  This paper assesses the effects of Ge-doping on the structural and optical
(band-to-band and intersubband (ISB)) properties of GaN/AlGaN multi-quantum
wells (QWs) designed to display ISB absorption in the short-wave, mid- and
far-infrared ranges (SWIR, MIR, and FIR, respectively). The standard c-plane
crystallographic orientation is considered for wells absorbing in the SWIR and
MIR spectral regions, whereas the FIR structures are grown along the nonpolar
m-axis. In all cases, we compare the characteristics of Ge-doped and Si-doped
samples with the same design and various doping levels. The use of Ge appears
to improve the mosaicity of the highly lattice-mismatched GaN/AlN
heterostructures. However, when reducing the lattice mismatch, the mosaicity is
rather determined by the substrate and does not show any dependence on the
dopant nature or concentration. From the optical point of view, by increasing
the dopant density, we observe a blueshift of the photoluminescence in polar
samples 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 of
the linewidth when using Ge as a dopant for high doping levels, whatever the
spectral region under consideration (i.e. different QW size, barrier
composition and crystallographic orientation).
