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 degrees C, the GaN layers were deposited at two different temperatures (690 degrees C, 1050 degrees C). Preliminary electrical and SIMS investigations have evidenced 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 mu m thick GaN layer. The rocking curve of the GaN layers grown epsilon-Ga2O3 /sapphire at standard high temperature (1050 degrees C) 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).