Excimer Laser Annealing of Hydrogen Modulation Doped a-si Film

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1 Materials Transactions, Vol. 48, No. 5 (27) pp. 975 to 979 #27 The Japan Institute of Metals Excimer Laser Annealing of Hydrogen Modulation Doped a-si Film Akira Heya 1, Naoto Matsuo 1, Tadashi Serikawa 2 and Naoya Kawamoto 3 1 University of Hyogo, Himeji , Japan 2 Osaka University, Ibaraki , Japan 3 Yamaguchi University, Ube , Japan A novel low-temperature crystallization method is proposed; the excimer laser annealing (ELA) of amorphous silicon (a-si) with a hydrogen-modulation-doped layer (ELHMD). The effects of hydrogen on low-energy crystallization by conventional ELA and ELHMD were investigated. As the hydrogen concentration increases, the crystallinity of the polycrystalline silicon (poly-si) prepared at a low energy density improves. It is considered that the nucleation is enhanced by the desorption energy of hydrogen from the Si- bond during the Si melting. In addition, the film exfoliation by burst can be suppressed using HMD a-si film. [doi:1.232/matertrans ] (Received September 29, 26; Accepted January 22, 27; Published April 25, 27) Keywords: excimer laser annealing, hydrogen modulation doped amorphous silicon, nucleation, desorption energy, polycrystalline silicon, amorphous silicon 1. Introduction The flexible organic light-emitting diode (OLED) display is one of promising next-generation displays because of its light weight and flexibility. A polycrystalline silicon (poly- Si) film prepared below 2 C is necessary as the material of the thin-film transistor (TFT) in flexible displays. Such poly- Si films prepared by the crystallization of amorphous silicon (a-si) by excimer laser annealing (ELA) have been examined. 1 4) It is known that hydrogen in a-si film causes the film exfoliation, and therefore, the H concentration is decreased to below 1 at% by thermal treatment at 5 C for several hours. However, hydrogen desorption does not occur for lowtemperature processes. The influence of H in a-si film on the properties of poly-si films prepared by ELA is not negligible. On the other hand, the H influences nucleation and grain growth, whose phenomenon can be discussed in terms of the secondary-grain-growth model. 4,5) Furthermore, the effects of H during ELA are important for high quality poly-si films during low-temperature processes. 6,7) We propose a new crystallization method using H-modulation-doped (HMD) a- Si film, namely, ELA of a-si film with a H-modulation-doped layer (ELHMD). In this study, the effects of hydrogen on the crystallization by ELA and ELHMD at a low energy density are investigated. The nucleation model of a-si with a high hydrogen concentration is considered by undercooling, and the advantage of ELHMD is inferred. 2. Experimental Fig. 1 Poly Si target Ar Substrate (2 2mm) Cooling water RF 35mm Main valve DP RP Schematic diagram of RF magnetron sputtering apparatus. 2.1 a-si film deposition by RF magnetron sputtering The a-si films were prepared by radio frequency (RF) magnetron sputtering. The RF magnetron sputtering apparatus used in this study is schematically illustrated in Fig. 1. A poly-si wafer of 76 mm in diameter was used as a target. The distance between the substrate and the target was 35 mm. The back pressure was below 1 4 Pa. The RF frequency and electrical power were MHz and 1 W, respectively. The gas pressure during a-si deposition was fixed at 1.3 Pa by controlling a main valve. A glass substrate was used for Raman scattering spectroscopy. A SiO 2 (1 nm)/si substrate was used for scanning electron microscopy (SEM). A Si substrate was used for Fourier transform infrared (FT-IR) measurement. The substrate temperature was below 1 C. The film thickness was 5 nm for Raman scattering spectroscopy and SEM. The H concentration in the a-si films was changed by controlling the atmospheric gas. The ratio of the /Ar flow rate was varied from to 3%. For the deposition of HMD a- Si films with low and high H concentrations, pure Ar gas and a mixture gas ( /Ar flow rate ratio: 3%) were used. In the case of HMD a-si films, the atmospheric gas was completely exhausted before the second-layer deposition. The H concentration in the a-si film with a thickness of 1 mm was estimated by FT-IR measurement. The H concentration was estimated from peaks due to the stretching mode at 2 cm 1 and the 2 stretching mode at 21 cm 1 using the equation

2 976 A. Heya, N. Matsuo, T. Serikawa and N. Kawamoto Samples A Top layer with high H conc. B Middle layer with high H conc. C Bottom layer with high H conc. D Uniform 11at% Substrate 1nm 4nm 2nm 1nm 2nm 4nm 1nm 5nm Absorbance (arb. units) 2 O (b) CO 2 (a) Wave number, ω /cm 1 Fig. 2 Structure of HMD a-si films (A, B and C) and a-si film with uniform H concentration (D). Total H concentrations are shown at the left of samples. 2 2 Z N ¼ A! d! ¼ A S ð1þ t! where N, A,,!, t,! and S are the H density, the conversion factor (9: 1 19 cm 2 at 2 cm 1 and 2: cm 2 at 21 cm 1 ), the absorbance, the frequency, the film thickness, the peak frequency and the peak area, respectively. 8) The structures of HMD a-si films are shown in Fig. 2. All samples were 5 nm in thickness. Samples A, B and C were HMD a-si films and sample D had a uniform H concentration. Sample D was prepared at a /Ar flow rate ratio of 5%. 2.2 Poly-Si film formation by ELA The poly-si films were prepared using a KrF excimer laser with an energy density of 1 mj/cm 2 at 1 Hz, at room temperature and under a pressure of approximately 1 3 Pa. The wavelength and pulse duration of the laser were 248 nm and 23 ns, respectively. The number of shots was 5. The area of laser irradiation was 5 1 mm 2. The evaluation was carried out at the center of the irradiated area. The properties of the poly-si films were measured by Raman spectroscopy and SEM. Raman scattering measurements using the exciting light of nm wavelength of an argon (Ar) ion laser were carried out at room temperature. The fraction of crystalline phase to amorphous phase was evaluated from the transverse optical (TO) phonon signal of the Raman spectra. The crystalline fraction was determined from the areal ratio of the signal due to the crystalline phase at around 521 cm 1 to the sum of the signals due to both the crystalline phase and the amorphous phase at around 48 cm 1. The poly-si films were treated by secco etching for SEM observation. 9) The depth profiles of the H concentration for the a-si film in the laser-irradiated area and the unirradiated area were measured by time-of-flight secondary ion mass spectroscopy (TOF-SIMS). A Au ion was used as the first ion. The accelerated voltage and measured area were 25 kv and 1 mm square, respectively. The depth profile was measured by Ar ion sputtering. The accelerated voltage and sputtered area were 2 kv and 3 mm square, respectively. (a) Fig. 3 FT-IR spectra of a-si films prepared at /Ar ratios of % (a) and 3% (b). The film thickness was 1 mm. 3. Results and Discussion 3.1 H concentration in a-si film The FT-IR spectra of a-si films prepared at /Ar ratios of and 3% are shown in Fig. 3. The signals due to at 2 cm 1 and 2 at 21 cm 1 were observed in the FT- IR spectra of both a-si films. The 2 peak of the a-si film prepared at a /Ar ratio of 3% is larger than that of the a-si film prepared at a /Ar ratio of %. It is known that an a-si film prepared at a low temperature has many 2 bonds. 1) The 2 bonds are unstable and is easily desorbed by the thermal energy. The H concentrations of the a-si films prepared at /Ar ratios of and 3% were 5 and 21 at%, respectively. The H concentration of HMD a-si films was estimated at 8 at% from the thickness ratio of the low- and high-h-concentration layers. The H concentration of the a-si film as a function of the /Ar flow rate ratio is shown in Fig. 4. As the /Ar ratio increases up to 1%, the H concentration related to the 2 bond increases. However, the H concentration estimated from the bond is lower than that estimated from the 2 bond, and it is independent of the /Ar ratio. It is shown that the H concentration is limited at 2 at% and that the 2 bond is dominant compared with the bond in an a-si film with a high H concentration deposited by RF magnetron sputtering using a and Ar mixture gas. 3.2 Influence of H concentration on poly-si properties The Raman spectra of poly-si films prepared from a-si films with various H concentrations are shown in Fig. 5. All poly-si films have peaks due to the a-si phase at 48 cm 1 and the crystalline silicon (c-si) phase at cm 1.A difference in Raman spectra was observed for the poly-si films prepared at various H concentrations. (b)

3 Excimer Laser Annealing of Hydrogen Modulation Doped a-si Film 977 H concentration (at%) Total(+ ) /Ar flow rate ratio (%) Fig. 4 H concentration as a function of /Ar flow rate ratio. The H concentration was estimated from the peak due to and 2 bonds in the FT-IR spectra. Fig. 6 Raman peak shift, R/cm 1 FWHM /cm 1 Crystalline fraction (%) H concentration (at%) Raman peak shift, FWHM and crystalline fraction of poly-si films. Intensity (arb. units) /Ar flow rate ratio 3% 14% 9% 5% % c Si H concentration 21at% 1 21at% 11at% 5at% Raman shift, R/cm 1 Fig. 5 Raman spectra of poly-si films prepared from a-si films with various H concentrations. The Raman peak shift, full width at half maximum (FWHM) and crystalline fraction of the poly-si films are shown in Fig. 6. As the H concentration increases, the Raman peak shift and the crystalline fraction increase and the FWHM decreases. The FWHM of the Raman peak due to the crystalline phase and the difference between the peak shift of poly-si and that of crystalline Si (521 cm 1 ) correspond to the defect density and the internal stress of the poly-si film, respectively. 11) As the H concentration in the a-si films increases, the Raman peak shift increases and the FWHM decreases. It is shown that the stress was relieved for a-si with a high H concentration. The SEM images of the poly-si films are shown in Fig. 7. Film exfoliation was observed and occurred even under lowenergy-density irradiation. As the H concentration in the a-si films increases, the exfoliation area increases. It is known that an a-si film prepared by sputtering using Ar gas contains Ar, and that the Ar causes the film exfoliation. In this study, the Ar concentration was not estimated. However, it is considered that as the /Ar ratio increases, the Ar concentration in the a-si films decreases. Therefore, it is considered that the film exfoliation occurs by H burst during ELA. 5% 21% 1µm Fig. 7 SEM images of poly-si films prepared from a-si films with H concentrations of 5 and 21 at%.

4 978 A. Heya, N. Matsuo, T. Serikawa and N. Kawamoto Intensity (arb. units) c Si Samples A High H conc. Top layer B Middle layer C Bottom layer D Uniform Raman shift, R/cm 1 Fig. 8 Raman spectra of poly-si films prepared from HMD a-si films shown in Fig. 2. Raman peak shift, R/cm 1 FWHM /cm 1 Crystalline fraction (%) A B C D Sample Fig. 9 Raman peak shift, FWHM and crystalline fraction of poly-si films prepared from HMD a-si films shown in Fig. 2. Sample A Sample C 1µm Fig. 1 SEM images of poly-si films prepared from HMD a-si films (samples A and C). 3.3 Properties of poly-si films prepared from HMD a-si films The Raman spectra of poly-si films prepared from HMD a-si films are shown in Fig. 8. Here, for comparison, the Raman spectrum of the poly-si film prepared from a-si with a uniform H concentration is also shown in this figure. The properties of the poly-si film changed with the initial a-si structure. The Raman peak shift, FWHM and crystalline fraction of the poly-si films prepared from HMD a-si films are shown in Fig. 9. As the high-h-concentration layer approaches from the film surface to the interface between the film and the substrate, the peak shift increases and the FWHM decreases. The samples prepared from HMD a-si films have low crystalline fractions compared with sample D. However, sample C has a very small FWHM and a large Raman peak shift. It is shown that this poly-si film has a low defect density and a low internal stress. These results indicate that high-quality crystal grains are obtained using HMD a-si films. The SEM images of poly-si films prepared from HMD a-si films are shown in Fig. 1. Film exfoliation was not observed in samples A and C. This shows that film exfoliation can be suppressed using HMD a-si films. The depth profiles of the H concentration for an a-si film on a glass substrate (sample C) with and without ELA by TOF-SIMS are shown in Fig. 11. For the a-si film without ELA, the H concentration in the region 1 nm from the interface of the a-si film and the glass substrate was higher than that at other regions. It is conformed that H-modulated a-si film was prepared by changing the atmospheric gas during a-si deposition. On the other hand, the H concentration in the a-si film with ELA decreased to a quarter of its preannealing concentration. In addition, the thickness of the a-si film, measured using a noncontact 3D surface profiling system, was decreased by ELA from 5 to 4 nm. This

5 Excimer Laser Annealing of Hydrogen Modulation Doped a-si Film 979 Intensity poly Si without ELA with ELA Glass sub. Glass sub Sputter time, t/s Fig. 11 Depth profile of H concentration for a-si film on glass substrate (sample C) with and without ELA by TOF-SIMS. implies that the Si thickness was decreased by H desorption and volume shrinkage. 3.4 Advantage of ELHMD Many 2 bonds are included in the a-si film with a high H concentration. The H is emitted to the vacuum outside the film during ELA, and thermal energy corresponding to the H-H bond energy of 7: J (4.5 ev) is generated locally. 12) Therefore, a temperature increase occurs around the 2 bond. It is considered that the initial temperature of a-si films with high H concentrations is higher than that of a-si films with low H concentrations because of the desorption energy. Therefore, the quench rate is increased by increasing the initial temperature. It is known that as the quench rate increases, the degree of undercooling increases. 13) The degree of undercooling of an a-si film with a high H concentration is larger than that of an a-si film with a low H concentration. Crystal nucleation occurs during undercooling. 14) As the degree of undercooling increases, the nucleation probability increases. Therefore, the crystallinity is improved by increasing the H concentration in the a-si film. The nucleation occurs at the lowest temperature site. Therefore, the nucleation site is at the interface between the a-si film and the glass substrate for the uniform-h-concentration a-si film. When the crystal nucleation occurs at the interface between the a-si film and the glass substrate, the glass substrate suppresses the grain growth toward the glass substrate. Therefore, the crystal grain has a high stress and a high defect density for crystallization of a-si film without H modulation. On the other hand, the nucleation site is inside the a-si film for HMD a-si films. In addition, the crystal grains are grown free from stress at the interface of the glass substrate. Therefore, the poly-si film prepared from the HMD a-si film (sample C) has a small FWHM and a large Raman peak shift, as shown in Fig. 9. It implies that H modulation doping has the effect of controlling the nucleation sites in the direction of the film thickness. 4. Conclusions The effects of H concentration and H distribution on crystallization by ELA were investigated. The following conclusions were obtained. (1) The H concentration in an a-si film can be changed by controlling the atmospheric gas used for RF magnetron sputtering. Many 2 bonds are included in the a-si film with a high H concentration. (2) The properties of poly-si films prepared at a low energy density changed with H concentration and H distribution in the a-si films. (3) High-quality poly-si films are obtained by ELHMD at a low energy density. It is considered that the desorption energy of the hydrogen from the 2 bond enhances crystal nucleation during the Si melting. REFERENCES 1) R. S. Sposili and J. S. Im: Appl. Phys. Lett. 69 (1996) ) C. H. Oh, M. Ozawa and H. Matsumura: Jpn. J. Appl. Phys. 37 (1998) L492 L495. 3) T. Sameshima, M. Hara and S. Usui: Jpn. J. Appl. Phys. 28 (1989) ) N. Matsuo, Y. Aya, T. Kanamori, T. Nouda, H. Hamada and T. Miyoshi: Jpn. J. Appl. Phys. 39 (2) ) N. Kawamoto, N. Matsuo, H. Abe, F. Anwar, I. Hasegawa, K. Yamano and H. Hamada: Jpn. J. Appl. Phys. 43 (24) ) N. Kawamoto, A. Masuda, N. Matsuo, Y. Seri, T. Nishimori, Y. Kitamon, H. Matsumura, H. Harada and T. Miyoshi: Jpn. J. Appl. Phys. 45 (26) ) A. Heya, N. Matsuo, H. Matsumura and N. Kawamoto: Jpn. J. Appl. Phys. 45 (26) ) A. A. Langford, M. L. Fleet and B. P. Nelson: Phys. Rev. B 45 (1992) ) F. Secco: J. Electrochem. Soc.: 119 (1972) ) W. Beyer and H. Wagner: J. Non-cryst. Solids 59 & 6 (1983) ) K. Kitahara, A. Moritani, A. Hara and M. Okabe: Jpn. J. Appl. Phys. 38 (1999) L1312 L ) X. Qi, Z. Chen and G. Wang: J. Mater. Sci. Technol. 19 (23) ) S. R. Stiffler, M. O. Thompson and P. S. Peercy: Phys. Rev. B 43 (1991) ) S. R. Stiffler, P. V. Evans and A. L. Greer: Acta Metal. Mater. 4 (1992)