臺灣博碩士論文加值系統

This thesis describes the huge positive magnetoresistance of GaAs/AlGaAs high- and low-mobility sample structures at high temperatures. This thesis comprises the following two parts:

1. Huge positive magnetoresistance of GaAs/AlGaAs high electron mobility transistor structures at high temperatures

We have shown experimental evidence for huge positive magnetoresistance (PMR) of our high-mobility sample structures at high temperatures. The huge PMR can be described by a model which is based on macroscopic inhomogeneity within a 2D semiconductor and permits us to evaluate the density inhomogeneity of about 5.5 % in our systems. Because a GaAs-based HEMT is nonmagnetic, it is not influenced by ferromagnetic noise which seems to appear an essential challenge to the scalability of magnetic MR devices to ultrahigh area densities. In order to prevent conductance fluctuations, our sample needs to be larger than the elastic mean free path of our device. Most importantly, our experimental results lay the foundation for the experimental realization and scalability of a future generation of MR devices based on nonmagnetic semiconductors which are not susceptible to ferromagnetic noise.

2. Huge positive magnetoresistance of a low-electron-mobility system in the GaAs/AlGaAs heterostructure at high temperatures

We have performed magnetoresistivity measurements on a GaAs two-dimensional electron system containing self-assembled InAs quantum dots at high temperatures. For the GaAs-based low-electron-mobility sample structure, we observe the negative magnetoresistance in intermediate magnetic fields (B < 2 T) at high temperatures. This low-mobility sample shows large PMR for B > 3 T. The MR value is larger than 125 % at T=80 K and less than 200 % at T=20 K. I used the model which is studied in chapter 4 to calculate the density inhomogeneity . And I found it is much larger than that in our high-mobility sample structures.

Chapter 1
Introduction to two-dimensional electron system 1

1.1 Two-dimensional electron system in a GaAs/AlGaAs heterostructure.....................1
1.2 Density of states........................................................................................................2
References.......................................................................................................................8


Chapter 2
Theoretical Background 9

2.1 Classical Hall effect..................................................................................................9
2.2 Quantum Hall effect..................................................................................................11
2.2.1 Landau levels..................................................................................................11
2.2.2 The Shubnikov-de Haas effect.......................................................................13
2.2.3 Edge states......................................................................................................16
2.2.4 Landauer-Buttiker formalism........................................................................18
References.......................................................................................................................21


Chapter 3
Sample fabrication and experimental instrument 22

3.1 Sample structure......................................................................................................22
3.2 Cryogenic system: Sorption Pumped 3He cryostat..................................................23
3.2.1 Condensation of 3He.......................................................................................24
3.2.2 The temperature control.................................................................................25
3.2.3 Measurement of the four-terminal resistance.................................................25
Reference.......................................................................................................................27


Chapter 4
Huge positive magnetoresistance of GaAs/AlGaAs high electron mobility transistor structures at high temperatures 28

4.1 Introduction.............................................................................................................28
4.2 Background: The effective-medium approximation...............................................29
4.3 Experiment...............................................................................................................31
4.4 Data analysis............................................................................................................32
4.5 Summary..................................................................................................................42
References.....................................................................................................................43


Chapter 5
Huge positive magnetoresistance of a low-electron-mobility system in the GaAs/AlGaAs heterostructure at high temperatures 44

5.1 Introduction.............................................................................................................44
5.2 Sample structure......................................................................................................45
5.3 Magnetoresistance in a low-mobility two-dimensional electron gas......................46
5.4 Density inhomogeneity............................................................................................51
5.5 Summary..................................................................................................................55
References.....................................................................................................................57

Chapter 6
Conclusions 58

Reference.......................................................................................................................60

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