Inversion of the Internal Electric Field due to Inhomogeneous Incorporation of Ge Dopants in GaN/AlN Heterostructures Studied by Off-Axis Electron Holography.

The engineering of the internal electric field inside III-nitride devices opens up interesting perspectives in terms of device design to boost the radiative efficiency, which is a pressing need in the ultraviolet and green-to-red spectral windows. In this context, it is of paramount importance to have access to a tool like off-axis electron holography which can accurately characterize the electrostatic potentials in semiconductor heterostructures with nanometer-scale resolution. Here, we investigate the distribution of the electrostatic potential and chemical composition in two 10-period AlN/GaN (20 nm/20 nm) multilayer samples, one of these being non-intentionally doped and the other with its GaN layers heavily doped with Ge at a nominal concentration ([Ge] = 2.0 ± 0.2 × 10 cm) which is close to the solubility limit. The electron holography experiments demonstrate the effects of free carrier screening in the case of Ge doping. Furthermore, in the doped sample, an inversion of the internal electric field is observed in some of the AlN layers. A correlated study involving holography, electron dispersive X-ray spectroscopy, and theoretical calculations of the band diagram demonstrates that the perturbation of the potential can be attributed to Ge accumulation at the heterointerfaces, which paves the way to the use of Ge delta doping as a design tool to tune the electric fields in polar heterostructures.