Structural characterization of porous GaN distributed Bragg reflectors using x-ray diffraction

Abstract

Porous GaN distributed Bragg reflectors (DBRs) provide strain-free, high-reflectivity structures with a wide range of applications across nitride optoelectronics. Structural characterization of porous DBRs is currently predominantly achieved by cross-sectional scanning electron microscopy (SEM), which is a destructive process that produces local data and has accuracy limited to around 3% by instrument calibration uncertainty. Here we show that high-resolution x-ray diffraction (XRD) offers an alternative, non-destructive method for characterizing porous nitride structures. XRD scans of porous GaN DBRs show that despite the constant lattice parameter across the DBR layers, characteristic satellite peaks still arise, which are due to interference between X-rays reflected from the porous and non-porous layers. By comparing the intensities and positions of the satellite peaks through diffraction patterns simulated from a kinematic model, the structural properties of the porous GaN DBRs can be analysed. Using our method we have measured a series of DBRs with stopbands from the blue wavelength region to the IR and compared their structural values with those from SEM data. Our results show that the XRD method offers improvements in the accuracy of determining layer thickness, although uncertainty for the value of porosity remains high. To verify the results gained from the XRD and SEM analysis we modelled the optical reflectivity using the structural values of both methods. We found that the XRD method offered a better fit to the optical data. XRD therefore offers accurate, non-destructive characterization of porous DBR structures, based on macro scale measurements and suitable for full wafer analysis. Methods Material Fabrication Layers of gallium nitride were grown by metalorganic vapour-phase epitaxy (MOVPE) in a Thomas Swan close-coupled showerhead reactor 16 using trimethylgallium, ammonia, and silane as precursors on c-plane sapphire substrates. Periodic layers of non-intentionally doped (NID) GaN and Si doped n-GaN were created by modulating the silane flow. The n-GaN layers were then porosified following a single etch step process described elsewhere 4. Three 10-pair DBR structures were grown with different layer thicknesses and a target doping density in the n-GaN layers of 10 19 cm-3. The three