Enhancing Photoresponse of GaAs-Based Photodetector by Plasmon Grating Structures

Nanostructured metal–semiconductor-metal photodetectors (MSM-PDs) can assist in future high-speed communication devices for achieving high responsivity characteristics. However, such devices suffer from low responsivity due to low absorption, and the large band gap limits its detection range. Herein, we propose a GaAs-based photodetector with enhanced photoresponse by plasmonic Au-GaAs grating structure. The design of a grating structure on the surface of n-GaAs can excite a plasmon mode to enhance the photoelectric performance of photodetectors. Consequently, under 795 nm incident light irradiation, the grating hybrid detector exhibits a nearly 4.2-fold increase in photocurrent compared to the bare GaAs device. The enhanced absorption can be up to 99% and a specific responsivity of 240 mA/W is realized. These results can thus provide a potential scheme to fabricate high-performance GaAs detector for numerous applications.


Introduction
Plasmonic photodetectors [1] are promising for many applications such as optical communications, optical interconnections, 3D imaging, and remote sensing [2][3][4][5]. With an increasing demand on the miniaturization and integration of optoelectronic devices, metal-semiconductor-metal (MSM) structures have been suggested as a promising approach for next-generation photodetectors due to their ease of fabrication, ultra-low intrinsic capacitance (high speed), ultralow dark current, and high spectral bandwidth compared to conventional PIN and APD photodetectors [6,7]. Gallium arsenide (GaAs) and its alloys are widely applied in optoelectronic devices because of advantages that include a direct band gap, easy refractive index modulation, and high carrier mobility, which is the superior candidate compared to other semiconductor materials [8]. However, conventional GaAs-based devices suffer greatly from low absorption coefficients, and accordingly, the photoelectric conversion efficiency is limited, which hinders their practical applications [9][10][11][12].
To date, many studies have been performed to address these challenges. One strategy is to form semiconductor heterojunctions with other photosensitive materials with a small band gap, for example, MoS 2 , graphene, and other two-dimensional materials [13][14][15][16][17], which can increase the carrier collection through a long-lived charge-trapping process. The other strategy is to leverage plasmonic structures. Metal components are an inherent feature of plasmonic photodetectors and plasmonic structures can concentrate the optical field in their vicinity to enhance the light-matter interaction. For example, Miao et al. [18] placed Au nanoparticles directly on the MoS 2 surface to boost the photocurrent, showing that periodic Au nanoparticle arrays exhibit an even stronger enhancement than randomly distributed nanoparticles with similar size and spacing. Gao et al. [19] designed a crossed-finger metal electrode to maximize the collection of photogenerated carriers, effectively avoiding carrier recombination before being collected, which has enabled the SiN waveguide graphene photodetector to reach a responsivity of 2.3 A/W. Dai et al. [20] used Au nanoparticle arrays to improve the performance of a InSe selfpowered photodetector by hybridized quadrupole plasmon resonance of Au nanoparticles, and comparing with pristine InSe devices the responsivities of the Au/InSe photodetector could reach up to 369 and 244 mA/W at wavelengths of 365 and 685 nm under zero bias voltage. It is easier to control the size and arrangement of the pattern through photolithography method. Although nanoparticles can excite plasmons, it is imperative to choose the right size and particle configuration to maximize the enhancement effect of the plasmon mode. Kwonet al. [21] adjusted the width of the conical array of the tapered array structures to gradually tailor the cut-off frequencies and group velocities of the tightly confined plasmonic modes for enhanced light absorption and suppressed reflection of the photonic mode in the silicon waveguide. The design of suitable structures can induce the coupling of multiple resonance effects to realize the enhancement of optoelectronic performance. Zhao et al. [22] placed the monolayer graphene directly on the Ag grating directly and used MPs (excitation of magnetic polaritons) and SPPs (surface plasmon polaritons) to enhance the light absorption of monolayer graphene.
Over the corresponding studied spectra, the optical response of these structures was dominated by the excitation of surface plasmons. This efficiency enhancement was mainly attributed to increased light absorption in the heterojunction; the metal nanostructure enhances the electromagnetic field intensity of the heterojunction and enhances the generation and collection of electron-hole pairs. Metal nanostructures enhanced the electromagnetic field density of the corresponding semiconductor material, which in turn enhances light absorption and the generation and collection of electron-hole pairs. These results indicate that a plasmon resonance-enhanced photodetector possesses a significant potential to achieve high performance. However, the fabrication of different plasmon structures such as interdigitated [19] or quantum-well [23] type photodetectors generally requires multiple, complicated lithography fabrication steps, and the fabrication of devices is mainly focused on metallic structures, which is time-consuming and limits the enhancement achieved. In addition, the superposition of multiple layers of materials produced by heterojunctions produces interesting photoelectric effects; it will also introduce many uncertain factors in the experiment, such as lattice matching, adhesion force, bonding gap, and surface cleanliness [24]. Hence, it is extremely imperative to propose a structure that can be prepared by simple experiments with high performance compared to complex metal nanostructures and heterojunction structures for photodetectors.
In this paper, we propose a new type of GaAs-based plasmonic photodetector that integrates a plasmonic Au-GaAs grating structure to enhance the photodetection properties. Compared with bare GaAs photodetector, the photodetector with Au-GaAs grating exhibits a higher light absorption and responsivity due to the localized surface polaritons (LSP).
In addition, the structure can be obtained by simple photolithography. By adjusting the position of the periodic metallic grating structure, the intensity of the electric field near the interface between the metallic grating and the absorption layer can be significantly enhanced. Finite element method (FEM) simulation suggests that the plasmonic grating structure can significantly enhance the absorption of GaAs layer by optimizing the geometric parameters of the grating structure, so that the absorption capacity, external quantum efficiency, responsivity, and spectrum of the heterostructure have been improved. In addition, we investigated the influences of modification of structural parameters on detector performance. A responsivity of 240 mA/W corresponding to an absorption rate of 99% is achieved in the proposed plasmonic device for the illumination wavelength of 795 nm. Additionally, the external quantum efficiency and photocurrent response of the detector in the corresponding detection wavelength are 4.2 times higher than the GaAs-based detector without plasmonic grating structure. This work thus presents the determined photoelectric properties of GaAs-based plasmonic detectors and provides a new solution to improve the detection efficiency of these devices and further enhance the photoelectric response peak to a larger dimension for a host of applications.

Model Design and Method
Here, GaAs-based plasmonic photodetectors with Au-GaAs gratings were established on a lightly n-type doped GaAs substrate with a commercial FEM software, COMSOL Multiphysics. Figure 1a shows a three-dimensional schematic diagram of the proposed detector structure. From the bottom to the top, the detector is composed of a metal electrode, GaAs absorber, and grating, and the Au grating layer was constructed at the top of the model. The thickness of the whole GaAs absorber, GaAs grating, and Au grating is 2 um (T), 300 nm (h1), and 70 nm (h2), respectively. In particular, as shown in Fig. 1b and c, we take the periodic cell as the theoretical simulation area to build the model that Au gratings are located on the top and the bottom of the GaAs grating array (noted top-bottom type structure), and the unit period and grating width are 300 nm (p) and 150 nm (w), respectively.
Doping of GaAs substrates can be grown by using metal organic chemical vapor deposition (MOCVD), and Au-GaAs grating was fabricated onto the GaAs surface by chlorine-based inductively coupled plasma etching. The fabrication procedure for this structure was similar to that implemented in Reference [25]. In order to demonstrate how the position of Au grating affects the photodetector performance, three additional different simulation models with top-grating, bottom-grating, and grating-free structures are constructed, as shown in Fig. 1d-f, respectively.
The perfect match layer (PML) was applied on the top and bottom of the simulation region, which absorbs tangential wave vector, and a normally incident plane wave acts as incident excitation source, and periodic boundary condition was used for the simulated infinite region. The optical properties of gold and GaAs material are chosen from Palik [26]. Additional information regarding the model is provided in Table 1. The FEM is designed to solve Maxwell's equations combined with the drift-diffusion equations to obtain the distribution of the electromagnetic field, photocurrent, and responsivity, respectively.
The absorptance (α) for the whole structure can be calculated by the ratio of the total power dissipation density w (x, y, z) (W/m 3 ) within a volume V (m 3 ) to the incoming power through the exposed surface area A (m 2 ) by (1) [27,28]; meanwhile, the power dissipation density is calculated by (2).
where E inc is the incident electric density, E is the electric field, imag(ε) is the imaginary part of dielectric function, θ is the incident angle, ε 0 is the vacuum permittivity, and c 0 is the speed of light. The photocurrent generation can be calculated by integrating the normal current density of the electrode surface.
To better understand the enhancement of the photodetector performance by the plasmonic metallic grating, two other wavelength-dependent parametric quantities, external quantum efficiency (EQE) and responsivity (R), were further calculated by (4) and (5). where h is Planck's constant, q is the electron charge, v is the frequency of light, I ph is the photocurrent generation, and P in is the incident light power.

Results and Discussion
The simulation results are presented in Fig. 2a and depict an interesting picture for the light absorption process in the four proposed photodetectors for the wavelength  between 450 and 900 nm. Within the wavelength of 500 to 800 nm, the absorptivity of the plain grating-free structure photodetector is slightly higher than 0.6, and the absorption line does not fluctuate sharply. As the position of the Au grating is changed, excitation of the plasmon mode leads to increasing absorption capacity of GaAs photodetectors. The other three structures all display double resonance absorption peaks compared to the gratingfree structure detector, and the highest absorptivity was 0.99 at 795 nm for the top-bottom grating structure with a plasmon mode. It can be found that when shifting the Au grating from the bottom to the top of the GaAs grating, the position of the double absorption peak shifts flat with increasing wavelength which indicates the tunability of the absorption peak position. In addition, the highest absorptivity is achieved at 0.99 for the top-grating structure, while the maximum absorptivity is 0.92 for the bottom-grating structure at 750 nm. In some studies [29,30], the light absorption ratios of semiconductor materials and metals in photodetectors with plasma-metal structures can be calculated separately. This also suggests that not only plasmonic grating structures can enhance light absorption but also semiconductor materials with photoelectric conversion properties, further demonstrating that the absorption enhancement effect can improve the electrical signal response. Under the excitation of incident light, the electron mobility in GaAs is much higher than that of holes, so photogenerated electrons preferentially reach the electrodes to be collected and generate a directional current. Figure 2b depicts the photocurrent-wavelength curves taken from the proposed four structure types of detectors, which are acquired by theoretical simulation at 0 bias voltage. The top-down grating structure demonstrates a current that is nearly 4.2 times higher than the grating-free structure, which is the highest photocurrent among the three detectors. Corresponding to the absorption peak, the photocurrent peaks also appear at the relative wavelength position, and the photocurrent of the top-down grating detector is about 15.9 nA at 795 nm, which further indicates that the plasmonic grating structure facilitates the photoelectric conversion capability of the detector.
To gain an insight regarding the location of the absorbed energy confinement, thus contributing to the photocurrent, the E-field spatial distribution and the absorption were numerically simulated at a resonant wavelength of 530 nm and 795 nm and a non-resonant wavelength of 650 nm, respectively. Figure 3a, b, and c show the profiles of the E-field intensity for top-down grating structure under 530 nm, 650 nm, and 795 nm wavelength light illumination, respectively. At the resonant wavelength of 795 nm, the electric field intensity is significantly increased around the Au/GaAs interface, indicating that the incident light is concentrated and "folded" at the Au/GaAs interface, whereas no significant increment of the electric field intensity is observed at the non-resonant wavelength of 650 nm, as shown in Fig. 3b and c. However, at the resonance wavelength of 530 nm as shown in Fig. 3a, it can be found that the electric field enhancement is mainly concentrated on the surface of the Au grating, compared with the enhancement position appearing on the surface of Au/GaAs, but the latter is more favorable to the absorption and photoelectric conversion of semiconductor materials. It illustrates that the localized surface polariton (LSP) mode is excited to enhance the absorption capacity of the metal grating. Localized surface polaritons refer exclusively to the mutual coupling of Fig. 2 a, b Absorption spectra and photocurrent under a plane light for the top-down grating structure photodetector and the other three structures electromagnetic waves with free electrons in metal with sizes much smaller than the wavelength. The enhancement of the magnetic field also occurs at the GaAs absorber; as show in Fig. 3f, the electric field flows along the magnetic field enhancement location to form a closed loop, which demonstrates a typical magnetic polariton (MP) resonance. The MP promotes the strongest mutual coupling of electromagnetic waves inside and outside the structure to achieve a strong magnetic resonance effect. In addition, the parallel currents generated by the photoelectric conversion effect inside the semiconductor material lead to electrical resonance, while the perpendicular currents form a circular current and produce a magnetic dipole effect [31][32][33]. These suggest that the enhancement of the detector absorption originated from the coupling of LSP and MP. As shown in Fig. 3d-f, compared to the bandwidth of 530 nm when the bandwidth increases to 795 nm, the MP effect is most obvious.
Moreover, according to previously published research studies [34][35][36], the absorption enhancement and transmission effects of the metallic gratings are induced by the combination of two mechanisms: the first is the resonant interaction between the incident radiation field and the LSP mode excited on the metal surface, and the second is that such a grating forms a Fabry-Perot cavity. The first mechanism predicts the resonant enhancement if the wave vector of the plasmatic polarization waves on the metal surface is phase matched with that of the incident plane wave. Under these circumstances, a stronger coupling occurs between the incident plane wave and the plasmatic polarization wave on the metal surface, giving rise to increased transmission. Meanwhile, the second mechanism considers the grating spacings as Fabry-Perot cavities, and this effect becomes prominent if the wavelength of the incident plane wave matches the grating depth. The results of the simulation are clearly consistent with this theory. Apparently in our study, the second mechanism also exists but the phenomenon is less pronounced compared to the first mechanism.
In addition to the absorption characteristics, some major electrical characteristics were calculated in order to better evaluate our proposed detector, as shown in Fig. 4.   Fig. 3 The distribution of electromagnetic field, a, b, and c and d, e, and f are the electric and magnetic field distribution for the top-down grating structure at 530 nm, 650 nm, and 795 nm, respectively; the red arrow represents the direction of the electric field When photons are incident on the inside of a semiconductor photoelectric structure, they can generate an electric current by the photoelectric conversion effect. As shown in Fig. 4a, we calculated the EQE and responsiveness of grating-free structure type and top-down structure type detectors respectively by the magnitude of photocurrent at zero bias. The results suggest that the top-down grating structure has higher external quantum efficiency than the plain grating-free structure in the range of 450-900 nm. The EQE of the heterojunction structure can show that due to the influence of the surface recombination of GaAs, the EQE of the two structures is less than the absorptivity. The EQE of the top-down grating structure is 0.37 at 795 nm, which is 4.2 times higher compared to gratingfree structure at the same position. Compared with conventional bulk semiconductors, the discrepancy between the absorptivity and EQE of the proposed Au-GaAs grating structure is significantly low, which indicates that the plasmonic grating structure is more efficient at converting the absorbed photons into electrical energy, because the amounts of carriers have been considerably increased. Figure 4b shows the distribution curve of responsivity with spectrum. It is evident that the responsivity increases with the light absorption capacity. The maximum responsivity is 240 mA/W at 795 nm; this is significantly higher than that of the grating-free structure which is 57 mA/W at the same wavelength position.
To analyze the power dependence of photocurrent responses, the variation of the photocurrents with different incident powers at different incident wavelengths is calculated. Figure 4c shows the measured relation curves between photocurrent and incident power at three incident wavelengths, whose slopes gradually flatten as the power increases. This phenomenon can be explained by Fig. 4 a, b Comparison of the electrical characteristics of top-down grating structure and grating-free structure at zero bias. c Photocurrent under laser illumination with different wavelengths and powers.
d Responsivity under laser illumination with different wavelengths and powers. The inset of d shows the variation of responsivity at a power of 0.01 to 200 mW the trap-induced photoconductive gain. Due to the higher mobility of electrons than holes in GaAs, the photocurrent is dominated by the collection efficiency of photogenerated electrons, which strongly depends on the trap-induced photoconductive gain [37,38]. The responsivity of the detector under varying light powers is calculated and shown in Fig. 4d, which also demonstrates the strong power dependence at the three different wavelengths. Under higher incident power, more electron traps are occupied by the photogenerated electrons; thus, the availability of vacant electron traps is reduced, which results in a gradual increase in photocurrent response. Once the electron traps are filled, the number of free electrons increases and the probability of electron-hole recombination increases, leading to a decrease in the responsivity, and the inset shows that the responsivity slowly increases before the input optical power is 200 mW and reaches its maximum value thereafter. By analyzing the electrical properties of our proposed device, it is obvious that the absorbed enhancement can boost the various electrical properties of the structure. Since different photon energies correspond to different spectra, as in our proposed plasmonic Au-GaAs grating photodetector, it produces the highest electrical properties at the point of maximum absorption.
The above analysis shows that the enhancement of light absorption can improve the performance of the detector. To further understand the effect of the structural parameters on the performance of the detector, the absorption spectra of the top-down type structure at different grating widths, grating heights, and periods are given in Fig. 5. As shown in Fig. 5a, when the grating width is increased from 120 to 180 nm, both resonance peaks are red-shifted, and the peak intensity changes slightly, but the trend of the first resonance peak changes gradually and becomes slower. By increasing the height of the GaAs grating, the double resonance absorption peaks of the device are clearly Fig. 5 Absorptivity contours for various geometric parameters using the parameters given in Fig. 2 as the base case. a Grating width. b Grating height, h1. c Period. d Incidence angle visible, and the peak position is significantly red-shifted. As shown in Fig. 5b, the peak intensity reaches a maximum approximately between 750 and 800 nm wavelength and decreases on both sides of this position. In addition, we also calculated the influence of the height change of the Au grating on the absorption of the structure, which is similar to the case of the GaAs grating and will not be described in detail here. As the period increases, two absorption peaks also appear, the upper one representing the excitation of the LSPs on the interface between Au and GaAs, which appears slightly red-shifted. As shown in Fig. 5c, the absorption peak appears when the period is greater than 0.2 um, and the position of the upper peak gradually disappears when the period is greater than 0.5 um. Based on the analysis of the effect of the above structural parameters on the peak intensity and position, it is observed that the peak intensity of the detector and position of the absorption peak can be efficiently controlled through the trench width and grating height, respectively, without significantly affecting the absorption intensity. The structure always has two absorption peaks and the period has a strong influence on the position and density of the peaks. As shown in Fig. 5d, there are two distinct absorption peaks when the incident angle changes within 60 degrees, which shows that the proposed structure has good angle insensitivity characteristics. The two peaks merge into one peak and the position of the two peaks gradually decreases as the angle increases when the incident angle is greater than 60 degrees.
Nanofabrication for the as-designed metal/semiconductor photodetector was carried out using an electron beam lithography-based process. The specific process can be described as, first of all, coating of a PMMA layer on n-type GaAs substrate for the electron beam lithography. Afterwards, the E-beam is shone on the PMMA coated on top of the GaAs substrate, and after development and metallization by Au and lift off. The next step is Cl-based inductively coupled plasma etching using the pre-patterned Au as etch mask to form the Au-GaAs grating. This step is also the most challenging part of the photodetector fabrication, and a suitable etching process is required to ensure the integrity of the grating. Finally, the electrodes connected to the wires are made by thermal evaporation deposition to form the photodetector. Hence, with this advanced nanofabrication, the device is easily fabricated with fewer experimental steps.

Conclusions
In conclusion, light trapping occurs and LSPs are excited in GaAs-based photodetectors with an optimized Au-GaAs grating structure, demonstrating that plasmonic enhanced light-matter interaction. Simulation results show a significant enhancement of responsivity across the wavelength ranging between 450 and 900 nm. The responsivity of this device goes up to 240 mA/W at zero bias voltage, corresponding to the external quantum efficiency of 37% for 795 nm wavelength light. The Au-GaAs grating structure boosts both the inter-band transition and the internal photoemission effect, which is supported by simulations including the distributions of the electromagnetic field density intensity, absorption, and photocurrent. The present findings confirm that the plasmon metallic grating structures have a significant potential as high-performance self-powered broadband photodetectors. Thus, the results obtained in this work are useful for designing high responsivity GaAs-based photodetectors without complicated etching process for a wide range of applications in communication, sensing, or solar cells.