Data are presented demonstrating that an exposed AlAs layer can be sealed against further decomposition as would otherwise occur due to, for example, wet oxidation. A critical processing step consists of a brief 500–600 °C anneal in forming gas of a previously room ambient exposed AlGaAssurface. In the brief high‐temperature anneal, a thin but dense surface barrier layer forms which further blocks diffusing oxygen species. The effectiveness of the surface layer as a barrier is demonstrated through its use as a mask against wet oxidation in the fabrication of an all‐epitaxial selectively oxidized multimode AlAs/AlGaAs vertical‐cavity laser.