Silicon/Electrolyte Interface - Electrochemistry of Silicon and Its Oxide - page 55

2.3.1. Hydrogen Termination

In HF solutions the silicon surface is predominantly terminated by hydrogen.
There may be small amounts of oxygen- and fluorine-terminated sites depending on the
conditions. The phenomenon of hydrogen termination of silicon surfaces has been
extensively investigated. Experimental results indicate the following general charac-
teristics: (1) hydrogen termination is essentially complete with immersion time on the
order of 1 min; (2) it is formed through chemical adsorption yielding a covalent bond
having a strength close to that of the Si-Si bond; (3) each surface silicon atom can be
terminated by one, two, or three hydrogen atoms depending on its geometric position
in the surface lattice; (4) the surface is stabilized (or passivated) by hydrogen termina-
tion. The extent of termination and type of hydrides depend on the surface condition,
solution composition, and preparation procedures. Table 2.8 shows the characteristics
of hydrogen adsorption on silicon surfaces prepared by different processes.
In general, unless special procedures are taken, the silicon surfaces of both (100)
and (111) orientations treated with HF solution are completely terminated by hydrogen
in the form of mono-, di-, and trihydride. It is rough at the atomic scale due to the pres-
ence of terraces, steps, kinks, adatoms, and vacancies displaying a mixture of facets
of (100), (111), and other orientations. 577,622 Whether the termination is dominated by

mono-, di-, or trihydride depends on the orientation and roughness of the surface. The

ideal (100) surface tends to be terminated by and (111) by SiH or due to the

577,820

difference in the number of dangling bonds between the two surfaces. It is more

difficult to obtain a flat (100) surface with a termination completely by due to the
more reactive nature of the (100) surface. The atoms on an ideal (100) silicon surface
have two dangling bonds, which is most favorable for the formation of dihydride. The
fact that appreciable amounts of mono- and trihydrides are generally also found sug-
gests that the (100) surface after treatment in HF is atomically rough since these bonds
will only be present at atomic steps where the silicon atoms may have single or triple
dangling bonds. 636 Thus, the form of hydrides can be used as an indication of surface
flatness at the atomic scale. The surface is characteristically rough on an atomic scale,
when all three types of hydrides are present on the surface.
The (111) surface with an almost complete termination by SiH, indicating atomic
scale flatness of the surface, can result from different treatment procedures including a