Effects of oxygen gas on the dry etching of high-Al-content AlGaN layers

The etching characteristics of AlGaN layers comprising varying Al compositions (50%, 60%, 70%, and 80%) were investigated using Cl2/Ar-based inductively coupled plasma at varying O2 flow rates. A GaN layer was used as a reference to evaluate the etching selectivity of GaN over AlGaN. The results showed that in the absence of O2, all samples exhibited comparable etch rates regardless of the Al content. However, at increasing O2 flow rates, the Al-rich AlGaN layers displayed substantially reduced etch rates, a finding attributed to the formation of chemically inert Al-containing oxides. The selectivity improved dramatically under O2-rich conditions, exceeding one order of magnitude in the high-Al samples, thereby enabling a precise etch stop at the GaN/AlGaN interface. Atomic force microscopy measurements showed that plasma exposure typically smoothened the surface, whereas excessive O2 led to increased roughness in the high-Al-content layers owing to nonuniform oxide formation. These results offer valuable insights into optimizing dry-etching conditions for the fabrication of high-Al-content AlGaN-based optical and electrical devices, including enhancement-mode high-electron-mobility transistors where etch precision and surface integrity are critical for device performance and reliability.