Hopping conduction and magnetoresistance of a GaAs/AlxGa1−xAs quantum well with embedded InAs dots

Abstract

Magnetoresistance and temperature-dependent conductance are measured in the sample made of a GaAs/Al x Ga 1−x As quantum well with self-assembled InAs dots. Conductance is analyzed by Mott's hopping theory; the localization lengths have been extracted at various gate voltages. The sample is in the transition from near to metal-insulator to the deeply hopping regime with the combined effect of the long-and short-range scattering potentials. The magnitude of the negative magnetoresistance increases with increasing negative gate voltage. The magnetic-field dependence of the resistance can be explained by the theory of the interference model of hopping electrons.The advancement of semiconductor technology allowed us to achieve a variety of electron systems, one of which is the self-assembled InAs quantum dots embedded in twodimensional (2D) electron gases formed in GaAs/Al x Ga 1−x As heterostructures. In recent years there has been increased interest in the quantum transport properties of these systems. The InAs dots can be considered as random repulsive scattering centers that interact with the electrons in the 2D system and modify the scattering potential. 1,2 It has been shown that this system has the charge trapping effect, which holds the potential to be used in low power memory devices. 3 In this work, we study the electron transport in the variable range hopping (VRH) regime. The conventional picture of hopping considers that localized states are formed by individual impurities. The lower the temperature, the larger the separation of the localized states between which a typical hop can take place. When lowering temperature further, the conductivity of a strongly localized systems tends to zero. Applying a magnetic field has several effects on the hopping process of an electron. First, the magnetic field shrinks the wave function of the localized states 4 and results in a positive magnetoresistance (MR). Second, the destructive interference effect from different possible tunneling paths can be destroyed by applying a magnetic field (B), causing negative MR linear or parabolic with B. 5,6 In an approach by the authors of Ref. 7 from the interference model, it is found that the magnetic field can lead to a correction to the localization length in 2D samples that results in negative MR with a field dependence ln( ρ(B) ρ(0) ) ∼ B 1/2 . The 2D electron system is formed in a GaAs quantum well (100 plane). The 20-nm-wide quantum well is modulationdoped from both sides and separated by spacer layers of thickness 40 nm. In the middle of the quantum well, a few monolayers of InAs are introduced. Due to lattice mismatch, a strain is produced in the material that results in the formation of InAs dots after an initial 2D wetting layer. 8 Figure 1 is the transmission electron microscope (TEM) image of the sample showing the interface where the InAs dots are formed. The majority of dots have a size of 28 nm in diameter. Dots are not evenly distributed in the whole area. In densely populated areas, the dot density is about 1 ...

Electron dephasing of a GaAs/AlGaAs quantum well with self-assembled InAs dots

“…When inserted into a twodimensional (2D) electron system the quantum dots form random repulsive scattering centres and reshape the scattering potential from ionized donors [1,2]. It has been reported that variable range hopping of electrons can take place when applying a large negative gate voltage [3], and Landau quantization and localization can coexist in this system [4].…”