Building 6H-SiC Question

[adkins.99]

My knowledge in solid state physics is limited. I'm trying to build 6H-SiC to run tests on the surfaces. I'm not sure if I'm doing this correctly. I start with the Hexagonal SiC in the data base, it consists of 4 atoms. I repeat it and then move atoms around to get the structure of the 6H-SiC unit cell (12 atoms) by going off schematics I've found from various sources. Then I repeat the 6H-SiC structure 5X5X1. I attached images of two of my stash items with the unit cell and the 5X5X1, I also attached two images I looked off of to help me.

Basically my question is...is this right?

[Julian Schneider]

It looks right, but it's difficult to tell exactly just from the picture.
I have attached a .cif file of the SiC-6H (http://rruff.geo.arizona.edu/AMS/minerals/Moissanite) structure to which you can compare your structure.

[adkins.99]

It looks right, but it's difficult to tell exactly just from the picture.
I have attached a .cif file of the SiC-6H (http://rruff.geo.arizona.edu/AMS/minerals/Moissanite) structure to which you can compare your structure.

I'm confused, doesn't 6H-SiC have 12 atoms in a unit cell? There's only 6 in the coordinates you gave for 6H.

Also, different sources seem to get different structures for the unit cell, is there more than one?

[adkins.99]

Another question I have about building the structures...I attached two excerpts from a paper, I don't see how they obtained that structure (i.e. what was the unit cell they repeated?) If I use the unit cell I described in my original post above (with 12 atoms), and repeat it, none of the surfaces appear similar to the figures below...

[Julian Schneider]

I'm confused, doesn't 6H-SiC have 12 atoms in a unit cell? There's only 6 in the coordinates you gave for 6H.

If you import the .cif file, attached in my previous post, into the Builder (via Add > From Files), you will see that it has 12 atoms, although the cif file has only 6 coordinates. The remaining coordinates are deduced from the symmetry.

Also, different sources seem to get different structures for the unit cell, is there more than one?

The two structures in the pictures might be equivalent within periodic boundary conditions, but it is really difficult to tell just from the flat picture.

My knowledge in solid state physics is limited. I'm trying to build 6H-SiC to run tests on the surfaces. I'm not sure if I'm doing this correctly. I start with the Hexagonal SiC in the data base, it consists of 4 atoms. I repeat it and then move atoms around to get the structure of the 6H-SiC unit cell (12 atoms) by going off schematics I've found from various sources. Then I repeat the 6H-SiC structure 5X5X1. I attached images of two of my stash items with the unit cell and the 5X5X1, I also attached two images I looked off of to help me.

Basically my question is...is this right?

If you take the unit cell from the .cif file and cleave the (0001) surface (using the Surface (Cleaver) plugin, and selecting a carbon atom as the topmost terminating atom) then you get the surface as in the picture (see attached figure). Then you can start introducing the modifications.

[adkins.99]

Thanks, that helped. It looks like the unit cell I was attempting to use was not twisted like yours is, and was actually a planar object. I'm undergraduate and have not taken solid state.

If you look at attachment "6H-SiC Unit Cell.png," that's the structure I repeated to obtain "6H SiC Example.png"

The "6H SiC Example.png" looks just like images of 6H-SiC that I find in literature sources. For instance, I've attached an excerpt from,

Jakse, N., Arifin, R., & Lai, S. K. (2011). Growth of graphene on 6H-SiC by molecular dynamics simulation. Condensed Matter Physics, 14(4), 43802-1-43802-7.

In the diagram the structure looks similar to " 6H SiC Example.png " in my original post. I'm guessing the structure in Jakse was not really meant to represent the physical appearance of the material from one face, but perhaps just a 2-d representation of the stacking sequence?

[Julian Schneider]

In the diagram the structure looks similar to " 6H SiC Example.png " in my original post. I'm guessing the structure in Jakse was not really meant to represent the physical appearance of the material from one face, but perhaps just a 2-d representation of the stacking sequence?

Yes, you are right. From the picture in the paper it could be both 6H-SiC or 2H-Sic.

Guessing from the side view in your picture " 6H SiC Example.png", it might rather be 2H-SiC (cf. attached picture), but it is not 6H-SiC.

[adkins.99]

Ok, I see. Interestingly when I go to surface cleave the (0001) surface bonds break. I use the default setting for the surface lattice.

I've been reading,


Ogasawara N, Norimatsu W, Irle S, Kusunoki M. Growth mechanisms and selectivity for graphene or carbon nanotube formation on SiC (0001¯): A density-functional tight-binding molecular dynamics study. Chemical Physics Letters [serial online]. March 18, 2014;595-596:266-271. Available from: ScienceDirect, Ipswich, MA. Accessed September 11, 2015.

They use 5x5x.5 unit cells, I wonder if that is a coincidence?

Edit: sorry had to fix the images, now you can see what I mean, the first pic is just showing how I cleaved the surface in the 0001 direction on the carbon atom, the second shows the output with a broken bond right in the middle.

[Julian Schneider]

The bond stub is still there, so it looks as if the structure is still intact, only that the one silicon atom is outside the box. Try repeating the cell in A- and B-direction, or using Bulk Tools > Wrap to wrap the atoms inside the box, and check if the bonds still is broken.
I don't see any of these problems when I use the cif-file structure and do the same cleaving operations.

I've been reading,


Ogasawara N, Norimatsu W, Irle S, Kusunoki M. Growth mechanisms and selectivity for graphene or carbon nanotube formation on SiC (0001¯): A density-functional tight-binding molecular dynamics study. Chemical Physics Letters [serial online]. March 18, 2014;595-596:266-271. Available from: ScienceDirect, Ipswich, MA. Accessed September 11, 2015.

They use 5x5x.5 unit cells, I wonder if that is a coincidence?

I guess that's just the choice the authors made to obtain a supercell of a suitable size.

[Julian Schneider]

BTW. We'll soon have a brand new tutorial on vapor deposition simulations on the SiC(0001) surface..

[adkins.99]

The bond stub is still there, so it looks as if the structure is still intact, only that the one silicon atom is outside the box. Try repeating the cell in A- and B-direction, or using Bulk Tools > Wrap to wrap the atoms inside the box, and check if the bonds still is broken.
I don't see any of these problems when I use the cif-file structure and do the same cleaving operations.

I think I may have been using an altered copy of the unit cell and didn't notice. It worked fine on a fresh copy

BTW. We'll soon have a brand new tutorial on vapor deposition simulations on the SiC(0001) surface..

Awesome, that's the topic of my senior research actually.