In vacuo atomic layer deposition and electron tunneling characterization of ultrathin dielectric films for metal/insulator/metal tunnel junctions

Abstract

Metal-insulator-metal tunnel junctions (MIMTJs) are an enabling technology for future electronics including advanced computing, data storage, sensors, etc. MIMTJs are formed by inserting an ultrathin insulating layer, known as the tunnel barrier (TB), between metal electrodes. Devices based on MIMTJs have advantages of enhanced quantum coherent transport, fast speed, small size, and energy efficiency. The performance of MIMTJs depends critically on the thickness and quality of the tunnel barrier. Specifically, the tunneling current, for example, the superconducting critical current in superconductor-insulator-superconductor Josephson junctions (JJs) or the spin tunneling current in ferromagnetic-insulator-ferromagnetic magnetic tunnel junctions (MTJs), decreases exponentially with the TB thickness. This means thinner TBs would enable stronger coherent tunneling in MIMTJs. In addition, the defects in the TBs can degrade the quantum coherence of electrons (spins) of JJs and MTJs, respectively, resulting in decoherence and degraded performance of the MIMTJs. This justifies the urgent need in research and development of ultrathin (subnanometers to 1 nm) pinhole-free and defect-free TBs beyond the current state-of-the-art TBs of larger thickness (>1–2 nm) and high defect concentration made using thermal diffusion of oxygen or physical vapor deposition (PVD) including magnetron sputtering and molecular beam epitaxy. Atomic layer deposition (ALD) can provide a unique resolution to achieving ultrathin and defect-free dielectric TBs for high-performance MIMTJs for future electronics. In this article, a review on their recent effort in the development of in vacuo ALD for the fabrication of ultrathin TBs for JJs and MTJs is presented. A custom-designed system that integrates high-vacuum/ultrahigh-vacuum PVD, ALD, and scanning probe microscopy was established for in vacuo fabrication of MIMTJs and characterization of the electronic properties of ALD TBs including Al2O3, MgO, and Al2MgO4 on both superconductor metals (Al) and ferromagnetic metals (Fe and FeCoB). Capacitors with ALD dielectric of thickness in the range of 1–5 nm were also constructed for the characterization of the dielectric properties of the ALD TBs. The authors have found that the metal-insulator interface plays a critical role in controlling the quality of the ALD TBs including the tunnel barrier height, dielectric constant, electric breakdown, and uniformity. They have shown that JJs and MTJs with 0.1–1.0 nm thick ALD Al2O3 TBs can be obtained with highly promising performance. The result obtained suggests that the in vacuo ALD may provide a unique approach toward MIMTJs with an atomic-scale control of the device structure required for high-performance future electronics.