Atomically-thin Al2O3 dielectric films for metal-insulator-metal tunnel junctions

Abstract

Metal-Insulator-Metal tunnel junctions (MIMTJ) are a core building block for a variety of microelectronics including Magnetic Tunnel Junctions (MTJs) for magnetic memory and Josephson Junctions (JJs) for quantum computers. The performance of MIMTJ devices critically depends on the insulator which should have few defects and an atomic-scale thickness. However, the current state of the art insulators are both high-defect and atomic-scale (thermal or plasma assisted AlOx), or low defect and ultrathin (epitaxial MgO or Al2O3). In this work, we develop a novel Atomic Layer Deposition (ALD) process which enables the growth of atomically-thin and low-defect density Al2O3 for MIMTJ devices. Exceptional control of the metal-insulator interface is required to achieve this end as any interfacial layer (IL) which develops is catastrophic, introducing defects and impairing the insulator growth. Specifically, two critical issues of pre-ALD IL formation and ALD nucleation on the metal surface were resolved by integrating ALD with sputtering in situ under High Vacuum (HV) along with a pre-ALD H2O pulse to hydroyxlate the Al surface. Ab-initio molecular dynamics simulations were run to shed light on the mechanisms of IL formation in the HV environment and the hydroxylation of the metal surface using this pre-ALD H2O pulse. In tandem, in situ Scanning Tunneling Spectroscopy (STS) quantified the quality of the Al2O3 as the IL was systematical reduced by optimizing the pre-ALD H2O pulse, sample temperature, and pre-ALD heating time. After optimizations, STS revealed a remarkably high ALD Al2O3 tunnel barrier height which was constant down to the single monolayer scale of 1 ALD cycle with a band gap comparable to ultrathin epitaxial Al2O3. In addition, the highest known ALD Al2O3 dielectric constant, in the ultrathin thickness range, was measured in fabricated capacitors. Amazingly, capacitance fittings along with STS imaging discovered that the IL thickness is sub-monolayer after our optimizations. Thus this work has achieved the first atomically-thin and low defect insulator for MIMTJ devices. Fabricated JJs show promise and preliminary tests reveal that this in situ ALD Al2O3 process can be grown on other metals such as Fe, which is essential for MTJ devices.