A combined confocal microscopy measurement and first-principle calculation for investigating the spatial distribution of neutral oxygen vacancies on ZnO nanowire surfaces

Introduction

Zinc oxide (ZnO) is a semiconductor which has generated a great deal of interest in device applications in recent years due to its several useful electrical and optical properties. Density functional theory (DFT) based calculations is a powerful method of investigating the ground state properties of materials. It is well known that the ZnO nanowire (NW) contain a large density of intrinsic defects such as oxygen deficiencies or vacancies (V_O), which influence their electronic and optical properties as well as their performance in device applications. Hence understanding the spatial distribution of the shallow intrinsic surface defects on the nanostructure’s surfaces and deep intrinsic bulk defects plays a crucial role in understanding and tailoring the electronic properties of the NW based device. The ZnO (0001) and (10-10) surfaces are the most important among the different surface structures of ZnO as they occur predominantly due to their relative stability.

Discussion

From multiple arguments along the theoretical considerations of the various intrinsic defects formation energies, the NWs’ synthesis temperature and photoluminescence response, it is likely that the green luminescence emitted by the synthesized NW arrays during the confocal photoluminescence (CPL) microscopy measurements (obtained with a PicoQuant MicroTime 200 confocal microscope) originates from neutral oxygen vacancies. In addition, the luminescence intensity or the magnitude of the measured CPL spectra is also directly correlated with the concentration of V_O in the ZnO NWs. Therefore the CPL measurements can provide a direct and non-destructive qualitative measurement of the directional dependence of the V_O concentration in the ZnO NWs. The spatial profile of the luminescence intensity indicates that the V_O concentration is largest near the edges of the (0001) and (10-10) surfaces of the NWs and the V_O concentrations decreases significantly in the interior parts of the NWs [Figs. 7(a) and 8(a) of J. Appl. Phys. 114, 034901 (2013)].

In order to gain insight into the physical mechanisms that govern the confocal optical properties of the ZnO NWs in the [0001] and [10-10] directions, DFT calculations using the full-potential linearized augmented plane-wave plus local orbitals method was also utilized. N x 1 x 1 supercell-slab (SS) models was utilized in the surface simulations by stacking the ZnO primitive unit cells along the a-axis of the hexagonal lattice which is followed by a vacuum region of several angstroms to decouple the interactions between the SSs. Self-consistency in the SS calculations was achieved by iterative convergence of the minimum total energy and Hellman-Feynman forces to a value below 0.0001 Ry and 1 mRy/a.u. respectively. Comparison with the CPL measurements demonstrates that the N x 1 x 1 SSs was suitable as an approximation of the ZnO NW cross sections where the confocal measurements are taken [Figs. 7(b) and 8(b) of J. Appl. Phys. 114, 034901 (2013)].

The calculations show that the spatial electronic profile of the neutral V_O formation energy in the [0001] and [10-10] directions is lower near the boundary and increases to bulk-like values in the SS center, which explains the behaviour of the spatial profile of the CPL measurements. For further details and discussions on the results as well as the in depth descriptions of the experimental procedures and first-principle calculations, the reader is referred to J. Appl. Phys. 114, 034901 (2013). Using this combined experimental and theoretical approach, a better qualitative understanding of the spatial distribution of the surface and deep V_O in the ZnO NWs was achieved, thus leading to a more efficient functionalization and improved integration of the ZnO NW in future nanodevices for better performance. These results would have important implications for ZnO nanostructures in potential nanoscale applications.

(a) Schematic diagram depicting the (0001) polar and (10-10) non-polar surfaces of a ZnO nanowire. (b) Spatial distribution of the CPL intensity along the blue line on the (0001) surface. (c) Spatial distribution of the oxygen vacancies defect formation energy along a 6 x 1 x 1 SS approximating the cross section of the ZnO NW [K. M. Wong, et al., J. Appl. Phys. 114, 034901 (2013).]

Reference

Wong, Kin Mun, Alay-e-Abbas, S. M., Fang, Y., Shaukat, A., & Lei, Y. Spatial distribution of neutral oxygen vacancies on ZnO nanowire surfaces: An investigation combining confocal microscopy and first principles calculations Journal of Applied Physics, 114 (3), 034901 (2013). DOI: 10.1063/1.4813517