Improved characteristics of near-band-edge and deep-level emissions from ZnO nanorod arrays by atomic-layer-deposited Al2O3 and ZnO shell layers
© Sun et al; licensee Springer. 2011
Received: 2 June 2011
Accepted: 17 October 2011
Published: 17 October 2011
We report on the characteristics of near-band-edge (NBE) emission and deep-level band from ZnO/Al2O3 and ZnO/ZnO core-shell nanorod arrays (NRAs). Vertically aligned ZnO NRAs were synthesized by an aqueous chemical method, and the Al2O3 and ZnO shell layers were prepared by the highly conformal atomic layer deposition technique. Photoluminescence measurements revealed that the deep-level band was suppressed and the NBE emission was significantly enhanced after the deposition of Al2O3 and ZnO shells, which are attributed to the decrease in oxygen interstitials at the surface and the reduction in surface band bending of ZnO core, respectively. The shift of deep-level emissions from the ZnO/ZnO core-shell NRAs was observed for the first time. Owing to the presence of the ZnO shell layer, the yellow band associated with the oxygen interstitials inside the ZnO core would be prevailed over by the green luminescence, which originates from the recombination of the electrons in the conduction band with the holes trapped by the oxygen vacancies in the ZnO shell.
PACS 68.65.Ac; 71.35.-y; 78.45.+h; 78.55.-m; 78.55.Et; 78.67.Hc; 81.16.Be; 85.60.Jb.
Because of large surface-to-volume ratio and spatial confinement of carriers, researches on one-dimensional (1D) nanostructures have attracted great interest [1–3], and remarkable progress has been achieved in various electronic, photonic, and sensing devices [3–7]. Novel synthetic approaches to the fabrication of high-quality semiconductor nanotubes have been reviewed by Yan et al. . Zinc oxide (ZnO) has been regarded as one of the most promising materials for 1D nanostructures due to its distinguished characteristics such as direct and wide band gap (approximately 3.37 eV), large excitonic binding energy (up to 60 meV), and high piezoelectricity [9–11]. The synthesis of well-aligned ZnO nanorod arrays (NRAs) is crucially important for the practical applications such as field emitters , nanogenerators , solar cells , and nanolasers . One of the popular techniques for fabricating ZnO NRAs is to use Au as catalyst on a lattice-matched substrate . Since the optical properties of ZnO NRAs are strongly dependent on surface conditions [17–20] and natural defect states [21–24], a large variety of surface modifications on ZnO NRAs have been carried out by depositing a shell layer. For instance, the enhancement of photoluminescence (PL) has been observed in ZnO/Er2O3 and ZnO/MgZnO core-shell NRAs [25, 26]. The enhanced surface-excitonic emission together with the suppression in deep-level emission has also been reported in ZnO/amorphous-Al2O3 core-shell nanowires . Apart from the enhancement of light emission, strong photoconductivity , photocatalytic activity , and quantum confinement  have been observed in various 1D ZnO nanostructures.
In this paper, vertically aligned ZnO NRAs were synthesized using an aqueous chemical method, which is beneficial for low reaction temperature, low cost, catalyst-free synthesis, and large-scale production. The growth of ZnO NRAs was assisted by a ZnO seed layer prepared by atomic layer deposition (ALD). The self-limiting and layer-by-layer growth of ALD contribute to many advantages such as easy and accurate thickness control, conformal step coverage, high uniformity over a large area, low defect density, good reproducibility, and low deposition temperature. Therefore, highly conformal Al2O3 and ZnO shell layers could be deposited upon the surface of ZnO nanorods by ALD to form the ZnO/Al2O3 and ZnO/ZnO core-shell NRAs in this study. PL measurements were conducted to investigate the optical characteristics of ZnO/Al2O3 and ZnO/ZnO core-shell NRAs. The near-band-edge (NBE) emission was significantly enhanced, and the deep-level band was suppressed by the Al2O3 and ZnO shells due to the flat-band effect and the reduction in the surface defect density. In addition, the shift of deep-level emissions from the yellow band to the green band in ZnO/ZnO core-shell structure was reported. The mechanisms of flat-band effect and the shift of deep-level emissions were elucidated in detail.
The ZnO NRAs were synthesized on (100) Si wafers by aqueous chemical growth. Before the synthesis, a 50-nm-thick ZnO seed layer was deposited on the wafer at a temperature of 180°C by ALD. Diethylzinc and H2O vapors were used as the precursors for zinc and oxygen, respectively. After the ALD deposition, the seed layer was treated by rapid thermal annealing at 950°C for 5 min in nitrogen atmosphere to improve its crystal quality. Afterwards, the ZnO NRAs were grown in 320 ml aqueous solution, containing 10 mM zinc nitrate hexahydrate and 5 ml ammonia solution, at 95°C for 2 h. More details of ZnO NRA synthesis have been described elsewhere [31, 32]. Finally, Al2O3 and ZnO shell layers were prepared by the ALD on the as-grown ZnO NRAs to fabricate ZnO/Al2O3 and ZnO/ZnO core-shell NRAs. The precursors for Al2O3 deposition were trimethylaluminum and H2O vapors, and the deposition temperature was 180°C. The Al2O3 shell layers were 2, 5, and 10 nm in thickness. The ALD condition of ZnO shell layers was the same as that of the ZnO seed layer. The thicknesses of ZnO shell layers were 5, 10, and 15 nm, respectively. The details of ZnO and Al2O3 ALD parameters can be found in our previous studies [33–35].
The structural characterization of ZnO NRAs was examined by Germini LEO 1530 field emission scanning electron microscopy (SEM) (Carl Zeiss Microscopy, Carl-Zeiss-Straße 56, 73447 Oberkochen, Germany) and FEI Tecnai G2 T20 transmission electron microscopy (TEM) (FEI Company, 5350 NE Dawson Creek Drive, Hillsboro, Oregon 97124 USA). X-ray diffraction (XRD) measurement was used to characterize the crystallinity and crystal orientation of ZnO NRAs. PL spectroscopy was measured in a standard backscattering configuration where the light emission from top surface of the ZnO NRAs was collected, using a continuous-wave He-Cd laser (λ = 325 nm) as the excitation source.
Results and discussion
In summary, the ZnO/Al2O3 and ZnO/ZnO core-shell NRAs have been prepared using the aqueous chemical synthesis and the conformal ALD technique. The deep-level emission around λ ≈ 550 nm from the oxygen interstitials at the surface of ZnO cores was suppressed by the Al2O3 and ZnO shell layers. The shell layers also reduce the surface band bending, leading to the increase in overlap of the wavefunctions of electrons and holes in the ZnO core. Therefore, the NBE emission at λ ≈ 380 nm and the deep-level band around λ ≈ 550 nm from the oxygen interstitials inside the core were enhanced by the shell layers. Furthermore, the shift of defect-related emissions from the ZnO/ZnO core-shell NRAs was observed due to the competition between light emissions from the oxygen interstitials inside the ZnO core and the oxygen vacancies in the ZnO shell. As the thickness of the ZnO shell layer increased, the green luminescence (λ ≈ 490 nm) originating from the oxygen vacancies in the shell dominated over the yellow band (λ ≈ 550 nm) associated with the oxygen interstitials inside the ZnO core due to the flat-band effect. The results indicate that the shell layers prepared by ALD have significant influence both on the NBE and defect-related emissions of the ZnO NRAs.
This work was financially supported by the National Science Council in Taiwan under contract number NSC98-2112-M-002-018-MY2 and NSC100-3113-E002-011.
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