Supplement Number 4

XPS of the Mo 3d spectrum a) after the 2nd cycle NbCl5 pulse, b) 2nd cycle O2 pulse, c) 3rd cycle NbCl5 pulse, and d) 3rd cycle O2 pulse, measured in vacuo in between etching a Mo thin film with NbCl5 and O2 at 300 {degree sign}C. NbCl5 pulse length was 4 s and O2 pulse length 6 s.