Unintentional Ca incorporation caused by Ca-contaminated substrate surfaces on as-purchased GaAs wafers are known to limit the efficiency of solar cells based on dilute nitride materials. This article focuses on further understanding the conditions and mechanisms by which these Ca impurities incorporate. Plasma-assisted molecular-beam epitaxy utilizing a 1% N-2 in Ar precursor gas mixture was used to grow GaAs at 400 and 580 degrees C, and GaN0.01As0.99 at 400 degrees C. Two plasma operating combinations of rf power and gas flow rate were used to generate different amounts and energies of both ions and other plasma species, while keeping nitrogen incorporation constant. The ions were characterized with a dual-grid, retarding-field ion energy analyzer, and the corresponding ion energy distributions are presented to correlate ions with Ca incorporation. When appropriate, dc-biased deflector plates were used to remove ions during growth. Secondary ion mass spectrometry was used to measure Ca in GaAs and GaN0.01As0.99. Ca incorporation was observed in the dilute nitride samples, but the effects of ions did not exceed other Ca incorporation mechanisms associated with defects due to both low temperature growth and nitrogen incorporation; however, different neutral active nitrogen species (atomic N and metastable N-2(*)) may be a factor. Ca incorporation measured in GaAs grown at 400 degrees C with a pure Ar plasma is predominantly due to defects associated with low temperature growth, as opposed to plasma damage caused by the ions. GaAs growths at 580 degrees C without a plasma did not exhibit Ca incorporation, but growth at 580 degrees C with ions from a pure Ar plasma caused Ca incorporation. (C) 2008 American Vacuum Society.