High Tunneling Magnetoresistance in Magnetic Tunnel Junctions with Subnanometer Thick Al₂O₃ Tunnel Barriers Fabricated Using Atomic Layer Deposition

Pinhole-free and defect-free ultrathin dielectric tunnel barriers (TBs) are a key to obtaining high-tunneling magnetoresistance (TMR) and efficient switching in magnetic tunnel junctions (MTJs). Among others, atomic layer deposition (ALD) provides a unique approach for the fabrication of ultrathin TBs with several advantages including atomic-scale control over the TB thickness, conformal coating, and a low defect density. Motivated by this, this work explores the fabrication and characterization of spin-valve Fe/ALD-Al2O3/Fe MTJs with an ALD-Al2O3 TB thickness of 0.55 nm using in situ ALD. Remarkably, high TMR values of ∼77 and ∼90% have been obtained, respectively, at room temperature and at 100 K, which are comparable to the best reported values on MTJs having thermal AlOx TBs with optimized device structures. In situ scanning tunneling spectroscopy characterization of the ALD-Al2O3 TBs has revealed a higher TB height (Eb) of 1.33 ± 0.06 eV, in contrast to Eb ∼ 0.3–0.6 eV for their AlOx TB counterparts, indicative of significantly lower defect concentrations in the former. This first success of the MTJs with subnanometer thick ALD-Al2O3 TBs demonstrates the feasibility of in situ ALD for the fabrication of pinhole-free and low-defect ultrathin TBs for practical applications, and the performance could be further improved through device optimization.