Metal-insulator transition at B=0 in an AlGaAs/GaAs two-dimensional electron gas under the influence of InAs self-assembled quantum dots

Abstract

We report the observation of a metal-insulator transition in zero magnetic eld in a two dimensional electron gas under the in uence of a plane of InAs self-assembled quantum dots located at 30 A below the AlGaAs GaAs heterointerface. The transition is observed as a function of temperature and electric eld at B=0. A scaling analysis yields exponents similar to those obtained for Si MOSFET's . We suggest that the disorder introduced by the quantum dots plays a crucial role. I IntroductionIn an important paper of 1979 based on scaling theory of localization, Abrahams et al. have shown that a t wo-dimensional system without interactions was expected to behave as an insulator in zero-magnetic eld 1 . Since then, it has been a general belief that there is no metallic regime in 2D at T = 0, contrary to what happens in the 3D case. The metal-insulator transition MIT at B = 0 , rst reported for a Si metaloxide-semiconductor eld-e ect transistor MOSFET by Kravchenko et al. 2 , has thus generated a great interest in the eld. Experimentally, the MIT was further con rmed not only in Si MOSFET's 3-6 but also for holes in SiGe quantum wells 7 and holes in AlGaAs GaAs structures 8,9 . All the samples used there were optimized towards low disorder clean samples and large electron-electron e-e interaction large effective carrier masses. It was also clear that a scaling analysis of the data as a function of temperature or electric eld is essential to characterize the metallic and the insulator states 4 , even when including interactions 10,11 . Coulomb i n teraction is believed to be in the heart of the MIT 12,13 and, in fact, the reported ratio E e,e =E F is of the order of 10 for a MIT 3-9 . Very recently, h o wever, we h a ve reported on the observation of a MIT in an AlGaAs GaAs two-dimensional electron gas 2DEG under the in uence of InAs self-assembled quantum dots SAQD 14 . This result is striking because our samples' parameters are rather di erent from the ones in the previous studies. First, the 2DEG is highly disordered due to the proximity of the SAQD. Second, E e,e E F m " p N s 1 and, for electrons in GaAs, one has a smaller e ective mass m and a slightly higher dielectric constant " than in the previous papers. Since the present MIT occurs for about the same carrier density N s , in our samples the e-e interaction energy E e,e is of the order of the electronic kinetic energy E F E e,e =E F ' 0:9. It seems unlikely that this weak e-e interaction could be solely responsible for the observed metallic phase. In this paper we show the results of a scaling analysis and suggest that the presence of the SAQD might drive the observed MIT.The samples used in this work are described in detail in Ref. 15. A layer of InAs SAQD is embedded 30 A from the interface AlGaAs GaAs where the 2DEG forms. Since the substrate was not rotated during the growth of the dot layer, we found a gradient of dot density across the wafer 15 . We will concentrate on the rst and second highest dot density samples, A 110