Anisotropy of subband structure of Si and Ge surface inversion layers and control of its physical properties by means of strain

Si和Ge的能带各向异性与不同取向的拉伸或压缩应变协同作用，显著改变了能带结构。对于Si价带，我们研究了应变引起的体能带或表面子带的各向异性，并寻找有助于提高空穴迁移率的应变类型。进一步阐明了晶胞中两原子相对位移的显著效应（内部应变）对能带结构的影响，找到了引起Ge间接-直接带隙转变的单轴拉伸方向和双轴拉伸平面取向，并估算了发生这种转变的应变大小，用角分辨光电子能谱观察了Si的价带，我们已经发现双轴张力将具有小的面内空穴有效质量的带提升到价顶。

Band anisotropy of Si and Ge and applied tensile or compressive strain with various orientations cooperate synergistically to change the band structure remarkably. As for the Si valence band, we have investigated the anisotropy of the bulk band or the surface subband induced by strain, and searched for those strain types which contribute to improve hole mobility. Furthermore, we have clarified the conspicuous effect of relative atom displacement of two atoms in the crystal unit cell (internal strain) on the band structure.We have found the direction of uniaxial tensility and the plane orientation of biaxial tensility which cause the indirect-direct band-gap transition of Ge, and evaluated the strain magnitude where the transition occurs.Observing the Si valence band by angle-resolved photoemission spectroscopy, we have found that biaxial tensility lifts up the band with a small in-plane hole effective mass to the valence top.