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Abstract
Due to the wavelength-selective absorption characteristic of indium gallium nitride (InGaN) ternary alloy, the InGaN-based photodetectors (PDs) show great potential as high signal-to-noise ratio (SNR) receivers in the visible light communication (VLC) system. However, the application of InGaN-based PDs with simple structure in the VLC system is limited by slow speed. Integration of graphene (Gr) with InGaN is an effective strategy for overcoming the limitation. Herein, we report on a high responsivity and fast response PDs based on Gr/InGaN heterojunctions. It finds that the three-layer Gr (T-Gr) can effectively improve the InGaN-based PDs photoelectric properties. The T-Gr/InGaN PDs show a high responsivity of 1.39 A/W@−3 V and a short rise/fall time of 60/200 µs, which are attributed to the combination of the high-quality InGaN epitaxial films and finite density of states of three-layer graphene. The fast response with high responsivity endows the T-Gr/InGaN PDs with great potential for selective detection of the VLC system.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapid development of communication technology, data security, and wireless spectrum resource is facing severe challenges [1–3]. Visible light communication (VLC) has drawn significant attention as a complementary technology to the traditional radio wave technique due to its higher-speed transfer rate, broader spectrum bandwidth, lower power consumption, and stronger data confidentiality [4–6]. It is known that photodetectors (PDs) are critical parts of the VLC system for achieving efficient photoelectronic conversion of signals, and directly determine the transmission rate and reliability of the VLC system [7,8]. To date, most of the commercially available PDs are Si-based PDs, which have the advantages of low cost and wide wavelength coverage [9,10]. However, limited by the broadband detection characteristics, the Si-based PDs exhibit a low signal-to-noise ratio (SNR), hampering the reliability of the VLC system [11,12].
As an adjustable band-gap semiconductor, InGaN can achieve wavelength-selective absorption, and the InGaN-PDs enable a response in the band of the light source to improve SNR [13,14]. Therefore, InGaN-based PDs is considered to be ideal candidates for VLC system receivers, and there are already many reports of high-speed InGaN-based PDs applied in VLC system [7,9,11,12]. However, these devices heavily rely on complex epitaxial structure and preparation processes. And the InGaN-PDs with simple structure, such as metal-semiconductor-metal PDs and heterostructures PDs, are still facing a challenge in “low response speed”. On the other hand, the metal electrode shielding effect reduces the effective photosensitive area of InGaN-PDs for reducing the responsivity [15–17]. Therefore, the introduction of a transparent semiconductor material into InGaN for achieving InGaN-based heterostructures PDs has been highly valued due to the enhancement in both responsivity and speed of devices.
Graphene (Gr), a typical two-dimensional (2D) materials with high carrier mobility and universal optical transmittance, have been used to construct high-responsivity PDs with GaN [18,19], Ga2O3 [20], and SiC [21]. However, the previous Gr-based PDs studies have a focus on the response, and the influencing mechanism of graphene layers is still unclear. In addition, the application of Gr/GaN-based PDs in the VLC system is limited by the unsatisfactory speed [22,23].
Herein, we design and fabricate a series of Gr/InGaN heterojunctions PDs with different numbers of graphene layers. The three-layer Gr (T-Gr)/InGaN PDs exhibits faster carrier transfer and stronger photoelectric conversion compared with the pure InGaN PDs and single-layer Gr (S-Gr)/InGaN PDs. Based on the experimental results, the charge transport behavior and underlying mechanism have been proposed. It shows that the introduction of T-Gr facilitates the construction of Schottky junction, which promotes the charge separation and transfer. As a result, the T-Gr/InGaN PDs exhibits a shorter rise/fall response time of 60/200 µs and a high responsivity of 1.39 A/W@−3 V. This work not only reveals the charge carriers transport mechanism within Gr/InGaN PDs, but also presents a valuable avenue for achieving high performance InGaN-based PDs for potential application in visible light communication through the integration of InGaN and T-Gr.
2. Experimental procedure
The structure for InGaN films on Si substrates in this study is displayed in Fig. 1(a), and the high-quality InGaN epitaxial films grown on Si substrates by the combination of low-temperature pulsed laser deposition (LT-PLD) and high-temperature metal organic chemical deposition (HT-MOCVD) [24]. Firstly, the Si substrate were taken through a cleaning process with acetone, alcohol and deionized water in order to remove surface contamination. Secondly, the as-cleaned substrates were transferred into the PLD growth chamber. Subsequently, an AlN layer layer were grown on Si substrates by PLD with a target substrate distance of 5 cm and an optimized laser density of 3.0 J/cm2 in a 4 mTorr nitrogen plasmas atmosphere produced by a radio-frequency plasma radical generator at temperatures of 450 °C, respectively. Thirdly, the template was transferred into a MOCVD growth chamber. A ∼1 µm GaN and 100 nm InGaN epitaxial films were grown at a temperature of 1100 and 750 °C, respectively.
Fig. 1. (a) Schematic diagram of epitaxial structure for InGaN. (b) HRTEM image for InGaN. (c)XRD 2θ-ω for InGaN. Typical XRCs image for (d) InGaN (0002) and (e) InGaN (10-12). (f) PL spectrum for InGaN.
The graphene with monolayer (ML) to trilayer (TL) were reported in our previous reports [25].
The device fabrication process is as follows: 250 nm Al2O3 layer was first deposited to cover a part of the InGaN surface using electron beam (e-beam) evaporation coating system and the surface pattern were defined by standard lithography and lift-off techniques. Afterward, Ti (40 nm)/Al (180 nm)/Ni (50 nm)/Au (60 nm) four-layer metal was deposited using e-beam evaporation on InGaN films and then rapidly thermal annealed at 800 °C for 30 s in N2 ambient to form good Ohmic contact. Then, The PMMA/graphene micro sheets were transferred onto the InGaN pieces. Finally, the graphene on InGaN was cut into a 100 µm×30 µm rectangle through oxygen plasma.
3. Results and discussion
The crystallinity quality of InGaN film directly affects the photodetection performance of devices, and it was examined by transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Figure 1(b) shows a highly crystalline structure for InGaN with the typical (0001) facet spacing of 0.52 nm, and the corresponding diffraction spots that appeared are as well found in the selected area electron diffraction (SAED) pattern [25,26]. Figure 1(c) shows the XRD pattern of the prepared InGaN film, the diffraction peaks at 28.4 °, 33.6 °, 34.5 °, and 36.0 °are ascribed to the Si (111), InGaN (0002), GaN (0002), and AlN(0002) crystal facets, corresponds to the epitaxial structure shown in Fig. 1(a). In addition, the crystal structure of the InGaN film was also characterized by high-resolution X-ray rocking curves (XRC). From Fig. 1(d)-(e), the full-width at half-maximums (FWHMs) value of InGaN (0002) and InGaN (10-12) is 275.48 and 319.98 arcsec, respectively. The small value of the FWHMs indicates the high-quality of InGaN film. The surface morphology of InGaN films is also critical to InGaN-based PDs because the high surface roughness would induce light scattering and surface state [27,28]. The surface morphology and the surface roughness of the InGaN film are characterized by AFM. The RMS roughness of InGaN is very small of ∼3.1 nm (as shown in Fig. S1). As discussed above, the high crystalline quality InGaN films lay the foundation for the development of efficient photodetector devices. Finally, photoluminescence (PL) is used to investigate the optical properties of InGaN films. Figure 1(f) shows the PL spectra for InGaN measured at room-temperature, it is observed a narrow PL peak at 448 nm with an FWHM of 20.5 nm. Furthermore, no defect peak is identified at the PL spectra, which indicates the low concentration of defects for InGaN film. According to the relationship between the photon energy (E, eV) and the photon wavelength (λ, nm), the bandgap value of InGaN is calculated as 2.76 eV. The In content in InGaN alloy can be estimated by the classic Vegard’s law, and the calculation formula is as follows [29]:
where Eg(GaN) = 3.4 eV, Eg(InN) = 0.7 eV, and bowing parameter b = 1.43 eV. According to Eq. (1), the value of x is 0.16.To investigate the photoelectric conversion performance of InGaN-based PDs with different numbers of graphene layers, the InGaN PDs, S-Gr/InGaN PDs, and T-Gr/InGaN PDs prepared separately. The structure schematic diagram for Gr/InGaN PDs is shown in Fig. 2(a), Al2O3 is deposited on the InGaN films as isolation lay under one of the electrodes, and the graphene covers the InGaN and the isolated electrode. Figure 2(b) illustrates the optical microscope (OM) photo for Gr/InGaN PDs, and the image is consistent with the structure shown in Fig. 2(a). The tailored Gr with less wrinkle can be observed as the junction area and transparent electrode. The Raman spectra are used to study the number of Gr layers for Gr/InGaN PDs. Figure 2(c) shows the room temperature Raman spectra of the InGaN films, the sharp peak at 521 cm−1 attributed to Si substrates. The peak at 568 and 734 cm−1 corresponds to E2 (high) and A1 (LO) phonon vibration modes of hexagonal InGaN, respectively [30,31]. The result also suggests that InGaN has an excellent crystalline quality. Figure 2(d) shows the typical Raman spectra for Gr transferred onto the InGaN surfaces. The Raman spectra present the G band and 2D band for Gr located at 1586 and 2697 cm−1, respectively. Furthermore, it is known that the layer number of Gr could be determined by the 2D to G ratio (I2D/IG). The I2D/G in this study for the S-Gr and T-Gr were 1.37 and 0.94, respectively, which confirms that Gr has been successfully transferred on InGaN films [32,33].
Fig. 2. (a)The structure for Gr/InGaN PDs. (b)OM image for Gr/InGaN PDs. (c)Raman spectroscopy for InGaN. (d)Raman spectroscopy for Gr on InGaN.
Figure 3(a) shows the semilogarithmic current versus voltage (I-V) curves under dark and light illumination (λ=405 nm) for InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs. The Idark-V curve of Gr/InGaN PDs exhibits a weaker rectifying behavior in the Gr/InGaN Schottky junctions, which are caused by the high density of interface states between Gr and InGaN. Compared with the InGaN PDs, the dark current of Gr/InGaN PDs is increased, which may be attributed to the defects formed by the wrinkle of Gr. It should be noted the dark current of S-Gr/InGaN is higher than that of T-Gr/InGaN, this behavior can be ascribed to the high-density recombination radiation center. And the center is caused by the wrinkles which formed through the S-Gr releases surface energy [34–38]. The corresponding model is shown in Fig. S2. The previous study indicates wrinkle of Gr is determined by the number of graphene layers, and the wrinkle decreases with the number of Gr layers increases. So, as shown in Fig. S2(a) and (c), there are high-density recombination radiation centers on the surface of S-InGaN, which is the reason for the higher dark current of S-Gr/InGaN PDs as compared with that of T-Gr/InGaN PDs. Moreover, the S-Gr and T-Gr all exhibit a p-doping due to adsorbing oxygen and water molecules in normal atmospheric conditions, as shown in Fig. S2(b) and (d).
Fig. 3. (a)The I-V curve, (b) The responsivity, (C)The photocurrent as a function of light power density at a bias of −3 V under 405 nm light illumination for InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs. (d)The interface and oxygen molecule desorption models for S-Gr/InGaN PDs, (e)The energy band diagram under light for S-Gr/InGaN PDs, The UPS spectra (f) and valence spectra (i) of InGaN, (g) The interface and oxygen molecule desorption models for T-Gr/InGaN PDs, (h)The energy band diagram under light for T-Gr/InGaN PDs.
The ideality factor (n) is used to describe the recombination mechanism, and the I-V characteristic of the Schottky junction can be described by the following function [20]:
where Js is the saturation current density, e is the electronic charge, n is the diode ideality factor, KB is the Boltzman constant and T is the absolute temperature. The ideality factor n for InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs derived through the logarithmic fitting of the I-V curves was 2.16, 2.44, and 1.62, respectively. The higher n also demonstrates a high-density recombination center for S-Gr/InGaN PDs caused by the wrinkle.The light current value at the reverse bias of −3 V for InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs is 1.48${\times} $10−7, 2.32${\times} $10−7, and 5.98${\times} $10−6 A, respectively. The results indicate the introduction of S-Gr to InGaN PDs does not significantly improve the light current. On the contrary, the dark current is increased. As control and as a comparison, the T-Gr/InGaN PDs show a higher light current than that of InGaN and S-Gr/InGaN PDs. The reason for this phenomenon can be explained by the reduction of the Schottky barriers caused by the high interface density of S-Gr and oxygen desorption, and the underlying mechanisms could be explained by the models and energy band diagram [19], as schematically illustrated in Fig. 3.
The energy band alignment of the obtained InGaN was investigated by ultraviolet photoelectron spectroscopy (UPS). According to the UPS spectra of InGaN for work function, given in Fig. 3(f), the value of work function of InGaN is 4.26 eV (the work function of InGaN is calculated through equation φ=hv-Ecutoff, where hv is 21.22 eV [27]). In addition, the valence band edges of InGaN is 1.83 eV obtained from the UPS spectra of InGaN, as shown in Fig. 3(i). Combining the valence spectra and PL spectra, the position of VB maximum of InGaN is shown in Fig. S2(b) and (d). When graphene is transferred onto the InGaN films and contacted together, a Schottky barrier height of ΦB will be formed between graphene and InGaN, which points from InGaN to Gr. Under the light illuminate, the oxygen molecules are desorbed, which releases electrons and contributes to n-doping [19]. It is known that the work function of graphene decreases about ∼0.15 eV under light illumination, therefore, the Schottky barrier height for Gr/InGaN PDs would decrease correspondingly. However, at the reverse bias, the interface states, defects play a more decisive role to reduce the Schottky barrier height for Gr/InGaN PDs.
The spectral responsivity (R) of our devices was evaluated by the following formula [39]:
Herein, Iph = Ilight-Idark is the photocurrent, P is the incident light power density, and S is the in-plane area of the device. As shown in Fig. 3(b), the maximum value of R for InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs is 0.42, 0.38, and 1.39 A/W under the condition of −3 V at 430 nm, respectively. The results prove that the S-Gr is unhelpful for photoelectric conversion of InGaN PDs, and the introduction of T-Gr increases the responsivity for InGaN PDs about three times. Moreover, the value of R for InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs is 0.01, 0.03, and 0.03 A/W under −3 V at 600 nm, respectively. The blue/600 nm rejection ratio for these devices is of 28.29, 11.56 and 122.79, respectively, which indicate high spectral selectivity of the blue region.
To further evaluate the photoelectric conversion characteristics of InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs, we also measured the photocurrent as a function of the incident light power density P, ranging from 0.05 to 300 µW/mm2, under light illumination of λ=405 nm, the calculation formula for α is as follows [40]:
The photocurrent of InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs increases with increasing P, which follows Iph∝Pα. As shown in Fig. 3(c), at −3 V bias, the α for InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs is 0.24, 0.14, and 0.57, respectively, which suggests a complex process of photogenerated carriers excitation, recombination, trapping, and diffusion.
The linear dynamic range (LDR) is used to characterize the linear response capability under a range of the light intensity for PDs, the LDR can be expressed by the following formula [41]:
Here, Iph is the light current and Id is the dark current of the PDs. According to Eq. (5), at −3 V bias, the LDR for InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs is 8.8, 5.4, and 19.4 dB, respectively, which prove that the introduction of T-Gr into InGaN is an effective way to improve the LDR for InGaN-based PDs.
A fast response speed of InGaN-based PDs is expected to be applied in the VLC system because that directly determines the transmission rate of the VLC system. The response speed of the InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs, is investigated by the pulse measurements system, which is composed of a blue laser (405 nm, 30 mW, 100 Hz) as the incident light source, an optical chopper as the ON/OFF switches, and an oscilloscope as the data recorder, as shown in Fig. 4(a). As shown in Fig. 4(b), the fast response signals of the T-Gr/InGaN PDs are observed, the rise times (τr)/fall times (τf) is measured to be 60/200 µs at −3 V bias. In the same way, the τr/τf of InGaN and S-Gr/InGaN PDs are measured to be 0.4/3.7 ms and 4/9 ms at −3 V bias, respectively, given in Fig. S3. Compared with the T-Gr/InGaN PDs, the τr/τf of InGaN and S-Gr/InGaN PDs is relatively long. The high response speed of T-Gr/InGaN PDs can be ascribed to the strong built-in electric field formed by the T-Gr and InGaN, which will facilitate charge carrier extraction. It is worth noting that the τr/τf of S-Gr/InGaN PDs is slower than that of InGaN and T-Gr/InGaN PDs, which is caused the high-density wrinkle of S-Gr.
Fig. 4. (a) The schematic illustration of the experimental measurement setup for temporal response time of the Gr/InGaN PDs device. (b)The responding speed at −3 V@405 nm, (c)The responding speed at −3 V@405 nm with different light densities, (d)The responding speed at 0 V@405 nm for InGaN, S-Gr/InGaN, and T-Gr/InGaN PDs.
Figure 4(c) shows light power density-dependent photoresponse properties of T-Gr/InGaN PDs, and it is observed the photocurrent can be switched on and off repeatedly with different power densities. After several cycles, the photocurrent remains constant, demonstrating that the T-Gr/InGaN PDs can be used as a sensitive and fast photodetector. In addition, the photoresponse properties of T-Gr/InGaN PDs at 0 V bias is also studied. As shown in Fig. 4(d), the current for T-Gr/InGaN PDs increases to a stable value and then drops to the dark current value under the light turn ON and OFF. Although the rise and fall process of current is slow and non-linear, the results still demonstrate a self-powered peculiarity for T-Gr/InGaN PDs. Consequently, the T-Gr/InGaN PDs was even better than that of many Gr-based and GaN-based PDs with a fast response, as shown in Table 1.
Table 1. Research progress of Gr-based and InGaN-based PDs
4. Conclusion
In summary, high responsivity and fast response PDs have been fabricated based on T-Gr/InGaN heterojunction PDs. The as-fabricated T-Gr/InGaN Schottky junction dramatically promotes the carrier generation and transfer, and significantly enhances the photocurrent and reduces the response time. By comparing the photoelectric characteristics of Gr/InGaN PDs with different numbers of graphene layers, we find the T-Gr can effectively improve the photoelectric properties of InGaN-based PDs. The T-Gr/InGaN PDs show a high responsivity of 1.39 A/W@−3 V and a short rise/fall time of 60/200 µs, and the performance of T-Gr/InGaN PDs is better than the previously reported InGaN-based PDs and Gr-based PDs. By correlating experimental results, we have carefully studied the underlying mechanism and highlighted the influence of graphene layers in trapped charges, which would better understand the changes in electrostatic potential between the InGaN and Gr. This work provides a deep insight to realize high speed and selective detection InGaN/Gr Schottky junction PDs for the future application in VLC system.
Funding
National Key Research and Development Program of China (2018YFB1801900, 2018YFB1801902); Distinguished Youth Foundation of Guangdong Scientific Committee (2021B1515020001); Young Elite Scientist Sponsorship Program by CAST (YESS20200016); Key Area Research and Development Project of Guangdong Province (2020B010170001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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