@phdthesis{Tarnawska2012, author = {Tarnawska, Lidia}, title = {Novel oxide buffer approach for GaN integration on Si(111) platform through Sc₂O₃/Y₂O₃ bi-layer}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-27904}, school = {BTU Cottbus - Senftenberg}, year = {2012}, abstract = {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.}, subject = {Lumineszenzdiode; Galliumnitrid; MOCVD-Verfahren; GaN auf Si; Oxidpuffer; Heteroepitaxie; GaN on Si; Heteroepitaxy; Oxide buffer}, language = {en} } @phdthesis{Kozlowski2012, author = {Kozlowski, Grzegorz}, title = {On the compliant behaviour of free-standing Si nanostructures on Si(001) for Ge nanoheteroepitaxy}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-24512}, school = {BTU Cottbus - Senftenberg}, year = {2012}, abstract = {Due to its superior optoelectronic properties, Germanium (Ge) is attracting increasing interest to build up future photonic modules within Si chip baseline technology. Despite clear advantages of Ge integration in Si technology (identical diamond crystal structure with Si, complementary metal oxide semiconductor (CMOS) compatibility due to no contamination risks etc.), it faces also some true challenges limiting the optoelectronic performance. Among them, thermal and lattice mismatch result in too high defect levels (which act as non-radiative recombination centres). Parasitic diffusion and formation of SiGe alloys at elevated temperatures is also a common problem. In this respect, selective chemical vapor deposition Ge heteroepitaxy approaches for high quality Ge nanostructure growth with reasonable thermal budget must be developed for local Ge photonic module integration. A promising vision is offered by the compliant substrate effects within nanometer scale Ge/Si heteroepitaxial structures. Here, in contrast to the classical Ge deposition on bulk Si substrates, the thermal and lattice mismatch strain energy accumulated in the Ge epilayer is partially shifted to the free-standing Si nanostructure. This strain partitioning phenomenon is at the very heart of the nanoheteroepitaxy theory (NHE) and, if strain energy levels are correctly balanced, offers the vision to grow defect-free nanostructures of lattice mismatched semiconductors on Si. In case of the Ge/Si heterosystem with a lattice mismatch of 4.2\%, the strain partitioning phenomenon is expected to be triggered when free-standing Si nanopillars with the width of 50 nm and below are used. In order to experimentally verify NHE with its compliant substrate effects, a set of free-standing Ge/Si nanostructures with diameter ranging from 150 to 50 nm were fabricated and investigated. The experimental verification of compliant substrate effects is challenging and requires sophisticated characterization techniques. The main limitation corresponds to a simultaneous detection of a) the strain partitioning phenomenon between Ge and Si and b) the absence of defects on the nano-scale. In this respect, synchrotron-based grazing incidence x-ray diffraction was applied to study the epitaxial relationship, defect and strain characteristics with high resolution and sensitivity in a non-destructive way. Raman spectroscopy supported by finite element method calculations were used to investigate the strain distribution within a single Ge/Si nanostructure. Special focus was devoted to transmission electron microscopy to determine the quality of the Ge epilayer. It was found, that although high quality Ge nanoclusters can be achieved by thermal annealing on Si pillars bigger than 50 nm in width, no proof of strain partitioning phenomenon was observed. In clear contradiction to the present NHE theory, no strain partitioning phenomenon was found even for ~50 nm wide Si pillars for which the compliant substrate effects are expected. The absence of the strain partitioning between Ge and Si is caused by the stress field exerted by the SiO2 growth mask on the Si nanopillar. In contrast to such nanostructures monolithically prepared from a Si(001) wafer, first results in this thesis clearly prove the strain partitioning phenomenon within Ge/Si nanostructures on Silicon-on-insulator substrate. Here, the compliant substrate effects were clearly observed for pillar widths even bigger than 50 nm. This experimental work demonstrates, that NHE with its compliant substrate effects, offers an interesting approach for high quality Ge nanostructures on Si, avoiding even the misfit dislocation network with its non-tolerable electrical activity in Ge nanodevices. However, the theory does not yet include important aspects of thin film growth on the nano-scale and must be further developed. It is the aim of this PhD thesis to provide this experimental basis for the Ge/Si heterosystem. Finally, it is noted that here developed growth approach is fully Si CMOS compatible and is not only relevant for Ge integration but also for other lattice mismatched alternative semiconductors (GaAs etc.) to enable higher performance / new functions in future Si microelectronics technologies.}, subject = {Heteroepitaxie; Germanium; Silizium; Heteroepitaxie; R{\"o}ntgenbeugung; Raman Spektroskopie; Germanium; Silicon; Heteroepitaxy; X-ray diffraction; Raman spectroscopy}, language = {en} }