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(OC 2 H 5 ) 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 Ta 2 O 5 film grown via thermal ALD using the identical Ta precursor and H 2 O. 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 (Ta 2 O 5 /TaO x ) where the actual switching occurred within the stoichiometric Ta 2 O 5 layer. The bilayer memory stack showed reliable resistance switching up to ∼10 6 switching cycles, whereas the single-layer Ta 2 O 5 memory showed only several hundred switching cycles.