Influence of processing parameters on the transport properties of quantum point contacts fabricated with an atomic force microscope

We describe a reliable technique for fabricating ballistic quantum point contacts (QPCs) with large energy separation between one-dimensional subbands. The technique is based on lithography with an atomic force microscope and wet chemical etching of a GaAs/AlGaAs heterostructure. The high-mobility two-dimensional electron gas located 55 nm below the surface is laterally confined by 20 nm or 50 nm deep grooves with separations ranging between 65-105 nm or 100-185 nm, respectively. The conductance characteristics at T = 4.2 K exhibit clear quasi-plateaux at multiples of 2e(2)/h. Both the conductance threshold voltage and the plateau widths are directly related to the QPC geometry. The energy separation DeltaE(1,2) of the lowest subbands is determined from the conductance under nonzero dc drain voltage. Upon reducing the QPC width, DeltaE(1,2) varies from 6 meV to 15 meV.