Metal-Semiconductor-Metal Near-Ultraviolet (~380 nm) Photodetectors by Selective Area Growth of ZnO Nanorods and SiO2 Passivation
© The Author(s). 2016
Received: 12 February 2016
Accepted: 5 July 2016
Published: 15 July 2016
Metal-semiconductor-metal near-ultraviolet (NUV) photodetectors (PDs) based on zinc oxide (ZnO) nanorods (NRs), operating at λ ~ 380 nm, were fabricated using conventional photolithography and hydrothermal synthesis processes. The vertically aligned ZnO NRs were selectively grown in the channel area of PDs. The performance of ZnO NR-based NUV PDs was optimized by varying the solution concentration and active channel width (W ch). For the fabricated samples, their electrical and photoresponse properties were investigated under the dark state and the illumination at wavelength of ~380 nm, respectively. For the device (W ch = 30 μm) with ZnO NRs at 25 mM, the highest photocurrent of 0.63 mA was obtained with the on/off ratio of 1720 at the bias of 5 V. The silicon dioxide passivation was also carried out to improve the photoresponse properties of PDs. The passivated devices exhibited faster rise and reset times rather than those of the unpassivated devices.
Zinc oxide (ZnO) nanostructures (NSs, i.e., thin films, nanowires, and nanorods) have been extensively investigated in various applications including optoelectronics, transparent electrodes, and sensors due to their high transparency in the visible range, low cost, radiation hardness, and thermal/chemical stabilities [1–12]. Particularly, ZnO is a promising metal oxide semiconductor material for short wavelength (ultraviolet (UV) and blue) applications including photodetectors (PDs), lasers, and light-emitting diodes due to its excellent optical properties such as wide direct bandgap of 3.37 eV and large exciton binding energy of 60 meV at room temperature. In addition, the ZnO NSs have larger surface-to-volume ratio and higher quantum efficiency compared to the bulk ZnO, which makes themselves as a promising candidate for UV PDs [1, 2, 5–9]. However, there is very little information on ZnO homojunction-based UV PDs because undoped ZnO generally exhibits a nature of n-type conductivity while i- or p-type ZnO with high quality and reproducibility is difficult to be obtained [1, 3]. Meanwhile, in most reports on ZnO-based UV PDs, metal-semiconductor-metal (MSM) structures which provide an easy control, stable operation, simple fabrication process, and compatibility with various materials have been employed [1–6].
However, there are still practical limitations to grow ZnO NSs on selected areas of MSM structures. In order to define the selective areas, a conventional photolithography process is required, causing complex fabrication processes with potential damages. Although there has been much effort to grow ZnO NSs efficiently by various methods, these issues still need to be discussed. Meanwhile, hydrothermal synthesis has been considered as a promising method due to its relatively simple process, low working temperature, and short growth time [2, 13–15]. In this work, we investigated the synthesis and characteristics of one-dimensional ZnO nanorods (NRs) hydrothermally formed on the Al-doped ZnO (AZO) layer in MSM structures by the selective area growth and their applications for near-ultraviolet (NUV) PDs. To understand the passivation effect on the photoresponse property, comparative studies between the devices with/without silicon dioxide (SiO2) passivation were also carried out.
Results and Discussion
Figure 2b shows the (i) EDX spectrum of the selectively grown ZnO NRs on the active channel and their corresponding (ii) zinc (Zn), (iii) oxygen (O), and (iv) silicon (Si) elemental mapping images. From the EDX spectrum, the amounts of zinc and oxygen atoms were quantified to be ~14.3 and 20.5 %, respectively, indicating a 1:1 ratio of Zn and O for ZnO NRs. The amount of oxygen atoms was a little higher than that of zinc atoms because the oxygen in SiO2 layer on Si substrate was also counted. The elemental mapping images exhibited that the zinc and oxygen atoms found at high concentrations were uniformly distributed over the active channel region while the silicon was rarely observed. This indicates that the ZnO NRs were densely grown on the active channel without any significant defects or vacancies.
Figure 2c shows the current-voltage (I-V) curves of the ZnO NR-based NUV PDs at (i) 15 mM, (ii) 25 mM, and (iii) 50 mM. Herein, the W ch was fixed to be 30 μm. At the bias of 5 V, very low dark currents were observed from all the devices while the photocurrents were measured to be ~0.35, 0.63, and 0.46 mA for the solution concentrations of 15, 25, and 50 mM, respectively. It is well known that the magnitude of photocurrent is closely related to the surface area and crystallinity of ZnO NRs. The ZnO NRs grown at 25 mM provided a relatively large surface area. At 50 mM, on the other hand, the photocurrent decreased due to the reduced surface area and low crystallinity of large ZnO NRs [2, 16, 17]. The inset of Fig. 2c shows the corresponding cross-sectional FE-SEM images of the ZnO NRs grown at 15, 25, and 50 mM using the hydrothermal synthesis. The diameter and height of ZnO NRs were increased as the solution concentration was increased. These values were estimated to be 40 ± 5 nm (diameter)/385 ± 15 nm (height), 65 ± 10 nm/1000 ± 50 nm, and 205 ± 20 nm/2600 ± 300 nm, indicating the aspect ratio of 9.6, 15.4, and 12.7, for the solution concentrations of 15, 25, and 50 mM, respectively. This can be explained by the fact that the diffusion of Zn2+ ions to nuclei is more enhanced at higher solution concentration. At 50 mM, however, the aspect ratio was slightly decreased because the excessive Zn2+ ions cause the higher growth rate on the overall facets of ZnO, i.e., the rapid isotropic growth of ZnO NRs [2, 18, 19].
Figure 3c shows the spectral responsivity (R λ ) of the unpassivated and passivated ZnO NR-based NUV PDs grown at 25 mM with the W ch of 30 μm as a function of wavelength of incident light. The SiO2 passivation was performed at different deposition times of 120, 300, and 600 s to reduce the surface defects of ZnO NRs and environmental effects. The R λ indicates the ratio of photocurrent to light intensity at certain wavelength. For measurements, the monochromatic light source was used from 350 to 450 nm with an interval of 5 nm. At the bias of 5 V, for all devices, the R λ exhibited relatively high values below 390 nm while it was gradually decreased above 400 nm, which exhibits similar tendency with typical absorption spectra of ZnO NRs. Though the passivation on ZnO NRs with dielectric or organic materials enables enhanced photoconduction and efficiency by reducing the probability of surface recombination [29–31], for the ZnO NR-based NUV PDs, the R λ was dramatically dropped down with increasing the SiO2 deposition time for passivation. This is mainly attributed to the high resistivity of SiO2 . The optical losses by SiO2 passivation are not considered because it is highly transparent (i.e., transmittance > 90 %) in the NUV and visible ranges. At the wavelength of 380 nm, the highest R λ values were obtained to be 101.68, 59.29, 46.80, and 29.62 mA/W for the devices passivated with SiO2 deposited at the deposition times of 0, 120, 300, and 600 s, respectively. The inset of Fig. 3c shows the external quantum efficiency (EQE) of the unpassivated and passivated ZnO NR-based NUV PDs grown at 25 mM with the W ch of 30 μm as a function of wavelength of incident light. The EQE of ZnO NR-based NUV PDs was calculated using the equation of EQE = 1240 × R λ × λ − 1. At 380 nm, as shown in the inset of Fig. 3c, the maximum EQE values were observed to be 33.2, 19.4, 15.3, and 9.7 % for the devices passivated with SiO2 at 0, 120, 300, and 600 s, respectively.
Comparison of device properties between this and other ZnO-based UV PDs
Rise/fall time (s)
Bias voltage (V)
2.0 × 107
ZnO NPs-graphene core-shell
640 × 103
13.28 × 103
ZnO NRs between asymmetry Au
ZnO NRs on AZO
The ZnO NR-based NUV PDs in the type of MSM structure were successfully fabricated by the conventional photolithography and hydrothermal synthesis processes. The device characteristics were analyzed and optimized with various solution concentrations for different W ch. The optimized device performance was achieved from the ZnO NRs grown at 25 mM with the W ch of 30 μm, indicating the photocurrent of 0.63 mA and on/off ratio of 1720. The SiO2 passivation enhanced the photoresponse properties (i.e., reduced rise and reset times), but it made the photocurrent reduced. These results may be helpful for the facile fabrication and optimization of device performance in ZnO NR-based UV PD applications.
This work was supported by the National Research Foundation of Korea (NRF) grant and funded by the Korea government (MSIP) (NRF-2013R1A1A2010037).
SHL fabricated the ZnO NRA-based NUV photodetectors by selective area growth and SiO2 passivation, and measured the device characteristics. SHK assisted the fabrication and measurements. JSY supervised the conceptual framework and drafted the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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