Structure–property relationships in porous GaN generated by Pt-assisted electroless etching studied by Raman spectroscopy

Porous GaN (PGaN) produced by Pt-assisted electroless etching has been studied by Raman spectroscopy. Three different unintentionally doped GaN films grown by hydride vapor phase epitaxy on sapphire were used for this study. Pt-assisted electroless etching takes place without external contact to a current source, simplifying the generation of porous semiconductors. Raman spectra were excited with both subband gap (441.6 nm) and above-band gap (325 nm) radiation, and collected in an unpolarized backscattering configuration. In both cases, a strong increase in scattering is observed for the Raman modes, depending on the depth of the porous layer. With visible excitation, symmetry-forbidden A1(TO) and E1(TO) are observed in PGaN, and spectral line shape analysis reveals the presence of the forbidden E1(LO) mode as well. Raman spectra were also analyzed using an effective medium theory, revealing extraordinary LO phonon scattering with visible, and Fröhlich scattering with UV, excitation. In addition a quasi-LO phonon is observed in the visible spectrum. All of the characteristic features of PGaN Raman scattering: symmetry-forbidden modes, quasi-LO scattering, and strong Fröhlich mode scattering, can be attributed to the dendritic porous morphology of PGaN.