Effect of carrier density on the surface morphology and optical properties of nanoporous GaN prepared by UV assisted electrochemical etching

Nanoporous GaN samples were prepared by UV-assisted electrochemical etching of Si-doped GaN using the HF-based electrolyte. The Si doping density in the as-grown GaN samples was varied from 1×10¹⁶ to 1×10¹⁹ cm^-3. The surface morphology and optical quality of these nanoporous GaN films are controlled by the doping density in the as-grown films. The scanning electron microscopy results reveal that the average pore size can be tuned from 100 to 45 nm by increasing the carrier density in the as-grown GaN films from 1×10¹⁶ to 1×10¹⁹ cm^-3. Photoluminescence (PL) measurements indicate that the nanoporous GaN films exhibit a higher PL intensity with redshifted band-edge PL peak compared to the as-grown GaN films. The nanoporous GaN prepared from highly doped sample gives highest PL intensity and stress relaxation due to the presence of smallest size nanopores. The redshift of the E₂(high) phonon mode in the Raman spectra of nanoporous GaN prepared from heavily Si-doped samples shows a significant relaxation of the compressive stress.