Effect of high-temperature postannealing atmosphere on the properties of BaSi2 films

We evaluated the effect of O atoms on the postannealed BaSi2 films grown by molecular beam epitaxy. Postannealing (PA) in an Ar atmosphere at a pressure of 1.9 × 105 Pa increased the O concentration to 7 × 1020 cm−3 in the bulk region and further increased to ∼1022 cm−3 at the BaSi2/Si interface. Cracks formed during the PA process, allowing O to enter more easily to the BaSi2 films. In the x-ray photoelectron spectroscopy spectrum of the Si 2s core level measured at 10 nm from the surface, a shift of the peak related to SiOx was detected, indicating a change in the bonding state of Si and O in this region. When PA was performed in vacuum at 10−3 Pa, the photoresponsivity in the short wavelength region was enhanced, with a maximum value of 6.6 A W−1 at 790 nm. The O concentration in the film decreased in the sample annealed in vacuum, and the PL peak intensity at 0.85 eV decreased, suggesting that this was due to a decrease in O-related defects compared to the Ar atmosphere. However, agglomeration of BaSi2 caused significant surface roughness, indicating the importance of PA conditions that minimize O uptake and keep the surface smooth for improved performance of BaSi2 solar cells.