  Attaining low resistivity AlGaN layers is the keystone to improve the
efficiency of light emitting devices in the ultraviolet spectral range. Here,
we present a microstructural analysis of Ge-doped AlGaN samples with Al mole
fraction from x=0 to 1, and nominal doping level in the range of 1E20 cm-3,
together with the measurement of Ge concentration and its spatial distribution
down to the nm scale. AlGaN:Ge samples with x smaller or equal to 0.2 do not
present any sign of inhomogeneity. However, samples with x > 0.4 display
micrometer-size Ge crystallites at the surface. Ge segregation is not
restricted to the surface: Ge-rich regions with a size of tens of nanometers
are observed inside the AlGaN:Ge layers, generally associated with Ga-rich
regions around structural defects. With this local exceptions, the AlGaN:Ge
matrix present an homogenous Ge composition which can be significantly lower
than the nominal doping level. Precise measurements of Ge in the matrix provide
a view of the solubility diagram of Ge in AlGaN as a function of the Al mole
fraction. The solubility of Ge in AlN is extremely low. Between AlN and GaN,
the solubility increases linearly with the Ga mole fraction in the ternary
alloy, which suggests that the Ge incorporation takes place by substitution of
Ga atoms only. The maximum percentage of Ga sites occupied by Ge saturates
around 1%. The solubility issues and Ge segregation phenomena at different
length scales likely play a role in the efficiency of Ge as n-type AlGaN
dopant, even at Al concentrations where Ge DX centers are not expected to
manifest. Therefore, this information can have direct impact in the performance
of Ge-doped AlGaN light emitting diodes, particularly in the spectral range for
disinfection (around 260 nm), which requires heavily-doped alloys with high Al
mole fraction.
