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

在這篇論文裡，我們研究了在0.3 K ~ 4 K溫度範圍內，砷化鋁鎵/砷化鎵二維電子系統其電子-聲子散射對於溫度改變的反應，而我們主要是量測電子-聲子散射機率τep-1 Te p的p值。在樣品LM 4646的量測中，由於其量子井中做了delta摻雜，使得我們能夠以隨溫度改變之縱向電阻 Rxx(T) 當作溫度計來求得特定電流所對應到的電子溫度。實驗結果顯示，在零磁場時p = 2.3 2、在負磁阻的範圍內p = 1.6~1.8 2，以及在SdH震盪範圍內p = 0.9~1.0 1。而由p 2變化至p 1似乎是以絕緣態-量子霍爾相變的交叉點為分界。在另一塊樣品LM 4640的量測中，其量子井並無做delta摻雜，因此我們是以隨溫度改變之SdH震盪的振幅來當作溫度計，並發現p = 0.7~1.0 1在此SdH範圍內。而與文獻上理論計算和實驗結果的比較下，我們可以解釋在樣品LM 4646中，在零磁場以及負磁阻的範圍內，系統中動態屏蔽的聲頻壓電散射為主要的電子-聲子散射機制因此導致了p 2的行為。而在樣品LM 4640中，在SdH範圍裡量測到p 1，也表示無屏蔽的聲頻壓電散射在此系統中為主要的電子-聲子散射機制。至於在樣品LM 4646中，由p 2(零磁場以及負磁阻範圍)至p 1(SdH範圍)的變化在過去並沒有被觀察到，其變化的行為似乎也可能與絕緣態-量子霍爾相變有某些相關。而我們也需要進一步的實驗來研究其背後可能的機制。In this thesis, we studied the temperature behavior of electron-phonon scattering in the temperature range of 0.3 K T 4 K, in AlGaAs/GaAs two-dimensional electron systems. We focused on determining the temperature exponent p of electron-phonon scattering rate τep-1 Te p. In sample LM 4646 which has delta doping in the quantum well, we used the temperature dependence of longitudinal resistance Rxx(T) as thermometer to find the corresponding electron temperature Te. The experimental results showed that p = 2.3 2 at B=0 T and p = 1.6~1.8 2 in the negative magnetoresistance (NMR) regime, and p = 0.9~1.0 1 in the SdH regime. The “critical point” of this transition from p 2 to p 1 is very close to the crossover of insulator to Quantum Hall transition. On the other hand, in sample LM 4640 which is a typical modulation-doped heterostructure, p = 0.7~1.0 1 was found in the SdH regime by using the temperature-dependent amplitude of SdH oscillations as a thermometer. Comparing with theoretical calculations and experimental results in literatures, we can explain the value of p 2 in sample LM 4646, at B=0 T and in the NMR regime, that dynamically screened piezoelectric scattering dominates. And p 1 in sample LM 4640, in the SdH regime, implies that the unscreened piezoelectric scattering is relatively important. But the transition from p 2 (small magnetic field regime) to p 1 (SdH regime) in sample LM 4646 has not been observed in the past, which is probably associated with the I-QH transition, and further works are needed to explore the underlying mechanisms.Chapter 1ntroduction to electron transport in semiconductors 1.1 Introduction..............................................................................................................1.2 The Drude model......................................................................................................1.3 Electrons in periodic potentials－Band structures...................................................3.4 Effective mass..........................................................................................................6.5 Densities of states.....................................................................................................7.6 Scattering process.....................................................................................................9.6.2 Electron-impurity scattering..........................................................................10 1.6.2 Electron-phonon scattering............................................................................12eferences....................................................................................................................15hapter 2ransport in a two-dimensional electron system 16.1 Semiconductors heterostructures............................................................................16.2 Classical Hall effect................................................................................................17.3 Two-dimensional electron transport under perpendicular magnetic field－Landau levels......................................................................................................................20.4 Shubnikov-de Haas Oscillations............................................................................23.5 Quantum Hall effect...............................................................................................25.6 Weak localization...................................................................................................29eferences....................................................................................................................31hapter 3lectron heating and electron-phonon scattering at zero magnetic field 32.1 Introduction............................................................................................................32.2 The two bath model................................................................................................33.3 Experimental setup.................................................................................................36.4 Experimental results I：Sample LM 4646.............................................................37.5 Experimental results II：Sample LM 4640............................................................41.6 Discussions.............................................................................................................42.7 Conclusions............................................................................................................49eferences....................................................................................................................50hapter 4lectron heating and electron-phonon scattering in a perpendicular magnetic field 51.1 Introduction............................................................................................................51.2 Current scaling.......................................................................................................52.3 Experimental setup.................................................................................................56.4 Experimental results I：Sample LM 4646.............................................................56.5 Experimental results II：Sample LM 4640............................................................61.6 Discussions.............................................................................................................65.7 Conclusions............................................................................................................71eferences....................................................................................................................72hapter 5onclusion 73onclusion....................................................................................................................7