posted on 2021-01-15, 18:20authored byCatherine Bougerol, Eric Robin, Enrico Di Russo, Edith Bellet-Amalric, Vincent Grenier, Akhil Ajay, Lorenzo Rigutti, Eva Monroy
Attaining low-resistivity AlxGa1–xN
layers is one keystone to improve
the efficiency of light-emitting devices in the ultraviolet spectral
range. Here, we present a microstructural analysis of AlxGa1–xN/Ge samples
with 0 ≤ x ≤ 1, and a nominal doping
level in the range of 1020 cm–3, together
with the measurement of Ge concentration and its spatial distribution
down to the nanometer scale. AlxGa1–xN/Ge samples with x ≤ 0.2 do not present any sign of inhomogeneity. However,
samples with x > 0.4 display μm-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
AlxGa1–xN/Ge layers, generally associated with Ga-rich regions around structural
defects. With these local exceptions, the AlxGa1–xN/Ge matrix presents
a homogeneous 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 AlxGa1–xN 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 an n-type AlGaN dopant, even at
Al concentrations where Ge DX centers are not expected to manifest.
Therefore, this information can have direct impact on the performance
of Ge-doped AlGaN light-emitting diodes, particularly in the spectral
range for disinfection (≈260 nm), which requires heavily doped
alloys with a high Al mole fraction.