Comparison of fast measurement methods for short-term negative bias temperature stress and relaxation

The initial degradation during negative bias stress is often assumed to be due to hole trapping, while the generation of interface states may dominate at longer stress times. We conduct a thorough study of short-time negative bias temperature stress and relaxation using ultra-fast measurement techniques in the micro-seconds regime to clarify the physical mechanisms behind the responsible hole trapping phenomenon. We observe that the extracted degradation of the drain-current DeltaI_D or the threshold-voltage DeltaV_TH can be well fitted by a logarithmic time dependence. Only for stronger stresses, that is, higher temperatures and/or voltages, the data shows a detectable deviation from the logarithmic behavior, allowing for a power-law fit. The exponent of this power-law is about 0.04 and thus considerably smaller than the typically reported long-term exponents of about 0.12 to 0.15. We finally observe a strong field- and temperature-dependence of the initial degradation, which is incompatible with the frequently assumed elastic hole trapping mechanism but favors a thermally activated hole trapping process.