Investigation of ultra-thin gallium compound layers fabricated by liquid metal chemistry for hybrid integrated photonics
Creators: name: Pedram, Panteha
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400909
subject: Photonic and electro-optical devices, sensors and systems (excl. communications)
classificationCode: 401605
subject: Functional materials

description: In today's digital age, the demand for internet connectivity is growing at an unprecedented rate. Conventional optical transceivers, although effective, are often bulky, costly and energy consuming, limiting their widespread implementation. It is imperative to seek out innovative technologies that can address this demand for high-speed data transmission while overcoming the limitations of traditional solutions. In the span of the past two decades, the landscape of photonic technology has undergone a dramatic transformation, with silicon photonics emerging as a mature technological platform. By integrating various optical components on a silicon substrate, it can provide advanced functionality while harnessing the advantages of silicon's well-established manufacturing processes. With its ability to transfer optical data at very high speeds, across a wide bandwidth, with its power efficiency and scalability, silicon photonics has created an effective and compact technology for telecommunications, data centers, and sensing systems. Nowadays, multiple optical functions can be integrated onto a single chip, resulting in the availability of optical systems including electro-optic modulators, Silicon germanium (SiGe) photodetectors, and low-loss silicon waveguides.

However, silicon encounters intrinsic limitations regarding light emission or nonlinear functions, calling for the use of complementary materials. The heterogeneous integration of III-V materials onto silicon has offered one route to overcome these limitations . Concurrently, several material candidates are being investigated for their nonlinear properties with an aim to integrate them onto the mature silicon photonic platform. However, despite the high application potential of nonlinear optics, no clear nonlinear material candidate has clearly emerged to complement silicon photonics. Hence, scientists continue to search for materials with strong nonlinearities even at a reduced scale, which could be readily integrated with silicon photonics.Enter two-dimensional (2D) materials, a promising contender in the quest for the ideal nonlinear material. These materials, compatible with planar photonic devices through post-processing techniques, bring new properties to the table that can potentially complement silicon photonics.

Therefore, our research endeavors began with the aim of identifying new 2D materials that have the potential to complement silicon photonics for applications at around 1,55um wavelength. Among the candidates, gallium oxide (Ga2O3) and gallium nitride (GaN) have gained significant attention due to their impressive optoelectronic. However, it is worth noting that the existing research primarily focuses on the properties of Ga2O3 and GaN in bulk or thick layers, leaving a significant knowledge gap regarding their behavior in the 2D or ultrathin dimensions. Recognizing this gap, we aim to shed light on the unique properties of these materials in the 2D realm.

My PhD research project was centred on three main focal points:
- the synthesis of ultrathin Ga2O3 and GaN through an inventive method known as liquid metal chemistry
- the structural and optical characterization of these ultrathin Ga-based materials
- the integration of these materials into photonic devices.

This project is part of an international cotutelle between Ecole Centrale de Lyon (ECL) and Royal Melbourne Institute of Technology (RMIT) under the umbrella of European Union and Marie Curie framework. The investigative endeavor this project presented unfolded across two state-of-the-art laboratories:
(i) Institute of Nanotechnologies of Lyon (INL) and NANOLYON nanotechnology platform
(ii) RMIT Micro Nano Research Facility (MNRF) and the assistance of RMIT Microscopy and Microanalysis Facility (RMMF).

In Chapter 1: 'State of the Art', we provide a thorough overview of research literature related to 2D materials, particularly emphasizing Ga2O3 and GaN.This chapter explores the ongoing journey to find a method that is affordable, scalable, and efficient in producing high-quality Ga2O3 and GaN. Moreover, we are looking for a technique that allows for easy integration of these materials into photonic devices. We also discussed the structure and characteristics of liquid metal as a method for synthesizing these materials. This discussion highlights the importance of finding new techniques to overcome current obstacles that prevent us from fully harnessing the potential of 2D materials and their integration on photonic devices.

Chapter 2: expands upon our exploration by tackling a central issue: large-area, ultra-thin layer fabrication. Here, we introduce an inventive approach in the form of liquid metal chemistry has been recently developed  at RMIT university and mainly relies on a two-step process . This method has exhibited encouraging outcomes in the scalable synthesis of exceedingly thin layers of Ga2O3 and GaN. Next, the chapter delves into intricate details, elaborating on the various characterization methods utilized to investigate the topography, spectroscopy, and optical traits of the synthesized ultra-thin Ga2O3 and GaN.

Chapter 3: presents the experimental results of the Ga-based ultra-thin layers synthesized by the method developed in chapter 2. It also turns our attention to a pivotal element of our research – exploring Gallium OxyNitride (GaOxNy) compounds, an intermediate stage between Ga2O3 and GaN. Additionally, this chapter exhibits the results from the characterization methods previously introduced in Chapter 2. It proffers an in-depth examination of the structural and optical attributes of these intermediate compounds and Ga2O3 and GaN. We combined sophisticated characterization methods, Raman measurement, and Density functional theory (DFT) calculations to achieve this.

In the final chapter (Chapter 4:) we develop an approach to integrate 2D Ga2O3 and GaN materials into photonic devices. We exploit here Mach-Zehnder Interferometers (MZIs) and selective patterning to study the optical properties of hybrid SiN waveguides covered with ultra-thin Ga-based compounds. This step is crucial, for it paves the way towards realizing hybrid integrated photonics with these 2D materials. We discuss the integration process in detail, the challenges faced, and the ways to overcome these challenges. We undertook precise linear measurements on the MZIs to characterize the properties of Ga2O3 and GaN in detail. The initial choice of these materials was driven by our aim to develop nonlinear 2D materials for chip-based devices. However, it's important to acknowledge that in this phase of our research, we focused on their linear properties and have not yet studied their nonlinear characteristics. Nonetheless, this investigation underscores the significant potential of these 2D materials in advancing the field of integrated photonics and paves the way for further studies into their nonlinear properties.