DIVACANCY ACCEPTOR LEVELS IN ION-IRRADIATED SILICON

High-purity n-type silicon samples have been irradiated with mega-electron-volt ions (H-1(+), He-4(2+), O-16(4+), S-32(7+), Br-79(8+), and I-127(10+), and the two divacancy-related acceptor levels approximately 0.23 and approximately 0.42 eV below the conduction band (E(c)), respectively, have been studied in detail using deep-level transient spectroscopy (DLTS). Depth concentration profiles show identical values for the two levels at shallow depths, while in the region close to the damage peak large deviations from a one-to-one proportionality are found. These deviations increase with ion dose and also hinge strongly on the density of energy deposited into elastic collisions per incoming ion. Evidence for a model of the two levels is presented and, in particular, the model invokes excited states caused by motional averaging and lattice strain associated with damaged regions. The divacancy center is known to exhibit a pronounced Jahn-Teller distortion at low temperatures (less-than-or-equal-to 20 K), and three equivalent electronic distortion directions exist. However, at higher temperatures (less-than-or-equal-to 30 K) reorientation (bond switching) from one distortion direction to another takes place; in a perfect lattice the reorientation rate ultimately becomes so high that the defect does not relax in the distorted configurations, and a motionally averaged state with an effective point-group symmetry of D3d appears. At the temperatures where the DLTS peaks at E(c) - 0.23 and E(c) - 0.42 eV are observed, the reorientation time for bond switching is several orders of magnitude smaller than the time for electron emission from the two levels. This implies strongly that the levels originate from the motionally averaged state and not from the distorted state. Consequently, a clear distinction must be made between these DLTS peaks and the charge-state transitions observed in low-temperature studies where the divacancy is frozen in one of the three equivalent distorted configurations. Finally, the association of electronic energy levels with motionally averaged states is expected to apply not only for the divacancy but also for other defects where dynamic effects occur, e.g., the monovacancy and the E center.