26.4: Questions

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Click on this link to launch the epitaxial growth simulation in a new window. You may need it to answer some of the questions,

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

At a medium temperature, say 0.5 Tm, which of these conditions favours layer (Frank-van der Merwe) growth?

Adatom-adatom bond strength

Adatom-substrate bond strength

Lattice parameter difference

0.6%

Answer: B

At a low temperature, say 0.3 Tm, which of these conditions favours island (Volmer-Weber) growth?

Adatom-adatom bond strength

Adatom-substrate bond strength

Lattice parameter difference

0.3%

0.1%

Answer: A

At a high temperature, say 0.8 Tm, which of these conditions favours Stranski-Krastanov growth?

Adatom-adatom bond strength

Adatom-substrate bond strength

Lattice parameter difference

0.1%

0.6%

Answer: D

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

At 0.7 Tm, what is the effect of lattice parameter difference in the following cases? Increase the number of atoms from 100 to 150 to see this effect.

Adatom-adatom bond strength

Adatom-substrate bond strength

Lattice parameter difference

Answer

What happens at 0.5 Tm in the following conditions? Is this physically reasonable?

	Adatom-adatom bond strength	Adatom-substrate bond strength	Lattice parameter difference
	100	0	0

Answer: Particles, some detached from substrate. Yes, they are nanoparticles which might indeed float free for a while (although eventually subject to gravity)

What happens as more and more atoms are added in the conditions of Q7? What drives this process?

Answer: The particles coarsen, driven by surface energy which is lower per unit of material (i.e. number of atoms) for larger particles.

Run the simulation with parameters which give you either pronounced SK growth or clear island formation. What shape are the resultant particles or islands?

Answer: Faceted but tending to be equi-axed

What would you expect to happen to the growing layer if the lattice parameter difference was 5%?

Answer: It would start as island growth, but the islands would not be in perfect alignment as they eventually met and defects (dislocations or twins) would have to form.

If the substrate had been polished so that its surface was not exactly a low-index plane (e.g. perhaps 0.5 degrees away from exact {111}, would there be any occasions during growth when there were no steps on the growing surface?

Answer: No, steps would move constantly across the surface, maintaining approximately the same spacing the whole time.

Open-ended questions

The following questions are not provided with answers, but intended to provide food for thought and points for further discussion with other students and teachers.

Try to explain the results you saw in the sixteen runs of Questions 1 to 4. Predict a set of parameters which should give Stranski-Krastanov growth at 0.2Tm. e.g. [50, 50, 1]

Predict and confirm a set of conditions which might give a gas rather than a deposit [0,0,0] or [0,0,1] at any temperature, because no bonding of adatoms either to substrate or each other]