J. Coutinho

Thermal double donors in Si and Ge

Abstract Three experiments appear to cast doubt on self-interstitial-free models of the family of thermal donors, based on oxygen defects of increasing size. These are (a) the rapid transformation of TDD(N) into TDD(N+1) with activation energies considerably lower than that of oxygen diffusion, (b) the lack of any appreciable spin-density on oxygen in TDD(N)+, and (c) the observation of only two oxygen related vibrational modes associated with each donor. However, we show that the oxygen-only model of the donors is compatible with experiment for a structure involving an insulating core with normal oxygen coordination, surrounded by over-coordinated oxygen atoms which are responsible for the donor activity. It is also shown that the calculated stress-energy tensors for the early donors are in good agreement with the measurements.

Isotopic effects on vibrational modes of thermal double donors in Si and Ge

Abstract The local vibrational modes of thermal double donors in Si and Ge are investigated by FTIR and ab initio modelling. At most, two oxygen modes are detected from each donor, which exhibit regular shifts with increasing donor number. By using mixtures of 16 O and 18 O , it is found that in Si the upper band does not yield any new modes suggesting that any oxygen atom is decoupled from any other. However, the lower frequency bands exhibit unique mixed modes proving for the first time that oxygen atoms are coupled together and in close spatial proximity. Ab initio calculations demonstrate that the modes and their isotopic shifts are consistent with a model involving two parallel chains of oxygen atoms linking second neighbour Si atoms, terminated by O-atoms close to the normal position for interstitial O.

Over-coordinated oxygen in the interstitial carbon–oxygen complex

Abstract The interstitial carbon–oxygen complex is one of the most prominent defects formed in e-irradiated Cz-Si containing carbon. Stress alignment investigations have shown that the oxygen atom only perturbs the carbon interstitial but the lack of a high frequency oxygen mode has been taken to imply that the oxygen atom is over-coordinated. Local vibrational mode spectroscopy and ab initio modeling are used to investigate the defect. We find new modes whose oxygen isotopic shifts, along with the piezoscopic stress-energy tensor support the trivalent model, thus providing evidence for oxygen over-coordination.

Local vibrational mode bands of V–O–H complexes in silicon

Abstract H 2 molecules, which are introduced into moderately doped silicon crystals by high-temperature in-diffusion from H 2 gas ambient followed by fast cooling to room temperature, are found to interact effectively with the defects induced by irradiation of the crystals with fast electrons. In Czochralski-grown silicon crystals, the interaction of the mobile H 2 molecules with vacancy-oxygen defects (A centers) leads to the creation of V–O–H 2 complexes. This complex gives rise to infrared (IR) absorption lines at 943.5, 2126.4, and 2151.5xa0cm −1 . Ab initio calculations showed that the most stable configuration of V–O–H 2 consists of one oxygen and two hydrogen atoms sharing a vacancy site. It is suggested that the interaction of the V–O–H 2 complexes with interstitial oxygen atoms results in the formation of V–O 2 –H 2 complexes, which are responsible for the IR absorption line at 891.5xa0cm −1 .