Distortion of donor properties in III-nitride based nano-scale systems.
Résumé
In order to achieve efficient doping of GaN layers, both incorporation and activation ratio of dopants must be carefully controlled. In thick polar GaN layers, n-doping is usually obtained by the incorporation of silicon or oxygen atoms. Silicon binding energy in bulk GaN is 30.4 meV, in good agreement with the 28.8 meV calculated in the effective mass approximation. However, impurity ground states in non-flat potential band profiles are strongly modified, as it is the case in heterostructures or in presence of electric fields. In this work, we have thus calculated in the effective potential formalism the energy of an electron confined on a shallow donor nucleus located in any given conduction band-profile. We study particularly the influence on donor binding energy of basal stacking fault (BSFs) in non-polar GaN and of surface in GaN nanocolumns, and then discuss some of their consequences on the electrical and the optical properties of these two systems. We first show that in non-polar GaN, a donor in the vicinity of a BSF can exhibit a binding energy increased up to a factor of 1.8 compared to the 30 meV expected for stacking fault free GaN. Donor activation ratio is thus decreased at room temperature, in agreement with the incomplete donors ionization reported by M. McLaurin et al., J. Appl. Phys. 100, 063707 (2006). We then discuss the consequences of D°-BSF complexes on the emission properties of non-polar GaN. In particular, we propose the statistical distribution of donors in the vicinity of the BSFs planes as origin of both the broad emission from BSF-excitons and intra-BSF exciton localization. In the case nanocolumns, we find that donor binding energy goes from 28.8 meV at the core of the structure to 7.2 meV at its surface. We show that such variation in binding energy is responsible of the broad D°X emission reported for strain-free GaN nanocolumn with high surface-to-volume ratio. We finally suggest that the alteration of D°X wave functions in the vicinity of the nanocolumn surface is responsible of the strong emission at 3.449 eV typically observed in these nanostructures.