Ion beam synthesis of silicon-carbon structures and related materials

[eng] The goal of this dissertation is the study and characterisation of high dose Carbon (C) implantation processes into Silicon (Si) and related materials for the synthesis of Silicon Carbide (SiC). The attainment of well-characterised multilayer structures useful to fabricate sensor and electronic devices based on them constitutes the main objective of this work. SiC constitutes a very promising semiconductor, thermally and chemically stable with excellent physical properties, which has started to be successfully applied in some of the most outstanding fields in electronics. Electronic devices of SiC can operate at high temperatures, in chemically aggressive conditions and even under extreme radiation, dissipating heat excesses, and as a result, proportioning enormous benefits over electronic devices based on the other available semiconductors. Among the amount of applications, the optoelectronic, high temperature electronic, hard radiation electronic, high power and high frequency devices are worth remarking. Operable devices such as three colour LEDs, power MOSFETs, bipolar transistors, photodiodes or rectifiers have successfully been fabricated. Additionally, the high mechanical and chemical stability, and the high stiffness of SiC in relation to Si, are also interesting properties to develop complete Micro-Electro-Mechanical systems (MEMS), where sensing devices, signal processing and even actuators are integrated in the same substrate. The synthesis of good quality and cheaper SiC structures is required to develop its technology. This implies the growth of SiC on the cheap and useful Si substrate. The traditional methods for obtaining SiC have been CVD and MBE, but some drawbacks such as the degree of reproducibility and the higher temperatures used in these processes can constitute a problem. In this sense, ion implantation constitutes a suitable alternative process for the direct synthesis of SiC, totally compatible with Si technology. This technique allows a high degree of control in the synthesis, offering enormous possibilities for obtaining complex structures. By ion implantation, the properties of the synthesised material strongly depend on the implantation and annealing parameters. The perfect design of the implantation process will have repercussions on the characteristics of the resulting structure. Moreover, the amount of implanted C plays an important role in the way in which C is incorporated in the system. The incorporated C can take different places in de Si network, what will finally affect the structural characterisation of the synthesised material. A complete group of analysis techniques has been used in order to characterise adequately the materials studied along this work. Among them, it may be mentioned the Raman and FTIR spectroscopies, mainly used in structural studies and to determine phases, XPS, which has allowed the measurement of the in-depth implanted C distribution and the study of the different bond configurations in the implanted layer, and finally TEM, which is used for the direct observation of the synthesised structures. The interpretation of all these data makes it possible to deep in the knowledge of the physical mechanisms involved in these processes. As far as the contents are concerned, the thesis is structured in five chapters, each one containing its own references to other works. The introductory part, chapter 1, is divided in sections. The first one contains all the basic data related to SiC. The second section deals with the ion implantation as a method of SiC synthesis. A description of the analysis techniques used along this work is presented in the last section. Chapter 2 is devoted to the synthesis and recrystallisation of amorphous SiC. Previously, it has been studied the behaviour of the 6H-SiC commercially available in front of implantation processes, taking into account the role played by the temperature of implantation, the implanted dose and the temperature of annealing. Once performed this characterisation, the study of amorphous Si implanted with C at room temperature (RT) is carried out. The detailed disposition of existing chemical bonds as a function of the amount of introduced C has been an object of interest. Finally, the recrystallisation of this material by thermal processes and by IBIEC (a dynamic process that involves high-energy implantation) has been considered. The direct synthesis of crystalline SiC by C implantation in a Si crystalline substrate is the subject of chapter 3. The procedure is to implant at a high enough temperature, 500°C, to avoid the Si amorphisation. This part of the chapter has been divided attending to the value of the implanted dose. Firstly, samples implanted at doses below the threshold dose needed to reach a stoichiometric SiC composition at the implanted peak have been analysed. The study of these samples has allowed to characterise the main processes involved in the ion beam synthesis of crystalline SiC. Then, a multiple step implantation has been defined to obtain a continuous stoichiometric SiC layer. The analysis of the implanted samples has corroborated the formation of such a layer, with abrupt interfaces with the surface and substrate Si regions. Chapter 3 finishes with a detailed study of the etch-stop properties of the ion beam synthesised layers. The excellent etch selectivity between these layers and the Si regions has allowed the fabrication of test micromechanic structures such as membranes, bridges and other self-standing simple structures. The attainment of these kinds of structures based on the ion beam synthesised layers constitutes an important conclusion, as it demonstrates its ability for the development of micromachined devices. The study of the ion beam synthesis of SiC is extended to SiGe substrates in Chapter 4. In principle, the aim of this study is to improve the understanding of the mechanisms involved, in order to determine the influence of parameters such as the chemical composition, strain and bond length of the target material. Moreover, the possible modification of the synthesised phases by the incorporation of Ge atoms in a Si(x_y)Ge(x)C(y) ternary alloy is also investigated. This study is performed in a way similar to that previously done in Si. Firstly, the synthesis and recrystallisation of amorphous layers is investigated. After that, the analysis of high temperature processes in crystalline substrates is performed. Finally, some general conclusions are written in chapter 5