兆赫輻射係指頻率約在1012 Hz附近的電磁波段，相較於其他波段的電磁波，人們對於兆赫輻射的瞭解與應用還是相當的有限。因此對於兆赫輻射的產生、偵測與應用，以及理論模型的提出與解釋等都是值得投入與探討的課題。在本論文中，除了對兆赫輻射產生與偵測的實驗系統與理論模型做一介紹外，我們將利用兆赫輻射產生與偵測實驗系統以及光調制光譜實驗系統，對臨界電場、共軛高分子材料(DB-PPV)與飽和效應對半導體微結構產生之兆赫輻射強度的影響做一研究與討論。

一般在光電導模式下認為兆赫輻射強度會與外加或內建電場的大小成正比，即電場愈大其產生的兆赫輻射強度愈強。但實驗中，當我們利用砷化鎵表面本徵N+型摻雜微結構(GaAs SIN+ Structure)的樣品做為產生兆赫輻射的輻射器(THz emitter)時，卻發現電場愈大所產生的兆赫輻射強度反而愈小，此現象與一般所認知的理論結果有所出入。主要的原因是: 當半導體材料中存在一外加或內建電場時，在低電場的情況下，自由載子的飄移速度會與電場的大小成正比；但是當電場強度超過某一個臨界值的時候，此時自由載子的飄移速度不再與電場的大小成正比，而會到達一飽和值，此一臨界值即稱之為臨界電場(critical electric field)。由參考文獻中得知砷化鎵(GaAs)的臨界電場值約為40kV/cm，實驗所使用的GaAs SIN+樣品其內建電場強度都已超過GaAs的臨界電場強度，故需考慮臨界電場對兆赫輻射強度的影響。可定義一有效電場( )為: 當內建電場( )強度小於臨界電場( )時，有效電場等於內建電場，即 ；當內建電場強度大於臨界電場時，有效電場等於臨界電場，即 。則兆赫輻射電場的強度可表示為 。經由上述的修正，我們得到的理論預期與實驗所發現的結果是相當吻合的，即臨界電場對兆赫輻射的產生具有重要的影響而且是必須考慮的重要因素。臨界電場的大小與半導體材料中的Γ能谷與L能谷的差值有關，若能利用兆赫輻射決定半導體的臨界電場，可反過來決定半導體Γ能谷與L能谷的差值(L valley offset) 。

在增強兆赫輻射強度的研究上，我們發現可以經由共軛高分子(conjugate polymer)材料2, 3- dibutoxy-1, 4-polyphenylenevinylene (DB-PPV) 對樣品表面電場的增強效應來達成增強兆赫輻射的目的。在本論文的研究中發現共軛高分子材料DB-PPV會造成半導體材料表面態密度(surface state density)的增加而使得樣品表面電場的增強，因而導致了兆赫輻射強度的增強。當我們以SI GaAs wafer作為兆赫輻射產生器時，若在其表面鍍上DB-PPV，則最大的增強幅度可達約50%左右。研究也發現共軛高分子材料DB-PPV對激發雷射光有非線性的雙光子吸收效應(two photon absorption effect)，此吸收效應造成激發雷射光強度減弱，間接導致了兆赫輻射強度的減弱。

最後，研究不同緩衝層摻雜濃度的InAlAs SIN+樣品的兆赫輻射隨激發光強度改變的關係。實驗發現，隨著激發光強度的改變，兆赫輻射會分別受到臨界電場、樣品內存能量與載子飄移率的限制，而導致兆赫輻射強度隨激發雷射光強度的增強，由線性增加而趨於飽和，最終有些許的減弱。當激發雷射光強度增強，光激發載子的密度增加導致載子飄移率降低，兆赫輻射的強度亦隨之減弱。

Terahertz radiation (THz) represents the electromagnetic waves with frequencies around 1012 Hz. In comparison with other electromagnetic waves, the characteristics of terahertz radiation have not been clearly understood and very few scientific research and practical applications have been reported. Therefore, the generation, detection, application and mechanism of terahertz radiation have become important area of studies. In this thesis we first introduce the generation and detection system and studies of the mechanism of terahertz radiation. The effects of critical electric field, conjugate polymer (DB-PPV) and saturation effect on the intensity of terahertz radiation radiated from semiconductor microstructures will also be reported.

It is widely believed that, in photoconductive mode, intensity of terahertz radiation is proportional to local field (bias) by the external applied or built-in electric field. However, when GaAs surface intrinsic-N+ (SIN+) structures are used as the terahertz emitter, the larger the bias electric field, the smaller the terahertz radiation intensity is observed. There exists the so-called “the critical electric field” in semiconductors. As the local field is below the critical electric field Ec, the maximum drift velocity of free charged photo-excited carriers in a semiconductor is proportional to the electric field in the semiconductor. However, as the field rises above the critical electric field Ec, the maximum drift velocity declines slightly as the field increases. The maximum drift velocity of the free charged carriers peaks at the critical electric field, which depends on the energy difference between the Γ to L valley (intervalley threshold, L valley offset) in the semiconductor. While the intrinsic layer thickness is less than 200 nm, the electric field of GaAs is larger than the critical field thus the drift velocity is approximately constant. The amplitude of THz radiation above the critical electric field Ec is not proportional to nphEloc but proportional to nphEc. An effective electric field Eeff can be defined, which equals to the critical field Ec as the Eloc is larger than the critical field and equals to Eloc as the Eloc is smaller than the critical field. Then, the terahertz radiation intensity can be expressed by . The dependence of THz and nphEeff on the thickness of the intrinsic layer obtained experimentally are almost identical to each other implying that there is indeed a critical electric field, Ec , in the semiconductor such that ETHz is dependent on Eloc when Eloc is smaller than Ec and is independent of Eloc when Eloc exceeds Ec. As the field is lower than the critical field, the amplitude is proportional to the product of the surface field and the number of photo-excited carriers. In the high field limit where the surface field exceeds the critical field, the amplitude of THz is independent of the surface field but is proportional to the product of the critical field and the number of the photo-excited carriers. Since the critical electric field depends on the energy difference between Γ and L valley or the L valley offset in semiconductors. The L valley offset can be estimated from critical electric field determined from THz radiation.

In the second part of this study, THz radiation from the surfaces of various semiconductor wafers and microstructures is investigated. Various polymer films are spin-cast on the surfaces of semiconductors and semiconductor heterostructures to enhance the terahertz radiation. The conjugate polymer (2, 3- dibutoxy-1, 4-polyphenylenevinylene (DB-PPV)) is found to effectively enhance the terahertz radiation intensity from the surface intrinsic GaAs wafer by as much as 50%. Changes in surface field and the density of interfacial states are detected associated with the enhancement of THz radiation by DB-PPV conjugate polymer surface-coating. For the semiconductors in which drift current dominates the diffusion current, THz radiation is enhanced by coating DB-PPV on their surfaces to increase their surface fields. For the semiconductors in which the diffusion current dominates the drift current, THz radiation is not enhanced by increasing in surface fields. The contactless and nondestructive modulation spectroscopy of photoreflectance is employed to determine the changes in surface field and the density of interfacial states which are closely related to the enhancement of THz radiation.

Finally, we study the dependence of intensity of terahertz radiation on pump beam power in InAlAs SIN+ samples with different doping concentrations in the buffer layer. The intensity of terahertz radiation increases linearly with the pump power; reaches its saturation intensity; and declines slightly in the high power region. It is found that the intensity of the terahertz radiation is restricted by the critical electric field and the energy stored in the semiconductors, which depends on the surface field within the surface intrinsic layer. The variation in the THz intensity with pump power can be attributed to the change in the mobility of the photo-excited charged carriers.

[1] James C. Wiltse, IEEE Trans. Microwave Theory and Tech., vol. MTT-32, no. 9 , p1118(1984).
[2] T. G. Phillips and J. Keene, Proceedings of the IEEE, vol. 80, p1662 (1992).
[3] M. C. Gaidis, 8th International Conference on Terahertz Electronics, p125(2000).
[4] H. Harde, R. A. Cheville and D. Grischkowsky, J. Opt. Soc. Am. B vol.14, p3282(1997)
[5] R. A. Cheville and D. Grischkowsky, J. Opt. Soc. Am. B vol.16, p317(1999)
[6] D. S. Venables, A. Chiu and C. A. Schmuttenmaer, J. Chem. Phys. vol. 113, p3243(2000)
[7] T. M. Nymand, C. Ronne and S. R. Keiding, J. Chem. Phys. vol. 114, p5246(2001)
[8] M. Li, J. Fortin, J. Y. Kim, G. Fox, F. Chu, T. Davenport, T. M. Lu and X.-C. Zhang, IEEE J. Sel. Top. Quantum Electron. vol. 7, p624(2001)
[9] D. Grischkowsky and S. Keding, Appl. Phys. Lett. vol.57, p1055(1990)
[10] C. Ronne, K. Jensby, B. J. Loughnane, J. Fourkas, O. F. Nielsen and S. R. Keiding, J. Chem. Phys. vol. 113, p3749(2000)
[11] A. G. Markelz, R. Roitberg and E. Heilweil, J. Chem. Phys. Lett. vol. 320, p42(2000)
[12] M. Brucherseifer, M. Nagel, P. H. Bolivar, H. Kurz, A. Bosserhoff and R. Buttner, Appl. Phys. Lett. vol. 77, p4049(2000)
[13] G. P. Gallerano, A. Doria, E. Giovenale and A. Renieri, Infrared Phys. and Tech. vol. 40, p161 (1999).
[14] G. Mourou, C. V. Stancampiano, and D. Blumenthal, Appl. Phys. Lett. vol.38, p470 (1981).
[15] G. Mourou, C. V. Stancampiano, A. Antonetti, and A. Orszag, Appl. Phys. Lett. vol. 39, p295 (1981).
[16] R. Heidemann, T. Pfeffer, and D. Jager, Electron Lett. vol. 19, p316 (1983).
[17] D. H. Auston, K. P. Cheung, and P. R. Smith, Appl. Phys. Lett. vol.45, p284 (1984).
[18] L. Xu, X. C. Zhang, D. H. Auston, Appl. Phys. Lett. vol.59, p3357 (1991).
[19] H. G. Roskos, M. C. Nuss, J. Shah, K. Leo, D. A. Miller, A. M. Fox, S. Schmitt-Rink, and K. Khler, Phys. Rev. Lett. vol.68, p2212 (1992).
[20] X. C. Zhang, B. B. Hu, J. T. Darrow, and D. H. Auston, Appl. Phys. Lett. vol.56, p1011 (1990).
[21] M. van Exter, C. Fattinger, and D. Grischkowsky, Appl. Phys. Lett. vol.55, p337 (1989).
[22] Y. Pastol, G. Arjavalingam, and J. M. Halbout, Elect. Lett. vol.26, p133 (1990).
[23] D. R. Dykar, B. I. Greene, J. F. Federci, A. F. J. Levi, L. N. Pfeffer, and R. F. Kopf, Appl. Phys. Lett. vol.59, p262 (1991).
[24] Q. Wu, and X. C. Zhang, Appl. Phys. Lett. vol.67, p2523 (1995)
[25] Q. Wu, and X. C. Zhang, Appl. Phys. Lett. vol.68, p1604 (1996)
[26] Q. Wu, and X. C. Zhang, Appl. Phys. Lett. vol.68, p2924 (1996)
[27] Q. Wu, T. D. Hewitt, and X. C. Zhang, Appl. Phys. Lett. vol.69, p1026 (1996)
[28] Q. Wu, and X. C. Zhang, Appl. Phys. Lett. vol.71, p1285 (1997)
[29] X. C. Zhang, B. B. Hu, J. T. Darrow, and D. H. Auston, Appl. Phys. Lett. vol.56, p1011 (1990).
[30] S. L. Chuang, S. Schmitt-Rink, B. I. Greene, P. N. Saeta, and A. F. J. Levi, Phys. Rev. Lett. vol.68, p102 (1992).
[31] N. Karpowicz, H. Zhong, C. Zhang , K. I. Lin , J. S. Hwang , J. Xu , and X.-C. Zhang, Appl. Phys. Lett. vol.86, p054105 (2005).
[32] G. Gagliardi, S. Viciani, M. Inguscio, P. De Natale, c. Gmachl, F. Capasso, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson and A. Y. Cho, Opt. Lett. vol. 27, p521(2002)
[33] A. Tredicucci, C. Gmachl, F. Capasso, M. C. Wanke, A. L. Hutchinson, D. L. Sivco, S.-N. G. Chu and A. Y. Cho, Opt. Mat. vol. 17, p211(2001)
[34] X.-C. Zhang and D.H. Asuton , J. Appl. Phys. vol. 71, p326(1992)
[35] X.-C. Zhang, Y. Jin and X. F. Ma, Appl. Phys. Lett. vol. 61, p2764(1992)
[36] A. Rice, Y. Jin, X. F. Ma and X.-C. Zhang, Appl. Phys. Lett. vol. 64, p1324(1994)
[37] J. D. Jackson, Classical Electrodynamics, 3rd ed.,1998
[38] X.-C. Zhang and D.H. Asuton , J. Appl. Phys. vol. 71, p326(1992)
[39] P. Gu, M. Tani, S. Kono, and K. Sakai, in 8th International Conference on Terahertz Electronics ~VDE Verlag, Berlin-Offenbach, pp. 63(2000)
[40] S. Kono, P. Gu, M. Tani and K. Sakai, Appl. Phys. B vol. 71, p901(2000)
[41] X.-C. Zhang and D. H. Auston, J. Appl. Phys. vol. 71, p326(1992)
[42] T. Dekorsy, T. Pfeifer, W. Ku¨tt and H. Kurz, Phys. Rev. B vol. 47, p3842(1993)
[43] J. N. Heyman, N. Coates and A. Reinhardt, Appl. Phys. Lett. vol. 83, p5476(2003)
[44] M. Bass, P. A. Franken, J. F. Ward and G. Weinreich, Phys. Rev. Lett. vol. 9, p446(1962)
[45] J. Morris and Y. R.. Shen, Opt. Commun. vol. 3, p81(1971)
[46] S. L. Chuang, S. Schmitt-Rank, B. I. Greene, P. N. Saeta and A. F. J. Levi, Phys. Rev. Lett. vol. 68, p102(1992)
[47] B. I. Greene, P. N. Saeta, D. R. Dykaar, S. Schmitt-Rank and S. L. Chuang, IEEE J. Quantum Electron. vol. 28, p2302(1992)
[48] Y. R. Shen, The Principles of Nonlinear Optic, Wiley-Interscience Publication, New York, 1984
[49] R.W. Boyd, Nonlinear Optic, Academic Press, 1992
[50] Hecht, Optics (third edition)
[51] G.Gallot and D. Grischkowsky, J. Opt. Soc. Am. B, vol. 16, No. 8 (1999)
[52] P. R. Smith, D. H. Auston and M. C. Nuss, IEEE J. Quantum Electron. QE-24, 255(1988)
[53] Q. Wu and X.-C. Zhang, Appl. Phys. Lett. vol. 68, p1604(1996)
[54] Q. Wu and X.-C. Zhang, Appl. Phys. Lett. vol. 70, p1784(1997)
[55] W. L. Faust and C. H. Henry, Phys. Rev. Lett. vol. 71, p1265(1966)
[56] P. C. M. Planken, H . K. Nienhuys, H. J. Baker and T. Wenckebach, J. Opt. Soc. Am. B vol. 18, p313(2001)
[57] 張仲志，光激發SIN+ GaAs、InAlAs與氧化物-GaAs半導體產生兆赫輻射性質之研究，國立成功大學博士論文 (2004)
[58] 黃文啟，光調制光譜及拉曼光譜研究光電材料之表面特性，國立成功大學博士論文 (2001)
[59] 林光儀，螢光光譜、光調制光譜及拉曼光譜研究氮磷化銦鎵/砷化鎵異質結構之光電特性，國立成功大學博士論文 (2006)
[60] F. H. Pollak, Proc. Soc. Photo-Optical Instrum. Eng. vol. 276, p142(1981)
[61] F. H. Pollak and H. Shen, J. Cryst. Growth vol. 98, p53(1989)
[62] D. E. Aspnes, Handbook on Semiconductor 2, 109(1980)
[63] D. E. Aspnes, Phys. Rev. B vol. 10, p4228(1974)
[64] D. E. Aspnes, Surf. Sci. vol. 37, p418(1973)
[65] K. Seeger, Semiconducto Physics,學風科學圖書出版社，新竹， 1986
[66] H. Shen and F. H. Pollak, Phys. Rev. B vol. 42, p7097(1990)
[67] N. Botta, D. K. Gaskill, R. S. Sillmon, R. Henry and R. Glosser, J. Electron. Mater. vol. 17, p161(1988)
[68] K. S. Viswanathan and J. Callaway, Phys. Rev. vol. 143, p564(1966)
[69] D. E. Aspnes and A. A. Studna, Phys. Rev. B vol. 7, p4605(1973)
[70] J. S. Hwang, W. C. Hwang, Z. P. Yang and G. S. Chang, Appl. Phys. Lett. vol. 75, p2467(1999).
[71] J. S. Hwang, K. I. Lin, H. C. Lin, S. H. Hsu, K. C. Chen, and Y. T. Lu, Y. G. Hong and C. W. Tu, Appl. Phys. Lett. vol. 86, p061103(2005).
[72] H. Shen, M. Dutta, R. Lux, W. Buchwald, and L. Fotiadis, Appl. Phys. Lett. vol. 59, p321(1991).
[73] H. Shen, M. Dutta, L. Fotiadis, P. G. Newman, R. Moerkirk, W. H.Chang, and R. N. Sacks, Appl. Phys. Lett. vol. 57, p2118(1990).
[74] X. Yin, H. M. Chen, F. H. Pollak, Y. Chen, P. A. Montano, P. D. Kirchner, G. D. Pettit, and J. M. Woodall, Appl. Phys. Lett. vol. 58, p260(1991).
[75] J. R. Hook & H. E. Hall, Solid State Physics, Second Edition, John Wiley & Sons
[76] 施敏，半導體元件物理與製作技術，新竹
[77] J. N. Heyman, N. Coates, and A. Reinhardt, Appl. Phys. Lett. vol. 83, p5476(2003).
[78] S. M. Sze, Semiconductor Devices Physics and Technology, Wiley, New York, 1985
[79] M. V. Fischetti, IEEE Tans. Electron Devices ED38, p634(1991)
[80] K. Brennan and K. Hess, Solid State Electron. vol. 27, p347(1984)
[81] R. Dittrich and W. Schroeder, Solid State Electron. vol. 43, p403(1999)
[82] A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss and W. H. Knox, Appl. Phys. Lett. vol. 74, p1516(1999).
[83] A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss and W. H. Knox, Phys. Rev. Lett. vol. 82, p5140(1999).
[84] A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss and W. H. Knox, Phys. Rev. B. vol. 61, p16642(1999).
[85] E. Budiarto, J. Margolies, S. Jeong, J. Son and J. Bokor, IEEE J. Quantum Electro. vol. 32, p1839(1996)
[86] J. H. Son, T. B. Norris and J. F. Whitaker, J. Opt. Soc. Am. B vol. 11, p2519(1994)
[87] M. L. Tu, Y. K. Su, W. C. Lu, H. Yang, T. F. Kuo and T. C. Wen, Jpn. J. Appl. Phys, vol. 44, p7482 (2005)
[88] J. Darmo, G. Strasser, T. Muller, R. Bratshitsch, and K. Unterrainer, Appl. Phys. Lett. vol. 81, p871 (2002).
[89] C. Weiss, R. Wallenstein, and R. Beigang, Appl. Phys. Lett. vol. 77, p4160 (2000).
[90] Alexander M. Sinyukov, Megan R. Leahy, and L. Michael Hayden, Appl. Phys. Lett. vol. 85, p5827(2004).
[91] M. Nakajima, Y. Oda and T. Suemoto, Appl. Phys. Lett. vol. 85, p2694(2004).
[92] J. S. Hwang, H. C. Lin, K. I. Lin, and X.-C. Zhang, Appl. Phys. Lett. vol. 87, p1107 2005).
[93] J. R. Hook & H.E. Hall, Solid State Physics (John Wiley & Sons, 1991), Chap. 5.
[94] X. Yin, H. M. Chen, F. H. Pollak, Y. Chan, P. A. Montano, P. D. Kirchner, G. D. Pettit and J. M. Woodall, J. Vac. Scl. Technol. A10, p131(1992)
[95] H. Shen and M. Dutta, J. Appl. Phys. vol. 78, p2151(1995)
[96] J. S. Hwang, C. C. Chang, M. F. Chen, C. C. Chen, K. I. Lin, F. C. Tang, M. Hong and J. Kwo, J. Appl. Phys. vol. 94, p348(2003)
[97] W. Sha, J. K. Rhee, IEEE J. Quantum Elec. vol. 28, p2445(1992)
[98] X. C. Zhang and D. H. Auston, J. Appl. Phys. vol. 71, p326(1992)
[99] M. Balu, J. Hales, D. J. Hagan, E. W. Van Stryland, Opt. Express vol. 12, p3820 (2004)
[100] A. Othonos, J. Appl. Phys. vol. 83, p1789(1998).
[101] A. J. Sabbah and D. M. Riffe, Phys. Rev. B. vol. 66, p165217 (2002).