Divacancy acceptor levels in ion-irradiated silicon

Abstract

High-purity n-type silicon samples have been irradiated with mega-electron-volt ions ('H+, He'+, ' 0, "S'+, ' Br"+, and '"I'"+), and the two divacancy-related acceptor levels -0.23 and -0.42 eV below the conduction band (E,. ), 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-toone 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 (~20 K), and three equivalent electronic distortion directions exist. However, at higher temperatures ( 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, -0.23 and E, -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.

Bulk damage effects in irradiated silicon detectors due to clustered divacancies

“…The energy levels for the electrons associated with the charge states of (0), (-) and (2-) are at E v +0.25 eV, E c -0.42 eV and E c -0.23 eV respectively. 50 The states closest to the conduction or valence band edges have the highest carrier emission rate probabilities, with an exponential dependence on E band -E trap . In high resistivity silicon under reverse bias, the neutral state of the divacancy is most probable, by a considerable amount.…”

Annealing kinetics of vacancy-related defects in low-dose MeV self-ion-implantedn-type silicon

“…These two centers dominate the majority carrier spectrum for n-type samples and are normally the prime defects after implantation at low doses, together with the interstitialcarbon-interstitial-oxygen complex (C i O i ) having a level at ϳ0.35 eV above the valence-band edge 7,32,33 ͑not shown here͒. The apparent deviation from a 1:1 ratio between the singly negative (V 2 Ϫ ) and doubly negative charge state (V 2 2Ϫ ) of V 2 is a characteristic feature of ion bombardment, 9,34 together with a highly nonexponential capture rate of the E C Ϫ0.23 eV level as a function of the fillingpulse duration.…”

In situ trap properties in CCDs: the donor level of the silicon divacancy

“…From an experimental viewpoint the divacancy has four charge states (+1, 0, -1, -2) [11] introducing in the band gap three deep energy levels [12]. These trap levels correspond to a singly ionised donor state (0/+), a singly ionised acceptor state (-/0) and doubly ionised acceptor state (=/-).…”