This TLP is designed to help you learn about epitaxial growth; the growth of a (usually thin) single crystalline layer in the same crystallographic orientation as its single crystal substrate. Epitaxial growth is widely used in the electronics industry to enable the deposition of precisely controlled thin layers of semiconductors or oxides for use in devices such as thin film transistors, diodes and lasers.
One of the major film deposition techniques, which is implied in this TLP, is molecular beam epitaxy (MBE). In this technique low energy gas phase beams of atoms or molecules are directed at the crystalline substrate, usually in a vacuum. The resultant thin films are required to be single crystalline and may, or may not, be atomically flat. The exact form of the thin film (flatness, composition, strain, band gap ...) depends sensitively on the growth parameters, of which the most important is temperature.
The aim of this TLP is to allow you to explore the significance of key parameters in the process, such as the substrate temperature and the various bond energies between the atoms involved. At the heart of the package is a two-dimensional simulation of the deposition of the atoms. Using this you can explore the effect of these parameters on the growth of a generic crystal which models many features of "real" systems involving semiconductors such as Si, GaAs and AlGaAs.
The perfection of the thin film depends crucially on the mode of growth of the deposit. Three such modes are usually identified:
- Frank-van der Merwe (one perfect monolayer at a time);
- Volmer-Weber (island growth), and a combination of the two called
- Stranski-Krastanov (one or more perfect layers followed by island growth).
You are encouraged to explore the simulation until you can identify the physical reasons why each of these occurs, and thus the conditions most likely to lead to the preferred growth mode and thin film. Your knowledge will be applicable to other epitaxial growth techniques such as MOCVD and VPE, and not simply MBE.
The questions associated with this TLP should help you to decide whether you have understood the key messages.
Before you start
You can view the epitaxy simulation simply as a rather pretty animation. However in order to fully appreciate what is going on you need to be familiar with the concepts of interatomic bonding, the migration of a single atom across the surface of a crystal (surface diffusion) and the thermal activation of processes (Arrhenius behaviour).
You do not need to know anything specifically about semiconductors or opto-electronics.