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Abstract
InGaN-based nanowires (NWs) have been investigated as efficient photoelectrochemical (PEC) water splitting devices. In this work, the InGaN/GaN NWs were grown by molecular beam epitaxy (MBE) having InGaN segments on top of GaN seeds. Three axial heterojunction structures were constructed with different doping types and levels, namely n-InGaN/n-GaN NWs, undoped (u)-InGaN/p-GaN NWs, and p-InGaN/p-GaN NWs. With the carrier concentrations estimated by Mott–Schottky measurements, a PC1D simulation further confirmed the band structures of the three heterojunctions. The u-InGaN/p-GaN and p-InGaN/p-GaN NWs exhibited optimized stability in pH 0 electrolytes for over 10 h with a photocurrent density of about –4.0 and –9.4 mA/cm2, respectively. However, the hydrogen and oxygen evolution rates of the Pt-treated u-InGaN/p-GaN NWs exhibited a less favorable stoichiometric ratio. On the other hand, the Pt-decorated p-InGaN/p-GaN NWs showed the best PEC performance, generating approximately 1000 µmol/cm2 hydrogen and 550 µmol/cm2 oxygen in 10 h. The band-engineered p-InGaN/p-GaN axial NWs-heterojunction demonstrated a great potential for highly efficient and durable photocathodes.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photoelectrochemical (PEC) water splitting is a promising process for hydrogen generation, which utilizes semiconductor materials to realize energy conversion from solar energy to hydrogen chemical energy. Among various semiconductor materials, III-nitrides have shown a great potential for higher performance PEC applications. In particular, InGaN has attracted significant attention because of its tunable bandgap that can cover the entire visible part of the solar spectrum [1–4]. Furthermore, by carefully controlling the In:Ga composition ratio of the InGaN material with its band edges capable of straddling the oxygen and hydrogen redox reactions [5–8], InGaN-based photoelectrodes can fulfill the thermodynamic criteria for water splitting (1.23 eV). Another unique feature of InGaN is its feasibility to grow in the form of nanowires (NWs), free from structure-related defects, on a Si substrate [9,10]; thus, it can be cost-effective and scalable for commercialization. The three-dimensional (3D) geometry of NWs would also result in a larger surface-to-volume ratio compared to planar structures and bulk electrode counterparts. Consequently, this helps to enhance the photogenerated charge carrier separation and collection processes [6,11], and increase the available sites for electrochemical reactions [12,13]. Despite the progress, there is still an urgent need to understand the basic properties concerning their photoelectrochemical (PEC) performance, such as the intentional design of doping and bandgap engineered heterojunction for facilitating extraction of the photogenerated carriers.
Because of different surface charge properties, n-type and p-type semiconductors might exhibit different stabilities in aqueous electrolytes [14]. Under illumination, electron-hole pairs would be generated and separated with help from surface band bending at the semiconductor/electrolyte interface. For n-type semiconductors, there is an upward band bending that helps holes accumulate at the n-type semiconductor/electrolyte interface and oxidize the water into oxygen. The photogenerated holes can behave as oxidizing agents, which can lead to the oxidation of the semiconductor photoelectrode surface, leading to associated photocorrosion. In contrast, the downward band bending in p-type semiconductors results in the accumulation of electrons under illumination, which serves as cathodic protection against photocorrosion [15]. Thus, p-type semiconductors are more promising for high-stability PEC devices [16]. Magnesium is a typical p-type dopant for III-nitride materials [16–19]. It has advantages including low formation energy and efficient dopant incorporation into NWs [19–21], and Mg-doped InGaN-based NWs have already been applied to PEC devices [2,17,22]. The previous study has been done on investigating the influence of doping concentration on tuning the surface Fermi level. However, the band alignment on the axial direction remains to be further studied since it is relevant to the carrier extraction for a PEC device [23,24].
The InGaN NWs that were grown by molecular beam epitaxy (MBE), however, requires the formation of GaN seeds prior to the growth of InGaN, which lead to the formation of axial GaN/InGaN heterojunctions. In addition to the surface charge properties, the interfacial charge kinetics at the GaN/InGaN heterojunction interface is crucial for improving the charge extraction/collection efficiency [25]. In this study, we investigate the influence of the variation of the doping profile of GaN and InGaN on the PEC performance, as well as the stability of the water splitting process. By varying the polarity of GaN and InGaN by Si- and Mg-doping, we aim to control the interface charge kinetics at the axial heterojunctions that can improve the overall PEC performance. The morphological and optical properties of NWs with different GaN/InGaN axial heterojunction designs are also studied. Mott–Schottky measurements were used to quantify the ionized carrier concentrations carefully, the stability of various photoelectrodes was evaluated, and finally, the O2 and H2 gas evolution were quantified. These characterizations are essential building blocks for complete performance evaluation of the doping-engineered heterojunction based photoelectrodes for PEC water splitting.
2. Experimental details
Figure 1(a) shows a schematic of the general structure of the InGaN-based NW photoelectrodes grown by a Veeco Gen930 plasma-assisted MBE system. The InGaN NW is typically grown on GaN seeds on bare silicon substrates and decorated with a suitable metal co-catalyst. Three different structures of different Si-doped (n-type) and Mg-doped (p-type) GaN seeds and InGaN NWs, and hence different heterojunction designs, were proposed in this study. A photoanode composed of an n-type InGaN grown on n-GaN decorated with Ir metal co-catalyst was compared to another two photocathodes with unintentionally doped (u-type) u-InGaN grown on p-GaN and p-InGaN grown on p-GaN structures. The u-InGaN/p-GaN and p-InGaN/p-GaN NWs were decorated with Pt metal co-catalysts to improve the surface charge extraction. Prior to the MBE growth, 20% HF soak, H2O rinse, and final high-temperature treatment of 850 °C in the growth chamber were applied to the silicon substrates to remove the native oxide layer. During growth, the nitrogen plasma source was operated at 350 W with a chamber pressure of 1.5 × 10−5 Torr. In the case of p-InGaN/p-GaN NWs, 50 nm of p-type GaN seed was grown on a p-Si substrate at 660 °C with a Ga beam equivalent pressure (BEP) of 5 × 10−8 Torr, and 150 nm of p-type InGaN NW was grown at 480 °C with a Ga and In BEP of 3 × 10−8 and 1.5 × 10−8 Torr, respectively. The p-type doping of GaN was realized by Mg doping at a cell temperature of 300 and 200 °C for GaN seeds and InGaN NWs, respectively. The u-InGaN/p-GaN sample was grown under the same conditions without Mg doping into InGaN. The reference n-type sample (n-InGaN/n-GaN) was grown on n-type Si (100) substrates using previously reported conditions [13] with a Si cell temperature of 1100 °C for n-type doping.
Fig. 1 General structure and band diagram of InGaN/GaN NWs. (a) Schematic of the InGaN/GaN NW photoelectrode with longer InGaN segments grown directly on GaN seeds. SEM images of: (b) n-InGaN/n-GaN, (c) u-InGaN/p-GaN, and (d) p-InGaN/p-GaN NWs; the insets are the corresponding top-view images at same scales; (e) room-temperature PL spectra.
The morphology of the InGaN/GaN NWs was determined by an FEI Nova NanoSEM 630 field-emission scanning electron microscope (SEM) at an accelerating voltage at 4 kV. The optical properties of the InGaN/GaN NWs were examined by photoluminescence (PL) spectroscopy (LabRAM ARAMIS, Horiba Jobin Yvon) at room temperature using a 325-nm He-Cd laser as an excitation source.
As for the PEC device characterizations, electrochemical impedance spectroscopy (EIS) techniques were performed in the dark to estimate the ionized carrier concentrations inside the NWs. Accordingly, the band structures of the three heterojunctions were simulated with PC1D [26]. It was assumed that the background doping concentrations of the n-InGaN/n-GaN NWs and p-InGaN/p-GaN NWs were homogeneous and equal to the values estimated by Mott–Schottky analysis. The unintentionally doped InGaN segments of the u-InGaN/p-GaN NWs were assumed to have an n-type background doping level of 5 × 1016 cm–3, whereas the doping level of the p-type GaN seeds was in accordance with the Mott–Schottky results. Other physical parameters in the PC1D model were either extracted from the literature [27–33] or specified from the SEM and PL results.
The corresponding PEC performance was studied by a systematic PEC characterization, including open-circuit potential (OCP) measurements, linear scan voltammetry (LSV), gas evolution measurements, and stability tests. All the PEC studies were performed in a three-electrode cell using InGaN/GaN-NWs as the working electrode (WE), a Pt coil as the counter electrode (CE), and an Ag/AgCl as the reference electrode (RE). The reference potential is + 0.197 V vs RHE in pH 0 solution and + 0.61 V vs RHE in pH 7 solution. The u-InGaN/p-GaN and p-InGaN/p-GaN samples were tested in 1 M sulfuric acid (H2SO4) solution (pH 0), whereas a 0.1 M potassium phosphate buffer solution (pH ~7) was used in the case of the n-InGaN/n-GaN sample. The three-electrode cell was connected to an Ametek PARSTAT4000 + potentiostat system. EIS characterizations (Nyquist and Mott–Schottky measurements) were conducted in dark conditions, whereas for the other PEC measurements, a solar simulator (HAL-320, Asahi Spectra) provided the simulated solar illumination, which was fixed at 600 mW/cm2 during the whole study. For gas evolution measurements, a leak-tight glass reactor with a quartz window was utilized, which was connected to a closed high-purity nitrogen gas circulation system and purged before illumination. The evolved gases were collected and quantified using two separate SRI 310C gas chromatographs. In the case of hydrogen gas, a HayeSep Q column and nitrogen carrier gas were used, whereas a molecular sieve (5Å) column and helium carrier gas were used for the oxygen gas measurements.
3. Results and discussion
Figures 1(b)–1(d) show the morphology of the different InGaN/GaN NW photoelectrodes. The average height of the InGaN/GaN NWs is approximately 210–260 nm. The height of n-InGaN/n-GaN NWs is slightly longer than that of the u-InGaN/p-GaN and the p-InGaN/p-GaN NWs since the Si dopant induce the vertical growth [19,34]. With introducing the Mg dopants, the InGaN NWs exhibit an increased lateral growth and slightly lower density over the substrate surface area. The average diameter of the p-InGaN/p-GaN NWs was ~45 nm, while that of the u-InGaN/p-GaN NWs is ~30 nm. This diameter increase has been commonly observed for Mg-doped III-nitride NWs [19,34,35]. The optical properties of the different InGaN/GaN NWs were evaluated using room-temperature PL, as shown in Fig. 1(e). The PL spectra of the three samples are dominated by broad peaks because of near-band-edge emission and the transmission between the conduction band and the shallow doping levels [33,34]. The PL peak emission energy is centered at about 2.15 eV for the u-InGaN/p-GaN NWs, while those of the pure Si-doped and Mg-doped InGaN/GaN NWs shift to lower energy values. The u-InGaN/p-GaN sample was supposed to have the lowest carrier concentration among the three samples as only the GaN segment in it was intentionally doped with Mg. The incorporation of shallow donors (Si) or acceptors (Mg) would introduce doping levels near the band-edge, reducing the energy of the emitted photons to be lower than the intrinsic bandgap of the material, thus redshifting the emission peaks [17,34]. These optimum bandgaps are larger than 2 eV, which enable the InGaN/GaN NWs to drive the redox reaction while overcoming the overpotential during PEC experiments [11,22].
The ionized carrier concentrations of the different InGaN/GaN NWs were quantified using Mott–Schottky plots, which were depicted by extracting the fitted capacitance of InGaN NWs from the Nyquist plots, as shown in Fig. 2. Details of EIS measurements can be found in our previous articles [13,20]. The positive slope of the curve for n-InGaN/GaN NWs and the negative slope of the curve for the u-InGaN/p-GaN and p-InGaN/p-GaN NWs were due to successful Si and Mg doping, correspondingly. A modified Mott–Schottky equation has been demonstrated in the literature [13,17,36], which considers the 3D geometry of NWs and hence better estimates the ionized carrier concentration within NW structures. The estimated ionized carrier concentrations of the u-InGaN/p-GaN and p-InGaN/p-GaN NWs were in the range of 1 × 1016 and 7.6 × 1015 cm–3, respectively, while that of n-InGaN/n-GaN NWs exhibited the highest concentration of 5.7 × 1017 cm–3. As III-nitride NWs are naturally n-type, p-type dopants must usually go through a compensation step before the NWs exhibit p-type characteristics [17,18,20]. The Mg doping may induce more structural defects, which can behave as recombination centers and decrease the ionized dopant concentration [17,20].
Fig. 2 Mott–Schottky quantification of carrier concentrations. (a) n-InGaN/n-GaN, (b) u-InGaN/p-GaN, and (c) p- InGaN/p-GaN NWs.
The OCP measurements were further used to identify the conduction type of NWs in Fig. 3(a) [10]. The samples were tested under dark and light illumination conditions with an illumination period of 30 s. The rapid photoresponse indicates the excellent material quality of the NWs. Upon illumination, the photogenerated electron-hole pairs would induce a shift in the Fermi level of the majority carriers. The OCP shifts toward more anodic potentials for a p-type semiconductor and shifts oppositely for an n-type semiconductor. The more significant the OCP shift, the higher the potential change within the space charge layer caused by Fermi level pinning at the NW surface [10,17]. The OCP results are consistent with the Mott–Schotty estimation. It is clear that both u-InGaN/p-GaN and p-InGaN/p-GaN NWs possess p-type conduction characteristics. The higher OCP difference (ΔOCP) of the p-InGaN/p-GaN NWs demonstrates that the net generation rates of electron-hole pairs in the Mg-doped NWs are higher than those of u-InGaN/p-GaN NWs. In contrast, the ΔOCP of the n-InGaN/n-GaN NWs is larger than those of the other two samples, indicating higher Si dopant concentrations than the Mg dopant concentrations.
Fig. 3 (a) OCP measurements, (b) LSV, and (c) the corresponding ABPE for the different InGaN/GaN NW photoelectrode designs (The photoanodes and photocathodes were tested in pH 7 and pH 0 electrolytes, respectively, under 6 suns AM 1.5G illumination).
The PEC performance of the InGaN/GaN NW photoelectrodes with different heterojunction designs was investigated using LSV and the applied bias photon-to-current conversion efficiency (ABPE). All samples exhibited a negligible dark current (Fig. 3(b)). Compared to the u-InGaN/p-GaN NWs, the p-InGaN/p-GaN NWs show a higher photocurrent density (~–17 mA/cm2 at an applied bias of –1.0 V vs RHE, reversible hydrogen electrode). These results are reasonable despite the higher estimated ionized dopant concentration of u-InGaN/p-GaN NWs, because the Mott–Schottky measurements assumed that the dopants distributed homogeneously in the NWs. However, the actual axial inhomogeneity of the carrier distribution may affect real functional PEC devices under illumination. The NWs would also suffer carrier losses because of the electron-hole recombination at the p–n junction in the u-InGaN/p-GaN NWs [36]. Although the ionized dopant concentration of the n-InGaN/n-GaN NWs was the highest among the three samples, the photocurrent density (~0.15 mA/cm2 at 0.5 V vs RHE) was very small, which can be partially attributed to the poor carrier collection/extraction efficiency [23] as well as the n-type semiconductor surface instability [15,16]. In Fig. 3(c), the ABPE conversion efficiency was calculated accordingly using Eq. (1) by assuming the Faradaic efficiency (ƞ) as 1. The highest ABPE was recorded for the p-InGaN/p-GaN NWs at 0.65% at an applied bias of –0.83 V vs RHE, which is about 2.5 times higher than that of the u-InGaN/p-GaN NWs and more than 50 times higher than that of the n-InGaN/n-GaN sample. The results are even higher than the typically reported ABPE (<0.5%) for similar GaN/InGaN nanostructures [1,37–43].
The reliability of the photoelectrode is crucial for PEC water splitting, which can also be reflected in the gas production quality. The stability of the InGaN/GaN NW photoelectrodes with different heterojunction designs was measured by conducting chronoamperometry (CA) tests. The Pt-decorated p-InGaN/p-GaN and u-InGaN/p-GaN NWs demonstrated superior chemical stability in 1 M H2SO4 under high-intensity illumination (6 suns) for over 10 h without noticeable degradation (Fig. 4(a)). A lower potential (–0.453 V vs RHE for p-type and + 0.81 V vs RHE for n-type) was applied to overcome the ohmic losses on the external circuit during the CA and gas evolution measurements [44]. The n-type sample degraded rapidly within 2 h because of the strong self-oxidation [14]. The amount of the gas evolved during the CA tests was continuously recorded and analyzed using a gas chromatography system. The leak-tight reactor used during these measurements is presented in Fig. 4(b). The amount of the gas produced by the n-InGaN/n-GaN photoanode was too small to be measured, which can be associated with its poor stability. In contrast, the p-type samples show superior chemical stability and evolution rates in the pH 0 electrolytes. The current density of the two photocathodes increase for the first two hours. The plausible explanation is the interfacial changes at the InGaN surface [4,45]. Then, the current density of the u-InGaN/p-GaN and p-InGaN/p-GaN saturated at approximately –4.0 and –9.4 mA/cm2, respectively. These photocathodes exhibit a superior longterm photoactivity, demonstrated by interrupting the light illumination after 9 h, as shown in Fig. 4(a). In spite of the increase in the dark current densities of these two photocathodes at the end of the measurements because of the unexpected corrosion reaction, the dark current densities are still negligible compared to the photocurrent densities. Although a stable current is produced by the u-InGaN/p-GaN sample and the progressive evolution of gases, the oxygen and hydrogen do not exhibit a good stoichiometry. In Fig. 4(c), the overall stoichiometric ratio is H2:O2 ~0.8, which can be partially attributed to the recombination of the photogenerated carriers at the p–n junction, which can reduce the carrier extraction efficiency. Another reason for the imperfect stoichiometry might be the excess oxygen evolution from the undoped segments and the back reaction of the generated gas in the PEC cell. An ideal evolution of hydrogen and oxygen gases with a good stoichiometric ratio (H2:O2 ~1.8) was then realized over the p-InGaN/p-GaN photocathode. The amount of evolved hydrogen and oxygen from the p-InGaN/p-GaN linearly increases with time and reaches a total of approximately 1000 µmol/cm2 for hydrogen and 550 µmol/cm2 for oxygen, which are higher than those of the previously reported Pt-decorated GaN NW-based photocathodes under similar experimental conditions [37].
Fig. 4 Stability and gas evolution measurements of InGaN/GaN NWs. (a) Chronoamperometry measurements, (photoanodes and photocathodes were tested in pH 7 and pH 0 electrolytes, respectively, under 6 suns AM 1.5G illumination) (b) leak-tight glass reactor used for gas evolution measurements (the inset figure shows the top view of the reactor), and (c) hydrogen and oxygen evolution for the u-InGaN/p-GaN and p-InGaN/p-GaN samples.
In order to understand the differences in the PEC performance of the InGaN/GaN NWs with different axial heterojunctions, the energy band diagram of the three photoelectrodes was simulated using PC1D software. The energy band diagram configuration shown in Figs. 5(a)–5(c) was constructed assuming the NWs are not in contact with the electrolyte and are illuminated with six suns (AM 1.5G) and assuming a uniform In concentration along the axis of the NW. The schematics shown in Figs. 5(d)–5(f) represent the surface band bending during the PEC water splitting process when the NW photoelectrodes are immersed in the electrolyte. The PEC performance of the InGaN/GaN photoelectrodes can be explained considering two main parameters, which are the surface band bending and the potential barrier at the InGaN/GaN heterointerfaces. Typically, as the n-InGaN NWs are brought into direct contact with the electrolyte under light illumination conditions, there will be an upward band bending at the semiconductor/electrolyte interface because of the transport of excess charge carriers from the semiconductor to the electrolyte to achieve electrochemical equilibrium. The photogenerated holes will then diffuse to the semiconductor surface to start the water oxidation, and the photogenerated electrons, in contrast, will transfer through the NW to the counter electrode to reduce the hydrogen ions into hydrogen molecules. The small conduction band offset between n-InGaN and n-GaN will allow electrons to readily tunnel through, while the transport of holes in the same direction will be hindered by the large valence band offset, leading to the accumulation of holes on the surface of the NWs [25]. The occurrence of the water oxidation reaction on the surface of the n-type NWs may lead to the formation of Ga2O3, which can be dissolved in aqueous electrolytes, leading to the degradation and instability of these photoanodes [15,46]. In the case of u-InGaN on the p-GaN heterojunction, the conduction band in the u-InGaN is tilted in a direction near the Fermi level of the p-GaN, forming a large potential barrier at the conduction band offset and moving the photogenerated electrons toward the surface, which may explain the cathodic behavior of these NWs [47]. A similar energy band diagram can be seen for the p-type InGaN/GaN photocathodes but with a perfect downward surface band bending. A large conduction band offset is formed drifting the photogenerated electrons to the NW/electrolyte interface, and a small valence band offset is formed, allowing the separated holes to pass through to the NWs. In this case, hydrogen reduction occurs on the NW surface, and water oxidation takes place on the Pt counter electrode. As compared to the other two photoelectrodes, this heterojunction design demonstrates long-term stability and the best gas evolution efficiency, which can be explained as follows. In the PA-MBE InGaN NWs grown under nitrogen-rich conditions, all the exposed surfaces (top polar (000-1) c-plane and non-polar (10-10) m-plane side walls) are nitrogen-terminated surfaces [14]. During the PEC water splitting, these surfaces are negatively charged, which, together with the downward surface band bending, can reduce the energy barrier for injecting holes into the electrolyte and generate hydrogen from the water oxidation reaction. These N-terminated surfaces were also reported to protect the InGaN NWs against photocorrosion and oxidation, leading to the observed long-term stability and perfect hydrogen and oxygen evolution [14].
Fig. 5 Energy band diagram of InGaN/GaN NWs. Energy band configuration as simulated by PC1D software for: (a) n-InGaN/n-GaN, (b) u-InGaN/p-GaN, and (c) p-InGaN/p-GaN. Schematic of the InGaN/GaN NW photoelectrodes in direct contact with the electrolyte under light illumination conditions: (d) n-InGaN/n-GaN, (e) u-InGaN/p-GaN, and (f) p-InGaN/p-GaN.
4. Conclusions
In summary, we demonstrated the effect of the axial doping engineering of InGaN/GaN NWs on the morphology, optical properties, and PEC performance. Ionized dopant concentrations were estimated by Mott–Schottky measurements, and the corresponding band diagrams were studied by simulations. The optimized band structures of the p-InGaN/p-GaN NWs led to enhanced PEC performance. By incorporating Pt co-catalysts, a photocurrent density of about –17 mA/cm2 at –1.0 V vs RHE was obtained. In addition, the Pt-treated p-InGaN/p-GaN axial NWs exhibited high stability over the 10 hr test period in 1 M H2SO4 electrolyte while maintaining a considerably high current density of –9.4 mA/cm2 at –0.453 V vs RHE, and a hydrogen evolution rate of 107 ± 1.8 µmol/cm2/hr.
Funding
King Abdulaziz City for Science and Technology (KACST) (TIC R2-FP-008); King Abdullah University of Science and Technology (KAUST) baseline funding (BAS/1/1614-01-01), and MBE equipment funding (C/M-20000-12-001-77).
References
1. M. Ebaid, J.-H. Kang, S.-H. Lim, J.-S. Ha, J. K. Lee, Y.-H. Cho, and S.-W. Ryu, “Enhanced solar hydrogen generation of high density, high aspect ratio, coaxial InGaN/GaN multi-quantum well nanowires,” Nano Energy 12, 215–223 (2015). [CrossRef]
2. M. G. Kibria, F. A. Chowdhury, S. Zhao, B. AlOtaibi, M. L. Trudeau, H. Guo, and Z. Mi, “Visible light-driven efficient overall water splitting using p-type metal-nitride nanowire arrays,” Nat. Commun. 6(1), 6797 (2015). [CrossRef] [PubMed]
3. M. Ebaid, J.-H. Kang, and S.-W. Ryu, “Controlled synthesis of GaN-based nanowires for photoelectrochemical water splitting applications,” Semicond. Sci. Technol. 32(1), 013001 (2017). [CrossRef]
4. M. Ebaid, D. Priante, G. Liu, C. Zhao, M. S. Alias, U. Buttner, T. K. Ng, T. T. Isimjan, H. Idriss, and B. S. Ooi, “Unbiased photocatalytic hydrogen generation from pure water on stable Ir-treated In0.33Ga0.67N nanorods,” Nano Energy 37, 158–167 (2017). [CrossRef]
5. G.-L. Su, T. Frost, P. Bhattacharya, and J. M. Dallesasse, “Physical model for high indium content InGaN/GaN self-assembled quantum dot ridge-waveguide lasers emitting at red wavelengths (λ ~ 630 nm),” Opt. Express 23(10), 12850–12865 (2015). [CrossRef] [PubMed]
6. B. AlOtaibi, M. Harati, S. Fan, S. Zhao, H. P. T. Nguyen, M. G. Kibria, and Z. Mi, “High efficiency photoelectrochemical water splitting and hydrogen generation using GaN nanowire photoelectrode,” Nanotechnology 24(17), 175401 (2013). [CrossRef] [PubMed]
7. N. U. H. Alvi, P. E. D. S. Rodriguez, P. Aseev, and V. J. Gómez, “A. u. H. Alvi, W. u. Hassan, M. Willander, and R. Nötzel, "InN/InGaN quantum dot photoelectrode: Efficient hydrogen generation by water splitting at zero voltage,” Nano Energy 13, 291–297 (2015). [CrossRef]
8. J. Juodkazytė, B. Šebeka, I. Savickaja, A. Kadys, E. Jelmakas, T. Grinys, S. Juodkazis, K. Juodkazis, and T. Malinauskas, “InxGa1−xN performance as a band-gap-tunable photo-electrode in acidic and basic solutions,” Sol. Energy Mater. Sol. Cells 130, 36–41 (2014). [CrossRef]
9. K. D. Goodman, V. V. Protasenko, J. Verma, T. H. Kosel, H. G. Xing, and D. Jena, “Green luminescence of InGaN nanowires grown on silicon substrates by molecular beam epitaxy,” J. Appl. Phys. 109(8), 084336 (2011). [CrossRef]
10. M. Ebaid, J.-W. Min, C. Zhao, T. K. Ng, H. Idriss, and B. S. Ooi, “Water splitting to hydrogen over epitaxially grown InGaN nanowires on a metallic titanium/silicon template: reduced interfacial transfer resistance and improved stability to hydrogen,” J. Mater. Chem. A Mater. Energy Sustain. 6(16), 6922–6930 (2018). [CrossRef]
11. M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, and N. S. Lewis, “Solar water splitting cells,” Chem. Rev. 110(11), 6446–6473 (2010). [CrossRef] [PubMed]
12. J. Kamimura, P. Bogdanoff, J. Lähnemann, C. Hauswald, L. Geelhaar, S. Fiechter, and H. Riechert, “Photoelectrochemical properties of (In,Ga)N nanowires for water splitting investigated by in situ electrochemical mass spectroscopy,” J. Am. Chem. Soc. 135(28), 10242–10245 (2013). [CrossRef] [PubMed]
13. H. Zhang, M. Ebaid, J.-W. Min, T. K. Ng, and B. S. Ooi, “Enhanced photoelectrochemical performance of InGaN-based nanowire photoanodes by optimizing the ionized dopant concentration,” J. Appl. Phys. 124(8), 083105 (2018). [CrossRef]
14. M. G. Kibria, R. Qiao, W. Yang, I. Boukahil, X. Kong, F. A. Chowdhury, M. L. Trudeau, W. Ji, H. Guo, F. J. Himpsel, L. Vayssieres, and Z. Mi, “Atomic-scale origin of long-term stability and high performance of p-GaN nanowire arrays for photocatalytic overall pure water splitting,” Adv. Mater. 28(38), 8388–8397 (2016). [CrossRef] [PubMed]
15. K. Fujii and K. Ohkawa, “Photoelectrochemical properties of p-type GaN in comparison with n-type GaN,” Jpn. J. Appl. Phys. 44(28), L909–L911 (2005). [CrossRef]
16. K. Aryal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, “Hydrogen generation by solar water splitting using p-InGaN photoelectrochemical cells,” Appl. Phys. Lett. 96(5), 052110 (2010). [CrossRef]
17. J. Kamimura, P. Bogdanoff, M. Ramsteiner, P. Corfdir, F. Feix, L. Geelhaar, and H. Riechert, “p-Type doping of GaN nanowires characterized by photoelectrochemical measurements,” Nano Lett. 17(3), 1529–1537 (2017). [CrossRef] [PubMed]
18. Q. Wang, X. Liu, M. G. Kibria, S. Zhao, H. P. T. Nguyen, K. H. Li, Z. Mi, T. Gonzalez, and M. P. Andrews, “p-Type dopant incorporation and surface charge properties of catalyst-free GaN nanowires revealed by micro-Raman scattering and X-ray photoelectron spectroscopy,” Nanoscale 6(17), 9970–9976 (2014). [CrossRef] [PubMed]
19. T. Stoica and R. Calarco, “Doping of III-nitride nanowires grown by molecular beam epitaxy,” IEEE J. Sel. Top. Quantum Electron. 17(4), 859–868 (2011). [CrossRef]
20. C. Zhao, M. Ebaid, H. Zhang, D. Priante, B. Janjua, D. Zhang, N. Wei, A. A. Alhamoud, M. K. Shakfa, T. K. Ng, and B. S. Ooi, “Quantified hole concentration in AlGaN nanowires for high-performance ultraviolet emitters,” Nanoscale 10(34), 15980–15988 (2018). [CrossRef] [PubMed]
21. E. Cimpoiasu, E. Stern, R. Klie, R. A. Munden, G. Cheng, and M. A. Reed, “The effect of Mg doping on GaN nanowires,” Nanotechnology 17(23), 5735–5739 (2006). [CrossRef]
22. R. T. ElAfandy, M. Ebaid, J.-W. Min, C. Zhao, T. K. Ng, and B. S. Ooi, “Flexible InGaN nanowire membranes for enhanced solar water splitting,” Opt. Express 26(14), A640–A650 (2018). [CrossRef] [PubMed]
23. M. G. Kibria, S. Zhao, F. A. Chowdhury, Q. Wang, H. P. T. Nguyen, M. L. Trudeau, H. Guo, and Z. Mi, “Tuning the surface Fermi level on p-type gallium nitride nanowires for efficient overall water splitting,” Nat. Commun. 5(1), 3825 (2014). [CrossRef] [PubMed]
24. F. A. Chowdhury, Z. Mi, M. G. Kibria, and M. L. Trudeau, “Group III-nitride nanowire structures for photocatalytic hydrogen evolution under visible light irradiation,” APL Mater. 3(10), 104408 (2015). [CrossRef]
25. M. Ebaid, J.-W. Min, C. Zhao, T. K. Ng, H. Idriss, and B. S. Ooi, “Water splitting to hydrogen over epitaxially grown InGaN nanowires on a metallic titanium/silicon template: reduced interfacial transfer resistance and improved stability to hydrogen,” J. Mater. Chem. A Mater. Energy Sustain. 6(16), 6922–6930 (2018). [CrossRef]
26. D. A. Clugston and P. A. Basore, "PC1D version 5: 32-bit solar cell modeling on personal computers," in Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference - 1997 (IEEE, 1997), pp. 207–210. [CrossRef]
27. F. Schwierz, “An electron mobility model for wurtzite GaN,” Solid-State Electron. 49(6), 889–895 (2005). [CrossRef]
28. M. Anani, H. Abid, Z. Chama, C. Mathieu, A. Sayede, and B. Khelifa, “InxGa1−xN refractive index calculations,” Microelectronics J. 38(2), 262–266 (2007). [CrossRef]
29. O. K. Jani, "Development of wide-band gap indium gallium nitride solar cells for high-efficiency photovoltaics," Ph.D. Thesis (Georgia Institute of Technology, 2008).
30. F.-M. Chen, B.-T. Liou, Y.-A. Chang, J.-Y. Chang, Y.-T. Kuo, and Y.-K. Kuo, “Numerical analysis of using superlattice-AlGaN/InGaN as electron blocking layer in green InGaN light-emitting diodes,” Proc. SPIE 8625, 862526 (2013). [CrossRef]
31. A. S. Kushwaha, P. Mahala, and C. Dhanavantri, “Optimization of p-GaN/InGaN/n-GaN double heterojunction p-i-n solar cell for high efficiency: simulation approach,” Int. J. Photoenergy 2014, 819637 (2014). [CrossRef]
32. A. Mesrane, F. Rahmoune, A. Mahrane, and A. Oulebsir, “Design and simulation of InGaN p-n junction solar cell,” Int. J. Photoenergy 2015, 594858 (2015). [CrossRef]
33. S. O. S. Hamady, A. Adaine, and N. Fressengeas, “Numerical simulation of InGaN Schottky solar cell,” Mater. Sci. Semicond. Process. 41, 219–225 (2016). [CrossRef]
34. J. Kamimura, M. Ramsteiner, L. Geelhaar, and H. Riechert, “Si doping effects on (In,Ga)N nanowires,” J. Appl. Phys. 116(24), 244310 (2014). [CrossRef]
35. F. Schuster, A. Winnerl, S. Weiszer, M. Hetzl, J. A. Garrido, and M. Stutzmann, “Doped GaN nanowires on diamond: structural properties and charge carrier distribution,” J. Appl. Phys. 117(4), 044307 (2015). [CrossRef]
36. I. Mora-Seró, F. Fabregat-Santiago, B. Denier, J. Bisquert, R. Tena-Zaera, J. Elias, and C. Lévy-Clément, “Determination of carrier density of ZnO nanowires by electrochemical techniques,” Appl. Phys. Lett. 89(20), 203117 (2006). [CrossRef]
37. P. Varadhan, H.-C. Fu, D. Priante, J. R. D. Retamal, C. Zhao, M. Ebaid, T. K. Ng, I. Ajia, S. Mitra, I. S. Roqan, B. S. Ooi, and J.-H. He, “Surface passivation of GaN nanowires for enhanced photoelectrochemical water-splitting,” Nano Lett. 17(3), 1520–1528 (2017). [CrossRef] [PubMed]
38. Z. Chen, H. N. Dinh, and E. Miller, Photoelectrochemical water splitting (Springer, New York, 2013), Chap. 2.
39. C. Sheng, L. Wei, Y. Yanfa, H. Thomas, S. Ishiang, W. Dunwei, and M. Zetian, “Roadmap on solar water splitting: current status and future prospects,” Nano Futures 1(2), 022001 (2017). [CrossRef]
40. C. Jiang, S. J. A. Moniz, A. Wang, T. Zhang, and J. Tang, “Photoelectrochemical devices for solar water splitting - materials and challenges,” Chem. Soc. Rev. 46(15), 4645–4660 (2017). [CrossRef] [PubMed]
41. L. Ji, M. D. McDaniel, S. Wang, A. B. Posadas, X. Li, H. Huang, J. C. Lee, A. A. Demkov, A. J. Bard, J. G. Ekerdt, and E. T. Yu, “A silicon-based photocathode for water reduction with an epitaxial SrTiO3 protection layer and a nanostructured catalyst,” Nat. Nanotechnol. 10(1), 84–90 (2015). [CrossRef] [PubMed]
42. S. Fan, B. AlOtaibi, S. Y. Woo, Y. Wang, G. A. Botton, and Z. Mi, “High efficiency solar-to-hydrogen conversion on a monolithically integrated InGaN/GaN/Si adaptive tunnel junction photocathode,” Nano Lett. 15(4), 2721–2726 (2015). [CrossRef] [PubMed]
43. A. A. Dubale, A. G. Tamirat, H.-M. Chen, T. A. Berhe, C.-J. Pan, W.-N. Su, and B.-J. Hwang, “A highly stable CuS and CuS–Pt modified Cu2O/CuO heterostructure as an efficient photocathode for the hydrogen evolution reaction,” J. Mater. Chem. A Mater. Energy Sustain. 4(6), 2205–2216 (2016). [CrossRef]
44. B. AlOtaibi, H. P. T. Nguyen, S. Zhao, M. G. Kibria, S. Fan, and Z. Mi, “Highly stable photoelectrochemical water splitting and hydrogen generation using a double-band InGaN/GaN core/shell nanowire photoanode,” Nano Lett. 13(9), 4356–4361 (2013). [CrossRef] [PubMed]
45. L. Caccamo, G. Cocco, G. Martín, H. Zhou, S. Fundling, A. Gad, M. S. Mohajerani, M. Abdelfatah, S. Estradé, F. Peiró, W. Dziony, H. Bremers, A. Hangleiter, L. Mayrhofer, G. Lilienkamp, M. Moseler, W. Daum, and A. Waag, “Insights into Interfacial Changes and Photoelectrochemical Stability of In(x)Ga(1-x)N (0001) Photoanode Surfaces in Liquid Environments,” ACS Appl. Mater. Interfaces 8(12), 8232–8238 (2016). [CrossRef] [PubMed]
46. S. H. Kim, M. Ebaid, J.-H. Kang, and S.-W. Ryu, “Improved efficiency and stability of GaN photoanode in photoelectrochemical water splitting by NiO cocatalyst,” Appl. Surf. Sci. 305, 638–641 (2014). [CrossRef]
47. S. A. Kazazis, E. Papadomanolaki, and E. Iliopoulos, “Polarization-engineered InGaN/GaN solar cells: realistic expectations for single heterojunctions,” IEEE J. Photovolt. 8(1), 118–124 (2018). [CrossRef]
