The impurities act in two ways
       i)             They create shallow bound states which can easily be ionized to ensure electrical conduction.
       ii)            When ionized, the impurities scatter the mobile carriers  

Replacing bulk materials by heterostructures (quantum wells (QW)/wires) leads to a lowering of the dimensionality of the free motion (quasi 2D/1D instead of 3D).  This leads to changes in the impurity binding energies.  More importantly, the location of the impurities in the heterostructure drastically affects all the physical properties associated to them: the velocity relaxation time or the level lifetime (modulation doping) but also the far infrared absorption (e.g. the lineshape of the inter-subband transitions in QW’s).

In quasi 2D materials, we shall explain how the existence of (quasi) discrete states located in energy below the edges of excited subbands for the z motion modifies the inter-subband scattering times.  Finally, we shall show that few impurities (10¹⁰cm^-2) are enough to scramble the subband quantum number in THz emitting quasi-2D heterostructures.

This work has been done in collaboration with C. Ndebeka-Bandou and F. Carosella.