The effect of post-annealing treatment on photoluminescence of ZnO nanorods prepared by hydrothermal synthesis
Introduction
Zinc oxide (ZnO) II–VI semiconductor is an important material. It presents interesting electrical, optical, acoustic and chemical properties, which find wide application in the fields of optoelectronic, sensors and catalysis. In the past decade, interest has been focused on 1D ZnO nanostructure having high surface area and specific crystalline orientation. ZnO nanorods and nanowires are particularly interesting since they can be used to tune electronic and optoelectronic devices that involve UV lasing action [1], [2]. In addition, knowledge of the electronic structure of the nanorod is crucial to understand the basic physics for these applications.
There are many ways to grow ZnO nanorods such as metal organic chemical vapor deposition (MOCVD) [3], pulse laser deposition [4], radio frequency magnetron sputtering [5] and anodized aluminum oxide membrane [6]. Recently, solution method such as cathodic electrodeposition [7], thermal decomposition [8] and the use of complex agents have been reported for ZnO nanorod production. Here, we report a new solution method for ZnO nanorod deposition which employs an aqueous solution containing ammonia hydroxide NH4OH and zinc acetate Zn(CH3COO)2.2H2O. This growth method showed some advantages compared with others, such as use of simple equipment, low-temperature deposition (100 °C), low cost, less hazardous, and no need for use of metal catalysts. The GaN film was used as a template for the ZnO nanorod growth since these materials have the same wurtzite crystal structure, low lattice constant misfit (1.9%) and similar thermal expansion. Due to similar crystal properties, the ZnO nanorods are expected to epitaxially grow on the GaN template. In addition, growing ZnO on GaN also offers promising approach to fabricate nZnO/pGaN heterostructure with nanojunction which is believed to give higher carrier injection efficiency than the normal junction [9], [10]. By using this growth method, highly ordered arrays of nanorods with micrometer length can be produced within an hour. However, due to low-temperature growth, the obtained ZnO nanorods have low crystal quality with lattice defects and surface defects. Post-annealing treatment is an effective method to enhance the crystalline and optical qualities of the ZnO nanorods by reducing nonradiative related defects. In this paper, the photoluminescence (PL) of the ZnO nanorods grown on GaN and annealed in different ambient (oxygen and nitrogen) was studied. The results showed that both annealing ambient and the PL measurement temperature strongly influenced the light emission behaviors of the ZnO nanorods. The UV emission can be greatly enhanced by annealing the as-grown ZnO nanorods in nitrogen ambient. From these experimental results, the effects of UV enhancement and temperature dependence of the light emission from the ZnO films were discussed.
Section snippets
Experiment procedure
The well-aligned ZnO nanorod arrays were grown directly on the GaN sample. The GaN sample was prepared by growing 3 μm GaN film on c plane sapphire at 1020 °C using an EMCORE D125 metal organic chemical vapor deposition (MOCVD) using two-step method. Trimethylgallium (TMGa) and NH3 were used as the Ga and N source, respectively. H2 gas was used as a carrier gas. The GaN epilayer was then used as a template to grow ZnO nanorods. The aqueous solution method was used to synthesize the ZnO nanorods.
Results and discussion
The high-density ZnO nanorod arrays were synthesized by using hydrothermal method. Fig. 1 shows the SEM image of the ZnO nanorods, grown on (0 0 0 1) GaN sample for 4 h. The nanorods uniformly covered the entire surface with hexagonal cross section and sharp tips. From the cross-section image, the rods grew vertically from the GaN substrate, having uniform thickness and length distribution. The diameter and length of the nanorods are 80–120 nm and ∼2 μm, respectively. The hexagonal shape of the
Conclusion
We have shown the annealing influences on the emission characteristic of the ZnO nanorods. The photoluminescence of the highly oriented ZnO nanorods annealed in nitrogen and oxygen ambient was characterized at room temperature and low temperature. The emission spectra can be divided into two categories. One is the near band edge emission at UV range, which is caused by the free exciton recombination and the other is the deep level emission at the visible range, resulting from the impurities and
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