Polarization Effects in III-Nitride Nanowire Heterostructures

Group III-nitride (III-N) nanowires (NWs) and nanowire heterostructures (NWHs) are a topic of current research as they offer the possibility of realizing novel, nanoscale optoelectronic devices with improved stability and efficiency or the perspective of improving electronic devices due to their low density of structural defects. Moreover, NWHs can also serve as an ideal model system for the analysis of carrier confinement, carrier dynamics and internal electric fields, as the influence of structural defects on the optical properties is remarkably low.

We report on the structural and optical properties of III-N NWHs with a focus on internal electric fields in GaN nanodiscs (NDs) axially embedded in NWHs. The presence of strain- induced radial and polarization-induced axial internal electric fields is demonstrated and the latter can be probed by studying the effect of superimposed external electric fields on the photoluminescence characteristics of single NWHs. The results are compared to those obtained from a recently developed direct measurement technique using transmission electron microscopy.

Efficient n-type doping Germanium as a donor facilitates electrostatic screening of polarization-induced internal electric fields. While the three-dimensional carrier confinement in non-intentionally doped AlN/GaN NWHs enables long exciton lifetimes up to 5 µs, this is reduced by almost two orders of magnitude in case of Ge-doping. Further control of the growth conditions allows the realization of self-organized GaN quantum wires with diameters below 5 nm on AlN/GaN NW templates that unique optical properties. As an outlook, possible applications of NWH-based devices are discussed.