## 1 Introduction  
As a group-III nitrides semiconductor, gallium nitride (GaN) materials with superior properties such as wide electronic energy band, wurtzite structural symmetry, strong atomic bonds, high thermal conductivity and so on, have shown major potential in the manufacturing field of optoelectronic devices [1, 2]. In recent decades, GaN has demonstrated broad development prospects in the application of high-brightness blue/green light emitting diodes (LEDs), blue laser diodes (LDs), photoelectric detectors and deep ultraviolet optoelectronic devices [3, 4, 5]. Although remarkable progress in the growth of GaN and the fabrication technologies of the GaN-based devices has been made, some basic properties of GaN such as phonon dynamics and interactions (phonon-electron and phonon-phonon) still need to be studied further. It is known that the the stress in the GaN epilayer caused by the lattice constants and thermal expansion coefficient mismatch between the substrate and GaN films is still an extremely critical issue in the current study [6, 7, 8]. The existence of stress will induce defects, which are considered as the primary factor restraining further development of high efficiency III-nitrideoptoelectronic devices. In addition, the mechanism of lattice vibration and phonon scattering inside GaN is also crucial for the performance improvement of high-speed and high-power GaN-based devices. Raman spectroscopy is a non-contact, damage-free characterization method that does not require preparation of the sample in advance compared with other characterization methods. More importantly, the stress and the interaction between phonon and carrier properties of GaN can be characterized by Raman spectra without damage to the sample. For this reason, it is greatly helpful to explore the internal information of GaN crystals with Raman spectra. In addition, being proficient in the knowledge of phonon dynamics and interactions (electron-phonon or phonon-phonon) with free carriers of GaN is also an essential foundation to the principle of lattice scattering and the molecular vibration. To date, in most of the Raman scattering studies performed for GaN, there have been research results showing that all symmetry-allowed optical phonons of the Raman spectra were detected in the sapphire substrate according to the polarization selection rules by [PERSON] [9]. Furthermore, a high-pressure structural phase transition was observed in GaN at 47 GPa by means of Raman spectroscopy [10]. The coupled modes of the longitudinal optical (LO) phonon and overdamped plasma in the GaN grown with aluminum nitride (AlN) buffer layer by Metal-organic Vapor Phase Epitaxy (MOVPE) have also been studied in sapphire substrate [11], which indicated the manner in which the different carrier concentrations changed with the A\({}_{1}\) (LO) scattering peak shifts. We already know that depositing different buffer layers on the single crystal sapphire substrate will greatly affect the quality of the GaN epitaxial layer above it, which further affects the phonon scattering process in the GaN epitaxial layer. By studying the Raman phonon behavior in GaN films, it can provide a support for subsequent analysis and improvement about the quality of GaN films. In recent years, the use of graphene as a buffer layer to grow GaN epitaxial layers has attracted significant interest. Graphene possesses many properties that can improve the quality of GaN epitaxial layers and GaN devices [6; 7; 8; 12; 13; 14]. However, there is no systematic theoretical study on the phonon scattering behavior in the GaN epilayer obtained by inserting graphene buffer layers. In this work, we have used Raman spectroscopy to investigate the A\({}_{1}\) (LO) and E\({}_{2}\) (high) phonons in high-quality GaN thin films using different AlN buffer layers with the metal-organic chemical vapor deposition (MOCVD) two-step method.