Measurement of valence-band offset at native oxide/BaSi₂ interfaces by hard x-ray photoelectron spectroscopy

Undoped n-type BaSi₂ films were grown on Si(111) by molecular beam epitaxy, and the valence band (VB) offset at the interface between the BaSi₂ 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 BaSi₂ 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 BaSi₂ and -4.9 eV for the native oxide. We found that the band bending did not occur near the native oxide/BaSi₂ 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-BaSi₂ (holes) was determined to be 3.9 eV. No band bending in the BaSi₂ close to the interface also suggests that the large minority-carrier lifetime in undoped n-BaSi₂ 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.