Page 420 - 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 505

TABLE II Typical Sensitivity of ICPMS for Various Metal able for critical applications requiring these sources, e.g.,
Elements in Metal Alkyls a the MOCVD growth of LEDs and injection lasers con-
Element/ Element/ Element/ Element/ taining InAlGaP and InAlGaN alloys.
sensitivity sensitivity sensitivity sensitivity The selection of the Column V precursor is of equal
(ppm) (ppm) (ppm) (ppm) importance. High-purity AsH 3 ,PH 3 , and NH 3 are most
commonly used and are now available from various ven-
Ag < 0.4 Cr < 0.4 Mn < 0.03 Se < 1.0
Al < 0.5 Cu < 0.05 Mo < 0.5 Si < 0.03 dors. These hydrides are extremely toxic and great care
As < 0.5 Fe < 0.1 Na < 0.5 Sn < 0.5 must be taken to handle them safely. Because the purity of
Au < 0.5 Ga < 0.5 Nb < 0.5 Sr < 0.1 the as-produced hydrides is not yet equal to the purity of
B < 0.4 Ge < 0.5 Ni < 0.5 Tb < 0.5
H 2 , point-of-use purifiers are normally used to ensure the
Ba < 0.1 Hg < 0.5 P < 0.5 Ti < 0.2
purity required for high-performance devices. The thresh-
Be < 0.02 In < 0.5 Pb < 1.0 U < 1.5
old limit values (TLVs) established by the American Con-
Bi < 0.1 K < 1.0 Pd < 0.5 V < 0.5
Ca < 0.02 La < 0.4 Pt < 0.5 W < 0.5 ference of Governmental Industrial Hygienists (ACGIH)
Cd < 0.02 Li < 0.4 Rh < 0.5 Y < 0.02 for the “safe” exposure to these gases for an 8-hr period
Co < 0.4 Mg < 0.02 Sb < 1.0 Zn < 0.2
are 0.050 ppm for AsH 3 , 0.3 ppm for PH 3 , and 50 ppm
a for NH 3 . Lethal concentrations for exposure of a few min-
Data from Air Products and Chemicals, Allentown, PA, United
States. utes are approximately AsH 3 ≥ 0.5 ppm, PH 3 ≥ 2 ppm,
and NH 3 ∼ 2000–3000 ppm. These values are listed in
the corresponding Material Safety Data Sheets (MSDSs),
ionized fragments that are then analyzed by a sensitive copies of which are shipped with each cylinder of gas.
mass spectrometer (typically a magnetic sector instru- Other materials commonly used in gaseous form for the
ment). At present, many manufacturers of electronic- doping of MOCVD-grown films are the hydrides silane
grade organometallic compounds employ ICPMS to rou- (SiH 4 ), disilane (Si 2 H 2 ), germane (GeH 4 ), hydrogen se-
tinely analyze each batch of precursors. This has greatly lenide (H 2 Se), hydrogen sulfide (H 2 S), diethyltelluride
reduced the variability of metal alkyl sources that are man- (DETe), and the halogens carbon tetrachloride (CCl 4 ), and
ufactured using the “same” process and equipment. Prior carbon tetrabromide (CBr 4 ). Typically, these dopant gases
to the use of ICPMS, the only useful way of testing the are supplied in high-pressure mixtures in hydrogen with
purity of “electronic grade” organometallics was the “use dopant precursor concentrations in the 10–200 ppm range.
test”—grow an epitaxial film using a “standard” growth All of these high-pressure gas sources are hazardous and
run recipe and analyze the resulting film for impurities. extra precautions for the safe handling of gas cylinders
In most cases, this involved using low-temperature pho- and the disposal of reaction by-products must be made.
toluminescence, variable-temperature Hall-effect mobil- As noted above, in the past few years, there has been
ity analysis, secondary-ion mass spectrometry (SIMS), or increasing interest in the use of “alternate Column V
photothermal ionization spectroscopy. All of these tech- precursors” to replace the hazardous Column V hydride
niques are costly and time-consuming. In many cases, the sources. Much of the recent work has been devoted to
sensitivity is inadequate to indicate the exact chemical As- and P-organometallics, specifically, the monoaklyl-
composition of the impurities. Furthermore, the impurity substituted hydrides tertiarybutylarsine (TBAs) and ter-
concentrations can depend upon the growth conditions tiarybutylphosphine (TBP). The growth of high-quality
and the other sources used in the growth, e.g., the hydride films of the III-As and III-P compound semiconductors us-
group V sources. ing TBAs and TBP has been demonstrated. These sources
Recently, many of the commonly used precursors, e.g., are liquids near room temperature and can be supplied
TEGa, TMGa, TMIn, and TMAl, have become available by bubbling a carrier gas through the storage vessel.
in special high-purity forms from a variety of vendors. An The compounds are relatively low-vapor pressure liquids
especially important consideration for the growth of many (see Table I) and thus they have inherently lower storage
high-quality semiconductor materials is the reduction of pressures at 300 K than the hydrides AsH 3 (220 PSIA,
the oxygen-containing species in these precursors, e.g., 1500 kPa) and PH 3 (607 PSIA, 4190 kPa), which are
unwanted residual alkoxide compounds. “Low-oxygen” liquids at 300 K. The lower storage pressure of TBAs
sources have now been developed, particularly, TMAl (∼110 Torr, 15 kPa) and TBP (∼200 Torr, 26.3 kPa) near
sources. In recent work, it has been shown that the use room temperature make them safer to handle since the
of low-oxygen TMAl leads to an increase in the PL in- exposure from accidental release is likely to be greatly
tensity for AlGaAs layers by a factor of 3–10 over the reduced. However, the absolute toxicities of these materi-
same alloy layers grown using “normal” grades of TMAl. als are still nearly that of the corresponding hydrides and
Low-oxygen TMGa and TMIn are also becoming avail- adequate procedures for the safe handling use of these