Biomedical Engineering Reference
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contingent upon the complete removal of crystal defects that arise during the
low temperature annealing of implanted GaAs. This is not to suggest that
complete removal of crystalline dislocation loops is sufficient to guarantee
good electrical behavior. Other factors may certainly influence the electrical
properties, following furnace annealing, such as point defect clusters and
material impurities, particularly Cr distributions, and the nature of encapsu-
lants used to protect the surface from decomposition.
Pulsed laser annealing of ion-implanted GaAs received considerable atten-
tion from several laboratories [88]. These studies indicated that uncapped
wafers be laser processed in air at room temperature to provide good crys-
tallinity, high substitutionality of implemented ions, and electrical activities
superior to those achievable by conventional furnace annealing although,
under certain conditions, decomposition of the surface was a problem.
Williams compared [89] the effectiveness of CW laser and furnace anneal-
ing in the removal of amorphous implant damage in GaAs, and found
that both CW (argon ion) laser and the low-temperature furnace anneal
produced good amorphous-to-crystalline recovery (stage-1 annealing). It
became apparent, in both the pulsed and CW laser annealing experiments,
that many of the problem areas associated with furnace processing of GaAs
either remained as difficulties or gave way to significant problems associated
with rapid heating and cooling effects [90]. Results obtained from both fur-
nace annealing and the various transient annealing methods for implanted
GaAs are summarized in Figure 5.25 [91]. Since generally only simple point
defects are produced on implanted GaAs, with no extended defects or loops,
only first-stage annealing (125°C-230°C) provides an effect, which is advan-
tageous to optical performance.
5.10.3 MOCVD: Growth and Evaluation
MOCVD and MBE are discussed and contrasted in an earlier section on
material growth. Recent improvements in crystal growth capabilities and
heteroepitaxial deposition of very low defect density structures, using either
MOCVD or MBE, have significantly advanced the optical circuit device pro-
ducibility and performance factors.
Figure 5.26 illustrates the operation of the vertical atmospheric-pressure
MOCVD reactor [92]. In this technique, “vapor-phase mixtures containing
the metalorganic compounds are pyrolyzed at or near the surface of a heated
substrate, where they combine to form the deposited layer. Most sources are
liquid near room temperature with relatively high vapor pressures, and so
may be carried to the reaction zone by bubbling a carrier gas such as high
purity hydrogen through the liquid source” [93]. As an example, GaAs is
usually grown using trimethylgallium plus pure arsine gas producing GaAs
and carbon tetrahydride (CH 4 ) by-product. “Deposition proceeds as reac-
tants decompose in a stagnant boundary layer just above the surface of the
wafers.” Commercial equipment is capable of processing up to twelve 3 in.
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