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Rapid thermal-treated transparent electrode for photodiode applications
Materials Letters 115 (2014) 45–48
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Rapid thermal-treated transparent electrode for photodiode applications Hyunki Kim a, Seung-Hyouk Hong b, Yun Chang Park c, Jaekuen Lee a, Chil-Hwan Jeon a, Joondong Kim b,n a
Department of Electrical Engineering, Kunsan National University, Kunsan 573701, Republic of Korea Department of Electrical Engineering, Incheon National University, Incheon 406772, Republic of Korea c Measurement and Analysis Division, National Nanofab Center (NNFC), Daejeon 305806, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 August 2013 Accepted 2 October 2013 Available online 14 October 2013
We demonstrate a high performing and cost-effective photodiode with a low thermal budget. A quality heterojunction photoelectric device provided extremely high photo-responses. Transparent conductive indium-tin-oxide (ITO) film was coated on a Si substrate at a room temperature and then, rapid thermal treatment was done at 300 1C for 10 min. This heterojunction (ITO/Si) spontaneously provides a rectifying junction that shows high photo-response values of 1920%, 3240%, and 2800% at wavelengths of 350 nm, 600 nm, and 1100 nm, respectively. Thermal treatment affects the solid-state oxidation reaction of ITO and Si and control the formation of an interfacial layer. & 2013 Elsevier B.V. All rights reserved.
Keywords: Transparent electrode Indium-tin-oxide (ITO) Oxidation reaction Rectifying contact Photodiodes
1. Introduction Transparent conductors are mostly used as ohmic contact layers of front or back transparent electrodes for photoelectric devices. However, a transparent layer can be uniquely applied as a rectifying junction [1] to a semiconductor, which also enables a transparent conductor to work as an electrode [2]. This may be a promising approach for cost-effective photoelectric applications, including solar cells [3] and photodiodes [4,5]. Indium-tin-oxide (ITO) has been widely used as a transparent conductor due to its excellent electrical conductivity and a high optical transparency. However, a high processing temperature induces cost burden and limits the use of ITO in flexible electronics [6]. Thus, a crucial issue is the fabrication of a high quality ITO film with a low thermal budget. Herein, we present a high-response photodiode of an ITO filmembedding Si heterojunction device. Rapid thermal treatment was effective to form a quality rectifying junction. An interfacial layer, between the ITO film and the Si substrate, is substantially affected by thermal treatment, which treatments are crucial for the performances of photodiodes.
2. Experimental procedure ITO films were deposited on p-type Si and glass substrates by DC sputtering. A DC power source (3.70 W/cm2) was applied to a n
Corresponding author: Tel.: þ 82 32 835 8770; fax: þ82 32 835 0773. E-mail address: [email protected] (J. Kim).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.10.017
4-inch ITO target (In2O2 containing 10 wt% SnO2) at room temperature (RT). Rapid thermal annealing (RTA) was subsequently performed for 10 min under vacuum condition. ITO films on glass samples, processed in the same manner as their Si counterparts, were used to measure the optical properties of the ITO films with a UV spectrophotometer (Scinco, Neosys-2000). A transmission electron microcope (TEM, JEM-ARM200F, JEOL) was employed to investigate the cross-sectional structures. The depth profile and content of the elements were measured by secondary ion mass spectroscopy (SIMS, Cameca, magnetic sector ims7f). A quantum efficiency measuring system (McScience, K3100) was used to measure the quantum efficiencies of the temperature-dependent ITO film devices.
3. Results and discussion We prepared 200 nm-thick ITO films at room temperature, 300 1C and 600 1C (hereafter referred to as ITO-RT, ITO-300 1C, and ITO-600 1C). Fig. 1(a–f) are TEM images of interfaces between an ITO film and a Si substrate. Due to the identical deposition conditions, all ITO films have the same thickness on the Si substrate (Fig. 1a–c). For an as-deposited ITO film, TEM clearly shows mixed structures of crystalline and amorphous phases. The interplanar distance was measured and found to be 0.29 nm, which is consistent with a cubic (222) plane of ITO (Fig. 1d). Scale bar was calibrated for precision using a single crystalline Si (111) plane. After RTA process, an ITO film was well crystallized. Fig. 1(e) is for ITO300 1C, showing crystalline ITO film growth along the (222) plane
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H. Kim et al. / Materials Letters 115 (2014) 45–48
Fig. 1. (a–c) Low-magnitude TEM images. (d–f) High-magnitude TEM images. (a)–(d) are for ITO-RT, (b)–(e) are for ITO-300 1C, and (c)–(f) are for ITO-600 1C. (g–i) Depth profiles. (g) is for ITO-RT, (h) shows Si signals and (i) shows Sn signals of ITO-RT, ITO-300 1C, and ITO-600 1C.
with a lattice space of 0.29 nm. Fig. 1(f) is an image of ITO-600 1C, having a lattice space of 0.41 nm along the (211) plane. It is interesting to observe the increase of thickness of an interface layer as a function of the applied RTA temperature. ITO-RT shows an interface thickness of 1.3 nm, which is thicker than the usual 0.2–0.6 nm native oxide layer. This is attributed to the formation of a SiOx layer by the implantation of negatively charged oxygen ions into the Si during the ITO deposition [4]. After RTA processes, the interface grew to 1.4 nm for ITO-300 1C and to 1.6 nm for ITO-600 1C. To investigate the interface transition, SIMS depth profiles were performed. For the ITO-RT sample, uniform distributions of In, O, and Sn atoms were found through the 200 nm-thick ITO film, as shown in Fig. 1(g). At a depth of 200 nm, abruptly changed signals were detected. A Si signal appeared above the interface, as shown in Fig. 1 (h). Different from the ITO-RT case, noticeable Si transitions toward ITO were found from ITO-300 1C and ITO-600 1C samples. In contrast, Sn signals were propagated into the Si substrate for ITO-300 1C and ITO-600 1C samples, as shown in Fig. 1(i). Both transition intensity and penetration depth of Si and Sn tend to be enhanced according to an
increased RTA-temperatures. This clearly shows the solid-state oxidation reaction of Si to ITO [4]. Limited Si diffusion occurs through the SiOx and Si continuously finds oxygen at an interface, incurred by the decomposition of SnO2 [1], resulting in the growth of the SiOx layer. Fig. 2(a) is the electrical resistivity of ITO films. As-deposited ITO-RT film has a resistivity of 8.56 10 3 Ω cm. The high resistive value was caused by insufficient activation of the impurity dopant (Sn4 þ ) at RT. Sn4 þ atoms replace In3 þ atoms in the In2O3 lattice and one substitutional replacement donates an electron to increase the carrier concentration, reducing the resistivity [7]. A proportional reduction of resistivity was achieved as annealing temperature increases. RTA processed at 300 1C reduced the resistivity value by 4.97 10 4 Ω cm. A further reduction was achieved at 600 1C–2.92 10 4 Ω cm. In comparison, the resistivity of a similar thick-ITO film, grown with substrate heating at 600 1C during 30 min of sputtering, showed a value of 2.39 10 4 Ω cm. This clearly indicates that the RTA process is an effective method to reduce the thermal budget, while maintaining a comparable performance. Fig. 2(b) shows optical transmittance profiles. Excellent transparency was achieved from ITO-600 1C, showing an average
H. Kim et al. / Materials Letters 115 (2014) 45–48
47
Fig. 2. (a) Resistivity values and (b) optical transmittance of ITO-RT, ITO-300 1C, and ITO-600 1C, respectively. (c) Schematic of a heterojunction device (the inset is a photoimage). Dark I–V characteristics of (d) linear and (e) semi-log plot.
Fig. 3. (a) IQE profiles. Photo-responses at wavelengths of (b) 350 nm, (c) 600 nm, and (d) 1100 nm.
transmittance value of 89.2% in the range of 400–1100 nm. At a wavelength of 600 nm, the transmittance reached 94.5%, which is highly improved value compared to 57.5% and 45.32% from the ITO-300 1C and the ITO-RT samples, respectively. To characterize the heterojunction, an aluminum (Al) grid was patterned onto an ITO film. Al was also sputtered on a Si substrate as a back electrode. A schematic of the ITO film-embedding
heterojunction device is presented in Fig. 2(c). Each device was tailored to a size 1 1 cm2. The dark I–V profiles are presented in Fig. 2(d). RTA-treated devices (ITO-300 1C and ITO-600 1C) clearly established rectifying current flows, different from the ITO-RT case. More details can be found from a semi-log plot (Fig. 2e). As-deposited sample (ITO-RT) seems to be an ohmic contact. However, RTA-treated devices rectify the current. An ideality
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H. Kim et al. / Materials Letters 115 (2014) 45–48
factor of 1.59 was obtained from the ITO-300 1C device, which shows an excellent junction quality compared to that of the ITO600 1C device, which had a value of 2.31. Current transport between the ITO film and the Si substrate is significantly affected by the recombination process. The ideality factor tends to be decreased with increased thermal treatments at moderate temperature ranges; however, excess thermal stress may degrade the junction quality [8]. To investigate the carrier collection performance, we measured the internal quantum efficiencies (IQE) as shown in Fig. 3(a). The ITO-RT showed the low carrier collection performance. A distinctively enhanced QE values were obtained from the ITO-300 1C device, similar to the case discussed in a previous report [8]. The remarkably low carrier collection efficiency from the ITO-600 1C, which is attributed to the degraded heterojunction quality according to the growth of interfacial oxide, is a distinctive factor of this material [1]. We have investigated heterojunction devices as photodiodes. We have achieved wavelength-dependent current values at zero bias. The photo-response is defined as the ratio of the lightreactive current to the initial current. Three-different wavelengths were chosen to monitor the photo-responses of heterojunction devices at short (350 nm), middle (600 nm), and long wavelengths (1100 nm). At a wavelength of 350 nm, photo-responses were obtained and found to be 516% and 379% for the ITO-RT and ITO-600 1C devices, respectively. However, the ITO-300 1C device showed a relatively high photo-response value of 1920% as shown in Fig. 3(b). The wavelength of 600 nm is crucial to Si-based photoelectric applications [9,10]. At 600 nm, due to the longer propagation of the corresponding wavelength photons, all ITO devices showed enhanced photo-responses, as can be seen in Fig. 3(c). High photoresponse values of 3100% and 3240% were obtained from the ITORT and ITO-300 1C devices, respectively. However, the ITO-600 1C device still has a relatively low value of 730%. At a long wavelength of 1100 nm (Fig. 3d), a significant difference was observed for ITO-embedding heterojunction devices. Substantially degraded performances were obtained from both the ITO-RT and ITO600 1C devices, which had photo-response values of 610% and 103%, respectively. Meanwhile, the ITO-300 1C device showed a high photoresponse value of 2800%, indicating that a fair heterojunction formation is crucial to enhance the carrier collection efficiency and photoresponse at long wavelengths. At longer wavelengths, light can
penetrate more deeply into a Si light-absorber. The photogenerated electrons are minority carriers in p-type Si and have a long pathlength to be collected to a front electrode [8]. The recombination loss at an interface is a critical factor in attempts to collect photo-generated carriers for long wavelengths [1].
4. Conclusions We have investigated ITO film-embedding Si heterojunction photodiodes. These devices are part of a promising approach to high performance photoelectric devices for wavelength-dependent responses. RTA is effective at establishing a quality heterojunction formation between an ITO film and a Si substrate, at a reduced thermal budget. Rectifying junctions are highly controlled by thermal treatment conditions. As temperature increased, a thicker interfacial oxide was formed that degraded the photo-responses of ITO/Si heterojunction devices. We suggest that an effective ITO-embedding Si heterojunction device can be achieved by forming a rectifying junction or an ohmic contact in corresponding applications.
Acknowledgments The authors acknowledge the financial support of the Korea Institute of Energy Technology Evaluation and Planning (KETEP20113030010110) and the Industry, University and Research Institute Core Technology development and Industrialized Supporting Business through the Jeonbuk province. Hyunki Kim and SeungHyouk Hong contributed equally to this work. References [1] Goodnick SM, Wager JF, Wilmsen CW. J Appl Phys 1980;51:527–31. [2] Kim J, Kim M, Kim H, Song K, Lee E, Kim DW, et al. Appl Phys Lett 2012;101:143904. [3] Kim J, Yun JH, Park YC, Anderson WA. Mater Lett 2012;75:99–101. [4] Ow-Yang CW, Shigesato Y, Paine DC. J Appl Phys 2000;88:3717–24. [5] Yun JH, Kim J. Mater Lett 2012;70:4–6. [6] Kim MG, Kanatzidis MG, Facchetti A, Marks TJ. Nat Mater 2011;10:382–8. [7] Facchetti A, Marks TJ. Transparent electronics: from synthesis to applications. 1st ed.. Chichester: Wiley; 2010. [8] Subrahmanyam A, Balasubramanian N. Semicond Sci Technol 1992;7:324–7. [9] Kim H, Kim J, Lee E, Kim DW, Yun JH, Yi J. Appl Phys Lett 2013;102:193904. [10] Han SE, Chen G. Nano Lett 2010;10:1012–5.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Rapid thermal-treated transparent electrode for photodiode applications Hyunki Kim a, Seung-Hyouk Hong b, Yun Chang Park c, Jaekuen Lee a, Chil-Hwan Jeon a, Joondong Kim b,n a
Department of Electrical Engineering, Kunsan National University, Kunsan 573701, Republic of Korea Department of Electrical Engineering, Incheon National University, Incheon 406772, Republic of Korea c Measurement and Analysis Division, National Nanofab Center (NNFC), Daejeon 305806, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 August 2013 Accepted 2 October 2013 Available online 14 October 2013
We demonstrate a high performing and cost-effective photodiode with a low thermal budget. A quality heterojunction photoelectric device provided extremely high photo-responses. Transparent conductive indium-tin-oxide (ITO) film was coated on a Si substrate at a room temperature and then, rapid thermal treatment was done at 300 1C for 10 min. This heterojunction (ITO/Si) spontaneously provides a rectifying junction that shows high photo-response values of 1920%, 3240%, and 2800% at wavelengths of 350 nm, 600 nm, and 1100 nm, respectively. Thermal treatment affects the solid-state oxidation reaction of ITO and Si and control the formation of an interfacial layer. & 2013 Elsevier B.V. All rights reserved.
Keywords: Transparent electrode Indium-tin-oxide (ITO) Oxidation reaction Rectifying contact Photodiodes
1. Introduction Transparent conductors are mostly used as ohmic contact layers of front or back transparent electrodes for photoelectric devices. However, a transparent layer can be uniquely applied as a rectifying junction [1] to a semiconductor, which also enables a transparent conductor to work as an electrode [2]. This may be a promising approach for cost-effective photoelectric applications, including solar cells [3] and photodiodes [4,5]. Indium-tin-oxide (ITO) has been widely used as a transparent conductor due to its excellent electrical conductivity and a high optical transparency. However, a high processing temperature induces cost burden and limits the use of ITO in flexible electronics [6]. Thus, a crucial issue is the fabrication of a high quality ITO film with a low thermal budget. Herein, we present a high-response photodiode of an ITO filmembedding Si heterojunction device. Rapid thermal treatment was effective to form a quality rectifying junction. An interfacial layer, between the ITO film and the Si substrate, is substantially affected by thermal treatment, which treatments are crucial for the performances of photodiodes.
2. Experimental procedure ITO films were deposited on p-type Si and glass substrates by DC sputtering. A DC power source (3.70 W/cm2) was applied to a n
Corresponding author: Tel.: þ 82 32 835 8770; fax: þ82 32 835 0773. E-mail address: [email protected] (J. Kim).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.10.017
4-inch ITO target (In2O2 containing 10 wt% SnO2) at room temperature (RT). Rapid thermal annealing (RTA) was subsequently performed for 10 min under vacuum condition. ITO films on glass samples, processed in the same manner as their Si counterparts, were used to measure the optical properties of the ITO films with a UV spectrophotometer (Scinco, Neosys-2000). A transmission electron microcope (TEM, JEM-ARM200F, JEOL) was employed to investigate the cross-sectional structures. The depth profile and content of the elements were measured by secondary ion mass spectroscopy (SIMS, Cameca, magnetic sector ims7f). A quantum efficiency measuring system (McScience, K3100) was used to measure the quantum efficiencies of the temperature-dependent ITO film devices.
3. Results and discussion We prepared 200 nm-thick ITO films at room temperature, 300 1C and 600 1C (hereafter referred to as ITO-RT, ITO-300 1C, and ITO-600 1C). Fig. 1(a–f) are TEM images of interfaces between an ITO film and a Si substrate. Due to the identical deposition conditions, all ITO films have the same thickness on the Si substrate (Fig. 1a–c). For an as-deposited ITO film, TEM clearly shows mixed structures of crystalline and amorphous phases. The interplanar distance was measured and found to be 0.29 nm, which is consistent with a cubic (222) plane of ITO (Fig. 1d). Scale bar was calibrated for precision using a single crystalline Si (111) plane. After RTA process, an ITO film was well crystallized. Fig. 1(e) is for ITO300 1C, showing crystalline ITO film growth along the (222) plane
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Fig. 1. (a–c) Low-magnitude TEM images. (d–f) High-magnitude TEM images. (a)–(d) are for ITO-RT, (b)–(e) are for ITO-300 1C, and (c)–(f) are for ITO-600 1C. (g–i) Depth profiles. (g) is for ITO-RT, (h) shows Si signals and (i) shows Sn signals of ITO-RT, ITO-300 1C, and ITO-600 1C.
with a lattice space of 0.29 nm. Fig. 1(f) is an image of ITO-600 1C, having a lattice space of 0.41 nm along the (211) plane. It is interesting to observe the increase of thickness of an interface layer as a function of the applied RTA temperature. ITO-RT shows an interface thickness of 1.3 nm, which is thicker than the usual 0.2–0.6 nm native oxide layer. This is attributed to the formation of a SiOx layer by the implantation of negatively charged oxygen ions into the Si during the ITO deposition [4]. After RTA processes, the interface grew to 1.4 nm for ITO-300 1C and to 1.6 nm for ITO-600 1C. To investigate the interface transition, SIMS depth profiles were performed. For the ITO-RT sample, uniform distributions of In, O, and Sn atoms were found through the 200 nm-thick ITO film, as shown in Fig. 1(g). At a depth of 200 nm, abruptly changed signals were detected. A Si signal appeared above the interface, as shown in Fig. 1 (h). Different from the ITO-RT case, noticeable Si transitions toward ITO were found from ITO-300 1C and ITO-600 1C samples. In contrast, Sn signals were propagated into the Si substrate for ITO-300 1C and ITO-600 1C samples, as shown in Fig. 1(i). Both transition intensity and penetration depth of Si and Sn tend to be enhanced according to an
increased RTA-temperatures. This clearly shows the solid-state oxidation reaction of Si to ITO [4]. Limited Si diffusion occurs through the SiOx and Si continuously finds oxygen at an interface, incurred by the decomposition of SnO2 [1], resulting in the growth of the SiOx layer. Fig. 2(a) is the electrical resistivity of ITO films. As-deposited ITO-RT film has a resistivity of 8.56 10 3 Ω cm. The high resistive value was caused by insufficient activation of the impurity dopant (Sn4 þ ) at RT. Sn4 þ atoms replace In3 þ atoms in the In2O3 lattice and one substitutional replacement donates an electron to increase the carrier concentration, reducing the resistivity [7]. A proportional reduction of resistivity was achieved as annealing temperature increases. RTA processed at 300 1C reduced the resistivity value by 4.97 10 4 Ω cm. A further reduction was achieved at 600 1C–2.92 10 4 Ω cm. In comparison, the resistivity of a similar thick-ITO film, grown with substrate heating at 600 1C during 30 min of sputtering, showed a value of 2.39 10 4 Ω cm. This clearly indicates that the RTA process is an effective method to reduce the thermal budget, while maintaining a comparable performance. Fig. 2(b) shows optical transmittance profiles. Excellent transparency was achieved from ITO-600 1C, showing an average
H. Kim et al. / Materials Letters 115 (2014) 45–48
47
Fig. 2. (a) Resistivity values and (b) optical transmittance of ITO-RT, ITO-300 1C, and ITO-600 1C, respectively. (c) Schematic of a heterojunction device (the inset is a photoimage). Dark I–V characteristics of (d) linear and (e) semi-log plot.
Fig. 3. (a) IQE profiles. Photo-responses at wavelengths of (b) 350 nm, (c) 600 nm, and (d) 1100 nm.
transmittance value of 89.2% in the range of 400–1100 nm. At a wavelength of 600 nm, the transmittance reached 94.5%, which is highly improved value compared to 57.5% and 45.32% from the ITO-300 1C and the ITO-RT samples, respectively. To characterize the heterojunction, an aluminum (Al) grid was patterned onto an ITO film. Al was also sputtered on a Si substrate as a back electrode. A schematic of the ITO film-embedding
heterojunction device is presented in Fig. 2(c). Each device was tailored to a size 1 1 cm2. The dark I–V profiles are presented in Fig. 2(d). RTA-treated devices (ITO-300 1C and ITO-600 1C) clearly established rectifying current flows, different from the ITO-RT case. More details can be found from a semi-log plot (Fig. 2e). As-deposited sample (ITO-RT) seems to be an ohmic contact. However, RTA-treated devices rectify the current. An ideality
48
H. Kim et al. / Materials Letters 115 (2014) 45–48
factor of 1.59 was obtained from the ITO-300 1C device, which shows an excellent junction quality compared to that of the ITO600 1C device, which had a value of 2.31. Current transport between the ITO film and the Si substrate is significantly affected by the recombination process. The ideality factor tends to be decreased with increased thermal treatments at moderate temperature ranges; however, excess thermal stress may degrade the junction quality [8]. To investigate the carrier collection performance, we measured the internal quantum efficiencies (IQE) as shown in Fig. 3(a). The ITO-RT showed the low carrier collection performance. A distinctively enhanced QE values were obtained from the ITO-300 1C device, similar to the case discussed in a previous report [8]. The remarkably low carrier collection efficiency from the ITO-600 1C, which is attributed to the degraded heterojunction quality according to the growth of interfacial oxide, is a distinctive factor of this material [1]. We have investigated heterojunction devices as photodiodes. We have achieved wavelength-dependent current values at zero bias. The photo-response is defined as the ratio of the lightreactive current to the initial current. Three-different wavelengths were chosen to monitor the photo-responses of heterojunction devices at short (350 nm), middle (600 nm), and long wavelengths (1100 nm). At a wavelength of 350 nm, photo-responses were obtained and found to be 516% and 379% for the ITO-RT and ITO-600 1C devices, respectively. However, the ITO-300 1C device showed a relatively high photo-response value of 1920% as shown in Fig. 3(b). The wavelength of 600 nm is crucial to Si-based photoelectric applications [9,10]. At 600 nm, due to the longer propagation of the corresponding wavelength photons, all ITO devices showed enhanced photo-responses, as can be seen in Fig. 3(c). High photoresponse values of 3100% and 3240% were obtained from the ITORT and ITO-300 1C devices, respectively. However, the ITO-600 1C device still has a relatively low value of 730%. At a long wavelength of 1100 nm (Fig. 3d), a significant difference was observed for ITO-embedding heterojunction devices. Substantially degraded performances were obtained from both the ITO-RT and ITO600 1C devices, which had photo-response values of 610% and 103%, respectively. Meanwhile, the ITO-300 1C device showed a high photoresponse value of 2800%, indicating that a fair heterojunction formation is crucial to enhance the carrier collection efficiency and photoresponse at long wavelengths. At longer wavelengths, light can
penetrate more deeply into a Si light-absorber. The photogenerated electrons are minority carriers in p-type Si and have a long pathlength to be collected to a front electrode [8]. The recombination loss at an interface is a critical factor in attempts to collect photo-generated carriers for long wavelengths [1].
4. Conclusions We have investigated ITO film-embedding Si heterojunction photodiodes. These devices are part of a promising approach to high performance photoelectric devices for wavelength-dependent responses. RTA is effective at establishing a quality heterojunction formation between an ITO film and a Si substrate, at a reduced thermal budget. Rectifying junctions are highly controlled by thermal treatment conditions. As temperature increased, a thicker interfacial oxide was formed that degraded the photo-responses of ITO/Si heterojunction devices. We suggest that an effective ITO-embedding Si heterojunction device can be achieved by forming a rectifying junction or an ohmic contact in corresponding applications.
Acknowledgments The authors acknowledge the financial support of the Korea Institute of Energy Technology Evaluation and Planning (KETEP20113030010110) and the Industry, University and Research Institute Core Technology development and Industrialized Supporting Business through the Jeonbuk province. Hyunki Kim and SeungHyouk Hong contributed equally to this work. References [1] Goodnick SM, Wager JF, Wilmsen CW. J Appl Phys 1980;51:527–31. [2] Kim J, Kim M, Kim H, Song K, Lee E, Kim DW, et al. Appl Phys Lett 2012;101:143904. [3] Kim J, Yun JH, Park YC, Anderson WA. Mater Lett 2012;75:99–101. [4] Ow-Yang CW, Shigesato Y, Paine DC. J Appl Phys 2000;88:3717–24. [5] Yun JH, Kim J. Mater Lett 2012;70:4–6. [6] Kim MG, Kanatzidis MG, Facchetti A, Marks TJ. Nat Mater 2011;10:382–8. [7] Facchetti A, Marks TJ. Transparent electronics: from synthesis to applications. 1st ed.. Chichester: Wiley; 2010. [8] Subrahmanyam A, Balasubramanian N. Semicond Sci Technol 1992;7:324–7. [9] Kim H, Kim J, Lee E, Kim DW, Yun JH, Yi J. Appl Phys Lett 2013;102:193904. [10] Han SE, Chen G. Nano Lett 2010;10:1012–5.