High quality aluminium doped zinc oxide target synthesis from nanoparticulate powder and characterisation of sputtered thin films P.J.M. Isherwood a, ⁎, N. Neves b , J.W. Bowers a , P. Newbatt b , J.M. Walls a a Centre for Renewable Energy Systems Technology, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK b Innovnano, S. A., Rua Coimbra Inovação Parque, IParque Lote 13, 3040-570 Antanhol, Coimbra, Portugal abstract article info Article history: Received 13 December 2013 Received in revised form 16 July 2014 Accepted 17 July 2014 Available online 26 July 2014 Keywords: Aluminium doped zinc oxide Transparent conducting oxide Sputter target synthesis Nanoparticle synthesis Compositional control Nanoparticulate aluminium-doped zinc oxide powder was synthesised through detonation and subsequent rapid quenching of metallic precursors. This technique allows for precise compositional control and rapid nano- particle production. The resulting powder was used to form sputter targets, which were used to deposit thin films by radio frequency sputtering. These films show excellent sheet resistance and transmission values for a wide range of deposition temperatures. Crystal structure analysis shows that crystals in the target have a random ori- entation, whereas the crystals in the films grow perpendicular to the substrate surface and propagate preferen- tially along the (002) axis. Higher temperature deposition reduces crystal quality with a corresponding decrease in refractive index and an increase in sheet resistance. Films deposited between room temperature and 300 °C were found to have sheet resistances equivalent to or better than indium tin oxide films for a given average trans- mission value. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Transparent conducting oxides (TCOs) are an unusual class of mate- rial with a wide range of uses. These include electronic screens and dis- plays including touchscreen panels through to electrochromic windows, light-emitting diodes (LEDs) and solar cells [1,2]. They are easily pro- duced, and techniques such as chemical vapour deposition, magnetron sputtering and spray pyrolysis are regularly employed [3,4]. The major concerns surrounding the metal oxide conductors are their brittleness and lack of flexibility, their inherent trade-off between transparency and conductivity [4,5] and in a few cases cost, rarity and environmental issues associated with obtaining the raw materials [3,4]. Despite these concerns, doped metal oxides are still the dominant type of transparent conductor, largely because of their good electrical properties, ease of for- mation and deposition and their relatively good chemical and thermal stability [6]. At present the most commonly used TCO is tin-doped indi- um oxide (indium tin oxide, or ITO) [1], but due to concerns over supply and cost of indium there has recently been an increasing interest in al- ternatives [7]. Aluminium-doped zinc oxide (AZO) is another commonly used TCO and is a cheaper alternative to ITO [7,8]. In order to compete with ITO, alternatives must have as good or better electrical and optical properties. This means an average visible light (400– 800 nm) transmission of around 80%, and sheet resistances of 20 Ω/sq or less [9]. The best quality ITO films are usually deposited at elevated tem- peratures, which can cause serious problems for any temperature- sensitive technology such as organic solar cells and LEDs [9]. Materials that can achieve similar transmission and sheet resistance values at lower deposition temperatures are therefore of significant interest. The electrical and optical properties of sputtered thin films depend not only on the deposition parameters but also on the characteristics of the sputter target. In particular, grain size, density, oxygen content and homogeneity play an important role in the determination of target qual- ity [10–14]. Innovonano S.A. has developed a patented nanoparticle- based route for the synthesis of AZO powders [15]. The following is a description and analysis of the material synthesis technique and charac- terisation of films sputtered from targets formed using this material. 2. Experimental details 2.1. Powder synthesis and target preparation A pyrolysis production method was used to synthesise AZO nano- structured powder containing 0.5 wt.% Al dopant [15]. Nanoparticulate powder was synthesised by detonation of an emulsion containing me- tallic Zn and Al precursors in the same concentrations as the desired ma- terial composition (Fig. 1). This method combines high pressures (N 1000 MPa), high tempera- tures (500–3000 °C) and ultrafast quenching (10 8 °C/s to 10 9 °C/s). Powder crystal size is determined by the precise conditions used. The resulting material shows a structure composed of both individual and agglomerated nanoparticles (Fig. 2). Subsequent to formation, the pow- der was disaggregated in order to improve sinterability [11]. Thin Solid Films 566 (2014) 108–114 ⁎ Corresponding author. Tel.: +44 1509 635306. E-mail address: P.J.M.Isherwood@lboro.ac.uk (P.J.M. Isherwood). http://dx.doi.org/10.1016/j.tsf.2014.07.032 0040-6090/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf