Origin of swift heavy ion induced stress in textured ZnO thin films: An in situ X-ray diffraction study
Introduction
Zinc oxide (ZnO) is a direct wide band gap semiconductor having wurtzite crystal structure. The large exciton binding energy of ∼60 meV ensures stable luminescence properties at room temperature (RT). In the last few years, ZnO based nanostructures have attracted much attention and research activities in view of their potential applications [1], such as short wavelength electro-optical devices [2], lasers [3], white light emitting devices [4], [5], spintronic devices [6], [7], [8] and solar cells [9]. A lot of studies are reported on the strain or stress effects on the properties of ZnO films, but to the best of our knowledge hardly any study exists on their manipulation. The study of strain in textured films is of considerable importance for understanding the response of dilute magnetic semiconductor devices, as the strain or stress in the films can tailor the magnetic and optical properties to a very large extent.
Swift Heavy Ion (SHI) irradiation is a very efficient means for modifying the structural and optical properties of materials [10], [11], [12], [13], [14]. Each SHI passes through the material and produces a perturbation of the electronic system of the target in a narrow cylinder along its trajectory resulting in changes in a narrow zone along its trajectory. This narrow cylinder with a variety of defects leads to interesting properties in the material. Therefore, in the present paper, SHI irradiation has been used as a tool to create stress and to understand their effect in the textured ZnO films grown by the pulsed laser deposition technique.
Section snippets
Experimental
The thin films of thickness around 200 nm were deposited on a silicon substrate using the pulsed laser deposition (PLD) technique. The target was synthesized using ZnO powder with a purity of 99.99%, by compaction under uniaxial pressure of 2 MPa followed by sintering in air for 6 h at 1100 °C. A pulsed KrF eximer laser having a wavelength of 248 nm, at an average fluence of 2.8 J/cm2, pulse repetition rate of 2 Hz and pulse duration of 25 ns was used for the ablation. The laser beam was
Results and discussion
Fig. 1 shows the (002) predominant peak observed in the XRD pattern of pristine and 120 MeV Ag9+ ion irradiated films (JCPDS-36-1451). Indeed these patterns reveal a strong texturing along the [002] axis for pristine and irradiated films, being the preferential orientation of ZnO films [16]. Other reflections were absent in the full pattern (not shown). Intensity of the (002) reflection keeps decreasing, and its full width at half-maximum (FWHM) increasing and shifting to higher diffraction
Conclusions
In conclusion, the issue of stress is addressed in view of its strong implications in the origin of ferromagnetism in oxide materials such as ZnO. A systematic induction of compressive stress in the textured ZnO film by swift heavy ion irradiation is demonstrated. The origin of the stress is attributed to the strong density of defects like dislocations at the grain boundaries created by dense electronic excitations.
Acknowledgements
The authors are grateful to Dr. A. Roy, Director, IUAC, Dr. D.K. Avasthi and Dr. D. Kanjilal for their encouragement and moral support. Experimental support from Ms. I. Sulania is also acknowledged. The authors are also grateful to the Pelletron group of IUAC for providing the stable beam during the SHI irradiation experiment and the Department of Science and Technology (DST), Govt. of India for providing the XRD facility to IUAC through the IRPHA project.
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