Modification of (Pb,La)(Zr,Ti)O3 thin films during pulsed laser liftoff from MgO substrates

Abstract

Pulsed excimer laser irradiation of an absorbing thin film through a transparent substrate has been employed to separate epitaxial (Pb,La)(Zr,Ti)O3 (PLZT) thin films from their MgO growth substrates. The morphology, microstructure, and dielectric properties of the laser-modified PLZT layer generated at the original PLZT/MgO interface by this laser liftoff (LLO) process was investigated. Atomic force microscopy measurements of the film surface after LLO indicated average surface roughness on the order of 90–100 nm, complementing previously reported scanning electron microscopy studies that revealed what appeared to be a rough glassy layer. The surface roughness is thought to form as the original growth substrate is ejected from the films transferred towards the end of the 38 ns pulse, pulling the molten near-interfacial portion of the PLZT film via surface tension, and followed by quenching of the liquid phase. Transmission electron microscopy of PLZT films transferred to Si wafers showed the surface layer consists of an irregular amorphous layer ranging in thickness from 50 to 100 nm, although some surface features protruded as far as 300 nm. Enhanced levels of lead were found to be present in the amorphous phase by Rutherford backscattering spectrometry, implying PbO evaporation and subsequent condensation during LLO. Electrical measurements determined the relative dielectric constant of the surface layer to be in the range of 40 – 70. Subsequent ion milling of the layer demonstrated that the ferroelectric and dielectric properties of the original PLZT films were largely recovered, while the surface roughness was reduced to below 40 nm. The observed differences between the dielectric and ferroelectric properties of the film transferred after complete removal of the amorphous layer compared to those of the film on the original MgO substrate are attributed to the removal of substrate clamping.