Influence of nitrogen on the growth of diamond thin films by microwave plasma assisted Chemical Vapour Deposition

Abstract

It is well known that nitrogen can seriously influence the growth of diamond films during Plasma Assisted-Chemical Vapour Deposition (PA-CVD) (Chapter 1). If the generic mechanisms by which diamond is deposited at pressures and temperatures at which it is thermodynamically metastable are well understood, generally accepted chemical mechanisms that lead to low-pressure growth of diamond are still lacking, even for the most commonly used binary methane-hydrogen feed gas mixture. Therefore, the mechanisms of nitrogen incorporation in the diamond film are still missing too. This is mainly due to the extreme complexity of the processes involved, as it is unlikely that there is a single and simple diamond growth mechanism that applies to all deposition systems and deposition conditions (Chapter 2). In this work, Optical Emission Spectroscopy (OES), proviso the needed precautions, is used as a non-intrusive analysis technique to monitor variations in the plasma chemistry during microwave PA-CVD. Semi-quantitative analysis is made possible by the use of actinometry, while the temperature of some plasma species is determined by using the so-called Boltzmann plot technique (Chapter 3). It is proven for the first time that small nitrogen fractions can seriously modify the plasma chemistry, such as the relative concentration and the temperature of the light emitting species in the plasma. A home-made optical probe was built to reduce drastically the volume captured by the optical set-up, allowing us to achieve some spatial resolution. With the optical probe, we show that the spatial distribution as well as the temperature of the light emitting species varies together with the distance from the substrate surface. We demonstrate that plasma chemical reactions as well as surface chemical reactions lead to the production of the presumed precursors responsible for the growth of CVD diamond (Chapter 4). Characterisation of the deposited films by Scanning Electron Microscopy, X-ray diffraction, micro-Raman analysis and Secondary-Ion Mass Spectrometry demonstrate respectively the strong evolution in morphology, preferred orientation, film quality and relative concentration of hydrogen and nitrogen in the diamond film as a function of the nitrogen content in the feed gas mixture (Chapter 5). The plasma and surface reaction paths proposed in this work try to explain how the various plasma emitting species are produced and under which form nitrogen could be incorporated in the diamond lattice (Chapter 6). Finally, we draw some conclusions and proposed solutions to the problems encountered in this work (Chapter 7).