Measurement of valence-band offset at native oxide/BaSi_2 interfaces by hard x-ray photoelectron spectroscopy

摘要

Undoped n-type BaSi_2 films were grown on Si(111) by molecular beam epitaxy, and the valence band (VB) offset at the interface between the BaSi_2 and its native oxide was measured by hard x-ray photoelectron spectroscopy (HAXPES) at room temperature. HAXPES enabled us to investigate the electronic states of the buried BaSi_2 layer non-destructively thanks to its large analysis depth. We performed the depth-analysis by varying the take-off angle (TOA) of photoelectrons as 15°, 30°, and 90° with respect to the sample surface and succeeded to obtain the VB spectra of the BaSi2 and the native oxide separately. The VB maximum was located at -1.0 eV from the Fermi energy for the BaSi_2 and -4.9 eV for the native oxide. We found that the band bending did not occur near the native oxide/BaSi_2 interface. This result was clarified by the fact that the core-level emission peaks did not shift regardless of TOA (i.e., analysis depth). Thus, the barrier height of the native oxide for the minority-carriers in the undoped n-BaSi_2 (holes) was determined to be 3.9 eV. No band bending in the BaSi_2 close to the interface also suggests that the large minority-carrier lifetime in undoped n-BaSi_2 films capped with native oxide is attributed not to the band bending in the BaSi2, which pushes away photogenerated minority carriers from the defective surface region, but to the decrease of defective states by the native oxide.