Change in Tunneling Spectrum of a Co/AlO x /Co Junction under Constant Voltage Stress
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1108-A09-04
Change in Tunneling Spectrum of a Co/AlOx/Co Junction Under Constant Voltage Stress Kouhei Horikiri and Kazuo Shiiki Department of Applied Physics and Physico-Informatics, Keio University, Yokohama 223-8522, Japan. ABSTRACT To investigate the aging effect of a tunneling junction under constant voltage stress, the tunneling resistance and inelastic electron tunneling (IET) spectra were measured. At a low voltage, up to about 0.5 V, the tunneling resistance of junctions increased gradually over time, while above about 0.6V, it decreased gradually. This change was observed independent of voltage polarity. When the applied voltage was positive, the IET spectrum did not change, whereas when it was negative, the IET spectrum changed—the asymmetric peak became symmetric. X-ray photoelectron spectroscopy (XPS) analysis showed that the junction exhibiting an asymmetric peak contained metallic Al at the AlOx/bottom electrode interface, and the junction exhibiting a symmetric peak contained a homogeneous barrier layer. An increase in the tunneling resistance indicates that the amount of metallic Al decreased in the AlOx barrier layer. Transformation from an asymmetric to a symmetric peak indicates that the metallic Al in the AlOx/bottom electrode interface was oxidized, which led to the AlOx layer becoming homogeneous. INTRODUCTION There has been considerable interest in using tunneling junctions in applications such as magnetic disk storage systems[1], magnetoresistive random access memories [2], and Josephson junctions. [3] However, the very thin insulator of tunneling junctions suffers from an aging effect and electrical breakdown under constant voltage stress [4–7]. Thus, reliability is an important issue for device applications. Although the tunnel resistance of a junction changes under constant voltage stress, the mechanism and polarity dependence of the aging effect is not clear. Inelastic electron tunneling (IET) spectroscopy is a powerful tool for detecting minute changes in the current–voltage (I–V) characteristics of devices [8–12]. In the present study, to investigate the aging effect of a tunneling junction under constant voltage stress, the tunneling resistance and IET spectra were measured. EXPERIMENT Tunneling junctions of Co (10 nm)/AlOx/Co (10 nm) were fabricated on a glass substrate by ion-beam sputtering. The bottom electrode was deposited as a stripe using a metal mask. After depositing a 2.5 nm thick Al layer on the bottom Co electrode, an insulator layer of AlOx was formed by thermal oxidation in pure O2 at 200°C. The O2 pressure during oxidation was 1 atm. The top electrode was deposited on the AlOx such that it crossed the bottom electrode. The junction area was approximately 0.04 mm2.
A constant voltage stress was applied to the tunneling junction and the time dependence of the tunneling resistance was observed. The tunneling resistance of junction was measured with the four-probe method. The I–V characteristics of the junction were measured by the four-probe method, before and after th
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