Energy level alignment at C_6o/Hf interface by using x-ray and ultraviolet photoelectron spectroscopy

We report the characterization of the interface formation of fullerene (C₆₀) on hafnium (Hf) using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The valence band, C 1s and Hf 4f spectra were measured during the deposition of C₆₀ on a clean Hf surface in a stepwise manner. After enough deposition of C₆₀ layers on Hf, XPS measurements indicate that there is no chemical reaction between C₆₀ on Hf, and band bending exists at the C₆₀/Hf interface. From UPS measurements, the energy level difference between Fermi level of Hf and the onset of the highest occupied molecular orbital of C₆₀ is 2.01 eV. The vacuum level of Hf was shifted toward low binding energy (0.95eV) as the C₆₀ deposition. These valence spectra results indicate that there exist an interface dipole and small electron injection barrier at the C ₆₀/Hf interface. We provide the complete energy band diagram of the C₆₀/Hf interface and confirm that a small electron injection barrier can be achieved by inserting a low work function metal in a C₆₀ field-effect transistor.