Internal photoemission of electrons from Ta-based conductors into SiO2 and HfO2 insulators

Abstract

The origin of annealing-induced variations in the effective work function of Ta-based conductors (TaC, Ta2C, and TaNx) at interfaces with SiO2 and HfO2 dielectrics is investigated using the spectroscopy of internal photoemission (IPE) of electrons. Two physical mechanisms were considered that could affect the Ta-based metal/oxide barrier: (a) oxidation of the metal conductor after thermal treatment by oxygen supplied from the oxide layer, resulting in a variation in the density of electron states inside the metal emitter, and (b) annealing-induced variations in the barrier at the interface. Field-dependent and differential IPE experiments allowed us to identify the latter as the dominant mechanism causing the barrier instability after thermal treatment in nonoxidizing ambient. The incorporation of negatively charged centers in the near-interface oxide layers is the most likely explanation for the significant increase (∼0.6 eV) of the barrier height after annealing, leading us to the hypothesis that Ta may diffuse into the oxide layer, and thereby be at the origin of the negative charge creation.