Lattice order in thulium-doped GaN epilayers: In situ doping versus ion implantation

We have investigated the crystalline quality of thulium-doped GaN obtained either by in situ doping during MBE growth or by ion-beam implantation of MOCVD GaN layers. Both types of samples display the typical sharp intra-4f shell emission lines of Tm3+ ions in the blue and infrared spectral regions. The Raman spectra of the MBE samples indicate a good crystalline quality, showing the narrow E2 and A1 phonon peaks characteristic of GaN, even for the highest Tm concentrations. In contrast, Raman peaks associated with vacancy-related defects, as well as low-frequency bands due to disorder-activated modes can be observed in the Raman spectra of the implanted samples. These results indicate that, for the implantation doses required to achieve Tm emission, some residual disorder remains in the implanted GaN layers which is not observed in Tm-doped MBE samples with higher Tm concentration.