Graphene—atomically thin carbon sheets with a two-dimensional hexagonal lattice structure—exhibits unusual electronic and optical properties. Photodetectors are a good prospective application of graphene because they should ideally exhibit a broadband photoresponse from the ultraviolet to terahertz regions and high-speed operation, as well as be inexpensive to produce. Numerous methods have been proposed in order to enhance the responsivity of graphene-based photodetectors. Among these methods, photogating is most promising because it can realize the highest performance. Photogating requires photosensitive layers at the vicinity of graphene in order to produce a voltage change. Various photosensitive layers, including quantum dots, Si, InSb, and LiNbO3, are used in the visible to near-, mid-, and long-wavelength IR (NIR, MWIR, and LWIR) regions, respectively. However, the operating wavelength region is determined by the photosensitive layer, which undermines the advantage of broadband operation of the graphene. In this work, graphene nanoribbon (GNR) was used as a photosensitive layer. Graphene transistors were prepared using Si substrates with a SiO2 layer and source and drain electrodes. Single-layer graphene fabricated by chemical vapor deposition was transferred onto this substrate and formed a channel, and GNR was formed on the graphene channel using a solution dispersion method. The photoresponse was measured in the mid- and long-wavelength infrared regions. The photoresponse was found to be enhanced by GNR photogating compared with the photoresponse of devices without GNR. These results are expected to contribute to the development of high-performance broadband IR photodetectors.
There is a growing interest in low-cost small-format infrared array sensors. In this study, we demonstrate the properties of small-format graphene infrared array sensors. The devices consisted of 9 x 9 pixels, which were composed of graphene field-effect transistors (FETs) and graphene/semiconductor Schottky barrier diodes (SBDs). The photoresponses of these devices were evaluated under middle-wavelength infrared (MWIR) light irradiation. The graphene FETs exhibited ultrahigh responsivity owing to modulation of the field-effect and surface carriers caused by photocarriers generated in photosensitizers. The MWIR photoresponse of the graphene FETs was enhanced by photogating. Compared to the FETs, the SBDs showed improved dark current characteristics. The photocarriers injected into the graphene were amplified by the photogating effect induced in the graphene/insulator region. Line-scan MWIR images and profiles were obtained; the devices were mounted in ceramic image sensor packages and vacuum-cooled. They were then exposed to a scanning blackbody light source, and the MWIR photoresponse was evaluated. The photocurrent linearly increased with the step shift of the blackbody source. The results obtained in this study will contribute to the development of high-performance graphene-based IR image sensors.
This study investigated the fabrication and performance of highly responsive photodetectors, constructed of turbostratic stacked graphene produced via chemical vapor deposition (CVD) and using the photogating effect. This effect was induced by situating photosensitizers around a graphene channel such that these materials coupled with incident light and generated large electrical changes. The responsivity of such devices correlates with the carrier mobility of the graphene, and so improved mobility is critical. This work assessed the feasibility of using turbostratic stacked CVD graphene to improve mobility since, theoretically, multilayers of this material may exhibit linear band dispersion, similar to monolayer graphene. This form of graphene also exhibits higher carrier mobility and greater conductivity than monolayer CVD graphene. The turbostratic stacking can be accomplished simply by the repeated transfer of graphene monolayers produced by CVD. Furthermore, it is relatively easy to fabricate CVD graphene layers having sizes suitable for the mass production of electronic devices. Unwanted carrier scattering that can be caused by the substrate is also suppressed by the lower graphene layers when turbostratic stacked graphene is applied. The infrared response properties of the multilayer devices fabricated in the present work were found to be approximately tripled compared with those of a monolayer graphene photodetector. It is evident that turbostratic stacked CVD graphene, which can be produced on a large scale, serves to increase the responsivity of photodetectors in which it is included. The results of this study are expected to contribute to the realization of low-cost, mass-producible, high-responsivity, graphene-based infrared sensors.
The photoresponse mechanism of graphene/InSb heterojunction middle-wavelength infrared (MWIR) photodetectors was investigated. The devices comprised a graphene/InSb heterojunction as a carrier-injection region and an insulator region of graphene on tetraethyl orthosilicate (TEOS) for photogating. The MWIR photoresponse was significantly amplified with an increase in the graphene/TEOS cross-sectional area by covering the entire detector with graphene. The graphene-channel dependence of the MWIR photoresponse indicated that the graphene carrier density was modulated by photocarrier accumulation at the TEOS/InSb boundary, resulting in photogating. The dark current of the devices was suppressed by a decrease in the graphene/InSb carrier-injection region and the formation of the heterojunction using an n-type InSb substrate. The results indicate that photocarrier transportation was dominated by the formation of a Schottky barrier at the interface of the graphene/InSb heterojunction and a Fermi-level shift under bias application. The high-responsivity and low-dark-current photoresponse mechanism was attributed to the graphene/InSb heterojunction diode behavior and the photogating effect. The devices combining the aforementioned features had a noise equivalent power of 0.43 pW / Hz1/2. The results obtained in our study will contribute to the development of high-performance graphene-based IR image sensors.
Graphene has unique optoelectronic properties and potential applications in improved infrared (IR) photodetectors. Due to its Dirac cone structure, graphene exhibits broadband light absorption and rapid responsivity. In addition, unlike conventional quantum photomaterials, graphene can be synthesized inexpensively via a non-toxic process. Although graphene has advantages in IR photodetector applications, graphene photodetectors have shown low responsivity due to their minimal IR absorption (just 2.3%) and also require cooling. Therefore, there is considerable interest in enhancing the responsivity of graphene photodetectors operating at room temperature so that their advantages can be employed in IR applications. The present work demonstrates room temperature, high-responsivity, long-wavelength infrared (LWIR) graphene photodetectors. These devices operate on the photogating effect, using a lithium niobate (LiNbO3) substrate with enhancement of the photogating via a pyroelectric effect in the substrate in conjunction with a SiN layer. This effect significantly modulates the back-gate voltage to increase the photoresponse by a factor of approximately 600 compared to that for a conventional graphene photodetector. This work also found a change in the type of charge carrier with variations in temperature, which was attributed to a large shift in the Dirac point owing to the strong photogating effect. The results of this study are expected to contribute to the future realization of high-responsivity, low-cost LWIR photodetectors for applications such as thermal imaging, medical care and gas analysis.
Graphene, an atomically thin carbon sheet, has drawn significant attention in many fields because of its unique electronic and optical properties. Graphene is a potential candidate for plasmonic metamaterial absorbers and emitters because of its optical tunability and extreme thinness. We have previously demonstrated graphene Salisbury screen metasurfaces. Although the absorption wavelength of such metasurfaces can be controlled by varying the graphene patch size, the absorbance is insufficient for practical applications. In this study, therefore, multilayer graphene metamaterial absorbers (MGMAs) were theoretically investigated in the middle- to long-wavelength infrared (IR) region. The MGMAs consist of graphene layers alternating with insulator layers formed on a bottom reflector. The spectral absorbance was calculated using the rigorous coupled-wave analysis method. The calculation results demonstrated that a high absorption of ~100% can be achieved because of the multiple plasmonic resonance between each graphene layer and the bottom reflector. The absorption wavelength can be controlled by regulating the graphene pattern size because of the plasmonic resonance of graphene. Furthermore, the absorption wavelength can be tuned by controlling the chemical potential of graphene, which allows for the development of electrically tunable wavelength-selective IR absorbers and emitters. These results will contribute to the development of high-performance wavelength-tunable graphene-based IR detectors and emitters.
Graphene infrared (IR) photodetectors are promising devices that take advantage of the unique optoelectronic properties of graphene, such as broadband light absorption, rapid response, and high chemical stability. Despite its advantages, graphene has a low absorbance of 2.3%, which limits its photoresponsivity. We have previously reported the responsivity enhancement of graphene middle wavelength IR (MWIR) photodetectors using the photogating effect. The photogating effect is induced by photosensitizers located around the graphene channel that generate a large electrical change. The MWIR photoresponse with the photogating effect was enhanced by 100-fold relative to conventional graphene field-effect transistors (FETs). Although our graphene FETs using photogating exhibited ultrahigh responsivity, the dark current was extremely high, as in the case of conventional graphene FETs, because the normally-OFF operation cannot be realized in graphene. Therefore, reducing the high dark current is essential for applying graphene photodetectors to IR applications. We demonstrate dark current reduction and high responsivity MWIR light detection in graphene MWIR photodetectors. The devices consist of graphene FETs with a carrier injection region. The dark current is reduced by applying a bias voltage. The photocarriers injected into the graphene are amplified by the photogating effect induced in the graphene/insulator region. The dark current of the devices was significantly suppressed compared with that of conventional graphene FETs. The photoresponse characteristics were investigated for devices of different structure sizes. The results obtained in this study will contribute to the development of high-performance graphene-based IR image sensors.
Disorderly stacked multilayer graphene, called turbostratic graphene, is a promising candidate for highly responsive infrared detectors due to its higher carrier mobility than well-ordered multilayer graphene, and facility to suppress the Coulomb scattering from the substrate. Such properties are expected to enhance photogating for high-responsivity infrared detection. The electronic structure of turbostratic graphene was investigated using first-principles calculations. The turbostratic graphene was modeled by introducing disorder to bilayer graphene in terms of the distance and the rotation angle between the graphene layers. The calculation results show that an increase in these parameters leads to linear band dispersion and a structure similar to monolayer graphene.
We demonstrated a middle-wavelength infrared (MWIR) graphene photodetector using the photogating effect. This effect was induced by photosensitizers situated around a graphene channel that coupled incident light and generated a large electrical charge. The graphene-based MWIR photodetector consisted of a top graphene channel, source–drain electrodes, an insulator layer, and a photosensitizer, and its photoresponse characteristics were determined by current measurements. Irradiation of the graphene channel of the vacuum cooled device by an MWIR laser generated a clear photoresponse, as evidenced by modulation of the output current during irradiation. The MWIR photoresponse with the photogating effect was 100 times greater than that obtained from conventional graphene photodetectors without the photogating effect. The device maintained its MWIR photoresponse at temperatures up to 150 K. The effect of the graphene channel size on the responsivity was evaluated to assess the feasibility of reducing the photodetector area, and decreasing the channel area from 100 to 25 μm2 improved the responsivity from 61.7 to 321.0 AW − 1. The results obtained in our study will contribute to the development of high-performance graphene-based IR imaging sensors.
Graphene-based transistors were investigated as simple photodetectors for a broad range of wavelengths. Graphene transistors were prepared using p-doped silicon (Si) substrates with a SiO2 layer, and source and drain electrodes. Monolayer graphene was fabricated by chemical vapor deposition and transferred onto the substrates, and the graphene channel region was then formed. The photoresponse was measured in the broadband wavelength range from the visible, near-infrared (NIR), and mid- to long-wavelength IR (MWIR to LWIR) regions. The photoresponse was enhanced by the photogating induced by the Si substrate at visible wavelengths. Enhancement by the thermal effect of the insulator layer became dominant in the LWIR region, which indicates that the photoresponse of graphene-based transistors can be controlled by the surrounding materials, depending on the operation wavelength. These results are expected to contribute to provide the key mechanism of high-performance graphene-based photodetectors.
Advanced functional infrared (IR) photodetectors with wavelength selectivity are promising for a wide range of applications, such as multicolor imaging, gas analysis and biomedical analysis. Graphene is considered to be a promising material for novel IR detectors. However, the absorption of graphene is constant at approximately 2.3% and rather small. We have developed multispectral high-performance graphene IR photodetectors using metal-insulator-metal (MIM) or single-layer (SL) plasmonic metasurfaces (PMs). MIM- or SL-PMs induce localized surface plasmons on their surfaces and enhance absorption at the wavelength, which can be controlled by their surface patterns, such as the period or the gaps between micropatches. The absorption of graphene with PMs was theoretically investigated for various structural parameters. The absorption wavelength can be controlled based on plasmonic resonance by varying the surface geometry of the PMs. Graphene-based IR photodetectors with SL-PMs were fabricated by the chemical vapor deposition of graphene and then transferred onto the PMs. Wavelength-selective enhancement of the optical absorption and detection by graphene could be achieved due to the effect of the PMs. The results obtained here are expected to contribute to the realization of multispectral graphene infrared image sensors.
Graphene is an atomically thin carbon sheet with a two-dimensional hexagonal lattice structure that has drawn significant attention in many fields due to its unique electronic and optical properties. In this study, graphene Salisbury screen metasurfaces (GSMs) were theoretically investigated as wavelength-selective plasmonic metamaterial absorbers. The GSMs consist of a top graphene sheet, a middle insulator layer and a bottom reflector. The absorption wavelengths of GSMs with a continuous graphene sheet are demonstrated to be controllable according to the insulator layer thickness, which is similar to the case for a conventional Salisbury screen. The insulator thickness can be used to control the optical impedance to incident light using the graphene as a resistive sheet. GSMs with a periodic micropatch array of graphene can be used to control the absorption wavelength, mainly based on the graphene micropatch size and symmetry in conjunction with the insulator thickness. This wavelength selectivity is mainly attributed to the plasmonic resonance in graphene. In both structures, the chemical potential of graphene can be used to tune the absorbance and the absorption wavelength. These results will contribute to the development of electrically tunable and high-performance graphenebased wavelength- or polarization-selective absorbers or emitters.
Graphene has remarkable optoelectronic properties and thus would represent a means to improve infrared (IR) photodetectors. As a result of its Dirac-cone structure, graphene exhibits broadband light absorption and a rapid response. Unlike quantum photomaterials, graphene can also be synthesized inexpensively via a non-toxic process. Despite these advantages, graphene-based photodetectors suffer from low responsivity due to the low absorption of graphene of around 2.3%. Therefore, there is a strong demand to enhance the IR responsivity of graphene photodetectors and expand the range of IR applications. In this study, enhancement of the middle-wavelength IR (MWIR) photoresponsivity of graphene photodetectors using the photogating effect was investigated. The photo-gating effect is induced by photosensitizers, which are located around the graphene channel and couple incident light and generate a large electrical change. The graphenebased MWIR photodetectors consisted of a top graphene channel, source-drain electrodes, insulator layer, and photosensitizer. The photoresponse characteristics were investigated through current measurements using a device analyzer. The device was vacuum-cooled and the graphene channel was irradiated with light from a MWIR laser. The device exhibited a clear MWIR photoresponse observed as modulation of the output current during irradiation. The MWIR photoresponse with the photo-gating effect was 100 times higher than that of conventional graphene photodetectors without the photo-gating effect. The device maintained its MWIR photoresponse at temperatures up to 150 K. The results obtained in this study will contribute to the development of high-performance graphene-based IR image sensors.
Graphene, which is carbon arranged in atomically thin sheets, has drawn significant attention in many fields due to its unique electronic and optical properties. Photodetectors are particularly strong candidates for graphene applications due to the need for a broadband photoresponse from the ultraviolet to terahertz regions, high-speed operation, and low fabrication costs, which have not been achieved with the present technology. Here, graphene-based transistors were investigated as simple photodetectors for a broad range of wavelength. The photoresponse mechanism was determined to be dependent on factors such as the operation wavelength, the components near the graphene channel of the photodetector, and temperature. Here, we report the detailed mechanism that defines the photoresponse of graphene-based transistors. Graphene transistors were prepared using doped silicon (Si) substrates with a SiO2 layer, and source and drain electrodes. Single-layer graphene was fabricated by chemical vapor deposition, transferred onto the substrates, and the graphene channel region was then formed. The photoresponse was measured in the visible, near-infrared (NIR), and mid- and long-wavelength IR (MWIR and LWIR) regions. The results indicated that the photoresponse was enhanced by the Si substrate gating at visible wavelengths. Cooling was required at wavelengths longer than NIR due to thermal noise. Enhancement by the thermal effect of the insulator layer becomes dominant in the LWIR region, which indicates that the photoresponse of graphene-based transistors can be controlled by the surrounding materials, depending on the operation wavelength. These results are expected to contribute to the development of high-performance graphenebased photodetectors.
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