Interface trap transformation in radiation or hot-electron damaged MOS structures

The interface traps created by ionising radiation or hot-electron injection in MOS capacitors have been found to undergo significant changes with time over an extended period (many months). Immediately after radiation or hot-electron damage, an interface trap peak above the midgap (∼Ev + 0.75 eV) invariably appears. This peak (designated peak 1), along with its background, would continuously change with time after damage, and the detailed time-dependent behaviour depends on the surface orientation of the Si substrate, processing history, gate bias and sample temperature. For samples made on (100) substrates, three separable regimes have been observed: (i) latent generation (peak 1 and its background increase with time), (ii) defect transformation (peak 1 gradually converts into peak 2 below the midgap, resulting in a double-peak interface trap distribution) and (iii) room-temperature annealing (the overall density of interface traps decreases with time). The focus of this paper is on the defect transformation process. For samples made on (111) samples, on the other hand, the most salient feature is the gradual shift of the energy position of peak 1 toward the valence band, and eventually a single-peak residing below the midgap is observed. In contrast to the (100) results, no discernible double-peak distribution has been found in (111) samples. Results on (110) samples are qualitatively similar to those on (111) samples, whole (311) samples are similar to (100) samples. The various experimental parameters that affect the defect transformation process in both (100) and (111) samples will be discussed. While the (100) results are too complex to be explained satisfactorily based on existing theories at the present time, the (111) results will be interpreted in terms of the atomic relaxation of the dangling-bond defect at the (111)Si/SiO2 interface. (author)