Magnesium Boron Oxide Tunnel Barriers
In this dissertation I investigate the materials physics of thin ?lm growth processes for magnetic tunnel junctions (MTJs) and Josephson junctions (JJs). The studies I present focus primarily on the chemical, electronic, and structural properties of the tunnel barriers and their interfaces with the adjacent electrodes. I developed a growth process for making MgO (MgBO)-based MTJs, studied this ?lm stack in detail, and also examined the materials properties of MgB2 oxidation processes and AlN tunnel barrier formation for JJ development. I conducted x-ray photoelectron spectroscopy (XPS) studies on CoFeB / MgO bilayers to explore the MgO growth process. MgO that is rf sputtered directly on CoFeB or on a thin Mg protective layer forms MgBO with promising physical and chemical properties. Post-growth annealing reduces Fe and Co oxides formed in the deposition process through a reaction of B from the electrode with O in the transition metal oxides. Annealing also causes an atomic rearrangement of the Mg, B, and O species in these barriers. In sputtered MTJs with thin barriers, reaction between B from the electrode and sputter deposited MgO is an inherent part of the formation of MgO-based MTJs. I studied the correlated results of scanning tunneling electron microscopy (STEM) utilizing electron energy-loss spectroscopy (EELS), scanning tunneling spectroscopy (STS), current-in-plane tunneling (CIPT), and magnetometry studies. These investigations show that MgBO barriers have fewer low energy defect states than MgO barriers and comparison of MTJs with MgBO barriers with MTJs with Mg/MgO bilayer barriers shows that MgBO barriers yield higher TMR values and lower RA values than MgO barriers of comparable thickness. MgBO MTJs are also compatible with permalloy-B (PyB) free electrodes that show desirable magnetic characteristics. The electrode B content is important for the formation of low defect MgBO barriers and for the use of superior Py-based electrode materials for spin torque magnetic random access memory (MRAM) and magnetic sensor applications. I also studied MgBO barriers in an exploration of the oxidation of MgB2 thin ?lms. The oxidation of the MgB2 ?lm surface forms MgBO that is chemically similar to the MgBO materials formed in MTJ structures. Exposure to N2 or O2 promotes formation of MgO on the MgB2 ?lm surface which becomes completely composed of MgO if the oxidation process is carried out at elevated temperatures for an extended time. Lower temperatures form oxides similar to the native surface oxide, and promote formation of elemental B, and B sub-oxide near the ?lm surface. These B species are likely to e?ect JJs made with these barrier formation processes. These chemical studies provide insights into optimal MgBO barrier formation techniques for future MgB2 -based JJ devices. Finally, I started developing growth processes for AlN tunnel barriers formed both by N beam exposure of Al ?lms and by reactive rf sputtering of AlN in either Ar or N2 for use in Nb / AlN / Nb JJs. Both AlN growth processes introduce O into the AlN ?lm, which could possibly be controlled with the use of a getter material during barrier deposition. These XPS studies show that when a pure N2 atmosphere is used for reactive rf sputtering of AlN, the ?lm growth nitridizes the underlying Nb ?lm, similar to the way MgO oxidizes CoFeB. These studies provide some insights as to optimal AlN barrier formation for JJ structures that can now be further developed.
dissertation or thesis