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

The plasma-enhanced atomic layer deposition (PEALD) process using Ta(OC2H5), as a Ta precursor and plasma-activated hydrogen as a reactant for the deposition of TaOx 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 TaOx 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 TaOx layers were studied for application as an oxygen-deficient layer in a resistance switching random access memory stack (Ta2O5/TaOx) where the actual switching occurred within the stoichiometric Ta2O5 layer. The bilayer memory stack showed reliable resistance switching up to similar to 10(6) switching cycles, whereas the single-layer Ta2O5 memory showed only several hundred switching cycles.