Silicon carbide has been realized to be one of powerful semiconductors. Various kinds of devices, including Schottky diodes, bipolar-transistors, MOSFETs and some IC units with good properties, have been manufactured using SiC. In these devices, SiO₂ films play important roles. However, since the existence of carbon atoms, qualities of thermal oxidation formed SiO₂ films on SiC wafers were far from as good as those on Si. The effects of impurities on the C-V characteristics of SiO₂/SiC have been investigated by using atoms sensitive probes such as the secondary ion mass spectrometry (SIMS) and the x-ray photoelectron spectroscopy (XPS). These analyzing techniques are not adequate for full understanding of characteristics of SiO₂/SiC interface and the structure of SiO₂ film since a large of void-type defects will form during thermal oxidation and post-annealing due to the removal or escape of C atoms and CO molecules.
In this work, the technique of Doppler broadening of positron annihilation spectroscopy (PAS) has been used to reveal information on void-type defects of SiO₂/SiC system. Variable-energy positron beam makes positrons to different depth in the sample. The value of the S parameter, which reflects the number of positrons annihilated with low momentum electrons, increases since the reduced electron density at open-volume defects narrows the momentum distribution.
The SiC materials used in the experiments were obtained from Cree. The wafers were 6H-SiC (0001) having basic nitrogen and aluminum dopant of 110¹⁷ cm^-3 in n- and p-type samples, respectively. Dry oxidation was carried out at 1150C for 6 hours. Post-annealing was done at 1100C in nitrogen gas. The thickness of the SiO₂ film was about 0.6 nm measured by ellipsometry. Variable energy (0-20 keV) positron beam made the positrons stop and annihilate at different depth from the SiO₂ surface to the SiC bulk.
The measurement results obviously showed that the S-parameters
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