Bodeau William, Otoge Kaisei, Yeh Wenchang, Kobayashi Nobuhiko P
Electrical and Computer Engineering Department, Baskin School of Engineering, University of California Santa Cruz, Santa Cruz, California 95064, United States.
Graduate School of Natural Science and Technology, Shimane University, Matsue, Shimane 690-8504, Japan.
ACS Appl Mater Interfaces. 2022 Oct 27;14(44):49919-27. doi: 10.1021/acsami.2c11412.
The use of a laser with a Gaussian-beam profile is frequently adopted in attempts of crystallizing nonsingle-crystal thin films; however, it merely results in the formation of polycrystal thin films. In this paper, selective area crystallization of nonsingle-crystal copper(II) oxide (CuO) is described. The crystallization is induced by laser, laser-induced crystallization, with a beam profile in the shape of a chevron. The crystallization is verified by the exhibition of a transition from a nonsingle-crystal phase consisting of small (∼100 nm × 100 nm) grains of CuO to a single-crystal phase of copper(I) oxide (CuO). The transition is identified by electron back scattering diffraction and Raman spectroscopy, which clearly suggests that a single-crystal domain of CuO with a size as large as 5 μm × 1 mm develops. The transition may embrace several distinctive scenarios such as a large number of crystallites that densely form grow independently and merge, and simultaneously, solid-state growth that takes place as the merging proceeds reduce the number of grain boundaries and/or a small number of selected crystallites that sparsely form grow laterally, naturally limiting the number of grain boundaries. The volume fraction of the single-crystal domain prepared under the optimized conditions─the ratio of the volume of the single-crystal domain to that of the total volume within which energy carried by the laser is deposited─is estimated to be 32%. 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 polycrystal, primarily depend on a geometrical profile with which melting of nonsingle-crystal regions takes place along the laser scan direction; concave-trailing profiles yield larger grains, which lead to a single crystal, while convex-trailing profiles result in smaller grains, which lead to a polycrystal, casting light on the fundamental question
在非单晶薄膜结晶的尝试中，常采用具有高斯光束轮廓的激光；然而，这只会导致多晶薄膜的形成。本文描述了非单晶氧化铜（CuO）的选择性区域结晶。这种结晶是由具有人字形光束轮廓的激光诱导的，即激光诱导结晶。通过展示从由小（约100 nm×100 nm）氧化铜晶粒组成的非单晶相到氧化亚铜（Cu₂O）单晶相的转变来验证结晶。通过电子背散射衍射和拉曼光谱确定了这种转变，这清楚地表明形成了尺寸高达5μm×1mm的氧化铜单晶畴。这种转变可能包含几种不同的情况，例如大量密集形成的微晶独立生长并合并，同时，随着合并过程中发生的固态生长减少了晶界数量，和/或少量稀疏形成的选定微晶横向生长，自然地限制了晶界数量。在优化条件下制备的单晶畴的体积分数——单晶畴体积与激光能量沉积区域总体积的比值——估计为32%。基于这些实验结果，开发了一种基于元胞自动机模型的理论评估方法，该模型具有局部再结晶和随机成核的行为。该理论评估可以定性地描述激光诱导结晶中重要可观测特征（如晶粒尺寸和总体几何形状）对激光束几何形状的依赖性。理论评估预测，所得结晶度（单晶或多晶）的差异主要取决于非单晶区域沿激光扫描方向发生熔化的几何轮廓；凹形尾迹轮廓产生较大的晶粒，从而形成单晶，而凸形尾迹轮廓导致较小的晶粒，从而形成多晶，这为一个基本问题提供了线索
