Single Atom Electronics

This thesis describes a series of experiments on the electronic properties of individual shallow dopant atoms in silicon. Shallow dopants are impurity atoms that bind either a single electron or hole and can therefore be con- sidered as the solid state analogue to the hydrogen atom. As the transistor density increases, critical device dimensions are fast approaching the effective Bohr radius of shallow dopant atoms. This offers the compelling possibility to utilize the quantum nature of dopant atoms to enhance the functionality if semiconductor nano-devices. The emphasis of the experimental work described in this thesis is on the interaction between single dopant atoms and their environment. When dopant atoms are embedded within a nano-structure quantum confinement at the interface will strongly perturb the wavefunction of the dopant-bound electron (or hole), as a consequence the wavefunctions and energies will no longer be that of hydrogenic states. Moreover, dielectric mismatch between the semicon- ductor and its surroundings will influence the energy spectra of dopant atoms near the interface. Generally speaking, the presence of interfaces will break the tetrahedral symmetry of the silicon crystal and will cause degeneracies to be lifted, drastically shifting the electronic states of dopant atoms with respect to dopant-bound states in bulk silicon. Scanning tunneling spectroscopy (STS) is a unique method that allows for the spatially resolved investigation of the electronic structure of sub-surface dopant atoms. Unlike in other transport measurements, both lateral position and depth of single dopant atoms can unambiguously be determined. Chapters 3 and 4 of this thesis describe experiments aimed at studying the energy spectra as a function of depth of individual sub-surface dopant atoms by means of electron transport through the localized dopant states. Interface enhancement of the ionization energy, and as a consequence deactivation of dopants near the interface, is a major concern for doping nano-structures. Chapter 3 describes the experimental investigation of the effect of the interface on the ionization energy of single sub-surface acceptors. It is worthwhile mentioning here that the vacuum-silicon interface yields the largest possible dielectric mismatch attainable for silicon. The depth of individual ac- ceptors is measured by the influence of the ionized acceptor nucleus on the local density of valence band states. The ionization energy is determined from the voltage at which resonant tunneling through the localized acceptor state occurs. An absolute energy scale is provided by the thermal broadening of the conductance peaks. It is explicitly demonstrated that acceptors in silicon less than a Bohr radius away from the interface maintain a bulk-like ionization energy. Building on the methods described in Chapter 3, measurements of the excited state spectra of single sub-surface acceptors are presented in Chapter 4. Interface induced spin-orbit splitting of the four-fold degenerate ground state of boron in silicon results in the formation of two Kramers doublets. The observed enhancement of this splitting for acceptors close to the interface, and moreover the ability to controllably tune this splitting will have strong implications for quantum computation schemes based on the spin of acceptor-bound holes. One of the key challenges in single atom electronics is the strict require- ments for dopant placement. Recent developments in scanning tunneling microscopy (STM) based bottom-up fabrication have paved the way for atomically precise dopant based electronic devices. Chapter 5 illustrates, for the first time, how low temperature scanning tunneling spectroscopy can be used in conjunction with bottom-up dopant engineering. Transport measurements on single phosphorus donors deliberately placed five monolayers beneath the surface of a p-type silicon substrate serve as a proof-of-principle for STS studies on atomically precise dopant structures. Chapter 6 describes an experiment where, for the first time, the quantum states of a single arsenic donor embedded in a nano-scale field-effect transistor are utilized to increase the device functionality of the transistor. By integrating two single-atom transistors in a circuit a classical logic operation, namely a full addition, is performed using only a fraction of the transistors required in a conventional complementary-metal-oxide-semiconductor circuit.Quantum NanoscienceApplied Science

Give-to-Get: Free-riding-resilient Video-on-Demand in P2P Systems

Centralised solutions for Video-on-Demand (VoD) services, which stream pre-recorded video content to multiple clients who start watching at the moments of their own choosing, are not scalable because of the high bandwidth requirements of the central video servers. Peer-to-peer (P2P) techniques which let the clients distribute the video content among themselves, can be used to alleviate this problem. However, such techniques may introduce the problem of free-riding, with some peers in the P2P network not forwarding the video content to others if there is no incentive to do so. When the P2P network contains too many free-riders, an increasing number of the well-behaving peers may not achieve high enough download speeds to maintain an acceptable service. In this paper we propose Give-to-Get, a P2P VoD algorithm which discourages free-riding by letting peers favour uploading to other peers who have proven to be good uploaders. As a consequence, free-riders are only tolerated as long as there is spare capacity in the system. Our simulations show that even if 20% of the peers are free-riders, Give-to-Get continues to provide good performance to the well-behaving peers. In particular, they show that Give-to-Get performs very well for short videos, which dominate the current VoD traffic on the Internet.Department of Computer ScienceElectrical Engineering, Mathematics and Computer Scienc

Drain current modulation in a nanoscale field-effect-transistor channel by single dopant implantation

We demonstrate single dopant implantation into the channel of a silicon nanoscale metal-oxide-semiconductor field-effect-transistor. This is achieved by monitoring the drain current modulation during ion irradiation. Deterministic doping is crucial for overcoming dopant number variability in present nanoscale devices and for exploiting single atom degrees of freedom. The two main ion stopping processes that induce drain current modulation are examined. We employ 500 keV He ions, in which electronic stopping is dominant, leading to discrete increases in drain current and 14 keV P dopants for which nuclear stopping is dominant leading to discrete decreases in drain current.Kavli Institute of NanoscienceApplied Science

Single Ion Implantation into Si-Based Devices

Deterministic doping is crucial for overcoming dopant number variability in present nano-scale devices and for exploiting single atom degrees of freedom. The development of determinisitic doping schemes is required. Here, two approaches to the detection of single ion impact events in Si-based devices are reviewed. The first is via specialized PiN structures where ions are directed onto a target area around which a field effect transistor can be formed. The second approach involves monitoring the drain current modulation during ion irradiation. We investigate the detection of both high energy He+ and 14 keV P+ dopants. The stopping of these ions is dominated by ionization and nuclear collisions, respectively. The optimization of the implant energy for a particular device and post-implantation processing are also briefly considered.QN/Quantum NanoscienceApplied Science