Ultrasonic Spray Pyrolysis Preparation Of Barium Strontium Titanate Thin Film Technology And Performance Research

Ferroelectric barium strontium titanate (Ba_xSr_1-xTiO₃, BST) has attracted much attention for its potential applications in devices such as dynamic random access memories (DRAM), thin-film capacitors and ferroelectric microbolometers. By adjusting the Ba/Sr ratio, the Curie point (Tc) of BST materials is close to room temperature at which it takes on the maximum pyroelectric coefficient. An uncooled infrared focal plane arrays (UFPAs) system using such elements will achieve the optimal sensitivity when operated at near ambient temperature.In this thesis, the ultrasonic spray pyrolysis (USP) technique is used to prepare BST thin films. We review the progress and the growth mechanism of USP method, and then discuss the reaction kinetics and transfers dynamics in the forming of BST thin films.The precursor of dry BST gel was tested by the differential thermal analysis (DTA) and the thermogravimetric analysis (TGA). The result showed the phase transition of perovskite structure was perfect after 600℃, which was propitious to the post experiments.Ba_0.65Sr_0.35TiO₃ thin films were prepared by ultrasonic spray pyrolysis deposition method on the Pt/Ti/SiO₂/Si substrate. The properties and the structure of the films were characterized by XRD, SEM and AFM etc. The effects of various deposition parameters on the structure, such as solution concentration, substrate temperature, deposition time and doping concentration etc, were discussed in details. The optimal parameters of the experiment as follows: the solution concentration was 0.1M, the substrate temperature was 500℃, the carrier gas flow rate was 5L/min, the deposition time was 60min, and the annealing temperature and time of the BST thin films were 650℃and 30min, respectively. The BST thin films deposited under such conditions displayed perfect performances in uniformity and density. The dielectric constant, dielectric loss and the Curie point of the BST thin films were among 260～360, 2.5%～4.9% and about 29℃respectively. Under the low frequency band, the BST thin films exhibited the good stability of the dielectric constant, dielectric loss. The Ce-doped BST thin films were also investigated in this thesis. An excessive amount of cerium ion would change the microcosmic structure of BST thin films, and cause the dielectric properties decreasing. The doping concentration of cerium ion was controlled within 2 mol% in our experiments. The result showed that the dielectric constant and dielectric loss decreased with the increasing of Ce³⁺ concentration. The dielectric constant and dielectric loss of doped BST thin films with the Ce³⁺ concentrations of 0.5 and 1 mol% were 324-258, 304-246 and 0.021-0.035,0.018-0.03, respectively.The relations between the Curie temperature and the Ce³⁺ concentration were discussed. The curve of the temperature dependence of dielectric constant was depressed with the increasing of the Ce³⁺ concentration, which may be mainly attributed to the ionic radius charge. The ionic radius of Ce³⁺ is lager than that of Ti⁴⁺, which leads the unit cell expanding, and causes the unit cells surrounding area condensed. Such result induces the decreasing of the ferroelectricity in local area.