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Photoelectrochemical behavior of mixed ZnO and GaN (ZnO:GaN) thin films prepared by sputtering technique
Applied Surface Science 270 (2013) 718–721
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Photoelectrochemical behavior of mixed ZnO and GaN (ZnO:GaN) thin films prepared by sputtering technique Sudhakar Shet a,b,∗ , Yanfa Yan c , Nuggehalli Ravindra b , John Turner a , Mowafak Al-Jassim a a b c
National Renewable Energy Laboratory, Golden, CO 80401, USA New Jersey Institute of Technology, Newark, NJ 07102, USA The University of Toledo, Toledo, OH 43606, USA
a r t i c l e
i n f o
Article history: Received 4 October 2012 Received in revised form 13 December 2012 Accepted 21 January 2013 Available online 26 January 2013 Keywords: Bandgap Sputter Ambient Photoelectrochemical ZnO GaN
a b s t r a c t Mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with significantly reduced bandgaps were synthesized by using zinc oxide and gallium nitride target at 100 ◦ C followed by post-deposition annealing at 500 ◦ C in ammonia for 4 h. All the films were synthesized by RF magnetron sputtering on Fluorinedoped tin oxide-coated glass. We found that mixed zinc oxide and gallium nitride (ZnO:GaN) thin films exhibited significantly reduced bandgap, as a result showed improved PEC response, compared to ZnO thin film. Furthermore, mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with various bandgaps were realized by varying the O2 mass flow rate in mixed O2 and N2 chamber ambient. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Transition-metal oxides are promising candidates for photoelectrochemical applications such as H2 production [1,2]. However, to date, only TiO2 has received extensive attention [3–6]. ZnO has similar bandgap and band-edge positions compared to TiO2 [1]. However, ZnO has a direct bandgap and higher electron mobility than TiO2 [7]. Thus, ZnO is expected to be an even better candidate for photoelectrochemical applications [8]. Like TiO2 , the bandgap of ZnO (3.3 eV) is too large to effectively use visible light, since the solar spectrum has maximum intensity at about 2.7 eV [3]. It is therefore critical to reduce the bandgap of ZnO. So far, impurity incorporation has been the main method to reduce the bandgap of TiO2 . It has been reported that N-, C-, and S-doping can successfully narrow the bandgap of TiO2 and push the photoresponse in the long-wavelength region [3–6]. Although bandgap reduction of TiO2 has been extensively studied, very limited research exists on bandgap narrowing of ZnO by impurity incorporation. Significant amounts of N can only be incorporated into ZnO and WO3 at low temperatures [9–11]. However, the film grown at low temperatures usually exhibit very poor crystallinity, which is extremely detrimental to the PEC performance. This dilemma hinders the PEC performance of N-incorporated ZnO
and WO3 films. A possible cause for the inferior crystallinity is due to the uncompensated charged N atoms. This problem could be overcome by using a heterogeneous photocatalysts: for example, GaN:ZnO solid solution. K. Maeda et al. [12], recently reported that GaN:ZnO solid solution is one of the effective methods of bandgap narrowing, which led to improved photoresponse in the long-wavelength region. However, to date, photoelectrochemical behavior of mixed zinc oxide and gallium nitride (ZnO:GaN) thin films prepared by sputtering technique have scarcely been studied. In this paper, we report on the synthesis of mixed zinc oxide and gallium nitride (ZnO:GaN) thin films by reactive RF magnetron sputtering in mixed O2 and N2 ambient. We found that a mixed ZnO and GaN (ZnO:GaN) thin films exhibit enhanced crystallinity and significantly reduced bandgap compared to ZnO thin film. As a result, mixed ZnO and GaN (ZnO:GaN) thin films presented improved PEC response, compared to ZnO film. All the mixed ZnO and GaN (ZnO:GaN) thin films showed n-type behavior. Furthermore, we found that the bandgap reduction in mixed ZnO and GaN (ZnO:GaN) thin films can be effectively controlled by varying the O2 mass flow rate in mixed O2 and N2 chamber ambient. Our results provide an insights on the utility of mixed ZnO and GaN (ZnO:GaN)-based PEC system. 2. Experimental details
∗ Corresponding author. Tel.: +1 303 384 7621; fax: +1 303 384 6491. E-mail address: [email protected] (S. Shet). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.134
The mixed ZnO and GaN (ZnO:GaN) thin films were deposited by using reactive RF magnetron sputtering of ZnO and GaN
S. Shet et al. / Applied Surface Science 270 (2013) 718–721
target. The chamber ambient was O2 /N2 gas mixture. Fluorine doped tin oxide (FTO, 20–23 /)-coated transparent glasses were used as substrates. The distance between the target and substrate was 8 cm. The base pressure was below 5 × 10−6 Torr, and the working pressure for all synthesis was 2 × 10−2 Torr. Substrates were rotated at 30 RPM during deposition to enhance film uniformity. Prior to sputtering, a pre-sputtering process was performed for 30 min to eliminate any contaminants from the target. Sputtering was then conducted with fixed RF power of 200 W for ZnO target and 35 W for GaN target at 100 ◦ C. For comparison, ZnO film was deposited at an RF power of 200 W at 100 ◦ C in pure Ar ambient. All samples were controlled to have similar film thickness of about 1000 nm as measured by stylus profilometry. The structure of synthesized films was characterized by X-ray diffraction (XGEN-4000, SCINTAG Inc., operated with a Cu K␣ radiation source at 45 kV and 37 mA) and The surface morphology was examined by atomic force microscopy (AFM) (Veeco D 3100) conducted in tapping mode with a silicon tip. The UV-VIS absorption spectra of the samples were measured by an n&k analyzer 1280 (n&k Technology, Inc.) to investigate optical properties. Photoelectrochemical measurements were performed in a three-electrode cell with a flat quartz-glass window to facilitate illumination to the photoelectrode surface [13–15]. The sputterdeposited films were used as the working electrodes with an active surface area of about 0.25 cm2 . Pt mesh and an Ag/AgCl electrode were used as counter and reference electrodes, respectively. A 0.5M Na2 SO4 aqueous solution with a pH of 6.8 was used as the electrolyte for the PEC measurements and scan rate was 5 mV/s in this experiment. The photoelectrochemical properties of the samples were measured using a fiber-optic illuminator (150-W tungsten-halogen lamp) with a UV/IR filter. The light intensity was measured by a photodiode power meter. The total light intensity with the UV/IR filter only was fixed at 125 mW/cm2 . Because our films were deposited on conducting substrates, measurements of electrical property by the Hall Effect were not possible. Instead, the electrical properties were measured by Mott-Schottky plots, which were obtained by AC impedance measurements. AC impedance measurements were carried out with a Solartron 1255 frequency response analyzer using the above three-electrode cells. Measurements were performed under dark conditions with an AC amplitude of 10 mV and frequency of 5000 Hz were used for the measurements taken under dark condition and the AC impedances were measured in the potential range of −0.7 V to 1.25 V (vs. Ag/AgCl reference). The series capacitor-resistor circuit model was used for Mott-Schottky plots [16,17].
3. Results and discussion Fig. 1 shows the X-ray diffraction curves of mixed ZnO and GaN (ZnO:GaN) and ZnO thin films deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively. It is seen that the ZnO film exhibits poor crystallinity, due to the low-temperature sputtering process. The mixed ZnO and GaN (ZnO:GaN) thin films grown at different O2 mass flow rate showed slightly better crystallinity than the pure ZnO film, despite faster deposition rate. Lack of significantly enhanced crystallnity for mixed ZnO and GaN (ZnO:GaN) thin films may be due to high RF power for ZnO target, resulting in faster deposition rate. It is known that sometimes faster deposition rate deteriorate the crystal structure. It can be seen that as the mass flow rate of O2 is decreased from 75–25%, (002) and (100) peak is enhanced and (102) peak is suppressed. Crystallite sizes were about 21 nm, 22 nm, 23 nm, and 26 nm for the ZnO, 75%, 50%, and 25% O2 mass flow rate for mixed ZnO and GaN (ZnO:GaN) thin
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Fig. 1. The X-ray diffraction curves of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
films, respectively, which were estimated by applying the DebyeScherrer equation to our XRD data. Fig. 2(a–d) shows AFM images from the ZnO, 75%, 50%, and 25% O2 flow rate for mixed ZnO and GaN (ZnO:GaN) thin films. AFM images also confirmed that ZnO, and mixed (ZnO:GaN) thin films have similar morphology and average crystal size is also same for (ZnO:GaN) films compared to ZnO films. Fig. 3 shows the optical absorption spectra of the mixed ZnO and GaN (ZnO:GaN) and ZnO thin films, deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively. The ZnO films showed optical absorption spectra and could absorb only light with wavelengths below 550 nm, due to their wide bandgap. However, the mixed ZnO and GaN (ZnO:GaN) thin films could absorb lower-energy photons, up to 1000 nm, indicating that the mixed ZnO and GaN (ZnO:GaN) can be used to shift the optical absorption into the visible region. Fig. 4 shows the optical absorption coefficients of the mixed ZnO and GaN (ZnO:GaN) and ZnO thin films, deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively. The direct electron transition from valence to conduction bands was assumed for the absorption coefficient curves, because ZnO and GaN films are known as direct-bandgap materials [1,18,19]. The optical bandgaps of the films were determined by extrapolating the linear portion of each curve. The bandgap of the ZnO film is 3.25 eV, which is consistent with the results reported elsewhere [10,13–15,20]. The direct optical bandgaps measured for ZnO:GaN solid solution thin films deposited at varying O2 mass flow rate from 75–25% gradually decreased from 3.15 to 2.8 eV. It is shown theoretically that the bottom of the conduction band for ZnO:GaN is mainly composed of 4s and 4p orbitals of Ga, while N2p orbitals followed by Zn3d orbitals situated on the top of the valence band. The presence of Zn3d and N2p electrons in the upper valence band provides p-d repulsion for the valence band maximum, which results in narrowing of bandgap [21]. The absorption from this impurity band cannot be characterized by direct band transitions and typically results in an absorption tail in the measured optical absorption curve. Such an absorption tail is clearly evident in Fig. 4 for the mixed (ZnO:GaN) thin films. This tail can be considered further bandgap reduction, which enables light harvesting in the much longer wavelength regions compared to the ZnO film. Fig. 5 shows Mott-Schottky plots of the mixed ZnO and GaN (ZnO:GaN) and ZnO thin films, deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
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S. Shet et al. / Applied Surface Science 270 (2013) 718–721
Fig. 2. AFM surface morphology (5 m × 5 m) of (a) ZnO, (b)–(d) mixed (ZnO:GaN) thin films deposited at O2 mass flow rate of 75%, 50% and 25%, respectively.
All the samples exhibited positive slopes, indicating n-type semiconductors. Our previous studies [22,23] reported that ZnO:N films deposited under a O2 /N2 plasma showed the n-type behaviors, due to substitutional N2 molecules which act as shallow double-donors. Perkins et al. [24] reported recently that O2 /N2 plasma can contain significant fraction of N2 molecules that can become incorporated into the ZnO films, leading to the n-type behavior. Fig. 6 shows the photocurrent–voltage curves of mixed ZnO and GaN (ZnO:GaN) and ZnO thin films, deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively under light on/off illumination with the UV/IR filter. It showed clearly that the mixed ZnO and GaN (ZnO:GaN) thin film exhibited significantly increased photocurrents, compared to the ZnO film. The mixed ZnO and GaN (ZnO:GaN) thin films deposited with O2 mass flow rate of 75% showed enhanced photocurrent compared
to films deposited at 50% and 25%. At the potential of 1.2 V, the photocurrents were 9, 21, 24, and 34 A cm−2 for the ZnO, 75%, 50%, and 25% O2 mass flow rate for mixed (ZnO:GaN) thin films, respectively. To investigate the photoresponse in the long-wavelength region, a green color filter (wavelength: 538.33 nm; FWHM: 77.478 nm) was used in combination with the UV/IR filter, as shown in Fig. 7. The ZnO film exhibited no clear photoresponse, due to its wide bandgap. The mixed ZnO and GaN (ZnO:GaN) thin films exhibited much higher photocurrent than the ZnO film, despite much less light absorption. It indicates that a mixed ZnO and GaN (ZnO:GaN) thin films with enhanced crystallinity, reduced bandgap using optimum O2 flow rate in the chamber ambient can shift the optical absorption into the visible light regions, thereby improve the PEC performance.
Fig. 3. The optical absorption spectra of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
Fig. 4. The Optical absorption coefficients of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
S. Shet et al. / Applied Surface Science 270 (2013) 718–721
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4. Conclusions
Fig. 5. Color online) The Mott-Schottky plots of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
Mixed ZnO and GaN (ZnO:GaN) thin films were synthesized on FTO substrates by reactive RF magnetron sputtering in mixed O2 and N2 gas ambient with varying O2 mass flow rate at 100 ◦ C followed by post-deposition annealing in NH3 gas. Mixed (ZnO:GaN) films exhibited slightly better crystallinity compared to ZnO film. Bandgap narrowing of mixed ZnO and GaN (ZnO:GaN) thin films was achieved by varying O2 /(O2 + N2 ) mass flow rate ratio. The n-type conductivity is revealed for the mixed (ZnO:GaN) thin films by Mott-Schottky plots as well as photocurrent polarity in I-V analysis. Mixed (ZnO:GaN) thin films exhibited improved photocurrents compared to ZnO film due to enhanced crystallinity and reduced bandgap. Our results suggest that the mixed solid solution approach is a potential method for synthesizing heterogeneous photocatalyst with both high crystallinity and bandgap reduction, which should help to improve their PEC performance. Acknowledgement This work was supported by the U.S. Department of Energy under Contract # DE-AC36-08GO28308. References
Fig. 6. The photocurrent–voltage curves of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively, under the illumination with an UV/IR filter.
Fig. 7. The photocurrent–voltage curves of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively, under the illumination with the combined green and UV/IR filters.
[1] M. Gratzel, Nature 414 (2001) 338. [2] T. Bak, J. Nowotny, M. Rekas, C.C. Sorrell, International Journal of Hydrogen Energy 27 (2002) 991. [3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [4] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Science 297 (2002) 2243. [5] S. Sakthivel, H. Kisch, Angewandte Chemie International Edition 42 (2003) 4908. [6] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Applied Physics Letters 81 (2002) 454. [7] K. Kakiuchi, E. Hosono, S. Fujihara, Journal of Photochemistry and Photobiology A: Chemistry 179 (2006) 81. [8] T.F. Jaramillo, S.H. Baeck, A. Kleiman-Shwarsctein, E.W. McFarland, Macromolecular Rapid Communications 25 (2004) 297. [9] S. Shet, K.-S. Ahn, T. Deutsch, H. Wang, N. Ravindra, Y. Yan, J. Turner, M. AlJassim, Journal of Power Sources 195 (2010) 5801. [10] S. Shet, K.-S. Ahn, H. Wang, N. Ravindra, Y. Yan, J. Turner, M. Al-Jassim, Journal of Materials Science 45 (2010) 5218. [11] D. Paluselli, B. Marsen, E.L. Miller, R.E. Rocheleau, Electrochemical and SolidState Letters 8 (2005) G301. [12] K. Maeda, T. Takata, M. Hara, N. Saito, Y. Inoue, H. Kobayashi, K. Domen, Journal of the American Chemical Society 127 (2005) 8286. [13] S. Shet, K.–S. Ahn, Y. Yan, T. Deutsch, K.M. Chrusrowski, J. Turner, M. Al- Jassim, N. Ravindra, Journal of Applied Physics 103 (2008) 073504. [14] S. Shet, K.-S. Ahn, T. Deutsch, H. Wang, N. Ravindra, Y. Yan, J. Turner, M. AlJassim, Journal of Materials Research 25 (2010) 69. [15] S. Shet, K.-S. Ahn, N. Ravindra, Y. Yan, J. Turner, M. Al-Jassim, Journal of Materials 62 (2010) 25. [16] S.–H. Kang, J.–Y. Kim, Y. Kim, H.–S. Kim, Y.–E. Sung, Journal of Physical Chemistry C 111 (2007) 9614. [17] J. Akikusa, S.U.M. Khan, International Journal of Hydrogen Energy 27 (2002) 863. [18] M. Futsuhara, K. Yoshioka, O. Takai, Thin Solid Films 317 (1998) 322. [19] S. Berrah, H. Abid, A. Boukortt, Physica Scripta 74 (2006) 104. [20] C.X. Xu, X.W. Sun, X.H. Zhang, L. Ke, S.J. Chua, Nanotechnology 15 (2004) 856. [21] H. Wei, A. Zunger, Physical Review B 37 (1988) 8958. [22] S. Shet, K.-S. Ahn, N. Ravindra, Y. Yan, J. Turner, M. Al-Jassim, Thin Solid Films 519 (2011) 5983. [23] K.-S. Ahn, Y. Yan, S. Shet, K. Jones, T. Deutsch, J. Turner, M. Al-Jassim, Applied Physics Letters 93 (2008) 163117. [24] C.L. Perkins, S.H. Lee, X. Li, S.E. Asher, T.J. Coutts, Journal of Applied Physics 97 (2005) 034907.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Photoelectrochemical behavior of mixed ZnO and GaN (ZnO:GaN) thin films prepared by sputtering technique Sudhakar Shet a,b,∗ , Yanfa Yan c , Nuggehalli Ravindra b , John Turner a , Mowafak Al-Jassim a a b c
National Renewable Energy Laboratory, Golden, CO 80401, USA New Jersey Institute of Technology, Newark, NJ 07102, USA The University of Toledo, Toledo, OH 43606, USA
a r t i c l e
i n f o
Article history: Received 4 October 2012 Received in revised form 13 December 2012 Accepted 21 January 2013 Available online 26 January 2013 Keywords: Bandgap Sputter Ambient Photoelectrochemical ZnO GaN
a b s t r a c t Mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with significantly reduced bandgaps were synthesized by using zinc oxide and gallium nitride target at 100 ◦ C followed by post-deposition annealing at 500 ◦ C in ammonia for 4 h. All the films were synthesized by RF magnetron sputtering on Fluorinedoped tin oxide-coated glass. We found that mixed zinc oxide and gallium nitride (ZnO:GaN) thin films exhibited significantly reduced bandgap, as a result showed improved PEC response, compared to ZnO thin film. Furthermore, mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with various bandgaps were realized by varying the O2 mass flow rate in mixed O2 and N2 chamber ambient. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Transition-metal oxides are promising candidates for photoelectrochemical applications such as H2 production [1,2]. However, to date, only TiO2 has received extensive attention [3–6]. ZnO has similar bandgap and band-edge positions compared to TiO2 [1]. However, ZnO has a direct bandgap and higher electron mobility than TiO2 [7]. Thus, ZnO is expected to be an even better candidate for photoelectrochemical applications [8]. Like TiO2 , the bandgap of ZnO (3.3 eV) is too large to effectively use visible light, since the solar spectrum has maximum intensity at about 2.7 eV [3]. It is therefore critical to reduce the bandgap of ZnO. So far, impurity incorporation has been the main method to reduce the bandgap of TiO2 . It has been reported that N-, C-, and S-doping can successfully narrow the bandgap of TiO2 and push the photoresponse in the long-wavelength region [3–6]. Although bandgap reduction of TiO2 has been extensively studied, very limited research exists on bandgap narrowing of ZnO by impurity incorporation. Significant amounts of N can only be incorporated into ZnO and WO3 at low temperatures [9–11]. However, the film grown at low temperatures usually exhibit very poor crystallinity, which is extremely detrimental to the PEC performance. This dilemma hinders the PEC performance of N-incorporated ZnO
and WO3 films. A possible cause for the inferior crystallinity is due to the uncompensated charged N atoms. This problem could be overcome by using a heterogeneous photocatalysts: for example, GaN:ZnO solid solution. K. Maeda et al. [12], recently reported that GaN:ZnO solid solution is one of the effective methods of bandgap narrowing, which led to improved photoresponse in the long-wavelength region. However, to date, photoelectrochemical behavior of mixed zinc oxide and gallium nitride (ZnO:GaN) thin films prepared by sputtering technique have scarcely been studied. In this paper, we report on the synthesis of mixed zinc oxide and gallium nitride (ZnO:GaN) thin films by reactive RF magnetron sputtering in mixed O2 and N2 ambient. We found that a mixed ZnO and GaN (ZnO:GaN) thin films exhibit enhanced crystallinity and significantly reduced bandgap compared to ZnO thin film. As a result, mixed ZnO and GaN (ZnO:GaN) thin films presented improved PEC response, compared to ZnO film. All the mixed ZnO and GaN (ZnO:GaN) thin films showed n-type behavior. Furthermore, we found that the bandgap reduction in mixed ZnO and GaN (ZnO:GaN) thin films can be effectively controlled by varying the O2 mass flow rate in mixed O2 and N2 chamber ambient. Our results provide an insights on the utility of mixed ZnO and GaN (ZnO:GaN)-based PEC system. 2. Experimental details
∗ Corresponding author. Tel.: +1 303 384 7621; fax: +1 303 384 6491. E-mail address: [email protected] (S. Shet). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.134
The mixed ZnO and GaN (ZnO:GaN) thin films were deposited by using reactive RF magnetron sputtering of ZnO and GaN
S. Shet et al. / Applied Surface Science 270 (2013) 718–721
target. The chamber ambient was O2 /N2 gas mixture. Fluorine doped tin oxide (FTO, 20–23 /)-coated transparent glasses were used as substrates. The distance between the target and substrate was 8 cm. The base pressure was below 5 × 10−6 Torr, and the working pressure for all synthesis was 2 × 10−2 Torr. Substrates were rotated at 30 RPM during deposition to enhance film uniformity. Prior to sputtering, a pre-sputtering process was performed for 30 min to eliminate any contaminants from the target. Sputtering was then conducted with fixed RF power of 200 W for ZnO target and 35 W for GaN target at 100 ◦ C. For comparison, ZnO film was deposited at an RF power of 200 W at 100 ◦ C in pure Ar ambient. All samples were controlled to have similar film thickness of about 1000 nm as measured by stylus profilometry. The structure of synthesized films was characterized by X-ray diffraction (XGEN-4000, SCINTAG Inc., operated with a Cu K␣ radiation source at 45 kV and 37 mA) and The surface morphology was examined by atomic force microscopy (AFM) (Veeco D 3100) conducted in tapping mode with a silicon tip. The UV-VIS absorption spectra of the samples were measured by an n&k analyzer 1280 (n&k Technology, Inc.) to investigate optical properties. Photoelectrochemical measurements were performed in a three-electrode cell with a flat quartz-glass window to facilitate illumination to the photoelectrode surface [13–15]. The sputterdeposited films were used as the working electrodes with an active surface area of about 0.25 cm2 . Pt mesh and an Ag/AgCl electrode were used as counter and reference electrodes, respectively. A 0.5M Na2 SO4 aqueous solution with a pH of 6.8 was used as the electrolyte for the PEC measurements and scan rate was 5 mV/s in this experiment. The photoelectrochemical properties of the samples were measured using a fiber-optic illuminator (150-W tungsten-halogen lamp) with a UV/IR filter. The light intensity was measured by a photodiode power meter. The total light intensity with the UV/IR filter only was fixed at 125 mW/cm2 . Because our films were deposited on conducting substrates, measurements of electrical property by the Hall Effect were not possible. Instead, the electrical properties were measured by Mott-Schottky plots, which were obtained by AC impedance measurements. AC impedance measurements were carried out with a Solartron 1255 frequency response analyzer using the above three-electrode cells. Measurements were performed under dark conditions with an AC amplitude of 10 mV and frequency of 5000 Hz were used for the measurements taken under dark condition and the AC impedances were measured in the potential range of −0.7 V to 1.25 V (vs. Ag/AgCl reference). The series capacitor-resistor circuit model was used for Mott-Schottky plots [16,17].
3. Results and discussion Fig. 1 shows the X-ray diffraction curves of mixed ZnO and GaN (ZnO:GaN) and ZnO thin films deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively. It is seen that the ZnO film exhibits poor crystallinity, due to the low-temperature sputtering process. The mixed ZnO and GaN (ZnO:GaN) thin films grown at different O2 mass flow rate showed slightly better crystallinity than the pure ZnO film, despite faster deposition rate. Lack of significantly enhanced crystallnity for mixed ZnO and GaN (ZnO:GaN) thin films may be due to high RF power for ZnO target, resulting in faster deposition rate. It is known that sometimes faster deposition rate deteriorate the crystal structure. It can be seen that as the mass flow rate of O2 is decreased from 75–25%, (002) and (100) peak is enhanced and (102) peak is suppressed. Crystallite sizes were about 21 nm, 22 nm, 23 nm, and 26 nm for the ZnO, 75%, 50%, and 25% O2 mass flow rate for mixed ZnO and GaN (ZnO:GaN) thin
719
Fig. 1. The X-ray diffraction curves of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
films, respectively, which were estimated by applying the DebyeScherrer equation to our XRD data. Fig. 2(a–d) shows AFM images from the ZnO, 75%, 50%, and 25% O2 flow rate for mixed ZnO and GaN (ZnO:GaN) thin films. AFM images also confirmed that ZnO, and mixed (ZnO:GaN) thin films have similar morphology and average crystal size is also same for (ZnO:GaN) films compared to ZnO films. Fig. 3 shows the optical absorption spectra of the mixed ZnO and GaN (ZnO:GaN) and ZnO thin films, deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively. The ZnO films showed optical absorption spectra and could absorb only light with wavelengths below 550 nm, due to their wide bandgap. However, the mixed ZnO and GaN (ZnO:GaN) thin films could absorb lower-energy photons, up to 1000 nm, indicating that the mixed ZnO and GaN (ZnO:GaN) can be used to shift the optical absorption into the visible region. Fig. 4 shows the optical absorption coefficients of the mixed ZnO and GaN (ZnO:GaN) and ZnO thin films, deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively. The direct electron transition from valence to conduction bands was assumed for the absorption coefficient curves, because ZnO and GaN films are known as direct-bandgap materials [1,18,19]. The optical bandgaps of the films were determined by extrapolating the linear portion of each curve. The bandgap of the ZnO film is 3.25 eV, which is consistent with the results reported elsewhere [10,13–15,20]. The direct optical bandgaps measured for ZnO:GaN solid solution thin films deposited at varying O2 mass flow rate from 75–25% gradually decreased from 3.15 to 2.8 eV. It is shown theoretically that the bottom of the conduction band for ZnO:GaN is mainly composed of 4s and 4p orbitals of Ga, while N2p orbitals followed by Zn3d orbitals situated on the top of the valence band. The presence of Zn3d and N2p electrons in the upper valence band provides p-d repulsion for the valence band maximum, which results in narrowing of bandgap [21]. The absorption from this impurity band cannot be characterized by direct band transitions and typically results in an absorption tail in the measured optical absorption curve. Such an absorption tail is clearly evident in Fig. 4 for the mixed (ZnO:GaN) thin films. This tail can be considered further bandgap reduction, which enables light harvesting in the much longer wavelength regions compared to the ZnO film. Fig. 5 shows Mott-Schottky plots of the mixed ZnO and GaN (ZnO:GaN) and ZnO thin films, deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
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S. Shet et al. / Applied Surface Science 270 (2013) 718–721
Fig. 2. AFM surface morphology (5 m × 5 m) of (a) ZnO, (b)–(d) mixed (ZnO:GaN) thin films deposited at O2 mass flow rate of 75%, 50% and 25%, respectively.
All the samples exhibited positive slopes, indicating n-type semiconductors. Our previous studies [22,23] reported that ZnO:N films deposited under a O2 /N2 plasma showed the n-type behaviors, due to substitutional N2 molecules which act as shallow double-donors. Perkins et al. [24] reported recently that O2 /N2 plasma can contain significant fraction of N2 molecules that can become incorporated into the ZnO films, leading to the n-type behavior. Fig. 6 shows the photocurrent–voltage curves of mixed ZnO and GaN (ZnO:GaN) and ZnO thin films, deposited in mixed O2 and N2 ambient with O2 mass flow rate of 75–25% and in Ar respectively under light on/off illumination with the UV/IR filter. It showed clearly that the mixed ZnO and GaN (ZnO:GaN) thin film exhibited significantly increased photocurrents, compared to the ZnO film. The mixed ZnO and GaN (ZnO:GaN) thin films deposited with O2 mass flow rate of 75% showed enhanced photocurrent compared
to films deposited at 50% and 25%. At the potential of 1.2 V, the photocurrents were 9, 21, 24, and 34 A cm−2 for the ZnO, 75%, 50%, and 25% O2 mass flow rate for mixed (ZnO:GaN) thin films, respectively. To investigate the photoresponse in the long-wavelength region, a green color filter (wavelength: 538.33 nm; FWHM: 77.478 nm) was used in combination with the UV/IR filter, as shown in Fig. 7. The ZnO film exhibited no clear photoresponse, due to its wide bandgap. The mixed ZnO and GaN (ZnO:GaN) thin films exhibited much higher photocurrent than the ZnO film, despite much less light absorption. It indicates that a mixed ZnO and GaN (ZnO:GaN) thin films with enhanced crystallinity, reduced bandgap using optimum O2 flow rate in the chamber ambient can shift the optical absorption into the visible light regions, thereby improve the PEC performance.
Fig. 3. The optical absorption spectra of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
Fig. 4. The Optical absorption coefficients of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
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4. Conclusions
Fig. 5. Color online) The Mott-Schottky plots of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively.
Mixed ZnO and GaN (ZnO:GaN) thin films were synthesized on FTO substrates by reactive RF magnetron sputtering in mixed O2 and N2 gas ambient with varying O2 mass flow rate at 100 ◦ C followed by post-deposition annealing in NH3 gas. Mixed (ZnO:GaN) films exhibited slightly better crystallinity compared to ZnO film. Bandgap narrowing of mixed ZnO and GaN (ZnO:GaN) thin films was achieved by varying O2 /(O2 + N2 ) mass flow rate ratio. The n-type conductivity is revealed for the mixed (ZnO:GaN) thin films by Mott-Schottky plots as well as photocurrent polarity in I-V analysis. Mixed (ZnO:GaN) thin films exhibited improved photocurrents compared to ZnO film due to enhanced crystallinity and reduced bandgap. Our results suggest that the mixed solid solution approach is a potential method for synthesizing heterogeneous photocatalyst with both high crystallinity and bandgap reduction, which should help to improve their PEC performance. Acknowledgement This work was supported by the U.S. Department of Energy under Contract # DE-AC36-08GO28308. References
Fig. 6. The photocurrent–voltage curves of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively, under the illumination with an UV/IR filter.
Fig. 7. The photocurrent–voltage curves of mixed (ZnO:GaN) thin films deposited in mixed N2 and O2 ambient with O2 mass flow rate of 75–25% and in Ar respectively, under the illumination with the combined green and UV/IR filters.
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