Electronic structure calculations for vacancies and oxygen-related defects in semiconductors

摘要

Quantum mechanical plane-wave pseudopotential (PWPP) calculations are used to study properties of vacancies and oxygen-related defects in Si and GaAs. Total energies, atomic geometries, charge states, ionization levels, and local vibrational modes for the defects are reported.The convergence of electronic structure calculations with respect to supercell size is studied for the vacancy (V) and divacancy (V2) in Si. The negative-U behaviour has been explored in the case of V in Si. The doubly negative charge state of V is shown to have a split structure which can lead to ionization-enhanced diffusion of V. V2 in Si is shown to have four separate charge states in agreement with experiments. It is found that the neutral (V20) and negative divacancy (V2-) have a mixed structure, including both pairing and resonant-bond characters, V20 being more of the pairing type and V2- more of the resonant-bond type.The ionization levels, microscopic structures and local vibration (LV) modes are calculated for vacancy-oxygen (VO) defects in Si. The ionization level for VO is found near the computational conduction band and the charge-state induced shift in the LV frequency is predicted to be upwards as a function of defect level occupation, in agreement with experiments. VO2 is found to have two degenerate asymmetric stretching modes and thus only one observable mode despite the two oxygen atoms.The LV frequencies for electrically inert oxygen interstitials (Oi) and oxygen dimers (O2i) are presented. The computed asymmetric stretching frequency of the puckered Oi is found to be slightly underestimated compared to the experimental value. Two competing structures for O2i are found: the staggered Oi-Si-Oi conguration and the skewed Oi-Si-Si-Oi conguration. The changes in the LV frequency spectrum with isotopic substitutions of O are calculated, and the experimental frequencies are shown to originate from the staggered form of O2i. The effects of external pressure on structures and vibrational frequencies are reported.Various oxygen chain models for thermal double donors (TDDs) in Si are presented. The first three TDDs (TDD0-TDD2) are found to consist of one four-membered ring where two O atoms are bonded to two common Si atoms with one or two adjacent interstitial O atoms. The following TDDs (TDD3-TDD7) are found to consist of similar rings with flanking Oi atoms. The anomalously fast aggregation of oxygen may be explained by the diffusion of these structures. At the later stages shallow donors with a central "di-Y-lid" core are found to become energetically competitive with the ring structures.An isolated oxygen atom in GaAs is shown to occupy an interstitial Ga-O-As position and to be electrically inactive. The properties of the substitutional off-centered oxygen in arsenic vacancy (OAs) are shown to be at variance with the experimental results. A close similarity of the (AsGa)2-OAs complex with the experimentally observed Ga-O-Ga defect is found. Especially the negative-U and charge-state induced shifts in local vibrational frequencies are in close agreement with the experiments.