Complete stabilization and improvement of the characteristics of tunnel junctions by thermal annealing

Abstract

We have observed that submicron sized Al-AlOx-Al tunnel junctions can be stabilized completely by annealing them in vacuum at temperatures between 350 • C and 450 • C. In addition, low temperature characterization of the samples after the annealing treatment showed a marked improvement of the tunneling characteristics due to disappearance of unwanted resonances in the current. Charging energy, tunneling resistance, barrier thickness and height all increase after the treatment. The superconducting gap is not affected, but supercurrent is reduced in accordance with the increase of tunneling resistance.Submicron sized metal-insulator-metal (MIM) tunnel junctions have been used widely in applications such as radiation detectors[1], single electron transistors (SET) [2], and tunnel junction coolers and thermometers [3]. They are also the central element in superconducting quantum bits [4,5,6,7,8], and magnetic junctions [9] are developed for memory applications. However, there are often problems with stability of the junctions in room air conditions, the most noticable being aging, i.e. a slowly creeping increase of the tunneling resistance R T with time. Instability of the junctions is an obvious drawback when considering practical and commercial applications of tunnel junctions.One possible reason for aging is the absorption of unwanted molecules into the barrier. Direct knowledge of these processes is sketchy, although it is known that vacuum conditions can slow down aging significantly. [10] Another possibility is the relaxation of the oxide barrier properties. Typically, tunnel barriers in MIM junctions are fabricated by room temperature thermal oxidation of a disordered base electrode. This procedure results in formation of a polycrystalline or amorphous oxide layer, in which atoms are not in their global equilibrium positions. The relaxation from this "glassy" state could take months at room temperature. Moreover, in AlO x barriers studied here, chemisorbed oxygen ions bound on the barrier surface during fabrication have also been shown to be a source of instabilities. [11] A typical remedy for finding a better energy minimum is thermal annealing, the artificial acceleration of the relaxation processes by heating up the sample for some time and then letting it cool again. For technologically important Nb/Al-AlO x /Nb junctions, annealing treatments in gas atmospheres (He, N 2 , Ar, air) were shown to change the barrier properties, always increasing R T . [12,13,14,15] On the other hand, for magnetic tunnel junctions, improvements in their performance were obtained with vacuum anneals. [16,17] In this letter we discuss how Al-AlO x -Al tunnel junctions can be completely stabilized, and their characteristics improved by thermal annealing in vacuum. The general trend of increase of R T with the annealing is reproduced. The bias voltage dependence of the tunneling conductance reveal that the junction characteristics become more ideal after the anneal. In addition, the charging energy, the effective barrier thickne...

Aging- and annealing-induced variations in Nb/Al–AlOx/Nb tunnel junction properties

“…. 275°C, the tunnel barrier gets thicker either due to ͑additional͒ oxidation of the Al layer by chemisorbed oxygen, 7,13 or by adsorbed water molecules ͑hydroxyl groups͒. 3,[14][15][16] This was supported by Raman spectroscopy study, 17 showing decay of Al-OH peak and corresponding rise of Al-O peak as a result of Nb/ Al-AlO x / Nb junction annealing.…”

Double oxidation scheme for tunnel junction fabrication

“…It relaxes to the stoichiometric stable Al 2 O 3 only after some time when oxygen has diffused from the surface into the underlying aluminum, 5 which has been brought forward as a plausible explanation for aging. It was recently reported that annealing of MIM tunnel junctions stabilizes the junctions, 3 lending some support to the idea that aging is related to relaxation of AlO x to Al 2 O 3 . One method to improve the quality of tunnel junctions is to form the dielectric layer in several steps.…”

“…Hence, a change of resistance of tunnel junctions over time ͑i.e., aging effects͒ has been observed for both large area tunnel junctions 2 and for small ones. 3,4 It is believed that the standard process for fabricating AlO x produces an oxygen rich compound near to the surface. It relaxes to the stoichiometric stable Al 2 O 3 only after some time when oxygen has diffused from the surface into the underlying aluminum, 5 which has been brought forward as a plausible explanation for aging.…”

Tunneling through Al/AlOx/Al junction: Analytical models and first-principles simulations

“…The process of oxidation consists of a quick chemisorption of oxygen on a clean Al surface which is followed by a complex diffusion process leading to various widths of the interface which is typically disordered 22,[32][33][34] . The model that we consider is on the other hand relatively simple and ordered.…”

“…Nevertheless, in many experiments 8,9,11,15,[20][21][22] the studied interfaces have widths within the reach of ab initio simulations so that the accuracy of the potential barrier model to the interpretation of tunneling data can be tested. Specifically, Jung et al 15 presented such a study comparing the character of the equilibrium projected density of states of the Al/AlO x /Al interface obtained by a first principles simulation with the potential barrier model.…”