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Abstract
In this paper, we presented a novel double-layer light-trapping structure consisting of nanopores and nanograting positioned on both the surface and bottom of a gallium oxide-based solar-blind photodetector. Utilizing the finite element method (FEM), we thoroughly investigated the light absorption enhancement capabilities of this innovative design. The simulation results show that the double-layer nanostructure effectively combines the light absorption advantages of nanopores and nanogratings. Compared with thin film devices and devices with only nanopore or nanograting structures, double-layer nanostructured devices have a higher light absorption, achieving high light absorption in the solar blind area.
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1. Introduction
Due to the strong absorption of the atmospheric ozone layer, ultraviolet radiation with a wavelength of 200-280 nm from the sun is almost non-existent on the surface of the earth, so this band is called solar blind area [1,2], and detectors working in this band are also called solar blind photodetectors. Due to the lack of interference from solar radiation, photodetectors in this band hold significant value in applications such as missile tracking and fire alarm prediction [3,4]. So far, many wide-band gap semiconductors including AlGaN [5,6], MgO [7,8], diamond [9,10], β-Ga2O3 [4,11,12], etc. have been used to manufacture high-performance solar-blind photodetectors with various geometric shapes. Among them, the β-Ga2O3 has a direct band gap of 4.9 eV, with the advantages of large absorption coefficient, high chemical stability and high thermal stability, which is an ideal solar-blind photodetector material [13–15].
In the past few years, researchers have prepared a large number of solar-blind photodetectors based on β-Ga2O3 crystals [4,16,17], thin films [11,18], and nanostructures [6,1920]. As known, the actual applications of the photodetectors require fast response speed, high signal-to-noise ratio, low energy consumption, and low manufacturing cost [21]. Photodetectors based on β-Ga2O3 single crystals or stripped wafers generally exhibit high responsivity and sensitivity. However the high cost of the β-Ga2O3 single crystal materials and the low repeatability of the stripped wafers hamper their practical applications [16,22], meanwhile, the photodetectors based on the β-Ga2O3 films usually have the disadvantages of high dark current and low sensitivity due to the poor crystallinity of films [23–25]. At present, it is still difficult to solve the cost problems of the β-Ga2O3 single crystal and the quality problems of the thin film materials in a short time [26]. Therefore, in order to optimize the performance of the Ga2O3 based photodetectors, many groups have focused their researches on the nanostructures. Their aim is to realize the interaction between photons and optoelectronic materials through nanostructures, by reducing the optical reflection and enhancing the absorption, thereby improving the performance of the optoelectronic devices [26,27]. The common nanostructures include nanowires [3,26], nanopores [6,26], and nanoparticles [20,28], etc. For example, Zhang et al. used the thermally annealed Ni nanoparticles as a hard mask to etch the β-Ga2O3 film, and obtained a nanowire array photodetector (PD) with higher responsivity than the thin film PD [19]; Xing et al. used the pre-deposited Ga metal particles to self-reactively etch the ε-Ga2O3 thin film at a high temperature, and obtained the gallium oxide nanoporous PD, which had a 49 times higher responsivity than the corresponding thin film PD [26]; Tang et al. obtained Rh nanoparticles by thermally annealing the Rh metal film on the surface of gallium oxide, and obtained a solar-blind PD with a low dark current and a high detectivity by using the plasmon resonance effect of the metallic nanoparticles [20]. Generally, the main parameters to evaluate a gallium oxide photodetector are the responsivity [29], detectivity, signal-to-noise ratio and response time etc. Among them, the optical absorption of the gallium oxide thin films affects the responsivity and the other parameters of the detector, by influencing the quantum efficiency (QE) of the detector. Specifically, when the gallium oxides absorb the photons, the electron-hole pairs will be generated, which is the first procedure in the photoelectric conversion process. Thus, the absorption of the photons by the gallium oxides, is an important factor in determining the QE and the responsivity of the device. In the work mentioned above, few systematic researches were carried out on the emphasis of the light absorption process of the Ga2O3 PDs. In addition, related studies have proved that double-layer nanostructures can effectively boost the performance of optoelectronic devices, compared to single-layer nanostructures, expanding the light absorption capabilities across a broader spectral range [30–33]. Therefore, we propose and study systematically a double-layer structure to enhance the optical absorption of the gallium oxide thin film and provide a guidance for the design of gallium oxide PDs from an optical perspective.
In this paper, we proposed a double-layer nanostructure composed of nanopores and aluminum nanograting on the front and back surfaces of Ga2O3 PD. The schematic diagram is shown in Fig. 1. Utilizing the Finite Element Method (FEM), we investigated the impact of various structural parameters of nanopores and nanogratings on the light absorption of the PD. By adjusting the nano-structure parameters, we aimed to enhance the light absorption of the Ga2O3 PD specifically in the solar-blind region of 200-280 nm. In addition, by analyzing the electric field distribution inside the Ga2O3 PD, the principle of the enhanced light absorption of the double-layer nanostructures was analyzed. This research provides a new method and a theoretical basis for the development of the efficient and sensitive Ga2O3 PDs.
2. Photodetector structure design
The photodetector based on gallium oxide nanopores and aluminium nanograting structure was shown in Fig. 1. It is composed of aluminium oxide substrate, aluminium metal grating at the bottom, and gallium oxide containing nanopores. The width, height, period, and duty cycle of the bottom nanograting were individually expressed as w, Hg, P, f, where f = w/P, and the depth, radius, and period of the top gallium oxide nanopores were Hs, r, and P respectively. In the simulation, the thickness of the β-Ga2O3 layer was fixed at 200 nm. Plane wave light sources ranging from 200 to 300 nm are employed to illuminate the structure with normal incidence. With the help of the COMSOL software, we used the Finite Element Method to find the reflectance R(λ) and transmittance T(λ), and used the absorptance A(λ) = 1-R(λ)-T(λ) to characterize the light absorption enhancement effect of the structure.
3. Design and optimization of double-layer structure
The various structural parameters mentioned in the previous section will affect the absorption characteristics of the β-Ga2O3 photodetector, However, due to the complexity of these parameters, it is difficult to correctly grasp the law of the influence of each parameter on the absorption zone of the device, so the top-down method is adopted. We first discussed the device containing only nanopores structure, including the period P, radius r, and depth Hs of the nanopores. After obtained the influence law of each parameter, add Al nano-gratings structure to discuss the effects of grating period P, height Hg, and duty cycle f on the absorption characteristics of the device.
3.1 Design and optimization of nanopores structure
The light trapping ability of the nanopores structure is related to structural parameters such as period P, depth Hs, radius r, and sidewall profile [34]. In order to understand the relationship between the nanopores structure parameters and the absorption characteristics of the photodetector, the reflectance, transmittance, and absorptance characteristics under different structure parameters are calculated. The result is shown in Fig. 2. From Fig. 2, it is evident that the light absorption enhancement area (highlighted in red) exhibits a discernible dependence on the nanopore structure parameters. The influence of nanostructure parameters on light absorption can be summarized as follows: (1) When the fixed nanopores period P = 160 nm and the nanopores depth Hs = 100 nm, when the radius of the nanopores is adjusted, the absorption enhancement position appears at the position where the pore diameter is close to the period. (2) Fixed nanopores period P = 160 nm and nanopores radius r = 50 nm.When adjusting the depth of the nanopores, the absorption first increases with the increase of the hole depth, and beyond a certain level, the absorption no longer changes with the hole depth. (3) Fixed nanopores radius r = 50 nm, depth Hs = 100 nm, when adjusting the period, the absorption enhancement occurs at the position where the period P is close to the size of the nanopore diameter.
Fig. 2. Tunability of the optical absorption. The absorption, reflectance, and transmittance as function of P, Hs and r. In the simulations, P = 160 nm, r = 50 nm, Hs = 100 nm, unless otherwise specified.
Through the influence of nanopores structure parameters on light absorption, adjusting the nanopores parameters can achieve the optimal light absorption curve. As illustrated in Fig. 3, the inclusion of nanopores significantly enhanced the light absorption of the device when compared to the structure lacking nanopores. The main reason for the absorption in the 200-250 nm band is that the incident light is diffracted on the surface of the nanopores, so that more light can be effectively coupled into the device [35–37]. The distinct high aspect ratio of nanopore structure gives them an unexpectedly high absorption cross-section, enhancing their capacity for sufficient photon harvest, especially in the short wavelength region [38]. In addition, the effect of nanopores on the absorptivity of the device is mainly realized by reducing the reflectivity, while the transmittance is basically unchanged. This is mainly because the ability of nanopores to fix light is limited under the thickness of 200 nm gallium oxide film. Light can still be transmitted through the nanopores, so other structures are needed to limit the transmission of light.
Fig. 3. Schematic diagram of the light absorption(A), reflectance(R), and transmittance(T) of the structure with added nanopores (Solid line) and thin film structures (Dashed line) in the solar blind area.
3.2 Design and optimization of the double-layer structure of the top nanopores and the bottom nanograting
According to the subwavelength structure (SWS) theory, when the structure period is less than λ /n (where n is the refractive index of the material at the wavelength λ), the nanostructure will be regarded as a thin film [39]. Therefore, when calculating the absorption of the nanograting structure parameter to the device We fixed the grating period at 150 nm and adjusted the grating height Hg, duty cycle f and other parameters to obtain the dependence of light absorption on the nanostructure, as shown in the Fig. 4. In our research, we found that when the grating period is 150 nm, the grating height is 55 nm, the nanopores radius is 40 nm, and the hole depth is 100 nm, the duty cycle of the grating is adjusted. When the duty cycle is between 0.3 and 0.4, there will be a high absorption in the full wavelength range of the solar blind spot at 200-280 nm; at the same time, the duty cycle is fixed at 0.35, and when the grating height is adjusted, the high absorption of solar blind in the whole wavelength range will still occur near Hg = 55 nm; in addition, the fixed grating height 55 nm, the duty cycle is 0.35, when the period is adjusted, the absorption of the solar blind in the whole wavelength range only appears near the 150 nm period, which indicates that the enhancement of light absorption requires the mutual matching of structural parameters.
Fig. 4. Tunability of the optical absorption. The absorption as function of f, Hg and P. In the simulations, P = 150 nm, r = 40 nm, Hs = 100 nm, f = 0.35, Hg = 55 nm, unless otherwise specified.
According to the influence law of grating structure parameters on the enhancement of light absorption, we designed the optimal absorption map with period P at 140/150/160/170 nm. As shown in the Fig. 5(a), the addition of nano-gratings greatly improves the device’s light absorption in the 200-280 nm band, with the reason is mainly related to the diffraction of the grating and the surface plasmon resonance effect [31,32]. The light scattered from the top through the nanopores interacts with the bottom gratings to enhance the light absorption. As shown in Fig. 5, the absorption of the photodetector after optimized design of double-layer structure is as high as 85% in the full band of solar blindness, and even as high as 90% in some bands.
4. Analysis and discussion
In order to further study the principle of light absorption enhancement of double-layer nanostructures, we analyzed the electric field distribution diagrams under different structures. As shown in the Fig. 6 (a) and Fig. 6 (b), the electric field distribution diagrams of the film, nanopores, grating, nanopores combined with a grating structure at a wavelength of 240 nm, it can be seen that compared with the structure of the thin film, the nanopore structure undergoes light scattering on the pore surface, increasing the adsorption of light on the pore surface, however some light still transmits through the bottom [40]; the grating structure changes the propagation direction of the incident light reaching the bottom through the diffraction effect, thereby increasing the propagation path of the light and enhancing the absorption [32]; The combination of nanopores and nanograting structures. The diffraction and scattering effects of the hole structure and bottom grating structure greatly reduce the reflection and transmission of light, obviously improving the absorption of light. The combination of the two further enhances the light absorption performance of the device, and the optimized structure can achieve high absorption in the whole wavelength range of solar blind.
Fig. 6. Schematic diagram of the electric field at a wavelength of 240 nm for thin films, nanopores, nanograting, and nanopores /nanograting bilayer structures (a), as well as the variation curves of reflectivity, transmittance, and absorption (b).
In addition, we analyze the electric field distribution diagram of the double-layer structure at different wavelengths to study the principle of the double-layer structure's light absorption enhancement. As shown in the Fig. 7, When the wavelength is 200 nm, the predominant light absorption is attributed to the scattering of nanopores [41]; As the wavelength increases to 240 nm, the plasmon resonance effect of the metal grating couples the incident light into the device, thereby enhancing light absorption within this range [30,32]; When the wavelength increases to 270 nm, the light absorption ability of gallium oxide is weak. Most of the incident light reaches the bottom layer and interacts with the bottom metal grating. Part of the light is absorbed due to the plasmon resonance effect of the metal grating, and the other part of the light is due to the waveguide effect of the metal grating and the top nanopores. Improved the light absorption of the device in this wavelength band.
Fig. 7. Electric field distribution at different wavelengths in the dual layer structure of nanopores and nanograting.
5. Conclusion
In short, we proposed a double-layer structure of nanopores and nanograting on the surface and bottom of a solar-blind photodetector based on gallium oxide, and studied the influence of structural parameters on the light absorption of the device through a top-down method. The results show that the double-layer nanostructure inherits the light absorption effect of nanopores and nanograting. Compared with pure thin film devices and devices with only nanopores or nanograting, the light absorption effect of double-layer nanostructures has been greatly improved. The optimized structure realized high absorption in the solar-blind region.
Funding
National Natural Science Foundation of China (11804191); Natural Science Foundation of Shandong Province (ZR2018BA033); China Postdoctoral Science Foundation (2018M632661, 2019T120581); Fundamental Research Funds of Shandong University (2020HW016); Future Plan Program for Young Scholars of Shandong University.
Disclosures
The authors 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.
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