Capture and release of electrons on Na⁺-related trapping sites in the SiO₂ layer of metal-oxide-semiconductor structures at temperatures between 77 and 296 °K

Abstract

Electron trapping and detrapping mechanisms on Na+ ions near the Si-SiO2 interface of metal-oxide-semiconductor (MOS) structures and the location of these captured electrons were studied as a function of temperature from 77°to 296 °K. Capacitance-voltage and photocurrent-voltage techniques have been used to show directly that the charge neutralization on the Na+ ions takes place spatially in the immediate vicinity of the ions near the Si-SiO2 interface. Experiments reported here indicated that Na+ ions neutralized at 77 °K by electron capture do not become charged again even under negative voltage stressing conditions if the sample is heated to 296 °K. This applies if the initial areal density of Na+ ions near the Si-SiO2 interface is less than ≊3.7×1012 cm-2. For densities greater than ≊3.7×1012 cm-2, Na + motion back to the Al-SiO2 interface under negative voltage bias can be observed due to incomplete neutralization. Other experiments have shown that the electron capture process on Na+ related sites (with capture cross sections of 2×10-19 and 5×10 -20 cm2) decreases with increasing temperature as the MOS is warmed from 77 °K and stops at ≊158 °K. However, electron capture resumes again when the sample is cooled. This phenomenon appears to depend somewhat on local electric fields near the Si-SiO2 interface and is reversed (trapping increases with temperature) for large initial Na+ ion densities (≊6.1×1012 cm-2). Several models are discussed which are consistent with the overall picture for the neutralization process, and a comparison of this work to the studies by others of the effect of Na+ on inversion layer electrons is made.