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

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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.