Analysis of composition fluctuations on an atomic scale in Al0.25Ga0.75N by high-resolution transmission electron microscopy

Composition fluctuations in the ${\mathrm{Al}}_{\mathrm{0.25}}{\mathrm{Ga}}_{\mathrm{0.75}}\mathrm{N}$ layer of an AlGaN/GaN transistor structure grown by plasma induced molecular beam epitaxy on ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}\text{(0001)}$ at a growth temperature of $\text{870\hspace{0.17em}°}\mathrm{C}$ were studied by digital analysis of lattice images (DALI) of high-resolution transmission electron microscopy (HRTEM) cross-section images. DALI exploits the linear dependence of the lattice parameters on the Al content by applying Vegard’s law. Detecting the distances between intensity maxima positions in the micrograph which can be considered as a fingerprint of the local lattice parameters quantitatively derives composition profiles on an atomic scale. In the HRTEM cross-section image different areas were observed in the ${\mathrm{Al}}_{\mathrm{0.25}}{\mathrm{Ga}}_{\mathrm{0.75}}\mathrm{N}$ layer with either homogeneous or “striped” contrast. In the striped areas the analyses indicate a strong periodic decomposition with a period of 1 nm consisting of 1 ML ${\mathrm{Al}}_{\mathrm{0.8}}{\mathrm{Ga}}_{\mathrm{0.2}}\mathrm{N}$ and about 3 ML ${\mathrm{Al}}_{\mathrm{0.07}}{\mathrm{Ga}}_{\mathrm{0.93}}\mathrm{N}.$ The regions with homogeneous contrast do not exhibit significant composition fluctuations.