Recently, copper oxide (CuO)-based visible-light photodetectors have attracted great interest due lớn their narrow bandgap (1.2 eV), low cost, và ease of fabrication. However, there has been insufficient theoretical analysis and study of CuO-based photodetectors, resulting in inferior performance in terms of responsivity, detectivity, and response speed. This work develops a method khổng lồ enhance the performance of CuO photodetectors by engineering a grain structure based on a newly-developed theoretical model. In the developed theoretical grain-structure model, the grain size and the connections between grains are considered because they can strongly affect the optoelectronic characteristics of CuO photodetectors. Based upon the proposed model, the engineered CuO device achieves enhanced optoelectronic performance. The engineered device shows high responsivity of 15.3 A/W và detectivity of 1.08 × 1011 Jones, which are 18 và 50 times better than those of the unoptimized device, and also shows fast rising & decaying response speeds of 0.682 s and 1.77 s, respectively. In addition, the proposed method is suitable for the mass-production of performance-enhanced, reliable photodetectors. By using a conventional semiconductor fabrication process, a photodetector-array is demonstrated on a 4-inch wafer. The fabricated devices show uniform, high, & stable optoelectronic performance for a month.
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Visible-light photodetectors are essential for various optoelectronic applications, such as imaging, environmental research, chemical analysis, và optical communications1,2,3. Conventionally, visible-light photodetectors have been demonstrated with hard film-type silicon (Si), which has low noise và high detectivity4. However, the conventional film-type đắm đuối photodetectors are undesirable for recently emerging flexible và transparent electronics và internet of things (IoT) applications, because of their bulky and brittle material properties and cost-inefficiency5. For these reasons, recent studies have been conducted khổng lồ develop simple & cost effective high-performance photodetectors based on material và structural approaches. For example, metal oxides6,7,8,9,10,11,12, transition metal dichalcogenides (TMD)3,5,13,14,15, và perovskites16,17,18,19,20, have been developed for the light-absorbing material of photodetectors lớn achieve high sensitivity và fast response speed. Also, mechanical flexibility & enhanced light absorption of sensing materials have been developed by introducing zero-dimensional (0-D) (quantum dots), 1-D (nanowires, nanotubes, nanorods), & 2-D (nanosheets, monolayers, nanofilms) nanostructures1,8,9,10,11,12,18,19,21,22,23,24,25,26.
Among various materials, copper oxide (CuO), is highly suitable for high-performance & cost-effective visible-light photodetectors because of its narrow band gap of 1.2 eV, abundance, & mechanical stability based on its atomic structures27,28. In particular, CuO has received considerable attention in both academic & industrial fields because of not only its versatile material properties, but also its fabrication compatibility with conventional semiconductor technology29,30,31,32,33,34,35,36,37. However, the research on CuO-based photodetectors is at an early stage, so there is still a lack of deep understanding. Previous studies on CuO-based photodetectors are about simple material quality34,35,37 and shape changes29,30,31,32,36; thus, rigorous study on the optoelectronic characteristics of CuO and significant performance enhancement in device-scale, such as responsivity, detectivity, and response time is necessary.
In this work, we investigated a method to demonstrate a performance-enhanced CuO photodetector supported by a newly-developed grain-structure model. We first theoretically developed a grain-structure model based on conventional photo-carrier generation/recombination & oxygen adsorption/desorption mechanisms. We also studied methods to lớn improve the optoelectronic characteristics of the CuO photodetector based on the developed grain-structure model. By controlling the grain size and contacting size between the CuO grains, the dark current and the photocurrent generation can be engineered, resulting in enhanced optoelectronic characteristics of CuO, such as responsivity, detectivity, and response time. We also experimentally confirmed that the optoelectronic characteristics of CuO photodetectors can be controlled by application of the proposed model. By fabricating various CuO nanofilms that have various grain sizes & contacting sizes of grains using a metallic copper (Cu) oxidation method, larger grain và smaller contacting-size-based CuO photodetectors were demonstrated; they successfully exhibited reduced dark current as well as enhanced photocurrent and response speed, which are highly consistent with the proposed grain-structure model. As a result, the film having the largest grain size and smallest neck kích cỡ successfully showed the most performance enhancement. Finally, we further confirmed that the proposed concept is highly suitable for industrial-level large-area fabrication. Using a conventional semiconductor process, the CuO photodetectors that showed the most performance enhancement were fabricated on a 4-inch wafer, and we confirmed their fabrication and performance reliability.
The metal-oxide-based photodetector is basically operated by the conventional photo-carrier generation/recombination và oxygen adsorption/desorption mechanisms30,38,39,40,41. Figure 1(a–c) shows the proposed model in detail. In case of CuO film, a p-type semiconductor, the grain can consist of an electrically resistive core & an electrically conductive layer formed by hole accumulation on the surface due lớn the adsorbed oxygen molecules in an air ambient (Fig. 1(a))30,42,43. As light illuminates, a photocurrent is generated abruptly by the large và fast generation of electron-hole pairs
Overview of the proposed CuO photodetector model. (a) Schematic illustration of a polycrystalline p-type CuO nanofilm photodetector and its cross-sectional view. (b) Typical time-dependent current curve of the photodetector under light illumination. (c) Cross-sectional schematic views of the photodetecting process in the CuO photodetector. Schematic mô tả tìm kiếm of the effect of (d) the grain kích cỡ and (e) the neck kích cỡ on a dark current.
To experimentally verify the proposed grain-structure model and khổng lồ demonstrate the performance-enhanced CuO photodetector, we fabricated various types of CuO nanofilm photodetectors. In particular, we employed the metallic Cu nanofilm oxidation method for formation of the CuO nanofilms, not only khổng lồ fabricate CuO grain structures with various shapes, from tens of nm lớn hundreds of nm, but also to lớn keep other chemical states và material properties the same (Fig. 2). Figure 2(a,b) schematically shows the overall fabrication of the CuO photodetectors & the process how to lớn control the grain structure, respectively. Fabrication starts with a silicon (Si) substrate on which a 1 μm-thick silicon dioxide (SiO2) layer is formed (Fig. 2(a,i)). Then, an 80 nm-thick Cu layer is deposited on the substrate by the conventional thermal physical vapor deposition (PVD) and lift-off methods (Fig. 2(a,ii,iii)). It should be noted that a strip-array configuration of Cu is patterned for the sensing film lớn alleviate the adhesion problem during the following high-temperature annealing process. The CuO nanofilm is formed & its grain structure is controlled through the thermal annealing process (Fig. 2(a,iv)). Conventionally, CuO is formed from Cu through oxidation at temperatures above 300 °C <2Cu + 1/2O2 → Cu2O, Cu2O + 1/2O2 → 2CuO>46. At such temperatures, the phase of Cu changes to lớn CuO without grain growth because of insufficient thermal energy, resulting in a nanocrystalline structure (Fig. 2(b,1,2)). However, as the temperature further increases, the CuO nanocrystalline structure begins lớn grow its grain size. Especially, this grain growth process is drastically accelerated when the annealing temperature goes more than half of the melting temperature of the material, & then, the grain form size can reach up to hundreds of nm from a few nm of the initial grain kích cỡ (Fig. 2(b,3)). The neck-size control, which is the other important parameter in the proposed grain structure model, is also controlled by adjustment of the annealing temperature. When the annealing temperature is sufficiently high, the material starts khổng lồ be wetting and form a granular CuO because of the surface tension40. Therefore, the granular CuO not only enhances its grain size, but also forms very small necks with adjacent grains (Fig. 2(b,4)). In our experiment, we obtained CuO nanofilms with various morphologies by annealing the Cu nanofilms at various temperatures of 400, 500, 600, & 700 °C for 1 hour in an air ambient with the heating rate of 10 °C/min & natural cooling. Finally, we formed 150 nm-thick gold (Au) electrodes on the oxidized CuO by the PVD và lift-off methods (Fig. 2(a,v,vi)). The fabricated CuO nanofilm photodetectors were investigated by optical microscope images (Fig. 2(c)). We confirmed that the device was successfully fabricated without any damage or adhesion problems, even after 700 °C high-temperature annealing.
Demonstration of the CuO nanofilm photodetector. (a) Fabrication process of the CuO photodetector. (b) Cross-sectional schematic illustration of the process of grain structure control according khổng lồ the increasing annealing temperature. (c) Photograph of the fabricated device annealed at 700 °C (left) và its magnified optical microscope images (middle, right).
The grain structures of the annealed CuO nanofilms were investigated by scanning electron microscope (SEM), and their top-surface and cross-sectional images are shown in the upper và lower panels in Fig. 3(a), respectively (See Method section). From the top-surface SEM image, the CuO grain kích thước of tens of nm is confirmed from the device with an annealing temperature of 400 °C (More information about top-surface SEM images and statistical histograms about grain-size variation in terms of the annealing temperature are provided in Supplementary Fig. S2). From the cross-sectional SEM images (lower panels in Fig. 3(a)), the film thickness is observed to be increased to 160 nm because of oxidation, within which CuO nanocrystals are stacked in multilayers. Increased grain size was obtained as the annealing temperature increased. As seen in the top-surface SEM images, the grain size significantly increased with increasing annealing temperature from 400 °C to lớn 700 °C. We measured the x- & y-axis directional form size of 200 units of grains in each specimen (Fig. 3(e)). The grain kích cỡ increased continuously as the annealing temperature increased, và the x-axial sizes for annealing temperatures of 400 °C và 700 °C were ~35 nm and ~236 nm, respectively. Cross-sectional SEM images also show the drastic grain structure changes with respect lớn the annealing temperature. While the CuO nanofilms formed at 400 °C và 500 °C consisted of multiple layers of small grains, the nanofilms annealed at temperatures above 600 °C were composed of nearly single layers of large grains. Notably, the film annealed at 700 °C showed a reduced neck size. Lớn ensure crystallinity in the single CuO grain, we performed transmission electron microscope (TEM) analysis of the CuO annealed at 700 °C (See Method section). As seen in the cross-sectional TEM image, the CuO film was composed of serially connected granular grains (Fig. 3(b,c)). Moreover, from high-resolution TEM (HRTEM) image (Fig. 3(d)) & diffraction patterns (Fig. 3(d) inset), we found that a single grain of CuO has high-quality single crystallinity. Lớn confirm that crystallinity & chemical states of the fabricated CuO nanofilms were kept, even though they were annealed at various temperatures, and to objectively analyze the optoelectronic properties of the fabricated devices using the proposed model, we performed X-ray photoelectron spectroscopy (XPS) (Fig. 3(f)) and X-ray diffraction (XRD) analysis (Fig. 3(g)). From the wide-scan spectra, we confirmed that the CuO nanofilm consisted of only Cu, O, and C atoms. The spectra for Cu 2p showed a Cu 2p3/2 peak at ~933 eV and satellites consistent with CuO XPS spectra, where there is no difference according to the annealing temperature47. Also, O1s peaks at 530 eV were observed in all samples no matter what the annealing temperature was, which were in agreement with the peaks of CuO48. The XRD analysis results showed that the crystalline chất lượng of the CuO nanofilm improved with increasing annealing temperature; however, all diffraction patterns showed high consistency with those of monoclinic CuO (PDF#00-041-0254) with no other phases (Fig. 3(g)). By the investigation of the CuO material properties, we confirmed that our fabrication method can control the grain structure precisely without any chemical disturbance.
Material property of the fabricated CuO nanofilm. (a) Surface SEM images (upper panels) và cross-sectional SEM images (lower panels) of the CuO nanofilms annealed at 400, 500, 600, and 700 °C, respectively. Insets of lower panels show cross-sectional schematics of the CuO nanofilms. (b) TEM image, (c) Magnified TEM image, và (d) HRTEM image of the CuO nanofilm annealed at 700 °C. (e) Measured grain sizes, (f) XPS spectra results (top: wide-scan spectrum, middle: spectra for Cu 2 p, bottom: spectra for O1 s), & (g) XRD results of the nanofilms annealed at various annealing temperatures.
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To characterize the electrical property of the fabricated photodetectors, the current-voltage (I–V) was measured in dark condition (Fig. 4(a)). The electrical measurement was performed using our customized probe station and semiconductor parameter analyzer (Keithley 2636B) at room temperature (See Method section). The gold (Au) electrical probes were contacted to Au pads of the fabricated device (inset in Fig. 4(a)). We confirmed that a lower dark current was achieved with a higher annealing temperature because of the increased volume fraction of the resistive vi xử lý core in the increased grain size. Using the proposed grain structure model, we calculated the electrical resistance of the fabricated CuO nanofilms và compared them with the experimental values (Fig. 4(b)) (See the Supplementary Information Table S1 for the information about all the calculated and measured results in detail). From the SEM images (Fig. 3(a)) và measured grain sizes (Fig. 3(e)), we modeled CuO nanofilms annealed at 400, 500, 600, & 700 °C into the simple electrical resistance system (calculations are described in detail in Fig. S3 & comparison of calculated và experimental values in Supplementary Information). We confirmed the calculated resistance increased with increasing annealing temperature, where the trend was in good agreement with that of the experimental results. Note that the lowest dark current was obtained from the CuO nanofilm annealed at 700 °C, which has the highest electrical resistance owing to lớn the combined effects of the largest grain kích cỡ and the reduced neck size. Based on the dark state result, we evaluated the optoelectronic properties of the photodetector annealed at 700 °C, which has the lowest dark current (the dark current of annealed CuO films at 400, 500, 600, & 700 °C was 774.5, 466.1, 233.7, & 79.05 μA, respectively, at 3 V) (Fig. 5(a,b)). Figure 5(a) shows the photocurrent (Iph = Ilight − Idark) as a function of the bias voltage under various intensities of white light ranging from 22.5 μW/cm2 to 2.92 mW/cm2. For the characterization of the proposed visible light photodetector, we used a commercial visible light halogen lamp (halogen-Fok-100W, Fiber Optic Korea Co., LTD.), which has a peak wavelength at 650 nm (The wavelength spectrum of the white light is provided in Supplementary Fig. S4). The Iph showed almost a linear relationship with the bias voltage & increased with higher light intensity. We also investigated the photosensitivity (defined as Iph/Idark × 100) under the condition of monotonic increase & decrease in light intensity (Fig. 5(b)). Although a slight hysteresis was observed, which can probably be attributed lớn the dark current that has not fully recovered lớn its initial state due khổng lồ the slow oxygen desorption reaction, the photodetector showed a stable & reversible photo-sensing property. To compare the performance of the photodetector annealed at 700 °C with other photodetectors annealed at 400, 500, và 600 °C, we measured the time-dependent photo-response under various light intensities at a bias voltage of 3 V (Fig. 5(c)). A larger photocurrent was observed with a higher annealing temperature (The Iph-V curves according to lớn the annealing temperature are shown in Supplementary Fig. S5). To lớn investigate the performance improvement according to the annealing temperature, we compared the responsivity and detectivity as a function of the light intensity at 3 V bias (Fig. 5(d,e)). The responsivity (R) represents the photocurrent per incident optical power, which is calculated as R = Iph/Pin S, where pin is the light intensity, & S is the illuminated area (Fig. 5(d))17. The increased Iph with higher annealing temperature contributed to lớn the increased R, which led to lớn the high responsivity from 15.3 khổng lồ 0.662 A/W obtained for the device annealed at 700 °C within the light intensity range of 0.0225 khổng lồ 2.92 mW/cm2. In particular, the responsivity of 15.3 A/W at the light intensity of 0.0225 mW/cm2 was 18 times higher than the value of 0.833 A/W obtained for the device annealed at 400 °C. In addition, another important figure of merit, the specific detectivity (D*) associated with the weakest detectable optical signal was obtained by calculating D* = R/(2eIdark/S)1/2 (Fig. 5(e))17. Because high detectivity is realized by high responsivity và low dark current, the photodetector annealed at 700 °C exhibited the highest detectivity, where the maximum value of 1.08 × 1011 Jones was 50 times higher than the value of 2.16 × 109 Jones obtained for the photodetector annealed at 400 °C at a light intensity of 0.0225 mW/cm2. The reduced responsivity và detectivity with the increased light intensity can be explained by the enhanced combination of excitons due khổng lồ the increased carrier concentration19. Figure 5(f) shows the response time obtained from the photo-switching result with the light-intensity of 2.92 mW/cm2. The rise time (τr) is defined as the time required to increase from 10% to lớn 90% of the maximum current after the light is turned on, & the decay time (τd) is defined as the time to decay from 90% khổng lồ 10% of the maximum current after the light is turned off40. In comparison to lớn the rise time and decay time of 42.3 s và 39.4 s, respectively, from the device annealed at 400 °C, we achieved greatly reduced rise time & decay time of 0.7 s and 1.8 s, respectively, from the device annealed at 700 °C (rise/decay time of device annealed at 400, 500, 600, and 700 °C was 42.3/39.4, 36.9/37.8, 15.4/22.8, & 0.7/1.8 s, respectively). These improvements according khổng lồ the annealing temperature can be explained by Fig. 5(g). As shown in Fig. 1(c,ii,iii), the photocurrent is generated by two mechanisms: photo-carrier generation at the resistive core và oxygen adsorption response at the hole accumulation layer. In the case of a small grain, the slow surface reaction is more dominant because of the large volume fraction of the shell, whereas for a large grain, fast và large photo-carrier generation is dominant due lớn the increased volume fraction of the core. As a result, a larger-grain-based CuO photodetector shows a larger photocurrent và faster response tốc độ than a smaller-grain-based photodetector, which results in improved performance. Meanwhile, as the neck size becomes smaller, there is an additional dark current suppression effect, which contributes lớn the performance enhancement (Fig. 1(e)). In this work, our CuO photodetector annealed at 700 °C showed high responsivity & detectivity as well as fast response time owing to lớn the large grain và small neck, which were superior lớn those of previously reported CuO film photodetectors, & even higher than some other types of CuO nanostructure (0D, 1D) photodetectors. We also studied about the relationship between responsivity và the device channel length, & we verified that the responsivity of the CuO device can be changed by the channel length because the carrier transit time, which determines the photo-current depends on the channel-length (See Supplementary Information Fig. S6)49. Furthermore, our film-based photodetector is more desirable for practical usage because of its higher process controllability and stability than other types of nanostructure based photodetectors (See Supplementary Table S2).