(Invited) Engineering Strain, Defects, and Electronic Properties of (110)-Oriented Strained Si
Authors: Arimoto Keisuke , Yamanaka Junji , Hara Kosuke O , Sawano Kentarou , Usami Noritaka , Nakagawa Kiyokazu 
Published: December 2020

Strain engineering of group IV semiconductors has been extensively investigated with an aim to produce high mobility platform for high performance electronic devices. In particular, combination of strain and 3D structures requires elaborate engineering of strain to obtain optimum effect. Effects of utilization of non-(001)-oriented channel and anisotropic strain are major factors to be considered. The lattice structure of a strained material is generally anisotropic. Symmetry of the lattice structure is expected to have significant influence on electrical properties of crystals, which should be taken into consideration for advanced application of the lattice deformation. In addition, anisotropy in the lattice structure significantly affects the generation of defects during heterostructure formations, which can result in even larger asymmetry in the lattice structure. As a matter of fact, it is found that strain components and morphology of defects are highly asymmetric in the strained Si/SiGe heterostructures formed on (110) substrates. Recently, significant enhancement of the hole mobility was demonstrated in the topmost strained Si film. In this presentation, formation mechanism of the highly asymmetric strain in this heterostructure system and its influence on the properties of strained Si pMOSFET are discussed.
Through a computational study of strain effect on the valence band structure, we found that the lowering of the hole effective mass due to strain is expected to be larger on (110) surface than on (001) surface. According to the calculation, the cyclotron effective mass of a hole on (110)-oriented strained Si can be lowered to ~ 0.2 m0 by strain. On the other hand, it was found that the minimum hole effective mass is ~ 0.3 m0 in the case of the (001)-oriented strained Si film. In addition, the result of the calculation shows that the hole effective mass on the (001)-oriented strained Si does not decrease with the increase of strain. These results motivated us to carry out experimental investigation on the growth and electrical properties of (110)-oriented strained Si. Strain-relaxed SiGe was employed as a buffer layer for strained Si layer. The growths were carried out using solid-source molecular beam epitaxy. On a Si(110) substrate, SiGe layer is subjected to biaxial compressive stress which induces glides of partial dislocations on (111) and (11-1) planes. Since the Burgers vectors of these partial dislocations are perpendicular to the in-plane axis [-110], the resultant strain relaxation is highly anisotropic. Accordingly, the surface morphology is also highly anisotropic. It was confirmed that strain in the topmost Si layer is also highly anisotropic in accordance with the lattice parameters of the SiGe layer. These anisotropic features play significant roles in electronic properties of the strained-Si MOSFET formed on this structure in the following manners. First, both the surface and the crystalline morphologies are almost homogeneous in the [-110] direction, for which the relaxation time is expected to be long for the motion in the [-110] direction. Secondly, the anisotropic strain shifts the valence band edges of the layers so that the holes are effectively confined to the high-mobility strained Si layer. A pMOSFET was fabricated on a (110)-oriented strained Si/SiGe structure and hole effective mobility was evaluated by IV and CV measurements. A significantly high hole effective mobility (480 cm2/Vs) was obtained in a pMOSFET with the [-110] channel. High hole mobility in the (110)-oriented strained Si with [-110] channel was also confirmed by gated Hall measurements.