We report pulsed atomic layer epitaxy growth of a very high crystalline quality, thick (~2 µm) and crack-free AlN material on c-plane sapphire substrates via a sandwich method using metal organic chemical vapor deposition. This sandwich method involves the introduction of a relatively low temperature (1050 °C) 1500 nm thick AlN layer between two 250 nm thick AlN layers which are grown at higher temperature (1170 °C). The surface morphology and crystalline quality remarkably improve using this sandwich method. A 2 µm thick AlN layer was realized with 33 arcsec and 136 arcsec full width at half maximum values for symmetric and asymmetric reflections of ω-scan, respectively, and it has an atomic force microscopy root-mean-square surface roughness of ~0.71 nm for a 5 × 5 µm2 surface area.
GaN-on-GaN light emitting diodes (LEDs) have been considered to present the path towards ultimate LED technology. However, at present, the performance of the LEDs is still lower in comparison to GaN-on-sapphire LEDs. This work attempted to identify the possible factors that limit the performance of a GaN-on-GaN LED. To perceive this clearly, a GaN-on-patterned sapphire substrate (PSS) LED was also included in this study for comparison. It was found that the GaN-on-GaN LED exhibited lower performance (e.g. external quantum efficiency) than the GaN-on-PSS LED, especially at higher currents, despite its dislocation density being much lower than the latter. This is because the GaN-on-GaN LED had a lack of V-pits formation and carriers localization. As a result, the LED showed higher operating voltage and lower radiative recombination in comparison to the GaN-on-PSS LED.
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