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Atomistic simulation of point defects and dislocations in bbc transition metals from first principles

Description: Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we have been studying point defects and dislocations in bcc transition metals, with molybdenum (Mo) as a prototype. For point defects in Mo, the calculated vacancy formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations in Mo have also been investigated. The <110> split dumb-bell is found to have the lowest formation energy, as is experimentally observed, but the corresponding migration energy is calculated to be 3--15 times higher than previous theoretical estimates. The atomic structure and energetics of <111> screw dislocations in Mo are now being investigated. We have found that the ``easy`` core configuration has a lower formation energy than the ``hard`` one, consistent with previous theoretical studies. The former has a distinctive 3-fold symmetry with a spread out of the dislocation core along the <112> directions, an effect which is driven by the strong angular forces present in these metals.
Date: January 19, 1996
Creator: Xu, W & Moriarty, J.A.
Partner: UNT Libraries Government Documents Department

Stress and Defect Control in GaN Using Low Temperature Interlayers

Description: In organometallic vapor phase epitaxial growth of Gail on sapphire, the role of the low- temperature-deposited interlayers inserted between high-temperature-grown GaN layers was investigated by in situ stress measurement, X-ray diffraction, and transmission electron microscopy. Insertion of a series of low temperature GaN interlayers reduces the density of threading dislocations while simultaneously increasing the tensile stress during growth, ultimately resulting in cracking of the GaN film. Low temperature AIN interlayers were found to be effective in suppressing cracking by reducing tensile stress. The intedayer approach permits tailoring of the film stress to optimize film structure and properties.
Date: December 4, 1998
Creator: Akasaki, I.; Amano, H.; Chason, E.; Figiel, J.; Floro, J.A.; Han, J. et al.
Partner: UNT Libraries Government Documents Department

Observation of photoexcited emission clusters in the bulk of KDP and laser conditioning under 355-nm irradiation

Description: Defect clusters in the bulk of large KDP crystals are revealed using a microscopic fluorescence imaging system and CW laser illumination. Exposure of the crystal to high power 355-nm, 3-ns laser irradiation leads to a significant reduction of the number of observed optically active centers. The initially observed defect cluster concentration is approximately 10<sup>4</sup>-10<sup>6</sup> per mm<sup>3</sup> depending on the crystal growth method and sector of the crystal. The number of defect clusters can be reduced by a factor of 10<sup>2</sup> or more under exposure to 355-nm laser irradiation while their average intensities also decreases. Spectroscopic measurements provide information on the electronic structure of the defects.
Date: December 15, 1998
Creator: De Yoreo, J. J.; Demos, S. G.; Radousky, H. B.; Staggs, M. & Yan, M.
Partner: UNT Libraries Government Documents Department

Compliant substrate technology for dissimilar epitaxy

Description: Strained-layer semiconductor films offer tremendous potential with regards to optoelectronic applications for high speed communications, mobile communications, sensing, and novel logic devices. It is an unfortunate reality that many of the possible film/substrate combinations that could be exploited technologically are off limits because of large differences in lattice parameters, chemical compatibilities, or thermal expansion rates. These mechanical, chemical, and thermal incompatibilities manifest themselves primarily in terms of lattice defects such as dislocations and antiphase boundaries, and in some cases through enhanced surface roughness. An additional limitation, from a production point of view, is money. Device manufacturers as a rule want the cheapest substrate possible. Freeing the heteroepitaxial world of the bonds of (near) lattice matching would vastly expand the types of working devices that could be grown. As a result, a great deal of effort has been expended finding schemes to integrate dissimilar film/substrate materials while preserving the perfection of the film layer. One such scheme receiving significant attention lately is the so-called compliant substrate approach.
Date: March 1, 2000
Partner: UNT Libraries Government Documents Department

Photonic Band Gap Micro-Cavities in Three-Dimension

Description: Localization of light to less than a cubic wavelength, {lambda}{sup 3}, has important quantum consequences. The creation of single mode cavities and the modification of spontaneous emission are two important examples. A defect formed inside a three-dimensional (3D) photonic crystal provides an unique optical environment for light localization. Single mode defect cavities were built, for the first time, from an infrared 3D photonic crystal. A cavity state with modal volume of less than one {lambda}{sup 3} was observed.
Date: April 26, 1999
Creator: Biswas, R.; Fleming, J.G.; Ho, K.M.; Lin, Shawn-Yu & Sigalas, M.M.
Partner: UNT Libraries Government Documents Department

Charged Local Defects in Extended Systems

Description: The conventional approach to treating charged defects in extended systems in first principles calculations is via the supercell approximation using a neutralizing jellium background charge. I explicitly demonstrate shortcomings of this standard approach and discuss the consequences. Errors in the electrostatic potential surface over the volume of a supercell are shown to be comparable to a band gap energy in semiconductor materials, for cell sizes typically used in first principles simulations. I present an alternate method for eliminating the divergence of the Coulomb potential in supercell calculations of charged defects in extended systems that embodies a correct treatment of the electrostatic potential in the local viciniq of the a charged defect, via a mixed boundary condition approach. I present results of first principles calculations of charged vacancies in NaCl that illustrate the importance of polarization effects once an accurate representation of the local potential is obtained. These polarization effects, poorly captured in small supercells, also impact the energetic on the scale of typical band gap energies.
Date: May 25, 1999
Creator: Schultz, Peter A.
Partner: UNT Libraries Government Documents Department

Final report for Grant DE-FG02-84ER45131

Description: This Final Report surveys the work done on understanding the properties and behavior of driven interfaces. It is presented under two topics: (1) interfaces driven in pure and perturbed Hele-Shaw cells; (2) gels, colloids, and polymer solutions as complex media for interface growth and motion. This work has contributed to the international effort to learn about nonlinear and pattern forming systems. The data have been influential as theoretical and computational groups have attempted to understand the dynamics and nonlinear processing steps and the structure-property relations of complex materials. The Hele-Shaw cell was especially productive during this period of intense interest in ''simple'' nonlinear pattern formation, providing the simplest and best understood pattern forming system which could then be complicated with changes of boundary condition or changes of fluid property to test in a controlled way the effect on pattern formation of added physical/mathematical complexity.
Date: June 7, 2001
Creator: Maher, James V.
Partner: UNT Libraries Government Documents Department

Te Inclusions in CZT Detectors: New Method for Correcting Their Adverse Effects

Description: Both Te inclusions and point defects can trap the charge carriers generated by ionizing particles in CdZnTe (CZT) detectors. The amount of charge trapped by point defects is proportional to the carriers’ drift time and can be corrected electronically. In the case of Te inclusions, the charge loss depends upon their random locations with respect to the electron cloud. Consequently, inclusions introduce fluctuations in the charge signals, which cannot be easily corrected. In this paper, we describe direct measurements of the cumulative effect of Te inclusions and its influence on the response of CZT detectors of different thicknesses and different sizes and concentrations of Te inclusions. We also discuss a means of partially correcting their adverse effects.
Date: October 25, 2009
Creator: Bolotnikov, A.E.; Babalola, S.; Camarda, G.S.; Cui, Y.; Egarievwe, S.U.; Hawrami, R. et al.
Partner: UNT Libraries Government Documents Department

Structural disorder produced in KH2PO4 by light-ion bombardment

Description: We study structural disorder produced in tetragonal KDP (KH{sub 2}PO{sub 4}) single crystals at room temperature by irradiation with MeV light ions. Results show that electronic energy loss plays a major role in the production of lattice defects in KDP. The effective diameters of ion tracks depend superlinearly on the electronic stopping power of energetic light ions. Structural lattice disorder is also accompanied by the formation of a network of cracks and blisters on the sample surface. Such irradiation-induced cracking and blistering typically evolves over extended periods of time (e.g., days) after bombardment, strongly affected by ion irradiation and sample storage conditions.
Date: December 16, 2003
Creator: Kucheyev, S O & Felter, T E
Partner: UNT Libraries Government Documents Department

An atomistic study of dynamic brittle fracture in silicon

Description: Dynamic fracture has been modeled using a modified embedded atom method (MEAM) potential for silicon. For Mode I dynamic fracture along (1 1 1) crystallographic planes, the molecular dynamics model predicts crack speeds and fracture energies in agreement with previous experimental results [l]. In this orientation, hcture occurs almost exclusively along (1 1 1) planes for energy release rates up to 30 J/m2. For Mode I fracture oriented initially on (1 10) planes, fracture occurs by cleavage on (1 10) planes for a static energy release rate (J,) less than 8 J/m2. For greater values of J,, the fracture surfaces switch to alternating (111) planes, which is in agreement with previous experimental results [2]. Crack speed predictions for the (1 10) orientation are somewhat In the atomistic simulations, the dynamically propagating cracks generate dislocations, which are primarily produced on the (1 1 1) and (1 10) planes. Differences in the type and quantity of dislocations produced have been observed for different orientations. Molecular dynamics has the ability to calculate the energy consumed by dislocations and other lattice defects produced during fracture and the total surface energy of the main crack, side branches and secondary cracks. The sum of the surface energy and the energy consumed by lattice defects determines the dynamic fracture less than the high speeds observed experimentally. toughness, J(v). The dynamic fkacture toughness has been found to vary linearly with J,. For the (111) orientation with cracks propagating in the [211] direction, J(v) asymptotically approached a value of 1/3 of J,. The remainder of the strain energy that is released during fracture is converted into kinetic energy at the crack tip during the fracture process, which occurs atom by atom.
Date: January 1, 2002
Creator: Swadener, J. G. (John G.); Baskes, M. I. (Michael I.) & Nastasi, Michael Anthony,
Partner: UNT Libraries Government Documents Department

Defects in p-GaN and their atomic structure

Description: In this paper defects formed in p-doped GaN:Mg grown with Ga polarity will be discussed. The atomic structure of these characteristic defects (Mg-rich hexagonal pyramids and truncated pyramids) in bulk and thin GaN:Mg films grown with Ga polarity was determined at atomic resolution by direct reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities were present inside the defects. The inside walls of the cavities were covered by GaN which grew with reverse polarity compared to the matrix. It was proposed that lateral overgrowth of the cavities restores matrix polarity on the defect base. Exchange of Ga and N sublattices within the defect compared to the matrix lead to a 0.6 {+-} 0.2 {angstrom} displacement between the Ga sublattices of these two areas. A [1{und 1}00]/3 shift with change from AB stacking in the matrix to BC within the entire pyramid is observed
Date: October 8, 2004
Creator: Liliental-Weber, Z.; Tomaszewicz, T.; Zakharov, D.; Jasinski, J. & and O'Keefe, M.
Partner: UNT Libraries Government Documents Department

Superconducting nanostructured materials.

Description: Within the last year it has been realized that the remarkable properties of superconducting thin films containing a periodic array of defects (such as sub-micron sized holes) offer a new route for developing a novel superconducting materials based on precise control of microstructure by modern photolithography. A superconductor is a material which, when cooled below a certain temperature, loses all resistance to electricity. This means that superconducting materials can carry large electrical currents without any energy loss--but there are limits to how much current can flow before superconductivity is destroyed. The current at which superconductivity breaks down is called the critical current. The value of the critical current is determined by the balance of Lorentz forces and pinning forces acting on the flux lines in the superconductor. Lorentz forces proportional to the current flow tend to drive the flux lines into motion, which dissipates energy and destroys zero resistance. Pinning forces created by isolated defects in the microstructure oppose flux line motion and increase the critical current. Many kinds of artificial pinning centers have been proposed and developed to increase critical current performance, ranging from dispersal of small non-superconducting second phases to creation of defects by proton, neutron or heavy ion irradiation. In all of these methods, the pinning centers are randomly distributed over the superconducting material, causing them to operate well below their maximum efficiency. We are overcome this drawback by creating pinning centers in aperiodic lattice (see Fig 1) so that each pin site interacts strongly with only one or a few flux lines.
Date: July 13, 1998
Creator: Metlushko, V.
Partner: UNT Libraries Government Documents Department

The evolution of deformation microstructures and local orientations

Description: A brief overview of the evolution of microstructures during deformation is presented within the framework of grain subdivision. Three aspects of the evolving microstructure that are related to recrystallization are emphasized. These include the formation of high angle dislocation boundaries during deformation, the local environment of crystallographic orientations and a new scaling method for modeling detailed microstructural data.
Date: December 31, 1995
Creator: Hughes, D.A.
Partner: UNT Libraries Government Documents Department

Sputtering induced changes in defect morphology and dopant diffusion for Si implanted GaAs: Influence of ion energy and implant temperature

Description: Experimental observations of dopant diffusion and defect formation are reported vs ion energy and implant temperature in Si-implanted GaAs. In higher energy implants (>100 keV), little or no diffusion occurs, while at energies less than 100 keV, the amount of dopant redistribution is inversely proportional to energy. Extended defect density shows the opposite trend, increasing with ion energy. Similarly, Si diffusion during post implant annealing decreases by a factor of 2.5 as the implant temperature increases from -2 to 40 C. In this same temperature range, maximum depth and density of extrinsic dislocation loops increases by factors of 3 and 4, respectively. Rutherford backscattering channeling indicates that Si- implanted GaAs undergoes an amorphous-to-crystalline transition at Si implant temperatures between -51 and 40 C. A unified explanation of the effects of ion energy and implant temperature on both diffusion and dislocation formation is proposed based on known differences in sputter yields between low and high energy ions and crystalline and amorphous semiconductors. The model assumes that the sputter yield is enhanced at low implant energies and by amorphization, thus increasing the excess vacancy concentration. Estimates of excess vacancy concentration are obtained by simulations of the diffusion profiles and are quantitatively consistent with a realistic sputter yield enhancement. Removal of the vacancy-rich surface by etching prior to annealing completely suppresses the Si diffusion and increases the dislocation density, lending further experimental support to the model.
Date: December 1, 1994
Creator: Robinson, H.G.; Deal, M.D.; Lee, C.C.; Haynes, T.E.; Allen, E.L. & Jones, K.S.
Partner: UNT Libraries Government Documents Department

Electrically inactive poly-silicon grain boundaries

Description: Structures, energies, and electronic properties of symmetric [001] tilt grain boundaries in Si have been studied using Stillinger-Weber and Tersoff classical potentials, and semi-empirical (tight-binding) electronic structure methods. The calculated lowest energy (310) grain boundary structure and electronic properties are consistent with previous TEM measurement and calculations. For the controversial (710) grain boundaries, the tight-binding calculations do not show any electronic energy levels in the band gap. This indicates that with every atom fully fourfold coordinated, the (710) grain boundary should be electrically inactive. Some high-energy metastable grain boundaries were found to be electrically active by the presence of the levels introduced in the band gap. Also, the vacancy concentration at the (310) GB was found to be enhanced by many orders of magnitude relative to bulk. The dangling bond states of the vacancies should be electrically active.
Date: May 1, 1996
Creator: Chen, S.P.; Kress, J.D.; Voter, A.F. & Albers, R.C.
Partner: UNT Libraries Government Documents Department

Transmission electron microscopy study in-situ of radiation-induced defects in copper at elevated temperatures

Description: Neutrons and high-energy ions incident upon a solid can initiate a displacement collision cascade of lattice atoms resulting in localized regions within the solid containing a high concentration of interstitial and vacancy point defects. These point defects can collapse into various types of dislocation loops and stacking fault tetrahedra (SFT) large enough that their lattice strain fields are visible under diffraction-contrast imaging using a Transmission Electron Microscope (TEM). The basic mechanisms driving the collapse of point defects produced in collision cascades is investigated in situ with TEM for fcc-Cu irradiated with heavy (100 keV Kr) ions at elevated temperature. The isothermal stability of these clusters is also examined in situ. Areal defect yields were observed to decrease abruptly for temperatures greater than 300 C. This decrease in defect yield is attributed to a proportional decrease in the probability of collapse of point defects into clusters. The evolution of the defect density under isothermal conditions appears to be influenced by three different rate processes active in the decline of the total defect density. These rate constants can be attributed to differences in the stability of various types of defect clusters and to different loss mechanisms. Based upon observed stabilities, estimations for the average binding enthalpies of vacancies to SFT are calculated for copper.
Date: December 1, 1996
Creator: Daulton, T.L.; Kirk, M.A. & Rehn, L.E.
Partner: UNT Libraries Government Documents Department

Positron beam lifetime spectroscopy at Lawrence Livermore National Laboratory

Description: Defect analysis is needed for samples ranging in thickness from thin films to large engineering parts. We are meeting that need with two positron beam lifetime spectrometers: on on a 3 MeV electrostatic accelerator and the second on our high current linac beam. The high energy beam spectrometer performs positron lifetime analysis on thick samples which can be encapsulated for containment or for in situ measurements in controlled environments. At our high current beam, we are developing a low energy, microscopically focused, pulsed positron beam to enable positron annihilation lifetime spectroscopy for defect specific, 3-D maps with sub-micron location resolution. The data from these instruments with the aid of first principles calculations of defect specific positron lifetimes.
Date: October 1, 1996
Creator: Howell, R.H.; Cowan, T.E.; Hartley, J.H. & Stern, P.A.
Partner: UNT Libraries Government Documents Department

Long-term (>30,000-40,000 h) creep behavior of an advanced Si{sub 3}N{sub 4} ceramic

Description: Long-term creep data are reported for a hot-isostatically-pressed (HIPed) silicon nitride (Si{sub 3}N{sub 4}) ceramic material tested at 1250 {degrees}C in air for 28,000 to 38,000 h (3.2 to 4.3 4 years) with no creep failure. Two specimens tested at 150 and 175 MPa each showed extensive primary creep in excess of 10,000 h, followed by well-defined steady-state creep. Because the primary creep range was extensively long and reasonably linear, the creep curves appeared to portray two stages of steady-state creep having occurred due to two different creep mechanisms: (1) formation and (2) growth of cavities. Approximately 57 to 70% of measured apparent creep strain was estimated due to the evolution of cavities, while the remaining was attributed to the viscous creep of grain boundary materials.
Date: March 1, 1997
Creator: Liu, K.C.; Lin, H.T.; Stevens, C.O. & Brinkman, C.R.
Partner: UNT Libraries Government Documents Department

Carrier localization in gallium nitride

Description: In wide bandgap GaN, a large number of interesting and important scientific questions remain to be answered. For example, the large free electron concentration reaching 10{sup 19} to 10{sup 20} cm{sup - 3} in nominally undoped material are ascribed to intrinsic defects because no chemical impurity has been found at such high concentrations. According to theoretical models, a nitrogen vacancy acts as a donor but its formation energy is very large in n-type materials, making this suggestion controversial. We have investigated the nature of this yet unidentified donor at large hydrostatic pressure. Results from infrared reflection and Raman scattering indicate strong evidence for localization of free carriers by large pressures. The carrier density is drastically decreased by two orders of magnitude between 20 and 30 GPa. Several techniques provide independent evidence for results in earlier reports and present the first quantitative analysis. A possible interpretation of this effect in terms of the resonant donor level is presented.
Date: September 1, 1996
Creator: Wetzel, C.; Walukiewicz, W. & Haller, E.E.
Partner: UNT Libraries Government Documents Department

The nature of twin boundaries in the high-temperature superconductor YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

Description: Twin boundaries are the most commonly observed lattice defect in the high-temperature superconductor YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. Furthermore, the region around a twin boundary for which the structure and composition are seriously affected is of a scale comparable to the coherence length for the superconducting order parameter. Thus, twin boundaries can be important in the behavior of magnetic vortices and the critical current density in this material. In this paper the authors review the results of a wide range of investigations of twin boundary structure and composition by advanced transmission electron microscopy methods, both imaging and analytical in nature. A simple Landau model of twin boundary energy and width is proposed.
Date: June 1, 1999
Creator: Zhu, Y. & Welch, D.O.
Partner: UNT Libraries Government Documents Department

Direct observations of atomic structures of defects in GaN by high-resolution Z-contrast STEM

Description: GaN/(0001) Sapphire grown by low pressure MOVPE is studied by high resolution Z-contrast imaging using STEM. First direct observation of the threading dislocation with edge character shows the atomic core structure, which appears to have a similar configuration to the {l_brace}10-10{r_brace} surface. The surfaces of the nanopipe walls are on {l_brace}10-10{r_brace} with the terminating layer between the atoms with one bond per pair. In addition, the high resolution Z contrast image of the prismatic stacking fault confirms the results by conventional HRTEM.
Date: December 1, 1997
Creator: Xin, Y.; Pennycook, S.J.; Browning, N.D.; Sivananthan, S.; Nellist, P.D.; Faurie, J.P. et al.
Partner: UNT Libraries Government Documents Department