Heterostructure formation of group-III sesquioxides via cation-exchange reactions with metal chloride gases

Thermodynamic analyses and experimental demonstrations were performed to investigate whether cation-exchange reactions (CERs) with metal chloride gases are a feasible technique for forming heterostructures of group-III sesquioxides. The thermodynamic analyses revealed the possibility of formation of aluminum oxide (Al2O3) layers on gallium oxide (Ga2O3) or indium oxide (In2O3) substrates via CERs with aluminum chloride gases, as well as formation of Ga2O3 layers on In2O3 substrates via CERs with gallium chloride gases, in a practical temperature range. However, CERs with gallium chloride gases or indium chloride gases did not occur on Al2O3 substrates, suggesting that heterostructures were not formed. On the basis of the results of the thermodynamic analyses, a CER was performed by the reaction of n-type β-Ga2O3(2̄01) substrates with aluminum trichloride gas, resulting in the formation of Al2O3 layers. Under optimal conditions, a dense (001)-oriented κ-Al2O3 layer with a thickness of 30 nm was formed and served as an excellent gate oxide in metal–oxide–semiconductor devices. The relative dielectric constant of κ-Al2O3 was obtained via capacitance–voltage measurements, and a high value of ∼22 was obtained in the several tens of kHz band. CERs using metal chloride gases open a new method for forming heterostructures of group-III sesquioxides.