Interface-layer formation in microcrystalline Si:H growth on ZnO substrates studied by real-time spectroscopic ellipsometry and infrared spectroscopy

By applying real-time spectroscopic ellipsometry and infrared attenuated total reflection spectroscopy (ATR), we have characterized interface-layer formation in microcrystalline silicon $\mathrm{(\mu c-}\mathrm{Si}:\mathrm{H})$ growth on ZnO substrates in a conventional rf plasma-enhanced chemical vapor deposition. With an increase in the hydrogen dilution ratio $\mathrm{(R=[}{\mathrm{H}}_2\mathrm{]/[}{\mathrm{SiH}}_4\mathrm{]),}$ we find a significant increase in the interface-layer thickness from $\text{0\hspace{0.05em}(R=0)}$ to $\text{48\hspace{0.05em}Å\hspace{0.05em}(R=100).}$ In contrast, no interface-layer formation was observed in $\mathrm{\mu c-}\mathrm{Si}:\mathrm{H}$ growth on ${\mathrm{SiO}}_2$ substrates. Detailed analyses show that the interface layer formed on ZnO is porous amorphous Si:H with a large amount of ${\mathrm{SiH}}_2$ bonds (16 at. %). During the early stage of interface-layer formation, we observed almost no film deposition and a dramatic increase in free-carrier concentration within the ZnO substrate due to a chemical reduction of ZnO by H-containing plasma. Real-time ATR spectra revealed the predominant formation of ${\mathrm{SiH}}_n{\mathrm{O}}_{\text{4−n}}\text{\hspace{0.05em}(n=1−2)}$ species on the ZnO surface prior to interface-layer formation. These ${\mathrm{SiH}}_n{\mathrm{O}}_{\text{4−n}}$ species are chemically inactive and remain at the $\mathrm{\mu c-}\mathrm{Si}:\mathrm{H}/\mathrm{ZnO}$ interface. Based on these findings, we propose that the ${\mathrm{SiH}}_n{\mathrm{O}}_{\text{4−n}}$ formation that results from ZnO reduction reaction by H suppresses chemical reactivity on the ZnO surface and induces porous interface-layer formation during the initial deposition process.