A systematic investigation of strain/stress effect on the dielectric properties of barium strontium titanate thin films

Abstract

Ba1-xSrxTiO3 (BST) based thin films have long been studied for both their fundamental properties and for its potential applications in tunable microwave and RF devices as well as high-density memories. However, compared with bulk BST ceramics, BST thin films exhibit a dramatic degradation of dielectric properties, which has limited the performance of devices based on thin film BST. Residual strain has been generally considered as a major cause for this degradation. Though many studies on this strain were reported, most of them were based on an assumption that a homogeneous strain exists throughout the film, especially in epitaxial or highly oriented thin films. However, due to the presence of point defects, dislocations, and strain gradient along the thickness direction, inhomogeneous strain is inevitably induced in BST thin films and its effect on the dielectric properties of BST thin films could not be neglected. The present dissertation focuses on a systematic investigation of strain effect including both macroscopic homogeneous strain effect and microscopic inhomogeneous strain effect on the dielectric properties of BST thin films deposited by pulsed laser deposition. Because the strain/stress in the thin films could be manipulated by doping, adding a buffer layer, changing film thickness and inserting an interlayer, four kinds of thin films were deposited. Highly (100)-oriented Mn doped (Ba0:7Sr0:3)(Sn0:2Ti0:8)O3 (BSSnT) films were deposited on LaAlO3 substrates. Up to a tunability of ~90% of the Mn-BSSnT thin film was obtained with a proper Mn dopant concentration. The Mn doping can change both the elastic residual strain and inhomogeneous strain in the thin films. However, the inhomogeneous strain has a dominant effect on the tunability compared to the elastic residual strain. The tunability changes inversely with inhomogeneous strain. This unusual behavior can be explained by the pinning effect of dipoles induced by oxygen vacancies in the films. Ba0:6Sr0:4TiO3 (BST) thin films were deposited on La0:7Sr0:3CoO3 (LSCO) buffered and unbuffered Pt (111)/Ti/SiO2/Si substrates. The former exhibits a (100) preferred orientation and the latter a random orientation, respectively. It was found that the LSCO buffered BST film has markedly lower tensile residual stress than the unbuffered BST film. As a result, the dielectric property of the LSCO buffered BST thin film is greatly improved, which shows a larger dielectric constant and tunability, smaller loss tangent and lower leakage current than those of the unbuffered BST thin film. The relaxation of the larger tensile residual stress is attributed to the larger grain size in the buffered BST thin film and to a closer match of thermal expansion coefficient between the BST and the LSCO buffer layer. Highly (001) oriented Ba0:6Sr0:4TiO3 (BST) thin films, grown on (001) LaAlO3 substrates, exhibit strong variation of strain over the thickness range of 20-800 nm. The tensile elastic residual strain reaches a minimum value at a thickness of 250 nm, while the inhomogeneous strain decreases gradually with increasing film thickness. The 250-nm-thick film has the largest in-plane dielectric constant due to a smaller tensile elastic strain and inhomogeneous strain in the film and the largest in-plane tunability of 40% is achieved in the thickest film due to a smaller dislocation density and less interface impact in the thin film. Ba0:6Sr0:4TiO3(BST)/MgTiO3(MT)/Ba0:6Sr0:4TiO3 multilayer thin films were deposited on LaNiO3(100)/Pt/Ti/SiO2/Si(100) substrates. Pure BST thin film exhibits (100) preferred orientation, while BST thin films with a MgTiO3 interlayer exhibit a random orientation. Residual stress was relaxed dramatically due to a closer match of thermal expansion coefficient between the BST and MT interlayer. The largest figure of merit (Tunability/tanδ) of 18.7 was achieved in the multilayer thin film with a 50-nm-thick MT interlayer which also exhibited a tunability of 30%. Dielectric constant and loss tangent decrease with increasing MT layer thickness due to a series dielectric dilution effect. Residual strain or stress plays an important role in manipulating the dielectric properties of BST thin films. Results of this dissertation show that strain/stress can be manipulated by doping, using a buffer layer, changing film thickness, and adding an interlayer with an appropriate thickness in order to optimize the dielectric properties of BST thin films.

Date of Award	17 Feb 2010
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhengkui XU (Supervisor)