Epitaxial growth of lead-free (K, Na)NbO3 – based thin films by rf sputtering and pulsed laser deposition

Abstract

Piezoelectric devices, since the first discovery of direct piezoelectric effect by Pierre Curie and Jacques Curie in 1882 [1], have been playing major roles in many industrial fields, medical diagnosis, medical treatment, etc [2-4]. Thin-film piezoelectricity is one of the key factors for MEMS technologies with novel applications in piezoelectric transducers, pyroelectric IR sensors, actuators, electro-optical devices, and nonvolatile ferroelectric memories [5]. The most successful and practical piezoelectric materials have been the lead-based type such as PZT or Pb(Zr,Ti)O3 due to the exceptional electromechanical properties [6-8]. However, since most lead-based materials contain higher than wt. 60% Pb, they are considered toxic, and poses a serious threat to the environment. Consequently, regulations of hazardous substances (RoHS) have been established by the European Union to restrict their usage in electronic devices [6]. Many studies have been carried out in finding the alternatives, and among the most promising candidates are K0.5Na0.5NbO3 (KNN)-based materials, with KNN ceramic’s piezoelectric strain coefficient d33 = 416 pC/N comparable to that of PZT-based. In the meantime, KNN-based ceramics have been considered to possess the closest characteristics with respect to PZT [8-12]. As lead-based materials had been continuously researched, developed and optimized for several decades since it was first discovered in 1950s until the numerous reports of its toxicity [6], KNN based materials are now constantly focused and improved. KNN thin films have the potentials for great applications, especially in medical science for being biocompatibility. Extensive efforts have been made to fabricate both KNN and KNN-based thin-films, however their qualities and piezoelectric properties are much inferior in compared to those of their ceramic counterparts. Up until recently, reports on d33 of all KNN-based thin-films are in the range of 40-74 pm/V, regardless of the fabricating process [9, 13-17]. The significant inferiority of thin-films KNN-based is reported to originate from the severe volatility of alkaline elements during the deposition process, leading to the deviation from compositional stoichiometry, the formation of secondary phases, and consequently high leakage current density. Many studies have been carried out to overcome the high leakage problem of KNN and KNN-based thin-film. Kizaki et al.,[18] in 2006, reported the improvement in leakage control in Mn-doped single crystal KNN. Kondo et at.,[19] and Lee et al.,[20] respectively reported on the improvement of chemically deposited KNN-based thin-films by Mn-doping. However, the drawback of chemical synthetic route involves early aging, formation of micro-cracks across the film, and the necessity of post annealing treatment for the crystallization of KNN-based film. Lopez-Juarez et al.,[21] and Wang et al.,[22] reported the successfully fabricated Mn-doped KNN ceramic, in 2015, with improvements in dielectric, ferroelectric and electrical properties. Consequently, this has opened an opportunity for physically fabricated Mn-doped KNN film. The scope of this work, firstly, involves successful synthesis of high density and good quality Lead-free Mn-doped Potassium Sodium Niobate (K,Na)NbO3-based ceramic targets. Next, Mn doped KNN thin films are grown by using the rf magnetron sputtering and pulse laser deposition (PLD). Insight studies on the influence of thin-film fabrication parameters, effect of substrate temperature, substrate orientation, O2 partial pressure, post deposited thermal treatment and protecting Al2O3 layer will be carried out. Investigation and analysis on structural phases, grain morphologies, electrical properties and ferroelectric properties such as dielectric permittivity, ferroelectric polarization, and piezoelectric responses, and electrical leakage current will be carried out for confirming the reduction in leakage current density and good piezoelectric properties. Finally, the applications of KNN-based films for the sensors, the energy harvesters, and energy storage devices are addressed, and current challenges and prospects for future work will be discussed.