In situ atomic layer deposition and electron tunneling characterization of monolayer Al 2 O 3 on Fe for magnetic tunnel junctions

Abstract

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 Al 2 O 3 TBs both directly on Fe films and with an ultrathin Al wetting layer. In situ characterization of the ALD Al 2 O 3 TB was carried out using scanning tunneling spectroscopy (STS). Despite a moderate decrease in TB height E b with reducing Al wetting layer thicknesses, a remarkable E b of ∼1.25 eV was obtained on 1 Å thick ALD Al 2 O 3 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.