Pablo D. Borges

Study of the oxygen vacancy influence on magnetic properties of Fe- and Co-doped SnO2 diluted alloys

Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both experimental and theoretical points of view due to their potential use in spintronics towards new nanostructured devices and new technologies. In the present work, we study the magnetic properties of Sn0.96TM0.04O2 and Sn0.96TM0.04O1.98(VO)0.02, where TM = Fe and Co, focusing in particular in the role played by the presence of O vacancies nearby the TM. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a ground state, respectively, with 2 and 1 μB/cell, for Fe and Co. Two metastable states, with 0 and 4 μB/cell were found for Fe, and a single value, 3 μB/cell, for Co. The spin-crossover energies (ES) were calculated. The values are ES0/2 = 107 meV and ES4/2 = 25 meV for Fe. For Co, ES3/1 = 36 meV. By creating O vacancies close to the TM site, we show that the metastablity and ES change. For iron, a new state appears, and the state with zero magnetic moment disappears. The ground state is 4 μB/cell instead of 2 μB/cell, and the energy ES2/4 is 30 meV. For cobalt, the ground state is then found with 3 μB/cell and the metastable state with 1 μB/cell. The spin-crossover energy ES1/3 is 21 meV. Our results suggest that these materials may be used in devices for spintronic applications that require different magnetization states.

Structural, electronic, vibrational and dielectric properties of selected high-shape K semiconductor oxides

The semiconductor oxides SnO2, HfO2, ZrO2, TiO2 and SrTiO3 are interesting materials for applications as high-K dielectric gate materials in silicon-based devices and spintronics, among others. Here we review our theoretical work about the structural, electronic and vibrational properties of these oxides in their most stable structural phases, including dielectric properties as derived from the electronic structure taking into account the lattice contribution. Finally, we address the recent role played by the presence of transition metal atoms in semiconductor oxides, considering in particular SnO2 as an example in forming diluted magnetic alloys.

Electronic and magnetic properties of SnO2/CrO2 thin superlattices.

In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2)n(SnO2)nsuperlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2)n(SnO2)nSLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2)n(SnO2)nSLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.

Optical Properties and Carrier Effective Masses of Rutile SnO2 as Obtained from Full Relativistic Ab Initio Calculations

The electronic and optical properties of rutile SnO2 are studied by means of the ab initio full‐potential linear augmented plane‐wave method and within a full relativistic approach. Relevant relativistic and spin‐orbit coupling effects are observed. We provide values for the dielectric constant and refraction index, as well as for conduction‐ and valence‐band effective masses. These later are shown to be highly anisotropic, and with the holes heavier than the electrons. The values obtained for the electronic part of the dielectric constant are exx(0) = 4.5 and ezz(0) = 4.8. For the refraction index nxx and nzz we obtained the values of 2.15 and 2.20, respectively.

Native defects as sources of optical transitions in MgAl2O4 spinel

The outstanding physical and chemical properties of the magnesium aluminate (MgAl2O4) spinel makes it an important material for novel technological applications. Considering that a presence of native defects can promote important changes in those properties, in this work we present a study of the structural, electronic and thermodynamic properties of the MgAl2O4 spinel. The calculated formation energy for isolated defects, such as the vacancies of magnesium (V Mg), aluminum (V Al) and oxygen (V O), oxygen interstitial (Oi), magnesium and aluminum antisites (MgAl, AlMg), as well as some complex defects (V O + Oi, V O + AlMg, V O + MgAl, MgAl + AlMg) in the most stable charge states are shown. Through experimental data, we obtained that complex defects centers, such as V O , V O + Oi, V O + AlMg and VO + MgAl at different charge states are good candidates for the observed optical transitions at 4.75, 5.3, and 6.4 eV. Our findings were obtained from ab initio electronic structure calculations performed by using density functional theory. The Perdew–Burke–Ernzerhof generalized gradient approximation was used for the exchange-correlation potential. Furthermore, a modified Becke-Johnson exchange potential (GGA-mBJ) correction to the exchange potential were used to obtain a suitable value for the band gap energy, 7.40 eV, in accordance with the experimental one of 7.8 eV.

Electronic structure and dielectric properties calculations of pure tin dioxide and of vacancies in tin dioxide

In this work we report the results of ab initio electronic structure calculations for pure SnO2 as well as for some defects, such as the oxygen vacancy and the interstitial tin impurity, and for the In and Sb as substitutional impurities in a configuration which corresponds to a concentration of 4.2%. We also show results for related optical properties which are derived directly from the band structure. Our findings are in good agreement with the available data, and the role of the studied systems is described in the present work.

Electronic and optical properties of antiferromagnetic iron doped NiO – A first principles study

Antiferromagnetic NiO is a candidate for next generation high-speed and scaled RRAM devices. Here, electronic and optical properties of antiferromagnetic NiO: Fe 25% in the rock salt structure are studied and compared to intrinsic NiO. From density of states and complex dielectric function analysis, the first optical transition is found to be at lower frequency than intrinsic NiO due to an Fe impurity level being the valence band maximum. The resulting effects on refractive index, reflectivity, absorption, optical conductivity and loss function for Fe-doped NiO are compared to those of intrinsic NiO, and notable differences are analyzed. The electronic component of the static dielectric constant of NiO: Fe 25% is calculated to be about 2% less than that of intrinsic NiO.

Ab initio Results for the Structural and Electronic Properties of Intrinsic Defects in PbTe

In this work, we have performed spin-polarized calculations for the structural and electronic properties of vacancies and anti-site defects in the rocksalt PbTe. Our obtained results have shown that both the Pb and Te antisites are the favorable defects in Pb and Te rich conditions, respectively. Moreover, in the perfect stoichiometry condition, the antisites, as well as the Te vacancy, are equally probable to find. Considering the charge injection in the system, all the defects change from the 2+ charge state to the 2one within 14 meV, at around 80 meV from the valence band edge within the bandgap. This feature makes it difficult to experimentally characterize these defects in PbTe.

Complex centers of hydrogen in tin dioxide

Tin dioxide is a wide band-gap semiconductor and is part of a class of promising transparent conducting oxides. It shows n-type conductivity, even when not intentionally doped, and is usually attributed to intrinsic defects. Theoretically, the unintentional doping with hydrogen, either at interstitials or at O sites, has been proposed to provide the shallow donors for the n-type conductivity of SnO2. Since H is an electrically active impurity present in many growth environments, a deeper theoretical understanding of the hydrogen and H-related complexes in SnO2 is highly welcome. We present here the results of ab initio studies, based on self-consistent electronic structure calculations, based on Perdew, Burke and Ernzerhof plus the on-site Coulomb correction and Heyd–Scuseria–Ernzerhof hybrid functional approaches, for several H-related defect centers in SnO2. Isolated substitutional (HO) and interstitial (Hi) impurities, as well as some complexes related to them, like 2H, HO–H VSn–H, VSn–2H, VO–H2, VO–2H and Sni–H, have been analyzed from structural and electronic properties, formation energy and vibrational frequencies. A comparison of our calculated vibrational frequencies with recent infrared measurements (IR) allowed us to ascribe the observed IR peaks to the H-related centers. This, added to the low formation energy of the VO–H2 center, and nudged-elastic band method-based calculations, is a strong indication for this center to be the source of hidden hydrogen in SnO2.