Extrinsic Point Defects Research Articles

Intrinsic stoichiometry and oxygen-inducedp-type conductivity of pyrite FeS2

The stoichiometry and ubiquitous observation of $p$-type conductivity of synthetic pyrite FeS${}_{2}$ thin films are investigated via first-principles computations of native (vacancies, interstitials, antisites) and extrinsic (O${}_{\mathrm{S}}$, O${}_{i}$) point defects. Native defects have high formation energies and are predicted to occur in low concentrations within the Fe- and S-rich limits, showing that pyrite should be intrinsically stoichiometric. Under sufficiently oxidizing conditions, O${}_{\mathrm{S}}$ becomes the most dominant defect type and induces $p$-type conductivity. At the experimental oxygen impurity concentration, the hole concentration is predicted to be $O({10}^{19})$ cm${}^{\ensuremath{-}3}$, in agreement with Hall measurements reported in the literature. Therefore, we attribute the unintentional $p$-type conductivity of pyrite to oxygen impurities and propose that improvements in device performance may be achieved under more reducing conditions.

First-principles study of native and extrinsic point defects in cubic boron nitride

Using first-principles total energy methods, we systematically calculated the formation energies and transition energy levels of intrinsic and extrinsic defects in cubic boron nitride (c-BN). We employed a mixed k-point sampling approach and brought the band-gap error into consideration. We find that under B-rich conditions boron vacancy (VB) and nitrogen vacancy (VN) act as the dominant compensating centers for additional dopants, while at N-rich limits, boron antisite (NB) becomes the main acceptor ‘killer’. The result is inaccessible if the impact of band-gap correction on the formation energies is ignored. Besides, all the calculated defect levels are too deep to account for the observed n- or p-type conductivity in as-grown samples. Among all the attempted dopants, substitutional Be (BeB) is the desirable acceptor in terms of lower formation energy and energy levels under N-rich conditions. However, n-type c-BN seems difficult to obtain under equilibrium conditions, because of the low solubility of dopants and the abundance of the compensating center VB3−.

Photoluminescence study of pure and Li-doped ZnO thin films grown by sol–gel technique

The effect of annealing atmosphere, temperature and aging on the photoluminescence of pure and Li-doped ZnO thin films has been investigated. Annealing the pure ZnO in N 2 and He above 800 °C results in green emission centered at ca. 500 nm; however annealing in air red-shifts the green emission to 527 nm. The visible emission of the Li-doped ZnO is found to be largely dependent on the annealing atmosphere. Warm-white photoluminescence with a broad emission band covering nearly the whole visible spectrum is obtained for the Li-doped ZnO films annealed in helium. The substitutional and interstitial extrinsic point defects created by lithium doping may mediate the relative concentration of the intrinsic defects and thereby tune the intrinsic-defect-related visible emission. The enhanced intensity ratio of near-band-edge ultraviolet emission to deep-level visible emission with aging time may be ascribed to both in-diffusion of oxygen from air and self-diffusion of oxygen interstitials to heal the oxygen vacancies during the aging process.

First-principles modeling of intrinsic and extrinsic defects in γ-Al2O3

The electronic properties of a set of intrinsic and extrinsic point defects in γ-Al2O3 are investigated using quasiparticle calculations within the G0W0 approximation. We find that the electronic signature of atomic vacancies lie deep in the band gap, close to the top of the valence band edge. The introduction of C, Si, and N impurities induces defective levels that are located close to the conduction band edge and near the middle of the band gap of the oxide. The comparison with electrical measurements reveals that the energy levels of some of these defects match with the electronic fingerprint of the defects reported in γ-Al2O3 based nonvolatile memories.

On intrinsic point defect cluster formation during Czochralski crystal growth

AbstractProgress in characterizing, understanding and controlling intrinsic point defect generation and clustering during Cz growth of single crystals is discussed. It is shown that intrinsic point defects in Si and Ge behave quite similar and that the observed clustering behaviour can be understood taking into account the impact of extrinsic point defects, doping and strain on the formation, recombination and diffusivity of the intrinsic point defects in both materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of direct sulfation of limestone by Na 2CO 3 addition

For an oxy-fuel circulating fluidized bed combustion system, the limestone calcination is normally prevented due to excessive CO 2 partial pressures and the limestone is subject to a direct sulfation reaction. The enhancement of the direct sulfation of limestone by Na 2CO 3 was investigated under high CO 2 partial pressure in a thermogravimetric apparatus (TGA) and scanning electron microscope (SEM) analysis method. A commercial limestone with a mean size of 18.8 μm was used. Experimental results indicate that the incorporation of Na + ions in solid product CaSO 4 lattice structures results in formation of more extrinsic point defects in the crystal lattices of CaSO 4 and a significantly increased solid-state diffusivity/mobility in the solid product. So the direct sulfation of Na 2CO 3-doped limestone shows higher rate and higher degree of conversion in the later stage of sulfation, in comparison with the direct sulfation of original limestone. The reaction changes from diffusional control to chemical reaction control in the presence of Na 2CO 3 because of the effect of foreign ions on accelerating the solid-state diffusion.

The effect of silica doping on neodymium diffusion in yttrium aluminum garnet ceramics: implications for sintering mechanisms

The Nd 3+ cation diffusion into transparent polycrystalline YAG (Y 3Al 5O 12) was investigated as a function of temperature and silica content. Thin neodymium oxide layers were deposited on sintered YAG substrates prior to annealing under air at temperatures from 1400 to 1600 °C. Bulk and grain boundary neodymium diffusion coefficients were measured by secondary ion mass spectrometry. The experimental results show that silica addition increases the diffusivity of Nd 3+ by a factor 10 whatever the diffusion path, probably as a result of extrinsic point defects formation, especially rare-earth vacancies. The experimental diffusion data were used to elucidate the sintering mechanism of Nd:YAG ceramics in the temperature range 1450–1550 °C. Firstly, it appeared that the intermediate stage of solid-state sintering should be controlled by the rare-earth diffusion along the grain boundary with an activation energy of about 600 kJ mol −1. Secondly, grain growth mechanism at the final stage of liquid-phase sintering was investigated for silica-doped Nd:YAG samples. Thus, the grain growth should be limited by the reaction at interfaces at a temperature lower than 1500 °C, with an activation energy of about 880 kJ mol −1. At higher temperature, it seems to be limited by the ionic diffusion through the intergranular liquid phase, with an activation energy of 250 kJ mol −1.

Gadolinium diffusion-doping for enhancing activation energy in superconducting YBa2Cu3O7−x

Abstract The resistance transitions of Gd-diffused polycrystalline Y123 samples have been measured as a function of temperature and magnetic field. It is deduced from experimental results that the activation energy increases when the diffusion annealing temperature increases and also the activation energy of Gd-diffused samples are larger than that undoped Y123 sample. The magnetic field dependence of U follows a power law, i.e. U ∝ H − α , with the values of α are 0.42, 0.56 and 0.61, respectively, for pure Y123, and Gd-diffused Y123 at 800 and 900 °C. The changing of α values from 0.42 to 0.61 imply that the thermally activated dissipative resistivity is directly associated with pinning mechanism changes from intrinsic planar defects pinning to extrinsic point defects pinning because of large Gd content into Y123 sample by the means of diffusion annealing temperature increasing. In this paper, the possible reasons responsible for the effect of Gd-diffusion on the pinning behaviors are discussed.

Defect–Domain Wall Interactions in Trigonal Ferroelectrics

Domains and domain walls are a fundamental property of interest in ferroelectrics, magnetism, ferroelastics, superconductors, and multiferroic materials. Unlike magnetic Bloch walls, ideal ferroelectric domain walls are well accepted to be only one to two lattice units wide, over which polarization and strain change across the wall. However, walls in real ferroelectrics appear to show unexpected property variations in the vicinity of domain walls that can extend over micrometer length scales. This chapter specifically reviews the local electrical, elastic, optical, and structural properties of antiparallel domain walls in the trigonal ferroelectrics lithium niobate and lithium tantalate. It is shown that extrinsic point defects and their clustering play a key role in the observed local wall structure and influence macroscale properties by orders of magnitude. The review also raises broader and yet unexplored fundamental questions regarding intrinsic widths, defect–domain wall interactions, and static versus dynamic wall structure.

Correlation effects and energetics of point defects in uranium dioxide: a first principle investigation

Density functional (DFT) calculations have been used to investigate the stability of point defects in uranium dioxide. Correlation effects are taken into account within the DFT + U approach as implemented in the Vienna ab initio simulation package (VASP). More particularly, the formation energies of both intrinsic and extrinsic point defects, i.e. vacancies, interstitials, Frenkel pairs and Schottky trio defects, are calculated. Our results are compared with available experimental data and are also discussed in relation to previous calculations based on conventional functionals, such as the local spin-density approximation and generalized gradient approximations.

(Fe,Mn)-Mg interdiffusion in natural diopside: effect of pO2

We investigated (Fe,Mn)-Mg interdiffusion along the [001] axis in single crystals of natural diopside at ambient pressure, at temperatures between 1000°C to 1200°C and at oxygen fugacities between 10 −18.5 atm to 10 −6 atm. Thin layers (~350–550 A) of ferro-johannsenite deposited by RF plasma sputtering provided iron and manganese for exchange with magnesium from the diopside substrates. Interdiffusion profiles of Fe-Mg and Mn-Mg were analysed by secondary ion mass spectrometry in depth profiling mode. We found that the (Fe,Mn)-Mg interdiffusion coefficient is strongly pO 2 sensitive. At 1100°C and between 10 −7 atm and 10 −15 atm the interdiffusion coefficient is proportional to ~(pO 2 ) m , indicative of an extrinsic point defects regime. We obtained m = 0.22 ± 0.02, indicating a vacancy mechanism. The data at 1100°C further suggest that at pO 2 × 10 −15 atm and at pO 2 > 10 −7 atm the interdiffusion coefficient is pO 2 -independent, possibly in relation with the appearance of Early Partial Melting (EPM, Jaoul & Raterron, 1994). At constant oxygen fugacity and without EPM the interdiffusion coefficient has an activation energy of 297 ± 31 kJ/mol. Our data significantly improve the constraints on the activation energy beyond the preliminary results reported earlier (Dimanov & Sautter, 2000). This work contributes towards a better understanding of the point defects chemistry and diffusion mechanisms in Fe-bearing diopside.

Compensation and defect passivation processes in polycrystalline CdTe: Cl layers

AbstractAn extensive optical spectroscopy investigation of Polycrystalline Cl doped CdTe layers brings new insights into the compensation and passivation processes typical of such structurally imperfect materials. 10 K Photoluminescence shows that if some of the main Cl related compensation processes are similar in poly‐ and mono‐crystalline materials, specific mechanisms are evidenced in polycrystalline layers involving complexes between chlorine atoms and extrinsic point defects in the vicinity of structural defects (e.g. extended defects). Using low temperature Cathodoluminescence imaging, it is also demonstrated that structural defects can be efficiently passivated by chlorine atoms, leading to improved electro‐optical properties. Finally, using cross‐section photoluminescence on thick layers, we correlate the evolution of the defect levels to that of the structural properties along the growth axis. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Intrinsic and extrinsic point-defects in vapor transport grown ZnO bulk crystals

Vapor transport grown ZnO bulk crystals were characterized by electrical, optical and magnetic resonance spectroscopy. The experiments show that the residual carrier concentration is caused by residual H, Al, Ga, and oxygen vacancies in the material. Annealing the samples in O 2 at about 1000 °C (2 atm, 20 h) reduces the H and V O donor concentration by typical one order of magnitude. The photoluminescence and DLTS results suggest a correlation between the broad unstructured emission at 2.45 eV (green band) and a donor level 530 meV below the conduction band (E4).

Application of wavelength‐resolved optically‐detected XAS methods to phase‐segregated silicates

AbstractWavelength‐resolved optically‐detected X‐ray absorption experiments are described that monitor the emission from naturally occurring phase‐exsolved aluminosilicate feldspars, when exciting across the Ca and KL2,3core levels. The results from a selection of example cases are presented that demonstrate the power of the technique in examining the structural and optical properties of phase segregated materials that could not easily be achievable by other methods. The results highlight that in these materials, extrinsic point defects such as Mn2+and Fe3+can also become preferentially associated with particular exsolved phases. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Formation of Defect Complexes in a ZnO Grain Boundary

The microscopic properties a ZnO grain boundary containing extrinsic point defects are studied using a density functional computational approach. The results show that the grain boundary acts as a sink for native defects, such as the zinc vacancy and the oxygen interstitial, and also for bismuth substitutional impurities. The defects tend to accumulate at under-coordinated sites in the boundary core and prefer to form small clusters. In particular the segregation of Bi promotes the formation of the other native defects by lowering their formation energies in the boundary. Individually, the native defects and the Bi impurity do not produce deep interface states in the band gap which are electrically active. However, when the defects cluster to form a BiZn-VZn-Oi complex, new gap states are created of acceptor type. It is suggested that these new states are caused by defect interactions which compensate one another resulting in the depletion of an occupied impurity state and new bond formation. The results are discussed in terms of the Schottky barrier model commonly used to describe the electrical characteristics of ZnO varistors.

Electronic structure calculations of intrinsic and extrinsic hydrogen point defects inKH2PO4

We report first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of H intrinsic (Frenkel pair) and extrinsic (H vacancy or interstitial) point defects in ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}.$ The relaxed atomic structures, the formation energy, the ionization energy, and electron and hole affinities for the various defects have been calculated. For the Frenkel pair, the additional hole leads to a decrease of the $\mathrm{O}---\mathrm{O}$ bond length between the two O atoms next to the H vacancy, while the effect of the additional electron is small. For the H vacancy, the added hole is trapped and shared by the two O atoms adjacent to the vacancy, reducing dramatically the $\mathrm{O}---\mathrm{O}$ bond length, thus forming a molecular-type polaron. We find that the positively charged H vacancy introduces states in the gap, in contrast with its neutral state, confirming the experimental suggestion that it is a relevant absorbing center. The negatively charged H vacancy leads to an increase of the two O atoms close to the H vacancy, and does not induce states in the gap. The H interstitial does not interact with the host atoms in the neutral state. However, the addition of an electron leads to the ejection of a H host atom and the subsequent formation of a ${\mathrm{H}}_{2}$ interstitial molecule and a H vacancy, in agreement with experimental suggestions. In the positively charged state the H interstitial binds to its nearest-neighbor O atom forming a hydroxyl bond. The H interstitial in both positive and negative charge states induces no defect states in the band gap, in contrast with its neutral state. The calculations provide insights into the role of the charged and neutral defects on the transient optical absorption under irradiation by high-intensity laser beam.

Correlations between photoluminescence and Hall mobility in GaN/sapphire grown by metalorganic chemical vapor deposition

We have investigated photoluminescence (PL) and electron Hall mobility for unintentionally doped GaN epitaxial layers grown by low-pressure metalorganic chemical vapor deposition on c-plane Al 2O 3 substrates. Four GaN films having identical dislocation density but remarkably different electron Hall mobility were exploited. At low temperature (12 K), a PL line associated with a bound exciton was observed and strong correlations were found between the Hall mobility and the PL intensity of the exciton transition. That is, relative PL intensity of the bound exciton to a donor-bound exciton monotonously increased with decreasing the electron mobility of the GaN films. This correlation was interpreted in terms of electrical compensation. Efforts to find the chemical origin of the PL line led to the conclusion that the BE line originated neither from threading dislocations nor from extrinsic point defects. Intrinsic acceptors such as Ga vacancy and GaN anti-site were suspected as plausible origin.

Formation Energies of Point Defects in Copper Indium Diselenide Using ab initio Methods

AbstractThe opto-electronic properties of CuInSe2 and related compounds depend on their defect chemistry in a way that is far from being understood and in which ab initio calculations could help by providing new insights as shown previously. Ab initio calculations of energy and electronic structure of various intrinsic (including defect pairs) and extrinsic (including potential dopants such as Zn) point defects have been performed in the chalcopyrite semiconductors CuInSe2, some of them being computed for the first time by advanced ab initio techniques. The method used is based on the density functional theory within the framework of pseudo-potentials and plane waves basis set. The results are discussed in view of the existing data, models and calculations.

Diffusion Mechanisms and Intrinsic Point-Defect Properties in Silicon

High-purity silicon used for the growth of single crystals is a material with a high resistivity. Small traces of foreign atoms, which are mainly substitutionally dissolved on lattice sites, make the material highly conductive and therefore suitable for electronic applications. The controlled incorporation of extrinsic point defects in silicon is the main task for the production of electronic devices. Homogeneous doping is generally achieved by adding a controlled amount of the dopant element to the silicon melt. However, the fabrication of electronic devices like diodes, transistors, and complex integrated circuits requires spatially inhomogeneous dopant distributions. Control of the inhomogeneous doping profiles demanded by the considerations outlined in the article by Packan in this issue requires a detailed knowledge of the atomic mechanisms of dopant diffusion in silicon, the properties of intrinsic point defects like vacancies (V) and self-interstitials (I), and the interactions among different point defects.

Effects of extended defects on the properties of intrinsic and extrinsic point defects in silicon

We investigated the interaction of intrinsic and extrinsic point defects with stacking faults in silicon. The calculations were carried out using ab initio total energy methods. The results show that the formation energies of intrinsic defects and impurities (P, As, and Al) are lower at the stacking fault as compared to the respective defects in crystalline environment. Therefore, stacking faults should have a large concentration of defects, and they should play an important role on the mechanisms of dislocation motion.