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

Abstract

The various scattering mechanisms induced by dislocations have been reviewed andadapted to the case of threading dislocations in AlGaN/GaN quantum wells. Thesescattering mechanisms can be classified into two categories, the first one issuingstraightforwardly from the dislocation strain field, the other one being due tothe Coulomb potential created by electrons trapped on the energy states thatdislocations 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 withthe so-called deformation potential, the piezoelectric coupling being ruled outbecause of the particular geometry of the threading dislocation strain fields. Weshow that the dislocation deformation potential can only be responsible for avery weak, even negligible, effect on the carrier mobility. Then, after a survey ofthe various results found in the literature concerning the possible existence ofdislocation energy states we conclude that dislocations are responsible for theexistence of shallow acceptor states below the conduction band and propose amodel for describing the potential associated with such states when filled byelectrons. More particularly, we show that the linear dislocation charge densityresulting from the trapped carriers at dislocation states can not be uniform, as it issystematically assumed in the literature, and we propose a description of this linearcharge density as a function of the dislocation energy state position and of thevarious features characterizing the quantum well. Using the scattering potentialinduced by such a spatially-dependent dislocation charge density together withthe usual scattering mechanisms allows us to give an estimation of their effecton 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 scatteringmechanisms and intrinsic scattering mechanisms. Finally we suggest that our model couldbe employed for determining the position of the dislocation energy level in the gap.