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We propose a method for obtaining crack-free fully-strained SiGe layers on Ge(111). To achieve the crack-free strained SiGe layers, we introduce patterned area with a sufficient depth (step height) of more than 1 µm on Ge(111) substrates. Because of the complete suppression of the crack propagation from the SiGe layer grown on the outside of the patterned area on Ge(111), we achieve crack-free fully strained SiGe layers on the inside of the patterned area. This approach will drastically expand applicability of the strained SiGe to the fields of Si photonics and spintronics.

Recently, silicon photonics has been attracting increased attention toward the realization of on-chip optical interconnection, which requires high-efficiency light-emitting devices to be integrated on Si substrates. Although Ge is an indirect-band-gap semiconductor like Si, it is expected that strong direct-transition luminescence can be obtained from tensile-strain induced Ge-on-Si. We have previously reported that highly efficient room temperature EL emission can be obtained from strained Ge-on-Si p-i-n diodes[1]. Further increase in luminescence intensity and control of emission wavelengths can be made possible by introducing a quantum well structure in the active layer. Although the fabrication of high-quality strained SiGe layers on Ge is essential for the fabrication of quantum well structures, critical thickness limits the total growth thickness and numbers of strained layers, which makes device applications difficult. We have succeeded in fabricating a strained SiGe layer that is much thicker than the critical thickness by using a patterning method, and this result is very promising for the fabrication of SiGe/Ge multi-quantum well structure[2]. It also makes it possible to increase the number of layers of MQW structure. In this work, we fabricate relatively thick strained SiGe/Ge MQW structures on Ge-on-Si by utilizing the patterning method, and evaluate their crystallinity and optical properties. In experiments, the crystal growth was carried out with solid source molecular beam epitaxy. The Ge-on-Si was fabricated using the so-called two-step method. 40 and 500 nm thick Ge layers were subsequently grown on a Si(100) or Si(111) substrate at 350 and 600℃, respectively, followed by annealing at 800℃ for 10 min. Subsequently we carried out the patterning of the Ge-on-Si(111) substrate by photolithography process. 50nm thick Ge buffer layer ware grown on a Ge-on-Si(100) or Ge-on-Si(111), 6nm thick Si0.1Ge0.9 barrier layer and 10nm thick Ge well layer ware grown with 10-20 cycles at 350℃. Laser microscopic measurements showed no clear roughness on the surface. X ray diffraction measurements show periodic peaks originated from the multi layers of SiGe/Ge, which indicates that the sample has high crystallinity and abrupt interface between the Ge and SiGe layers. TEM observation indicated that no defect was found inside the crystal in the MQW structure. Photoluminescence (PL) measurements were carried out at room temperature and a PL peak originated from the quantum-confinement in the SiGe/Ge MQW was obtained. This PL peak wavelength was also found to shift depending on the Ge concentration of the SiGe. PL intensity of the peak was increased compared with Ge-on-Si without the SiGe/Ge MQW overgrowth. From these results, it is demonstrated that high quality SiGe/Ge multiple quantum structures can be fabricated on Ge-on-Si by utilizing the patterning method and strong room temperature PL was obtained from the MQW, indicating that SiGe/Ge MQW is promising for the applications to light-emitting devices integrated on the Si platform. [1] K.Yamada et al. Appl. Phys. Express 14 045504(2021) [2] Y.Wagatsuma et al. Appl. Phys. Express 14 025502(2021) Figure 1

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 Si0.2Ge0.8 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. Express14 025502(2021). Figure 1

We fabricate thick strained Si0.1Ge0.9/Ge multiple quantum wells (MQWs) structures on Ge-on-Si, and evaluate their crystallinities and optical properties. As a result, it is found that highly crystalline Si0.1Ge0.9/Ge MQWs are grown both on Ge-on-Si(100) or Ge-on- Si(111), where surface roughness slightly differs between the two. From both MWQs we obtain strong room-temperature photoluminescence (PL) via quantum confinements of carriers in the MQWs. It is also found that the PL peak positions and intensities are different between (100) and (111). Surface roughening is considered to cause reduction in the PL intensity and the peak shift for the (111) case whereas the MQWs on Ge-on-Si(100) shows very smooth surface and resultantly strong PL intensity, indicating that Si0.1Ge0.9/Ge MQWs are promising for applications to light-emitting devices that can be integrated on the Si platform.