| The quantum cascade laser (QCL) is a unipolar device in that it uses only electrons or holes to generate coherent light. The wavelength of the light emitted can be tailored through bandgap engineering of a heterostructure, and can theoretically be tailored to any wavelength subject to the conduction-band offset of the semiconductor used. The active medium of conventional QCLs is composed of a superlattice of quantum wells and barriers of the same composition, respectively. As a result, for devices emitting in the 4.5-5.5mum range there is substantial thermionic carrier leakage from the upper laser level to the continuum given the relatively small (∼ 200meV) energy differential, deltaE, between the upper lasing level and the top of the exit barrier.;To solve this problem we have designed and fabricated a QCL device for which carrier leakage is suppressed due to deep wells and tall barriers in each active region. For our deep-well design deltaE has a value of ∼ 400 meV, and thus the carrier-leakage current is substantially suppressed. Another advantage of the proposed deep-well design is that the highly strained layers are located only in a portion of each stage, thereby holding the potential to improve the device long-term reliability.;This deep-well concept uses three or more different compositions for well and barrier materials and thus metal-organic chemical vapor deposition (MOCVD) epitaxial crystal growth is ideally suited to implement it, since changing the composition of the grown materials involves only changing the relative flow rates of the constituent gases.;Laser designs were made for GaAs- and InP-based deep-well devices. Monte-Carlo simulations of a GaAs-based, deep-well device show that the such a design can be used to lower the emission wavelength without penalty in device performance, compared to a state-of-art GaAs-based QCLs. Deep-well, InP-based 4.8mum QCLs provided low threshold-current density, Jth, and Jth characteristic-temperature, T0, values as high as 218K as the heatsink temperature varied from 250K to 340K. |
