We determine types and concentrations of free carriers in GaAs layers under organometallic chemical vapor deposition growth conditions from the linear electro‐optic structure observed near 3 eV in reflectance‐difference spectroscopy. The sensitivity is about 1017cm−3at 400 °C and 1018cm−3at 600 °C, sufficient to measure common doping levels at or near growth temperatures. We observed the transition betweenn‐ andp‐type doping during atomic layer epitaxy of a carbon‐dopedp‐type layer on ann‐type substrate at 470 °C.