Insight on the Silver Catalyst Distribution During Silicon Nanowire Array Formation: an X-ray Reflectivity Study

Although metal-catalysts are commonly used to form nanoscale surface structures, little is quantitatively known about the vertical distribution of these metal nanoparticles. Using X-ray specular reflectivity, we report the first quantitative investigation, with nanoscale resolution, of the Ag catalyst depth distribution profile during Si nanowire (SiNW) array formation by metal-assisted chemical etching (MACE). Despite challenges from the extraordinarily rough interfaces, it is revealed that Ag nanoparticles unexpectedly distribute along the length of the nanowires, which are not tapered or pitted. Coarsening over the course of etching indicates that the Ag ions are highly mobile – interestingly the Ag remain in the SiNW region rather than migrating into the etch bath. The Ag mobility and vertical distribution suggests a strong chemical force that attracts Ag towards the etch front. Results of this study provide important new insight into the poorly-understood metal-assisted chemical etching process.