(Digital Presentation) Fabrication of Strained Ge Microbridges with Mesh-Patterned Pads and Their Optical Properties
Authors: Ikegaya Reo , Inoue Takahiro , Wagatsuma Youya , Okada Kazuya , Sawano Kentarou 
Published: November 2022

Introduction
Recently, Ge has attracted attention as a luminescent material on Si. As fabrication of Ge-on-Si structures can induce tensile strain into Ge owing to thermal mismatch between Si and Ge and resultantly modify its band structure, the direct transition probability can be improved, offering strong luminescence. However, amounts of the strain induced in the Ge-on-Si are quite small (~0.2 %) and the larger strain is highly desired. And then, microbridge structures have been studied to increase the tensile strain and luminescence intensity [1,2]. Usually, electron beam lithography, which is not suitable for mass production, is used for making sub mm scale microbridges. We attempt to fabricate relatively large, several mm scale, microbridges by simple photolithography process. To provide large tensile strain in the Ge microbridge, it is necessary to enlarge areas of the pad region (both side areas of the bridge) due to the relatively large bridge size. However, the lower etching rate of the (111) plane makes it difficult to completely remove the Si underneath the Ge in the pad area. Therefore, we attempt to fabricate microbridge structures with mesh-type holes on the pads to enhance the etching of the Si in the pad areas and obtain a significant increase in room temperature PL luminescence intensity.
Experimental
A low-temperature Ge layer (Tg=350℃,40 nm) and high-temperature Ge layer (Tg=600℃, 500nm) were grown on Si(100) substrates using solid source MBE, and annealed to improve crystallinity. Subsequently, microbridge structures with mesh holes on the pad areas were patterned with the bridge along <110> direction by standard photolithography. The Ge and Si were etched down by dry etching. Then the Si underneath the Ge was removed by selective etching with KOH to fabricate freestanding microbridges. In addition, since amounts of strain are expected to vary depending on the area of the holes in the pad area, samples with different areas of holes were fabricated and compared. PL measurements at room temperature (excitation wavelength: 980nm) were performed to evaluate luminescent characteristics. Moreover, Raman measurements were performed to evaluate the amounts of strain (Laser wavelength: 514.5nm).
Results and discussions
Laser microscope measurements confirmed that microbridges with mesh-holes on the pads were fabricated and that the Si underneath the Ge in the pad areas was fully etched and the pad areas are freestanding. From the PL spectrum obtained from the central position of the microbridge, a strong PL peak is observed, and a PL intensity of the microbridge with the mesh structure is significantly higher than that without the mesh structure. The peak is seen to shift to longer wavelength around 1800 nm, indicating that the tensile strain in the Ge microbridge with the mesh pattern is larger than that without the mesh structure, which was confirmed by Raman measurements. It is notable that the Ge-on-Si without patterning hardly yield light emission, indicating that the fabrication of microbridges with mesh-pattern by the simple photolithography process is expected to realize Si-based high-efficiency light sources.
This work was supported in part by Grant-in-Aid for Scientific Research (19H02175, 19H05616, 20K21009).
Ref
1.M.J.Süess et al. Nature Photonics7 46-472(2013)
2. F.T. Armand Pilon et al. Nature Communications10 2742 1-8(2019)

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