Disorder, defects and bandgaps in ultra thin (001) MgO tunnel barrier layers

TL;DR

This study uses scanning tunneling spectroscopy to analyze how disorder, defects, and strain affect the electronic bandgap and defect states in ultra-thin MgO tunnel barriers grown on Fe, revealing the impact of thickness and lattice mismatch.

Contribution

It provides detailed insights into the electronic structure and defect states of ultra-thin MgO layers, highlighting the effects of thickness and strain on their bandgap and defect formation.

Findings

Thick MgO layers have a bulk-like band gap of 5-7 eV.

Thin MgO layers show defect-induced band tails extending to 0.5 V.

Vacancy defects are linked to lattice mismatch-induced strain.

Abstract

We report scanning tunneling spectroscopy studies of the electronic structure of 1.5 to 3 nm (001) textured MgO layers grown on (001) Fe. Thick MgO layers exhibit a bulk-like band gap, approximately 5-7 eV, and sparse, localized defect states with characteristics attributable to oxygen and, in some cases, Mg vacancies. Thin MgO layers exhibit electronic structure indicative of interacting defect states forming band tails which in the thinnest case extend to approximately 0.5 V of the Fermi level. These vacancy defects are ascribed to compressive strain from the MgO/Fe lattice mismatch, accommodated as the MgO grows.