  The precise control of a bilayer system consisting of two adjacent
two-dimensional electron gases (2DEG) is demonstrated by using a novel planar
back-gate approach based on ion implantation. This technique overcomes some
common problems of the traditional design like the poor 2DEG mobility and
leakage currents between the gate and the quantum well. Both bilayers with and
without separate contacts have been prepared and tested. Tuning the electron
density in one layer while keeping the second 2DEG at fixed density, one
observes a dramatic increase of the carrier concentration. This tunneling
resonance, which occurs at equal densities of both layers, demonstrates the
separated contacts to each individual layer. In another sample with a smaller
tunneling barrier and parallel contacted 2DEGs, the transition from a single
2DEG to a bilayer system is investigated at 50 mK in magnetic fields up to 12
T, showing the gate stability in high magnetic fields and very low
temperatures. Transitions into an insulating (Wigner crystal) phase are
observed in the individual layers in high fields at filling factors below 1/3.
The absence of a fractional quantum Hall liquid at filling factor 1/5 in our
structure seems to be a consequence of confining the electrons in quantum wells
rather than at interfaces. The observed metal-insulator transitions appear to
be nearly unaffected by the presence of the second layer separated by a barrier
which is only 3 nm thick. We believe that this planar back-gate design holds
great promise to produce controllable bilayers suitable to investigate the
exotic (non-abelian) properties of correlated states.
