Extended InGaAs Photodiode Integrated on SOI Waveguide Circuit for 2 μm Waveband

Introduction

The short-wave infrared (SWIR) wavelength range from 2-2.5 μm holds great potential for sensing applications due to the strong absorption of important molecules like H2O, CO2, CH4, NH3, and lactate in this waveband. Silicon photonics integrated circuits can leverage this by incorporating photodetectors sensitive to the 2 μm wavelength region. In this work, researchers from Tyndall National Institute and Munster Technological University demonstrate the micro transfer printing of 2 μm bandgap InGaAs photodiodes onto a silicon-on-insulator (SOI) waveguide circuit.

Design and Fabrication

The photodiode structure consists of a p-i-n junction with 25 strain-balanced InGaAs quantum wells in the intrinsic region, grown by MOCVD on an InP substrate. The quantum well bandgap is designed for 1975 nm (2 μm waveband). A 500 nm InAlAs sacrificial layer enables device release for transfer printing.

Fabrication begins with mesa etching to define diameters from 50-100 μm, followed by ohmic contact formation. A grating structure is etched around 500x300 μm coupons, which are undercut using wet etching to leave tethers for transfer printing.

The SOI waveguide circuit uses 400 nm thick silicon with a 3 μm buried oxide layer. E-beam lithography and reactive ion etching define 225 nm minimum feature sizes. The waveguides use a 400 nm etch depth, while grating couplers have a 157 nm shallow etch for 1960 nm peak response with -6.5 dB coupling efficiency and 42 nm 3dB bandwidth.

The 50 μm diameter photodiodes are transfer printed and aligned to the waveguide grating couplers, with 1 μm intervia coating promoting adhesion, as shown in Figure 1.

Figure 1: (a) Schematic of transfer printed PD on SOI (b) Characterization setup of printed PD and inset optical image of transferred device.

Measurement and Results

The photodiode absorption spectrum is measured by illuminating the top surface while sweeping the wavelength from a Xe/quartz-tungsten source and monochromator. Figure 2(a) shows a strong excitonic peak at 1962 nm under zero bias, which red-shifts and broadens under reverse bias due to the quantum-confined Stark effect (QCSE), validating quantum well absorption.

Figure 2: (a) Absorption vs wavelength sweep under different bias condition (b) Measured dark current before and after printing on SOI (c) Response of the grating coupler (d) Reverse bias characteristics under illumination with 1967 nm laser light through the grating coupler (e) Measured photocurrent for different input power at a reverse bias of 2 V (f) Measured change in absorption of printed device with bias due to QCSE under illumination of 2000 nm light.

The dark current, measured before and after transfer printing in Figure 2(b), improves after printing on SOI, confirming the process does not degrade performance.

To characterize the printed photodiodes integrated with the SOI waveguides, the fiber-coupled grating response in Figure 2(c) is used, with -6.6 dB coupling efficiency at 1967 nm.

Under illumination at 1967 nm through the grating coupler, the photocurrent increases linearly with reverse bias in Figure 2(d), with 0.45 A/W responsivity at 2V reverse bias.

The linear photocurrent dependence on input optical power at 1967 nm is shown in Figure 2(e).

Finally, the QCSE-induced change in absorption amplitude with applied reverse bias is measured at 2000 nm wavelength when illuminating through the grating coupler in Figure 2(f).

Figure 2: (f) Measured change in absorption of printed device with bias due to QCSE under illumination of 2000 nm light.

Conclusion

This work successfully demonstrates the micro transfer printing integration of InGaAs photodiodes designed for 2 μm waveband absorption onto SOI waveguide circuits. The printed photodiodes exhibit low dark current below 15 nA and 0.45 A/W responsivity at 1967 nm wavelength when coupled through grating couplers. The compact footprint enabled by transfer printing shows Promise for dense integration of SWIR photodetectors with silicon photonics for on-chip molecular sensing applications in healthcare, environmental monitoring and more.

Reference

[1] Y. Arafat, B. Roycroft, J. Justice, L. O’Faolain, A. Gocalińska, E. Pelucchi, F. Atar, F. Gunning, E. Russel, B. Corbett, "Extended InGaAs Photodiode Integrated on SOI Waveguide Circuit for 2 µm Waveband," Tyndall National Institute, Cork, Ireland; Munster Technological University, Cork, Ireland, 2024.