AlAs/n-GaAs superlattice and its application to high-quality two- dimensional electron gas systems

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Abstract
A novel AlAs/n‐GaAs superlattice structure, in which the GaAs midlayers are selectively doped with Si, has been designed as an electron supplying layer for two‐dimensional electron gas (2DEG) systems to obtain a high sheet carrier concentration, using a simple Kronig–Penney model calculation. The optimized structure is a very short period superlattice (∼40 Å), in which minibands are formed. This design realizes the elimination of DX centers and obtains a shallow donor level, in contrast with alloyed n‐AlxGa1−xAs (x≂0.3), which has the same band gap as the superlattice. A 2DEG existence at the undoped GaAs and the AlAs/n‐GaAs superlattice heterointerface is confirmed by Shubnikov–de Haas measurements for normal and inverted structures. High sheet carrier concentrations (8×1011 cm−2) were obtained for both structures with a one period undoped AlAs/GaAs spacer layer. Namely, total spacer layer thickness is about 60 Å. The 2DEG concentrations showed no variation with decrease in temperature from 300 to 4.2 K and with room light illumination at 4.2 K. Electron mobilities at 4.2 K are as high as 190 000 and 110 000 cm2/V s for the normal structure and the inverted structure, respectively.