Titanium in silicon: Lattice positions and electronic properties

摘要

Secondary ion mass spectroscopy (SIMS) and deep level transient spectroscopy measurements were carried out on Czochralski (Cz)- and float-zone-grown (FZ) Si crystals, which were implanted with Ti ions and annealed in the temperature range 600-900 ℃. The electrical behavior of Ti atoms is found to be different in Cz- and FZ-Si annealed at 650 ℃, although the Ti SIMS profiles are similar. It is argued that interstitial Ti atoms (Ti_i) in FZ-Si crystals interact with implantation-induced vacancies and take a substitutional position (Ti_s). No energy levels which can be assigned to Ti_s have been detected in this work or in previous experimental literature. However, previous calculations suggest that Ti_s is a deep acceptor in Si. We show from density functional calculations that by taking proper account of interactions within the d-shell of the Ti impurity the electronic structure of Ti_s has no levels in the band gap. The calculations show that Ti_i is more energetically favorable than Ti_s and that Ti_i binds more strongly to the silicon vacancy than interstitial oxygen does, explaining the observed differences between FZ- and Cz-irradiated materials.