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In bulk semiconductor, it is well documented that doping can induce a semiconductor-metal phase transition by forming impurity bands. When the doping density is high enough, the conductivity of the bulk semiconductor materials no longer follows the normal semiconductor behavior to decrease as the temperature decreases. While the Si(111)-7x7 surface is well known to be metallic at room temperature, its electronic properties at low temperatures were seldom studied. Using scanning tunneling spectroscopy, we have systematically studied the electronic structure of the Si(111)7x7 surface as a function of temperature (5-80 K) for variously doped Si crystals. We have observed an energy gap near Fermi level at low temperatures when the doping density is high enough. This energy gap is consistently observed at different surface positions and at different tip-sample distances, indicating that the gap is genuine from the surface rather than induced by tip or defect. This gap also strongly depends on temperature and dopant type. The observed energy gap at 5K is as large as ~0.3 eV for p-type semiconductor at a doping level of 8x10(18)/cm3 and ~1 eV for a n-type semiconductor at a doping level of 4x10(19)/cm3. As the temperature increases, the width of the observed energy gap decreases monotonically to zero. The null observation of an energy gap for samples at lower doping levels even at low temperatures indicates that the Si(111)7x7 surface undergoes a metal-insulator transition when the dopant density increases. The talk will further discuss the mechanisms of this phase transition and its relation to the tunneling spectroscopic measurements. |