Question

1. (a) With the aid of an annotated sketch describe the growth technique known as Molecular Beam Epitaxy (MBE). Briefly describe how this growth technique has been used to grow Strans ki-Krastanov quantum dot nanostructures Metal Organic Chemical Vapor Deposition (MOCVD) is an alternative technique for growing epitaxial material. What is the principal advantage of MOCVD over MBE? (b) Figure 1 below shows a shutter sequence for a Il-V MBE reactor, where 1 denotes an open shutter, and 0 denotes a closed shutter Sketch the conduction band and valence band edges of the heterostructure material produced by this shutter sequence, also annotating the material layers produced. Assume non-degenerate doping such that Er is at the conduction band edge and valence band edge for n and p type doping respectively, and assume a growth rate of um/hr). [S In 0 3 Time (min) Figure 1. Molecular Beam Epitaxy (MBE) shutter sequence for an active device layer growth What might this structure be used for when a suitable voltage is applied? Assuming the material quality is good, how might you increase the efficiency of such a device? If the barrier material is comprised of Ina Gao sAs, estimate using Vegard's law what the turn on voltage would be for such a device 141 Note: Bandgap (RT) E, (InAs) 0.36ev, E, (GaAs)-1.42ev

Answer 1

1. (a) Here is an extract on Molecular Beam Epitaxy from my thesis on alloy preparation:

1. Molecular beam Epitaxy

Molecular beam epitaxy or MBE is a method of deposition of a crystalline overlayer on a crystalline substrate, with a deposition rate less than 3,000 nanometre per hour that allows the crystals to grow epitaxial. The thickness of each layer is as much as that of a single layer of atoms. This is done by computer-controlled shutters.

Advantages over other techniques:

• Takes place in Ultra-high vacuum ~10^-8-10^-12 Torr. Therefore, no carrier gases are present and films are highly pure.
• High-quality i.e. low-defect and high uniformity films.
• Films can be made from more than one element.
• Extremely thin films are grown in carefully controlled way.

Effusion cells element A*element B atoms atoms Substrate Figure 5. Molecular beam Epitaxy.

The major advantage of MOCVD over MBE is that it can deposit almost anything, from metals and semiconductors to dielectrics, while MBE only works well for semiconductors.