氧化銦錫奈米材料在兆赫波段光電特性之量測及其應用

Characterization and Applications of Indium-Tin-Oxide Nanomaterials in the Terahertz Frequency Band

楊承山(Chan-Shan Yang)

指導教授 ： 潘犀靈

國立清華大學/理學院/物理學系/博士(2014年)

https://doi.org/10.6843/NTHU.2014.00394

摘要

我們分別利用穿透式以及反射式兆赫波時域光譜技術來研究氧化銦錫奈米晶鬚和其濺鍍薄膜這兩種重要材料結構的遠紅外光學特性和電性，並分析其在非Drude電學模型 (Drude-Smith模型) 下所存在的反向散射與局限效應。氧化銦錫奈米晶鬚之複數導電率在兆赫波段的非Drude電學行為可以歸咎為載子在晶界間與雜質離子間之散射。然而，在氧化銦錫薄膜中，因為缺少明顯的晶界，主要只有雜質離子所造成的載子散射。考慮相同高度的氧化銦錫奈米晶鬚和其薄膜，前者具有較高的電子遷移率 (~125 cm2V-1s-1)，比薄膜的 (~27 cm2V-1s-1)高出許多。這個現象是由於奈米晶鬚擁有較長的載子散射時間。另外，雖然氧化銦錫奈米晶鬚之直流導電率 (~250 ?-1cm-1)和兆赫波段之實部導電率都低於氧化銦錫薄膜的 (~800 ?-1cm-1)，但其導電能力，已經足以用來當作電極。較突出的導電率，也間接反映出氧化銦錫薄膜具備較高的電漿頻率。更重要的是，由於兆赫輻射可以輕易地通過氧化銦錫奈米晶鬚中空氣所填滿的區域，故其在兆赫波段之穿透率 (? 60~70 %)優於薄膜13倍以上。
此外，為了獲得材料更高頻的光電學資訊，並比較不同氧化銦錫奈米結構 (奈米柱和奈米晶鬚)，我們整合了雷射光激發電漿與光導天線兆赫波時域光譜儀來得到0.15~9.00兆赫波段的頻譜資訊。也因此，我們可以很準確地獲得0.20~4.00兆赫之間奈米材料之光學與電學之資訊。同時，利用傅里葉轉換紅外光譜，我們發現在整個遠紅外波段中(0.2~15.0 兆赫)，奈米結構可以維持70 %以上的穿透率，而傳統濺鍍薄膜只能維持在9 %左右。氧化銦錫奈米柱,奈米晶鬚與濺鍍薄膜之寬頻複數導電率皆可以被Drude-Smith模型所擬合。考慮一樣的氧化銦錫組成體積比，由於奈米晶鬚之載子侷限行為較輕微，故無論是電子遷移率或直流導電率皆優於奈米柱。另一方面，濺鍍薄膜由於具有較高之載子與雜質離子濃度，故造成嚴重的散射行為，進而使電子遷移率較奈米柱和奈米晶鬚皆來的低。在此寬頻兆赫波段的研究中，我們也考慮複數導電率在兆赫頻率範圍之極值與曲折點，這些資訊提供了額外的標準來評斷利用兆赫波時析光譜儀來獲得非Drude模型的材料之電學特性準確度。目前的研究結果顯示高度在 ~1000 奈米左右的奈米晶鬚具有最優異的穿透率和電學特性，因此最有潛力當作在兆赫波段之高透明電極材料。
利用前面所研究之氧化銦錫奈米晶鬚，我們設計了兩種同時具備高兆赫波穿透率與低操作電壓之新型電控液晶兆赫波相位延遲器。首先，利用奈米晶鬚取代傳統濺鍍薄膜當作一高穿透率之電極材料。更進一步地，我們首創使用氧化銦錫奈米晶鬚當作液晶配向之媒介。此兩種液晶層厚度約513微米之兆赫波相位延遲器同時展現高兆赫波穿透率 (~77%)，此外，皆可在低操作電壓(分別為17.68 and 2.83 V (rms))成為1/4波長相位延遲元件(操作頻率為1.0兆赫)。同時具備當作透明電極與液晶配向能力的特性，使得氧化銦錫奈米晶鬚除了在可見光波段的廣泛應用外，也成為兆赫波段元件之不可或缺的材料。

關鍵字

兆赫波 ； 奈米材料 ； 光譜儀 ； 液晶 ； 相位延遲器 ； 氧化銦錫

並列摘要

In this thesis, we firstly studied the non-Drude behavior of indium-tin-oxide (ITO) nanowhiskers (NWhs) and thin film by using the transmission-type THz-TDS (THz-TDTS) and reflection-type THz-TDRS (THz-TDRS). Their electrical properties, such as plasma frequencies, carrier scattering times, were found to be fitted well by the Drude-Smith model over 0.1~1.4 THz. The non-Drude behavior of complex conductivities in ITO NWhs can be attributed to carrier scattering from grain boundaries and impurity ions. On the other hand, in ITO thin films, non-Drude behavior observed is ascribed to scattering by impurity ions only. Under the condition of the same height, the mobility of NWhs (~125 cm2V-1s-1) is much larger than that of the ITO thin films (~27 cm2V-1s-1), which is due to the longer carrier scattering time of the NWhs. The DC conductivities (~250 ?-1cm-1) or real conductivities in the THz frequency region of ITO NWhs is, however, lower than those of the ITO thin films (~800 ?-1cm-1) but adequate for use as electrodes. Significantly, the transmittance of ITO NWhs (? 60~70 %) is much higher (? 13 times) than that of ITO thin films in the THz frequency range. The underneath basic physics is that the THz radiation can easily propagate through the air-space among NWhs.
In order to realize the THz information in higher frequencies, two different types of THz -TDSbased on the photoconductive antenna, and laser-induced gaseous plasma, respectively, with combined spectral coverage from 0.15 to 9.00 THz were applied. These catalyze accurate determination of the optical and electrical properties of such ITO nanomaterials in the frequency range from 0.20 to 4.00 THz. When the volume filling factors of both type of nanomaterials, NWhs, and nanorods (NRs), are nearly same, mobilities and DC conductivities of ITO NWhs are higher than those of NRs due to less severe carrier localization effects on the NWhs. On the other hand, mobilities of sputtered ITO thin films are lower than ITO nanomaterials because of larger concentration of dopant ions in films, which causes stronger carrier scattering. To date, our study indicates that ITO NWhs at the height of ~1000 nm exhibit superb transmittance and adequate electrical characteristics for the applications of transparent conducting electrodes of THz Devices.
Utilizing THz characteristics of ITO NWhs as transparent electrodes, we have also demonstrated two novel schemes of high-transmittance low-operative-voltage THz phase shifters by electrically tuning liquid crystals (LCs) cell. In place of traditional ITO thin film, NWhs with graded-refractive-index (GRIN) in the THz region act as transparent electrodes. Meanwhile, a new method of LCs alignment by using ITO NWhs is also presented. For two schemes with THz transmittance ~77%, phase shift of more than ?/2 at 1.0 THz is achieved in a ~513 ?m-thick cell with pretty low driving voltages, 17.68 and 2.83 V (rms), respectively. The ITO NWhs obliquely evaporated by electron-beam glancing-angle deposition can serve simultaneously as transparent electrodes and alignment layer for LC devices in the THz frequency range.

並列關鍵字

Terahertz ； Nanomaterials ； Spectroscopy ； Liquid crystal ； Phase shifter ； Indium-tin-oxide

參考文獻

[1] X.-C. Zhang and J. Xu, Introduction to THz Wave Photonics, New York: Springer, 2010.

[2] Y.-S. Lee, Principles of Terahertz Science and Technology, New York: Springer, 2009.

[3] D. H. Auston, K. P. Cheung, and P. R. Smith, “Picosecond photoconducting Hertzian dipoles,” Appl. Phys. Lett., vol. 45, 284-286 (1984).

[4] C.-K. Lee, C.-S. Yang, S.-H. Lin, S.-H. Huang, O. Wada, and C.-L. Pan, “Effects of two-photon absorption on terahertz radiation generated by femtosecond-laser excited photoconductive antennas,” Opt. Express, vol. 19, no. 24, pp. 23689-23697 (2011).

[5] C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron., in accepted pending minor revisions.