Micro-Raman investigation of strain in GaN and AlxGa1−xN/GaN heterostructures grown on Si(111)

Using micro-Raman spectroscopy, we have studied the vibrational properties of GaN and ${\mathrm{Al}}_{\text{0.5}}{\mathrm{Ga}}_{\text{0.5}}\mathrm{N/GaN}$ long period superlattices (SLs) grown on Si(111). Crack-free areas of GaN layers grown on Si(111) exhibit residual tensile stress, which is evidenced by the red shift of the frequency of $E_2(\mathrm{TO})$ phonon. We have derived the strain cartography in GaN and ${\mathrm{Al}}_{\text{0.5}}{\mathrm{Ga}}_{\text{0.5}}\mathrm{N/GaN}$ long period SLs, which shows that cracking leads to strain relaxation. In addition, the AlGaN layers on GaN introduce an additional component of compressive strain into the GaN layers in these SLs. The amount of strain is quantified using micro-Raman analyses and by taking into account the elastic properties of GaN and AlGaN. By introducing a thin, low temperature InGaN interlayer, we could significantly reduce the crack density of the GaN layer.