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Maria Spies

Correlated electro-optical and TEM studies on single III-N nanowire heterostructures

Published on 17 October 2019
Thesis presented October 17, 2019

Nanowires (NWs) are promising building blocks for future optoelectronic architectures requiring advanced miniaturization. The strain relaxation, high surface sensitivity and optical properties of these structures bring additional advantages over traditional two-dimensional designs. NWs are also a potential platform to study quantum phenomena. Considering quantum dots (QDs) embedded in NWs, the QD location, size and density can be easily controlled, and QDs with high crystalline quality can be synthesized due to elastic strain relaxation at the surfaces of the NW. All NWs studied in this project were III-nitride NWs grown by plasma-assisted molecular-beam epitaxy.
For the study of quantum and nano-scale phenomena it is crucial to analyze the same object with a multi-technique approach. In this project, this is achieved through the fabrication of Si3N4 membrane chips compatible with transmission electron microscopy (TEM). The NWs are placed and contacted on these membranes prior to characterization. Results of electrical, photocurrent, micro-photoluminescence (µPL) and scanning TEM measurements are correlated and compared with theoretical calculations.
In view of the application of NWs as photodetectors, we studied a NW design incorporating an axial GaN/AlN superlattice. The presence of the heterostructure results in reduced dark current and enhancement of the photocurrent under ultraviolet excitation. The heterostructure dimensions and doping profile were likewise designed in such a way that the application of positive or negative bias leads to an enhancement of the collection of photogenerated carriers from the GaN/AlN superlattice or from the GaN base, respectively. Thus, the devices display enhanced response in the ultraviolet B (~ 280-330 nm) / A (~ 330-360 nm) spectral windows under positive/ negative bias.
On the other hand, the linearity of the photodetector response as a function of the incident optical power is crucial for the quantification of the incoming light. NWs of many material systems, however, were shown to have a sublinear dependence on impinging light. In this work, we showed that a linear dependence can be achieved in NWs with an AlN/GaN/AlN insertion if they are below a critical diameter, which corresponds to the total depletion of the NW due to the Fermi level pinning at the sidewalls. In the case of NWs that are only partially depleted, their nonlinearity is explained by a nonlinear variation of the diameter of their central conducting channel under illumination.
Finally, the manipulation of the electric field in a QD is important for potential application and in-depth understanding of their electronic properties. In this project, we studied the spectral tunability of the emission of an AlN/GaN/AlN QD in a GaN NW by application of external bias. Measurements of µPL on contacted single NWs showed a single emission line in the spectral region from 283 nm to 321 nm (depending on the dot size), which shifts with bias at a rate of 1.0 nm/V. It blue shifts when the external electric field compensates the internal electric field generated by the spontaneous and piezoelectric polarization. In small QDs, we observed a spectral switch of the emission with bias, which is attributed to the transition of the exciton to other charged states. Correlation of TEM, µPL and theoretical calculations show that the NW stem and cap are highly conductive, and the applied voltage drops at the edges of the AlN/GaN/AlN heterostructure.

GaN, nanowire, photodetector, heterostructure, quantum dot

On-line thesis.