Influence of intentionally strained sapphire substrate on GaN epilayers

The structural and optical properties of GaN epilayers grown on intentionally strained sapphire (0001) substrates by metalorganic chemical vapor deposition (MOCVD) were investigated. Intentionally tensile strained sapphire substrates were prepared by the implantation of 2.4 MeV energy Cl ⁺ and As⁺-ions with a 10¹⁵cm^-2 dose where the projection ranges (R_p) were 1.16 μm and 0.95 μm, respectively. It was found from Raman spectroscopy that the compressive stress normally existed in the GaN epilayer/sapphire was decreased by the use of a Cl⁺-ion-implanted sapphire substrate. The intentionally tensile strained sapphire surface can result in a superior crystalline GaN epilayer on top of it. However, excessively roughened and modified surface by As ⁺-ions degraded the GaN epilayer. Not only the ionic radius but also the chemical species of implanted ions in sapphire affected the crystal quality of GaN epilayers. Therefore, we concluded that the properly tensile-strained sapphire (0001) substrate could improve the properties of GaN epilayers grown by MOCVD.