Electron heating and electron transport in AlGaAs/GaAs two-dimensional electron system

在這篇論文的第一部分，我們主要在研究電子在外加磁場下在砷化鎵量子井裡面傳輸的特性。在這樣的情況之下，電子處在非常多雜質的環境，而形成非常Disordered系統，因此電子並不是如往常一樣的傳輸，而是利用一種稱作Hopping 的機制去進行傳播。電子在低溫的時候容易被許多的雜質給束縛住，在這樣的情形下，我們去看是否有符合Variable-range hopping 和 Nearest-neighbor hopping 的模型。另一方面，磁阻的行為是我們另一個重點，我們去觀察40 K ~ 80 K 的溫度範圍，並且發現了在低磁場之下巨有負磁阻的現象，在高磁場有正磁阻的現象。第二部分是我們去研究熱電子效應在一個比較輕微Disordered 系統下。我們去研究電子跟聲子的散射特性在0.3 K ~ 1.5 K之間。如果在一個非常低溫的系統之下，電子會受到外加電場而加速，這樣的行為可以等下電子的溫度上升，這樣的效應跟我們去固定一個小的外加電場，然後去調變樣品的溫度是具有等效性的，這個等效性結果可以用T ~ I^a來表示。這兩個情形都直接的去影響到Localized states 和Extending states。藉著電流的尺度效應，我們得到了p ~ 3 在負磁阻的範圍，p ~ 2 在SdH的範圍，並且在霍爾平台2跟4之間求得 k=0.252。In the first part of the thesis, I studied electron transport in a delta-doped GaAs single quantum well in magnetic field. The hopping conduction is the major behavior in this situation. Electrons are strongly localized by the impurities and then we fit variable range hopping and nearest neighbor hopping models to our experimental result. Magnetoresistance behavior is another focus in a strongly disordered system. We measure magnetoresistance (MR) between 40 K ~ 80 K. We observed negative MR in low magnetic fields and positive MR in the high magnetic regime. In the second part of this thesis, I studied current scaling and electron heating in a weakly disordered system. We studied the electron-phonon scattering rate in the temperature range 0.3 ~ 1.5 K. If electrons are accelerated by the electric field at a fixed lattice temperature, it is also likely to increase lattice temperature at fixed current. By the phenomenon we studied the relation temperature and current. The two conditions directly affect the distribution of the localized states and extending states. By current scaling, we are able to determine the relations:(dRxy/dB)max~I^b, T^k and k=p/2v.The Hall resistivity is used to find k. We found k =0.252 between plateau v=2 and v=4, and p ~ 3 in the NMR regime and p ~ 2 in the SdH regime.Contentshapter 1ntroduction to electron 1.1 Introduction to electron transport in semiconductors..............................................1.2 The Drude model.....................................................................................................1.3 Effective mass.........................................................................................................3.4 Density of states......................................................................................................4.5 Two-dimensional electron system...........................................................................7hapter 2lectron transport in two-dimensional systems 12.1 Theoretical background..........................................................................................12.2 Landau levels..........................................................................................................15.3 Shubnikov-de Haas oscillations.............................................................................18.4 Integer quantum Hall effect....................................................................................20.5 Weak localization...................................................................................................23hapter 3 ample fabrication and experimental techniques 27.1 Ohmic contact........................................................................................................27.2 Optical lithography.................................................................................................28.3 Sample packaging and handling.............................................................................30.4 Experimental set-up................................................................................................30.5 Condensation of 3He...............................................................................................32.6 Four-terminal resistance measurement...................................................................33hapter 4lectron transport in a delta-doped GaAs single quantum well 35.1 Introduction............................................................................................................35.2 Hopping conduction...............................................................................................36.3 Sample structure.....................................................................................................39.4 Experiment results..................................................................................................40.5 Conclusion..............................................................................................................49hapter 5lectron heating and current scaling at a weakly disordered system1.1 Introduction............................................................................................................51.2 Theoretical background and previous work...........................................................52 5.2.1 The two bath model.......................................................................................52 5.2.2 Current scaling..............................................................................................53.3 Sample structure.....................................................................................................57.4 Experiment result and discussion...........................................................................58.5 Conclusions............................................................................................................65hapter 6onclusionsonclusion....................................................................................................................6