Nanoelectronics Part II Single-Electron and Few-Electron Phenomena and Devices Chapter 6 Tunnel Junctions and Applications of Tunneling

Consider a CNT is cut by AFM AFM cut

Creating a gap in CNTs

In this chapter, we connect a quantum dot to wires via tunnel junctions, in order to form an electronic device such as a transistor

6.1 Tunneling Through a Potential Barrier 123 From classical mechanics, for E > V 0, the particle will simply move past the potential barrier. (this is not the case for quantum mechanics.)

6.1 Tunneling Through a Potential Barrier

7

8

9

10

This explains that, for gate leakage of MOSFET, as oxide thickness decreased, tunneling can become a significant problem.

6.2 Potential Energy Profiles for Material Interfaces Modified work function of metal-SiO 2 Metal-Insulator

Metal-Semiconductor

Metal-Semiconductor Schottky barrier

Metal-Semiconductor The depleted region is called a space charge layer, defined as W. This is called Schottky diode Ohmic contact: very little resistance in either direction (metal to semiconductor or semiconductor to metal.)

Metal-vacuum-metal junction

Metal-Insulator-Metal Total potential energy

6.3 Applications of Tunneling Field Emission

6.3.1 Field Emission or (or called cold emission)

6.3.2 Gate-Oxide Tunneling and Hot Electron Effects in MOSFETs In an ideal classical MOSFET, electrons do not travel between the channel and gate.

Hot Electrons and Fowler-Nordheim Tunneling Electrons is accelerated by the source-drain voltage. As they gain sufficient kinetic energy, they may tunnel through the oxide at the drain end. This is called hot-electron effect. Fowler-Nordheim Tunneling: if a strong gate voltage is applied, electrons will be energetic enough to tunnel through the oxide.

Hot Electrons

Fowler-Nordheim Tunneling

Direct tunneling vs. FN tunneling

6.3.3 Scanning Tunneling Microscope Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. (invented in 1981)

6.3.3 Scanning Tunneling Microscope

6.3.3 Scanning Tunneling Microscope

6.3.3 Scanning Tunneling Microscope

6.3.3 Scanning Tunneling Microscope STM modes: (1) constant height, measure the tunneling current; (2) keep the current constant by varying the tip height. It can also be used to measure local DOS.

6.3.4 Double Barrier Tunneling and the Resonant Tunneling Diode

Double Barrier Tunneling

Resonant Tunneling 0aa + L2a + L There are 8 unknowns: A, B, C, D, E, F, G and H There are 8 boundary conditions (4 boundary interface): ψ and dψ/dx are continuous across the boundary.

Resonant Tunneling

1. Why we only see one peak at E 1, how about E 2 and E 3 ? 2. How to change E? Negative resistance

Resonant Tunneling

Resonant Tunneling

Resonant Tunneling The work of Leo Esaki, Ivar Giaever and Brian Josephson predicted the tunnelling of superconducting Cooper pairs, for which they received the Nobel Prize in Physics in While Dr. Esaki was involved in the development of a transistor with advanced high-frequency performance at Tokyo Telecommunications Engineering Corporation (currently Sony Corporation) in 1957, he discovered a phenomenon called a negative resistance: electric current decreases with the increase of voltage in a p-n junction for which a large amount of impurity is added. He proved that the phenomenon occurs by the jump of electrons from an n-type region to a p-type region, which is caused by a quantum-mechanical tunneling effect. The element developed by this phenomenon is called the Esaki-Diode and is applied to the microwave oscillation circuit.

Supperlattice Supperlattice is a periodic array of barriers.

Supperlattice In 1969, while working at the IBM Watson Research Center in the U.S., Dr. Esaki proposed an artificial material called semiconductor superlattice that has a periodic structure of layered semiconductors with different compositions. The concept of artificial superlattice proposed by Dr. Esaki opened up a new field in solid-state physics and made a major impact on the subsequent study.

6.4 Main Points Quantum particle tunneling through a simple energy barrier Material junctions Field emission, applications to CNT Gate-oxide tunneling in MOSFETs: direct tunneling, FN tunneling, hot electron Principle of STM Tunneling through double barriers Idea of supperlattice Try to work on all the problems: 6.1 – typical examples: 6.3, 6.11 and 6.12