Ion transport phenomena in oxide layer on the silicon surface and electron–ion exchange effects at the SiO2/Si interface

Abstract

The results of ion depolarization experiments for SiO2 insulating layers on Si under isothermal and thermostimulated conditions have been analyzed. Additionally, the ion currents under the continuous change of gate voltage (dynamic current–voltage characteristics) have been investigated. It is shown that, depending on the type of experiment, an insulating layer shows itself either as an ion trap system with the wide distribution of lifetimes (energies of trap activation are distributed in an interval of 0.75–1.5 eV), or as a medium with free ions, which have a mobility μ (423 K)=2.7×10−8 cm2/V s and an activation energy of mobility Eμ=0.80±0.05 eV. These values correspond to those found for Na+ ions. The model resolving this apparent contradiction has been offered. Each ion and isolator matrix surrounding it are expected to generate a localized electronic state, but ion+electron neutral associates (NAs) are formed by filling these states owing to electron tunnel transitions from the semiconductor. NAs play the role of ion traps. The wide distribution of ion lifetimes on such traps is connected with spread of an electron tunnel length at the associate decay. The increase of NA ionization degree at the quasistationary electrical field change provides a smooth transition from a set of a few mobile NAs to an ensemble of free ions. The theory developed on the basis of model given and taking into account both NAs diffusion and their formation and decay processes allows the experimental data to be described qualitatively and quantitatively. Undamped circulation of particles in an oxide layer of a metal–oxide–semiconductor system under a stationary polarization electrical field is predicted and found: NAs formed as a result of ion neutralization at the semiconductor surface diffuse into the insulator volume, where they decay thermally to ions and electrons; the latter leave for a gate almost instantly compared to ion transport times; the ions formed again come back to the insulator–semiconductor interface under the electrical field action. The NA diffusion coefficient D(423 K)≈8.6×10−18 cm2 s−1 is a minimum on 8 orders of magnitude less than the free ion diffusion coefficient at the temperature 423 K.