A comparison of free carrier absorption and capacitance voltage methods for interface traps measurements

Abstract: This project aims at establishing a new method to characterize the interface between 4H-SiC and passivating dielectric layers. The investigations are made on metal-oxide-semiconductor (MOS) test structures. The oxides are made of Al2O3, deposited by atomic layer deposition (ALD), and SiO2, deposited by plasma-enhanced chemical vapour deposition (PECVD) and the semiconductor is lowly doped 4H-SiC n-type epitaxial layers. These structures have been exposed to different fluencies of Ar ion irradiation to induce damage at the interface and then measured by well-established electrical techniques, as well as the new method utilizing optical free carrier absorption (FCA) technique to assess the interface traps. We have furthermore developed an analytical tool using Matlab that can extract surface recombination velocity (SRV) from the optical data. This tool is developed for an idealized epi-layer between two surfaces, but could also be applied, for instance, to the region between the emitter and base contacts on SiC mesa etched BJT’s. First, optical free carrier absorption (FCA) measurements are carried out to measure the effective lifetimes in the structures. The data obtained from FCA is then fitted with the analytical tool and SRVs, for samples with both types of dielectrics with various Ar ion fluencies, are extracted. Standard electrical measurements using capacitance-voltage (CV) measurements are also undertaken and the density of interface traps (Dit) is extracted using the Terman method. Extracted SRV values are then compared to Ditvalues. It is observed that SiO2 samples show a large rise of SRVs, from 0.5x104 cm/s for a reference sample to 8x104 cm/s for a fluence of 1x1012 cm-2, whereas Al2O3 samples show more stable SRV, changing from 3x104 cm/s for the un-irradiated reference sample to 6x104 cm/s for a fluence of 1x1012 cm-2. A very similar trend is observed for Dit values extracted from CV measurements and it can therefore be concluded that the FCA method is very suitable for characterization of the interface, and together with CV, it should be possible to obtain quantitative values on charge carrier trapping dynamics.