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Abstract
The fabrication and characterization of two-terminal devices using solution-synthesized Cu-doped ZnO nanorods and their photosensitive properties are reported in this work. Hydrothermally prepared ZnO and Cu:ZnO nanorods were characterized by SEM, EDS and PL studies. Two-terminal devices of ZnO and Cu:ZnO nanorods were prepared using a simple drop-casting on E-beam evaporated metal patterns. The aluminum metal patterns were produced using lithography and the lift-off technique. The current–voltage (I–V) characteristics of the nanorod devices show different shapes and current levels depending on the contact resistance and size of the nanorods. The photoresponse of the nanorods under different illumination conditions was analyzed. Both ZnO and Cu-doped ZnO show enhanced response under UV light excitation compared to halogen light excitation. The Cu-doped ZnO nanorods show an enhanced photosensitive property compared to undoped ZnO. This work demonstrates a simple fabrication technique of ZnO nanorod-based planar two-terminal devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nano-dimensional semiconductors are expected to play an important role in nano-level devices because their electro-optical properties can be tuned by altering the size of the nanostructures. Among various oxide semiconducting materials, zinc oxide (ZnO) has attracted immense interest because of its promising properties [1, 2]. It is a wide-band-gap semiconductor with unique physical and chemical properties that are suitable for various technological applications [3]. In addition, it is a low-cost and environmentally friendly material compared to other oxides. Nanostructured ZnO plays multifunctional roles depending on the shape, size, and doping. Its band gap varies depending on the size of the nanostructure and dopant-induced variations. Hence, it is used in the applications of solar cells, photodiode, light-emitting devices, gas sensors, biosensors and photocatalysis [4–7]. Particularly, it is a very interesting material for the fabrication of UV photodetectors as very sensitive to UV radiation [8]. The one-dimensional ZnO nanorods show high efficiency in all applications because of their large surface-to-volume ratio [9]. Moreover, its property can be tailored to be more suitable for a specific application by doping a preferred material and changing the size and shape. The doping of ZnO can also yield new properties such as p-type conductivity, ferromagnetism, diluted magnetic semiconduction (DMS), light emission and spintronic effects [10, 11]. Transition-metal-doped ZnO nanostructures recently attracted much attention because they are useful in diluted magnetic semiconductors, spintronic and other innovative applications because of the change in spin states.
Amongst various dopants, Copper (Cu) is highly notable for ZnO because Cu has similar physical and chemical properties to Zn. Cu doping can create localized impurity levels, which improves the electrical properties. The Cu impurity ions that are diffused into the ZnO lattice replace either substitutional or interstitial zinc ions and cause structural changes. In addition, Cu doping can noticeably modify the ZnO band gap and consequently its luminescent and optical properties [12]. Hence, the inclusion of Cu in the ZnO lattice can significantly affect the properties of ZnO nanorods [13]. Although undoped ZnO nanorods are prepared using various techniques including hydrothermal approaches, most Cu-doped ZnO nanorods for device fabrication are prepared by expensive high-temperature vapor deposition techniques, which restricts the application of Cu-doped ZnO nanorods [1].
In solution-based methods, ZnO nanostructures are mostly prepared using sol-gel synthesis, chemical precipitation and solvothermal or hydrothermal methods. However, the hydrothermal method has several advantages compared to other methods, such as the synthesis at low temperature with inexpensive experimental facilities, catalyst-free growth, and environmentally friendly synthesis. Because this method permits control of the morphology and size of the structures by varying the growth parameters such as time, temperature and concentration, it can be successfully used to prepare technologically important semiconductors with device quality purity. This study focuses on the two-terminal device fabrication and characterization of facile hydrothermally prepared Cu-doped ZnO nanorods. There are few reports on Cu-doped ZnO thin films and their ferromagnetic property [14], and to our knowledge, no report is available on the hydrothermal synthesis of Cu-doped ZnO nanorods and their two-terminal studies. Generally, vertically grown nanorod arrays were largely studied [15, 16]. However, works on planer devices of individual nanorods are scarce. Only very few reports are available on ZnO nanorod-based planer two-terminal devices. Wan et al. fabricated single nanowire FET in planar configuration and studied its FET characterization [1]. In their work, the vertically grown ZnO nanowires were separated chemically and used for the fabrication of individual nanowire device by conventional lithographic method. Chow et al. deposited vertically grown Cu-doped and undoped ZnO nanorods on silicon substrates by solution method. The grown nanorods were separated from the substrate using chemical process, and using the separated nanorods two-terminal sensors were fabricated by in situ lift-out and focused ion beam (FIB) procedure [13]. Chai et al. fabricated and studied the photoresponse of crossed ZnO nanorods synthesized by hydrothermal method. For the device fabrication they used an in situ lift-out technique in FIB instrument [17]. Compared to all other available methods our method is a facile and new approach, where the solution-dispersed ZnO nanorods are drop-cast onto the pre-fabricated metal patterns to align the nanorod between the metal contacts.
This solution-based method is easy and inexpensive, so it can be used to fabricate metal contacts for two-terminal nanorod devices. In this work, we report the fabrication of two-terminal devices using chemically synthesized Cu-doped ZnO nanorods, their photosensing property, and the FET characterization in the back-gate configuration. There are several works on ZnO nanorod-based photodetectors, but all of them are vertically grown ZnO nanorod arrays and/or heterostructures [18–20]. Here, we demonstrate the facile fabrication of planar two-terminal devices using individual undoped and Cu-doped ZnO nanorods.
2. Experimental details
All chemicals used to synthesize Cu-doped and undoped ZnO nanorods were of analytical-grade. In this hydrothermal synthesis, zinc nitrate [Zn(NO3)2.6H2O], Copper (II) nitrate, Cu(NO3)2, hexamethylenetetramine (HMT) (C6H12N4) were used to prepare doped and undoped ZnO nanorods. For undoped ZnO nanorods, 100 mL solution of zinc nitrate hexahydrate and HMT (50 mM each) were used. The mixed precursor solution was continuously stirred for 30 min; then, it was left undisturbed and tightly closed for 5 h at a constant temperature (80 °C). After the set period, the white precipitate was filtered, washed several times using DI water and ethanol to remove the residual solvents and other impurities, and finally dried the product.
To dope Cu with ZnO, copper nitrate was added to the precursor solution with a concentration of 5 mM. The synthesized ZnO and Cu:ZnO nanorods were subjected to field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDAX) and photoluminescence (PL) studies to analyze the morphological, compositional and optical properties. To fabricate two-terminal devices using the prepared nanorods, electrode structures were fabricated on p-silicon covered with 500 nm SiO2 layer. The finger-type pattern was formed using the standard electron beam lithography and a lift-off technique [21]. The thermally evaporated aluminum electrodes consisted of three 10-μm-wide fingers pointed head to head with a gap of 2–5 μm. These fingers were connected to 100 × 100 μm2 contacting pads for the probe contacts as shown in Fig. 1.
Fig. 1 Al metal pattern evaporated on the SiO2 covered silicon substrate by the E-beam evaporator; Insets show the devices of nanorods aligned with metal contacts.
To prepare two-terminal nanorod devices with metal contacts, the prepared nanorods were dispersed in ethanol and sonicated for 10 min. Then, the nanorod-dispersed solution was drop-casted onto the metal fingers. The nanorod-spread substrate was annealed at 400 °C for 1 h by keeping a weight on the substrate to press the nanorods against the metal contacts, so that the contact between nanorods and metal electrodes was improved. After drop-casting and annealing, the substrates were examined through an optical microscope to identify the nanorods aligned with the metal contacts. Then, the identified nanorod devices were subjected to current-voltage measurements. Some of the nanorods aligned with metal electrodes are shown in the inset of Fig. 1. The red arrow in the top-left inset shows the aligned ZnO nanorod with one set of fingers. Current–voltage (I–V) measurements of the fabricated two-terminal devices were performed using a Keithley electrical parameter analyzer (4200 SCS). The I–V measurements were conducted in the dark and under the illumination of halogen (200 W) and UV light (325 nm). The fabricated Cu-doped ZnO device was also studied as a back-gated FET with the silicon substrate as the back gate.
3. Result and analysis
Figures 2(a) and (b) show the FESEM images of the undoped and Cu-doped ZnO nanorods prepared at 80°C for 5 h. The undoped ZnO nanorods (Fig. (2)a) have different lengths of 3-6 µm and diameter approximately 50-70 nm with a perfect hexagonal shape. Compared to undoped ZnO nanorods, the Cu- doped ZnO nanorods have smaller length and diameter, as shown in the Fig. 2(b). It also exhibits a smooth surface with a hexagonal shape. The inset of Fig. 2(b) shows an individual Cu-doped ZnO nanorod of length 4.8 µm.
Fig. 2 Scanning electron microscopy images of (a) ZnO nanorods and (b) Cu:ZnO nanorods; Inset shows a length of individual nanorod.
The elemental compositions of undoped and Cu-doped ZnO nanorods are shown in Fig. 3(a) and (b), respectively. They show only the presence of expected elements such as Zn and O in the undoped ZnO nanorods and Zn, Cu and O in the Cu-doped ZnO nanorods, which confirms the growth of Cu-doped ZnO nanorods and the incorporation of Cu into the ZnO lattice.
Figure 4 shows the room temperature photoluminescence (PL) spectrum of ZnO and Cu:ZnO nanorods. These PL spectra show two bands: a sharp UV emission band at approximately 380 nm and a broad green-red emission band centered at 600 nm. The UV emission is attributed to the near-band-edge (NBE) emission that corresponds to the exciton recombination. The broad green emission band obtained at approximately 550 nm corresponds to the oxygen vacancies in ZnO nanorods. The Cu-doped ZnO nanorods also shows the identical trend with a slight shift of the UV emission band with reduced intensity. As shown in Fig. 4, the UV emission band of Cu: ZnO nanorods is shifted to a higher wavelength from the undoped ZnO nanorods. There is also a broad emission band at approximately 600 nm, which indicates the oxygen vacancy and Cu-impurity-related emission. In addition, the Cu-doped ZnO shows a small extra band at 530 nm, which indicates the green emission because of Cu impurities or defects. This PL study confirms the growth of Cu-doped ZnO nanorods [14].
The doped and undoped ZnO nanorods aligned with metal contacts were subjected to a current-voltage (I-V) measurement at room temperature. Some of the nanorod devices were not stable, i.e., they became unresponsive after few measurement steps, which may result from the breakdown of the devices because of the high resistive heating. Therefore, to check the field-dependent effect of the devices, the potential was increased step-wise as shown in Fig. 5(a). Figure 5(a) shows the field-dependent variation on the device current for the biases of 1 V (i) and 2 V (ii). When the field was increased to 4 V (iii), a breakdown was produced. Figure 5(b) shows the breakdown of another device at 4 V bias, which demonstrates the high resistive heating and break down of the nanorod devices at high voltage. Figure 6 shows the I-V plots of the two-terminal devices fabricated using undoped ZnO nanorods. Figure 6 shows the I-V plots of three different devices with different characteristics, which depend on the nature of the nanorods and their alignment with metal contacts. The variation of the current levels may depend on the contact resistance variation and thickness variation of the nanorods. The nonlinear asymmetric I-V nature of the rods may result from the asymmetric contact resistance of the nanorods. When we invert the polarity of the applied field or poles of the nanorods, the shape of the I-V curve is also inverted, as shown in Fig. 7, which indicates the pole-dependent contact resistance variation. As shown in Fig. 7, the length of the nanorod that is in contact with the metal layers is different on either side of the nanorods, which can alter the contact resistance.
Fig. 5 I-V plots of two different ZnO nanorod devices with high voltage breakdown; (i) −1 V to + 1 V, (ii) −2 V to + 2 V, and (iii) −4 V to + 4 V.
Fig. 6 I-V plots of different devices, which show the shape-dependent nature; Inset shows a magnified view of plot (a).
To study the photosensitivity of the fabricated devices, the devices were illuminated with halogen and UV lamps, and the current variation was recorded. The I-V variation of the ZnO nanorod devices under different illuminated conditions are shown in Fig. 8: (a) dark, (b) halogen light and (c) UV light. The inset of Fig. 8 shows the enlarged view of plots (a) and (b). The high forward and low reverse leakage current indicate the rectifying diode-like property of the ZnO nanorod device. Figure 8 also shows the current variation with respect to the illumination intensity. The UV irradiation produces a larger current than halogen light, which confirms the UV light sensing property of ZnO nanorods. The current is 1.00 x 10−8, 2.6 x 10−8 and 1.7 x 10−7 A for the devices under dark, halogen light and UV light conditions, respectively.
Fig. 8 I-V plots of the ZnO nanorod device in the dark and under different light illuminations: (a) dark; (b) 200-W halogen light; (c) UV light of 325-nm wavelength.
Compared to the undoped ZnO nanorods, Cu-doped ZnO nanorods have produced increased current levels; hence, the Cu doping improves the conductivity of the nanorods. Figure 9 shows the I-V characterization of Cu-doped ZnO nanorods under different illuminated conditions: (a) dark, (b) halogen light, and (c) UV light of 325-nm frequency. Figure 9 shows that the Cu-doped ZnO nanorods have produced higher currents than the undoped ZnO nanorods regardless of the illuminated conditions. The current is 1.8 x 10−8, 5.2 x 10−8 and 4.8 x 10−7 A at 2 V for the devices under dark, halogen light and UV light conditions, respectively. Thus, the Cu-doped ZnO nanorod can act as a better UV detector. Sarangi et al. have also reported the UV light sensing ability of hydrothermally grown ZnO nanorod films as heterostructures formed with n-Si [22].
Fig. 9 I-V plots of Cu:ZNO nanorod device in the dark and under different light illuminations: (a) dark; (b) 200-W halogen light; (c) UV light of 325-nm wavelength.
Figure 10 (a & b) shows the photoresponse and recovery time of the Cu-doped ZnO nanorod device. Under the illumination of a particular type of light, the lamp was switched ON and OFF for an interval (~20 sec); then, the current was recorded with zero-volt applied bias. Figure 10 (a) shows the transient current response of the device under 200-W halogen light excitation for 0-100 s; the current suddenly increases to a maximum when the light is ON and remains constant at this level as long as the light is ON. Similarly. When the light is switched off, the current is suddenly reduced to low, which indicates the fast sensing of the device. The constant current level in both low (OFF) and high (ON) states indicates the stability of the device in light sensing. Figure 10(b) shows the transient current response of the device under UV light excitation. It also shows the stability and reproducibility of the device with higher ON current than halogen light. Compared with halogen light, UV light produced delayed growth and decay times for the current both in ON and OFF conditions. This current obtained under UV light illumination is high because extra charge carriers are generated under UV illumination, which attributes to the UV light sensitivity of the Cu doped ZnO nanorod.
Fig. 10 Photoresponse and recovery time of the Cu:ZNO nanorod device under (a) halogen and (b) UV light excitation.
The delayed growth and decay of the current can result from the defects produced in the solution-synthesized nanorods and the defect-related slow charge collection and charge recombination under UV illumination. Moreover, in the dark condition, the free electrons of Cu-doped ZnO nanorods are captured by surficial oxygen molecules and forms low conducting depletion near the surface [18]. When the nanorods are illuminated, the captured electrons are liberated, which improves the conduction. In addition, under UV illumination, extra carriers (hole-electron pairs) are generated, so the current increases. These observations demonstrate the ability of Cu-doped ZnO in UV light detection. Figure 11 shows the Ids vs. Vds response of the back-gated Cu:ZnO nanorod FET for different Vgs values. The change in drain current at 2V Vds indicates the effect of the gate on the drain current. The drain current variation is shown in the left-bottom inset of this figure; its decrease with the increasing Vgs denotes the p-type nature of Cu-doped ZnO nanorods. This type of ZnO-nanorod-based devices has good potential in biosensor applications [23].
4. Summary
Two-terminal devices were successfully fabricated on SiO2-covered p-Si substrates using hydrothermally grown Cu-doped and undoped ZnO nanorods. The morphology, composition, and photoluminescence properties of the prepared nanorods were investigated using SEM, EDAX, and PL spectra studies. The PL measurements show a sharp emission band at 380 nm and a broad emission band at approximately 600 nm with a shift for Cu-doped ZnO, which confirm the growth of doped and undoped ZnO nanorods. The I-V measurement of the fabricated nanorod devices shows the shape- and size-dependent variations with the rectifying nature. Both doped and undoped ZnO nanorods show good photoresponses with enhanced responses for UV light. The Cu-doped ZnO nanorods have better light-sensing responses than the undoped ZnO nanorods. In general, this study demonstrates an easy fabrication of nanorod-based two-terminal devices.
Funding
Ministry of Trade, Industry and Energy (MOTIE, Korea) under Sensor Industrial Technology Innovation Program (No. 10063682).
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