Mechanically tunable conductivity in piezoelectric semiconductors
Final Report Abstract
Within this project, the piezotronic performance of metal/semiconductor and bicrystal interfaces were simultaneously studied. For both, the fundamental theory of the piezotronic effect could be confirmed. In comparison, it is found that bicrystals with varistor-type potential barriers feature a much higher stress sensitivity compared to metal/semiconductor Schottky contacts. The low sensitivity for the Schottky contacts can be rationalized by an imperfect screening mechanism of the piezoelectric potential due to the rather small spatial separation between piezoelectric charges and free screening charges within the metal electrode. Due to the symmetric nature of varistor-type potential barriers as well as the different physical mechanism behind the formation and the piezotronic modulation of the electrostatic potential barriers, ZnO-ZnO bicrystal interfaces do not feature a sensitivity limiting mechanism as in Schottky contacts. One main achievement within this project was the successful preparation of individual bicrystal interfaces by an epitaxial solid-state transformation process. It allows to define the piezoelectric tensor and control the defect chemistry at the interface, as verified by detailed (S)TEM investigations. The perfect orientation of the piezoelectric tensor maximizes the stress induced piezoelectric charges and thereby leads to a maximized stress sensitivity. The existence of only one potential barrier results in predictable electrical properties and low operating voltages for future applications. Hence, ZnO bicrystal interfaces are the ideal system to study the piezotronic effect which could lead to the development of multifunctional electronic devices based on the piezotronic effect of varistor-type potential barriers. An attempt to further increase the stress sensitivity was realized by preparing single crystal–polycrystal structures. By decreasing the time of the high-temperature treatment, the conductivity change could be increased by around two orders of magnitude, as compared to ZnO bicrystals. A second important finding in this project is the relationship between interfacial coherency and dopant segregation as concluded from the comparative (S)TEM studies. It was shown that the stable and effective incorporation of dopants such as bismuth at the interface, which are necessary for the creation of potential barriers, require a certain degree of interfacial incoherency. In other words, the varistor performance of bicrystals with coherent alignment cannot be increased by simply adding more dopants, it also requires an appropriate synthesis procedure such as the epitaxial solid-state transformation, which maintains the macroscopic alignment of the polarization vectors, but nevertheless leads to sufficiently incoherent boundaries. Piezoelectric measurements of ZnO single crystals with ohmic or Schottky contact were performed to shed light on the internal screening of the piezoelectric potential by free charge carriers. By decreasing the temperature, the number of free charge carriers within the ZnO single crystal could be artificially decreased, which resulted in an increase of the attainable piezoelectric polarization. The results demonstrate the general concept of the screening mechanism. Regarding piezotronic applications, a comparison between the response of a crystal with only ohmic contacts and a crystal with one Schottky contact is highly insightful. For the crystal with Schottky contact, the piezoelectric response can be already detected at room temperature and at low loading frequencies. This is a result of the depletion region which is formed in the vicinity of the rectifying Schottky contact. Microprobe measurements of individual grain boundaries in polycrystalline varistor ceramics were performed under the application of mechanical stress. A statistical evaluation of a multitude of potential barriers could be performed. The expected broadening of potential barrier characteristics within a polycrystalline material could be confirmed. These results are seen as a valuable input for the theoretical modeling of piezotronic devices based on polycrystalline varistor ceramics.
Publications
- Gauge factors for piezotronic stress sensor in polycrystalline ZnO; Journal of Physics D: Applied Physics 2017, 50, 175106
P. Keil, R. Baraki, N. Novak, J. Rödel, T. Frömling
(See online at https://doi.org/10.1088/1361-6463/aa65f6) - Piezotronic effect at Schottky barrier of a metalZnO single crystal interface; Journal of Applied Physics 2017, 121, 155701
P. Keil, T. Frömling, A. Klein, J. Rödel, N. Novak
(See online at https://doi.org/10.1063/1.4981243) - Piezotronic Tuning of Potential Barriers in ZnO Bicrystals; Advanced Materials 2018, 30, 1705573
P. Keil, M. Trapp, N. Novak, T. Frömling, H-J. Kleebe, J. Rödel
(See online at https://doi.org/10.1002/adma.201705573) - Finite element simulations on piezoelectric modulation of ZnO grain boundary barrier height; J. Appl. Phys. 2019, 127, 205101
Z.-Q. Zhou, K. Taylor, E. Gjonaj, T. Frömling, B.-X. Xu
(See online at https://doi.org/10.1063/1.5109666) - Influence of metal/semiconductor interface on attainable piezoelectric and energy harvesting properties of ZnO; Acta Materialia 2019, 162, 277-283
N. Novak, P. Keil, T. Frömling, F.H. Schader, A. Martin, K.G. Webber, J. Rödel
(See online at https://doi.org/10.1016/j.actamat.2018.10.008) - Mechanically tuned conductivity in piezoelectric semiconductors
Peter Keil
- ZnO-based single crystal-polycrystal structures for piezotronic applications; Journal of the American Ceramic Society 2019, 102, 2640- 2647
P. Keil, M. Gehringer, T. Frömling, N. Novak, J. Rödel
(See online at https://doi.org/10.1111/jace.16120) - Mechanically tuned conductivity at individual grain boundaries in polycrystalline ZnO varistor ceramics, J. Appl. Phys. 2020, 127, 034101
D. Bremecker, P. Keil, M. Gehringer, D. Isaia, J. Rödel and T. Frömling
(See online at https://doi.org/10.1063/1.5131003) - Segregation and properties at curved versus straight (0001) inversion boundaries in piezotronic ZnO bicrystals; Journal of the American Ceramic Society 2020, 103, 2817-27
M. Trapp, P. Keil, T. Frömling, J. Rödel and H.-J. Kleebe
(See online at https://doi.org/10.1111/jace.16912) - Synthesis and assembly of zinc oxide microcrystals by a low temperature dissolution–reprecipitation process: Lessons learned about twin formation in heterogeneous reactions; Chemistry A. European Journal 2020
R.C. Hoffmann, M. Trapp, E. Erdem, M. Kratzer, C. Teichert, H.-J. Kleebe, J.J. Schneider
(See online at https://doi.org/10.1002/chem.201904638)
