ABSTRACT
Contemporarily, we analyze the semiconductor systems by a substrate materials class, i.e., ‘‘Si’’ or
‘‘III-Vs.’’ The III-V classification is ultimately split into ‘‘GaAs’’ or ‘‘InP,’’ for example. This nomenclat-
ure contains two subtle but important connotations. First, we recognize the importance of the material
on the performance of the system. Second, it is assumed inherently that materials choices are confined to
a particular bulk substrate material, e.g., a Si substrate. Bulk is defined here as a substrate grown by a
bulk crystal growth technique, which in general employs pulling a crystal from a melt. Semiconductor
compounds that form elemental or binary compounds, referred to below as ‘‘bulk semiconductors,’’ are
amenable to bulk crystal growth, but miscible alloys of the bulk compounds are not. Thus, all current
semiconductor-based systems are built on particular lattice constants allowed by nature. Early in
epitaxial growth research, many lattice-mismatched films (i.e., films composed of materials that are
alloys of bulk semiconductors) were deposited on bulk semiconductors, but the lattice-mismatch
between the film and substrate led to poor material quality in the thin film. A consequence of this
epitaxial research was that lattice-mismatched epitaxy, i.e., achieving lattice constants in-between bulk
semiconductors, was considered impractical. Thus, nearly all electronic and optoelectronic systems are
built on lattice constants of the bulk semiconductors. These materials combinations can be seen in
Figure 6.4.1 by following the vertical lines of constant lattice-constant up and down the diagram. In Si
technology, today a ‘‘silicon wafer’’ is often a silicon epitaxial layer on a bulk substrate. In CD lasers and
other applications involving optoelectronics near the 870 nm wavelength, the AlGaAs alloy system on
GaAs was employed, as heterostructure devices could be designed without creating lattice mismatch.
And for telecommunications applications, InP substrates became the bulk semiconductor of choice, as
InGaAsP alloys could be grown lattice-matched to the InP lattice constant and also achieve the desired
1.3 and 1.55mm wavelength emission required for low-loss transmission in optical fiber. Thus, all
commercial and defense semiconductor systems have been developed on bulk semiconductors using
lattice constants of the substrate.
