Quantum Transport in Epitaxial Ultra Wide Bandgap Aluminum Gallium Oxide Tunnel Heterostructures*

In this work, we studied the epitaxial growth of the wide-band gap, monoclinic beta-Ga₂O₃ (~4.4-4.9 eV) and (Al_xGa_1-x)₂O₃ (~4.4-9.0 eV, Al% = 0-100) using molecular beam epitaxy on (010) beta-Ga₂O₃ substrates. We will discuss the optimum growth conditions for both materials and how the formation of Ga interstitial-divacancy complexes in unoptimized growths of (Al_xGa_1-x)₂O₃ can lead to the formation of unwanted phases of Ga₂O₃ in our heterostructures. Additionally, tunnel barrier structures made with an (Al_xGa_1-x)₂O₃ layer sandwiched between two n+ Ga₂O₃ layer are studied to explore tunneling behavior in this material system. The current-voltage characteristics are measured for a varying Al composition and the (Al_xGa_1-x)₂O₃ barrier thickness. Using the Wentzel–Kramers–Brillouin approximation and Non-equilibrium Green’s Function formalism, the current-voltage characteristics of these tunnel barrier devices are simulated and compared to experimental data. The above study helps identify the conduction band offset ΔE_c a critical unknown in this material family directly from transport. The ΔE_c thus found between beta-(Al_xGa_1-x)₂O₃ and beta-Ga₂O₃ is compared to those extracted from XPS, capacitance-voltage measurements, and also by DFT.