Unveiling two-dimensional electron systems on ultra-wide bandgap semiconductor ${\hat{\mathrm{I}}}^2$-Ga${}_2$O${}_3$

Abstract

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 ${\hat{I}}^2$-Ga${}_2$O${}_3$ single crystals. Alkaline-metal-induced electron doping forms an isotropic circular Fermi surface, achieving a sheet carrier density of up to $1.0\times 10^{14}$ ${\mathrm{cm}}^{-2}$. 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 $2\times 10^{13}$ to $1.0\times 10^{14}$ ${\mathrm{cm}}^{-2}$, the effective mass anomalously increases, nearly doubling to a final value of 0.48 ${\text{m}}_{\mathrm{e}}$. This trend is notably opposite to that reported for other oxide semiconductors, pointing towards a unique renormalization mechanism in ${\hat{I}}^2$-Ga${}_2$O${}_3$. Our findings establish the interfacial electronic structure of ${\hat{I}}^2$-Ga${}_2$O${}_3$ and demonstrate that UWBG materials provide fertile ground for exploring carrier-density-driven electronic phenomena, opening new avenues for future quantum and power devices.

18 pages, 3 figures