title: Enhanced free exciton and direct band-edge emissions at room temperature in ultrathin ZnO films grown on Si nanopillars by atomic layer deposition.
abstract: Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films grown on silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200 °C, an intensive UV emission corresponding to free-exciton recombination (~3.31 eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25 °C, albeit the appearance of the defect-associated visible emission, the UV emission peak was observed to shift by ~60 meV to near the direct band edge (3.37 eV) recombination emission. The high-resolution transmission electron microscopy (HRTEM) showed that the ZnO films obtained at 25 °C were consisting of ZnO nanocrystals with a mean radius of 2 nm embedded in a largely amorphous matrix. Because the Bohr radius of free-exictons in bulk ZnO is ~2.3 nm, the size confinement effect may have occurred and resulted in the observed direct band edge electron-hole recombination. Additionally, the results also demonstrate order of magnitude enhancement in emission efficiency for the ZnO/Si-NP structure, as compared to that of ZnO directly deposited on Si substrate under the same conditions.
authors.affiliation: Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan. ymchang7@gmail.com
authors.forename: Yuan-Ming
authors.initials: YM
authors.lastname: Chang
authors.forename: Jiann
authors.initials: J
authors.lastname: Shieh
authors.forename: Pei-Yuan
authors.initials: PY
authors.lastname: Chu
authors.forename: Hsin-Yi
authors.initials: HY
authors.lastname: Lee
authors.forename: Chih-Ming
authors.initials: CM
authors.lastname: Lin
authors.forename: Jenh-Yih
authors.initials: JY
authors.lastname: Juang