Abstract
Hydrogen plasma reduction of fluorine doped tin oxide is a beneficial method to form tin nanodroplets on the sample surface directly in the plasma-enhanced chemical vapor deposition reactor. The formation of catalyst droplets is a crucial initial step for vapor-liquid-solid growth of silicon nanowires for radial junction solar cells and solar fuel cell technology. We present an original optical model which allows us to trace the formation process on fluorine doped tin oxide on soda-lime glass substrate from the in situ data and is in a good agreement with the spectroscopic ellipsometry data measured before and during the reduction process. The model reproduces well the phase shift introduced by a transition double layer in fluorine doped tin oxide which acts as a barrier against the sodium diffusion. Furthermore, we study the process of tin reduction from fluorine doped tin oxide in a real time and compare estimated amount of produced metallic tin with images from scanning electron microscopy.The proposed approach is very important for in situ real-time monitoring of the one-pump-down fabrication process used to grow nanowires and form radial junction devices.
Highlights
Tin nanodroplets which catalyze silicon nanowire (SiNW) growth can be formed from thermally evaporated tin film on substrate
The presented optical study of the Sn reduction from fluorine doped tin oxide (FTO) by the hydrogen plasma in a plasma-enhanced chemical vapor deposition (PECVD) reactor is divided into two main parts
We have developed the layered model of the FTO films with barrier against the sodium diffusion prepared on soda-lime glass substrate
Summary
Tin nanodroplets which catalyze silicon nanowire (SiNW) growth can be formed from thermally evaporated tin film on substrate. Misra et al [1] achieved 9.2% efficiency on radial junction solar cell built on silicon nanowires grown on a glass substrate using thermally evaporated 1 − 5 nm thick Sn layer This approach require extra process step of Sn thermal evaporation. Their method can be further improved to one-pump-down process when direct tin reduction from fluorine doped tin oxide (FTO) commercial substrates in plasma-enhanced chemical vapor deposition (PECVD) reactor instead of thermal evaporation is employed [2]. This approach is well-compatible with industrial fabrication processes and lowers a risk of sample contamination. We show how this optical model can be further extended to trace the amount of reduced tin in the real time and to better control the fabrication process
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