Blue and UV light emitting diodes (LEDs) and lasers based on AlN and GaN semiconductor compounds represent one of the most important breakthroughs in electronics and optoelectronics of recent years. By varying the composition of ternary Al_xGa_(1-x)N alloys and by the achievement of controlled doping, the optical and electrical properties can be tuned over a wide spectral range.

So far, n-type conductivity is commonly obtained by the incorporation of Si, which is supposed to substitute for Ga or Al in the GaN or AlN lattice, respectively. The dependence of the AlGaN material properties on Si doping has already been reported, but some observed trends are not well understood yet, e.g. the effect of Si on the defect formation, alloy disorder, and stress relaxation in AlGaN is still under discussion. The observed strong correlation between electrical and optical properties is generally explained by a random distribution of impurities and defects that may produce local strain and potential fluctuations. However, little direct information on the spatial compositional distribution and local structure of Si in AlGaN lattice is available.

In this study, we report the application of synchrotron radiation X-ray microprobe to the study of Si impurities in plasma-induced molecular bema epitaxy (PIMBE) grown Al_0.32Ga_0.68N. The synchrotron-related data were collected at the LUCIA beamline of Soleil installed at the Swiss Light Source (Villigen, Switzerland). Elemental maps obtained by micro-x-ray fluorescence spectrometry show inhomogeneous distributions of Si, Al, and Ga on the micron scale. X-ray absorption near-edge structure spectra taken at the Si and Al K edges provided information about their local chemical environment and revealed the change of the spectral features as depending on the position compared to the sample surface and on the concentration of Si.

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