(Invited) Fabrication of Ge Waveguides by Epitaxial Lateral Overgrowth toward Monolithic Integration of Light Sources
Authors: Oda Katsuya , Okumura Tadashi , Kasai Junichi , Kako Satoshi , Ishida Satomi , Iwamoto Satoshi , Arakawa Yasuhiko 
Published: February 2020

The Ge laser is one of the most promising devices as a monolithic light source for high-speed optical interconnections due to its compatibility with Si processes. Although optical gain has been observed [1, 2], further improvements of crystallinity are required [3, 4] to ensure continuous wave operation of Ge lasers. In this work, we fabricated high-quality Ge waveguides using epitaxial lateral overgrowth on a SiO2 layer and chemical mechanical polishing [5] , and we investigated its crystallographic and optical properties.
An eight-inch Si wafer was used as a substrate, and a SiO2 window was fabricated as a mask for Ge selective epitaxial growth (SEG). A Ge layer was selectively grown by using low-pressure chemical vapor deposition along with germane (GeH4) as a source of Ge with H2 carrier gas. To prevent indirect transition by filling electrons into the L-valley in the conduction band of Ge [6], we also conducted in-situ phosphorus (P) doping by supplying phosphine (PH3). First, a Ge buffer layer was deposited within the SiO2 window at 400°C and annealed at 750°C, then an additional Ge layer was selectively grown only on the Ge buffer layer at relatively high temperature. Finally, rapid thermal annealing was carried out at 850°C. By optimizing the growth pressure, the length of the epitaxial lateral overgrowth (ELO) on the SiO2 layer was increased to more than 5 mm. Although the dislocation and stacking faults were observed around a region of the Ge on the Si substrate by transmission electron microscopy, no dislocations were evident on the ELO-Ge region grown on the SiO2 layer.
Since the thickness of the SEG-Ge layer increased as the length of the ELO increased, chemical mechanical polishing (CMP) was applied to remove the top part of the SEG-Ge layer. By using measurements of micro-Raman spectroscopy, it was confirmed that the tensile strain was remained in the ELO-Ge on SiO2 layer even after the CMP process. Then, additional P doping was carried out by spin-on-dopant (SOD) process. The SOD solution [Filmtronics, P8545SF] was coated on the CMP-Ge layer, and the annealed at 750°C, for 10min. The maximum P concentration of 3.2×1019cm-3 was achieved, which was measured by secondary ion mass spectroscopy. Finally, Ge waveguides (Ge-WGs) were fabricated by reactive ion etching of the CMP-Ge layer to remove a part of the Ge layer that contained a lot of crystal defects due to the lattice mismatch between Ge and Si. Figure 1(a) shows a bird's-eye SEM image of the Ge-WG after dry etching with an offset (Dx) of 3.1 mm to the SiO2 window. Photoluminescence (PL) spectra from the Ge-WGs with various Dx are shown in Fig. 1(b). Obvious PL spectra were observed from the Ge-WGs with peak wavelength around 1600 nm. The PL peak intensity from the Ge-WG with Dx = 3.1 mm was four-times higher than that corresponding to Dx = 0.1 mm. This result indicates that the better crystallinity of the Ge-WG was obtained at the position apart from the Ge/Si interface.
This work was supported by Project for Developing Innovation Systems of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

References

[1] R. E. Camacho-Aguilera, et al., Optics Express 20 (2012) 11316.
[2] R. Koerner, et al., Optics Express 23 (2015) 14815.
[3] Q. Li, et al., Appl. Phys. Lett. 85 (2004) 1928.
[4] H. -Y. Yu, et al., Electron Dev. Lett. 33 (2012) 579.
[5] K. Oda, et al., Jpn. J. Appl. Phys. 55 (2016) 04EH06.
[6] J. Liu, et al., Opt. Lett. 35 (2010) 679.

Figure 1