The Schottky barrier of Co on strained and unstrained SixGe1−x alloys

In this study, metal films of Co were deposited in situ on strained and unstrained SixGe1−x alloys, and the Schottky barrier (EF-EV) was determined by angle resolved ultraviolet photoemission spectroscopy (ARUPS). Measurements were obtained as a function of Ge composition. Strained and unstrained epitaxial SixGe1−x alloys were grown on Si(100) wafers using electron beam evaporation in an ultra-high vacuum molecular beam epitaxy (UHV MBE) chamber. The ARUPS experiments were performed to measure the Schottky barrier heights of Co on a series of SixGe1−x alloys, and to observe the surface states. The surface states of clean SixGe1−x alloys were observed and were extinguished as Co thickness increased to ∼0.4 Å. The p-type Schottky barrier of Co on Si was found to be 0.52 eV. The measured barrier heights of Co on strained SixGe1−x alloys ranged from 0.33 eV to 0.46 eV as x increased from 0.40 to 0.80. The Schottky barrier of Co on unstrained SixGe1−x alloys ranged from 0.23 eV to 0.41 eV as x increased from 0 to 0.60. In fact, the p-type Schottky barrier was essentially identical for strained and unstrained SixGe1−x alloys of the same concentration. This indicates that the n-type Schottky barrier is substantially different for strained and unstrained alloys. ARUPS was also conducted to measure the electron affinities of the series of SixGe1−x alloys and the work function of Co. The results show that the barrier does not follow the work function model.