Shallow energy levels induced by γ rays in standard and oxygenated floating zone silicon

Shallow defect levels in floating zone (FZ) and diffusion oxygenated FZ (DOFZ) silicon, before and after irradiation with a 60Co γ-source up to 300 Mrad, have been studied by thermally stimulated currents (TSC) and deep level transient spectroscopy (DLTS) in the temperature range 4.2–110 K. Besides vacancy oxygen (VO) and interstitial-substitutional carbon (CiCs) emissions, several TSC peaks have been observed. A trap with an activation energy of 11 meV has been observed at 6 K only in irradiated DOFZ. Two hole traps at 80 meV and 95 meV have been observed both in irradiated FZ and DOFZ, while a trap at 100 meV, related to an interstitial-oxygen (IO2) complex, has been revealed only in irradiated DOFZ. A TSC peak close to 24 K has been resolved into two components, whose concentrations are independent of irradiation fluence: a trap at 55 meV and a level which remains charged after emission at 80 meV. Our measurements confirm the formation, only in DOFZ, of a radiation induced donor at 230 meV. It appears to be responsible for the improved radiation hardness of oxygenated Si together with the suppression of deep acceptors, since no shallower radiation-induced donors have been detected in DOFZ samples.