(Invited) Selective Epitaxy of Submicron Ge Wire Structures for Photodetectors and Optical Modulators in Si Photonics

Abstract

Ge epitaxial layer on a Si-on-insulator substrate has been applied to waveguide photodetectors (PDs) in Si photonics, which operate at the optical communication wavelengths (1.3–1.6 µm). For enhancing the operating frequency over 50 GHz with keeping a high photodetection efficiency, a sub-micron-wide Ge wire structure with a lateral pin junction is effective because of the RC delay smaller than that for conventional vertical pin PDs. Such a Ge wire structure allows a single-mode light propagation, being also applicable to electro-absorption optical modulators (EA-MODs) based on the Franz-Keldysh effect. Here, in terms of higher-performance PDs and EA-MODs, sub-micron Ge wire structures by selective epitaxial growth on Si are presented. Bandgap engineering is discussed utilizing a control of grown-in tensile strain in sub-micron Ge.Figure 1(a) shows a typical cross-sectional scanning electron microscope image of a Ge wire structure with the width as narrow as 0.6 µm. The Ge wire in the [110] direction was selectively grown on an exposed Si (001) surface in a SiO2-covered Si substrate by ultrahigh-vacuum chemical vapor deposition at 700°C. The wire surface was surrounded mainly with inclined {113} and {111} facet planes, remaining a narrow (001) top plane. For protecting the fragile Ge surface, a Si cap layer was subsequently grown at 600°C, whereas a surface roughening was unfavorably induced on the {113} planes near the boundary with the {111} plane, as in Fig. 1(b). Raman spectra show that a SiGe alloy was accompanied on the wire surfaces, although no such SiGe was detected on the (001) plane of wide mesa top [1]. Based on theoretical analyses, a mass transport together with a Si-Ge interdiffusion would be induced by a stress concentration at the {113}/{111} facet boundaries. It is important that the roughening and alloying were significantly suppressed by decreasing the growth temperature for the Si cap layer to 530°C, as in Fig. 1(c).Typical photoluminescence (PL) spectra are shown in Fig. 1(d). For the Ge wire structure, the PL peak due to the direct transitions was located at the wavelength of 1.55 µm, which is shorter than 1.57 µm for the blanket Ge film on Si. The peak position is similar to that for a bulk Ge wafer. The blue shift for the wire corresponds to a widening of direct bandgap, which results from an elastic relaxation [2] of grown-in tensile strain generated by the thermal expansion mismatch with the Si substrate. The blue shift plays a significant role for Ge EA-MODs. As reported recently [3], in contrast to the operating wavelength range of > 1.60 µm in previous studies, EA-MODs of a sub-micron Ge wire favorably operates in the 1.55 µm range of C band (1.530–1.565 µm).[1] Y. Ishikawa et al., Proc. SPIE 11193, 1119309 (2019).[2] M. Nishimura et al., ECS Trans. 86, 3 (2018).[3] J. Fujikata et al., Opt. Express 28, 33123 (2020). Figure 1