Nondestructive investigation of microstructures and defects at a SrTiO₃ bicrystal boundary

The effects of thermal annealing on the microstructure at the grain boundary of a 36.8° symmetric [100] tilt SrTiO₃ bicrystal were studied. Scanning tunneling microscopy and atomic force microscopy were used for nondestructive observation of the boundary structures. Annealing the bicrystalline substrates at temperatures as low as 780°C led to the formation of grooves at their boundaries. This provides direct evidence that the thickness depression of YBa₂Cu₃O_7-δ films at the bicrystal boundaries originates from the underlying grooved substrates. Defects characterized as holes with diameters ranging from ∼30nm to ∼200nm were also observed.