Unreconstructed intrinsic semiconductor (111) surfaces are, to a good approximation, two-dimensional metals. The Fermi surface has flat portions, leading to a very large density of states at the Fermi energy, and to a quasi-1 dimensional electron dynamics. The condition for the stability of the surface metallic state is examined and shown to be violated; a surface phase transition to an insulating state follows. The insulating state can be either a magnetically ordered state (spin density wave) or a Peierls superlattice (charge density wave), depending on the relative strength of the surface electron-hole and electron-phonon interaction. The periodicities of many of the higher n × n reconstructions observed on Si and Ge (111) agree with those expected for the charge density wave case. It is pointed out that SDW magnetic ordering on surfaces might be detectable by polarized LEED, or otherwise
