Oxidation of freestanding silicon nanocrystals probed with electron spin resonance of interfacial dangling bonds

The oxidation of freestanding silicon nanocrystals (Si-NCs) passivated with Si-H bonds has been investigated for a wide range of oxidation times (from a few minutes to several months) by means of electron spin resonance (ESR) of dangling bonds (DBs) naturally incorporated at the interface between the NC core and the developing oxide shell. These measurements are complemented with surface chemistry analysis from Fourier transform infrared spectroscopy. Two surface phenomena with initiation time thresholds of 15 min and 30 h are inferred from the dependence of ESR spectra on oxidation time. The first initiates before oxidation of surface Si-Si bonds and destruction of the NC hydrogen termination takes place (induction period) and results in a decrease of the DB density and a localization of the DB orbital at the central Si atom. Within the Cabrera-Mott oxidation mechanism, we associate this process with the formation of intermediate interfacial configurations, resulting from surface adsorption of water and oxygen molecules. The second surface phenomenon leads to a steep increase of the defect density and correlates with the formation of surface Si-O-Si bridges, lending experimental support to theoretically proposed mechanisms for interfacial defect formation involving the emission of Si interstitials at the interface between crystalline Si and the growing oxide.