Titanium in silicon: Lattice positions and electronic properties

Abstract

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 °C. The electrical behavior of Ti atoms is found to be different in Cz- and FZ-Si annealed at 650 °C, although the Ti SIMS profiles are similar. It is argued that interstitial Ti atoms (Tii) in FZ-Si crystals interact with implantation-induced vacancies and take a substitutional position (Tis). No energy levels which can be assigned to Tis have been detected in this work or in previous experimental literature. However, previous calculations suggest that Tis 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 Tis has no levels in the band gap. The calculations show that Tii is more energetically favorable than Tis and that Tii binds more strongly to the silicon vacancy than interstitial oxygen does, explaining the observed differences between FZ- and Cz-irradiated materials.