Electrical and Optical Properties of Radio-Frequency Magnetron Sputtered High Dielectric Constant Oxide Films

• Main Authors: Yu-Fu Kuo, 郭有福
• Other Authors: Tseung-Yuen Tseng
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/19886623688540545409

博士 === 國立交通大學 === 電子工程系 === 87 === In this study, we present the physical, electrical and optical properties of Ba(Ti0.8Sn0.2)O3 and (Ba0.7Sr0.3)TiO3 films deposited by RF magnetron sputtering. The effects of deposition temperature and film thickness on the physical properties (grain size, orientation, composition, crystallinity), electrical (dielectric constant, leakage current, conduction mechanism, breakdown voltage, polarization, dielectric degradation, dielectric relaxation) properties were investigated in details. In addition, a post-deposition thermal treatment by rapid thermal annealing in O2 and N2O ambient was used to improve the electrical characteristics. At first, the Ba(Ti0.8Sn0.2)O3 paraelectric thin films were successfully prepared on Pt/TiO2/SiO2/Si(100) substrates by RF magnetron sputtering deposition. The films deposited at 650 oC exhibited good crystallinity and had a dielectric constant of 210 and a leakage current density of 3.9x10-8 A/cm2 at an electric field of 100kV/cm with a delay time of 30 seconds. This indicates that the BTS films can be applied to the dielectrics of DRAM capacitors. It was also found that the dielectric constant and leakage current of the films closely correlated with the peak intensity ratio of (110)/(111) in the XRD patterns of the films. Secondary, the dielectric relaxation phenomenon and the defect analysis of RF magnetron sputtered Ba(Ti0.8Sn0.2)O3 thin films, were investigated using complex capacitance, impedance and admittance analysis as a function of frequency (100 Hz ￡ f ￡ 10 MHz), and temperature (27°C ￡ T ￡ 125°C). An equivalent circuit was proposed to well explain the AC response of Ba(Ti0.8Sn0.2)O3 thin films in the frequency range between 100 Hz to 1 MHz. However, at frequencies above 1 MHz, the films exhibit resonance in capacitance. The complex admittance measurements proved to be very useful to identify the presence and contribution of defect states on the frequency dependence resistance which in turn influence the electrical properties of Ba(Ti0.8Sn0.2)O3 thin films. On the basis of the complex plane analysis, we envisage that the effects of shallow defect traps are negligible whereas the grain boundary and interface defects provide significant contribution for the origin of dielectric relaxation. Present studies also demonstrate that dielectric relaxation of Ba(Ti0.8Sn0.2)O3 films is less pronounced compared with the other dielectric films. Third, the dielectric properties of Ba(Ti0.8Sn0.2)O3 films have been studied with and without post-annealing. Dielectric constant of the films increased with increasing thickness and with post-annealing. The improvement in film crystallinity attained through the post-annealing process is envisaged to be responsible for the better dielectric properties of the BTS films. The dielectric dispersion with the capacitance, which is due to the dielectric relaxation, has been observed for the as-deposited film and the effects of this phenomenon can be suppressed by the post-annealing in the O2 and N2O ambient. From comparison of the electrical characteristics of as-deposited and post-annealed films, it is concluded that electrons from oxygen vacancies in the interfacial layer are the origin of the dielectric relaxation. The leakage current and time dependent dielectric breakdown (TDDB) properties were investigated for the reliability of BTS thin films with and without post-annealing. The leakage current decreases with increasing film thickness and with post-annealing. Both tBD versus E and tBD versus 1/E plots showed straight lines, independent of the film thickness, and projected a lifetime longer than 10 years at 1V for 50 to 100 nm BTS films. Furthermore, the post-annealed film showed a longer lifetime than the as-deposited film leading to a longer dielectric breakdown time and higher dielectric breakdown field. Finally, rf-sputtered thin films of Ba0.7Sr0.3TiO3 (BST) were prepared on the SiO2/Si substrates with radio frequency magnetron sputtering. This work also examined how the O2 atmosphere annealing temperature affects the crystalline structure, the grain growth and the optical and optoelectronic properties. According to our results, the packing density, the grain size and the refractive index of the films increased with an increasing annealing temperature. The films exhibit the highest dense packing growth and near-bulk optical properties as the O2 atmosphere annealing temperature increases. The optical band gap energy depends on the grain size and becomes smaller for films with a larger grain size. The dispersion of the refractive index was also analyzed in terms of the parameters Eo and Ed using the single oscillator dispersion model of Wemple and DiDomentico. Our experimental results about Wemple oscillator energies Eo (?1.49Egap) and the empirical factor b ? 0.23 of the 750oC annealing films closely correspond with reported bulk data (Eo ? 1.5Egap and b ? 0.26 ± 0.04 eV).