Date Published: January 18, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Seo‐Hyeon Jo, Hae Won Lee, Jaewoo Shim, Keun Heo, Minwoo Kim, Young Jae Song, Jin‐Hong Park.
In recent years, various van der Waals (vdW) materials have been used in implementing high‐performance photodetectors with high photoresponsivity over a wide detection range. However, in most studies reported so far, photodetection in the infrared (IR) region has not been achieved successfully. Although several vdW materials with narrow bandgaps have been proposed for IR detection, the devices based on these materials exhibit notably low photoresponsivity under IR light illumination. Here, highly efficient near‐infrared (NIR) photodetection based on the interlayer optical transition phenomenon in a vdW heterojunction structure consisting of ReS2 and ReSe2 is demonstrated. In addition, by applying the gate‐control function to the two‐terminal vdW heterojunction photodetector, the photoresponsivity is enhanced to 3.64 × 105 A W−1 at λ = 980 nm and 1.58 × 105 A W−1 at λ = 1310 nm. Compared to the values reported for previous vdW photodetectors, these results are the highest levels of photoresponsivity in the NIR range. The study offers a novel device platform for achieving high‐performance IR photodetectors.
Since the graphene photodetector was first implemented in 2009,1 various van der Waals (vdW) materials, such as graphene,1, 2, 3, 4 transition metal dichalcogenides (TMDs),5, 6, 7, 8, 9, 10, 11, 12 and black phosphorus (BP),13, 14, 15 have been utilized to achieve high‐performance photodetectors with high photoresponsivity and a wide detection range. In the early graphene‐based photodetectors, photodetection in a wide range from ultraviolet to terahertz wavelengths was possible, owing to the zero‐bandgap nature of graphene.16 However, this zero‐gap nature worked negatively in terms of photocarrier lifetime and electron–hole recombination, thereby decreasing the collection probability of photocarriers.17 As a result, graphene photodetectors with broad detection ranges have presented relatively low photoresponsivity values between 10−6 and 10 A W−1.1, 2, 3, 4 After that, by replacing graphene with TMD materials having moderate energy bandgaps between 1 and 2 eV, the photoresponsivity of the vdW photodetector was further enhanced.18, 19 Although the photoresponsivity values of photodetectors based on group VI‐TMDs (e.g., MoS2, MoSe2, WS2, and WSe2) and group VII‐TMDs (e.g., ReS2 and ReSe2) were reported to be between 104 and 107 A W−1 under exposure to light with λ = 520 nm,5, 6, 7, 8, 10, 20, 21, 22 it was not easy to detect infrared (IR) light with a wavelength greater than 900 nm in most of the TMD photodetectors owing to their energy bandgap properties.18, 19 In light of this limitation, the ternary metal chalcogenide synthesis technique, which can be used to design various energy band structures through stoichiometric alteration, has recently been proposed for IR photodetection.23, 24 The narrow energy bandgaps of such designed ternary metal chalcogenides enabled vdW photodetectors to operate in a considerably wider spectrum range.23, 24 However, compared to the conventional TMD‐based devices, these vdW photodetectors exhibited relatively low photoresponsivity between 3 × 10−1 and 6 × 103 A W−1.23, 24, 25, 26, 27 This is because of the poor crystallinity resulting from the complicated synthesis process that needs to precisely control the relative quantities of the three different constituent atoms.27, 28 In the case of recently reported BP with an intermediate bandgap (0.35 eV) between graphene and TMDs, it was possible to detect lights in a broader wavelength range. The BP devices also exhibited appreciably higher photoresponsivity values between 5 × 10−3 and 9 × 104 A W−1.13, 14, 15 However, it was difficult to use BP for the fabrication of vdW photodetectors because BP is highly hygroscopic and tends to condense moisture on its surface, and consequently being well decomposed in air.29
To understand the roles of metal contacts on the performance of ReS2‐ and ReSe2‐based electronic devices, we fabricated back‐gated field‐effect transistors (FETs) with two different metal contacts (Ti and Pt) and then performed electrical measurements. Figure1a presents the schematic of a FET device (both channel length and width are 5 µm) fabricated using a vdW material. According to the ID–VG curves (at VDS = 5 V) of the ReS2‐ and ReSe2‐based devices (Figure 1b,c), the on‐current (Ion) and threshold voltage (VTH) were higher and lower, respectively, in the Ti‐contacted devices, as compared to the Pt‐contacted devices. As shown in Figure 1d, Ti‐contacted ReS2 (VTH_Ti–ReS2: −32.8 V) and ReSe2 (VTH_Ti–ReSe2: −28.8 V) devices both have smaller threshold voltages than Pt‐contacted ReS2 (VTH_Pt–ReS2: −30.0 V) and ReSe2 (VTH_Pt–ReSe2: −20.8 V) devices, respectively. Owing to the smaller work function of Ti (WTi = 4.33 eV < WPt = 6.35 eV), relatively low electron barriers (ΦTi) are formed at the junctions between Ti and vdW materials, compared to Pt–vdW junctions. These lower barriers increase the probability of electron injection from the source metal to vdW materials, thereby reducing VTH and increasing Ion in both the Ti‐contacted devices (Figure 1e). Additionally, for the same contact metal, when compared to the ReSe2 devices (Figure 1c), the ReS2 devices (Figure 1b) required a smaller gate voltage (VGS) bias to turn on the device (VTH_ReS2 < VTH_ReSe2) and consequently exhibited a higher on‐current (Ion_ReS2 > Ion_ReSe2), although ReSe2 has a smaller energy bandgap than ReS2.
We presented a highly efficient NIR photodetection based on the interlayer optical transition phenomenon in a vdW heterojunction structure consisting of ReS2 and ReSe2. In particular, by applying a new gate terminal to the two‐terminal vdW heterojunction photodetector, we controlled the heterojunction interface and consequently achieved high photoresponsivity over a wide detection range. First, we investigated the role of metal contacts (Ti and Pt) on the device performance of single‐vdW‐material‐based photodetectors (ReS2 and ReSe2). By using Ti contacts on the vdW materials, we achieved a high photoresponsivity (3.23 × 106 A W−1 for ReS2 and 1.13 × 104 A W−1 for ReSe2); however, the photoresponse was slightly slow (rising/decaying times: 8.74/13.1 s for ReS2 and 5.20/5.30 s for ReSe2). In contrast, the Pt‐contacted devices exhibited a relatively fast photoresponse (rising/decaying times: 4.94/3.20 s for ReS2 and 0.032/0.053 s for ReSe2) and low photoresponsivity characteristics (1.70 × 104 A W−1 for ReS2 and 6.12 × 102 A W−1 for ReSe2), owing to the high contact resistance between Pt and the vdW materials. In addition, the photodetection ranges of the ReS2 and ReSe2 devices were limited by their energy bandgap properties (up to 850 nm for ReS2 and 980 nm for ReSe2). By exploiting these single vdW materials, we then implemented a vdW heterojunction with a staggered bandgap alignment that induces an interlayer optical transition; this bandgap alignment was experimentally confirmed by KPFM analysis. The use of this vdW heterojunction structure resulted in the extension of the previous photodetection range up to the NIR region (λ = 1310 nm). Furthermore, by applying an additional gate terminal to the two‐terminal vdW heterojunction photodetector, we could control the heterojunction interface and thereby achieve excellent photoresponsivity over a broad detection range (6.78 × 106 A W−1 for 405 nm and 1.58 × 105 A W−1 for 1310 nm). This study on a gate‐controllable vdW heterojunction is expected to offer a novel device platform for achieving high‐performance IR photodetectors.
Fabrication of Device: Mechanically exfoliated ReS2 and ReSe2 layers were transferred onto the 90 nm thick SiO2 on a heavily boron‐doped Si substrate. The source and drain electrode regions (channel length and width are both 5 µm) were patterned using an optical lithography process. Then, 10 nm thick Ti (for low work function metal contact) or Pt (for high work function metal contact) and 40 nm thick Pd were deposited by an e‐beam evaporator.
The authors declare no conflict of interest.