at 1580°C and a Si to C ratio of 0.3. Results of X-ray diffraction, RHEED, SEM and AFM characterization demonstrated good surface quality of the films grown on porous material. PL data indicate a greatly improved defect structure in the epi layers grown on PSC as compared to control samples. Preliminary etch pit experiments to estimate the dislocation density indicate a three-fold reduction in defect density in the epi material grown on PSC substrates as compared to epi grown conventional substrates.
Single Crystal Growth of Gallium Nitride Substrates Using an High Pressure High Temperature Process
Authors: Singh Rajiv K. , Gilbert Donald R. , Kelly Francis , Chodelka Robert , Abbaschian Reza , Pearton Stephen , Novikov Alexander , Patrin Nikolay , Budai John 
Published: April 2011

ABSTRACTThe use of standard bulk semiconductor crystal growth processes for the production of GaN is prohibited by both the high melt temperature of GaN and thermal decomposition of the compound into Ga metal and N2 gas. We have employed a novel hydrostatic pressure system to grow GaN crystals. A high temperature, ultra-high pressure process was developed using a solid-phase nitrogen source to form GaN crystals in a Ga metal melt. Using a thermal gradient diffusion process, in which nitrogen dissolves in the high temperature region of the metal melt and diffuses to the lower temperature, lower solubility region, high quality crystals up to ∼1 mm in size were formed, as determined by SEM, X-ray diffraction and micro-Raman analysis.
Measurement of Transit Time and Carrier Velocity Under High Electric Field in III-Nitride P-I-N Diodes
Authors: Wraback M. , Shen H. , Carrano J.C. , Li T. , Campbell J.C. 
Published: April 2011

ABSTRACTWe present an optically-detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in a p-i-n diode, and show how it may be used to determine the electron transit time and velocity-field characteristic in GaN at room temperature. The transit time drops with increasing electric field E in the intermediate field regime (50-100kV/cm), and the electron velocity possesses a weak, quasi-linear dependence on E attributed to polar optical phonon scattering. In the high field regime the transit time and the electron velocity gradually become independent of E. The peak electron velocity of 1.9×107 cm/sec, corresponding to a transit time of ∼2.5 ps across the 0.53 μm depletion region, is attained at ∼ 225 kV/cm. The experimental results are in qualitative agreement with theoretical steady-state velocity-field characteristics found in the literature.
Real Time Observation and Characterization of Dislocation Motion, Nitrogen Desorption and Nanopipe Formation in GaN
Authors: Stach Eric A. , Kisielowski C.F. , Wong W.S. , Sands T. , Cheung N.W. 
Published: April 2011

ABSTRACTDespite the considerable attention focused on GaN and related alloys during the past decade, many outstanding questions remain regarding the mechanisms of defect formation in these materials. In this work, we take advantage of a recently developed processing technique known as laser lift-off to examine the behavior of thin, free-standing, nearly stress-free single crystals of GaN subjected to thermal stimulus. GaN layers of 7 νm thickness were removed from their sapphire growth substrate using the laser lift-off method, then ion-milled to electron transparency. The samples were then annealed at temperatures between 850 and 1025 °C within the objective lens of a 200 kV transmission electron microscope. This allowed real time observation of defect formation via diffraction contrast imaging. Above 925 °C nitrogen desorption first becomes visible at the thinnest edges of the TEM sample. Concomitant with this is the preferential desorption of nitrogen along the cores of those dislocations which are pure screw in character, resulting in the formation of nanopipes. In regions with small residual stresses, those dislocations with mixed edge and screw components propagate parallel to the basal plane in the three close-packed {1 1 00} directions, leaving a hollow tube in their wake. At these temperatures, no motion of dislocations with pure edge character is observed. These results indicate that desorption and plasticity may occur simultaneously in these materials. Additionally, it appears that nanopipe formation may be unavoidable during heteroepitaxial growth by chemical vapor deposition and hydride vapor phase epitaxy, as these deposition methods require substrate temperatures of this magnitude.
Transient Photoresponse from Co Schottky Barriers on AlGaN
Authors: Schwarz R. , Niehus M. , Melo L. , Brogueira P. , Koynov S. , Heuken M. , Meister D. , Meyer B.K. 
Published: April 2011

ABSTRACTCo on AlGaN is expected to form a large barrier Schottky contact due to its high work function. We have used this material combination with 18 % of Al in AlxGaN for the study of transient photoresponse in the photovoltaic mode and in secondary photocurrent measurements after pulsed laser excitation. In reverse bias and in short- circuit mode a fast decay with a characteristic time of a few microseconds is dominant at room temperature. This mode is appropriate for UV detector operation. At elevated temperature, a much slower tail extending to several milliseconds is also observed. In forward bias operation the slow tail is dominating at any temperature. We discuss this asymmetry with respect to fast minority carrier collection within the space charge region for primary photocurrents and the slower majority carrier transport in forward bias.
The Temperature Dependent Breakdown Voltage For 4H- and 6H-SiC Diodes
Authors: Lee Y. S. , Han M. K. , Choi Y. I. 
Published: April 2011

ABSTRACTThe breakdown voltages of 6H- and 4H-SiC rectifiers as function of temperature were modeled analytically in both non-reachthrough diode and reachthrough diode. The breakdown voltage was derived by the ionization integral employing accurate hole impact ionization coefficient. The breakdown voltage of SiC rectifiers was increased with increasing temperature and the positive temperature coefficient of breakdown voltage indicates that SiC rectifiers are suitable for high temperature applications. The breakdown voltages of both 6H- and 4H-SiC diodes were increased by M(T)-1/4 in NRDs and M(T)-1/8 in RDs.
Improved Sensitivity SiC Hydrogen Sensor
Authors: Muntele C. I. , Ila D. , Williams E. K. , Muntele Iulia C. , Evelyn A. L. , Poker D. B. , Hensley D. K. 
Published: April 2011

ABSTRACTWe have produced silicon carbide sensors by two techniques: palladium coating and low energy palladium implantation. The palladium implantation was done at 550°C into the Si face of 6H, n-type SiC at various energies and at various fluences. The sensitivity of each sensor was measured at temperatures between 20°C to 400°C. The response of the SiC sensors produced by Pd implantation has revealed a completely different behavior than the SiC sensors produced by Pd deposition. In the Pd deposited SiC sensors, as well as in the ones reported in the literature [1, 2], the current rises in the presence of hydrogen at room temperature as well as at elevated temperatures. In the case of Pd implanted SiC sensors, the current decreases in the presence of hydrogen whenever the temperature is raised above 100°C [3].
Heteroepitaxial Barium Hexaferrite Films on (111) Magnesium Oxide Substrates
Authors: Oliver Steven A. , Yoon Soack Dae , Kozulin Izabella , Chen Ming Ling , Zuo Xu , Vittoria Carmine 
Published: April 2011

AbstractHigh quality films of barium hexaferrite (BaFe12O19) were deposited by pulsed laser ablation onto MgO (111) substrates. In contrast to previous films deposited onto c-plane sapphire (Al203), these films were expected to have compressive biaxial stress, and indeed showed no indications for either cracking or delamination to a film thickness of 32 νm. All films were found to be highly c-axis textured by both x-ray diffraction measurements and magnetization results. The saturation magnetization (4πMs = 4.2 kG) and uniaxial anisotropy field (HA = 16 kOe) values for these films approach bulk values. Ferrimagnetic resonance measurements on a calcined 3 μm thick film show a narrow linewidth (ΔH ∼ 100 Oe) for the uniform resonance mode. The properties of these films approach those required for self-biased millimeter wavelength devices.
Phase Transformations in Sol-Gel PZT Thin Films
Authors: Eakin D.P. , Norton M.G. , Bahr D.F. 
Published: April 2011

AbstractThin films of PZT were deposited onto platinized and bare single crystal NaCl using spin coating and sol-gel precursors. These films were then analyzed using in situ heating in a transmission electron microscope. The results of in situ heating are compared with those of an ex situ heat treatment in a standard furnace, mimicking the heat treatment given to entire wafers of these materials for use in MEMS and ferroelectric applications. Films are shown to transform from amorphous to nanocrystalline over the course of days when held at room temperature. While chemical variations are found between films crystallized in ambient conditions and films crystallized in the vacuum conditions of the microscope, the resulting crystal structures appear to be insensitive to these differences. Significant changes in crystal structure are found at 500°C, primarily the change from largely amorphous to the beginnings of clearly crystalline films. Crystallization does occur over the course of weeks at room temperature in these films. Structural changes are more modest in these films when heated in the TEM then those observed on actual wafers. The presence of Pt significantly influences both the resulting structure and morphology in both in situ and ex situ heated films. Without Pt present, the films appear to form small, 10 nm grains consisting of both cubic and tetragonal phases, whereas in the case of the Pt larger, 100 nm grains of a tetragonal phase are formed.
Electric and Magnetic Properties of Epitaxial Fe2−xTixO3+σ Films
Authors: Fujii T. , Ayama K. , Nakanishi M. , Sohma M. , Kawaguchi K. , Takada J. 
Published: April 2011

AbstractSolid solution films of the σ-Fe2O3-FeTiO3 series are one of the candidates for noble half-metallic oxides. They were epitaxially formed on σ-Al2O3(001) single crystalline substrates by O2 -reactive evaporation method. The Fe2−x TixO3+σ films prepared at higher Ts=973 K and with larger Ti content x≤0.4 had the ilmenite structure with R3 symmetry. Other films at lower Ts or with smaller x possessed the corundum structure with R3c. Only the films with R3 symmetry had large ferrimagnetic moments, though the observed spontaneous magnetization was less than half of the ideal value expected from the fully ordered structure. Room temperature resistivity of intermediate composites dropped to 10−lΔcm due to the formation of the mixed valence states between Fe2+ and Fe3+. However the Fe2+ content of the films was rather small as compared with stoichiometric Fe2−xTixO3. The Ti-rich films had large oxygen nonstoichio netry of about σ=0.3.
Density-of-States Effective Mass and Scattering Parameter Measurements on Transparent Conducting Oxides Using Second-Order Transport Phenomena
Authors: Young D. L. , Coutts T. J. , Li X. , Keane J. , Kaydanov V. I. , Gilmore A. S. 
Published: April 2011

AbstractTransparent conducting oxides (TCO) have relatively low mobilities, which limit their performance optically and electrically, and which limit the techniques that may be used to explore their band structure via the effective mass. We have used transport theory to directly measure the density-of-states effective mass and other fundamental electronic properties of TCO films. The Boltzmann transport equation may be solved to give analytic solutions to the resistivity, Hall, Seebeck, and Nernst coefficients. In turn, these may be solved simultaneously to give the density-of-states effective mass, the Fermi energy relative to either the conduction or valence band, and a scattering parameter, s, which characterizes the relaxation time dependence on the carrier energy and can serve as a signature of the dominate scattering mechanism. The little-known Nernst effect is essential for determining the scattering parameter and, thereby, the effective scattering mechanism(s). We constructed equipment to measure these four transport coefficients on the same sample over a temperature range of 30 – 350 K for thin films deposited on insulating substrates. We measured the resistivity, Hall, Seebeck, and Nernst coefficients for rf magnetron-sputtered aluminum-doped zinc oxide. We found that the effective mass for zinc oxide increases from a minimum value of 0.24me, up to a value of 0.47me, at a carrier density of 4.5 × 1020 cm−3, indicating a nonparabolic conduction energy band. In addition, our measured density-of-states effective values are nearly equal to conductivity effective mass values estimated from the plasma frequency, denoting a single energy minimum with a nearly spherical, constant-energy surface. The measured scattering parameter, mobility vs. temperature, along with Seebeck coefficient values, characterize ionized impurity scattering in the ZnO:AI and neutral impurity scattering in the undoped material.
Growth and Chemical Substitution of Transparent p-Type CuAlO2
Authors: Stauber R.E. , Parilla P.A. , Perkins J. D. , Ginley D.S. 
Published: April 2011

AbstractCuAlO2 is one of several materials currently being investigated for application as a p-type transparent conducting oxide. In this paper we report a method of making c-axis oriented CuAlO2 thin films which eliminates previously reported phase-purity and surface morphology problems. Thin film precursors of CuA1O2 were deposited on YSZ (100) substrates by roomtemperature radio frequency sputtering. Subsequent annealing in 10mT of oxygen at temperatures as low as 800°C resulted in phase-pure, textured CuAlO2. The films were p-type and transparent with a gap of 3.5eV, but typical carrier concentrations were low (on the order of 1016 cm−3). Oxygen anneals at 700°C in 1 atm of O2 raised this to 1018 cm−3 in some samples. In order to increase the carrier concentration further, we are testing chemical substitution on the metal sublattice. Initial experiments were done with Mg as a dopant, either by direct solid-state synthesis for bulk materials or by alternating deposition of MgO and CuAlO2 during the pulsed laser deposition of thin films. At Mg molar concentrations of 1% or less, the bulk material was phase-pure CuAlO2 by x-ray diffraction, but was mixed-phase CuAl2O4 and CuO when the Mg concentration was increased to 2%. Thin films sputtered from the 1% Mg substituted target crystallized to CuA102 after a 940°C anneal at 10T 02, but were more than 30 times more resistive than the undoped films.
Characteristics of Ultra violet-Assisted Pulsed Laser Deposited Thin Oxide Films
Authors: Craciun V. , Singh R.K. 
Published: April 2011

AbstractThe properties of thin oxide films such as Y2O3, ZnO and Ba0.5Sr0.5TiO3 grown using an in situ ultraviolet (UV)-assisted pulsed laser deposition (UVPLD) technique have been studied. With respect to films grown by conventional PLD under similar conditions but without UV illumination, the UVPLD grown films exhibited better structural and optical and electrical properties, especially for lower substrate temperatures. They also exhibited a better stoichiometry and contained less physisorbed oxygen than the conventional PLD grown layers. These improvements can be traced to several factors. Firstly, deep UV photons and ozone ensure a better in situ cleaning of the substrate prior to the deposition. Secondly, the presence of more reactive gaseous species like ozone and atomic oxygen formed by photodissociation of molecular O2 promotes the growth of more oxygenated films. Thirdly, absorption of UV photons by adatoms could result in an increased of their surface mobility. All these factors have a beneficial effect upon crystalline growth, especially for moderate substrate temperatures. For optimised growth conditions, the crystalline quality and properties of ultraviolet-assisted pulsed laser deposited films was similar to that of films grown using conventional PLD at substrate temperatures of at least 200°C higher.