Unveiling two-dimensional electron systems on ultra-wide bandgap semiconductor β-Ga₂O₃

Ultra-wide bandgap (UWBG) semiconductors promise to revolutionize power electronics, yet a fundamental understanding of their interfacial electronic structure has been hindered by the absence of direct experimental observation. Here, we report the first momentum-resolved observation of two-dimensional electron systems on a UWBG material, enabled by angle resolved photoemission spectroscopy (ARPES) on high-purity $β$-Ga₂O₃ single crystals. Alkaline-metal-induced electron doping forms an isotropic circular Fermi surface, achieving a sheet carrier density of up to 1.0×10¹⁴ cm⁻². Self-consistent Poisson-Schrödinger calculations show that the electrons are confined within 1.2 nm of the surface and reveal an internal electric field of $18$ MV cm⁻¹. Crucially, our measurements reveal a pronounced renormalization of the electronic band structure: a series of carrier-density-dependent ARPES measurements shows that as the carrier density increases from 2×10¹³ to 1.0×10¹⁴ cm⁻², the effective mass anomalously increases, nearly doubling to a final value of 0.48 mₑ. This trend is notably opposite to that reported for other oxide semiconductors, pointing towards a unique renormalization mechanism in $β$-Ga₂O₃. Our findings establish the interfacial electronic structure of $β$-Ga₂O₃ and demonstrate that UWBG materials provide fertile ground for exploring carrier-density-driven electronic phenomena, opening new avenues for future quantum and power devices.