Chemistry Reference
In-Depth Information
2
Silicon/Electrolyte Interface
2.1. BASIC PROPERTIES OF SILICON
Silicon is a semiconductor with an intrinsic conductivity of
band gap of 1.12eV at 300K. It has a diamond crystal structure characteristic of the
elements with four covalently bonded atoms. As shown in Fig. 2.1, the lattice constant,
and a
a, is 5.43 Å for the diamond lattice of silicon crystal structure. The distance between
that is, 2.35 Å, and the radius of the silicon atom is
1.18 Å if a hard sphere model is used. Some physical parameters of silicon are listed
in Table 2.1.
The silicon materials that are used in the electronic industry are normally doped
to increase the conductivity. The common donors for silicon are P, As, and Sb and the
acceptors are B, Al, and Ga. They are substitutional impurities with ionization levels
located in the range of 0.04 to 0.07eV from the corresponding bands. Table 2.2 lists
the resistivity, which is the reciprocal of the conductivity, of
n-
and
p-
type silicon doped
with phosphorus and boron, respectively.
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For single-crystalline silicon the surface properties depend on the orientation.
Table 2.3 lists surface properties of silicon for the three principal crystal planes. The
(111) plane has the highest atomic density and the lowest surface energy, while the
(100) plane has the lowest atomic density and the highest surface energy. Table 2.4
shows the density of surface bonds on various crystal planes.
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The (100) plane has
the highest surface bond density although it has the lowest atomic density. If the in-
plane bonds, which are zero for the (100) and (111) planes but high on the (110) plane,
are also included, the (110) plane is found to have the highest total bond density. Table
2.4 also shows the surface free energy of the different crystal planes based on the values
of bond density and bond strength. The silicon surface is not perfectly flat at the atomic
scale but contains defects such as steps, vacancies, kinks, etc.
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The step formation
energy, which is the energy for creating a straight step from a flat surface at T = 0K,
has been found to be 0.162eV/Å for
the nearest two neighbors is
step on (111) plane, and 0.22eV/Å for a kink
on
step.
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