Epitaxial Growth of GaN/Ga2O3 and Ga2O3/GaN Heterostructures for Novel High Electron Mobility Transistors

Heterostructures made of GaN and epsilon-Ga2O3 epitaxial layers may be very interesting because they could exploit the high electron mobility of GaN combined with the ferroelectric character of epsilon-Ga2O3. We have explored the possibility of using epsilon-Ga2O3 templates, deposited by metalorganic chemical vapor deposition on sapphire substrates, in order to reduce the lattice mismatch of GaN with sapphire. Considering that epsilon-Ga2O3 is metastable and undergoes a first phase transition at around 700 °C, the GaN layers were deposited at two different temperatures (690°C, 1050 °C). Preliminary electrical and SIMS investigations have enced the diffusion of oxygen from the epsilon-Ga2O3 to the GaN epitaxial layer, which results in an n-type conductivity and a sheet resistance as low as 70 Ohm/sq in a 1 ?m thick GaN layer. The rocking curve of the 2 GaN layers grown epsilon-Ga2O3/sapphire at standard high temperature (1050ºC) indicates a crystal quality worse than for GaN deposited directly on sapphire. In parallel, we studied the nucleation of epsilon-Ga2O3 on GaN templates. We evidenced that epsilon-Ga2O3 nucleates in 3D islands on the surface of GaN grown on on-axis sapphire, with coalescence taking place as the layer grows thicker. The use of off-cut sapphire substrates, instead, permits to inhibit islands formation, resulting in a smoother layer. The possibility of obtaining uniform and very thin epsilon-Ga2O3 layers on GaN layers opens interesting possibilities for the development of novel high electron mobility transistors (HEMT).