Superlateral Growth of Silicon by Artificially Designed Spatial Intensity Laser Beam Profile

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1 J /2009/156 7 /J192/7/$25.00 The Electrochemical Society Superlateral Growth of Silicon by Artificially Designed Spatial Intensity Laser Beam Profile Eok Su Kim and Ki-Bum Kim*,z Department of Materials Science and Engineering, Seoul National University, Seoul, , Korea A location-controlled superlateral grain growth of polycrystalline silicon was achieved by double irradiation with the excimer laser using an artificially designed spatial intensity beam profile with periodic maximum and minimum intensities. With the energy density corresponding to the partial melting regime at the location of minimum intensity, the first irradiation formed the large polycrystalline silicon grains at controlled locations through explosive crystallization and lateral growth. The second irradiation was performed using the same spatial intensity profile and energy density as the first irradiation but was shifted spatially from the first irradiation. Superlateral growth occurred from the unmolten crystalline seeds formed from a single grain at the location of minimum intensity, which had been formed by the first irradiation. The array of polycrystalline grains with a size of one period of minimum intensity was obtained uniformly over the entire irradiated area. The double-irradiation process confirmed that superlateral growth occurred uniformly with a wide range of energy densities from 0.80 to 1.00 J/cm 2, whereas superlateral growth occurred randomly at a singular energy density of 1.05 J/cm 2 by the single irradiation The Electrochemical Society. DOI: / All rights reserved. Manuscript submitted January 18, 2009; revised manuscript received April 9, Published May 13, Polycrystalline silicon poly-si films have been investigated extensively and are widely utilized as active channel materials for thin-film transistors TFTs in active matrix liquid crystal displays and active matrix organic light-emitting diodes on account of their higher carrier mobility and stability against a threshold voltage shift than amorphous silicon a-si. As most of these applications are typically manufactured on transparent glass or flexible substrates, it is important to develop a low temperature process for the formation of poly-si to prevent damage to the substrates but still obtain high carrier mobility and low leakage current device properties. In this respect, excimer laser crystallization ELC of as-deposited a-si films has been proposed as a promising way of fabricating high quality poly-si films. 1,2 Various methods of ELC have been suggested to increase the poly-si grain size and to control their location using lateral growth or superlateral growth SLG with the major aim of improving the carrier mobility and device-to-device uniformity The evolution of the poly-si microstructure during the ELC of an a-si film has been well investigated, particularly the SLG phenomenon. 20,21 It is known that the SLG phenomenon occurs in the near-complete melting regime where a discrete Si island remains without complete melting and serves as a single-crystalline seed during a subsequent solidification. In previous work, an ELC method was proposed based on the modulation of the spatial intensity beam profile of the excimer laser with periodic maximum I max and minimum I min intensities using a specially designed mask structure. 22 Using a single irradiation of this spatial beam intensity modulation on a-si films, the melt depth can be varied spatially, which can result in the evolution of a range of microstructures. In particular, it was reported that SLG grains as large as 7.0 m can be obtained in the near-complete melting regime. However, in practice, the SLG phenomenon does not occur uniformly over the entire irradiated area due to the nonuniform distribution of crystalline seeds and fluctuations in the incident energy density of the excimer laser beam. This paper reports that this problem can be overcome by using double irradiation. Here, a second irradiation was performed on the microstructure formed by the first irradiation. In this case, the second irradiation was spatially shifted to a distance less than the half period of I min with the same spatial intensity beam profile and energy density as the first irradiation. The results show that double irradiation not only increases the grain size but also improves the crystalline quality of the resulting poly-si film. In particular, the process window is much wider because a partial melting regime is used instead of a near-complete melting regime. Experimental As shown in Fig. 1, the spatial intensity beam profile of a periodic variation of the I max and I min intensities was formed by designing and fabricating an opaque mask pattern with a size less than the optical resolution of the projection lens, which was calculated by optics simulations. The wavelength of the XeCl excimer laser beam was 308 nm, and the pulse duration time had a full width at half-maximum of 240 ns. The substrate was prepared using the following process. A 300 nm thick silicon oxide SiO 2 buffer layer was deposited on a glass substrate, and then a 50 nm thick hydrogenated a-si a-si:h film was deposited by plasma-enhanced chemical vapor deposition. The as-deposited a-si:h film was dehydrogenated by furnace annealing at 500 C for 2 h to prevent the rapid evolution of hydrogen during laser beam irradiation. The dehydrogenated a-si film was first irradiated with the spatial intensity profiles of I max and I min. A second irradiation was then performed by * Electrochemical Society Active Member. z kibum@snu.ac.kr Figure 1. Schematic of the ELC method with an artificially designed spatial intensity profile beam of periodic I max and I min.

2 J193 spatially shifting to a distance less than the half period of I min or I max but with the same spatial intensity profile and energy density as the first irradiation. To examine the microstructure of poly-si in more detail, Secco-etched scanning electron microscopy SEM images were obtained and the Raman spectra were obtained by a Jobin-Yvon LabRam HR with a liquid-nitrogen-cooled chargecoupled device multichannel detector at room temperature in conventional backscattering geometry. The spectra were excited with the nm line of an Ar-ion laser. Results and Discussion Optics simulation. To generate a spatial intensity beam profile, mask patterns first need to be designed. When the mask pattern is composed of transparent and opaque regions, the aerial image and spatial intensity beam profile are calculated using Fourier series analysis and scalar diffraction theory as follows The electric field of the diffraction pattern M x,y on the entrance to the objective lens is given by the Fraunhofer diffraction integral for projection lithography M x,y = m x,y exp 2 i f x x + f y y dxdy 1 where f x = x / z and f y = y / z are the spatial frequencies of the diffraction pattern here, z is the distance from the mask to the objective lens, and m x,y is the electric-field transmittance of a mask pattern with a value of 1 under a transparent pattern and 0 under an opaque pattern. The diffraction pattern is just the Fourier transform of the mask pattern 26 M f x, f y = F m x,y 2 The electric field at the image plane can be obtained from the inverse Fourier transform of the diffraction pattern, which is collected only by the objective lens of a finite numerical aperture NA E x,y = F 1 M f x, f y P f x, f y 3 where P is a pupil aperture function of the objective lens, which is 1 inside the aperture for fx 2 + f y 2 NA/ and 0 outside for fx 2 + f y 2 NA/. However, because the illumination of the mask is really composed of light coming in from a range of angles rather than just a single one coherent, i.e., partially coherent, Eq. 3 becomes E x,y, f x, f y = F 1 M f x f x, f y f y P f x, f y 4 where f x and f y are the shift in the spatial frequency due to the tilted illumination. The aerial image is defined as the intensity distribution at the substrate and is simply the square of the magnitude of an electric field I x,y = E x,y 2 5 The full aerial image can be determined by calculating the coherent aerial image from each point on the source by Eq. 4 and then by integrating the intensity with respect to the source. Based on this, the aerial image and spatial intensity profiles with various opaque and transparent pattern sizes can be calculated in a variety of arrays, i.e., line and space, square, and hexagon arrays. The wavelength of the excimer laser beam, the NA, and the partial coherence used for the optics simulation were 308 nm, 0.12, and 0.7, respectively. Figure 2 shows the mask concept of each array, where the dotted line denotes the predicted boundaries between the poly-si grains. Screened by the mask composed of periodic opaque and transparent pattern arrays, the excimer laser beam is diffracted by these mask patterns. However, the projection lens with NA of 0.12 cannot collect enough of the diffracted beams when the opaque pattern size is less than the optical resolution of the projection lens. Therefore, considerable beam intensity appears under the opaque pattern with I min, whereas I max appears under the transparent pattern. Figure 3 shows the spatial intensity profile of the Figure 2. Mask design concept and predicted poly-si grains of a line and space array, b square array, and c hexagon array. line and space array with an opaque pattern size of 0.6 m and that of square and hexagon arrays with an opaque pattern size of m. In all cases, I min increased with decreasing opaque pattern size, gradually approaching I max, whereas I max was relatively

3 J194 Figure 4. The comparison of modulation values with the various a opaque pattern sizes and b transparent pattern sizes among the different arrays. M = I max I min 6 I max + I min Figure 4 shows the modulation values with various opaque and transparent pattern sizes. Similar modulation values were obtained at a larger opaque pattern size in the square and hexagon arrays compared to those in the line and space array due to the twodimensional geometric effect. Similar modulation values were obtained in the two-dimensional geometry of the square and hexagon at the same opaque pattern size, as shown in Fig. 4a. The transparent pattern size over the resolution and the array structure do not affect the modulation value, as shown in Fig. 4b. The aerial image and spatial intensity profiles of the line and space array were adopted to experimentally irradiate on Si film in this study, but the experimental results adopting those of the square and hexagon arrays will be reported elsewhere. Figure 3. Spatial intensity profile of a the line and space array with an opaque pattern size of 0.6 m, b the square array with an opaque pattern size of m, and c the hexagon array with an opaque pattern size of m. unaffected and showed a uniform intensity. The quantitative relationship between I max and I min can be expressed as the modulation function Microstructural evolution on single irradiation. Figure 5 shows a series of Secco-etched SEM images of the poly-si microstructure evolved after the single irradiation at different energy densities at a modulation value of 0.50, which corresponds to a transparent pattern size of 5.0 m and an opaque pattern size of 0.6 m in the line and space array, as shown in Fig. 3a. The mechanism of the microstructure evolution was reported in our previous work. 22 Overall, the evolution of the microstructure with the energy density can be classified into the following three regimes based on the melt depth of a-si at the location of I min : i in the partial melting regime 1.00 J/cm 2, explosive crystallization 27,28 occurs vertically at the location of I min and the grains grow laterally Fig. 5a ; ii in the near-complete melting regime =1.05 J/cm 2, the grains grow bilaterally by SLG at the location of I min Fig. 5b ; and iii in the

4 J195 Figure 6. Color online Schematic of the simulated three-dimensional system. Figure 5. Top-view Secco-etched SEM images of the microstructure after the first irradiation at a modulation value of 0.50 with energy densities of a 1.00, b 1.05, c 1.10, and d 1.20 J/cm 2. complete melting regime 1.10 J/cm 2, nucleation occurs at the location of I min and the grains grow laterally Fig. 5c. As the energy density is further increased, secondary nucleation occurs at the location of I max. Therefore, the lateral growth of the first nucleated grains is impinged by the secondary nucleated grains Fig. 5d. The temperature profile calculated from the heat flow simulation also supported the mechanism of microstructural evolution, including secondary nucleation. The morphological evolution was investigated graphically with the heat flow simulation developed by Columbia University. 29,30 The numerical model calculates the three-dimensional heat conduction equation by incorporating the effects of heat flow, interface motion, and random nucleation c p T t = x k T x + y k T y + z k T z + S laser + S Hm 7 where is the density, c p is the specific heat, k is the thermal conductivity of Si, T is the absolute temperature, S laser is the source term for the absorbed laser energy, and S Hm is the source term for the latent heat released or absorbed during the phase change. Figure 6 shows the three-dimensional system simulated in this study. The stack of materials was a 50 nm thick a-si and a 300 nm thick SiO 2 layer on a 10.0 m thick glass substrate in the y direction. The dimension of the z direction was only 0.5 m to reduce the calculation time, which is reasonable due to the symmetry in the z direction. The dimension of the x direction was 45.0 m corresponding to 8 times the period of I min. The spatial intensity profile for the modulation of 0.50 corresponds to the transparent and opaque patterns of 5.0 and 0.6 m, respectively, in the line and space array, as shown in Fig. 3a. Figure 7 shows the simulated microstructures as a function of the energy density in the x-z plane, where the different colors denote the different crystallographic orientations of the poly-si grain. At energy densities of 0.90 and 1.00 J/cm 2, as shown in Fig. 7a and b, respectively, the molten region only solidifies without nucleation. Figure 8 shows a cross-section view of solidification in the x-y plane with time at an energy density of 1.00 J/cm 2.Upto these energy densities, the total energy, including the energy density of I min and the energy flowing from the location of I max, is still lower than the critical energy for the complete melting of the a-si film at I min. Therefore, the a-si film at I min is partially melted, and solidification occurs laterally from the unmolten solid portion at I min to the completely molten region of I max. As mentioned previously, the evolution of the microstructure up to this energy density can be explained as belonging to the partial melting regime. The nearcomplete melting regime is considered to be the utmost limits of the partial melting regime in this simulation. When the energy density was increased to 1.10 J/cm 2, as shown in Fig. 7c, a-si film was melted completely at I min and I max. Therefore, during the cooling stage, nucleation occurs at the location of I min, and these grains grow laterally to the completely molten region of I max, which is the case of the complete melting regime. As shown in Fig. 9, the experimental microstructure at an energy density of 1.10 J/cm 2 was compared with the simulated microstructure, which was expanded to 5.0 m in the z direction. When the energy density was increased further to 1.20 J/cm 2, as shown in Fig. 7d, nucleation occurs first at the location of I min and the grains grow laterally to the completely molten region of I max. However, these grains are impinged by secondary nucleation occurring at the location of I max. The simulated microstructures were well matched to the experimental microstructures. A characteristic Raman spectrum of a-si is typified by the broad transverse acoustic phonon bands at 160 cm 1, the strong transverse optical phonon bands at 480 cm 1, and the longitudinal acoustic modes at 300 and 400 cm 1, as shown by the deconvoluted lines in Fig. 10a. 31 Figure 10b shows the Raman spectra of the microstructure at the location of I min with the different energy densities. At an energy density of 0.90 J/cm 2, the Raman spectrum has an asymmetric profile deconvoluted into two peaks at cm 1 apparently coming from the microcrystalline structure as a result of explosive crystallization and cm 1 coming from the large poly-si grain due to the lateral growth, as shown by the dotted lines. At 1.00 J/cm 2, the intensity of the peak at cm 1 almost disappeared relative to the intensity of the peak at cm 1. At 1.05 J/cm 2, the Raman spectrum has a symmetric profile at cm 1 due to the enhanced crystallinity by SLG. At 1.10 J/cm 2, the Raman spectrum also has a symmetric profile. The change in Raman spectra clearly shows the evolution of different microstructures with the energy density. The size and the crystallinity of poly-si grains significantly affect the carrier mobility of poly-si TFT because the grain boundary and the defect within the poly-si TFT channel act as an energy barrier to the transport of carriers The overall grain size and the crystallinity can be determined from the SEM images and Raman spectra as follows: i in the partial melting regime, the microstructure has a region of small poly-si grains at I min and the lateral grain size is less than the half period of I min 2.8 m in this case ; ii in the near-

5 J196 Figure 7. Color online The simulated microstructures with energy densities of a 0.90, b 1.00, c 1.10, and d 1.20 J/cm 2. complete melting regime, the microstructure has the best crystallinity and the grain size is almost the same as one period of I min ; and iii in the complete melting regime, the microstructure has a nucleation region at I min and the lateral grain size is half the period of I min. At higher energy densities, the microstructure has the first nucleation region at I min and the secondary nucleation region at I max. The lateral grain size is less than the half period of I min. Therefore, the best poly-si microstructure by a single irradiation is formed in the near-complete melting regime. However, as mentioned above, SLG does not occur uniformly over the entire irradiated area due to the difficulty in obtaining uniform surviving crystalline seeds over Figure 8. Color online Cross-sectional view of solidification with time at an energy density of 1.0 J/cm 2. Figure 9. Color online Comparison of a the simulated microstructure and b the experimental microstructure at an energy density of 1.10 J/cm 2. Figure 10. Raman spectra of a the dehydrogenated a-si film and b the microstructure at the location of I min at an energy density of i 0.90, ii 1.00, iii 1.05, and iv 1.10 J/cm 2.

6 J197 Figure 12. Raman spectra of the microstructure at the location of I min i by the first irradiation and ii after the second irradiation at an energy density of 0.90 J/cm 2. Figure 11. Top-view Secco-etched SEM images of the microstructure after the first irradiation with an energy density of a 0.90 J/cm 2 and after the second irradiation shifted to a distance of b 0.2, c 1.2, d 1.8, and e 2.8 m half period of I min. the irradiated area. The process window for obtaining the nearcomplete melting regime is quite narrow because this is a transitional regime between the partial melting and the complete melting. Microstructural evolution on double irradiation. To overcome these difficulties, the second irradiation was performed with the same spatial intensity beam profile and energy density as the first irradiation, only spatially shifted to a distance less than the half period of I min from the first irradiation. In the complete melting regime, microstructural evolution by the second irradiation is similar to that by the first irradiation because the energy density is high enough to completely melt the microstructure formed by the first irradiation. In the near-complete melting regime, the situation is similar to that by the first irradiation. The second irradiation does not improve the situation of obtaining uniform surviving seeds over the entire irradiated area and SLG does not occur uniformly. The double irradiation in the complete melting and near-complete melting regimes does not improve the microstructural evolution. However, in the partial melting regime, the results of double irradiation are very different from those of single irradiation depending on the shift length of the second irradiation. Figure 11 shows a series of Secco-etched SEM images of the microstructure by the first irradiation at an energy density of 0.90 J/cm 2 Fig. 11a and after the second irradiation Fig. 11b-e with the shift lengths of 0.2, 1.2, 1.8, and 2.8 m, respectively. Due to the relatively short shift length, I min of the second irradiation is now located inside the region of the small poly-si grains in the shift length of 0.2 m Fig. 11b. Therefore, the final microstructure is similar to that formed by the first irradiation in the partial melting regime. The microstructural evolution was totally different when the shift length was increased to 1.2 m, and I min of the second irradiation was located in the large poly-si grains resulting from the first irradiation, as shown in Fig. 11c. The small grain size of poly-si disappeared completely, and large poly-si grains appeared uniformly over the entire irradiated area. The grain size became double than that formed by the first irradiation, corresponding to one period of I min. The configuration of the microstructure was similar after further increase in the shift length to 1.8 m Fig. 11d, but there were fewer isolated grains occluded by the surrounding large poly-si grains. Finally, with a shift length of 2.8 m, which corresponds to the half period of I min Fig. 11e, the isolated grains almost disappeared and the grain size became equal to the half period of I min. Figure 12 shows the Raman spectra of the microstructure Fig. 11a by the first irradiation and of the microstructure Fig. 11d after the second irradiation. As elucidated in Fig. 10, the Raman spectrum of the microstructure after the first irradiation has two peaks at cm 1, coming from the microcrystalline structure, and at cm 1, coming from the large poly-si grain. However, after the second irradiation, the Raman spectrum has a symmetric profile at cm 1, which shows the same profile as SLG in the near-complete melting regime by the single irradiation. These results clearly demonstrate that the microstructural evolution is considerably improved by double irradiation in the partial melting regime. Figure 13 shows a schematic cross-sectional view of the poly-si growth mechanism by double irradiation, where the filled line types denote the different crystallographic orientations. By the first irradiation, the poly-si grains grow vertically at the location of I min through explosive crystallization. They then grow laterally toward the completely molten region of I max during subsequent cooling, as shown in Fig. 13a. The poly-si grains formed uniformly during the first irradiation grow bilaterally when the second irradiation is shifted to less than the half period of I min. Therefore, SLG occurs from the unmolten single-crystalline seed at I min to the completely molten region of I max and results in the formation of an array of poly-si grains with a size of one period of I min, as shown in Fig. 13b. As the second irradiation is shifted further, the width of the SLG grains increases and the number of isolated grains decreases due to the lower density of single-crystalline seeds and the larger space between them. Therefore, an array of poly-si grains with a size of one period of I min 5.6 m in this case can be formed over a wide range of energy densities from 0.80 to 1.00 J/cm 2 in the partial melting regime. When the second irradiation step is finally

7 J198 shifted to a half period of I min, there is a grain boundary where two poly-si grains with a different crystallographic orientation impinged in the first irradiation. In this case, poly-si grains grow laterally from the unmolten two distinct crystalline seeds at I min to the completely molten region of I max, and an array of poly-si grains is formed with a size equal to the half period of I min Fig. 13c. Conclusion SLG occurs more or less randomly over the entire area irradiated by single irradiation due to the nonuniform distribution of crystalline seeds and the fluctuations of the incident energy density of the excimer laser beam. To overcome these problems of single irradiation and to obtain uniform SLG grains, a double-irradiation process was proposed using the artificially designed spatial intensity profile of the periodic I max and I min. The microstructural evolution and the SLG mechanism by double irradiation were investigated and elucidated. The single-crystalline seeds for the SLG were distributed uniformly by the first irradiation with the energy density of the partial melting regime. SLG then occurred from the unmolten singlecrystalline seeds by the second irradiation, which was shifted spatially to a distance less than half the period of I min with the same spatial intensity profile and energy density as the first irradiation step. SLG occurred uniformly with a wide range of energy density from 0.80 to 1.00 J/cm 2 in the partial melting regime. Finally, an array of SLG grains with a size of one period of I min was formed uniformly over the entire irradiated area. Figure 13. The schematic cross-section view of the growth mechanism by a the first irradiation, b the second irradiation shifted less than the half period of I min, and c the second irradiation shifted to the half period of I min. Acknowledgment This study was supported by the National Program for Tera- Level Nano Devices, which is one of the 21st Century Frontier R&D Programs funded by the Ministry of Science and Technology of Korea. Seoul National University assisted in meeting the publication costs of this article. References 1. T. Sameshima, S. Usui, and M. Sekiya, IEEE Electron Device Lett., 7, K. Sera, F. Okumura, H. Uchida, S. Itoh, S. Kaneko, and K. Hotta, IEEE Trans. Electron Devices, 36, R. S. Sposili and J. S. Im, Appl. Phys. Lett., 69, J. S. Im, R. S. Sposili, and M. A. Crowder, Appl. Phys. Lett., 70, C.-H. Oh, M. Ozawa, and M. Matsumura, Jpn. J. Appl. Phys., Part 2, 37, L L. Mariucci, A. Pecora, R. Carluccio, and G. Fortunato, Thin Solid Films, 383, J.-Y. Park, C. I. Im, T. Hofmann, and D. S. Knowles, SID Int. Symp. Digest Tech. Papers, 36, K.-C. Park, S. H. Jung, W.-J. Nam, and M.-K. Han, Mater. Res. Soc. Symp. Proc., J.1.5.1, C.-W. Lin, L.-J. Cheng, Y.-L. Lu, Y.-S. Lee, and H.-C. Cheng, IEEE Electron Device Lett., 22, A. Pecora, L. Mariucci, S. Piperno, and G. Fortunato, Thin Solid Films, 427, T.-F. Chen, C.-F. Yeh, C.-Y. Liu, and J.-C. Lou, IEEE Electron Device Lett., 25, H. J. Kim and J. S. Im, Appl. Phys. Lett., 68, C.-H. Kim, I.-H. Song, W.-J. Nam, and M.-K. Han, IEEE Electron Device Lett., 23, D.-H. Choi, E. Sadayuki, O. Sugiura, and M. Matsumura, Jpn. J. Appl. Phys., Part 1, 33, H. J. Song and J. S. Im, Appl. Phys. Lett., 68, P. Ch. van der Wilt, B. D. van Dijk, G. J. Bertens, and R. Ishihara, Appl. Phys. Lett., 79, H. Kumomi, Appl. Phys. Lett., 83, J.-H. Jeon, M.-C. Lee, K.-C. Park, and M.-K. Han, Jpn. J. Appl. Phys., Part 1, 39, M. Nakata, H. Okumura, H. Kanoh, and H. Hayama, in Proceedings of the Asia Display/IMID 04, p J. S. Im, H. J. Kim, and M. O. Thompson, Appl. Phys. Lett., 63, J. S. Im and H. J. Kim, Appl. Phys. Lett., 64, E. S. Kim and K.-B. Kim, J. Electrochem. Soc., 154, J B. M. Watraslewicz, Opt. Acta, 12, B. Richards and E. Wolf, Proc. R. Soc. London, Ser. A, 253, J. R. Sheats and B. W. Smith, Microlithography Science and Technology, Marcel Dekker, New York J. W. Goodman, Introduction to Fourier Optics, McGraw-Hill, New York M. O. Thompson, G. J. Galvin, J. W. Mayer, P. S. Peercy, J. M. Poate, D. C. Jacobson, A. G. Cullis, and N. G. Chew, Phys. Rev. Lett., 52, W. Sinke and F. W. Saris, Phys. Rev. Lett., 53, J. P. Leonard and J. S. Im, Appl. Phys. Lett., 78, H. Kisdarjono, A. T. Voutsas, and R. Solanki, J. Appl. Phys., 94, L. Tay, D. J. Lockwood, J.-M. Baribeau, X. Wu, and G. I. Sproule, J. Vac. Sci. Technol. A, 22, N. Yamauchi, J.-J. J. Hajjar, and R. Reif, IEEE Trans. Electron Devices, 38, S. D. Brotherton, D. J. McCulloch, J. P. Gowers, J. R. Ayres, and M. J. Trainor, J. Appl. Phys., 82, Y. Kuo, Thin Film Transistors: Materials and Processes, Vol. 2, Kluwer Academic, Boston 2004.