MOLECULAR-BEAM EPITAXY FOR HIGH-ELECTRON-MOBILITY MODULATION-DOPED 2-DIMENSIONAL ELECTRON GASES

Spatial separation of the electrons participating in conduction from the corresponding donor impurities is possible by modulation doping of a heterojunction. The most common modulation-doped heterojunction that has been studied is n-AlGaAs/i-GaAs. The electrons transfer from the AlGaAs to the GaAs, where they collect in the triangular potential well at the AlGaAs/GaAs heterojunction, forming a two-dimensional electron gas (2DEG). This spatial separation of the conduction electrons and the ionized donors results in a reduction of the ionized impurity scattering. In addition, the scattering due to phonons can be reduced by cooling the sample. Several groups, using the advanced film growth technique of molecular beam epitaxy, have reported 2DEG mobilities in the range (1-10) x 10(6) cm2 V-1 s-1 with carrier concentrations of (1-5) x 10(11) cm-2 at temperatures of 0.35-4.2 K. These electron mobilities and carrier densities translate into electrons traveling distances greater than 10 mum between collisions. Since photolithographic capabilities allow much smaller device geometries, devices based on the wave nature of the electron are now possible and several have been demonstrated. A discussion of molecular beam epitaxy techniques to obtain such high mobility modulation-doped 2DEGs will be presented, together with a review of what other groups have reported.