Towards a GaN-on-sapphire photonic integrated circuit via micro-transfer printing
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Date
2024
Authors
O'Brien, Megan
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Publisher
University College Cork
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Abstract
Photonic integrated circuits (PICs) can alleviate the pressure on existing infrastructure for increasing bandwidth requirements by potentially offering faster data transmission with low energy consumption all on the same chip. The ability to integrate active lasing sources to waveguide platforms while also maintaining low-loss coupling is key for efficient PICs.This research focuses on the development of an AlGaN/GaN-on-sapphire platform and the integration of 1.3 µm edge-emitting InP-based multi-quantum well (MQW) and GaAs-based quantum dot (QD) lasing devices to the platform via micro transfer printing (µTP). Gallium nitride (GaN) can be used alternatively to traditional Si-based photonics due to its broad spectral transmission, birefringence, refractive index suitable for fiber coupling, and the possession of both the linear electro-optic effect and non-linear properties.
First, we design an AlGaN/GaN structure that creates a single-mode, broad passive spot size for relaxed alignment tolerance of the transfer-printable
laser-to-waveguide. Coupling losses as low as 0.6 dB were calculated for the optimal positioning of the laser. We also outline a double-tapered mode
adapter that converts the dimensions to more suitable structures for light-routing. The integration of LiNbO3 coupon to the AlGaN/GaN platform is
explored through simulations in order to enhance the electro-optic property, and a Mach-Zehnder modulator (MZM) design is proposed. Additionally, the simulations of (i) directional couplers, (ii) polarization converters and (iii) grating couplers on an AlGaN/GaN platform are presented in this work.
Next, the fabrication of AlGaN/GaN waveguides devices through BCl3-based etching recipes, along with the challenges of achieving smooth and vertical waveguide sidewalls, are discussed. A BCl3 dry etch for forming a trench into sapphire is also developed for µTP alignment purposes. The AlGaN/GaN waveguides were then characterized, and we distinguish the differences in propagation losses between the TE and TM polarizations.The average propagation losses ranged between 2.4-14.7 dB/cm for both polarizations. Following on from this, both the InP-based MQW and the GaAs-based QD laser coupons were characterized and transfer-printed and coupled to the fabricated GaN waveguides. A coupling output of 2.3 mW at 80 mA was achieved for the QD laser, giving a coupling efficiency of 10.3 %. The results show promise for GaN to be a suitable platform for integrated photonics.
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Keywords
Photonics , Integrated photonics , Heterogenenous integration , Micro-transfer printing , Gallium nitride , Sapphire , Photonic integrated circuits , Quantum dot lasers
Citation
O'Brien, M. R. 2024. Towards a GaN-on-sapphire photonic integrated circuit via micro-transfer printing. PhD Thesis, University College Cork.