Structure and optical properties of nanocrystalline hafnium oxide thin films

Hafnium oxide (HfO2) films were grown by sputter-deposition by varying deposition temperature (T-s) in a wide range of 25-700 degrees C. The deposited HfO2 films were characterized by studying their growth behavior, microstructure, and optical properties. Characterization of the films employing a wide range of analytical techniques indicate a clear functional relationship between processing conditions, structure, morphology, and optical properties. HfO2 films were amorphous at T-s <= 200 degrees C, at which point a structural transformation occurs. HfO2 films grown at T-s >= 200 degrees C were nanocrystalline, stabilized in a monoclinic structure. The nanocrystalline HfO2 films exhibit a strong ((1) over bar 11) texturing. The average crystallite size of HfO2 films increased from similar to 10 nm to similar to 20 nm with increasing T-s Electron and atomic force microscopy measurements also correlated with the crystalline behavior, as well as an evenly distributed network of spherical shaped crystallites for the nanocrystalline HfO2 films. Density (rho) of HfO2 films probed using X-ray reflectivity and ellipsometry data analysis indicate that the rho values are strongly dependent on T-s; rho varied in the range of 7.36-9.14 g/cm(3). The higher end of rho values were noted only for HfO2 films grown at relatively higher T-s indicating the crystalline nature accounts for the rho improvement. The band gap values of the films varied in the range of 5.78-6.17(+/- 0.03) eV for T-s = 25-700 degrees 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, packing density of HfO2 films with increasing T-s. Based on the observed results, a correlation between growth conditions, microstructure and optical constants is established. (C) 2014 Elsevier B.V. All rights reserved.