B. S. Avset

Annealing of defects in irradiated silicon detector materials with high oxygen content

A deep level transient spectroscopy (DLTS) study of electrically active defects in electron irradiated silicon detectors has been performed. Two types of materials have been studied and compared: carbon-lean magnetic Czochralski (MCZ-) Si, and high purity, diffusion oxygenated float-zone (DOFZ-) Si. In both materials we observed an earlier reported shift in position of peaks associated with the divacancy (V2) at 250–325 °C, indicating a gradual transition from V2 to the divacancy–oxygen complex (V2O). Heat treatments at higher temperatures reveal a difference in annealing behaviour of defects in DOFZ- and MCZ-Si. It is observed that VO and V2O anneal with a higher rate in DOFZ-Si. The appearance of a hydrogen related level only in the DOFZ-Si reveals a small presence of H and it is suggested that the difference in annealing behaviour is due to defect interaction with H in the DOFZ-Si. Our findings also suggest that dissociation may be a main mechanism for the annealing of V2O in MCZ-Si.

On the formation of the L-centre in silicon during heat treatment in the temperature range 205–285°C

Annealing kinetics of electron-irradiation induced defects in n-type diffusion oxygenated float-zone silicon has been studied in the temperature-range 205–285°C. Previous deep level transient spectroscopy (DLTS) reports have established that an observed shift in the positions of two peaks related to the divacancy (V2), is due to the annealing of the divacancy and the formation of the divacancy–oxygen complex (V2O). In parallel to this transformation from V2 to V2O, a new defect of unknown identity, the so-called L-centre, forms with a level located at 0.36 eV below the conduction band edge. The L-level has a first order formation-kinetics in the temperature region studied; at 245–285°C the formation rate is very similar to the annealing rate of V2, while at lower temperatures the formation rate becomes lower with a relative difference by a factor two at 205°C. The Arrhenius plot for the L-level formation rate is not a straight line, indicating that the formation is controlled by at least two different processes. Kinetic modelling shows that the experimental data can be reproduced by a sequence of defect dissociation and migration, where the former limits at low temperatures (activation energy ~1.75 eV) and the latter at high temperatures (Ea~1.0 eV). Based on these results and other findings, the identity of the L-centre is discussed.

Rapid annealing of the vacancy-oxygen center and the divacancy center by diffusing hydrogen in silicon

In hydrogenated high-purity Si, the vacancy-oxygen (VO) center is shown to anneal already at temperatures below 200 deg. C and is replaced by a center, identified as a vacancy-oxygen-hydrogen complex, with an energy level 0.37 eV below the conduction-band edge and a rather low thermal stability. At long annealing times, the process is reversed and the concentration of the latter defect is reduced, while the VO center partly recovers. The divacancy (V{sub 2}) center anneals in parallel with the initial annealing of the VO center, and the loss in V{sub 2} exhibits a one-to-one proportionality with the appearance of a hole trap 0.23 eV above the valence-band edge attributed to a divacancy-hydrogen (V{sub 2}H) center.

Defect behaviour in deuterated and non-deuterated n-type Si

Deep level transient spectroscopy (DLTS) has been performed on deuterated and non-deuterated p + -n - -n + Si-diode detectors made from oxygenated, low-doped, high-purity FZ-Si wafers. The deuterated samples were exposed todeuterium plasma at 150 °C for 2 and 4 h, respectively. All the samples were then irradiated with 6-MeV electrons to a dose of 2 x 10 1 2 cm - 2 . In the as-irradiated state the concentration of electrically active defects is the same in all groups of samples. The samples were subsequently annealed isochronally for 15 minutes at temperatures from 100 to 400 °C. No difference in the DLTS spectra is seen for the samples after annealing up to 200 °C. After annealing at 200 °C and above, however, pronounced differences in the annealing behaviour of the defects are observed. In the non-deuterated samples, the peaks of the two known divacancy levels (V 2 ) shift slightly during the annealing steps up to 300 °C. This shift is, from previous reports, ascribed to the interaction of V 2 with interstitial oxygen which results in the divacancy-oxygen centres (V 2 O). In the deuterated samples the two V 2 centres are fully annealed at 250 °C, without the transition to V 2 O. In the sample deuterated for 4 h the disappearance of the VO peak, starting during the annealing at 200 °C, coincides with the appearance of two peaks with positions at 0.17 and 0.58 eV below the conduction band, respectively.