Formation of quasi-two-dimensional electron gas at the interface of ZnO/Ga2O3 heterostructure

Abstract

A two-dimensional electron gas (2DEG) was formed at the interface of an ultrathin ZnO/Ga2O3 heterostructure deposited by atomic layer deposition (ALD) at 200 °C on a SiO2/Si substrate. It is found that the sheet resistance decreases steeply to ∼104 Ω/□ as the thickness of the ZnO layer increases to a certain extent, and an optimal thickness of the top layer is determined. An ∼50-nm-thick β-phase Ga2O3 layer was formed after annealing at 800 °C, and a 12-nm-thick amorphous ZnO layer was observed by x-ray diffraction and transmission electron microscopy. The oxygen defects were analyzed by x-ray photoelectron spectroscopy. The defect energy levels of oxygen vacancies in Ga2O3 and ZnO thin films were calculated by first-principles simulation, and oxygen vacancies were generated at the interface of the ZnO/Ga2O3 thin film heterostructure to form 2DEG, which is demonstrated experimentally and theoretically. An assumption is proposed that the oxygen vacancy plays a role of the donor of free electrons at the ZnO/Ga2O3 interface because of the small energy level between the oxygen vacancy defect level and conduction band minimum (CBM) of the ZnO layer, which could easily trigger the charge transfer in the interface region. The 2DEG can be realized on the partially ordered microstructure of the Ga2O3 layer. With the assistance of the ALD technique, the thickness of the ZnO/Ga2O3 heterostructure can be as thin as ∼65 nm, which is favorable for devices of stack or ultrathin structures. Moreover, the low temperature deposition by ALD can be adopted for flexible or stretchable devices.