Microscopic Stress Analysis of BaTiO3 Using Optical Activity of Its Point Defects

Abstract

Microscopic stress analysis of BaTiO3 thin films was performed on the basis of the optical activity of their point defects by means of piezo-spectroscopic (PS) method applied to cathodoluminescence (CL) measurements. Accordingly, the interfacial residual stress that accumulates during manufacturing between a BaTiO3 film and its sapphire substrate could be estimated. Taking advantage of the dependence of the penetration depth of the electron beam in CL measurement on accelerating voltage, CL spectra collected as a function of accelerating voltage could be considered to represent an in-depth profile. In this context, an approximately 2 nm CL spectral shift between the film surface and interface was observed. The accelerating voltage that corresponded to the CL spectrum in the vicinity of the interface was chosen on the basis of the appearance of the sapphire F-band at approximately 330 nm. A PS coefficient was measured using CL spectra from a BaTiO3 single crystal, and a three-dimensional PS coefficient, Π=21.8 nm/GPa, could be retrieved. Because of the biaxial nature of interfacial stress, an averaged biaxial PS coefficient (2/3Π=14.5 nm/GPa) was employed for calculating interfacial stress from the observed spectral shift. This procedure allowed us to measure a maximum tensile stress of 140 MPa near the film/substrate interface in the BaTiO3 film. FEM simulation was also performed, and the results were compared with those of CL measurements.