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Martin Wolf «  Rosaria Brescia »  Wolfgang Krätschmer
Rosaria Brescia
Transmission electron microscopy studies and simulations of heteroepitaxial diamond nucleation on Ir/YSZ/Si
Supervisor: Titel Vorname Nachname [Experimental physics IV]
Date of oral examination: 12/11/2013
151 pages, english
During the last decades diamond has emerged as a promising material for many technological fields, some of them requiring large area, high purity and single-crystal quality. These characteristics are attainable by plasma-assisted chemical vapour deposition (PACVD), with a suitable choice of nucleation methods, substrates and process parameters. Bias-enhanced nucleation (BEN), consisting in the application of a voltage difference between the PACVD chamber walls and the substrate during plasma ignition, is by far superior to other nucleation methods in terms of areal fraction of epitaxially oriented diamond grains on a variety of substrates. Among them, iridium (Ir) allows unmatched structural quality of diamond since its initial growth stages. The present work is centred on the Ir / yttria-stabilised zirconia (YSZ) / silicon (Si) multilayer system, which shows suitable characteristics for the synthesis of large-area single-crystal diamond. In the first part of this work, diffraction-contrast transmission electron microscopy (TEM) has been employed for the analysis of epitaxial YSZ films of Si(001) substrates. First of all, the dislocation density within cubic YSZ has been monitored as a function of their thickness and high temperature treatments. The results of these analyses suggest conditions for the improvement of the structural quality of the films. In tetragonal YSZ on Si(001), different texture components are included in the films, as formerly deduced by XRD analyses. In this work these are addressed to specific features in the material and have been monitored under different conditions. The deduced conclusions concerning the volume fraction of the different components and their relative arrangement show some significant differences compared to former reports. The second part of the work is focussed on the peculiar mechanisms taking place during BEN on Ir surfaces. High-resolution TEM (HRTEM) and reflection high-energy electron diffraction (RHEED) analyses of the BEN product and of the early stages of diamond growth on Ir/YSZ/Si give a precious complement to the former analyses on analogous systems. The few nm carbonaceous layer deposited on top of Ir via the BEN procedure does not present any crystalline diamond features in HRTEM and RHEED analyses. Only after few seconds deposition a clear evidence of epitaxial crystalline diamond is demonstrated by these two techniques. The monitoring of the first two minutes of diamond deposition by HRTEM shows an arrangement of isolated epitaxial crystalline diamond grains within well-defined regions (the domains) and their coalescence during 3D growth with ongoing deposition. This also provides the unique opportunity to extrapolate to the ordering and structure of the diamond nuclei formed by BEN, which cannot be observed directly. Test experiments show that the effects of the bombardment of hydrocarbon ions taking place during BEN on Ir are qualitatively similar, in terms of ordering of the diamond phase, to the ones obtained from implantation of relatively low energy ions into thick diamond films. However, the BEN procedure is confirmed to be very peculiar in its ability to produce at the same time highly oriented, structurally perfect epitaxial grains and a highly defective epitaxial diamond phase, which is etched off at the beginning of growth. The arrangement of the epitaxial diamond nuclei within the domains, unique for BEN on Ir, can be explained by the combination of few real nucleation events with lateral growth via addition of atoms from the plasma, diffusing on the surface. However, experimental results show evidence that the nuclei are well isolated within the domains. This can be explained via some kind of interaction causing subdivision of an initial extended diamond layer into smaller units and repulsion between them. One possibility is represented by the strain energy developing in the Ir substrate due to the lattice mismatch between diamond and Ir. The results of finite element simulations conducted in this work explain qualitatively all these experimental results. Moreover, the different shapes observed for the domains can be explained by a simple Monte Carlo model assuming the combination of lateral growth and etching mechanisms. Indeed, experimental results clearly show that these processes play a fundamental role during BEN. The results of this work allow to complete the picture of the unique BEN process on Ir surfaces. The Ir/YSZ/Si multilayer is surely recognized by the scientific community as a promising system in view of the technological application of diamond, which however needs deep knowledge and careful control of several aspects concerning the different components and processes involved in order to give its optimum results.