Effect of compositionally graded AlGaN buffer layer grown by different functions of trimethylaluminum flow rates on the properties of GaN on Si (111) substrates

•A series of functions of TMAl flow rate are used.•Linearly graded AlGaN is obtained.•Compressive stress of GaN on Si substrate is obtained.•The relation of stress and dislocations is discussed.•3-μm-thick crack-free GaN film on Si substrate is obtained.

In the present paper, compositionally graded AlGaN buffer layer was employed to compensate the tensile stress and improve the crystalline quality of GaN grown on silicon substrate. During the growth of the graded buffer layer, the flow rate of trimethylaluminum (TMAl) was gradually decreased according to a series of functions, while the flow rate of trimethylgalliumm (TMGa) was increased with the same linear function. Therefore, these graded AlGaN layers showed different distributions of Al, which was confirmed by the high-resolution X-ray diffraction (HR-XRD) and the secondary ion mass spectroscopy (SIMS). A linear compositionally graded AlGaN layer was obtained by using the convex parabolic TMAl flow rate, which leads to a large compressive stress in GaN layer due to a linearly gradual change of lattice parameters and thermal expansion coefficient (TEC) from AlN seeding layer to GaN layer. Using the linear compositionally graded buffer layer, a 3-μm-thick crack-free GaN film with a low dislocation density was grown on silicon substrate. The dislocation evolution associated with stress was investigated by the cross-sectional micro-Raman scattering and transmission electron microscopy (TEM).