Ultra-shallow quantum dots in an undoped GaAs/AlGaAs 2D electron gas
  We report quantum dots fabricated on very shallow 2-dimensional electron gases, only 30 nm below the surface, in undoped GaAs/AlGaAs heterostructures grown by molecular beam epitaxy. Due to the absence of dopants, an improvement of more than one order of magnitude in mobility (at 2E11 /cm^2) with respect to doped heterostructures with similar depths is observed. These undoped wafers can easily be gated with surface metallic gates patterned by e-beam lithography, as demonstrated here from single-level transport through a quantum dot showing large charging energies (up to 1.75 meV) and excited state energies (up to 0.5 meV). 
W. Y. Mak, F. Sfigakis, K. Das Gupta, O. Klochan, H. E. Beere, I.   Farrer, J. P. Griffiths, G. A. C. Jones, A. R. Hamilton,...
cond-mat.mes-hall