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Magnetic tunnel junctions (MTJs), formed through sandwiching an ultrathin insulating film (so-called tunnel barrier or TB), with ferromagnetic metal electrodes, are fundamental building blocks in magnetoresistive random access memory (MRAM), spintronics, etc. The current MTJ technology employs physical vapor deposition (PVD) to fabricate either amorphous AlOx or epitaxial MgO TBs of thickness around 1 nm or larger to avoid leakage caused by defects in TBs. Motivated by the fundamental limitation in PVD in, and the need for atomically thin and defect-free TBs in MTJs, this work explores atomic layer deposition (ALD) of 1-6 Å thick Al2O3 TBs both directly on Fe films and with an ultrathin Al wetting layer. In situ characterization of the ALD Al2O3 TB was carried out using scanning tunneling spectroscopy (STS). Despite a moderate decrease in TB height Eb with reducing Al wetting layer thicknesses, a remarkable Eb of ∼1.25 eV was obtained on 1 Å thick ALD Al2O3 TB grown directly on an Fe electrode, which is more than twice of that of thermal AlOx TB (∼0.6 eV). Achieving such an atomically thin low-defect TB represents a major step towards improving spin current tunneling in MTJs. 

As metal/insulator/metal tunnel junctions (MIMTJs), such as magnetic tunnel junctions and Josephson tunnel junctions, push the insulating tunnel barrier (TB) towards the ultrathin regime (<1 nm) defects inherent in current physical vapor deposition methods become a fundamental obstacle to create pinhole-free and defect-free MIMTJs. Atomic layer deposition (ALD) could offer a solution by providing a conformal, leak-free tunnel barrier with low defect density and atomic thickness as demonstrated recently in ALD Al2O3 tunnel barriers. A question arises on the viability of the ALD TBs in practical circuits of multilayer structures on which increased roughness may occur. To answer this question, this work investigates electron tunneling properties of ALD Al2O3 tunnel barriers of 1.1 –1.2 Å in thickness on half-cell MIMTJs of Al/Fe/Nb fabricated on multilayer structures of different surface roughness using in situ scanning tunneling spectroscopy. Remarkably, the tunnel barriers grown on the raised multilayer device analogue only show a moderate decrease in barrier height from 1.63 eV, to 1.51 eV and to 1.27 eV as the surface roughness increases from 0.9 nm to 2.3 nm, and to 15 nm, alongside a slight decrease in ALD coverage from ∼96%, to ∼93% and 84% on these samples. Overall, these results validate the ALD TBs of atomic thickness for future 3D arrays of devices.