HETEROSTRUCTURES

Question 1

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

Answer 1

A heterostructureis defined as a semiconductor structure in which the chemical composition changes with position.The definition can be expanded to include other types of materials. In a heterostructure two (or more) materials are intimately stacked during the growth process.
One may observe changes in structure, symmetry, charge (oxidation states)and chemistry (composition) at an interface

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 quite similar (x,y) unit cell dimensionsfor the two surfaces that join at the interface. The difference must be limited to a very few percent (say 1-2%)

Question 3

Why can the 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 3

We will have KO- and NbO2+ possible surface terminations. These are yet a different category than those discussed for LaAlO3 and SrTiO3.

Question 4

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 4

Ferro-or ferrimagnetism: these may have open M(H) hysteresis loops. No, two diamagnetic substances alone are expected to result in a diamagnetic composite/ heterostructure, and cannot explain ferro-/ferrimagnetism. So, special physics must occur owing to mismatch in charge states at the interface.

Question 5

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

Answer 5

Yes; in principle clearly possible. As in all these cases, an intermixing may occur at the interface. The tendency to intermixing will be larger when the two materials have the same structure, the same cation and anion charges, very similar ionic radii. This means that if deposition techniques are used that involve a large energy budget, then more intermixing will occur than for corresponding low temperature depositions.