Seminarium Fizyki Ciała Stałego

Gallium nitride (GaN) nanowires (NWs) grown by molecular beam epitaxy on Si substrates hold significant potential for optoelectronic applications due to their exceptional structural quality, high luminescence efficiency, reduced lattice mismatch strain to the substrate, and a high thermal stability. However, their advantages are often compromised by external conditions. To mitigate this issue, shells are frequently employed to protect the NWs from air exposure, and thus ensuring high system efficiency. In this study, core-shell GaN-AlOx/HfOx NWs were investigated, with oxide coatings applied through atomic layer deposition. Scanning electron microscopy was used to examine the morphology and structure of the NWs. To analyze the changes in the GaN core’s crystal lattice, strain was calculated based on outcomes of X-ray diffraction, photoluminescence (PL), and Raman spectroscopy. Consistent results were obtained by all these techniques, supported by statistical analysis, which revealed statistically significant differences in the calculated strain. The potential of NWs for optoelectronic applications was highlighted through PL and cathodoluminescence (CL) spectra and maps at room- and lower temperatures. Both experiments showed that the shells effectively protect NWs from photodegradation and enhance luminesce efficiency by passivating surface states, enabling core field screening through carrier accumulation, preventing carrier tunneling to surface states, minimizing strain, and inducing a flat-band effect. Interestingly, the highest emission intensity was observed for NWs with the thinnest shells (1-5 nm), while thicker shells (over 5 nm up to 20 nm) resulted in lower PL and CL signals. The reduction in signal intensity may be due to an increasing strain gradient, generated defects, and light scattering. Our results clearly show that the discussion on the optimal shell thickness is essential for proper design of the NW structures. R. Szymon, E. Zielony, et al., Small 2024, 2401139, 1-10.