Most GaN-based optoelectronic devices suffer from low electrical conductivity of the p-doped GaN. To achieve uniform current injection, transparent current spreading layers such as indium tin oxide (ITO) are often used on top of the p-doped GaN layer. Achieving high quality ITO films requires very precise process control and p-GaN is very sensitive to plasma-induced damage, making it difficult to deposit a layer on top by sputtering (which is used to deposit ITO). Moreover, the absorption in ITO rapidly increases when the emission wavelength approaches 400 nm, and finding alternative at these short wavelengths is thus necessary.
A promising candidate to replace ITO is graphene, which has high transmittance over a very wide wavelength range and also a low sheet resistance. To ensure a high quality interface between graphene and GaN, direct growth of graphene on GaN is preferable instead of transferred graphene.
Initial tests show that it is possible to grow graphene-like films directly on the GaN-surface, but the sheet resistance of graphene as well as its contact resistance to p-GaN are too high for the intended application. This master thesis project will therefore be focused on optimizing the process for growing graphene by chemical vapour deposition (CVD) directly on GaN. The electrical properties of the graphene films will be evaluated by Hall measurements and the contact resistance between graphene and GaN by transmission line method (TLM) measurements. The main parts of this master thesis will thus be
· Direct growth of graphene on GaN by plasma-enhanced CVD
· Evaluation of the graphene by Raman spectroscopy
· Evaluation of optical transmittance and electrical properties by Hall and TLM measurements
Most of the research activities will be carried out using an Aixtron CVD system in Chalmers’s world-class clean room. This is a one-year or six-month thesis project jointly supervised by Drs. Jie Sun at the Quantum Device Physics Laboratory (firstname.lastname@example.org) and Åsa Haglund (email@example.com) at the Photonics Laboratory.