Investigation of (110) SnO2 growth mechanisms on TiO2 substrates by plasma-assisted molecular beam epitaxy.

We investigated the growth mechanisms of (110) SnO2 on (110) TiO2 by varying tin fluxes under a fixed oxygen pressure by plasma-assisted molecular beam epitaxy. A growth diagram was constructed that consists of two distinct growth regimes: an increase in growth rates in an oxygen-rich environment and a decrease in growth rates in a tin-rich environment. The excess tin played an important role for the unexpected decreasing growth rate in a tin-rich regime. Without a buildup of the tin adlayer coverage, the accumulation of macroscopic tin droplets on the SnO2 film surface was confirmed by the spotty high energy electron diffraction pattern and the absence of intensity oscillations. No SnO2 growth was observed when the impinging tin flux was larger than twice the stoichiometric tin flux, suggesting that all active oxygen atoms were consumed by evaporated tin atoms to form volatile tin suboxide (SnO). The formation of volatile SnO, causing the decreasing growth rate in a tin-rich condition, was monitored by in situ quadrupole mass spectrometry. [ABSTRACT FROM AUTHOR]