Ferroelectric Na0.5K0.5NbO3 as an electro-optic material

Abstract: Ferroelectrics are a group of advanced electronic materialswith a wide variety of properties useful in applications suchas memory devices, resonators and filters, infrared sensors,microelectromechanical systems, and optical waveguides andmodulators. Among the oxide perovskite-structured ferroelectricthin film materials sodium potassium niobate or Na0.5K0.5NbO3(NKN) has recently emerged as one of the most promisingmaterials in microwave applications due to high dielectrictunability and low dielectric loss. This licentiate thesispresents results on growth and structural, optical, andelectrical characterization of Na0.5K0.5NbO3 thin films. Thefilms were deposited by rf-magnetron sputtering of astoichiometric, high density, ceramic Na0.5K0.5NbO3 target ontosingle crystal LaAlO3 and Al2O3, and polycrystalline Pt80Ir20substrates. By x-ray diffractometry, NKN films on c-axisoriented LaAlO3 substrates were found to grow epitaxially,whereas films on hexagonal sapphire and polycrystallinePt80Ir20 substrates were found to be preferentially (00l)oriented. Optical and waveguiding properties of theNa0.5K0.5NbO3/Al2O3 heterostructure were characterized using aprism-coupling technique. Sharp and distinguishable transversemagnetic (TM) and electric (TE) propagation modes wereobserved. The extraordinary and ordinary refractive indiceswere calculated to ne = 2.216±0.003 and no =2.247±0.002 for a 2.0 μm thick film at λ = 632.8nm. This implies a birefringence Δn = ne - no =-0.031±0.003 in the film. The ferroelectric state inNKN/Pt80Ir20 films at room temperature was indicated by apolarization loop with polarization as high as 33.4 μC/cm2at 700 kV/cm, remnant polarization of 9.9 μC/cm2 andcoercive field of 91 kV/cm. Current-voltage characteristics ofvertical Au/NKN/Pt80Ir20 capacitive cells and planar Au/NKN/LaAlO3 interdigital capacitors (IDCs) showed very goodinsulating properties, with the leakage current density for anNKN IDC on the order of 30 nA/cm2 at 400 kV/cm. Rf dielectricspectroscopy demonstrated low loss, low frequency dispersion,and high voltage tunability. At 1 MHz NKN/LaAlO3 showed adissipation factor tan δ of 0.010 and a tunability of 16.5% at 200 kV/cm. For the same structure the frequencydispersion, Δεr, between 1 kHz and 1 MHz was 8.5%.Key words:ferroelectrics, sodium potassium niobates,thin films, rf-magnetron sputtering, waveguiding, refractiveindex, prism coupling, dielectric tunability