Enhanced electromechanical coupling in piezoelectric MEMS vibration energy harvesters via strain-induced phase transition in Mn-doped bismuth ferrite epitaxial films

Mn-doped BiFeO₃ (BFMO) epitaxial films grown on (100) Si wafers delivered enhanced electrical and piezoelectric properties under systematically optimized growth conditions, realized through a biaxial combinatorial sputtering method. The dielectric constant and dielectric loss of the resulting BFMO films were approximately 140 and 1%, respectively, considerably lower than those of undoped BiFeO₃. Most notably, the effective transverse piezoelectric coefficient was -6.0 C/m², the highest yet reported for this material system. According to detailed structural and electrical characterizations, the improved piezoelectric performance stems from a strain-induced phase transition from the rhombohedral to the monoclinic structure. To demonstrate this enhancement beyond the material level, the optimized films were successfully integrated into piezoelectric MEMS vibration-energy harvesters. The films demonstrated device-level performance improvements with a generalized electromechanical coupling factor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo> <mml:msup><mml:mrow><mml:mi>K</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msup> <mml:mo>)</mml:mo></mml:mrow> </mml:math> of 0.5%, fivefold that of (100) oriented BFO films.