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.