Mechanically tuned conductivity in piezoelectric semiconductors
In zinc oxide (ZnO) varistor ceramics, the non linear electrical characteristics are attributed to back to back double Schottky barriers along grain boundaries. These potential barriers are formed due to the trapping of charge carriers by dopants and associated defect states at the interface. Zinc oxide’s non centrosymmetric wurzite structure provides piezoelectric properties, which can be utilized to mechanically tune the height of these barriers. In doing so, mechanical stress causes compensating or accumulating piezoelectric charges at the grain boundaries, depending on the orientation of the crystal structure and if tensile or compressive strain is applied.
In close cooperation with the Nonmetallic-Inorganic Materials group of Prof. Jürgen Rödel, ZnO bicrystals with specific orientations and doping strategies are chemically and structurally investigated by atomic resolved transmission electron microscopy (TEM) methods with an emphasis on the characteristics of the grain boundaries and their interdependence with dopant segregation and synthesis procedures. The experimental results are compared and correlated with load depended electrical measurements. Since bicrystals are perfect model systems for the investigation of specific grain-boundary configurations and hence specific potential barriers, these studies contribute to both the development of novel piezotronic devices and the fundamental research on zinc oxide varistor ceramics.
In addition, the applied TEM characterization methods are also performed on related ZnO systems, such as nanowires, nano sized inversion twins and hexagonal platelets.
P. Keil, M. Trapp, N. Novak, T. Frömling, H. J. Kleebe, J. Rödel,
Piezotronic Tuning of Potential Barriers in ZnO Bicrystals
Adv Mater, 30, 1705573, (2018).
M. Trapp, M. M. Müller, Z. Nazarpoor, H. J. Kleebe,
Full reoxidation of CuMn2O4 spinel catalyst triggered by epitaxial Mn3O4 surface nanocrystals
J Am Ceram Soc, 100, 5327-34, (2017).