STM (Scanning Tunneling Microscopy) observation on structural changes of Si(111)-7×7 during K and Cs adsorption has been extended to high coverages up to the saturation coverage at room temperature. Up to the alkali-metal coverage of about 0.3 ML on Si(111)-7×7 surfaces, the ordered structures such as the 2D gas phase and the magic clusters known to turn into a disordered phase around 0.25 ML were well reproduced. For coverages beyond 0.3 ML, clusters (or islands) of identical size have been identified to appear and increase the number with coverage. K and Cs atoms are suggested to form 2D islands inside the half unit cell at coverage close to the saturation coverage.
