Thermoelectric properties of higher manganese silicide films deposited by radio frequency magnetron co-sputtering

Higher manganese silicide (HMS) MnSi_γ films with varying γ (1.65 ≤ γ ≤ 3.23) were deposited on oxidized Si wafers at 703 K by radio frequency (RF) magnetron co-sputtering of Mn and Si targets. As-deposited films were annealed at 1273 K for 3min using a lamp annealing system, and the thermoelectric (TE) properties were measured in the temperature range from room temperature to 1073 K. All the MnSi_γ films were p-type below about 550 K, but the polarity converted to n-type at higher temperatures. Moreover, the power factor (PF) of the polarity-inverted films was exceptionally high, reaching the maximum value of 12.3 mW/K²m in the annealed MnSi_2.04 film. In case of the as-deposited films, the PF in the p-type regime (PF_p) grew larger as the Si/Mn molar ratio γ approached the stoichiometric value of 1.73, while the PF_n became vanishingly small. In contrast, the PF_n of the annealed films remained very high even in the Mn-rich film (γ < 1.73), such that 9.6 mW/K²m was obtained from the MnSi_1.65 film. Hall measurement at elevated temperatures (≤773 K) showed that electron mobility was greatly increased after the annealing, which has produced unusually high PF_n. The HMS phase was composed of Mn₄Si₇ and Mn₁₁Si₁₉, but regions of unidentified phase were also confirmed by transmission Kikuchi diffraction (TKD) analysis, the proportion of which was larger in the Si-rich films. The modification of the band structure induced by defects and structural disorder of the Mn and/or Si sublattice are supposed to be the origin of this novel phenomenon.