Laser-induced crystallization of copper oxide thin films: A comparison made between Gaussian and chevron-beam profiles provides a clue for the failure of Gaussian-beam profile

Abstract

The use of laser with a Gaussian-beam profile is frequently adopted in attempts of crystallizing non-single-crystal thin films; however, it merely results in the formation of poly-crystal thin films. In this paper, selective area crystallization of non-single-crystal copper(II) oxide (CuO) is described. The crystallization is induced by laser, laser-induced crystallization, with a beam profile in the shape of chevron. The crystallization is verified by the exhibition of a transition from a non-single-crystal phase consisting of small 100 nm x 100 nm grains of CuO to a single-crystal phase of copper(I) oxide (Cu2O). The transition is identified by electron back scattering diffraction and Raman spectroscopy, which clearly suggests that a single-crystal domain of Cu2O with size as large as 5 {\mu}m x 1 mm develops. Provided these experimental findings, a theoretical assessment based on a cellular automaton model, with the behaviors of localized recrystallization and stochastic nucleation, is developed. The theoretical assessment can qualitatively describe the laser beam geometry-dependence of vital observable features (e.g., size and gross geometry of grains) in the laser-induced crystallization. The theoretical assessment predicts that differences in resulting crystallinity, either single-crystal or poly-crystal, primarily depend on a geometrical profile with which melting of non-single-crystal regions takes place along the laser scan direction; concave-trailing profiles yield larger grains which lead to single-crystal while convex-trailing profiles results in smaller grains which lead to poly-crystal, casting light on the fundamental question Why does a chevron-beam profile succeed in producing single-crystal while a Gaussian-beam profile fails?