Challenges and Opportunities of Near and Mid-Infrared Photonics Based on SiGe and Ge
Authors: Takenaka Mitsuru , Kim Younghyun , Han Jae-Hoon , Kang Jian , Takagi Shinichi 
Published: February 2020

Silicon-germanium (SiGe) and germanium (Ge) have been investigated as high-mobility channel materials for high-performance metal-oxide-semiconductor field-effect transistors (MOSFETs). The intense development of heterogeneous integration of SiGe and Ge on Si has given us opportunities to integrate new functionalities on Si platform. Since SiGe and Ge have narrow bandgap energies which match to photon energies used in optical fiber communication, SiGe/Ge photodetectors monolithically integrated on Si are available, evoking the emergence of Si photonics technology as a standard photonics integrated circuit (PIC) platform. In addition to photodetector (PD) applications, we have investigated the heterogeneous integration of SiGe and Ge on Si to extend the functionalities for Si photonics platform. In this paper, we present challenges and opportunities of SiG and Ge for near and mid-infrared integrated photonics.
We have investigated strained SiGe to enhance the modulation efficiency of Si-based optical modulators. Since there are no significant electro-optic effects in Si, free-carrier modulation is a common method to modulate optical properties such as refractive index and absorption coefficient. However, the weak free-carrier effects such as the plasma dispersion effect and free-carrier absorption result in low modulation efficiency. To cover the shortcoming of Si, we have proposed to introduce strained SiGe. We have predicted that biaxial compressive strain enhances the free-carrier effect because of the light hole effective mass in SiGe [1]. Owing to the lattice-mismatch between Ge and Si, biaxial compressive strain is introduced into SiGe grown on Si. According to the Drude model, we have estimated approximately three times greater plasma dispersion effect in SiGe than in Si when a Ge content in SiGe is 0.5.
To demonstrate the enhanced free-carrier effects in SiGe, carrier-injection SiGe optical modulators have been explored. First we have examined the free-carrier absorption by measuring the optical attenuation in the SiGe modulator with varied injection current. We have observed approximately two times greater absorption in the SiGe device than in the Si device [2]. We have also fabricated a SiGe Mach-Zehnder interferometer (MZI) optical modulator to evaluate refractive index change precisely. The enhanced plasma dispersion effect in SiGe have bee also confirmed [3], proving the concept of the strain-induced enhancement in the free-carrier effect. We have also investigated SiGe optical modulators based on accumulation [4] and depletion [5]. Thus, strained SiGe is also very promising for optical modulator applications as well as transistor application.
We have also investigated Ge-based photonics by using CMOS compatible process technologies. First we have examined the surface passivation impact on the dark current of Ge PDs. Owing to the high-quality Ge oxide/Ge interface, we have achieved extremely low dark current density of 0.032 mA/cm2 in conjunction with the gas phase doped n+/p junction [6]. We have also revealed the impact of fixed charged in the passivation layer on the dark current [7].
Since Ge is transparent in mid-infrared wavelengths, Ge photonics is emerging as mid-infrared integrated photonic platform for bio/medical and environmental sensor applications. We have proposed the Ge CMOS photonics platform by using a Ge-on-insulator (GeOI) wafer [8]. We have fabricated a high-quality GeOI wafer by wafer bonding. Ge waveguide devices including rib waveguides, bend waveguides, MMI couplers, ring resonators, optical modulators have been successfully demonstrated on the GeOI wafer [9, 10].
In conclusion, the heterogeneous integration of SiGe and Ge on Si opens up many possibilities for optoelectronic application for near and mid-infrared wavelengths as well as electronics applications.
This work was supported by a Grant-in-Aid for Young Scientists (S) from MEXT, and NEDO “PECST” project.
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