s. kugler: lectures on amorphous semiconductors 1 structural constrains

S. Kugler: Lectures on Amorphous SemicoS. Kugler: Lectures on Amorphous Semiconductorsnductors

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Structural Structural constrainsconstrains


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Constrains for a-SiConstrains for a-Si

S. Kugler and Z. Varallyay, Phil. Mag. Lett. 81 (2001) S. Kugler and Z. Varallyay, Phil. Mag. Lett. 81 (2001) 569, and 569, and

Z. Varallyay and S. Kugler, J. Non-Cryst. Solids Z. Varallyay and S. Kugler, J. Non-Cryst. Solids 299-302 (2002) 265.299-302 (2002) 265.


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CAMBRIDGE STRUCTURAL DATABASECAMBRIDGE STRUCTURAL DATABASE

During the growth of amorphous samples During the growth of amorphous samples it might be expected that covalently it might be expected that covalently bonded atoms tend to form "bonded atoms tend to form "natural natural configurationsconfigurations". The only question is ". The only question is what the possible "what the possible "natural natural configurationsconfigurations" are? " are?

There is no direct experimental method There is no direct experimental method having atomic resolution for determining having atomic resolution for determining the atomic distributions in 3 dimensions; the atomic distributions in 3 dimensions; therefore efforts to develop new techniques therefore efforts to develop new techniques for analyzing the atomic scale structure are for analyzing the atomic scale structure are continuously being made. continuously being made.


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One way of ascertaining these One way of ascertaining these natural configurations in amorphous natural configurations in amorphous silicon is to consider configurations silicon is to consider configurations of silicon fragments embedded inside of silicon fragments embedded inside large molecules. The environment of large molecules. The environment of silicon atoms might be considered as silicon atoms might be considered as "white noise" around such fragments. "white noise" around such fragments.


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We carried out a systematic analysis We carried out a systematic analysis of structural data using the Cambridge of structural data using the Cambridge Structural Database (CSD), this being Structural Database (CSD), this being the world's largest database of the world's largest database of experimentally determined crystal experimentally determined crystal structures containing the results of X-structures containing the results of X-ray and neutron diffraction studies. ray and neutron diffraction studies.

CSD is designed as a critically CSD is designed as a critically evaluated numerical resource, evaluated numerical resource, containing three- dimensional atomic containing three- dimensional atomic coordinates.coordinates.


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We have collated from the CSD the We have collated from the CSD the experimentally determined structural experimentally determined structural data of molecules containing Si-Si-Si data of molecules containing Si-Si-Si fragments. fragments. Our recent search found Our recent search found 2559 targets2559 targets. .

SiSi

Si


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Since in one Si-Si-Si fragment two Since in one Si-Si-Si fragment two bond length belong to one bond bond length belong to one bond angle, we have 5118 bond lengths. angle, we have 5118 bond lengths. The overwhelming majority of bond The overwhelming majority of bond lengths and bond angles fall in the lengths and bond angles fall in the expected region, i.e. around 2.35 A expected region, i.e. around 2.35 A and 109.47 deg. The minimum bond and 109.47 deg. The minimum bond length is 2.217 A while the length is 2.217 A while the maximum is 2.647 A. maximum is 2.647 A.

Except for one fragment, the Except for one fragment, the bond angles lie inside the bond angles lie inside the interval 53.9-145.8 deg.interval 53.9-145.8 deg.


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There are some significant peaks There are some significant peaks in the interval 75-96 deg. which is in the interval 75-96 deg. which is an unexpected region (314 an unexpected region (314 angles). Most of them belong to a angles). Most of them belong to a near planar square arrangement. near planar square arrangement. 68 such squares were found in 68 such squares were found in the database.the database. Bond angles Bond angles around 60 deg. can be observed around 60 deg. can be observed corresponding to nearly corresponding to nearly equilateral triangles. equilateral triangles. 29 29 triangles were found.triangles were found. This is This is another unexpected regionanother unexpected region. .


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Constrains for a-SeConstrains for a-Se

J. Hegedüs and S. Kugler: J.Phys: Condens.

Matter, 17 pp6459-6468 (2005)


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s. kugler: lectures on amorphous semiconductors 1 structural constrains

Documents

amorphous semiconductorss

amorphous silicon

growth of amorphous

bond angles

si fragments

minimum bond length

structural data of molecules

structural constrainss