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 ALDAl2O3 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 lowdefect ultrathin TBs for practical applications, and the performance could be further improved through device optimization.
KEYWORDS: atomic layer deposition, ultrathin film, magnetic tunnel junction, tunneling magnetoresistance,
scanning tunneling spectroscopy