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Abstract
We report an ultraviolet (UV) photodetector with a universally transferable monolayer film with ordered hollow TiO2 spheres on p-GaN. After forming a TiO2 monolayer film by unidirectional rubbing of hollow TiO2 spheres on a polydimethylsiloxane (PDMS) supporting plate, we used a 5% polyvinyl alcohol (PVA) aqueous solution to transfer the film onto the target substrate. The PVA/TiO2 monolayer film was detached from the PDMS film and transferred to the p-GaN/Al2O3 substrate. To investigate the effects of crystallized phases of the TiO2 hollow spheres, anatase and rutile TiO2 sphere monolayers prepared by combining template synthesis and thermal treatment. The responsiveness of the UV photodetectors using anatase and rutile hollow n-TiO2 monolayer/p-GaN was 0.203 A/W at 312 nm and 0.093 A/W at 327 nm, respectively.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For ultraviolet (UV) detection, wide-band-gap semiconductors such as ZnO, SiC, ZnSe, and TiO2 have been used for photoresponse wavelength matching between the bandgap and absorption of UV radiation [1–3]. Among the wide-band-gap compound semiconductors, TiO2 has received a great deal of attention due to its high power-conversion efficiency, relative ease with which the structure can be controlled, low cost, and resistive switching properties, relative to other compound semiconductors [4,5]. In order to fabricate the UV photodetectors, the hetero pn junction using metal oxide nanostructures formed on various templates or substrates were attempted [6]. Recently, the UV photodetectors using TiO2 nanorods and nanoparticles based heterojunction formed on p-Si substrates were reported [7,8]. Although TiO2 thin films consisting of nanoparticles formed by a spin-coating process has been widely used for a wide range of applications due to its many advantages, the main drawback affecting its application to optoelectronic devices is the problem of surface cracks forming in the TiO2 films [9,10]. After the formation of a TiO2 thin film on a target substrate by a simple spin-coating method, a post-annealing process is essential to remove the binders and impurities which may be incorporated into the TiO2 nanoparticles during the synthesis reaction in a solution-based process. During a heat-treatment process, tensile stress builds up in a TiO2 thin film due to the shrinkage of the xerogel film. This adversely affects the stability of the TiO2 film due to the easy formation of surface cracks [11]. The cracks that form in the TiO2 films of a pn-junction-based device can act as leakage current paths in operation mode, resulting in the degradation of the optoelectronic device performance, including the optical and electrical characteristics.
In this study, we fabricated UV photodetectors using a transferable n-TiO2 monolayer film/p-GaN based pn junction structure. The formation of the ordered TiO2 thin film with nano-sized hollow TiO2 spheres and its transfer onto the target substrate were achieved by a simple unidirectional rubbing method and a PVA supporting film [12]. The control and the effects of the crystal phases of TiO2 hollow spheres on the performance of UV detectors were investigated using current-voltage (I-V) probing and photo-responsiveness. Our suggested transferrable n-TiO2 hollow spheres monolayer/p-GaN pn junction structure has great potential for application to a wide range of optoelectronic devices including UV photodetectors.
2. Experimental details
A transferable hollow TiO2 nanosphere monolayer was used for an n-type film placed on a p-GaN template to fabricate a pn junction for application to UV detectors. Hollow nanostructures can effectively increase the light-scattering effect in an inner free space covered by a shell layer, resulting in the enhancement of the photo-responsiveness of a UV detector [13]. To form hollow TiO2 spheres, a template method was used as described in the literature [14]. The sacrificial template, used to make the hollow TiO2 spheres, consisted of poly(MAA/EGDMA) microspheres that were synthesized by a distillation precipitation polymerization process [15]. To maintain the size of the final hollow TiO2 spheres at about 350 nm, poly(MAA/EGDMA) microspheres with 610-nm-diameter were used. Methacrylic acid (MAA, 14 g) and ethyleneglycol dimethacrylate (EGDMA, 6 g) were polymerized at 88 °C for 150 min in a medium consisting of acetonitrile (AN) and deionized (DI) water, as well as α, α’-azobis (isobutyronitrile) (AIBN). EGDMA and AIBN were used as a cross-linker and initiator, respectively [16]. After polymerization, the remaining residues were removed and purified by centrifugation with ethanol and DI water. The TiO2 shell layer was formed over the poly(MAA/EGDMA) microspheres by a formation reaction conducted in a 1-L four-neck glass reactor with a stirring bar using titanium isopropoxide (TTIP) as the TiO2 precursor. We dispersed 2 g of each of the poly(MAA/EGDMA) microspheres into 200 mL of ethanol in a reactor for 2 h. A mixture, consisting of 1.4 g of TTIP and 100 mL of ethanol, was poured into the glass reactor. The solution was then stirred in the reactor for 24 h and then centrifuged at 3600 rpm for 10 min to filter out the residues. The final products were then dried in a vacuum oven. The as-prepared core/shell poly(MAA/EGDMA)/amorphous TiO2 microspheres were annealed at 450 °C and 700 °C for 4 h to control the crystallized phases of, respectively, the anatase and rutile TiO2 submicron spheres with hollow structures.
Figure 1 is a schematic diagram of the pn junction formation process using a transferable TiO2 monolayer and a p-GaN template. To make TiO2 monolayers using hollow TiO2 spheres, we applied the unidirectional rubbing method as described in the literature [12]. First, two flat polydimethylsiloxane (PDMS) flexible plates were formed in a petri dish. Then a film of hollow TiO2 spheres was supported on one of the PDMS plates and rubbed with the other PDMS supporting plate. After sufficient rubbing, the extra TiO2 nanospheres were pushed to the edge of the PDMS plate and were then removed by blowing with N2 gas. Then, 3 g of 5% polyvinyl alcohol (PVA) solution was poured onto the TiO2 monolayer/PDMS supporting plate to form a PVA/TiO2 monolayer/PDMS film. The 5% PVA solution spread uniformly to create a PVA/TiO2 monolayer film after aging for 24 h under a vacuum. After the formation of the TiO2 monolayer film, covered with PVA, the PVA/TiO2 monolayer film was peeled from the PDMS supporting plate. The detached PVA/TiO2 monolayer films were then transferred onto p-GaN/Al2O3 substrates to fabricate pn junction-based UV photodetectors. The TiO2 monolayer/PVA/p-GaN structures were heated to 450°C for 4 h to evaporate the PVA film. After the transfer of the ordered-monolayer of hollow TiO2 spheres onto the p-GaN, an indium electrode was deposited onto the p-GaN and a 100-nm ITO film, formed on glass, was used as the electrode for the n-TiO2 monolayer.
The I-V characteristics of the n-TiO2 monolayer/p-GaN junction structures were examined using a Keithley 2400 source meter and a UV lamp. Photo-responsivity measurements with respect to a change in the wavelength were carried out using a K3100 incident photon conversion efficiency (IPCE) measurement system.
3. Results and discussion
To investigate the effects of the crystal phase of the TiO2 on the device performance, we modified the crystal phase of the as-synthesized amorphous hollow TiO2 spheres to the anatase and rutile phases by changing the post-annealing process conditions. To confirm the phase change of the TiO2 spheres after annealing at different temperatures, we observed the XRD patterns. Figures 2(a)-2(c) show the XRD spectra of the as-synthesized, anatase-, and rutile-phase hollow TiO2 spheres deposited on quartz glass. For the as-synthesized core/shell TiO2/poly(MAA/EGDMA) spheres, there are no strong peaks in the XRD pattern, indicating that there is no crystallized phase, as shown in Fig. 2(a). By increasing the annealing temperature to 450 °C for 4 h, 2θ peaks appeared at 25.281°, 48.049°, and 55.060°, as shown in Fig. 2(b). These dominant XRD peaks are in good agreement with the (101), (200), and (211) plane peaks of the anatase-structured TiO2 crystal (JCPDS no. 21-1272). Moreover, after annealing the as-synthesized TiO2 at 700 °C, the XRD peaks mainly exhibited (110), (211), and (101) plane-related peaks at 2θ = 27.446°, 54.322°, and 36.085°, respectively, as shown in Fig. 2(c). This is in good agreement with the rutile-structured TiO2 crystal (JCPDS no. 21-1276). According to the JCPDS references, as-synthesized amorphous TiO2 spheres change to anatase- and rutile-phase TiO2 as a result of heat treatment performed at 450 °C and 700 °C, respectively [17]. In addition, no poly(MAA/EGDMA) peaks were observed in the XRD patterns of either the anatase or rutile TiO2 due to the removal of the carbon, hydrogen, and oxygen content. The carbon and hydrogen components of the organic materials combined with O2 during annealing in an air atmosphere such that the carbon and hydrogen were removed.
Fig. 2 XRD spectra obtained from TiO2 spheres with (a) as-synthesized, (b) anatase, and (c) rutile structure.
The surface morphology and size of the core/shell structures of the TiO2/poly(MAA/EGDMA) spheres, both before and after the thermal annealing process, were evaluated by SEM measurements. Figure 3 shows the as-synthesized TiO2/poly(MAA/EGDMA) spheres, as well as the hollow anatase and rutile TiO2 spheres after annealing at 450 °C and 700 °C for 4 h, respectively. The hollow structure is well retained after the annealing process due to the interconnection of the TiO2 nanoparticles [18]. The size of the core/shell TiO2/poly(MAA/EGDMA) decreased from about 610 nm to 360 and 335 nm after the formation of the anatase and rutile hollow TiO2 spheres, as a result of the shrinkage of the core/shell structures by thermal annealing. The higher-temperature annealing process used to attain rutile hollow TiO2 structures causes the hollow TiO2 spheres to be broken into small pieces.
Fig. 3 SEM images of poly(MAA/EGDMA)/α-TiO2 particles (a) before annealing process, after annealing process at (b) 450 °C and (c) 700 °C in air atmosphere. TiO2 monolayer transferred onto p-GaN template using (d) anatase TiO2 spheres (e) rutile TiO2 spheres.
To fabricate a well-arranged TiO2 thin film, after transfer of TiO2 monolayer film covered with PVA solution to enable transfer to a targeted substrate, the PVA was solidified by the application of a small amount of heat, after which the PVA/TiO2 monolayer film could be transferred to the p-GaN. The second annealing process is necessary to remove the PVA. SEM images were obtained for the TiO2 monolayer films with anatase and rutile hollow TiO2 spheres, as shown in Figs. 3(d) and 3(e), respectively. After the second annealing process in air, the PVA was removed by the reaction between the carbon in the PVA and oxygen in the air.
During the second annealing process, the outer TiO2 spheres melted slightly and became connected to other spheres. These slightly melted TiO2 structures with carbon residue between the TiO2 spheres produce a strong connection with the other spheres. The electrical conductivity through the carbon-wrapped TiO2 monolayer was improved by reducing the number of disconnections and maintaining the special distance between the TiO2 spheres [19].
From the EDS results (not shown here), we observed that some carbon residue remained after the burning of the PVA during the annealing process in an air atmosphere. To better understand the carbon residue originating from the PVA after the second annealing process, the surface chemistry was examined by using XPS, as shown in Figs. 4(a) and 4(b). The C 1s spectrum consists of four peaks. For the anatase TiO2 monolayer shown in Fig. 4(c), the peaks located at 284.38, 284.98, and 288.08 eV were attributed to the C-C, C-O, and C = O bonds, respectively. Furthermore, in the case of the rutile TiO2 monolayer shown in Fig. 4(d), the peaks located at 284.53, 285.98, and 288.58 eV were assigned to the C-C, C-O, and C = O bonds, respectively. These results are acceptable when compared with previously reported values [18,20]. Among three carbon bonds (C-C, C-O, and C = O), C-C bonding contributes to the enhanced electrical conductivity of carbon [21]. Figures 4(a) and 4(b) show that C-C bonding is dominant in the TiO2 monolayer, while the peak intensity of the oxidized carbon was relatively low, which can lead to a decrease in the electrical conductivity.
Therefore, TiO2 spheres connected by carbon (C-C) exhibit a better electrical conductivity than pure TiO2. Therefore, the carbon residues influence the electrical properties of the hollow TiO2 spheres and the performance of UV photodetectors due to the superior conductivity of n-TiO2 monolayers [22].
To investigate the optical properties of the anatase and rutile TiO2 monolayers, the absorbance was measured by applying UV-VIS spectroscopy. Figure 5(a) shows the absorbance spectra of the anatase (red line) and rutile (green line) TiO2 monolayers. It clearly shows that the anatase and rutile TiO2 monolayers absorb only UV light due to their wide band gap. We calculated the optical band gap of the TiO2 monolayers and p-GaN from a Tauc plot, shown in Fig. 5(b), as a plot of the versus hv curves [23].
Here, A is a constant, hv is the incident photon energy, Eg is the optical band gap, is an absorption coefficient, and the value of m is 2 for indirect transitions. The calculated band gaps for the anatase and rutile TiO2 monolayer are about 3.27, and 2.95 eV, respectively [24].Fig. 5 (a) Absorbance spectra of TiO2 monolayers using anatase and rutile TiO2 spheres. (b) Tauc plot for band gap calculation of TiO2 monolayer with different phases.
The electrical properties of the anatase- and rutile-structured n-TiO2 monolayer/p-GaN junctions were measured by using an I-V probing system under both UV light illumination and darkness. Figures 6(a) and 6(b) show the good rectifying behavior of both anatase and rutile TiO2 monolayer junction structures under reverse-bias conditions. The reverse currents of the anatase- and rutile-structured TiO2 monolayer were −0.58 µA and −0.029 µA, respectively, at −8.8 V under the no-light condition. After illuminating the pn junction structures with UV light, the reverse currents for the anatase and rutile TiO2 monolayer structures were increased to −4.29 µA and −0.44 µA, respectively, at −8.8 V. The increased absolute current of the anatase TiO2 monolayer/p-GaN junction was 3.71 µA upon the change from the dark state to UV illumination, which was higher than that of the rutile TiO2 monolayer junction (0.411 µA). The pn junction consisting of the transferred n-TiO2 monolayer film onto the p-GaN template effectively generated electron-hole pairs (EHPs) under UV light, which may be the source of the additional current flow through the pn junction under reverse-bias conditions. Moreover, the anatase TiO2 monolayer/p-GaN structure provides a more efficient means of generating excess carriers under UV illumination than a pn junction based on a rutile TiO2 monolayer. The performances of n-TiO2 monolayers/p-GaN photodetectors consisting of anatase and rutile phases were evaluated in terms of their photo-responsiveness by applying a spectral IPCE measurement system. As shown in Fig. 6(c), the peak responsivity of the anatase- and rutile-structured TiO2 monolayer/p-GaN pn junctions are 0.203 A/W at 312 nm and 0.093 A/W at 327 nm. The responsivity of a pn junction based on an anatase TiO2 monolayer junction is better than that of a device based on a rutile TiO2 monolayer. This phenomenon can be attributed to the different electron transport behaviors in the different phases of the TiO2 hollow spheres. The effective diffusion coefficient of anatase TiO2 is greater than that of rutile-structured TiO2, meaning that the electron transport in anatase TiO2 is easier than in rutile TiO2. Another possible reason for the superior performance of a detector based on an anatase TiO2 monolayer is related to the different effective masses of the electrons with respect to the phase change. Anatase-structured TiO2 has a smaller electron effective mass as well as a longer recombination lifetime than rutile TiO2. This means that additionally generated EHPs in anatase TiO2 resulting from UV irradiation of the devices can survive longer with higher mobility than ones in rutile TiO2 monolayer. Therefore, an anatase TiO2 monolayer photodetector exhibits a photo-response performance that is superior to that of one based on a rutile-structured TiO2 monolayer.
Fig. 6 I-V characteristics of TiO2 monolayer/p-GaN pn junction using (a) anatase TiO2 spheres (b) rutile TiO2 spheres. (c) Responsiveness of TiO2 monolayer/p-GaN pn junction using anatase and rutile TiO2 spheres (−9 V bias).
4. Conclusions
In summary, we successfully transferred a n-TiO2 monolayer consisting of hollow TiO2 spheres onto p-GaN to form a pn junction to demonstrate the new TiO2 film type and investigate its application to optoelectronic fields. The effects of the TiO2 monolayer with different phases of sphere on the performance of a fabricated UV photodetector were investigated. The I-V characteristics of all the n-TiO2 monolayer/p-GaN junctions exhibit good rectifying properties. To demonstrate the feasibility of our suggested structures for application to UV photodetectors, we compared the spectral responsiveness of anatase and rutile TiO2 monolayer/p-GaN pn junctions. The anatase TiO2 monolayer/p-GaN junction performed better as a UV photodetector than a pn junction based on a rutile TiO2 monolayer due to the faster electron transport of the anatase TiO2. The lighter effective mass and longer lifetime of the excess electrons in the anatase TiO2 spheres enable a slower recombination rate between the electrons and holes than is the case with rutile TiO2.
Funding
The Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF-2015R1A1A1A05027848)
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