Detecting anomalous behavior in Si MOS capacitors

Si MOS capacitors are fabricated in order to assess electrical behavior. High-frequency capacitance-voltage measurements, performed in the 60 – 500 K range exhibit anomalous behavior both at low (60 – 200 K) and high (460 – 500K) temperatures. “Inversion” points on the C-V curves are identified for these ranges, which move towards positive values as temperature rises. This behavior is explainable, for the high-temperature region, by the significant increase in minority carriers. In order to elucidate low-temperature behavior, the flat-band voltage variation is correlated to the shift in relative Fermi level position. The analysis suggests an important density of interface states in the Si band gap, on different energy levels, mostly near the conduction band. These traps can be activated even at cryogenic temperatures and have a dominant effect on MOS structure capacitance, especially as the dopant carrier concentration drops, with freeze out occurring under 65K.