Surface States on Clean and on Cesium‐Covered Cleaved Silicon Surfaces

The surface conductivity of p‐type Si crystals cleaved in UHV and covered with Cs up to a monolayer was measured. With increasing Cs coverage θ the surface conductivity, Δσ, initially decreases from the value of the clean surface and reaches the minimum at θ = 0.005. After a subsequent increase Δσ remains constant from θ = 0.015 to 0.25 and exhibits a steep increase for further coverage up to θ = 1. From these surface conductivities the positions of the Fermi level E F at the surface are calculated. At the clean surface the intrinsic level E i lies 0.27 eV above E F and shifts with increasing θ to 0.27 eV below E F at θ = 1. From the dependence of ( E F – E i ) s on Cs coverage the distribution of surface states can be calculated since the adsorbed Cs atoms are ionized, giving one electron to a Si surface state. At the clean surface there are two surface state bands, one with donor states 0.31 eV below E i and a density of 8.5 × 10 13 cm −2 eV −1 , and another one with acceptor states 0.13 eV below E i , 0.05 eV wide and with a density of 5.7 × 10 13 cm −2 eV −1 . For each adsorbed Cs atom a new surface acceptor state is created in a band 0.1 eV above E i and 0.03 eV wide.