Degradation of MgB2 Ultrathin Films Under Different Environmental Conditions

MgB2 ultrathin films have been recently recognized to have potential for making sensitive superconducting devices working at relatively high temperatures. Investigation on the stability of MgB2 ultrathin films under various environmental conditions should be helpful in evaluating their prospect in practical device applications. Here, we present a comparative study on the degradation of MgB2 films of thicknesses of similar to 15 nm grown by hybrid physical-chemical vapor deposition method and then placed at room temperature under three different kinds of environments, i.e., in air, in a desiccator with reduced humidity, and in a vacuum chamber with further reduced atmospheric pressure. The study was performed by measuring the normal-state resistance and the superconducting properties such as the superconducting critical temperature T-c and the upper critical field H-c2 of the films after different storage times, in conjunction with supplemental scanning electron microscopy and X-ray diffraction measurements. It has been found that, compared with the film exposed to air that started to show apparent degradation when the storage time was beyond about one month, the films kept in a desiccator or low vacuum exhibited less degradation. In particular, the film in low vacuum remained stable throughout the measured time period of similar to 4 months. The results suggest that the degradation of MgB2 ultrathin films in ambient environment may mainly arise from the interaction of the sample with the moisture in the air and that, in a dry air or preferably vacuum or an N-2 environment, MgB2 ultrathin films can maintain the reasonable long-time stability for superconducting applications.