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Selective growth of gallium nitride nanowires by femtosecond laser patterning
Journal of Alloys and Compounds 449 (2008) 250–252
Selective growth of gallium nitride nanowires by femtosecond laser patterning D.K.T. Ng a,b , M.H. Hong a,b,∗ , L.S. Tan a , Y. Zhou b,c , G.X. Chen a a
Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore b Data Storage Institute, Agency for Science, Technology and Research, DSI Building, 5 Engineering Drive 1, Singapore 117608, Singapore c Department of Mechanical Engineering, National University of Singapore, 2 Engineering Drive 3, Singapore 117576, Singapore Received 4 November 2005; received in revised form 27 December 2005; accepted 11 February 2006 Available online 29 December 2006
Abstract We report on gallium nitride (GaN) nanowires grown using pulsed laser ablation, adopting the vapor–liquid–solid (VLS) growth mechanism. The GaN nanowires are obtained based on the principle that a catalyst is required to initiate the nanowires growth. Locations of the GaN nanowires are patterned using femtosecond laser and focused ion beam. Scanning electron microscopy (SEM) is used to characterize the nanowires. This patterning of GaN nanowires will enable selective growth of nanowires and bottom-up assembly of integrated electronic and photonic devices. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanopatterning; Nanowire; Femtosecond laser
1. Introduction Microelectronics today has been driven by innovations in “top-down” manufacturing processes to shrink bulk semiconductor materials for improving processing power and reducing cost of manufacturing. However, this trend of miniaturization is facing a technical challenge due to fundamental physical and/or economic limitations. On the other hand, “bottom-up” approaches have the potential for fabrication of nanomaterials for electronic and photonic applications. Among such semiconducting nanomaterials, gallium nitride (GaN) nanostructures have attracted intense worldwide attention because of its potential for new visible and UV optoelectronic applications. Nanowires with low defect density are very important for preparation of nanoelectronic devices. Because the processing, dispersal and patterning techniques, such as ultrasonication and lithography, can introduce defects in nanowires, growing nanowires suitable for in situ fabrication of nanowire devices represents a better method. To fabricate nanowire devices in situ,
∗ Corresponding author at: Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore. Tel.: +65 6874 8707; fax: +65 6777 1349. E-mail address: HONG [email protected] (M.H. Hong).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.02.086
it is necessary to selectively grow the nanowires at desired locations. Therefore, patterning techniques need to be developed that are capable of precisely delivering nanowire catalysts to these desired locations. Recently, it was reported that electron-beam lithography [1] and atomic force microscope-based “dip-pen” nanolithography [2–5] can pattern catalysts on a substrate for the controlled synthesis of GaN nanowires. However, electron-beam lithography systems are generally expensive and highly complex machines requiring substantial maintenance. In addition, the electron beam must be scanned across patterned areas pixel by pixel, therefore requiring many hours to complete the exposure. As for atomic force microscope-based “dip-pen” nanolithography, the main disadvantage is that the writing species need to be replenished periodically, requiring the atomic force microscopy probe to be dismounted, which interrupts the writing process. On the other hand, patterning using femtosecond laser ablation is a faster and more cost-effective method in producing arrays of patterns over a large area. The choice of femtosecond laser over nanosecond pulse lasers in achieving smaller feature size is due to the higher peak power intensity and smaller heat-affected zone of femtosecond laser. The pulse width of femtosecond laser is also shorter than the electron–phonon-coupling time (∼10−12 s) which is sufficiently shorter compared with the thermal diffusion time. As a result, the energy of pulses can be efficiently and precisely deposited into a material without
D.K.T. Ng et al. / Journal of Alloys and Compounds 449 (2008) 250–252
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heat transformation [6,7]. Furthermore, the rapid development of femtosecond lasers over the last decade has opened up a wide range of new applications in materials science. In this study, patterned GaN nanowires are grown by pulsed laser ablation, adopting the vapor–liquid–solid (VLS) [8] growth mechanism. Since a catalyst is needed to initiate nanowire growth, in this report, patterns are made by femtosecond laser patterning on the layer of Au catalyst on the sapphire substrate before synthesis. In addition to obtaining nanopatterns, the formation of metallic nanoblisters in GaN nanowires under self-ion implantation is also demonstrated simultaneously with a Ga+ focused ion beam (FIB) used to ‘bombard’ the synthesized nanowires. This patterning of GaN nanowires will enable bottom-up assembly of integrated electronic and photonic devices. At the same time, the formation of nanoblisters in FIB patterning provides potential in application involving swelling induced embrittlement of materials in nuclear reactors [9]. 2. Experimental Sapphire substrates coated with gold (Au) film (thickness 10 nm) by an ebeam evaporator, were first patterned by a femtosecond laser (100 fs, 800 nm). They were then placed on a substrate holder in a pulsed laser deposition chamber. The target was first prepared using GaN powders (99.99+% purity; Aldrich) pressed into a solid tablet at a pressure of 10 MPa and loaded into the chamber. The patterned substrates were positioned 3.5 cm opposite to the GaN target. After evacuating the chamber to a base pressure of 2 × 10−6 Torr, the chamber was purged with 99.999% purified N2 at a flow rate of 60 sccm and maintained at a deposition pressure of 30 mTorr. The temperature of the substrates was kept at 700 ◦ C during the synthesis of the nanowires. The laser ablation process was performed using a KrF (248 nm, 30 ns) excimer laser (Lambda Physik Germany COMPEX 102) at a laser fluence of 5 J/cm2 focused on the rotating GaN target for 10 min. The pulse repetition rate was 10 Hz. The plasma plume produced was delivered directly to the substrate, and thus initiated the VLS mechanism [8]. The substrates were allowed to cool down to 100 ◦ C in an N2 ambient to prevent oxidation. The patterned GaN nanowires by femtosecond laser were examined with a field emission scanning electron microscopy (FESEM). Meanwhile, FIB was also used to pattern regions on the substrate with nanowires. The nanopatterns obtained by FIB were also observed under the FESEM.
3. Results and discussion Fig. 1a shows the FESEM image of the GaN nanowires synthesized by pulsed laser ablation on Au-coated sapphire substrate after the substrate had reached a temperature of 700 ◦ C. The Au film deposited on the substrate before the growth of nanowires was patterned by 800 nm femtosecond laser at a fluence of around 5.3 J/cm2 . The word ‘NANO’ is written on the substrate by selectively removal of the Au film, resulting in no catalyst to initiate growth of the nanowires in these areas. Thus, no nanowires are seen growing on these laser ablated surfaces. Fig. 1b shows higher magnification of this pattern by focusing on the alphabet ‘A’. The line width of each alphabet is measured to be around 3 m, indicating the region where Au catalyst is removed by femtosecond laser ablation. However, femtosecond laser ablation has the potential to obtain nanopatterns under the principle of near-field enhancement [10]. Fig. 1c shows a closeup FESEM image of the GaN nanowires grown on regions where the Au catalyst is not removed by femtosecond laser ablation. A high density of randomly grown smooth nanowires can be
Fig. 1. FESEM images: (a) the substrate with ‘NANO’ written by the femtosecond laser, (b) showing part of the letter ‘A’ patterned by the femtosecond laser with the nanowires grown in the unpatterned region and (c) showing high density of GaN nanowires grown on sapphire substrate (Inset shows the TEM image of a single GaN nanowire obtained from these grown GaN nanowires).
seen with lengths ranging from 300 to 500 nm and diameters of around 50 nm. Each nanowire is terminated with a nanoparticle at its end which is the Au catalyst that initiated the growth of the nanowires [11]. The inset further shows the TEM image of a single GaN nanowire. The darker tip is where the Au catalyst resides and this is the point of nucleation of the GaN nanowires follows the well-known VLS mechanism [8]. The VLS growth concerns the existence of a catalytic metal (Au) that defines the nucleation sites of the nanowires. When Au is heated up to a temperature of 700 ◦ C, pulsed laser ablation of the GaN target
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pattern. These nanowires around 200 nm outside the pattern that are not removed showed obvious enlargement compared to the nanowires that are more than 200 nm away from the pattern. This enlargement in the nearby nanowires is due to the formation of nanoblisters resulting from Ga+ implantation as a result of FIB ‘bombardment’ [9]. FIB has removed all the nanowires in the targeted region and serves as another technique for nanopatterning applications. 4. Conclusions We have synthesized patterned GaN nanowires using pulsed laser ablation. The growth of these nanowires follows the VLS mechanism [8]. Patterns are made using femtosecond laser ablation. In addition, FIB is also used to pattern the nanowires, achieving a line width of 100–150 nm. Nanoblisters are also formed in nanowires near the patterns caused by Ga+ implantation from the FIB. The patterned GaN nanowires are observed using the SEM. Though the use of FIB in patterning achieves a smaller dimension in this report, it is a much more costly technique compared to femtosecond laser ablation. Nevertheless, femtosecond laser ablation can still be used to obtain even smaller nanoscale feature size when combined with near-field effect. The ability to pattern nanowires in the nanoscale dimension will open up the possibility of controlled selective growth of nanowires and bottom-up assembly of integrated electronic and photonic devices. Fig. 2. (a) FESEM images of the word ‘NANO’ patterned by the FIB and (b) higher magnification FESEM image of the alphabet ‘A’ made by using the FIB.
delivers the plasma plume (containing Ga vapor) to the substrate. Au and Ga will then form eutectic liquid alloy droplets at the nanowires nucleation sites. Upon supersaturation of these liquid alloy droplets, subsequent influx of the plasma plume (containing Ga and N vapor in N2 environment) will dissolve in the liquid alloy droplets of Au and Ga to form GaN nanowires. Once pulsed laser ablation stops, the nanowires will solidify, terminating the growth process. FIB is used to draw small patterns on the substrate after the GaN nanowires were grown. This patterning method makes use of a finely focused beam of gallium (Ga+ ) ions which is rastered on the surface of the nanowires. As the ions hit the predetermined targeted surface, nanowires are selectively removed, resulting in patterning of the substrate covered with nanowires and the formation of nanoblisters in the nearby GaN nanowires that are not removed. Fig. 2a shows the word ‘NANO’ patterned by the FIB. The line width of each alphabet is around 100–150 nm. The pattern is clearly made and there are no stray nanowires within the pattern. Fig. 2b shows a close-up on the alphabet ‘A’ and nanowires are observed at regions outside the
Acknowledgement Ms. D. K. T. Ng is grateful to National University of Singapore for the financial support. References [1] S. Han, W. Jin, T. Tang, C. Li, D. Zhang, X. Liu, J. Han, C.W. Zhou, J. Mater. Res. 18 (2003) 245. [2] R.D. Piner, J. Zhu, F. Xu, S. Hong, C.A. Mirkin, Science 283 (1999) 661. [3] Y. Li, B.W. Maynor, J. Liu, J. Am. Chem. Soc. 123 (2001) 2105. [4] B.W. Maynor, Y. Li, J. Liu, Langmuir 17 (2001) 2575. [5] L.M. Demers, D.S. Ginger, S.J. Park, Z. Li, S.W. Chung, C.A. Mirkin, Science 296 (2002) 1836. [6] H. Varel, D. Ashkenasi, A. Rosenfeld, M. Wahmer, E.E.B. Campbell, Appl. Phys. A 65 (1997) 367. [7] B.N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, A. T¨unnermann, Appl. Phys. A 63 (1996) 109. [8] X. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188. [9] A. Datta, S. Dhara, S. Muto, C.W. Hsu, C.T. Wu, C.H. Shen, T. Tanabe, T. Maruyama, K.H. Chen, L.C. Chen, Y.L. Wang, Nanotechnology 16 (2005) 2764. [10] Y. Lin, M.H. Hong, W.J. Wang, Y.Z. Law, T.C. Chong, Appl. Phys. A 80 (2005) 461. [11] J. Liu, X.M. Meng, Y. Jiang, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 83 (2003) 4241.
Selective growth of gallium nitride nanowires by femtosecond laser patterning D.K.T. Ng a,b , M.H. Hong a,b,∗ , L.S. Tan a , Y. Zhou b,c , G.X. Chen a a
Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore b Data Storage Institute, Agency for Science, Technology and Research, DSI Building, 5 Engineering Drive 1, Singapore 117608, Singapore c Department of Mechanical Engineering, National University of Singapore, 2 Engineering Drive 3, Singapore 117576, Singapore Received 4 November 2005; received in revised form 27 December 2005; accepted 11 February 2006 Available online 29 December 2006
Abstract We report on gallium nitride (GaN) nanowires grown using pulsed laser ablation, adopting the vapor–liquid–solid (VLS) growth mechanism. The GaN nanowires are obtained based on the principle that a catalyst is required to initiate the nanowires growth. Locations of the GaN nanowires are patterned using femtosecond laser and focused ion beam. Scanning electron microscopy (SEM) is used to characterize the nanowires. This patterning of GaN nanowires will enable selective growth of nanowires and bottom-up assembly of integrated electronic and photonic devices. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanopatterning; Nanowire; Femtosecond laser
1. Introduction Microelectronics today has been driven by innovations in “top-down” manufacturing processes to shrink bulk semiconductor materials for improving processing power and reducing cost of manufacturing. However, this trend of miniaturization is facing a technical challenge due to fundamental physical and/or economic limitations. On the other hand, “bottom-up” approaches have the potential for fabrication of nanomaterials for electronic and photonic applications. Among such semiconducting nanomaterials, gallium nitride (GaN) nanostructures have attracted intense worldwide attention because of its potential for new visible and UV optoelectronic applications. Nanowires with low defect density are very important for preparation of nanoelectronic devices. Because the processing, dispersal and patterning techniques, such as ultrasonication and lithography, can introduce defects in nanowires, growing nanowires suitable for in situ fabrication of nanowire devices represents a better method. To fabricate nanowire devices in situ,
∗ Corresponding author at: Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore. Tel.: +65 6874 8707; fax: +65 6777 1349. E-mail address: HONG [email protected] (M.H. Hong).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.02.086
it is necessary to selectively grow the nanowires at desired locations. Therefore, patterning techniques need to be developed that are capable of precisely delivering nanowire catalysts to these desired locations. Recently, it was reported that electron-beam lithography [1] and atomic force microscope-based “dip-pen” nanolithography [2–5] can pattern catalysts on a substrate for the controlled synthesis of GaN nanowires. However, electron-beam lithography systems are generally expensive and highly complex machines requiring substantial maintenance. In addition, the electron beam must be scanned across patterned areas pixel by pixel, therefore requiring many hours to complete the exposure. As for atomic force microscope-based “dip-pen” nanolithography, the main disadvantage is that the writing species need to be replenished periodically, requiring the atomic force microscopy probe to be dismounted, which interrupts the writing process. On the other hand, patterning using femtosecond laser ablation is a faster and more cost-effective method in producing arrays of patterns over a large area. The choice of femtosecond laser over nanosecond pulse lasers in achieving smaller feature size is due to the higher peak power intensity and smaller heat-affected zone of femtosecond laser. The pulse width of femtosecond laser is also shorter than the electron–phonon-coupling time (∼10−12 s) which is sufficiently shorter compared with the thermal diffusion time. As a result, the energy of pulses can be efficiently and precisely deposited into a material without
D.K.T. Ng et al. / Journal of Alloys and Compounds 449 (2008) 250–252
251
heat transformation [6,7]. Furthermore, the rapid development of femtosecond lasers over the last decade has opened up a wide range of new applications in materials science. In this study, patterned GaN nanowires are grown by pulsed laser ablation, adopting the vapor–liquid–solid (VLS) [8] growth mechanism. Since a catalyst is needed to initiate nanowire growth, in this report, patterns are made by femtosecond laser patterning on the layer of Au catalyst on the sapphire substrate before synthesis. In addition to obtaining nanopatterns, the formation of metallic nanoblisters in GaN nanowires under self-ion implantation is also demonstrated simultaneously with a Ga+ focused ion beam (FIB) used to ‘bombard’ the synthesized nanowires. This patterning of GaN nanowires will enable bottom-up assembly of integrated electronic and photonic devices. At the same time, the formation of nanoblisters in FIB patterning provides potential in application involving swelling induced embrittlement of materials in nuclear reactors [9]. 2. Experimental Sapphire substrates coated with gold (Au) film (thickness 10 nm) by an ebeam evaporator, were first patterned by a femtosecond laser (100 fs, 800 nm). They were then placed on a substrate holder in a pulsed laser deposition chamber. The target was first prepared using GaN powders (99.99+% purity; Aldrich) pressed into a solid tablet at a pressure of 10 MPa and loaded into the chamber. The patterned substrates were positioned 3.5 cm opposite to the GaN target. After evacuating the chamber to a base pressure of 2 × 10−6 Torr, the chamber was purged with 99.999% purified N2 at a flow rate of 60 sccm and maintained at a deposition pressure of 30 mTorr. The temperature of the substrates was kept at 700 ◦ C during the synthesis of the nanowires. The laser ablation process was performed using a KrF (248 nm, 30 ns) excimer laser (Lambda Physik Germany COMPEX 102) at a laser fluence of 5 J/cm2 focused on the rotating GaN target for 10 min. The pulse repetition rate was 10 Hz. The plasma plume produced was delivered directly to the substrate, and thus initiated the VLS mechanism [8]. The substrates were allowed to cool down to 100 ◦ C in an N2 ambient to prevent oxidation. The patterned GaN nanowires by femtosecond laser were examined with a field emission scanning electron microscopy (FESEM). Meanwhile, FIB was also used to pattern regions on the substrate with nanowires. The nanopatterns obtained by FIB were also observed under the FESEM.
3. Results and discussion Fig. 1a shows the FESEM image of the GaN nanowires synthesized by pulsed laser ablation on Au-coated sapphire substrate after the substrate had reached a temperature of 700 ◦ C. The Au film deposited on the substrate before the growth of nanowires was patterned by 800 nm femtosecond laser at a fluence of around 5.3 J/cm2 . The word ‘NANO’ is written on the substrate by selectively removal of the Au film, resulting in no catalyst to initiate growth of the nanowires in these areas. Thus, no nanowires are seen growing on these laser ablated surfaces. Fig. 1b shows higher magnification of this pattern by focusing on the alphabet ‘A’. The line width of each alphabet is measured to be around 3 m, indicating the region where Au catalyst is removed by femtosecond laser ablation. However, femtosecond laser ablation has the potential to obtain nanopatterns under the principle of near-field enhancement [10]. Fig. 1c shows a closeup FESEM image of the GaN nanowires grown on regions where the Au catalyst is not removed by femtosecond laser ablation. A high density of randomly grown smooth nanowires can be
Fig. 1. FESEM images: (a) the substrate with ‘NANO’ written by the femtosecond laser, (b) showing part of the letter ‘A’ patterned by the femtosecond laser with the nanowires grown in the unpatterned region and (c) showing high density of GaN nanowires grown on sapphire substrate (Inset shows the TEM image of a single GaN nanowire obtained from these grown GaN nanowires).
seen with lengths ranging from 300 to 500 nm and diameters of around 50 nm. Each nanowire is terminated with a nanoparticle at its end which is the Au catalyst that initiated the growth of the nanowires [11]. The inset further shows the TEM image of a single GaN nanowire. The darker tip is where the Au catalyst resides and this is the point of nucleation of the GaN nanowires follows the well-known VLS mechanism [8]. The VLS growth concerns the existence of a catalytic metal (Au) that defines the nucleation sites of the nanowires. When Au is heated up to a temperature of 700 ◦ C, pulsed laser ablation of the GaN target
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pattern. These nanowires around 200 nm outside the pattern that are not removed showed obvious enlargement compared to the nanowires that are more than 200 nm away from the pattern. This enlargement in the nearby nanowires is due to the formation of nanoblisters resulting from Ga+ implantation as a result of FIB ‘bombardment’ [9]. FIB has removed all the nanowires in the targeted region and serves as another technique for nanopatterning applications. 4. Conclusions We have synthesized patterned GaN nanowires using pulsed laser ablation. The growth of these nanowires follows the VLS mechanism [8]. Patterns are made using femtosecond laser ablation. In addition, FIB is also used to pattern the nanowires, achieving a line width of 100–150 nm. Nanoblisters are also formed in nanowires near the patterns caused by Ga+ implantation from the FIB. The patterned GaN nanowires are observed using the SEM. Though the use of FIB in patterning achieves a smaller dimension in this report, it is a much more costly technique compared to femtosecond laser ablation. Nevertheless, femtosecond laser ablation can still be used to obtain even smaller nanoscale feature size when combined with near-field effect. The ability to pattern nanowires in the nanoscale dimension will open up the possibility of controlled selective growth of nanowires and bottom-up assembly of integrated electronic and photonic devices. Fig. 2. (a) FESEM images of the word ‘NANO’ patterned by the FIB and (b) higher magnification FESEM image of the alphabet ‘A’ made by using the FIB.
delivers the plasma plume (containing Ga vapor) to the substrate. Au and Ga will then form eutectic liquid alloy droplets at the nanowires nucleation sites. Upon supersaturation of these liquid alloy droplets, subsequent influx of the plasma plume (containing Ga and N vapor in N2 environment) will dissolve in the liquid alloy droplets of Au and Ga to form GaN nanowires. Once pulsed laser ablation stops, the nanowires will solidify, terminating the growth process. FIB is used to draw small patterns on the substrate after the GaN nanowires were grown. This patterning method makes use of a finely focused beam of gallium (Ga+ ) ions which is rastered on the surface of the nanowires. As the ions hit the predetermined targeted surface, nanowires are selectively removed, resulting in patterning of the substrate covered with nanowires and the formation of nanoblisters in the nearby GaN nanowires that are not removed. Fig. 2a shows the word ‘NANO’ patterned by the FIB. The line width of each alphabet is around 100–150 nm. The pattern is clearly made and there are no stray nanowires within the pattern. Fig. 2b shows a close-up on the alphabet ‘A’ and nanowires are observed at regions outside the
Acknowledgement Ms. D. K. T. Ng is grateful to National University of Singapore for the financial support. References [1] S. Han, W. Jin, T. Tang, C. Li, D. Zhang, X. Liu, J. Han, C.W. Zhou, J. Mater. Res. 18 (2003) 245. [2] R.D. Piner, J. Zhu, F. Xu, S. Hong, C.A. Mirkin, Science 283 (1999) 661. [3] Y. Li, B.W. Maynor, J. Liu, J. Am. Chem. Soc. 123 (2001) 2105. [4] B.W. Maynor, Y. Li, J. Liu, Langmuir 17 (2001) 2575. [5] L.M. Demers, D.S. Ginger, S.J. Park, Z. Li, S.W. Chung, C.A. Mirkin, Science 296 (2002) 1836. [6] H. Varel, D. Ashkenasi, A. Rosenfeld, M. Wahmer, E.E.B. Campbell, Appl. Phys. A 65 (1997) 367. [7] B.N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, A. T¨unnermann, Appl. Phys. A 63 (1996) 109. [8] X. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188. [9] A. Datta, S. Dhara, S. Muto, C.W. Hsu, C.T. Wu, C.H. Shen, T. Tanabe, T. Maruyama, K.H. Chen, L.C. Chen, Y.L. Wang, Nanotechnology 16 (2005) 2764. [10] Y. Lin, M.H. Hong, W.J. Wang, Y.Z. Law, T.C. Chong, Appl. Phys. A 80 (2005) 461. [11] J. Liu, X.M. Meng, Y. Jiang, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 83 (2003) 4241.