Effect of Dopants on the Band Structure of Barium Strontium Titanate Thin Films
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J14.5.1
Effect of Dopants on the Band Structure of Barium Strontium Titanate Thin Films Yuebing Zheng, Shijie Wang and Cheng Hon A. Huan Institute of Materials Research and Engineering, 3 Research Link, Singapore, 117602 ABSTRACT The effect of dopants on the band structure and crystal structure of Ba0.5Sr0.5TiO3 thin films on (100) LaAlO3 substrates has been investigated. The dopants include Ti, Mg and Al. The bandgap energies of the thin films were determined from the transmission spectra measured by UVVIS spectrophotometer and increased with the increase of dopant concentration regardless of the type of dopants. The crystal structure was studied by using transmission electron microscopy, atomic force microscopy, x-ray diffraction and micro-Raman spectroscopy. The relation between band structure and crystal structure was discussed. INTRODUCTION There has been tremendous interest in the preparation and characterization of BaxSr1-xTiO3 (0 ≤ x ≤ 1) (BST) thin films for a wide range of applications, such as high-density dynamic random access memory (DRAM) devices, millimeter microwave integrated circuits (MMICs) and electro-optic devices [1-6]. Controlling the ratio of Ba/Sr could tune BST to meet specific requirements for devices in terms of electrical, dielectric, and optical properties. However, the small band-gap energy around 3.5 eV of BST cannot meet the requirement of high barrier height and low leakage current density for DRAM applications. This puts certain demands on BST in relation to its band structure. Recently, we have demonstrated the enhanced band-gap energies of doped BST thin films [7]. In this study, we chose to present the band structure studies of Ti-, Mg- and Al-doped BST thin films grown on (100) LaAlO3. It was found that the dopants could tailor the band structure by modifying the grain size, stress and crystallinity in the thin films. EXPERIMENTAL DETAILS The Ti-, Mg- and Al-doped BST thin films were grown on (100) LaAlO3 single crystal substrates by pulsed laser deposition (PLD) with a KrF excimer laser. The targets were sintered BST with a TiO2, MgO or Al2O3 plate on its surface. The coverage area of TiO2/MgO/Al2O3 plate over BST determined the concentration of Ti/Mg/Al in the deposited thin films. Totally, ten samples were prepared, including one pure BST thin films, three Ti-doped BST thin films, three Mg-doped BST thin films and three Al-doped BST thin films. The pure BST thin film was abbreviated to BST0 and the Ti-, Mg- and Al-doped BST thin films were sequentially abbreviated to BSTT1, BSTT2, BSTT3, BSTM1, BSTM2, BSTM3, BSTA1, BSTA2 and BSTA3. The Ti, Mg and Al content in the samples was characterized by x-ray photoelectron spectroscopy (XPS) and analyzed by using peak fitting. The measurement shows relative concentration of dopants: BSTT1: BSTT2: BSTT3 = 1:4:8, BSTM1: BSTM2: BSTM3 = 1:5:8 and BSTA1: BSTA2: BSTA3 = 1:4:6.The thickness of grown thin films was around 500 nm measured by scanning electron microscopy (SEM). A Philips CM 300 transmission electron microscopy (TEM) and a Digit
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