Recently, it has been shown that Ge and SiGe with (111) orientation are very attractive for applications to spintronic devices because high quality ferromagnetic materials can be epitaxially grown on the Ge(111)[1]. Moreover strain introduction is expected to improve the spin lifetime of electrons. Previously we studied surface morphologies and strain states for the strained SiGe(111) layers grown on Ge(111), and found that cracks and related ridge roughness appear on the surfaces[2]. Moreover, we have demonstrated that such crack formation can be drastically suppressed by patterning of Ge-on-Si substrates [3]. In this work, we study mechanisms of the crack generation, propagation and network formation by evaluating strained SiGe layers grown on the patterned Ge substrates with various etching thicknesses.

In experiments, the crystal growth was carried out with solid source molecular beam epitaxy. The Ge-on-Si(111) was fabricated using the so-called two-step growth method. 40 and 650 nm thick Ge layers were subsequently grown on a Si(111) substrate at 400 and 700 °C, respectively, followed by annealing at 800 °C for 10 min. Subsequently, the 80 µm × 80 µm square mesa-patterning of the Ge-on-Si(111) and Ge(111) substrates were performed by the standard photolithography process. Depending on etched depth, partially etched Ge-on-Si (PE-GOS), fully etched Ge-on-Si (FE-GOS) and partially etched Ge substrates (PE-Ge) were prepared where the etched depth outside of mesa was from 350 to 600 nm for PE-GOS and 350 nm to 1 µm for PE-Ge. Then strained Si_0.2Ge_0.8 layers with thicknesses of 250 nm were grown on these templates at 350 °C.

It is found that completely no crack is formed on the surface of the SiGe layers on FE-GOS, whereas high density crack network appears both inside and outside of the mesa for the SiGe on PE-GOS. We consider that the cracks are firstly generated in the SiGe grown on Ge outside of the mesa, and the generated cracks propagate into the mesa, leading to the crack network on the mesa. It is also demonstrated that the crack generation does not occur in the strained SiGe grown on Si, that is, fully etched region outside of the mesa, and that the density of the crack generation sources are so low that the mesa area (80 µm × 80 µm square) is free from the crack generation. Thus, we can say that the Ge layer has to be completely etched down in order to suppress crack formation.

A behavior of the crack propagation across the mesa boundary was next investigated in more detail in terms of the mesa etching thickness, that is, the step height of the mesa. We compared several samples of the strained SiGe layers that were grown on the PE-Ge substrates with various etching thickness. As a result, it is found that the crack network appears both inside and outside of the mesa in the case of the etching thickness below 1 µm. By contrast, for the sample with the etching thickness of 1 µm, the crack network arises only outside of the mesa and a crack-free SiGe is formed on the mesa. This observation indicates that the crack propagation across the mesa boundary takes place when the step height is below 1 µm. Since the strained SiGe layer is not continuous between inside and outside of the mesa for all samples studied here, not only the crack within the SiGe layer but also the crack penetrating into the Ge substrate is speculated to contribute to the crack propagation behavior.

This work was supported in part by Grant-in-Aid for Scientific Research (Nos. 19H02175, 19H05616, 20K21009) from MEXT, Japan

[1] K. Hamaya et al., J. Phys. D: Appl. Phys. 51, 393001(2018). [2] Md. M. Alam et al., Appl. Phys. Express 12, 081005 (2019). [3] Y. Wagatsuma et al., Appl. Phys. Express 14 025502(2021).