The Lattice Constant of a semiconductor describes the spacing (in angstroms) of the atoms in the semiconductor crystal of that material. Growing different semiconductor material on top of each other creates a boundary region where the two crystal structures meet. If the lattice constants are different enough, the mismatching positions of the atoms in the crystal at the boundaries can cause strain, leading to cracks or dislocations that propagate through the structure. This can be disastrous for the active region, as it interrupts the carefully engineered superlattice that defines the quantum structure.
When growing a material on top of another with a significantly different lattice constant, a buffer layer can be utilized. This layer eases the lattice constant of the structure from one value to another in gradual steps. It can also trap travelling striations, protecting the laser core. In QCLs, we have used this technique to grow an InP waveguide on a GaAs substrate.
Continuing along this trend, GaAs can be further grown on Si. This would allow for the growth of QCLs on Si and integration into computer chips. The end result being tunable infra-red sources and sensors integrated in computer systems.