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The structure of the Na-induced 3ϫ1 reconstruction of the Ge͑111͒ surface has been examined using scanning tunneling microscopy ͑STM͒. The STM images reveal significant differences from those of the metal-induced Si(111)3ϫ1 surfaces. Our interpretation of the images leads us to conclude that unlike the Si(111)3ϫ1 surfaces, there exist no GevGe double bonds on Ge(111)3ϫ1-Na despite the similarity in structure. This raises a serious question about a recent proposal that the metal-induced 3ϫ1 reconstruction of the Si͑111͒ surface is stabilized by the formation of a SivSi double bond. We propose that surface electrostatic energy due to the charge transfer accompanying the surface relaxation plays an important role in stabilizing the Ge(111)3ϫ1-Na surface, and possibly the metal-induced Si(111)3ϫ1 surfaces as well.

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Li-induced 3ϫ1 reconstruction of the Ge͑111͒ surface is studied using a scanning tunneling microscopy ͑STM͒. Both filled-and empty-state STM images of the Ge͑111͒3ϫ1-Li surface appear quite different at first sight from those reported for other metal-induced 3ϫ1 phases of Si͑111͒ and Ge͑111͒ surfaces. We propose that the STM images are interpretable by a model containing GeϭGe double bonds, similar to the model recently proposed for the Si͑111͒3ϫ1 reconstruction.