| AlxInyGa1−x−yN quaternary alloys with 0 < x < 0.26 and 0 < y < 0.15 were grown in a modified Thomas Swan MOCVD reactor. The growth of uniform, high-quality quaternary alloys required relatively high growth temperatures (T > 870°C), high gallium flow rates (>1.5 μmols/min), and high ammonia flows (4 slm). Quaternary growth at non-optimal growth conditions sometimes resulted in the formation of self-assembled superlattices (SASLs), with alternating layers of high and low Al+In content. It was found that the band gap and lattice constant of uniform AlInGaN films can be controlled independently for a finite range of compositions. This has allowed, for the first and only time in reported literature, the growth of InGaN quantum wells (QWs) subject to a full range of pseudomorphic strain (compressive, tensile, and no strain).; Capacitance-voltage (C-V) measurements show that the 2DEG properties in strained and unstrained heterostructures are markedly different. Depending on the type of strain, AlInGaN cladding can either enhance or reduce the 2DEG density in InGaN QWs. Similarly, the photoluminescence (PL) intensity of InGaN QWs is highly dependent on strain, with unstrained QWs showing much higher light emission than their strained counterparts. These observations can be explained by accounting for the effects of strain-induced piezoelectric fields, which are expected to be particularly high in III–V nitrides. Indeed, self-consistent calculations of strained and unstrained InGaN QWs, accounting for the effects of strain-induced polarization, closely match the optical and electrical characterization presented in this thesis.; Strain-induced polarization can have a particularly strong effect on the device properties of heterostructure field-effect transistors (HFETs). Self-consistent modeling, combined with data acquired in the lab, suggests that AlInGaN quaternary barrier layers can be used to enhance the 2DEG density in GaN-based HFETs beyond that achievable with current ternary technology. C-V carrier profiling of various III-nitride HFET structures has shown that AlInGaN strain engineering can enhance 2DEG density, reduce leakage current, deter the formation of parasitic conducting channels, and alter threshold voltage. |
