Page 173 - Electrical Properties of Materials

Preparation of pure and controlled-impurity single-crystal semiconductors 155

violently. As a result silane is extremely dangerous, and handling precautions
such as the purging of gas handling systems and the use of concentric stainless
steel pipework to surround a silane-carrying pipe with an inert N 2 blanket are
mandatory. Other hydrides, such as arsine (AsH 3 ) and phosphine (PH 3 ), which
may be used as precursors of the dopants As and P, are even more dangerous.
Not only are they pyrophoric, they are also toxic in extremely low concentra-
tions and will kill before their smell (something between garlic and rotting fish)
is noticed. After that, they may also explode.
The epitaxial layer can be made very pure by controlling the purity of the
chemicals; or more usefully it can be deliberately doped to make it n- or p-type
by bubbling the hydrogen through a weak solution of (for example) phosphorus
trichloride or boron trichloride, respectively, before it enters the epitaxy fur-
nace. In this way epitaxial layers around 2–20 μm thick can be grown to a
known dimension and a resistivity that is controllable to within 5% from batch
to batch.
Liquid Phase Epitaxy (LPE) has also been used, mainly with compound
semiconductors. The substrate crystal is held above the melt on a quartz plate
and dipped into the molten semiconductor (Fig. 8.26). By accurately con-
trolling the cooling rate a single-crystal layer can be grown epitaxially on the
crystal.
In recent years liquid phase epitaxy has been the workhorse in growing
semiconductors for lasers (semiconductor lasers will be discussed in Section
12.7). It is simple and quite fast, and it has coped heroically with the problem
of stacking semiconductors of differing bandgaps when there was no alternat-
ive method, but it cannot really produce the sharply defined layers needed for
the latest devices. Some new techniques were bound to come. They are repres-
ented by Molecular Beam Epitaxy (MBE), Metal–Organic Chemical Vapour
Deposition (MOCVD), Metal–Organic Vapour Phase Epitaxy (MOVPE), and
Hydride Vapour Phase Epitaxy (HVPE).

Raise or lower

Quartz holder.
Semiconductor seed
Seed slice slice is held on to quartz
plate by a quartz spring

Fig. 8.26
Liquid phase epitaxy. The
semiconductor slice is held on the
plate by a quartz spring clip and
Crucible
containing Furnace wall lowered into the molten
semiconductor semiconductor alley. By correct
melt cooling procedures the pure
semiconductor is encouraged to
SiO tube containing precipitate onto the surface of the
2
inert gas flow slice.