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Energy Express

  • Editor: Bernard Kippelen
  • Vol. 20, Iss. S2 — Mar. 12, 2012
  • pp: A190–A196
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Immersed finger-type indium tin oxide ohmic contacts on p-GaN photoelectrodes for photoelectrochemical hydrogen generation

Shu-Yen Liu, J. K. Sheu, M. L. Lee, Yu-Chuan Lin, S. J. Tu, F. W. Huang, and W. C. Lai  »View Author Affiliations


Optics Express, Vol. 20, Issue S2, pp. A190-A196 (2012)
http://dx.doi.org/10.1364/OE.20.00A190


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Abstract

In this study, we demonstrated photoelectrochemical (PEC) hydrogen generation using p-GaN photoelectrodes associated with immersed finger-type indium tin oxide (IF-ITO) ohmic contacts. The IF-ITO/p-GaN photoelectrode scheme exhibits higher photocurrent and gas generation rate compared with p-GaN photoelectrodes without IF-ITO ohmic contacts. In addition, the critical external bias for detectable hydrogen generation can be effectively reduced by the use of IF-ITO ohmic contacts. This finding can be attributed to the greatly uniform distribution of the IF-ITO/p-GaN photoelectrode applied fields over the whole working area. As a result, the collection efficiency of photo-generated holes by electrode contacts is higher than that of p-GaN photoelectrodes without IF-ITO contacts. Microscopy revealed a tiny change on the p-GaN surfaces before and after hydrogen generation. In contrast, photoelectrodes composed of n-GaN have a short lifetime due to n-GaN corrosion during hydrogen generation. Findings of this study indicate that the ITO finger contacts on p-GaN layer is a potential candidate as photoelectrodes for PEC hydrogen generation.

© 2012 OSA

1. Introduction

2. Experiments

In this study, Mg-doped GaN (p-GaN) epitaxial layers were grown on a c-plane sapphire substrate. Before the growth of p-GaN epitaxial layers, a 30-nm thick low-temperature GaN nucleation layer and a 2 μm-thick undoped GaN were in subsequently grown on the sapphire substrate. The p-GaN epitaxial layers had a carrier concentration of ~5.8 × 1017/cm3 and a thickness of 0.2 μm. To determine whether immersed finger-type ITO ohmic contacts enhance photocurrent density and hence gas generation rate, we designed two different p-type GaN photoelectrodes for PEC water-splitting experiments. PEC1 consisted of the p-GaN photoelectrode with finger-type ITO ohmic contacts covered with a SiO2 layer to prevent the ohmic contact from contacting the electrolyte and generating current leakage. Figure 1(a)
Fig. 1 Schematic diagram of the photoelectrochemical cells (a) PEC1, (c) PEC2. Figure 1(b) shows the schematic structure of ITO/SiO2 staked layers on the p-GaN.
and 1(b) show the schematic diagram and the cross-section of a local area of PEC1, respectively. The schematic structure of the ITO ohmic contact and SiO2 protection layer on the p-GaN layer are also shown. PEC2 was composed of p-GaN photoelectrodes with ITO ohmic contacts outside of the working area, so that the ITO ohmic contacts were not immersed in the electrolyte during PEC experiments (Fig. 1(c)). For PEC1, the space between the ITO stripe contacts was 200 μm and the width of each stripe contact was 20 μm. Furthermore, two p-GaN photoelectrodes (e.g. PEC3 and PEC4) with layer thickness greater than that of PEC1 were also prepared. The p-GaN epitaxial layers of PEC3 and PEC4 had thicknesses of 1.5 and 2.5 μm, respectively. The photoelectrodes of PEC3 and PEC4 had the same schematic structure as PEC1. A potentiostat (Autolab-PGSTAT128N) was used to supply the external bias and to measure the current density in order to evaluate the electrical properties of the PEC cells. A platinum (Pt) wire was used as counterelectrode. A 300 W Xe lamp was used as light source and 1 mol/L NaCl was used as electrolyte.

3. Results and discussions

Figure 2
Fig. 2 Typical photocurrent density-bias curves of the experimental PEC cells.
shows photocurrent density as a function of external bias voltage (VCE). The VCE refers to the applied voltage between the p-GaN working electrode and the Pt counterelectrode. With an illumination intensity of 1.38 W/cm2, the photocurrent densities with the applied voltages ranged from 0.4 to −2.0 V. Photocurrents of PEC1 were markedly higher than that of PEC2 when the negative bias voltages were higher than −0.4 V. This result can be attributed to the beneficial effect of immersed finger-type ITO ohmic contacts of PEC1 to the collection of photo-generated holes. The photocurrent density of PEC1 was 96-fold of that of PEC2 when the illumination intensity and VCE were 1.38 W/cm2 and −0.6 V, respectively. The enhancement of photocurrent density increased rapidly with the increase of applied bias. For instance, when VCE was increased to −1 V, the photocurrent density of PEC1 increased significantly to 464-fold of that of PEC2. This finding can be attributed to the uniform distribution of the applied fields in PEC1 over the working area due to the immersed finger-type ITO ohmic contacts, thereby leading to shorter transit time of the holes compared with PEC2 [16

16. S. Y. Liu, J. K. Sheu, C. K. Tseng, J. C. Ye, K. H. Chang, M. L. Lee, and W. C. Lai, “Improved hydrogen gas generation rate of n-GaN photoelectrode with SiO2 protection layer on the ohmic contacts from the electrolyte,” J. Electrochem. Soc. 157(2), B266–B268 (2010). [CrossRef]

]. This observation also implies that immersed finger-type ITO ohmic contacts are indeed pathways for photo-generated holes, since the holes in PEC1 can reach the external circuit more readily under the same bias. Although immersed finger-type ITO ohmic contacts could effectively enhance the photocurrent of PEC cells with p-GaN as photocathode, the cells still required considerable external bias to generate enough gas. When light intensity was 1.38 W/cm2, the critical biases for hydrogen generation were −1.2 and −3 V for PEC1 and PEC2, respectively (Table 1

Table 1. Critical Bias for Hydrogen Generation of the Experimental PEC Cells

table-icon
View This Table
). These findings are consistent with the bias-dependent photocurrents shown in Fig. 2. In principle, p-type photocathodes should not be susceptible to photocorrosion [13

13. 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]

], so that illumination intensity on the photocathodes can be increased to improve photocurrent density and hydrogen generation rate. Clearly, the critical bias for hydrogen generation was reduced from −1.2 to −0.6 V when illumination intensity was increased from 1.38 to 1.83 W/cm2.

As mentioned above, photocurrent densities could be increased obviously under more dense illumination. In fact, p-GaN photoelectrodes associated with neutral electrolyte (e.g., NaCl solution) did not suffer from the corrosion during the photoelectrolysis process because of the reduction reaction rather than the oxidation reaction. Therefore, worrying about the consumption of the p-GaN layer due to corrosion may be not be necessary when gas generation rate is enhanced by increasing light illumination intensity. Furthermore, the immersed ITO finger ohmic contacts can overcome the inherent weakness of high resistivity, low mobility, and short diffusion length of carriers in p-GaN. Our findings suggest that p-InxGa1-xN/ITO photoelectrodes associated with neutral electrolytes is a promising scheme for hydrogen generation from water splitting. The design of the immersed ITO finger ohmic contacts can reduce the critical applied bias for hydrogen generation from −3 to −1.2 V. However, the photocurrent observed from PEC1 or PEC2 was not high enough for the massive photoelectrolysis of water. In general, we expect that photocurrent can be increased by moderately increasing the thickness of p-GaN layer to enable the absorption of more photons.

In theory, the reduction reaction at the p-GaN working electrode should protect it from corrosion during the PEC reaction. To evaluate the morphologies of the p-GaN working electrodes, scanning electron microscopy (SEM). Figure 4(a)
Fig. 4 SEM images of the surface of PEC1 (a) before and (b) after photoelectrochemical measurements. The insets of Fig. 4(a) and 4(b) show the top-view of the AFM images of PEC1 before and after photoelectrochemical measurements, respectively.
and 4(b) correspond to the p-GaN surfaces of PEC1 before and after hydrogen generation, respectively. No detectable change between p-GaN surfaces before and after hydrogen generation was observed. To further inspect p-GaN surface morphology, atomic force microscopy (AFM) images were also taken from the surface of p-GaN. The insets of Fig. 4(a) and 4(b) show the top-view AFM images. The root-mean-square (RMS) roughnesses of p-GaN surfaces for PEC1 before and after hydrogen generation were 0.45 and 0.48 nm, respectively. Only a tiny change between p-GaN surfaces before and after hydrogen generation was observed. In contrast, n-GaN photoelectrodes have a short lifetime due to corrosion during hydrogen generation [6

6. W. Luo, B. Liu, Z. Li, Z. Xie, D. Chen, Z. Zou, and R. Zhang, “Stable response to visible light of InGaN photoelectrodes,” Appl. Phys. Lett. 92(26), 262110 (2008). [CrossRef]

8

8. K. Fujii, M. Ono, T. Ito, Y. Iwaki, A. Hirako, and K. Ohkawa, “Band-edge energies and photoelectrochemical properties of n-Type AlxGa1−xN and InyGa1−yN alloys,” J. Electrochem. Soc. 154(2), B175–B179 (2007). [CrossRef]

, 12

12. R. Dimitrova, L. Catalan, D. Alexandrov, and A. Chen, “Evaluation of GaN and In0.2Ga0.8N semiconductors as potentiometric anion selective electrodes,” Electroanalysis 19(17), 1799–1806 (2007). [CrossRef]

]. Findings of this study suggest that the ITO contacts on p-GaN epitaxial layers scheme can serve as potential reliable photocathodes for hydrogen generation from the photoelectrolysis of water.

4. Conclusions

In this study, we utilized immersed ITO finger ohmic contacts to overcome the inherent weaknesses of high resistivity and poor carrier mobility of p-GaN. Immersed ITO finger ohmic contacts were beneficial to the collection of photo-generated carriers, leading to higher photocurrent density and lower critical bias for hydrogen generation. In addition, we found that the thickness of the p-GaN layer is not the key factor for improving photocurrent during water splitting reaction. Photocathodes with the ITO/thin p-GaN/u-GaN scheme did not suffer from significant corrosion and the photocurrent density increased remarkably with higher illumination intensity. Our findings indicate that the use of p-GaN working electrodes with finger-type ITO contacts is a promising approach for hydrogen generation using PEC cells.

Acknowledgments

Financial support from the Bureau of Energy, Ministry of Economic Affairs of Taiwan, ROC through grant No. 99-D0204-6 is appreciated. The authors would also like to acknowledge the National Science Council for the financial support of the research Grant Nos. NSC 97-2221-E-006-242-MY3, 98-2221-E-218-005-MY3 and 100-3113-E-006-015.

References and links

1.

A. J. Nozik and R. Memming, “Physical chemistry of semiconductor-liquid interfaces,” J. Phys. Chem. 100(31), 13061–13078 (1996). [CrossRef]

2.

J. A. Turner, “A realizable renewable energy future,” Science 285(5428), 687–689 (1999). [CrossRef] [PubMed]

3.

A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature 238(5358), 37–38 (1972). [CrossRef] [PubMed]

4.

A. J. Nozik, “Electrode materials for photoelectrochemical devices,” J. Cryst. Growth 39(1), 200–209 (1977). [CrossRef]

5.

R. C. Kainthla and B. Zelenay, “Significant efficiency increase in self-driven photoelectrochemical cell for water photoelectrolysis,” J. Electrochem. Soc. 134(4), 841 (1987). [CrossRef]

6.

W. Luo, B. Liu, Z. Li, Z. Xie, D. Chen, Z. Zou, and R. Zhang, “Stable response to visible light of InGaN photoelectrodes,” Appl. Phys. Lett. 92(26), 262110 (2008). [CrossRef]

7.

J. Li, J. Y. Lin, and H. X. Jiang, “Direct hydrogen gas generation by using InGaN epilayers as working electrodes,” Appl. Phys. Lett. 93(16), 162107 (2008). [CrossRef]

8.

K. Fujii, M. Ono, T. Ito, Y. Iwaki, A. Hirako, and K. Ohkawa, “Band-edge energies and photoelectrochemical properties of n-Type AlxGa1−xN and InyGa1−yN alloys,” J. Electrochem. Soc. 154(2), B175–B179 (2007). [CrossRef]

9.

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]

10.

I. Waki, D. Cohen, R. Lal, U. Mishra, S. P. DenBaars, and S. Nakamura, “Direct water photoelectrolysis with patterned n-GaN,” Appl. Phys. Lett. 91(9), 093519 (2007). [CrossRef]

11.

J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager III, E. E. Haller, H. Lu, and W. J. Schaff, “Small band gap bowing in In1−xGaxN alloys,” Appl. Phys. Lett. 80(25), 4741 (2002). [CrossRef]

12.

R. Dimitrova, L. Catalan, D. Alexandrov, and A. Chen, “Evaluation of GaN and In0.2Ga0.8N semiconductors as potentiometric anion selective electrodes,” Electroanalysis 19(17), 1799–1806 (2007). [CrossRef]

13.

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]

14.

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]

15.

S. Y. Liu, J. K. Sheu, J. C. Ye, S. J. Tu, C. K. Hsu, M. L. Lee, C. H. Kuo, and W. C. Lai, “Characterization of n-GaN with naturally textured surface for photoelectrochemical hydrogen generation,” J. Electrochem. Soc. 157(12), H1106–H1109 (2010). [CrossRef]

16.

S. Y. Liu, J. K. Sheu, C. K. Tseng, J. C. Ye, K. H. Chang, M. L. Lee, and W. C. Lai, “Improved hydrogen gas generation rate of n-GaN photoelectrode with SiO2 protection layer on the ohmic contacts from the electrolyte,” J. Electrochem. Soc. 157(2), B266–B268 (2010). [CrossRef]

17.

J. K. Sheu, Y. K. Su, G. C. Chi, M. J. Jou, C. M. Chang, and C. C. Liu, “Indium tin oxide ohmic contact to highly doped n-GaN,” Solid-State Electron. 43(11), 2081–2084 (1999). [CrossRef]

18.

T. Margalith, O. Buchinsky, D. A. Cohen, A. C. Abare, M. Hansen, S. P. DenBaars, and L. A. Coldren, “Indium tin oxide contacts to gallium nitride optoelectronic devices,” Appl. Phys. Lett. 74(26), 3930 (1999). [CrossRef]

19.

J. F. Muth, J. H. Lee, I. K. Shmagin, R. M. Kolbas, H. C. Casey Jr, B. P. Keller, U. K. Mishra, and S. P. DenBaars, “Absorption coefficient, energy gap, exciton binding energy, and recombination lifetime of GaN obtained from transmission measurements,” Appl. Phys. Lett. 71(18), 2572 (1997).

OCIS Codes
(310.3840) Thin films : Materials and process characterization
(310.4925) Thin films : Other properties (stress, chemical, etc.)
(310.6845) Thin films : Thin film devices and applications
(310.7005) Thin films : Transparent conductive coatings

ToC Category:
Solar Fuel

History
Original Manuscript: November 11, 2011
Revised Manuscript: January 4, 2012
Manuscript Accepted: January 5, 2012
Published: January 11, 2012

Citation
Shu-Yen Liu, J. K. Sheu, M. L. Lee, Yu-Chuan Lin, S. J. Tu, F. W. Huang, and W. C. Lai, "Immersed finger-type indium tin oxide ohmic contacts on p-GaN photoelectrodes for photoelectrochemical hydrogen generation," Opt. Express 20, A190-A196 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-S2-A190


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References

  1. A. J. Nozik and R. Memming, “Physical chemistry of semiconductor-liquid interfaces,” J. Phys. Chem.100(31), 13061–13078 (1996). [CrossRef]
  2. J. A. Turner, “A realizable renewable energy future,” Science285(5428), 687–689 (1999). [CrossRef] [PubMed]
  3. A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature238(5358), 37–38 (1972). [CrossRef] [PubMed]
  4. A. J. Nozik, “Electrode materials for photoelectrochemical devices,” J. Cryst. Growth39(1), 200–209 (1977). [CrossRef]
  5. R. C. Kainthla and B. Zelenay, “Significant efficiency increase in self-driven photoelectrochemical cell for water photoelectrolysis,” J. Electrochem. Soc.134(4), 841 (1987). [CrossRef]
  6. W. Luo, B. Liu, Z. Li, Z. Xie, D. Chen, Z. Zou, and R. Zhang, “Stable response to visible light of InGaN photoelectrodes,” Appl. Phys. Lett.92(26), 262110 (2008). [CrossRef]
  7. J. Li, J. Y. Lin, and H. X. Jiang, “Direct hydrogen gas generation by using InGaN epilayers as working electrodes,” Appl. Phys. Lett.93(16), 162107 (2008). [CrossRef]
  8. K. Fujii, M. Ono, T. Ito, Y. Iwaki, A. Hirako, and K. Ohkawa, “Band-edge energies and photoelectrochemical properties of n-Type AlxGa1−xN and InyGa1−yN alloys,” J. Electrochem. Soc.154(2), B175–B179 (2007). [CrossRef]
  9. 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]
  10. I. Waki, D. Cohen, R. Lal, U. Mishra, S. P. DenBaars, and S. Nakamura, “Direct water photoelectrolysis with patterned n-GaN,” Appl. Phys. Lett.91(9), 093519 (2007). [CrossRef]
  11. J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager, E. E. Haller, H. Lu, and W. J. Schaff, “Small band gap bowing in In1−xGaxN alloys,” Appl. Phys. Lett.80(25), 4741 (2002). [CrossRef]
  12. R. Dimitrova, L. Catalan, D. Alexandrov, and A. Chen, “Evaluation of GaN and In0.2Ga0.8N semiconductors as potentiometric anion selective electrodes,” Electroanalysis19(17), 1799–1806 (2007). [CrossRef]
  13. 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]
  14. 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]
  15. S. Y. Liu, J. K. Sheu, J. C. Ye, S. J. Tu, C. K. Hsu, M. L. Lee, C. H. Kuo, and W. C. Lai, “Characterization of n-GaN with naturally textured surface for photoelectrochemical hydrogen generation,” J. Electrochem. Soc.157(12), H1106–H1109 (2010). [CrossRef]
  16. S. Y. Liu, J. K. Sheu, C. K. Tseng, J. C. Ye, K. H. Chang, M. L. Lee, and W. C. Lai, “Improved hydrogen gas generation rate of n-GaN photoelectrode with SiO2 protection layer on the ohmic contacts from the electrolyte,” J. Electrochem. Soc.157(2), B266–B268 (2010). [CrossRef]
  17. J. K. Sheu, Y. K. Su, G. C. Chi, M. J. Jou, C. M. Chang, and C. C. Liu, “Indium tin oxide ohmic contact to highly doped n-GaN,” Solid-State Electron.43(11), 2081–2084 (1999). [CrossRef]
  18. T. Margalith, O. Buchinsky, D. A. Cohen, A. C. Abare, M. Hansen, S. P. DenBaars, and L. A. Coldren, “Indium tin oxide contacts to gallium nitride optoelectronic devices,” Appl. Phys. Lett.74(26), 3930 (1999). [CrossRef]
  19. J. F. Muth, J. H. Lee, I. K. Shmagin, R. M. Kolbas, H. C. Casey, B. P. Keller, U. K. Mishra, and S. P. DenBaars, “Absorption coefficient, energy gap, exciton binding energy, and recombination lifetime of GaN obtained from transmission measurements,” Appl. Phys. Lett.71(18), 2572 (1997).

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