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Attaining low resistivity AlGaN layers is the keystone to improve theefficiency of light emitting devices in the ultraviolet spectral range. Here,we present a microstructural analysis of Ge-doped AlGaN samples with Al molefraction 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 distributiondown to the nm scale. AlGaN:Ge samples with x smaller or equal to 0.2 do notpresent any sign of inhomogeneity. However, samples with x > 0.4 displaymicrometer-size Ge crystallites at the surface. Ge segregation is notrestricted to the surface: Ge-rich regions with a size of tens of nanometersare observed inside the AlGaN:Ge layers, generally associated with Ga-richregions around structural defects. With this local exceptions, the AlGaN:Gematrix present an homogenous Ge composition which can be significantly lowerthan the nominal doping level. Precise measurements of Ge in the matrix providea view of the solubility diagram of Ge in AlGaN as a function of the Al molefraction. The solubility of Ge in AlN is extremely low. Between AlN and GaN,the solubility increases linearly with the Ga mole fraction in the ternaryalloy, which suggests that the Ge incorporation takes place by substitution ofGa atoms only. The maximum percentage of Ga sites occupied by Ge saturatesaround 1%. The solubility issues and Ge segregation phenomena at differentlength scales likely play a role in the efficiency of Ge as n-type AlGaNdopant, even at Al concentrations where Ge DX centers are not expected tomanifest. Therefore, this information can have direct impact in the performanceof Ge-doped AlGaN light emitting diodes, particularly in the spectral range fordisinfection (around 260 nm), which requires heavily-doped alloys with high Almole fraction.

Solubility limit of Ge Dopants in AlGaN: a Chemical and Microstructural Investigation down to the Nanoscale