- Nano Express
- Open Access
Fabrication and Optimization of Vertically Aligned ZnO Nanorod Array-Based UV Photodetectors via Selective Hydrothermal Synthesis
© Ko et al. 2015
- Received: 21 April 2015
- Accepted: 1 August 2015
- Published: 12 August 2015
Vertically aligned ZnO nanorod array (NRA)-based ultraviolet (UV) photodetectors (PDs) were successfully fabricated and optimized via a facile hydrothermal process. Using a shadow mask technique, the thin ZnO seed layer was deposited between the patterned Au/Ti electrodes to bridge the electrodes. Thus, both the Au electrodes could be connected by the ZnO seed layer. As the sample was immersed into growth solution and heated at 90 °C, the ZnO NRAs were crystallized and vertically grown on the ZnO seed layer, thus creating a metal-semiconductor-metal PD structure. To investigate the size effect of ZnO NRAs on photocurrent, the PDs were readily prepared with different concentrations of growth solution. For the ZnO NRAs grown at 25 mM of concentration, the PD with 10 μm of channel width (i.e., gap distance between two electrodes) exhibited a high photocurrent of 1.91 × 10−4 A at an applied bias of 10 V under 365 nm of UV light illumination. The PD was optimized by adjusting the channel width. For 15 μm of channel width, a relatively high photocurrent on-off ratio of 37.4 and good current transient characteristics were observed at the same applied bias. These results are expected to be useful for cost-effective and practical UV PD applications.
- Zinc oxide nanorod arrays
- Ultraviolet photodetectors
- Hydrothermal method
Zinc oxide (ZnO) nanorod array (NRA)-based photodetectors (PDs) are alternative ultraviolet (UV) sensors because they have several advantages such as wide direct band gap (3.37 eV) and large surface area [1–3]. Compared to conventional ZnO thin-film UV PDs, it has been revealed that the nanostructured ZnO UV PDs can offer higher photocurrents [4–6]. In particular, one-dimensional (1D) ZnO nanostructures such as nanowires and nanorods have exhibited many advantages including large surface to volume ratio, high quantum efficiency, and direct pathway of charge transport [7, 8]. Furthermore, the reduced dimensionality enhances carrier lifetime and photoresponse properties . Currently, the most popular approach for the fabrication is based on a metal-semiconductor-metal (MSM) structure by direct growth of lateral ZnO NRAs because the MSM photodetector is simple for easy fabrication and compatible with various semiconductor nanomaterials [10–13]. However, there are still technical difficulties for practical fabrication processes of PDs. In order to grow ZnO NRAs as an active channel between electrodes, the ZnO seed layer should be selectively etched after photolithography process. Although the ZnO nanorods are easily grown by chemical synthesis methods, it requires many procedures for exposing the selective ZnO seed surface.
On the other hand, hydrothermal synthesis has been considered as one of the promising methods for growing 1D ZnO nanostructures because it allows relatively low temperature (75–90 °C) and scalable manufacturing process [14–16]. Especially, this process is highly practical for various applications including field-effect transistors, solar cells, UV PDs, and piezoelectric devices [17–19]. By emerging the ZnO seed layer-coated substrates into aqueous solution, the ZnO NRAs are grown selectively on the seed layer, which enables to fabricate various device structures for specific device applications [20, 21]. In this letter, we demonstrated a facile fabrication and optimization of ZnO NRA-based MSM UV PDs by a hydrothermal growth. By the selective deposition of ZnO thin film using a shadow mask, the active channel for PDs was directly formed for the device fabrication. As compared with previous works of vertically aligned ZnO NRA-based UV PDs, this is a relatively convenient and controllable fabrication approach.
Figure 2b shows the XRD patterns of ZnO NRAs in the device structure. According to the standard JCPDS card no. 89-1397, the ZnO XRD peaks are in good agreement with a hexagonal wurtzite crystal structure. Among the ZnO XRD peaks, the dominant (002) peak was observed at 2θ = 34.4°, indicating that the ZnO NRAs were grown and crystallized along the direction of the c-axis in the crystal structure. From the metal electrodes, the Au (111) XRD peak also appeared. To examine the photocurrent of the ZnO NRAs in this structure, the measured current-voltage (I-V) curves were compared, as shown in Fig. 2c, for the same channel width (i.e., gap distance between two electrodes) of 10 μm. Without ZnO NRAs, the ZnO seed layer between the two electrodes did not sufficiently generate a photocurrent. In contrast, the ZnO NRA-based PD exhibited a large photocurrent under the light illumination of 365 nm. In this condition, the device exhibited an on-off ratio of 25.3 at 5 V of bias voltage and a high photocurrent of 1.91 × 10−4 A at 10 V. As well-known in previous works , the photocurrent is attributed to the photogenerated electron-hole pairs in ZnO NRAs. Among them, the holes intend to be captured with trap levels at the surface of ZnO NRAs, and thereby the electrodes mainly contribute to enhancing the photocurrent. Here, the concentration of growth solution is closely related with the morphology of ZnO NRAs because it determines their size and height.
The ZnO NRA-based MSM PDs were facilely fabricated by the shadow mask technique and hydrothermal process. The selective deposition of ZnO seed layer offered a convenient and controllable fabrication method. For the optimization, the ZnO NRA-based PDs were prepared and characterized by changing the growth condition of ZnO nanorods and the W ch. For 15 μm of W ch, the ZnO NRAs grown at 25 mM exhibited a relatively high photocurrent on-off ratio of 37.4 in typical transient characteristics. These results can provide a deep insight into the simple fabrication for practical and efficient UV PD applications.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015-023255).
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