Achievement of Systematically Higher Transport Mobilities for a Large Range of Two-dimensional Electron Densities in GaAs Quantum Wells*

For the range of two-dimensional (2D) electron densities between 1.5 x 10¹⁰ cm^-2 and 1.4 x 10¹¹ cm^-2, we report measurements that confirm a ~60 to 70 % increase in the transport mobility of remotely-doped GaAs quantum wells compared to all previous GaAs material. A characteristic example of this development is the 2D transport mobility at an electron density of 1 x 10¹¹ cm^-2 in a 50 nm quantum well. Previously, in this type of sample the electron mobilities were limited to ~ 1.8 x 10⁷ cm²V^-1s^-1; now we have grown several with transport mobilities reaching ~3.0 x 10⁷ cm²V^-1s^-1. We attribute these results to our program of high-temperature in-situ bakes that have cleaned our AlGaAs barrier material of oxygen impurities, and to the use of large-area cryogenic cold-plates operating at 17 K in the vacuum sump of our molecular beam epitaxy chamber. When cooled to temperatures below 100 mK, the magnetotransport data of the 1 x 10¹¹ cm^-2 samples described above are remarkable, displaying clear quantum Hall features at the Landau level filling factors ν = 11/23, 12/25, and even 13/27.