Design and operation of mid-infrared light-emitting devices (λ ≈ 11 μm) based on a chirped superlattice

The optical and electrical characteristics of a set of devices in which the periodicity of the superlattice layers has been chirped are presented. The operation of these devices can be explained through the delocalization of electron wavefunctions which satisfy the Bragg transmission condition across a portion of our structures, under the influence of an electric field. This groups together discrete states in sets of manifolds, each manifold separated by an energy gap of the order of the emitted photon energy under bias. Thus a quantum cascade scheme of electrons jumping between each manifold of states forms the basis of these structures. The operation of electroluminescent devices working at approximate to 11 mu m has been measured over the temperature range from 77 to 300 K. Line widths of 13 meV and slope efficiencies of 15 nW A(-1) have been measured at 77 K. Limiting factors in the operation of these devices can be understood from carrier overflow into higher-energy states, an effect that can be seen from light-current (L-I), current-voltage (I-V) and spectra characteristics.