Polarization engineering of two-dimensional electron gas at ϵ-(Al_xGa_1-x)₂O₃/ϵ-Ga₂O₃ heterostructure

In this study, we present an investigation of the spontaneous and piezoelectric polarization in the ϵ-(Al_xGa_1-x)₂O₃/ϵ-Ga₂O₃ heterostructures using density functional theory calculations. The spontaneous polarization (P_sp) was found to increase from 23.93 to 26.34 μC/cm² when Al-content increase from 0% to 50%. With Al-content increasing, the strain-induced piezoelectric polarization (P_pe) increases, which negates the P_sp, causing the total polarization (P_tot) of the epitaxy layer in the AlGaO/GaO heterostructure to remain almost constant across all Al compositions. Additionally, due to the ferroelectric nature of ϵ-Ga₂O₃, a high-density polarization-induced two-dimensional electron gas (2DEG) can be formed at the interface of polarization reversed ϵ-(Al_xGa_1-x)₂O₃/ϵ-Ga₂O₃ when an electric field is applied. Using the 1D Schrödinger-Poisson model, the 2DEG of polarization reversed ϵ-(Al_0.125Ga_0.875)₂O₃/ϵ-Ga₂O₃ was found to be 2.05 × 10¹⁴ cm⁻², which is nearly ten times larger than that of GaN-based structures. Our work indicates that ϵ-(Al_xGa_1-x)₂O₃/ϵ-Ga₂O₃ heterostructure could play a key role attributed to the large polarization and capability in modulating the polarization for high-power electronic and radio frequency device applications.