Formation Of Intrinsic Point Defects Research Articles

Formation of Grown-In Nitrogen Vacancies and Interstitials in Highly Mg-Doped Ammonothermal GaN.

The formation of intrinsic point defects in the N-sublattice of semi-insulating Mg-doped GaN crystals grown by the ammonothermal method (SI AT GaN:Mg) was investigated for the first time. The grown-in defects produced by the displacement of nitrogen atoms were experimentally observed as deep traps revealed by the Laplace transform photoinduced transient spectroscopy in the compensated p-type crystals with the Mg concentrations of 6 × 1018 and 2 × 1019 cm-3 and resistivities of ~1011 Ωcm and ~106 Ωcm, respectively. In both kinds of materials, three closely located traps with activation energies of 430, 450, and 460 meV were revealed. The traps, whose concentrations in the stronger-doped material were found to be significantly higher, are assigned to the (3+/+) and (2+/+) transition levels of nitrogen vacancies as well as to the (2+/+) level of nitrogen split interstitials, respectively. In the material with the lower Mg concentration, a middle-gap trap with the activation energy of 1870 meV was found to be predominant. The results are confirmed and quantitatively described by temperature-dependent Hall effect measurements. The mechanism of nitrogen atom displacement due to the local strain field arising in SI AT GaN:Mg is proposed and the effect of the Mg concentration on the charge compensation is discussed.

New insights into intrinsic point defects of delafossite CuAlO2 as optoelectronic materials

AbstractDelafossite CuAlO2 has great potential in optoelectronics applications. The intrinsic point defects play the decisive roles in the optoelectronic properties and application of CuAlO2. However, there are still disputes about their relevant underlying physical mechanisms at the atomic scale. Here, the formation of intrinsic point defects of CuAlO2 and their effects on optical absorption spectra and electronic structure have been systematically investigated using first‐principles calculations. In oxygen‐rich and oxygen‐poor chemical environments, acceptor‐type defects such as Cu vacancy and Cu substitution on the Al site can spontaneously form and produce p‐type conductivity at room temperature. This is the important physical basis for CuAlO2 to be widely used as transparent conductive oxide films. The shallow acceptor‐type levels produced by Cu substitution on Al site and O vacancy, and the shallow donor‐type levels produced by interstitial Cu, are favorable for the separation of photo‐generated electron–hole pairs, which can be suitable for applications in photocatalysis and photovoltaics. The Cu or Al vacancy can produce delocalized energy band at the top of valence band and pinning Fermi level at the same time, which may make CuAlO2 a degenerate semiconductor and produce local surface plasmon resonance effect. Al substitution on Cu site and interstitial Al or O form complex deep defect energy bands in the forbidden band and produce significant multiband optical absorption in the visible–NIR region. They are expected to become alternative materials for photoluminescence and photoelectric detection. This work demonstrates the importance of intrinsic defect engineering for optimizing optoelectronic properties and thus promoting the discovery of novel optoelectronic materials.

The Role of Carbon in the Formation of Intrinsic Point Defects and Hydrogen Migration in α‐Al2O3‐Based Tritium Permeation Barriers

The impurity effect is one of the major concerns for α‐Al2O3‐based tritium permeation barriers (TPBs). The effect of carbon, a common impurity element in α‐Al2O3, on the formation of intrinsic point defects and hydrogen migration behaviors is investigated by first‐principles calculations. It is found that carbon has a significant influence on the existing form, charge state, and relative stability in α‐Al2O3. In view of the hydrogen migration, carbon is not beneficial for the hydrogen permeation resistance in α‐Al2O3‐based TPBs because carbon can remarkably reduce the hydrogen migration barrier in α‐Al2O3.

Understanding copper diffusion in CuInSe2 with first-principles based atomistic and continuum models

We investigate the diffusion of copper in CuInSe2 using thermodynamic and kinetic models based on density functional theory calculations, attempting to reconcile large differences in reported experimental diffusivities. We find that observations of rapid chemical diffusion can be explained by large thermodynamic factors, which we calculate using a compositionally constrained model of intrinsic point defect formation. We further characterize how copper diffusion coefficients depend on material synthesis conditions and exhibit their variation across the CuInSe2 secondary phase diagram. In doing so, we identify stable off-stoichiometries that are dominated by either vacancy- or interstitial-mediated diffusion mechanisms. These results are employed in the development of a continuum reaction–diffusion model, which we use to simulate experimental depth profiles.

Conductivity of SbxSey films grown by CMBD from Sb and Se precursors for use in solar cells

Antimony selenide (Sb2Se3) has been developed as attractive, non-toxic and earth-abundant solar absorber candidate among the thin-film photovoltaic devices. The growth of SbxSey thin films, by atmospheric pressure chemical molecular beam deposition (CMBD) method, from separate Sb and Se precursors has been reported. The conductivity of the films was investigated as a function of the vapor phase mixture of Sb and Se. By the precise control of the Sb/Se ratio we succeeded in obtaining stoichiometric Sb2Se3 films. It is also found out that we can control the conductivity by deliberately introducing the deviation from the stoichiometry. The conductivity was varied in the wide range of 10−5 ÷ 102 (Ohm × cm)−1 and samples had p- and n-type conductivity depending on Sb/Se ratio. The obtained results were explained by the formation of intrinsic point defects.

Intrinsic point defects and charge carrier trapping in monolayer BiOX (X = I, Br, and Cl)

Two-dimensional (2D) layered bismuth oxyhalides, BiOX (X = I, Br, and Cl), have great potential in optoelectronics and photocatalysis applications. The intrinsic point defects are crucial for carrier conductivity and transport. However, the understanding for defect physics of 2D atomic-scale BiOX are still unclear. Herein, through the first-principles calculations, we investigate the formation of intrinsic point defects and their effect on charge carrier trapping in 2D monolayer BiOX. Under a O-poor condition, the donor defects, such as the Biad, BiX, VO, and VX, can form spontaneously and induce a high n-type conductivity. The VX shows a shallow transition level and has no defect states. In contrast, the Biad, BiX, and VO display deep transition levels and obvious localized defect states that are responsible for the charge carrier trapping. As O becomes richer, the concentration of acceptor defects increases. Nevertheless, the donor and the acceptor defects can strongly compensate each other, pinning the Fermi energy in the band gap. The dominant acceptor defects, such as the BrBi, OBr, and OCl, show the deep transition levels and serve as carrier traps due to the charge localized around the defect sites. Our work gives an insight into the defect physics of atomic-scale 2D BiOX and provides a guidance for their optoelectronics and photocatalysis applications.

Thermoelectric optimization of AgBiSe2 by defect engineering for room-temperature applications

The hexagonal phase of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ has been discovered as a promising thermoelectric material for room-temperature applications. However, its basic conduction type is still ambiguous, and its current ZT value is pretty low. To improve the thermoelectric performance of $\mathrm{AgBiS}{\mathrm{e}}_{2}$, we apply band engineering to modify its band structure by introducing defects to increase the band degeneracy. From the calculated intrinsic point defect formation energies of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ at different growth conditions, we clarify that the conducting behavior of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ is a $p$-type semiconductor, and the Ag vacancy is the dominated acceptor. Based on scrutinizing the band structure of $\mathrm{AgBiS}{\mathrm{e}}_{2}$, two kinds of methodologies can be used to modify its band structure to achieve high band degeneracy: (i) shifting the Fermi level into the valence band using intrinsic defects, and (ii) converging several valence-band maxima by introducing extrinsic defects. We find that the intrinsic Ag vacancy is helpful to significantly increase the power factor, leading to a large ZT for Ag vacancy-doped $\mathrm{AgBiS}{\mathrm{e}}_{2}$: the maximum ZT value is increased to 0.3--0.5 at near room temperature. Based on analyzing the bonding characters and atomic energy levels in the compound, we predict several extrinsic dopants (Cu, Rh, and Pd) that can be used to converge three valence-band maxima. Our work provides methodologies to improve the room-temperature thermoelectric applications of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ by tuning its band structures using intrinsic or extrinsic defects.

Impurity-free, mechanical doping for the reproducible fabrication of the reliable n-type Bi2Te3-based thermoelectric alloys

Precise control of carrier density is essential to synthesize high-performance thermoelectric materials. Doping by impurities is often frustrated in n-type Bi2Te3 alloys by incomplete activation, bipolar doping, the formation of secondary phases, and prevailing intrinsic point defects such as vacancies. This weakens the reproducibility of synthesis processes and reduces the long-term reliability of material's performance, hence aging. Here, we explore an impurity-free doping technique to synthesize n-type bismuth tellurium selenides, combining a cold deformation and a hot extrusion. The cold deformation enables controlling the electron density in the range of ∼1019/cm3 via the formation of intrinsic point defects, and the hot extrusion allows texturing the microstructure to enhance the electrical conductivity, hence a large power factor of >5 × 10−3 W-m−1-K−2. We confirm that our process is very reproducible, and the properties of the samples are stable without aging even after thermal stresses. Using this method, we can decouple the relationship between bandgap, carrier density, and composition to improve the high-temperature thermoelectric property. Moreover, we demonstrate the fabrication of high-performance thermoelectric materials from low-graded, raw materials by modifying the degree of the mechanical deformation to reach an optimum carrier density. Our work provides a promising approach to synthesizing n-type thermoelectric materials in the reproducible and adaptable way.

Influence of anisotropic thermal stress during Si crystal growth on intrinsic point defect formation

Influence of anisotropic thermal stress during Si crystal growth on intrinsic point defect formation

Cr Effect on Hydrogen Interactions with Intrinsic Point Defects and Hydrogen Diffusion in α-Al2O3 as Tritium Permeation Barriers

Hydrogen transport in the outer Al2O3 scale is the rate-limiting step for H-permeation resistance of FeAl/Al2O3-type aluminide tritium permeation barriers (TPBs) in fusion reactors. With first-principle calculations, the effects of Cr, a main impurity element in the Al2O3 scale, on intrinsic point defect formation, hydrogen interactions with intrinsic point defects, and hydrogen diffusion in α-Al2O3 have been investigated under the working conditions of aluminide TPBs. It is found that Cr addition is favorable for the formation of VO, yet unfavorable for VAl formation in α-Al2O3. Compared with the α-Al2O3–H case, Cr is beneficial for the formation of H-related defects in α-Al2O3, whereas it is unfavorable for the Hi trapping ability of VAl and VO. HO– will dominate among Hi–, VAl3–, VO0, and [VAl3––H+]2–, and only one step of Hi reorientation will be involved for the Hi diffusion in Cr-doped α-Al2O3. Hi is the dominant diffusion species in both pure and Cr-doped α-Al2O3, whereas the activation energy of H...

Si単結晶中の点欠陥形成に与える熱応力とドーパントの複合効果に関する第一原理解析

Si単結晶中の点欠陥形成に与える熱応力とドーパントの複合効果に関する第一原理解析

Existence of off-stoichiometric single phase kesterite

In this work the flexibility of the kesterite structure is discussed. It is shown that kesterite type CZTS and CZTSe can deviate from stoichiometric composition by the formation of intrinsic point defects leading to different off-stoichiometric kesterite types. For this study off-stoichiometric CZTS and CZTSe powder samples have been synthesized by solid state reaction from pure elements. Their chemical composition was determined by WDX spectroscopy and the structural characterization was conducted by X-ray diffraction. Amongst the studied 49 samples, a number of them have been identified to contain only an off-stoichiometric kesterite type phase which exhibit Cu-poor/Zn-rich composition. On the other hand, Cu-rich/Zn-poor kesterite type CZTS and CZTSe always coexists with secondary phases. Thus it has been demonstrated, that the kesterite type structure can self-adapt to Cu-poor and Cu-rich compositions in CZTS and CZTSe without any structural change except the cation distribution.

First principles study of defect formation in thermoelectric zinc antimonide, β-Zn4Sb3

Understanding the formation of various point defects in the promising thermoelectric material, β-Zn4Sb3, is crucial for theoretical determination of the origins of its p-type behavior and considerations of potential n-type dopability. While n-type conductivity has been fleetingly observed in Te:ZnSb, there have been no reports, to the best of our knowledge, of stable n-type behavior in β-Zn4Sb3. To understand the origin of this difficulty, we investigated the formation of intrinsic point defects in β-Zn4Sb3 density functional theory calculations. We found that a negatively charged zinc vacancy is the dominant defect in β-Zn4Sb3, as it is also in ZnSb. This explains the unintentional p-type behavior of the material and makes n-doping very difficult since the formation of the defect becomes more favorable at higher Fermi levels, near the conduction band minimum (CBM). We also calculated the formation energy of the cation dopants: Li, Na, B, Al, Ga, In, Tl; of these, only Li and Na are thermodynamically favorable compared to the acceptor Zn vacancy over a range of Fermi levels along the band gap. Further analysis of the band structure shows that Li:Zn4Sb3 has a partially occupied topmost valence band, making this defect an acceptor so that Li:Zn4Sb3 is indeed a p-type thermoelectric material. The introduction of Li, however, creates a more orderly and symmetric configuration, which stabilizes the host structure. Furthermore, Li reduces the concentration of holes and increases the Seebeck coefficient; hence, Li:Zn4Sb3 is more stable and better performing as a thermoelectric material than undoped β-Zn4Sb3.

Morphology of Oxygen Precipitates in RTA Pre-Treated Czochralski Silicon Wafers Investigated by FTIR Spectroscopy and STEM

Rapid thermal annealing (RTA) is a commonly used technique to control precipitation of oxygen in Czochralski silicon wafers (1,2). RTA pre-treatment leads to the formation of intrinsic point defects, mainly vacancies, which are responsible for the enhanced precipitation of oxygen. Oxygen precipitates are widely known as getter sites for metal impurities in the silicon microelectronic technology (3). The getter efficiency of oxygen precipitates depends on their density and morphology (4,5). The density of the precipitates can be controlled by the temperature of RTA and it can be easily determined by preferential etching. The morphology of oxygen precipitates depends on the type and the concentration of dopants, the thermal history, and the annealing temperature and time (6-8). Recently, it was shown that the supersaturation of vacancies impacts the morphology of oxygen precipitates (9). Since RTA pre-treatments introduce vacancy supersaturation in silicon wafers its usage before nucleation annealing could modify the morphology of precipitates compared to wafers without RTA. The morphology of oxygen precipitates can be determined by transmission electron microscopy, however the preparation of samples is demanding. An alternative is the Fourier transform infrared (FTIR) spectroscopy at liquid helium temperature, which also allows distinguishing different types of oxygen precipitates. The oxygen precipitates give rise to absorption bands in the range between 1000 and 1300 cm-1. There are two characteristic broad bands at 1095 cm-1 and at 1225 cm-1 which belong to spherical and plate-like precipitates, respectively (10-12). The sharp bands (1136 cm-1 and satellites and 1205 cm-1) are attributed to interstitial oxygen.In this work, we investigate the morphology of oxygen precipitates by FTIR spectroscopy at liquid helium temperature and by scanning transmission electron microscopy (STEM) in silicon samples pre-treated by RTA at 1250°C for 30s in O2 containing Ar atmosphere subsequently annealed in the temperature range between 800°C and 1000°C in N2. The results show that the morphology of the oxygen precipitates in RTA pre-treated samples depends on the nucleation temperature. Fig. 1 presents absorption spectra of silicon samples pre-treated by RTA at 1250°C and annealed at 800°C (a), 900°C (b) and 1000°C (c) for 64h. As can be seen from the deconvolution of the spectra, the number and position of the bands is different for each annealing temperature. Based on the effective medium theory (EMT) (13) and STEM investigations the absorption bands can be matched to relevant morphologies.

Comment on "Experimental Study of the Impact of Stress on the Point Defect Incorporation during Silicon Growth" [ECS Solid State Lett., 3, N5 (2014)

The authors would like to comment on the choice of intrinsic point defect formation and migration energies in the above mentioned letter. The values that were used are extracted from experimentally observed defect formation and distribution in as-grown Si crystals and therefor already implicitly contain to some extent the impact of stress. It is shown that this "built-in" stress effect can be taken into account to some extent by fitting the experimental data using a stress dependent Voronkov criterion yielding a consistent estimate of the pre-factors of the exponential equations describing the diffusivity and thermal equilibrium concentration of the intrinsic point defects.

Composition dependent intrinsic defect structures in SrTiO₃.

Intrinsic point defect complexes in SrTiO3 under different chemical conditions are studied using density functional theory. The Schottky defect complex consisting of nominally charged Sr, Ti and O vacancies is predicted to be the most stable defect structure in stoichiometric SrTiO3, with a relatively low formation energy of 1.64 eV per defect. In addition, the mechanisms of defect complex formation in nonstoichiometric SrTiO3 are investigated. Excess SrO leads to the formation of oxygen vacancies and a strontium-titanium antisite defect, while a strontium vacancy together with an oxygen vacancy and a titanium-strontium antisite defect are produced in an excess TiO2 environment. Since point defects, such as oxygen vacancies and cation antisite defects, are intimately related to the functionality of SrTiO3, these results provide guidelines for controlling the formation of intrinsic point defects and optimizing the functionality of SrTiO3 by controlling nonstoichiometric chemical compositions of SrO and TiO2 in experiments.

Interdiffusion of Mg–Fe in olivine at 1,400–1,600 °C and 1 atm total pressure

The interdiffusion coefficient of Mg–Fe in olivine (D Mg–Fe) was obtained at 1,400–1,600 °C at the atmospheric pressure with the oxygen fugacity of 10−3.5–10−2 Pa using a diffusion couple technique. The D Mg–Fe shows the anisotropy (largest along the [001] direction and smallest along the [100] direction), and its activation energy (280–320 kJ/mol) is ~80–120 kJ/mol higher than that estimated at lower temperatures. The D Mg–Fe at temperatures of >1,400 °C can be explained by the cation-vacancy chemistry determined both by the Fe3+/Fe2+ equilibrium and by the intrinsic point defect formation with the formation enthalpy of 220–270 kJ/mol depending on the thermodynamical model for the Fe3+/Fe2+ equilibrium in olivine. The formation enthalpy of 220–270 kJ/mol for the point defect (cation vacancy) in olivine is consistent with that estimated from the Mg self-diffusion in Fe-free forsterite. The increase in the activation energy of D Mg–Fe at >1,400 °C is thus interpreted as the result of the transition of diffusion mechanism from the transition metal extrinsic domain to the intrinsic domain at the atmospheric pressure.

Theoretical study of the impact of stress on the behavior of intrinsic point defects in large-diameter defect-free Si crystals

For the development of the crystal pulling process for 450mm-diameter defect-free Si crystals, it is important to evaluate the impact of thermal stress on intrinsic point defect behavior during crystal growth. In this study, we evaluated the impact of thermal stress between −30MPa (tensile) and 30MPa (compressive), which covers the complete range of stress levels that can be expected in mass production for 450mm-diameter crystals, on the so-called Voronkov criterion. This criterion predicts the dominant point defect based on a critical pulling speed over axial temperature gradient ratio. The internal pressure dependencies of both the formation and the migration enthalpies of intrinsic point defects were obtained with a density functional theory study, and it was found that assuming internal stress only, the thermal stress shifts the growing Si crystal towards more vacancy-rich under compressive and towards more self-interstitial-rich under tensile stresses. Also Si crystals containing interstitial clusters and heavily doped crystals were studied to evaluate the effect of thermal and dopant-related stresses on the intrinsic point defect formation and diffusivity. It was concluded that the experimentally observed increase in the number of interstitial clusters and the increase in the concentration of self-interstitials in heavily doped Si cannot be explained by the thermal and/or dopant-related stresses.

An atomistic approach to self-diffusion in uranium dioxide

Abstract The formation and mobility of point defects in UO 2 have been studied within the framework of the Density Functional Theory. The ab initio Projector Augmented Wave method is used to determine the formation and migration energies of defects. The results relative to intrinsic point defect formation energies using the Generalized Gradient Approximation (GGA) and GGA+U approximations for the exchange-correlation interactions are reported and compared to experimental data. The GGA and GGA+U approximations yield different formation energies for both Frenkel pairs and Schottky trios, showing that the 5 f electron correlations have a strong influence on the defect formation energies. Using GGA, various migration mechanisms were investigated for oxygen and uranium defects. For oxygen defects, the calculations show that both a vacancy and an indirect interstitial mechanism have the lowest associated migration energies, 1.2 and 1.1 eV respectively. As regards uranium defects, a vacancy mechanism appears energetically more favourable with a migration energy of 4.4 eV, confirming that oxygen atoms are much more mobile in UO 2 than uranium atoms. Those results are discussed in the light of experimentally determined activation energies for diffusion.

Formation of Intrinsic Point Defects in Fluorine-doped Synthetic SiO2Glass by60Co γ-ray Irradiation

Intrinsic point defect formation in a fluorine-doped synthetic silica (SiO2) glass by 60Co γ-ray irradiation was examined. The most abundantly formed defects are oxygen vacancies (Si–Si bonds). The concentrations of Si–Si bonds and interstitial oxygen molecules increase almost linearly with the γ-ray dose. These observations indicate that the primarily intrinsic defect process in SiO2 glass irradiated with 60Co γ-rays is the Frenkel pair formation, rather than a simple cleavage of an Si–O bond into a pair of silicon and oxygen dangling bonds.