臺灣博碩士論文加值系統

為了應用在自旋電子學的領域，這篇論文完成了自旋極化的氮化鋁鎵/氮化鎵高電子遷移率電晶體結構的設計、製造和分析。利用能帶計算裡的原子軌域線性結合法和雙能帶k．p法，本論文檢視了不同應力下纖維鋅礦結構的自旋分裂能量和最小自旋分裂面與費米波向量的關係。根據這些結果，非彈道自旋電晶體的主體材料設計也在被提出。藉由最佳化鋁含量和二維電子氣載子密度, 二維電子氣的費米面將會達到最小自旋分裂面使自旋生命週期共振.

為了實現自旋電晶體，高品質的氮化鋁鎵/氮化鎵高電子遷移率電晶體結構是必要的。利用電漿輔助分子束磊晶法，本篇論文裡研究了磊晶條件對於氮化鎵磊晶層極化和結構品質的影響。在c方向上的藍寶石基板上成長一層鋁飽和的氮化鋁成核層，鎵極化的氮化鋁鎵/氮化鎵異質結構可被成功的製造；藉由成長鎵飽和的氮化鎵磊晶層，二維電子氣的介面粗糙度和差排散射也可以減少。此外，不同類型的差排對於氮化鋁鎵/氮化鎵高電子遷移率電晶體結構的電子遷移率的影響也被檢視。在低溫的環境裡，氮化鋁鎵/氮化鎵高電子遷移率電晶體結構的電子遷移率主要是被刃差排所散射而非混合錯位。

在c方向上的藍寶石基板上，利用有機金屬氣相磊晶法成長不同鋁含量 (x = 0.191~0.397) 的自旋極化氮化鋁鎵/氮化鎵高電子遷移率電晶體結構，前面提出的自旋電晶體的材料設計也被實現。高電子遷移率 (在4K下10682 cm2/Vs)、平整的介面 (表面粗糙度 < 0.5 nm)、和高品質的氮化鎵磊晶層 ((102)面的X光繞射搖擺曲線為417 arcsec) 提供了一個良好的環境來研究自旋分裂。利用Shubnikov-de Haas量測, 本論文得到了自旋分裂能量與費米波向量的關係。在鋁含量為0.39的氮化鋁鎵/氮化鎵高電子遷移率電晶體結構中，大自旋分裂能量 (10.76 meV)與最小自旋分裂面已經被成功的製造與控制，將可應用在Datta-Das自旋電晶體與非彈道自旋電晶體的主體材料。

The design, fabrication, and characterizations of the spin-polarized AlxGa1-xN/GaN HEMT structure have been achieved for spintronic application. By band calculation within linear combination of atomic orbitals and two-band k·p methods, the theoretical spin-splitting energy and minimum-spin-splitting surface of wurtzite structure have been investigated as a function of the Fermi wavevector with various strain-relaxations. Base on these results, the design of host material of the nonballistic spin-FET has also been proposed. By optimizing the Al composition and n2DEG, the Fermi surface of two-dimensional electron gas is supposed to reach the minimum-spin-splitting surface to produce resonant spin-lifetime.

Because the high quality AlxGa1-xN/GaN HEMT structure is necessary for realizing the spin-FET, the influence of the growth conditions on the polarity and structure quality of the GaN epilayer have been studied on the sample grown by plasma-assisted molecular beam epitaxy. Ga-polar AlGaN/GaN heterostructures on c-Al2O3 has been realized by growing over the Al-rich AlN nucleation layer. And the reduction of interface roughness and threading dislocation scatterings of the electrons in two-dimensional electron gas has also been achieved by growing GaN epilayer under slightly Ga-rich condition. Furthermore, the effect of different types of threading dislocation on the electron mobility of the AlxGa1-xN/GaN HEMT structure has been investigated as well. At low temperature, the electron mobility of two-dimensional electron gas in AlGaN/GaN heterostructures is majorly scattered by the edge type dislocation rather than the screw type.

The designs of proposed host material for spin-FETs have been realized through growing high quality spin-polarized AlxGa1-xN/GaN HEMT structures with various Al composition (x= 0.191 – 0.397) grown on c-Al2O3 by metalorganic vapor phase epitaxy. The high mobility (10682 cm2/Vs at 0.4 K), flat interface (surface roughness < 0.5 nm), and high quality HEMT provide a good environment to study the spin-splitting energy. To investigate the spin-splitting energy as functions of the Fermi wavevector, the Shubnikov-de Haas measurements were performed. A large spin-splitting energy (10.76 meV) has been fabricated in Al0.390Ga0.61N/GaN HEMT structure with kf = 8.14 × 108 m-1 for the host material of the Datta-Das spin-FET. And for the first time, the minimum-spin-splitting surface has been experimentally generated in Al0.390Ga0.61N/GaN HEMT structure with kf = 8.33 × 108 m-1 for the host material of the nonballistic spin-FET.

Acknowledgementt......................................................................................................................................................................ii
Abstract................................................................................................................................................................................vi
List of papers....................................................................................................................................................................viii
Acronyms............................................................................................................................................................................xi

1 Introduction..........................................................................................................................................................................1
1.1 Background......................................................................................................................................................................1
1.2 Motivation..........................................................................................................................................................................4
1.3 Outline of the thesis........................................................................................................................................................5

2 Design of the spin-FETs....................................................................................................................................................6
2.1 LCAO method and the Hamiltonian..............................................................................................................................6
2.2 Spin-splitting energy and minimum-spin-splitting surface....................................................................................13
2.3 Design of the host material..........................................................................................................................................17
2.4 Summary.........................................................................................................................................................................18

3 Polarity and quality control of the AlGaN/GaN HEMT structure grown by PA-molecular beam epitaxy..............19
3.1 Growth procedure...........................................................................................................................................................19
3.2 Polarity control.................................................................................................................................................................21
3.3 Quality control..................................................................................................................................................................25
3.4 Influence of different types of threading dislocation on the electron mobility......................................................28
3.5 Summary..........................................................................................................................................................................33

4 Characterizations of the HEMT structures with various Al compositions..................................................................35
4.1 Metalorganic vapor phase epitaxial growth..................................................................................................................35
4.2 Characterizations of the HEMT structures with various Al compositions.................................................................39
4.3 Study of the zero-field spin-splitting by Shubnikov-de Haas effect.............................................................................42
4.4 Summary................................................................................................................................................................................53

5 Summaries, conclusions and future works.......................................................................................................................54

References..................................................................................................................................................................................55
Appendix.......................................................................................................................................................................................60

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