Fabrication and Application of Atomic-scale Silicon Structures

  • Author / Creator
    Achal, Roshan
  • On a perfect hydrogen-terminated Si(100)-2x1 surface, each surface silicon atom is capped with exactly one atom of hydrogen. When one of the capping hydrogen atoms is removed, the now unsatisfied orbital of the underlying silicon atom is exposed at that site. This site is better known as a dangling bond (DB), and it is an atom-sized building block that has been at the centre of numerous proposals for the next generation of technology over the last two and a half decades. This is because DBs have ideal electronic, chemical, and thermal properties for nanoscale device applications. In one approach, DBs can be used to create ultra-low power electronic circuitry. Atomically defined DB structures for such circuitry can be created with the use of a scanning tunneling microscope (STM). This is achieved by removing hydrogen atoms from the surface one at a time with the STM to make DBs at selected sites. However, due to limitations of the apparatus, it has not yet been possible to build sufficiently error-free structures to realize any of the technological proposals. Without a means to correct errors, this has continued to remain true.

    In this work, STM techniques were developed to improve the atomic precision of the automated fabrication of DB structures, hydrogen lithography (HL), on silicon. A scalable method to correct errors and edit DB structures, hydrogen repassivation (HR), was also developed. In this method, single hydrogen atoms on the STM tip are transferred back to the surface, erasing DBs by reforming bonds with them. Using these techniques in conjunction, a perfect sculpture of a maple leaf was fabricated from just 32 DBs. The techniques were then used to create two rewriteable atomic memory arrays, achieving the highest solid-state storage density (1.1 petabits per square inch). The first array was used to store the alphabet letter-by-letter in 8 bits. The second array was used to store and play back the first 24 notes of the Mario theme in 192 bits. The applicability of these techniques was then explored in different conditions, including at varying temperatures and with deuterium substituted for hydrogen. The nature of the transfer process of hydrogen from the tip to the surface during HR was also examined.

    To improve upon HR further, a new, faster and simplified form of error correction was developed, molecular hydrogen repassivation (M-HR), which is able to direct single molecules of hydrogen to erase DB sites (without the use of atoms attached to a scanned probe). This capability was achieved by changing the DBs to be erased into tailored reactive sites for ambient hydrogen molecules. The atomic memory arrays were redesigned to accommodate M-HR as the primary means of rewriting information (0.88 petabits per square inch). It was then used to rewrite information in a 24-bit memory array. In addition to M-HR, two other techniques were developed to bring new functionality to the STM. A charge characterization technique, which reduces the influence of the typically perturbative STM tip field, was created to characterize the charge of structures on the surface. Also, by combining this with the ability to create tailored reactive sites for particular molecules, a technique to electronically detect single molecule binding events was demonstrated.

    Now that a full suite of improved fabrication tools is available to manipulate single atoms and molecules of hydrogen, including the ability to easily and reliably correct fabrication errors, the stage is set for DB-based technologies to flourish. The tools and techniques described in this thesis may also help uncover deeper insights into chemical reactivity and dynamics at the atomic scale.

  • Subjects / Keywords
  • Graduation date
    Spring 2020
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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