A study on the electronic and structural properties of semiconductor heterostructures

Electronic structures of two-dimensional heterostructures of van der Waal’s materials show interesting properties and it is useful to contrast them with the electronic structure of the conventional semiconductor heterostructures. We have examined the type-I to type-II electronic structure transition occurring in classical GaAs/AlAs heterostructure as a function of the thickness of the GaAs layers, which is consequent upon quantum confinement effects, to find out whether the transition happens abruptly or whether it happens gradually over few monolayers.
ReS2 is found to be semiconducting in its ground state, which is quite surprising, as Re has a formal d electron count of 3 in ReS2 which should have led to metallic property if we follow the same scheme of level splitting as observed in Mo and W based transition metal dichalcogenides. The structure of ReS2 has four Re atoms clusters connected at the corners to form parallelly running chains. Through a tight-binding analysis we find that the formation of these clusters alone are responsible for the semiconducting state. Moreover ReS2 shows signature of weak interlayer coupling as the bandgap is found to vary weakly with the number of layers.  This is accounted for by the fact that the band extrema emerge from interactions of the Re atoms within each cluster resulting in strong localization of the associated wavefunctions primarily in the Re layer with very little extension into the van der Waal’s gap. 
We also probed the electronic structure changes in group-IVB TMDs as a function of changing the anion. Experiments as well as density functional theory based calculations reported ZrSe2 to be semiconducting and ZrTe2 as metallic. We find that the scaling of the hopping interaction strengths with Zr-X bondlengths is responsible for differing electronic ground states for the two materials.