A route to atomically flat TiO₂ terminated surfaces in SrTiO₃ avoiding HF acid

SrTiO₃, known as a quantum paraelectric, is a very interesting quantum system. At the same time it is a widely used substrate to grow epitaxial oxide thin films. For the growth of high quality films and superlattices, it is imperative to have an atomically flat starting surface with a known surface termination. While the main technique for obtaining quasi-ideal SrTiO₃ surfaces for growth of epitaxial thin films has been via an etching step that involves HF acid¹ followed by an anneal in flowing oxygen to around 1000 C, here we discuss an alternate process for obtaining atomically flat surfaces. Our process involves etching in a dilute HCl acid solution, which is known as a polishing etch for SrTiO₃, and eliminates the use of HF acid and the need for the high temperature (1000 C) anneal. We will compare atomic force microscopy and RHEED images of SrTiO₃ substrates prepared with our process involving dilute HCl acid with those prepared using the method involving HF acid. Our controlled experiments suggest that once heated to typical film-growth temperatures, this process provides extremely smooth surfaces with very sharp step edges that are mostly oriented parallel to the crystallographic axes.