Controllable growth of ZnO nanorod arrays with different densities and their photoelectric properties
© Wang et al.; licensee Springer. 2012
Received: 1 February 2012
Accepted: 26 March 2012
Published: 6 May 2012
Since the photoelectric response and charge carriers transport can be influenced greatly by the density and spacing of the ZnO nanorod arrays, controlling of these geometric parameters precisely is highly desirable but rather challenging in practice. Here, we fabricated patterned ZnO nanorod arrays with different densities and spacing distances on silicon (Si) substrate by electron beam lithography (EBL) method combined with the subsequent hydrothermal reaction process. By using the EBL method, patterned ZnO seed layers with different areas and spacing distances were obtained firstly. ZnO nanorod arrays with different densities and various morphologies were obtained by the subsequent hydrothermal growth process. The combination of EBL and hydrothermal growth process was very attractive and could make us control the geometric parameters of ZnO nanorod arrays expediently. Finally, the vertical transport properties of the patterned ZnO nanorod arrays were investigated through the microprobe station equipment, and the I-V measurement results indicated that the back-to-back Schottky contacts with different barrier heights were formed in dark conditions. Under UV light illumination, the patterned ZnO nanorod arrays showed a high UV light sensitivity, and the response ratio was about 104. The controllable fabrication of patterned ZnO nanorod arrays and understanding their photoelectric transport properties were helpful to improve the performance of nanodevices based on them.
There is a growing interest in designing new architectures for enhancing the performance of photoelectric devices. Due to the uniquely combined optical and electrical characteristics, ordered nanorod arrays have received considerable attentions for this goal. Photoelectric devices based on ZnO, CdS, ZnS, InP, SnO2, and Si nanowires or nanorod arrays offer the advantages of enhanced light absorption, improved carrier collection efficiency, and longer lifetime for minority carriers compared to conventional planar photoelectric devices, which can find many applications from field emission devices, sensors, solar cells, nanogenerators to UV photodetectors with the significantly improved performances[3–9]. Among them, ZnO nanorod array is one of the most promising materials for photoelectric devices due to its large exciton binding energy (60 meV), versatile synthesis, high mechanical and thermal stabilities, and nontoxic n-type nature.
Recently, various synthetic methods have been developed for the growth and fabrication of vertically aligned ZnO nanorod arrays, which can be classified into two categories: vapor-phase and hydrothermal synthesis. The hydrothermal synthesis method is more favorable for the practical applications due to its low growth temperature, low cost, and good potential for scale-up. More importantly, this method avoids the usage of gold catalyst, which is commonly used in vapor-phase methods and may introduce the residual catalyst atoms into the ZnO rod arrays[11–13]. Based on the hydrothermal method, three dimensional (3D) ZnO hybrid architectures and “nanoforest” hierarchical ZnO arrays have been synthesized[14, 15]. Theoretical and experimental works also have been carried out on the ZnO nanorod arrays based photoelectric devices and shown that the photoelectric response and charge carriers transport can be influenced greatly by the density and spacing of the ZnO nanorod arrays. Wang et al. report the field emission properties of ZnO arrays are correlative with the rod density. Subsequently, Spencer et al. prove theoretically that varying the spacing will affect the sensing property of ZnO nanorods using the density functional theory. Therefore, ZnO nanostructure arrays fabricated following a designed pattern, with a high degree of control in density and spacing is highly desirable. The development of nanofabrication techniques and equipments such as electron beam lithography (EBL), nanoimprint lithography, laser interference lithography, and nanosphere lithography provide us the potential opportunity to fulfill this goal.
However, precise control of these geometric parameters is rather challenging in practice. The e-beam lithography is the most reliable technique which can define the exact positions of the nanostructures with high precision. Therefore, the spacing distance between the patterned ZnO seed areas can be easily controlled in nanometer scales by using the EBL method. On the other hand, the hydrothermal growth technique of ZnO is versatile, large scale, and not confined to the inorganic substrate due to its low growth temperature as mentioned above. Therefore, combining these two methods together to fabricate patterned ZnO nanorod arrays with controllable geometric parameters is possible and attractive.
In this paper, patterned ZnO nanorod arrays with well-defined positions and spacing distance are fabricated onto Si substrate by a combination of EBL and hydrothermal growth process. The influences of spacing distance and growth time on the morphology of the nanorod arrays are also investigated. Finally, the transport properties of our ZnO nanorod arrays with suitable spacing distance and density are investigated.
The hydrothermal method is used for the growth of ZnO nanorods on the patterned areas as shown in Figure 1e. For details, the Si substrate is immersed vertically in the nutrient solution of 0.035 M zinc nitrate [Zn(NO3)2 · 6H2O] and 0.65 M NH3. The reaction temperature is kept at 80°C with different reaction times. The morphologies of the samples are characterized by scanning electron microscopy (SEM) (JEOL, JSM-5600LV, Akishima, Tokyo, Japan), high-resolution transmission electron microscopy (high-resolution transmission electron microscopy (HRTEM) (JEOL JEM 2010). The current–voltage characteristics measurements are carried out on the probe station (Lake Shore) equipped with a Keithley 4,200 semiconductor characterization system (Cleveland, OH, USA). The light used in our experiment is an UV light source (λ = 350 nm) with an average power of 0.8 mW.
Results and Discussion
The asymmetrical I-V curves mean that the-two Schottky barriers heights are different in dark condition. In nanometer scales, the surfaces states on the semiconductor surface will play a crucial role in determining the electron transport properties of nanostructures. The previous studies by our group and others have demonstrated that the surface states on the ZnO nanostructure surface can dominate the photoelectric properties of the ZnO nanorod arrays, and different surface states which caused by O2 adsorption or vacancies on the surfaces of semiconductor nanowires will cause the asymmetrical behavior of I-V curves[25–30]. In this experiment, one end of the Au electrode is deposited on the top of the ZnO nanorods, and the other end is surrounding the patterned ZnO nanorods on the Si substrate. In the middle of the ZnO nanorod patterns, there might have no Au deposited because of the high density of the rod arrays. Therefore, different contact properties are formed between the two ends of the Au electrode, accordingly different surface states will formed between ZnO nanorod and Au electrodes. Different heights of the Schottky barriers at the interface of metal and semiconductor which dominate by the surface states will make the asymmetrical I-V curves in dark condition. Under UV light irradiation, the photogenerated electrons and holes are quickly separated through the strong local electric fields formed at the reverse bias Schottky barrier, resulting in the current increased greatly. Since the barrier height and build-in electric field of Schottky diodes are different, the separation efficiency of photogenerated electron–hole pairs in the depletion layer will be different. The higher build-in electric field can separate more photogenerated holes to the surface of the ZnO nanorod, and the Schottky barriers height will be degraded larger also. As a result, the I-V curve changes from asymmetrical to symmetrical nonlinear I-V behavior under UV light illumination.
In summary, we have demonstrated an effective approach for controllable fabrication of ZnO nanorod arrays with different geometric parameters through the combination of EBL and hydrothermal growth process. EBL is employed to fabricate the patterned ZnO seed layers with different areas and spacing distances with high precise, while a hydrothermal growth method is used to control the density and morphologies of ZnO nanorod arrays. This combined nanofabrication approach provide a possibility to integrate the patterned ZnO nanorod arrays into real devices. The vertical transport properties of the patterned ZnO nanorod arrays are investigated, and the I-V curve measurement indicates that the back-to-back Schottky contacts with different barrier heights are formed between the Au electrodes and ZnO nanorods in dark conditions. Under UV light illumination, the patterned ZnO nanorod arrays show a high UV light sensitivity, and the response ratio is about 104. The controllable fabrication of patterned ZnO nanorod arrays and understanding their photo-electric transport properties are helpful to fabricate novel nanodevices based on them.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 20903034 and 10874040), and the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No. 708062), and supported by Program for Changjiang Scholars and Innovative Research Team in University, No. PCS IRT1126.
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