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

Heterostructures made of GaN and ɛ-Ga₂O₃ epitaxial layers may be very interesting because they could exploit the high electron mobility of GaN combined with the ferroelectric character of ɛ-Ga₂O₃.

We have explored the possibility of using ɛ-Ga₂O₃ templates, deposited by metalorganic chemical vapor deposition on sapphire substrates, in order to reduce the lattice mismatch of GaN with sapphire. Considering that ɛ-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 ɛ-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 ɛ-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 ɛ-Ga₂O₃ on GaN templates. We evidenced that ɛ-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 ɛ-Ga₂O₃ layers on GaN layers opens interesting possibilities for the development of novel high electron mobility transistors (HEMT).