Role of sample thickness and self-absorption effects in simultaneous XEOL-XAS measurements on single crystalline ZnO and GaN

The x-ray excited optical luminescence (XEOL) for defect and near band edge (NBE) transitions combined with simultaneous x-ray absorption measurements are experimentally and theoretically studied on single crystalline ZnO and GaN across the Zn and Ga <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>K</a:mi></a:mrow></a:math> edges, respectively, in a wide range of sample thicknesses. Increasing the sample thickness leads to the appearance of an inverted line shape and negative edge jump for the XEOL defect response, whereas the line shape of the XEOL NBE edge remains positive. A one-dimensional transport model is developed, which includes experimental geometry, the creation of x-ray generated excitations, diffusion and recombination of the carriers, and reabsorption of x-ray fluorescence and XEOL photons. The model calculations reproduce the experimentally observed changes of the edge shape in the XEOL spectra caused by variation of the sample thickness and reveal surface recombination and optical absorption as the main factors determining the XEOL edge shape for a given sample thickness.