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

Abstract

Higher manganese silicide (HMS) MnSiγ 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 MnSiγ 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 MnSi2.04 film. In case of the as-deposited films, the PF in the p-type regime (PFp) grew larger as the Si/Mn molar ratio γ approached the stoichiometric value of 1.73, while the PFn became vanishingly small. In contrast, the PFn 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 MnSi1.65 film. Hall measurement at elevated temperatures (≤773 K) showed that electron mobility was greatly increased after the annealing, which has produced unusually high PFn. 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.