Antimonide-based type-II superlattices for infrared detection

The vast array of applications for the detection of mid- to long-wave infrared radiation has spurred continuous interest in new and novel technologies to replace the current generation of state-of-the-art devices such as mercury cadmium telluride (MCT) detectors. One of the more promising alternatives is the type-II superlattice (T2SL), which was first proposed for infrared detection in 1978 and has been theoretically predicted to perform better than any MCT detector. Some advantages of the T2SL are its vastly improved electrical properties, material growth quality and cost, and the large number of degrees of freedom in tailoring the band structure to maximize performance at any given wavelength when compared to MCT detectors. However, the performance of T2SLs has been limited due to growth and fabrication problems, though the past decade has seen devices demonstrated with less than an order of magnitude difference in performance metrics when compared to commercially available MCT products, which is very promising for this material system. This thesis presents only the second generation of T2SL devices grown via metal-organic chemical vapor deposition (MOCVD), and the first generation of devices grown on an InAs substrate by either molecular beam epitaxy (MBE) or MOCVD, an important fact for flip-chip bonding applications due to the lower absorption coefficient of InAs in the infrared when compared to the more common GaSb substrates. MOCVD is the preferred growth method in the industry, when available, due to its fast deposition rates with only a minimal sacrifice in growth precision when compared to MBE, so it is imperative that MOCVD T2SL devices quickly demonstrate performances similar to MBE grown T2SLs. A peak specific detectivity of 7.62 x 10^9 Jones at approximately 8 um is reported, a 4.8 times increase from the value of 1.6 x 10^9 Jones for the first MOCVD grown T2SL (which was grown on a GaSb substrate). This thesis also addresses the reduction and elimination of surface leakage current, a dark current method that can be debilitating to device performance if not properly addressed. Promising results are achieved through the use of an ammonium sulfide soaking solution. In one instance, an almost one order of magnitude reduction of dark current densities is achieved using a neutralized ammonium sulfide solution, indicating that surface passivation is an important and necessary processing step. Future improvements, such as the usage of an encapsulating layer of polyimide or silicon nitride, are suggested in order to maintain the integrity of the ammonium sulfide passivation scheme and to physically protect the device itself

Mode control in VCSELs using patterned dielectric anti-phase filters

A novel transverse mode control method to achieve single-fundamental-mode lasing and higher-order-mode suppression using a multi-layer, patterned, dielectric anti-phase (DAP) filter is employed on the top of oxide-confined and proton-implanted vertical-cavity surface-emitting lasers (VCSELs). Dielectric layers are deposited and patterned on individual VCSELs in a wafer-scale process to modify (increase/decrease) the mirror reflectivity across the oxide aperture via anti-phase reflections, creating spatially-dependent threshold material gain and VCSEL lasing mode control. A one-dimensional (1D) plane-wave propagation method is used to calculate the dielectric layer thicknesses in each spatial region needed to facilitate or suppress lasing. A Quasi-3D oxide-confined VCSEL model is formulated using a combination of variations of the propagation matrix method, the weighted effective index method, and the step-index fiber mode dispersion (BV) curves to properly calculate the effect of the DAP filter on the calculated cavity modes as well as determine the optimal radial proportions of the filter. A single-fundamental-mode, continuous-wave output power greater than 4.0 mW is achieved on an oxide-confined VCSEL at a lasing wavelength of 850 nm with a side-mode suppression ratio (SMSR) greater than 25 dBm. Proton-implanted VCSELs achieve a single-fundamental-mode, continuous-wave output power of up to 3.5 mW with a SMSR of 25 dBm. The behavior of the proton-implanted devices both with and without the DAP filter illuminates an unobserved annular thermal guiding mechanism even in smaller device sizes, contrary to historical models which have calculated or assumed a parabolic refractive index or gain-guided profile. A finite difference, self-consistent thermal, electrical, and optical model is developed and agrees well with the observed results both with and without the DAP filter. The dielectric anti-phase filter is an additive, non-destructive method that allows for mode selection at any lasing wavelength and for any VCSEL layer structure or design without the need for destructive etching techniques or epitaxial regrowth. It also offers the capability of a tailored filter design based on available materials and deposition methods

Breaking down barriers to consistent, climate-smart regulation of invasive plants - a case study of northeast states

Efforts to prevent the introduction and spread of new invasive plants are most effective when regulated species are consistent across jurisdictional boundaries and proactively prohibit species before they arrive or in the earliest stages of invasion. Consistent and proactive regulation is particularly important in the northeast U.S. which is susceptible to many new invasive plants due to climate change. Unfortunately, recent analyses of state regulated plant lists show that regulated species are neither consistent nor proactive. To understand why, we focus on two steps leading to invasive plant regulation across six northeast states (Connecticut, Maine, Massachusetts, New Hampshire, New York, and Vermont): which sets of species are evaluated and how risk is assessed. Our analysis confirms previous findings that invasive plant regulations are inconsistent and reactive. Of the 128 plants regulated by one or more states, 54 were regulated by a single state and only 16 were regulated by all six states; regulated species tended to be widespread across the region (not proactive). These outcomes are largely driven by different sets of evaluated species. For example, neighboring states Vermont and New Hampshire evaluated 92 species in total, but only 26 overlapped. In addition, states rarely evaluated species that were absent from the state. Risk assessment protocols varied considerably across states, but consistently included criteria related to ecological impact, potential to establish, dispersal mechanisms, and life history traits. While none of the assessments explicitly consider climate change, they also did not contain language that would preclude regulating species that have not yet arrived in the state. To increase consistency and proactivity, states would benefit from 1) evaluating species identified as high risk by neighboring states as well as high risk, range-shifting invasives, both of which we compiled here and 2) explicitly considering climate change when assessing ‘potential distribution’ or ‘potential impact’ of target species. Additionally, a mechanism for sharing knowledge and risk assessments regionally would benefit states with fewer resources to address invasive species threats. Presenting a unified defense against current and future threats is critical for reducing impacts from invasive species and is achievable with better state-to-state coordination

Phenological mismatch in Arctic-breeding shorebirds: Impact of snowmelt and unpredictable weather conditions on food availability and chick growth

The ecological consequences of climate change have been recognized in numerous species, with perhaps phenology being the most well-documented change. Phenological changes may have negative consequences when organisms within different trophic levels respond to environmental changes at different rates, potentially leading to phenological mismatches between predators and their prey. This may be especially apparent in the Arctic, which has been affected more by climate change than other regions, resulting in earlier, warmer, and longer summers. During a 7-year study near Utqiaġvik (formerly Barrow), Alaska, we estimated phenological mismatch in relation to food availability and chick growth in a community of Arctic-breeding shorebirds experiencing advancement of environmental conditions (i.e., snowmelt). Our results indicate that Arctic-breeding shorebirds have experienced increased phenological mismatch with earlier snowmelt conditions. However, the degree of phenological mismatch was not a good predictor of food availability, as weather conditions after snowmelt made invertebrate availability highly unpredictable. As a result, the food available to shorebird chicks that were 2–10 days old was highly variable among years (ranging from 6.2 to 28.8 mg trap−1 day−1 among years in eight species), and was often inadequate for average growth (only 20%–54% of Dunlin and Pectoral Sandpiper broods on average had adequate food across a 4-year period). Although weather conditions vary among years, shorebirds that nested earlier in relation to snowmelt generally had more food available during brood rearing, and thus, greater chick growth rates. Despite the strong selective pressure to nest early, advancement of nesting is likely limited by the amount of plasticity in the start and progression of migration. Therefore, long-term climatic changes resulting in earlier snowmelt have the potential to greatly affect shorebird populations, especially if shorebirds are unable to advance nest initiation sufficiently to keep pace with seasonal advancement of their invertebrate prey