Page 422 - 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

Metalorganic Chemical Vapor Deposition 507

(TMAA), and tritertiarybutylaluminium (TTBAl). Oxy- at λ ∼ 1.3 µm. The growth of InAlGaAs/InP strained-
gen contamination in AlGaAs films has been a continuing quantum-well lasers by MOCVD has the potential to in-
problem. In general, O incorporation is a function of the crease the performance of low-cost optical communica-
growth temperature, substrate orientation, and V/III ra- tions links, which do not require “active” temperature
tio, with larger values of T g , substrate misorientation, and control, and cooling of the laser itself.
V/III ratio resulting in lower O contamination. Typically, The ternaries In x Ga 1−x As and In x Al 1−x As are other im-
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AlGaAs is grown at 720 C < T g < 800 C and V/III ≥150. portant compounds, which can be grown lattice matched
With the advent of “low-alkoxide” grades of TMAl, in situ to InP substrates. These materials can be used to
purification of AsH 3 , and the improved performance char- grow a variety of high-speed optoelectronic devices,
acteristics of current-generation MOCVD reactors, the including strained-quantum-well lasers, p-i-n photo-
O concentration (as measured by SIMS) of Al x Ga 1−x As diodes, heterojunction avalanche photodetectors, high-
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(x ∼ 0.20) films is typically below ∼2 × 10 cm . electron-mobility transistors (HEMTs), pseudomorphic
The first high-performance III–V devices grown by high-electron-mobility transistors (PHEMTs), and hetero-
MOCVD were AlGaAs–GaAs double-heterostructure junction bipolar transistors (HBTs). Great success has
(DH) and quantum-well (QW) injection lasers. Since this been achieved in growing these device structures by
early work, MOCVD has become the materials technol- MOCVD.
ogy of choice for the large-scale growth of high-quality
AlGaAs–GaAs injection lasers. For example, most of 3. InGaAsP/GaAs
the compact-disc injection lasers, and virtually all of the
Thin films in the In x Ga 1−x As y P 1−y quaternary system
high-power semiconductor lasers, are manufactured from
have also been grown lattice matched to GaAs substrates
MOCVD-grown materials.
using MOCVD. For growth by MOCVD, the commonly
used sources are again TMGa, TEGa, TMIn, AsH 3 , and
2. InAlGaAsP/InP PH 3 . One of the most important potential applications for
thesematerialsistothegrowthof“Al-free”injectionlasers
The epitaxial growth of thin films of the quaternary al-
operating at λ ∼ 0.980 µm, a spectral region that is well
loys In x Ga 1−x As y P 1−y lattice matched to InP substrates is
suited to the fabrication of semiconductor lasers designed
of interest for a variety of commercially important semi-
to pump solid-state lasers.
conductor devices, including injection lasers and high-
speed photodiodes used in high-speed long-distance op-
4. InAlGaP/GaAs
tical communications systems. The MOCVD growth of
these materials is normally accomplished using reactors The In x (Al y Ga 1−y )P quaternary has been grown by
operating at low-pressure owing to the tendency for adduct MOCVD to fabricate a variety of visible light-emitting de-
formation between TMIn and PH 3 at atmospheric pres- vices. Growth of InAlGaP films lattice matched to GaAs
sure. The most commonly used sources are TMGa, TEGa, substrates has been accomplished by both atmospheric-
TMIn, AsH 3 , and PH 3 . The “alternate” precursors TBAs pressure (AP) and low-pressure (LP) MOCVD. Currently,
and TBP have also been used. In early work, Duchemin most work is carried out at pressures of 60–76 Torr
et al. used TEIn, TEGa, AsH 3 , and “precracked” PH 3 to (∼10 kPa). The Column III and P precursors are carried
grow epitaxial quaternary films on InP. Subsequently, it into the growth zone by a high flow of H 2 carrier. Typically,
was discovered that the precracking was not necessary TMIn, TEGa, TMAl, and PH 3 are used as precursors. In
since the pyrolysis efficiency of PH 3 is greatly enhanced most cases, the best layer quality is obtained when a thin
by surface kinetics and by the presence of TMIn. Alloy (∼20–100 nm) “buffer layer” of GaAs is grown first. It has
films have been grown throughout the composition range been found that high V/III ratios (>400) and high growth
having a close lattice match to InP. In particular, InGaAsP temperatures (T g > 700 C) are important for the reduction
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alloys with bandgap energies corresponding to emission of O incorporation and the activation of Mg acceptors.
wavelengths of λ ∼ 1.2, 1.33, and 1.55 µm have been Si and Te are commonly used donors, usually supplied
grown by MOCVD, and are of great interest for devices by silane (SiH 4 ), and DETe, respectively. At this time,
for optical communications. MOCVD is the technology of choice for the production
of InAlGaP materials for high-performance red and yel-
Another quaternary in this system is In 1−y (Al x Ga 1−x ) y
As, which can be grown lattice matched to InP sub- low LEDs and injection lasers emitting in the red spectral
strates. These materials are grown by MOCVD using the region (λ ∼ 630–670 nm). These high-brightness LEDs
precursors cited above with the addition of TMAl. Re- have luminous efficiencies that actually exceed the output
cently, this quaternary has shown promise for the growth efficiency (i.e., > 40 lumens/watt) of 30 W halogen lamps
of advanced high-performance injection lasers operating for the production of light. The application of MOCVD