Epitaxial growth of GaN/Ga2O3 and Ga2O3/GaN heterostructures for novel high electron mobility transistors

Heterostructures made of GaN and ε-Ga2O3 epitaxial layers may be very interesting because they could exploit the high electron mobility of GaN combined with the ferroelectric character of ε-Ga2O3. We have explored the possibility of using ε-Ga2O3 templates, deposited by metalorganic chemical vapor deposition on sapphire substrates, in order to reduce the lattice mismatch of GaN with sapphire. Considering that ε-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 evidenced the diffusion of oxygen from the ε-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 μm thick GaN layer. The rocking curve of the GaN layers grown ε-Ga2O3/sapphire at standard high temperature (1050 °C) indicates a crystal quality worse than for GaN deposited directly on sapphire. In parallel, we studied the nucleation of ε-Ga2O3 on GaN templates. We evidenced that ε-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 ε-Ga2O3 layers on GaN layers opens interesting possibilities for the development of novel high electron mobility transistors (HEMT).