Hochauflösende mikroskopische und spektroskopische Untersuchungen zur strukturellen Ordnung an MgO-CoFeB-Grenzflächen

Abstract

Magnetic tunnel junctions (MTJ), with crystalline MgO tunnel barrier sandwiched between amorphous CoFeB electrodes, received much attention due to their high tunnel magneto resistance (TMR) and flexible integration into mass-production processes. Previous calculations showed that the structural and chemical properties of the interface have a significant impact on the TMR. In the thesis presented here, this interface is analyzed by quantitative, high resolution and analytical transmission electron microscopy. In these systems, it is essential to have a crystalline transition zone between the tunnel barrier and the electrodes in order to achive high TMR. Calculations showed that already several monolayers of crystalline material at the interface will lead to high TMR. Based on these theoretical works, the microstructure of the crystalline/amorphous MgO/CoFeB-interface is investigated on the sub-nanometer scale. The experimental data is acquired by aberration-corrected, high resolution transmission electron microscopy (HRTEM) on a model system. The MgO-induced crystalline order at the interface to the CoFeB is quantified by an iterative image series matching process using simulated HRTEM-images. For the simulation of HRTEM interface images, the “averaged-projected-potential” approach is used and extended in order to analyze monoatomic steps at the interface in beam direction of the microscope. It could be demonstrated that this method is very suitable to describe the order at the MgO-CoFeB-interface of non-annealed samples. In annealed samples, boron diffusion leads to crystallization of CoFeB using the MgO as a template. In the second part of this thesis, the boron diffusion and crystallization of the CoFeB in dependence on the cap layer material and the MgO deposition method in model systems, as well as in functional MTJs is investigated. Electron energy loss spectroscopy (EELS) showed that the cap layer material as well as the MgO deposition method have a crucial influence on the boron-diffusion in this material.