The objective of the proposed work is to demonstrate devices that utilize intersubband transitions in arrays of quantum boxes (QBs), to achieve room-temperature, continuous-wave (CW) oscillation in the mid-infrared (IR) wavelength range, with low heat-dissipation powers.
Intellectual Merit: The novel approaches proposed are: 1) the use of block-copolymer lithography for the realization of arrays of 20-25 nm-diameter QBs; 2) the use of deep quantum wells for the QB active material for strong carrier confinement; 3) the use of selective regrowth of high-bandgap barriers to confine carriers in the plane of the QBs..
Broader Impacts: The realization of uncooled, CW mid-IR sources will make possible hand-held, real-time sensors for a wide range of noninvasive medical diagnostics and enable exciting developments such as practical sensing of chemical agents and explosives. A central goal and natural outcome of this project is the research-based education of both undergraduate and graduate students. The students will be involved in all aspects of the research within an established collaborative team-based environment. The proposed program provides opportunities for outreach to underrepresented groups through our ongoing partnering with the existing college and NSF Center-based programs at the University of Wisconsin. The realization of the proposed QB lasers will have a transformational impact on both the science and technology of mid-IR lasers because reducing the heat dissipation by two orders of magnitude will lead to uncooled devices, which in turn will revolutionize the mid-IR sensors industry since hand-held sensors would become available for a vast array of applications.
The project mainly focused on the tailoring of the active regions of mid-infrared (IR)-emitting quantum cascade lasers (QCLs) for maximizing their overall electrical-to-optical power conversion efficiency, so-called wallplug efficiency. The basic problem with conventional QCL structures is that they are composed superlattices of quantum wells (QWs) and barriers of fixed alloy compositions; i.e., the device design consists of only varying the thicknesses of the superlattice layers, something we call one-dimensional conduction-band engineering. This restriction causes QCLs emitting in the 4.5-5.0 μm range to suffer from severe carrier leakage out of their active regions which, in turn, manifests itself in low continuous-wave (CW) wallplug efficiency values (10-12 %) compared to theoretical expectations of ~ 30 %; and is the main reason why statistically relevant lifetest data exist only for low (< 0.2 W) CW output powers. By employing the crystal-growth technique of metal-organic vapor-phase epitaxy (MOVPE) we have the flexibility to control the composition of each layer within superlattice structures; thus, we can design novel QCL active-region structures for which the composition of QWs and barrier layers can be adjusted at will. As a result, we have developed and employed multidimensional CB engineering for designing devices that achieve performances approaching theoretical limits. The project outcomes and findings are as follows: A novel QCL structure design: Step-Taper, Tapered Active-region (STA); leads to both virtually complete suppression of carrier leakage (i.e., less than 2 % of the threshold current is leakage current) as well as reduction of the non-leakage-part of the lasing threshold current. As a result, the room-temperature threshold current does decrease by ~ 25 % compared to that of conventional QCLs. At a heatsink temperature of 90 oC the threshold current of a STA-type QCLs is as much as ~ 40 % lower than that of conventional QCLs. This, in turn, will lead to much less heating and to higher CW operating temperatures than for conventional mid-IR QCLs. The STA-QCL concept has been presented at several conferences, and has triggered Invited Talks at two prestigious international conferences. It has also triggered an Invited Paper in the special issue of semiconductor lasers of the prestigious journal IEEE Journal of Selected Topics in Quantum Electronics (JSTQE), which paper has been downloaded 185 times since it appeared on line in January 2013; and thus represents the most-downloaded QCL-related paper from the special JSTQE issue on semiconductor lasers. Finally, a patent application covering the STA-QCL concept was filed with the U. S. Patent Office in August 2012. Considering 4.5-5.0 μm-emitting STA QCLs, the projected maximum pulsed room-temperature wallplug efficiency is 29 %; that is, within 1 % of the theoretical limit. The projected maximum CW room-temperature wallplug efficiency is 27 %. In turn, long-term reliable operation becomes possible at high (> 0.5 W) CW output powers as needed for a variety of applications from environmental monitoring to missile- avoidance systems. The STA-QCL concept was extended to low strain-differential 3.0-4.0 μm-emitting QCL structures grown atop virtual substrates deposited on GaAs substrates. STA designs of negligible leakage current (i.e., less than 2 % of the threshold current is leakage current) have been reached for 3.1 μm-, 3.6 μm- and 4.0 μm-emitting QCL structures. Due to reduced thermal conductance as well as suppressed carrier leakage such devices will result in substantially improved CW electro-optical characteristics compared to conventional 3.0-4.0 μm-emitting QCLs as well as will allow, for the first time, long-term reliable operation. These findings led to two Invited Talks at international conferences and an Invited Paper in the prestigious journal IET Optoelectronics. The realization of high-power, reliable 3.0-4.0 μm-emitting QCLs is relevant for cost-efficient marking of plastic food packages and for remote detection of many environmental chemical agents and gases such as methane which is 25 times more abundant in the atmosphere than carbon dioxide. In prior years, by using multidimensional conduction-band engineering, we have demonstrated a QCL whose slope efficiency varied with temperature five times slower than in conventional QCLs. That was experimental proof of carrier-leakage suppression. The novel concept was patented in a patent issued in December 2012. We have also demonstrated the successful fabrication of large-area, high-density nickel nanopillar arrays on GaAs substrates using diblock-copolymer lithography and electrodeposition. The results were published in an article in the Journal of Vacuum Science & Technology (JVST) which was one of the Top 20 Most Downloaded JVST articles for the month of April 2013. We have also developed, in 2010, the first accurate model for electron leakage in QCLs, which has proven a valuable design tool for QCLs emitting in both the mid- and far-IR spectral ranges. The model was published in the high-impact journal Applied Physics Letters (APL) and was one of the Top 20 Most Downloaded APL articles for the month of August 2010. Ever since it has been full-text downloaded over 1750 times.
