Sven Öberg

Theoretical and isotopic infrared absorption investigations of nitrogen-oxygen defects in silicon

The vibrational spectroscopy of NNO defects in Si introduced by 16O, 14N and 15N ion implantation is studied, and especially the N-isotopic shifts of the localized vibrational modes. These investigations show that the local modes of the three impurity atoms comprising the defect are only weakly coupled dynamically. Ab initio cluster calculations of the local mode frequencies of the defect are performed. Several models are investigated, and the model consisting of a bridging O atom adjacent to the N pair defect accounts for its dynamic properties.

Vacancy- and acceptor-H complexes in InP

It has been suggested that iron in InP is compensated by a donor, related to the local vibrational mode and previously assigned to the fully hydrogenated indium vacancy, . Using AIMPRO, an ab initio local density functional cluster code, we find that acts as a single shallow donor. It has a triplet vibrational mode at around this value, consistent with this assignment. We also analyse the other hydrogenated vacancies , and determine their structure, vibrational modes, and charge states. Substitutional group II impurities also act as acceptors in InP, but can be passivated by hydrogen. We investigate the passivation of beryllium by hydrogen and find that the hydrogen sits at a bond-centred site and is bonded to its phosphorus neighbour. Its calculated vibrational modes are in good agreement with experiment.

Structure and energy level calculations of dislocations in gallium arsenide

Abstract The geometry of dislocations in the sphalerite structure is described and illustrated. Particular attention is given to the dissociated set of dislocations. Calculations of the energy levels of the 30° and 90° glide partials are made using a tight-binding Hamiltonian and a supercell containing a pair of dislocations of opposite sign. Two sets of parameters describing the Hamiltonian are used. Several models of the core structure of each partial are analysed. The results suggest that, at low temperature, core reconstructions occur leading to full bands separated from empty ones. These conclusions are compared with experimental results based on electrical, photo-conductivity, and particularly, velocity measurements on doped plastically deformed gallium arsenide. In general, the agreement between the theoretical results and experiments is satisfactory.

The nitrogen-pair oxygen defect in silicon

The nitrogen-pair oxygen defect in silicon has been studied by infrared absorption spectroscopy on samples implanted with various combinations of 14N, 15N, 160 and 170. The measurements give direct evidence for the involvement of nitrogen and oxygen in the defect and show that the impurity atoms comprising the defect are only weakly coupled. Ab initio cluster calculation on several models of the nitrogen-pair oxygen defect have been performed and are compared with experiment. Based on these investigations a model consisting of a bridging oxygen atom adjacent to the nitrogen pair is suggested.

Neutralisation of group vi donors by hydrogen in gallium arsenide

Abstract The infrared absorption of GaAs:S and GaAs:Te samples partially neutralised by hydrogen show two local modes with very similar frequencies. These modes are comparable to the ones already reported in GaAs:Se. These results are interpreted by assuming that neutralisation takes place by the formation of a bond between a Ga atom first neighbour of the chalcogen and a H atom in an antibonding location. This assumption is strengthened by ab initio calculations that provide also frequencies of the right order of magnitude.

Instabilities of simple models of C-Asi complexes in gallium arsenide

Irradiation of GaAs:C produces C-related defects called C(1) centres. These are analysed using first-principles theory. The authors find that the stable defect has C3v symmetry, in agreement with FTIR results, but two obvious models, one with Asi lying at an anti-bonding site to CAs and the other with the interstitial anti-bonding to an on-site Ga neighbour, are unstable. They find that the interstitial displaces one of the Ga atoms bonded to CAs into a bond-centred configuration, leaving Asi as an anti-site defect. The calculated vibrational modes of this structure are in reasonable agreement with the observed ones.

Theory of substitutional carbon and boron in silicon

Local density functional theory is used to evaluate the structures, vibrational frequencies and effective charges of the substitutional impurities C and B- in silicon. The method uses a 71 atom H-terminated cluster with Gaussian basis functions. The authors find the B-Si length to be 2.097 AA and the triplet local mode at 635 cm-1, which is within 12 cm-1 of the observed one. The C-Si length and triplet mode are 2.101 AA and 561 cm-1 respectively. This frequency is within 50 cm-1 of the observed local mode. The effective charges on the impurities show that substitutional C leads to twice as much local mode absorption as an equal concentration of B-.

Oxygen-Carbon, Oxygen-Nitrogen and Oxygen-Dimer Defects in Silicon

An ab initio local density functional cluster method, AIMPRO, is used to examine a variety of oxygen related point defects in silicon. In particular results are given for X-O n complexes where X is interstitial C, N or O. For n = 2, the first defect, C-O2 has been assigned to the P-centre giving a PL line at 0.767 eV and seen in Cz-Si annealed around 450°C. The second, N-O2, has properties consistent with a nitrogen related shallow thermal donor. We have also found that a (C-H)02i defect has very similar electronic properties, and this implies that shallow thermal donors do not have a unique composition. The structure and migration energy of the oxygen dimer is considered and the dimer is found to migrate very much faster than a single oxygen atom.

Theory of Si donor-acceptor complexes in GaAs

Local density functional cluster calculations are carried out on SiGa-As-X complexes where X=Li, Be, Cu, Zn and a vacancy. We find that the donor and acceptor move away from each other contrary to a simple argument based on a Coulomb attraction between them. This result explains the insensitivity of the Si-related local vibrational modes to the identity of the impurity and their similarity to the case when X is a vacancy.

Ab initio calculations of the structure and dynamical properties of boron double acceptors in GaAs

Ab initio local density functional cluster calculations are used to investigate the structure and dynamic properties of BAs in its various charge states in GaAs. The authors find B- - is an on-site defect with a triplet mode at 604 cm-1 (11B). This is to be compared with an observed mode at 601 cm-1. B- moves slightly off-site along (111) away from one of its Ga neighbours, whereas the neutral defect, B0, suffers a large distortion along this direction. E-vibratory modes of the neutral defect are higher than the modes of B - - but their effective charge is only about half that of B --. -. The singly charged defect, B-, may tunnel between its four equivalent locations. Explanations are advanced for the loss of the infrared absorption due to BAs when the Fermi level drops below about Ev+0.2 eV.