V.J.B. Torres

Calculation of deep carrier traps in a divacancy in germanium crystals

We present an ab initio density functional study on the electronic structure and electrical properties of divacancies in Ge. Although suffering essentially different Jahn-Teller distortions when compared to the analogous defect in Si, the relative location of the electrical levels in the gap does not differ radically in both materials. We propose a V2 model that is responsible for a donor level at Ev+0.03eV, a first acceptor state at Ev+0.3eV, and a second acceptor level at Ec−0.4eV. The latter is only 0.1eV deeper than an electron trap that has been recently linked to a divacancy in proton implanted material.

Vibrational properties of elemental hydrogen centres in Si, Ge and dilute SiGe alloys

The local vibrational modes arising from single interstitial hydrogen centres in Si, Si-rich SiGe, Ge-rich SiGe, and Ge crystals are modelled by an ab initio supercell method. The stress response of the 1998 and 1794 cm -1 bands that appear in proton-implanted Si and Ge samples is well reproduced, further confirming their assignment to bond-centred H + defects. It is shown that H - in Ge is anti-bonded to a Ge atom, and is likely to be considerably less mobile than in Si. Although H + is not trapped by the minority species in both Si-rich and Ge-rich alloys, we find that H - can be stabilized by forming anti-bonded H-Si structures.

Ab initio calculations of the structure and dynamics of C60 and C60 3

A local-density-functional cluster method is used to calculate the structure and vibrational modes of C60. We find C–C lengths in good agreement with observed values. The effect of doping the molecule with three extra electrons is investigated and shown to result in a surprising shortening of the longer bonds. The second derivatives of the energy are evaluated and have enabled, for the first time, all the normal modes of the molecule to be found. We find these to be in fair agreement with the available experimental results.

Early Stages of Silicon Oxidation

Experiments have shown that the early stages of silicon oxidation proceed layer by layer, so that one layer is essentially complete before another develops. Other experiments show that the mechanism does not involve step growth, the most obvious mechanism. We use a new approach to modelling the growth to show that these two observations can be understood when there is a rate-determining step which depends strongly on the local oxide thickness. The rate in question might be the sticking probability, or the rate of incorporation of adsorbed oxygen species into the oxide network. Such mechanisms are possible when transport by an ionic species dominates, contrary to the situation for thicker films. Our modelling suggests the mechanisms are driven by the image interaction, as in earlier suggestions by Stoneham and Tasker, rather than an effect of the electric field central to the Mott-Cabrera mechanism.

Metastable VO2 complexes in silicon: experimental and theoretical modeling studies

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.

Nitrogen–Hydrogen Defects in GaP

Models of the nitrogen–hydrogen defect in GaP, which contain one and two H atoms, are investigated using ab initio density functional cluster theory. We find that a single H atom binding to N possesses two infrared absorption frequencies close to those attributed to an NH2 defect. The modes shift with its charge state consistent with the photo-sensitivity found for the defect. A third mode observed for this centre is assumed to be an overtone of the bend mode. The isotope shifts of the calculated modes are in excellent agreement with experiment in contrast with the model which contains two H atoms.

Hydrogen passivation of titanium impurities in silicon: Effect of doping conditions

While the contamination of solar silicon by fast diffusing transition metals can be now limited through gettering, much attention has been drawn to the slow diffusing species, especially the early 3d and 4d elements. To some extent, hydrogen passivation has been successful in healing many deep centers, including transition metals in Si. Recent deep-level transient spectroscopy (DLTS) measurements concerning hydrogen passivation of Ti revealed the existence of at least four electrical levels related to TiiHn in the upper-half of the gap. These findings challenge the existing models regarding both the current level assignment as well as the structure/species involved in the defects. We revisit this problem by means of density functional calculations and find that progressive hydrogenation of interstitial Ti is thermodynamically stable in intrinsic and n-doped Si. Full passivation may not be possible to attain in p-type Si as TiiH3 and TiiH4 are metastable against dissociation and release of bond-centered pr...

GeSi Raman spectra vs. local clustering/anticlustering: Percolation scheme and ab initio calculations

We formalize within the percolation scheme that operates along the linear chain approximation, i.e., at one dimension (1D), an intrinsic ability behind Raman scattering to achieve a quantitative insight into local clustering/anticlustering in an alloy, using GeSi as a case study. For doing so, we derive general expressions of the individual fractions of the six GeSi percolation-type oscillators [1×(Ge-Ge), 3×(Ge-Si), 2×(Si-Si)], which monitor directly the Raman intensities, via a relevant order parameter κ. This is introduced by adapting to the 1D oscillators of the GeSi-diamond version of the 1D-percolation scheme, i.e., along a fully consistent 1D treatment, the approach originally used by Verleur and Barker for the three-dimensional (3D) oscillators of their 1D-cluster scheme applying to zincblende alloys [H. W. Verleur and A. S. Barker, Phys. Rev. 149, 715 (1966)], a somehow problematic one in fact, due to its 3D–1D ambivalence. Predictive κ-dependent intensity-interplays between the Ge0.5Si0.5 Raman ...

Electronic properties and structure of a complex incorporating a self-interstitial and two oxygen atoms in silicon

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.

Electronic structure of divacancy-hydrogen complexes in silicon

Divacancy–hydrogen complexes (V2H and V2H2) in Si are studied by ab initio modelling using large supercells. Here we pay special attention to their electronic structure, showing that these defects produce deep carrier traps. Calculated electrical gap levels indicate that V2H2 is an acceptor, whereas V2H is amphoteric, with levels close to those of the well known divacancy. Finally our results are compared with the available data from deep level transient spectroscopy and electron paramagnetic resonance experiments.