bluuebunny/crossref_metadata_2025_split huggingface dataset:
Extended Defects in GaN: a Theoretical Study
Authors: Eisner J. , Frauenheim Th. , Haugk M. , Gutierrez R. , Jones R. , Heggie M. I. 
Published: April 2011

AbstractWe present density-functional theory studies for a variety of surfaces and extended defects in GaN. According to previous theoretical studies {1010} type surfaces are electrically inactive. They play an important role in GaN since similar configurations occur at open-core screw dislocations and nanopipes as well as at the core of threading edge dislocations. Domain boundaries are found to consist of four-fold coordinated atoms and are also found to be electrically inactive. Thus, except for full-core screw dislocations which possess heavily strained bonds all investigated extended defects do not induce deep states into the band-gap. However, electrically active impurities in particular gallium vacancies and oxygen related defect complexes are found to be trapped at the stress field of the extended defects.
Statistical Analysis of Local Composition and Luminescence in InGaN Grown by Molecular Beam Epitaxy
Authors: Einfeldt S. , Böttcher T. , Hommel D. , Selke H. , Ryder P. L. , Bertram F. , Riemann T. , Rudloff D. , Christen J. 
Published: April 2011

AbstractInGaN layers grown by molecular beam epitaxy are investigated in terms of their compositional homogeneity using transmission electron microscopy and cathodoluminescence spectroscopy performed with high spatial resolution. Strong fluctuations of the indium content were found in bulk-like layers, which could be partially reduced by modulating the indium flux during growth, i. e. by nominally growing a short period GaN/InGaN superlattice. For indium compositions above x ≠ 0.1 this approach fails. Strained InGaN in quantum wells exhibits lateral fluctuations on an atomic scale and on a scale of several hundred nanometers. The results are discussed in view of the origin of inhomogeneous indium incorporation.
Disordering of InGaN/GaN Superlattices After High-Pressure Annealing
Authors: McCluskey M.D. , Romano L.T. , Krusor B.S. , Hofstetter D. , Bour D.P. , Kneissl M. , Johnson N.M. , Suski T. , Jun J. 
Published: April 2011

AbstractInterdiffusion of In and Ga is observed in InGaN multiple-quantum-well superlattices for annealing temperatures of 1250 to 1400°C. Hydrostatic pressures of up to 15 kbar were applied during the annealing treatments to prevent decomposition of the InGaN and GaN. In as-grown material, x-ray diffraction spectra show InGaN superlattice peaks up to the fourth order. After annealing at 1400°C for 15 min, only the zero-order InGaN peak is observed, a result of compositional disordering of the superlattice. Composition profiles from secondary ion mass spectrometry indicate significant diffusion of Mg from the p-type GaN layer into the quantum well region. This Mg diffusion may lead to an enhancement of superlattice disordering. For annealing temperatures between 1250 and 1300°C, a blue shift of the InGaN spontaneous emission peak is observed, consistent with interdiffusion of In and Ga in the quantum-well region.
Enhanced GaN Decomposition at Movpe Pressures
Authors: Koleske D.D. , Wickenden A.E. , Henry R.L. , Twigg M.E. , Culbertson J.C. , Gorman R.J. 
Published: April 2011

AbstractGaN decomposition was studied above 800°C in flowing H2 and N2 for pressures ranging from 10 to 700 torr. From careful weighings of the GaN film on sapphire before and after annealing, the rates for GaN decomposition, Ga surface accumulation, and Ga desorption were obtained. An enhancement in the GaN decomposition rate was observed in H2 pressures greater than 100 torr. Even with this enhanced GaN decomposition, the Ga desorption rate is nearly constant at higher pressures. As a result, Ga droplets accumulate on the surface. For N2 pressures ranging from 76 to 400 torr no net enhancement in the GaN decomposition rate is observed and the GaN decomposition rate is reduced compared to identical annealing conditions in H2. This suggests that HI- is acting chemically to reduce the barrier for GaN decomposition. This may occur through a surface mediated dissociation of H2 followed by the formation of more mobile and volatile hydrogenated N and Ga species. The significance of this study for GaN growth is that by increasing the GaN decomposition, the Ga atoms diffuse farther and subsequently re-incorporate into the growing lattice, increasing the GaN crystal quality. Connections between the enhanced GaN decomposition rate and the coalescing of nucleation layer during the ramp to high temperature and the consequences for the high temperature growth are discussed.
Relaxation Phenomena in GaN/ AlN/ 6H-SiC Heterostructures
Authors: Edwards N.V. , Batchelor A.D. , Buyanova I.A. , Madsen L.D. , Bremser M.D. , Davis R.F. , Aspnes D.E. , Monemar B. 
Published: April 2011

AbstractWe have developed a method to modulate the strain state (normally > 4 kbar, tensile) of moderately thick (∼2 μm) GaN based structures grown on 6H-SiC to a range 0 to -2 kbar of compressive stresses by introducing a strain-mediating layer (SML) above the standard high-temperature AlN buffer layer. The strain characteristics of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML layer. This is achieved by in-situ techiniques during crystal growth without degrading the optical and structural properties of the deposited layers.
Optical and Structural Properties of Er3+-Doped GaN Grown by MBE
Authors: Birkhahn R. H. , Hudgins R. , Lee D. S. , Lee B.K. , Steckl A. J. , Saleh A. , Wilson R. G. , Zavada J. M. 
Published: April 2011

AbstractWe report the morphological and compositional characteristics of Er-doped GaN grown by MBE on Si(111) substrates and their effect on optical properties. The GaN was grown by molecular beam epitaxy using solid sources (for Ga and Er) and a plasma gas source for N2. The films emit by photoexcitation in the visible and near infrared wavelengths from the Er atomic levels. The morphology of the GaN:Er films was examined by AFM. Composition was determined by SIMS depth profiling that revealed a large Er concentration at 4.5 × 1021 atoms/cm3 accompanied by a high oxygen impurity concentration.
Characteristic Temperature Estimation for GaN-Based Lasers
Authors: Honda T. , Kawanishi H. , Sakaguchi T. , Koyama F. , Iga K. 
Published: April 2011

AbstractWe have estimated the characteristic temperature T0 of GaN-based vertical-cavity surface-emitting lasers. The density matrix theory including intraband relaxation broadening has been taken into account. The estimated T0 is about 300 K, which suggests a good temperature characteristic in GaN-based lasers.
Focused ION Beam Etching of GaN
Authors: Flierl C. , White I.H. , Kuball M. , Heard P.J. , Allen G.C. , Marinelli C. , Rorison J.M. , Penty R.V. , Chen Y. , Wang S.Y. 
Published: April 2011

AbstractWe have investigated the use of focused ion beam (FIB) etching for the fabrication of GaN-based devices. Although work has shown that conventional reactive ion etching (RME) is in most cases appropriate for the GaN device fabrication, the direct write facility of FIB etching - a well-established technique for optical mask repair and for IC failure analysis and repair - without the requirement for depositing an etch mask is invaluable. A gallium ion beam of about 20nm diameter was used to sputter GaN material. The etching rate depends linearly on the ion dose per area with a slope of 3.5 × 10-4 μm3/pC. At a current of 3nA, for example, this corresponds to an etch rate of 1.05μm3/s. Good etching qualities have been achieved with a side wall roughness significantly below 0.1μm. Changes in the roughness of the etched surface plane stay below 8nm.
Monitoring and Controlling of Strain During Mocvd of AlGaN for UV Optoelectronics
Authors: Han Jung , Crawford M. H. , Shul R. J. , Hearne S. J. , Chason E. , Figiel J. J. , Banas M. 
Published: April 2011

AbstractThe grown-in tensile strain, due to a lattice mismatch between AlGaN and GaN, is responsible for the observed cracking that seriously limits the feasibility of nitride-based ultraviolet (UV) emitters. We report in-situ monitoring of strain/stress during MOCVD of AIGaN based on a wafer-curvature measurement technique. The strain/stress measurement confirms the presence of tensile strain during growth of AlGaN pseudomorphically on a thick GaN layer. Further growth leads to the onset of stress relief through crack generation. We find that the growth of AlGaN directly on low-temperature (LT) GaN or AIN buffer layers results in a reduced and possibly controllable strain.
Normal and Inverted Algan/Gan Based Piezoelectric Field effect Transistors Grown by Plasma Induced Molecular Beam Epitaxy
Authors: Murphy M. J. , Foutz B. E. , Chu K. , Wu H. , Yeo W. , Schaff W. J. , Ambacher O. , Eastman L. F. , Eustis T. J. , Dimitrov R. , Stutzmann M. , Rieger W. 
Published: April 2011

AbstractHigh quality Ga-face and N-face AlGaN/GaN based heterostructures have been grown by plasma induced molecular beam epitaxy. By using Ga-face material we are able to fabricate conventional heterojunction field effect transistors. Because the N-face material confines electrons at a different heterojunction, the resulting transistors are called inverted. The Ga-face structures use a high temperature AIN nucleation layer to establish the polarity. Structures from these materials, relying only on polarization induced interface charge effects to create the two-dimensional electron gases, are used to confirm the polarity of the material as well as test the electrical properties of the layers. The resulting sheet concentrations of the two dimensional electron gases agree very well with the piezoelectric theory for this materials system. Hall mobilities of the two-dimensional gases for the N-face structures are as high as 1150 cm2/Vs and 3440 cm2/Vs for 300 K and 77 K respectively, while the Ga-face structures yield room temperature mobilities of 1190 cm2/Vs. Both structures were then fabricated into transistors and characterized. The inverted transistors, which were fabricated from the N-face material, yielded a maximum transconductance of 130 mS/mm and a current density of 905 mA/mm. Microwave measurements gave an ft of 7 GHz and an fmax of 12 GHz for a gate length of 1 μm. The normal transistors, fabricated from the Ga-face material, produced a maximum transconductance of 247 mS/mm and a current density of 938 mA/mm. Microwave measurements gave an ft of 50 GHz and an fmax of 97 GHz for a gate length of 0.25 μm. This shows that using plasma induced molecular beam epitaxy N-face and Ga(A1)-face AlGaN/GaN heterostructures can be grown with structural and electrical properties very suitable for high power field effect transistors.
Molecular Dynamics Simulations of Nanoindentation of Silicon Nitride
Authors: Walsh Phillip , Omeltchenko Andrey , Kikuchi Hideaki , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya 
Published: April 2011

AbstractThis is a report of work in progress on 10 million atom Molecular Dynamics (MD) simulations of nanoindentation of crystalline and amorphous silicon nitride (Si3N4). Nanoindentation is used to determine mechanical properties of extremely thin films such as hardness and elastic moduli. We report load-displacement curves for several Si3N4 configurations using an idealized non-deformable indenter and analyze the local stress distributions in the vicinity of the indenter tip. Preliminary results for surface adhesion using Si3N4 for both tip and substrate are also reported.
Studies of Crack Pattern in Two Dimensional Media as Resulting From Ion Beam like or Freezing Destruction
Authors: Ausloos M. , Vandewalle N. , D‘Hulst R. 
Published: April 2011

AbstractWe have numerically studied various models for crack formation in the slow dynamical regime. Two different aspects are presented herein: (i) sputtering with reconstruction and (ii) internal destruction of a porous medium through the invasion of a secondary phase.
Deposition of Pb(Zr, Ti)O3-Films on High-Grade Steel Hastelloy and Modification by Ion Implantation
Authors: Klarmann R. , Biegel W. , Wörz B. , Hemberger J. , Stritzker B. 
Published: April 2011

AbstractPulsed Laser Deposition (PLD) was used to deposit Pb(Zr,Ti)O3 thin films on high-grade steel Hastelloy. This material is suitable as substrate and electrode material as well. The deposited films were characterized with respect to their structural (XRD, RBS), topological (AFM) and their ferroelectric properties (P(E) hysteresis, retain and fatigue) which were found to be excellent. The ferroelectric properties in general, are sensitive to structural modifications and chemical doping and could therefore be influenced by ion implantation. Several species of ions (Ca+, O+) show different influence on the ferroelectric behavior and on the structural defects of the films. A subsequent annealing process can partially restore these modifications. The implanted Ca+ films were characterized by dielectric spectroscopy dependend on frequency and temperature to get information about ferroelectric domains.
Composition Dependence of Dielectric Anomaly in Strontium Bismuth Tantalate Thin Films
Authors: Takemura Koichi , Noguchi Takehiro , Hase Takashi , Kimura Hidekazu , Miyasaka Yoichi 
Published: April 2011

AbstractEffects of off-stoichiometry and Nb substitution on the dielectric anomaly and ferroelectric properties have been investigated for SrBi2Ta2O9(SBT) thin films. Local atomic environment for the stoichiometric and off-stoichiometric SBT films has been also measured. The features of the dielectric anomaly, the Curie temperature (Tc), and the temperature dependence of the spontaneous polarization (Ps) are independent of the film thickness, and are governed by the nature of the crystal. For the stoichiometric Sr content SBT films, the grain size increases and the temperature dependence of the remanent polarization (Pr) decreases with increasing thickness. For the films with Sr/Bi/Ta = 0.8/2/2 and 0.8/2.2/2, Sr deficient local structure was observed, and such lattice structure probably leads to higher Tc than the stoichiometric crystal. Nb substitution for Ta also raises Tc, and makes the dielectric anomaly sharper. The phase transition associated with the dielectric anomaly for these films is thought to be first-order.
Studies of Electrical Polarization Fatigue in SrBi2Ta2O9 Thin Films
Authors: Lee K. M. , Thomas D. , Kim S. H. , Maria J. P. , Kingon A. I. , Jang H. M. 
Published: April 2011

AbstractThe polarization suppression and electrical properties directly associated with the electrical polarization fatigue in SrBi2Ta2O9system were systematically investigated using Pt/SBT/Pt capacitors. Three general observations were made after 109 switching cycles: (i) ∼95% of the remanent polarization was conserved, (ii) both high and zero bias field capacitance decreased, and (iii) leakage current density increased from approximately 10−7 to 10−5 A/cm2at ∼30kV/cm2. In addition, the “knee” field, at which the leakage abruptly increases, assumed smaller values with cumulative switching cycles. Temperature dependent leakage data was collected for both as-deposited and field-cycled samples. Based on these results, we propose the possibilities of enhanced concentration of charge carriers or additional reductions in interfacial conduction barriers. Motion of oxygen vacancies to less-shallow energy levels near electrode/ferroelectric interface may allow this mechanism to occur.
Synthesis and Dielectric Properties of SrBi2Nb2O9 Layered Perovskite by Sol-Gel Processing
Authors: Wu Y. , Ohuchi F. S. , Cao G. Z. 
Published: April 2011

AbstractFerroelectric layered perovskite strontium bismuth niobium oxide SrBi2Nb2O9(SBN) ceramics and films were made by sol-gel processing. A multiple step hydrolysis-condensation process was applied to the synthesis of the SBN sol by using inorganic salts as precursors with citric acid as a complexing agent. Single phase polycrystalline SBN was obtained after heat-treatment at 650°C. It was found that the dielectric constants of SBN ceramics were dependent on heat-treatment. SEM, XPS, TGA/DTA and XRD analyses indicated that bismuth-deficiency may contribute to the relatively low dielectric constants. While a prolonged firing time at high temperatures may lead to a lattice expansion resulting in a significant decrease of the dielectric constant. C-oriented SBN films were obtained by dip-coating onto SrTiO3 substrates and the films are dense and crack-free with a thickness of 900 nm.
Studies of Metallic Species Incorporation During Growth of SrBi2Ta2O9 Films on YBa2Cu3O7−x Substrates Using Mass Spectroscopy of Recoiled Ions
Authors: Dhote A.M. , Krauss A.R. , Auciello O. , Im J. , Gruen D.M. , Ramesh R. , Pai S.P. , Venkatesan T. 
Published: April 2011

AbstractThe incorporation of metallic species (Bi, Sr and Ta) during the growth of layered perovskite SrBi2Ta2O9 (SBT) on a-axis oriented YBa2Cu3O7−x (YBCO) conducting oxide substrates has been investigated using in situ low energy mass spectroscopy of recoiled ions (MSRI). This technique is capable of providing monolayer-specific surface information relevant to the growth of single and multi-component thin films and layered heterostructures. The data show a temperature dependence of metallic species incorporation during co-deposition of Sr, Bi and Ta on YBCO surfaces. At high temperatures (400 < T ≤ 700°C), negligible incorporation of Bi is observed as compared to Ta and Sr. At low temperatures (≤ 400°C), there is a substantial incorporation of Bi, Sr and Ta on the surface of YBCO, and the MSRI signal intensities for Sr, Bi and Ta are nearly independent of substrate temperature. According to thermodynamic calculations, the presence of Ba and Y on the YBCO surface inhibit the incorporation of Bi due to competition for oxygen required to establish bonding of metallic species to the surface. This may be the explanation for the observed Bi deficiency in films grown on YBCO surfaces at temperatures > 400°C. SBT films grown at temperatures ≤ 400°C and annealed in oxygen or air at 800°C exhibit a polycrystalline structure with partial a-axis orientation.
Low Temperature MOCVD of Thin Film PZT
Authors: Chen Ing-Shin , Roeder Jeffery F. 
Published: April 2011

AbstractIncorporation of ferroelectric thin films into integrated microelectronics requires deposition temperatures compatible with back-end processing. Pb(Zr,Ti)O3 (PZT) thin films have been deposited at low temperatures by a thermal chemical vapor deposition process using flash-vaporized metalorganic precursors. Deposition temperatures, A-site to B-site ratios in the precursor solution, and bottom electrodes have been surveyed. Stoichiometric perovskite films with (Zr/Ti) ratios ranging from 20/80 to 50/50 were obtained. Films deposited at temperatures below 500°C possess very fine grain structures. Capacitor structures were fabricated by depositing top electrodes using electron-beam evaporation, followed in some cases by a post-electrode anneal at 650°C. Remanent polarizations greater than 20 μC/cm2 were observed on films deposited on Pt at temperatures as low as 475°C. Perovskite films were obtained on Ir electrodes at temperatures as low as 450°C, with remanent polarization greater than 10μC/cm2 in the as-deposited condition.
Correlation Between Large Electrostrictive Strain and Relaxor Behavior with Structural Changes Induced in P(VDF-TrFE) Copolymer by Electron Irradiation
Authors: Bharti Vivek , Ye Yaohong , Xu T.-B. , Zhang Q. M. 
Published: April 2011

AbstractThe effect of high energy electron irradiation is investigated on poly(vinylidene fluoridetrifluoroethylene), P(VDF-TrFE), copolymers in the broad range of irradiation doses and temperature. It was found that after a suitable electron irradiation condition, ferroelectric polymer materials show exceptionally high electrostrictive response (> 4%), high dielectric constant (>60) with low dielectric loss over a broad temperature range. In addition, the material after irradiation exhibits many features resembling those of a relaxor ferroelectric, suggesting that the electron irradiation breaks up the coherent polarization domain in a normal ferroelectric material into micro-polar regions that transform the material into a relaxor ferroelectric. These distinct features make this material very attractive for a broad range of applications. In this paper we will review the experimental results on various characteristics of material properties including the dielectric and electrostrictive strain response induced at different temperatures under different electron irradiation conditions and their relation with structural changes observed by X-ray and DSC.
Target Crystallography and the Growth of Pb(Mg1/3Nb2/3)O3 (PMN) Thin Films
Authors: Corbett M.H. , Catalan G. , Gregg J.M. , Bowman R.M. 
Published: April 2011

AbstractLead magnesium niobate thin films have been grown on {100} MgO substrates using pulsed laser deposition (PLD). Several series of films were made using targets of varying perovskite and PbO content. The conditions necessary to produce almost 100% perovskite Pb(Mg1/3Nb2/3)O3 (PMN) films were most sensitive to the growth temperature rather than the starting target crystallography. The electrical properties were determined by fabricating planar thin film capacitor structures and monitoring capacitance and dielectric loss as a function of temperature. Our best capacitors yielded a loss of 0.1 at 1 kHz over a wide temperature range.
Electrical Properties of PZT Thin Film Capacitors with Novel Pt-Ir Based Electrode Barriers for Nonvolatile Memories
Authors: Desu Chandra S. , Vedula Ramakrishna , Lee Kwang B. , Desu Seshu B. 
Published: April 2011

AbstractA new electrode barrier structure based on noble metals Pt and Ir alloys and their oxides was proposed for the integration of PZT capacitors into high density nonvolatile memories. The proposed PtIrOx/PtIr/PtIrOx structure with Pt rich compositions (∼90%) is an excellent conducting electrode for the ferroelectric capacitor and a very good diffusion barrier for species like oxygen and Si. These structures are thermally stable up to the PZT processing temperatures of 650°C. PZT deposited on these electrodes crystallized predominantly in pervoskite phase. The test capacitors showed well saturated hysteresis loops with Pr and Ec of 21.3 µC/cm2 and 43 kV/cm, respectively. An extremely low polarization fatigue of 3% after 1× 1011 repetitions and leakage currents close to those on Pt were observed.
Glass Transition in Sub-nanometer Confinement
Authors: Huwe A. , Kremer F. , Arndt M. , Behrens P. , Schwieger W. , Ihlein G. , Akdogan Ö. , Schüth F. 
Published: April 2011

AbstractBroadband dielectric spectroscopy (10−2 Hz - 109Hz) is employed to study the molecular dynamics of low-molecular-weight glassforming liquids being confined to nanopores. For the H-bond forming liquid propylene glycol being confined to (uncoated and silanized) nanopores (pore size: 2.5 nm, 5.0 nm and 7.5 nm) a molecular dynamics is observed which is comparable to that of the bulk liquid. Due to surface effects in uncoated nanopores the relaxation time distribution is broadened on the long term side and the mean relaxation rate is decreased by about half a decade. This effect can be counterbalanced by lubricating the inner surfaces of the pores resulting in a relaxation rate which is slightly faster compared to the bulk liquid. For the H-bonded liquid ethylene glycol (EG) embedded in zeolites of different pore size and topology one observes a sharp transition from a single-molecule dynamics to that of a liquid depending on the coordination number of the confined molecules. While EG in silicalite (showing a single molecule relaxation) has four neighboring molecules, EG in zeolite beta or AIPO4-5 has a coordination number of five and behaves like a bulk liquid.