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This paper assesses the effects of Ge-doping on the structural and optical(band-to-band and intersubband (ISB)) properties of GaN/AlGaN multi-quantumwells (QWs) designed to display ISB absorption in the short-wave, mid- andfar-infrared ranges (SWIR, MIR, and FIR, respectively). The standard c-planecrystallographic orientation is considered for wells absorbing in the SWIR andMIR spectral regions, whereas the FIR structures are grown along the nonpolarm-axis. In all cases, we compare the characteristics of Ge-doped and Si-dopedsamples with the same design and various doping levels. The use of Ge appearsto improve the mosaicity of the highly lattice-mismatched GaN/AlNheterostructures. However, when reducing the lattice mismatch, the mosaicity israther determined by the substrate and does not show any dependence on thedopant nature or concentration. From the optical point of view, by increasingthe dopant density, we observe a blueshift of the photoluminescence in polarsamples due to the screening of the internal electric field by free carriers.In the ISB absorption, on the other hand, there is a systematic improvement ofthe linewidth when using Ge as a dopant for high doping levels, whatever thespectral region under consideration (i.e. different QW size, barriercomposition and crystallographic orientation).

Effect of Ge-doping on the short-wave, mid- and far-infrared intersubband transitions in GaN/AlGaN heterostructures