Distributed Bragg reflectors with porous GaN: Experimental assessment of effective medium approximations

Porous GaN offers a versatile platform for refractive index engineering in functional layers that have been defined by epitaxial growth. While low-refractive-index layers based on electrochemical porosification of GaN have readily been integrated in a multitude of devices, the relationship between the layers' refractive indices and their porosity has only been investigated by simulations so far. Based on numerical investigations, the volume averaging theory effective medium approximation (EMA) has commonly been applied for porous GaN distributed Bragg reflectors (DBRs). In this work, we present an independent determination of porosity together with an optical extraction of the refractive index, allowing us to test the most common EMAs for DBRs designed for peak reflectance in the blue spectral range. The measurements show that the parallel model EMA describes the obtained data most accurately. Consequently, the parallel model is found to yield a better identification between refractive index and porosity in ultrathin porous GaN layers.