Direct evidence of spontaneous polarization effect in GaN grown on SiC(0001): Heterojunction electronic properties

The band alignment at the SiC/GaN interface is of relevance for electronic devices such as the nide-gap emitter (GaN layer) SiC based heterostructure bipolar transistor. We extracted the valence band offset (VBO) at this heterojunction by depositing thin layers of GaN on 6H-SiC(0001) by means of plasma enhanced MBE and measuring in-situ X-ray photoemission spectra. Differently, thick samples were considered, showing a trend in the shift of the Ga(3p) core level position. This effect is connected to a band bending induced by internal polarization fields. EXAFS measurements on the same samples showed a relaxed growth already for the thinnest layer. We therefore infer that the field in the GaN originates from the spontaneous polarization, without piezoelectric contributions. The extracted value for the valence band discontinuity is 0.9 +/- 0.1 eV, in good agreement with previous theoretical results. The fits of the EXAFS data do not exclude a possible C/N intermixing at the otherwise, when abrupt, unstable GaN/SiC interface.