Electronic and structural variations of 2D metal-chalcogenide and 3D oxide semiconductors

Abstract

Electronic structure methods based on the hybrid density functional theory (DFT) are powerful tools for studying the electronic and structural properties of materials. Starting from only the atomic numbers and a guess for the atomic positions and crystal structure, DFT calculations predict lattice parameters typically within 1% of the experimental values. It can be used to determine enthalpies of formation, defect formation energies, surface and interface energies, migration energy barriers, phonon spectra, and electronic band structure. The screened hybrid functional, as proposed by Heyd, Scuseria, and Enzerhof (HSE06), can describe most metals, semiconductors, and insulators quite accurately, allowing the study of a wide range of materials, covering almost all the Periodic Table. Here, we use hybrid DFT to study two classes of materials: 2D metal dichalcogenides and metal oxides. The sensitivity of these materials to external stimuli, such as strain or doping, provides a powerful tool for engineering their properties. Besides, these are important materials in semiconductor research due to their exotic electronic, optical, and structural properties that enable a wide range of applications, including transistors, photonic memory devices, and solar cells. ☐ In this dissertation, the effects of uniaxial stress and biaxial strain on the electronic structure and optical properties of the metal dichalcogenides, and the effects of doping on lattice expansion and band alignments in the metal oxides are explored.Transition-metal dichalcogenides (TMDs) are promising materials for nano- and optoelectronics due to their tunable electronic properties, especially when strained. We employed the HSE06 hybrid functional to study the effects of uniaxial and biaxial strain on monolayer TMDs (MX2 , with M = Mo, W, and X=S, Se, Te). The study focuses on how strain can modulate the band structures of these materials, providing valuable insights into strain engineered devices. The results show that uniaxial tensile stress breaks the ideal hexagonal symmetry and strongly affects the conduction band, lowering it with respect to the vacuum level. For strains greater than 2%, the conduction band minimum and valence-band maximum are shifted from the high symmetry K points, leading to significant distortion and anisotropy of the band structure near K, with important implications to luminescence efficiency and, thus, optoelectronic device applications. ☐ These results emphasize the potential of TMDs for hosting defects that act as single-photon emitters. The ability to change TMDs’ electronic properties using stress opens up new opportunities for controlling their emission properties. Fine-tuning the strain levels allows for the optimization of emitting wavelengths and increased photon generation rates. Furthermore, the inclusion of controlled defects within TMDs can result in confined states that allow the emission of single photons. This dual approach, which combines strain and defect engineering, provides an effective approach for creating efficient, scalable single-photon sources, which are important for the advancement of quantum information and communication technologies. ☐ In the case of In2Se3 , a phase-change material suitable for photonic memories, experiments explored the reversible transitions between its α and β crystalline phases. These transitions, triggered by a single nanosecond pulse, enable low-energy switching, which is essential for non-volatile memory applications. We investigated the atomic mechanisms driving these phase transitions using DFT calculations in combination with experimental observations. The results show that the low configurational entropy and distinct optical properties of In2Se3 make it an excellent candidate for reconfigurable photonic devices. The study also reveals how phase stability and refractive index variations contribute to the performance of the material in optical systems. ☐ In the study of oxides, we investigate the effects of high doping levels on the lattice expansion of CaSnO3 , which is a promising wide bandgap oxide with high electron mobility at room temperature. CaSnO3 has long been considered undopable due to previous challenges in achieving effective n-type doping. In this study, our collaborators have successfully demonstrated n-type doping with lanthanum (La) using hybrid molecular beam epitaxy. These results open new possibilities for the use of CaSnO3 in high-performance electronic devices. Although the ionic radii of La and Ca are comparable, increasing the doping concentration led to an expansion of the out-of-plane lattice parameters, a phenomenon attributed to an electronic effect, as revealed by our DFT calculations. In addition, CdTe-related oxides are studied to understand the role of surface and grain boundary oxidation on solar cell efficiency, as CdTe is an important technology for photovoltaic applications. We are investigating how different CdTe-related oxides, such as CdO and CdTeO3 , affect the band alignment with that of CdTe. ☐ Finally, we systematically evaluated the performance of different DFT functionals, including DFT+U and hybrid functionals, to compare their accuracy in predicting important properties such as band gaps, ionization potentials (IP), and electron affinities (EA) for metal oxides. While DFT+U tends to improve band gap predictions in many cases, it often falls short in accurately predicting IP and EA. On the other hand, hybrid functionals, particularly the Heyd-Scuseria-Ernzerhof (HSE) functional with the right mix of parameters, provide more reliable predictions for a wider range of materials. This emphasizes the need for careful selection of computational methods, especially for high-throughput material screening in fields such as catalysis, photovoltaics, and batteries, where accurate description of band gaps, IP, and EA is critical to designing devices.