Positive Temperature Coefficient of Resistance in MOCVD (Ba 0.75 Sr 0.25 )Ti 1+y O 3+z Films
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Positive Temperature Coefficient of Resistance in MOCVD (Ba0.75Sr0.25)Ti1+yO3+z Films S. Saha†, D. Y. Kaufman§, S. K. Streiffer†, R. A. Erck§, and O. Auciello†, † Materials Science Division and §Energy Technology Division, Argonne National Laboratory, Argonne, IL-60439, USA ABSTRACT The leakage and dielectric properties of a thickness series (90-480 nm) of {100} fibertextured MOCVD (Ba0.75Sr0.25)Ti1+yO3+z (BST) thin films on Pt/SiO2/Si were investigated. The temperature and voltage dependence of the permittivity were consistent with previous observations, where thinner films demonstrated a suppressed temperature and electric field response that transitioned to a more bulk-like response with increasing film thickness. The current-voltage characteristics showed two distinct regimes. At low fields the current displayed weak field dependence and a monotonic increase with increasing temperature. In contrast, positive temperature coefficient of resistance (PTCR) was observed in the high-field leakage current behavior. The PTCR behavior was more pronounced for thicker BST films. Factors contributing to the observed PTCR effect are outlined and contrasted with the description for bulk BaTiO3 ceramics.
INTRODUCTION The high dielectric constant, low dielectric loss, low leakage current, and high breakdown strength of (Ba,Sr)TiO3 (BST) thin films have provided a unique niche for this technology in the rapidly developing microelectronics world [1,5-7]. Understanding the electrical properties of BST thin films has attracted significant attention for the past several years [1-4]. Although conclusive understandings of the electrical phenomena have been obtained, the thickness regime explored has predominantly been below 200nm. Since present and future applications demand greater film thicknesses, it is both scientifically and technologically important to explore the electrical response of films greater than 200nm. Previous investigations on Pt/BST/Pt heterostructures confirmed the presence of a Schottky barrier at the BST/Pt interface, which dominates the leakage behavior in thin films [2, 3, 8]. Several theories have been put forward to account for the leakage current response in BST thin films, including Schottky conduction [2], a modified Schottky theory [3], Fowler-Nordheim tunneling [8], etc. More recently, photoemission measurements on Ti rich MOCVD BST films indicated conduction band bending at the PT/BST interface of the order of 0.5eV, with an estimated depletion width of 70nm [9]. In this paper the dielectric and leakage properties of a thickness series between 90-480 nm of Pt electroded {100} fiber-textured MOCVD (Ba0.75Sr0.25)Ti1+yO3+z films have been investigated.
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EXPERIMENTAL DETAILS {100} fiber-textured BST thin films were deposited by metalorganic chemical vapor deposition (MOCVD), details of which can be found elsewhere [10]. The BST films were deposited at 675oC on Pt-coated SiO2/Si wafers. The Ba:Sr composition was fixed at 75:25 while the Ti content was maintained at 53.5% (metals). The com
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