Oxygen annealing modification of conduction mechanism in thin rf sputtered Ta2O5 on Si

The effect of oxygen annealing at high temperature (873, 1123 K; 30 min) on the insulating properties and conduction mechanism of rf sputtered Ta2O5 (25-80 nm) on Si has been investigated. It is found that the oxygen heating significantly reduces the oxide charge (Q(f) < 10(10) cm(-2)) and improves the breakdown characteristics (the effect is more pronounced for the higher annealing temperature). It is accompanied by an increase of the effective dielectric constant (up to 37 after 1123 K treatment). It is established that the influence of the oxygen treatment on the leakage current is different depending on the film thickness, namely: a beneficial effect for the thinner and a deterioration of leakage characteristics for thicker (80 nm) films. A leakage current density as low as 10(-7) A/cm(2) at I MV/cm applied field for 26 nm annealed layers has been obtained. The current reduction is considered to be due to a removal by annealing of certain structural nonperfections present in the initial layers. Generally, the results are discussed in terms of simultaneous action of two opposite and competing processes taking place at high temperatures-a real annealing of defects and an appearance of a crystal phase and/or a neutral traps generation. The contribution of the neutral traps also is involved to explain the observed weaker charge trapping in the as-fabricated films compared to the annealed ones. The conduction mechanism of the as-deposited films is found to be of Poole-Frenkel (PF) type for a wide range of applied fields. A change of the conduction mechanism for the annealed films at medium fields (0.8-1.3 MV/cm) is established. This transition from PF process to the Schottky emission limited current is explained with an annealing of bulk traps (oxygen vacancies and nonperfect bonds). It is concluded that the dominant conduction mechanism in the intermediate fields can be effectively controlled by appropriate technological steps. (C) 2002 Elsevier Science Ltd. All rights reserved.