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Birefringence measurements of MnPc thin film by polarization microscopy
Applied Surface Science 254 (2008) 7947–7949
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Birefringence measurements of MnPc thin film by polarization microscopy T. Hashimoto a,*, T. Kaito a, S. Yanagiya b, A. Mori b, N. Goto b a b
Course in Optical Science and Technology, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan Institute of Technology and Science, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
We have studied optical properties of near-infrared (NIR) spectra and birefringence of the manganese phthalocyanine (MnPc) thin films. The morphology of the MnPc thin film grown on KCl (0 0 1) substrates was observed by using an atomic force microscope. The NIR spectral range of 1.0–1.7 mm was studied in this study, because that of 1.3–1.5 mm is known as an optical communication wavelength. The birefringence was measured with changing the growth condition of a deposition rate and a substrate temperature. The birefringence of the film was most affected by the deposition rate. Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved.
Available online 4 April 2008 Keywords: Manganese-phthalocyanine KCl Birefringence
1. Introduction Epitaxial thin films of organic materials have attracted considerable interests from electronic and optical properties. Studies of the metal-phthalocyanine (MPc) have been carried out actively for a long times. It is known that the metal-phthalocyanine grow epitaxially on a potassium chloride substrate [1–8]. Manganese-phthalocyanine (MnPc) is known as a material to show a saturable absorption in the wavelength region of 1.5 mm [9]. So, MnPc has potential to be used as optical functional devices such as an optical switching device and a noise cut filter. The optical properties of thin films are important for improvement of optical communication devices. MnPc molecule has molecular polarizability anisotropy and therefore shows a birefringence. MnPc crystal is a monoclinic system (biaxial crystal), which also shows birefringence. The birefringence has usually measured by using ellipsometry [10]. However, there is measurement difficulty when the sample is very thin. Therefore, in this study, we have measured the birefringence by a polarization microscopy of the MnPc crystals epitaxially grown on KCl (0 0 1) substrate. Optical properties of absorption characteristics have also been measured. Finally, we have discussed the relationship between the optical properties and MnPc growth conditions. 2. Experiments MnPc films were grown on KCl (0 0 1) substrates by a physical vapor deposition. MnPc powder produced by SIGMA–ARDRICH Co.
* Corresponding author. Tel.: +81 88 656 9418. E-mail addresses: [email protected] (T. Hashimoto), [email protected] (S. Yanagiya).
was used as the deposition source after once purification. The KCl substrates were prepared by cleavage through a (0 0 1) plane using a KCl single crystal grown by Czochralski (CZ) method. The MnPc powder was placed in a Knudsen cell of a molybdenum boat in the lower part of a vacuum chamber and the KCl substrate in the upper part of the chamber so that its surface was facing downward. The distance between the boat and the substrate was 150 mm. Before the vapor deposition, the substrate was thermally treated under the condition of 393 K for 2 h in order to avoid deliquescence. This thermal treatment has been reported to significantly affect the epitaxial growth [11]. The MnPc films were grown under conditions of a base pressure of about 104 Pa and the substrate temperature at 313 or 373 K. The source temperature was set to be 653–723 K, which resulted in deposition rates of 0.1, 1, and 5 1010 m/s. The six kinds of samples were consequently prepared. The MnPc film thickness was monitored using a quartz crystal microbalance. After vapor deposition, the MnPc films were picked up from the vacuum chamber. Their surface morphologies were observed using an atomic force microscope (AFM, NanoScope III) operated in air in tapping mode. Optical properties of the nearinfrared (NIR) spectra and the birefringence of the samples were also investigated as described below. The NIR spectra were measured in air using a spectrophotometer (G250, Nikon). The measured wavelength region was 1.0–1.7 mm. The birefringence was measured by determining the optical retardation by means of a traditional Senarmont method [12]. A polarizer for the microscope (OPTIPHOTO2-POL, Nikon) was adjusted for the transmission light to be brightest. The samples were set between the two polarizers and the transmitted light intensity was measured as a function wavelength using a spectrometer (USB4000, Ocean Optics). A quarter wavelength plate (Senarmont compensator) was set between the sample and the analyzer. This quarter wavelength plate converts elliptical
0169-4332/$ – see front matter . Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.171
7948
T. Hashimoto et al. / Applied Surface Science 254 (2008) 7947–7949
Fig. 1. AFM images of MnPc films on the KCl (0 0 1) substrate. The condition of the substrate temperature and the deposition rate were 313 K and 0.1 m/s (a), 373 K and 0.1 m/s (b), 373 K and 0.02 1010 m/s (c), and 373 K and 10.0 1010 m/s (d).
polarization into linear polarization. The plane of polarization of the converted linear polarization depends on a phase difference d. Therefore, the phase difference was determined from measured light intensity by rotating the analyzer. The birefringence dn is demanded by an expression.
dn ¼ d½rad
l 2pd
;
where l is an incident wavelength (546 nm) and d is the film thickness. 3. Results and discussion The effects of growth conditions on the morphology of MnPc thin films were firstly investigated. Fig. 1 shows AFM images of the MnPc film on a KCl (0 0 1) substrate. The substrate temperature was 313 K and 373 K in Fig. 1(a) and (b), respectively. It is found that the grain size of MnPc becomes larger on higher temperature substrates. At the higher substrate temperature, the MnPc molecules move more widely on the substrate, and their probability of encounters becomes higher. The grains are therefore considered to be larger. The grain size was also dependant on the deposition rate as shown in Fig. 1(c) and (d). Fig. 2 shows a near-infrared absorption spectrum of the MnPc film on KCl (0 0 1). The MnPc film on KCl (0 0 1) showed a wide absorption from 1.3 mm to 1.7 mm. It is apparent that there are two small peaks in the wide peak. The absorption peak of the sample prepared at 313 K and 373 K was 1.38 mm and 1.5 mm, respectively. It is interesting that the peak wavelength shifts with changing the growth condition. In order to determine the birefringence, the angular dependency of the light intensity was measured. A typical result is shown in Fig. 3. The peak-shift can be regarded as the change of the birefringence of the MnPc thin film. In order to quantify the birefringence, the plots are fitted by the approximation curve of I=A cos (2u d) + B. Calculated birefringence dn under all growth
Fig. 2. Near-infrared spectra of the MnPc thin films. Substrate temperature of the samples were 313 K and 373 K, respectively.
conditions are summarized in Table 1. It is found that dn is larger at the lowest deposition rate of 0.1 1010 m/s. On the other hand, an effect of the substrate temperature was negligibly small. Table 1 The summary of birefringence dn of the samples grown under various condition Samples
Deposition rate (1010 m/s)
Substrate temperature (K)
Film thickness (nm)
dn
A B C D E F
0.1 0.1 1 1 5 5
313 373 313 373 313 373
100 100 100 100 100 100
2.77 102 2.62 102 0.97 102 1.11 102 0.89 102 1.47 102
T. Hashimoto et al. / Applied Surface Science 254 (2008) 7947–7949
7949
In our AFM study, the grain size depends not only on the substrate temperature but also on the deposition rate. However, the birefringence measurements indicate that the dn is not dependent on the substrate temperature in spite of the deposition rate dependency. The reason is considered to be their essential difference in the role of crystallization. The substrate temperature affects thermodynamics of crystallization and 2D diffusivity on the substrate. On the other hand, the deposition rate affects the volume diffusion and, consequently, the available time period for the crystallization of the grain crystal. Under the condition of this study, the substrate temperature was considered to be effective in the size but ineffective in a crystal quality of the hetero-epitaxial film. The dn therefore was not changed. On the other hand, a high deposition rate causes a nucleation of non-epitaxial grain, which induces different index ellipsoids. And this would result in the deposition dependency.
4. Conclusions Fig. 3. Angle dependencies of the polarized light intensity. The peak of MnPc/KCl was shifted from the reference intensity of KCl and the birefringences were calculated from these peak-shift at various conditions.
In earlier studies, the Senarmont method was employed for an anisotropic substance such as a single crystal and a liquid crystal. If the grains of MnPc had been larger than the optical beam, the birefringence would have been measured as that of the single crystal. However, the diameter of the optical fiber (1.5 mm) is much larger than the grain size of MnPc (less than 3 mm). We therefore discuss the interpretation of the Senarmont method for hetero-epitaxial poly-crystallites films and the relationship between the birefringence and the MnPc surface morphology. If each MnPc grain has the same crystal structure, index ellipsoids are naturally the same. And if the molecular orientations on a substrate are the same, an elliptical section of refractive index could be the same shapes. On the other hand, even if perfect epitaxy, the orientations of the grains have some directions because of the symmetry of the substrate. The KCl, for instance, has 4-fold symmetry and the ellipses of MnPc have two directions whose angle is 908. The birefringence of dn is therefore determined by the sum of the refractive index of each grain in the beam area.
We have investigated birefringence of MnPc thin film grown on a KCl substrate by vapor deposition, and investigated the absorption characteristic in near infrared. In this study, useful absorption in optical communication appeared in 1.5 mm region when the substrate temperature was increased, and we found that dn has a large value when a high quality epitaxial film was provided at a low deposition rate. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
M. Ashida, Bull. Chem. Soc. Jpn. 39 (1966) 2625. M. Ashida, Bull. Chem. Soc. Jpn. 39 (1966) 2632. T. Morioka, H. Tada, A. Koma, J. Appl. Phys. 73 (1993) 5. H. Tada, K. Saiki, A. Koma, Jpn. J. Appl. Phys. 30 (1991) L306. H. Hoshi, A.J. Dann, Y. Maruyama, J. Appl. Phys. 67 (1990) 6871. H. Hoshi, Y. Maruyama, H. Masuda, T. Inabe, J. Appl. Phys. 68 (1990) 1396. S. Yanagiya, S. Nishikata, G. Sazaki, A. Hoshino, K. Nakajima, T. Inoue, J. Cryst. Growth 254 (2003) 244. S. Yanagiya, H. Wakamatsu, O. Nishikata, T. Inoue, Jpn. J. Appl. Phys. 43 (2004) 11A. P. Day, G. Scregg, R.J.P. Williams, Nature 197 (1963) 589. F.C. Peiris, B.D. Hatton, G.A. Ozin, Appl. Phys. Lett. 87 (2005) 241902. A. Andreev, T. Haber, D.-M. Smilgies, R. Resel, H. Sitter, N.S. Sariciftci, L. Valek, J. Cryst. Growth 275 (2005) e2037. K. Iizuka, Elements of Photonics, vol. 1, Wiley-interscience, 2002 Chapter 6.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Birefringence measurements of MnPc thin film by polarization microscopy T. Hashimoto a,*, T. Kaito a, S. Yanagiya b, A. Mori b, N. Goto b a b
Course in Optical Science and Technology, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan Institute of Technology and Science, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
We have studied optical properties of near-infrared (NIR) spectra and birefringence of the manganese phthalocyanine (MnPc) thin films. The morphology of the MnPc thin film grown on KCl (0 0 1) substrates was observed by using an atomic force microscope. The NIR spectral range of 1.0–1.7 mm was studied in this study, because that of 1.3–1.5 mm is known as an optical communication wavelength. The birefringence was measured with changing the growth condition of a deposition rate and a substrate temperature. The birefringence of the film was most affected by the deposition rate. Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved.
Available online 4 April 2008 Keywords: Manganese-phthalocyanine KCl Birefringence
1. Introduction Epitaxial thin films of organic materials have attracted considerable interests from electronic and optical properties. Studies of the metal-phthalocyanine (MPc) have been carried out actively for a long times. It is known that the metal-phthalocyanine grow epitaxially on a potassium chloride substrate [1–8]. Manganese-phthalocyanine (MnPc) is known as a material to show a saturable absorption in the wavelength region of 1.5 mm [9]. So, MnPc has potential to be used as optical functional devices such as an optical switching device and a noise cut filter. The optical properties of thin films are important for improvement of optical communication devices. MnPc molecule has molecular polarizability anisotropy and therefore shows a birefringence. MnPc crystal is a monoclinic system (biaxial crystal), which also shows birefringence. The birefringence has usually measured by using ellipsometry [10]. However, there is measurement difficulty when the sample is very thin. Therefore, in this study, we have measured the birefringence by a polarization microscopy of the MnPc crystals epitaxially grown on KCl (0 0 1) substrate. Optical properties of absorption characteristics have also been measured. Finally, we have discussed the relationship between the optical properties and MnPc growth conditions. 2. Experiments MnPc films were grown on KCl (0 0 1) substrates by a physical vapor deposition. MnPc powder produced by SIGMA–ARDRICH Co.
* Corresponding author. Tel.: +81 88 656 9418. E-mail addresses: [email protected] (T. Hashimoto), [email protected] (S. Yanagiya).
was used as the deposition source after once purification. The KCl substrates were prepared by cleavage through a (0 0 1) plane using a KCl single crystal grown by Czochralski (CZ) method. The MnPc powder was placed in a Knudsen cell of a molybdenum boat in the lower part of a vacuum chamber and the KCl substrate in the upper part of the chamber so that its surface was facing downward. The distance between the boat and the substrate was 150 mm. Before the vapor deposition, the substrate was thermally treated under the condition of 393 K for 2 h in order to avoid deliquescence. This thermal treatment has been reported to significantly affect the epitaxial growth [11]. The MnPc films were grown under conditions of a base pressure of about 104 Pa and the substrate temperature at 313 or 373 K. The source temperature was set to be 653–723 K, which resulted in deposition rates of 0.1, 1, and 5 1010 m/s. The six kinds of samples were consequently prepared. The MnPc film thickness was monitored using a quartz crystal microbalance. After vapor deposition, the MnPc films were picked up from the vacuum chamber. Their surface morphologies were observed using an atomic force microscope (AFM, NanoScope III) operated in air in tapping mode. Optical properties of the nearinfrared (NIR) spectra and the birefringence of the samples were also investigated as described below. The NIR spectra were measured in air using a spectrophotometer (G250, Nikon). The measured wavelength region was 1.0–1.7 mm. The birefringence was measured by determining the optical retardation by means of a traditional Senarmont method [12]. A polarizer for the microscope (OPTIPHOTO2-POL, Nikon) was adjusted for the transmission light to be brightest. The samples were set between the two polarizers and the transmitted light intensity was measured as a function wavelength using a spectrometer (USB4000, Ocean Optics). A quarter wavelength plate (Senarmont compensator) was set between the sample and the analyzer. This quarter wavelength plate converts elliptical
0169-4332/$ – see front matter . Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.171
7948
T. Hashimoto et al. / Applied Surface Science 254 (2008) 7947–7949
Fig. 1. AFM images of MnPc films on the KCl (0 0 1) substrate. The condition of the substrate temperature and the deposition rate were 313 K and 0.1 m/s (a), 373 K and 0.1 m/s (b), 373 K and 0.02 1010 m/s (c), and 373 K and 10.0 1010 m/s (d).
polarization into linear polarization. The plane of polarization of the converted linear polarization depends on a phase difference d. Therefore, the phase difference was determined from measured light intensity by rotating the analyzer. The birefringence dn is demanded by an expression.
dn ¼ d½rad
l 2pd
;
where l is an incident wavelength (546 nm) and d is the film thickness. 3. Results and discussion The effects of growth conditions on the morphology of MnPc thin films were firstly investigated. Fig. 1 shows AFM images of the MnPc film on a KCl (0 0 1) substrate. The substrate temperature was 313 K and 373 K in Fig. 1(a) and (b), respectively. It is found that the grain size of MnPc becomes larger on higher temperature substrates. At the higher substrate temperature, the MnPc molecules move more widely on the substrate, and their probability of encounters becomes higher. The grains are therefore considered to be larger. The grain size was also dependant on the deposition rate as shown in Fig. 1(c) and (d). Fig. 2 shows a near-infrared absorption spectrum of the MnPc film on KCl (0 0 1). The MnPc film on KCl (0 0 1) showed a wide absorption from 1.3 mm to 1.7 mm. It is apparent that there are two small peaks in the wide peak. The absorption peak of the sample prepared at 313 K and 373 K was 1.38 mm and 1.5 mm, respectively. It is interesting that the peak wavelength shifts with changing the growth condition. In order to determine the birefringence, the angular dependency of the light intensity was measured. A typical result is shown in Fig. 3. The peak-shift can be regarded as the change of the birefringence of the MnPc thin film. In order to quantify the birefringence, the plots are fitted by the approximation curve of I=A cos (2u d) + B. Calculated birefringence dn under all growth
Fig. 2. Near-infrared spectra of the MnPc thin films. Substrate temperature of the samples were 313 K and 373 K, respectively.
conditions are summarized in Table 1. It is found that dn is larger at the lowest deposition rate of 0.1 1010 m/s. On the other hand, an effect of the substrate temperature was negligibly small. Table 1 The summary of birefringence dn of the samples grown under various condition Samples
Deposition rate (1010 m/s)
Substrate temperature (K)
Film thickness (nm)
dn
A B C D E F
0.1 0.1 1 1 5 5
313 373 313 373 313 373
100 100 100 100 100 100
2.77 102 2.62 102 0.97 102 1.11 102 0.89 102 1.47 102
T. Hashimoto et al. / Applied Surface Science 254 (2008) 7947–7949
7949
In our AFM study, the grain size depends not only on the substrate temperature but also on the deposition rate. However, the birefringence measurements indicate that the dn is not dependent on the substrate temperature in spite of the deposition rate dependency. The reason is considered to be their essential difference in the role of crystallization. The substrate temperature affects thermodynamics of crystallization and 2D diffusivity on the substrate. On the other hand, the deposition rate affects the volume diffusion and, consequently, the available time period for the crystallization of the grain crystal. Under the condition of this study, the substrate temperature was considered to be effective in the size but ineffective in a crystal quality of the hetero-epitaxial film. The dn therefore was not changed. On the other hand, a high deposition rate causes a nucleation of non-epitaxial grain, which induces different index ellipsoids. And this would result in the deposition dependency.
4. Conclusions Fig. 3. Angle dependencies of the polarized light intensity. The peak of MnPc/KCl was shifted from the reference intensity of KCl and the birefringences were calculated from these peak-shift at various conditions.
In earlier studies, the Senarmont method was employed for an anisotropic substance such as a single crystal and a liquid crystal. If the grains of MnPc had been larger than the optical beam, the birefringence would have been measured as that of the single crystal. However, the diameter of the optical fiber (1.5 mm) is much larger than the grain size of MnPc (less than 3 mm). We therefore discuss the interpretation of the Senarmont method for hetero-epitaxial poly-crystallites films and the relationship between the birefringence and the MnPc surface morphology. If each MnPc grain has the same crystal structure, index ellipsoids are naturally the same. And if the molecular orientations on a substrate are the same, an elliptical section of refractive index could be the same shapes. On the other hand, even if perfect epitaxy, the orientations of the grains have some directions because of the symmetry of the substrate. The KCl, for instance, has 4-fold symmetry and the ellipses of MnPc have two directions whose angle is 908. The birefringence of dn is therefore determined by the sum of the refractive index of each grain in the beam area.
We have investigated birefringence of MnPc thin film grown on a KCl substrate by vapor deposition, and investigated the absorption characteristic in near infrared. In this study, useful absorption in optical communication appeared in 1.5 mm region when the substrate temperature was increased, and we found that dn has a large value when a high quality epitaxial film was provided at a low deposition rate. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
M. Ashida, Bull. Chem. Soc. Jpn. 39 (1966) 2625. M. Ashida, Bull. Chem. Soc. Jpn. 39 (1966) 2632. T. Morioka, H. Tada, A. Koma, J. Appl. Phys. 73 (1993) 5. H. Tada, K. Saiki, A. Koma, Jpn. J. Appl. Phys. 30 (1991) L306. H. Hoshi, A.J. Dann, Y. Maruyama, J. Appl. Phys. 67 (1990) 6871. H. Hoshi, Y. Maruyama, H. Masuda, T. Inabe, J. Appl. Phys. 68 (1990) 1396. S. Yanagiya, S. Nishikata, G. Sazaki, A. Hoshino, K. Nakajima, T. Inoue, J. Cryst. Growth 254 (2003) 244. S. Yanagiya, H. Wakamatsu, O. Nishikata, T. Inoue, Jpn. J. Appl. Phys. 43 (2004) 11A. P. Day, G. Scregg, R.J.P. Williams, Nature 197 (1963) 589. F.C. Peiris, B.D. Hatton, G.A. Ozin, Appl. Phys. Lett. 87 (2005) 241902. A. Andreev, T. Haber, D.-M. Smilgies, R. Resel, H. Sitter, N.S. Sariciftci, L. Valek, J. Cryst. Growth 275 (2005) e2037. K. Iizuka, Elements of Photonics, vol. 1, Wiley-interscience, 2002 Chapter 6.