Novel oxide buffer approach for GaN integration on Si(111) platform through Sc₂O₃/Y₂O₃ bi-layer ; Innovativer Oxidansatz auf Basis von Sc₂O₃/Y₂O₃ Heterostrukturen zur Galliumnitrid-Integration auf der Si(111) Plattform

Motivation: Preparation of GaN virtual substrates on large-scale Si wafers is intensively pursued as a cost-effective approach for high power/high frequency electronics (HEMT's etc.) and optoelectronic applications (LED, LASER). However, the growth of high quality GaN layers on Si is hampered by several difficulties mainly related to a large lattice mismatch (-17%) and a huge difference in the thermal expansion coefficient (56%). As a consequence, GaN epitaxial layers grown on Si substrates show a high number of defects (threading dislocations etc.), which severely deteriorate the overall quality of the GaN films. Additionally, due to the different thermal expansion coefficients of the substrate and the film, µm-thick GaN layers crack during post-growth cooling. To solve these integration problems, different semiconducting (e.g. AlN, GaAs, ZnO, HfN) and insulating (e.g. Al₂O₃, MgO, LiGaO₂) buffer layers, separating the Si substrate from the GaN film, are applied. Goal: In this thesis, a novel buffer approach for the integration of GaN on Si is proposed and investigated. The new approach employs Sc₂O₃/ Y₂O₃ bilayer templates as a step-graded buffer to reduce the lattice mismatch between GaN and the Si(111) substrate. According to the bulk crystal lattices, since the Y₂O₃ has an in-plane lattice misfit of -2% to Si, Sc₂O₃ -7% to Y₂O₃, the lattice misfit between GaN and the substrate can be theoretically reduced by about 50% from -17% (GaN/Si) to -8% (GaN/Sc₂O₃). Experimental: The GaN/Sc₂O₃/ Y₂O₃/Si(111) heterostructures are prepared in a multichamber molecular beam epitaxy system on 4 inch Si(111) wafers. In order to obtain complete information on the structural quality of the oxide buffer as well as the GaN layer, synchrotron- and laboratory-based x-ray diffraction, transmission electron microscopy and photoluminescence measurements are performed. The topography of the films is characterized by scanning electron microscopy and chemical inter-diffusion is investigated by energy-dispersive x-ray spectroscopy. The ...