Authors: A.G. Marinopoulos, P. Santos, J. Coutinho
Abstract: Early transition metals (TMs) of the 3d and 4d rows are undesired contaminants in solarandelectronic-grade Si. From the theoretical standpoint, understanding the properties of these TMs insilicon still remains a challenging problem owing to the strong correlations among the TM d-electrons.The present study proposes a first-principles Hubbard-corrected DFT+U approach, with on-site parametersaccounting separately for electron Coulomb (U) and exchange (J) effects. We use this approachtogether with conventional DFT to determine electrical levels and migration barriers of early3d (Ti, V and Cr) and 4d (Zr, Nb and Mo) TMs in Si. Comparisons with experimental data allowedus to uniquely assign the deep levels in the gap appraising also the effect of on-site correlation. Ourresults also resolve existing controversies in the literature concerning the type and origin of the donorlevels of Cr and Mo. For all the metals, with the exception of Cr, high barriers of interstitial diffusionare obtained, thus confirming that most of these TMs are slow diffusers in silicon.
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Authors: J. Coutinho, C. Janke, A. Carvalho, Sven Öberg, Vitor 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.
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Authors: Lyudmila I. Khirunenko, Yu.V. Pomozov, Mikhail G. Sosnin, A.V. Duvanskii, Vitor 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.
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Authors: Vitor 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.
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