In Situ Control of Oxygen Vacancies in TaO_x Thin Films via Plasma-Enhanced Atomic Layer Deposition for Resistive Switching Memory Applications

The plasma-enhanced atomic layer deposition (PEALD) process using Ta(OC₂H₅)₅ as a Ta precursor and plasma-activated hydrogen as a reactant for the deposition of TaO_x films with a controllable concentration of oxygen vacancies (V_O) is reported herein. The V_O concentration control was achieved by varying the hydrogen volume fraction of the hydrogen-argon mixture in the plasma, allowing the control of the leakage current density in the tantalum oxide films within the range of 5 orders of magnitude compared with the Ta₂O₅ film grown via thermal ALD using the identical Ta precursor and H₂O. Temperature-dependent current-voltage measurements combined with Poole-Frenkel emission modeling demonstrated that the bulk trap depth decreases with the increasing hydrogen volume fraction, which could be attributed to the increase of the V_O concentration. The possible chemical change in the PEALD TaO_x films grown under different hydrogen volume fractions was confirmed by the in situ X-ray photoelectron spectroscopy (XPS) measurements of the Ta 4f core and valence band spectra. The comparison of the XPS-measured nonstoichiometry and the secondary ion mass spectrometry analysis of the hydrogen content allowed this study to conclude that the nonstoichiometry is largely related to the formation of Ta-V_O sites rather than of Ta-H sites. Such oxygen-deficient TaO_x layers were studied for application as an oxygen-deficient layer in a resistance switching random access memory stack (Ta₂O₅/TaO_x) where the actual switching occurred within the stoichiometric Ta₂O₅ layer. The bilayer memory stack showed reliable resistance switching up to ∼10⁶ switching cycles, whereas the single-layer Ta₂O₅ memory showed only several hundred switching cycles.