Abstract
The non-centrosymmetricity of III-nitride wurtzite crystals enables metal or nitrogen polarity with dramatically different surface energies and optical properties. In this work, III-polar and N-polar nanostructured ultraviolet multiple quantum wells (UV-MQWs) were fabricated by nanosphere lithography and reactive ion etching. The influence of KOH etching and rapid thermal annealing treatments on the luminescence behaviors were carefully investigated, showing a maximum enhancement factor of 2.4 in emission intensity for III-polar nanopillars, but no significant improvement for N-polar nanopillars. The discrepancy in optical behaviors between III- and N-polar nanopillar MQWs stems from carrier localization in III-polar surface, as indium compositional inhomogeneity is discovered by cathodoluminescence mapping, and a defect-insensitive emission property is observed. Therefore, non-radiative recombination centers such as threading dislocations or point defects are unlikely to influence the optical property even after post-fabrication surface treatment. This work lays solid foundation for future study on the effects of surface treatment on III- and N-polar nanostructured light-emitting-diodes and provides a promising route for the design of nanostructure photonic devices.
Highlights
III-nitride based ultraviolet light-emitting-diodes (UV-LEDs) are useful for many applications including UV curing, photo therapy and UV disinfection, due to the direct bandgap property of wurtzite phase III-nitride crystals [1,2,3,4]
The UV emission intensity increased significantly after post-fabrication treatment and by as much as 2.4 times after rapid thermal annealing (RTA) annealing for III-polar nanopillars, whereas almost no effect on the luminescence spectra was seen for N-polar samples
CL intensity maps indicate carrier localization effects due to In-rich clusters in the N-polar nanopillars, which dominated the light output mechanism because of the reduced carrier diffusion length scales. This led to reduced probability of carrier recombination with threading dislocations or point defects, meaning that despite the fact that surface treatment passivation methods might reduce the trap defects, they did not contribute to the PL efficiency
Summary
III-nitride based ultraviolet light-emitting-diodes (UV-LEDs) are useful for many applications including UV curing, photo therapy and UV disinfection, due to the direct bandgap property of wurtzite phase III-nitride crystals [1,2,3,4]. Challenges in light extraction have encouraged research and development of small length scale devices for new applications, like nanostructured LEDs, for example [8,9,10]. The three-dimensional geometry of the nanopillars or nanoholes allows light to be extracted from the sidewalls of the nanostructures. This becomes increasingly important as the emission wavelength of the LEDs moves from visible to ultraviolet wavelength region due to increasing proportion of TM-polarized light, as well as the growing impact of light absorption in the p-AlGaN contact layer [11,12]. The benefits from strain relaxation in nanostructured LEDs means reduced influence from quantum confined stark effect (QCSE), which has an adverse impact on the electron and hole wavefunction overlap [13]
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