Why do we need a substrate while growing?

[Weimin]

Mismatching is the problem in growing materials. I wonder why they did not use the same crystal to grow the sample. For example, GaN is grown on Al203, why not take GaN as the seed?

 

[Cthugha]

That depends on what exactly you want to grow.
It is possible to use the material you want to grow as substrate (homoepitaxy), but sometimes mismatching is not a problem, but highly welcome.
For example the growth of low dimensional heterostructures (quantum dots, quantum wells, ...) requires some mismatching to work.

 

[ZapperZ]

Mismatching is the problem in growing materials. I wonder why they did not use the same crystal to grow the sample. For example, GaN is grown on Al203, why not take GaN as the seed?

But in many cases, what you want is a pristine surface with a particular crystallographic orientation. If that's the case, you can't just use a pre-made surface that have been exposed to air. So you have to make the film in situ and then study it.

The lattice mismatch for GaN, for example, is mostly solved by depositing a buffer layer first such as AlN. Furthermore, in films such as GaAs, the lattice mismatch is actually desirable because it produces a strain in the film that breaks the degeneracy of the crystal, allowing for the production of spin-polarized photoelectrons.

Zz.

 

[Weimin]

I got it, thank you.

 

[leright]

Also, usually the reason for growing using a deposition method such as MBE, VPE, or CVD as opposed to the Czochralsky method is because the Czochralsky method does not work. Such is the case with AlN, since at temperatures where Al is molten nitrogen is of course a gas so Czochralsky isn't possible. Therefore, AlN wafers are simply not commercially available for epitaxial growth and you need to use a substrate such as sapphire (Al2O3) to grow AlN. However, as mentioned before, the lattice mismatch is not significant usually, and the strain can be minimized by first growing a annealed buffer layer. Then the epitaxial film is grown on the buffer layer.