Ingeniería de estructura y composición de nitruros componentes de transistores HEMT de nueva generación

Abstract

The present Ph. D. thesis entitled “Ingeniería de estructura y composición de nitruros componentes de transistores HEMT de nueva generación” (Engineering of structure and composition of nitrides for their implementation in novel HEMT transistors) has been carried out in the Department of Material Science and Metallurgical Engineering, and Inorganic Chemistry of the University of Cádiz, within the activities of the research group “Material Science and Engineering”. It has been supervised by the Associate Professor, Dr. Francisco Miguel Morales Sánchez. The work has been financially supported by the European Union, the Spanish Ministry of Economy and Competitiveness, the Andalusian Regional Government, and the University of Cádiz (UCA). The main objective of the Ph. D. thesis, which results are presented in this manuscript, has been to carry out the engineering of novel single phase alloyed nitride semiconductor materials, through the application of our knowledge and know-how on analytical techniques, mostly those based on accelerated electron beams (both image and spectroscopy), X-rays and ion beams, in order to achieve an improvement in the electronic properties of novel devices consisting of high electron mobility transistors (HEMTs). On this purpose, different methodologies are applied for studying the structure, the chemical composition and the electronic structure (bandgap), at different scales (even sub-angstrom and sub-electronvolt), associated to the buffer, active, barrier and cap layers and their interfaces, all involved in the developed HEMT heterostructures. These studies included advanced transmission electron microscopy techniques, such as inline electron holography, or valence electron energy loss spectroscopy. The electronic properties of the systems are also tested, which allow establishing a solid and fluent feedback between the developers of materials and devices and the researchers in charge of their structural and compositional characterization. The studied heterosystems in the context of the present doctoral thesis are, in some cases, single junctions, and, in other ones, more complex multilayered structures, integrated in HEMT devices, which are partially or almost completely assembled (having together pseudosubstrate, spacer, barrier, cap). The analyses have been focused on heterostructures containing (i) InAlN, AlGaN, InGaN and InAlGaN barrier layers, grown on GaN (0001), for the optimization of epitaxial processes in high efficiency GaN based HEMTs (GaN-HEMTs) grown on sapphire or silicon carbide substrates; and (ii) buffer and active polar GaN layers, grown on Si (111) substrates, which strain states were designed a priori in order to get commercially viable GaN-HEMTs which were competitive in their performances with respect to those currently using SiC substrates. Along with this primary objective, other goals of this thesis have been the search, development and tuning of methodologies that support these planned studies, based on: (i) preparation of complex multi-layered samples until reaching their electron-transparency; and (ii) the analysis and treatment of results from transmission electron microscopy (TEM) related experimental techniques.