Ge has been attracting attention for the realization of intra chip optical integrated circuits. In particular, the introduction of tensile strain by epitaxial growth of Ge on Si substrate improves the direct transition probability by shifting the Γ-valley, resulting in an increase in luminescence efficiency. Furthermore, by forming Ge microbridge structure, the larger uniaxial tensile strain can be induced into the freestanding Ge, which is expected to further improve the luminescence efficiency [1]. However, when the stronger tensile strain is applied to Ge, the emission wavelength shifts significantly to the longer wavelength. In order to arbitrary control the emission wavelength to the shorter, especially within the telecommunication wavelength band, introduction of SiGe/Ge heterostructures in the microbridge is considered to be one of the most proper ways. In this work, we attempt to grow the strained SiGe layer on the Ge microbridge and observe the shorter wavelength light emission from the strained SiGe. Moreover, it has been shown that Fabry-Perot resonance takes place in the Ge microbridge structure owing to the side wall reflection [2], leading to the strong resonant luminescence peaks. Here also we observe the resonant light emission from the SiGe/Ge microbridge structure.
Experimental Methods
A Ge layer was grown on a Si(100) wafer by solid-source molecular beam epitaxy (MBE) with using so-called two-step growth method. First, a 40-nm-thick buffer layer was grown at a low temperature of 350 °C. Then, 500 nm thick Ge layer was grown at 600 °C. Finally, thermal annealing was performed in situ at a temperature of 800 °C for 10 min to reduce the threading dislocation density in Ge layers. After the MBE growth, patterning of the microbridge structure was performed by standard photolithography process and inductively coupled plasma reactive-ion dry etching. Si beneath the microbridge was then removed to form freestanding structures by KOH wet etching. After fabrication of the Ge microbridge, 60 nm thick Si0.2Ge0.8 layer was directly grown on the Ge microbridge at 350 °C.
Results and Discussion
SEM observation showed that fine microbridge structure remains unchanged after the Si0.2Ge0.8 layer overgrowth on the Ge microbridge. Raman measurements confirmed that the grown SiGe has tensile strain originated from the lattice mismatch between the Si0.2Ge0.8 and underneath Ge, indicating that elastic strain relaxation did not occur. A PL spectrum obtained from the central position of the Si0.2Ge0.8 microbridge exhibits strong light emission in the wavelength range from 1500 to 2000 nm. This is thought to be originated both from SiGe and Ge layers. The PL intensity around shorter wavelength region is significantly increased after the Si0.2Ge0.8 epitaxial growth compared to that before the growth, indicating that the SiGe growth is contributing to the light emission at the telecommunication band. It is noted that a PL spectrum obtained in the unpatterned area from the same sample has almost no light emission, indicating that the microbridge structure is very promising for obtaining the strong light emission. Also it is remarkable that the resonance peaks can be obviously observed. The calculation confirmed this resonance is due to the reflection between side walls of the microbridge. From these results, we can say that the SiGe/Ge heterostructure based microbriges serve as wavelength-controllable high efficiency light sources on Si.
This work was partially supported by JSPS KAKENHI (Nos. 19H02175, 19H05616 and 20K21009).
[1] M. J. Suess et al, Nat. Photonics 7: 466, 2013
[2] Peiji Zhou et al 2018 Jpn. J. Appl. Phys. 57 04FH10