Optoelectronic properties and polar nano-domain behavior of sol–gel derived K_0.5Na_0.5Nb_1−xMn_xO_3−δnanocrystalline films with enhanced ferroelectricity

Abstract

High-quality lead-free piezoelectric K 0.5 Na 0.5 Nb 1Àx Mn x O 3Àd (KNNMx, 0 r x r 0.10) films have been successfully deposited on Pt(111)/Ti/SiO 2 /Si(100) substrates by a modified sol-gel method. The effects of Mn substitution on the microstructure, morphology, lattice vibrations, and optical and ferroelectric properties of the KNNMx films have been investigated in detail. All films are polycrystalline, crack-free and show a pseudo-cubic (pc) structure with a thickness of about 215 nm. Raman analysis indicates that the characteristic frequency of n 1 , n 5 and n 1 + n 5 modes shifts towards lower wavenumbers with increasing Mn concentration. The optimal ferroelectric properties were obtained in the film doped with x = 0.06, whose remnant polarization (2P r ) and coercive field (2E c ) values at the applied electric field of 1000 kV cm À1 are 51 mC cm À2 and 265 kV cm À1 , respectively. The increased valence of Mn 2+ , which is substituted at the Nb 5+ site as Mn 3+ , plays an important role in reducing the amount of both oxygen vacancies and holes. In addition, the dielectric functions of the KNNMx films have been uniquely extracted by fitting ellipsometric spectra with the Adachi dielectric function model and a four-phase layered model (air/surface rough layer/KNNMx/Pt) in the photon energy range of 1.5-5.5 eV. The optical band gap (E g ) slightly decreases, while the high-frequency dielectric constant (e N ) linearly increases with increasing Mn concentration. Moreover, temperature dependent optical dispersion behavior of the KNNM0.06 film has been investigated from 300 K to 800 K. The analysis of E g and the extinction coefficient (k) reveals the correlation between optical properties and structural phase transition. Furthermore, a distinct in-plane (1801) polar nano-domain pattern with a well-defined rectangular phase hysteresis loop has been observed in the KNNM0.06 film from piezoresponse force microscopy (PFM) experiments. The present results could be crucial for potential multifunctional KNN-based device applications.high Curie temperature (T C B 420 1C), excellent ferroelectric and piezoelectric properties (d 33 B 416 pC N À1 ), as well as good biocompatibility. [5][6][7] Most of the current research on KNN is for bulk ceramics and single crystals. 5,6,8,9 However, there are a few reports on KNN films, which could offer an unexceptional way to build miniaturized devices, such as sensors and micro-electromechanical systems (MEMS). KNN films have been prepared by various methods, such as pulsed laser deposition (PLD), RF-magnetron sputtering and the sol-gel method. 10-12 Among them, the sol-gel method is a well-accepted method due to some evident advantages, such as its chemical homogeneity, ease of stoichiometry control, low-cost and suitability for mass production. 4,13 Unfortunately, the sol-gel derived KNN films show disappointing ferroelectric and piezoelectric properties due to their high leakage current. Many studies have suggested that the oxygen and alkali-ion vacancies in perovskite oxides pla...

Exploring optoelectronic properties and mechanisms of layered ferroelectric K4Nb6O17 nanocrystalline films and nanolaminas

“…It was noteworthy that, KN was easily formed as a secondary phase caused by the volatilization of alkali metal ions, when preparing the K 0.5 Na 0.5 NbO 3 film in our previous study 33 . Moreover, this phenomenon also frequently occurs in the preparation of ferroelectric KNbO 3 and K x Na 1− x NbO 3 based ceramic and film 34–36 .…”

Structure and piezo-ferroelectricity relationship study of (K0.5Na0.5)0.985La0.005NbO3 epitaxial films deposited on SrTiO3 by sputtering

“…3), the three peaks describing the peak profile at 2 θ  = 70.20° (Fig. 1a) can be associated to the possible coexistence of monoclinic phases found in the vicinity of the MPB or polymorphic regions for x = 0.5 5 or to the coexistence of such monoclinic phases and a tetragonal one 9,11–13,15 . We consider that the coexistence of tetragonal and monoclinic phases is the best picture to explain the observed results.

Figure 3Structural simulation, using the calculated lattice parameters, of: ( a ) commensurate tetragonal (top and lateral views), ( b ) incommensurate monoclinic growth (top view), and ( c ) maze-like pattern (top view) in the KNNLa film grown on the NSTO100 substrate.

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“…5) probably due to the coexistence of tetragonal and monoclinic phases where the spontaneous polarization lies along the directions [001] parallel and [101] 90° away from the out-of-plane direction, respectively (Fig. 3) 9,12,13,15 . The influence of the coexistence of different crystalline phases on the coercive electric field and the piezoelectric response have been reported for KNN and PZT systems 8,11,16,17,41 .…”

“…(K x Na 1−x )NbO 3 (KNN), belonging to the ABO 3 perovskite family, is a promising candidate owing to its high Curie temperature ( T C around 400 °C) and excellent piezoelectric properties 1–5 . Similar to PZT, the phase diagram of the KNN system, resulting from the combination of the ferroelectric KNbO 3 and the antiferroelectric NaNbO 3 compounds, has temperature independent morphotropic phase boundaries (MPB) in the phase diagram, specially, around x = 0.5 and x = 0.82 where the dielectric constant, electromechanical coupling, piezoelectric coefficients and remanent polarization are maximized due to the coexistence of different crystalline phases 2–13 . However, from the crystallographic point of view, there is plenty of inconsistence in the literature about the KNN structures reported at room temperature around the MPB regions.…”

“…In the meantime, driven by the miniaturization and integration tendencies 22,23 , great efforts have been made to fabricate high-quality KNN or KNN-based thin films using different growth techniques such as chemical solution deposition (CSD) via sol-gel procedure 3,13,18,19,24–26 , pulsed laser deposition (PLD) 27,28 , and rf-magnetron sputtering 29–31 . Among them, rf-magnetron sputtering is a particularly attractive technique for applications because it allows large area deposition and is commonly used in actual microelectronic manufactures.…”