First of all there are two types of quaternaries:
As a first approximation one could try to use simple linear interpolation from the binary parameters (Bs).
If relationships for the ternary parameters are available one should use the weighted sum of the related ternary values (Ts):
The lattice constant is known to obey Vegard's law well, i.e. to vary linearly with composition.
The band gaps in many ternary alloys (AxB1-xC or CAxB1-x) can be approximated in the form of the usual quadratic function
where CABC is the bowing parameter.
Lattice matched quaternaries
If one considers lattice matched quaternaries (lattice matched to the common substrate materials GaAs, InP, InAs or GaSb) one can express Q(x,y) as Q(x,y(x)). As we only have one free parameter in this case, our simulator will simply use the same scheme as in the case of ternary alloys that also have only one free parameter. The quaternary will be represented as a combination of two lattice-matched constituents, one of which must be a ternary while the other may be either a binary or a ternary. The treatment is considerably simplified by the usual absence of any strong bowing of the band parameters for such an alloy, which is expected on theoretical grounds because the two constituents have identical lattice constants.
In the following we will assume two lattice matched binary or ternary end points, E and F, which are combined with arbitrarily composition z to form the lattice matched quaternary alloy EzF1-z. we then employ the expression
QEF(z) = zTE + (1-z)TF - z(1-z)CEF (5)
where TE and TF are the values at the end poits and CEF is the additional bowing associated with combining the two end point materials to form a quaternary. For lattice matched quateraries, using equation (5) with the available experimental evidence to determine CEF for each property should lead to a better representation then either the procedure of (3) and (4) or simple linear interpolation (1, 2).
For this procedure, unfortunately, the materials
(This text is taken party from Vurgaftman et al., JAP 89, 5815 (2001))