Abstract
•Thulium oxide layers on germanium were investigated.•The valence band offset was found to be 2.95eV.•The band gap of thulium oxide was found to be 5.3eV from the Tauc method.•A sub-band gap absorption at 3.2eV points to a bulk thulium oxide defect.•A permittivity of 14 was derived from electrical measurements.
This paper investigates the band line-up and optical properties (dielectric function) of Tm2O3/Ge gate stacks deposited by atomic layer deposition. X-ray photoelectron spectroscopy has been performed to ascertain the shallow core levels (Ge3d and Tm4d) in ultra-thin and bulk Tm2O3/Ge stacks as well as valence band maxima in Ge and bulk Tm2O3. The valence band offset of Tm2O3/Ge has been found to be 2.95±0.08eV. Vacuum ultra violet variable angle spectroscopic ellipsometry studies reveal the indirect band gap nature of Tm2O3, with the value extracted from the Tauc method of 5.3±0.1eV. A distinct absorption feature is observed at ∼3.2eV below the band gap of Tm2O3, and clearly distinguished from the Si and Ge critical points. A dielectric constant of 14 to 15 has been derived from the electrical measurements on 5nm Tm2O3/epi Ge/Si gate stacks. The band line-up study of Tm2O3/Ge implies an acceptable barrier for holes (2.95eV) and electrons (greater than 1.7eV) for Ge MOSFET engineering.