High‐Efficiency ZnO‐Based Ultraviolet Photodetector with Integrated Single‐Walled Carbon Nanotube Thin‐Film Heater

Recently, sol–gel‐derived ZnO thin films have been explored for high performance photodetectors (PDs). However, the crystallinity of sol–gel‐derived ZnO films is inferior to vacuum‐based grown ZnO film, leading to poor photoreaction in the PDs. This study combines a single‐walled carbon nanotube (SWCNT) heating system with sol–gel ZnO‐based ultraviolet (UV) PDs. A SWCNT heater is fabricated on sapphire substrates using spin coating technique, resulting in a transparent heater with a transmittance of ≈80% by controlling spin speed. In addition, increasing the number of SWCNT spin coatings from 1 to 3 raises the heater temperature to over 170 °C, resulting in a response time of less than 1 min. The sol–gel ZnO‐based UV PDs with the SWCNT heater shows a 137% increase in photocurrent as the SWCNT heater temperature increases from 25 to 112 °C. In addition, the increase in temperature of the embedded‐SWCNT heater significantly shortens the rise and decay times of the sol–gel‐derived ZnO PDs.


Introduction
ZnO has unique properties such as high transparency, good electrical conductivity and high electron mobility, making it attractive for optoelectronic applications. [1][2][3] In particular, ZnO-based UV PDs have gained a lot of attention due to their numerous advantages over other materials, including low cost, improved by grafting these CNTs heater to the PDs. The use of the CNTs heating film is expected to improve the photoresponsiveness of UV PDs. This study aimed to investigate the electrical and optical characteristics of a sol-gel ZnO PDs that includes a nanostructured heater manufactured using SWCNTs spin coating and is temperature controllable without an external temperature control system. Figure 1a shows the schematic diagram of hybrid sol-gelderived ZnO PDs on sapphire substrate with SWCNT heater system to control the operation temperature of the PDs. In the hybrid sol-gel-derived ZnO PDs with heating system, only SWCNT heaters are depicted on the back side of sapphire substrates as shown in Figure 1b. The gray and yellow regions represent SWCNT-deposited heating area and the copper electrodes, respectively. Figure 1c shows photographs of the SWCNT film heaters fabricated on glass substrates to analyze the optical transmittance of the SWCNT film heater with different spin coating speeds and numbers. It indicates that the transparency of the SWCNT film heater on the transparent substrates becomes more transparent as the spin coating speed increases and the number of spin coatings decreases. Under these spin coating conditions, the SWCNT films have high optical transparency, allowing the image of the backside to be clearly seen. The optical transmittances of SWCNT film heaters were characterized by UV-visible spectroscopy as shown in Figure 1d,e. Figure 1d shows the optical transmittance of SWCNT film heaters fabricated using spin coating speeds of 2000 to 4000 rpm. It appears that increasing the spin coating speed for SWCNTs led to an increase in transmittance in both the 365 and 550 nm regions. In particular, as the speed increased from 2000 to 4000 rpm, the transmittance at 365 nm increased from 65% to 75%, and the transmittance at 550 nm increased from 71% to 80%. In addition, it seems that changing the number of SWCNT spin coatings at a constant speed of 3000 rpm led to a reduction in optical transmittance at both the 365 and 550 nm regions. Specifically, as the number of spin coatings increased from 1 to 3 times, the transmittance at 365 nm decreased from 73% to 43%, and the transmittance at 550 nm decreased from 88% to 52%, as shown in Figure 1e. It implies that low spin www.advmatinterfaces.de coating rates and multiple coats of SWCNTs cause the material to form a thicker film that can scatter light and absorb more light, resulting in reduced optical transmittances. In particular, it can be seen that the optical transmittance of the SWCNT film is almost inversely proportional to the number of spin coating, which means that the thickness of the SWCNT film is well controlled by the number of spin coatings. [35] Based on these results, we suggest that these spin coating conditions are expected to achieve a transmittance of more than 50% for both UV and visible light, which can then be detected in a ZnO PD with a SWCNT film heater at the bottom, in addition to the UV light coming from the top.

Raman Spectroscopic Analysis of SWCNT Film Heating System
Raman spectroscopy is a common method for analyzing the structural defects in SWCNT. [36] The higher frequency D (defect), G (graphite), and radical breaking mode (RBM) bands in the Raman spectrum can provide information about the defects in SWCNT film. [37] Figure 2a,c show the Raman spectra of SWCNTs films coated on sapphire substrates with different spin coating speeds and the coating numbers. Regardless of the spin coating speed and coating number, it clearly indicates four peaks at 200, 1350, 1580, and 2350 cm −1 , which are associated with RBM, higher frequency D, G, and G′ (second-order family scattering from G-band variation) modes, respectively. [38] In particular, the RBM of CNT is a low frequency mode that is well-known for its strongest intensity in the Raman spectrum of CNTs. The RBM is a unique feature of CNTs and is not observed in other carbon-related material systems, making it a useful tool for identifying and characterizing CNTs. [39] It is observed that the peak intensity of RBM decreases with increasing spin coating speed, whereas that of RBM increases with increasing the coating number. This behavior may be due to the fact that a higher spin coating speeds result in a thinner and more dispersed distribution of CNTs on the substrate, whereas multiple spin coating cycles result in a more densely packed and ordered arrangement of CNTs. In addition, the ratio of the D/G band is commonly used as an indicator of the defect density in CNTs. [40] Calculating the area of each spectrum from the fitting of the D and G peaks as shown in Figure S1 (Supporting Information), it can be seen that the peak and integration intensities of the D and G peaks increase with the increase in thickness of the spincoated SWCNT film formed by the lower spin coating speed and the multiple coating process. However, the area ratios of D/G bands are almost the same regardless of the increase in the spin coating speed and the coating number. It implies that the spin coating process does not introduce additional defects into the SWCNTs films. Therefore, it is believed that this spin coating conditions only affect the film thickness and not the formation of defects in the SWCNTs films.  Figure 3a shows the temperature formed by applying voltage to SWCNT film heater fabricated with different spin coating speeds. The temperature of the SWCNT film heater fabricated at a spin coating speed of 4000 rpm increased from 24.6 to 37.4 °C, as the applied voltage was increased from 10 to 70 V. The SWCNTs film heater fabricated with a spin coating speed of 2000 rpm showed a higher temperature increase compared to the heater fabricated with a spin coating speed of 4000 rpm. When the voltage applied to the heater fabricated at 2000 rpm was increased to 70 V, the temperature of the SWCNT film heater rose to 80.5 °C, which is higher than the temperature increase observed in the heater fabricated at 4000 rpm. It indicates that the temperature of the SWCNT film heater is highly sensitive to the applied voltage, and that the spin coating speed can also play a role in affecting the temperature of the SWCNT film heater. Figure 3b shows the temperature profiles of SWCNT film heater which is made by spin coating at a speed of 2000 rpm with 1 time coating. Regardless of the applied voltages, the maximum heating temperatures are reached within 60 s. It indicates that the SWCNT film heater has a rapid heating capability and can reach its maximum temperature quickly. In addition, the heating temperature of the SWCNT film heater fabricated at a 2000 rpm with one spin coating increases from 26.6 to 53.1 °C as the applied voltage increases from 10 to 50 V. Similarly, the heating temperature of the SWCNT film heater fabricated with 3000 rpm and three spin coatings increases significantly from 31.8 to 170 °C under the same applied voltage conditions, as shown in Figure 3c. This means that the number of spin coatings rather than the spin coating speed can have a significant effect on the heating temperature of the SWCNT film heater, and that the heating temperature can be controlled by adjusting the number of spin coatings along with the applied voltage. Figure 3d shows the heating temperature profiles over time of the SWCNT film heater that was fabricated by increasing the number of spin coatings from 1 to 3 at a constant spin coating speed of 3000 rpm. When a voltage of 50 V was applied, the heating temperature immediately increased to 170 °C within 60 s and maintained a constant temperature. In addition, when the applied bias was removed, the SWCNT film heater decreased from 170 to 25 °C within 60 s. It indicates that the SWCNT film heater has a fast response time with rise and recovery times less than 1 min. As a result, the fast response time and efficient control of the SWCNT film heater are believed to make them suitable for temperature control system in high sensitivity applications such as PD heating system. [34] Figure 4a, the operating temperature was measured using a Fourier transformation infrared thermal imaging camera on top of the ZnO UV PD. The dark current of the ZnO hybrid PD was measured to be 5.44 µA when a bias voltage of 1.0 V was applied to the ZnO UV PD at room temperature, with no voltage applied to the SWCNT film heater. When a voltage of 40 V was applied to the SWCNT film heater, the temperature of SWCNT heater was measured to be 112 °C, and the dark current of ZnO PD increased to 8.99 µA. It has been determined that an increase in temperature of the SWCNT film heater results in a corresponding increase in the dark current of the ZnO PDs. This is because higher temperature causes an increase in the carrier concentration and mobility of the ZnO film produced through the sol-gel method, which contributes to an increase in the dark current. [41] As a result, within this operating temperature range, the sol-gel-derived ZnO hybrid PD has a relatively low dark current, which is believed to result in reduced noise and improved performance. The photocurrent of the ZnO PDs (I photo = I UV -I dark ) can be determined by subtracting the dark current (I dark ) from the UV current (I UV ) in the presence of UV light. Figure 4b shows the photocurrents of the ZnO PD as the voltage applied to SWCNT film heater is changed from 0 to 40 V, with the ZnO PDs being exposed to UV light with a wavelength of 365 nm and a bias voltage of 1.0 V. At room temperature with no voltage applied to the SWCNT film heater, a photocurrent of 17.9 µA was measured. When a voltage of 40 V was applied to the SWCNT film heater, the temperature of the ZnO UV PD increased to 112 °C, and the photocurrent of the ZnO UV PDs increased to 42.7 µA as shown in the inset of Figure 4b. In addition, the operating temperature increases (25-112 °C) as the voltage of the SWCNT film heater increases, which further improves the photoresponsivity (18-42.7 mA W −1 ) and external quantum efficiency (4.2-18%) of the ZnO PDs, as shown in Figure 4c. These increases in photocurrent, photoresponsivity and external quantum efficiency are likely due to the increased temperature of the ZnO UV PD, which could have improved its photoresponse properties and its ability to generate photocurrent in response to UV light. In general, the UV detection mechanism of ZnO PDs can increase the absorption of oxygen atoms on the surface in the depletion region, which reduces the electrical conductivity. [42] The reduction in electrical conductivity can affect photoresponse of the ZnO PDs. However, when the ZnO PD is exposed to UV light, photogenerated carriers can participate in a reduction www.advmatinterfaces.de reaction with the adsorbed oxygen atoms, leading to their desorption and an increase in photocurrent. [43] This is consistent with the results in Figure S3 (Supporting Information), where the photoresponse was measured at atmospheric and vacuum conditions. Therefore, as the voltage applied to the SWCNT film heater increases, the temperature of the PD increases, and the increase in photocurrent indicates the enhanced electrical conductivity of the ZnO PDs. Figure 4d shows the photocurrent of the ZnO PD measured at 1.0 V as a function of the reciprocal of the temperature of the ZnO PD, obtained by applying a voltage of 0 to 40 V to the bottom-mounted SWCNT film heater. The photocurrent of the ZnO PD is directly proportional to the temperature of the SWCNT film heater, indicating that the heat generated by the bottom-mounted SWCNT film heater is effectively transferred to the ZnO PD, resulting in an improved photocurrent of ZnO PD. The activation energy of 92 meV for the increase in photocurrent was calculated using the Arrhenius equation (I photo = I o exp(−E a /kT)), where I photo , k, and T are the photocurrent, Boltzman constant and temperature of the ZnO PD, respectively. This activation energy is lower value than previously reported results. [44,45] It implies that, compared to a conventional external heater, the bottom-mounted SWCNT film heater being attached direct to the ZnO PD can transfer heat more efficiently, leading to a faster oxygen desorption and reduction reactions in hybrid ZnO PD. Figure 5a shows the time-dependent photocurrent of the ZnO PD with UV on and off as the operating temperature increases from 25 to 112 °C by applying voltage to the backside SWCNT film heater. It indicates that the photocurrent of ZnO PD increases, and the photocurrent saturation time decreases as the operating temperature increases by applying a voltage to the SWCNT film heater. It implies that the performance of ZnO PD is positively affected by the increase in temperature of the backside-mounted SWCNT film heater. Despite the positive effect of increasing the operation temperature on the photocurrent and its saturation time within 1000 s after UV exposure, the maximum photocurrent was obtained at the operating temperature of the SWCNT film heater at an operating temperature of 80 °C, rather than the highest operating temperature of 112 °C. It is believed that the decrease in photocurrent above 80 °C is due to thermal oxidation becoming dominant above this temperature. [18] The thermal oxidation process is thought to accelerate the desorption of surface oxygen, which negatively affects the photocurrent reduction of ZnO PD. Figure 5b illustrates the relationship between the saturation time for the maximum photocurrent of the ZnO PD and the reciprocal temperature of the SWCNT film heater. As the operating temperature of the

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SWCNTs film heater increases, the saturation time for the maximum photocurrent of the ZnO PD decreases. It means that the ZnO PD becomes more efficient at higher temperatures, reaching the maximum photocurrent faster. Using Arrhenius equation, the activation energy for the photocurrent saturation time of ZnO PDs is calculated to be 203 meV, which is lower than values obtained from conventional temperaturedependent measurement system. [18,46] It implies that the hybrid ZnO PDs with SWCNT film heater are more efficient in adding thermal energy to improve photocurrent, compared to conventional separated thermal heating system. Figure 5c shows the rise and recovery times of the hybrid ZnO PDs as a function of the operating temperature of the SWCNT film heater, which is controlled by applying a voltage for 0-40 V. The photoresponse rise time is defined as the time required to increase the photocurrent from 10% to 90% of the peak intensity, while the photo-recovery time is achieved by measuring the time required to decrease the photocurrent from 90% to 10% of its peak value. [47] As the operating temperature of the SWCNTs film heater increased from 25 to 112 °C, the photo-response rise time decreased from 76.6 to 13.9 s, and the photo-recovery time decreased from 812 to 245 s. It is believed that the faster photoresponse rise time and photo-recovery time are attributed to acceleration of the desorption and adsorption process of surface oxygen atoms caused by the increase in the operating temperature of SWCNT film heater.

Conclusion
High performance hybrid ZnO PDs embedded with SWCNT heating system were fabricated by sol-gel process and spin coating technique. A transparent (60-80%) SWCNTs film heater was formed on sapphire substate by spin coating process and could reach the maximum temperature of 170 °C by applying a voltage of 50 V. The operation temperature reached its maximum temperature within 1 min and quickly return to room temperature after the voltage was turned off. The operation temperature of the SWCNT film heater has a significant effect on the photocurrent, photoresponsivity, external quantum efficiency and photoresponse rise and fall time of ZnO UV PDs, with improvements observed at higher temperatures. It suggests that the performance of ZnO UV PDs can be improved by controlling the operation temperature using the embedded SWCNTs heating system fabricated through a simple spin coating process.

Experimental Section
Fabrication of SWCNT Film Heater: The sapphire substrates were cleaned using the conventional RCA cleaning method before depositing the SWCNT film using a spin coater. The process involved ultrasonically treating both substrates with acetone for 10 min, followed by isopropyl alcohol for 10 min, and deionized water for 10 min to remove organic impurities from the surface. Thereafter, an additional cleaning process was performed at 70 °C for 20 min with a washing solution in which ammonium hydroxide, hydrogen peroxide, and deionized water were mixed in a ratio of 1:1:5. This RCA cleaning process has the advantage of removing particle by changing the surface of the sapphire substrate from hydrophobic to hydrophilic. After surface cleaning, SWCNTs were coated on the sapphire substate for 30 s by controlling the number of coatings and RPM through spin coating. After changing the coating RPM to 2000, 3000, and 4000 rpm, the number of coatings was adjusted to 1, 2, and 3 times at 3000 rpm. Then, the solvent was evaporated through annealing at 200 °C and the surface adhesion of the SWCNTs was increased. After depositing a silver paste on the edge of the substrate, copper electrodes were bonded to fabricate the SWCNT heater.
The Fabrication of Sol-Gel ZnO-Based PDs with a CNT Heating System: The sol-gel-derived ZnO films were grown on c-plane sapphire substrate using spin coating process. Zinc acetate dihydrate (ZAD, C 4 H 6 O 4 Zn⋅2H 2 O) was used as the precursor for the sol-gel ZnO film. The sol-gel solution for the ZnO film was made using 2-methoxy ethanol (C 3 H 8 O 2 ) the solvent and monoethanolamine (MEA, C 2 H 7 NO) as the stabilizer. The mixed solution has a molar concentration of 0.1 m for both ZAD and MEA, and a 1:1 molar ratio of ZAD to MEA. The prepared mixture was stirred at 1000 rpm and 100 °C for 30 min, and after a little aging, a uniform sol solution was deposited on a c-plane sapphire substrate at 6000 rpm for 30 s using a spin coating method. The spin-coated ZnO thin film was pre-baked at 200 °C for 5 min to remove solvent and enhance adhesion, then post-baked at 800 °C for 1 h to recrystallize the ZnO thin film made from sol-gel method. A solgel-derived ZnO PD with a metal-semiconductor-metal (MSM) structure was fabricated by depositing Al/Au (50/10 nm) electrodes. A hybrid high efficiency ZnO PD system is a combination of sol-gel-derived ZnO PD and a SWCNT heater. The ZnO MSM PD was deposited on the front side of a c-plane sapphire substrate with a thermal conductivity of 25.2 W mK −1 , [48] while the SWCNT heater was placed on the back side of the substrate, as shown in Figure 1a.
Characterization of Sol-Gel-Derived Hybrid ZnO PDs with SWCNT Film Heaters: The surface structure and roughness of the SWCNT film heater and the sol-gel-derived ZnO films were analyzed using atomic force microscopy in non-contact mode. The optical transmittance and structural defects of SWCNTs films were analyzed using an UV-visible spectroscopy and Raman spectroscopy, respectively. The electrical and thermal properties of the SWCNT heater were measured using a Keithley 2400 source meter and a Fourier transformation infrared thermal imaging camera. I-V characteristics and UV photocurrent of sol-gel ZnO PDs with a SWCNT heating system were measured by an Agilent 4155A parameter analyzer. In addition, the UV light response of sol-gelderived ZnO PDs was measured using a Xenon lamp as an excitation light source.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.