Quantum-cascade-laser structures
Light emission from quantum cascade laser (QCL) structures is based on intersubband transitions. QCLs consisting of GaAs/(Al,Ga)As grown on GaAs may emit light ranging from about 8 to 300 µm, depending on their design.
The active zone in a QCL structure, cladded between waveguide and contact layers, contains a large number of periods which in turn consist of a chirped superlattice. The total number of layers may exceed 1000, and the total thickness of the QCL structure is about 10 µm. Some layers within a period approach a thickness equivalent to two monolayers.
Therefore, the fabrication of a QCL structure requires a high stability of all fluxes, particularly group III and dopant, during growth (about 12 hours).
The structural properties are examined by X-ray diffraction. An example is shown in the image.

XRD curve of a quantum cascade laser structure using the (002) reflection. From the positions of the numerous satellites and the dynamics of their intensities the period lenght, the average Al composition and with the constraint of constant fluxes the thicknesses of the individual layers are determined.

