Silicon-Germanium Stressors for Germanium Photonic Devices on Silicon
Authors: Nishimura Michiharu , Kawashita Kazuki , Ishikawa Yasuhiko 
Published: February 2020

Ge epitaxial layers on Si have been widely studied in Si photonics for active photonic devices such as photodetectors operating in the communication wavelengths (1.3–1.6 µm). The control in the direct bandgap of Ge or the fundamental optical absorption edge is important for the wavelengths of device operation [1]. One of the promising approaches is to apply a stress to Ge, and SiNx has been examined as an external stressor [2,3]. In order to change the absorption edge by several 10 nm, a uniaxial stress applied to Ge is required more than 0.5 GPa [2]. This indicates that the built-in stress in the SiNx stressor should be even larger (> 1 GPa). However, such a highly stressed SiNx is not easily deposited. In this work, an alternative stressor of strained SiGe epilayer is investigated. An advantage lies in the amount of stress, which is controlled by the alloy composition; e.g., a tensile stress in Si0.2Ge0.8 on Ge is as large as 1.4 GPa.
Figures 1(a) and 1(b) schematically show the concepts to apply compressive and tensile stresses, respectively, to Ge mesa structures on Si. As in Fig. 1(a), a stressor of SiGe epitaxial overlayer on Ge possesses a built-in tensile stress due to the lattice mismatch, as long as the SiGe layer is pseudomorphically grown. For narrow mesa structures, the tensile stress in SiGe is partially relaxed by an elastic deformation, inducing a compressive stress in the underlying Ge mesa. This results in an increase of direct bandgap, i.e., a blue shift in the absorption edge. With increasing the thickness of SiGe as well as the Si composition, a larger stress is induced in Ge, although the critical layer thickness of SiGe should be taken into account to prevent the strain relaxation due to the generation of dislocations. In turn, a reduction in the direct bandgap, i.e., a red shift in the absorption edge is induced when a compressive SiGe overlayer is formed on Ge. As in Fig. 1(b), such a compressive SiGe is obtained when a relaxed Si layer is inserted between the SiGe overlayer and the Ge mesa structure. The Si layer can be as thin as 10 nm because of the critical thickness on Ge as small as a few nm.
In order to experimentally examine the applicability of this concept, two types of epitaxial SiGe overlayers were prepared on trapezoidal Ge mesa structures running in the [110] direction, which were selectively grown on Si (001) wafers with SiO2 masks by ultrahigh-vacuum chemical vapor deposition. The Ge mesa was composed of a (001) top surface and (113) facet sidewalls. The height/thickness was 500 nm, and the base width was typically 5 µm. For the built-in tensile stress in Fig. 1(a), a 40-nm-thick Si0.2Ge0.8 layer was grown on the Ge mesa. On the other hand, for the built-in compressive stress in Fig. 1(b), a 10-nm-thick relaxed Si layer was grown on the Ge mesa, followed by a growth of 40-nm-thick Si0.8Ge0.2. X-ray diffraction measurements revealed that strained SiGe overlayers were successfully formed in both cases. As a reference, a Ge mesa was prepared with a 20-nm-thick relaxed Si overlayer. Photoluminescence (PL) spectra were measured in order to examine the change in the bandgap. Typical results are shown in Fig. 1(c). The PL peak positions were successfully shifted, depending on the stress in SiGe; a blue shift was observed for the tensile SiGe overlayer, while a red shift was observed for the compressive SiGe overlayer. Although the shift was as small as 10 nm in the present study, further shift would be realized by optimizing the thickness and the alloy composition of SiGe.
[1] Y. Ishikawa et al., Appl. Phys. Lett. 82, 2044 (2003).
[2] R. Kuroyanagi et al., Opt. Express 21, 18553 (2013).
[3] G. Capellini et al., J. Appl. Phys. 113, 013513 (2013).

Figure 1