(Invited) Anisotropic Strain Introduction into Si/Ge Hetero Structures
Authors: Sawano Kentarou , Konoshima Shiori , Yamanaka Junji , Arimoto Keisuke , Nakagawa Kiyokazu 
Published: February 2020

Ge is one of the most attractive materials toward ultra-low-power and high-performance complementary metal oxide semiconductor (CMOS) circuits owing to its higher hole mobility than Si and various III-V materials. Moreover, it is possible to enhance the mobility further by introducing strain into the channel. Especially an uniaxial strain is expected to provide the highest hole mobility [1,2]. So far, several approaches have been researched to create uniaxially strained SiGe and Ge channels.
We have been developing the selective ion implantation technique to induce uniaxial strain into SiGe buffer layers [3,4]. In this method, a Si substrate is selectively subjected to the ion implantation and followed by the SiGe overgrowth. A subsequent post-growth-annealing promotes the strain relaxation of the SiGe selectively in the implanted region as the implantation-induced defects formed act as dislocation nucleation sources. And hence, with proper configurations of the selective implantation, the anisotropic strain can be induced in the SiGe in the unimplanted area since the neighboring relaxed SiGe films in the ion-implanted areas provide shear stress at the interfaces. In previous works, we succeeded in fabrication of uniaxially strained SiGe buffer layers with Ge concentrations below 30% [3,4], which can serve as proper templates for uniaxially strained Si/Ge heterostructures, such as uniaxially strained Si and Si1-xGex (x < 0.5) channels. For the formation of uniaxially strained Ge and Si1-xGex(x > 0.5) channels, however, SiGe buffers with higher Ge concentrations are required. In this study, therefore, we fabricate uniaxially strained SiGe buffer layers with high Ge concentrations with the selective ion implantation technique and overgrew uniaxially strained Ge layers on the SiGe buffers.
The sample fabrication procedure is as follows (Fig.1). Ar+ ions were selectively implanted into the Ge substrate through the SiO2 mask window. Line widths of the ion-implanted and unimplanted regions were varied from 2 to 20 μm. A SiGe layer was pseudomorphically grown on the selectively ion-implanted Ge substrate with solid source molecular beam epitaxy. Subsequently, postgrowth annealing was carried out to promote the strain relaxation of the SiGe only in the implanted region. Finally a Ge layer with the thickness of 10 nm was pseudomorphically grown on the SiGe.
Figure 2 shows a Raman mapping image for the sample with 3-μm-width stripe lines, where a Ge-Ge mode peak wavenumbers obtained from the top Ge layer were scanned. A clear contrast is observed corresponding to the large and small compressive strain of the Ge layer in the implanted and unimplanted regions, respectively. This demonstrates that the strain of the Ge is laterally controlled by the selective implantation. Since XRD measurements confirmed that the SiGe buffer layer has uniaxial strain, it can be said that the top Ge layer is also uniaxially strained. This uniaxially strained Ge layer can be transferred onto the insulator substrate via wafer bonding process, where the ion-implanted defective region can be removed. Finally defect-free uniaxially strained GOI substrate can be obtained, which is applicable to high mobility uniaxilly strained Ge channel MOSFETs.
[1] Y. Sun, et al., J. Appl. Phys. 101, 104503 (2007). [2] T. Krishnamohan et al., IEDM Tech. Dig., 899 (2008). [3] K. Sawano et al., Appl.Phys.Express 1, 121401 (2008). [4] K. Sawano et al., J. of Crystal Growth 378, 251 (2013).

Figure 1