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Research Paper

Applied Science and Convergence Technology 2025; 34(1): 27-30

Published online January 30, 2025

https://doi.org/10.5757/ASCT.2025.34.1.27

Copyright © The Korean Vacuum Society.

Investigation on Response Characteristics of Flexible Photosensors Fabricated with High-Crystalline InN Nanowires

Jaehyeok Shin , Siyun Noh , Seunghwan Jhee , Sumin Kang , Yumin Lee , and Jin Soo Kim

Department of Electronic and Information Materials Engineering, Division of Advanced Materials Engineering, Research Institute of Materials and Energy Science, and Research Center of Advanced Materials Development, Jeonbuk National University, Jeonju 54896, Republic of Korea

Correspondence to:kjinsoo@jbnu.ac.kr

Received: November 28, 2024; Revised: December 27, 2024; Accepted: December 27, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.

Herein, we report the dynamic characteristics of flexible photosensors fabricated with InN nanowires and graphene. To investigate the effect of the number of graphene layers on the response characteristics, the top graphene layer was set as a single layer, and the bottom graphene layers were varied from single to quadruple layers to fabricate flexible photosensors. The response and recovery times of the as-fabricated flexible photosensor, with the triple-layer bottom graphene, were measured to be 20.6 and 24.2 ms, respectively, at an optical pulse illumination frequency of 5 Hz. The cross-point percentage was calculated to be 48.9 %, indicating a low probability of distortion in the pulse signal. The response and recovery times calculated from an equivalent circuit model composed of capacitances and resistances are in good agreement with the measured values. The flexible photosensors with fast-response characteristics demonstrated in this study could contribute to the implementation of the light-fidelity systems.

Keywords: InN nanowire, Graphene, Flexible photosensor, Dynamic property

Near-infrared (NIR) photosensors are essential components for optoelectronic applications, including health monitoring systems, industrial inspection, security, and optical communication [13]. Particularly, photosensors that operate at a wavelength of 1.3 µm are especially important for implementing light-fidelity (Li-Fi) system, which is attracting attention as a next-generation wireless communication system owing to fast transmission of information, high security, and eye safety [4]. In Li-Fi communication environments, for the effective transmission and reception of optical communication, it is desirable for photosensors to be attached to the human body, clothing, and machines, which leads to the conclusion that photosensors should have flexible characteristics. The III-V compound semiconductor-based commercial 1.3 µm photosensor does not possess flexible characteristics owing to its inherently brittle nature. To overcome this limitation, various polymers with effective infrared absorbance, such as poly(triarylamine), have been proposed for the fabrication of flexible photosensors [5]. However, the low photocurrent of their device (30 nA) makes the device unsuitable for practical applications. Alternatively, organicinorganic hybrid nanocomposites have been proposed to improve the electrical properties of flexible photosensors [6,7]. For example, Saran et al. [7] reported a phenyl-C61-butyric acid methyl ester/PbS quantum dot (QD)-based flexible photosensor; however, its photoresponsivity is still very low at 0.051 A/W, indicating the need for improvement. Recently, the tunability of the composition of group-III-nitride semiconductor materials makes them promising materials for developing high-efficiency optoelectronic devices that operate across wavelengths ranging from ultraviolet to NIR [8]. Among the group-IIInitride semiconductor materials, InN is well-suited for optoelectronic devices that operate in the NIR wavelength range because of its high electron mobility, saturation velocity, and a band gap from 0.6 to 1.1 eV [9,10]. In addition, the nanowire (NW) structure can enhance device performance because of its large surface area per unit volume and enables the implementation of flexible photosensors owing to its flexibility compared to its bulk or film counterparts [11]. In our previous work, we reported flexible photosensors working at the 1.3 µm wavelength with highly crystalline InN NWs embedded in a graphene sandwich structure [12]. For practical applications of the Li-Fi system, it is essential to consider not only the static properties of the photosensors but also their dynamic properties, as on-off keying relies on repetitive on/off switching of the light source [13]. Many research groups have recently focused on the dynamic properties of 1.3 µm photosensors. Wang et al. [14], for example, reported a ZnO/PbS QD-based photosensor that exhibited response and recovery times of 4.3 and 7.63 s, respectively. Zhen et al. [15] reported response and recovery times of 0.43 and 0.41 s, respectively, for a Ta2Ni3Se8-based photosensor. However, their practical applicability to Li-Fi systems is rather limited due to their insufficient response characteristics. Howlader et al. [16] recommended the coating of carbon nanomaterials (CNMs) onto metal halide perovskites (MHPs) to improve the response characteristics of the photosensors. The response and recovery times of their MHP/CNM-based photosensor were measured to be 738 and 912 µs, respectively. However, although the MHP/CNM-based photosensor has a fast response characteristic, the large difference between the response and recovery times causes distortion in the pulse signals. Considering these aspects, it is essential to develop a flexible photosensor with fast response characteristics and without signal distortion for practical applications in Li-Fi systems.

In this study, we investigated the dynamic properties of flexible photosensors using an equivalent circuit model composed of resistances and capacitances. To fabricate flexible photosensors, the graphene sandwich structure with InN NWs, exhibiting emission wavelength of 1.3 µm, embedded laterally was transferred onto a polyethylene terephthalate (PET) substrate. The static properties of the as-fabricated flexible photosensor were evaluated by measuring the photocurrent as a function of the number of graphene layers and bending conditions. Also, the dynamic properties of the flexible photosensors were evaluated by the photocurrent under optical pulse illumination conditions.

Figure 1(a) schematically illustrates the fabrication process of the flexible photosensor using InN NWs and graphene. The InN NWs and graphene were used as the photoresponsive medium and carrier channel, respectively. Details of the growth method and characterizations of the InN NWs are provided in our previously reported work [17]. The emission wavelength of the InN NWs at room temperature was 1.33 µm, as reported in our previous study [18].

Figure 1. (a) Fabrication process of the flexible photosensor with InN NWs and graphene. (b) Raman spectra of graphene layers transferred onto the PET substrate. (c) I−V characteristic curves of the flexible photosensors with respect to the number of bottom graphene layers at the light intensity of 60 mW/cm2. (d) Photocurrent and photoresponsivity of the photosensors under bending tests with respect to the degree of strain. (e) Photocurrent of the photosensor after device fabrication: measured immediately, 4, 7, 11, and 15 days.

First, single- to quadruple-layer graphene was transferred onto a PET substrate with Au electrodes using a wet-transfer method to form the bottom carrier channel. Subsequently, separated InN NWs from a Si substrate via an ultrasonic process were laterally dispersed onto the bottom graphene layers. Finally, to form the top carrier channel for the flexible photosensors, a single graphene layer was transferred onto the dispersed InN NWs. The flexible photosensors fabricated with single, double, triple, and quadruple layers of graphene in the bottom carrier channel are represented as FPBS, FPBD, FPBT, and FPBQ, respectively. For the structural characterization of graphene, Raman spectroscopy (Nanobase XperRAM-S) was used. The electrical characteristics of the as-fabricated flexible photosensors, including current (I)−voltage (V) curves and dynamic properties, were investigated using a source meter (Keithley 2400). A xenon lamp (McScience MAX-303) was used as the light source.

The Raman spectra of multilayer graphene without InN NWs are shown in Fig. 1(b). To protect the graphene layers from structural deterioration during the measurements, the light intensity of the 532-nm laser was set to a low level of 1 mW. The D peak, typically positioned at approximately 1,355 cm−1 was rarely observed, indicating that graphene exhibits high crystallinity and fewer defects [19]. The two peaks observed at 1,586 and 2,683 cm−1 are attributed to the G and 2D bands, respectively. The intensity ratio (IG/I2D) of single- to quadruple-layer graphene is measured as 0.41, 0.62, 0.85, and 1.14, respectively. IG/I2D increased with the number of graphene layers due to increased intensity of the G peak, which originated from the doubly degenerate phonon mode [20]. The 2D peak of the multilayer graphene exhibits a symmetric shape, indicating that the multilayer graphene exhibits no physical damage during the repetitive transfer process [21]. Figure 1(c) shows the I−V curves of the flexible photosensors with respect to the number of bottom graphene layers measured at light intensity of 60 mW/cm2. The photocurrents of the FPBS, FPBD, FPBT, and FPBQ samples were measured as 0.37, 0.51, 1.16, and 0.74 mA, respectively, at the voltage of 1 V. The increase in photocurrent as the number of bottom graphene layers varies from single to triple layers can be explained by the large effective volume of the graphene layers, which enables the photogenerated carriers to contribute more efficiently to the electrical signal [22]. However, a further increase in the number of bottom graphene layers causes a reduction in the photocurrent, which is attributed to carrier scattering effects, such as surface phonon scattering, short-range scattering, and Coulomb scattering, in multilayer graphene [23]. Figure 1(d) shows the photocurrent and photoresponsivity of the FPBT sample with respect to the degree of strain measured at light intensity of 60 mW/cm2 and 1 V. The photocurrent and photoresponsivity at the degree of strain of 3 % (bending state) were measured to be 1.161 mA and 0.486 A/W, respectively. These results correspond to 99.2 and 99.4 % compared to the value obtained before bending, which demonstrates that the degradation of the device performance with increasing the degree of strain is negligible. A summary of the photocurrent variations in the FPBT sample at different times over a 15-day operational period is shown in Fig. 1(e). The photocurrents of the photosensor were measured in the released state. The photocurrents measured immediately and at 4, 7, 11, and 15 days after device fabrication were 1.167, 1.166, 1.166, 1.165, and 1.163 mA, respectively, indicating negligible decrease over time. This demonstrates that the flexible photosensor can operate stably for extended periods.

The photoresponse curves of the FPBS, FPBD, FPBT, and FPBQ samples measured at a frequency of 2.5 Hz are shown in Figs. 2(a)–(d). The light intensity and voltage were set at 60 mW/cm2 and 1 V, respectively. The response times (recovery times) of FPBS, FPBD, FPBT, and FPBQ samples were calculated to be 63.5 (65.3), 48.5 (51.7), 20.4 (23.9), and 29.3 ms (33.5 ms), respectively. As the number of bottom graphene layers increased from single to triple layers, the response and recovery times decreased; however, they slightly increased when further increased to quadruple layers, with an overshoot observed due to the carrier trapping effect [24]. The variation in the response and recovery times with respect to the number of bottom graphene layers was investigated by using an equivalent circuit model. Figure 2(e) shows the photoresponse curves for the FPBT sample measured at a frequency of 5 Hz to evaluate the response characteristics at higher frequency. At a frequency of 5 Hz, the response and recovery times of the FPBT sample were calculated to be 20.6 and 24.2 ms, respectively, and were approximately the same to those measured at a frequency of 2.5 Hz, without degradation in the photocurrent. Figure 2(f) shows an eye diagram of the FPBT sample at a frequency of 5 Hz. The crosspoint percentage, which is used to investigate the degree of distortion in the pulse signals, was obtained by calculating the relative ratio of the photocurrent at the crossing points to the one and zero levels [25]. The cross-point percentage of the FPBT sample was calculated to be 48.9 %, which is nearly equal to the ideal value, indicating superior response characteristics of the as-fabricated flexible photosensor.

Figure 2. Photoresponse curves of (a) FPBS, (b) FPBD, (c) FPBT, and (d) FPBQ under an optical pulse illumination frequency of 2.5 Hz. (e) Photoresponse curves of FPBT at a frequency of 5 Hz. (f) Eye diagram of FPBT at a frequency of 5 Hz.

From the perspective of carrier transport in the graphene layer, the influence of varying graphene layer numbers on the response characteristics was analyzed using an equivalent circuit model composed of capacitances and resistances. The schematic diagram and equivalent circuit of the as-fabricated photosensor with InN NWs and graphene layers are shown in Fig. 3(a). The capacitance and resistance can be classified into two components: capacitance (Cmetal−graphene) and resistance (Rmetal−graphene) at the Au electrode-graphene interface, and capacitance (CNW−graphene) and resistance (RNW−graphene) at the InN NW-graphene interface. Because the electrode and graphene form an Ohmic contact, Cmetal−graphene and Rmetal−graphene can be neglected; therefore, the measured capacitance and resistance can be approximated as CNW−graphene and RNW−graphene, respectively [26]. Figure 3(b) shows CNW−graphene and RNW−graphene of the photosensor with respect to the number of bottom graphene layers. The CNW−graphene and RNW−graphene were measured by using an LCR meter (GWINSTEK LCR-6002) at 60 mW/cm2 and 1 V. The CNW−graphene (RNW−graphene) of FPBS, FPBD, FPBT, and FPBQ samples was 34.6 (4,280), 36.7 (2,864), 68.4 (362), and 35.4 nF (3,516 Ω), respectively. Up to FPBT, the CNW−graphene (RNW−graphene) increases (decreases) with an increasing number of bottom graphene layers, which can be explained by enhanced carrier accommodation in the multilayer graphene. However, for FPBQ sample, an enhanced carrier-trapping effect in the multilayer graphene layers causes a decrease (increase) in the CNW−graphene (RNW−graphene). Figures 3(c)–(f) shows the photoresponse curves for the FPBS, FPBD, FPBT, and FPBQ samples, as calculated by the following Eqs. (1) and (2) with the CNW−graphene and RNW−graphene measured by the LCR meter.

Figure 3. (a) Schematic diagram (top) and equivalent circuit (bottom) of a photosensor with InN NWs and graphene layers. (b) CNW−graphene and RNW−graphene of the photosensor with respect to the number of bottom graphene layers. Photoresponse curves of (c) FPBS, (d) FPBD, (e) FPBT, and (f) FPBQ, as calculated using RNW−graphene and CNW−graphene obtained from LCR measurements.

iresponse(t)=1e1RNWgrapheneCNWgraphenet
irecovery(t)=e1RNWgrapheneCNWgraphenet

where iresponse(t) and irecovery(t) correspond to the photocurrents over time after pulse illumination and after blocking illumination, respectively. The RNW−graphene·CNW−graphene is time constant for the photosensor. The response times (recovery times) of FPBS, FPBD, FPBT, and FPBQ samples were calculated to be 72.4 (74.2), 57.3 (58.5), 22.2 (24.3), and 31.6 ms (35.3 ms), respectively, and these results correspond well to the behaviors observed in the experimental results. These fast response characteristics indicate the practical applicability of the proposed flexible photosensor in Li-Fi system.

We investigated the dynamic characteristics of a flexible 1.3 µm photosensor fabricated with InN NWs and graphene. The photocurrent of the flexible photosensor was measured as 1.16 mA under light intensity of 60 mW/cm2, indicating that our flexible photosensors in this work show improved device performances compared to previously reported results. The response and recovery times of the as-fabricated flexible photosensor with the triple-layered bottom graphene were calculated to be 20.6 and 24.2 ms, respectively, at a frequency of 5 Hz. The cross-point percentage was calculated to be 48.9 %, indicating a low probability of distortion in the pulse signal. Response characterization was performed by approximating the flexible photosensors with an equivalent circuit model composed of resistance and capacitance. The results demonstrate that the proposed flexible photosensor with fast response characteristics can contribute to the realization of nextgeneration wireless communication Li-Fi systems.

This work was supported by Jeonbuk National University and Global-Learning & Academic research institution for Master’s·PhD students and Postdocs (LAMP) Program of the National Research Foundation of Korea funded by the Ministry of Education (No. RS-2024-00443714).

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