Structural and Electronic Properties of Flexible ZnO and Ti/Mn:ZnO Thin Films
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Structural and Electronic Properties of Flexible ZnO and Ti/Mn:ZnO Thin Films Manish Kumar,∗ Jitendra Pal Singh and Hyun Hwi Lee† Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 37673, Korea
Keun Hwa Chae Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 02792, Korea (Received 8 January 2020; revised 14 April 2020; accepted 16 April 2020) Thin films of ZnO, Ti doped ZnO and Mn doped ZnO were prepared on flexible polyimide substrates by means of the RF sputtering technique for different sputtering parameters, and their structural and electronic properties were investigated. Grazing incidence X-ray diffraction (GIXRD) results confirmed that the grown samples followed the hexagonal wurtzite symmetry. The lattice parameters and the crystallite size showed a dependence on doping as well as deposition condition. The presence of oxygen during sputtering growth significantly suppressed the crystallite size and increased the number of oxygen defect states. The effect of doping and the deposition parameters on the electronic structure of flexible ZnO thin films was also realized through X-ray absorption measurements. Keywords: Zinc oxide, Flexible substrate, Flexible electronics, X-ray diffraction, X-ray absorption spectroscopy DOI: 10.3938/jkps.77.452
I. INTRODUCTION
Zinc oxide (ZnO) is among the highly studied semiconducting materials in past decades owing to its inimitable optical properties, for instance its wide band gap and large exciton binding energy. It is a potential candidate to be used in modern solid state technology (e.g., solar cells, optoelectronic, spintronic and surface devices, sensors, nonlinear optical devices etc.) [1–7]. As device realization requires stabilization of ZnO in the thin film form hence consistent efforts were carried out across the globe investigating for ZnO-based thin-film growth. For fabrication of ZnO thin films, various techniques can be employed, such as the sol-gel method, molecular beam epitaxy, metal organic chemical vapor deposition, pulsed laser deposition (PLD), RF sputtering, etc. [8–12]. The functional properties of ZnO thin films can be significantly tailored by controlling the growth parameters during deposition. Moreover, doping ZnO with different dopants has led to modified properties [12–15]. Especially transition metals (TMs) are chosen as dopants to tailor the functional properties of ZnO [16]. Among the different TMs for doping ZnO, Mn and Ti have some advantages, their ionic radii are nearly the same as that ∗ E-mail: † E-mail:
of Zn2+ , Mn2+ : 0.066 nm, Ti4+ : 0.061 nm) and Zn2+ : 0.060 nm [12,17]. Since the work of Hoffman et al. on ZnO thin-film transistors in 2003, significant flexible and transparent electronics have emerged [18]. The key requirement in flexible device fabrication is a low synthesis temperature. In this aspect, ZnO has an edge over other inorganic wide-bandgap semiconductors, such as gallium nitride (GaN) and silicon carbide (SiC), which require comparatively higher temperature for fabrication. High electr
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