Fabricating shallow 2D systems in GaAs/AlGaAs heterostructures towards the creation of Artificial Graphene

Patterning ordinary GaAs/AlGaAs heterostructures with a triangular nano-array of quantum anti-dots has been shown to yield Artificial Graphene. The lattice spacing of the patterned quantum anti-dot array must be in the order of 100 nm to enable the observation of Dirac physics. This implies that the GaAs/AlGaAs interface has to be considerably shallower than 100 nm to enable strong modulation of the 2D carriers with surface gates gates. Fabricating low disorder shallow 2D systems is experimentally challenging due to the close proximity to the surface and the thin barrier to the surface gate, determined by the AlGaAs layer. This thinner barrier increases leakage between the gate and the 2D system, and increases scattering at the interface, damaging the mobility of the 2D carriers.
Here we present results on the fabrication of 2D systems in accumulation mode heterostructures as shallow as 35 nm. We use aluminium gates grown in-situ in the MBE chamber, to avoid exposing the GaAs surface to air. Surprisingly, we find that the 2D mobility with an ‘in situ’ gate is lower than with an ‘ex situ’ gate. We investigate the density dependence of the mobility to identify the underlying scattering mechanisms behind this observation.