Marked enhancement of the photoresponsivity and minority-carrier lifetime of BaSi2 passivated with atomic hydrogen

Passivation of barium disilicide ($\mathrm{BaS}{\mathrm{i}}_{2}$) films is very important for their use in solar cell applications. In this paper, we demonstrated the effect of hydrogen (H) passivation on both the photoresponsivity and minority-carrier lifetime of $\mathrm{BaS}{\mathrm{i}}_{2}$ epitaxial films grown by molecular beam epitaxy. First, we examined the growth conditions of a 3-nm-thick hydrogenated amorphous silicon (a-Si) capping layer formed on a 500-nm-thick $\mathrm{BaS}{\mathrm{i}}_{2}$ film and found that an H supply duration (${t}_{\mathrm{a}\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}}$) of 15 min at a substrate temperature of 180 \ifmmode^\circ\else\textdegree\fi{}C sizably enhanced the photoresponsivity of the $\mathrm{BaS}{\mathrm{i}}_{2}$ film. We next supplied atomic H to $\mathrm{BaS}{\mathrm{i}}_{2}$ epitaxial films at 580 \ifmmode^\circ\else\textdegree\fi{}C and changed supply duration (${t}_{\mathrm{BaSi};\mathrm{H}}$) in the range of 1--30 min, followed by capping with an a-Si layer. The photoresponsivity of the films changed considerably depending on ${t}_{\mathrm{BaSi};\mathrm{H}}$ and reached a maximum of 2.5 A/W at a wavelength of 800 nm for the sample passivated for ${t}_{\mathrm{BaSi};\mathrm{H}}=15$ min under a bias voltage of 0.3 V applied to the front-surface indium-tin-oxide electrode with respect to the back-surface aluminum electrode. This photoresponsivity is approximately one order of magnitude higher than the highest value previously reported for $\mathrm{BaS}{\mathrm{i}}_{2}$. Microwave photoconductivity decay measurements revealed that the minority-carrier lifetime of the $\mathrm{BaS}{\mathrm{i}}_{2}$ film with the highest photoresponsivity was 14 \ensuremath{\mu}s, equivalent to its bulk carrier lifetime ever reported. We performed theoretical analyses based on a rate equation including several recombination mechanisms and reproduced the experimentally obtained decay curves. We also calculated the total density of states of $\mathrm{BaS}{\mathrm{i}}_{2}$ by ab initio studies when one Si vacancy existed in a unit cell and one, two, and three H atoms occupied Si vacancy or interstitial sites. A Si vacancy caused a localized state with two energy bands to appear close to the middle of the band gap. In certain cases, H passivation of the Si dangling bonds can markedly decrease trap concentration. From both experimental and theoretical viewpoints, we conclude that an atomic H supply is beneficial for $\mathrm{BaS}{\mathrm{i}}_{2}$ solar cells.