Synthetic and Spectroscopic Study of the Mechanism of Atomic Layer Deposition of Tin Dioxide

This study details the surface reaction chemistry relevant to the vapor deposition mechanism of SnO2 thin films by atomic layer deposition. The mechanism was elucidated by combining different spectroscopic studies. Initial nucleation of cyclic N,N-di-tert-butyl-butane-2,3-diamido tin(II) (1) consists of surface SiOH protonation of ligands as shown by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This SiO2-surface-bound stannylene was further characterized by X-ray absorption (XAS) and resonance Raman spectroscopy. XAS, DRIFTS and Raman spectroscopy were then used to follow the further reaction of the surface-bound stannylene with different oxygen sources and a second equivalent of 1. It was observed that water does not oxidize the initial surface bound tin site and a well-defined, three-coordinate tin(II) species, with two surface oxygen bonds and coordinated water molecule, was characterized. Treatment of the surface stannylene with protic oxidants such as H2O2 or tBuOOH fully oxidizes tin to 4+ and coordination of additional oxygen ligands is observed. When a second equivalent of 1 is added to surface bound Sn, the resulting surface tin is also found to be in the +4 oxidation state, contrary to the nonoxidative nucleation step. As such, surface peroxide species provide a probable growth mechanism of SnO2 through oxidation, while nucleation occurs through protic ligand exchange.