Structural properties of GaN layers on Si(001) grown by plasma-assisted molecular beam epitaxy

We report on the growth and microstructure of GaN films deposited on Si(001) substrates by plasma-assisted molecular beam epitaxy. GaN films grown directly on Si(001) are found to be phase mixtured, containing both cubic (β) and hexagonal (α) modifications. The origin of this phase mixture is identified to be due to the formation of amorphous ${\mathrm{Si}}_x{\mathrm{N}}_y$ at the GaN/Si interface during the nucleation stage. Therefore, a GaAs buffer layer is employed to prevent the formation of ${\mathrm{Si}}_x{\mathrm{N}}_y.$ GaN films grown on this $\mathrm{GaAs/Si}\text{(001)}$ structure are in fact predominantly cubic and exhibit the characteristic band-edge photoluminescence (PL) of β-GaN up to room temperature. However, the PL efficiency from these samples is low compared to that of β-GaN layers directly grown on GaAs(001). We explain the lower PL efficiency by the presence of additional structural defects, which are observed in the GaN/GaAs/Si(001) heterostructure by transmission electron microscopy.