Increased Critical Thickness for Strained SiGe on Ge-on-Si(111)
Authors: Youya Wagatsuma , Alam Md. M , Okada Kazuya , Hoshi Yusuke , Yamada Michihiro , Hamaya Kohei , Sawano Kentarou 
Published: December 2020

Since a Ge has been recognized as one of the promising alternative materials to Si, many researches have been pursued mostly based on a bulk Ge with a (100) surface orientation. Recently, it has been shown that a Ge with a (111) orientation is more favorable for applications to spintronics devices because high quality ferromagnetic materials can be epitaxially grown on the Ge(111)[1]. Also a strained SiGe(111) is expected to improve the spin lifetime of electrons. So far, however, epitaxial growth of tensile-strained SiGe on Ge has not been explored sufficiently.
Recently we systematically investigated critical thickness of the strained SiGe/Ge-on-Si(111) [2], and found that unusual line-shaped ridge roughness appears on the SiGe surface at the initial stage of strain relaxation. Moreover, we observed generation of cracks extending deep into underlying Ge layer in the ridge region, imposing essential requirements to suppress the formation of cracks and related ridges. For this purpose, we attempted the growth of a strained SiGe on a Ge-on-Si(111) that was patterned in line & space mesa and succeeded in suppression of the ridge roughness formation [3]. Toward device applications, such as spintronics devices, we need to widen the area of the crack-free strained SiGe and to increase critical thickness of the strained SiGe/Ge-on-Si.
In this work, we employ much larger (~ 50 um) square-shaped mesa-pattern of the Ge-on-Si and succeed in the growth of crack-free strained SiGe with the thickness which largely exceeds the expected critical thickness.
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. The 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. For experiments of the SiGe growth on the patterned Ge(111) and Ge-on-Si(111), one of the Ge(111) samples and grown Ge-on-Si(111) samples was subjected to photolithography process[3]. Subsequently the 200 to 250 nm thick SiGe layer was grown on the patterned Ge(111) and Ge-on-Si(111) substrate at 350 °C.
Figures 1(a)-1(d) show laser microscope (LM) surface images of 200 or 250 nm thick strained SiGe layers grown on (a) the un-patterned Ge-on-Si(111), (b) the patterned Ge-on-Si(111), (c) the un-patterned Ge(111) substrate and (d) patterned Ge(111) substrate. For the 250 nm thick Si0.2Ge0.8 on the un-patterned GOS [Fig. 1(a)], we can clearly observe high-density ridge roughness running along three equivalent [1-10], [10-1] and [01-1] directions, which are corresponding to intersections of surface and (111) slip planes. The ridge roughness involves cracks along the plane. In contrast, for the 250 nm thick Si0.2Ge0.8 on the patterned GOS [Fig. 1(b)], we can find the surface is very flat without such ridge roughness. This result implies that the strain energy accommodated in the strained SiGe is effectively released at mesa edges, which can eliminate a driving force for the ridge roughness formation, that is, the crack formation. It is very interesting to compare these results with those obtained for the SiGe on Ge substrates. Figs. 1(c) and (d) shows ridge roughness appears for both 200 nm thick SiGe layers grown on the patterned and un-patterned Ge(111) substrates. This means that the patterning cannot suppress the ridge formation for the case of the strained SiGe on the Ge(111) substrate. Note that the ridge roughness is seen to be connected between the mesa patterned and etched areas, implying that the SiGe film grown on the etched area affects the ridge formation mechanism.
In conclusion, we can say that the critical thickness of the strained SiGe can be significantly increased by means of growing the SiGe layer on the patterned Ge-on-Si(111) and the 250 nm thick strained Si0.2Ge0.8 can be grown without any roughness and crack formation. This thickness largely exceeds an expected critical thickness for the Si0.2Ge0.8/Ge(111) [2]. It can be said that this method can highly widen the applicability of the strained SiGe(111) heterostructures to spintronics and high performance electronic devices.
This work was supported in part by MEXT Supported Program for the
Strategic Research Foundation at Private Universities 2015–2019 and by Grant-in-Aid for Scientific Research (Nos. 16H02333, 19H02175 and 19H05616) 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., Mater. Sci. Semicond. Process. 117, 105153 (2020).

Figure 1