Surface recombination in ZnO nanorods grown by chemical bath deposition

Abstract

Vertically well-aligned ZnO nanorods on Si substrates were prepared by a two-step chemical bath deposition ͑CBD͒ method. The optical properties of the grown ZnO nanorods were investigated by time resolved photoluminescence spectroscopy. It was found that the effective decay time of the near bandgap recombination in the CBD grown ZnO nanorods strongly depends on the diameter of the ZnO nanorods. Typically, the decay curves obtained from these ZnO nanorods show a combination of two exponential decays. The experimental results show that the fast exponential decay is related to the surface recombination and the slow decay is related to the "bulk" decay. The measured decay time of the effective surface recombination decreases with decreasing diameter, while the bulk decay time remains unchanged. The results also show that an annealing treatment around 500°C significantly reduces the surface recombination rate. A simple carrier and exciton diffusion equation is also used to determine the surface recombination velocity, which results in a value between 1.5 and 4.5 nm/ps.

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

“…23, in which theoretical calculated results show a well consistency with these assignments, especially for  S . The value of  S is 14 ps and 21 ps for the as-grown and 500 o C annealed ZNAs, respectively, which clearly indicates that the decay constant that arises from surface processes increases after annealing, i.e.…”

“…surface defects and intrinsic defects (also called as deep level defects) [23][24]. To further clearly distinguish the contribution from the surface defects and intrinsic defects, for the prolonged decay time after 500oC annealing, we also investigated the effect of annealing temperature on the optical properties of ZnO nanorods [24].…”

“…a substrate treatment prior to the ZNAs and some other controllable growth conditions can be found elsewhere [21][22][23][24].…”

“…by first spin coating the substrate with a seed layer of ZnO nanoparticles and then using the CBD growth. And we also investigated in detail the surface recombination of ZNAs with different diameters by time-resolved photoluminescence measurement and revealed that the surface recombination can be strongly suppressed by annealing at 500 o C at ambient conditions [23,24]. We pointed out that the surface recombination maybe originate from the functional groups absorbed by ZNAs surface during the chemical reaction in the solution, which should be related to the elements such as carbon, nitrogen and hydrogen, but we could not identify the origin of the surface recombination due to the limitation of the used characterization techniques [24].…”

Long lifetime of free excitons in ZnO tetrapod structures

“…The desire for efficient light emitters has motivated considerable interest in exciton dynamics [1][2][3][4][5][6][7][8][9][10] as it provides information both on efficiency of radiative recombination, determined by oscillator strength of the corresponding optical transitions, and also on material quality reflected by contributions of nonradiative recombination in lifetimes of photogenerated carriers. The decay time of excitonic photoluminescence ͑PL͒ reaches several nanoseconds at RT in high quality ZnO single crystals 1,5 but is substantially shorter ͑ϳhundreds of picoseconds͒ in ZnO nanostructures.…”