Characteristics of deep-well 4.8 μm-emitting quantum-cascade lasers grown by MOCVD

In this work we present the characteristics of a novel type of quantum-cascade (QC) laser: the deep-well (DW) QC device, which, unlike conventional QC lasers, contains a superlattice of quantum wells and barriers of different composition, respectively. The fabrication of DW-QC devices is made possible by the use of metal-organic chemical vapor deposition (MOCVD), a crystal growth technique which allows one to easily vary the composition of wells and barriers within QC structures, thus providing significantly increased flexibility in optimizing the device design. We have designed such varying-composition QC structures to have deep quantum wells in and tall barriers in and around the active region. DW-QC laser structures have fabricated into 19 mu m-wide ridges and 3 mm-long chips. Threshold-current densities as low as 1.5 kA/cm(2) are obtained at room temperature in the 4.6-4.8 mu m wavelength region. In conventional QC lasers emitting in the 4.5-5.5 mu m range there is substantial thermionic carrier leakage from the upper laser level to the continuum, as evidenced by a significant decrease in the slope efficiency above 250 K, which is understandable given the relatively small (i.e., similar to 200 meV) energy differential, delta E, between the upper lasing level and the top of the exit barrier. For the DW design carrier leakage is suppressed due to deep active wells and tall barriers, such that dE reaches values in excess of 400 meV. Preliminary results include a threshold-current characteristic temperature, T-0, value of 218 K over the temperature range: 250-340 K.