Photoemission investigation of the alkali-metal-induced two-dimensional electron gas at the Si(111)(1 X 1):H surface

We investigate the formation of a two-dimensional electron gas (2DEG) at the hydrogen-passivated silicon (111) surface exposed to tiny amounts of alkali metals (K and Cs) by means of high-resolution ultraviolet photoemission spectroscopy. The accumulation layer derives from the quantization and filling of bulk states located around the conduction-band minimum. Direct photoemission from these states results in a well-defined and structureless feature, which becomes huge when the alkali metal is deposited at low temperature (150 K), indicating a much higher charge transfer associated with the growth at low temperature. Its shape is characterized by a tail which is attributed to the broadening of the 2DEG density of states due to the random fields set up by the ionized alkali metal at the surface, providing direct experimental evidence of tailing of two-dimensional subbands at a free semiconductor surface. © 2003 The American Physical Society.