Semiconductor Quantum Dots Flashcards
What occurs to the surface of a semiconductor in a large lattice spacing when approaching critical thickness?
Small islands grow on the surface, relaxing outwards and reducing the elastic energy but increasing the surface area.
Why is MBE the preferred method of growing quantum dots?
RHEED can be used to monitor the formation of the islands.
Why do dots have very good structural and optical properties?
There are no defects associated with broken atomic bonds.
What is the main issue with self-assembly quantum dots and why does this occur?
There is a spread of sizes. This is due to randomness in formation or defects on surface that cause dots to form at some points slightly earlier.
Why is the spread in dot size and resultant variation in energy levels a disadvantage?
Need maximum possible amplification at one photon energy, having an energy distribution spreads this amplification over a range so some of the amplification is wasted.
How can a quantum dot convert light (with a spread of photon number) to a regular series of single photons using a system of excitons?
The quantum dot converts the incoming laser photons, which all have the same energy, to a series of photons of different energies (emitted from exciton decays) from which a filter can be used to select just the final emitted photon.
What application does a source of single photons have?
Quantum cryptography. To intercept data you have to measure the polarisation of photon which changes the state and so the interception is detected.
What is a Coulomb blockage?
An electron in a quantum dot blocks a second electron with an energy lower than the energy required to push like-charges together.
What is a Coulomb blockage used for?
Can be used to make an electronic switch (single electron transistor) where the presence of not of an electron controls whether other electrons are able to flow though the quantum dot.
Why can a Coulomb blockage only be observed if the charging energy exceeds the thermal energy (kT)?
The thermal energy of the electrons will allow them to overcome the charging energy.
