A synchrotron‐radiation photoemission study of K evaporated on rf‐sputtereda‐Si:H,a‐Si, andc‐Si(001) semiconductor surfaces at room temperature is reported. Thea‐Si substrate was obtained from sputtering of thec‐Si(001) surface. It is found that the change in work function upon K adsorption is smallest on the most disordered surface,a‐Si:H. Astrongcovalent K–Si interface state at about 5.9 eV binding energy appears only in disordered surfaces. Surface structure plays a major role in its occurrence. Further, the amorphous surfaces stay semiconducting despite large doses of K atoms, while the K/c‐Si(001) surface becomes metallic at the instance of a work‐function minimum. ©1995 American Institute of Physics.