Elsevier

Optical Materials

Volume 45, July 2015, Pages 97-103
Optical Materials

Effect of sputtering power on the electrical and optical properties of Ca-doped ZnO thin films sputtered from nanopowders compacted target

https://doi.org/10.1016/j.optmat.2015.03.015Get rights and content

Highlights

  • Ca-doped ZnO films grown by rf-magnetron on glass substrates.

  • (40–100 W) CZO films grown by rf-magnetron on glass substrates.

  • Transmittance up to 85% in visible region, sharp absorption edge at 380 nm.

  • Blue shift of optical band gap: Eg = 3.30 eV for rf = 60 W and d = 400 nm; Egmax = 3.49 eV for rf = 100 W.

  • Good physical properties suited for films integration in optoelectronic devices.

Abstract

In the present work, we have deposited calcium doped zinc oxide thin films by magnetron sputtering technique using nanocrystalline particles elaborated by sol–gel method as a target material. In the first step, the nanoparticles were synthesized by sol–gel method using supercritical drying in ethyl alcohol. The structural properties studied by X-ray diffractometry indicates that Ca doped ZnO has a polycrystalline hexagonal wurzite structure with a grain size of about 30 nm. Transmission electron microscopy (TEM) measurements have shown that the synthesized CZO is a nanosized powder. Then, thin films were deposited onto glass substrates by rf-magnetron sputtering at ambient temperature. The influence of RF sputtering power on structural, morphological, electrical, and optical properties were investigated. It has been found that all the films deposited were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (0 0 2) crystallographic direction. They have a typical columnar structure and a very smooth surface. The as-deposited films show a high transmittance in the visible range over 85% and low electrical resistivity at room temperature.

Introduction

Zinc oxide (ZnO) is an unintentional n-type II–VI semiconductor material with an hexagonal wurtzite structure. ZnO has a direct band gap of ∼3.3 eV, high transmittance in visible region, high conductivity and free exciton binding energy of 60 meV at room temperature [1], [2]. ZnO has also several substantial advantages such as being abundant, inexpensive, nontoxic and exhibiting active ultraviolet (UV) response. As a result of these superior characteristics, ZnO has been suggested for many important large area electronic applications including transparent thin film sensors, transistors (TFTs), solar cells and window insulation systems [3], [4], [5], [6], [7]. Sputtered ZnO films are especially attractive since they promise lower cost than ITO films and higher conductivities and transparencies than SnO2-based TCO films. Doping of ZnO using Ca species in particular is more effective for the stabilization of lattice systems and increases the ionicity of chemical bonds in ZnO films. Generally, ZnO films are fabricated by magnetron sputtering [8], sol–gel [9], MOCVD [10], chemical vapor deposition [11] and spray pyrolysis [12]. The doping of ZnO is being considered for manufacturing transparent electrodes thanks to their high luminous transmittance, good electrical conductivity, good adhesion to the substrate and the fact that they are chemically inert.

In this paper, transparent conducting Ca doped ZnO (CZO) films deposited on glass substrates by rf-magnetron sputtering technique are reported using a nanocrystalline powder synthesized by the sol–gel method as a target material. The effects of rf power on structural, electrical and optical properties for the CZO films are investigated in detail.

Section snippets

Experimental techniques

Nanocrystalline CZO aerogels were first prepared by sol–gel method using 10 g of zinc acetate dehydrate [Zn(CH3COO), 2H2O] as a precursor in 70 ml of methanol. After 30 min under magnetic stirring at room temperature, an adequate quantity of calcium chloride-6-hydrate [CaCl2, 6H2O] corresponding to [Ca]/[Zn] ratios of 0.03 was added. After 15 min under magnetic stirring, the solution was placed in an autoclave and dried in a supercritical condition of ethanol (EtOH). Then, the CZO films were

Structural studies

Fig. 1 shows the XRD diffractogram of CZO nanoparticles. Three pronounced ZnO diffraction peaks appear at 2θ = 36.77°, 40.02° and 43.15°, corresponding to (1 0 0), (0 0 2) and (1 0 1) planes respectively, which are very close to wurtzite ZnO ones [16]. The obtained XRD diffractogram matched well with the space group P63mc (No. 36-1451) of wurtzite ZnO structure [16], [17]. For our samples, the (0 0 2) diffraction line was narrower than the (1 0 1) line, which is in turn broader than the (1 0 0) line. This

Conclusion

Ca-doped zinc oxide films were deposited on glass substrates at room temperature by rf-magnetron sputtering process using aerogel nanopowders prepared by sol–gel technique as a target material. The effect of the RF power on the structural, electrical and optical properties of CZO films was investigated. All of the obtained films were polycrystalline with a hexagonal structure and have a preferred orientation with c-axis perpendicular to the substrate. Flat and smooth surface morphologies with

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