Center for Artificial Low Dimensional Electronic Systems

Geometrical Separation of Defect States in ZnO Nanorods and Their Morphology- Dependent Correlation between Photoluminescence and Photoconductivity

Cheolmin Park

October 25(Tue) - October 25(Tue), 2016

Geometrical Separation of Defect States in ZnO Nanorods and Their Morphology-Dependent Correlation between Photoluminescence and Photoconductivity

Cheolmin Park

Korea Institute of Machinery & Materials (KIMM)

An understanding of the morphology-dependent correlation between photoluminescence and photoconductivity in nanostructured ZnO is important for elucidating the carrier dynamics and expanding its use in optoelectrical applications. In this study, we investigated this relationship using distinctly different ZnO nanorods with diameters greater than 100 nm,

which were produced by a hydrothermal method. Furthermore, in order to study the effects of its defect states on the correlation, we thoroughly characterized the defect states of the ZnO nanorods in terms of the light-penetration depth during photoluminescence. The photoconductivities of the nanorods were measured using light sources with wavelengths of

355, 405, and 532 nm to confirm the influences of the visible-emission-generating defects on carrier transport. We found that the intensity of the near-band-edge emission was almost comparable to the amount of photocurrent generated under ultraviolet (UV) light; this could be attributed to the crystallinity of the inside of ZnO nanorods. However, the concentration of

the surface defects resulting from the size and morphology probably had an effect, leading to the observed differences in the photocurrent sensitivity under low-intensity UV light, the dark current level, the amount of photocurrent under a specific wavelength of light within the visible range, and the persistent photoconductivity. The results of this study could aid

research on carrier dynamics in nanostructured ZnO and further its use in optoelectronics. 

Carrier transport behaviors depending on the two orthogonally directional energy bands in the ZnO nanofilm affected by oxygen plasma

 An oxygen plasma treatment of ZnO nanostructures has frequently been used for obtaining a desired optoelectrical property. Nevertheless, a detailed study regarding carrier transport behaviors affected by the plasma has scarcely been managed, especially in the thin film structure, owing to its more complex physics than those of a one-dimensional nanostructure.

Herein, we demonstrate an analysis of carrier transport behaviors on an oxygen plasmatreated ZnO nanofilm (50 nm thick) on a SiO2/Si substrate. By comparison with the as-grown sample, we observed drastic changes in carrier transport behavior according to the short exposure times of 30 s and 60 s. The plasma effect leading to the distinction was confirmed to

originate from the bombardment of energetic ions near the surface and the diffusion of various oxygen ions and radicals into the host. The mechanism of the resulting carrier transport was comprehended through the revelation of two orthogonally directional energy band structures (surface band bending in the surface layer and localized energy bending at the

grain boundary). Furthermore, we experimentally observed that the increased electrical barrier of the grain boundary, due to negatively absorbed oxygen ions, could be helpful in impeding persistent photoconductivity and in reducing dark current