Porous gallium nitride generated by electroless etching: Morphology, optical properties, and sensing applications

Since the discovery of visible luminescence in porous Si in 1990, porous semiconductors have attracted considerable interest for possible applications in opto-electronic devices. Porous GaN is of particular interest for opto-electronics, due to its large direct bandgap (3.4 eV). It is also of interest for use as a substrate for epitaxial growth and for chemical sensing. Porous GaN was produced by a novel Pt-assisted electroless etching technique. The morphology of porous GaN, as revealed by SEM, is dominated by the formation of a ridge-and-valley structure with a deep porous network of anisotropic columnar pores ∼100 nm in diameter extending through the entire GaN film. The ridge structures on the surface may be removed by sonication, resulting in a uniform porous morphology. Cathodoluminescence and photoluminescence studies revealed an increase in the luminescence efficiency for certain porous GaN films, but not others, likely due to a decreased defect density of the starting bulk GaN. Raman spectroscopy studies revealed increased scattering efficiency, scattering by geometry forbidden phonon modes, and strong Frohlich scattering with above bandgap excitation.; In addition to characterization, several possible applications for porous GaN have been explored. Currently, because there is not a route for the growth of bulk GaN, GaN films must be grown heteroepitaxially. Substrates for GaN growth are not ideal, and introduce strain and defects. Porous GaN has been studied as an alternative substrate for high-quality epitaxial GaN film growth, and has shown modest improvement in crystal quality over conventional substrates. Additionally, porous GaN has been functionalized for use as a surface enhanced Raman spectroscopy (SERS) substrate by solution-based electroless deposition and vacuum evaporation of Ag and Au. SERS enhancement factors up to 10 8 have been observed for Ag-coated porous GaN, allowing the spectroscopic detection of trace amounts of analytes. The advantages of using porous GaN as a SERS substrate are the simplicity of substrate preparation and the consistency of the SERS response across the substrate. Ongoing studies are focused on using the innate properties of crystalline GaN and the high surface area inherent to the porous film as conductivity or hydrogen sensors.