is related to other physical lithographic techniques, with two important advances. First, this method does not rely on photolithography or EBL to define features, hence it can produce thinner wires with much smaller pitches than can be fabricated even with EBL. Second, a metal lift-off step is not needed because fully separated metal lines are directly deposited. Avoiding this step greatly enhances our ability to fabricate wires at very small separations, where previously metal adhesion limited the possible wire pitch. In addition, the metal wires themselves can be used as etch masks to transfer the nanowire pattern to an underlying semiconductor substrate, such as silicon-on-SiO2 (SOI) wafers, thereby generalizing this technique for both metal and semiconductor wires. Finally, the selectively etched GaAs/ AlGaAs superlattice may also find uses as a master stamp for other physical lithographic techniques such as nanoimprinting.
The SNAP synthetic procedure is illustrated in Fig. 1. A GaAs/Al0.8Ga0.2As superlattice was grown on the [100] surface of a GaAs substrate with the use of MBE techniques. A dilute mixture of buffered hydrofluoric acid (15 ml of 6:1 buffered oxide etchant, 50 ml of H2O) was used to selectively etch the AlGaAs layers roughly 20 to 30 nm deep (4). Metal wires were then evaporated onto the top of the GaAs layers by orienting the superlattice at 36° with respect to the evaporative flux within the vacuum chamber of an electron beam metal evaporation system. This tilt caused the metal to be deposited only on the GaAs layers because of their elevation relative to the etched AlGaAs layers. Gold, chromium, aluminum, titanium, niobium, platinum, and nickel wires were all used in this study; however, a variety of other metals are also expected to work.