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Abstract
In this paper, a facile and cost effective hydrothermal method was employed to fabricate high performance ZnO/β-Ga2O3 core-shell heterostructures. A highly sensitive ultraviolet (UV) sensor was fabricated based on vertically aligned β-Ga2O3 decorated ZnO core-shell nanorod arrays (NRs). The ZnO/β-Ga2O3 heterostructures showed an excellent improvement in UV response characteristics comparing with bare ZnO NRs. It can be attributed to the lower interface barrier and larger surface area-to-volume ratio for the ZnO/β-Ga2O3 heterostructures. Meanwhile, a higher concentration of surface oxygen vacancies has been observed in ZnO/β-Ga2O3 heterostructures, which helps to increase the charge separation efficiency for the UV detector. The results suggested that the ZnO/β-Ga2O3 core-shell heterostructure should be suitable for practical applications of UV photodetectors.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, one-dimensional (1D) nanostructured ZnO with diverse morphologies such as nanorods, nanowires and nanotubes has been extensively investigated [1–3]. In particular, well-aligned ZnO nanorod or nanowire arrays are highly desirable for their potential application on UV photodetectors [4]. Likewise, Gallium Oxide (β-Ga2O3) is a wide band gap compound and exhibits good conduction properties due to a donor band related to oxygen vacancies [5]. It has attracted much attention for the last few years due to its potential applications for deep UV light sensors [6]. ZnO and β-Ga2O3 are both promising materials for fabricating photonic, optical, and electronic devices.
Meanwhile, the heterostructures consisting of two functional materials are of prime importance for revealing unique properties and essential for developing potential nanoelectronic and optoelectronic devices [7,8]. Zhao et al. [9] proposed single β-Ga2O3/ZnO structure through high temperature vapor solid method and quite well UV sensitivity was obtained. However, the junction area of such structure was too small, which is difficult to the fabrication of UV devices.
Moreover, different methods have been applied to synthesize various kinds of ZnO or β-Ga2O3 nanostructures. Among them, the hydrothermal growth method is considered as cheap, simple, achievable at low temperature and environmentally friendly [10, 11].
In this work, β-Ga2O3/ZnO NRs were prepared using aqueous method by two-step. The morphological study shows that the β-Ga2O3 nanostructures are densely packed on the ZnO nanorods. Meanwhile, the UV response of the decorated ZnO NRs was remarkably enhanced by the presence of the β-Ga2O3. To our knowledge, this was the first report for the fabricate β-Ga2O3/ZnO core-shell structures in large scale by aqueous method. Meanwhile, the results showed the ZnO/β-Ga2O3 core-shell heterostructures would make it highly suitable for practical applications of UV photodetectors.
2. Experiment
The ZnO NRs were fabricated via a simple hydrothermal technique as described in our previous reports [12]. The quarz substrate was used as template to grow ZnO NRs. ZnO NRs were prepared by the following two step process. Firstly, zinc acetate dehydrate (Zn(CH3COO)22H2O) was dissolved in ethanol with a concentration 30 mM. Then a droplet of solution was coated onto quarz substrates. The coated substrates were annealed at 200 °C for 15 min in air to yield ZnO seed layer. In a typical experiment an equimolar (0.03M) aqueous solution of zinc acetate (Zn(CH3COO)2) and hexamethyltetramine (C6H12N4) was prepared. And they were dissolved in distilled water and the solution was stirred at room temperature. The quarz substrate was putted into the above solution of 90°C for 3h. After deposition, the sample was cleaned with deionized water and then dried in an air atmosphere. Then the sample was placed face up in an aqueous solution of 0.001M gallium nitrate hexahydrate (Zn(NO3)26H2O) and 0.001M C6H12N4 with reaction temperature at 70°C for 24h. Afterwards, the sample coated by GaOOH was thoroughly rinsed and dried in air at room temperature. For the fabrication of the β-Ga2O3 shells, it was annealed under a temperature of 800 °C at 3h.
The morphology of the as-deposited material was examined using scanning electron microscopy (FE-SEM, Hitachi S-4800). The crystal structures were investigated using transmission electron microscopy (TEM, Philips CM-200). Spatial-resolved energy dispersive X-ray spectroscopy (EDS) analyses were also performed. The PL measurement was performed using a He-Cd laser line of 325 nm as the excitation source. X-ray photoelectron spectrum (XPS) was obtained by using a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectrometer. The device performance was characterized by current-voltage (I-V) characteristics and time response as UV light switching on and off. For the wide range of wavelength measurement, a Xe-arc lamp and monochromator combination was used as the light source. And the photoresponse characteristics of the sample were measured using a Keithley 2400 voltage source instrument. The ITO with width of 0.1 mm was formed to separate two ITO electrodes. Then, the as-grown β-Ga2O3/ZnO and ZnO NRs were reversed and placed on the prepared ITO/glass substrate were contacted with the top of the sample. UV photo response measurement was carried out with a fixed bias voltage of 5 V by switching the light from a portable UV lamp (λ = 365 nm, 0.3 mW/cm2) on and off. The distance between the sample and the UV lamp was fixed. All of the measurements were carried out at room temperature in ambient condition.
3. Results and discussion
Figure 1 (a) displayed the fabrication scheme to obtain the β-Ga2O3/ZnO NRs. The reaction process can be described below:
The GaOOH precursors are firstly accumulated on the surface of the ZnO NRs. Further annealing of ZnO NRs with GaOOH nanostructures results in the formation of α-Ga2O3 in low temperature(>300°C) and β-Ga2O3 in high temperature(>800°C) [13]. Fig. 2(a) displayed the plan-view SEM image of β-Ga2O3/ZnO NRs, it can be clearly observed that the ZnO nanorods is completely coated by β-Ga2O3 nanocrystals. Figures 2(b) and 2(c) displayed the β-Ga2O3/ZnO NRs with tilt-view with low and high magnification. It showed that the β-Ga2O3 nanostructures are densely packed on the ZnO nanorods. The size of the β-Ga2O3 nanostructures around 30nm. Figure 2(d) demonstrated the EDS spectrum of the β-Ga2O3/ZnO structure, only Ga, Zn, O were detected. The EDS spectrum indicated the presence of Ga in addition to the Zn and O atoms.
Fig. 2 (a) Plan view SEM image of ZnO/β-Ga2O3 core-shell heterostructures. (b) Tilt view SEM image of the ZnO/β-Ga2O3 core-shell heterostructures. (c) Tilt view SEM image of the ZnO/β-Ga2O3 core-shell heterostructures with high magnification. (d) EDS spectra of the β-Ga2O3/ZnO structure.
Figure 3(a) displayed a TEM image of typical individual β-Ga2O3/ZnO structure. It was clear seen that the ZnO nanorods were homogeneously coated by β-Ga2O3 nanocrystals. A high-resolution TEM image at the interface of the structure was shown in Fig. 3(b). It showed that the core possesses the single-crystal structure of ZnO with lattice spacing of around 0.26 nm, while the shell is of monoclinic β-Ga2O3 with lattice spacing of around 0.28 nm corresponds to the d spacing of (002) plane. The spatial distribution of the atomic composition across the β-Ga2O3/ZnO structure was detected by EDS line scan of a single ZnO nanorod covered with β-Ga2O3 nanocrystals, which was shown in the Fig. 3(c). Figure 3(d) showed the elemental maps of Ga and Zn elements, which was taken across an individual core-shell nanorod. The Ga elements were present everywhere in the heterostructures. The result confirmed the formation of the core-shell structure. Meanwhile, it can be seen that the Zn elements are also present in the shell region. It was speculated the formation of ZnGa2O4 is likely to take place in ZnO:Ga during the shell layer growth and annealing process.
Fig. 3 (a) TEM image of typical individual β-Ga2O3/ZnO structure. (b) lattice-resolved TEM image enlarging an area near the interface. (c) EDX line scan of a single ZnO nanorods covered with β-Ga2O3 nanocrystals. (d) Elemental maps Ga and Zn elements taken across an individual core-shell nanorod.
The characteristics of the chemical bonding states of the as-grown β-Ga2O3/ZnO nanostructures were performed by XPS, which were shown in the Fig. 4(a)-(d). And bonding states of the ZnO NRs were shown in the Fig. 4(e)-(f). Figures 4(a) and 4(b) displayed the binding energy of Ga 2p and Ga 3d, it agreed well with that of β-Ga2O3 reported previously which stem from the Ga-O bonding. Figures 4(c) and 4(e) showed the Zn 2p binding energy of ZnO NRs and as-grown β-Ga2O3/ZnO nanostructures, it can be seen the growth of β-Ga2O3 layer has no effect on Zn 2p state. Figures 4(d) and 4(f) displayed the O 1s in both structures. The peak around 530eV can be attributed to Zn-O and Ga-O bonds. The higher binding energy (532 eV) peak is usually attributed to chemisorbed or dissociated oxygen or hydroxyl (OH) species on the surface during the high temperature annealing [14]. It can be clearly seen for the β-Ga2O3/ZnO nanostructures, there are more oxygen vacancies on the surface than that on the bare ZnO NRs. Since the oxygen vacancies are very active species, easily combined with other groups to become stable [15].
Fig. 4 XPS spectrum of β-Ga2O3/ZnO structures: (a) Ga2p, (b) Ga3d, (c) Zn2p, (d) O1s and ZnO NRs:(e) Zn2p, (f) O1s.
Figure 5(a) presented the room temperature PL spectra of as-grown ZnO nanorod arrays. The PL spectrum of ZnO nanorod arrays displayed a common feature that consists of an UV PL peak and a visible emission band. Generally, the UV peak is due to the recombination of photo-generated electrons and holes, while the visible emission is associated with oxygen vacancies. Meanwhile, β-Ga2O3 powders with 30mM concentration growth was also measured as reference, which was shown in Fig. 5(b). The blue luminescence in β-Ga2O3 can be attributed to the recombination of an electron on a donor formed by oxygen vacancies (VO) and a hole on an acceptor formed by gallium vacancies (VGa) [16].
For the ZnO/β-Ga2O3 heterostructures, it consisted a weaker band in the UV region and visible light peak at 520 nm. It was shown in the Fig. 5(c). Considering the inevitable defects occurring in the annealing process, a mass quantity of oxygen vacancies would be created in the growth of β-Ga2O3 nanostructures. These oxygen vacancies generally act as deep defect donors and would induce the formation of new energy levels in the band gap [17]. The PL results show that the β-Ga2O3/ZnO nanostructures have more oxygen vacancies than that of bare ZnO NRs, which is in agreement with XPS results.
The schematic structure of the β-Ga2O3/ZnO photodetector was shown in the inset of Fig. 6(a). A ditch with width of 0.1 mm was firstly formed to separate two ITO electrodes. Then, the as-grown β-Ga2O3/ZnO sample was reversed and placed on the prepared ITO/glass substrate. The tips of the β-Ga2O3/ZnO nanostructures are put in direct contact with the ITO/glass substrate. The UV response of the devices was characterized by a portable UV lamp at room temperature in ambient condition. The photoresponse spectra of the device based on β-Ga2O3/ZnO heterostructures and ZnO NRs were shown in Fig. 6(b). The device based on β-Ga2O3/ZnO heterostructures with wide range responsivity in the UV region, which is much better than pure ZnO NRs for UV detection. The results confirmed that the performance of the device combined the optoelectrical signals from both ZnO and β-Ga2O3. The current-voltage characteristics of the photodetector based on β-Ga2O3/ZnO sample under dark and UV illumination (365 nm, 0.3mW/cm2) are shown in Fig. 6 (c). Meanwhile, pure ZnO nanorods arrays I-V characteristics under dark and UV illumination were also tested for reference, which were shown in Fig. 6 (d). Clear rectifying behavior can be observed with and without UV illumination for both samples. It was well known that ZnO NRs were sensitive to UV illumination. The measured current of the nanorods arrays was increased from 3.04 × 10−7 to 1.41 × 10−6 under UV illumination at an applied voltage of 5V. For the detector based β-Ga2O3/ZnO, the current was increased from 5.14 × 10−7 to 1.32 × 10−5. It showed higher photo response compared to pure ZnO NRs.
Fig. 6 (a) The schematic structure of β-Ga2O3/ZnO photodetector. (b) Photo-response spectra of the device based on p β-Ga2O3/ZnO heterostructures and ZnO NRs. (c) I-V curve of β-Ga2O3/ZnO heterostructures with and without UV illumination (d) I-V curve of ZnO NRs with and without UV illumination.
To examine the repeatability and response speed of the ZnO UV detectors, the time-resolved photocurrent at 5 V bias with multiple UV on/off cycles was measured, in which both the turn-on and turn-off times of the UV light are 60s. Meanwhile, the same device structure based on ZnO NRs was also tested for reference. The photodetectors can reversibly be turned on and off by switching the UV illumination, respectively, as shown in Fig. 7 (a) (b). To further study the mechanism of the response, the decaying and rising of the photocurrent of the both the devices were fitted biexponentially with the following equation [18, 19]:
where τ1 and τ2 are two relaxation time constants. For device based on ZnO NRs, the rising time are τr1 = 6.6s, τr2 = 41.2s, respectively. As shown in Fig. 7 (c) (d), after the UV light was turned off, the decay process was fast initially and then becoming slower. The decaying time are τd1 = 8.0s, τd2 = 66.8s, respectively. For the device based on ZnO/β-Ga2O3 core-shell heterostructures, the rising time are τr1 = 2.5s, τr2 = 18.4s, the decaying time are τd1 = 4.6s, τd2 = 32.4. Both the rising and decaying time were shortened compared to the ZnO NRs.Fig. 7 The time-resolved photocurrent at 5 V bias with multiple UV on/off cycles of (a) β-Ga2O3/ZnO (b) ZnO NRs. The experimental curve and fitted rising and decaying process curve of (c) β-Ga2O3/ZnO heterostructures (d) ZnO NRs.
It is well known that the surface states of ZnO play an important role to the fast decaying and rising times. The UV response and recovery of ZnO NRs are known to be generally governed by water molecules and ionized oxygen adsorption and desorption at the surface defect sites [20]. In our case, as the ZnO NRs were annealed at relatively high temperature, the influence of water molecules on the photoresponse could be eliminated [21]. Both ZnO and β-Ga2O3 are n-type semiconductors, it means that the contact is n-n heterojunction and electrons dominate the conductivity of the device. The energy band diagram of the β-Ga2O3/ZnO structure is shown in Fig. 8. According to the diagram, the band gaps of β-Ga2O3 and ZnO are 4.9 and 3.37 eV, respectively. The energetic barrier for electrons (ΔEc) and holes (ΔEv) are 1.85 and 0.32 eV, respectively. The ΔEc between ZnO and β-Ga2O3 is much larger than the ΔEv, it facilitates electron transfers from β-Ga2O3 to ZnO due to the lower interface in the barrier region [9]. Moreover, as the ZnO nanorods was covered by β-Ga2O3 nanostructures. Consequently, the increases in the surface area of the nanorods lead to collection of more UV light, thereby increasing the photocurrent [22]. Meanwhile, the large surface area-to-volume ratio enhances the recombination of electron-hole pairs, which contributes to a shorter decay time [23]. Moreover, it is well known that the oxygen vacancy plays a very important role in the processes of photocatalytic degradation and UV performance [24]. In our case, the β-Ga2O3/ZnO heterostructures contain more oxygen vacancies than that of bare ZnO NRs according to the PL and XPS results. The origin of such improvement is mostly attributed to the formation of disordered states and co-generation of zinc and oxygen vacancies during the β-Ga2O3 growth which create transition levels within the band gap and facilitate more efficient transfer of photoexcited charge carriers [24]. Meanwhile, concerning the importance of active surface area, Ga2O3/ZnO has high surface-to-volume ratio which will absorb more oxygen molecules. Thus, it will cause high concentration of free electron due to oxygen vacancies. Thus, the β-Ga2O3/ZnO heterostructures showed a significant improvement in UV responses.
4. Conclusion
In conclusion, we provide a facile and cost effective hydrothermal method to fabricate high performance ZnO/β-Ga2O3 core-shell heterostructures. The β-Ga2O3/ZnO heterostructures showed an excellent improvement in UV response characteristics compared to bare ZnO NRs. The UV response and recovery speed of β-Ga2O3/ZnO heterostructures were enhanced. The method would provide in large scale production for practical applications.
Funding
National Natural Science Foundation of China (Grant No. 61474031, 11474045, and 61764001); Guangxi Natural Science Foundation (2016GXNSFDA380021); Guangxi Key Laboratory of Precision Navigation Technology and Application (PF17052x, DH201701); Liaoning Provincial Natural Science Foundation of China (201602202); Guangxi District Education Office projects to enhance the basic ability of young teachers (2017KY0201).
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