Abstract
Photodetectors based on one-dimensional structures have recently attracted great interest due to their high surface-to-volume ratio and light-trapping efficiency. In this study, a self-powered photodetector based on vertically aligned WO3 nanorod with SiO2 as an interfacial layer has been fabricated using glancing angle deposition. Scanning electron microscope (SEM) analysis confirms the successful growth of WO3 nanorod and SiO2 thin film with a thickness of ~125 nm and ~70 nm, respectively. The device’s optical properties were also analysed using UV-visible spectroscopy, which revealed a wide bandgap and an intense absorption peak in the ultraviolet region. The electrical analysis showed a nonlinear rectifying current-voltage behaviour with high photosensitivity. Additionally, the photodetector exhibits a fast response of 0.31 s rise time and 0.32 s fall time at 0 V. Moreover, the devices possess a high detectivity and a responsivity of 0.68 mA/W. Thus, the obtained finding reveals a potential candidate for self-powered photodetector application.
1. Introduction
In recent years, metal oxide one-dimensional (1D) nanostructures, including nanowires (NW) and nanorods (NR), have gained significant attention in the field of electronics and optoelectronics due to their unique combination of electrical, optical, and structural properties, such as a high surface-to-volume ratio and tunable electrical and optical properties [1–3]. These properties make 1D structures suitable for various applications such as energy conversion and storage, sensing, and optoelectronics [3–6]. 1D self-powered devices using various techniques and materials, such as V2O5 [7–9] and InN [10, 11], are already reported. Among the commonly used metal oxide such as TiO2, ZnO, NiO2, Gd2O3, and Ga2O3 [12–17] for the study of optoelectronics, tungsten oxide (WO3) is a promising candidate for the construction of high-performance ultraviolet (UV) photodetectors due to its high bandgap with good chemical and physical properties [18]. Among the wide variety of photodetector structures reported, a metal semiconduction (MS) junction is widely used due to its ease of preparation. Studies have shown that an MS-based photodetector is an effective method to produce a comparable performance that facilitates photogenerated electron-hole at the interface of the junction [19, 20]. However, the MS-based photodetector structure also faced a problem of slow responses due to its photoconductive effect [21]. In recent years, numerous researchers have focused on improving the performance of devices based on WO3 nanostructures. For instance, a study on WO3 NW by Rajkumari and Singh performed annealing to reduce the device defect and improve the device response time (rise time/fall time) from 1.78 s/1.09 s to 1.33 s/0.94 s [18]. Another study by Rajkumari et al. showed improved photoresponse (rise time/fall time) of 0.84 s/0.78 s after incorporating Ag nanoparticles on top of WO3 NW [22]. In order to enhance the device’s performance further, an updated model of metal semiconduction with an interfacial layer is introduced. The introduction of the interfacial layer strongly impacts the device’s photoconductive characteristics. Studies have shown that adding an interfacial layer enhances the response speed of the photodetector [23]. Among the existing materials for the use of interfacial layer, SiO2 is a highly suitable material due to its low cost, ease of manufacture, and high chemical stability [24].
In this study, our primary objective is to investigate the effects of inserting a SiO2 interfacial layer between n-Silicon (Si) and WO3 NR in a hybrid structure (HS) UV photodetector. We aim to comprehensively analyse how this interfacial layer influences the photoconductive properties of the photodetector, specifically focusing on its impact on response speed and overall efficiency. By fabricating the HS using glancing angle deposition (GLAD), we intend to uncover insights into the performance enhancement achievable through configuration. GLAD, known for its precision in controlling nanostructured film growth, allows us to tailor the morphology and properties of the WO3 NR layer in our hybrid structure. Ultimately, this research is aimed at contributing to advancing photodetector technology by paving the way for developing high-performance devices with enhanced response capabilities for various applications. Further, Keithley 4200 scs and monochromator (New Port TLS-300XU) were used to analyse the device’s current (I)–voltage (V) characteristics. Finally, our fabricated HS photodetector showed enhanced photosensitivity and fast response with a self-powered ability.
2. Experimental Details
The WO3/SiO2 HS was prepared using n-type silicon (Si) substrate (resistivity of 1-10 ohm-cm with μm thickness, Ultra nanotech) with an electron beam (e-beam) evaporation technique and a glancing angle deposition technique. Before the SiO2 thin film (TF) deposition, the Si substrate was ultrasonically cleaned (MTI, KJ Group) with acetone, methanol, and deionized water for 3 min. After that, a SiO2 (99.99% purity, Ultra nanotech) TF of 70 nm was deposited on top of the Si substrate. For the deposition of WO3 NR, the substrate was kept at 85° normal to the crucible containing WO3 granule of 99.99% purity (Ultra nanotech). The deposition was carried out with a base pressure of mbar, and the substrate was azimuthally rotated at 30 rpm with a constant deposition rate of 1 Å/s. Figure 1 illustrates the schematic representation of the GLAD model. It comprises two motors: the ϕ motor, responsible for executing azimuthal rotations, and the θ motor, regulating the angle formed between the substrate’s normal and the incident flux. By employing these motors, alterations to the structural configuration can be achieved. For thin film deposition, the angle θ is kept at 0°. Afterward, the angle θ is changed to 85° along with the rotation of the substrate azimuthally at a constant speed for the growth of vertical nanocolumnar structure. During the growth of vertical nanoclumnar structure, the incident flux () can be divided into two components: a lateral component denoted as and a vertical component denoted as . As the substrate maintains a consistent rotational pace, particles are uniformly deposited onto it from the component. However, over the course of a full revolution, the cumulative effect of cancels out due to opposing directions. This outcome results in the prominence of the vertical growth influenced by the component. Finally, gold (Au) contact of 0.1 cm diameter ( cm2) was deposited on top of WO3 NR to form the Schottky contact.
3. Results and Discussion
3.1. Optical Analysis
The UV-Vis spectra of the WO3/SiO2 HS were recorded using UV-Vis spectrophotometer (Hitachi UH4150) as shown in Figure 2(a). The spectra reveal a strong absorption in the UV region. The maximum absorption in the UV region can be ascribed to the band-to-band transition of WO3 and SiO2 [25–27]. A broad peak in the absorption above 400 nm was also observed. This absorption can be attributed to oxygen vacancies creating a midgap state within the bandgap [28]. Further, the optical bandgap () for the HS was calculated using the Tauc plot [29] given by where = absorption coefficient, = photon energy, and = ½ for indirect transition. By extrapolating , as shown in Figure 2(b), the bandgap was obtained as ~3.9 eV.
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3.2. Structural Analysis
Figure 3 shows the X-ray diffraction (XRD) pattern of WO3/SiO2 HS obtained using a Rigaku X-ray diffractometer from 15° to 80° with a scan rate of 0.02 o/s. The diffraction peak was indexed as (110), (102), (211), and (-212) plane, displaying a WO3 monoclinic NR structure with JCPDS: 872378. Also, a peak with (111) plane was observed corresponding to the SiO2 cubic structure. In addition, the distinct peak observed in the XRD pattern confirms the polycrystalline nature of the as-grown WO3/SiO2 HS. The average crystallite size was calculated from the WO3 NR peak by employing Scherrer’s equation [30] as follows: where Å is the wavelength of the incident X-rays, is the FWHM, is the Bragg diffraction angle, and is the average crystallite size. The average crystallite was found to be 11.18 nm along with a dislocation density (δ) of lines/m2 calculated using the equation given by Hassan et al. [31].
The cross-sectional Field Emission Scanning Electron Microscope (FESEM) image was performed using Zeiss Sigma, NIT Durgapur, as shown in Figure 4(a). The cross-sectional image confirms the successful growth of the SiO2 TF and WO3 NR. A uniform structure with a thickness of 70 nm for SiO2 and 125 nm for WO3 was obtained. Figure 4(b) shows the top view FESEM image of the WO3 NR. It can be seen that the as-grown NR are densely packed with fully grown WO3 NR. Some undergrown NR are also visible along with the fully grown WO3 NR due to the shadowing effect associated with the GLAD technique [32]. Further, the average particle size of the WO3 NR was obtained as ~25 nm, as shown in Figure 4(b) inset. In addition, energy-dispersive X-ray spectroscopy (EDS) analysis was used to determine the purity and the presence of W, Si, and O in the sample. Figures 5(a)–5(d) shows the EDS spectrum and elemental mapping of WO3/SiO2. It can be seen that the elements of W, Si, and O are uniformly distributed in the as-grown sample.
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3.3. Electrical Analysis
Figures 6(a) and 6(b) show the I-V linear and log scale of WO3/SiO2 HS PD under dark and light conditions using xenon light with an intensity of 2.5 mW/cm2. The device displayed rectifying characteristics under a biasing voltage of -6 V to 6 V, indicating the formation of a good Schottky junction between the Au electrode and the WO3 NR. For nonideal Schottky diodes with a series resistance, it is assumed that the total current flowing through the device is primarily attributed to the thermionic emission (TE) current given by [33] where , , , , , , , , and represent the temperature (300 K), the electron charge, the biasing voltage, the ideality factor, the Boltzmann constant, the saturation current at reverse bias, the area of the contact, the Richardson constant (112 Acm-2 K-2 for n-type Si), and effective barrier height, respectively. The value of under dark and light conditions using equation (4) was obtained as 0.78 eV and 0.70 eV. The interfacial layer of SiO2 acts as a tunnel barrier that does not hinder electron flow at low voltages. Therefore, the as-grown device exhibits a very low dark current of ~46 pA due to the effect of the SiO2 layer [23] and photocurrent of ~0.11 μA at 0 V, respectively. In addition, the device displayed photovoltaic characteristics with an open voltage () of 0.25 V and short-circuit current () of 0.11 μA, as shown in Figure 6(a) inset.
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The mechanism of the WO3/SiO2 HS photodetector can be comprehended with the help of an energy band diagram, as illustrated in Figures 6(c) and 6(d). Surface states frequently influence the performance of optoelectronic responses. These surface properties give rise to a phenomenon where the upward band undergoes a bending effect close to the surface, thereby capturing and trapping holes. Under dark conditions, oxygen molecules (O2) from the atmosphere get absorbed on WO3 NR. These O2 react with the electron (e-) from the conduction band and form adsorbed oxygen ions . These ionized oxygen at the Au and WO3 NR interface forms a thin depletion layer with low conductivity [34], as shown in Figure 6(c). Under light conditions, the photon energy excites the electrons from the valence band to the conduction band; as a result, more electron-hole pairs are generated in the device. The adsorbed ionized oxygen traps the photogenerated holes at the surface of WO3 NR, thereby releasing the oxygen through the desorption process . As a result, the depletion zone becomes narrower and reduces the band bending, causing an increased conductivity (Figure 6(d)). However, the recombination of the electrons and holes can limit the response time of the photodetector. Therefore, adding a SiO2 interfacial layer between WO3 and Si substrate improved the device insulation as SiO2 will act as the electrical insulator. This improved insulation reduces the charge carrier recombination rate, increasing the number of available charge carriers for conduction [23]. As a result, the response time of the photodetector is reduced.
In addition, the devices showed high photosensitivity (/) of ~2371 at 0 V, as shown in Figure 7(a). Other crucial parameters for testing the photodetector’s capabilities, such as photoswitching and photoresponsivity, were also performed. Figure 7(b) shows the photoswitching of the WO3/SiO2 HS PD at 0 V. It can be seen that the device exhibits a good stability switching response with two distinct states revealing the response and recovery speed of the device. To calculate the rise time () and fall time () of the device, fitting was performed with the following equations (5) and (6) on the switching response at 0 V. where and I(t)f are the photocurrent time, is the dark current, is the scaling constant, and and are the rise and fall time. From Figure 7(c), the rise time of 0.31 s and fall time of 0.32 s were obtained from the fitted curve. The obtained result significantly outperformed the previously reported PD device using WO3 [18, 22]. The existence of SiO2 as an interfacial layer significantly improves the device’s response speed. Finally, the responsivity of the device at 0 V was calculated using the following [35]: where is the photocurrent and is the power of incident light. The calculated value of is plotted as shown in Figure 7(d). The device exhibits a larger photocurrent response in the UV region and lower as its wavelength increases, indicating a good spectrum selectivity response. To validate the detection of signal in a noise environment, the detectivity () and noise equivalent power (NEP) was calculated using the following [35]: whereis the responsiveness,is the charge of the electron,is the effective area of radiation of the photodetector, andis taken as 1 KHz bandwidth for flicker noise. The calculated and NEP were plotted as shown in Figure 7(e). Table 1 shows the comparison of our work with the existing reported literature. Our device exhibits an excellent ability to detect weak signals from the noise environment.
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4. Conclusion
In summary, this study successfully demonstrated the WO3/SiO2 HS photodetector fabrication using a glancing angle deposition technique incorporated inside an e-beam evaporator. The optical analysis confirmed a strong absorption in the UV region with a bandgap of ~3.9 eV. This underscores its potential for applications requiring sensitivity to ultraviolet light, such as UV photodetectors and sensors. As confirmed by SEM and EDS analysis, the successful growth of distinct WO3 NR and SiO2 TF underpins the precisely tuned and controlled deposition process. The electrical analysis of the WO3/SiO2 HS device revealed a self-powered characteristic with and . In addition, the device showed high photosensitivity and fast switching response of rise/fall time of 0.31 s/0.32 s at 0 V. Further, our device also displayed high responsivity of 0.68 mA/W with low NEP and high detectivity. These results demonstrate a cost-effective and easy fabrication way of improving the device performance by adding an interfacial layer.
Data Availability
The datasets used and/or analysed during the current study are available from the corresponding authors upon reasonable request.
Conflicts of Interest
The authors declare no conflict of interest.
Authors’ Contributions
We confirm that all listed authors have made a significant scientific contribution to the research in the manuscript, approved its claims, and agreed to be the authors of this manuscript. We declare that no further changes to authorship will be made after this point.
Acknowledgments
This work was supported by the Deanship of Scientific Research, Vice Presidency for Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia (Project No: GRANT4,408). The authors would like to acknowledge Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R230), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.