Experimental observations of temperature-dependent flat band voltage transients on high-k dielectrics

In this work, we show the existence of flat band voltage transients in ultrathin Gd2O3 dielectric films on silicon, one of the high-k dielectrics nowadays proposed to substitute silicon oxide as the dielectric gate on the future complementary metal-oxide-semiconductor circuit generations. These transients were obtained by recording the gate voltage while keeping the capacitance constant at the value measured at initial flat band condition. The dependencies of transient time constant and amplitude on dielectric thickness and temperature suggest that there are tunnelling assisted processes involved. Time constant appears to be independent on the temperature, whereas the amplitude of the transients is thermally activated with energies in the range of soft-optical phonons usually reported for high-k dielectrics. In the case of gadolinium oxide a phonon energy of 55 ± 10 meV has been obtained. Leakage current behaviour at high electric fields confirms that conduction is governed by a phonon-assisted tunnelling mechanism between localized states in the band gap of the insulator.