The addition of light rare-earth (RE) metal elements (La, Pr, and Nd) to Al thin films with about 2-7 at. % markedly decreases the grain size of the Al matrix more than 50% compared with those of pure Al. Such addition largely suppresses growth of thermal defects of hillocks and whiskers at high temperatures (350-450 degrees C), A large number of fine metallic compounds of Al(11)RE(3) and/or Al(3)RE (RE=La, Pr, and Nd) were segregated in an Al matrix, mostly at grain boundaries, after annealing at 350 degrees C. The resistivities of the films after annealing at the above temperatures show low values of less than 6 mu Omega cm compared with those of current thin-film-transistor liquid-crystal displays gate electrode materials (more than about 15 mu Omega cm), without the salient formation of hillocks or whiskers on the surfaces.