PhD Project: Developing next generation cleanroom processing technologies for Silicon Carbide power electronic devices

Supervisor: Dr Vishal Shah

Students must have a minimum Bachelors degree of 2.1 (UK) or international equivalent qualifications and a degree in experimental engineering/chemistry/physics.

Wide bandgap (WBG) semiconductor materials such as Silicon Carbide (SiC) and Gallium Nitride (GaN) have recently been introduced into the power electronics market in the last 5 years, with early adoption of SiC devices in hybrid and fully electric cars as the main driver of industrial uptake. Predictions state that the power market will be worth $43bn by 2030 with direct relevance to achieving the green energy adoption for many governments’ net zero carbon emissions commitment. A major argument for the adoption of SiC device manufacturing is the ability to use existing mature fabrication processes for Silicon device fabrication.

However, a major challenge is that SiC’s newly recent uptake has not allowed some of these transferred processes to be investigated in detail, with a view on how it affects the device performance or reliability. The importance here is that SiC is a “harsh environment” material – it takes a lot of energy to fabricate important components like MOS layers, ohmic contacts or Schottky contacts. Things like mesa etching, oxide growth or dopant activation are fundamentally very different and requires radical new approaches to improve the device technology.

In this studentship, the student will develop semiconductor processes for SiC power electronics and combine this with both electrical and materials characterisation. The student will learn, implement and develop fabrication techniques within the NanoFab-RTP cleanroom with physical characterisation (e.g. XPS, XRD, electron microscopy or spectroscopy). Then they will evaluate these processes in electrical characterisation to evaluate these new fabrication technologies for use in SiC power device technology.

Applications are open and rolling throughout the year, contact Dr Shah in the first instance (vishal.shah@warwick.ac.uk).

PhD Project: Photoelectrochemical Generation of Green Hydrogen using Silicon Carbide

Supervisor: Dr Vishal Shah

Students must have a minimum Bachelors degree of 2.1 (UK) or international equivalent qualifications and a degree in experimental engineering/chemistry/physics.

In this proposal we intend on developing integrated semiconductor-electrocatalyst devices for the generation of green hydrogen using Silicon Carbide materials as the photoabsorber.

Silicon Carbide (SiC) is a wide bandgap semiconductor which has high critical electrical fields up to 2.8 MV/cm that result in advantages in power electronics as well as a high thermal conductivity (up to 4.5W/ (cm × K)) to allow high temperature operation. Moreover, it is extremely chemically stable. SiC has around 250 polymorphs. From this, only 4H-SiC (3.3 eV bandgap) has emerged as a commercial product on the market as the replacement to Silicon in power electronics. However, 4H-SiC requires its own dedicated substrate (up to 150 mm available), which are still expensive at ~£700 per wafer and which require a significant manufacturing thermal budget. 3C-SiC is a polytype with a slightly smaller bandgap (2.3 eV) and breakdown electric field (1.4 MV/cm), but can be grown on top of cheap Silicon substrates, up to 450mm in diameter. The development of 3C-SiC on Si is limited by defects, bow and thickness limitations due to the lattice mismatch between the Si and 3C-SiC.

3C-SiC has also shown promise in the photoelectrochemical (PEC) generation of H₂, as the conduction and valence band positions of 3C-SiC ideally overlap the water redox potentials [1], allowing it to be used as both anode and cathode with photogenerated carriers and without external bias. The other advantages of 3C-SiC PEC are that it can be fabricated sustainably and at low costs, only requiring modest system complexity. It can moreover achieve an acceptable solar-to-hydrogen conversion efficiency > 10%, with current densities up to 8 mA/cm²[2]. The other main advantageous consideration is that since SiC is extremely chemically stable, hence has a chance of long-term, low maintenance implantation in the field.

For the use of only a standalone 3C-SiC layer, both PEC applications require thicknesses above 40 µm [1], which have previously required the use of expensive bulk growth. Hence, this proposal is timely due to the knowledge which has recently been developed at UoW for 3C-SiC epitaxy.

This studentship will expand on work being performed on a £100k grant won from the Henry Royce Institute. The successful student will both 1) develop 3C-SiC materials through nanostructuring the materials via conversion (e.g., SiC nanoparticles, SiC nanowires, SiC nanopores, trenching etc.) and 2) developing the overall SiC PEC system for hydrogen evolution and water-splitting through optimisation of a multi-material system with catalysts.

References:

[1] Jian et al. Sol. RRL 2020,4, 2000111

[2] Li et al. Catal. Sci. Technol., 2015, 5, 1360

Applications are open and rolling throughout the year, contact Dr Shah in the first instance (vishal.shah@warwick.ac.uk).

PhD Project: 3C-SiC - the renaissance of a wide bandgap material (x2 projects available)

Supervisor: Dr Vishal Shah

Students must have a minimum Bachelors degree of 2.1 (UK) or international equivalent qualifications and a degree in experimental engineering/chemistry/physics.

3C-SiC is a material which has potential as a replacement for 4H-SiC, due to its ability to be epitaxially grown on cheaper Silicon substrates, whilst having better largely reducing material costs. Furthermore compared to 4H-SiC, 3C-SiC has a potential larger mobility and a the larger conduction band offset at the 3C-SiC/SiO2 interface theoretically enables MOS devices to have considerably lower leakage currents and interfacial charge levels than unipolar MOS devices fabricated on 4H-SiC.

However, the large mismatch in physical, thermal and crystallographic properties between 3C-SiC and the underlying Si substrate hampers the development of this material due to the larger defect levels in the material. Even though high mobility 3C-SiC MOSFETs have been reported in the past decade, in-depth studies of 3C-SiC MOS devices still show high leakage levels and positive charge at the 3C-SiC/SiO2 interface.

There are two projects in this area:

1. New processing techniques will be developed at Warwick to address these materials issues and will be implemented in a 3C-SiC power electronics device.

2. The student will develop SiC materials and fabrication methodologies to contribute to micro electronic mechanical systems (MEMS) structures for power devices and sensors

Applications are open and rolling throughout the year, contact Dr Shah in the first instance (vishal.shah@warwick.ac.uk).

Propose your own

I am happy for students to propose any subjects in the wide bandgap semiconductor area to do with: material fabrication, materials characterisation, device fabrication techniques, electrical characterisation or new functionalisation (sensors, quantum, chemical processing, harsh environment electronics etc). Please get in touch!