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Abstract
Self-powered photodetectors (SPDs) have great application potential in next-generation instruments that operate wirelessly and independently. Hence, there is a strong market demand for more efficient, easier, cheaper, and faster SPDs fabrication methods and tools. Specially, spray pyrolysis (SS) is an extensively used technique because of its simplicity, cost-effectiveness, and high deposition rate. Furthermore, in a SS system, parameters such as the chamber temperature, air pressure, substrate temperature, and nozzle-substrate distance must be precisely selected since they determine the deposited film quality. Otherwise, pressurized air blast spraying is a standard atomization method implemented in economical and multi-use micro airbrush guns that can be used in conjunction with a substrate heater to complete the SS process. In this study, a fabrication procedure, complemented with a micro airbrush gun as an easy, inexpensive, and efficient tool for film deposition was developed and implemented to obtain a single and a parallel connection of four SPDs based on cadmium chalcogenides (CC). A comprehensive analysis of the optical and electrical properties of the obtained devices confirms the functionality of the implemented fabrication procedure. All fabricated SPDs devices show responsivity and specific detectivity in the visible spectrum demonstrating their capability for self-powered photodetection applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photodetection devices (PDs) are essential components for sensing and signal processing in applications such as optical modulators [1–3], heterodyne mixers of coherent optical radiation [4], radio-on-fiber technology [5,6], biomedical sensing systems [7,8] surface plasmon resonant measurements [9], and optical scanning systems [10–12]. Worthy of note are PDs capable to work in photovoltaic mode generating voltage under light illumination with very low dark and noise currents. These photodiodes are known as self-powered photodetectors (SPDs) and enable photocurrent generation without an external power source [13]. SPDs have great application potential in next-generation instruments that operate wirelessly and independently [14,15]. PDs and SPDs have been fabricated using different material technologies. Although PDs fabricated using crystalline silicon (Si) have dominated the total market, Si is clearly not an ideal material for PDs because of its indirect band-gap [16]. Though compound materials from the group III–V such as gallium-arsenide (GaAs) and indium-phosphide (InP) with direct band-gap are ideal for photodetection applications, they are too expensive for large-scale commercial applications because of the high cost of the necessary precursors for the deposition systems themselves [17]. An alternative materials system is the uses of cadmium chalcogenides (CC) which are semiconducting materials comprising compounds of elements II and VI groups of the periodic table [18]. Among other materials, CC compounds have received substantial research interest because of the electrical, magnetic, optical, chemical, and physical properties that are unique in their bulk and nanostructure form [19–22]. Owing to their direct band-gap, high light sensitivity, and quantum efficiency, they are known for great characteristics for photodetecting applications. However, the dissimilar band-gap for individual materials restricts the flexibility of such applications. To circumvent this restriction, structural arrangements of CC are made to form heterostructures that improve the performance by offering great band-gap engineering opportunities. Currently, heterostructures such as cadmium-sulfide/cadmium-telluride (CdS/CdTe), cadmium-sulfide/cadmium-selenite (CdS/CdSe), and cadmium-selenite/cadmium-selenite-telluride (CdSe/ CdSeTe), fabricated by low-temperature deposition techniques have chemical structures where the internal and external surfaces are intrinsically well passivated and are characterized by a low recombination velocity for the excess carriers [23,24]. This property allows the use of these materials to build a multi-junction structure with a band-gap gradient aiding the collection of excess carriers and hence facilitating the development of optoelectronics and PDs. In addition, depending on the solubility, the type, and the cost-effectiveness of solution precursors, CC can be dissolved with nitrates, chlorides, and acetates in aqueous and alcoholic solvents prepared for various types of deposition techniques [25].
Heterojunctions of CC materials such as CdS, CdSe, CdTe, and CdSeTe have been obtained by deposition and synthesis techniques such as chemical bath deposition [26,27], electrochemical deposition [28], solvothermal deposition [29,30], and sol-gel deposition [31,32]. Although these heterojunctions are being obtained by different techniques, there is a strong market demand for more efficient, easier, cheaper, and faster deposition and synthesis methods. Spray pyrolysis is an extensively explored technique for this purpose because of its simplicity, cost-effectiveness, high deposition rate, and ability to deposit on broad surface areas [33–35]. In this process, thin films are deposited by spraying a solution on a heated substrate, where the constituents react to form a chemical compound [36]. Figure 1 shows the schematic diagram of spray pyrolysis deposition equipment. The main parts are the precursor in solution form and the atomizer for obtaining aerosol from the precursor solution. In addition, it consists of a thermally insulated chamber, a substrate holder, an electrical heater, and a temperature controller [37]. The principal parameter to be controlled is the substrate temperature because the drying of droplets, decomposition, crystallization, and grain growth depend strongly on this parameter [38]. In addition, adjusting the carrier gas flow rate, the opening of nozzle tip, and the nozzle-substrate distance offers the control on particle size of the product, which spotlights the suitability of this technique for thin film deposition [39].
By contrast, a micro airbrush is a small, air-operated tool that sprays a liquid solution on a surface by nebulization. Standard airbrushes use pressurized air blast spraying for atomization. Figure 2 shows an image of a commercial micro airbrush gun. Most of commercial micro airbrush guns are gravity-fed with dual action and internal mixing. Some micro airbrush guns are fed by suction or are side-fed, which allows the user to work at difficult angles such as pointing upward; however, this is not needed for face-up uniform single layer deposition. The dual action is activated by the double-action trigger, which allows the control of the amount of output air and solution independently. The spray pattern defined as the figure seen on the cross-sectional shape of the spray is established by the nozzle and needle size. Some commercial micro airbrushes come with a few nozzle and needle sets of different calibers; hence, the user can change the spray pattern size depending on the area to be deposited.
In this study, a fabrication procedure, complemented with a micro airbrush gun as an easy, inexpensive, and efficient tool for film deposition was developed and implemented to obtain over Indium tin oxide (ITO) coated gasses a single and a parallel connection of four SPDs based on CdS/CdSe, CdS/CdTe, and CdS/CdSe/CdSeTe/CdSe heterojunctions. The airbrush gun used employs a double-action pressurized air blast with a 0.35 mm nozzle to spray the solution on a heater substrate. A comprehensive analysis of the optical and electrical properties of the obtained devices confirmed the easy functionality of the developed fabrication procedure. Practical measurements in all fabricated devices demonstrated responsivity and specific detectivity for wavelengths between 300 and 800 nm, which demonstrate their effectiveness in self-powered photodetection applications. The best values of Voc of 0.76 V, Jsc of 14.9 mA/cm2, and PCE of 6.2% were obtained for a parallel connection of four CdS/CdSe/CdSeTe/CdSe heterojunction based SPDs.
2. Practical implementation
For practical implementation, two groups of SPDs were fabricated. The first group comprises the device arrangements Glass/ITO/CdS/CdTe/Ag, Glass/ITO/CdS/ CdSe/Ag, and Glass/ITO/CdS/CdSe/CdSe0.4Te0.6/CdSe/Ag, each arrangement fabricated on a single substrate. In the second group, on each substrate, the parallel connection of the four same SPD arrangement was fabricated. All device arrangements were fabricated using the procedure shown in Fig. 3. An Eclipse HP-CS pressurized air blast airbrush from IWATA was used to spray the semiconductor solutions. The main adjusted parameters are shown in Table 1. The temperature of the substrate was set at 160 °C in the heater plate, a 0.35 mm diameter nozzle was used, and a nozzle-substrate distance of 12 cm was established for the deposition.
Table 1. Technical parameters used in the deposition process
The CdS, CdSe, and CdTe powder of 99.99% purity were obtained from Sigma-Aldrich. To prepare the CdSe0.4Te0.6 powder, an alloy was prepared following a stoichiometric ratio reported previously [40]. Both the binary compounds of CdTe and CdSe were mixed, and no sintering was applied to the target because of the applied deposition method. 50 × 25 mm ITO coated glasses were obtained from TechInstro and used as substrates. Each of the coated glasses have a width of 1.1 mm and transparent ITO layer width of 200 nm, with 10 Ω of resistivity and transmittance > 90%. Before the deposition, the substrates were chemically and ultrasonically cleaned. For the deposition of the CdS layer, 1g of CdS, 0.8 g of ethylene glycol (C2H6O2), and 0.2 g of ethanol (C2H6O) were mixed to obtain a homogeneous solution, as shown in Fig. 4(a). The homogeneous solution was poured in the atomizer chamber [Fig. 4(b)] and the substrate was heated at 160 °C by 40 s. Subsequently, eight spraying-cycles were performed to cover the desired area on the heated substrate. The deposited film was cooled at room temperature for 24 h. The parallel connection of the four same SPD arrangement was fabricated by the deposition pattern establishing using a mask, as shown in Fig. 4(c) and 4(d).
Fig. 4. Pattern in the fabrication of the four same SPD arrangements on the same substrate; (a) manual mixing of reagents, (b) manual pouring of solution in the atomizer chamber, (c) mask to establish the deposition pattern, and (d) CdS deposited pattern.
To deposit the CdSe or CdTe layer, 0.5 g of CdSe or CdTe, 0.4 g of C2H6O2 and 0.2 g of ethanol C2H6O were mixed to obtain a homogeneous solution. This solution was poured in the atomizer chamber and ten spraying-cycles were performed on the desired area over the previously deposited CdS. Then, the deposited film was cooled at room temperature for 24 h. For the deposition of the ternary compound layer, 0.5 g of CdSe0.4Te0.6, 0.4 g of C2H6O2 and 0.2 g of ethanol C2H6O were mixed to obtain a homogeneous solution. Similar to the previous deposits, the homogeneous solution was poured in the atomizer chamber and three spraying-cycles were performed on the desired area. Furthermore, the deposited film was cooled at room temperature for 24 h. For the deposition of the Ag layer, liquid Ag obtained from Sigma-Aldrich was poured in the atomizer chamber and sprayed on the desired area. To characterize the electrical performance of the fabricated SPD arrangements, all devices were annealed in a vacuum at 100 °C for 30 min to improve the contact between the structure layers, reduce grain boundaries, and improve electron mobility.
Figure 5 shows the layer-by-layer fabrication of a SPD with the glass/ITO/CdS/CdTe/Ag arrangement, following the previously described procedure. The average thickness of all layers was obtained using a film thickness measurement instrument (Filmetrics F20-W). Figure 5(a) shows a schematic representation of the fabricated device. The layer of ITO in the coated glass having a thickness of 200 nm serves as the front contact; the layer of a high photoconductivity n-type CdS semiconductor having a thicknesses of 1.4 µm serves as a window material; the layer of a p-type CdTe semiconductor material having a thicknesses of 1.5 µm serves as the principal photon absorber; and the Ag layer having a thicknesses of 2 µm serves as the back contact. Figures 5(b), (c) and (d) show the coated glass with the deposited Cd, CdTe, and Ag, layers, respectively, whereas Figs. 5(e) and (f) show the wired process and the final device, respectively.
Fig. 5. Layer-by-layer fabrication of a SPD with arrangement Glass/ITO/CdS/CdTe/Ag; (a) schematic of the SPD, (b) deposited CdS, (c) deposited CdTe, (d) deposited Ag, (e) wire connection, (f) obtained SPD.
Figure (6) shows the layer-by-layer fabrication of the parallel connection of four SPDs with the arrangement Glass/ITO/CdS/CdTe/Ag. Figure 6(a) shows schematic representation of the multiple-device fabricated devices. Similar to the single fabricated SPD, the arrangement is composed of a coated ITO glass and four islands of CdS semiconductor layers, followed by a layer of CdTe and Ag, respectively. Figure 6(b), 6(c) and 6(d) show the CdS, CdTe and Ag deposited pattern respectively, Fig. 6(e) shows the wire connection, Figs. 6(f) and 6(h) show the back side and the front side of the four PDs arrangement, and Fig. 6(h) the completed four PD arrangement.
Fig. 6. Layer-by-layer fabrication of the parallel connection of four SPD with arrangement Glass/ITO/CdS/CdTe/Ag; (a) Schematic of the SPDs, (b) CdS deposited pattern, (c) CdTe deposited pattern, (d) Ag deposited pattern, (e) wire connection, (f) back side of the arrangement, (g) front side of the arrangement and (h) obtained four PD arrangement.
3. Results and discussions
Figure 7 shows the photocurrent density (Jph) versus the wavelength (λ) of the fabricated SPDs. The measurements were realized at illumination power Pin = 100 mW/cm2, the wavelengths were measured using a spectrometer SP215i from Princeton Instruments, and the optical power were measured using a PM100D from Thorlabs. Specifically, Fig. 7(a) shows the plot of Jph versus λ for the arrangements Glass/ITO/CdS/CdSe/Ag, Glass/ITO/CdS/CdTe/Ag and Glass/ITO/CdS/CdSe/CdSe0.4Te0.6/CdSe/Ag, each fabricated on a single substrate. Figure 7(b) shows the Jph versus λ plots of the parallel connection of the four same PD arrangements fabricated on single substrates. Figures 7(a) and (b) shows that the generated Jph of all fabricated PDs are given by wavelengths between 300 and 750 nm, which cover the visible spectrum. All fabricated devices have the maximum generated photocurrent between 350 nm and 600 nm, which demonstrates their capability in photovoltaic applications. In addition, the arrays of parallel connection of the four same SPDs generated almost three times the photocurrent of individual devices. This is attributed to the addition of photocurrent generated by each SPD in the parallel connection.
Fig. 7. Photocurrent density (Jph) versus the wavelength (λ) of the fabricated devices, (a) individual devices fabricated on a single substrate, and (b) parallel connection of four same SPDs fabricated on single substrates.
Figure 8 shows the photodiode responsivity R(λ) and Fig. 9 the specific detectivity D*(λ) for the PD arrangements each fabricated on a single substrate and the parallel connection of the same four PDs arrangements fabricated also in a single substrates. R(λ) and D(λ) were calculated using Eqs. (1) and (2), respectively [41,42].
Here, Jp = Ip/A, where Ip is the photocurrent and A the sensitivity area of the pn junction. k is the Boltzmann’s constant, T is the temperature, and rA is the resistance-area product of the photodetector, given by rA = (dJ(λ,V)/dV)−1, The Jph, R(λ), and D*(λ) are characterized within 300 - 900 nm. Figures 8 and 9 show the color- sensitive characteristics of all the fabricated PDs. A maximum responsivity (R) of 20.23 mA/W and maximum specific detectivity (D*) of 4.85×109 cmHz1/2/W of the single fabricated devices were achieved at wavelengths of 380 nm and 360 nm, respectively, while for the arrays of four PVDs, R and D* of 18.20 mA/W and 4.88×109 cmHz1/2/W, respectively, were achieved at wavelengths of 385 nm and 390 nm, respectively. In addition, Figs. 8 and 9 suggest a high sensitivity to the visible wavelength range, which allows absorbance required for photovoltaic applications.
Fig. 8. Responsivity R(λ) versus wavelength (λ) of the fabricated devices; (a) individual devices fabricated each on a single substrate and (b) parallel connection of four same SPD arrangements fabricated on single substrates.
Fig. 9. Specific detectivity D*(λ) versus wavelength (λ) of the fabricated devices; (a) individual devices fabricated each on a single substrate and (b) parallel connection of four same SPD arrangements fabricated on single substrates.
The J-V curves of the fabricated SPDs were obtained in a chamber under controlled environment at 25 °C using a Keithley source meter system. A light of a wavelength range between 280 and 800 nm obtained from a 100 mW/cm2 Xenon arc lamp with filters installed to approximate the AM1.5G spectrum was used as illumination source. Figure 10(a) shows the obtained J-V curves for the single SPDs, and Table 2 summarizes their performance characteristics. Similarly, Fig. 10(b) shows the obtained J-V curves for the parallel connection of the four same SPD arrangements fabricated on single substrates, and Table 3 summarizes their performance characteristics. Tables 2 and 3 indicate a significant increment in Jsc and PCE from the single devices to the parallel connection of four devices. The most significant increment of 8.4 mA/cm2 in Jsc and 2.15% in PCE are achieved by the arrangement Glass/ITO/CdS/CdSe/ CdSe0.4Te0.6/CdSe/Ag. In addition, no significant increment in Voc is observed from the single devices to the parallel connection of four devices. The increment in Jsc and the PCE of the arrays of four SPDs compared with individual SPDs is attributed to the increase in the generated photocurrent due to the parallel connection of four SPDs on the same substrate. As expected, the parallel connection of four devices maintained Voc but increased Jsc.
Fig. 10. J-V characteristics of the fabricated SPDs; (a) individual devices fabricated each on a single substrate and (b) parallel connection of four same SPD arrangements fabricated on single substrates.
Table 2. Electrical characterization of single SPDsa
Table 3. Electrical characterization of the array of four SPDsa
Figure 11 shows the optical absorbance profile for the wavelength 250-800 nm of the SPD arrangements Glass/CdS/CdSe/Ag, Glass/CdS/CdTe/Ag, and Glass/CdS/CdSe/CdSe0.4Te0.6/ CdSe, each fabricated on a single substrate. The optical absorbance profile of all arrangements is strong around the violet region of the spectra and gradually decreases in the visible range. Specially, the absorbance profile of the Glass/CdS/CdSe/CdSe0.4Te0.6/CdSe arrangement shows a bump due to the absorbance effect of the CdSe0.4Te0.6 layer in the QW. The absorbance bump is centred near 742 nm which is the effective transition wavelength (λeff) of the QW. The λeff can be theoretically obtained using the relation λeff = hc/Eeff where h is the Plank constant, c is the speed of light, Eeff = Eg3 + Ec1 +Ev1 is the effective bandgap of the QW, Eg3 (≈ 1.51 eV) is the bandgap of the CdSe0.4Te0.6 layer, and Ec1 (≈ 0.159 eV) and Ev1 (≈ 0.001 eV) are the first energy levels in the conduction and valence bands of the confined system respectively. Ec1 and Ev1 are estimated by solving the standard Schrödinger equation with effective mass and envelope function approximations for a CdSe0.4Te0.6 layer width of 75 nm [43,44].
Fig. 11. Absorbance profiles of the arrangements GlassITO//CdS/CdTe/Ag, Glass/ITO/CdS/CdSe/Ag and Glass/ITO/CdS/CdSe/CdSe0.4Te0.6/CdSe/Ag fabricated each on a single substrate.
An intrinsic loss mechanism that limits the generation of photocurrent in conventional CdS/CdSe and CdS/CdTe SPDs is governed by the inability of those structures to absorb energies less than the bandgap energy of the CdSe or CdTe materials. Incorporating a thin layer of CdSexTe1−x in the depletion region of the SPD structure to form a CdS/CdSe/CdSexTe1-x/CdSe QW arrangement provides a means of adjusting the photon absorption edge below that of a conventional structure. Tuning the molar fraction x and the thickness of CdSexTe1-x layers is the principal mechanism to precisely adjust the position of the photon absorption edge. Since R(λ) and D*(λ) depends in Jp [Eqs. (1) and (2)] their improvement in the SPD with QW arrangement is due to the increases in Jp given by the introduction of the QW. In addition, the improvement of Jsc in the QW arrangement is also attributed to the increases in Jp given by the introduction of the QW and the crystallinity reached by materials in the annealing process during fabrication.
The optical bandgap energy (Bg) of all single SPD was determined using Fig. 12 [45,46]. Figure 12 plots the absorption spectrum (αhv)2 versus photon energy of all single fabricated SPD. Specifically, the plot of the Glass/CdS/CdSe/CdSe0.4Te0.6/CdSe arrangement shows a first absorption transition at the bandgap of 1.74 eV (0.713 µm) which is the optical bandgap of the CdSe, the first material absorbing light. In addition, the plot of the same arrangement shows a second absorption transition at the bandgap of 1.67 eV (0.742 µm) which is the effective optical bandgap, Eeff, of the CdSe/CdSe0.4Te0.6/CdSe of QW.
Fig. 12. Absorption spectrum (αhv)2 versus photon energy hv of the arrangements Glass/ITO/CdS/CdTe/Ag, Glass/ITO/CdS/CdSe/Ag, and Glass/ITO/CdS/CdSe/CdSe0.4Te0.6/CdSe/Ag fabricated each on a single substrate.
Figure 13 shows the incident light irradiance (I) versus the photocurrent density (Iph) of the individual SPD device and the parallel connection of four same SPD devices with arrangement Glass/ITO/CdS/CdSe/CdSe0.4Te0.6/CdSe/Ag. To obtain the plots of Fig. 13 a variable neutral density filter was used to attenuate I from the illumination source. Both plots show a very good linear relationship between the output Iph of photodiode arrangements and I. The difference between the measured values of Iph for each tested I and the corresponding value in the ideal line is smaller than 0.4 mA/cm2.
Fig. 13. Incident light irradiance I versus photocurrent dencity Iph of the individual SPD device and the parallel connection of four same SPD devices with arrangement Glass/ITO/CdS/CdSe/CdSe0.4Te0.6/CdSe/Ag.
A SPD achieves light detection and low dark currents in the absence of external voltage. Hence, beside the photoresponsivity and D* its performance can also be judged by the light on/off ratio (S) [47,48]. The S is the photoelectric conversion performance evaluated by the ratio of photocurrent (Ip = Ion − Ioff) to dark current (Ioff): S = (Ion − Ioff)/(Ioff).
Furthermore, Table 4 summarizes the performance of the CdS/CdSe, CdS/CdTe and CdSe/CdSe0.4Te0.6/CdSe based SPDs fabricated in this work and other reported materials-based SPDs. Most of the listed SPDs are used from ultraviolet (Uv) to visible (Vis) or to near infrared (Nir) region of the spectrum. The reported D* and S range from 2.0×1011 to 1.93×1013 cmHz1/2W−1 and 6.0×102 to 2.2×107 respectively [49–59], while the highest D* and S achieved in this work are 4.8×109 cmHz1/2W−1 and 3.0×104 respectively.
Table 4. Summary of characteristics performance of the CdS/CdSe, CdS/CdTe and CdSe/CdSe0.4Te0.6/CdSe fabricated SPDs and other reported materials-based SPDs.
Beside that the fabricated SPDs show better self-powered photovoltaic performance since they generate more Voc and Isc than the other reported SPDs. Accordingly, the fabricated SPDs can reach PCE > 3.5 with a balance in all performance parameter. In addition, it has been demonstrated that the CdSe/CdSe0.4Te0.6/CdSe based SPD has the highest photovoltaic self-powered performance due to the passivation of the thin CdSe0.4Te0.6 layer in the implemented QW.
4. Conclusions
In this study, a fabrication procedure, complemented with a micro airbrush gun as an easy, inexpensive, and efficient instrumentation tool for film deposition, was used to obtain two groups of SPDs. The fabricated SPDs were based on CC compounds. The first group of SPDs comprised the arrangements Glass/ITO/CdS/CdTe/Ag, Glass/ITO/CdS/ CdSe/Ag, and Glass/ITO/CdS/CdSe/CdSe0.4Te0.6/CdSe/Ag, where each arrangement was fabricated on a single substrate.
An analysis of the optical and electrical properties of the obtained devices confirmed the easy functionality of the proposed deposition instrumentation tool and fabrication procedure. Practical measurements in all fabricated devices demonstrated responsivity and specific detectivity for wavelengths between 300 nm and 800 nm which demonstrates their capability for photodetection applications. The proposed fabrication procedure and instrumentation tool for film deposition are suitable for multi-layer preparation, which is appealing for fabricating functionally graded layers, where the film composition can be adjusted by varying the starting solutions. In addition, significant limitations are the low flexibility to scale-up the system for large number of SPDs integration in the same substrate, sulfides can suffer oxidation when processed in air atmosphere and there are difficulties determining the optimal growth temperature.
Funding
Mexican Council for Science and Technology (CONACYT); Program for Professional Development of Teachers (PRODEP);.
Acknowledgments
The Author would like to thank the graduate students Vianey A. Candelas, Monserrat Vargas, Carlos Tamayo and Daniel O. Baez from the Applied Physics lab. of the Engineering Institute at Autonomous University of Baja California for allowing the use of equipment and facilities for the development of the present study.
Disclosures
The author declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. V. J. Sorger and R. Maiti, “Roadmap for gain-bandwidth-product enhanced photodetetctors: opinion,” Opt. Mater. Express 10(9), 2192–2200 (2020). [CrossRef]
2. H. D. Jahromi, A. Binaie, M. H. Sheikhi, A. Zarifkar, and H. Nadgaran, “Response of colloidal quantum dot infrared photodetectors to modulated optical signals,” IEEE Sens. J. 15(6), 3274–3280 (2015). [CrossRef]
3. R. Mesleh and A. AL-Olaimat, “Acousto-optical modulators for free space optical wireless communication systems,” J. Opt. Commun. Netw. 10(5), 515–522 (2018). [CrossRef]
4. F. L. Constantin, “Phase-coherent heterodyne detection in the terahertz regime with a photomixer,” IEEE J. Quantum Electron. 47(11), 1458–1462 (2011). [CrossRef]
5. J. Yu, Z. Jia, T. Wang, and G. Chang, “A novel radio-over-fiber configuration using optical phase modulator to generate an optical mm-wave and centralized lightwave for uplink connection,” IEEE Photonics Technol. Lett. 19(3), 140–142 (2007). [CrossRef]
6. R. C. Williamson and R. D. Esman, “RF Photonics,” J. Lightwave Technol. 26(9), 1145–1153 (2008). [CrossRef]
7. H.-Y. Tsai, F.-C. Su, C.-H. Chou, Y.-H. Lin, K.-C. Huang, Y.-J. J. Yang, L.-W. Kuo, L.-D. Liao, and H.-S. Yu, “Wearable inverse light-emitting diode sensor for measuring light intensity at specific wavelengths in light therapy,” IEEE Trans. Instrum. Meas. 68(5), 1561–1574 (2019). [CrossRef]
8. M. Norgia, A. Magnani, D. Melchionni, and A. Pesatori, “Drop measurement system for biomedical application,” IEEE Trans. Instrum. Meas. 64(9), 2513–2517 (2015). [CrossRef]
9. J. D. Hwang, F. H. Wang, C. Y. Kung, and M. C. Chan, “Using the surface plasmon resonance of au nanoparticles to enhance ultraviolet response of zno nanorods-based Schottky-Barrier photodetectors,” IEEE Trans. Nanotechnol. 14(2), 318–321 (2015). [CrossRef]
10. W. Flores-Fuentes, M. Rivas-Lopez, O. Sergiyenko, J. C. Rodríguez-Quiñonez, D. Hernández-Balbuena, and J. Rivera-Castillo, “Energy center detection in light scanning sensors for structural health monitoring accuracy enhancement,” IEEE Sens. J. 14(7), 2355–2361 (2014). [CrossRef]
11. W. Flores-Fuentes, J. E. Miranda-Vega, M. Rivas-Lopez, O. Sergiyenko, J. C. Rodríguez-Quiñonez, and L. Lindner, “Comparison between different types of sensors used in the real operational environment based on optical scanning system,” Sensors 18(6), 1684–1699 (2018). [CrossRef]
12. C. Villa-Angulo, “CdSe0.4Te0.6 Quantum well based photodetector toward imaging vision sensors,” IEEE Sens. J. 20(22), 13357–13363 (2020). [CrossRef]
13. H. Schneider, C. Schönbein, M. Walther, K. Schwarz, J. Fleissner, and P. Koidl, “Photovoltaic quantum well infrared photodetectors: The fourzone scheme,” Appl. Phys. Lett. 71(2), 246–248 (1997). [CrossRef]
14. Y. Luo, Z. Dong, Y. Chen, Y. Zhang, Y. Lu, T. Xia, L. Wang, S. Li, W. Zhang, W. Xiang, C. Shan, and H. Guo, “Self-powered NiO@ZnO-nanowire-heterojunction ultraviolet micro-photodetectors,” Opt. Mater. Express 9(7), 2775–2784 (2019). [CrossRef]
15. N. Youngblood, C. Chen, S. Koester, and M. Li, “Waveguide-integrated black phosphorus photodetector with high responsivity and low dark current,” Nat. Photonics 9(4), 247–252 (2015). [CrossRef]
16. J. Tauc, “Optical properties and electronic structure of amorphous Ge and Si,” Mater. Res. Bull. 3(1), 37–46 (1968). [CrossRef]
17. P. Yu and M. Cardona, Fundamentals of Semiconductors (Springer, 1996), p. 566.
18. A. Majid and M. Bibi, Cadmium based II-VI Semiconducting Nanomaterials (Springer International Publishing AG, 2018).
19. W. W. Yu, L. Qu, W. Guo, and X. Peng, “Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS NCs,” Chem. Mater. 15(14), 2854–2860 (2003). [CrossRef]
20. L. Zhu, C. Li, Y. Li, C. Feng, F. Li, D. Zhang, Z. Chen, S. Wen, and S. Ruan, “Visible-light photodetector with enhanced performance based on a ZnO@ CdS heterostructure,” J. Mater. Chem. C 3(10), 2231–2236 (2015). [CrossRef]
21. Y. Zhang, L. W. Wang, and A. Mascarenhas, “Quantum coaxial cables for solar energy harvesting,” Nano Lett. 7(5), 1264–1269 (2007). [CrossRef]
22. Y. Heng, L. Jingobo, A. L. Richard, W. Lin-Wang, and E. B. William, “Two-versus three-dimensional quantum confinement in indium phosphide wires and dots,” Nat. Mater. 2(8), 517–520 (2003). [CrossRef]
23. D. Esparza, T. Lopez-Luke, J. Oliva, A. A. Cerdán-Pasarán, A. Martinez-Benitez, I. Mora-Sero, and E. De la Rosa, “Enhancement of efficiency in quatum dot sensitized solar cells based on CdS/CdSe/CdSeTe heteorestructures by improving the light absorption in the VIS-NIR region,” Electrochim. Acta 247, 899–909 (2017). [CrossRef]
24. X. Zheng, D. Kuciauskas, J. Moseley, E. Colegrove, D. S. Albin, H. Moutinho, J. N. Duenow, T. Ablekim, S. P. Harvey, A. Ferguson, and W. K. Metzger, “Recombination and bandgap engineering in CdSeTe/CdTe solar cell,” APL Mater. 7(7), 071112 (2019). [CrossRef]
25. L. Mädler, “Liquid-fed aerosol reactors for one-step synthesis of nano-structured particles,” KONA Powder Part. J. 22(0), 107–120 (2004). [CrossRef]
26. C. A. VanderHyde, S. D. Sartale, J. M. Patil, K. P. Ghoderao, J. P. Sawant, and R. B. Kale, “Room temperature chemical bath deposition of cadmium selenide, cadmium sulfide and cadmium sulfoselenide thin films with novel nanostructures,” Solid State Sci. 48, 186–192 (2015). [CrossRef]
27. Y. Tian, Y. Zhang, Y. Lin, K. Gao, Y. Zhang, K. Liu, Q. Yang, X. Zhou, D. Qin, H. Wu, Y. Xia, L. Hou, L. Lan, J. Chen, D. Wang, and R. Yao, “Solution-processed efficient CdTe nanocrystal/CBD-CdS hetero-junction solar cells with ZnO interlayer,” J. Nanopart. Res. 15(11), 2053 (2013). [CrossRef]
28. S. S. Fomanyuk, V. N. Asaula, G. Y. Kolbasov, and T. A. Mirnaya, “Electrochemical synthesis and optical properties of ultra-fine CdSe nanoparticles,” J. Nanostruct. Chem. 6(4), 289–297 (2016). [CrossRef]
29. Y. Guo, J. Wang, L. Yang, J. Zhang, K. Jiang, W. Li, and L. Jiang, “Facile additive-free solvothermal synthesis of cadmium sulfide flower-like three dimensional assemblies with unique optical properties and photocatalytic activity,” CrystEngComm 13(16), 5045–5048 (2011). [CrossRef]
30. Q. Wang, D. Pan, S. Jiang, X. Ji, L. An, and B. Jiang, “A solvothermal route to size-and shape-controlled CdSe and CdTe NCs,” J. Cryst. Growth 286(1), 83–90 (2006). [CrossRef]
31. C. Li and N. Murase, “Synthesis of highly luminescent glasses incorporating CdTe NCs through Sol–Gel processing,” Langmuir 20(1), 1–4 (2004). [CrossRef]
32. M. Nogami, K. Nagasaka, and T. Suzuki, “Sol-gel synthesis of cadmium telluride-microcrystal-doped silica glasses,” J. Am. Ceram. Soc. 75(1), 220–223 (1992). [CrossRef]
33. M. G. Faraj, M. Z. Pakhuruddin, and P. Taboada, “Structural and optical properties of cadmium sulfide thin films on flexible polymer substrates by chemical spray pyrolysis technique,” J. Mater. Sci.: Mater. Electron. 28(9), 6628–6634 (2017). [CrossRef]
34. B. Malaman, R. Bagtache, and K. Abdmeziem, “Synthesis and structure of Cu3PO4[1,2,4-triazole]2OH with a hybrid layered structure: A new organically templated copper (II) hydroxyphosphate,” Solid State Sci. 12(7), 1178–1182 (2010). [CrossRef]
35. S. Tewari and A. Bhattacharjee, “Synthesis and characterization of Cadmium Chalcogenide CdX (X = S, Te) thin films,” Int. J. Chem. Sci. 7(1), 105–115 (2009).
36. J. B. Mooney and S. B. Radding, “Spray pyrolysis processing,” Annu. Rev. Mater. Sci. 12(1), 81–101 (1982). [CrossRef]
37. S. Kozhukharov and S. Tchaoushev, “Spray pyrolysis equipment for various applications,” Journal of Chemical Technology and Metallurgy 48(1), 111–118 (2013).
38. L. Filipovic, S. Selberherr, G. C. Mutinati, E. Brunet, S. Steinhauer, A. Köck, J. Teva, J. Kraft, J. Siegert, and F. Schrank, “Methods of simulating thin film deposition using spray pyrolysis Techniques,” Microelectron. Eng. 117, 57–66 (2014). [CrossRef]
39. O. J. Ilegbusi, S. M. N. Khatami, and L. I. Trakhtenberg, “Spray pyrolysis deposition of single and mixed oxide thin films,” Mater. Sci. Appl. 08(02), 153–169 (2017). [CrossRef]
40. L. Kumar, B. P. Singh, A. Misra, S. C. K. Misra, and T. P. Sharma, “Characterization of CdSexTe1-x sintered films for photovoltaic applications,” Phys. B 363(1-4), 102–109 (2005). [CrossRef]
41. Y. Kumar, H. Kumar, G. Rawat, C. Kumar, A. Sharma, B. N. Pal, and S. Jit, “Colloidal ZnO quantum dots based spectrum selective ultraviolet photodetectors,” IEEE Photonics Technol. Lett. 29(4), 361–364 (2017). [CrossRef]
42. H. Kumar, Y. Kumar, B. Mukherjee, G. Rawat, C. Kumar, B. N. Pal, and S. Jit, “Electrical and Optical Characteristics of Self-Powered Colloidal CdSe Quantum Dot-Based Photodiode,” IEEE J. Quantum Electron. 53(3), 1–8 (2017). [CrossRef]
43. J. R. Villa-Angulo, R. Villa-Angulo, and C. Villa-Angulo, “Photon absorption coefficient (α) for a low-dimensional CdS/CdTe absorber by a partial k-selection approach,” J. Nanophotonics 7(1), 073099 (2013). [CrossRef]
44. J. R. Villa-Angulo, R. Villa-Angulo, K. Solorio-Ferrales, S. E. Ahumada-Valdez, and C. Villa-Angulo, “Effect of effective mass mismatch in CdS/CdTe heterojunctions on the fundamental design parameters of nanophotonic devices,” J. Nanophotonics 8(1), 083096 (2014). [CrossRef]
45. A. R. Zanatta, “Revisiting the optical bandgap of semiconductors and the proposal of a unified methodology to its determination,” Sci. Rep. 9(1), 11225 (2019). [CrossRef]
46. A. Z. Johannes, R. K. Pingak, and M. Bukit, “Tauc plot software: calculating energy gap values of organic materials based on Ultraviolet-Visible absorbance spectrum,” IOP Conf. Ser.: Mater. Sci. Eng. 823, 012030 (2020). [CrossRef]
47. S. C. Dhanabalan, J. S. Ponraj, H. Zhang, and Q. Bao, “Present perspectives of broadband photodetectors based on nanobelts, nanoribbons, nanosheets and the emerging 2D materials,” Nanoscale 8(12), 6410–6434 (2016). [CrossRef]
48. W. Zhang, C. Chih-Piao, H. Jing-Kai, C. Chang-Hsiao, T. Meng-Lin, C. Yung-Huang, L. Chi-Te, C. Yu-Ze, C. Yu-Lun, H. Jr-Hau, C. Mei-Yin, and L. Lain-Jong, “Ultrahigh-gain photodetectors based on atomically thin graphene-MoS2 heterostructures,” Sci. Rep. 4(1), 3826 (2015). [CrossRef]
49. P. Dharmaraj, G. Paulraj, V. Purushothamn, H. Jr-Hau, and K. Jeganathan, “High performance, self-powered photodetectors based on a graphene/silicon Schottky junction diode,” J. Mater. Chem. C 6(35), 9545–9551 (2018). [CrossRef]
50. R. Md, F. Abu, D. Toan, N. T. Viet, T. Philip, P. Hoang-Phuong, N. Tuan-Khoa, H. Ben, W. S. Erik, L. Mirco, and V. D. Dzung, “Self-powered broadband (UV-NIR) photodetector based on 3C-SiC/Si heterojunction,” IEEE Trans. Electron Devices 66(4), 1804–1809 (2019). [CrossRef]
51. J. Jiansheng, S. Zhibin, Z. Qing, Z. Xiaohong, W. Yuming, S. Zheng, and L. Shuit-Tong, “MoS2/Si heterojunction with vertically standing layered structure for ultrafast, high-detectivity, self-driven visible–near infrared photodetectors,” Adv. Funct. Mater. 25(19), 2910–2919 (2015). [CrossRef]
52. L. Lin-Bao, H. Han, W. Xian-He, L. Rui, Z. Yi-Feng, Y. Yong-Qiang, and L. Feng-Xia, “A graphene/GaAs near-infrared photodetector enabled by interfacial passivation with fast response and high sensitivity,” J. Mater. Chem. C 3(18), 4723–4728 (2015). [CrossRef]
53. J. Cheng, W. Di, W. Enping, G. Jiawen, Z. Zhihui, S. Zhifeng, X. Tingting, H. Xiaowen, T. Yongtao, and L. Xinjian, “A self-powered high-performance photodetector based on a MoS2/GaAs heterojunction with high polarization sensitivity,” J. Mater. Chem. C 7(13), 3817–3821 (2019). [CrossRef]
54. Z. Zheng, J. Yao, B. Wang, Y. Yang, G. Yang, and J. Li, “Self-assembly high-performance UV–vis–NIR broadband β-In2Se3/Si photodetector array for weak signal detection,” ACS Appl. Mater. Interfaces 9(50), 43830–43837 (2017). [CrossRef]
55. D. Mingjin, C. Hongyu, W. Fakun, L. Mingsheng, S. Huiming, H. Yunxia, L. Wen, G. Chuanyang, Z. Jia, J. Zhang, Z. Tianyou, F. Yongqing, and H. PingAn, “Ultrafast and sensitive self-powered photodetector featuring self-limited depletion region and fully depleted channel with van der Waals contacts,” ACS Nano 14(7), 9098–9106 (2020). [CrossRef]
56. M. C. Aru, C. Greeshma, P. Rohit, R. Basanta, K. S. Deependra, K. K. Nanda, and S. B. Krupanidhi, “Self-powered, broad band, and ultrafast InGaN-Based photodetector,” ACS Appl. Mater. Interfaces 11(10), 10418–10425 (2019). [CrossRef]
57. L. Qin, Z. Xuanting, W. Xudong, B. Wei, Y. Jing, Z. Yuanyuan, Q. Ruijuan, H. Rong, H. Weida, T. Xiaodong, W. Jianlu, and C. Junhao, “Ultrahigh-detectivity photodetectors with Van der Waals epitaxial CdTe single-crystalline films,” Small 15(17), 1900236 (2019). [CrossRef]
58. P. Zhiqiang, Z. Jiyue, L. Qin, B. Wei, Y. Jing, Z. Yuanyuan, Q. Ruijuan, H. Rong, T. Xiaodong, and C. Junhao, “The dependence of optoelectronic performance on channel length in photodetectors with MBE CdTe single-crystalline thin films on perovskite SrTiO3,” Opt. Mater. 109, 110276 (2020). [CrossRef]
59. L. Qin, Z. Lisa, Z. Jiyue, W. Hongzhu, B. Wei, Y. Jing, Z. Yuanyuan, Q. Ruijuan, H. Rong, T. Xiaodong, W. Jianlu, and C. Junhao, “Nanometer-thick metastable zinc blended γ-MnTe Single-crystalline films for high-performance ultraviolet and broadband photodetectors,” ACS Appl. Nano Mater. 3(12), 12046–12054 (2020). [CrossRef]
