What is the role of CdTe in CdTe/HgTe/CdTe Topological insulator?

[dufigo]

to get a 2D mercury telluride topological insulator,
one has to construct a quantum well structure to get a bulk gap
and most people use sandwiched structure
with mercury cadmium telluride on top and the bottom. (so CdTe/HgTe/CdTe)

and my question is
can we get same or similar quantum spin hall effect
when we use other normal insulator other than mercury cadmium telluride
or can we just use freestanding mercury telluride and see the same effect?

if CdTe is necessary, what is special about using mercury cadmium telluride?

 

[Cthugha]

to get a 2D mercury telluride topological insulator,
one has to construct a quantum well structure to get a bulk gap
[...]
and my question is
can we get same or similar quantum spin hall effect
when we use other normal insulator other than mercury cadmium telluride
or can we just use freestanding mercury telluride and see the same effect?

if CdTe is necessary, what is special about using mercury cadmium telluride?

First, free mercury telluride will not work because it simply is no quantum well. You need a second material with different band gap to get the necessary confinement for creating a quantum well as you get trapping of carriers inside the material with lower band gap.

The special thing about using CdTe is simply that it works. For quantum wells you need two materials with different band gap grown on top of each other. However, if these do not have a rather similar lattice structure, you get lots of strain by lattice mismatch which significantly lowers the quality of the structure.

The lattice parameters of HgTe and CdTe are as follows (at room temperature):
HgTe 0.646 nm CdTe: 0.648 nm

So they are a really good match.