The role of interfacial contamination on antiphase domain boundary (APB) formation in GaAs grown epitaxially on Si(100) by metal-organic chemical vapor deposition was investigated. The pre-growth cleaning of the Si substrate was varied to affect the relative amount of residual surface contamination across the wafer. APB density in the as-grown GaAs film was examined with the aid of a selective APB etchant. The interfacial oxygen and carbon concentrations were determined using secondary ion mass spectrometry. It was found that the APB density increased significantly from 0.14 to 3.2 mu m(-1) between the center and the edge of the wafer. Over this distance, the integrated carbon concentration at the GaAs/Si interface increased by an order of magnitude and the integrated oxygen concentration, although two orders of magnitude lower than carbon, also increased by a factor of six. Analysis of the GaAs/Si interface with high-resolution transmission electron microscopy and high-angle annular dark field scanning transmission electron microscopy showed a direct association of APBs with amorphous interfacial particles. The particles are likely carbon-based, possibly SiC, and thus residual carbon contamination is believed to be the primary contributor to increased APB formation.