Structure and optical properties of nanocrystalline hafnium oxide thin films

Abstract

Hafnium oxide (HfO2) films were grown by sputter-deposition by varying deposition temperature (Ts) in a wide range of 25–700°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 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 (1¯11) texturing. The average crystallite size of HfO2 films increased from ∼10nm to ∼20nm with increasing Ts. 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 (ρ) of HfO2 films probed using X-ray reflectivity and ellipsometry data analysis indicate that the ρ values are strongly dependent on Ts; ρ varied in the range of 7.36–9.14g/cm3. The higher end of ρ values were noted only for HfO2 films grown at relatively higher Ts indicating the crystalline nature accounts for the ρ improvement. The band gap values of the films varied in the range of 5.78–6.17(±0.03)eV for Ts=25–700°C. Index of refraction at 550nm 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 Ts. Based on the observed results, a correlation between growth conditions, microstructure and optical constants is established.