Deep-well 4.8μm -emitting quantum-cascade lasers grown by MOCVD

A novel design for a deep-well InP-based QC-laser structure has been realized using MOCVD growth for emission wavelengths of 4.6μm-4.8μm with low threshold current density (1.5 KA/cm²) at room temperature (RT) and reduced temperature sensitivity. The active medium of conventional quantum-cascade lasers is composed of a superlattice of quantum wells and barriers of the same compositions. One consequence is that for devices emitting in the 4.5-5.5μm range there is substantial thermionic carrier leakage from the upper laser level to the continuums, evidenced by a strong decrease in the slope efficiency above 250 K. That is, the slope-efficiency characteristic- temperature coefficient, T₁, drops from a reasonably high value of 600K for heatsink temperatures below 250K, to a low value of 120K for heatsink temperatures between 250K and 298K, which is understandable given the relatively small (≈ 200meV) energy differential, δE, between the upper lasing level and the top of the exit barrier [1]. Thus, even though such BH devices show high wallplug efficiency [1], the strong temperature sensitivity of their electro-optic characteristics raises strong doubts about their reliability and viability as practical devices. To solve this problem, we designed and fabricated a device (Fig.1) for which carrier leakage is suppressed due to deep wells and tall barriers in the active regions. For our deep-well design δE has a value of ∼ 400 meV, and thus the carrier-leakage current proportional to exp(-δE/kT) is suppressed. Another advantage of the proposed deep-well design is that the highly strained layers are located only in one portion of a stage, thereby reducing the overall strain within each stage. A deep-well QC laser structure has been grown by MOCVD and fabricated into 19μm-wide mesas and 3mm-long chips. The devices lase at 4.8μm (Fig.2) with a threshold-current density of 1.5kA/cm² at 298K. This value is comparable to the state-of-the-art for 4.6-4.8μm conventional QCLs with the same cavity length (and uncoated facets) [2], even though the squared matrix element, [z₄₃]², is only 66% of the typical value for a conventional QCL. Preliminary tests under pulsed operation yield T₀ values of ∼ 230K, as compared to ∼ 150K for conventional QCLs, although narrower-ridge (< 10μm) devices are needed to accurately measure T ₀ and T₁. Another advantage of the deep-well devices is that, because of different Bragg mirror and injector regions, the quantity Δ, the energy differential between the quasi-Fermi level in the injector region and the lower lasing level in the prior stage, can be increased to suppress thermal backfilling [3] and in turn allow for better CW operation. For the current deep-well design the Δ value is 146meV, compared to 120meV for conventional 4.8μm QC devices [4]. In conclusion we present a novel type of QCL device designed for tight carrier confinement to the active quantum wells as well as reduced backfilling. Such structures are expected to provide room-temperature wallplug efficiencies as high as 15% with watt-range CW output powers.