Complex impedance spectroscopy of high-k HfO_2 thin films in Al/HfO_2/Si capacitor for gate oxide applications

摘要

The dielectric responses of ultrathin (～ 6.65 nm) HfO_2 films, in the form of Al/HfO_2/Si capacitors were prepared by rf sputtering technique, has been studied in the wide frequency range as a function of deposition temperatures. Deposition temperatures were varied from room temperature (30 ℃) to 500 ℃. Thickness and the interfacial and surface roughness of heterostructures were extracted by fitting the specular X-ray reflectivity data. The impedance analysis combined with modulus spectroscopy was performed to get insight of the microscopic features like grain, grain boundary and film-electrode interfaces and their effects in the film properties. The films exhibited maximum frequency dispersion in both real and imaginary part of impedance at low frequency range. The frequency analysis of the modulus and impedance studies showed the distribution of the relaxation times due to the presence of grains and grain boundaries in the films. Impedance analysis revealed that the interfacial polarization caused by space charges in the film/electrode interfaces plays an important role in the dielectric behavior of the capacitor. In order to explain effectively that the impedance plots contain one or two arcs due to more than one relaxation contributions, the results are interpreted using the approach proposed by Abrantes (Z′ vs. |Z″|/f representation). The dielectric loss (tan δ) curves exhibited the fact that there is possibility of existence of a Schottky barrier at the insulator semiconductor interface, which is due to traps distributed throughout the semiconductor-insulator interface and it is believed to be due to auto doping during deposition process. The ac conductivity, σ_(ac) (ω), varies as σ_(ac) (ω) = Bω~n with n in the range 0.06-0.71.