Two-dimensional electron gas formation in undoped In0.75Ga0.25As/In0.75Al0.25As quantum wells

We report on the achievement of a two-dimensional electron gas in completely undoped In0.75Al0.25As/In0.75Ga0.25As metamorphic quantum wells. Using these structures we were able to reduce the carrier density, with respect to reported values in similar modulation-doped structures, to about 2-3×1011 cm-2 with mobilities of up to 2.15×105 cm2 (V s)-1. We found experimentally that the electronic charge in the quantum well is likely due to a deep-level donor state in the In0.75Al0.25As barrier band gap, whose energy lies within the In0.75Ga0.25As/In0.75Al0.25As conduction band discontinuity. This result is further confirmed through a Poisson-Schrödinger simulation of the two-dimensional electron gas structure.