利用寬頻兆赫波光譜儀研究氧化銦錫奈米結構的光電特性

The studies of indium-tin-oxide nanostructures by broadband terahertz spectroscopy

張展銘(Chang, Chan Ming)

指導教授 ： 潘犀靈

國立清華大學/理學院/物理學系/碩士(2012年)

摘要

氧化銦錫由於具有高的光學穿透率與導電率這兩種優異的性質，已經被廣泛使用於各種光電元件，像是太陽能電池、液晶顯示器、和發光二極體。最近，氧化銦錫的奈米結構被證明具有良好的抗反射特性；這種新材料也成功的被使用在提高光伏電池與發光二極體的效率。在本論文中，我們利用光導天線與雷射光激發電漿的兆赫波時域光譜儀，研究不同厚度之氧化銦錫薄膜、奈米柱、奈米晶鬚的兆赫波段光學常數與導電率。
氧化銦錫薄膜是用直流活性磁控濺鍍法成長在高阻值矽晶圓基板上，而奈米柱、奈米晶鬚則是利用掠射角電子束蒸鍍法沉積於高阻值矽晶圓基板。
為了得到在兆赫波段下更寬頻的光學與電學資訊，我們整合了光導天線與雷射光激發電漿兆赫波時域光譜儀所測量的光學常數，而寬頻的導電率可以由計算折射率得到。因此，藉由寬頻的氧化銦錫薄膜、奈米柱、奈米晶鬚導電率擬合Drude-Smith模型可以得到更準確的光電材料電性參數，像是直流的遷移率與載子濃度。我們推算氧化銦錫薄膜的電漿頻率在1547-3170 rad•THz的範圍，散射時間為4.32-9.19 fs。而奈米柱與奈米晶鬚的電漿頻率分別為751-853與561-1006 rad⋅THz，散射時間為13.2-39.6和13.5-31.7 fs。氧化銦錫薄膜的電子遷移率、載子濃度分別為2.14-16.2 cm2 V−1 s−1和2.26-9.49 × 1020 cm−3，奈米晶鬚為20.26-92 cm2 V−1 s−1和5.33-6.86 × 1019 cm−3，而奈米柱為9.13-53.6 cm2 V−1 s−1 與2.97-9.56 × 1019 cm−3。
我們的結果指出奈米柱、奈米晶鬚展現比薄膜更長的載子散射時間，表示這兩種奈米結構具有優異的結晶與較大的晶粒尺寸。除此之外，我們也討論在不同奈米結構下反向散射與局限效應，造成了奈米晶鬚的導電率優於奈米柱。

關鍵字

兆赫波時域光譜 ； 氧化銦錫薄膜 ； 氧化銦錫奈米晶鬚 ； 氧化銦錫奈米柱 ； 雷射光激發電漿

並列摘要

Indium tin oxide (ITO) exhibits two outstanding properties which are the high optical transparency and electrical conductivity. It has been widely used for various optoelectronic devices, such as solar cells, liquid crystal displays, and light emitting diodes (LED). Recently, because of the broadband and omnidirectional antireflection (AR) characteristics, ITO nanostructures have been successfully employed to enhance efficiency of photovoltaics and LED. In this thesis, we aimed to study the frequency-dependent complex refractive indices and conductivities of ITO thin films, nanorods, and nanowhiskers by THz time-domain spectroscopy (THz-TDS) based on photoconductive (PC) antennas and laser-induced gas plasma, respectively.
ITO thin films were grown on the high resistivity silicon substrate by DC reactive magnetron sputtering. On the other hand, ITO nanorods and nanowhiskers were deposited on the high resistivity silicon substrate using glancing-angle electron-beam evaporation.
In order to obtain the complete optical and electrical information, we combined the experimental results of complex refractive indices measured from the PC antenna (0.15-1.4THz) and laser-induced gas plasma (0.5-4THz) THz-TDS system. Because the complex conductivities can be extracted from the refractive indices, the important electrical parameters for optoelectronic materials, such as DC mobilities and carrier densities fit complex conductivities of the ITO thin films, nanorods, and nanowhiskers by Drude-Smith model, will be much more accurate. We have determined that the plasma frequencies of the ITO films are in the range of 1547-3170 rad•THz, while the corresponding scattering times are 4.32-9.19 fs. For nanowhiskers and nanorods, the plasma frequencies are 751-853 versus 561-1006 rad⋅THz, and carrier scattering time are 13.2-39.6 versus 13.5-31.7 fs, respectively. The mobility, electron density of the thin films were determined to be 2.14-16.2 cm2 V−1 s−1 and 2.26-9.49 × 1020 cm−3, while 20.26-92 cm2 V−1 s−1 and 5.33-6.86 × 1019 cm−3 in nanowhiskers, 9.13-53.6 cm2 V−1 s−1 and 2.97-9.56 × 1019 cm−3 in nanorods.
Our results showed that the ITO nanowhiskers and nanorods exhibit longer carrier scattering times than ITO thin films. This denotes that the two kinds of nanostructures have an excellent crystallinity with large grain size. In addition, we also discussed the backscattering and localization effect in the different nanostructures, while it caused the nanowhiskers to show the more remarkable conductivity than nanorods.

並列關鍵字

THz-TDS ； ITO thin film ； ITO nanowhisker ； ITO nanorod ； laser-induced gas plasma

參考文獻

[1] Jared H. Strait, Paul A. George, Mark Levendorf, Martin Blood-Forsythe, Farhan Rana, and Jiwoong Park, “Measurements of the Carrier Dynamics and Terahertz Response of Oriented Germanium Nanowires using Optical-Pump Terahertz-Probe Spectroscopy,” Nano Lett., Vol. 9, No. 8, pp. 2967-2972, June 2009.

[2] Katsuhiro Ajito and Yuko Ueno, “THz Chemical Imaging for Biological Applications,” IEEE Transactions on Terahertz Science and Technology, Vol. 1, No. 1, pp. 293-300, September 2011

[4] D. H. Auston, K. P. Cheung, and P. R. Smith, “Picosecond photoconducting Hertzian dipoles,” Appl. Phys. Lett., Vol. 45, No. 3, pp. 284-286, May 1984.

[5] A. Rice, Y. Jin, X. F. Ma, X.C. Zhang, D. Bliss et al., “Terahertz optical rectification from 110 zincblende crystals,” Appl. Phys. Lett., Vol. 64, No. 11, pp. 1324-1326, March 1994.

[6] 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, No. 11, pp. 1011-1013, March 1990.