Various forms of the InGaN micro-LED concept have been reported in recent years, all of which indicate that an enhanced efficiency can be achieved with such microstructures. The overall external quantum efficiency of an InGaN LED is heavily dependent on the internal quantum efficiency (which is limited by the build-in piezoelectric field) and the extraction efficiency (due to the high refractive index of nitride materials). A significant increase in extraction efficiency is certainly achieved in the current generation of micro-LEDs, where the dimension of an individual element of typically larger than 4mm. However, in order to achieve a reduction of the piezoelectric field in the InGaN quantum wells, the dimension of the micro-LED should be further reduced. In fact, Demangeot et al observed significant strain relaxation in InGaN microstructures of less than 1 μ m through micro-Raman scattering, suggesting that micro-LEDs should be scaled down to sub-micron dimensions in order to benefit from an increased in internal quantum efficiency.

We report on the fabrication of sub-micron InGaN ring microstructures using standard micro-fabrication techniques. 1mm disk structures are patterned using photoresist as a masking material and standard photolithography. The ring structures are spontaneously formed after photoresist development. The exact mechanism for the spontaneous ring formation is unknown at this time. The ring structures, as shown in the SEM image of Figure 1, have an external diameter of 1.5 μ m with a wall width of 750nm. Preliminary evidence of strain relaxation in these structures is provided by the spectroscopic results shown in Figure 2. From the normalized cathodoluminescene spectra, a consistent blue shift is observed as the dimension of the InGaN structure is scaled down. High-efficiency InGaN micro-LEDs can potentially be fabricated from these sub-micron ring structures.

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1. Electron Channelling Contrast Imaging of Defects in Nitride Semiconductor Thin Films 
2. Luminescence and structural studies of In-containing III-nitrides