On the origin of the thermal-field asymmetry in ionic polarization/depolarization of oxide in Si-MOS structures

Abstract

Based on the concepts of the significant role of the surface neutralization of positive ions at the boundaries of the oxide layer in the processes of ion transport in an insulating gap of MOS structures, the origin of a well-pronounced asymmetry in the temperature and temporal characteristics of the volume-charge ionic polarization/depolarization of an insulator are analyzed. The neutralization of ions occurs owing to the tunneling capture of electrons from semiconducting and metallic contacts. Experimental data obtained in a wide range of variations in the gate potential Vg and related to the thermally stimulated and isothermal polarization of oxide in Si-MOS structures consistently support the asymmetry model that accounts for a higher degree of neutralization of ions and a higher coupling of ions to electrons at the metal surface (gate) than at the semiconductor surface. The transients exhibit three stages during polarization. The first of these is related to the transport of unneutralized (free) ions; in the initial stages of thermally stimulated and isothermal polarization for Vg=const, the ions move in the oxide ballistically. In the second stage, a transition from the mode of free-ion drift to the modes of hyperbolic and (or) exponential kinetics of relaxation is observed; in the latter case, the current becomes virtually independent of the field, temperature, and the rates of the field or temperature scans and becomes a single-valued function of actual time. In this case, the law of relaxation is defined by the rate of tunneling ionization of neutral associations of ion + electron and (or) by their diffusion and thermal decomposition in the bulk of the insulator.