Optical and Compositional Properties of SiOx Films Deposited by HFCVD: Effect of the Hydrogen Flow

In this work, the effect of hydrogen flow and thermal annealing on the compositional and optical properties of non-stoichiometric silicon oxide (SiOx) films with embedded silicon nanocrystals is reported. The SiO (x) films are obtained by hot filament chemical vapor deposition technique at three different hydrogen flow levels, namely, 50 sccm, 100 sccm, and 150 sccm. The SiOx films are characterized by different techniques. It is found by x-ray photoelectron spectroscopy (XPS) that with increasing hydrogen flow, the SiOx films contain higher silicon (Si) concentration. When the hydrogen flow decreases, the absorption edge of the as-grown SiOx films, as obtained from the transmittance spectra, shifts from 300 nm to 500 nm, and this opens the possibility of band gap tuning. Increasing the hydrogen flow level in turn means that the SiOx films contain higher Si concentration, as confirmed by the XPS profile composition measured in the SiOx films. After thermal annealing, the SiOx films transmittance spectra showed a further shift of the absorption edge toward larger wavelengths. The Fourier transform infrared (FTIR) spectroscopy reveals film composition changes induced by the hydrogen flow variations. In addition, the FTIR spectra reveal the bands attributed to the hydrogen presence in the as-grown SiOx films. The bands become more intense with increasing hydrogen flow, but they rapidly disappear after the thermal annealing. The as-grown SiOx films exhibit wide band photoluminescence (PL) spectra with the main components at 688 nm, 750 nm, and 825 nm. The SiOx film deposited at 100 sccm hydrogen flow level shows the strongest PL intensity. According to PL results, the thermal annealing of the SiOx films generates the PL quenching in all samples due to hydrogen evaporation. The defects such as OH and Si-H groups in the as-grown SiOx films not only modify the optical band gap structure, but they also play the role of passivating non-radiative defects, which enhances the PL emission.