Correlation between phase and optical properties of yttrium-doped hafnium oxide nanocrystalline thin films

• The effect of thickness on the structure of yttrium-doped HfO2 films is described.
• A correlation between phase and optical properties is established.
• Yttrium-doped HfO2 films of 26–70 nm are monoclinic while thicker films are cubic.
• Higher band gap (6.03 eV) in cubic yttrium-doped HfO2 films is demonstrated.

Yttrium-doped hafnium oxide (YDH) nanocrystalline films were produced by sputter-deposition at a substrate temperature of 400 °C. The deposition was made onto Si (1 0 0) and optical grade quartz substrates with variable deposition time to produce YDH films in a wide range of thickness, dYDH ∼ 25 nm to 1.1 μm. The effect of dYDH on the crystal structure, surface/interface morphology and optical properties of YDH films was investigated. X-ray diffraction analyses revealed the formation of monoclinic phase for relatively thin films (<150 nm). The evolution towards stabilized cubic phase with increasing dYDH is observed. The scanning electron microscopy results indicate the dense, columnar structure of YDH films as a function of dYDH. Spectrophotometry analyses indicate that the grown YDH films are transparent. The band gap was found to be ∼5.60 eV for monoclinic YDH films while distinct separation and an increase in band gap to ∼6.03 eV is evident with increasing dYDH and formation cubic YDH films. A correlation between growth conditions, phase evolution, and optical properties of the YDH nanocrystalline films is established.