Microfabrication of an in-plane gated quantum wire using low energy ion damaging for pattern transfer

Abstract

A transmission electron microscope has been converted for electron beam lithography applications where computer control of the stage and electron beam are required. Supporting modifications and extensive software complete the system that is capable of exposing patterns with linewidths well below 100nm. The modifications, interface, and software are included in this dissertation. This lithography system has been used to pattern nanoscale masks on the surface of MBE grown GaAs-AlGaAs heterojunction two-dimensional electron gas (2-DEG) material. Flood exposure to low energy argon ions transfers the pattern to the 2-DEG by destroying conduction in the unprotected regions. Each device consists of two perpendicular wires, typically 20μm long and 110nm wide, with their ends making ohmic contact to bonding pads. One wire is patterned with a 300nm gap at its center for the second wire (the conduction channel) to pass through. By applying a negative potential to the split wire a potential barrier is formed in the channel through the gap region, effectively "gating" conduction through the channel. The devices were designed "in-plane" meaning both the conduction channel and gates (split wire) were fabricated in the plane of the 2-DEG, with metal only being used for ohmic contacts. The in-plane design drastically reduces gate capacitance, compared to traditional metal gated devices, promising ultra-fast switching. Cryogenic measurements have shown the ion damaged regions to be extremely isolated, with sheet resistance approaching 10^14Ω/α at IV, and breakdown voltages exceeding 10^6V/cm. The devices exhibit typical I-V behavior for field effect transistors; having a low bias linear portion followed by a saturation region, with overall conductance decreasing with increasing (negative) gate potential. Unique to these devices however, is the observation of negative differential conductance, i.e., the conduction through the device decreases upon increasing the applied bias. When operated near pinchoff the devices exhibit very large resistance switching, attributed to single electron traps modulating the barrier height.