Nanometric structure-property relationship in hafnium oxide thin films made by sputter-deposition
Hafnium oxide (HfO2) is technologically an important material, which exhibits a unique set of properties such as a high dielectric constant (k∼25) and wide band gap (∼5.7 eV) which make this material attractive in the fields of microelectronics and optoelectronics. HfO2 has become the leading candidate to replace SiO2 dielectrics in gate oxides due to a higher permittivity and reportedly lower electron tunneling effects. HfO2 exhibits various polymorphs; the thermodynamic stability and phase existence depends on the temperature and pressure conditions. In addition, the controlled growth and manipulation of specific HfO2 crystal structures at the nanoscale dimensions is the driving force for current and emerging technological implications. Therefore, it is important to characterize and obtain a correlation between physio-chemical and optical properties in nanocrystalline HfO2 films as a function of growth conditions. The present work entails a detailed analysis of growth behavior, microstructure, and optical properties of monoclinic HfO2 films as a function of growth temperature. Amorphous and nanocrystalline HfO2 films were grown by sputter-deposition by varying deposition temperature (Ts) in a wide range from 25-700 °C. Characterization of the films employing an array of analytical techniques indicate a clear functional relationship between processing conditions, structure, morphology, and optical properties. HfO2 films were amorphous at Ts≤200 °C, at which point a structural transformation occurs. HfO2 films grown at Ts≥200 °C were nanocrystalline, stabilized in a monoclinic structure. The nanocrystalline HfO2 films exhibit a strong (111) texturing. The average crystallite size of HfO2 films increased from ∼10 nm to ∼20 nm with increasing Ts. Electron and atomic force microscopy measurements also correlated with the crystalline behavior, as well as an evenly distributed network of crystals spherical in shape for the nano-crystalline films. Density of HfO2 films probed using X-ray reflectivity and ellipsometry data analysis indicate that the values vary in the range of 7.36-9.14 g/cm3. The higher end of the density values were noted only for HfO2 films grown at relatively higher Ts. The band gap values of the films vary in the range of 5.75-6.19 (±0.03) eV for Ts=25-700 °C. Index of refraction at 550 nm increased from 1.80 to 2.09, which also correlates with the characteristic feature of improved structural order.
Engineering|Mechanical engineering|Materials science
Vargas, Mirella, "Nanometric structure-property relationship in hafnium oxide thin films made by sputter-deposition" (2014). ETD Collection for University of Texas, El Paso. AAI1558937.