1. Structural constrains
S. Kugler: Lectures on Amorphous Semiconductors

2. Constrains for a-Si
S. Kugler and Z. Varallyay, Phil. Mag. Lett. 81 (2001) 569, and
Z. Varallyay and S. Kugler, J. Non-Cryst. Solids (2002) 265.
S. Kugler: Lectures on Amorphous Semiconductors

3. CAMBRIDGE STRUCTURAL DATABASE
During the growth of amorphous samples it might be expected that covalently bonded atoms tend to form "natural configurations". The only question is what the possible "natural configurations" are?
There is no direct experimental method having atomic resolution for determining the atomic distributions in 3 dimensions; therefore efforts to develop new techniques for analyzing the atomic scale structure are continuously being made.
S. Kugler: Lectures on Amorphous Semiconductors

4. S. Kugler: Lectures on Amorphous Semiconductors
One way of ascertaining these natural configurations in amorphous silicon is to consider configurations of silicon fragments embedded inside large molecules. The environment of silicon atoms might be considered as "white noise" around such fragments.
S. Kugler: Lectures on Amorphous Semiconductors

5. S. Kugler: Lectures on Amorphous Semiconductors
We carried out a systematic analysis of structural data using the Cambridge Structural Database (CSD), this being the world's largest database of experimentally determined crystal structures containing the results of X-ray and neutron diffraction studies.
CSD is designed as a critically evaluated numerical resource, containing three- dimensional atomic coordinates.
S. Kugler: Lectures on Amorphous Semiconductors

6. S. Kugler: Lectures on Amorphous Semiconductors
We have collated from the CSD the experimentally determined structural data of molecules containing Si-Si-Si fragments. Our recent search found 2559 targets. Si
S. Kugler: Lectures on Amorphous Semiconductors

7. S. Kugler: Lectures on Amorphous Semiconductors

8. S. Kugler: Lectures on Amorphous Semiconductors
Since in one Si-Si-Si fragment two bond length belong to one bond angle, we have 5118 bond lengths. The overwhelming majority of bond lengths and bond angles fall in the expected region, i.e. around 2.35 A and deg. The minimum bond length is A while the maximum is A.
Except for one fragment, the bond angles lie inside the interval deg.
S. Kugler: Lectures on Amorphous Semiconductors

9. S. Kugler: Lectures on Amorphous Semiconductors

10. S. Kugler: Lectures on Amorphous Semiconductors
There are some significant peaks in the interval deg. which is an unexpected region (314 angles). Most of them belong to a near planar square arrangement. 68 such squares were found in the database. Bond angles around 60 deg. can be observed corresponding to nearly equilateral triangles. 29 triangles were found. This is another unexpected region.
S. Kugler: Lectures on Amorphous Semiconductors

11. S. Kugler: Lectures on Amorphous Semiconductors
Constrains for a-Se
J. Hegedüs and  S. Kugler:  J.Phys: Condens. Matter, 17 pp (2005)
S. Kugler: Lectures on Amorphous Semiconductors

12. S. Kugler: Lectures on Amorphous Semiconductors

13. S. Kugler: Lectures on Amorphous Semiconductors

14. S. Kugler: Lectures on Amorphous Semiconductors