Excitonic Effects and Mott Transition in Semiconductor Heterostructures

We present a study of excitonic effects and Mott transition in two different types of heterostructures: polar AlGaN/AlN quantum wells and MoS₂ bilayers. Narrow AlGaN quantum wells are important elements of ultra-violet light emitting devices and the excitonic effects in radiative lifetimes were found to be significant. We included the density dependent screening of polarization fields in our theory of luminescence, and employed Green’s function formalism with self-consistent T-matrix approximation. This allows to include effects of exciton formation and dissociation. Exciton formation was found to have a strong effect on the density dependence of the radiative recombination rates. We evaluated the ionization ratio as a function of carrier density for different values of well width, from low to high densities, through the excitonic Mott transition where the Pauli blocking and screening raise the ionization rate to unity. In case of MoS₂ bilayer we have n and p type conduction channels separated by a thin dielectric barrier allowing strong electron-hole Coulomb correlation and formation of indirect excitons. For the case of gated MoS₂ monolayers separated by a few-monolayer BN we evaluated the transition from insulating excitonic state to the conducting electro-hole plasma state.