Impact of Ga deposition and desorption on the properties of high Al-fraction AlGaN grown by plasma-assisted molecular beam epitaxy

We have grown two sets of 500-nm-thick n+-Al0.85Ga0.15N layers on AlN templates at various substrate temperatures between 700 and 875 °C by plasma-assisted molecular beam epitaxy, differing only in the detailed surface treatment prior to growth. In one set, Ga deposition and desorption (Ga D&D) on the template surface is performed at the substrate temperature used for growth prior to film deposition, while in the other set, the Ga D&D is performed at 875 °C for all the samples. These layers were doped with Si at a nominal concentration of 3 × 1019 cm−3. We observe significantly red-shifted (∼850 meV) photoluminescence for sub-bandgap optical pumping, the intensity of which increases as the growth temperature of the layer increases. We attribute this sub-bandgap emission to the presence of nanoscale compositional inhomogeneities in the layers and speculate that they contribute to the reduced Hall mobility. Reciprocal space mapping indicates that most of the AlGaN layers are under compressive strain and are pseudomorphic to the AlN template, while the samples with the highest Hall mobility and least intense sub-bandgap emission are under smaller tensile strain. We attribute the presence of tensile strain to layer growth on a Ga-rich compliant pseudosubstrate that presents for Ga D&D and growth at 700 and 750 °C, for which Ga desorption is incomplete prior to growth initiation.