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A model of the atomic arrangement of the Si͑100͒4ϫ3-In surface phase is proposed on the basis of the known data and the appearance of the Si͑100͒4ϫ3-In surface found in the present STM study at low bias voltages. The model incorporates a 4ϫ1-reconstructed Si͑100͒ substrate having every second top Si atom double row missing. Indium atoms of the overlayer occupy the sites where each In atom is bonded to one Si atom of top Si dimer and to two Si atoms in the lower bulklike Si͑100͒ substrate layer. Three Si dimers and six In atoms form a unit cluster of the Si͑100͒4ϫ3-In surface reconstruction. The model successfully accounts for all available experimental data. ͓S0163-1829͑98͒01820-7͔

Using scanning tunneling microscopy ͑STM͒, we have studied the formation and structure of the Si(100)2ϫ3-Na surface phase. It has been found that both the formation process and atomic arrangement of the Si(100)2ϫ3-Na surface differ substantially from those known for 2ϫ3 reconstructions induced by other alkali and alkaline earth metals, since the Si͑100͒2ϫ3-Na phase is the only one in which the top Si layer undergoes a substantial reordering. The Si-atom density in the Si(100)2ϫ3-Na surface phase has been determined to be 1/3 ML. A structural model of the Si͑100͒2ϫ3-Na reconstruction has been proposed, and its registry to STM images has been discussed. ͓S0163-1829͑98͒01131-X͔

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