Ion irradiation of AZO thin films for flexible electronics

https://doi.org/10.1016/j.nimb.2016.11.037Get rights and content

Highlights

  • Evidence of electrical good quality AZO ultra thin films without thermal annealing.

  • Evidence of the main role of Oxygen vs. structural parameters in controlling the electrical performances of AZO.

  • Evidence of the role of the ion irradiation in improving the electrical properties of AZO ultra thin films.

  • Synthesis of AZO thin films on flexible/plastic substrates with good electrical properties without thermal processes.

Abstract

Aluminum doped Zinc oxide (AZO) is a promising transparent conductor for solar cells, displays and touch-screen technologies. The resistivity of AZO is typically improved by thermal annealing at temperatures not suitable for plastic substrates. Here we present a non-thermal route to improve the electrical and structural properties of AZO by irradiating the TCO films with O+ or Ar+ ion beams (30–350 keV, 3 × 1015–3 × 1016 ions/cm2) after the deposition on glass and flexible polyethylene naphthalate (PEN). X-ray diffraction, optical absorption, electrical measurements, Rutherford Backscattering Spectrometry and Atomic Force Microscopy evidenced an increase of the crystalline grain size and a complete relief of the lattice strain upon ion beam irradiation. Indeed, the resistivity of thin AZO films irradiated at room temperature decreased of two orders of magnitude, similarly to a thermal annealing at 400 °C. We also show that the improvement of the electrical properties does not simply depend on the strain or polycrystalline domain size, as often stated in the literature.

Introduction

Transparent conductive oxides (TCO) are strategic materials due to their combination of high optical transparency and good electrical conductivity, which make them essential components for a large variety of key devices such as displays, solar cells, E-paper, touch-screens [1], [2], [3]. The expanding use of tin-doped indium oxide (In2O3:Sn or ITO) for the production of transparent electrodes is endangered by the scarcity and increasing price of In. Nowadays, aluminum-doped zinc oxide (Al:ZnO or AZO) is considered a true alternative to ITO due to the lower cost of the source materials, non-toxicity and good electro-optical properties [2]. For these reasons AZO has been extensively investigated and employed as transparent electrode. Although over the past years significant progresses have been achieved, further intensive studies are needed to better understand some of the main properties influencing the performance of this material, e.g. the role of the elastic strain and defects on the optical and electrical performances [4], [5], [6], [7], or the band structure of heavily doped zinc oxide [1].

Stoichiometric ZnO has a direct optical band gap Eg = 3.2–3.4 eV, and an intrinsic n-type behavior due to crystal defects as O vacancies and Zn interstitials [1], [4]. Extrinsic n-type doping of ZnO can be achieved by a few percent (1–3%) of Al, so obtaining AZO. The presence of intrinsic and extrinsic n-type dopants, and the consequent filling of the conduction band, not only affects the structural properties of the materials (e.g. a tensile strain along the c-axis), but it also increases the optical band gap value, Eg, as described by the Moss-Burnstein (M-B) effect [8]. This is why the reduction of the electrical resistivity is often accompanied by an increment of Eg. However, the M-B effect can be strongly reduced by many field interactions [9], [10]. Finally, the electrical properties of this material are also related to other effects which directly depends on its structural properties, such as the scattering at the grain boundaries for polycrystalline films (as those grown by sputtering) and with ionized impurities (as Al3+) for doping level higher than 1020 cm−3. In these cases, the carrier mobility is another critical parameter influencing the electrical resistivity. Thermal processes are an efficient way to modify structural and electrical properties of materials and, in particular, can strongly reduce the electrical resistivity of AZO and other TCO films [1], [11] This effect is attributed to several concurrent modifications such as, for example, the size increasing of crystalline domains in polycrystalline thin films, the positioning of Al atoms into Zn substitutional sites and the strain release. Thus, thermal processes at around 250–300 °C are often mandatory for industrial applications of AZO as transparent electrodes. However, thermal budgets can be incompatible with the use of plastic or organic substrates, and alternative routes must be considered in this case. The use of energetic electron or ion beams for the modification of ZnO-based TCO is also reported in the literature, mainly focusing on the effect of light and heavy ion irradiation at very high energy on the electro-optical properties [12], [13], [14], [15], [16], [17], [18], [19].

This paper deals with the modifications of structural, optical and electrical properties of AZO polycrystalline thin films induced by medium energy ion beam irradiations and/or thermal annealing. Ion beams and thermal processes produced similar modifications of the lattice strain, crystalline domain size and optical band-gap. The electrical resistivity also strongly depends on the post growth treatments and a reduction of more than 2 orders of magnitude was obtained by using high temperature thermal annealing or appropriate ion beam irradiations at room temperature. The latter process allowed us to produce AZO thin films deposited on plastic substrates (polyethylene naphthalate, PEN), without any thermal process during or after the growth, with electro-optical performance comparable to those of AZO films deposited on glass and annealed at 400 °C.

Section snippets

Experimental

AZO films were deposited on soda lime glass (about 1 mm thick) or on PEN (0.13 mm thick) substrates by RF magnetron sputtering (room temperature, Ar atmosphere, working pressure of 1 Pa). After cleaning in de-ionized water and absolute ethanol, the substrates were placed in front of the target source material (2 wt% Al2O3 and 98 wt% ZnO) at a distance of 7 cm. The sputtering power was 225 W and the AZO film thicknesses were in the range 60–80 nm. This kind of deposition produces highly (0 0 2) textured

AZO on glass

Fig. 1 displays the XRD spectra of various AZO samples deposited on glass, recorded in the θ-2θ configuration and normalized to the same arbitrary intensity. In particular, we report diffraction pattern of the films before and after the ion irradiation processes.

All samples exhibit only the (0 0 2) peak, which indicates a hexagonal wurtzite structure strongly textured, with the c–axis along the growth direction. Two main features can be discussed for the (0 0 2) peak: the Full Width Half Maximum

Conclusions

We investigated the modification of structural, optical and electrical properties of AZO thin films deposited on rigid glass or flexible PEN substrates upon ion irradiation with O+ or Ar+ ions. Samples were also characterized before and after thermal treatments up to 400 °C (only for the glass substrates). For all samples we report a clear improvement of the structural quality upon ion implantation, with a significant growth of the polycrystalline grain size and strain release. A remarkable

Author contributions

SB prepared samples, performed optical and electrical characterizations, contributed to data interpretation and prepared the draft of the paper. GT prepared samples and contributed to electrical and optical characterization. IC contributed to the electrical characterization and data interpretation. AA performed XRD measurements and data interpretation. SM performed RBS analyses and gave major contributions to the revision of the paper. FR performed AFM measurements and data analyses. FS

Funding sources

This work has been partially funded by the MIUR project PON02_00355_3391233 ENERGETIC

Notes

The Authors declare no competing financial interest.

Acknowledgement

We greatly acknowledge the technical supports of S. Tatì and G. Pantè (CNR-IMM MATIS) for ion implantation processes and thermal annealing, respectively.

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