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Since mid-infrared (MIR) wavelengths have a great potential for optical communication, sensing, and quantum information, Si-based MIR photonic integrated circuits (PICs) have been developed by leveraging Si photonics technology for near-infrared wavelengths. However, the transparency wavelength window of Si is from 1.2 μm to 8 μm, limiting the available wavelengths in the MIR spectrum. Ge is emerging as a waveguide material to overcome this difficulty because Ge is transparent in the entire MIR spectrum. We have developed a Ge-on-insulator (GeOI) platform for MIR integrated photonics. The strong optical confinement in a GeOI waveguide enables an ultracompact MIR PIC. Using wafer bonding and Smart-cut, a GeOI wafer was successfully fabricated. As a result, we have demonstrated various Ge passive devices, thermo-optic phase shifters, modulators, and photodetectors on a GeOI platform.

We propose a III–V metal-oxide-semiconductor (MOS) optical modulator with a graphene gate electrode along with the analysis of the modulation properties. With p-type doped graphene used as a transparent gate electrode, we can fully utilize the electron-induced refractive index change in an n-type InGaAsP waveguide with the reduction of the hole-induced optical absorption observed in a III–V/Si hybrid MOS optical modulator. Numerical analysis displays that up to the phase modulation efficiency of 0.82 V·cm and 0.22 dB optical loss for π phase shift can be achieved when the gate oxide thickness is 100 nm. With the elimination of the unnecessary parasitic capacitance found in the overlapping of graphene on the slab part of the waveguide, in conjunction with the high electron mobility in InGaAsP, the device also enables a modulation bandwidth of greater than 200 GHz.

Since mid-infrared (MIR) wavelengths have a great potential for optical communication, sensing, and quantum information, Si-based MIR photonic integrated circuits (PICs) have been developed by leveraging Si photonics technology for near-infrared wavelengths. However, the transparency wavelength window of Si is from 1.2 µm to 8 µm, limiting the available wavelengths in the MIR spectrum. Ge is emerging as a waveguide material to overcome this difficulty because Ge is transparent in the entire MIR spectrum. A Ge-on-Si waveguide is one of the promising platform for a MIR PICs. We have also proposed a Ge-on-insulator (GeOI) platform for MIR integrated photonics [1]. The strong optical confinement in a GeOI waveguide enables an ultracompact MIR PIC. Using wafer bonding and smart-cut, a GeOI wafer was successfully fabricated [2]. As a result, we have demonstrated various Ge passive devices, thermo-optic phase shifters, and modulators on a GeOI platform [3][4]. The propagation loss of a GeOI waveguide is one of the issues. The crystal defects induced by hydrogen implantation for smart-cut generates holes in a Ge film. As a result, evening using an n-type Ge donor wafer for smart-cut, a final Ge layer tends to be p-type. Since free-hole absorption in Ge is significant, the propagation loss of a p-type GeOI becomes large. To suppress hole generation, the optimization of the hydrogen implantation conditions was conducted. We found that the higher the implantation energy was, the deeper the center of the defect position was from a Ge surface [5]. When the implantation energy increases from 80 keV to 160 keV, the defects can be removed from a 300-nm-thick Ge device layer, enabling an n-type GeOI wafer. As a result, a low-loss GeOI waveguide with a propagation loss of 2.3 dB/cm was demonstrated. The monolithic integration of Ge passive waveguides and photodetectors (PDs) is also essential for a MIR PIC. We examined the defect-assist photodetection mechanism in a GeOI waveguide with a lateral PIN junction. When a reverse voltage was applied to the PIN junction, the substantial photocurrent was observed at a wavelength of 2 µm for which Ge is transparent. The defect-assist photodetection is expected to be enhanced due to the large intrinsic carrier density in Ge. As a result, the responsivity of 0.25 A/W was obtained at -5 V. In conclusion, we have developed a GeOI photonics platform for MIR wavelengths. We have achieved a low-loss Ge waveguide, PDs, and optical modulators, the indispensable building blocks for large-scale MIR PICs. The functionality of a GeOI platform can be extended with emerging materials including graphene and phase change materials. This work was partly based on the results from the project JPNP13004, commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and supported by JST-Mirai Program Grant Number JPMJMI20A1, JSPS KAKENHI Grant Number JP20H02198, and the Canon Foundation. Reference [1] Z. Zhao, C.-M. Lim, C. Ho, K. Sumita, Y. Miyatake, K. Toprasertpong, S. Takagi, and M. Takenaka, “Low-loss Ge waveguide at the 2-µm band on an n-type Ge-on-insulator wafer,” Opt. Mater. Express, vol. 11, no. 12, pp. 4097–4106, 2021. [2] J. Kang, X. Yu, M. Takenaka and S. Takagi, “Impact of thermal annealing on Ge-on-Insulator substrate fabricated by wafer bonding,” Materials Science in Semiconductor Processing, vol. 42, Part 2, pp. 259-263, 2016. [3] M. Takenaka et al., “Heterogeneous CMOS Photonics Based on SiGe/Ge and III–V Semiconductors Integrated on Si Platform,” IEEE J. Sel. Top. Quantum Electron., vol. 23, no. 3, pp. 64–76, May 2017. [4] T. Fujigaki, S. Takagi, and M. Takenaka, “High-efficiency Ge thermo-optic phase shifter on Ge-on-insulator platform,” Opt. Express, vol. 27, no. 5, p. 6451, Mar. 2019. [5] Z. Zhao et al., “Low-loss Ge waveguide at the 2-µm band on an n-type Ge-on-insulator wafer,” Opt. Mater. Express, OME, vol. 11, no. 12, pp. 4097–4106, Dec. 2021. [6] Z. Zhao, C.-P. Ho, K. Toprasertpong, S. Takagi, and M. Takenaka, “Monolithic Germanium PIN Waveguide Photodetector Operating at 2 μm Wavelengths,” Optical Fiber Communication Conference (OFC2020), W4G.3, San Diego, 8–12 March 2020.

We propose an optical phase modulator with a hybrid metal-oxide-semiconductor (MOS) capacitor, consisting of single-layer graphene and III–V semiconductor waveguide. The proposed modulator is numerically analyzed in conjunction with the surface conductivity model of graphene. Since the absorption of graphene at a 2 μm wavelength can be suppressed by modulating the chemical potential of graphene with the practical gate bias, the phase modulation efficiency is predicted to be 0.051 V·cm with a total insertion loss of 0.85 dB when an n-InGaAs waveguide is used, showing the feasibility of the low-loss, high-efficiency graphene/III–V hybrid MOS optical phase modulator, which is useful in the future 2 μm optical fiber communication band.

New high-quality thermoelectric devices with amorphous silicon/magnesium silicide (a-Si/Mg2Si)-stacked hetero-structure thin film layers have been fabricated using both radio frequency sputtering and rapid thermal annealing techniques. The thermoelectric properties of the a-Si/Mg2Si hetero-structure layers were found to be much superior to those of the a-Si layer.