Optical properties of thermally annealed hafnium oxide and their correlation with structural change

We studied the optical properties of hafnium oxide as its structure changed. The shoulderlike feature that appears in crystalline HfO2 near the energy trough of the optical absorption edge after thermal anneal was found to have a direct correlation with the crystallization of the film. A luminescence feature near 4.1 eV could also be observed for some of the crystallized hafnium oxide films, but it did not have a direct correlation with crystallization or the shoulderlike feature. Our experimental results do not support the recently proposed self-trapped exciton (STE) model, where the shoulderlike absorption and the UV luminescence features are both attributed to STE. Simultaneous measurements of structural and optical properties of thermally annealed hafnium oxide films revealed that the shoulderlike feature is not related to defects, but rather is intrinsic to crystallized hafnium oxides. We discuss some possible origins of the shoulderlike feature based on our experimental results. Nevertheless, the UV luminescence feature seems to be related to oxygen defects in crystalline hafnium oxide. (C) 2008 American Institute of Physics.