1. Introduction

Solar hydrogen (H₂) generation via water splitting poses an advantageous challenge for developing clean eco-friendly energy systems [1]. The photoelectrochemical method of generating H₂ is one of the most desirable renewable routes to address this challenge [2]. Fujishima et al. have reported water splitting into H₂ and O₂ with TiO₂ electrode under ultraviolet (UV) light illumination [3]. Since then, several metal-oxide semiconductors have been widely investigated [4]. Besides metal-oxide semiconductors, various semiconductors have been demonstrated as water splitting photoelectrochemical cells (PECs) photoelectrode or photocatalyst, such as GaN [5–7], MoS₂/Si [8], GaP [9], and CdS [10]. Semiconductors for use in PECs H₂ generation should possess a conduction band potential that is less than that of the cathode reduction half-reaction, and its valence band-edge potential must be higher than that of the anode oxidation half-reaction [11]. GaN-based materials have suitable band gap that fits the criteria for photoelectrochemical H₂ generation [12,13]. In addition, III-nitride materials are potentially resistant to aqueous solutions [14]. Tuning the band gap of GaN-based material from 3.4 eV to 0.7 eV by adding indium content would result in the PECs fitting the solar spectrum for enhancing light absorption. Moreover, Ono et al. have also reported that n-GaN electrode PECs split water without extra bias [15]. However, the photocurrent density is not sufficiently high to produce adequate hydrogen gas within a short time. Increasing the area of GaN surface would be one method to enhance the photocurrent of PECs for water splitting. Waki et al. have reported on GaN area increase by introducing a select area GaN growth technique [16]. The group created faceted GaN strips on a patterned n-GaN template to increase the area of GaN and, consequently, the photocurrent of PECs. However, Liu et al. have reported that PECs with a naturally textured rough n-GaN electrode did not exhibit photo-current enhancement compared with a smooth surface n-GaN electrode under illumination [17]. This result should be attributed to the highly defective low-temperature growth of textured n-GaN electrode. Moreover, Tu et al. have reported that the photocurrent of PECs with p-GaN electrode could be enhanced by introducing Au nanoparticles on the p-GaN electrode, because the Au nano-particles could bend down the energy band of p-GaN larger than p-GaN in direct contact with the solution [18]. Therefore, the band bending of the GaN electrode would also affect the photocurrent of the PECs. In this study, we aimed to demonstrate a faceted AlGaN/n-GaN heterostructure with linear gradient Al composition (LGAC) AlGaN layer as the electrode of PECs. The optoelectrical characteristics of the fabricated PEC cell with flat n-GaN, faceted n-GaN, and facet LGAC AlGaN/n-GaN heterostructure electrode will be discussed.

2. Experiments

All samples were grown on a (0001) sapphire substrate by vertical MOCVD. In n-GaN and heterostructure AlGaN/n-GaN epitaxy, trimethylgallium (TMGa), trimethylaluminum (TMAl), and ammonia are the source materials for Ga, Al, and N, respectively. Silane is an n-type doping source of the III-nitride material. A 25 nm thick, low-temperature GaN nucleation layer was first deposited on the substrate at 520 °C after a standard H₂ etch-back process at 1060 °C. A 1.5 μm thick, high-temperature undoped GaN (u-GaN) was deposited at 1050 °C after the NH₃ / H₂ etch-back process. A 2 μm thick, Si-doped GaN layer (n-GaN) with doping concentration of 1 × 10¹⁹cm⁻³ was then grown on the u-GaN layer. The samples were then removed from reactor and then a 250 nm thick SiO₂ deposited on n-GaN. Standard photolithography was then used to define a circular disk pattern of 3 μm diameter and 3 μm spacing between two neighboring disks. An inductively coupled (ICP) plasma etcher was used to etch the exposed SiO₂ regions to form a SiO₂-pillar mask. Subsequently, a faceted n-GaN layer was grown on the n-GaN template with the SiO₂ pillar mask where the faceted n-GaN has thickness and doping concentration of 5 μm and 1 × 10¹⁹cm⁻³, respectively. A 100 nm thick undoped AlGaN layer with and without gradient Al composition was grown on the faceted n-GaN layer to form the faceted AlGaN/n-GaN heterostructure. We varied the TMAl flow rate linearly from 40 sccm to 5 sccm to create AlGaN layers with a linear gradient Al composition from 20% at the AlGaN/GaN interface to 2.5% on the surface. And we keep the TMAl flow rate of 40 sccm to create constant 20% Al composition of AlGaN layer. We performed scanning electron microscopy (SEM) to observe the surface morphology of samples. Transmission electron microscope (TEM) was performed to index dodecagonal planes of faceted n-GaN structure.

For ohmic contact of the faceted AlGaN/n-GaN heterostructure, a portion of the AlGaN layer was removed via ICP dry etching to expose the n-GaN layer. A bilayer metal Cr (50 nm) / Au (80 nm) was then deposited on the n-GaN and exposed n-GaN to form an ohmic contact. The PECs with flat n-GaN electrode was named as PECs I for comparison. The PECs with faceted n-GaN, constant Al composition (CAC) AlGaN/n-GaN, and LGAC AlGaN/n-GaN electrode were named as PECs II, III, and VI, respectively. A potentiostat (Autolab-PGSTAT128N) was employed to supply the external bias. The current density was measured to evaluate the electrical properties of the PECs. A 300 W Xe lamp was utilized as light source, and 1 mol/L NaCl was used as electrolyte at room temperature. The light illumination power at position of PECs is 2.2 W/cm². An Ag/AgCl reference electrode and a platinum (Pt) wire counter electrode were used to measure the potential. The bias voltage was applied to the working electrode with respect to the Ag/AgCl reference electrode. Hydrogen gas was generated at the Pt wire counter electrode.

3. Results and discussions

Figure 1 shows surface morphology SEM images of faceted n-GaN, CAC AlGaN/n-GaN, and LGAC AlGaN/GaN electrode. The n-GaN layer with facets presented epitaxy lateral overgrowth (ELO) on top of the SiO₂ pillars at standard growth conditions, as shown in Fig. 1. Therefore, faceted dodecagon V-shaped pits were created on top of the SiO₂ pillars. As the growth time elapsed, the bottom of the dodecagon V-shaped pits decreased because of the ELO process. However, the growth rate of dodecagon V-shaped pit facet surfaces is considerably less than that of the (0001) surface. Therefore, the facet surfaces of dodecagon V-shaped pits would eventually coalesce with the facet surfaces of the neighboring V-shaped pits, and the (0001) surface disappeared. Finally, we could obtain a dodecagon faceted n-GaN structure layer having the average size of the bottom and top dodecagon V-shaped pits of 1.02 and 6.2 μm, respectively. Moreover, the dodecagon faceted n-GaN structure layer was consisted with rough and smooth facets as indicated in Fig. 1(a). We have done the TEM on the sample of faceted GaN structure to index the faceted GaN planes. Two TEM samples were prepared by Focus Ion Beam (FIB) to cut along line 1 and line 2 from the dodecagon faceted n-GaN structure as indicated by the Fig. 1(a). TEM analysis was then performed in a FEI Tecnai F20 TEM operated at 200 kV. TEM bright field (BF) images and their corresponding selected area diffraction patterns (SADPs) were recorded. The rotation angle between images and SADPs was calculated to be 90 clockwise (SADP fixed) from the over-focused shadow image which contains the image inside the transmitted beam. The rough surface of the dodecagon faceted n-GaN structure is identified to be {1 1 −2 2} by a pair of TEM BF cross-section image cut along the line 1 presented in Fig. 1(a) and its corresponding SADP, as indicated in Fig. 1 (d). The smooth surface of the dodecagon faceted n-GaN structure is identified to be {-1 1 0 1} by a pair of TEM BF cross-section image cut along the line 2 presented in Fig. 1(a) and its corresponding SADP, as indicated in Fig. 1(e). Both {1 1 −2 2} and {-1 1 0 1} should belong to the semi-polar surface. The {1 1 −2 2} facets have a rougher surface than the {-1 1 0 1}. To date, the details for the formation of rough {1 1 −2 2} surfaces are not yet understood and need to be investigated. The following AlGaN layer growth for the CAC AlGaN/GaN and LGAC AlGaN/GaN heterostructures should not change the dodecagon facet shape of the faceted n-GaN layer.

 

Fig. 1 Surface morphology SEM images of (a) faceted n-GaN, (b) CAC AlGaN/n-GaN, and (c) LGAC AlGaN/GaN electrode (d) The TEM cross-section image cut on the rough surface of the dodecagon faceted n-GaN structure and its corresponding SADP with z = [1-100]. (e) The TEM cross-section image cut on the smooth surface of the dodecagon faceted n-GaN structure and its corresponding SADP with z = [11–20].

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Figure 2 demonstrates the dark and illuminated current densities of PECs I, II, III, and VI as a function of external bias voltage (V_ext). All PECs show similar dark current density (J_d) – V_ext characteristics and possess cathodic currents associated with proton reduction onset potential of around –0.8 V. Under illumination and at a photocurrent density of 2 mA/cm², the voltage of PECs II (–0.035 V) is larger than that of PECs I (–0.27 V). In addition, the zero-bias voltage photocurrent density of PEC II (2.34 mA/cm²) is less than that of PECs I (6.06 mA/cm²). However, the photocurrent of PECs II is larger than PECs I when the applied V_ext is larger than 0.6 V. The PECs with dodecagon faceted n-GaN electrode have the least and the largest zero bias photocurrent density and voltage at 2 mA/cm², respectively. Lin et al. have reported that the polarization direction of GaN would affect the energy band bending of the GaN and electrolytes junction [19]. Ga-polar n-GaN with polarization pointing to GaN would have larger band bending than the N-polar n-GaN with polarization pointing out off the GaN surface. This trend imparts Ga-polar n-GaN with more effective carrier separation and less carrier recombination than the N-polar n-GaN. Moreover, Jung et al. have reported that the surface Fermi level of semipolar (11-22) n-GaN/metal contact pinned around 0.82 eV below the conduction band energy [20]. The reduced Schottky barrier height of the semipolar n-GaN/metal contact would suppress the band bending of n-GaN. The dodecagon faceted n-GaN electrode consisted of semipolar surface would have less polarization than the smooth surface n-GaN electrode. In addition, the dodecagon faceted n-GaN electrode/electrolyte interface would be similar to the semipolar n-GaN/metal contact, which reduces the band bending of the dodecagon faceted n-GaN electrode and the electrolyte junction because of the polarization reduction and surface Fermi level pinning effect. The carrier recombination process then increases. Therefore, PECs with dodecagon faceted n-GaN electrode have lower zero bias photocurrent density than PECs with smooth surface n-GaN electrode. Fuji revealed the PECs properties on (11-20) nonpolar and (11-22) semipolar n-GaN layer [21], where PECs with (11-20) nonpolar and (11-22) semipolar n-GaN layer electrodes have lower zero bias photocurrent density than that with (0001) polar n-GaN electrode.

 

Fig. 2 (a) Dark and (b) illuminated current densities of PECs I, II, III, and IV as a function of Potential vs. Ag/AgCl (V_ext).

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However, the photocurrent density (J_p)-V_ext characteristics of PECs with dodecagon faceted n-GaN electrode could be improved by adding an AlGaN layer. PECs III presents a voltage at 2 mA/cm² and zero bias photocurrent of –0.32 V and 8.62 mA/cm², respectively. PECs III shows more negative voltage at 2 mA/cm² than of PECs II and I. Furthermore, the zero-bias photocurrent density of PECs III is around 3.7 and 1.5 times larger than those of PECs II and I, respectively. In addition, the photocurrent density of PECs III is larger than those of PECs I and II for an applied bias ranging from –0.6 V to 2.0 V. Fujii et al. [22] have reported that the energy differences between the conduction band-edge energy and the energy of the H₂ evolving half-reaction increases with Al composition. The electrons in the conduction band of Al_xGa_1−xN are expected to have higher energy for H₂ gas production than those in GaN. Thus, easier H₂ gas evolution at the counter electrode is expected when Al_xGa_1−xN is used than when GaN is used with the same number of electrons. Moreover, the group also revealed that the onset voltage of PECs with AlGaN electrode would have negative voltage with increasing Al composition of AlGaN. Although the larger band gap of AlGaN is directly linked to lower photo absorption, the internal field of AlGaN/GaN heterostructure from the piezoelectric polarization effect would enhance the generated electron–hole pair separation and suppress the carrier recombination process in the AlGaN layer. Therefore, the PECs with dodecagon faceted AlGaN/n-GaN electrode (PECs III) has considerably better zero-bias photocurrent density and more negative voltage at 2 mA/cm² than the PECs with flat n-GaN and dodecagon facet n-GaN electrodes.

The J_p-V_ext characteristic of PECs with dodecagon facets AlGaN/n-GaN heterostructure electrode could be further improved by band engineering of the AlGaN layer with linear gradient Al composition. The PECs IV has the largest zero-bias photocurrent density of 13.77 mA/cm² and has almost the same voltage of −0.36 V at 2 mA/cm² as PECs III. The zero-bias photocurrent density of PECs IV is around 1.6, 5.9, and 2.3 times larger than those of PECs III, II, and I, respectively. Therefore, the PECs IV should have larger amount gas generation than the PECs I at zero-bias as indicated in Fig. 3. Figure 4 (a) shows the numerical simulated schematic band diagrams of electrolyte and dodecagon faceted n-GaN, dodecagon faceted CAC AlGaN/n-GaN heterostructure, and dodecagon faceted LGAC AlGaN/n-GaN heterostructure electrode interface at zero-bias. And we have taken the derivation of conduction band (E_c) and valance band (E_v) to find the electric field profiles at conduction band and valence band of all PECs samples as shown in Fig. 4(b). The details of the simulation software have been reported by the Wu et al. and Li et al. [23,24]. The Fermi level is zero in the whole junction at zero bias. The band-offset ratio ($\Delta E_c/\Delta E_g$) is 63%. The basic parameters especially the setting of polarization value can be found in report of Ambacher et al. [25]. According to Ref. 25, since GaN is the substrate, the polarization of different x% Al composition is the function of x%. For the linear gradual Al composition AlGaN layer case, we divided 100nm thick AlGaN layer into 33 sections where each section’s Al composition gradually changes from 20% to 5% in the growth direction. As shown in the insect of Fig. 3, each section of E_c and E_v of LGAC AlGaN has small band offsets $\Delta E_c$ and $\Delta E_v$ due to the composition changes. The polarization charge difference induced at each interface can be obtained by taking the divergence of P(x) along the growth direction, i.e. $\nabla P(x)={\rho }_{pol}$. In our setting, the equivalent negative polarization charge density is induced as Simon et al. [26] and Zhang et al. [27–30] suggested. And the polarization charge at the semipolar plane is changed as well due the induced of shear strain and projection angle [31,32]. Two {-1 1 0 1}, {1 1 −2 2} planes have close polarization charge because of similar tilt angle of 60 degree as shown in Fig. 1 (d) and (e). The flat n-GaN electrode/electrolyte interface shows a rapid band bending upward with depletion thickness of 27 nm and the largest electric field. The dodecagon faceted n-GaN electrode/electrolyte interface has less upward band bending and electric field than that of flat n-GaN electrode/electrolyte interface because of polarization reduction and surface Fermi level pinning effect. The dodecagon faceted CAC AlGaN/n-GaN heterostructure electrode/electrolyte interface demonstrated the whole 100 nm thick CAC AlGaN layer having linear band bending up and constant E_c and E_v electric field because of the assistance of the internal spontaneous and piezoelectric polarization field. Therefore, PECs with dodecagon faceted CAC AlGaN/GaN electrode having much thicker thickness of the constant E_c and E_v electric field region than that with n-GaN and dodecagon faceted n-GaN electrode would improve the light absorption and separation efficiency of the photo-generated electron–hole pair in the AlGaN layer, which would consequently improve the zero-bias current density and have more negative voltage at 2 mA/cm² of PECs III.

 

Fig. 3 Gas generation picture of the PECs I and IV under illumination. Left side of picture is PECs IV and right side of picture is PECs I. Both PECs are generating H2 gas at the Pt cathode (see Media 1).

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Fig. 4 Schematic (a) band diagrams, (b) E_c and E_v electric field of electrolyte and dodecagon faceted n-GaN, dodecagon faceted CAC AlGaN/n-GaN heterostructure, and dodecagon faceted LGAC AlGaN/n-GaN heterostructure electrodes interface.

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Unlike the linear upward bending band of CAC AlGaN/GaN heterostructure electrode of PECs, the LGAC AlGaN/GaN hetero-structure electrode PECs have a curve-bending band as shown in Fig. 3 (a). The LGAC AlGaN/GaN heterostructure electrode shows more rapid band bending up at the interface of the AlGaN/GaN than CAC AlGaN/GaN heterostructure electrode. In addition, the band bending of LGAC AlGaN/GaN heterostructure electrode slows down and flattens near the electrode surface. Therefore, the E_c and E_v electric field is not constant in the whole LGAC AlGaN layer but gradually reduces from AlGaN/GaN interface toward the surface. Since small band offsets $\Delta E_c$ and $\Delta E_v$ due to the composition changes in each section of LGAC AlGaN layer, there were E_c and E_v electric field pikes at each section junction after taking the derivation of E_c and E_v. However, the pikes electric field directions at conduction band and valence band are different. The overall electric field of LGAC AlGaN/GaN heterostructure is larger than that of CAC AlGaN/GaN heterostructure in the region between 1000 nm and 1045 nm. For the conduction band, the overall E_c electric field is smaller including those E_c electric field pikes due to the band offset. However, because the electron mobility is much higher than holes, the diffusion length is much longer to avoid recombination. The advantage part is in the valence band, the E_v electric field pikes direction (induced by the valence band offset) is the same as the depletion field direction. As mentioned, the hole mobility is much lower than electron. Therefore, the hole needs a larger electric field to be pulled away from the AlGaN/GaN interface especially near the AlGaN/GaN interface where a large two dimensional electron gas is there. In the LGAC case, the hole is much easier to move away due to the larger electric field near AlGaN/GaN interface and also those additional E_v electric field pikes. Therefore, the PECs with LGAC AlGaN/GaN hetero-structure electrode should have better photo-generated electron–hole pair separation efficiency than that with CAC AlGaN/GaN heterostructure electrode. Moreover, the LGAC AlGaN layer might have better light absorption than the AlGaN layer because of the extra light absorption wavelength from 334 nm to 362 nm. Therefore, PECs IV has the largest zero-bias photocurrent density and has almost the same voltage at 2 mA/cm²as PECs III.

Figure 5 shows the surface morphology SEM images of PECs I to IV after J_p-V_ext characteristics measurement. High-density tiny pits were formed on the surface of PEC cells with flat n-GaN electrode (PECs I) after photoelectrochemical etching. In addition, the photoelectrochemical-etched facets n-GaN electrode also show high density tiny pits on all facets of {-1 1 0 1}, {1 1 −2 2}, and (0001) surface. The {-1 1 0 1} surfaces of the etched dodecagon faceted n-GaN electrode present similar surface morphology to etched flat n-GaN electrode. However, the {1 1 −2 2} surfaces of etched dodecagon faceted n-GaN electrode seem to present a rougher surface than {-1 1 0 1}. Rougher photoelectrochemical etched {1 1 −2 2} facets should be attributed to the rough surface of the as grown {1 1 −2 2} facets. Furthermore, adding AlGaN layer on the dodecagon faceted n-GaN enlarged the photoelectrochemical etching pits size of the facet surface. The enlarged pits size of dodecagon faceted AlGaN/n-GaN heterostructure indicates high photoelectrochemical reaction rate on the facet surface. This finding also implies that PECs III has a larger photo current density with applied bias than PECs II and I. Moreover, the dodecagon faceted AlGaN/n-GaN heterostructure electrode also shows rougher PEC etched surfaces of {1 1 −2 2} than that of {-1 1 0 1}. Finally, the dodecagon faceted LGAC AlGaN/n-GaN hetero-structure electrode PECs have the roughest etched facet surfaces. In addition, the boundary of the etched {-1 1 0 1} and {1 1 −2 2} faceted LGAC AlGaN/n-GaN hetero-structure electrode was barely recognized. The roughest dodecagon facet surface of the LGAC AlGaN/n-GaN heterostructure electrode indicates that PECs IV underwent the most severe photoelectrochemical reaction. Therefore, PECs IV should have the largest photocurrent density with applied bias.

 

Fig. 5 Surface morphology SEM images of (a) PECs I, (b) PECs II, (c) PECs III, and (d) PECs IV after the J_p-V_ext characteristics measurement.

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4. Conclusions

In summary, we demonstrated PECs with dodecagon faceted n-GaN, CAC AlGaN/GaN hetero-structure and LGAC AlGaN/n-GaN heterostructure electrode for H₂ generation. The separation efficiency of the photo-generated electron–hole pairs in the electrode performs a key function for H₂ generation efficiency of PECs. Dodecagon faceted n-GaN electrode has less zero-bias photocurrent density than flat n-GaN electrode because of the reduced polarization field and surface Fermi level pinning effect to reduce photo-generated electron–hole pair separation efficiency. Adding an AlGaN layer on the dodecagon faceted n-GaN electrode would help improve the zero-bias photocurrent density by 3.7 times because of the assistance of internal spontaneous and piezoelectric polarization fields. Furthermore, the LGAC AlGaN could create more internal field and light absorption because of the linear graded band gap. Therefore, zero-bias photocurrent density of PECs with dodecagon faceted LGAC AlGaN/n-GaN heterostructure electrode is around 5.9 times larger than that with dodecagon faceted n-GaN electrode.