Effect of a nitrogen electron‐cyclotron‐resonance (ECR) microwave plasma on near‐surface composition, crystal structure, and morphology of the As‐stabilized GaAs (100) surface is investigated with the use of digitally image‐processedinsitureflection high energy electron diffraction. Nitridation is performed on molecular beam epitaxially (MBE) grown GaAs surfaces near 600 °C under typical conditions for ECR microwave plasma‐assisted MBE growth of GaN films on GaAs. Brief plasma exposures (≊3–5 s) are shown to result in a specular, coherently strained, relatively stable, GaN film approximately one monolayer in thickness, which can be commensurately overgrown with GaAs while longer exposures (up to 1 min) result in incommensurate zincblende epitaxial GaN island structures. Specular and nonspecular film formations are explained in terms of N‐for‐As surface and subsurface anion exchange reactions, respectively. Commensurate growth of ultrathin buried GaN layers in GaAs is achieved. ©1995 American Institute of Physics.