The goal of the present project is the direct quantitative determination of internal electric fields (IEFs) in semiconductor nanostructures using up-to-date techniques based on transmission electron microscopy (TEM) in combination with controlled superposition of external electric fields. As an ideal experimental model system we focus on group III-nitride nanowire (III-N NW) pn-junctions and axial group III-nitride nanowire heterostructures (III-N NWHs) synthesized by plasma-assisted molecular beam epitaxy. Such NWHs can be grown with high precision and reproducibility and exhibit high polarization-induced IEFs of the order of several MV/cm that can be controlled in magnitude by adjusting the geometrical dimensions, the chemical composition and the dopant concentration. Furthermore, the IEF strength and spatial distribution in individually contacted NWs and NWHs can be modified in a controlled manner by superposition of an externally applied bias. As the optical characteristics of III-N NWHs are influenced by IEFs due to the quantum-confined Stark effect we will investigate the IEFs of specific NWHs by the novel TEM-based techniques and we will characterize the same individual nanostructures by bias-dependent photocurrent and micro-photoluminescence analysis. Comparison of the TEM and optical characterization results for different electron beam intensities and excitation powers, respectively, in combination with simulation of the electron/hole states will allow for quantitative determination of the IEFs as well as an improved understanding of the electronic band profiles, shielding of internal fields by free charge carriers and influence of preparation-induced surface states. The availability and optimization of a TEM-based technique to reliably quantify IEFs is not only important for analyzing polarization-induced IEFs in the AlN/GaN and InGaN/GaN nanostructures in this study. It is a promising and useful method that is applicable for other complex nanostructures and nanoscale devices of other semiconductor materials (such as GaAs(AlGaAs) but also for the analysis different systems such as ferroelectric tunnel junctions.
DFG Programme
Research Grants
