Ab Initio Study of Structural, Electronic, and Hyperfine Properties of n-type SnO2:Ta Semiconductor

A detailed theoretical first-principles study of structural, electronic, and hyperfine properties at Sn and Ta sites of undoped and Ta-doped rutile SnO2 is presented, using the Full-Potential Augmented Plane Wave plus local orbitals (FP-APW+lo) method. In the Ta-doped systems, we performed calculations for two different charge states. The predicted electric-field-gradient (EFG) tensor, the key magnitude in this study, for both charge states of the impurity result to be almost equal and in good agreement with Time-Differential Perturbed γ–γ Angular Correlation (TDPAC) results in 181Ta-doped SnO2 thin films. This study enables at present to discuss the origin of the EFG and the role played by the structural anisotropic contractions introduced by the Ta atom and the impurity charge state on the hyperfine properties. To determine the correct charge state of the impurity, we performed energetic studies, predicting the metallic behavior of degenerate semiconductors, in agreement with resistivity experimental results obtained in samples with the same Ta dilution.Fil: Darriba, German Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: Muñoz, Emiliano Luis. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: Errico, Leonardo Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentina. Universidad Nacional del Noroeste de la Provincia de Buenos Aires; ArgentinaFil: Rentería, Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentin

Electric field gradient study on pure and Cd-doped In(111) surfaces: Correlation between experiments at the atomic scale and first-principles calculations

We present a complete all-electron density functional theory/ab initio study of structural and electronic properties at pure In(111) and Cd-doped In(111) surfaces, enabling a deep analysis of the electric field gradient (EFG) in these systems. We explained, from first-principles, results of previously performed key time-differential perturbed γ-γangular correlations experiments [W. Körner et al., Phys. Rev. Lett. 49, 1735 (1982)10.1103/PhysRevLett.49.1735] on In111 isotopes (which decay to Cd111 impurity probe-atoms) deposited onto the In(111) surface of thin films under ultrahigh vacuum, adding inactive indium layer by layer, carefully designed to determine the EFG at different depths from the surface. We confirmed the existence of only two hyperfine interactions, one related with Cd111 probes localized at the two more superficial In sites (HFIS), and the other related with probes localized in any of the other inequivalent In sites existing from the surface towards the bulk (HFIB). In the case of HFIS, V33 is oriented normal to the (111) surface, and for HFIB we found a V33 orientation parallel to the [001] axis and coincident with the orientation predicted for both the pure and Cd-doped In bulk (not determined experimentally at present), enabling us to confirm the experimental assignment of HFIB. The axial symmetry that the EFG has in pure and Cd-doped In bulk systems is broken when the surface is generated and is recovered as the probe (In or Cd, respectively) goes deeper from the surface into the bulk. We separated the structural and electronic effects and their sources in the pure and Cd-doped In(111) surface. For the Cd-doped systems we confirm the experimental ratio |V33S/V33B|≈4, showing that light structural modifications have in important impact on the Cd p-states distribution, which governs the V33 behavior. Finally, from the combination of the predicted V33 for the Cd-doped systems as a function of the depth of the Cd localization from the surface with the experimental fractions of HFIS and HFIB, we demonstrated that a single 3-Å active monolayer was enough to explain the origin of these fractions (in discrepancy with the previous interpretation of the experiments), proposing a deposition rate for the inactive In layers, in agreement with the experimental fraction evolution as a function of inactive In deposition.Fil: Darriba, German Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Faccio, Roberto Jose. Universidad de la República; UruguayFil: Rentería, Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin

Surfactant and dopant addition effect on optical and structural properties of ZnSe (Te) nanostructured semiconductors

ZnSe and ZnTe nanoparticles were obtained by mechanical milling. The influence of surfactant and doping agent addition on structural and optical properties were analyzed. X-ray diffraction (XRD) confirmed cubic crystalline structure is maintained for all milling times and the progressive substitutional incorporation of Al atoms into the Zn-based semiconductor structure. The agglomerate state of milled powders was disclosed by scanning electron microscopy (SEM), and Dispersion Light Scattering (DLS). Positron annihilation lifetime (PALS) spectroscopy allowed to identify the defects induced by mechanical work. A slight shift on the energy band gap with respect to pure semiconductor nanoparticles was revealed from optical diffuse reflectance measurements for all studied samples. The obtained results agree with first-principles calculations based on the Density Functional Theory (DFT). The calculations predict that Al substitute the Zn atom in the ZnTe and ZnSe lattice and a zinc vacancy must appear in order to recover the semiconductor character, as shown (or suggest) the experimental results. The characteristic positron annihilation lifetimes for the doped samples are obtained for both systems and compared with those measured. This theoretical approach helps us to deeper understand the origin and characteristics of different positrons traps.Fil: Valluzzi, Lucas Gabriel. Universidad Nacional de Tierra del Fuego; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Valluzzi, Marcos Gabriel. Universidad Nacional de Tierra del Fuego; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Damonte, Laura Cristina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Meyer, Marcos. Universidad Nacional de La Plata. Facultad de Ciencias Exactas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Darriba, German Nicolas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin

First-principles study of Cd impurities localized at and near the (001) α-Al2O3 surface

Abstract A combination of two first-principles electronic structure calculation methods in the framework of density-functional theory was applied to investigate the (001) α-Al2O3 surface reconstruction and its structural, electronic, magnetic, and hyperfine properties when doped with Cd impurities at different depths from the surface. The SIESTA approach was used to obtain the equilibrium positions of all atoms and the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) method was employed in order to obtain the electronic structure at these equilibrium positions and all the other physical properties. For the most stable (001) α-Al2O3 surface, we have demonstrated that the inclusion of the Cd atom at substitutional Al sites at and near the surface produces a ground state magnetic behavior. The largest principal component V33 of the electric-field-gradient (EFG) tensor at the Cd atom localized just above the α-Al2O3 terrace showed the same [001] orientation and a dominating p-character as Cd does when it is localized at bulk α-Al2O3, but exhibits an anomalous V33 magnitude four times larger than its value in bulk. Just below the surface, the non symmetric structural relaxation around the Cd impurity is responsible for the strong change in the asymmetry, magnitude, and orientation of the EFG tensor. The changes in the hyperfine properties have been correlated with the modifications observed on the electronic charge density at the different Cd sites and on the p-states of the Cd-projected partial density of states. Accordingly, the significant differences on the hyperfine parameters showed for different depths suggest that 111Cd probe-atoms used in Perturbed γ-γ Angular Correlation experiments could be used for evaluating geometrical and electronic distortions, particularly for positions quite close to the reconstructed surface, as well as contributing to studies of growth, adsorption, and diffusion of atoms in oxide surfaces and interfaces.Fil: Darriba, German Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Faccio, Ricardo. Universidad de la República; UruguayFil: Rentería, Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin

Structural and electronic properties of Al-doped ZnO semiconductor nanopowders: Interplay between XRD and PALS experiments and first-principles/DFT modeling

A combined experimental and novel theoretical ab initio structural and electronic study was performed in order to characterize ZnO semiconductor nanopowders doped with Al atoms. For this, powder mixtures of ZnO and metallic Al in adequate proportions yielding different contents of Al (5, 10, and 30 at. %) were prepared by mechanical milling. The systems were characterized by X-ray diffraction and positron annihilation lifetime spectroscopy measurements. Additionally, combining two first-principles methods based on the Density Functional Theory (DFT) we calculated the final equilibrium structures for different concentrations of Al dopants and Zn vacancies in ZnO, predicting afterwards the characteristic positron annihilation lifetimes at these equilibrium structures. In addition to the structural relaxations, the ab initio predictions of the electronic properties in the studied systems help us to understand deeper the origin and characteristics of different positrons traps. This experimental and ab initio/DFT combined study allows to verify the dopant incorporation into the ZnO wurtzite structure and to extract the maximum information from the experimental data, giving an insight into the different defect complexes and their influence in the structural and electrical properties.Fil: Damonte, Laura Cristina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Darriba, German Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Rentería, Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin

Ionic exchange and the local structure in the HfO2/Ho 2O3 system studied by PAC spectroscopy

The ionic exchange of Hf and Ho atoms in the HfO2/Ho 2O3 system was studied at the atomic level applying the nuclear solid-state Time-Differential γ-γ Perturbed-Angular- Correlation (PAC) spectroscopy. This exchange was promoted by a ball-milling-assisted solid-state reaction between Ho2O3 and m-HfO2 initial powders. In order to follow and to elucidate the effect of different variables (milling time, temperature, pressure) on the exchange process and the appearance of new phases, 181Hf(→ 181Ta) ions were used as local probes in the PAC experiments. The measured hyperfine interactions enabled the electric-field gradient tensor (EFG) characterization at Hf sites at each step of the process. At the final stages of the solid-state reaction, 75-90% Hf-doping at both substitutional defect-free cation sites of Ho2O3 was achieved, being the EFG measured at these sites in excellent agreement with those determined in 181Hf-implanted Ho2O3 samples and to those predicted by the EFG systematics established in rare-earth bixbyites doped by ion-implantation of 181Hf(→181Ta) ions. Ab initio electronic structure calculations of the EFG at Ta impurities localized at both cation sites in Ho2O3 also confirm the 181Hf cationic substitution in both PAC experiments. Additional ab initio calculations at Hf impurity sites in Ho2O3 and Tm2O 3 were performed to study the relative Hf preference for the symmetric site of the structure. We showed that high-energy milling plus high temperature treatments are both necessary to achieve a high degree of Hf substitution in the cation sublattice of the Ho2O3 structure. Also, we found that the pressure effect on the crystal structure favors the impurity substitution at cationic sites closer to a homogenous distribution of the probes and with much less local and far disorder. Additional spurious hyperfine interactions that were always present in 181Hf-implanted Ho2O3 samples were not observed when using this solid-state reaction method. The appearance of a modified m-HfO2 phase produced after heavy Ho-doping was also discussed.Fil: Richard, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Darriba, German Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Muñoz, Emiliano Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Errico, Leonardo Antonio. Universidad Nacional del Noroeste de la Provincia de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Rentería, Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin

Experimental and First-Principles Theoretical Study of Structural and Electronic Properties in Tantalum-Doped In2O3 Semiconductor: Finding a Definitive Hyperfine Interaction Assignment

In this work we present an experimental and theoretical study from first-principles of the structural, electronic, and hyperfine properties of Ta-doped In2O3 semiconductor. The ab initio electronic structure calculations in the Ta-diluted In2O3 system enabled to obtain the structural lattice distortions and the hyperfine parameters when the Ta atom is placed at each cationic site of the bixbyite crystal structure. To this purpose we used the full-potential augmented plane wave plus local orbital (FP-APW+lo) method, within the density functional theory. The obtained results indicate that the substitutional Ta probe-impurity produces strong changes on the local structure. In addition, we performed accurate time-differential perturbed γ-γ angular correlations (TDPAC) key experiments in 181Hf(→181Ta)-implanted In2O3 samples with high crystallinity, in order to obtain high quality measurements of the electric-field gradient tensor (EFG) that unraveled the controversy settled in the literature and overcome dissimilar interpretations of previously reported TDPAC experiments. The experiments were performed at room temperature in air, after each step of a series of thermal annealing treatments in air at increasing temperatures in order to remove radiation damage and locate the 181Hf probes at substitutional cationic sites. We succeeded to obtained two well-defined hyperfine interactions that were assigned to 181Ta probes located at the two defect-free inequivalent cationic sites of the In2O3 crystal structure. The EFG calculations are in excellent agreement with the results of these TDPAC measurements, and show that the largest component of the diagonalized EFG, V33, at the Ta site has mainly p character. The accuracy of the experiments together with the reliable and precise ab initio results allowed a definitive determination of the EFG at both cationic sites in this system. Formation energy calculations of defects were needed to determine the charge state of the 181Ta impurity, which agrees with a semiconducting character for the In2O3:Ta doped system.Fil: Richard, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Darriba, German Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Muñoz, Emiliano Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Errico, Leonardo Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Eversheim, Paul D.. Universitat Bonn; AlemaniaFil: Rentería, Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin