DETERMINING PERIOD VARIATIONS IN A DISTRIBUTED-BRAGG-REFLECTOR THROUGH HIGH-RESOLUTION X-RAY-ANALYSIS

We studied the structural properties of a GaAs/AlAs distributed Bragg reflector using high resolution X-ray techniques. We evaluated diffraction and reflection scans by considering several possible structural defects including period and individual thickness variations, compositionally graded interface layers, and interfacial roughness. Diffraction scans provided information on layer thicknesses and crystalline quality. Both X-ray diffraction and reflectivity curves were compared to calculated curves using dynamical theory and Fresnel multilayer theory, respectively. X-ray reflectivity measurements and triple axis reciprocal space measurements showed the interfaces to be abrupt, and the superlattice to deviate from perfect periodicity. Together, these techniques provided a detailed analysis of superlattice parameters. The standard deviation of layer variations was shown to be 1.5-2.0% for the quarter-wave stacks. We also present for the first time, the sensitivity of X-ray reflectivity measurements to period deviation, and use reciprocal space maps to distinguish period dispersion from interface roughness. The results indicate the accuracy of high resolution X-ray techniques for quantitative superlattice analysis.