GaN with Laterally Aligned Nanopores To Enhance the Water Splitting

solar water splitting. Rami T. ElAfandy , Mohamed Ebaid , Jung-Wook Min , Chao Zhao , Tien Khee Ng , Boon S. Ooi. Optics Express 2018 26 (14), A64...
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GaN with Laterally Aligned Nanopores To Enhance the Water Splitting Chao Yang,†,‡ Lei Liu,†,‡ Shichao Zhu,†,‡ Zhiguo Yu,† Xin Xi,†,‡ Shaoteng Wu,†,‡ Haicheng Cao,†,‡ Jinmin Li,†,‡ and Lixia Zhao*,†,‡ †

Semiconductor Lighting Research and Development Center, Institute of Semiconductors, Chinese Academy of Sciences, No. A35, Qinghua East Road, Haidian District, Beijing 100083, China ‡ College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, No. 19A, Yuquan Road, Shijingshan District, Beijing 100049, China S Supporting Information *

ABSTRACT: GaN with aligned nanopores was fabricated using a lateral anodic etching process in HNO3 solution. This laterally porous structure can be modified from triangular pores to quasi-circular pores with increasing the voltage, indicating the transformation from anisotropic etching gradually toward isotropic etching. Furthermore, we have established the correlation between the etching current and pore trajectories based on the in situ chronoamperometry and find that the lateral etching is initially driven by the avalanche effect, then enter a steady state as a balance between the oxidation and dissolution of GaN at the pore tips. The water splitting properties of the laterally porous photoanode have also been studied. Compared with the as-grown GaN film, nearly 3.4 times enhancement of self-driven photocurrent was achieved for the porous GaN with triangular pores. Our findings not only reveal the formation kinetics of porous GaN but also pave a way for the application of solar water splitting using laterally porous GaN.

1. INTRODUCTION Water splitting using solar power has drawn increasing attention because of the requirement of the clean and renewable technique for hydrogen production.1−3 Till now, many semiconductor materials, such as TiO2,4 GaP,5 and GaAs,6 have been explored for the solar hydrogen production, but most of them have the limitations either in light absorption or material stability.7−9 As an extremely promising material for optoelectronics,10 GaN has a wide direct bandgap and the bandgap is tunable after alloying with indium to cover the complete solar spectrum, which will enable more efficient absorption for solar water splitting.11 The self-driven water splitting can also be realized because the energy bandgap of GaN can straddle with the water redox potentials.12−14 Furthermore, both the GaN-based materials and underlying sapphire substrate (widely used) are chemically inert to the harsh electrolyte, leading to an improved device durability.15,16 However, until now, the solar-to-hydrogen conversion efficiency for GaN photoanode is still low (∼1.8%)17 and cannot meet the requirements for practical hydrogen production. Porosification of GaN offers a potential cost-effective solution for high efficient water splitting. With the embedded air-gaps, the surface-to-volume ratio of porous GaN is increased,18 so the active area can be increased during the water splitting reactions. Furthermore, after porosification, the migration distance of photogenerated carriers toward the semiconductor/solution interface will be reduced. It will suppress the recombination of the photogenerated carriers and increase the photocurrent significantly. However, for the porous GaN photoanode, most © XXXX American Chemical Society

studies on water splitting have been focused on the vertical pores by anodic etching.11,13,15 As the vertical etching process is restricted to the thickness of a GaN epilayer, we cannot get longrange pores to sufficiently use the pore walls for water splitting reactions. In addition, the anodic etching route for the formation of vertical pores will leave a low-porosity (nucleation) layer at the top surface, which is a bottleneck for the electrolyte diffusion into the pores.19 Recently, III-nitride photonic devices based on the laterally porous GaN have been reported.20−23 With a surface passivation layer, the lateral pores can propagate into the GaN layer from the exposed side walls. Different from the vertical etching, the propagation depth of lateral pores is not restricted to the thickness of GaN and can be simply controlled by the etching time. The pore morphology and etching trajectory can be identified easily, and the lateral etching process can be monitored continuously to understand the pore formation. This laterally porous GaN can also effectively increase the surface-to-volume ratio of GaN film, with almost no degradation of the electrical conductivity.20 In addition, defects can be introduced significantly during the exposure of GaN side walls, and the initial surface pits originated from these defects will increase. Thus, the porosity of the nucleation layer can be effectively increased, which will help to further improve the electrolyte diffusion for the water splitting. However, the Received: January 23, 2017 Revised: March 5, 2017 Published: March 9, 2017 A

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The lateral etching process was carried out in the twoelectrode cell at room temperature with 65 wt % HNO3 solution (Figure 1d), where a constant voltage was supplied by a Keithley 2420 source meter. Considering the breakdown effect of SiO2 at higher voltages (Supporting Information Figure S1), etching voltages from 10 to 25 V were used to fabricate the laterally porous GaN. During the etching process, the HNO3 solution was stirred magnetically to remove the bubbles from LS channels, and the real-time current during the etching process was in situ measured. All the chips have been fully etched to form the porous structures along the 300 μm-spacing GaN layer, as indicated by the overlapping etched area between the adjacent side walls. The etched area showed a different color from the initial area so that it can be easily distinguished using optical microscopy. Finally, SiO2 of the GaN chip was removed (Figure 1e), and all samples were ultrasonically cleaned using deionized water and dried in N2. The pore morphologies and etching trajectories of the laterally porous GaN were characterized using scanning electron microscopy (SEM, Hitachi S-4800). The optical reflectance properties of different GaN chips were studied by an UV−vis− NIR spectrophotometer (Varian Cary-5000). The water splitting experiment was performed in a quartz cell using the three-electrode configuration with an Ag/AgCl reference electrode. A 300 W Xe arc lamp was used as the light source, and the light intensity illuminated at the GaN chip was 100 mW/cm2 (one Sun illumination), as measured using a radiometer. Voltammogram and chronoamperogram were performed using a CHI 660E electrochemical workstation, with 1 M NaOH solution as the electrolyte solution12,25 during the water splitting reactions.

influence of purely lateral GaN pores on the water splitting has not been explored yet. In this work, GaN materials with different laterally porous arrays have been fabricated using anodic etching. We find that the formation of lateral pores is strongly dependent on the crystal plane and anodic voltage. Anisotropic etching process can occur at lower voltages, but it will gradually transform toward isotropic etching with increasing the voltage. Furthermore, we have established the correlation between the etching current and pore trajectories using in situ chronoamperometry, and systematically revealed the kinetics mechanism behind the lateral etching process. The water splitting properties of the laterally porous photoanode have also been studied. Compared to the as-grown GaN, nearly 3.4 times increase of self-driven photocurrent has been achieved for the porous photoanode with triangular pores, suggesting the laterally porous GaN can be served as a promising material to improve the water splitting efficiency.

2. EXPERIMENTAL SECTION The investigated GaN epilayers were grown on c-plane sapphire using metal−organic chemical vapor deposition (MOCVD). First, the sapphire was thermally cleaned under H2 at 1100 °C for 10 min, and then a 30 nm nucleation layer was deposited onto the substrate at 525 °C. Afterward, a 1 μm undoped GaN layer was grown at 1060 °C, followed by a 2 μm Si-doped nGaN layer grown at 1045 °C. Typical electron concentrations of the n-GaN layer are 2.5 × 1018 cm−3 with a mobility of around 320 cm2 V−1 s−1, as determined using Hall measurements. Figure 1 shows the schematic process to fabricate the laterally porous GaN. First, a GaN epilayer was grown on c-plane

3. RESULTS AND DISCUSSION 3.1. Pore Morphology. Figure 2a shows the cross-sectional SEM images of the laterally porous GaN etched at different voltages. The morphologies of the lateral pores are strongly influenced by the applied etching voltage (Ue). At higher voltages of 20 and 25 V, quasi-circular pores were obtained, but triangle and raindrop pores formed at lower voltages of 10 and 15 V. The triangle pore morphologies indicate that when applying a lower Ue, there is a strong anisotropic etching speed depending on the crystal planes. With increasing the voltage, the anisotropic etching is gradually replaced by the isotropic etching. Figure 2b shows the statistics of the pore size distribution in the cross-sectional SEM images. The pore size increases significantly with increasing the Ue, and the distributions of the pore size also gradually become wide. The porosity of laterally porous GaN chips etched at 10, 15, 20, and 25 V were calculated to be ∼6%, 15%, 24%, and 40%, respectively, which increases with increasing the Ue as well. We have further investigated the anisotropic etching and evolution of the pore morphologies with increasing the Ue. Typically, there are three kinds of triangle pores observed for the porous GaN etched at 10 V during the SEM measurements, as summarized in the bottom part in Figure 3. The typical angles between the pore wall and the bottom (0001) Ga plane is about ∼75° for the left-most pore but ∼58° for other triangular pores. The slow-etching planes with ∼75° and ∼58° angle are attributed to the {202̅1} and {101̅1̅} family of planes, respectively, which are resistant to the broadening of pores at lower Ue. The anisotropic etching behaviors can also explain the preferential c axis growth, as always observed for the GaN nanowires.26 Considering the smaller surface energies of {1011̅ }̅

Figure 1. Schematic process diagram for the laterally porous GaN. (a) Epitaxial growth of GaN. (b) Deposition of SiO2. (c) Laser scribing for the etching channels. (d) Anodic etching. (e) Removal of SiO2.

sapphire (Figure 1a). Then, a passivation layer of 800 nm SiO2 was deposited on top of the epilayer using plasma-enhanced chemical vapor deposition (Figure 1b). To expose the side walls of n-GaN film, parallel etching channels with a 300 μm-spacing were defined along the cleavage plane (a-plane) using a laser scribing (LS) process, as shown in Figure 1c. Afterward, the GaN epilayer was cut into a chip with an area of 0.45 × 1.5 cm2, and the front contact area (∼2 mm-wide) of the chip was clamped directly by a Pt sheet to serve as the anode. Another Ptelectrode (1 cm2) was served as the counter cathode. B

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Figure 2. (a) Cross-sectional SEM images of the pore morphologies etched at different voltages. (b) Statistics of the pore size distribution and the mean pore diameter dm.

etching. With further increasing the Ue, crystal planes with high surface energies will become unstable and gradually disappear. Therefore, the anisotropic etching perpendicular to the pores gradually transforms toward the isotropic etching, and results in the raindrop pores. Finally, the pore tips become wide all around and most of the raindrop pores transform into the quasi-circular pores. 3.2. Etching Trajectory. To evaluate the pore trajectories along the etching direction, the laterally porous GaN chips were peeled off from the top surface using the inductively coupled plasma (ICP) with a low bias power (15 W, 300 s). Figure 4a shows the top-view SEM images of the lateral etching trajectories after the ICP process. For the porous GaN etched at 10 V, the trajectories parallel to each other along the etching direction show an obvious curved morphology with some side branches. With increasing the Ue, the trajectories become aligned with each other and show a highly parallel morphology at 20 V. These porous patterns are highly uniform across the length of the porous GaN chip, as shown in the Supporting Information (Figure S2). With further increasing the Ue to 25 V,

Figure 3. Evolution of the typical pore morphologies with increasing the voltage. The scale bar is 40 nm.

planes than that of {202̅1} planes, it suggests that the triangular pores with different angles and walls reflect the different stages of the pore formation at lower Ue. At 10 V, the energy of the reaction ionic is not enough to break all the covalent bonds surrounding the pore tips, which leads to the anisotropic

Figure 4. (a) Top-view SEM images of the etching trajectories at different voltages, located at the middle etched area. (b) The micrograph of the laterally porous GaN etched at 15 V for 5 min. (c,d) Top-view SEM images of (b) at the pore initiation (A) and the pore tip (B), respectively. (e) The current density−time (J−t) plots at different voltages. Inset is the current density in the first 1 s. C

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The Journal of Physical Chemistry C the pore trajectories start to deform but still maintain a longrange parallel morphology. Furthermore, we focused the lateral propagation properties of the pore initiation and the pore tip (area A and B in Figure 4b). Figure 4c,d shows the typical trajectories at the corresponding area A and B, respectively. The porous patterns around the side walls are different from the inner etching trajectories. At the pore initiation (Figure 4c), there are lots of radiated porous patterns acting as the terminal distributed discharge. Details of the initial trajectories at different etching voltages can be found in the Supporting Information (Figure S3). While during the pore growth (Figure 4d), the propagation of the lateral pores is highly aligned at the tips. The morphology and size of the etching trajectories are highly uniform along the etching direction, which will not be affected during the sequent propagation of the pore tips. To investigate the formation kinetics behind these initial trajectories, in situ chronoamperometry has also been carried out during the etching process. Figure 4e shows the typical current density as a function of the etching time (J−t plots). Obviously, the J−t plots of lateral etching for all voltages can be divided into three stages. At the beginning, the current density increases dramatically to the maximum, followed by a continuous decrease and finally stabilizes at a constant current. Here, the current behaviors before and under steady state can reflect the kinetics process of pore initiation and pore growth, respectively. Therefore, we can investigate the influence of the etching current on the pore trajectories in more detail, which is desirable to further analyze the kinetics mechanism of lateral pores. 3.3. Kinetics Mechanism. For Si and III−V semiconductors,24 the electrochemical oxidation and the chemical dissolution can take place simultaneously or successively during the etching. The reactions for the formation of laterally porous GaN can be illustrated as follows.13 When a positive voltage is applied to the GaN immersed in an electrolyte, oxidation of nGaN by holes would occur near the interface of GaN/ electrolyte: GaN + 3H 2O + 3h+ → Ga(OH)3 +

1 N2 + 3H+ 2

Figure 5. (a) Energy band diagram of lateral etching at the GaN/ solution interface. (b−e) Graphic illustration of the etching mechanism in HNO3 solution (top-view images). (b) Formation of SCR. (c) Initial etching at the surface pits. (d) Avalanche effect and multiple current flows branch off from the surface pits. (e) Steady-state growth of lateral pores.

process,13,28 and they can influence the local electric field distribution at the n-GaN/electrolyte interface. Therefore, the holes generated within the SCR will transfer to the interface and react preferentially at the specific initiation sites, resulting in the formation of surface pits, as shown in Figure 5c. Subsequently, strong band bending occurs at the inner edge of the SCR around the pits. The electrons inside of the SCR will be accelerated and finally get sufficient energy to create the impact ionization, leading to the avalanche breakdown29 at the pore initiation. As majority of holes flush into the GaN bulk, multiple current flows generate from the avalanche site and then branch off with the pore propagation, which allows the subsequent pores growth (the current line oriented model24). Therefore, the radiated etching trajectories appear around the side walls (Figure 5d). Because of the avalanche process, a large number of carriers will generate exponentially throughout the depth of SCR instantly, resulting in the dramatic increase of current density in the J−t plots. In this case, generation of holes is not a rate-limiting step anymore for the whole etching reaction, and the holes will accumulate at the GaN surface, which will dramatically accelerate the oxidation of GaN (eq 1). With the fast oxidation rate, the time is not sufficient enough for the oxide product to dissolve (eq 2). Therefore, the contact between GaN and solution is locally blocked by the broadening oxide layer, which will make the current density continuously decrease. However, when the oxide layer gradually dissolves with increasing the etching time, the oxidation (eq 1) and the dissolution (eq 2)

(1)

At higher voltages, the holes may participate in the oxidation of water as well.13 Since the oxidized product of GaN is not thermodynamically stable in the acid electrolyte, it will dissolve into Ga3+ for continual etching of GaN: Ga(OH)3 + 3H+ → Ga 3 + + 3H 2O

(2)

The lateral etching starts at the pore initiation along the side wall, then the lateral pores propagate into GaN for a steady-state growth. When the n-GaN chip is immersed in the acid solution, Schottky junction forms at the n-GaN/electrolyte interface initially. Considering the band bending (Figure 5a), when the etching voltage Ue is higher than the flat-band potential, the electrons will rapidly deplete at the interface, and the space charge region (SCR) will form accordingly, as shown in Figure 5b. The slight current drop in the first 0.1 s (inset of Figure 4e) just corresponds to the formation of SCR, which is also consistent with the previous result of the impedance spectra at the low frequency range of 102−105 Hz.27 Since the GaN layer based on the heteroepitaxial growth normally contains various defects, and during the LS process, some defects can also be introduced across the side wall. These defects will have a high chemical activity during the etching D

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The Journal of Physical Chemistry C would balance with each other at the pore tips and finally reach a steady-state current. Thereby, at a suitable Ue, the pores could maintain a highly steady propagation at the tips, resulting in the successive anodization of GaN (Figure 5e) as well as the aligned etching trajectories. The formation of branching or aligned pores is determined by the space between adjacent pores. Based on the depletion model,28 when the space between adjacent pores is too close, it would become completely depleted as SCRs overlap, and the holes cannot be supplied from the pore walls. When the branch of the etching current is cut off, the pores can be allowed to extend only at the tip. Otherwise, if the space is large enough, etching can take place at the pore walls besides the pore tips. Therefore, the lateral propagation of pores parallel and perpendicular to the etching direction are both available, and side branching effect appears.13,28 3.4. Water Splitting Performance. Figure 6a shows the water splitting performance of the laterally porous GaN as a

laterally porous GaN decreases, but it still shows a significant enhancement after porosification. Since the laterally porous GaN provides large surface−volume ratio for the water splitting reactions, it allows a greater current flow between GaN and solution.15 The increased reaction area can reduce the current crowding at the GaN/solution interface, thus enhancing the water splitting efficiency especially when the density of photogenerated carriers is high.7 In addition, the small interpore distance can also efficiently suppress the recombination possibility of photogenerated carriers during their transportation toward the GaN/solution interface.11,30 Therefore, the charge separation efficiency is strongly increased for the porous GaN, and it can get a much higher photocurrent compared with the as-grown GaN chip. The gradual increase of photocurrent with increasing the Uw is mainly because of the porosity-induced interface traps. A larger voltage Uw is required to fully compensate the trapped charges by these interface states, and then all the photogenerated carriers can start to contribute to the photocurrent. The maximum photocurrent density was achieved for the laterally porous GaN etched at Ue = 10 V rather than higher Ue. This is due to the balance effect between the increased active area and the reduced optical absorption. With increasing the Ue, the porosity of GaN photoanode increased, which will provide more reaction areas for water splitting reactions. However, compared to the as-grown GaN photoanode, the optical reflectance increased between 300 and 370 nm for the porous GaN etched at 15 and 20 V, and no significant change was observed for the porous GaN etched at 10 V (Supporting Information Figure S4). Therefore, the optical absorption decreased after lateral etching at higher voltages, which will prevent the further enhancement of the photocurrent density. In addition, the photocurrent density−voltage characteristics were measured for the porous GaN and the as-grown GaN using different incident wavelengths over a range of 320−370 nm. Compared to the as-grown GaN, the incident photon conversion efficiency (IPCE) at Uw = 0 V increased to ∼28% at 345 nm for the porous GaN device etched at 10 V. Furthermore, no obvious degradations in the photocurrent density were observed for both the as-grown and porous GaN photoanodes etched at 10 and 15 V after 60 min, suggesting a very good stability for these porous GaN devices, and details can be found in the Supporting Information (Figures S5 and S6).

Figure 6. (a) Photocurrent density−voltage (J−Uw) plots for the asgrown GaN (ref.) and the laterally porous GaN etched at different voltages. (b) Time-dependent photocurrent density of different GaN photoanodes at Uw = 0 V under static conditions, and the illumination period is 10 s.

4. CONCLUSIONS In summary, we have fabricated the aligned porous GaN using a simple lateral etching process. The pore morphologies can be controlled from branching triangular pores to highly parallel quasi-circular pores by applying different anodic voltages. Anisotropic etching can occur at lower voltages, but it will gradually transform toward isotropic etching with increasing the voltage. Based on the in situ chronoamperometry, we find that the formation of lateral pores is initially driven by the avalanche effect, then enters a steady-state pore growth as a balance between the oxidation and dissolution of GaN at the pore tips. This two-stage etching result has also been confirmed by both the etching current and the pore trajectories. In addition, there was a significant enhancement of water splitting performance for the GaN photoanode after porosification. Compared with the as-grown GaN, nearly 3.4 times enhancement of the self-driven photocurrent density was achieved for the porous photoanode with triangular pores. The photocurrent density of laterally porous GaN increases with decreasing the pore size as a balance

function of the water splitting voltage (Uw). As a reference, photocurrent of the as-grown GaN photoanode with identical size and LS lines was measured as well. For the as-grown GaN film, the photocurrent density saturates at ∼0.07 mA/cm2 above −0.25 V. While for the porous GaN photoanode, the saturation photocurrents increase for all etching voltages and the porous photoanode etched at 10 V demonstrates a significantly higher photocurrent density of ∼0.32 mA/cm2. To evaluate the selfdriven water splitting performance, measurements of timedependent photocurrents under static conditions were also carried out, as shown in Figure 6b. The highest photocurrent density at Uw = 0 V is observed for the laterally porous GaN etched at 10 V (∼0.24 mA/cm2), which is ∼3.4 times higher than the as-grown GaN photoanode (∼0.07 mA/cm2). With increasing the etching voltage Ue, the photocurrent density of E

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result of the increased active area and improved optical reflection. Our findings not only reveal the formation kinetics of porous GaN structures but also pave a way for the application of solar water splitting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00748. SEM images of laterally porous GaN; optical reflectance spectra; photocurrent density−voltage curves depending on the incident wavelength; photocurrent density as a function of water splitting time (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-10-82305476. ORCID

Lei Liu: 0000-0001-8157-0088 Lixia Zhao: 0000-0002-0466-247X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (11574306), the National High Technology Program of China (2015AA03A101 and 2015AA033303), and the National Key Technology Support Program of China (2014BAK02B08).

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REFERENCES

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