Crossover from direct to trap assisted Fowler Nordheim (FN) tunneling in CoFeB/MgO/CoFeB magnetic tunnel junctions

Abstract

Electron conduction mechanisms in CoFeB (0.8-10 nm)/MgO (3nm)/CoFeB (4.2 nm) magnetic tunnel junctions (MTJs) have been investigated in detail. A clear crossover from direct tunnelling to trap assisted Fowler Nordheim (FN) tunneling is observed with increasing bias voltage in all the pristine MTJs. In contrast, FN-like tunneling is completely suppressed in vacuum annealed MTJs, which plausibly indicates diffused interfaces and enhanced trap state density in MgO barrier. In annealed MTJs, beyond the direct tunneling regime, bulk-limited Pool Frenkel emission followed by interface-limited Schottky emission are found to be the dominant transport mechanisms. Simulations of tunnel current density J (V) and differential conductance (dJ/dV) of MTJs have been carried out using Simmons (symmetric rectangular barrier) and Brinkman (asymmetric trapezoidal barrier) model, which provides valuable insights on the barrier height and interface property at the MgO/CoFeB interface. Results of this study might be helpful to further improve CoFeB/MgO based MTJs for efficient implementation in sensors and memory devices.