Transistors form the backbone of our technological world, and these days thanks to modern silicon processing we can fit thousands of transistors on a single small chip, but silicon is not great at everything, and the search for new materials that can be used to make other useful transistors is ceaseless. More recently there is a demand to redesign our electrical grid into one that is adaptive, a so called “smart grid”. The grid must be able to regulate and distribute all the renewable energy sources we are adding, facilitate charging of electrical vehicles and power our homes and workplaces etc. For this we need transistors that can handle much higher voltages and power than silicon and current state-of-the-art power electronic technologies can offer.
The Diamond and gallium Oxide interfaces for Power Electronic Devices (DOPED) project aims to utilise two semiconductors that fall into a category of materials classed as ultra-wide bandgap semiconductors, namely diamond, and gallium oxide. These two materials should give 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, but there is a long way to go to bring these materials to the forefront of transistor technologies.
We first need to gain a lot of knowledge about how the charge carriers in each material behave when we form an interface between the two. We need to answer questions such as, how does one material grow on the other? 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? We aim to build this knowledge base and move the concept to a higher technologically relevant level.
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.