Two-dimensional electron gas formation in undoped   In[0.75]Ga[0.25]As/In[0.75]Al[0.25]As quantum wells
  We report on the achievement of a two-dimensional electron gas in completely undoped In[0.75]Al[0.25]As/In[0.75]Ga[0.25]As metamorphic quantum wells. Using these structures we were able to reduce the carrier density, with respect to reported values in similar modulation-doped structures. We found experimentally that the electronic charge in the quantum well is likely due to a deep-level donor state in the In[0.75]Al[0.25]As barrier band gap, whose energy lies within the In[0.75]Ga[0.25]As/In[0.75]Al[0.25]As conduction band discontinuity. This result is further confirmed through a Poisson-Schroedinger simulation of the two-dimensional electron gas structure. 
F. Capotondi, G. Biasiol, I. Vobornik, L. Sorba, F. Giazotto, A.   Cavallini, and B. Fraboni