Interdiffused Multilayer Processing (IMP) in Alloy Growth
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INTERDIFFUSED MULTILAYER PROCESSING (IMP) IN ALLOY GROWTH J B MULLIN, J GIESS, S J C IRVINE, J S GOUGH AND A ROYLE RSRE, St Andrews Road, Malvern, Worcs, UK ABSTRACT The Interdiffused Multilayer Processing (IMP) technique has been developed as a way of growing uniform layers of cadmium mercury telluride (CMT).
The principle of
the technique is discussed in the context of an interdiffusion coefficient D so as to assess the potential applicability of the technique to the growth of other II-VI or III-V alloys. The development of IMP for the growth of CMT layers, together with the properties of the layers and their suitability for use in the fabrication of IR detectors, are reviewed.
INTRODUCTION
The Interdiffused Multilayer Processing (IMP) technique [1,21 was developed as a way
of
growing
uniform
(CdxHg,1 - xTe or CMT).
epitaxial
layers
of
the
alloy
cadmium
mercury
telluride
The principle of the technique involves the successive growth
of alternate layers, of the binary compounds CdTe and HgTe permit their complete interdiffusion
under conditions which
in the course of a growth run. In this paper we
analyse the reasons for the technique as well as the kinetics of the process in order to assess the potential suitability of IMP for the growth of alloys in general. that have been studied are reconsidered in the light of this analysis.
Those systems
Finally the results
and achievements that have followed the application of the IMP technique to CMT are reviewed.
REACTION MECHANISMS
The original choice [3] of reactants for the MOVPE of CMT were Hg (vapour) Me 2 Cd and Et 2Te.
The thermal decomposition characteristics of the alkyls [4] and the
general features of the growth process have been reviewed [5] previously.
Essentially,
the formation of HgTe requires the independent decomposition of Et 2 Te which achieves maximum efficiency in H 2 around 410"C whereas CdTe forms by what appears to be an adduct-like catalytic reaction between Et 2Te and Me 2 Cd, a reaction which occurs with maximum efficiency for the formation of CdTe down to
350"C.
The composition of
the alloy CdxHg,.xTe is found to be critically dependent on gas flow and temperature, making it very difficult to achieve good lateral uniformity of x when depositing over large substrate areas.
Control of composition in the epitaxial CMT layers is constrained by the gas phase composition of the heterogeneously decomposing gaseous mixture at the substrate surface
Mat. Res. Soc. Symp. Proc. Vol. 90.
1987 Materials Research Society
368
but in the incorporation process during crystal growth the proportion of Cd to Hg in the alloy is ultimately determined by the interface kinetics and the thermochemistry of alloy formation.
The
vapour
been
has
equilibrium.
incorporation process shown
[6]
to
This characteristic
in a
occur
number of III-V
under
could well be
a
condition
alloys grown from the close
to
thermodynamic
a feature of CdxHg1 _xTe
The weak bond between Hg and Te (AHV
298
=
-33.9
kJ mole-
the strong bond
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