Ultra-low-power photodetector based on a high-photoresponse, plasmonic-effect-induced gateless quasi-freestanding graphene device

Highlights

• Transfer/contamination/physical damage free one-pot growth of QFSG/vicinal SiC heterostructure.

• Achieved 20-fold enhanced photoresponse on AuNP-assisted QFSG/vicinal SiC.

• QFSG device with low-bias voltage achieved higher photoresponsivity (60.8 mA/W) than existing devices (0.2 to 20 mA/W) at 10 mV.

• Gateless QFSG/vicinal SiC achieved 30-fold increment in photodetection response than EG/SiC device.

• Demonstrated self-powered QFSG/vicinal SiC photodetector with high photoresponsivity.

Abstract

Large-area metal-graphene-metal UV–Visible photodetectors fabricated on quasifreestanding graphene (QFSG)/vicinal SiC (8° off-axis) wafers are applicable to future low-power consumption systems. They demonstrate effective photoresponse under 365 nm (UV) and 405 nm (Visible) light irradiation upon the application of a built-in electric field (self-biased) and ultra-low bias (−10 mV). Photocurrent gain and responsivity under UV–Visible light are more significant on QFSG/vicinal SiC than on epitaxial graphene (EG)/SiC(0 0 0 1) because of the freestanding nature of the topmost layer, absence of a buffer layer, and primary carrier scattering/trapping centers. Further, they are tuned by localized surface plasmon resonance using gold nanoparticles (AuNPs). In the self-power mode and low-bias mode in QFSG/vicinal SiC, the photocurrent is enhanced by 9-fold and 120-fold, respectively, compared to the photocurrent in EG/SiC(0 0 0 1). The responsivity of QFSG/vicinal SiC after AuNP treatment is ≈1.65 mA/W (at zero bias) and ≈20 mA/W (at −10 mV) under 365 nm light illumination (intensity = 18 mW/cm⁻²), significantly higher than that of EG/SiC (0 0 0 1). This device shows a similar trend of photoresponse under 405 nm light illumination. These results confirm that this QFSG/vicinal SiC combined with AuNPs possesses potential for application in UV–Visible detection with minimum power consumption.

Introduction

Graphene photodetectors have considerable applications in optoelectronic devices because of the high carrier mobility, broadband optical absorption, mechanical flexibility, and the ability to tune the carrier density of graphene [1], [2], [3], [4], [5], [6], [7]. However, photodetectors prepared from mechanically exfoliated and chemical vapor deposition (CVD) graphene yield low photoresponse because of the weak optical absorption [6] and ultrafast recombination of hot-generated carriers [8]. There have been several efforts to increase the photoresponse of graphene; some of these efforts include engineering the graphene surface into nano patterns [9], [10], laser irradiation [11], gate modulation [12], [13], and using a plasmonic active substrate pattern [14]. In conventional graphene/semiconductor photodetectors, the semiconductor acts as a light-absorption layer instead of graphene because of its linear band structure [15]. Furthermore, previous reports showed that wide-band-gap semiconductors (WBGS) such as oxides, nitrides, chalcogenides, and carbides exhibit excellent photo-sensing, optically transparency, and photovoltaic properties, photo-neuromorphic engineering compared to conventional semiconductor [16], [17], [18], [19]. Recently, heterojunctions of these materials incorporating other WBGS, 2D materials have been extensively investigated for cost-effective self-biased UV–Visible detectors [16], [20]. However, previous reports showed that graphene, as a transport layer, helps enhance the photoresponsivity of devices combined with photoresponsive nanostructures such as metal oxide semiconductors like ZnO, PbS [21], [22]. The coupling between surface plasmons of graphene with that of several nanomaterials such as quantum dot [23], fullerene [24], various shaped plasmonic nanoparticles (e.g., gold nanoparticles (AuNPs), silver nanoparticles (AgNPs), Cu-Ni nanoparticles) plays a vital role in increasing the photoresponsivity of the heterostructure [25], [26], [27]. Conversely, graphene shows the light response on forming heterojunctions with wide band-gap semiconductors (WBGS) such as GaN [28] and SiC [29], [30] because of the manipulation of the bandgap, which is derived from the asymmetry of the graphene sub-lattice with the underlying substrates [31]. Graphene is either transferred to or directly grown on the WBGS substrate to construct such heterojunction devices. The complex transfer process of graphene obtained by CVD or mechanical exfoliation degrades the efficiency of the photodetectors by chemical impurities contamination and mechanical damages; this increases the cost of the fabrication process [32], [33]. However, the direct growth of graphene on WBGS like GaN [34] and SiC [30] to form the heterojunction is a reliable and cost-effective method. In addition, thus grown mono/few layers graphene is suitably oriented on an on-axis SiC (0 0 0 1) [0.05° off-axis] and referred to as epitaxial graphene. Considering the epitaxial graphene on SiC [35], the graphene/SiC heterojunction can be fabricated by the high-temperature thermal decomposition process without any transfer process.

Thus far, there are various reports that focus on photodetectors based on the epitaxial graphene on SiC; most of these reports investigate UV photodetectors [36], [37], [38]. EG/SiC heterostructure is the optimal choice for UV photodetector fabrication because 4H-SiC possesses a bandgap of 3.26 eV and is a UV-sensitive material. Some reports demonstrated that visible, IR, and broadband photoresponses were realized in the EG/SiC heterojunction [39], [40], [41]. The low sheet resistance and zero bandgap make graphene a semi-metal electrode. Unlike a metal–semiconductor junction, the Schottky junction is formed at the interface between epitaxial graphene and the SiC substrate [42], [43]. Single-layer epitaxial graphene has a work function of ${\varnothing }_G=4.5eV$on 4H-SiC with an electron affinity of ${\chi }_e=3.3eV.$ As per the Schottky–Mott model, the barrier height is defined as ${\varphi }_{\mathit{SBH}}={\varphi }_G-{\chi }_e$, and it is about 1.2 eV for EG on SiC [44]. The graphene-like layer arising from partial bonding to the unsaturated Si atoms remains trapped at the interface during the graphitization process between EG and SiC(0 0 0 1), which is known as the buffer layer (BL), and acts as charged impurities and contributes to fluctuating both the interface density of states and Schottky barrier height (SBH). Compressive strain is exerted on the EG via the buffer layer because the lattice mismatch plays a significant role in SBH fluctuation. Such fluctuations enhance carrier scattering, and this reduces carrier conductivity [45]. Further, SBH on the EG/SiC heterojunction causes high power consumption because carriers in the circuit loop have to overcome it [46]. Many reports claimed a very good photoresponse of EG/SiC; it is measured either using the high biasing voltage or gate tunning voltage with the various engineered designs of EG [47]. Some studies attempt to reduce the effect of the Schottky barrier height using H-intercalation on the buffer layer[48] or growing graphene on SiC(0 0 0 −1) [49]. It is essential to control the barrier height precisely to employ it for the application. One possible approach to achieve such improvement involves investigating growth conditions and site preferences that can lead to the formation of multilayer graphene (MLG) without a buffer layer and compressive strain [35]. Recently, Khadka et al. highlighted the fabrication of such nature graphene on Si face of high miscut angled vicinal SiC [8° off-axis] and termed as quasi freestanding graphene [50].

The photovoltaic (PV) effect is the primary mechanism that explains the synergistic role of the photoresponse and semiconducting behavior in the metal-graphene-metal photodetector. The graphene Fermi level is shifted upward or downward according to the different work functions of the materials in the graphene-electrode interface. Various doped regions in graphene introduced by the interface result in the built-in electric field; this separates the optically excited electron–hole pairs that generate photocurrents [51], [52]. Similarly, the injection of a charged carrier from the LSPR of metal nanoparticles creates inhomogeneity in the density of states causes photocurrent generation [26], [53]. The above literature suggests that a cost-effective, high photoresponsivity by low-power consumption (or self-biased) based graphene/SiC heterojunction is required and in high demand in the optoelectronics field.

In this study, a gate-free metal-graphene-metal photodetector was fabricated using QFSG on vicinal SiC, and a significant photoresponse was achieved at zero (self-biased mode) and low-bias voltage conditions under the excitation of the UV–Visible light source. The QFSG/vicinal SiC device delivers an enhanced photodetector response than the monolayered EG/SiC device because of the growth of the MLG on the vicinal SiC without a buffer layer; the topmost layer acts as a quasi-freestanding layer. In contrast, the buffer layer in EG/SiC acts as a hindrance for carrier conduction, which reduces to lower photodetector response. Furthermore, the QFSG/vicinal SiC device photocurrent gain and responsivity are tuned and enhanced many folds by the deposition of gold nanoparticles (AuNPs) via the LSPR effect. The study paves the way for the proposed AuNPs-QFSG/vicinal SiC device to be a potential candidate for use in zero- or ultra-low-power UV–Visible photodetectors in various fields.