Effects of Ge Doping on Micromorphology of MnSi in MnSi∼1.7 and on Their Thermoelectric Transport Properties

Abstract

MnSi layers in Ge-doped MnSi∼1.7 increased with increasing Ge content up to x=0.00133, began to break at x=0.00265 and finally disappeared at x=0.00530. An experimental equation for the growth of MnSi was proposed for the interval between the MnSi layers and amount of doped Ge content. The crystallinity of Ge-doped MnSi∼1.7 increased initially with increasing doped Ge content and saturated at high Ge content. Thermoelectric transport properties along the c-axis of Ge-doped MnSi∼1.7 were measured as a function of Ge content at room temperature. Electrical conductivity and thermoelectric power of Ge-doped MnSi∼1.7 were compared to those of Al-doped MnSi∼1.7 in our previous work. A maximum in the electrical conductivity and a minimum in the thermoelectric power of Ge-doped MnSi∼1.7 were observed at x=0.00133, reflecting a change in hole density which was influenced by the volume ratio of MnSi. Hole mobility depended on the existence of MnSi layers and/or of interfaces between MnSi∼1.7 and MnSi and on the crystallinity of MnSi∼1.7. The thermal conductivity of Ge-doped MnSi∼1.7 had a maximum at x=0.00053. The increase in thermal conductivity at low Ge doping can be explained by the increase in the amount of MnSi segregated in doped MnSi∼1.7, while the decrease at high Ge content was caused by the increase in phonon scattering of Ge. A maximum figure of merit of Ge-doped MnSi∼1.7 was obtained at x=0.00974, reflecting a maximum power factor.