A comparative investigation of the Hall effect, conductivity, and thermopower properties of molecular-beam-epitaxy-grown GaN is presented. In unintentionally doped n-type GaN, a negligible thermal activation of the thermopower is observed above 300 K. In as-grown GaN:Mg, a thermopower activation energy of 280 meV is observed at high temperatures, as well as a scattering factor $A=3.\mathrm{}$ At temperatures below 120 K, the Seebeck coefficient of p-type GaN changes sign and indicates n-type conductivity. These results show that hopping in the acceptor band contributes significantly to the electronic transport properties. After hydrogenation of GaN:Mg, both conductivity and thermopower have an activation energy of 520 meV, which is at variance with the presence of potential fluctuations in the material. This demonstrates that hydrogen passivates Mg-doped GaN by the formation of electrically inactive Mg-H complexes, in contrast to the formation of compensating H-related donors, which should lead to noticeable potential fluctuations.

• Received 14 April 1998

DOI:https://doi.org/10.1103/PhysRevB.58.7786