Transparent conducting oxide (TCO) films, which is one of the widely used as window electrodes at the various devices, fulfil high electrical conductivity and optical transparency in the visible region of spectrum. Conventional TCO films are formed by the tuning charge transport of wide band gap oxides like indium gallium zinc oxide (IGZO), zinc oxide (ZnO), tin dioxide (SnO₂), and indium oxide (In₂O₃). Among the various TCO candidates, In₂O₃ is an attractive TCO candidate since In₂O₃ can satisfied the visible transparency and high carrier density via doping processes. To achieve the high mobility In₂O₃ films, doping processes with metal dopants such as Ti, Zr, Mo, and W have been suggested by many previous study. Intrinsic In₂O₃, however, has n-type semiconductor due to its native oxygen vacancy act as n-type donor and indium interstitial. As another approach to gain highly conductivity TCOs, two-dimensional electron gas (2DEG) channel can be suggested since 2DEG channel can be applied to increase the electrical conductivity of oxide semiconductors up to the metallic conduction level. The tuning of electrical and optical properties in oxides via surface and interfacial 2DEG channels is of great interest, as they reveal the extraordinary transition from insulating or semiconducting characteristics to metallic conduction or superconductivity enabled by the ballistic transport of spatially-confined electrons. Although most previous studies have been carried out in crystalline phase like single crystal SrTiO₃ or III-V compound semiconductor, realizing practical aspects of this exotic phenomenon toward short-range ordered and air-stable 2DEG channels remains a great challenge.

In this study, we fabricated Al₂O₃/In₂O₃ heterojunction system on soda-lime glass consisting of In₂O₃ deposited by RF reactive sputter with a thickness from 8 to 100 nm and Al₂O₃ deposited by ALD with a thickness below 100 nm. We suggest that Al₂O₃/In₂O₃ heterojunction acts as short-range ordered 2DEG channel with a low temperature (~200 ℃). After deposition of Al₂O₃ layer, the heterojunction two-dimensional metallic channel was fabricated between Al₂O₃ and In₂O₃ interface as showing the metallic conduction with sheet charge density > 10 ¹⁴/cm², sheet resistance of 850 Ω/cm², room temperature Hall mobility of 20.5cm² V^-1 sec^-1 depending on the In₂O₃ thickness. To prove existence of 2DEG channel at the interface, we performed the I-V measurement, time of flight secondary ion mass spectrometry, ultraviolet-visible spectrophotometer, tunneling electron microscope–electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and Hall measurement.