Page 413 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN009J-427 July 6, 2001 20:25

498 Metalorganic Chemical Vapor Deposition

FIGURE 1 Schematic illustration of the primary reactions involved in MOCVD growth. The processes that result in
reduced growth rates are in dashed boxes. Homogeneous reactions occur in the vapor phase and heterogeneous
reactions occur at the interface between the vapor phase and solid phase. For some precursors, adduct formation in
the vapor phase can greatly reduce the growth efficiency of the process.

1. Fundamental MOCVD Reactions example, typical “generic” net reaction employed for the
MOCVD growth of GaAs is given below in Eq. (2). While
The detailed chemical reactions occurring under “stan-
this reaction seems to be composed of very simple pyrol-
dard” MOCVD conditions have only recently begun to be
ysis reactions, more complete reaction models have been
understood. A schematic diagram of the important fea-
developed that include more than 39 individual interme-
tures of the MOCVD process occurring in various “re-
diate reactions and byproducts.
gions” in the MOCVD process is shown in Fig. 1. The
specific reaction kinetics and detailed thermodynamics are (CH 3 ) 3 Ga (g) + AsH 3 (g) → GaAs (s) + 3CH 4 ↑ (g).
strong functions of the precursors and substrate employed, (2)
as well as the growth pressure, temperature, carrier gas, As noted above, the MOCVD growth of III–V semi-
and reactor geometry. The hydrodynamic characteristics conductor films can be complicated by homogeneous re-
of the reactor chamber can also play a significant role in actions in the gas phase, precursor-dependent activation
the outcome of growth experiments. These complications energies and pyrolysis efficiencies, and surface kinetics.
have contributed to the general lack of a detailed under- With the advent of advanced computer modeling codes
standing of the MOCVD process. However, it is generally and the experimental verification of the general predic-
accepted that the net chemical reactions for the growth of tions of these models, it has recently become possible
III–V binary compounds by MOCVD are pyrolysis-driven to use the results of computational fluid dynamics tech-
reactions of the form: niques to determine the most favorable operating regime
for some reactor systems. However, for the study of spe-
R 3 M (g) + EH 3 (g) → ME (s) + 3RH ↑ (g), (1) cific materials and device parameters, the crystal grower
is required to explore the growth parameter space peculiar
where M is a Column III metal atom, e.g., Ga, Al, or In; to the specific reactor employed in order to determine the
R is an organic radical, typically CH 3 or C 2 H 5 ;andEis optimum conditions for the growth of epitaxial thin films.
a Column V atom e.g., As, P, or N. The reactions of the This is especially important for the commonly employed
type described in Reaction (1) are generally called “Lewis large-scale production reactors having growth chambers
acid–Lewis base” reactions. The Lewis acid (electron pair with both a vertical geometry (the “rotating-disk reactor,”
acceptor) in this case is the hydride and the Lewis base or RDR), and a horizontal geometry (the “Planetary Re-
(electron pair donor) is the metal atom in the metal alkyl. actor”) as described below.
While this greatly simplified net reaction ignores inter-
mediate reactions or “addition compound” formation that
2. Homogeneous and Heterogeneous Reactions
might occur, it provides a basic framework that can be
used to describe the more complicated cases where more The precursor reactions that are present under most com-
than one organometallic or hydride are involved, e.g., for monly used MOCVD growth conditions generally consist
the growth of quaternary compound semiconductors. For of chemical processes that occur in both the gas phase