Characterization of e-beam evaporated hafnium oxide thin films on post thermal annealing

• Published in: Applied surface science 2013-10, Vol.283, p.617-622
• Main Authors: Ramzan, M., Wasiq, M.F., Rana, A.M., Ali, S., Nadeem, M.Y.
• Format: Article
• Language: English
• Subjects: AFM; Annealing; Coefficients; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Crystallites; Evaporation; Exact sciences and technology; Hafnium oxide; Morphology; Optical energy gap; Optical properties; Orientation; Phase transition; Physics; Refractive index; Thin films; Urbach energy
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2013.07.001

•The effect of post thermal annealing on the optical properties of HfO2 thin films was studied.•The phase transition from amorphous to polycrystalline state occurred due to annealing.•The substrate temperature is responsible for film morphology and crystallite orientation.•The phase transition and preferred orientation strongly affect the optical energy gap.•In the region of absorption edge, the absorption coefficient obeys Urbach's rule. HfO2 thin films (80nm thick) were fabricated using electron beam evaporation technique at various substrate temperatures ranging from 25° to 120°C. These films were then thermally annealed at 500°C for one and half hour in vacuum. After thermal annealing, films were characterized through XRD, AFM and Spectrophotometry. In this regard, it was observed that the as-deposited HfO2 films were mostly amorphous in nature and transformed to polycrystalline with monoclinic structure after annealing at 500°C. Moreover, films fabricated at different substrate temperatures revealed different morphologies and crystallite orientations on thermal annealing. Such different morphologies and crystallite orientations appear to be responsible for any variations in the surface roughness and the optical properties e.g. optical band gap energy (3.4–3.65eV), refractive index (1.25–2.55), extinction coefficient (0.25–0.46) etc. These optical properties demonstrate oscillatory behavior with different substrate temperatures due to crystallite growth along different preferred orientations. On the basis of above mentioned facts, it can be concluded that the post thermal annealing demonstrates better tendency to change the structural and optical properties of HfO2 thin films. In addition, annealed HfO2 films showed better reflectivity (5–10%) in the NIR region which can further be improved by inserting a metallic layer into the oxide–metal–oxide (O–M–O) structure. Hence, such O–M–O structures can be useful for heat mirror applications.