稀磁性半導體結合了具有磁性以及可操控電子自旋方向的特性。氧化鋅是其中一個非常具有潛力能夠在室溫應用的材料。鈷摻雜氧化鋅曾被提及其鐵磁性的機制以及氧缺陷能有有效的增益鐵磁交互作用。然而，自旋電流以及氧缺陷之間的關聯性卻鮮少被提及。在我們的研究中，我們自己備製鈷摻雜氧化鋅靶材，利用射頻磁控濺鍍系統並控制成長氣氛中的氬氣以及氫氣的比例來成長一系列不一樣氧缺陷濃度的鈷摻雜氧化鋅薄膜。(5%, 10%, 20% and 30%比例氫氣分壓以及氬氣氛，並以CZO -#%來代表樣品)。
我們利用點接觸Andreev reflection來探測薄膜各處並同時量測電導對電壓的曲線。獲得曲線後利用彈道式以及擴散式的MBTK模型來模擬我們的數據，藉由此模擬來獲得自旋即化率。考慮到我們的樣品電阻很大，導致電導對電壓的曲線寬化。因此我們引入兩個額外的參數-等效溫度、額外電阻。模擬結果指出我們引入的額外輛個參數能有效地增進數據的擬合度，且彈道式的MBTK模型跟我們的數據擬合較好。CZO-30%的樣品具有高達73%的自旋極化率，CZO-20%也有63%的自旋極化率。這些結果指出電導對電壓圖譜被抑制是因為氧缺陷的濃度，且CZO-20%所包含的氧缺陷濃度剛好能夠形成充足的通道而被探測到最低濃度。因此我們可以推論自旋即化率的增益主要是因為載子以跳耀式的傳導機制沿著氧缺陷所形成的通道的增加並被氧缺中的磁性離子偏極化產生更多自旋電流。

Dilute magnetic semiconductor (DMO) combines spin-polarization and magnetic properties in semiconducting state. One of the compound base on ZnO have potential for application due to it can work at room temperature. Co doped ZnO (CZO) have been reported its ferromagnetic coupling mechanism and including oxygen vacancies can effectively enhance the ferromagnetic coupling. However, it’s rare to mention how the spin-polarized current associate with oxygen vacancies. For this study, we fabricate the CZO films via Zn0.95Co0.05O target and standard sputtering in various mixture atmosphere. (5%, 10%, 20% and 30% H2 partial and Ar atmosphere, and denoted the films as CZO -#%).
We use Point-contact Andreev reflection probe various spots of film and obtained conductance(G)-voltage(V) curves and extract the spin polarization values (P) from G-V curves via modified Blonder–Tinkham–Klapwijk (MBTK) model in ballistic and diffusive regime. Considering our present samples accompany relative high resistance which lead to the broadening G-V curves, we include two extra fitting parameters, spreading resistance (Γ) and effective temperature (Teff). The simulations indicate the extra parameters are curial for our G-V curves and have better fitting agreement in ballistic limit. The extracted P in ballistic regime are up to 73% and 60% for CZO-30% and CZO-20%, respectively. This result suggests G-V spectra are suppressed as oxygen vacancy concentration increasing and indicates effective percolating paths have been formed as concentration of oxygen vacancies reach to the level as CZO-20% contained. This reinforcement of P is attribute to hopping transport with localized states initiated by part of oxygen vacancies.

Contents
論文審定書 i
博士論文公開授權書 ii
中文摘要 iii
Abstract iv
List of Figures ix
List of Tables xxi
Chapter 1 introduction 1
1.1 Ferromagnetism and exchange interaction 3
1.1.1 Molecular field theory 3
1.1.2 Exchange interaction 4
1.1.3 Exchange in insulators 5
1.1.4 Exchange in metals 7
1.2 The role of oxygen vacancies in Dilute Magnetic Oxide 9
Chapter 1 References 15
Chapter 2 Determination of spin polarization 17
2.1 Distinction of magnetization and spin polarization 17
2.2 Mott scattering 19
2.3 Spin-dependent tunnel junction 22
2.4 Spin-LED 27
Chapter 2 References 30
Chapter 3 Point contact Andreev reflection 32
3.1 Point contact in different regions 32
3.2 Superconductivity & Bogoliubov Equation 36
3.3 Andreev reflection 40
3.4 BTK and Modified BTK 42
3.4.1 BTK Model 42
3.4.2 Modified BTK Model 50
3.4.3 The role of the P Z T parameter in ballistic and diffusive limit 55
Chapter 3 References 61
Chapter 4 Spectra broadening effect 63
4.1 Quasiparticle lifetime effect 63
4.2 Comparison of probing spin polarization via planner and point contact geometry setup 65
4.3 Additional resistance in Andreev reflection spectroscopy 70
Chapter 4 References 72
Chapter 5 Experimental and fitting Methodology 74
5.1 Spin polarization of metal state ZnO 74
5.2 Methodologies of fabrication of VO CZO 75
5.2.1 Thin film fabrication and verification of the existence of VO 75
5.3 Superconducting tip fabrication and PCAR setup 79
5.3.1 Tip fabrication 79
5.3.2 Superconducting tip fabrication, PCAR setup and G-V measurement 82
5.3.3 Methodology of getting effective G-V curve 84
5.4 Modeling and fitting 85
Chapter 5 References 93
Chapter 6 Result and discussion 95
6.1 The correlation between G-V spectra features and various oxygen vacancies concentration 95
6.2 Fitting approach of CZO-30% and CZO-20% data 97
6.3 P-Z relation of CZO-30% and -20% data 101
Chapter 6 References 106
Chapter 7 Summary 108
Supplement: 110
Part A 110
Part B 115

Chapter 1 References
[1.1] V. Srikant and D. R. Clarke, J. Appl. Phys. 83, 5447 (1998).
[1.2] M. N. Kamalasanan and S. Chandra. Thin Solid Films 288, 112 (1996).
[1.3] Anderson Janotti and Chris G Van de Walle, Rep. Prog. Phys. 72, 126501 (2009).
[1.4] D. C. Reynolds, D. C. Look and B. Jogai, Solid State Commun. 99, 873 (1996).
[1.5] T. Dietl, H. Ohno, F. Matsukura,1 J. Cibert, D. Ferrand Science 287, 1019 (2000).
[1.6] Pearton, S. J. et al. Wide band gap ferromagnetic semiconductors and oxides J. Appl. Phys. 93, 1 (2003)
[1.7] Qingyu Xu, Lars Hartmann, Shengqiang Zhou, Arndt Mcklich, Manfred Helm, Gisela Biehne, Holger Hochmuth, Michael Lorenz, Marius Grundmann, and Heidemarie Schmidt, Phys. Rev. Lett., 101, 076601 (2008).
[1.8] Karen A. Yates, Anthony J. Behan, James R. Neal, David S. Score, Harry J. Blythe, Gillian A. Gehring, Steve M. Heald, Will R. Branford, and Lesley F. Cohen, Phys. Rev. B 80, 245207 (2009).
[1.9] G. Ciatto, A. Di Trolio, E. Fonda, P. Alippi, A. M. Testa, and A. Amore Bonapasta Phys. Rev. Lett. 107, 127206 (2011).
[1.10]H. S. Hsu, J. C. A. Huang, Y. H. Huang, Y. F. Liao, M. Z. Lin, C. H. Lee, J. F. Lee, S. F. Chen, L. Y. Lai, and C. P. Liu Appl. Phys. Lett. 88, 242507.
[1.11]L. Li, Y. Guo, X. Y. Cui, R. Zheng, K. Ohtani, C. Kong, A. V. Ceguerra, M. P. Moody, J. D. Ye, H. H. Tan, C. Jagadish, H. Liu, C. Stampfl, H. Ohno, S. P. Ringer, and F. Matsukura, Phys. Rev. B 85, 174430.
[1.12]P. Weiss, L’hypoth`ese du champ mol´eculaire et la propri´et´e ferromagn´etique, J. de Phys. 6, 661 (1907).
[1.13]Heisenberg, W. Z. Physik 38, 411(1926).
[1.14]P. W. Anderson, Phys. Rev. 79, 350 (1950).
[1.15]J. M. D. Coey "Magnetism and Magnetic Materials" (Cambridge Univ. Press).
[1.16]G. Zener, Phys. Rev. 81, 446 (1951).
[1.17]M. A. Ruderman, C. Kittel, Physical Review. 96, 99 (1954).
[1.18]J. M. D. Coey, M. Venkatesan and C. B. Fitzgerald Nature materials 4, 173 (2005).
[1.19]Mott, N. F. "Conduction in Noncrystalline Materials" (Oxford Univ. Press, Oxford, 1987).
[1.20]H. Chou, C. P. Lin, J. C. A. Huang, and H. S. Hsu, Phys. Rev. B 77, 245210 (2008).

Chapter 2 References
[2.1]Attila Geresdi’s graduation thesis from Department of Physics Budapest University of Technology and Economics.
[2.2]N. F. MOTT, H. S. W. MASSEY: The Theory of Atomic Collisions (Clarendon Press, Oxford 1965) Chapt. IX
[2.3] Joachim Kessler Polarized Electrons Springer-Verlag Berlin Heidelberg New York in (1976)
[2.4] Tedrow P M, Meservey R, and Fulde P 1970 Phys. Rev. Lett. 25 1270; Tedrow P M and Meservey R 1971 Phys. Rev. Lett. 26 192; 1973 Phys. Rev. B 7 318.
[2.5] Meservey R and Tedrow P M 1994 Phys. Rep. 238 173.
[2.6] Moodera J S, Kinder L R, Wong T M, and Meservey R 1995 Phys. Rev. Lett. 74 3273.
[2.7] Qingyu Xu, Lars Hartmann, Shengqiang Zhou, Arndt Mcklich, Manfred Helm, Gisela Biehne, Holger Hochmuth, Michael Lorenz, Marius Grundmann, and Heidemarie Schmidt, Phys. Rev. Lett., 101, 076601 (2008).
[2.8] Meservey R and Tedrow P M 1994 Phys. Rep. 238 173.
[2.9] F. Meier and B. P. Zachachrenya, Optical Orientation (North-Holland Amsterdam, 1984), Vol. 8.
[2.10]B. T. Jonker, Y. D. Park, R. Bennett, H. D. Cheong, G. Kioseoglou, and A. Petrou, Phys. Rev. B 62, 8180 (2000).

Chapter 3 References
[3.1]L. Wang, T. Y. Chen, and C. Leighton, Phys. Rev. B 69, 094412 (2004).
[3.2]S. Gardelis, J. Androulakis, P. Migiakis, J. Giapintzakis, S. K. Clowes, Y. Bugoslavsky, W. R. Branford, Y. Miyoshi and L. F. Cohen, J. Appl. Phys. 95, 8063 (2004).
[3.3]G.Wexler, Proc. Phys. Soc. London 89,927 (1966)
[3.4]J. Bardeen, L.N. Cooper, J.R. Schrieffer: Phys. Rev. 108, 1175 (1957)
[3.5]http://inspirehep.net/record/1181776/plots,https://www.pinterest.com/pin/322851867015772623/
[3.6]N. N. Bogoliubov. On the theory of super°uidity. J. Phys. (USSR), 11:23, 1947.
[3.7]Thomas Schaipers: Superconductor/Semiconductor Junctions (Springer tracts in modern physics; Vol. 174) Chapt. II
[3.8]A.F. Andreev: Zh. ’Eksp. Teor. Fiz. 46, 1823 (1964) [Sov. Phys. JETP 19,1228 (1964)]
[3.9]G.E. Blonder, M. Tinkham, T.M. Klapwijk: Phys. Rev. B 25, 4515 (1982).
[3.10]Soulen Jr R J, Byers J M, Osofsky M S, Nadgorny B, Ambrose T, Cheng S F, Broussard P R, Tanaka C T, Nowak J, Moodera J S, Barry A, and Coey J M D Science 282 85 (1998)
[3.11]G. J. Strijkers, Y. Ji, F. Y. Yang, C. L. Chien, and J. M. Byers, Phys. Rev. B 63,104510 (2001).
[3.12]I. I. Mazin, A. A. Golubov, and B. Nadgorny, J. Appl. Phys. 89, 7576 (2001).

Chapter 4 References
[4.1]G. T. Woods, R. J. Soulen, Jr., I. Mazin,1 B. Nadgorny, M. S. Osofsky, J. Sanders, H. Srikanth, W. F. Egelhoff, and R. Datla, Phys. Rev. B. 70, 054416 (2004).
[4.2]T. W. Chiang, Y. H. Chiu, S. Y. Huang, S. F. Lee, J. J. Liang, H. Jaffrès, J. M. George, and A. Lemaitre, J. Appl. Phys. 105, 07C507 (2009).
[4.3]R. P. Panguluri, K. C. Ku, N. Samarth, T. Wojtowicz, X. Liu, J. K. Furdyna, Y. Lyanda-Geller, and B. Nadgorny, Phys. Rev. B 72, 054510 (2005).
[4.4]I. Žutić, and S. Das Sarma, Phys. Rev. B 60, R16322 (1999).
[4.5]A. Pleceník, M. Grajcar, Š. Beňačka, P. Seidel and A. Pfuch, Phys. Rev. B 49, 10016 (1994).
[4.6]J. Braden, J. Parker, P. Xiong, S. Chun, and N. Samarth, Phys. Rev. Lett. 91, 056602 (2003).
[4.7]I. Zutic and S. Das Sarma, Phys. Rev. B 60, R16 322 (1999).
[4.8]T. W. Chiang, Y. H. Chiu, S. Y. Huang, S. F. Lee, J. J. Liang, H. Jaffrès, J. M. George, and A. Lemaitre, J. Appl. Phys. 105, 07C507 (2009).
[4.9]T. Y. Chen, S. X. Huang, and C. L. Chien, Phys. Rev. B 81, 214444 (2010).
[4.10]G. T. Woods, R. J. Soulen, Jr., I. I. Mazin, B. Nadgorny, M. S. Osofsky, J. Sanders, H. Srikanth, W. F. Egelhoff, and R. Datla, Phys. Rev. B 70, 054416 (2004).
[4.11]J. S. Parker, S. M. Watts, P. G. Ivanov, and P. Xiong, Phys. Rev. Lett. 88, 196601 (2002).

Chapter 5 References
[5.1]Karen A. Yates, Anthony J. Behan, James R. Neal, David S. Score, Harry J. Blythe, Gillian A. Gehring, Steve M. Heald, Will R. Branford, and Lesley F. Cohen, Phys. Rev. B 80, 245207 (2009).
[5.2]Hsiung Chou, Kung-Shang Yang, Yao-Chung Tsao, G. D. Dwivedi, Cheng-Pang Lin, Shih-Jye Sun, L. K. Lin, and S. F. Lee, Appl. Phys. Lett. 108, 142404 (2016).
[5.3] 邱亦欣 “Determination of Spin Polarization of Ferromagnetic Fe3Si Thin Film”清大物理系碩士班應用物理組 畢業論文
[5.4]Adam Daire, An Improved Method for Differential Conductance Measurements, Keithley White Paper.
[5.5]K. S. Yang , T. Y. Huang , G. D. Dwivedi, L. K. Lin, S. F. Lee, S. J. Sun, and H. Chou “Direct observation of hopping induced spin polarization current in oxygen deficient Co-doped ZnO by Andreev reflection technique” under reviewing.
[5.6]G. J. Strijkers, Y. Ji, F. Y. Yang, C. L. Chien, and J. M. Byers, Phys. Rev. B 63, 104510 (2001).
[5.7]Ya-Fen Hsu, Tian-Wei Chiang, Guang-Yu Guo, Shang-Fan Lee, Jun-Jih Liang, J. Phys. Soc. Jpn. 81, 084704 (2012)
[5.8]N. Auth, G. Jakob, T. Block, and C. Felser, Phys. Rev. B 68, 024403 (2003).
[5.9]I. I. Mazin, A. A. Golubov, and B. Nadgorny, J. Appl. Phys. 89, 7576 (2001).
[5.10]T. Y. Chen, S. X. Huang, and C. L. Chien, Phys. Rev. B 81, 214444 (2010).
[5.11]G. T. Woods, R. J. Soulen, Jr., I. Mazin,1 B. Nadgorny, M. S. Osofsky, J. Sanders, H. Srikanth, W. F. Egelhoff, and R. Datla, Phys. Rev. B. 70, 054416 (2004).
[5.12]I. Zutic and S. Das Sarma, Phys. Rev. B 60, R16 322 (1999).

Chapter 6 References
[6.1]I. Zutic and S. Das Sarma, Phys. Rev. B 60, R16 322 (1999).
[6.2]Ji Y, Strijkers G J, Yang F Y, Chien C L, Byers J M, Anguelouch A, Xiao G, and Gupta A 2001 Phys. Rev. Lett. 86 5585.
[6.3]J.G. Braden, J. S. Parker, P. Xiong Chun, and N. Samarth, Phys. Rev. Lett. 91, 056602 (2003).
[6.4]T. W. Chiang, Y. H. Chiu, S. Y. Huang, S. F. Lee, J. J. Liang, H. Jaffrès, J. M. George, and A. Lemaitre, J. Appl. Phys. 105, 07C507 (2009).
[6.5]Y. Ji, G. J. Strijkers, F. Y. Yang, C. L. Chien, J. M. Byers, A. Anguelouch, G. Xiao, and A. Gupta, Phys. Rev. Lett. 86, 5585 (2001).
[6.6]G. J. Strijkers, Y. Ji, F. Y. Yang, C. L. Chien, and J. M. Byers, Phys. Rev. B 63, 104510 (2001).
[6.7]C. H. Kant, O. Kurnosikov, A. T. Filip, P. LeClair, H. J. M. Swagten, and W. J. M. de Jonge, Phys. Rev. B 66, 212403 (2002).
[6.8]R. P. Panguluri, K. C. Ku, N. Samarth, T. Wojtowicz, X. Liu, J. K. Furdyna, Y. Lyanda-Geller, and B. Nadgorny, Phys. Rev. B 72, 054510 (2005).
[6.9]Hsiung Chou, Kung-Shang Yang, Yao-Chung Tsao, G. D. Dwivedi, Cheng-Pang Lin, Shih-Jye Sun, L. K. Lin, and S. F. Lee, Appl. Phys. Lett. 108, 142404 (2016).
[6.10]H. Chou, C. P. Lin, J. C. A. Huang, and H. S. Hsu, Phys. Rev. B 77, 245210 (2008).