Optimization of porous GaN distributed Bragg reflectors via low-concentration oxalic acid electrochemical etching

Distributed Bragg reflectors (DBRs) are essential components of resonant cavities in optoelectronic devices and have proven difficult to manufacture in the nitride epitaxial system due to a lack of lattice-matched materials with significantly different refractive indices. Porous gallium nitride (GaN) DBRs, fabricated by a defect-mediated electrochemical etching (ECE) process, offer an alternative basis for achieving high reflectance using alternating porous/non-porous GaN layers. These are prepared in ECE by selectively porosifying the doped layers in a doped/undoped GaN layer stack. Improving reflectance of these devices involves the maximization of porosity (%) in the porous layers. Few attempts to optimize the parameter space of the ECE experiment have been made, aside from variations in applied voltage. In this work, we report porous GaN DBRs prepared on both sapphire and silicon substrates etched in low-concentration oxalic acid. Previously, we reported findings on thick doped layer GaN samples where lower concentrations of oxalic acid produce increases in porosity due to a change in the relative concentrations of singly and doubly charged anions. Here, we report that this can also lead to significant increases in porosity in porous layers of DBRs by changing the morphology of pores and increasing the number of dislocations that participate in etching. This leads to significant increases in reflectance of simple five-layer pair devices when prepared on sapphire substrates, but the same devices prepared on silicon substrates suffer from mechanical delamination at the substrate interface. Suggestions for circumventing delamination include exploitation of buffer solutions and compensatory doping in buffer layers.