Both Al and Si substitutional defects have been observed in the 7 X 7 domain and the square-root 3 X square-root 3 domain, respectively, on the specimen prepared by higher temperature annealing up to 900-degrees-C by scanning tunneling microscopy. At positive sample bias (+1 V) the protrusions in the square-root 3 X square-root 3 domain appear approximately 1 angstrom higher on the average than those in the 7X7 domain, though slight modulation in brightness can be observed in each domain. The image taken at negative sample bias (-1 V) reveals clearly the substitutional defects in both domains. The Si adatoms substituting Al in the square-root 3 X square-root 3 domain appear in almost the same contrast as Si adatoms in the 7X7 domain. The Al adatoms substituting Si in the 7X7 domain appear angstrom 1.5 angstrom higher than Al atoms in the square-root 3 X square-root 3 domain. These results suggest a distinct difference in the electronic structure between the Al (or Si) adatoms in both domains derived from the surrounding atomic geometry. In addition, Al preferentially substitutes the center adatom site in the 7X7 unit cell.