Extremely high-mobility two dimensional electron gas: Evaluation of scattering mechanisms

Abstract

We report on the characterization of selectively doped GaAs/AlGaAs heterostructures, grown by an extremely clean molecular beam epitaxy system, which exhibit a Hall mobility of a two dimensional electron gas exceeding 10ϫ10 6 cm 2 /Vs for a wide range of undoped spacer layer thickness ͑50-100 nm͒. A maximum electron mobility of 14.4ϫ10 6 cm 2 /Vs was measured at 0.1 K in a structure with a 68 nm spacer thickness and an areal carrier density of 2.4ϫ10 11 cm Ϫ2. This is the highest electron mobility ever reported, leading to a momentum relaxation mean-free path of ϳ120 m. We present experiments that enable us to distinguish between the main scattering mechanisms. We find that scattering due to background impurities limits electron mobility in our best samples, suggesting that further improvement in structure quality is possible.

Role of background impurities in the single-particle relaxation lifetime of a two-dimensional electron gas

“…It is now possible to achieve low temperature mobilities in excess of 10 7 cm 2 /Vs in high quality GaAs-AlGaAs heterostructures [3,4], and there is continued interest in further improving 2DEG mobilities [5]. Phenomena that are only observed in ultra-high mobility 2DEGs include anisotropies, stripe and bubble phases in higher Landau levels [6], microwave induced resistance oscillations [7], and even denominator fractional quantum Hall states [8].…”

“…Remote ionised impurity scattering can be reduced with the use of large undoped AlGaAs 'spacer' layers between the 2DEG and the modulation doping [14], or eliminated entirely with accumulation mode devices in which the carriers are introduced electrostatically rather than through doping [15]. Finally there are always 'background impurities' incorporated in the crystal during the epitaxial growth process, and these will limit the mobility in both the very cleanest modulation doped samples [4] and in electrostatically doped samples.…”

Spin pumping into two-dimensional electron systems

“…A 2DEG system may be formed at the interface between two different semiconductors, such as modulation-doped GaAs|AlGaAs heterojunctions or quantum wells that at low temperatures support electron mobilities of >10 7 cm 2 V −1 s −1 corresponding to transport mean-free paths of 100 μm [9]. While spin injection into thin semiconductor layers has been reported [10], we are not aware of spin injection experiments into high-mobility 2DEGs, presumably due to the conductance mismatch problem.…”

Optical conductivity of a 2DEG with anisotropic Rashba interaction at the interface of LaAlO3/SrTiO3

“…The mobility at oxide interfaces is relatively less than that of III-V semiconductor heterostructures 16 . Unlike the conventional III-V semiconductor heterojunctions, the 2DEGs at LaAlO 3 /SrTiO 3 interfaces are characterized by very strong spin-orbit interaction, high carrier densities, higher effective mass 15 .…”