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Photoconductive characteristics of ZnO: Al network films sputter-deposited at different deposition temperatures
Materials Letters 106 (2013) 218–221
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photoconductive characteristics of ZnO: Al network films sputter-deposited at different deposition temperatures Jinping Liu, Hong Qiu n, Guoshou Zou, Bin Hu, Zhiwei Yang Department of Physics, School of Mathematics and Physics, University of Science and Technology Beijing, 30 Xueyuanlu, Haidian District, ĆBeijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 19 February 2013 Accepted 29 April 2013 Available online 7 May 2013
50 nm-thick ZnO: Al network films were sputter-deposited on nanochannel Al2O3 substrates at 300 K, 423 K and 623 K. A photoconduction of the network films was measured by using a metal– semiconductor–metal planar configuration with Ag contact electrodes. Both the dark current and the photocurrent increase linearly with the applied voltage, meaning an ohmic contact between ZnO: Al network film and Ag electrode. The photocurrent of the network films increases with increasing deposition temperature. The network films show a slow photo-response. The rising process time constant is almost independent of deposition temperature. The photosensitivity of the network films decreases with increasing deposition temperature. & 2013 Elsevier B.V. All rights reserved.
Keywords: ZnO: Al network film Photoconduction Structure Sputtering Deposition temperature
1. Introduction ZnO and ZnO-based films have attracted considerable attention for their potential applications in optoelectronics, magnetoelectronics, piezoelectric devices and chemical sensors [1–3]. Al-doped ZnO (ZnO: Al) films are often used as transparent conductive electrodes and have been actively investigated for scientific and practical interests. Besides, a few studies on the photoconductive characteristics of ZnO: Al films have been reported. Xu et al. [4] used a sol–gel method to prepare ZnO: Al films and obtained a photocurrent of 58 μA at a bias voltage of 6 V. Mamat et al. [5] prepared ZnO: Al films with different Al doping concentrations by the sol–gel method. They found that the film with 1 at% Al had the highest photocurrent under ultraviolet illumination. Mamat et al. [6] prepared nanorod–nanoflake ZnO: Al network films by ultrasonic-assisted sol–gel and immersion methods. They found that the photo-response of the network film was higher than that of the continuous film. Ganesh et al. [7] prepared ZnO: Al films with different Al concentrations by sol–gel spin coating. An optimal Al doping concentration for photoconductive applications was 1–2 wt%. As mentioned above, the chemical methods were used to prepare the ZnO: Al films for the photoconductive applications. On the other hand, nanochannel Al2O3 is an attractive template to prepare nanomaterials [8]. Recently, we have sputter-depo sited ZnO: Al network films on nanochannel Al2O3 substrates [9].
n
Corresponding author. Tel.: +86 10 62333786; fax: +86 10 62332993. E-mail address: [email protected] (H. Qiu).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.04.105
A photoluminescence of the network film was better than that of the continuous film. In the present work, photoconductive characteristics of the ZnO: Al network films sputter-deposited on nanochannel Al2O3 substrates are studied as a function of the deposition temperature. The photoconductive ZnO: Al films have potential applications in optoelectronic devices. This work reports the photocurrent and the photosensitivity of the sputter-deposited ZnO: Al network films. It is significant for fundamental and practical viewpoints. 2. Experimental procedure The DC magnetron sputtering system with the target inclined to the substrate at an angle of 451 was used [9]. The commercial nanochannel Al2O3 substrate (Whatman) had an average pore diameter of 100 nm. The ZnO: Al films were sputter-deposited on nanochannel Al2O3 substrates at 300 K, 423 K and 623 K using a sintered ZnO+2 wt% Al2O3 target (99.99% purity). Prior to deposition, the working chamber was evacuated to a pressure of 4 10−4 Pa by a turbo molecular pump. The distance between target and substrate was 100 mm. During sputter-deposition, the Ar gas (99.9995% purity) pressure was 0.4 Pa and the sputtering power was 150 W. The deposition rate was 13 nm/min. The substrate holder was rotated by using a stepping motor during deposition in order to obtain the uniform films. Field emission scanning electron microscope (FE-SEM) of SUPRA 55 (Zeiss) was used to observe the film structure. The film thickness measured by the cross-sectional FE-SEM microphotograph of the film is 50 nm. Photoconductive characteristics of the
J. Liu et al. / Materials Letters 106 (2013) 218–221
network films were measured by using the metal–semiconductor– metal planar configuration with Ag contact electrodes. The Ag electrodes were prepared on the film surface by Ag paste. The distance between the Ag electrodes was 5 mm. A relationship of current and voltage (I–V) was measured by Keithley 4200-SCS system (Keithley) in order to determine the dark current and the photocurrent. ENF-280C/FE light source, which had a wavelength of 365 nm and a power of 0.47 mW/cm2, was used to obtain the photoconductive behaviors of the films. A relationship of photocurrent and time (I–t curve) was measured by using an electrochemical instrument SI1287 (Solartron Analytical), which was used as the voltage source and the current monitor.
3. Results and discussion Figure 1 shows FE-SEM microphotographs of the ZnO: Al films grown on nanochannel Al2O3 substrates at 300 K, 423 K and 623 K. As shown in Fig. 1, all the films have a network structure. For the films grown at 300 K and 423 K, the network is formed by connecting granules. The granule consists of many small grains. For the film grown at 623 K, the network is formed by connecting grains. The grain size increases with increasing deposition temperature. The high deposition temperature enhances the surface diffusion of adatoms, promoting the grain growth and the decrease in vacancies.
200nm
219
Figure 2 shows dark and photo-illuminated I–V characteristics of the ZnO: Al network films grown at various deposition temperatures. As shown in Fig. 2, all the network films exhibit the photoconductive behavior. Furthermore, both the dark and photoilluminated currents increase linearly with increasing bias voltage, indicating that the Ag electrode forms the ohmic contact on the ZnO: Al film. It was reported that the Schottky contact was formed between Ag and n-type ZnO [10,11]. Furthermore, the Schottky contact could become the ohmic contact with increasing the carrier concentration in the ZnO film [11]. Thus, in the present work, the ohmic contact between the Ag electrode and the ZnO: Al film can be attributed to the high carrier concentration in the film. Moreover, according to the I–V relationship of the dark current, it is said that the resistivity of the ZnO: Al network film decreases with increasing deposition temperature. It is attributed to the increase in the grain size and the decrease in the vacancies with the deposition temperature. It was reported that the oxygen chemisorption mechanism was used to explain the photoconduction of the ZnO and ZnO-based films [6,11,12]. The oxygen molecules adsorbed on the film surface play an important role in the oxygen chemisorption mechanism. In the present work, the ZnO: Al network films have a large ratio of surface area to volume. The large surface area of the network films leads to a large number of oxygen molecules on the film surface. As the network film is illuminated by the ultraviolet light, its photoconductive behavior can be explained by the oxygen chemisorption mechanism.
100nm
Fig. 1. FE-SEM microphotographs of the ZnO: Al films grown at (a, b) 300 K, (c, d) 423 K, and (e, f) 623 K.
220
J. Liu et al. / Materials Letters 106 (2013) 218–221
Fig. 3. Temporal photocurrent response of the ZnO: Al network films. Fig. 2. Dark and photo-illuminated I–V characteristics of the ZnO: Al network films.
The photo-generated photocurrent Iph is obtained by Iph ¼I−Id, where I is the photo-illuminated current and Id is the dark current. Fig. 3 shows a temporal photocurrent response of the ZnO: Al network films grown at various deposition temperatures. During measuring the photo-response, a voltage of 10 V was applied between the Ag electrodes. As shown in Fig. 3, the photocurrent of the network film increases with increasing deposition temperature. According to the oxygen chemisorption mechanism [6,11,12], the photo-generated carriers are free electrons. Generally, a current measured by the metal–semiconductor–metal planar configuration is an effective one which flows out the semiconductor and flows in the external electronic circuit. Then the photocurrent Iph is expressed as [13] I ph ¼
q w d Dn me V L
ð1Þ
where q is the electron charge, w is the width of the metal electrode, d is the semiconducting film thickness, Δn is the photo-generated electron concentration, μe is the electron mobility, V is the bias voltage applied between the electrodes and L is the spacing between the electrodes. The high deposition temperature improves the crystal quality as shown in Fig. 1, weakening a scattering of electrons. Thus the electron mobility is enhanced by increasing deposition temperature. Furthermore, the resistivity of the network film decreases with increasing deposition temperature as shown in Fig. 2. It can also be attributed to the increase in the electron mobility. Therefore, it is considered that the electron mobility in the network films increases with increasing deposition
temperature. According to Eq. (1), the photocurrent of the network films increases with increasing deposition temperature. The network film shows the photo-response with a long rising process time, as shown in Fig. 3. According to the photo-response in Fig. 3, the rising process time constant can be estimated. As the network film is photo-illuminated, the relationship between photocurrent Iph and time t is expressed as [6,14] t I ph ¼ I s 1−exp − ð2Þ τr where Is is the saturation photocurrent and τr is the rising process time constant. According to Eq. (2), for all the network films, a variation of −ln(1−Iph/Is) with t is plotted and exhibits a good linear dependence having a correlation coefficient better than 0.991. The rising process time constant is calculated in terms of the slope of the fitted straight line. The rising process time constants are 730 s for the film grown at 300 K, 725 s for the film grown at 423 K and 735 s for the film grown at 623 K. The network films exhibit the slow photo-response with the long rising process time constant. The rising process time constant is almost independent of deposition temperature. It means that the same mechanism dominates the photocurrent for the network films grown at various deposition temperatures. The photosensitivity S ¼Iph/Id, where Id is the dark current [15]. Fig. 4 shows a variation of photosensitivity with photo-illuminated time for the ZnO: Al network films grown at various deposition temperatures. As shown in Fig. 4, the photosensitivity of the network films decreases with increasing deposition temperature. It is mainly attributed to a marked increase in the dark current with the deposition temperature although the photocurrent increases with the deposition temperature.
J. Liu et al. / Materials Letters 106 (2013) 218–221
221
Acknowledgment The financial support from the Fundamental Research Funds for the Central Universities 2012 is gratefully acknowledged.
References
Fig. 4. Variation of photosensitivity with photo-illuminated time for the ZnO: Al network films.
4. Summary 50 nm-thick ZnO: Al network films were sputter-deposited on nanochannel Al2O3 substrates at 300 K, 423 K and 623 K. Both the dark current and the photocurrent increase linearly with the applied voltage. The photocurrent of the network films increases with increasing deposition temperature. The network films show the slow photo-response. The rising process time constant is almost independent of deposition temperature. The photosensitivity of the network films decreases with increasing deposition temperature.
[1] Ozgur U, Alivov YI, Liu C, Teke A, Reshchikov MA, Dogan S, et al. J Appl Phys 2005;98:041301. [2] Comini E, Baratto C, Faglia G, Ferroni M, Vomiero A, Sberveglieri G. Prog Mater Sci 2009;54:1–67. [3] Kapilashrami M, Xu J, Biswas A, Tamaki T, Sharma P, Rao KV, et al. Mater Lett 2010;64:1291–4. [4] Xu ZQ, Deng H, Xie J, Li Y, Zu XT. Appl Surf Sci 2006;253:476–9. [5] Mamat MH, Sahdan MZ, Khusaimi Z, Ahmed AZ, Abdullah S, Rusop M. Opt Mater 2010;32:696–9. [6] Mamat MH, Khusaimi Z, Musa MZ, Malek MF, Rusop M. Sens Actuators A 2011;171:241–7. [7] Ganesh T, Rajesh S, Xavier FP. Mater Sci Semicond Process 2012. http://dx.doi. org/10.1016/j.mssp.2012.10.008. [8] Banerjee N, Krupanidhi SB. Nat Sci 2010;7:688–93. [9] Zuo YP, Qiu H, Chen XB, He JP. Mater Chem Phys 2012;133:507–14. [10] Liang S, Sheng H, Liu Y, Huo Z, Lu Y, Shen H. J Cryst Growth 2001;225:110–3. [11] Al-Hardan NH, Abdullah MJ, Ahmad H, Aziz AA, Low LY. Solid-State Electron 2011;55:59–63. [12] Liu MJ, Kim HK. Appl Phys Lett 2004;84:173–5. [13] Razeghi M, Rogalski A. J Appl Phys 1996;79:7433–73. [14] Shinde SS, Rajpure KY. Appl Surf Sci 2011;257:9595–9. [15] Fathy N, Ichimura M. Solar Energy Mater Solar Cells 2005;87:747–56.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photoconductive characteristics of ZnO: Al network films sputter-deposited at different deposition temperatures Jinping Liu, Hong Qiu n, Guoshou Zou, Bin Hu, Zhiwei Yang Department of Physics, School of Mathematics and Physics, University of Science and Technology Beijing, 30 Xueyuanlu, Haidian District, ĆBeijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 19 February 2013 Accepted 29 April 2013 Available online 7 May 2013
50 nm-thick ZnO: Al network films were sputter-deposited on nanochannel Al2O3 substrates at 300 K, 423 K and 623 K. A photoconduction of the network films was measured by using a metal– semiconductor–metal planar configuration with Ag contact electrodes. Both the dark current and the photocurrent increase linearly with the applied voltage, meaning an ohmic contact between ZnO: Al network film and Ag electrode. The photocurrent of the network films increases with increasing deposition temperature. The network films show a slow photo-response. The rising process time constant is almost independent of deposition temperature. The photosensitivity of the network films decreases with increasing deposition temperature. & 2013 Elsevier B.V. All rights reserved.
Keywords: ZnO: Al network film Photoconduction Structure Sputtering Deposition temperature
1. Introduction ZnO and ZnO-based films have attracted considerable attention for their potential applications in optoelectronics, magnetoelectronics, piezoelectric devices and chemical sensors [1–3]. Al-doped ZnO (ZnO: Al) films are often used as transparent conductive electrodes and have been actively investigated for scientific and practical interests. Besides, a few studies on the photoconductive characteristics of ZnO: Al films have been reported. Xu et al. [4] used a sol–gel method to prepare ZnO: Al films and obtained a photocurrent of 58 μA at a bias voltage of 6 V. Mamat et al. [5] prepared ZnO: Al films with different Al doping concentrations by the sol–gel method. They found that the film with 1 at% Al had the highest photocurrent under ultraviolet illumination. Mamat et al. [6] prepared nanorod–nanoflake ZnO: Al network films by ultrasonic-assisted sol–gel and immersion methods. They found that the photo-response of the network film was higher than that of the continuous film. Ganesh et al. [7] prepared ZnO: Al films with different Al concentrations by sol–gel spin coating. An optimal Al doping concentration for photoconductive applications was 1–2 wt%. As mentioned above, the chemical methods were used to prepare the ZnO: Al films for the photoconductive applications. On the other hand, nanochannel Al2O3 is an attractive template to prepare nanomaterials [8]. Recently, we have sputter-depo sited ZnO: Al network films on nanochannel Al2O3 substrates [9].
n
Corresponding author. Tel.: +86 10 62333786; fax: +86 10 62332993. E-mail address: [email protected] (H. Qiu).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.04.105
A photoluminescence of the network film was better than that of the continuous film. In the present work, photoconductive characteristics of the ZnO: Al network films sputter-deposited on nanochannel Al2O3 substrates are studied as a function of the deposition temperature. The photoconductive ZnO: Al films have potential applications in optoelectronic devices. This work reports the photocurrent and the photosensitivity of the sputter-deposited ZnO: Al network films. It is significant for fundamental and practical viewpoints. 2. Experimental procedure The DC magnetron sputtering system with the target inclined to the substrate at an angle of 451 was used [9]. The commercial nanochannel Al2O3 substrate (Whatman) had an average pore diameter of 100 nm. The ZnO: Al films were sputter-deposited on nanochannel Al2O3 substrates at 300 K, 423 K and 623 K using a sintered ZnO+2 wt% Al2O3 target (99.99% purity). Prior to deposition, the working chamber was evacuated to a pressure of 4 10−4 Pa by a turbo molecular pump. The distance between target and substrate was 100 mm. During sputter-deposition, the Ar gas (99.9995% purity) pressure was 0.4 Pa and the sputtering power was 150 W. The deposition rate was 13 nm/min. The substrate holder was rotated by using a stepping motor during deposition in order to obtain the uniform films. Field emission scanning electron microscope (FE-SEM) of SUPRA 55 (Zeiss) was used to observe the film structure. The film thickness measured by the cross-sectional FE-SEM microphotograph of the film is 50 nm. Photoconductive characteristics of the
J. Liu et al. / Materials Letters 106 (2013) 218–221
network films were measured by using the metal–semiconductor– metal planar configuration with Ag contact electrodes. The Ag electrodes were prepared on the film surface by Ag paste. The distance between the Ag electrodes was 5 mm. A relationship of current and voltage (I–V) was measured by Keithley 4200-SCS system (Keithley) in order to determine the dark current and the photocurrent. ENF-280C/FE light source, which had a wavelength of 365 nm and a power of 0.47 mW/cm2, was used to obtain the photoconductive behaviors of the films. A relationship of photocurrent and time (I–t curve) was measured by using an electrochemical instrument SI1287 (Solartron Analytical), which was used as the voltage source and the current monitor.
3. Results and discussion Figure 1 shows FE-SEM microphotographs of the ZnO: Al films grown on nanochannel Al2O3 substrates at 300 K, 423 K and 623 K. As shown in Fig. 1, all the films have a network structure. For the films grown at 300 K and 423 K, the network is formed by connecting granules. The granule consists of many small grains. For the film grown at 623 K, the network is formed by connecting grains. The grain size increases with increasing deposition temperature. The high deposition temperature enhances the surface diffusion of adatoms, promoting the grain growth and the decrease in vacancies.
200nm
219
Figure 2 shows dark and photo-illuminated I–V characteristics of the ZnO: Al network films grown at various deposition temperatures. As shown in Fig. 2, all the network films exhibit the photoconductive behavior. Furthermore, both the dark and photoilluminated currents increase linearly with increasing bias voltage, indicating that the Ag electrode forms the ohmic contact on the ZnO: Al film. It was reported that the Schottky contact was formed between Ag and n-type ZnO [10,11]. Furthermore, the Schottky contact could become the ohmic contact with increasing the carrier concentration in the ZnO film [11]. Thus, in the present work, the ohmic contact between the Ag electrode and the ZnO: Al film can be attributed to the high carrier concentration in the film. Moreover, according to the I–V relationship of the dark current, it is said that the resistivity of the ZnO: Al network film decreases with increasing deposition temperature. It is attributed to the increase in the grain size and the decrease in the vacancies with the deposition temperature. It was reported that the oxygen chemisorption mechanism was used to explain the photoconduction of the ZnO and ZnO-based films [6,11,12]. The oxygen molecules adsorbed on the film surface play an important role in the oxygen chemisorption mechanism. In the present work, the ZnO: Al network films have a large ratio of surface area to volume. The large surface area of the network films leads to a large number of oxygen molecules on the film surface. As the network film is illuminated by the ultraviolet light, its photoconductive behavior can be explained by the oxygen chemisorption mechanism.
100nm
Fig. 1. FE-SEM microphotographs of the ZnO: Al films grown at (a, b) 300 K, (c, d) 423 K, and (e, f) 623 K.
220
J. Liu et al. / Materials Letters 106 (2013) 218–221
Fig. 3. Temporal photocurrent response of the ZnO: Al network films. Fig. 2. Dark and photo-illuminated I–V characteristics of the ZnO: Al network films.
The photo-generated photocurrent Iph is obtained by Iph ¼I−Id, where I is the photo-illuminated current and Id is the dark current. Fig. 3 shows a temporal photocurrent response of the ZnO: Al network films grown at various deposition temperatures. During measuring the photo-response, a voltage of 10 V was applied between the Ag electrodes. As shown in Fig. 3, the photocurrent of the network film increases with increasing deposition temperature. According to the oxygen chemisorption mechanism [6,11,12], the photo-generated carriers are free electrons. Generally, a current measured by the metal–semiconductor–metal planar configuration is an effective one which flows out the semiconductor and flows in the external electronic circuit. Then the photocurrent Iph is expressed as [13] I ph ¼
q w d Dn me V L
ð1Þ
where q is the electron charge, w is the width of the metal electrode, d is the semiconducting film thickness, Δn is the photo-generated electron concentration, μe is the electron mobility, V is the bias voltage applied between the electrodes and L is the spacing between the electrodes. The high deposition temperature improves the crystal quality as shown in Fig. 1, weakening a scattering of electrons. Thus the electron mobility is enhanced by increasing deposition temperature. Furthermore, the resistivity of the network film decreases with increasing deposition temperature as shown in Fig. 2. It can also be attributed to the increase in the electron mobility. Therefore, it is considered that the electron mobility in the network films increases with increasing deposition
temperature. According to Eq. (1), the photocurrent of the network films increases with increasing deposition temperature. The network film shows the photo-response with a long rising process time, as shown in Fig. 3. According to the photo-response in Fig. 3, the rising process time constant can be estimated. As the network film is photo-illuminated, the relationship between photocurrent Iph and time t is expressed as [6,14] t I ph ¼ I s 1−exp − ð2Þ τr where Is is the saturation photocurrent and τr is the rising process time constant. According to Eq. (2), for all the network films, a variation of −ln(1−Iph/Is) with t is plotted and exhibits a good linear dependence having a correlation coefficient better than 0.991. The rising process time constant is calculated in terms of the slope of the fitted straight line. The rising process time constants are 730 s for the film grown at 300 K, 725 s for the film grown at 423 K and 735 s for the film grown at 623 K. The network films exhibit the slow photo-response with the long rising process time constant. The rising process time constant is almost independent of deposition temperature. It means that the same mechanism dominates the photocurrent for the network films grown at various deposition temperatures. The photosensitivity S ¼Iph/Id, where Id is the dark current [15]. Fig. 4 shows a variation of photosensitivity with photo-illuminated time for the ZnO: Al network films grown at various deposition temperatures. As shown in Fig. 4, the photosensitivity of the network films decreases with increasing deposition temperature. It is mainly attributed to a marked increase in the dark current with the deposition temperature although the photocurrent increases with the deposition temperature.
J. Liu et al. / Materials Letters 106 (2013) 218–221
221
Acknowledgment The financial support from the Fundamental Research Funds for the Central Universities 2012 is gratefully acknowledged.
References
Fig. 4. Variation of photosensitivity with photo-illuminated time for the ZnO: Al network films.
4. Summary 50 nm-thick ZnO: Al network films were sputter-deposited on nanochannel Al2O3 substrates at 300 K, 423 K and 623 K. Both the dark current and the photocurrent increase linearly with the applied voltage. The photocurrent of the network films increases with increasing deposition temperature. The network films show the slow photo-response. The rising process time constant is almost independent of deposition temperature. The photosensitivity of the network films decreases with increasing deposition temperature.
[1] Ozgur U, Alivov YI, Liu C, Teke A, Reshchikov MA, Dogan S, et al. J Appl Phys 2005;98:041301. [2] Comini E, Baratto C, Faglia G, Ferroni M, Vomiero A, Sberveglieri G. Prog Mater Sci 2009;54:1–67. [3] Kapilashrami M, Xu J, Biswas A, Tamaki T, Sharma P, Rao KV, et al. Mater Lett 2010;64:1291–4. [4] Xu ZQ, Deng H, Xie J, Li Y, Zu XT. Appl Surf Sci 2006;253:476–9. [5] Mamat MH, Sahdan MZ, Khusaimi Z, Ahmed AZ, Abdullah S, Rusop M. Opt Mater 2010;32:696–9. [6] Mamat MH, Khusaimi Z, Musa MZ, Malek MF, Rusop M. Sens Actuators A 2011;171:241–7. [7] Ganesh T, Rajesh S, Xavier FP. Mater Sci Semicond Process 2012. http://dx.doi. org/10.1016/j.mssp.2012.10.008. [8] Banerjee N, Krupanidhi SB. Nat Sci 2010;7:688–93. [9] Zuo YP, Qiu H, Chen XB, He JP. Mater Chem Phys 2012;133:507–14. [10] Liang S, Sheng H, Liu Y, Huo Z, Lu Y, Shen H. J Cryst Growth 2001;225:110–3. [11] Al-Hardan NH, Abdullah MJ, Ahmad H, Aziz AA, Low LY. Solid-State Electron 2011;55:59–63. [12] Liu MJ, Kim HK. Appl Phys Lett 2004;84:173–5. [13] Razeghi M, Rogalski A. J Appl Phys 1996;79:7433–73. [14] Shinde SS, Rajpure KY. Appl Surf Sci 2011;257:9595–9. [15] Fathy N, Ichimura M. Solar Energy Mater Solar Cells 2005;87:747–56.