(A) study on magnetic tunnel junctions with insulating barriers formed by ozone oxidation

Abstract

A magnetic tunnel junction (MTJ) essentially consists of two magnetic electrodes separated by an insulating layer through which electrons can tunnel. The crucial part in the making of a tunnel junction, whether for application in magnetic random access memories or in read heads for high density magnetic recording, is the growth of the extremely thin tunnel barrier layer in a range of 1-2 nm thickness. The present method for forming an insulating layer is plasma oxidation. Although it has been the most successful and widely used method until now, energetic particles such as $O^{2-}$ ions may result in difficulty in controlling the optimum oxidation time and in structural damage at the interface between Al and a bottom magnetic layer. In this study, for more precise control of oxidation of Al layers and maintenance of clean interfaces, an alternate method of growing the tunnel barriers was attempted, namely ozone oxidation. As the ozone oxidation is a plasma free oxidation process, it could eliminate the possible structural damage by an oxygen ion bombardment and give improved control over the tunnel barrier growth. In the first part, we report on the results of a study on the ozone oxidation of thin Al layers. The tunnel barriers of Al oxide were formed by ozone oxidation, and we investigated the growth characteristics of the Al oxide on the basis of the Cabrera-Mott theory. The Al oxide layers were formed under different bias condition; zero, +100 V or -100 V bias conditions to study the oxidation mechanism in ozone atmosphere. Both positive and negative bias accelerated Al oxidation. This indicated that both electron tunneling and ion migration were rate limiting steps in this oxidation process and the ion migration was more dominant than the electron tunneling. In the second part, magnetic tunnel junctions with tunnel barriers formed by ozone oxidation have been studied. The ozone oxidized junctions with a 1.8 nm Al layer thickness showed a TMR of about 3...