Elsevier

Thin Solid Films

Volume 519, Issue 13, 29 April 2011, Pages 4293-4298
Thin Solid Films

Doping induced structural and compositional changes in ZnO spray pyrolysed films and the effects on optical and electrical properties

https://doi.org/10.1016/j.tsf.2011.02.018Get rights and content

Abstract

Highly transparent, conductive ZnO films have been deposited by spray pyrolysis of a zinc acetate based solution. Quality films are yielded as our process is analogous to an aerosol assisted chemical vapour deposition rather than a droplet deposition spray pyrolysis technique. The properties of the films are governed by the additives to the base precursor solution. When aluminium acetylacetonate is added to the precursor solution, ZnO:Al films are grown with free charge carrier concentrations of more than 1020 cm−3. The carrier density and mobility are measured by both Hall probe and near infrared spectroscopy. Film growth and grain size, morphology and orientation are altered using an increased percentage of ZnCl2 in the precursor, which results in a 10 fold increase in charge carrier mobility. An investigation is presented correlating the composition of the precursor solution with the chemical, structural, electrical and optical properties of the grown films.

Introduction

High quality doped ZnO is grown using reactive and non-reactive sputtering [1], metal organic chemical vapour deposition [2] and pulsed laser ablation deposition [3]. Thereby resistivities can be achieved down to 2 × 10−4 Ω cm. The highest reported Hall mobilities are ~ 60 cm2V−1 s−1 [4]. With non-vacuum methods, however, such as spray pyrolysis [5] and sol–gel deposition [6] resistivities are normally much higher (5 × 10−3 to 10 Ω cm) since it is more difficult to control grain growth, stoichiometry and purity with such deposition techniques.

In this work we use a simple non-vacuum spray pyrolysis method to deposit ZnO. Extremely high quality films are achieved as the method is not droplet deposition spray pyrolysis, where the droplets wet the substrate, but is an aerosol assisted chemical vapour deposition (CVD) [7].

Low resistivity ZnO film can be achieved by the creation of intrinsic donors by lattice defects, for example oxygen vacancies or zinc atoms on interstitial lattice sites, or by introduction of extrinsic dopants such as Al3+ on substitutional metal lattice sites or halogens, such as Cl on oxygen lattice sites [8]. ‘Intrinsic’ doping can be achieved during the deposition by control of the oxygen source [8] and temperature. In reality, in doped oxide films both intrinsic and extrinsic doping mechanisms occur simultaneously. The films presented here have been ‘extrinsically’ doped by adding aluminium acetylacetonate and/or zinc chloride to the zinc acetate precursor solution. In spray pyrolysis the dopants affect not only the free charge carrier density but the composition, structure and growth mechanism of the film [9].

Previously [10], it was shown that low resistivities and significant deposition rates can be achieved only above a temperature of 400 °C. This paper determines which precursor solutions at a set temperature of 400 °C produce both high charge carrier density and mobility, and correlates measured electrical and optical properties to the structure and chemical composition of the film.

Section snippets

Experimental details

The set up of the laboratory spray pyrolysis was based on apparatus described in detail elsewhere [11]. The spray chamber is purged thoroughly with nitrogen to ensure an oxygen and water-deprived atmosphere. A precursor solution is then nebulised using a MHz ultrasonic source to form an aerosol which passes at an angle over a heated substrate surface, arranged such that the precursor droplets do not impact the surface but evaporate to gaseous reactants which subsequently react and deposit on

Results and discussion

Firstly the microstructure and chemical composition of the films are discussed and then, in the following sections, correlated to the results of the electrical and optical measurements.

Summary

ZnO is grown by a spray pyrolysis process. In our setup the aerosol of the precursor solution evaporates before landing on the substrate and hence is in principle an aerosol assisted CVD method. For a non-vacuum technique this produces particularly high quality films.

Without intentionally extrinsically doping the films and using only Zn(ac)2 in the precursor solution, films with a relatively high carrier concentration (~ 8 × 1018 cm−3) are obtained. The films exhibit poor carrier mobility due to

Acknowledgments

We would like to thank Kathrin Schatke for the use of the Hall probe at the Technical University of Berlin and Dr. Michael Gledhill for proof reading.

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