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研究生:林明谷
研究生(外文):Ming-Gu Lin
論文名稱:砷化鎵之自旋極化與氮化鎵之微波調制傳輸
論文名稱(外文):Spin polarization in GaAs and microwave-modulated transport in GaN
指導教授:梁啟德
指導教授(外文):Chi-Te Liang
學位類別:碩士
校院名稱:國立臺灣大學
系所名稱:物理研究所
學門:自然科學學門
學類:物理學類
論文種類:學術論文
論文出版年:2004
畢業學年度:92
語文別:英文
論文頁數:65
中文關鍵詞:砷化鎵微波調制氮化鎵自旋極化
外文關鍵詞:GaAsspin polarizationmicrowave-modulatedGaN
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本論文研究砷化鎵之自旋極化與氮化鎵之微波調制傳輸。內容包括以下二部分:

1.在外加平行磁場下稀薄的砷化鎵電子系統之遷移率與載子濃度相關性:

第一部分的結果是有關高品質具閘極二維電子氣的低溫磁阻測量。在稀薄電子濃度極限下,我們發現在平行磁場下自旋極化明顯。使用簡單的模型,我們估計此稀薄二維電子氣的藍道g係數大概是3.32。這個藍道g係數比一般砷化鎵二維電子系統(0.44)還大的原因是認為在極低電子濃度之電子與電子的交互作用效應所造成的,而且在整個測量範圍內庫侖位能與動能比值rs並沒有值得注意地變化。此外我們還研究在不同平行磁場下稀薄電子氣的遷移率μ與電子濃度n之相關性。我們發現在 關係下指數項?H著平行磁場增加而變大。我們認為此效應的原因是由於散射增加與隨著平行磁場增加的自旋極化所產生。

2.微波調制技術之氮化鋁鎵/氮化鎵的類似Shubnikov-de Haas振盪

第二部分的結果是有關在微波調制技術下氮化鎵/氮化鋁鎵類似Shubnikov-de Haas(SdH)振盪訊號測量。在保持載子濃度不變下此技術可以大大地增加類似SdH振盪的明顯性。此增大的SdH振盪圖紋歸因於電子熱效應。我們發現類似SdH振盪的振幅會隨著微波必v變大而增加。在我們這個實驗中,類似SdH振幅的對數值與外加垂直磁場的倒數值成正比,且明顯地與傳統Dingle圖相似。而且這個類似Dingle圖的斜率對於不同的微波必v幾乎是定值。然而,在我們這個實驗中,在微波產生脈衝過程中電子具有的等效溫度比晶格溫度還高。在微波脈衝結束後電子又回到原本與晶格溫度相同時之溫度。所以在整個測量過程中電子溫度不是定值。因此我們新實驗數據說明了現有理論的缺乏且希望有更進一步的理論研究在現有微波調制技術之電子傳輸。
This dissertation describes the measurements on the spin polarization in GaAs and microwave-modulated transport in GaN electron systems. This dissertation consists of the following two parts.

1.Mobility dependence on carrier density in a dilute GaAs electron gas in an in-plane magnetic field

I report low-temperature magnetoresistivity measurements of a high-quality gated two-dimensional electron gas (2DEG). In the dilute electron density limit, we show evidence for spin polarization in an in-plane magnetic field. Using a simple model, we estimate the Landé g-factor in this dilute 2DEG to be about 3.32. This enhanced Landé g-factor compared with that of a bulk GaAs 2D electron system (0.44) is ascribed to electron-electron interaction effects at ultra-low electron densities and the fact that over the whole measurement range rs does not vary significantly. Moreover, we report the mobility μ dependence on electron density n of a dilute electron gas at different in-plane magnetic fields. It is found that exponent ?in the relation increases with increasing in-plane magnetic field. We ascribe this effect to the combination of increasing scattering and spin polarization with increasing parallel magnetic field.

2.Microwave-modulated Shubnikov-de Haas-like oscillations in an Al0.4Ga0.6N/GaN electron system

I report measurements of microwave-modulated Shubnikov-de Haas (SdH)-like oscillations in a GaN/AlGaN heterostructure. This technique greatly enhances the visibility of the SdH-like oscillations while keeping the carrier density constant. The enhanced SdH pattern is attributed to the hot electron effect. We find that the amplitudes of the SdH-like oscillations increases with increasing microwave power. In our case, the logarithm of the SdH-like amplitudes is proportional to the inverse of the applied perpendicular magnetic field, strikingly similar to a conventional Dingle plot. Moreover, the slopes of the Dingle-like plots are approximately constant at different applied microwave powers. However, in our case, during the microwave pulse the electrons possess an equivalent temperature higher than the lattice temperature. After the end of the microwave pulse the electrons relax to the lattice temperature. Therefore the electron temperature is not constant over the whole measurement range. Thus our new experimental results point to deficiency in existing theory and urge further theoretical studies on electron transport in the presence of microwave modulation.
Chapter 1 Introduction......................................1
1.1 GaAs two-dimensional electron gas ......................1
1.1.1 Properties of GaAs devices ...........................1
1.1.2 The modulation doped GaAs/AlGaAs heterostructure......2
1.2 Density of states.......................................7
1.3 AlGaN/GaN electron system...............................9

Chapter 2 Theoretical background............................12
2.1 Classical Hall effect...................................12
2.2 Hall bar mesa patterned in a heterostructure wafer......14
2.3 Quantum Hall effect.....................................15
2.3.1 Landau levels and Shubnikov-de Haas oscillations......16
2.3.2 Quantum Hall effect...................................20

Chapter 3 Sample fabrication and measurement techniques.....24
3.1 Sample fabrication......................................24
3.1.1 Sample structure......................................24
3.1.2 Optical lithography...................................25
3.2 Cryogenic system: Sorption pumping 3He cryostat.........27
3.2.1 Condensation of He....................................28
3.2.2 Controlling the temperature...........................29
3.3 Measurement set-up and four-terminal resistance.........29

Chapter 4 Mobility dependence on carrier density in a dilute GaAs electron gas in an in-plane magnetic field.............33
4.1 Introduction............................................33
4.2 Spin polarization.......................................34
4.3 Density of state varies by applying an in-plane magnetic field.......................................................38
4.4 Changing the carrier densities..........................40
4.5 The ratio of Coulomb energy to kinetic energy...........42
4.6 Results and discussions.................................43
4.6.1 Measurements of the diagonal resistivity in an in-plane magnetic field..............................................43
4.6.2 Estimating the g-factor in an in-plane magnetic field model.......................................................47
4.6.3 Mobility dependence on electron density of a dilute electron gas at different in-plane magnetic fields..........49
4.7 Summary.................................................50

Chapter 5 Microwave-modulated Shubnikov-de Haas-like oscillations in an Al0.4Ga0.6N/GaN electron system......................................................53
5.1 Introduction............................................53
5.2 Experiment..............................................54
5.3 Results and discussions.................................56
5.3.1 Microwave-modulated SdH oscillations..................56
5.3.2 Microwave-modulated SdH oscillations dependence on microwave power.............................................58
5.4 Summary.................................................61

Chapter 6 Conclusions.......................................64
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