Page 419 - 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

504 Metalorganic Chemical Vapor Deposition

prepared by reactions with Al-based metalorganics, as in conductor film. Some of the impurities in the hydrides
example Reaction (3) above. Metal alkyls containing Al also contribute to this problem, e.g., water vapor in the
can be prepared through the use of an organo-lithium com- hydrides may enhance the incorporation rates of certain
pound and aluminum trichloride, e.g., impurities in the metal alkyls, particularly O.
Besides the Column III metalorganics, several “alter-
natealkyl-containingColumnVprecursors”havebeende-
AlCl 3 + 3(CH 3 )Li → (CH 3 ) 3 Al ↑+ 3LiCl. (5)
veloped to replace the hazardous As and P hydrides, arsine
Such organo-aluminum compounds are used in large (AsH 3 ) and phosphine (PH 3 ). The most practically suc-
quantities in the chemical industry as catalysts in the man- cessful of these are tertiarybutylarsine (C 4 H 9 AsH 2 , TBAs)
ufacture of plastics, polyethylene, etc., and low-purity ver- and tertiarybutylphosphine (C 4 H 9 PH 2 , TBP). While these
sions of these metal alkyls are manufactured in industrial molecules have only one of the parent hydride’s H atoms
plants in “railroad car” quantities (thousands of kilograms replaced with a butyl group (C 4 H 9 ), they are consider-
per month). They are also used in the manufacture of phar- ably safer to use because of the much lower vapor pres-
maceuticals, flavor agents, and fragrances. sure of these liquid sources compared to the higher va-
Simple fractional distillation processes for purification por pressures of the pure hydrides. Furthermore, the TB
of metalorganics can be employed to remove some of these compounds decompose more readily at lower tempera-
impurities, but this is a very inefficient approach. A dra- tures than the pure hydrides, making lower V/III ratios
matic improvement in the yield of many high-purity metal more practical, resulting in a smaller usage rate for the
alkyl compounds resulted from the development of the toxic chemicals during growth. However, these sources
“adduct-purification” scheme for the purification of metal initially were not as pure as the AsH 3 and PH 3 parents, and
alkyls, which was commercially developed by A. C. Jones they were not readily accepted in production. While cur-
and coworkers. This process uses the strong tendency of rently there are large-scale users of TBAs and TBP (par-
many metal alkyls to form stable adduct compounds with ticularly in Japan), these precursors have not gained wide
other reactants, thus making a difficult problem that is en- acceptance and, as a result, the cost per gram is still quite
countered in the epitaxial growth arena into an useful ad- high.
vantage in the synthetic arena. Actual synthetic and purifi- In addition to these Columns III and V “primary precur-
cation routes employed in the manufacture of metal alkyls sors,” vapor-phase sources of dopant atoms—e.g., Zn and
are proprietary. It is a challenge to develop an optimized Mg from Column II; carbon from Column IV for p-type
synthetic process that has the required purity, efficiency, doping; and S, Si, Se, Te from Column V—for n-type dop-
volume, reproducibility, and yield. ing are required for the growth of epitaxial device struc-
For any crystal growth process, an extremely impor- tures. In most practical applications, these dopants can be
tant consideration in the growth of high-quality epitaxial readily obtained from the corresponding precursors listed
device structures is the purity of the sources. This is es- in Table I. Of particular note is the metal alkyl source
pecially true for MOCVD since the organometallic pre- for Mg, (bis)cyclopentadienylmagnesium [(C 2 H 5 ) 2 Mg,
cursors are extremely reactive and are thus difficult to Cp 2 Mg] (a solid source) that is commonly used to provide
purify. In addition, the hydrides are toxic and, in the case Mg acceptor atoms to make p-type wide-bandgap III–V
of PH 3 , also can react with air to form hazardous mate- materials (e.g., materials in the InAlGaP and InAlGaN
rials. Owing to these difficulties, it is only recently that systems).
techniques have been developed to directly measure the The availability of inductively coupled plasma mass
impurities in the metal alkyls with the necessary sensitiv- spectrometry (ICPMS) has provided a method of detection
ities in the range of parts per billion (ppb) by weight. This of many impurities at very low concentrations directly in
level of detection is required because “unintentional” im- the organometallic compound itself. ICP mass spectrom-
purity concentrations in the solid films grown with these etry is a relatively recently developed chemical analysis
sources directly influence the electronic properties of the technique that is useful in the detection of trace element
epitaxial film. For example, a GaAs epitaxial layer hav- concentrations in a liquid or solid matrix. ICPMS can mea-
3
22
ing an atomic density of ∼2 × 10 atoms/cm and a total sure the presence of almost all elements simultaneously,
impurity concentration of 10 ppb of one specific element thus giving a detailed, semiquantitative picture of the im-
would have an unintentional concentration of “unwanted” purity distribution in the sample. This technique has sensi-
−3
14
atoms of nearly 2 × 10 cm . In many cases, these unin- tivities for many elements in the parts-per-billion to parts-
tentional impurities are present in organometallic sources per-trillion range. It has the advantage that it is extremely
in much higher concentrations, in the range of 2–5 ppm sensitive and can analyze small samples (10 ml or less) of
by weight. This results in a much higher concentration of organometallics directly. The ICPMS technique employs a
unintentional impurities being incorporated into the semi- plasma to dissociate the material to be characterized into