Heterostructures

Question 1

Define a heterostructure. What changes do we find at the interface in a heterostructure?

Answer 1

• Heterostructure: Two or more materials are intimately stacked during growth process.
• At the interface changes in structure, symmetry, charge (ox. state) and chemistry (composition) can be observed.

Question 2

In the example we considered on perovskite type oxides forming a heterostructure, we observed very good epitaxy.

• Which parameters are important for having a good lattice matching?
• How can you do an experiment for observing the epitaxy at the interface?

Answer 2

• Good lattice matching require similar unit cell dimensions (xy). Good match = Difference limited to 1-2 %
• TEM and EDS

Question 3

Consider the perovskite type structure. Draw the unit cell (assume cubic structure). Explain how the outer surface perpendicular to the c-axis could have two different chemistries? Sketch the two possibilites

Answer 3

LaAlO3 and SrTiO3, nice example =)

Question 4

Why can terminating surface be either charged or neutral for the same basic crystal structure, but for two different chemical compounds? Use SrTiO3 and LaAlO3 as examples.
What would be the corresponding situation for the perovskite oxide KNbO3?

Answer 4

AlO2(-) and LaO(+) are charged layers (the compound itself is neutral), while SrO and TiO2 are neutral.

Question 5

Magnetization measurements of a heterostructure of SrTiO3 and LaAlO3 shows an M(H) hysteresis loop. What type of magnetism does then exist? Can this be explained on basis of the two diamagnetic oxide components of the heterostructure?

Answer 5

An open M(H) hysteresis loop indicate ferro- or ferrimagnetism.

Question 6

Would it be possible to make a heterostructure based on two rock salt oxides with relatively similar unit cell dimensions? E.g. MgO and NiO? Justify your answer

Answer 6

Possible in principle, but an intermixing at the interface may occur. The more similar the two materials are, the larger tendency of intermixing.