Effect of annealing temperature and ambience on roughened GaN substrate

Improving light extraction of GaN-on-GaN LEDs is still a challenge. This work attempted to address this issue by investigating the effect of temperature and ambience of annealing in obtaining high hexagonal pyramids density of roughened backside (N-face) GaN substrates for the light extraction improvement. Here, the GaN substrates were annealed prior to the roughening. The annealing temperature was varied at 600 °C and 700 °C in nitrogen, air, and oxygen ambiences. Results showed that the hexagonal pyramids density could be increased up to 4.3 × 1010 cm-2 by pre-annealing the GaN substrate in oxygen at 600 °C. In comparison to other ambiences, oxygen promoted more Ga–O compounds on the surface during the annealing. This enhanced the etching effect and hence, increased the hexagonal pyramids density. Meanwhile, with oxygen pre-annealing at 700 °C, the pyramids density for the GaN substrate was around 1.0 × 1010 cm-2. This result was almost comparable to the substrates annealed in other ambiences at 700 °C. The reason is that annealing at 700 °C was not only improved the GaN crystals, but also the Ga–O compounds. This turned the compounds to be in more stable structure and therefore, difficult to be dissolved by the etching. Moreover, the roughened GaN substrate with the highest pyramids density obtained in this work (through the 600 °C pre-annealing in oxygen) led to higher optical power of GaN based LED as compared its counterparts.