TUNABLE IN-PLANE-GATED (IPG) QUANTUM WIRE STRUCTURES FABRICATED WITH DIRECTLY WRITTEN FOCUSED ION-BEAMS

A new tunable quantum wire structure directly written by focused ion beams has been fabricated and characterized. We define a conducting channel with a width of several micrometers by locally destroying the conductivity of a two-dimensional electron gas (2DEG) of a GaAs-AxGa1-xAs heterojunction by accelerated 100 keV Ga+ ions. The electrically isolated 2DEG regions on both sides of the channel are used as gates, when a gate voltage Vg is applied simultaneously at both gates with respect to the channel. By varying Vg, the channel changes its resistance R proportional to 1 (Vg- Vth), Vth being the threshold voltage of typically -5 V. The Vg-induced change in R covers the wide range of 103-109Ω. At low temperatures, steps in the resistance, which are characteristic for the depopulation of one-dimensional (1D) subbands, are observed. The striking new aspect of this in-plane-gate (IPG) structure is that the confining electric field is parallel to the 2DEG, and the distorted, insulating region is taken as a dielectric. In contrast to the conventional split-gate configuration, where first the 2DEG under the gates has to be depleted, the new IPG structure is inherently quasi-one-dimensional and the whole gate potential is fed into the 1D confinement. The IPG exhibits a substantial lower gate capacitance than conventional planar gates, which is important for applications in ultrafast logical and linear devices. © 1990.