Microstructure related influence on the electrical properties of pulsed laser ablated (Ba, Sr)TiO3 thin films

The microstructural dependence of electrical properties of (Ba, Sr)TiO3(BST ) thin films were studied from the viewpoint of dc and ac electrical proper ties. The films were grown using a pulsed laser deposition technique in a t emperature range of 300 to 600 degrees C, inducing changes in grain size, s tructure, and morphology. Consequently, two different types of films were r ealized, of which type I, was polycrystalline, multigrained, while type II was [100] oriented possessing a densely packed fibrous microstructure. Leak age current measurements were done at elevated temperatures to provide evid ence of the conduction mechanism present in these films. The results reveal ed a contribution from both electronic and ionic conduction. In the case of type I films, two trapping levels were identified with energies around 0.5 and 2.73 eV, which possibly originate from oxygen vacancies V-O and Ti3+ c enters, respectively. These levels act as shallow and deep traps and are re flected in the current-voltage characteristics of the BST thin films. The a ctivation energy associated with oxygen vacancy motion in this case was obt ained as 1.28 eV. On the contrary, type II films showed no evidence of deep trap energy levels, while the identified activation energy associated with shallow traps was obtained as 0.38 eV. The activation energy obtained for oxygen vacancy motion in type II films was around 1.02 eV. The dc measureme nt results were further elucidated through ac impedance analysis, which rev ealed a grain boundary dominated response in type I in comparison to type I I films where grain response is highlighted. A comparison of the mean relax ation time of the two films revealed three orders of magnitude higher relax ation time in the case of type I films. Due to smaller grain size in type I films the grains were considered to be completely depleted giving rise to only grain boundary response for the bulk of the film. The activation energ y obtained from conductivity plots agree very well with that of dc measurem ents giving values 1.3 and 1.07 eV for type I and type II films, respective ly. Since oxygen vacancy transport have been identified as the origin of re sistance degradation in BST thin films, type I films with their higher valu e of activation energy for oxygen ion mobility explains the improvement in breakdown characteristics under constant high dc field stress. The role of microstructure in controlling the rate of degradation is found useful in th is instance to enhance the film properties under high electric field stress es. (C) 2000 American Institute of Physics. [S0021-8979(00)00418-7].