Study of in-depth strain variation in ion-irradiated GaN

The strain state of (0001) GaN layers grown on 4H-SiC by Metal Organic Vapour Phase Epitaxy (MOV- PE) and its change upon ion irradiation was studied by Raman microscopy (RM) and X-ray diffraction (XRD). The layers were irradiated with light (H, C, O) and heavy (Fe, Kr) ions at high (68 and 120 MeV) and low (2 MeV) energies in order to simulate the situation in space. The in-depth profiles of the E2 phonon frequency of the layers recorded across the cleaved edge show a pronounced gra- dient between surface and interface. Taking the in-plane strain data calculated from XRD into account, the layers are generally tensile-strained but nearly strain-compen- sated at the interface. This is in line with the strain development during the growth process as measured by the wafer curvature. The E2 frequencies and their gradient over the layer thickness are not significantly changed by irradiation with high-energy ions. Small changes are seen for layers irradiated with heavy low-energy ions that pen- etrate into the layer only a few microns. Consequently, GaN can be considered as hard against space-like ion irradiation in view of change of the strain state.