Page 421 - 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

506 Metalorganic Chemical Vapor Deposition

materials must be followed. The TLVs for TBAs are TBP As noted above, the first high-performance heterojunc-
not yet established. However, some toxicity data on TBAs tion devices grown by MOCVD were AlGaAs/GaAs so-
and TBP has been obtained. The lethal concentrations for lar cells and injection lasers reported by Dupuis et al.
in 1977. Since that time, MOCVD has been used to
which 50% of the exposed rat population dies (the LC 50
values) for TBAs and TBP are ∼45 and ∼1100 ppm, re- produce a variety of other important devices including
spectively, whereas AsH 3 has an LC 50 of ∼45 ppm. Thus, light-emitting diodes, heterojunction field-effect transis-
these tests show that TBAs is about as toxic as AsH 3 , tors (HFETs), heterojunction bipolar transistors (HBTs),
however, because of the lower storage pressure, the use p-i-n photodetectors, metal–semiconductor–metal pho-
of TBAs amounts to a significant safety advantage during todetectors, waveguides, light modulators, and more so-
storage and usage. An additional advantage of TBAs and phisticated integrated device structures containing mul-
TBP from a production viewpoint is that they provide ex- tiple devices grown in one or more successive growth
cellent performance at relatively low input V/III ratios in runs. One particularly important recent development is
the vapor phase. This offers a distinct advantage in reduced the expansion of the MOCVD growth of III-N materials,
consumption of precursors as well as a reduced volume a process also pioneered by Manasevit et al. in 1971. This
of toxic waste byproducts. While the cost per gram is still application of MOCVD will soon lead to the dramatic ex-
quite high for TBP and TBAs, the increased volume of pansion of LED-based lighting products into many of the
production, which has occurred in recent years has led to “mass-market” lighting applications, including the devel-
somewhat reduced pricing. Other potential advantages of opment of high-efficiency white-light solid-state lamps.
these precursors are their somewhat lower pyrolysis tem-
peratures compared to AsH 3 and PH 3 , and possibly, the A. III–V Compound Semiconductors
ability to purify them using various organometallic purifi-
Epitaxial films of the compound semiconductors from
cation routes.
Another emerging alternate Column V source is Columns IIIA and Column VA (also called Columns 13
and 15 according to the IUPAC labeling) of the Periodic
the N compound unsymmetric 1,1-dimethylhydrazine
Table are of interest for a variety of electronic and opto-
(CH 3 ) 2 N-NH 2 (DMHy), which can be used as a low-
electronic applications. GaAs was the first of the III–Vs
temperature precursor for N. The vapor pressure of DMHy
to be identified as a semiconductor in about 1950. First
at 300 K is ∼150 Torr (19.7 kPa). Using DMHy and
produced in 1967, thin films of GaAs were also the first
TMGa, films of GaN have been grown at temperatures
epitaxial layers of the III–V semiconductors to be grown
◦
in the range from 425 to 960 C and at V/III ratios as low
by MOCVD. These materials can be grown in binary,
as 10. These conditions are quite different from those com-
ternary, quaternary, and pentanary forms. Descriptions of
monly used for GaN growth using TMGa and NH 3 where
the MOCVD growth processes for the most commercially
temperatures ∼1050 C and V/III ratios ∼3000–5000
◦
important III–Vs are given below.
are used. Another application for the use of DMHy is
for the MOCVD growth of GaAsN and InGaAsN alloys.
1. GaAs and AlGaAs
These III–V compounds are potentially useful for the real-
ization of GaAs-based injection lasers and photodetectors As noted above, thin films of GaAs were the first epi-
working in the 1.33 µm <λ< 1.55 µm range. taxial semiconductor layers grown by MOCVD. The
most commonly used metal alkyl Ga sources are TMGa
and TEGa, and the As precursors predominantly used
III. GROWTH OF III–V COMPOUND are AsH 3 and TBAs. Growth temperatures are in the range
SEMICONDUCTORS BY MOCVD 600 C < T g < 800 C. Typically, V/III ratios in the range
◦
◦
of 50–100 are used for AsH 3 growth. Lower ratios in the
Virtually all of the III–V compound semiconductors have 20 < V/III < 40 are used for TBAs. Generally, higher con-
been grown by MOCVD, in many cases, using a variety centrations of unintentional C acceptors are incorporated
of organometallic precursors for Column III sources. In when TMGa is used compared to TEGa. This is because
addition, in some cases, “all organometallic” processes TMGa pyrolysis occurs by successive dissociation of CH 3
have been demonstrated where both the Columns III and radicals, leading to C incorporation, while TEGa under-
V sources are metalorganics. A general overview of the goes β-hydride elimination reactions.
details of these processes, as well as papers describ- In 1971, Manasevit, using TMGa, TMAl, and AsH 3 ,
ing recent advances in the field are given in publica- was the first to report the growth of AlGaAs alloys by
tions listed in the Bibliography. Brief summaries of the MOCVD. Since this time, AlGaAs has been grown with
processes for various specific III–V materials are given a variety of organometallic Column III sources, includ-
below. ing TMGa, TEGa, TMAl, TEAl, trimethylamine alane