Stabilization of Silicon Surfaces by Thermally Grown Oxides

In this introduction we will give a qualitative discussion of the problem of stabilization of semiconductor surfaces. 1.1 Atomically Clean Surfaces Fig. 1(a) is a schematic diagram of a clean [100] silicon surface showing the surface dangling bonds or unfilled orbitals. Since two electrons can occupy a free orbital, surface atoms may become negatively charged, thus acting as acceptor surface states [Fig. 1(b)]. The existence of these states in crystals was first proposed by Tamm, 1 based on a special onedimensional model. A more general treatment by Shockley 2 showed that surface states can occur only if there is a separate potential trough at the surface or if the energy bands arising from separate atomic levels overlap. He further concluded that surface states should occur in the forbidden gap and their number should be approximately equal to the number of surface atoms. Experimentally, therefore, one expects to find that an atomically clean * T h i s work was s u p p o r t e d in p a r t by t h e D e p a r t m e n t of the A r m y under Cont r a c t DA 36-039 sc-64618. 749