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Novel oxide buffer approach for GaN integration on Si(111) platform through Sc₂O₃/Y₂O₃ bi-layer
(2012)
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 nucleation processes of the GaN onSc₂O₃ buff er are followed in-situ by reflection high energy electron diffraction and the interface chemistry is analyzed by means of x-ray photoelectron spectroscopy. Results: It is found, that the Sc₂O₃/ Y₂O₃ buffer approach provides a template of high structural quality for GaN overgrowth. The bi-layer buffer plays a lattice match mediator role between GaN and Si and acts as a barrier against impurity diffusion. GaN grown on Sc₂O₃/ Y₂O₃/Si(111) templates is single crystalline with a wurtzite structure and (0001) oriented. Due to the -8% lattice mismatch between GaN and Sc₂O₃, GaN growth proceeds by the nucleation of 3D islands. The size of the islands, coalescence time and the relaxation process depend on the GaN growth conditions and have a strong influence on the topography of closed layers, crystalline quality (defect density) as well as optical properties. The best GaN material parameters are obtained for the layers grown in Ga-rich regime when the Ga/N ratio is slightly higher than unity. The main three defects found in the µm-thick GaN layers are a) threading dislocation, with density in the order of 10^10 cm-2, b) stacking faults, resulting in cubic inclusions in the hexagonal matrix and c) inversion domain boundaries causing Ga-polar regions in the mainly N-polar film. A theoretical GaN/Sc₂O₃ interface model is discussed to explain these experimental findings. Despite the relatively large number of structural defects, photoluminescence shows sharp and strong donor-bound exciton transition and very low intensity yellow emission, which indicate that GaN layers grown on Sc₂O₃/ Y₂O₃/Si(111) are promising for future optoelectronic applications. Outlook: Future growth strategies will focus on interface engineering approach to further reduce the lattice mismatch between GaN(0001) and Sc₂O₃ (111) surfaces, enable growth of unipolar GaN films and trigger the occurrence of an early 2D-like growth mode to avoid cubic GaN inclusions and guarantee low threading dislocation densities.