Substitutional Impurities Research Articles

Stabilizing p-type conductivity in CuYO2 by co-doping: A first-principles study

This work figures out the importance of p-type conductive materials through first-principle calculations. It is shown that the co-doping mechanism can enhance the p-type conductivity of CuYO2. Substituting Boron at the Yttrium (BY) site, together with Oxygen at interstitial positions (Oi), introduces shallow acceptor levels just above the valence band maximum (VBM). From the thermodynamical calculations, the stability of defects was also confirmed in O-rich synthesis conditions. A detailed analysis of the electronic band structure shows that the VBM and CBM can be tuned by introducing suitable impurities. Likewise, the substitutional impurities Ge and In at Y sites (GaY and InY) are shown to have high electronic conductivity. We have demonstrated the possibility of ambipolar charge carriers in CuYO2 by co-doping.

Impurity Combination Effect on Oxygen Absorption in α2-Ti3Al

The effect of substitutional impurities of the transition metals of VB–VIIB groups on the oxygen absorption in the doped α2-Ti3Al alloy was studied by the projector-augmented wave method within the density functional theory. It is established that all considered impurities prefer to substitute for a Ti atom rather than an Al atom. Changes in the absorption energy due to impurities being in the first neighbors of the oxygen atom were estimated. It was demonstrated that the doping resulted in a decrease in the oxygen absorption energy, which is mainly caused by the chemical contribution to it. The interaction energy between impurity atoms was calculated in the dependence on the interatomic distance. It was shown that the configuration with the impurity atoms being in the second neighbors of each other was stable in comparison with other possible configurations. The influence of two impurity atoms being in the first neighbors of oxygen is additively enhanced. It was revealed that the effect of two impurity atoms on the oxygen absorption energy can be estimated as the sum of the effects of separate impurities with an accuracy of more than ~90%.

Enhancement of the creation yield of NV ensembles in a chemically vapour deposited diamond

In this work we investigate the properties of negatively charged nitrogen-vacancy (NV−) centres created during single crystal diamond growth by chemical vapour deposition (CVD) on [113]-oriented substrates and with N2O as a dopant gas. The use of spin echo and double electron-electron resonance (DEER) allows us to assess NV− ratio with respect to substitutional nitrogen impurities (Ns0) incorporated during growth, a critical figure of merit for quantum technologies. We demonstrate that, at moderate growth temperatures (800 °C), dense NV− ensembles of several hundreds of ppb (800 ppb for 50 ppm of added N2O) and with exceptionally high NV−/ Ns0 ratios of up to 25% can be achieved. This NV− creation yield is higher by at least an order of magnitude to that typically obtained on standard [100]-grown diamonds and comparable to the best values reported for electron-irradiated diamonds. The material obtained here thus advantageously combines a high NV− density, high creation yield, long coherence times of several tens of μs together with a partial preferential orientation. These are highly desirable requirements for diamond-based quantum sensors that may spur new developments with this crystalline orientation leading to improved performance and sensitivity.

Study of the structural stability and electronic properties of the C-doped boron nanomaterials

In the present study, the structural stability and electronic properties of boron nanostructures were analyzed using first-principles calculations. The main focus was concentrated in boron monolayers and nanotubes including substitutional carbon impurities. The obtained results indicate that all doped structures are more stable than the pristine ones. It was found that the doped planes have a metallic electronic behavior, similar to the undoped sheets. For boron nanotubes, pristine structures exhibit a semiconductor behavior for small diameters, in close agreement with previous studies. However, both types of doped tubes have a null gap, regardless of the diameter. It was also found, from calculations including spin polarization, a non-zero magnetization for tubes with a specific condition of doping.

Effect of Sc and Zn doping on structure and electro-optical behavior in c-BiAlO3: A DFT trial

Density Functional Theory (DFT) is an attractive route to search for new substitute lead free perovskite materials for linear and non-linear optics, with superior properties similar to PZT materials. We consider environment friendly facile c-BiAlO3 piezoelectric material, where Bi is substituted by transition metals (Sc, Zn). The effect of these dopant on the structural, electronic and optical properties of c-BiAlO3 are investigated by using generalized gradient approximation and ultrasoft pseudopotential method as implemented in CASTEP. Structural properties experience reciprocal effect on lattice constant as a function of ionic size of Impurities. In addition, a transition of indirect band gap of c-BiAlO3 to the direct one is obtained after TM impurity substitution. The optical properties such as absorption, energy loss, refractive index, dielectric function, function are computed for the energy range of 30eV. The static refractive index for pure and TM-doped obtained are 2.87, 2.99 and 3.5 respectively. Upsurges in static dielectric constants are also sighted, predicting a shift from semiconducting to metallic behavior after TM doping in c-BiAlO3. TM-doping has also boosted the absorption spectra response and reflectivity in the UV-region. Sc and Zn-doped c-BiAlO3 can be considered as potential materials in photovoltaic application.

Electronic structure properties of boron–doped and carbon–boron–codoped TiO2(B) for photocatalytic applications

A path to enhance the efficiency of photocatalytic devices is by doping the active semiconductor in order to maximize the light absorption. However, impurities could increase the recombination of photoinduced charge carriers, in order that a careful analysis on the effects of those impurities it is necessary. On this research, density functional theory calculations were applied to analyze the structure and electronic characteristics of B–doped and C+B–codoped TiO2(B) considering the lowest formation energy of substitutional impurities. Our calculations suggest that the swapping of a boron or carbon atom for an oxygen atom (B@O or C@O) is energetically more favorable than the replacement of titanium atoms. The obtained results seem to indicate that the boron substitution into O2C and O3C1 sites leads to its migration towards interstitial sites in both mono and codoping, thus causing a great geometrical distortion compared to pure TiO2(B). Furthermore, impurity states in the band gap were obtained after B@O doping and C+B–codoping, producing an improvement of the optical properties, so it can be assumed that this nanomaterial might act as a photoelectrode after an adequate design optimization that facilitates the charge carrier separation.

Common defects in diamond lattices as instances of the general T ⊗ (e+t2) Jahn-Teller effect

Finding the ground-state configuration of point defects in semiconductors is in general a high-dimensional optimization problem. However, their adiabatic potential energy surfaces (APES) can be described with few effective coordinates by exploiting symmetry arguments within the Jahn-Teller (JT) theoretical framework. In this paper, we propose a general framework, that combines group theory and ab initio calculations, to build a full picture of common defects in diamond lattices (vacancies and substitutional impurities) as instances of the general T $\ensuremath{\bigotimes}\phantom{\rule{4pt}{0ex}}(e+{t}_{2})$ APES. Starting from the original tetrahedral symmetry, we consider all possible JT symmetries and we numerically investigate the shape of the APES by targeting the minimum energy path between equivalent JT distortions taking as examples the vacancy (V) and the metallic impurities Pd, Pt, and Au in crystalline silicon.

The use of lattice defects in quartz to determine provenance and conditions for the formation of sedimentary deposits (using the example of quartz from sedimentary rocks of the Central Kyzylkum)

Research subject. The composition and distribution patterns of lattice defects in the clastic quartz of sedimentary rocks.Material and methods. Quartz collected from the core of two wells uncovering the Neogene, Paleogene and Cretaceous deposits in the rocks of the Central Kyzylkum. The registration and determination of the concentrations of lattice defects in detrital quartz were carried out by the EPR spectroscopy. Their stability in quartz under natural conditions was evaluated by comparing the concentrations of lattice defects in the near-surface and deep zones of quartz grains. The etching of quartz grains in HF was used to study the spatial location of lattice defects in the mineral. The genetic analysis of detrital quartz was carried out using the methods tested in the study of quartz from crystalline rocks.Results. The presence of substitutional Al, Ti, and Ge impurities, radiation induced E1-, Al–O−- and Ti-centers, as well as other paramagnetic centers possessing genetic information was found in quartz. The distribution of substitutional impurities in clastic quartz is close to their original distribution formed at the time of the mineral formation. This creates the prerequisites for identifying genetically similar quartz samples and attributing them to the certain provenance. The formation of sedimentary rocks of the Central Kyzylkum occurred due to two main provenance sources of terrigenous material. The expediency of using radiation induced centers with different thermal stability for studying the radiation prehistory of quartz and the conditions for the formation of uranium deposits is demonstrated.Conclusion. The clastic quartz of sedimentary deposits retains the main part of the genetically significant lattice defects formed during crystallisation. The most important of them are Al, Ti, and Ge isomorphic impurities. The regularities of their distribution, along with the distribution of radiation induced lattice defects in quartz, are able to carry information about the provenance of detrital material and the conditions of sedimentation and associated formation of mineral deposits.

Template‐Guided C8‐BTBT/MAPbBr3/C8‐BTBT Heterostructures for Broadband Bipolar Phototransistors

AbstractBipolar junction transistor (BJT) has had an unprecedented impact on the electronic industry and helped to usher in the digital revolution prominently. However, the critical part of BJT, p–n junction, is formed by carefully incorporating substitutional impurities in traditional bulk semiconductors and it is difficult to implement in nanostructured device. In this work, a floating‐base BJT device is demonstrated by growing C8‐BTBT/MAPbBr3/C8‐BTBT PNP heterostructure in liquid phase. Growth of two semiconductor microwires into a crisscross heterojunction is achieved through template‐guided printing and antisolvent crystallization method. The proof‐of‐concept phototransistor demonstrates broadband light absorption from ultraviolet (UV) to visible spectral region, which exhibits enhanced photoresponsivities of 1.28 A W–1 to UV and 0.425 A W–1 to visible light, respectively. In addition, the improved photoelectric amplification behavior of C8‐BTBT/MAPbBr3/C8‐BTBT phototransistor is observed by comparing with the performance of C8‐BTBT/MAPbBr3 two‐component photodiode. The maximal gain can reach up to ≈600 and can be maintained ≥200 at the positive bias. The device provides new attractive prospects in the investigation of integrated circuits based on micro‐structured semiconductors.

Density functional theory studies on C20 with substitutional TinNn impurities

In this paper, we have performed systematic theoretical surveys of C20 and its C20-2nTinNn nanocages with n = 1-8 at DFT. Full optimization indicates none of the structures collapse to open deformed as segregated heterofullerene. Also, in order to avoid the resulted strain of fused five-pentagon configuration, some of them deform their cage at the Ti-N bonds and appear cubic-like. Binding energy (Eb) increases, and the absolute heat of atomization │ΔHat│ of the designed C20-2nTinNn structures decreases, respectively, as the number of substituting Ti-N units increases. The calculated Eb of 57.05eV/atom and │ΔHat│ of 2437.40kcal/mol display C4Ti8N8 as the most thermodynamic stable heterofullerene where including eight separated Ti-N units through two double C═C bonds. In contrast, the calculated band gap of 2.06eV shows C18Ti1N1 as the best-insulated heterofullerene. Here isolable or extractable open-shell C18Ti1N1 heterofullerene must be kinetic stable species, and closed-shell C4Ti8N8 should be thermodynamic stable species. Compared to the suggested Ti-decorated B38 fullerene as a high capacity hydrogen storage material with large Eb (5.67eV/atom), our studied C20-2nTinNn heterofullerenes show the higher Eb with a range of 13.78 to 57.05eV/atom, the higher stability, and the higher capacity hydrogen storage. Each Ti-N unit can bind up to two hydrogen molecules with an average adsorption energy of 0.073eV/H2. While the C4Ti8N8 fullerene substituted with 8 Ti-N units can store 16 H2 molecules, the hydrogen gravimetric density (the hydrogen storage capacity) reaches up to 5.61 wt% with an average adsorption energy of 0.587eV/H2. Based on these results, we infer that C4Ti8N8 fullerene is a potential material for hydrogen storage with high capacity and might motivate active experimental efforts in designing hydrogen storage media.

Hydrogen four-level tunnel systems in substitutional body-centred cubic alloys

Abstract A brief account is provided of the main results of a study of the tunneling states of H trapped by substitutional (S) impurities in Nb, mainly consisting in anelasticity experiments. The phenomenology is rather complex when the concentration of S atoms is higher than a few parts per thousand, and various and contrasting interpretations are possible. The complication arises from the destruction of the symmetry of the S–H pair by the elastic interactions among the defects. The situation becomes, however, clear for an S content around 0.1 %, when the anelastic spectra reveal the relaxation processes due to the now nearly undistorted S–H complexes. In this case H delocalizes over four tetrahedral sites of a face of the cube containing the S atom, giving rise to a four-level tunnel system (FLS). The parameters of tunneling and coupling to phonons and electron excitations are similar to those found for the two-level system of H near an interstitial impurity, but new effects are found, due to the symmetry of the additional eigenstates of a centrosymmetric FLS.

Impurity-hydrogen complexes in β-Ga2O3: Hydrogenation of shallow donors vs deep acceptors

Substitutional impurities in β-Ga2O3 are used to make the material n-type or semi-insulating. Several O–H and O–D vibrational lines for complexes that involve impurities that are shallow donors and deep acceptors have been reported recently. The present article compares and contrasts the vibrational properties of complexes that involve shallow donors (OD-Si and OD-Ge) with complexes that involve deep acceptors (OD-Fe and OD-Mg). Theoretical analysis suggests that these results arise from defect complexes based on a shifted configuration of the Ga(1) vacancy with a trapped H atom and a nearby impurity.

Full orbital decomposition of Yu-Shiba-Rusinov states based on first principles

We have implemented the Bogoliubov-de Gennes (BdG) equation in a screened Korringa-Kohn- Rostoker (KKR) method for solving, self-consistently, the superconducting state for 3d crystals including substitutional impurities. In this report we extend this theoretical framework to allow for collinear magnetism and apply it to fcc Pb with 3d magnetic impurities. In the presence of magnetic impurities, there is a pair-breaking effect that results in sup-gap Yu-Shiba-Rusinov (YSR) states which we decompose into contributions from the individual orbital character. We determine the spatial extent of these impurity states, showing how the different orbital character affects the details of the YSR states within the superconducting gap. Our work highlights the importance of the first principles based description which captures the quantitative details making direct comparisons possible with experimental findings.

Effects of transition metal dopants on the electronic structure of potassium niobate

Potassium niobate (KNbO3) is an important ferroelectric and photorefractive material that finds multiple nonlinear optical and photorefractive applications such as two-beam coupling and dynamic holography. We employ standard Density Functional Theory to examine two defect complexes in KNbO3 doped with 3-d transition metals. These defect complexes involve the substitution of a Nb with a 3-d transition metal element and (1) a coordinating oxygen vacancy induced in the nearest-neighbor oxygen shell or (2) a non-coordinating oxygen vacancy induced in the supercell as far away as possible from substitution impurity. With the exception of Ti and V, the 3-d transition metal dopants studied here may represent viable dopants for purposes of extending the photorefractive spectral response of KNbO3. Our results indicate that each of these complexes introduces defect states into the gap of KNbO3. Furthermore, we notice that the distribution of impurity states changes quite systematically across the 3-d transition metal row, and we note the effects of the disruption of the crystal field in the case of the coordinating O vacancy on the position and ordering of defect states within the gap. Substituting KNbO3 with transition metal ions can, therefore, significantly affect its optical and infrared properties, exemplifying how a detailed understanding of the electronic structure can be an important tool in tailoring the properties of this material for ferroelectric and photorefractive applications.

Effect of Point Defects on Electronic Properties and Structure of Talc (001) Surface by First Principles

The surface structure and electronic properties of Mg vacancy defects on talc (001) and impurity defects with Fe, Mn, Ni, Al, and Ca replacing Mg atoms were calculated by using density functional theory. The calculation results show that the order of impurity substitution energy is Mn < Ni < Al < Ca < Fe. This indicates that Fe impurity defects are most easily formed in talc crystals. The covalent bonding between Si atoms and reactive oxygen atoms adjacent to impurity atoms is weakened and the ionic property is enhanced. The addition of Fe, Mn, and Ni atoms makes the surface of talc change from an insulator to a semiconductor and enhances its electrical conductivity. The analysis of electron state density shows that surface states composed of impurity atoms 4S orbital appear near the Fermi level.

Измерение коэффициентов диффузии примесей внедрения в сплаве Zr – 1 масс. % Nb методом внутреннего трения

In alloys with a GPU lattice, it is possible to observe a relaxation of the Snukovsky type if there is a certain amount of substitution impurities in it. However, it is impossible to calculate the diffusion coefficients of the introduction impurities by the measured relaxation time by analogy with BCC alloys, just because of the possible interaction with substitution atoms. However, in some cases, if reliable diffusion data are available for an alloy close in composition to the studied one, the relaxation time can be calculated from the ratio τr = τr0·exp(Hm/RTm), where Hm, Tm are the parameters of the relaxation maximum of the impurity from the experiment, and τr0 can be calculated from the formula D0 = c02/(16·τr0) for the GPU lattice, taking D0 from the corresponding diffusion expert data. In this way, the diffusion coefficients of the introduction impurities in the alloy Zr – 1 wt. % Nb are calculated for several states of samples.

The first-principle study of substitutional impurities’ effect on elastic properties of TlInS2layered crystal

The elastic and vibrational properties of the TIInS2 and TlIn(S0.75Se0.25)2 crystals were first calculated applying DFT/PBE method augmented by dispersion correction (DFT/PBE-D). The elastic constants Cij, bulk modulus B, Young's modulus E, shear modulus G, Poisson's ratio σ, and coefficient B/G were calculated for the studied crystals. The phonon energy dispersion curves and partial densities of the phonon states for the TIInS2 and TlIn(S0.75Se0.25)2 were also calculated. The obtained data are comparable with the experimental results. Anomalies occurring in the phonon energies observed for the TlIn(S0.75Se0.25)2 crystal were explained.

First-principles studies of the concentration-dependent tritium diffusion in the zirconium hydrides with and without Sn impurity

We performed first-principles calculations to study the 3H diffusion in the zirconium hydrides with or without Sn impurity. Our results show that the formation of Zr3Hx becomes preferable as the 3H concentration increases. Based on the most stable configurations of Zr3Hx (x = 0.5, 1.0, 1.5, 2.0), we studied the interstitial 3H diffusion with or without Sn impurity. The results show that the interstitial 3H diffusion becomes less probable in the hydrides with a higher 3H concentration. When introducing a substitutional Sn impurity, the diffusion barriers increase. Therefore, the substitutional Sn in the diffusion pathway hampers the 3H diffusion in Zr hydrides.

Easy-plane magnetic anisotropy of a single dopant in a single semiconductor nanowire

The magnetic anisotropy of an isolated substitutional Co impurity in a single ZnO nanowire is studied by performing density-functional supercell calculations with the Hubbard U correction. The variation of the magnetic anisotropy energy with the location of the Co atom in different regions of the nanowire is explored. Contrary to usual expectation, it is found that single Co-doped ZnO nanowire possess an easy plane (as opposed to easy axis) of magnetization, which is neither parallel nor perpendicular to the nanowire axis; the orientation of the easy plane is determined by the local geometry around the dopant. Our results also show that the magnetic anisotropy energy is enhanced by almost an order of magnitude when the Co impurity moves to a surface site from an interior site. This enhancement is elucidated on the basis of local bonding characteristics, because a pronounced variation of the orbital moment of Co with the magnetization direction is observed only when Co is located at the nanowire surface. The complementary analysis of these findings shows that the magnetic anisotropy of a single dopant in a single nonmagnetic semiconductor nanowire can be directionally manipulated and substantially enhanced by controlling the local chemical environment of the dopant in the absence of magnetic coupling. Our findings thus imply that the directional control of magnetic anisotropy needs to be achieved for applications of magnetic ion doped semiconductor nanowires.

Spin accumulation in metallic thin films induced by electronic impurity scattering

In order to explore the spin accumulation, evaluating the spin galvanic and spin Hall effect, we utilize the semi-classical Boltzmann equation based on input from the relativistic Korringa-Kohn-Rostoker Green's function method, within the density functional theory. We calculate the spin accumulation including multiple contributions, especially skew-scattering (scattering-in term) and compare this to three different approximations, which include the isotropic and anisotropic relaxation time approximation. For heavy metals, with strong intrinsic spin-orbit coupling, we find that almost all the effects are captured within the anisotropic relaxation time approximation. On the other hand, in light metals the contributions from the vertex corrections (scattering-in term) are comparable to the induced effect in anisotropic relaxation time approximation. We put a particular focus on the influence of the atomic character of the substitutional impurities on the spin accumulation as well as the dependence on the impurity position. As impurities will break space inversion symmetry of the thin film, this will give rise to both symmetric and antisymmetric contributions to the spin accumulation. In general, we find the impurities at the surface generate the largest efficiency of charge-to-spin conversion in case of the spin accumulation. Comparing our results to existing experimental findings for Pt we find a good agreement.