Grafting to process: from fundamentals to applications in semiconductor doping

The regulation of the electrical conductivity of silicon is accomplished by introducing impurities such as phosphorus or boron into a silicon crystal. This process is commonly referred to as silicon doping, a fundamental technique that is the foundation of the entire semiconductor industry1 . Moreover, doping technologies have applications beyond transistors. In the realm of solar cell development, nanostructured surfaces have demonstrated the ability to enhance light absorption and improve overall performance efficiency2 . Over several decades, there has been an unceasing effort to reduce the dimensions of microelectronic devices, a trend famously predicted by Moore's Law3 . But, this pursuit of smaller devices, coupled with improved performance and lower power consumption, remained highly desirable until 2012 when the physical limits of planar transistors were reached3 . Now, there is an ongoing effort to further reduce transistor size using innovative 3D designs4 . Within the most important research objectives in this regard, the concentration and homogeneity of dopant atoms is a concern. In small devices only a few dopant atoms are present, and statistical fluctuations in their concentration modify their performance. Even if technologies able to precise placement of single dopant atoms are available, they remain extremely time-consuming and not yet suitable for largescale applications5 , 6 , 7 . In this thesis, the literature review, presented in Chapter 1, starts by taking a theoretical approach to the doping process, exploring both emerging trends in transistor design and traditional methods used in industrial applications. Following this, the focus shifts to Monolayer Doping (MLD) as the first bottom-up strategy for dopant amount control, encompassing its associated challenges. Subsequently, the concept of Polymeric Precision Doping (PPD) is introduced as a strategic solution to overcome the limitations of the traditional MLD method. In PPD, the small dopant molecules used in MLD are replaced with polymeric carriers that offer the possibility of controlling the dopant amount and distribution by macromolecules with tuneable molecular weights. The process to chemically bond these carriers to silicon, the grafting to method is explained in detail. Afterward, the first documented approach to PPD employing dopant polymers is described based on existing literature. Finally, the challenges related to polymer dispersity in PPD processes are explored, and the introduction of precision polymers, such as polypeptoids is elucidated as a promising solution. Subsequently, the research questions that support this thesis and the motivation driving our investigation are introduced. Chapter 2 serves as a thesis outline, providing a structured overview of the subsequent discussions, all supported by the relevant publications made throughout this study. In Chapter 3 a reliable and robust method was developed to quantify the molecular weight discrimination that can occur in grafting to reactions via indirect MALDI-TOF quantification. In which the molecular weights of grafted chains are compared with the polymer before the grafting reaction and the unreacted material recovered after grafting, establishing a method to study molecular weight partitioning during grafting to reactions. Chapter 4 investigates the impact of the reaction environment following the grafting of identical hydroxy-terminated polymers onto both deglazed and non-deglazed surfaces. The differences in the reaction kinetics and their implications on the brush thickness, the amount of polymer grafted, and IV the reversibility of the process are discussed. Besides, the reaction over the two substrates confirmed the partitioning by molecular weight already described by other authors. Chapter 5 explores the grafting process of phosphorus-functionalized polystyrene onto deglazed and not deglazed silicon surfaces, highlighting the differences between the two substrates. This study demonstrates that PS-P can graft even on deglazed surfaces forming a thick and uniform brush layer. Still, substantial differences are observed between the two substrates in terms of grafting mechanism and efficiency of dopant diffusion and activation rate. Finally, in Chapter 6, monodisperse polypeptoids are proposed as a tool to guarantee the control of the dopant concentration with clear advantages for the integration of PPD into advanced technological platforms.