Accurate determination of the grain size of annealed poly-Si is an important task for its application in TFT fabrication. Meanwhile, fast determination of the grain size is another issue. Typically, Hall measurement is used to detect the mobility of undoped poly-Si and SEM images of the annealed samples to identify the grain size. However, both methods have the intrinsic restrictions, such as the limited imaging area and destructive sample preparation procedure. In this thesis, we introduced Optical-Pump-THz-Probe method and THz-TDS technique to directly identify the annealing quality of poly-Si in a large area without any preparation process.
By using OPTP system, we observed the photoexcited carriers dynamics in poly-Si film.
A double-exponential function shows the excellent agreement to photo-excited THz response.
The relaxation times of poly-Si with average grain size (~500 nm) and poly-Si with average grain size (~50 nm) are 32.69 ps and 24.42 ps, respectively. Longer relaxation time of poly-Si with average grain size (~500 nm), due to less trap density indicates that it has better annealing quality. And indeed, bigger average grain size and less grain boundary in the area of diameter 2 mm are observed for poly-Si with average grain size (~500 nm). In THz-TDS experiment, the complex conductivity and refractive index measured at 10 ps after photo excitation could be well described by Drude model. From the best fits for sample A and sample B, we get the carrier mobility of μ=175±19.4 cm2/Vs and μ=94.5±20.2 cm2/Vs, respectively.
To investigate the temporal evolution of the mobility and carrier concentration, we measured THz-TDS of poly-Si with average grain size (~500 nm) at different optical time delay. A slight decrease in the carrier concentration and the increase in mobility are observed as increasing the time delay of optical pump delay. The lack of significant dependence of carrier concentration on time delay indicates that available trapping sites are filled up as soon
as the carriers are excited and recombination of the photoexcited carriers is followed. The gradually increase mobility indicates the reduced carrier-carrier scattering due to the decrease in carrier concentration.
The stability of THz-TDS measurement is observed by comparing optically measured mobility of poly-Si to that of electrically measured by poly-Si TFT devices. Similar larger field-effect mobility with increasing annealed fluence, is observed for both THz-TDS and TFT methods.
In the future, we are planning to investigate several poly-Si samples with different uniformity or grain size. By using a combined optical system of OPTP and THz-TDS, we would be able to distinguish the different uniformity of grain size in poly-Si, which could be very useful for TFT industrial application.
References
[1] Mourou G, Stancampiano C V, Antonetti A and Orszag A, “Picosecond microwave pulse generation,” Appl. Phys. Lett., vol. 38, no. 6, pp. 470-472, 1981.
[2] Auston D H, Cheung K P and Smith P R, “Picosecond photoconducting Hertzian dipoles,” Appl. Phys. Lett., vol. 45, no. 3, pp. 284-286, 1984.
[3] Ch. Fattinger, and D. Grischkowsky, “Point source terahertz optics,” Appl. Phys. Lett., vol. 53, pp. 1480-1482, 1988.
[4] N. Sarukura, H. Othtake, S. Izumida, and Z. Liu, “High average-power THz radiation from femtosecond laser-irradiated InAs in a magnetic field and its elliptical polarization characteristics,” J. Appl. Phys., vol. 84, pp. 654-656, 1998.
[5] X.-C. Zhang, Perspectives in Optoelectronics, Ed. By Sudhanshu S. Jha, Would Scientific, chapter 3, 1995.
[6] Q. Wu and X. C. Zhang, “Ultrafast electro-optic field sensors,” Appl. Phys. Lett., vol.
68, no. 12, pp. 1604-1606, 1996.
[7] D.M. Mittleman, R.H. Jacobsen, R. Neelamani, R.G. Baraniuk and M.C. Nuss, “Gas sensing using terahertz time-domain spectroscopy,” Appl. Phys. B-Lasers and optics., vol. 67, pp. 379-390, 1998.
[8] S. Sriraman, S. Agarwal, E. S. Aydil, and D. Maroudas, “Mechanism of Hydrogen-Induced Crystallization of Amorphous Silicon,” Nature, vol. 418, pp. 62-65, 2002.
[9] G. A. Bhat, Z. Jin, H. S. Kwok, and M. Wong, “Effects of longitudinal grain boundaries on the performance of MILC-TFTs,” IEEE Electron Device Lett., vol. 20, no. 2, pp. 97-99, 1999.
[10] J. S. Im, H. J. Kim, and M. O. Thompson, “Phase transformation mechanisms involved
in excimer laser crystallization of amorphous silicon films,” Appl. Phys. Lett., vol. 63, pp. 1969-1971, 1993.
[11] G. K. Giust and T. W. Sigmon, “Microstructural characterization of solid-phase crystallized amorphous silicon films recrystallized using an excimer laser,” Appl. Phys.
Lett., vol. 70, pp. 767-769, 1997.
[12] S. D. Brotherton, D. J. McCulloch, J. P. Gowers, J. R. Ayres, and M. J. Trainor,
“Influence of melt depth in laser crystallized poly-Si thin film transistors,” J. Appl.
Phys., vol. 82, pp. 4086-4094, 1997.
[13] J. S. Im, M.A. Crowder, R. S. Sposili, J. P. Leonard, H. J. Kim, J. H. Yoon, V. V. Gupta, H. J. Song, and H. S. Cho, “Controlled Super-Lateral Growth of Si Films for Microstructural Manipulation and Optimization,” Phys. Stat. Sol. (a)., vol. 166, pp.
603-617, 1998.
[14] A. T. Voutsas, “A new era of crystallization: advances in polysilicon crystallization and crystal engineering,” Appl. Surf. Sci., vol. 208, pp. 250-262, 2003.
[15] A. Hara, F. Takeuchi, and N. Sasaki, IEEE Electron Devices Society, Proc. of 2000 International Electron Device Meeting, p. 209, 2000.
[16] S. K. Sundaram and E. Mazur, “Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses,” Nat. Mater., vol. 1, pp. 217-224, 2002.
[17] A. Rousse, C. Rischel, S. Fourmaux, I. Uschmann, S. Sebban, G. Grillon, Ph. Balcou, E. Förster, J. P. Geindre, P. Audebert, J. C. Gauthier, and D. Hulin,
“Non-thermalmelting in semiconductors measured at femtosecond resolution,” Nature., vol. 410, pp. 65-68, 2001.
[18] K. Sokolowski-Tinten, J. Biakowski, and D. von der Linde, “Ultrafast laser-induced order-disorder transitions in semiconductors,” Phys. Rev. B, vol. 51, pp. 14186-14198,
1995.
[19] T. Y. Choi and C. P. Grigoropoulos, “Plasma and ablation dynamics in ultrafast laser processing of crystalline silicon,” J. Appl. Phys., vol.92, no. 9, pp. 4918-4925, 2002.
[20] X. Liu, D. Du, and G. Mourou, “Laser Ablation and Micromachining with Ultrashort Laser Pulses,” IEEE J. Quantum Electron., vol. 33, no. 10, pp. 1706-1716, 1997.
[21] T. Q. Jia, Z. Z. Xu, X. X. Li, R. X. Li, B. Shuai, and F. L. Zhao, “Microscopic mechanisms of ablation and micromachining of dielectrics by using femtosecond lasers,” Appl. Phys. Lett., vol. 82, pp. 4382-4384, 2003.
[22] X. C. Zhang, B. B. Hu, J. T. Darrow, and D. H. Auston, “Generation of femtosecond electromagnetic pulses from semiconductor surfaces” Appl. Phys. Lett. vol. 56, pp.
1011-1013, 1990.
[23] X. C. Zhang, J. T. Darrow, B. B. Hu, D. H. Auston, M. T. Schmidt, P. Tham, E. S.
Yang, “Optically induced electromagnetic radiation from semiconductor surfaces,”Appl. Phys. Lett., vol. 56, pp. 2228-2230, 1990.
[24] X.-C. Zhang, B. B. Hu, S. H. Xin, D. H. Auston, “Optically induced femtosecond electromagnetic pulses from GaSb/AlSb strained-layer superlattices,” Appl. Phys. Lett., vol. 57, pp. 753-755, 1990.
[25] T. Dekorsy, H. Auer, C. Waschke, H. Bakker, H. Roskos, H. Kurz, V. Wanger, P.
Grosse, “Emission of Submillimeter Electromagnetic Waves by Coherent Phonons,”
Phys. Rev. Lett., vol. 74, pp. 738-741, 1995.
[26] P. Gu, M. Tani, S. Kono, K. Sakai, and X. C. Zhang, “Study of terahertz radiation from InAs and InSb,” J. Appl. Phys., vol. 91, pp. 5533-5537, 2002.
[27] P. Gu, and M. Tani, “Terahertz Radiation from Semiconductor Surfaces,” Topics Appl.
Phys., vol. 97, pp. 63-98, 2005.
[28] For review papers, see, for example, X.-C. Zhang and D.H. Auston: J. Appl.
Phys. 71, 326 (1992), S.C. Howells and L.A. Schlie: Appl. Phys. Lett. 67, 3688 (1995), T. Kondo, M. Sakamoto, M. Tonouchi, and M. Hangyo: Jpn. J. Appl. Phys. 38, L1035 (1999), M. Hangyo, M. Migita, and K. Nakayama: J. Appl. Phys. 90, 3409 (2001), M.B. Johnston, D.M. Whittaker, A. Corchia, A.G. Davies, and E.H. Linfield: J. Appl.
Phys. 91, 2104 ( 2002), M.B. Johnston, D.M. Whittaker, A. Corchia, A.G. Davies, and E.H. Linfield: Phys. Rev. B65, 165301 ,2002.
[29] N. Sarukura, H.Ohtake, S. Izamida, Z. Liu: J. Appl. Phys. 84, 1 (1998) according to a recalibration of the bolometer sensitivity by Sarukura et al., the total radiation power from an InAs surface under a magnetic field of 1 T is corrected to be about 50 μW with a pump power of 1W. 1398 (2000) I.; P.N. Saeta, D.R. Dykaar, S. Schmitt-Rink, and S.L. Chuang, IEEE J. Quantum Electron. 28, 2302, 1992.
[30] T. Dekorsy, H. Auer, H. Bakker, H. Roskos, and H. Kurz, “THz electromagnetic emission by coherent infrared-active phonons,” Phys. Rev. B., vol. 53, pp. 4005-4014, 1996.
[31] S. Kono, P. Gu, M. Tani, and K. Sakai, “Temperature dependence of terahertz radiation from n-type InSb and n-type InAs surfaces,” Appl. Phys. B-Lasers and optics., vol. 71, pp. 901-904, 2000.
[32] T. -R. Tsai, S. -J. Chen, C. -F. Chang, S. -H. Hsu, T. -Y. Lin, and C. -C. Chi, "Terahertz response of GaN thin films," Opt. Express., vol. 14, pp. 4898-4907, 2006.
[33] H. Kakinuma, M. Mohri, and T. Tsuruoka, “Mechanism of low-temperature polycrystalline silicon growth from a SiF4/SiH4/H2 plasma,” J. Appl. Phys., vol. 77, pp.
646-652, 1995.
[34] http://www.toppoly.com/Toppoly/tw/Technology/LTPS_Tech.asp
[35] Y. J. Chang, K. H. Kim, J. H. Oh, and Jin Janga, “Ni-mediated crystallization of amorphous silicon with a SiO2 nanocap,” Electrochemical and Solid-State Letters., vol.
7, pp. 207-209, 2004.
[36] W. G.. Hawkins, “Polycrystalline-silicon devices technology for large-area electronics,”
IEEE Trans. Electron Devices, vol. 33, pp. 477-481, 1986.
[37] http://www.mems-exchange.org/MEMS/processes/deposition.html
[38] T. Y. Choi and C. P. Grigoropoulos, “Plasma and ablation dynamics in ultrafast laser processing of crystalline silicon,” J. Appl. Phys., vol. 92, no. 9, pp. 4918-4925, 2002.
[39] J. M. Shieh, Z. H. Chen, B. T. Dai, Y. C. Wang, A. Zaitsev, and C. L. Pan,
“Near-infrared femtosecond laser-induced crystallization of amorphous silicon,” Appl.
Phys. Lett., vol. 85, pp. 1232-1234, 2004.
[40] K. P. H. Lui, and F. A. Hegmann, “Ultrafast carrier relaxation in radiation-damaged silicon on sapphire studied by optical-pump-terahertz-probe experimentts,” Appl. Phys.
Lett., vol. 78, pp. 3478-3480, 2001.
[41] W. H. Press, S. A. Teukolsky, W. T. Vetterling, and B. P. Flannery, Numerical Recipes in Fortran (Cambridge Unversity Press, NewYork, 1986), second edition ed.
[42] S. S. Prabhu, S. E. Ralph, M. R. Melloch, and E. S. Harmon, “Carrier dynamics of low-temperature-grown GaAs observed via THz spectroscopy,” Appl. Phys. Lett., vol.
70, pp. 2419-2421, 1997.
[43] P. Uhd Jepsen, W. Schairer, I. H. Libon, U. Lemmer, N. E. Hecher, M. Birkholz, K.
Lips, and M. Schall, “Ultrafast carrier trapping in microcrystalline silicon observed in optical pump-terahertz probe measurements,” Appl. Phys. Lett., vol. 79, pp.
1291-1293, 2001.
[44] RF Lyon, and PM Hubel, “Eyeing the Camera: into the Next Century,” Tenth Color Imaging Conference, 2002.
[45] E.D. Palik (ed.), “Handbook of Optical Constants of So!ids” (Academic Press, London, 1998).
[46] Neil W. Ashcroft, and N. David Mermin, “Solid State Physics” (International edition) [47] T.-I. Jeon, and D. Grischkowsky, “Nature of Conduction in Doped Silicon,” Phys. Rev.
Lett., vol. 78, pp. 1106-1109, 1997.
[48] Y. C. Wang, J. M. Shieh, H. W. Zan, and C.L. Pan, “Near-infrared femtosecond laser crystallized poly-Si thin film transistors,” OPTICS EXPRESS, vol. 15, no. 11, pp.
6981-6986, 2007.