Investigation of the Mechanism for Ohmic Contact Formation in Ti/Al/Ni/Au Contacts to β-Ga2O3 Nanobelt Field-Effect Transistors.

The issue of contacts between the electrode and channel layer is crucial for wide-bandgap semiconductors, especially the β-Ga2O3 due to its ultra-large bandgap (4.6-4.9 eV). It affects the device performance greatly and thus needs special attention. In this work, the high-performance β-Ga2O3 nanobelt field-effect transistors with Ohmic contact between multilayer metal stack Ti/Al/Ni/Au (30/120/50/50 nm) and unintentionally doped β-Ga2O3 channel substrate have been fabricated. The formation mechanism of Ohmic contacts to β-Ga2O3 under different annealing temperatures in an N2 ambient is systematically investigated by X-ray photoelectron spectroscopy. It is revealed that the oxygen vacancies at the interface of β-Ga2O3/intermetallic compounds formed during rapid thermal annealing are believed to induce the good Ohmic contacts with low resistance. The contact resistance (Rc) between electrodes and unintentionally doped β-Ga2O3 reduces to ∼9.3 Ω mm after annealing. This work points to the importance of contact engineering for future improved β-Ga2O3 device performance and lays a solid foundation for the wider application of β-Ga2O3 in electronics and optoelectronics.