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Abstract
Energy-saving photodetector (PD) with fast response speed is a key component of the next-generation photonic systems. In this work, self-powered photoelectrochemical (PEC) PD based on vertical (In,Ga)N nanowires (NWs) has been proposed and demonstrated successfully. With deionized water solution, the (In,Ga)N NWs are stable and the PEC PD is eco-friendly. The PEC PD has a good stability in terms of good on/off switching behaviors after continuously working for a few hours. The PD exhibits a high sensitivity under very low light illumination intensity of 6.4 μW/cm2. A fast rise/fall time of ∼54/55 ms with good symmetry can also be achieved. Moreover, the NW core-shell structure is proposed to provide an additional way for electron-hole carrier transport, which could play a key role in accelerating the response speed. This work paves a way to develop high-performance PEC PDs for the wide applications in wireless visible photodetection and communication.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Self-powered device can work independently and sustainably, which is one of the most effective approaches for the development of next-generation electronic systems [1–5]. As an indispensable part of a high-efficiency photoelectric sensing system, self-powered photodetectors (PDs) have attracted enormous attention [6,7]. Thanks to the ability to convert photons into electrical signals, photoelectrochemical (PEC) PDs are crucial to be utilized as the signal receivers for optoelectronic systems, such as the medical system [8], communication system and environmental monitoring system, etc [9,10]. With the advantages of simple equipment and low cost [11–14], PEC PDs are promising to construct a system without an external energy supply. In recent years, some materials have been utilized in the PEC field, such as silicon (Si) and III-V semiconductors [15–17]. Due to the advantages of tunable bandgap (0.7–3.4 eV), being non-toxic, long lifetime and superior stability against radiation, indium gallium nitride [(In,Ga)N] is emerging as an excellent candidate material for making PEC devices [18–20]. Thus, (In,Ga)N is suitable for fabricating an efficient and sustainable PDs, which can expand their applications in wireless visible photodetection and communication [21].
Compared with conventional planar films, nanowires (NWs) have a larger surface-to-volume ratio, which will increase optical absorption and photogenerated carrier density [21]. By relaxing strain and reducing defects, NWs are beneficial to achieve high In compositions [22], which are good for engineering the visible detection range of PDs. However, very few works have been reported to utilize (In,Ga)N NWs for PEC PDs. In our previous work, (In,Ga)N NWs have been applied in the electrochemical (EC) reaction [23,24]. After a very short time (<10 min), (In,Ga)N NWs can be detached from the epitaxial wafers. The EC solution can penetrate into the space among NWs to accelerate the etching process. In other words, when fabricating (In,Ga)N NWs for PEC PDs as that way shown in Ref. [25], the PD lifetime should be very short, resulting in the very low stability. Furthermore, the acid and alkaline etching solutions have potential safety hazards, which can hinder the applications of PEC PDs based on (In,Ga)N NWs. Therefore, an effective approach to fabricate self-powered (In,Ga)N NW PDs with high stability and safety is still very challenging but pretty attractive and promising.
In this work, we demonstrate a self-powered PEC PD based on (In,Ga)N NWs structure grown by molecular beam epitaxy. The environmental-friendly PD exhibits high stability and fast response speed. The underlying mechanism contributing to the fast response speed has also been studied.
2. Experimental section
Prior to the molecular beam epitaxy (MBE, Vecco G20) growth, the Si (111) substrates in the growth chamber should be heated up to about 900 °C for 15 min to eliminate native oxides by observing the 7×7 reconstruction. Then the substrate temperature was set to be 830 °C. Initially, GaN NWs were grown with a Ga flux of ∼7.5 × 10−9 Torr for 100 min. After the growth of the GaN section, the (In,Ga)N section was grown for about 50 min with an In/Ga flux ratio of ∼1.1. For the better In incorporation, the substrate temperature was decreased from 830 °C to 650 °C. When starting and finishing growing the (In,Ga)N segment, the shutters of In and Ga cells were opened and closed simultaneously controlled by the software. To increase the NW uniformity, samples were rotated with a rate of 120 °/s during the growth process.
A typical PEC system was used to evaluate the detector performance by an H-type electrolytic cell, which was made from quartz with high transparency in the visible range. The NW samples were used as the working electrode (anode). Cu plate was utilized as the counter electrode (cathode). A piece of In/Au/Al alloys was melted on the Si backside via a welding torch to form an ohmic contact. Then a conducting wire was used to connect the alloys and the electric source. Except the NW surface, these electric contacts were coated by epoxy resin to avoid the current leakage and EC corrosion. The cost for making a PEC PD (except MBE epitaxial cost) was estimated to be less than ${\$}$0.5, including In/Au/Al alloys and conducting wires. Such an inexpensive fabrication procedure should be beneficial for wide applications [23].
NW samples were characterized by scanning electron microscopy (SEM, S-4800, Hitachi). The scanning transmission electron microscopy (STEM) and high-resolution energy dispersive X-ray (EDX) mapping were utilized to measure the morphology and element distribution of NWs. Atomic force microscopy (AFM) was utilized to study the surface roughness. A photoluminescence (PL, PLE-2355, PI-Acton) system was also conducted to study the optical properties by using a 405 nm laser as the excitation source.
3. Results and discussion
As discussed in Refs. [23] and [24], the acid or alkaline solutions can accelerate the EC etching progress, and a thin AlN buffer layer can act as the sacrificial layer (Fig. 1(a)). In other words, the acid or alkaline solutions, as well as the AlN buffer layer can play the key roles in detaching (In,Ga)N NWs, which makes the NWs unstable to be lifted off during the EC processes. As a result, in this work, deionized water was used as the electrolyte instead of the acid and alkaline solutions to slow down the etching rate. Furthermore, to make the PD more stable, (In,Ga)N NWs were designed to be grown by MBE without AlN buffer layer (Fig. 1(b)).
Fig. 1. Schematic diagram of vertical (In,Ga)N NWs (a) with and (b) without AlN buffer layer. (c) Experimental PL spectra of (In,Ga)N NWs. (d) Schematic illustration of a self-powered PEC PD based on (In,Ga)N NWs.
To characterize the content of the In component, the PL measurement was performed. As shown in Fig. 1(c), the PL spectrum is centered at ∼680 nm. According to the physical properties with Vegard’s law [25,30], the PL peak corresponds to about 47% In composition. Figure 1(d) illustrates the working processes of self-powered PEC PD. The (In,Ga)N NW sample acts as the photoelectrode and the H-type reaction cell undertakes the photoelectric detection.
As illustrated in Figs. 2(a) and 2(b), NWs are vertically formed and aligned on the Si substrate. The NW diameters are in a range of 40∼80 nm and the lengths vary from 160 to 200 nm. The STEM images in Fig. S1 (Supporting content) prove that the top thick NWs are (In,Ga)N sections, indicating that the NW structure agrees well with the epitaxial design. The weak In signal (Fig. S1b) indicates the existence of (In,Ga)N core-shell structure. As illustrated in Fig. S2, the root-mean-square roughness (RMS) result of the top NW surface is 18.5 nm. Moreover, Figs. 2(c) and 2(d) show the NW morphology after the 3 h measurement of PEC detection. Compared to the etching results of lift-off NWs in Refs [23] and [25], the NWs within the PD are not lifted off and they remain on the wafer. Thus, the etching effect is limited and the PD is still stable after 3 h EC etching.
Fig. 2. (a) Top-view and (b) side-view SEM images of as-grown (In,Ga)N NWs before PEC detection. (c) Top-view and (d) side-view SEM images of (In,Ga)N NWs after 3 h PEC detection.
As illustrated in Fig. 3(a), the current-voltage (I-V) curve under dark condition avoids the obvious rectification effect, indicating the formation of ohmic contact and an open-circuit potential between the electrodes [26]. The current under 630 nm illumination exhibits an overall upward shift due to the photogenerated carriers. Moreover, the key parameters of the rise time (trise, defined as the time required for photocurrent increases from 10% to 90% of the maximum value) and fall time (tfall, defined as the time required for the photocurrent falls from 90% to 10% of the maximum value) are shown in Fig. 3(b) and Table 1. According to the enlarged curves in Fig. 3(b), trise is around 54 ms and tfall is about 55 ms. The nearly equal trise and tfall indicate the almost same carrier separation/recombination rates and the similar response speeds of PEC PD. To better compare this work with the recent results, Table 1 lists some data of PD response time and PD responsivity. By comparison, our PEC PD has advantages in the transient response, as well as the symmetrical characteristic of trise/tfall.
Fig. 3. (a) I-V curves of the PD in the dark and under the 630 nm illumination. (b) Transient response of the PD under 630 nm illumination and an open-circuit potential. Insets are the enlarged curves of rise time and fall time, respectively.
Table 1. Comparison of response time and responsivity between this work and some recent reports.
Figure 4(a) shows the photo-switching behaviors of the PEC PD under different illumination power. Fig. S3 and Eq. S1 illustrate the method of calibrating the light power density illuminating on the PD during the detection experiments. The current curves show sharp positive and negative spikes when the power density is stronger than 92 μW/cm2, while the spikes are not obvious under lower light power density. At the power density of about 92 μW/cm2, a light/dark current ratio of ∼22.8 can be achieved. To gain further insight into the photocarrier trapping and recombination processes of PEC PD, a critical relationship between the photocurrent and the light power density has been measured and shown in Fig. 4(b). Photoelectric current with incident illumination displays a well linear relationship with increasing light power. Responsivity (R) is another critical index for PDs, which is calculated from the following equation [13,41]:
where Iph is the photocurrent density and Pinc is the incident light power density. The effective area of our incident light is around 0.25 cm2. The calculated $R$ values under different light powers are summarized in Fig. 4(b). R of PEC PD can reach 0.75 mA/W with an illumination power of 6.4 µW/cm2.Fig. 4. (a) Photo-switching behaviors of the self-powered PEC PD under illuminations with different powers and an open-circuit potential. The unit of incident light power density is μW/cm2. (b) Photocurrent density and responsivity as a function of incident light power density.
As illustrated in Fig. 5(a), 50 on/off light cycles have almost no effects on the I-V curve of PEC PD, while the 100 on/off light cycles have a very limited effect. Figure 5(b) presents that the photocurrent density can be effectively switched by controlling the on/off processes of the light source. The periodic on/off illumination can result in a periodic charge and discharge cycle. To further evaluate the stability of the PEC PD, the additional long-time response measurements are carried out under open-circuit potentials for 2 h and 3 h. Figures 2(c), 2(d), 5(c) and 5(d) confirm that this PEC PD has a good stability, including the on/off switching behaviors.
Fig. 5. (a) Photocurrent stability of PEC PD based on (In,Ga)N NWs. Photocurrent response measurement of PEC PD under open-circuit potential when continuously working (b) 300 s, (c) 2 h and (d) 3 h.
To better study the underlying mechanism of the PEC PD, the schematic illustrations are plotted in Fig. 6. Compared with planar films, NWs could absorb photons and generate carriers more easily because of the larger surface-to-volume ratio. As the (In,Ga)N/GaN NWs grown by MBE are normal core-shell structures, which are possible to provide two ways for carrier transports (Fig. 6(a)). The top NW surfaces can absorb photons and the photogenerated electron-hole (e--h+) carriers can transport in the vertical direction as the common way (Way I in Fig. 6(a)). Apart from the top NW surfaces, the (In,Ga)N shell (sidewall) can also absorb photons and generate e--h+ carriers. Such carriers can transport in both the vertical (sidewall) and horizontal (core/shell heterojunction) directions (Way II). Furthermore, due to the light trapping in the NW assembly, the core/shell NWs could potentially optimize the light absorption and carrier transfer [40,42]. Therefore, the core-shell structure could allow a more efficient and faster carrier separation and collection [42–44], leading to the fast response speed of the PD (Table 1).
Fig. 6. Schematic illustrations of (a) the NW structure and (b) the corresponding energy band diagram under 630 nm illumination.
Figure 6(b) shows the electronic energy levels. When the (In,Ga)N section is in contact with the electrolyte, an EC equilibrium is established by conveying electrons from the NWs to the electrolyte, resulting in an upward band bend at the (In,Ga)N NW/electrolyte interface [45]. In dark conditions, the band bending at the interfaces acts as energy barriers to block the carrier transport, leading to a low dark current of the PEC PD (Fig. 3(b)). When the PEC PD is under illumination, the strong built-in electric field caused by band bends may lead to the rapid separation of electrons and holes in the (In,Ga)N section. The production of photoelectric current is likely to promote the water splitting into hydrogen and oxygen via the following reactions [46,47]:
The whole circuit with both light harvest and carrier transport can be completed in the absence of external bias. As shown in Fig. 3(b), the current shifts positively when the light is on (Process I). This positive current indicates that photogenerated holes transfer to the top NW surface while the electrons transfer to bottom NWs (Fig. 6(b)). However, the junction barrier at the interface obstructs the electrons passing through. These electrons are blocked and accumulated around the surface of the NWs. Thus, the flow of electrons through the external circuit is possible to be detected as an electrical pulse in Fig. 4(a) [2]. As time goes on, the current density gradually decreases to a new steady state under continuous illumination (Process II). When the light is off, electrons transfer to the top NW surface (Process III), which is the opposite direction of process I, leading to the recovery of dark state (Process IV). On the other hand, strong light intensity causes a large number of carrier accumulation, the width of the interface depletion area and the internal electric field may be affected by the light intensity [48,49]. From Fig. 4(a), the peak value of the positive charge is slightly larger than the negative one, which indicates that the separation of electron-hole pairs in the depletion layer plays a more significant role under illuminations.In the further study, the NW density, NW morphology and NW energy band can be engineered and optimized to enhance the photocurrent and responsivity densities based on this work. It is beneficial to further improve the PD performance and promote its applications.
4. Conclusion
In conclusion, an environmentally-friendly PEC PD based on (In,Ga)N NWs has been fabricated successfully with a low cost. Due to the band bend at the NW/electrolyte water interface, the PD can operate with a fast response speed of ∼54/55 ms. The NW core-shell structures could allow an efficient and fast carrier transfer. The on/off light behaviors of this PEC PD are stable. A responsivity of around 0.75 mA/W under 630 nm illumination can be achieved. In addition, the stabilities of on/off light cycles and photocurrent have also been demonstrated. Therefore, the proposed PEC PD is promising for wide applications requiring low cost, low power consumption, eco-friendly and excellent stability, such as the self-powered detection and communication systems, etc.
Funding
National Key Research and Development Program of China(2018YFB0406900, 2018YFB0406902); Natural Science Foundation of Jiangsu Province (BK20180252); Jiangsu Provincial Key Research and Development Program (BE2018005);Key Research Program of Frontier Science, Chinese Academy of Sciences (ZDBS-LY-JSC034); National Natural Science Foundation of China (61804163, 61827823, 61875224); Natural Science Foundation of Jiangxi Province (20192BBEL50033); National Key Scientific Instrument and Equipment Development Projects of China CAS (YJKYYQ20200073); National Key Scientific Instrument and Equipment Development Projects of China SINANO (Y8AAQ21001); Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics (B2006).
Acknowledgments
We are thankful for the technical support from Platform for Characterization & Test of SINANO, CAS.
Disclosures
The authors declare no conflict 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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