Electroluminescence (EL) and photoluminescence have been measured from thin film structures of heavily carbon‐doped GaAs on crystalline semi‐insulating GaAs substrates. EL is only detected when thep‐type film is biased positively. From this and the spectral distribution, it is concluded that there are heterojunction interface states at about 50 meV below the conduction band, with a concentration roughly estimated as 1017cm−3. ©1995 American Institute of Physics.