Investigation of thermal annealing effects on microstructural and optical properties of HfO ₂ thin films

In the present paper, we investigate the effect of thermal annealing on optical and microstructural properties of HfO ₂ thin films (from 20 to 190 nm) obtained by plasma ion assisted deposition (PIAD). After deposition, the HfO ₂ films were annealed in N ₂ ambient for 3 h at 300, 350, 450, 500 and 750 °C. Several characterisation techniques including X-ray reflectometry (XRR), X-ray diffraction (XRD), spectroscopic ellipsometry (SE), UV Raman and FTIR were used for the physical characterisation of the as-deposited and annealed HfO ₂ thin films. The results indicate that as-deposited PIAD HfO ₂ films are mainly amorphous and a transition to a crystalline phase occurs at a temperature higher than 450 °C depending on the layer thickness. The crystalline grains consist of cubic and monoclinic phases already classified in literature but this work provides the first evidence of amorphous-cubic phase transition at a temperature as low as 500 °C. According to SE, XRR and FTIR results, an increase in the interfacial layer thickness can be observed only for high temperature annealing. The SE results show that the amorphous phase of HfO ₂ (in 20 nm thick samples) has an optical bandgap of 5.51 eV. Following its transition to a crystalline phase upon annealing at 750 °C, the optical bandgap increases to 5.85 eV.