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

• Published in: Journal of applied physics 2019-12, Vol.126 (21)
• Main Authors: Griffin, P. H., Frentrup, M., Zhu, T., Vickers, M. E., Oliver, R. A.
• Format: Article
• Language: English
• Subjects: Applied physics; Bragg reflectors; Computer simulation; Diffraction patterns; Gallium nitrides; Mathematical models; Nondestructive testing; Optoelectronics; Porosity; Reflectance; Scanning electron microscopy; Structural analysis; Thickness; Uncertainty; X-ray diffraction; X-rays
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.5134143

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, nondestructive 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 the interference between x-rays reflected from the porous and nonporous 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 analyzed. Using our method, we have measured a series of DBRs with stop bands 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 modeled 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, nondestructive characterization of porous DBR structures based on macroscale measurements and is suitable for full wafer analysis.