1 Introduction

Short-wave infrared (SWIR) photodetectors are essential in various military and civilian applications, including optical telecommunications, target imaging, remote sensing, surveillance, and environmental monitoring.^[1-6] Traditionally, these devices have relied on optoelectronic materials such as single-crystalline germanium (Ge), and III-V semiconductors including indium gallium arsenide (InGaAs), indium antimonide (InSb), and mercury cadmium telluride (MCT).^[7-10] Although these materials are highly effective, their widespread adoption in commercial SWIR photodetectors is hampered by the high costs associated with complex manufacturing processes such as molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), and metal-organic chemical vapor deposition (MOCVD).^[11-14] Recent advancements have highlighted the potential of 2D materials in crafting high-performance SWIR detectors.^[15-18] However, hurdles related to large-scale fabrication, device uniformity, and overall reliability continue to pose significant challenges. These issues underscore the urgent need for novel semiconductor materials that can achieve a balance between cost-effective manufacturing and superior device performance.

Tellurium (Te), a 1D crystalline material, is characterized by its indirect bandgap of ≈0.31 eV,^{[19, 20]} making it a compelling candidate for photodetector applications.^[21-23] One of its defining features is its unique structure, which largely lacks dangling bonds, except at the termination points of molecular chains. This structural attribute minimizes surface-induced performance degradation -a significant issue in traditional III-V semiconductors, which often require surface passivation to improve operational stability. However, Te's relatively small bandgap tends to produce high dark currents, which can impair its effectiveness in photoconductive applications.^{[21, 24, 25]} To address this, alloying selenium (Se) with tellurium to form Se_xTe_1-x alloys has emerged as a viable strategy to enhance performance. These alloys allow for adjustable bandgaps ranging from 0.31 to 1.8 eV by modifying the Te to Se ratio.^[26] Moreover, the alloys retain a 3D structure with fewer dangling bonds and demonstrate a lower evaporation temperature below 300 °C. These characteristics make Se_xTe_1-x suitable for vacuum evaporation techniques and scalable manufacturing processes, presenting a significant step forward in developing efficient and cost-effective SWIR photodetectors.^[27-29] Tan et al. reported for the first time a Te_xSe_1-x thin-film infrared field-effect transistor (FET), demonstrating that at x = 0.68, the bandgap of the Te_xSe_1-x film approximated 0.8 eV.^[30] The Te_0.68Se_0.32 FET achieved a remarkable switching ratio of 10⁴ and a responsivity of 1.5 A W⁻¹ at 1.55 µm. Subsequently, Tang's group reported a low-cost, high-performance, and high-stability Te_xSe_1-x short-wave infrared photodiode detector utilizing an FTO/ZnO/Te_0.7Se_0.3/Au detection structure.^[31] This device features an ultra-wide detection wavelength range from 300 to 1600 nm, a high specific detectivity of 10¹⁰ Jones, and boasts the fastest responding Te-based photodiode device. Moreover, they further developed a high-performance flexible Te_0.7Se_0.3 photodetector that exhibited a broad-spectrum response from 365 to 1650 nm and a rapid response time of 6 µs.^[32] These groundbreaking investigations highlight the substantial promise of Te_xSe_1-x photodetectors in enhancing short-wave infrared detection capabilities. Despite these significant advancements, the creation of a Te_xSe_1-x thin-film photodiode capable of near-nanosecond-level high-speed responses, particularly at key optical communication wavelengths like 1310 and 1550 nm, would constitute a monumental advancement in the fields of high-speed optical communications and imaging technology.

In this work, we successfully fabricated high-crystallinity Se_xTe_1-x thin films using a vacuum evaporation process. A high-speed broadband photodetector based on Gr/Se_0.2Te_0.8/GaAs vertical heterostructure was achieved. This device is capable of detecting light across a broad spectral range, from visible to short-wave infrared (SWIR), and is specifically responsive at key optical communication wavelengths of 1310 and 1550 nm. The utilization of the vertical heterostructure, coupled with graphene as the carrier collection interface, enables the photodetector to achieve rapid response times of 800 ns/1µs at the 1310 nm wavelength. Additionally, we have engineered a 5 × 5 photodetector array that exhibits extensive and uniform SWIR imaging capabilities at ambient temperature. This work provides a viable pathway for the development of highly sensitive SWIR detectors, paving the way for next-generation high-throughput, wide-bandwidth optical communications, and room-temperature light detection and imaging applications.

2 Results and Discussion

In this study, Se_xTe_1-x powders were synthesized by meticulously mixing Te and Se powders in predetermined ratios and sintering the mixture at 600 °C for 5–6 h within a vacuum-treated quartz tube furnace, as outlined in Figure S1 (Supporting Information). This approach minimizes the introduction of impurities during synthesis. The atomic structure of the Se_xTe_1-x alloy, resembling Te with selective Se substitution, forms triangular spiral chains of Se/Te atoms. These chains are organized into hexagonal arrays via van der Waals forces, as depicted in Figure 1a. Each atom within these chains is covalently bonded to its two nearest neighbors, with dangling bonds present only at the chain ends, highlighting the 1D nature of the material. Photographs of alloy crystals at varying x ratios obtained after calcination are shown in Figure S2 (Supporting Information). Using these synthesized alloys, Se_xTe_1-x thin films were then produced through vacuum thermal evaporation, with evaporation temperatures adjusted according to the Se proportion, detailed in Figure S3 (Supporting Information). The Raman spectra of these thin films, presented in Figure 1b, exhibit shifts in vibrational modes toward higher wavenumbers as Se content increases, indicating the gradual dominance of Se characteristics in the alloy. Visible-infrared absorption spectra of the films, as shown in Figure 1c, cover a broad spectral range and demonstrate varying band gaps and optical absorption cutoffs with different Te/Se ratios. XPS analysis (Figure 1d) reveals the presence of both metallic and oxidized states of Te, with variations in peak intensities correlating with Se content, further explored in Figure S4 (Supporting Information). The X-ray diffraction (XRD) analysis of the post-calcination crystals, as shown in Figure 1e, reveals that the diffraction peaks progressively shift to larger angles with an increase in selenium (Se) content. This shift confirms the formation of an alloy, evidenced by the substitution of selenium atoms for tellurium (Te) atoms and the associated change in interlayer spacing. This behavior, consistent with findings reported in the literature,^[33] supports the structural transformation within the alloy. For thin films with a low Te/Se ratio, the XRD patterns, depicted in Figure 1f, display an amorphous structure. However, when the Te/Se ratio reaches 8:2, distinct diffraction peaks are observed, indicating that the films are polycrystalline. Transmission electron microscopy (TEM) was employed to examine the surface lattice structure of the Se_0.2Te_0.8 thin films across a test area measuring ≈150 × 200 nm², as depicted in Figure 1g. The TEM images reveal clear lattice fringes, which are characteristic of a localized single-crystalline nature. This finding is corroborated by high-resolution transmission electron microscopy (HRTEM) images shown in Figure 1h, which display a high-quality crystalline structure with lattice fringes measuring 0.29 nm, corresponding to the (100) plane. These observations indicate the formation of larger single-crystal grains within the films. Further microscopic analysis was conducted using atomic force microscopy (AFM), with results presented in Figure 1i. The AFM data show a root mean square (RMS) surface roughness as low as 0.2 nm, confirming the films' exceptionally smooth surface. Additionally, Hall effect measurements, detailed in Table S1 (Supporting Information), categorize these films as p-type conductive, with an electron mobility of ≈32.9 cm² V⁻¹ s⁻¹. This mobility rate is consistent with those reported in prior studies, affirming the films' effectiveness in applications requiring high electron mobility.

Furthermore, we designed and fabricated a vertical heterojunction detector structure, as illustrated in Figure 2a, which comprises a Gr/Se_0.2Te_0.8/GaAs configuration. We selected an n-type GaAs semiconductor to form the heterojunction with the p-type Se_0.2Te_0.8 thin film, which has a measured thickness of ≈140 nm (Figure S5, Supporting Information). The elemental composition of the deposited film closely matched our expectations, as detailed in Figure S6 (Supporting Information). A graphene layer was strategically applied to the surface of the device, serving dual functions as both the carrier collection layer and the top electrode. Indium tin oxide (ITO) was employed as the bottom electrode. TCAD simulations were conducted to analyze the potential distribution under zero bias. As shown in Figure 2b, the depletion region of the vertical heterojunction is mainly distributed on the side of the n-GaAs near the heterojunction interface, with the electric field direction from n-GaAs to p-Se_0.2Te_0.8. Furthermore, the photogenerated carrier of Se_0.2Te_0.8 passes through the ultra-thin depletion layer into the conduction band of the n-GaAs (Figure S7, Supporting Information), forming a rise of photoresponse. The band structure of the Se_0.2Te_0.8 thin film, as shown in Figure S8 (Supporting Information), correlates with ultraviolet photoelectron spectroscopy (UPS) results, which indicate a Fermi level at 4.58 eV. Figure 2c displays the corresponding band diagrams of the heterojunction. Following the formation of the heterojunction, a depletion region develops on the GaAs side, where the inherent electric field assists in the separation of photogenerated carriers. Under illumination, this built-in electric field rapidly separates photogenerated carriers within the heterojunction; electrons are transferred from the Se_0.2Te_0.8 to the GaAs, while holes move in the opposite direction from GaAs to Se_0.2Te_0.8. These are then efficiently collected by the graphene top electrode. The exceptional carrier mobility of graphene significantly accelerates this carrier separation process.

Figure 3 illustrates the response speed characterization of the device and its efficacy in optical communication applications within the near-infrared spectrum. The device benefits from an atomically short charge extraction channel, which significantly enhances light responsiveness and speed, as demonstrated in Figure 3a. Light irradiation experiments using sources at 1310 and 1550 nm wavelengths were conducted to measure the temporal response, with the current–time (I–t) curves and response speeds displayed in Figure 3b,c. Specifically, the device exhibits rise and fall times at 1310 nm of 800 ns and 1 µs, respectively. At the 1550 nm wavelength, the response times are notably slower, recorded at 10 and 5 µs. The high-speed response of the detector is attributed to several critical factors: the high crystallinity of the Se_0.2Te_0.8 thin films, the ultra-short migration channel afforded by the vertical heterojunction structure, and the optimized graphene layer functioning as both the carrier collection layer and the top electrode. Figure 3d highlights the device's robust broadband detection capability, presenting typical time-resolved optical response curves under a 1310 nm pulsed laser at modulation frequencies of 100 Hz, 10 kHz, and 50 kHz. To extensively evaluate the optical response across a broad frequency spectrum, the laser was modulated using a square wave from 1 Hz to 100 kHz. The 3 dB bandwidth of the device, a critical measure of frequency response, was subsequently determined and illustrated in Figure 3e. This measure involved normalizing the device's response relative to the pulse frequency and identifying the point where the response amplitude dropped to 70.7% of its peak value, revealing a 3 dB bandwidth of ≈55 kHz. As shown in Figure S10 (Supporting Information), a control device of Se_0.2Te_0.8/GaAs was also investigated. The Se_0.2Te_0.8/GaAs vertical heterojunction exhibited response speeds of less than 10 µs at 1310 and 1550 nm, with the high-speed response primarily attributed to the dissociation and rapid collection of photogenerated carriers by the heterojunction photovoltaic effect. Compared to the Se_0.2Te_0.8/GaAs device, the Gr/Se_0.2Te_0.8/GaAs detector showed a slight improvement in response speed and 3 dB bandwidth, mainly due to the high-speed carrier transport and collection capabilities of graphene, further enhancing the detector's response speed. Furthermore, the noise current spectra of the device were examined under various reverse biases, as shown in Figure 3f. The findings indicate that 1/f noise,^[34] typically associated with the diffusion current across the depletion region of the heterojunction, predominates. This noise characteristic offers valuable insights into the mechanisms influencing the device's performance and reliability.

Thanks to its rapid response speed, a system utilizing Gr/Se_0.2Te_0.8/GaAs heterojunction photodetectors as signal receivers has been developed to showcase their potential in infrared communication systems.^{[39, 40]} As demonstrated in Figure 3g, an information transmission process was established where a binary data stream, encoding the letters “TESE,” was generated. This stream was produced by combining the infrared communication band of a 1310 nm laser with a timing control module to function as a shutter. The Gr/Se_0.2Te_0.8/GaAs heterojunction was then used to detect and capture the resulting output electrical signals. These signals were decoded back into the corresponding letters based on ASCII codes, confirming the integrity of the signal without any distortion, as illustrated in Figure 3h. Within the near-infrared range, Gr/Se_0.2Te_0.8/GaAs heterojunctions demonstrate rapid and broad-band photodetection capabilities. This performance suggests significant potential for their application in efficient, non-cryogenic infrared target detection and optical communication systems in the future.

Exploiting the photosensitive attributes and outstanding infrared detection capacities of Gr/Se_0.2Te_0.8/GaAs heterojunction photodetectors, we developed a miniature 5 × 5 array device. This device demonstrates proficient photoelectric imaging capabilities at the 1310 nm wavelength, as illustrated in Figure 4. The device uses GaAs as a universal substrate. Within the array, each pixel unit comprises a heterostructure formed between the substrate and Se_0.2Te_0.8, with alumina employed as an insulating layer to prevent electrical contact between the electrodes and the substrate. The comprehensive fabrication procedure is detailed in Figure S11 (Supporting Information). Photocurrent responses from each unit, depicted in Figure 4a, show consistent photocurrent response curves with minimal fluctuation across the array. Figure 4b documents the photocurrent values for each of the 25 array units, confirming the device's exemplary uniformity in photoelectric response. As demonstrated in Figure 4c, the device, when exposed to incident light at 1310 nm through a patterned mask, effectively generates a photocurrent in the photoreceptive units that maps to the mask's configuration. Figure 4d presents a physical photograph of the device. In order to demonstrate the applicability of the photoelectric imaging capabilities of this device in different wavelengths, we select five different wavelength laser sources, including 405, 650, 808, 1310, and 1550 nm, and the related experimental results correspond to five patterns of “UESTC” respectively. This remarkable imaging performance at room temperature, combined with the device's uniform response, highlights its significant potential for applications in infrared imaging.

3 Conclusion

In summary, we employ a CMOS-compatible slow vacuum thermal evaporation technique to fabricate large-area, high-quality short-wave infrared (SWIR) thin film materials. This approach optimally refines the crystallinity of Se_xTe_1-x thin films, thereby striking a balance between quality and cost-efficiency. Our comprehensive investigation focuses on the electronic and optical properties of these thermally evaporated Se_xTe_1-x thin films, particularly the Se_0.2Te_0.8 films, which demonstrate superior crystalline quality and enhanced transport properties. Subsequently, we have engineered a high-speed, broad-spectrum photodetector using a Gr/Se_0.2Te_0.8/GaAs vertical heterostructure, achieving rapid response times of 800 ns/1 µs at the 1310 nm wavelength and 10 µs/5 µs at 1550 nm. This heterostructure was further utilized to develop a 5 × 5 array device, which exhibits high sensitivity, rapid response, CMOS compatibility, and cost-effectiveness, establishing it as an ideal candidate for high-speed, uncooled SWIR imaging applications.

4 Experimental Section

Conflict of Interest

The authors declare no conflict of interest.