Amphoteric properties of substitutional carbon impurity in GaN and AlN

Electronic structure and formation energies of substitutional carbon impurities in hexagonal GaN and AlN were studied by quantum molecular dynamics. Substitutions on both cation and anion sites were considered. Ccation is a shallow donor, while CN is a shallow acceptor. A DX-like configuration is metastable for CGa and CAl0, and stable for CAl−. The solubility of carbon is excellent, but it is accompanied by efficient self-compensation of carbon acceptors by carbon donors. This is due to two factors: (i) the large energy gain induced by electron transfer from Ccation to CN, which is close to the band gap energy, and (ii) the large binding energy (∼1 eV) of nearest neighbor Ccation+-CN− pairs.