Dielectric loss and current—Voltage measurements in sodium-contaminated Si—SiO2—Cr structures

A detailed study of the small-signal ac response of ntentionally sodium-contaminated Si-SiO2-Cr structures has been made with mobile ion concentrations of 10¹⁰-10¹²ions/cm², in the frequency range of 0.05-100 Hz, between temperatures of 300 and 450°C. The time dependence of the observed relaxation phenomena, attributed to the existence of deep traps near the SiO2-Cr interface, is slow enough to consider the measured relaxation spectra as quasi static. The dc bias dependence of the relaxation mechanism, manifested by a loss maximum around 0.6 V and by a monotonic decrease of the absorption frequency with increasing bias, is ascribed to shallow traps located at the SiO2-Cr interface. The trapping as well as the long range migration of Na⁺ions in SiO2has been carefully studied by I-V and C-V measurements carried out on both contaminated and on ultraclean samples. The long-range motion of the ions leads to the thickness dependence of the relaxation time. However, since the ac response is determined in the two halves of the measuring cycle alternately by the migration of ions in the bulk of the SiO2and by their emission from the shallow traps, the relaxation time is also dependent on the effect of the traps. Consequently, the observed bias-dependent activation energies, ranging from 0.91 to 1.21 eV, may also be interpreted as intermediate values, arising from the conduction of Na⁺ions in SiO2films with an activation energy of less than 0.91 eV, and from the release of ions from the traps having activation energies equal to or greater than 1.21 eV.