Hydrogen and fundamental defects in electron-irradiated high-purity silicon

The dynamics of various fundamental defects in electron-irradiated high-purity silicon detectors (diodes) were investigated by deep-level transient spectroscopy (DLTS). Samples with various oxygen concentrations were used and hydrogen was intentionally introduced into some samples. The defect dynamics was investigated by first creating defect centres by irradiating the diodes with 6-MeV electrons and subsequently annealing the samples, isochronally or isothermally, at increasingly higher temperatures up to 400 °C while measuring the concentration of the various electrically active defects by DLTS. Based on the temperature- and time-dependent changes of the concentration of the defects, models explaining the observations were suggested. In one study, the annealing of the di-vacancy-oxygen (V2O) centre was investigated and modelled, and it was concluded that this centre anneals out through a dissociation resulting in a vacancy-oxygen (VO) centre. The binding energy between the vacancy and the VO centre was estimated to be ~1.7 eV. In another investigation, a defect centre annealing out after a few weeks at room temperature was found to have two energy levels in the band gap: one, labelled E4, 0.37 eV below the conduction-band edge (Ec) and the other, labelled E5, 0.45 eV below Ec. Comparison with annealing studies performed with Fourier-transform infra-red spectroscopy (FTIR) suggested that the defect may be a di-interstitial-oxygen (I2O) complex. E5 is known to correlate with the leakage current of silicon detectors, and it was suggested that the oxygen concentration should be minimised to reduce the formation of I2O centres and thus reduce the leakage current. Several annealing studies with hydrogenated samples were performed. These resulted in the identification of a hydrogen-related level at Ec - 0.37 eV as a vacancy-oxygen-hydrogen centre, which was labelled VOH*. This centre was seen to form when positively charged hydrogen diffused in from the surface of the silicon diodes and reacted with the VO centre at depths with locally high hydrogen concentration. VOH* was seen to break up when the hydrogen diffusion had resulted in a lower, more uniform hydrogen concentration. Possible identification of other hydrogen-related defect levels were also put forward; in particularly a hole trap located 0.23 eV above the valence-band edge which is suggested to be a di-vacancy-hydrogen (V2H) centre. Secondary-ion mass spectrometry (SIMS) was used to measure the depth profile of hydrogen in some of the samples