Oxygen-related radiation-induced defects in SiGe alloys

Electronic properties of the vacancy-oxygen (V-O) complex and the interstitial oxygen and carbon atom pair (C_i-O_i) in unstrained Si_1-xGe_x crystals (0 < × ≤ 0.06) have been studied by means of capacitance transient techniques. The Si_1-xGe_x crystals were grown by the Czochralski method and were doped with either phosphorus or boron during growth. The V-O and C_i-O_i centres were introduced into the crystals by room temperature irradiation with 4 MeV electrons or with γ-rays from a ⁶⁰Co source. The enthalpy of electron ionization for the single acceptor level of the V-O centre relative to the conduction band edge, ΔH_n, was found to increase with a rate d(ΔH_n)/dx = 0.56 eV upon increase in Ge content. The enthalpy of hole ionization for the single donor level of the C_i-O_i centre relative to the valence band edge, ΔH_p, was found to decrease with a rate d(ΔH_p)/dx = -0.96 eV. For both the V-O- and C_i-O_i-related levels no significant changes in the values of the electron (hole) capture cross section or entropy of ionization with the changes in Ge content were observed. The levels are not pinned to the conduction band edge or to the valence band edge. It is argued that the value of the enthalpy of V-O ionization can be correlated with the lattice parameter or the Si-Si bond length: the larger this parameter, the bigger the enthalpy of the ionization.