Light extraction efficiency of thin-film GaN-based light-emitting-diode (LED) chip can be effectively improved by surface roughening. The film transfer is an indispensable process in the manufacture of thin-film LED chip, which means transferring the LED film from the growth substrate to a new substrate, and then removing the growth substrate. After the growth substrate is removed, the buffer layer is used to cushion the mismatch between the substrate and the n-GaN exposed, which has a significant influence on the roughening behavior of n-GaN. Unlike the GaN buffer layer grown on sapphire substrate, AlN buffer layer is usually used when n-GaN is grown on Si substrate. In this paper, the surface treatment of the AlN buffer layer by reactive ion etching (RIE) is used to improve the surface roughening effect of N-polar n-GaN grown on the silicon substrate in the hot alkali solution (85 ℃, 20% KOH mass concentration of solution), and the mechanism of the influence of the surface treatment on the roughening behavior is discussed by X-ray photoelectron spectroscopy (XPS) and other advanced methods. The degree of etching surface AlN buffer layer is detected by energy dispersive spectrometer (EDS), the sample surface state after RIE etching is analyzed by XPS, the morphology of the surface roughening is observed by scanning electron microscope (SEM) and the effect of surface roughening on the optical power of LED devices is verified by the photoelectric performance test. The EDS results show that the AlN buffer layer remains after RIE etching 10-30 min and the AlN disappears after RIE etching for 40 min. The SEM results show that surface states of AlN buffer layer have a great influence on the roughening behavior of n-GaN in KOH solution. The sample with part of AlN buffer layer has a good roughening effect and proper size hexagonal pyramid distributing uniformly. In addition, the rate of coarsening is too fast for the samples with AlN buffer layer completely removed, while the rate is too slow for the samples without any etching process. In summation, using RIE etching to remove a part of the AlN buffer layer can effectively improve the roughening effect of N-polar n-GaN in KOH solution. We believe that lots of N-vacancies are produced on the surface of the sample after RIE etching, which provides the electrons, thereby causing the surface Fermi level to be elevated. The XPS analysis shows that the RIE etching can improve the electronic binding energy of Al 2p of AlN buffer layer, resulting in a shift of the surface Fermi level near to the conduction band, and reducing the Schottky barrier between the KOH solution and the surface of the sample, which is beneficial to the surface roughening. To remove a part of the AlN buffer by using plasma etching layer can improve the roughening effect of N-polar n-GaN in KOH solution, resulting in the output power of the corresponding LED device being improved obviously.