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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

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

Implantation processing of Si: A unified approach to understanding ion-induced defects and their impact

Description: A model is presented to account for the effects of ion-induced defects during implantation processing of Si. It will be shown that processing is quite generally affected by the presence of defect excesses rather than the total number of defects. a defect is considered excess if it represents a surplus locally of one defect type over its compliment. Processing spanning a wide range of implantation conditions will be presented to demonstrate that the majority of the total defects played little or no role in the process. This is a direct result of the ease with which the spatially correlated Frenkel pairs recombine either dynamically or during a post-implantation annealing. Based upon this model, a method will be demonstrated for manipulating or engineering the excess defects to modify their effects. In particular high-energy, self-ions are shown to inject vacancies into a boron implanted region resulting in suppression of transient enhanced diffusion of the dopant.
Date: May 1997
Creator: Holland, O. W. & Roth, E. G.
Partner: UNT Libraries Government Documents Department

Sixth workshop on the role of impurities and defects in silicon device processing

Description: The Sixth Workshop on the Role of Impurities and Defects in Silicon Device Processing was held in Snowmass Village, August 12-14, 1996. The workshop was attended by 87 participants from academic institutions and photovoltaic industry representatives, from the United States, Australia, Belgium, Canada, France, Germany, Italy, Japan, Belgium, and The Netherlands. The workshop consisted of nine sessions that addressed different aspects of impurities and defects in silicon and applications to solar-cell processing. Each session opened with some review talks summarizing recent advances in this field and introduced important issues for further discussions during a subsequent panel discussion session. In addition, the latest research results were presented in two poster sessions.
Date: September 1, 1996
Creator: Tan, T.; Swanson, R. & Sopori, B.
Partner: UNT Libraries Government Documents Department

Investigation of damage in KDP using scattering techniques

Description: Interest in producing high damage threshold KH{sub 2}PO{sub 4} (KDP) and (D{sub x}H{sub 1-x}){sub 2}PO{sub 4} (DKDP)(also called KD*P) for frequency conversion and optical switching applications is driven by the requirements of the National Ignition Facility (NIF). Presently only the best crystals meet the NIF system requirements at the third harmonic (351 nm) and only after a laser conditioning process. Neither the mechanism for damage in bulk KDP nor the mechanism for conditioning is understood. As part of a development effort to increase the damage thresholds of KDP and DKDP, we have been developing techniques to pinpoint the locations where damage will initiate in the bulk material. After we find these locations we will use other measurement techniques to determine how these locations differ from the other surrounding material and why they cause damage. This will allow crystal growers to focus their efforts to improve damage thresholds. Historically damage thresholds have increased it is believed as a consequence of increased purity of the growth solution and through the use of constant filtration during the growth process. As a result we believe that damage is caused by defects in the crystals and have conducted a series of experiments using light scatter to locate these defects and to determine when and where damage occurs. In this paper we present results which show a low correlation between light scatter from bulk defects in KDP and the initiation sites for damage. We have also studied the effects of thermal conditioning on light scatter, strain induced birefringence and damage threshold. We have seen evidence that regions of high strain also exhibit lower damage threshold than the surrounding lower strain material. When thermally conditioned, these crystals show a decrease in some of the strong linear scattering features and a decrease in the strain birefringence while the ...
Date: February 12, 1997
Creator: Woods, B.; Runkel, M.; Yan, M.; Staggs, M.; Zaitseva, N.; Kozlowski, M. et al.
Partner: UNT Libraries Government Documents Department

Seventh workshop on the role of impurities and defects in silicon device processing

Description: This workshop is the latest in a series which has looked at technological issues related to the commercial development and success of silicon based photovoltaic (PV) modules. PV modules based on silicon are the most common at present, but face pressure from other technologies in terms of cell performance and cell cost. This workshop addresses a problem which is a factor in the production costs of silicon based PV modules.
Date: August 1, 1997
Partner: UNT Libraries Government Documents Department

Tuning ground states and excitations in complex electronic materials

Description: Modern electronic materials are characterized by a great variety of broken-symmetry ground states and excitations. Their control requires understanding and tuning underlying driving forces of spin-charge-lattice coupling, critical to macroscopic properties and applications. We report representative model calculations which demonstrate some of the richness of the phenomena and the challenges for successful microscopic modeling.
Date: September 1, 1996
Creator: Bishop, A.R.
Partner: UNT Libraries Government Documents Department