In this study, we demonstrated AlN films with unexpected N-polarity on Si-face 4H−SiC substrates using RF reactive magnetron sputtering and post high-temperature annealing (HTA). It is found that HTA-AlN sputtered with pure nitrogen or mixed gases of nitrogen and trace oxygen exhibits undoubtable N polarity, which appears to contradict the common assumption of Al-polar AlN on Si-face SiC. The interfacial studies of HTA-AlN sputtered with pure N 2 reveal that the naturally oxidized SiC surface induces the formation of Si−O�Al−N bonds rather than conventional Si�N−Al bonds, thereby causing N-polar AlN. Sputtering with the mixed gases results in 5 nm thick complex transition layers at the AlN/SiC interface, including two types of oxygen-participated energy-unstable inversion domain boundaries and multiple polarity inverted AlN layers. Compared to that sputtered with the mixed gases, the N-polar HTA-AlN film sputtered with pure nitrogen presents a smoother surface morphology for the absence of AlON islands and lower threading dislocation density due to closer X-ray rocking curve full widths at half-maximum for (0002) and (101̅ 2) reflections. This work proves a new approach to prepare high-quality N-polar AlN/SiC templates for Al-rich AlGaN-based N-polar optoelectronic and power devices.
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