Valence band intersubband electroluminescence from Si/SiGe quantum cascade structures

The principle of intersubband emission is applied to the Si/SiGe material system, using hole intersubband transitions in structures grown pseudo-morphically on Si substrate by molecular beam epitaxy. Cascade structures consisting of three times four repetitions of a five quantum well sequence are investigated. The design constraints are found to be imposed mainly by the total amount of strain, giving limitations to the number of wells per cascade and the total number of cascade periods. Despite the close approach to the critical thickness for misfit dislocations, requiring low temperature growth, these structures reveal intersubband electro-luminescence with a linewidth as narrow as 22meV. Peak energies between 125 and 154meV are obtained by tuning the well width and Ge content of the single active quantum well. By comparison with the emission from a III–V cascade structure, the non-radiative lifetime of the upper emission state is determined. It is found to depend strongly on the structureʹs design, but can reach values comparable to those in similar III–V cascade structures. A discussion of the importance of carrier escape to the continuum and the injection efficiency, as well as the relaxation via the light hole state is given.