Defect-induced Photocurrent Gain for Carbon Nanofilm-based Broadband Infrared Photodetector

Our work provides a carbon nanofilm for IR detection at a broad-spectrum region, with tunable defective structure, uniform thickness, and wafer-scale production.
Defect-induced Photocurrent Gain for Carbon Nanofilm-based Broadband Infrared Photodetector

In this Article, we reported a defective macro-assembled graphene nanofilm (D-nMAG)/Silicon (Si) photodetector using trap-assisted gain to optimize the photoelectric response. This wafer-scale environmentally friendly carbon material can be easily compatible with the complementary-metal-oxide-semiconductor (CMOS) technical. Noteworthy, the defective states in D-nMAG trap carriers and then enter the conduction (CB) and valence band (VB) again to be thermalized, generating a gain in photocurrent. Thus, our D-nMAG-based line array image sensor exhibits high-resolution infrared imaging of the target. This device displays a high responsivity at room temperature within a broad-spectrum region, i.e., 0.156 A/W @ 900 nm in the near-infrared (NIR) region and 3.7 mA/W @ 4 μm in the mid-infrared (MIR) region. Our work provides a carbon nanofilm for IR detection at a broad-spectrum region, with tunable defective structure, uniform thickness, and wafer-scale production.

With the continuousadvancement of materials and device structures, infrared photodetectors have been effectively used in industry, science, the military, and daily life. Nevertheless, as the demand for practical applications evolves, researchers are engaged in finding new materials and integrating them with existingplatforms to push the limits of the energy efficiency of conventional infrared photodetectors. For instance, III-V semiconductor (e.g., InAs, GaAs) quantum dots and quantum-well-based structures revolutionalized high-performance infrared photoelectric detection. Nevertheless, the spectral range for photodetection is limited by the bandgap of the quantum dots or the quantum well. The response time is tens of milliseconds or even up to a few seconds because of the slow chargediffusion process. Although III-V semiconductor-based photodetectors have extended the detection bandwidth to the mid-infrared range, these junctions still suffer from low detection rates (D*), especially at room temperature. In addition, ill-defined barrier heights and Fermi level pinning due to metal-induced gap states further worsen the signal-to-noise ratio (SNR). Recently, considerable works have been reported around carbonbased optoelectronics due to the unique physical structure, chemical stability, and green synthesis of carbon materials. Graphene is considered one of the most promising candidates for optoelectronic materials for its superior optical and electrical properties. Single-layer graphene (SLG) shows only 2.3% light absorption, limiting its application in infrared detection. Our previous work proposed a macro-assembled graphene nanofilm (nMAG) with uniform and controllable thickness and high crystalline orientation through a wafer-scale preparation process. The nMAGs raise the light absorption to 45% in the wavelengthranging from 2 to 10μm and drastically broaden the detection range of graphene-based infrared detectors. However, the fabrication of highly crystallized AB-stacked nMAG required high-temperature thermal treatment (2 h of sintering at 2800 C in the argon atmosphere), which significantly increases energy consumption and production cost. Nevertheless, nMAG/Si suffers from low photodetection. Recently, some works have reported that trapped carriers can be thermally reexcited to the conduction band (CB) or valence band (VB) of the materials, thus leading to the re-establishment of carrier equilibrium and the gain of the photocurrent. It is difficult to tune the defect states for conventional bulk materials because the modulation of semi conductor trap states is mainly concentrated on surface/interface treatments. 2D materials afford an excellent platform to explore these issues  because their electronic properties can be flexibly modulated by defect and interface engineering to attain a high photo electric transformation rate. Numerous techniques have trig gered defect engineering in 2D materials, including electron beams (e-beams), plasma, chemical treatment, ozone, and laser. To further improve the infrared detection performance of the carbon-based device, we modulate the material structure to evaluate the effect of material defect states on device performance.

We propose a macro-assembled graphene nanofilm with tunable defect states (D-nMAG) for broadband infrared photodetection and explore the relationship between defect sates content and the photo detection. We synthesized D-nMAG through thermal treatment at a low temperature and enhanced the responsivity of the device by introducing defects to the carbon materials simultaneously. Based on the outstanding characteristics of controllable uniform thickness and strong light absorption in ultra-wide spectral range, high-performance DnMAG/Si photodetectors were established in this work. Defect states were fully utilized to achieve an ultra-fast (~132 ns) broad spectral response with high responsivity in NIR (0.156 A/W @ 900 nm) and MIR (3.7 mA/W @ 4 μm). The carbon-based defective materials can be fabricated at a large scale and are easily compatible with the CMOS technical. These findings provide a solid approach for nanofilm modifications and a theoretical basis for constructing low-cost, high-performance, broad-spectrum infrared photodetectors.

D-nMAG is superior to infrared detection from the above detection results than SLG. D-nMAG (45 nm) has a stronger light absorption in a wide spectral range than SLG due to more layers. Silicon and D-nMAG are used simultaneously as the main functional layer at λ < 1.1 μm, where the light is excited through D-nMAG and silicon to produce carriers. For the photonenergy of 0.5 hν > SBH (i.e., 2.1 μm > λ > 1.1 μm), the photon energy is not sufficient to excite carriers in Si. The carriers excited in D-nMAG dominate the photoresponse. Photo-excited electrons directly transfer over the barrier between D-nMAG and Si and contribute to the photocurrent through the internal photoemission (IPE) effect, as shown in Fig. 5a. Under laser illumination at 4 μm wavelength (0.5 h ν < SBH), the energy of photo-excited electrons is lower than SBH, preventing the direct injection of photogenerated carriers into silicon. On this occasion, the photo-excited electrons in D-nMAG thermalize into a Fermi-Dirac distribution (Fig. 5b). The thermalized high-energy hot electrons (hν > SBH) can emit into silicon via PTI (photo-thermionic) emission. This is an essential characteristic of the D-nMAG/Si photodetector since the detection limit of photon energy is no longer governed by the bandgap of semi conductors but by the barrier height of the heterostructure. These two mechanisms can be confirmedby the relationshipbetween photocurrent (Iph) and laser power,as shown in Fig. 5c–d. The photocurrentwas fitted with a power-law of I ph∝αPβ, which shows a linear to super-linear transition as the photon energy decreases. It demonstrates a shift in the dominance of the photoelectric effect from the IPE effect to the PTI effect [48]. Moreover, new scattering centers can be introduced due to the defective states in D-nMAG.It can effectivelysolve the challengesof low charge separation efficiency and severe carrier compounding caused by the zero-band gap of D-nMAG, thus further improving the detection capability of D-nMAG-based infrared devices. In addition, the photogenerated carriers get trapped in defect states and enter CB or VB again to be thermalized. Then a new thermal equilibrium of carriers is established between the CB and the trap states, which process we defined as decompounding. This decompounding process can generate a photocurrent gain and furtherimprove the performance of the detectors. As a result, high infraredabsorption and trap-assisted gain increase EQE and responsivity of the D-nMAG-based infrared photodetector. Noteworthy, excessive defect density can substantially reduce the state ’s density near the Fermi energylevel of the materialand decrease infrared detection performance. The performance of the devices is maximized only at a specificdefect density, which can be confirmedin Fig. 5c–f. The ranking of β values obtained by fitting the normalized Iph of D-nMAG/Si as a function of normalized illumination power is listed as follows: 28% D-nMAG/Si (β = 1.5) < 14% D-nMAG/Si (β = 1.8) < 2% D-nMAG/Si (β = 2.2) < 85% D-nMAG/Si (β = 3.1), which indicates that more high-energy electrons are available to leap into silicon in the 28% D-nMAG device. This result is consistent with the responsivity and EQE in Figs. 3 and 4.


Carbon,Volume 198, 15 October 2022, Pages 244-251

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