Full Length ArticleEffect of Li doping on the electric and pyroelectric properties of ZnO thin films
Graphical abstract
Introduction
Wurtzite zinc oxide still attracts great interest due to a peculiar combination of piezoelectric, optical and electrical properties that recommends it for a wide range of applications. It was proven that ZnO has a great versatility in terms of electrical properties, going from wide band-gap semiconductor (intrinsic ZnO) [1], [2], to transparent electrode [3], [4], [5], or to high resistive layer [6], [7], [8], by simply controlling the doping nature/level or the growth conditions. Moreover, Onodera et al. [7], [8], [9], [10], [11] have found a new functionality of ceramic ZnO, namely ferroelectricity, by doping it with adequate amount of lithium atoms. The ferroelectricity seems to occur as a result of difference in ionic radii of host Zn and dopant Li, which occupies off-center position in ZnO lattice (small structural distortion) and thus generating polarization. But more important than the ionic radius difference is that the substituent lithium which has no d-electrons produces a modification in the bond length/character along c-axis. Onodera et al. spotted ferroelectricity in ceramic Li:ZnO (LZO) since the 90′s (with transition temperature between 330 K − 470 K), but ferroelectricity in LZO thin films deposited by chemical or physical methods (e.g. sol-gel or pulsed laser deposition) is still questionable due to the fact that the published hysteresis loops on LZO did not acquire saturation [12], [13], [14], [15], looking more like a dielectric with significant losses. However, the quality of the hysteresis loops is enhanced on Li implanted samples or in co-implanted samples including Li as implanted atoms [16], [17].
As a ferroelectric material, ZnO attracted interest for applications based on ferroelectrics, such as non-volatile memories [18], [19] or pyroelectric detectors [20]. In many applications, the ferroelectric material is used in capacitor geometry. In this paper, the structural and functional properties of un-doped ZnO (UDZO) and LZO capacitors with top and bottom platinum electrodes are investigated. Electrical measurements were performed both on as-grown and annealed samples. On as-grown samples it was found that the introduction of Li changes the properties compared to un-doped ZnO films, leading to higher resistivity and possible presence of ferroelectricity. The properties of LZO capacitors seem not to change after annealing, while the properties of un-doped ZnO capacitors change drastically after annealing, suggesting that the Pt contacts transform from ohmic to rectifying. It was also found that pyroelectric properties are considerably enhanced by Li doping.
Section snippets
Materials deposition
Un-doped ZnO (UDZO) and Li doped ZnO (LZO) films were grown by pulsed laser deposition (PLD) on platinized silicon substrate (Pt–100 nm/TiO2–15 nm/SiO2–450 nm/Si–500 μm) that were previously ultrasonically cleaned in acetone and isopropanol. The targets were dense ceramic disks of ZnO and Li0.1Zn0.9O; the PLD workstation (Surface GmbH) is equipped with a KrF pulsed excimer laser (λ = 248 nm) operated with a frequency of 10 Hz. The fluency of the laser beam was set to 1 J/cm2, while the
Structural properties of the as-grown films
ZnO films obtained by physical methods on different type of substrates (glass, silicon, or even mono-crystals) have the natural tendency to grow along c-axis [21]. In Fig. 1(a) are shown the X-ray diffractograms acquired in symmetric geometry of UDZO and LZO thin films deposited on platinum covered substrate. The diffractograms contain the two peaks corresponding to the 002 and 004 reflections of hexagonal wurtzite structure of ZnO (indexed according ICDD pattern 00-036-1451), while the other
Discussions
The results presented in previous sections need a more detailed discussion. First, the relatively curious results obtained in the case of annealed UDZO samples will be commented and explained. The butterfly shape of the C-V characteristic after annealing can be explained by the fact that the nature of the Pt contacts has changed from ohmic to rectifying (Schottky-like). However, the butterfly shape observed for short delay times during C-V measurements is not due to ferroelectricity but is due
Conclusion
Electric and pyroelectric properties of un-doped and Li-doped ZnO layers were investigated on samples with capacitor geometry having bottom and top Pt electrodes. Measurements were performed on as-grown films and on thermally annealed capacitor structures. The main results are:
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As-grown Li-doped samples show higher electrical resistivity compared to un-doped ZnO films, with butterfly shape of the C-V characteristic suggesting ferroelectric-like behavior.
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The annealed un-doped samples show,
Acknowledgement
The authors acknowledge the CORE Program project PN16-480102. M. Botea and L. Pintilie thank also to M-ERA NET project NOPYDET.
The authors wish to thank to G. Aldica for helping with ceramic target preparation, L. Trupina for platinum electrodes deposition and M. Enculescu for SEM images.
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