Page 93 - Semiconductor For Micro- and Nanotechnology An Introduction For Engineers

The Crystal Lattice System
is only a few atomic diameters thick, and if it separates the crystal from a
liquid or gas, we usually denote it as the surface of the crystal.
A new crystal surface can be formed by etching into an existing crystal,
or by growing a crystal from a seed, or by cleaving an existing crystal
along one of its planes. An interface can be created by depositing another
material onto the crystal, or by performing a reaction on the crystal sur-
face that results in another compound being formed (e.g., by oxidation).

This section will only introduce a few important concepts. Surface phys-
ics is a large and active discipline and the interested reader is encouraged
to refer to a specialized text such as [2.2].

Dangling Immediately after a crystal surface is formed in a vacuum by cleaving,
Bond the atoms at the surface have dangling bonds. In time, the atoms rear-
range themselves to assume a more energetically favorable configuration.
For example, the atoms, still bound to the underlying bulk material, will
relax to a lattice constant smaller (or larger) than the bulk value, to reflect
the fact that the bonding forces are one-sided in a direction normal to the
surface. Furthermore, the surface could buckle in shape so as to enable
the dangling bonds to create bonds with each other. In silicon, atoms on
the 111 > surface can form a so-called 2 × 1 (or 7 × 7 ) reconstruction
<
π
with -bonded chains [2.2], see Figure 2.30. In the atmosphere, we can
expect various gas atoms and molecules to attach to the dangling bonds.
For example, it appears that hydrogen preferentially bonds to silicon sur-
faces, a fact that influences for example the etch rate of silicon. We will
consider a simple model of a crystal surface to describe the modifications
to the bulk dispersion relation in Section 7.6.3.

Superlattices A superlattice is a term used for any lattice-like structure with a lattice
constant larger than that of the underlying crystalline material. Thus, e.g.,
when a crystal surface forms atomic rearrangements due to relaxation
effects, a superlattice is formed with a lattice constant that is often twice
as large as before. Epitaxially formed heterostructures such as found in

90 Semiconductors for Micro and Nanosystem Technology