Defects and Disorder in Nitride Semiconductors

Nitrides are emerging as a promising class of materials for advanced opto-electronic and energy conversion applications. Compared to the extensively studied oxides, nitride semiconductors offer several advantages, including narrower band gaps, stronger bond covalency, and improved carrier-transport properties, all of which are crucial for efficient sunlight harvesting in photovoltaic and photoelectrochemical systems. Moreover, the unique mixed covalent/ionic properties of nitrides, resulting from the moderate electronegativity of nitrogen, contribute to their defect tolerance, making them more resilient to crystallographic native defects compared to other pnictides and oxides. In this work, we focus on three different nitride materials and evaluate the impacts of defects and disorder on their physical properties. A central goal of this work is to understand the specific nature of defects that have a large influence on the optoelectronic properties of nitride semiconductors as well as to establish approaches to control them. In Chapter 2, we first describe the state of the art based on literature devoted to nitrides and structural disorder. Thereafter, Chapter 3 describes the experimental and theoretical methods used to study the three nitride semiconductors investigated in this thesis. The scope of this work covers a broad range of structures, from epitaxial and polycrystalline thin films to amorphous compounds. In Chapter 4, we study the calcium zinc nitride (Ca-Zn-N) material system in its amorphous and crystalline phases. We show the highly tunable optical and electrical properties of the amorphous compound with variation in the Ca and Zn contents. The amorphous structure shows a greater tolerance to defects compared to the crystalline counterpart, paving the way for a novel field of research targeting amorphous nitride semiconductors as tailored functional materials. Further, in Chapter 5, we investigate zinc nitride (Zn3N2) and the influence of O impurities on the materials properties. We show that small concentrations of O can dope the material, but can also help to achieve non-degenerate electrical conductivities by passivation of native defects and the formation of defect complexes. These first two experimental results chapters are then followed by Chapter 6, which is devoted to amorphous GaN thin films that are optimized for use as capping layers on a broad range of compounds. We show that the material properties enable the stabilization of underlying layers without altering their electrical properties over a period longer than 3 years. Finally, the results presented in this thesis conclude with Chapter 7, which describes a computational investigation of the influence of cation disorder and cation site occupation on the material properties of ZrTaN3 with ab-initio modeling. The results lead to a general concept for tuning optoelectronic properties via site symmetry, which is expected to be applicable to a broad range of increasingly investigated ternary materials. Interestingly, despite the variety of structures, very similar results are reported regarding the roles of primary defect types and internal disorder on the optoelectronic properties of nitride semiconductors.