Optical properties of epitaxial lateral overgrowth GaN structures studied by Raman and cathodoluminescence spectroscopies

Abstract

The properties of epitaxial lateral overgrowth (ELO) GaN layers were studied by means of cathodoluminescence (CL), micro-Raman spectroscopies, and transmission electron microscopy (TEM). CL shows a strong enhancement of the luminescence emission in the ELO regions, where TEM showed the absence of dislocations. The CL enhancement observed is mostly due to the yellow luminescence (YL) band, which contrasts with the good crystal quality observed in the ELO regions by TEM and Raman spectroscopy. Local CL spectra in the ELO regions showed a different behavior of the near band edge emission in relation to the buffer layers and central part of the vertical growth region. Donor-acceptor pair recombination is enhanced in the ELO regions. The Raman spectra indicate a low strain level in the ELO regions. Plasmon-coupled modes are not observed in these areas, indicating the absence of free carriers. CL and Raman observations could thus be explained in terms of the good crystalline quality of the ELO regions, in which the incorporation of impurities is electrically compensated by the formation of deep acceptors, probably VGa, responsible as well for the enhancement of the YL band.