Cite this: CrystEngComm, 2013, 15, 7222 Alkali ratio control for lead-free piezoelectric thin films utilizing elemental diffusivities in RF plasma Received 22nd March 2013, Accepted 10th July 2013 DOI: 10.1039/c3ce40508g www.rsc.org/crystengcomm Hussein Nili, a Ahmad Esmaielzadeh Kandjani, b Johan Du Plessis, c Vipul Bansal, b Kourosh Kalantar-zadeh, a Sharath Sriram* a and Madhu Bhaskaran* a High performance piezoelectric thin films are generally lead-based, and find applications in sensing, actuation and transduction in the realms of biology, nanometrology, acoustics and energy harvesting. Potassium sodium niobate (KNN) is considered to be the most promising lead-free alternative, but it is hindered by the inability to control and attain perfect stoichiometry materials in the thin film form while using practical large area deposition techniques. In this work, we identify the contribution of the elemental diffusivities in the radio frequency (RF) plasma in determining the alkali loss in the KNN thin films. We have also examined the effect of the substrate temperature during the RF magnetron sputtering deposition on the crystal structure of the substrate and KNN thin films, as well as the effect of the post- annealing treatments. These results indicate the need for well-designed source materials and the potential to use the deposition partial pressure to alter the dopant concentrations. Introduction The alkali-based potassium sodium niobate, K x Na 12x NbO 3 (KNN), material system is one of the most promising lead-free substitutes for the dominant ferroelectric lead zirconate titanate (PZT) system owing to its good piezoelectric proper- ties, high Curie temperature, and modifiability by various dopants. 1–4 KNN-based thin films have been investigated extensively during the past ten years for applications ranging from optical waveguides 5,6 and ferroelectric random-access memories (FRAMs) 7 to piezoelectric-based devices for energy harvesting applications. 8–10 KNN-based thin films have been processed by a variety of techniques encompassing chemical deposition such as the sol–gel technique 11,12 and chemical solution deposition (CSD), 13,14 and physical deposition such as pulsed laser deposition (PLD) 3,15,16 and RF magnetron sputtering. 5,7–10,17–22 Among these, RF magnetron sputtering is promising due to its capability for large area depositions with lower growth tempera- tures that makes it viable for integration with fabrication processes. Major challenges in the RF magnetron sputtering of KNN-based thin films are non-stoichiometry due to the volatility of the alkali metals, 18,23 the formation of secondary phases, 24 and a high leakage current density. 17,18 The loss of alkali metals during the KNN thin film synthesis was noted in the early attempts of PLD and RF magnetron sputtering. 17,23,25 Further studies on the effects of different sputtering parameters on the composition, crystal structure and growth rate of the KNN thin films revealed that while the crystalline phases and growth rate are largely dependent on the sputtering pressure and growth temperature, the alkali loss cannot be controlled by varying the sputtering para- meters. 17,18,24 It was suggested then that the alkali volatility can be addressed by excess amounts of the K and Na species (usually in the ratio of 1.5 : 1.5 : 1 for K : Na : Nb) in the sputtering targets. 23 Most recently, Kim et al. 22 have studied the effect of the post-annealing treatment on the crystal structure and electrical properties of KNN thin films grown on Pt/Ti/SiO 2 /Si substrates at a substrate temperature of 300 uC utilizing a stoichiometric target, in different annealing atmo- spheres. To the best of our knowledge, until now, no comprehensive study has been carried out on the alkali loss and the compositional homogeneity of the KNN thin films as a function of the oxygen partial pressure during the sputtering process. Such a study offers the potential to control elemental diffusivity during deposition to tailor the alkali content and composition ratios. This creates the opportunity to attain different alkali ratios from a single target and also create tailored deficiencies in films for ionic conduction for memristive applications. This investigation aimed to study the correlation between varying the oxygen partial pressure and the post-annealing treatment on the alkali loss, crystal structure and surface morphology of KNN thin films, as well as to design methods to a Functional Materials and Microsystems Research Group, School of Electrical and Computer Engineering, RMIT University, Melbourne, Victoria 3001, Australia. E-mail: sharath.sriram@gmail.com; madhu.bhaskaran@gmail.com b NanoBiotechnology Research Lab, School of Applied Sciences, RMIT University, Melbourne, Victoria 3001, Australia c School of Applied Sciences, RMIT University, Melbourne, Victoria 3001, Australia CrystEngComm PAPER 7222 | CrystEngComm, 2013, 15, 7222–7229 This journal is ß The Royal Society of Chemistry 2013 Published on 11 July 2013. Downloaded by RMIT Uni on 22/09/2013 12:39:56. View Article Online View Journal | View Issue