Micro-Transfer Printing of O-band InP-InGaAs Photodiodes on Silicon Nitride Photonic Platforms

Introduction

Photonic integrated circuits (PICs) are essential components in various applications, including transceivers, LiDAR systems, optical sensing, and computing systems. While silicon-on-insulator (SOI) silicon photonics (SiPh) platforms have been widely used, silicon nitride (SiN) platforms offer several advantages, such as low propagation loss, a broad transparency window, and high power handling capability, making them an attractive alternative.

However, active components cannot be directly integrated onto low-pressure chemical vapor deposition (LPCVD) SiN platforms. This limitation can be overcome using micro-transfer printing (μTP), an emerging heterogeneous integration technology. μTP has demonstrated great versatility, enabling the integration of various devices, including III-V, silicon, and lithium niobate components, onto SiN platforms, significantly extending their functionality.

In this article, we will explore the heterogeneous integration of O-band InP-InGaAs photodiodes on an LPCVD SiN photonic platform using μTP technology.

Fabrication Process

The fabrication process can be divided into three main parts: the fabrication of transfer-printable photodiode coupons, μTP, and the post-printing process.

1. Fabrication of Transfer-Printable Photodiode Coupons

As illustrated in Fig. 1(a), photodiodes are fabricated on an InP-InGaAs layer stack, with an additional sacrificial InGaAs layer to release the device coupons from the InP substrate. A SiN layer is deposited and patterned for passivation and to create tethers to anchor the photodiodes on the substrate. After the devices are released, a PDMS stamp is laminated onto the fabricated device coupon, causing the SiN tethers to break, allowing the devices to be picked up by the stamp.

Fig. 1. (a) Simplified fabrication process flow, (b) Microscope image of a resulting waveguide-coupled photodiode.

2. Micro-Transfer Printing (μTP)

The photodiode is then transfer-printed onto a prepared SiN PIC (300 nm SiN waveguide layer thickness, 1.1 μm waveguide width).

3. Post-Printing Process

The post-printing process includes spin-coating BCB cladding and final metallization for GSG (ground-signal-ground) electrodes. Fig. 1(b) shows a microscope image of a resulting waveguide-coupled photodiode. The device features a pair of grating couplers for light input and GSG electrodes for electrical probing. A reference waveguide is included next to the device to calibrate the input optical power.

Simulation and Characterization

1. Dark Current Measurements

IV curves were measured to investigate the dark currents of the fabricated photodiodes. For a photodiode with a 6 × 25 μm^2 active region, as presented in Fig. 2(a), the dark currents were found to be 47.5 nA and 0.5 μA at -1 V and -3 V bias voltages, respectively.

Fig. 2. (a) IV curve for a photodiode with a 6 × 25 µm2 active region, (b) The side-view of the fundamental TE modelaunched into a photodiode with an active region length of 25 µm, (c) The measured responsivities of photodiodes with active region length varying from 11 µm to 25 µm at -3 V bias voltage at 1310 nm wavelength, compared with the simulation result.

2. Optical Mode Simulation

Fig. 2(b) shows the simulated side-view of the fundamental TE mode launched into a photodiode with a 25 μm long active region. The optical power is evanescently coupled from the SiN waveguide into the mesa of the photodiode and is rapidly absorbed without much propagation.

3. Responsivity Simulation and Measurement

The simulated responsivity at 1310 nm remains at 0.9 A/W when the active region length increases from 11 μm to 25 μm, as shown in Fig. 2(c). Responsivity measurements were carried out on photodiodes with different active region lengths at a wavelength of 1310 nm and at a -3 V bias voltage. The input optical power was calibrated using reference waveguides. The responsivity measurement results are plotted in Fig. 2(c), showing responsivities up to 0.9 A/W. Compared with the simulation results, the responsivity varies across different device lengths, which can be attributed to differences in the coupling efficiency between grating couplers.

Conclusion

In this article, we have demonstrated the heterogeneous integration of waveguide-coupled O-band InP-InGaAs photodiodes on an LPCVD SiN photonic platform using micro-transfer printing technology. The fabricated photodiodes exhibited responsivities up to 0.9 A/W at a wavelength of 1310 nm and a bias voltage of -3 V.

The micro-transfer printing technique showcases great versatility in enabling the integration of various active components on SiN photonic platforms, significantly extending their functionality. This technology paves the way for the development of advanced photonic integrated circuits with enhanced capabilities for a wide range of applications.

Reference

[1] Senbiao Qin, Laurens Bogaert, Jing Zhang, and Gunther Roelkens, "Micro-transfer printing of O-band InP-InGaAs photodiodes on a silicon nitride photonic platform," Ghent University - imec, Ghent, Belgium, 2024.