## 3 Results
### Raman Spectrum of Graphene  
Figure 2 shows a typical Raman spectrum of graphene transferred with the sputtered AlN layer or MOCVD AlN interlayer on the sapphire substrate. Under the laser light, the electrons in the graphene jumped from the valence band to the conduction band, and the electrons interacted with the phonons to scatter, resulting in different Raman characteristic peaks. In both growth conditions, the peaks at 1350, 1584 and near 2700 cm\({}^{-1}\) were characteristic peaks related to the D, G, and 2D modes of graphene, respectively. The observation of the so-called G band, which was the graphene Raman active mode related to the stretching of the C-C bonds, appearing at 1584 cm\({}^{-1}\), was also important. In addition, the existence of an obvious and sharp G scattering peak indicated that the two-dimensional graphene layer was well present in the two structures. According to the principle of Raman phonon scattering of graphene, the G scattering peak (1584 cm\({}^{-1}\)) was generated by the in-plane vibration of the SP\({}_{2}\) carbon atom and the interaction between the dual degenerate in-plane transverse optical (iTO) and in-plane longitudinal optical (iLO) optical phonons in the center of the Brillouin zone [20; 21]. In addition, pure and single graphene layer did not contain disordered states, and the Raman spectra contained only G mode, D + D\({}^{\prime\prime}\) mode and 2D mode [22]. If the lattice size decreased, the long-range periodicity of the graphene was destroyed, proving that there appeared some defects and disorders in the graphene layer. Therefore, the D mode would be induced as a result of the disorders and defects existing in the graphene. Furthermore, the greater the degree of disorder, the higher the intensity of the D mode as well as the ratio of I\({}_{\text{D}}\)/I\({}_{\text{G}}\). In addition, the type of defects could also be judged by the ratio of the intensity of the I\({}_{\text{D}}\)/I\({}_{\text{D}^{\prime}}\) mode. (If the density of defects is very high, the D\({}^{\prime}\) mode will appear at 1620 cm\({}^{-1}\)). However, the ratio of I\({}_{\text{D}}\)/I\({}_{\text{G}}\) would decrease if the carbon structure was more amorphous, which led to the attenuation of all Raman peaks [23; 24]. Moreover, defects were not only characterized by the appearance of new defect-induced peaks but also the broadening of the peak in the Raman spectrum. As shown in Figure 2, the D band appeared in the both growing conditions, indicating that the disorders emerged in graphene in both growing conditions. Furthermore, using the basic principle of Raman spectroscopy to characterize the defects caused by the destroyed long-rangeperiodicity in a two-dimensional (2D) system, we mainly used the intensity of D + D\({}^{\prime\prime}\) mode to estimate the density of the defects. Therefore, from the Raman spectrum, the intensity of D + D\({}^{\prime\prime}\) scattering peak was stronger as graphene on the sputtered AlN layer, indicating that the high density of defects on the graphene/sputtered AlN buffer was more beneficial for nucleation during the growth of GaN. Additionally, according to experiment method, before the growth process of GaN films, the N source (NH\({}_{3}\)) etched the graphene surface and introduced defects and nucleation sites for subsequent epitaxial layer growth, so the intensity of the defect-induced D scattering peak was not very low. Besides, it is acknowledged that the intensity of 2D in the Raman spectra of graphene reflected the number of graphene layers. Thus, we came to the conclusion that the graphene transferred on the sputtered AlN interlayer was a single layer due to the intensity of 2D being large and sharp, and that when the GaN epilayer was grown on graphene/MOCVD AlN/sapphire composite substrate, the wet-transferred graphene was a not a monolayer layer due to the wrinkles caused by the wet-transferred process of graphene. It is known that single graphene and III nitrides were combined with each other by van der Waals forces.