Diamond and Gallium Oxide Interfaces for Power Electronics Devices

Transistors form the backbone of our technological world. Though modern silicon processing enables the integration of thousands of transistors onto a single small chip, silicon has its limitations, especially for high-voltage and high-power applications. As demands grow for an adaptive “smart grid”—an electrical network capable of regulating and distributing renewable energy, charging electric vehicles, and powering homes and workplaces—there is a need for transistors that can manage higher voltages and power levels than existing silicon-based and current state-of-the-art technologies. The Diamond and Gallium Oxide interfaces for Power Electronic Devices (DOPED) project focuses on two ultra-wide bandgap semiconductors: diamond and gallium oxide. These materials hold potential for creating advanced power electronic devices due to their inherent high-voltage capabilities and should give rise to the ultimate power electronic device, i.e. it is hard to see further potential for increasing the high voltage capability with any other elements of the periodic table. However, significant research is required to fully harness their potential and integrate them into mainstream transistor technologies. Over the past year, the DOPED project has made several notable advancements. We have expanded our team with the recruitment of a PhD student who has already contributed significantly to our research efforts. She has presented our work at both national and international conferences and received a "Best Poster" prize at the 3rd Annual TNNN conference held in Oslo this year, organized by the Research School for Training the Next Generation of Micro- and Nanotechnology Researchers in Norway (TNNN). Our investigations into the growth of the Beta-phase gallium oxide using remote plasma deposition are showing clear progress, we have investigated the effects of the substrate temperature during growth as well as the pressure of the growth chamber and the oxygen/argon ratio used. These projects take place by starting two master’s projects in collaboration with the Center for Materials Science and Nanotechnology (SMN). One of our master's students also gained recognition by winning the "Best Poster" prize at the centre's Winter Seminar. We have also advanced our project through access to international synchrotron research facilities. Our team has been awarded a week of synchrotron beam-time at the ISA facility in Aarhus, Denmark. The results we gain from our experiments originate from the upper most atomic layers of a material. For us, they are providing invaluable insights into the chemistry of gallium oxide formation on oxygen-terminated diamond surfaces. These studies focus on evaluating the critical parameters required for a gallium oxide crystal layer to seed its growth on the diamond surface. The goal is to gain the knowledge required achieve a low density of defects at the interface between the two materials, crucial to eventually achieving high quality electrical devices. Additionally, we attended the 37th European Conference on Surface Science (ECOSS37) in the UK this year, where we had the opportunity to meet with members of our international advisory board. These meetings facilitated the planning of collaborative experiments focusing on generating ultra-flat diamond surfaces, which provide an ideal starting point for our gallium oxide layers. This collaboration aims to enhance the quality and performance of the interfaces we are investigating. As we continue to explore these tow ultra-wide bandgap semiconductors, we aim to build a comprehensive knowledge base that will advance the technological relevance of diamond and gallium oxide as a material combination for power electronic devices. Key questions remaining are, could alloying help make a better crystal match between the diamond and gallium oxide? What are the main defects that form, and do they influence the electrical properties? How do we make electrical contacts to withstand the high fields and temperatures the device will be subjected to eventually? The progress made over the past year highlights measured steps towards obtaining our objectives and offers a solid foundation for the work to come.

Recently the beta phase of Ga2O3 has attracted significant attention as an emerging ultra-wide bandgap (UWBG) semiconductor for applications in power electronics. Most of the attention stems from its potential to operate at higher voltages and frequencies than the current front runners SiC and GaN. However, Ga2O3 suffers from two main drawbacks; 1) The intrinsic n-type nature of the defects means that p-type material is difficult to achieve, and 2) Poor thermal conductivity results in self-heating during operation. Diamond is another promising UWBG semiconductor that has potential to operate at high voltages, whilst its unparalleled thermal conductivity allows for devices to operate at elevated temperatures. However, the synthesis of a n-type material still remains challenging. The difficulty in preparing materials with either p or n-type doping for Ga2O3 and diamond respectively, prohibits the creation of efficient homojunctions, however their combination will result in heterojunction that allows for the excellent properties of both materials to be fully realised and exploited. To accomplish this, we plan to explore the value chain from fundamental science to prototype heterojunction fabrication and benchmarking, by utilising advanced synthesis and materials characterisation/modelling techniques to provide continuous feedback for the synthesis of epitaxial layers and development of contacts. Photoemission spectroscopy/microscopy, x-ray and electron diffraction as well as junction specific electronic spectroscopies will form the main analysis techniques, complemented by DFT calculations of the optimised crystal structures, electronic structure and device parameters. The ambition of the DOPED project is that the results will directly feed into the preparation of a working heterojunction device, whilst outlining the requirements for these materials to be utilised in bipolar junction transistors, a highly sought after technological step for both materials.