Control of optoelectronic properties via aliovalent disorder in ternary nitrides and oxynitrides

Abstract

The III-V family of semiconductor compounds have revolutionized optoelectronics. From record-efficiency GaAs solar cells to the GaN-based light-emitting diodes that won the 2014 Nobel Prize, these compounds and alloys have enabled contemporary optoelectronic technology. However, there are challenges that cannot be solved with the III-Vs; there is still an outstanding need for Earth-abundant and non-toxic optoelectronics, especially in the green-emitting regime. The underexplored II-IV-V2 family of materials offers the possibility of groundbreaking optoelectronic properties similar to the III-Vs yet with much broader chemical flexibility. In particular, the II-IV-Vs enable the use of aliovalent site ordering to tune materials properties, an entirely new design space which is predicted to control the optical band gap by up to 1 eV with very little corresponding change in bond length. The chemical and structural diversity of the II-IV-Vs massively opens up the property space of the achievable materials and alloys, enabling control and tunability far beyond the III-Vs. However, with this diversity comes complexity. Cation site ordering in the II-IV-Vs is challenging to control and to characterize in thin film form due to the very small structural changes involved. Consequently, the impact of cation site ordering on optoelectronic properties has yet to be understood. Additionally, the anion sublattice can also exhibit disorder which interplays with the cation sublattice; this is just as difficult to control and characterize. Finally, aliovalent disorder raises questions of the local coordination environments that enable this disorder, which have implications for extended defects and carrier localization but have never been investigated. Due to the complexity of this structural space, in this dissertation we develop directed synthesis and characterization routes to map the II-IV-Vs and concurrently leverage computational techniques to guide and frame experiments. Using the suite of high-throughput combinatorial synthesis techniques at the National Renewable Energy Laboratory (NREL), we orthogonalize the impact of structure and composition in order to elucidate fundamental structure-property relations, building a deeper understanding of knobs such as long-range and local structure as well as non-idealities such as oxygen contamination and off-stoichiometry. We first investigate the fundamental properties of ZnGeN2 in thin film form. To begin, we find that at high temperatures, cation- and anion-disordered ZnGeN2xOx grows epitaxially on c-Al2O3 despite significant incorporation of oxygen. Optical characterization, particularly room-temperature photoluminescence, reveals bandgap tuning consistent with a disordered and oxygen-containing film. This study, reported in Chapter 2, demonstrates that optically active epitaxial ZnGeN2xOx can form with a significant degree of disorder, indicating a path toward commercialization. Armed with the knowledge that unintentional oxygen has historically convoluted its reported properties, we then carefully map the phase space of cation-disordered ZnGeN2 to minimize oxygen contamination and characterize the fundamental properties of this material system. We find that disordered ZnGeN2 exhibits a decreased absorption threshold energy with cation disorder. An additional discovery enabled by combinatorial techniques is that ZnGeN2 stabilizes with significant cation off-stoichiometry in both the Zn- and Ge-rich directions, corresponding to an alloy-like structural shift and consequent tuning of the optical absorption. Off-stoichiometry is also framed from a theoretical perspective using defect complex calculations. These results, reported in Chapter 3, provide a thorough fundamental understanding of oxygen-free, cation-disordered ZnGeN2 and offer an entirely new knob for property tuning. Finally, we report in Chapter 4 a high-throughput study of the cation-disordered ZnSnN2-ZnO alloy system to understand the interactions between long-range disorder and local coordination environment. With the support of DFT calculations and FEFF simulations, we characterize octet-rule-violating bonding to both N and O anions with X-ray absorption near-edge structure (XANES). Upon annealing, we demonstrate a shift toward local ordering while long-range cation disorder is maintained, which correspondingly tunes the optical absorption onset. This study adds XANES to the characterization toolkit for II-IV-V2 materials for understanding local coordination environment, and begins to unravel the complex interactions between bonding environment and long-range disorder. Ultimately, this thesis re-affirms the potential of the II-IV-V2 semiconductor family and paves the way for the use of local and long-range aliovalent ordering and cation off-stoichiometry as novel knobs to tune materials properties.