Electronic transport in p-type and n-type β-rhombohedral boron

Controlled doping is required for the promising thermoelectric application of boron-rich solids. β-rhombohedral boron, which is p-type in pure form, can be made n-type by interstitial doping with V, Cr, Fe and Ni, while it remains p-type in the case of Cu and Co doping. Seebeck coefficient, dc electrical conductivity, dynamical conductivity and optical absorption of B:V, B:Fe and B:Co are presented. Irrespective of the carrier type, in all these cases the electronic transport can be described by a superposition of Drude-type and hopping type conduction. Doping to n-type requires the overcompensation of the unoccupied valence and gap states in pure boron by a doping level positioned between the conduction band and the uppermost intrinsic electron trap. While Fe atoms occupy the suitable interstitial sites statistically, V atoms prefer the A site at lower and preferably occupy D sites at higher metal contents. Irrespective of the lower ionisation energy of the V atoms compared with Fe, the doping efficiency of V is much higher and differs qualitatively from that of Fe