Origin of the surface recombination centers in ZnO nanorods arrays by X-ray photoelectron spectroscopy

Abstract

The surface composition of as-grown and annealed ZnO nanorods arrays (ZNAs) grown by a two-steps chemical bath deposition method has been investigated by Xray photoelectron spectroscopy (XPS). XPS confirms the presence of OH bonds and specific chemisorbed oxygen on the surface of ZNAs, as well as H bonds on 0) 1 (10 surfaces which has been first time observed in the XPS spectra. The experimental results indicated that the OH and H bonds play the dominant role in facilitating surface recombination but specific chemisorbed oxygen also likely affect the surface recombination. Annealing can largely remove the OH and H bonds and transform the composition of the other chemisorbed oxygen at the surface to more closely resemble that of high temperature grown ZNAs, all of which suppresses surface recombination according to time-resolved photoluminescence measurements.

Low temperature near band edge recombination dynamics in ZnO nanorods

“…Hydrogen in ZnO can be incorporated in three known positions: (i) interstitially (H i ) at the bond-centered site between Zn 2þ and O À or at the anti-bonding site bound to the O À ion, 22,23 (ii) in an oxygen vacancy (H O ), 22 and (iii) in the form of surface adsorbed H and species, such as OH and H 2 O. 8,14 In bulk ZnO, H i at the bond-centered (H BC ) site (which gives rise to a PL line at 3.3601 eV at 4.2 K (Ref. 24)) out-diffuses below 200 C, [24][25][26] whereas H O (which gives rise to the 3.3628 eV PL line (I 4 ) at 10 K) has been found to anneal out at temperatures from 500 C to 600 C. 1 Surface adsorbed hydrogen-related impurities are also removed at these temperatures.…”

“…[1][2][3][4][5][6][7][8] In the context of utilizing ZnO in the optoelectronics industry, a thorough understanding of its optical and electrical properties is essential. In particular, a detailed understanding of carrier relaxation processes, which strongly affect its electronic and optoelectronic characteristics, is a prerequisite for the inclusion of ZnO in the optoelectronics sector.…”

“…3,6,7 Secondly, surface recombination strongly affects the luminescence decay profiles. 3,8,9 The effect of the surface often manifests in a bi-exponential luminescence decay profile, given by I(t) ¼ A s exp(Àt/s s ) þ A b exp(Àt/s b ), with a fast component (s s ) and slow component (s b ). 3,9,10 Here, I denotes the PL intensity and the pre-factors A s /A b describe the contributions of the fast/slow component.…”

“…3 The slow component is related to recombination in the bulk 3,9 of the material, whereas the origin of the fast component is still under debate. 3,8,9,11 For D o X related transitions, Wagner et al 7 have suggested that the fast decay component (non-radiative) is due to the capture of excitons on deeper traps and saturates when all traps are occupied. Recently, Chen et al 3 studied the recombination dynamics of shallow donor bound excitons in bulk ZnO and suggested that the fast component is associated with surface recombination.…”

“…In particular, for nanostructures grown from solution (which is the case in this study); these species incorporate easily during growth. For example, Yang et al 8 (with a binding energy of 533 eV) and OH-related surface species after exposing hydrothermally grown ZnO nanorods to air for one year. Hence, the presence of surface adsorbed impurities on CBD-ZnO is expected, as various chemicals can be adsorbed to the surface at the relatively low growth temperature (85 C) and due to the very nature of the CBD method.…”