Nanochemistry, nanostructure, and electrical properties of Ta₂O₅ film deposited by atomic layer deposition and plasma-enhanced atomic layer deposition

Ta₂O₅ films were deposited by plasma-enhanced atomic layer deposition (PEALD) and thermal ALD on native oxide surface (SiO_x/Si). The properties of as-deposited and forming gas annealed films were examined and qualitatively compared with respect to nanostructural, nanochemical, capacitance-voltage and leakage-current–voltage (J_L-V), and oxide breakdown characteristics. Although high-resolution transmission electron microscopy showed structurally sharp Ta₂O₅/SiO_x interfaces in forming gas annealed PEALD Ta₂O₅/SiO_x/Si stacks, electron energy loss spectroscopy revealed interdiffusion of Ta and Si across this interface, the indiffusion length of Ta being higher than the outdiffusion length of Si. The consequent formation and enhancement of Ta–O–Si bond linkages in thicker Ta₂O₅ films were clearly reflected in the J_L-V data. Moreover, the fixed charge density (Q_f=5×10¹¹q C/cm^-2) was thickness invariant in PEALD Ta₂O₅. For similar PEALD and ALD Ta₂O₅ thickness in Ta₂O₅/SiO_x/Si stacks, the latter showed a lower D_it and higher defect density, - - results attributed to protons and hydroxyl groups, respectively, which stem from water used as an oxidant for the thermal ALD process.