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Energy band alignments between series band of Al-rich high- k materials (HfO 2 ) x (Al 2 O 3 ) 1-x and β-Ga 2 O 3 are investigated using X-Ray Photoelectron Spectroscopy (XPS). The results exhibit sufficient conduction band offsets (1.42–1.53 eV) in (HfO 2 ) x (Al 2 O 3 ) 1-x /β-Ga 2 O 3 . In addition, it is also obtained that the value of E g , △ E c , and △ E v for (HfO 2 ) x (Al 2 O 3 ) 1-x /β-Ga 2 O 3 change linearly with x , which can be expressed by 6.98-1.27 x , 1.65-0.56 x , and 0.48–0.70 x , respectively. The higher dielectric constant and higher effective breakdown electric field of (HfO 2 ) x (Al 2 O 3 ) 1-x compared with Al 2 O 3 , coupled with sufficient barrier height and lower gate leakage makes it a potential dielectric for high voltage β-Ga 2 O 3 power MOSFET, and also provokes interest in further investigation of HfAlO/β-Ga 2 O 3 interface properties. [ABSTRACT FROM AUTHOR]