Comparative study of zinc oxide and aluminum doped zinc oxide transparent thin films grown by direct current magnetron sputtering

doi:10.1016/j.tsf.2006.11.151

Thin Solid Films

Volume 515, Issue 16, 4 June 2007, Pages 6562-6566

https://doi.org/10.1016/j.tsf.2006.11.151 Get rights and content

Abstract

Pure and aluminum (Al) doped zinc oxide (ZnO and ZAO) thin films have been grown using direct current (dc) magnetron sputtering from pure metallic Zn and ceramic ZnO targets, as well as from Al-doped metallic ZnAl2at.% and ceramic ZnAl2at.%O targets at room temperature (RT). The effects of target composition on the film's surface topology, crystallinity, and optical transmission have been investigated for various oxygen partial pressures in the sputtering atmosphere. It has been shown that Al-doped ZnO films sputtered from either metallic or ceramic targets exhibit different surface morphology than the undoped ZnO films, while their preferential crystalline growth orientation revealed by X-ray diffraction remains always the (002). More significantly, Al-doping leads to a larger increase of the optical transmission and energy gap (E_g) of the metallic than of the ceramic target prepared films.

Introduction

ZnO thin films are highly attractive in the development of materials area, due to their interesting physical properties as high transparency in the visible and near-ultraviolet (UV–VIS) spectral regions, as well as their wide conductivity range and conductivity changes under photoreduction/oxidation condition. The conductivity and its changes under specific conditions for ZnO thin films depends upon several factors, such as the preparation technique, the in situ preparation parameters, the doping agent, the annealing temperature and atmosphere, and even the measurement conditions [1], [2], [3], [4]. When ZnO films are doped with the appropriate metal atoms, such as Al, Sn, Cd, Ga, In, etc., their conductivity can be changed from values as low as 10^− 10 (Ω cm)^− 1 to values as high as 10⁴ (Ω cm)^− 1. The wide range of conductivities and conductivity changes upon different environmental conditions make ZnO films suitable materials for oxidant gas sensing layers [5], [6], [7]. Dopant presence determines significant changes of film physical properties as crystalline structure (associated to stoichiometry), surface topology (associated to adsorption of species onto surface), optical properties (associated with photoconductivity) which reflect directly on film ability to act as a sensing layer. In the case of ZAO films, previous works have reported that the presence of the dopant determines usually crystallinity alterations [8] or decreases of the transmittance significantly, to values under 70% [9], facts that are highly inconvenient for sensing applications. In this work ZnO and ZAO thin films prepared by dc magnetron sputtering and their structural and optical properties were studied comparatively.

Section snippets

Experimental

The deposition of the ZnO and ZAO films was carried out in an Alcatel dc magnetron sputtering system using 99.999% pure Zn, ZnAl2 at% (ZnAl2%) metallic and ZnO, ZnO: 2 at.%Al₂O₃ (ZnAl2%O) ceramic targets all fabricated by Heraeus TMD. The base pressure in the chamber was 5 × 10^− 7 mbar (5 × 10^− 5 Pa). Films with thickness about 100 nm were deposited onto Corning 1737F glass and silicon substrates in an oxygen–argon atmosphere (O₂/Ar). The deposition constant parameters were the total pressure (8 × 10^− 3

Results and discussions

It is well known that sputtered ZnO films are highly textured with the c axis perpendicular to the substrate surface. The changes in crystal structure for films sputtered from different targets were investigated by X-ray diffraction (XRD). Fig. 1a displays XRD patterns of ZnO and ZAO films deposited on Corning glass. The XRD spectra are dominated by the hexagonal ZnO (002) plane confirming the strong (002) textures. The films exhibit only the (002) peak in the displayed 2θ/ω region and no

Conclusions

ZnO and ZAO thin films have been grown using dc magnetron sputtering from pure metallic Zn and ceramic ZnO targets, as well as from Al-doped metallic ZnAl2% and ceramic ZnAl2%O targets. All films have (002) texture, however for metallic targets the (002) peak shifts to lower angles and there is a decrease of film crystallinity. All films exhibit only the (002) peak and no metallic Zn or Al characteristic peaks were observed. The (002) diffraction peak position approaches but never reaches that

Acknowledgments

This work was partially supported by the ASSEMIC MRTN-CT-2003–504826 European founded project and National Funded Project PENED 2003–03ED733.

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Comparative study of zinc oxide and aluminum doped zinc oxide transparent thin films grown by direct current magnetron sputtering

Abstract

Introduction

Section snippets

Experimental

Results and discussions

Conclusions

Acknowledgments

Thin Solid Films

Thin Solid Films

Sens. Actuators, B

Thin Solid Films

Jpn. J. Appl. Phys.

Jpn. J. Appl. Phys.

Sol. Energy Mater. Sol. Cells