Vapour-phase passivation of Ge(100) using alkanethiols for future CMOS devices
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Date
2021-03-31
Authors
Garvey, Shane
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University College Cork
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Abstract
The First Industrial Revolution, facilitated by the steam engine, transformed agrarian economies into ones dominated by industry and machine manufacturing. With the advent of the internal combustion engine in the late 19th century, the Second Industrial Revolution began, enabling the mass production of goods. Computers powered by microchips instigated the Third Industrial Revolution, which has been roaring since the second half of the 20th century. We now stand at the precipice of the Fourth Industrial Revolution, still powered by computers. Industry 4.0, as it is known, is characterised by automation, the internet of things, cloud computing and artificial intelligence. As the population of the world approaches 8 billion people in 2021, equipped with high-performance computers and a sense of hope, we have set our sights on the most complex problems facing humanity, problems such as climate change, pandemics, food and water shortages, drought and even interplanetary travel. The microprocessor has played a central role in the rapid development of modern technologies since it began to be widely used in the latter part of the 20th century and it will continue to play this role as we tackle the problems mentioned. Silicon has served as the bedrock of modern microprocessors and has, for many years, supported the trend known as Moore’s Law. Moore’s Law holds that the number of transistors in an integrated circuit (IC) doubles roughly every two years. Initially, Moore’s Law was facilitated by the miniaturisation of the Si transistor. A smaller transistor footprint allowed for more to be contained in an IC, increasing the processing power. However, continued device-scaling has resulted in material and architectural limitations being reached. In an effort to keep pace with the technological demands of modernity, the use of novel materials and device architectures are being adopted to improve device performance. Germanium has long been considered a viable candidate for use in modern processors and has seen some inclusion in the form of SiGe alloys used as the channel material in CMOS devices. With that said, before 100% Ge gains widespread industry acceptance, there are certain issues, mostly to do with its oxide, that must be overcome. This thesis aims to highlight key advancements that have been made in relation to the passivation of Ge(100) surfaces such that Ge becomes a viable material for inclusion in modern CMOS devices.
Chapter 1 serves as an introduction to the work presented in this thesis. Key concepts such as the structure and properties of Ge(100) surfaces are outlined to highlight why Ge is seriously being considered as a future channel material for CMOS devices. An in-depth review of literature on Ge passivation is included also in an effort to set the foundation for and highlight the significance of the work that follows in the coming chapters.
Chapter 2 details the characterisation methods that are used to probe the Ge(100) surfaces that are discussed in Chapters 3, 4 and 5. Since XPS is the characterisation method most used throughout this body of work, particular attention is given to it. WCA and AFM are introduced also; however, since they serve as complementary characterisation methods, only a brief introduction is presented. DFT analysis performed by Dr. Barbara Maccioni and Dr. Michael Nolan is presented in this chapter also; however, a more detailed discussion of how the DFT simulations are used to help elucidate the behaviour of Ge-SAM systems can be found in the work chapters in which they are implemented.
Chapter 3 discusses the method developed to achieve thiol-SAM passivation of Ge(100) surfaces using a novel vapour-phase approach. This method improves on the current state-of-the-art by reducing the time required to form stable alkanethiol-SAMs on Ge(100) from 24 hours to ~200 minutes while still forming SAMs that inhibit reoxidation of the Ge for up to 1 week. Many of the vapour-phase passivation methods for Ge that are found in the literature rely on the use of high-vacuum. That is not the case for the novel process developed and documented in this chapter. Ultimately, the vapour-phase passivation method serves as the foundation for the remainder of the work presented in this thesis since it provided a reliable and reproducible method for Ge passivation. Thus, this process appears in both chapters 4 and 5 also.
Chapter 4 highlights the significant effect humidity in air has on the longevity of thiol-SAMs on Ge(100) surfaces. Although the thiol-SAM-passivated Ge surfaces that are prepared exhibit resistance to reoxidation, upon exposure to ambient conditions, reoxidation of the Ge does eventually occur. Thus, a natural progression from Chapter 1 is to determine what factors are significant in the reoxidation of Ge and the destruction of the passivating SAM. SAMs of different thiol molecules, prepared by both vapour- and liquid-phase passivation methods are prepared and exposure to various levels of humidity at constant temperature to elucidate what effect, if any, humidity in air has on the reoxidation of thiol-SAM-passivated Ge. It is found that irrespective of the passivating thiol molecule and the method used to achieve passivation, reoxidation of the Ge trends with relative humidity. DFT simulations are presented which help elucidate how water molecules interact with the SAM-Ge system and potential mechanisms for the reoxidation of the Ge and the loss of the SAM are outlined.
Chapter 5 explores what effect the length of the passivating alkanethiol molecule has on the stability of the SAM and the reoxidation of the Ge upon exposure to ambient conditions. Only alkanethiol molecules with an even number of –CH2 units in the C backbone are selected for the study to ensure parity-related effects are avoided. Thiol chain length is found to be an important factor in the stability of the SAM since long-chain thiols inhibit reoxidation of the Ge more effectively than their shorter-chain counterparts. With that said, SAMs comprised of short-chain thiol molecules, such as 1-butanethiol, still inhibit Ge reoxidation albeit less effectively than long-chain thiols such as 1-dodecanethiol.
Finally, Chapter 6 presents the conclusions made and provides an outlook on future prospects.
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Keywords
Germanium , Semiconductor , Passivation , CMOS , Surface chemistry , Material science
Citation
Garvey, S. 2021. Vapour-phase passivation of Ge(100) using alkanethiols for future CMOS devices. PhD Thesis, University College Cork.