While IV-VI materials were some of the first semiconductors ever studied, they have yet to reach widespread commercial application in optoelectronics. Unfortunately, the peculiar bonding and structure of the IV-VI rocksalts that provides them with unique electrical characteristics also makes them very difficult to incorporate on foreign substrates. The rocksalt IV-VIs, PbSe, PbTe, and SnTe, have a mixed-character bonding, which contributes to a curious band structure with a direct L-valley bandgap, low auger recombination, and high dielectric constant, all beneficial features for defect-tolerant optoelectronics, however achieving high structural quality has always been a problem. While the first PbSe and PbTe lasers were demonstrated almost 60 years ago, research on infrared materials has slowly shifted towards II-VIs, III-Vs, HgCdTe, and quantum cascade lasers, with many fundamental materials problems in the IV-VI family left unsolved. In this dissertation, I hope to address some of these questions, specifically related to material growth, and lay groundwork for further exploration of PbSe bonding, optical, and electronic properties on a more robust growth platform.As the field of epitaxial crystal growth inexorably advances, materials scientists are forced to tackle ever more challenging heteroepitaxial combinations. Beyond direct device applications, PbSe growth on III-V substrates is a compelling model system for understanding high energy interfaces incorporating changes in valency, interface charge, coordination, and lattice mismatch. This dissertation focuses on molecular beam epitaxy of PbSe on zincblende GaSb, InAs, and GaAs substrates. We find that on arsenic-terminated surfaces in both the (001) and (111) orientations, nucleation is controllable. By exposing a III-V surface to PbSe flux at a temperature above the PbSe re-evaporation temperature, the surface is converted to a better template for growth: subsequent low-temperature initiation of PbSe flux results in single-orientation films. The resulting heterovalent and heterocrystalline interfaces are mapped using high resolution scanning transmission electron microscopy, and possible mechanisms of interface formation are discussed, with emphasis on interfacial symmetry. As interfacial energetics have not yet allowed for layer-by-layer growth of PbSe on any foreign substrate, understanding and controlling island coalescence is the key to synthesizing high-quality PbSe films. By systematically varying lattice mismatch and comparing films grown in different orientations, different relaxation mechanisms can be activated and studied. We find PbSe island shape and coalescence behavior is a function of surface mismatch, chemistry, and local strain from extended defects. Extensive defect characterization using electron channeling contrast imaging and transmission techniques has allowed for direct characterization of these complex dislocation structures. Through these studies, we present multiple avenues towards high-quality PbSe film growth on III-V substrates. Fascinatingly, even in structurally defective PbSe and PbSnSe films, strong photoluminescence can be observed, speaking to the future of these materials as defect-tolerant infrared emitters and detectors.

Direct crystal growth approaches for III-V/silicon integration are of great technological interest for photonic integrated circuit applications. However, historically, these approaches have been hindered due to the generation of large numbers of interfacial dislocations that form to mediate materials property mismatches between GaAs/InP based films and the silicon substrate. The presence of dislocations at this interface, far from critical device regions, is not inherently problematic. But their ends, termed threading dislocations, rise upward through all the subsequent device layers, facilitating non-radiative recombination losses where they cross the light-emitting ‘active’ region. Further, the energy released in these defect-assisted processes facilitates ongoing dislocation growth during laser operation. As a result, these runaway ‘recombination enhanced dislocation climb’ and ‘recombination enhanced dislocation glide’ (REDC and REDG, respectively) processes culminate in device failure. Thus, laser reliability engineers have traditionally focused on efforts to reduce threading dislocation densities and to develop dislocation tolerant infrared emitters. However, even with decades of research seeking to reduce threading dislocation densities and to develop more dislocation tolerant gain materials such as p-modulation doped InAs quantum dots (QD), device lifetimes at industrially relevant temperatures (near 80°C) remained too short by orders of magnitude.Seeking to better understand the mechanism by which even relatively low densities of threading dislocations led to failure in InAs QD lasers, my colleagues and I identified that the threading dislocations give rise to in plane misfit dislocations lying at the QD layers. This however was puzzling: these layers are designed to be too thin for misfit dislocations to form due to lattice mismatch during growth. We therefore proposed that these specific misfit dislocations formed instead during post-growth cooldown due to (1) thermal expansion mismatch between the (Al)GaAs layers and the silicon and (2) local mechanical hardening effects in the QD layers. To test this hypothesis, we then used metallurgical principles to intentionally widen the mechanically hardened region and thus displace misfit dislocation formation to less critical regions of the device. This was very successful. Placing a single, thin, alloy-hardened ‘misfit dislocation trapping layer’ approximately 100-200 nm to either side of the quantum dot active region reduces the total QD adjacent misfit dislocation length by >10x and trapped misfit dislocations show no evidence of recombination enhanced dislocation climb (REDC) after more than 1000 hours of continuous operation at 60°C. Furthermore, lasers with misfit dislocation trapping layers operating 80°C demonstrate median extrapolated lifetimes significantly greater than 100,000 hours—meeting the requirements for data center operation. Thus, these results simultaneously emphasize both the importance of moving in earnest towards the advanced photonic integration schemes and the importance of continuing fundamental materials studies on III-V semiconducting systems.