Ca impurities are observed in GaInNAs grown by both molecular-beam epitaxy and metal organic chemical-vapor deposition by high-mass-resolution secondary-ion mass spectrometry measurements. The Ca appears to originate from the surface of the as-received GaAs substrates, and presumably comes from the polishing process used in substrate preparation. Ca was observed in samples grown by different growth methods and in different laboratories. No detectable Ca incorporates into epitaxial GaAs, although Ca levels as high as 10(17) cm(-3) are observed in GaInNAs. There are indications that the Ca impurities act as shallow acceptors in GaInNAs and may, at least in part, control the background acceptor concentrations. This has implications for high-current, p-i-n GaInNAs solar cells that require very low background doping to enhance current collection. (c) 2006 American Vacuum Society.