Epitaxial growth of BaSi2 light absorbers by molecular beam epitaxy and significant photoresponsivity enhancement by increased growth temperatures

摘要

We have paid special attention to Barium disilicide (BaSi2), which shows great promise as a new material for thin-film solar cells. In this study, we fabricated 500-nm-thick BaSi2 layers by molecular beam epitaxy on n-Si (111) substrates and investigated the effect of increasing growth temperature on optical properties and crystalline quality of BaSi2 light absorbers. By raising growth temperature from 580 to 650 °C, the optimum Ba to Si deposition rate ratio $(R_{mathrm{Ba}}/R_{mathrm{Si}})$ shifted from 3 to 4 from the viewpoint of crystalline orientation along the surface normal. This shift is interpreted to be determined by the actual Ba/Si atomic ratio. On the other hand, photoresponsivity reached a maximum at $R_{mathrm{Ba}}/R_{mathrm{Si}}=1.2$, meaning that the optimum $R_{mathrm{Ba}}/R_{mathrm{Si}}$ shifted to a Si-rich side from the viewpoint of minority-carrier properties. The achieved photoresponsivity is about 3 times higher than that ever reported. Raman spectroscopy suggested that Si vacancies decreased without forming Si precipitates for BaSi2 films grown at $R_{mathrm{Ba}}/R_{mathrm{Si}}=1.2$. However, further decrease in $R_{mathrm{Ba}}/R_{mathrm{Si}}$ generated Si precipitates proved by Raman spectroscopy. Based on these results, we can state that the highest photoresponsivity is obtained for the smallest $R_{mathrm{Ba}}/R_{mathrm{Si}}$ regardless of growth temperature, wherein Si precipitates do not form.