Correlation between infrared transmission spectra and the interface trap density of SiO2 films

This work is an attempt to estimate the electrical properties of SiO2 thin films by recording and analyzing their infrared transmission spectra. In order to study a big variety of films having different infrared and electrical properties, we studied SiO2 films prepared by low pressure chemical vapor deposition (LPCVD) from SiH4 + O2 mixtures at 425 °C and annealed at 750 °C and 950 °C for 30 min. In addition thermally grown gate quality SiO2 films of similar thickness were studied in order to compare their infrared and electrical properties with the LPCVD oxides. It was found that all studied SiO2 films have two groups of Si-O-Si bridges. The first group corresponds to bridges located in the bulk of the film and far away from the interfaces, the grain boundaries and defects and the second group corresponds to all other bridges located near the interfaces, the grain boundaries and defects. The relative population of the bulk over the boundary bridges was found equal to 0.60 for the LPCVD film after deposition and increased to 4.0 for the LPCVD films after annealing at 950 °C. Thermally grown SiO2 films at 950 °C were found to have a relative population of Si-O-Si bridges equal to 3.9. The interface trap density of the LPCVD film after deposition was found equal to 5.47 × 1012 eV−1 cm−2 and decreases to 6.50 × 1010 eV−1 cm−2 after annealing at 950 °C for 30 min. The interface trap density of the thermally grown film was found equal to 1.27 × 1011 eV−1 cm−2 showing that films with similar Si-O-Si bridge populations calculated from the FTIR analysis have similar interface trap densities.