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First-principles study of point-defect production in Si and SiC

Description: The authors have calculated the displacement-threshold energy E(d) for point-defect production in Si and SiC using empirical potentials, tight-binding, and first-principles methods. They show that -- depending on the knock-on direction -- 64-atom simulation cells can be sufficient to allow a nearly finite-size-effect-free calculation, thus making the use of first-principles methods possible. They use molecular dynamics (MD) techniques and propose the use of a sudden approximation which agrees reasonably well with the MD results for selected directions and which allows estimates of Ed without employing an MD simulation and the use of computationally demanding first-principles methods. Comparing the results with experiment, the authors find the full self-consistent first-principles method in conjunction with the sudden approximation to be a reliable and easy method to predict E{sub d}. Furthermore, they have examined the temperature dependence of E{sub d} for C in SiC and found it to be negligible.
Date: March 1998
Creator: Windl, W.; Lenosky, T. J.; Kress, J. D. & Voter, A. F.
Partner: UNT Libraries Government Documents Department

Point defect production, geometry and stability in silicon: A molecular dynamics simulation study

Description: We present results of molecular dynamics computer simulation studies of the threshold energy for point defect production in silicon. We employ computational cells with 8000 atoms at ambient temperature of 10 K that interact via the Stillinger-Weber potential. Our simulations address the orientation dependence of the defect production threshold as well as the structure and stability of the resulting vacancy-interstitial pairs. Near the <111> directions, a vacancy tetrahedral-interstitial pair is produced for 25 eV recoils. However, at 30 eV recoil energy, the resulting interstitial is found to be the <110> split dumbbell configuration. This Frenkel pair configuration is lower in energy than the former by 1.2 eV. Moreover, upon warming of the sample from 10 K the tetrahedral interstitial converts to a <110> split before finally recombining with the vacancy. Along <100> directions, a vacancy-<110> split interstitial configuration is found at the threshold energy of 22 eV. Near <110> directions, a wide variety of closed replacement chains are found to occur for recoil energies up to 45 eV. At 45 eV, the low energy vacancy-<l 10> split configuration is found. At 300 K, the results are similar. We provide details on the atomic structure and relaxations near these defects as well as on their mobilities.
Date: February 1, 1994
Creator: Caturla, M.J.; Rubia, T.D. de la & Gilmer, G.H.
Partner: UNT Libraries Government Documents Department

On the application of the weak-beam technique to the determination of the sizes of small point-defect clusters in ion-irradiated copper.

Description: We have made an analysis of the conditions necessary for the successful use of the weak-beam technique for identifying and characterizing small point-defect clusters in ion-irradiated copper. The visibility of small defects was found to depend only weakly on the magnitude of the beam-convergence. In general, the image sizes of small clusters were found to be most sensitive to the magnitude of Sa with the image sizes of some individual defects changing by large amounts with changes as small as 0.025 nm{sup {minus}1}. The most reliable information on the true defect size is likely to be obtained by taking a series of 5-9 micrographs with a systematic variation of deviation parameter from 0.2-0.3 nm{sup {minus}1}. This procedure allows size information to be obtained down to a resolution limit of about 0.5 nm for defects situated throughout a foil thickness of 60 nm. The technique has been applied to the determination of changes in the sizes of small defects produced by a low-temperature in-situ irradiation and annealing experiment.
Date: October 30, 1998
Creator: Jenkins, M. L.
Partner: UNT Libraries Government Documents Department

Fermi level stabilization energy in group III-nitrides

Description: Energetic particle irradiation is used to systematically introduce point defects into In{sub 1-x}Ga{sub x}N alloys over the entire composition range. Three types of energetic particles (electrons, protons, and {sup 4}He{sup +}) are used to produce a displacement damage dose spanning five decades. In InN and In-rich InGaN the free electron concentration increases with increasing irradiation dose but saturates at a sufficiently high dose. The saturation is due to Fermi level pinning at the Fermi Stabilization Energy (E{sub FS}), which is located at 4.9 eV below the vacuum level. Electrochemical capacitance-voltage (ECV) measurements show that the pinning of the surface Fermi energy at E{sub FS} is also responsible for the surface electron accumulation in as-grown InN and In-rich InGaN alloys. The results are in agreement with the amphoteric defect model that predicts that the same type of native defects are responsible for the Fermi level pinning in both cases.
Date: January 7, 2005
Creator: Li, S. X.; Yu, K. M.; Wu, J.; Jones, R. E.; Walukiewicz, W.; AgerIII, J. W. et al.
Partner: UNT Libraries Government Documents Department

Point Defect Incorporation During Diamond Chemical Vapor Deposition

Description: The incorporation of vacancies, H atoms, and sp{sup 2} bond defects into single-crystal homoepitaxial (100)(2x1)- and(111)-oriented CVD diamond was simulated by atomic-scale kinetic Monte Carlo. Simulations were performed for substrate temperatures from 600 C to 1200 C with 0.4% CH{sub 4} in the feed gas, and for 0.4% to 7% CH{sub 4} feeds with a substrate temperature of 800 C. The concentrations of incorporated H atoms increase with increasing substrate temperature and feed gas composition, and sp{sup 2} bond trapping increases with increasing feed gas composition. Vacancy concentrations are low under all conditions. The ratio of growth rate to H atom concentration is highest around 800-900 C, and the growth rate to sp{sup 2} ratio is maximum around 1% CH{sub 4}, suggesting that these conditions are ideal for economical diamond growth under the simulated conditions.
Date: August 2, 1999
Creator: Battaile, C.C.; Srolovitz, D.J. & Butler, J.E.
Partner: UNT Libraries Government Documents Department

Modeling the effect of irradiation and post-irradiation annealing on grain boundary composition in austenitic Fe-Cr-Ni alloys.

Description: Many irradiation effects in Fe-Cr-Ni alloys such as radiation-induced segregation, radiation-enhanced diffusion, and void swelling are known to vary with bulk alloy composition. The development of microstructural and microchemical changes during irradiation and during post-irradiation annealing is determined by the rate of diffusion of point defects and alloying elements. To accurately predict the changes in grain boundary chemistry due to radiation-induced segregation and post-irradiation annealing, the composition dependence of diffusion parameters, such as the migration energy, must be known. A model has been developed which calculates migration diffusivity. The advantages of this calculational method are that a single set of input parameters can be used for a wide range of bulk alloy compositions, and the effects of local order can easily be incorporated into the calculations. A description of the model is presented, and model calculations are compared to segregation measurements from seven different iron-chromium-nickel alloys, irradiated with protons to doses from 0.1 to 3.0 dpa at temperatures between 200 C and 600 C. Results show that segregation trends can be modeled using a single set of input parameters with the difference between model calculation and measurement being less than 5 at%, but usually less than 2 at%. Additionally, model predictions are compared to grain boundary composition measurements of neutron irradiated 304 stainless steel following annealing. For the limited annealing data available, model calculations correctly predict the magnitude and time scale for recovery of the grain boundary composition.
Date: March 5, 1998
Creator: Allen, T.; Busby, J. T.; Kenik, E. A. & Was, G. S.
Partner: UNT Libraries Government Documents Department

Self- and zinc diffusion in gallium antimonide

Description: The technological age has in large part been driven by the applications of semiconductors, and most notably by silicon. Our lives have been thoroughly changed by devices using the broad range of semiconductor technology developed over the past forty years. Much of the technological development has its foundation in research carried out on the different semiconductors whose properties can be exploited to make transistors, lasers, and many other devices. While the technological focus has largely been on silicon, many other semiconductor systems have applications in industry and offer formidable academic challenges. Diffusion studies belong to the most basic studies in semiconductors, important from both an application as well as research standpoint. Diffusion processes govern the junctions formed for device applications. As the device dimensions are decreased and the dopant concentrations increased, keeping pace with Moore's Law, a deeper understanding of diffusion is necessary to establish and maintain the sharp dopant profiles engineered for optimal device performance. From an academic viewpoint, diffusion in semiconductors allows for the study of point defects. Very few techniques exist which allow for the extraction of as much information of their properties. This study focuses on diffusion in the semiconductor gallium antimonide (GaSb). As will become clear, this compound semiconductor proves to be a powerful one for investigating both self- and foreign atom diffusion. While the results have direct applications for work on GaSb devices, the results should also be taken in the broader context of III-V semiconductors. Results here can be compared and contrasted to results in systems such as GaAs and even GaN, indicating trends within this common group of semiconductors. The results also have direct importance for ternary and quaternary semiconductor systems used in devices such as high speed InP/GaAsSb/InP double heterojunction bipolar transistors (DHBT) [Dvorak, (2001)]. Many of the findings which will ...
Date: March 26, 2002
Creator: Nicols, Samuel Piers
Partner: UNT Libraries Government Documents Department

The role of point defects and defect complexes in silicon device processing. Summary report and papers

Description: This report is a summary of a workshop hold on August 24--26, 1992. Session 1 of the conference discussed characteristics of various commercial photovoltaic silicon substrates, the nature of impurities and defects in them, and how they are related to the material growth. Session 2 on point defects reviewed the capabilities of theoretical approaches to determine equilibrium structure of defects in the silicon lattice arising from transitional metal impurities and hydrogen. Session 3 was devoted to a discussion of the surface photovoltaic method for characterizing bulk wafer lifetimes, and to detailed studies on the effectiveness of various gettering operations on reducing the deleterious effects of transition metals. Papers presented at the conference are also included in this summary report.
Date: August 1, 1994
Creator: Sopori, B. & Tan, T. Y.
Partner: UNT Libraries Government Documents Department

Role of point defects/defect complexes in silicon device processing. Book of abstracts, fourth workshop

Description: The 41 abstracts are arranged into 6 sessions: impurities and defects in commercial substrates: their sources, effects on material yield, and material quality; impurity gettering in silicon: limits and manufacturability of impurity gettering and in silicon solar cells; impurity/defect passivation; new concepts in silicon growth: improved initial quality and thin films; and silicon solar cell design opportunities.
Date: June 1, 1994
Partner: UNT Libraries Government Documents Department

Synthesis and characterization of a nanocrystalline diamond aerogel

Description: Aerogel materials have myriad scientific and technological applications due to their large intrinsic surface areas and ultralow densities. However, creating a nanodiamond aerogel matrix has remained an outstanding and intriguing challenge. Here we report the high-pressure, high-temperature synthesis of a diamond aerogel from an amorphous carbon aerogel precursor using a laser-heated diamond anvil cell. Neon is used as a chemically inert, near-hydrostatic pressure medium that prevents collapse of the aerogel under pressure by conformally filling the aerogel's void volume. Electron and X-ray spectromicroscopy confirm the aerogel morphology and composition of the nanodiamond matrix. Time-resolved photoluminescence measurements of recovered material reveal the formation of both nitrogen- and silicon- vacancy point-defects, suggesting a broad range of applications for this nanocrystalline diamond aerogel.
Date: July 6, 2011
Creator: Pauzauskie, Peter J.; Crowhurst, Jonathan C.; Worsley, Marcus A.; Laurence, Ted A.; Kilcoyne, A. L. David; Wang, Yinmin et al.
Partner: UNT Libraries Government Documents Department

High-temperature treatment of In-doped CZT crystals grown by the high-pressure Bridgman method

Description: We evaluated the effect of high-temperature treatment of Cd0.9Zn0.1Te:In single crystals using Hall-effect measurements, medium- and high-temperature annealing under various deviations from stoichiometry, and infra-red (IR) transmission microscopy Annealing at ~730 K sharply increased the electrical conductivity (by ~1-2 orders-of-magnitude). Plots of the temperature- and cadmium-pressure dependences of the electrical conductivity, carrier concentration, and mobility were obtained. Treating previously annealed Cd-samples under a Te overpressure at 1070 K allowed us to restore their resistance to its initial high values. The main difference in comparing this material with CdTe was its lowered electron density. We explained our results within the framework of Kröger’s theory of quasi-chemical reactions between point defects in solids.
Date: August 12, 2012
Creator: A., Bolotnikov; Fochuk, P.; Nakonechnyi, I.; Kopach, O.; Verzhak, Ye.; Panchuk, O. et al.
Partner: UNT Libraries Government Documents Department

First Principles Study of Metastable Beta Titanium Alloys

Description: The high temperature BCC phase (b) of titanium undergoes a martensitic transformation to HCP phase (a) upon cooling, but can be stabilized at room temperature by alloying with BCC transition metals such as Mo. There exists a metastable composition range within which the alloyed b phase separates into a + b upon equilibrium cooling but not when rapidly quenched. Compositional partitioning of the stabilizing element in as-quenched b microstructure creates nanoscale precipitates of a new simple hexagonal w phase, which considerably reduces ductility. These phase transformation reactions have been extensively studied experimentally, yet several significant questions remain: (i) The mechanism by which the alloying element stabilizes the b phase, thwarts its transformation to w, and how these processes vary as a function of the concentration of the stabilizing element is unclear. (ii) What is the atomistic mechanism responsible for the non-Arrhenius, anomalous diffusion widely observed in experiments, and how does it extend to low temperatures? How does the concentration of the stabilizing elements alter this behavior? There are many other w forming alloys that such exhibit anomalous diffusion behavior. (iii) A lack of clarity remains on whether w can transform to a -phase in the crystal bulk or if it occurs only at high-energy regions such as grain boundaries. Furthermore, what is the nature of the a phase embryo? (iv) Although previous computational results discovered a new wa transformation mechanism in pure Ti with activation energy lower than the classical Silcock pathway, it is at odds with the a / b / w orientation relationship seen in experiments. First principles calculations based on density functional theory provide an accurate approach to study such nanoscale behavior with full atomistic resolution, allowing investigation of the complex structural and chemical effects inherent in the alloyed state. In the present work, a model Ti-Mo ...
Date: August 2015
Creator: Gupta, Niraj
Partner: UNT Libraries


Description: Passivity breakdown of 316L SS in the presence of aggressive Cl{sup -} and inhibitive NO{sub 3}{sup -} anions has been experimentally studied and the results have been interpreted in terms of the Point Defect Model (PDM). By introducing the competitive adsorption of Cl{sup -} and NO{sub 3}{sup -} into surface oxygen vacancies at the passive film/solution interface, the PDM yields a critical breakdown potential (V{sub c}) that is predicted to vary linearly with log[Cl{sup -}], or with log ([Cl{sup -}]/[NO{sub 3}{sup -}]) [1] when nitrate ions are present, which is shown in Fig. 1. The Point Defect Model also explains the fact that the slope of V{sub c} vs. log[Cl{sup -}] does not change in the presence of NO{sub 3}{sup -}, which is attributed to the quasi-equilibrium ejection of a cation from the barrier layer to form the vacancy pair V{sub M}V{sub O}{sup (2-{chi})} at the barrier layer/solution interface. The Point Defect Model predicts that measured V{sub c} increases linearly with the square root of voltage scan rate {nu}{sup 1/2} [1]. From this correlation, the critical, areal concentration of cation vacancies at the metal/barrier layer interface, {zeta}, has been estimated and found to be comparable to that calculated from the concentration of sites on the cation sublattice at the same location based on the presumed Cr{sub 2}O{sub 3} composition of the barrier layer. The Point Defect Model also explains the near normal distribution of V{sub c} in terms of a normal distribution of breakdown sites on the surface with respect to the vacancy diffusivity (D) [2]. The calculated distribution agrees with the experimental results very well and this agreement is used to estimate the cation vacancy diffusivity. Chronoamperometric studies have been performed on Type 316L SS at different voltages, [Cl{sup -}], [NO{sub 3}{sup -}] and temperatures to study the transition ...
Date: October 23, 2006
Creator: S. Yang, G. Engelhardt, and D. D. Macdonald
Partner: UNT Libraries Government Documents Department


Description: CdZnTe (or CZT) crystals can be used in a variety of detector-type applications. This large band gap material shows great promise for use as a gamma radiation spectrometer. Historically, the performance of CZT has typically been adversely affected by point defects, structural and compositional heterogeneities within the crystals, such as twinning, pipes, grain boundaries (polycrystallinity) and secondary phases (SP). The synthesis of CZT material has improved greatly with the primary performance limitation being attributed to mainly SP. In this presentation, we describe the extensive characterization of detector grade material that has been treated with post growth annealing to remove the SPs. Some of the analytical methods used in this study included polarized, cross polarized and transmission IR imaging, I-V curves measurements, synchrotron X-ray topography and electron microscopy.
Date: July 9, 2008
Creator: Duff, M
Partner: UNT Libraries Government Documents Department

Room-temperature dislocation climb in copper-niobium interfaces

Description: Using atomistic simulations, we show that dislocations climb efficiently in metallic copper-niobium interfaces through absorption and emission of vacancies in the dislocation core, as well as an associated counter diffusion of Cu atoms in the interfacial plane. The high efficiency of dislocation climb in the interface is ascribed to the high vacancy concentration of 0.05 in the interfacial plane, the low formation energy of 0.12 e V with respect to removal or insertion of Cu atoms, as well as the low kinetic barrier of 0.10 eV for vacancy migration in the interfacial Cu plane. Dislocation climb in the interface facilitates reactions of interfacial dislocations, and enables interfaces to be in the equilibrium state with respect to concentrations ofpoint defects.
Date: January 1, 2008
Creator: Wang, Jian; Hoagland, Richard G; Hirth, John P & Misra, Amit
Partner: UNT Libraries Government Documents Department

Point-detect production and migration in plutonium metal at ambient conditions

Description: Modeling thermodynamics and defect production in plutonium (Pu) metal and its alloys, has proven to be singularly difficult. The multiplicity of phases and the small changes in temperature, pressure, and/or stress that can induce phase changes lie at the heart of this difficulty, In terms of radiation damage, Pu metal represents a unique situation because of the large volume changes that accompany the phase changes. The most workable form of the metal is the fcc (6.) phase, which in practice the 6 phase is stabilized by addition of alloying elements such as Ga or AI. The thermodynamically stable phase at ambient conditions is the between monoclinic (a-) phase, which, however, is approximately 20 % lower in volume than the 6 phase. In stabilized Pu metal, there is an interplay between the natural swelling tendencies of fcc metals and the volume-contraction tendency of the underlying phase transformation to the thermodynamically stable phase. This study explores the point defect production and migration properties that are necessary to eventually model the long-term outcome of this interplay.
Date: January 1, 2001
Creator: Baskes, M. I. (Michael I.); Stan, M. (Marius); Sickafus, K. (Kurt E.) & Valone, S. M. (Steven M.)
Partner: UNT Libraries Government Documents Department

Fermi level stabilization energy in cadmium oxide

Description: We have studied the effects of high concentrations of native point defects on the electrical and optical properties of CdO. The defects were introduced by irradiation with high energy He+, Ne+, Ar+ and C+ ions. Increasing the irradiation damage with particles heavier than He+ increases the electron concentration until a saturation level of 5x1020 cm-3 is reached. In contrast, due to the ionic character and hence strong dynamic annealing of CdO, irradiation with much lighter He+ stabilizes the electron concentration at a much lower level of 1.7x1020 cm-3. A large shift of the optical absorption edge with increasing electron concentration in irradiated samples is explained by the Burstein-Moss shift corrected for electron-electron and electron-ion interactions. The saturation of the electron concentration and the optical absorption edge energy are consistent with a defect induced stabilization of the Fermi energy at 1 eV above the conduction band edge. The result is in a good agreement with previously determined Fermi level pinning energies on CdO surfaces. The results indicate that CdO shares many similarities with InN, as both materials exhibit extremely large electron affinities and an unprecedented propensity for n-type conductivity.
Date: April 8, 2010
Creator: Speaks, D. T.; Mayer, M. A.; Yu, K. M.; Mao, S. S.; Haller, E. E. & Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

Point defects in metals

Description: A review is presented of the knowledge of point defects in pure metals. Data are tabulated for vacancies and divacancies in fcc metals, and for defects in bcc metals. Production of Frenkel defects by irradiation is also discussed; data are tabulated for both fcc and bcc metals. Questions that need answering are posed. (16 references). (DLC)
Date: January 1, 1973
Creator: Koehler, J.S.
Partner: UNT Libraries Government Documents Department


Description: In this study, we present nano-scale investigations of point defect dynamics in perovskite oxides by correlated atomic resolution high angle annular dark field imaging (HAADF) and electron energy loss spectroscopy (EELS). The point defect dynamics and interactions during in-situ reduction in the microscope column are analyzed. In particular, oxygen vacancy creation, diffusion and clustering are studied, as oxygen vacancies comprise the majority of the point defects present in these perovskite oxide systems [1]. The results have been acquired using the JEOL2010F, a STEM/TEM, equipped with a 200 keV field emission gun, a high angle annular dark field detector and a post column Gatan imaging filter (GIF). The combination of the Z-contrast and EELS techniques [2] allows us to obtain direct images (spatial resolution of 2 {angstrom}) of the atomic structure and to correlate this information with the atomically resolved EELS information (3s acquisition time, 1.2 eV energy resolution). In-situ heating of the material is performed in a Gatan double tilt holder with a temperature range of 300 K-773 K at an oxygen partial pressure of P{sub O{sub 2}} = 5 * 10{sup -8} Pa.
Date: August 4, 2002
Creator: KLIE,R.F.; BROWNING,N.D. & ZHU,Y.
Partner: UNT Libraries Government Documents Department

Transmission electron microscopy study of cascade collapse in copper during in-situ ion-irradiation at elevated temperatures.

Description: The basic mechanisms driving the collapse of point defects produced in collision cascades are investigated by transmission electron microscope (TEM) characterization of defect microstructure produced in fcc-Cu irradiated with low-fluences of heavy (100 keV Kr) ions at elevated temperature (23--600 C). Areal defect yields are determined from direct TEM observation of the total defect production integrated over the duration of the in-situ ion-irradiation. They are unequivocally demonstrated to decrease with increasing lattice temperature. This decrease in defect yield indicates a proportional decrease in the probability of collapse of cascade regions into defects of size where visible contrast is produced in a TEM.
Date: January 29, 1998
Creator: Daulton, T. L.; Kirk, M. A. & Rehn, L. E.
Partner: UNT Libraries Government Documents Department

Atomic scale simulations of arsenic ion implantation and annealing in silicon

Description: We present results of multiple-time-scale simulations of 5, 10 and 15 keV low temperature ion implantation of arsenic on silicon (100), followed by high temperature anneals. The simulations start with a molecular dynamics (MD) calculation of the primary state of damage after 10ps. The results are then coupled to a kinetic Monte Carlo (MC) simulation of bulk defect diffusion and clustering. Dose accumulation is achieved considering that at low temperatures the damage produced in the lattice is stable. After the desired dose is accumulated, the system is annealed at 800{degrees}C for several seconds. The results provide information on the evolution for the damage microstructure over macroscopic length and time scales and affords direct comparison to experimental results. We discuss the database of inputs to the MC model and how it affects the diffusion process.
Date: January 23, 1995
Creator: Caturla, M.J.; Diaz de la Rubia, T. & Jaraiz, M.
Partner: UNT Libraries Government Documents Department

Point defect concentrations and solid solution hardening in NiAl with Fe additions

Description: The solid solution hardening behavior exhibited when Fe is added to NiAl is investigated. This is an interesting problem to consider since the ternary Fe additions may choose to occupy either the Ni or the Al sublattice, affecting the hardness at differing rates. Moreover, the addition of Fe may affect the concentrations of other point defects such as vacancies and Ni anti-sites. As a result, unusual effects ranging from rapid hardening to solid solution softening are observed. Alloys with varying amounts of Fe were prepared in Ni-rich (40 at. % Al) and stoichiometric (50 at. % Al) compositions. Vacancy concentrations were measured using lattice parameter and density measurements. The site occupancy of Fe was determined using ALCHEMI. Using these two techniques the site occupancies of all species could be uniquely determined. Significant differences in the defect concentrations as well as the hardening behavior were encountered between the Ni-rich and stoichiometric regimes.
Date: August 1, 1997
Creator: Pike, L.M.; Chang, Y.A. & Liu, C.T.
Partner: UNT Libraries Government Documents Department

Determination of the symmetry of the superconducting pairing state and formation of a low-temperature normal metallic state in YBCO by electron irradiation

Description: A key difference between a d-wave and an anisotropic s-wave superconductor is the expected effect of nonmagnetic point defects on T{sub c}. T{sub c} of the former becomes zero whereas T{sub c} of the latter attains a nonzero constant value beyond a critical concentration of impurities. We report the first observation that T{sub c} of an YBCO single crystal becomes zero after 4.1% of the planar oxygens O(2,3) are displaced by 400keV electron irradiation along the c-axis. Our data therefore indicate that YBCO is a d-wave superconductor. We also report evidence of a disorder-induced superconductor-to-normal-metal transition.
Date: October 1, 1995
Creator: Giapintzakis, J.; Ginsberg, D.M. & Kirk, M.A.
Partner: UNT Libraries Government Documents Department