单晶硅的纳米力学响应及其相变机制

硅在大规模集成电路、MEMS/NEMS、半导体工业中具有不可替代作用,但是目前对硅的塑性变形及其相变机制的理解远未成熟.采用大规模分子动力学模拟研究(100)面的单晶硅在球形金刚石压头纳米压入过程中的纳米力学响应、相变过程和相分布规律.结果表明:在弹性变形阶段载荷-压深曲线与Hertz接触理论预测结果相吻合.两者的分离点准确地预示了塑性变形的发生.金刚石结构的Si-I相向体心结构的BCT5相转变导致了单晶硅初始的塑性变形.初始形成的BCT5相在次表面形成了一个倒置的金字塔形结构.Si-II相的形成则稍微滞后一些.在较大的载荷下BCT5在压入面上形成一个四重对称的图案分布.相对于小压头条件下大的BCT5相区,大压头更有利于SiII相的发展.卸载后生成的高压Si-II相和BCT5相全部转变为非晶硅.研究结果确认了单晶硅纳米压入中BCT5相的存在;揭示了单晶硅塑性变形的相变机理,即Si-I转变为BCT5和Si-II相;并强调了Si-I相向BCT5相转变对于单晶硅塑性变形的重要作用.

Nano-Mechanical Behavior and Phase Transformation Mechanism of Monocrystalline Silicon

Silicon (Si) plays an irreplaceable role in large scale integrated circuit, micro/nanoelectromechanical systems (MEMS/NEMS), and semiconductor industry. Its nanomechanical behavior and pressure phase transformation are always of immense interest and have been a focus of extensive experimental and theoretical researches for a few decades. We performed a large-scale molecular dynamics simulation of the nanoindentation on Si(100) surface to examine the nano-mechanical behavior and phase transformation mechanism of monocrystalline silicon. In the simulations, a large indenter with radius of ~21.73nm was utilized in order to approach the experimental indenter size. Benefit from the large indenter, the detailed phase transformation process and phase distribution were analyzed, and the structure of the high pressure phase was characterized by radial distribution function (RDF) and bond angle distribution function (ADF). The results showed that the load-depth curve in elastic stage was agreed well with the prediction of the Hertz contact theory. The mismatch between simulated load-depth curve and Hertz contact accurately indicated the onset of plastic deformation, which was corresponding with the initial phase transition from Si-I phase with diamond structure to bct5 phase with body-centered cubic (bcc). The initial bct5 phase generated an inverted pyramid on the subsurface. Increasing the indentation load, the Si-II phase was generated from the Si-I phase, and enlarged beneath the indenter. The bct5 phase formed a fourfold pattern along the indentation orientation. Compared with the small indenter, the large indenter prompted the grown of the Si-II phase, which is the one reason why the BCT5 phase almost cannot be probed in experiment. After unloading, both the Si-II and bct5 phases transformed into amorphous phase. The results validated the existence of bct5 in the nano-indentation process of monocrystal silicon; and revealed the phase transformation mechanism of the plastic deformation.