Of Bst Nonlinear Ferroelectric Thin Film Materials Research

Barium strontium titanate (BaxSr1-xTiO3,short for BST) thin films are potentialmaterials for the applications of microwave devices such as dielectric phase shiftersand tunable filters due to their high dielectric nonlinearity, low leakage current andadjusting Curie temperature. Ba0.6Sr0.4TiO3 thin films were prepared by rf-magnetronsputtering system. The deposition conditions, microstructures, ferroelectric propertiesof Ba0.6Sr0.4TiO3 thin films and BST ceramics targets were investigated. Thefollowing results are obtained:(1) When x≤0.6, the a-and c-axis lattice constant values are very close to eachother (i.e., c/a=1), therefore the crystal structures of the ceramics are cubic and therearen't hysteresis loop. However, when x>0.6, the c-axis lattice constant is larger thana-axis lattice constant (i.e., c/a>1), therefore at room temperature the crystalstructures are tetragonal phase and well-behaved hysteresis loops can be observed.The Weiss temperature (T0) is always lower than Curie temperature (TC) and thelinear relation between the Curie temperature and the stoichiometric percentage ofbarium can be observed and expressed as TC (℃) =-195.0+322.2x. The values ofcoercive electric field (EC) and remanent polarization (Pr) increase as Ba/Sr ratio ofBaxSr1-xTiO3 ceramics increases except that x≤0.5, which are due to the decrease ingrain size and the difference between the radius of Ba2+ and that of Sr2+. BSTceramics targets with ф120mm are dense and uniform.(2) The optimum conditions of RF magnetron sputtering have been obtained asfollows: substrate temperature is 400?C, sputtering power 90~100W, sputtering gasO2:Ar =5:1, deposition pressure 0.1Pa, target-substrate distance 71mm.(3) Nanometer-sized a-domains and c-domains in BST thin films were observedby piezoresponse force microscopy (PFM). It is found that the critical size from amulti-domain to a single-domain is between 28nm and 33nm.(4) The composition of BST films is almost uniform in the inner and theconcentration of oxygen vacancy in the surface is lower than in the inner. It is alsofound that Pt diffuses from bottom electrodes to BST films and substrates. Therefore,...