Detailed Study of Atomic Silicon Dangling Bond Charge State Dynamics on the Surface of Hydrogen Terminated Silicon (100)–2 × 1

• Author / Creator

  Achal, Roshan

• Atomic Silicon Dangling Bonds (DBs), which are natural quantum dots, are sites where silicon atoms on a hydrogen-terminated silicon surface have no bound hydrogen atom (leaving an un-reacted sp3 hybrid orbital). These DBs hold the promise of new ultra-low power devices based on the Quantum Cellular Automata (QCA) architecture proposed for larger quantum dots decades earlier. The need for new devices is growing rapidly as the end of the current technological roadmap approaches. The advancement of these ultra-low power devices, however, is hindered by the fact that many theoretical models and estimations used to design them have yet to be experimentally verified. In order to move towards the effective development of DB prototype devices, fundamental physical properties of DBs must first be characterized and studied. One of the properties inherent to DBs on the surface of a hydrogen-terminated n-type silicon (100) wafer (with a 2x1 surface reconstruction) is their ability to store electrons. They can exist in a positive, neutral, or negative charge state when storing zero, one, or two electrons respectively. At low temperatures (4 K) when imaging a DB with a Scanning Tunneling Microscope (STM) at particular bias voltages, fluctuations of the DB charge state can be observed in the image that are driven through influence of the STM tip. An analysis framework was developed based on a correlation analysis method borrowed from biophysics to study the manifestations of these fluctuations between charge states in the STM tunneling current itself. Through these studies the dependence of the transition rates between charge states on the radial distance of the STM tip (from the DB centre) was uncovered, along with preliminary studies on the sensitivity of these rates to applied bias voltage. The charging pathway between states was also identified, showing a linear pathway between negative--neutral--positive charge states with no direct transition between the negative and positive charge states. It was found that the filling rate of electrons into the DB from an STM tip had an exponential relationship with radial separation between the tip and the DB centre. The rates were found to be 50 Hz--3000 Hz within the scope of the experiments. These rates also showed little dependence on applied sample bias voltage over the range of 1.30 V to 1.45 V. The behaviour of the emptying rates of electrons into the silicon bulk showed a very different relationship with radial tip distance. It was found that the emptying rates were almost independent of radial tip separation, having a flat and constant value between 50 Hz--1000 Hz. The value of the emptying rates depended quite sensitively on applied sample bias voltage, however, and it was found that these rates increase with increasing bias voltage. The exact form of this dependence still requires further investigation. The results of these experiments also hint at what the dynamics of the DB charging might be in the absence of the STM tip. Using the calculated rates for both the filling and emptying processes, stochastic simulations of the DB charging dynamics were created in order to investigate both the success of the correlation method, and the validity of the results themselves. The simulations of charge state dynamics using the experimental rates showed good agreement with these experimental data. The simulations also showed strong support for the linear charging pathway determined experimentally. The results and analytical techniques described here open the doors to study more complex systems of interacting DBs as well, which is another important step towards making practical ultra-low power devices.

• Subjects / Keywords

  • Silicon (100)
  • Dangling Bond
  • Charge Dynamics
  • Hydrogen Terminated

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3MG7G936

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.