Investigation of thickness-dependent stress in PbTiO3 thin films

Abstract

X-ray diffraction (XRD) and Raman spectroscopy were used to investigate stress dependence on thickness in PbTiO3 (PTO) films grown by pulsed liquid injection metalorganic chemical vapor deposition on a LaAlO3 (001) substrate (LAO). Films on sapphire substrate (R plane) were used as the polycrystalline film reference. Epitaxial PTO films with a dominant c domain structure are grown on LAO substrate, whereas the films on sapphire are polycrystalline. A detailed investigation of the PTO/LAO film microstructure by XRD gives evidence of PTO twinning. Both techniques reveal that PTO films are under tensile in-plane stresses. The study of the film thickness dependence of microstrains, grain size, volume fraction of a domains, as well as surface morphology of PTO/LAO films indicates that these parameters are clearly correlated. A change in the relaxation mechanism between 65 and 125 nm of film thickness has been evidenced. A c parameter gradient occurs throughout the film depth; it originates in stress relaxation due to an increase in thickness. Raman spectra of PTO films allowed in-plane residual stress values to be estimated from the Raman shifts, and are in good agreement with those determined by XRD. Both techniques also indicate that thinner films are more stressed and residual stresses are partially relaxed with increasing thickness. Moreover, a domains are more stressed than c domains. The two components of the large A1(2TO) and A1(3TO) Raman modes have been associated with a and c domains and their intensity ratio clearly correlated with the volume fraction of a domains.