bluuebunny/crossref_metadata_2025_split huggingface dataset:
Optical Investigations of AlGaN on GaN Epitaxial Films
Authors: Steude G. , Christmann T. , Meyer B.K. , Goeldner A. , Hoffmann A. , Bertram F. , Christen J. , Amano H. , Akasaki I. 
Published: April 2011

AbstractWe have investigated AlxGal-xN/GaN heterostructures (0<x<0.22) grown by metal organic vapor phase epitaxy on sapphire with photoluminescence (PL), reflexion and cathodo-luminescence experiments. The energetic positions of the free A-exciton in GaN and AIGaN as a function of the alloy compositions are deduced from temperature dependent PL and from reflexion measurements. We obtain a small bowing parameter and no evidence for a Stokes shift between absorption and emission. The AlxGal-xN films induce additional compressive strain on the underlying GaN film. Compositional inhomogeneities are present, but the fluctuations are too small to be important for carrier localisation. The broadening of the luminescence line width in the alloy can be described by statistical disorder of a random alloy.
Pendeo-Epitaxy - A New Approach for Lateral Growth of Gallium Nitride Structures
Authors: Zheleva Tsvetanka S. , Smith Scott A. , Thomson Darren B. , Gehrke Thomas , Linthicum Kevin J. , Rajagopal Pradeep , Carlson Eric , Ashmawi Waeil M. , Davis Robert F. 
Published: April 2011

AbstractA new process route for lateral growth of nearly defect free GaN structures via Pendeoepitaxy is discussed. Lateral growth of GaN films suspended from {1120} side walls of [0001] oriented GaN columns into and over adjacent etched wells has been achieved via MOVPE technique without the use of, or contact with, a supporting mask or substrate. Pendeo-epitaxy is proposed as the descriptive term for this growth technique. Selective growth was achieved using process parameters that promote lateral growth of the { 1120) planes of GaN and disallow nucleation of this phase on the exposed SiC substrate. Thus, the selectivity is provided by tailoring the shape of the underlying GaN layer itself consisting of a sequence of alternating trenches and columns, instead of selective growth through openings in SiO2 or SiNx mask, as in the conventional lateral epitaxial overgrowth (LEO).Two modes of initiation of the pendeo-epitaxial GaN growth via MOVPE were observed: Mode A - promoting the lateral growth of the {1120} side facets into the wells faster than the vertical growth of the (0001) top facets; and Mode B - enabling the top (0001) faces to grow initially faster followed by the pendeo-epitaxial growth over the wells from the newly formed {1120} side facets. Four-to-five order decrease in the dislocation density was observed via transmission electron microscopy (TEM) in the pendeo-epitaxial GaN relative to that in the GaN columns. TEM observations revealed that in pendeo-epitaxial GaN films the dislocations do not propagate laterally from the GaN columns when the structure grows laterally from the sidewalls into and over the trenches. Scanning electron microscopy (SEM) studies revealed that the coalesced regions are either defect-free or sometimes exhibit voids. Above these voids the PEGaN layer is usually defect free.
Properties of Epitaxial ZnO Thin Films for GaN and Related Applications
Authors: Shen H. , Wraback M. , Pamulapati J. , Liang S. , Gorla C. , Lu Y. 
Published: April 2011

AbstractIn this paper, we present a detailed study of high quality (110) ZnO films, epitaxially grown on R-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The epitaxial relationships are (1120) ZnO//(0112) Al2O3 and {0001} ZnO//[0111] A12O3 as confirmed by X-ray diffraction (θ-2θ, and φ-scan) and high-resolution cross-sectional transmission electron microscopy (HR-TEM). Low temperature photoluminescence (PL) indicates the ZnO thin films are almost strain free. Optical absorption and reflection measurements with linearly polarized light indicate a strong optical anisotropy. The polarization rotation towards the C-axis associated with the optical anisotropy is utilized to demonstrate an optically addressed ultra-fast, ultraviolet light modulator.
GaN CVD Reactions: Hydrogen and Ammonia Decomposition and the Desorption of Gallium Michael
Authors: Bartram E. , Creighton J. Randall 
Published: April 2011

AbstractIsotopic labeling experiments have revealed correlations between hydrogen reactions, Ga desorption, and ammonia decomposition in GaN CVD. Low energy electron diffraction (LEED) and temperature programmed desorption (TPD) were used to demonstrate that hydrogen atoms are available on the surface for reaction after exposing GaN(0001) to deuterium at elevated temperatures. Hydrogen reactions also lowered the temperature for Ga desorption significantly. Ammonia did not decompose on the surface before hydrogen exposure. However, after hydrogen reactions altered the surface, N15H3 did undergo both reversible and irreversible decomposition. This also resulted in the desorption of N2 of mixed isotopes below the onset of GaN sublimation. This suggests that the driving force of the high nitrogen-nitrogen bond strength (226 kcal/mol) can lead to the removal of nitrogen from the substrate when the surface is nitrogen rich. Overall, these findings indicate that hydrogen can influence GaN CVD significantly, being a common factor in the reactivity of the surface, the desorption of Ga, and the decomposition of ammonia.
Effect of Buffer Layer and III/V Ratio on the Surface Morphology of Gan Grown by MBE
Authors: Piquette E. C. , Bridger P. M. , Beach R. A. , McGill T. C. 
Published: April 2011

AbstractThe surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and “loop” defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than l nm RMS.
Behavior of Crack Growth Resistance in Toughened Silicon Nitride Ceramics
Authors: Kawamoto H. , Hiramatsu K. , Takigawa Y. , Okada A. , Usami H. 
Published: April 2011

AbstractTesting methods for crack-growth resistance-curve (R-curve) behavior were investigated and developed to analyze the toughened mechanism in polycry stalline ceramics. These methods are a biaxial-flexure method for small-scale disc-shaped specimens with micro-indentation cracks and a single-edged notched beam flexural method with crack stabilizers. In both methods, the growing crack length is measured directly as a function of applied stress, using the system that consist of a microscope and a CCD camera. Applying these testing methods, R-curve behavior of a toughened silicon nitride with a preferred orientation of elongated grains was evaluated to characterize the toughened mechanism, comparing with the behavior in a commercially available silicon nitride. The behavior having these rising R-curves is discussed with emphasis on the effects of microstructure such as grain-growth and grain-orientation, and resultant grain-bridgings behind the crack-tip.
The Fracture of Composite Materials: A Chaotic Approach
Authors: Tonelli R. , Meloni F. , Aymerich F. 
Published: April 2011

AbstractIn the present work, we analyze the behaviour of graphite-reinforced composite materials under periodic loading. We reconstruct the phase space of these physical systems by means of the tools of nonlinear system theory. Experimental measurements are used to calculate chaotic parameters as fractal and embedding dimensions. We use these quantities to characterize differences in the physical samples and modifications in their internal structure. This analysis can provide useful indications about the fracture process.
Scanning Force Microscopy as a Tool for Fracture Studies
Authors: Thome F. , Göken M. , Große Gehling M. , Vehoff H. 
Published: April 2011

AbstractDynamic simulations of the fracture toughness as a function of the orientation and tem- perature were carried out and compared with experimental results obtained by in-situ loading pre-cracked NiAl single crystals inside a scanning force microscope (SFM). In order to compare the simulations with the experiments the problem of the short crack with dislocations was solved for general loading and arbitrary slip line directions. The stress and strain field obtained could be directly connected to FEM calculations which allowed the examination of the stability of micro cracks at notches. The effect of different fracture conditions for biaxial loading were studied in detail.The dynamic simulation yielded predictions of K1C, slip line length and dislocation distri- butions as a function of loading rate, temperature and orientation. These predictions were tested by in-situ loading NiAl single crystals inside a SFM at various temperatures. The local COD, slip line length and apparent dislocation distribution at the surface were measured as a function of the applied load and the temperature. The experiments clearly demonstrated that dislocations emit from the crack tip before unstable crack jumps occur. The local COD could be directly related to the number of dislocations emitted from the crack tip. With increasing temperature the number of dislocations and the local COD increased before unstable crack jumps or final fracture occurred.
Barium Strontium Titanate Capacitors for Embedded Dram
Authors: Zurcher Peter , Tracy C.J. , Jones R.E. , Alluri P. , Chu P.Y. , Jiang B. , Kim M. , Melnick B.M. , Raymond M.V. , Roberts D. , Remmel T.P. , Tsai T.L. , White B.E. , Zafar S. , Gillespie S.J. 
Published: April 2011

AbstractLong recognized as the best potential solution to the continued scaling of the onetransistor/one-capacitor standalone dynamic random access memory (DRAM) beyond a gigabit, barium strontium titanate (BST) and other high permittivity dielectrics are fast becoming enablers to embedding large amounts of memory into a high performance logic process. System requirements such as granularity, bandwidth, fill frequency, and power pose major challenges to the use of high density standalone DRAM, leading to the current push for embedded solutions where very wide buses are possible. As a result, projected embedded memory sizes are rapidly approaching that of the standalone products, and with the high wafer cost of the combined logic plus memory process, bit cell scaling is critical. The BST memory cell, with its low thermal budget processing, very high charge storage density, and high conductivity metal electrodes has the potential to be efficiently embedded with traditional logic flows if the materials and integration challenges of the required three dimensional (3D) bit cell capacitors can be solved. BST materials properties such as dielectric relaxation, interface capacitance, and resistance degradation and their impact on capacitor scaling will be reviewed along with the electrode materials issues associated with certain 3D capacitor designs. The scaling limits of BST bit cells in the deep sub-micron regime will be discussed.
Defect Identification in (La,Sr)CoO3−δ Using Positron Annhiiilation Spectroscopy
Authors: Friessnegg T. , Nielsen B. , Ghosh V. J. , Moodenbaugh A.R. , Madhukar S. , Aggarwal S. , Keeble D.J. , Poindexter E.H. , Mascher P. , Ramesh R. 
Published: April 2011

AbstractVacancy type defects in bulk La1−xSrxCoO3−δ samples were investigated by positron lifetime spectroscopy. The effects of Sr-doping as well as the effect of oxygen deficiency were determined. Comparing the resolved lifetimes with calculated values permits defect identification.
Preparation and Properties of Barium Incorporated Strontium Bismuth Tantalate Ferroelectric Thin Films
Authors: Lu Chung-Hsin , Wen Cheng-Yen 
Published: April 2011

AbstractSrBi2Ta2O9thin films doped with barium ions were studied, in which Ba/(Sr+Ba) = 0.3 and 0.5, meanwhile the content of (Sr+Ba) remains unity to keep the stoichiometry of SrBi2Ta2O9. Films were deposited using metalorganic decomposition method with spin-on coating. Crystallinity, surface morphology, and ferroelectric properties of prepared thin films were investigated. From X-ray diffraction (XRD) analysis, barium ions substituted strontium ions in the SrBi2Ta2O9 lattice. Shift of diffraction peaks was observed, indicating a slight distortion of the lattice while barium ions incorporated into. The observation of prepared films indicated that the grain size of films annealed at 750 °C was about 0.7∼0.8 μm. Such barium incorporated SrBi2Ta2O9 thin films exhibited higher remanent polarization than the intrinsic SrBi2Ta2O9thin films.
Raman Study of SrBi2Ta2O9 Bi3TiNbO9 Thin Films
Authors: Pérez W. , Ching-Prado E , Dobal P.S. , Reynés-Figueroa A. , Katiyar R.S. , Tirumala S. , Desu S. B. 
Published: April 2011

AbstractThin films of ferroelectric (SrBi2Ta2O9)x(Bi3TiNbO9)1−x layered structure were prepared for x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, by metal organic solution deposition method on Pt/TiO2/SiO2/Si substrates. The micro-Raman studies reveal that the samples are mostly homogeneous. The Raman sprectrum for the film with x = 0 shows bands around 60, 170, 232, 256, 337, 569, and 839 cm−1, which indicate Bi3TiNbO9 (BTN) formation. The prominent band around 839 cm−1, which is an A1g mode of the orthorombic symmetry, corresponds to symmetric stretching of the BO6 octahedra. The frequency of this band is found to shift significantly as SrBi2Ta2O9(SBT) material is added to the BTN compound. In this paper, the evolution of the Raman bands with the inclusion of SBT material are presented and discussed. Finally, The Raman spectra of SBT- BTN films are compared with those obtained in SBT-BTN bulk ceramics.
Preparation of Sr0.7Bi2+xTa2O9 Thin Films on SiO2/Si at Low Temperature by Pulsed Laser Deposition and Fatigue-Tolerant C-V Characteristics with Large Memory Window
Authors: Okuyama Masanori , Matsumuro Yoshinori , Sugiyama Hideki , Noda Minoru 
Published: April 2011

AbstractPreferentially (105)-oriented Sr0.7Bi2.8Ta2O9(SBTO) thin films on SiO2/n-Si(100) have been prepared by pulsed laser deposition at temperatures as low as 350°C, which is the lowest process temperature for growing SBTO ferroelectric thin films. Dielectric properties of the SBTO films have been improved by increasing the Sr/Bi atomic ratio from 0.7/2.8 to 0.7/2.0. Memory windows of as large as 3.6V and 4.V in the MFIS capacitor have been obtained at Sr/Bi ratios of 0.7/2.0 and 1.0/2.0 respectively and are the largest values reported in the MF(I)S diode structures. Little C-V degradation is observed up to with the 1010 cycles for the process temperature of 400°C, especially for Sr/Bi of 0.7/2.0, with the memory window remaining at more than 3.2V. Memory retention time was about 3×103 sec at which the difference in capacitance between “ON” and “OFF” states reduces to half of its initial value. Optimum hold bias voltage exists in the retention test. Improvement in memory characteristics is strongly related to the insulating properties of the ferroelectric and dielectric thin films rather than the value of dielectric constant.
Controlling the Chemical Order in PMN-Type Relaxors
Authors: Davies P. K. , Akbas M. A. 
Published: April 2011

AbstractInvestigations of pure and chemically substituted tantalate and niobate members of the Pb(Mg1/3 Nb2/3)O3 (PMN) family of perovskite relaxors demonstrate that the degree of cation ordering and size of the chemically ordered domains can be extensively modified by thermal treatment. These observations and refinements of the cation occupancies in well ordered samples conflict with the predictions of the widely accepted “space charge model”, and instead support a “random site” model for the 1:1 B-site ordering. In this model one of the positions in the ordered structure is occupied by Ta (or Nb) and the other contains a random distribution of Mg and the remaining Ta cations. The stability of the order and magnitude of the domain growth can be enhanced by relatively small concentrations of selected solid solution additives (e.g. Zr in Pb(Mg1/3Ta2/3)O3 (PMT); Th or Sc in PMN). Correlations between the stability of the 1:1 order and the size of the solid solution additive can be used to understand the different responses of the cation order in PMN and PMT to thermal treatment. Results are also presented for the dielectric response of fully ordered relaxors in the (l–x) Pb(Mg1/3Ta2/3)O3 – (x) Pb(Sc1/2 Ta1/2)O3 system. While large domain samples of PMT-rich compositions exhibit a relaxor response, a crossover to normal ferroelectric behavior is observed for x = 0.5. The change in the dielectric response can be rationalized in terms of the effect of the chemistry of the random site sub-lattice on the correlation length of the ferroelectric coupling.
Thermally Induced Imprint of PZT and SBT Thin Films
Authors: Kim Dong-Joo , Kim Seung-Hyun , Maria Jon-Paul , Kingon Angus I. 
Published: April 2011

AbstractThe imprint characteristics of chemical solution deposited Pb(Zr,Ti)O3 (PZT) and SrBi2Ta2O9(SBT) thin films have been investigated. The imprint properties were evaluated by polarization-electric field hysteresis measurements. Imprint was measured at the elevated temperatures to accelerate imprint rate. Several parametric variables such as voltage and DC bias were also used to investigate possible imprint mechanisms. To quantify imprint, two different standards for evaluation of ferroelectric capacitors were compared: voltage shift and asymmetric polarization shift. Two types of PZT thin films with the same Zr/Ti composition were prepared to investigate the influence of loop shape on the rate of imprint. In addition, the imprint behavior of SBT was also investigated. It is shown that the imprint rate is strongly dependent upon the method of evaluation as well as the loop shape.
Electrochemical Potential Gradients in Pb(Zr,Ti)O3 Thin Films
Authors: Brazier M. , Mansour S. , McElfresh M. 
Published: April 2011

AbstractThin film PbZr0.55Ti0.45O3 (PZT) capacitors were observed to develop a dc voltage offset when driven with an ac applied electric field. This voltage offset displayed a strong dependence on the ambient oxygen partial pressure, PO2, of the atmosphere above the film, in addition to applied electric field and temperature dependencies. A scenario is proposed wherein a chemical potential gradient is established in the film, the magnitude of which is determined by the ambient PO2. The subsequent redistribution of oxygen vacancies is thought to create the observed voltage offset. This hypothesis was supported by the observation of similar voltage offsets in compositionally graded PZT films. This effect shows promise for novel low-temperature oxygen sensing applications.
Crystal Structures of SrBi2Ta2O9 and Sr0.8Bi2.2Ta2O9 Ferroelectric Materials
Authors: Shimakawai Y. , Nakagawa Y. , Kubo Y. , Kamiyama T. , Asano H. 
Published: April 2011

AbstractStructural and ferroelectric properties of stoichiometric SrBi2Ta2O9 and Sr-deficient-and-Bi-excess Sr0.8Bi2.2Ta2O9 bulk ceramics materials are studied. The ferroelectric Curie temperatures for SrBi2Ta2O9 and Sr0.8Bi2.2Ta2O9 are 300 and 400°C, respectively. Structure analysis by neutron powder diffraction reveals that Bi2O2 layer and TaO6 octahedra are considerably distorted and that atomic displacement along the a-axis causes ferroelectric spontaneous polarization. In Sro8Bi22Ta2O9, both Bi-substitution and cation-vacancies exist at the Sr-site. The calculated polarization of Sr0.8Bi2.2Ta2O9 is higher than that of the stoichiometric sample, which is consistent with observations of remanent polarization in thin-film capacitors. The Bi-substitution and the cation-vacancies at the Sr site enhance structural distortion in the TaO6 octahedron and lead to the higher Curie temperature and the higher ferroelectric spontaneous polarization.
Self-Poling of Thin BaTiO3 Films by Contact Potential Difference
Authors: Lubomirsky Igor , Chang David T. , Stafsudd Oscar M. 
Published: April 2011

AbstractThe substrate dependence of the ferroelectric properties of sol-gel derived BaTiO3 thin films was investigated. Thin (0.35 µm) films were deposited on Si substrates of various orientations and doping. Films deposited on p+ Si (1020 cm−3 B, 2% Ge) (110) were self-poled. The poling direction is self-restoring even after being reversed by an external bias or heated above the Curie temperature. The pyroelectric coefficient was not zero above the Curie point, which indicated that the films were permanently subjected to an electric field, which originates from the contact potential between the film and the substrate. Thisconclusion was confirmed by surface photo voltage spectroscopy. The possibility of a paraelectric to ferroelectric transition driven by an electric field due to the contact potential difference must be taken into account for dynamic random access memory (DRAM) applications.
Thick PLZT Films Grown by Sol-gel for Optical Device Applications
Authors: Linnik M. , Wilson O. , Christou A. 
Published: April 2011

AbstractThe preparation and characterization of thick PLZT films for spatial phase modulator applications are reported. Films were fabricated on LSCO/LAO substrates by a sol-gel technique using multiple heat-treatment parameters. The crystal quality of PLZT 9/65/35 films was investigated by X-ray diffraction and scanning electron microscopy.
Optical Phase and Amplitude Modulation in (9/65/35) Pb-La-Zr-Ti-O Thin Films
Authors: Young D. , Li H. , Choopun S. , Wang L. , Salamanca-Riba L. , Ramesh R. , Christou A. 
Published: April 2011

AbstractElectro-Optic Pb0.91gLa0.09Zr0.65Ti0.35O3 (PLZT) thin films are studied for transverse optical modulator applications. PLZT thin films were grown by pulsed laser ablation on MgO and GaAs substrates. Structural characterization by x-ray diffraction, transmission electron microscopy, Rutherford backscattering, and sputter Auger electron profiling indicate polycrystalline, phase pure PLZT with evidence of interdiffusion and approximately 20nm RMS surface roughness.Single PLZT thin films, grown on MgO substrates with a SrTiO3 (STO) buffer layer, were fabricated into simple modulator structures for electro-optical characterization.Measurements were performed in normal transmission mode. Application of an oscillating electric field across the PLZT layer produced optical amplitude and phase modulation.The application of an oscillating electric field of frequency f across the PLZT has been observed to produce f and 2f frequency modulation in both the phase and amplitude of the output light. Theoretical considerations show that this can be attributed to the quadratic nature of the electro-optic effect in PLZT.