Molecular Beam Epitaxy Growth and Characterization of Germanium‐Doped Cubic Al x Ga 1− x N

In cubic (c‐)GaN Ge has emerged as a promising alternative to Si for n‐type doping, offering the advantage of slightly improved electrical properties. Herein, a study on Ge doping of the ternary alloy c‐Al x Ga 1− x N is presented. Ge‐doped c‐Al x Ga 1− x N layers are grown by plasma‐assisted molecular beam epitaxy. In two sample series, both the Al mole fraction x and the doping level are varied. The incorporation of Ge is verified by time‐of‐flight secondary ion mass spectrometry. Ge incorporation and donor concentrations rise exponentially with increasing Ge cell temperature. A maximum donor concentration of 1.4 × 10 20  cm −3 is achieved. While the incorporation of Ge is almost independent of x , incorporation of O, which acts as an unintentional donor, increases for higher x . Dislocation densities start increasing when doping levels of around 3 × 10 19  cm −3 are exceeded. Also photoluminescence intensities begin to drop at these high doping levels. Optical emission of layers with x  > 0.25 is found to originate from a defect level 0.9 eV below the indirect bandgap, which is not related to Ge. In the investigated range 0 ≤  x  ≤ 0.6, Ge is a suitable donor in c‐Al x Ga 1− x N up to the low 10 19  cm −3 range.