|AVS 55th International Symposium & Exhibition|
|Electronic Materials and Processing||Thursday Sessions|
|Session:||Electronic Materials and Processing Poster Session|
|Presenter:||E.E. Hoppe, University of Wisconsin-Milwaukee|
|Authors:||E.E. Hoppe, University of Wisconsin-Milwaukee
M. AlMomani, University of Wisconsin-Milwaukee
C.R. Aita, University of Wisconsin-Milwaukee
|Correspondent:||Click to Email|
Nanolaminate films of polycrystalline HfO2 and amorphous Al2O3 were grown by reactive sputter deposition on unheated fused SiO2 and the nascent oxide of <111> Si. Many different bilayer architectures were examined. The films were air annealed at 573 K to 1273 K, and analyzed by double angle x-ray diffraction and ultraviolet-visible spectrophometry. X-ray diffraction showed the amount of monoclinic HfO2 (the bulk equilibrium phase) decreased with decreasing HfO2 layer thickness. Previous high resolution transmission electron microscopy results1 showed that the initial HfO2 phases present in ultrathin layers were tetragonal and orthorhombic, consistent with a finite crystal size effect. Annealing of the as-grown nonmonoclinic structure at temperature up to 973 K produced no gross structural change. Annealing at 1173 K produced a metastable mixed cation tetragonal phase, Hf1-xAlxO2-z, that is isomorphous with tetragonal HfO2. This mixed cation phase became unstable at 1273 K and phase-separated into monoclinic HfO2 and amorphous Al2O3. In all cases, the fundamental optical absorption edge was found to consist of two distinct regions with respect to incident photon energy. Significant O 2p -> Hf 5d interband absorption occurred at energy E ≥ 6.2 eV. For E < 6.2 eV, films with nonmonoclinc structure retained a (desirable) featureless optical absorption edge despite further (nano)crystallization, including the formation of Hf1-xAlxO2-z. Films with a monoclinic structure developed a pre-gap absorption band found to be characteristic of the seven-fold Hf-O coordination in the single layer monoclinic HfO2.2 We relate this undesirable pre-gap absorption band to self-trapped excitons that can form in the monoclinic structure.3
1 E.E. Hoppe, M. Gajdardziska-Josifovska, and C.R. Aita, Appl. Phys. Lett. 91, 203105 (2007).
2 E.E. Hoppe, R.S. Sorbello, C.R. Aita, J. Appl. Phys. 101, 123534 (2007).
3 D. Muñoz Ramo et al. PRL 99, 155504 (2007).