Smith, Richard; (2021) A DFT investigation of Al-based atomically precise epitaxy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is about the growth and placement of dopants in silicon semiconductor devices and specifically acceptor dopants as device dimensions enter the nanoscale. Single-atom donor dopant devices have already been demonstrated in the laboratory. Using density functional theory (DFT) and the aluminium atom we now show how acceptor sites might be fabricated and characterize their electronic behaviour. The thesis opens with a review of the physical basis of statistical doping and the operation of the silicon CMOS transistor which is the most widespread microfabricated device by a wide margin. We show how downscaling requires ever-increasing accuracy in dopant placement and illustrate using some current process techniques. Next, we describe some prototype single-dopant devices and the chapter concludes with a description of a phosphorus nuclear spin qubit and its application. Chapter 2 outlines the theoretical basis of the DFT nanostructure models found in later chapters and chapter 3 presents some elementary calculations intended to validate the local DFT environment. Chapters 4, 5 and 6 are based on published papers produced during this work and listed on page 11. In chapter 4 we introduce patterned atomic layer epitaxy (PALE), an experimental fabrication technique for Si nanostructures. Chapters 5 and 6 describe how PALE could be applied to locate Al dopant atoms in an Si substrate. The final chapter offers some calculations indicating the electronic behaviour of this dopant when embedded in Si nanostructures of various kinds.

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