Chemistry at silicon crystalline surfaces

The chemistry at silicon crystalline surfaces often results in seemingly extravagant terminations: fluorine termination after gaseous HF attack, hydrogen termination after etching with HF solutions or with KOH solution at pH ⋍ 11.5, and oxygen termination after H2O rinsing. The physico-chemical factors producing the observed terminations after different attacks are discussed. It is concluded that the termination resulting after HF attack, either in gas phase or in water solution, is controlled by thermodynamic rather than kinetics factors, while hydrogen termination resulting after alkaline attack at pH ⋍ 11.5 is mainly controlled by kinetic factors. The surface structures resulting on (111) and (100) silicon after HF etching or water rinsing are somewhat different, but can be understood in the frame of two unifying concepts: nucleophilic attack of weak Si-Si backbonds, and instability of isolated mono- and di-silanol groups.