Near-infrared (NIR) photodetectors^[1−4] have demonstrated exceptional efficacy in a myriad of applications, including optical imaging, optoelectronic sensing, spectroscopy, optoelectronic switches^{[5, 6]} and biomedical imaging^{[7, 8]}. Two-dimensional(2D) materials^[9], composed of single or few atomic layers, possess unique physical and chemical properties such as high electron mobility, exceptional mechanical strength and so on. 2D Materials^[10] like graphene, hexagonal boron nitride, and transition metal dichalcogenides have garnered widespread attention for their unparalleled optical and electrical characteristics. It makes them prominent in optoelectronics^[11−14], energy storage and conversion^{[14, 15]}, biomedical^{[16, 17]} and sensing applications. Graphene, distinguished by its unique hexagonal single-atom layer structure, exhibits a number of excellent properties, such as zero bandgap, ultra-high carrier mobility (2 × 10⁵ cm²∙V⁻¹∙s⁻¹)^{[18, 19]}, and broad-spectrum absorption spanning from ultraviolet to NIR^[20−22]. These attributes render it a compelling candidate for encompassing transparent electrodes^[23] and surface plasmon devices^[24]. Nonetheless, its zero bandgap poses a limitation to light absorption in the visible and NIR regions. The inherent lack of a bandgap in graphene constrains its capabilities in light absorption. Despite this limitation, graphene boasts a naturally passivated surface, which effectively mitigates leakage current^[25]. Furthermore, the absence of surface dangling bonds facilitates reasonable stacking with other semiconductor materials to form van der Waals (vdW) heterostructures^{[26, 27]}. As a member of second-generation semiconductor material, GaAs is celebrated for its high electron mobility and wide direct bandgap, making it exceptionally effective for photodetection. The integration of graphene with GaAs to form a graphene/GaAs heterostructure capitalizes on GaAs’s advantages of strong light absorption. Thus, it addresses graphene’s shortcomings of low light absorption.

Diverse strategies have been investigated to elevate the performance of graphene/GaAs-based photodetectors. Luo et al. notably enhanced a bilayer graphene/GaAs (BLG/GaAs) Schottky junction NIR photodetector through the application of an AlO_x surface passivation method, which concurrently improved response time and detection rate^[28]. Furthermore, by leveraging the localized surface plasmon resonance (LSPR) effect of silver nanoparticles and the dielectric confinement effect of MoS₂ quantum dots (QDs), Chen et al. enhanced detector performance, leading to reduced dark current, accelerated response times, and increased detectivity and responsivity^[29]. In 2018, Wu et al. expanded the detection range of the graphene/GaAs heterostructure by incorporating upconversion nanoparticles (UCNPs), achieving significant responsivity and detectivity at a wavelength of 980 nm^[30]. However, these studies have overlooked the optimization of carrier collection and electric field distribution, an aspect that has not been adequately addressed in prior investigations.

Herein, we introduce a novel device design that optimizes the graphene/GaAs heterostructure through the utilization of interdigitated electrodes^[31]. This design enhances carrier collection efficiency and refines electric field distribution, leading to substantial enhancements in photoresponse and detectivity. By optimizing the electric field distribution, the photoelectric conversion efficiency and curtailed response time have been effectively improved. In summary, the synergistic combination of graphene with GaAs, coupled with the implementation of interdigitated electrodes, stands for a promising strategy for advancing photodetector performance. Continued research and optimization in this domain are poised to drive significant advancements in photodetection technologies.

Initially, n-type GaAs semiconductor substrates were diced into 1cm × 1cm squares using a Disco dicing saw. The surfaces were then treated to remove surface oxides with a cleaning solution consisting of acetone, isopropanol, and 5% hydrochloric acid. Subsequently, a 10 nm thick layer of chromium and a 90 nm thick layer of gold were deposited on the back of the substrates via magnetron sputtering, serving as electrodes. Following this, the graphene was then transferred to the device substrate through a wet transfer technique.

Subsequently, patterns for interdigitated electrodes with different finger widths were fabricated utilizing standard photolithographic processes, including spin-coating resist, exposing the pattern, and developing the resist. The transferred graphene was then coated with a 10/90 nm bilayer of Cr/Au through sputtering.

Graphene patterning was achieved through photolithography, followed by etching with an oxygen plasma etcher. The optimal etching parameters were ascertained through numerous trials, resulting in a power of 100 W, an etching time of 180 s, and an O₂ flow rate of 200 standard cubic centimeters per minute. It was observed that excessive power or prolonged etching times could lead to degradation of the photoresist or its residues on the graphene film. For comparative analysis, the same fabrication processes without interdigitated structures were also performed.

A Keithley 4200A-SCS system, which is designed for semiconductor characterization, was employed to assess the I−V characteristics and I−T responses of the photodetectors. The optical power density of an 808 nm light source (TTT-PC-Laser-NSC) was quantified using a digital handheld optical power meter (PM100) in conjunction with a photodiode power sensor (S122C). To analyze the current noise density spectrum (SID) of the photodetectors, measurements were conducted using a low-noise test system (LFN-2000). All measurements were performed in ambient air without any device encapsulation.

Fig. 1(a) depicts a three-dimensional (3D) schematic of the graphene/GaAs heterojunction detector with interdigitated electrodes. The analysis of Fig. 1(b) indicates the presence of well-formed interdigitated electrodes. High-quality and intact graphene exists between each pair of electrodes. The interface between graphene and GaAs is visible. The Raman spectrum of the graphene used is present in Fig. 1(c), featuring a G peak at 1585 cm⁻¹ and a 2D peak at 2673 cm⁻¹. The ratio of I_2D/I_G exceeds 1, confirming that the graphene is monolayer. Fig. 1(d) illustrates the band diagram of a graphene/GaAs Schottky junction under illumination. Upon contact with graphene, the GaAs band bends, creating a depletion region. Here, E_C, E_V, and E_F represent the conduction band, valence band, and Fermi level of GaAs, respectively, while E_gGaAs is the bandgap of GaAs. The work functions of graphene and GaAs are denoted as Φ_G and Φ_GaAs, and χ_GaAs is the electron affinity of GaAs. Under illumination, carriers absorb light energy, leading to the flow of electrons from graphene to GaAs and that of holes from GaAs to graphene, thus generating photocurrent.

Under dark conditions and 808 nm laser illumination, the I−V response of a graphene/GaAs heterojunction photodetector with interdigitated electrodes is shown in Fig. 2(a). The I−V characteristics of the graphene/GaAs heterojunction devices were measured and compared. We observed that the graphene/GaAs heterojunction device exhibited good current rectification at ±2 V. In the darkness condition, it had a rectification ratio of 5.46 × 10², indicating high-quality vdW heterojunctions between graphene and GaAs. This is supported by the excellent Ohmic contact between the Au/graphene and GaAs/Au interfaces. Notably, this value is approximately one order of magnitude higher than that of the device with interdigitated electrodes (5.46 × 10²). Several factors may account for this discrepancy. First, the interdigitated electrode structure introduces multiple current pathways, which lead to carrier losses and recombination during transport, thus reducing rectification efficiency. Otherwise, the fabrication of interdigitated electrodes might introduce more interface defects, increasing the interface state density and enhancing carrier recombination at the interface.

Under 808 nm illumination (7.23 mW/cm²), both devices exhibit a sharp increase in reverse current. Furthermore, the photovoltaic effect was remarkable in the graphene/GaAs heterojunction device with interdigitated electrodes. It shows an open-circuit voltage (VOC) of 0.79 V and a short-circuit current (ISC) of 253.3 μA, which were higher than those of the planar electrode device (0.69 V, 10.5 μA). The larger photo−dark switching ratio observed in the device with interdigitated electrodes can be ascribed to the increased effective illuminated area, which considerably enhances the generation and collection of photogenerated carriers. Despite the reduced rectification ratio, the recombination and trapping effects are less pronounced, contributing to the higher switching ratio.

Interface defects affect carrier recombination, potentially leading to an increase in effective carriers under illumination, further enhancing the switching ratio. The prominent photovoltaic properties allow the heterojunction device to detect light under zero bias voltage, eliminating the need for external power consumption. The time-resolved photovoltaic response of these two devices under 808 nm illumination is presented in Fig. 2(b). Remarkably, the graphene/GaAs heterojunction photodetector with interdigitated electrodes generated a photocurrent of 2.12 × 10⁻⁴ A, which is two orders of magnitude higher than that of the device without interdigitated electrodes (1.37 × 10⁻⁵ A). Simultaneously, the dark current in the device with interdigitated electrodes remained lower, yielding an impressive on/off current ratio (I_on/I_off) of 3.31 × 10⁷. This improvement is likely due to the interdigitated electrode structure, which shortens the path of photogenerated carriers to the electrodes, reducing series resistance and enhancing detector performance. It effectively captures photogenerated carriers, minimizes recombination, and increases collection efficiency. Additionally, it increases the contact area between light and the detector material, which improves light absorption efficiency. To further investigate the effect of interdigitated electrodes on graphene/GaAs devices, we fabricated graphene/GaAs heterojunction detectors with interdigitated electrodes of a = 2, 4, 6, and 8 μm widths. As shown in the I−V characteristics in Fig. 2(c) and the I−T characteristics in Fig. 2(d), we can see a clear trend that the photoresponse increases as the width of the interdigitated electrodes decreases. Specifically, the channel width of the devices we fabricated is fixed at 2 μm, while the electrode width varies from 2 to 10 μm. Under the condition of a constant device area, the channel width remains fixed, and the smaller the electrode width, the larger the photosensitive area of the device, which increases the effective light-exposure area and enhances the carrier extraction rate. Next, we conducted performance tests on the graphene/GaAs heterojunction detector with 2 μm width interdigitated electrodes.

The photodetection performance of the graphene/GaAs heterojunction with interdigitated electrodes strongly depends on light intensity. As light intensity rises from 1.55 μW/cm² to 7.23 mW/cm², both ISC and VOC increase together. The device reaches maximum values of 2.12 × 10⁻⁴ A and 0.79 V at 7.23 mW/cm², as shown in the I−V curves in Fig. 3(a). Fig. 3(b) shows the photodetector’s response under 808 nm illumination varies with light intensity. It is evident that the graphene/GaAs heterojunction detector with interdigitated electrodes responds rapidly and consistently to light signals, achieving a high switching ratio of 3.31 × 10⁷ at a laser illumination intensity of 7.23 mW/cm². In Fig. 3(c), it provides a detailed depiction of the photocurrent changes in response to varying levels of light intensity. The power-law equation, I_l = A × P^θ closely aligns with the experimental data, resulting in a power index θ of 0.98. This value is close to the ideal of 1, indicating that the vdW heterojunctions in the device with interdigitated electrodes have excellent quality and few traps and defects^[32]. As critical metrics for photodetectors, the responsivity (R) and specific detectivity (D*) of the graphene/GaAs vdW heterojunction device were analyzed based on the following equations, assuming that dark current noise is the predominant source of noise^[33]:

R=Il−IdPλ,

D*=A1/2R(2eId)1/2,

where I_l and I_d represent the photocurrent and dark current, P_λ is the incident light power, A represent the active device area, and e represents the elementary charge, respectively^[34]. Accordingly, the responsivity (R) and detectivity (D*) are demonstrated in Fig. 3(d). It is demonstrated that both parameters steadily increase with the increase in light intensity. They attain a peak responsivity of 41.4 mA/W and a detectivity of 2.89 × 10¹³ Jones at 7.23 mW/cm². These values are also higher than those of the graphene/GaAs device (5 mA/W, 2.88 × 10¹¹ Jones).

Next, the photoresponse characteristics of the graphene/GaAs heterojunction photodetector with interdigitated electrodes were further assessed at a wavelength of 1064 nm. Fig. 3(e) illustrates the I−V curves of the photodetector in the wavelength ranges of 808 and 1064 nm. Notably, the reverse bias current of the device exhibits a significant increase under illumination with two wavelengths of light. Although the absorption limit of GaAs is 850 nm, the detection range of the graphene/GaAs heterojunction photodetector with interdigitated electrodes extends to 1064 nm. The variation in Fermi levels between the two materials leads to free electrons and holes migrating in different directions when exposed to light. This movement results in the creation of photocurrent in the external circuit.

Response speed is important for photodetectors, as it defines their capability to identify pulsed light signals. This capability is essential for high-speed imaging and detection, biomedical applications, and security and surveillance systems. The frequency response of the graphene/GaAs heterojunction photodetector with interdigitated electrodes was assessed by adjusting the frequency of pulsed laser illumination. Fig. 4(a) and 4(b) display the temporal photoresponse to 808 nm pulsed illumination at frequencies of 2 kHz and 9581 Hz, respectively. Clearly, the heterojunction device demonstrates fast and stable responses to rapidly varying signals. Additionally, the graphene/GaAs heterojunction photodetector with interdigitated electrodes shows rise and fall times of 17.8 and 18.2 µs, respectively, when measured under 9581 Hz pulsed illumination (Fig. 4(d)). The key metrics of the graphene/GaAs heterostructure device with interdigitated electrodes compared with some previously studied graphene/GaAs-based devices are provided in Table 1. It summarizes the performance of the devices developed in this study alongside those of other devices with similar structures. The detectivity and switch radio of the fabricated device surpasses those of comparable structures. It also possesses good responsivity and response time. Photodetectors of our design are anticipated to be suitable for applications in environments characterized by weak and high-speed varying signals.

In summary, we have successfully created a self-propelled, high-efficiency graphene/GaAs heterojunction photodetector featuring interdigitated electrodes. The Type Ⅱ heterojunction expands its detection range to 1064 nm and ensures high sensitivity. The finger electrode structure shortens the path of photo-generated carriers to the electrodes, reducing series resistance. This design effectively captures photo-generated carriers, minimizes recombination, and increases collection efficiency. Additionally, the larger contact area between light and the detector material improves light absorption and enhances the efficiency of charge separation. This process is further accelerated by the presence of the heterojunction. As a consequence, the graphene/GaAs heterojunction photodetector with interdigitated electrodes exhibits a remarkable responsivity of 40.1 mA/W, superior detectivity reaching up to 2.9 × 10¹³ Jones, rapid response times of 18.5/17.5 µs, and a high switching ratio of up to 10⁷. These metrics surpass most graphene/GaAs heterojunction-based photodetectors. This work provides a novel strategy for designing high-performance NIR photodetectors, demonstrating significant potential for infrared detection and imaging applications.