Strain dependence of electronic transport in semiconducting oxide thin films
Final Report Abstract
This project was part of the Paketantrag PAK 928, dedicated to the mechanically tunable electrical conduction in semiconducting oxides. Within this consortium, this project was aiming at observing strain dependent electrical properties in ZnO thin films. Two approaches have been considered: i) the tunablity of electrical potential barrier heights at grain boundaries in polycrystalline films and ii) the strain dependent electrical conduction of ZnO thin film Schottky diodes. Polycrystalline ZnO thin films have been prepared by magnetron sputtering and, in collaboration with EPFL in Neuchâtel, Switzerland, by low-pressure chemical vapor deposition (LPCVD). The LPCVD films can be prepared with in-plane orientation of the crystallographic c-axis, which results in polarization axes terminating at the grain boundaries. As expected by fundamental considerations and confirmed by polycrystalline bulk ceramics and bicrystals within the PAK 928 consortium, such films should exhibit strain dependent potential barriers and thus a strain dependent electrical conductivity. However, the LPCVD films had an unstable electrical conductivity, likely related to a persistent photoconductance. The corresponding variation of conductivity was higher than any strain dependent effects. Temperature dependent Hall effect measurements have been set up and were studied in order to establish a new approach for quantifying grain boundary potential barrier heights. While this approach worked well for doped In2O3 films, it turned out that it cannot be applied to ZnO, due to the strong temperature dependence of the involved scattering mechanisms. Schottky diodes have been prepared on glass substrates with different configurations. Highly rectifying diodes could be reproducibly prepared with Al-doped ZnO ohmic bottom contacts, a bi-layer of ZnO deposited without (1st layer) and with (2nd layer) addition of oxygen to the process gas during deposition of the ZnO films by magnetron sputtering. RuO 2 served as rectifying top contact. The diode curves exhibited a pronounced hysteresis when cycled between negative and positive polarity. The hysteresis can be explained by an electrically programmable barrier height. We assign this observation to (metastable) charge trapping in oxygen vacancies, which has also been reported to account for persistent photoconductivity. Strain dependent measurements could not be completed due to the dominating effect of electrically modified barrier modification, which can potentially be applied in memristive devices. X-ray photoelectron spectroscopy analysis of ZnO/RuO 2 Schottky barriers confirmed the strong variation of barrier height with oxygen vacancy concentration. For low defect concentrations, the contacts exhibit barrier heights >1.3 eV, which are amongst the highest reported for ZnO. Measurements on c-axis oriented single crystals revealed for the first time a dependence of barrier height on surface polarity. The dependence of barrier height is consistent with a partial screening of the polarization by the electrodes and amounts to ~250 meV for the studied interfaces. This result indicates that the difference in barrier height for nominally Zn- and O-terminated ZnO surfaces depends crucially on sample preparation and electrode material, due to the influence of the atomic configuration at the interface on polarization screening.
Publications
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The piezotronic effect on Schottky barrier at the metal-ZnO single crystal interface, J. Appl. Phys. 121 (2017), 155701
P. Keil, T. Frömling, A. Klein, J. Rödel, N. Novak
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Polarization dependence of ZnO Schottky barriers revealed by photoelectron spectroscopy, Phys. Rev. Mater. 4 (2020), 084604
P. Wendel, S. Periyannan, W. Jaegermann, A. Klein
