Surface mound formation and growth via step destabilization under a direct current electric field on (001) strontium titanate single crystals

Abstract

The specific heat-treatment conditions are crucial for facilitating surface-diffusion processes that lead to the reconstruction of surface structures in oxide single crystals. This reconstruction gives rise to a step-terrace structure, which is characterized by steps with heights equivalent to the unit cell and atomically flat terraces. A method using a power spike (i.e., a flash event) generated under an applied electric field above the threshold value has emerged as a promising approach for promoting such surface reconstruction. In this study, we performed Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) to examine the (001) surface structure of a strontium titanate single crystal treated with a flash event induced by a Direct Current (DC) electric field. We observed the formation of a step-terrace structure on the (001) surface even with a short holding time of 1 min under the flash event (initial voltage of 300 V/cm at a limiting current of 35 mA). However, a surface mound with a height of approximately 5.5 nm also formed on the surface, the height of which increased with holding time. SEM and AFM observations of the same surface location revealed that the formation of the surface mound is due to the formation of anti-bands as well as the bunching of surface steps, which is caused by their destabilization under the DC electric field. To avoid the formation of surface mounds, we suggest using an alternating electric field to trigger the flash event, which does not destabilize the steps.