Photochemical Organometallic Vapor Phase Epitaxy of Mercury Cadmium Telluride
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Because of the difficulty in preparing single-crystalline Hg 1 x CdxTe with
adequate control of properties for detector array manufacturing, the emphasis in material development has shifted to epitaxial techniques, such as molecular beam epitaxy (MBE), epitaxy (OMVPE,
liquid phase epitaxy (LPE),
sometimes called MDCVD).
and organometallic vapor phase
Each of the epitaxial techniques
thus far reported has fundamental limitations such as a requirement for high growth temperature (LPE and OMVPE) or low growth rates (MBE).
We report here
preliminary results on a new technique, called photochemical organometallic vapor phase epitaxy (POMVPE),
which apparently does not have the limitations
of other epitaxial methods for growing Hg1_x CdxTe films. The basis for this POMVPE approach is a radical departure from conventional OMVPE technology.
We investigated a reaction scheme which,
in
principle, decouples deposition from substrate temperature and thus permits growth at the lowest temperature required for good crystal quality. The first step of any OMVPE method is the decomposition of volatile organometallic compounds into the nonvolatile elements of interest plus volatile byproducts.
Since all suitable organometallic compounds for OMVPE of
Group 12-Group 16 intermetallic compounds have the form MR2, where M is the element of interest and R is an organic ligand, the decomposition reaction simply involves the breaking of the two metal-carbon bonds.
In conventional
OMYPE, this decomposition is accomplished thermally at the surface of a heated
Mat. Res. Soc. Symp. Proc. Vol. 54. '1986 Materials Research Society
386
substrate. 1- 3
In POMVPE,
the same decomposition is accomplished
photochemically. Two basic requirements must be met for photochemical decomposition. First, the incident radiation must be sufficiently energetic to effect decomposition, and second, the compound in question must have a sufficiently high absorption coefficient at the incident radiation wavelength for significant decomposition to occur. Available data clearly indicate that Hg, Cd, and Te alkyls can be decomposed by 193 nm radiation with a single photon process.3-13
Decomposition,
however,
is not the only factor required for a practical epitaxial growth
method.
Decomposition of the organometallics must be uniform across the
substrate surface and sufficiently rapid to permit reasonable epitaxial growth rates.
Simple modeling to assess the practicality of POMVPE for Hg 1_xCd xTe 1'4
was performed for a geometry described by Boyer et al.
for the deposition of
Si0 2 , in which ultraviolet radiation passes parallel to the substrate. For this geometry,
the intensity of the incoming radiation decreases
along the optical axis because of absorption by the various alkyls used for growth.
Thus, one would expect the growth rate to decrease in the direction
of the optical axis, which is clearly not acceptable.
A simple solution to
this problem is to photolyze all the molecules in the illuminated volume near the substrate.
The
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