Dangling bonds, etching

Fig. 8.15. Model for etching mechanism of acidic and alkaline solutions on polar ZnO faces [109]. The dangling bonds at the surfaces exhibit partially positive (<5+ for Zn) or partially negative (

The kinetics of the reaction of germanium with dissolved oxygen can be related to the surface structure. Thus it was found (2) that the dissolution rates for the three principal low-index planes vary in the order of the densities of dangling bonds as shown in Fig. 4 and Table I. It was speculated that the intermediates Ge-OH and Ge-C H are formed on the surface during dissolution. Orientation effects have also been observed in fast etching media (4), although high rates usually mask relatively small differences due to orientation. 

The difference in etch rate between (111) and (100) surfaces was related to the bond densities on the two surfaces in the early surface kinetics models. According to Hesketh et the etch rate difference between (100) and (111) planes is due to the difference in the surface free energy of the crystal planes which is proportional to the number of bonds on the surface. The (111) plane, which has the lowest surface free energy measured in vacuum, has the lowest bond density and thus has the lowest etch rate. They postulated that the etch rate of crystal planes is a function of the total number of bonds at the surface, that is, the sum of the in-plane, lateral bonds between atoms in the plane of the surface, and surface bonds, dangling bonds. It was recognized however, that this effect alone will not cause etch rate differences of more than a factor of two. ° ... 

Si(lOO) reconstmcts as well, yielding a (1 x 2) surface phase that is formed when adjacent silicon atoms bond through their respective dangling bonds to form a more stable silicon dimer. This reconstructed bonding results in a buckling of the surface atoms. Furthermore, because Si-Si dimer bonds are weaker than bulk silicon bonds, the reconstruction actually facilitates etching. For a comprehensive discussion on STM studies... 

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