(Digital Presentation) Fabrication of Microbridges Based on Ge-on-SOI and Observation of Strong Resonant Light Emission
Authors: Odashima Ayaka , Inoue Takahiro , Wagatsuma Youya , Ikegaya Reo , Nagao Masaki , Sawano Kentarou 
Published: November 2022

Introduction

Ge is one of the most attractive material for silicon-based optoelectronics because it can be epitaxially grown on a Si substrate, allowing us to integrate Ge based light emitters and photodetectors monolithically on the Si platform. Although Ge is an indirect band gap semiconductor like Si, by epitaxially growing Ge on the Si substrate, tensile strain is introduced into Ge, resultantly leading to the Γ-valley shifts and increase in the light emission efficiency via the direct transition. The amount of the strain induced in Ge-on-Si, however, is not sufficiently large, demanding other methods of applying large tensile strain to Ge. Fabrication of Ge microbridge structure is expected to induce the larger strain and further improve light emission efficiency [1]. In general, microbridges reported so far have been fabricated by the electron beam lithography process, which is not appropriate for mass production. In this study, we fabricate Ge microbridge by simple photolithography process based on Ge-on-Si and Ge-on-SOI structures and very strong room temperature photoluminescence is obtained and strong resonant peaks can be observed.

Experimental Method

Ge layers were grown on a Si (110) wafer (Ge-on-Si) or a SOI (100) wafer (Ge-on-SOI) using a two-step growth method by solid source MBE. Firstly, a 40nm thick buffer layer at a low temperature of 350 ~ 400℃. Secondly, a 500 nm thick Ge layer was grown at a high temperature of 550 ~ 650℃. Post-growth-annealing was carried out at 800℃ for 10 minutes. The microbridge structures were defined by the standard photolithography process and mesa etching was performed by the reactive ion etching. Subsequently, the Si under the bridge and pads was selectively removed by KOH etchant, leading to free-standing Ge microbridge. For the Ge-on-SOI, the underlying SiO2 layer (buried oxide) was additionally removed by HF.

Results and Discussion

It was observed by SEM measurements that the Ge microbridges fabricated from Ge-on-Si were slightly warped from both ends to the center of the bridges. Hydrogen gases generated due to the reaction of Si and KOH during the selective etching are considered to be responsible for the warping of the microbridges, which makes it difficult to control the strain of the Ge. The selective etching time, therefore, has to be minimized to reduce the effects of the gas generation. For this purpose, the microbridges were fabricated from Ge-on-SOI, where the floating structure can be easily formed by the etching of the buried SiO2 layer with HF and the etching time of Si with KOH was reduced by a factor of four. As a result, the warping was completely suppressed and very flat microbridges were fabricated without any warping.
Photoluminescence measurements at the center of the microbridge on Ge-on-SOI were performed at room temperature. Strong PL emissions were obtained in the wavelength range of 1630-2000 nm, where several strong resonant peaks appear. This resonance is considered to occur due to reflection at the side wall of the bridge and the peaks can be well fitted with the cavity length of the bridge width. Also, we carried out PL measurements from one end to another end of the bridge, and investigated the bridge position dependence of the PL emission. It was found that the light emission intensity increased toward the center of the bridge, and the highest intensity was obtained at the center. Moreover, the strain amounts were measured by micro Raman line scan along the bridge and strain dependence of PL emission was investigated. It was also shown that the strain amount increased toward the center of the bridge and became the highest at it. It was demonstrated that the higher PL intensity can be obtained due to the higher tensile strain at the center of the microbridge.
In summary, we fabricated Ge microbridge structures by photolithography process and obtained strong room temperature resonant light emission from the Ge microbridges formed from Ge-on-SOI where bridge-warping was completely suppressed by shortening the etching time. From this result, we can say that the Ge microbridge light emitting devices are very promising for applications to the light sources on the Si platform.
This work was partially supported by JSPS KAKENHI (Nos. 19H02175, 19H05616 and 20K21009).
[1] M. J. Suess et al, Nat. Photonics 7: 466, 2013

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