Paradoxes of steady-state and pulse operational mode characteristics of silicon detectors irradiated by ultra-high doses of γ-rays

Detectors processed from standard and oxygenated Si are compared with respect to radiation hardness to ultra-high dose of γ-rays up to 1.76 Grad. The detectors are processed by different manufactures: Silicon Detector Development and Processing Lab (SDDPL) of BNL (USA), CIS (Germany) and Micron Semiconductor (UK). Oxygenation was performed either by using High Temperature, Long Time (HTLT) oxidation in oxygen-based ambient, or by short-time high-temperature oxidation in O2 followed by a long-time drive-in diffusion in N2. It has been shown that radiation hardness improvement in oxygenated detectors irradiated by γ-rays actually extends now up to ultra-high dose of 1.76 Grad. Effects of space charge sign inversion (SCSI) and linear build-up of negative space charge with γ-ray dose have been observed in standard Si detectors, similar to the case of neutron/proton irradiation. In contrast to standard Si in detectors, it has been revealed that in oxygenated Si detectors, positive space charge is accumulated with increasing dose up to 1.76 Grad with no SCSI (“positive space charge detectors”), which is unique for γ-irradiation. The advantage for using oxygenated Si detectors, as compared to standard Si detectors in practical applications in the ultra-high dose range of 1–1.76 Grad, has been demonstrated in terms of the profit in the reduction of full depletion voltage by a factor of 3–4, and in the reduction of leakage current by a factor of 3.2–5. In the pulse operational mode, however, polarization effect has been observed for oxygenated detectors irradiated to this ultra-high dose range of 1–1.76 Grad at room temperature, indicating the upper dose limit for a “damageless” oxygenated Si detector is about 1 Grad. The fact that this polarization occurs at RT is a paradox, since for other types of radiations it occurs only at cryogenic temperatures.