Effect of threading dislocations on carrier mobility in AlGaN/GaN quantum wells

The various scattering mechanisms induced by dislocations have been reviewed and adapted to the case of threading dislocations in AlGaN/GaN quantum wells. These scattering mechanisms can be classified into two categories, the first one issuing straightforwardly from the dislocation strain field, the other one being due to the Coulomb potential created by electrons trapped on the energy states that dislocations may create in the GaN band gap. For the first category of mechanisms (strain field effects), we indicate that edge dislocations can only be connected with the so-called deformation potential, the piezoelectric coupling being ruled out because of the particular geometry of the threading dislocation strain fields. We show that the dislocation deformation potential can only be responsible for a very weak, even negligible, effect on the carrier mobility. Then, after a survey of the various results found in the literature concerning the possible existence of dislocation energy states we conclude that dislocations are responsible for the existence of shallow acceptor states below the conduction band and propose a model for describing the potential associated with such states when filled by electrons. More particularly, we show that the linear dislocation charge density resulting from the trapped carriers at dislocation states can not be uniform, as it is systematically assumed in the literature, and we propose a description of this linear charge density as a function of the dislocation energy state position and of the various features characterizing the quantum well. Using the scattering potential induced by such a spatially-dependent dislocation charge density together with the usual scattering mechanisms allows us to give an estimation of their effect on the free carriers' mobility. We particularly show that at low carrier density (∼1012 cm-2) the mobility is mainly determined by the combination of dislocation scattering mechanisms and intrinsic scattering mechanisms. Finally we suggest that our model could be employed for determining the position of the dislocation energy level in the gap