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

Heterostructures made of GaN and e-Ga(2)O(3) epitaxial layers may be very interesting because they could exploit the high electron mobility of GaN combined with the ferroelectric character of e-Ga(2)O(3). We have explored the possibility of using e-Ga(2)O(3) templates, deposited by metalorganic chemical vapor deposition on sapphire substrates, in order to reduce the lattice mismatch of GaN with sapphire. Considering that e-Ga(2)O(3) 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 e-Ga(2)O(3) to the GaN epitaxial layer, which results in an Il-type conductivity and a sheet resistance as low as 70 Ohm/sq in a 1 gm thick GaN layer. The rocking curve of the GaN layers grown e-Ga(2)O(3)/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 e-Ga(2)O(3) on GaN templates. We evidenced that e-Ga(2)O(3) 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 e-Ga(2)O(3) layers on GaN layers opens interesting possibilities for the development of novel high electron mobility transistors (HEMT).