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

TL;DR

This study presents the first direct momentum-resolved observation of two-dimensional electron systems on ultra-wide bandgap $eta$-Ga$_2$O$_3$, revealing unique electronic properties and potential for advanced electronic applications.

Contribution

It provides the first experimental ARPES measurements of 2D electron systems on UWBG $eta$-Ga$_2$O$_3$, uncovering carrier-density-dependent band renormalization and surface confinement effects.

Findings

Isotropic Fermi surface with high sheet carrier density

Electrons confined within 1.2 nm of the surface

Effective mass nearly doubles with increasing carrier density

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 $\beta$-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 $1.0\times10^{14}$ $\mathrm{cm}^{-2}$. Self-consistent Poisson-Schr\"odinger 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 $2\times10^{13}$ to $1.0\times10^{14}$ $\mathrm{cm}^{-2}$, the effective mass anomalously increases, nearly doubling to a final value of 0.48 $\textit{m}_{\mathrm{e}}$. This trend is notably opposite to that reported for other oxide semiconductors, pointing towards a unique renormalization mechanism in $\beta$-Ga$_2$O$_3$. Our findings establish the interfacial electronic structure of $\beta$-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.