Evaluation of band offset at amorphous-Si/BaSi_2 interfaces by hard x-ray photoelectron spectroscopy

摘要

The 730nm-thick undoped BaSi_2 films capped with 5nm-thick amorphous Si (a-Si) intended for solar cell applications were grown on Si(111) by molecular beam epitaxy. The valence band (VB) offset at the interface between the BaSi_2 and the a-Si was measured by hard x-ray photoelectron spectroscopy to understand the carrier transport properties by the determination of the band offset at this heterointerface. We performed the depth-analysis by varying the take-off angle of photoelectrons as 15°, 30°, and 90° with respect to the sample surface to obtain the VB spectra of the BaSi_2 and the a-Si separately. It was found that the barrier height of the a-Si for holes in the BaSi_2 is approximately -0.2 eV, whereas the barrier height for electrons is approximately 0.6 eV. This result means that the holes generated in the BaSi_2 layer under solar radiation could be selectively extracted through the a-Si/BaSi_2 interface, promoting the carrier separation in the BaSi_2 layer. We therefore conclude that the a-Si/BaSi_2 interface is beneficial for BaSi_2 solar cells.