Papers by Author: Vitor J.B. Torres

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Authors: A. Carvalho, R. Jones, J. Coutinho, Vitor J.B. Torres, Patrick R. Briddon
Abstract: We report on the energetics, electrical and optical activity of small self-interstitial (I3 and I4) clusters in Si, found from ab-initio density-functional modeling studies. I4 possesses nine local vibrational modes above the Raman edge, which account for up to three dipole-allowed vibronic transitions observed in recent experiments associated with the X-photoluminescent line. Another prominent photoluminescent line (known as the W-line) that shows a trigonal stress-induced splitting pattern, has been previously assigned to I3. Our analysis of the LVMs of a metastable form of I3 support this assignment.
Authors: Ivana Capan, Tomislav Brodar, Takeshi Ohshima, Shinichiro Sato, Takahiro Makino, Željko Pastuovic, Rainer Siegele, Luka Snoj, Vadimir Radulović, José Coutinho, Vitor J.B. Torres, Kamel Demmouche
Abstract: We present a study of electrically active radiation-induced defects formed in 4H-SiC epitaxial layers following irradiation with fast neutrons, as well as 600 keV H+ and 2 MeV He++ ion implantations. We also look at electron emission energies and mechanisms of the carbon vacancy in 4H-SiC by means of first-principles modelling. Combining the relative stability of carbon vacancies at different sites with the relative amplitude of the observed Laplace-DLTS peaks, we were able to connect Z1 and Z2 to emissions from double negatively charged carbon vacancies located at the h- and k-sites, respectively.
Authors: Vladimir P. Markevich, L.I. Murin, Stanislav B. Lastovskii, I.F. Medvedeva, J. Lennart Lindström, Anthony R. Peaker, J. Coutinho, R. Jones, Vitor J.B. Torres, Sven Öberg, Patrick R. Briddon
Abstract: The electronic properties and structure of a complex incorporating a self-interstitial (I) and two oxygen atoms are presented by a combination of deep level transient spectroscopy (DLTS), infrared absorption spectroscopy and ab-initio modeling studies. It is argued that the IO2 complex in Si can exist in four charge states (IO− 2 , IO02 , IO+ 2 , and IO++ 2 ). The first and the second donor levels of the IO2 complex show an inverted location order in the gap, leading to a E(0/ + +) occupancy level at Ev + 0.255 eV. Activation energies for hole emission, transformation barriers between different structures, and positions of LVM lines for different configurations and charge states have been determined. These observables were calculated by density-functional calculations, which show that they are accounted for if we consider at least two charge-dependent defect structures.
Authors: Lyudmila I. Khirunenko, Yu.V. Pomozov, Mikhail G. Sosnin, A.V. Duvanskii, Vitor J.B. Torres, J. Coutinho, R. Jones, Patrick R. Briddon, Nikolay V. Abrosimov, H. Riemann
Abstract: The interstitial carbon impurity (CI) vibrational modes in monocrystalline Si-rich SiGe were investigated by Fourier Transform Infra Red spectroscopy and density functional modelling. The two absorption bands of CI are found to be close to those in silicon, but show shifts in opposite directions with increasing Ge content. The transversal mode band at 932 cm-1 shifts slightly to the high frequency side, while the longitudinal mode at 922 cm-1 suffers a pronounced red-shift. Each Ci-related band is found to consist of two components. An annealing of CI in Si1-xGex occures in two stage. During the first stage (210-250 K) the main components of bands anneals and revealed components grow in intensity. At T>250 K all components disappear. Two component structure of bands is suppose most likely correspond to different combinations of Si and Ge atoms in the neighbourhood of the carbon atom. The interstitial carbon defect was modelled by a supercell density-functional pseudopotential method (AIMPRO) for alloys with 4.69% Ge concentration. From energetics, it has been found that each Ge-C bond costs at least 0.4 eV in excess of a Si-C bond. However, structures where Ge atoms are second neighbors to the C atom are marginally bound, and may explain the two-component band structure in the absorption measurements. The vibrational mode frequencies taken from several randomly generated SiGe cells produce the observed opposite shifts for the transverse and longitudinal modes.
Authors: J. Coutinho, C. Janke, A. Carvalho, Sven Öberg, Vitor J.B. Torres, R. Jones, Patrick R. Briddon
Abstract: Vacancies and interstitials in semiconductors play a fundamental role in both high temperature diffusion and low temperature radiation and implantation damage. In Ge, a seri- ous contender material for high-speed electronics applications, vacancies have historically been believed to dominate most diffusion related phenomena such as self-diffusivity or impurity mi- gration. This is to be contrasted with silicon, where self-interstitials also play decisive roles, despite the similarities in the chemical nature of both materials. We report on density func- tional calculations of the formation and properties of vacancy-donor complexes in germanium. We predict that most vacancy-donor aggregates are deep acceptors, and together with their high solubilities, we conclude that they strongly contribute for inhibiting donor activation levels in germanium.
Authors: Vitor J.B. Torres, J. Coutinho, Patrick R. Briddon
Abstract: We have investigated the hydrogenation of the zinc acceptor in GaP and InP, and of the phosphorus acceptor in ZnTe, by computer modeling. We used a density-functional supercell code and pseudopotentials to deal with the core electrons. However zinc 3d electrons were explicitly taken to be valence electrons. We have determined the relaxed atomic geometry for seven hydrogen sites. We have found that, in the lowest total energy configuration, hydrogen sits in a bond centered position between zinc and arsenic atoms in all GaP, InP and ZnTe semiconductors and is bonded to the phosphorus atom. We found metastable states, by 0.4, 0.4 and 0.5 eV, for structures where H is antibonding to the phosphorus atom for GaP, InP and ZnTe, respectively. The calculated local vibrational modes (LVM) for the bond-centered configuration agree, within 1%, with the experimental values of 2379.0 cm-1 for GaP:Zn-H, 2287.7 cm-1 for InP:Zn-H and 2193 cm-1 for ZnTe:P-H. The isotopic shift due to the replacement of deuterium by hydrogen is reproduced by less than 2.5% using experimental data. The decrease in the LVM when going from GaP to ZnTe, as the perfect bond length increases, is also well-reproduced. A wag mode at 496 cm-1 and lower LVM, a doublet at 329 cm-1 and a singlet at 242 cm-1, are predicted for P-H in ZnTe.
Authors: Lyudmila I. Khirunenko, Yu.V. Pomozov, Mikhail G. Sosnin, Vitor J.B. Torres, João A.P. Coutinho, R. Jones, Nikolay V. Abrosimov, H. Riemann, Patrick R. Briddon
Abstract: The influence of Ge content on the local vibrational mode of substitutional carbon in Si-rich Si1-xGex single crystals has been investigated by infrared Fourier-transform spectroscopy and ab initio modeling methods. Czochralski-grown Si1-xGex samples doped with boron and carbon have been studied. The Ge fractional content was varied from x=0.004 to x=0.044. To reveal the CS-related absorption band in the Si1-xGex the difference spectra between carbon-lean and carbon-reach Si1-xGex samples with the same Ge content were studied. We have found that the CS-related absorption band in the Si1-xGex alloys red-shifts and broadens with increasing Ge content. It has been found that at x0.015 the CS absorption band consists of two overlapping lines corresponding to different combinations of Si and Ge atoms neighboring the CS atom. The calculations show that substitutional carbon atoms avoid Ge ligand atoms, and should be found in Si-rich regions. These results also reveal that the softening of the CS mode frequency arises from the SiGe volumic expansion.
Authors: L.I. Murin, J. Lennart Lindström, Vladimir P. Markevich, I.F. Medvedeva, Vitor J.B. Torres, J. Coutinho, R. Jones, Patrick R. Briddon
Abstract: We report on a combined experimental and theoretical study of the metastable form of the vacancy-dioxygen defect in Si labeled VO∗ 2. Important new experimental observations are the detection of mixed local vibrational modes of VO∗ 2 in 16O,18O co-doped samples, the determination of the position of LVM bands for the negatively charged defect, and an assignment of an acceptor level at about Ec − 0.05 eV. Defect energetics, electrical levels and LVM frequencies of the VO∗ 2 complex are also investigated by ab-initio density-functional modeling.We find it to be a bistable defect which accounts well for the experimental data. The metastable form produces an acceptor state at 0.05 eV below Ec, and can be thought of as a VO defect perturbed by interstitial oxygen.
Authors: R. Jones, A. Carvalho, J. Coutinho, Vitor J.B. Torres, Sven Öberg, Patrick R. Briddon
Abstract: The donor and acceptor levels of defects in Ge as well as in Si are found using a local density functional method applied to large H-terminated defective clusters. The surfaces of the clusters are modified so that their band gaps are aligned with experimental values. It is shown that the resulting energies of the first donor and acceptor levels are within about 0.2 eV of the experimental values.
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