I think your question is not clear because the answer is highly correlated to the growth mechanism. For example if you have invoked VLS technique to attain such structure it should be keep in mind that VLS technique is absolutely substrate dependent and crystallographic orientation of the achieved NWs could be widely affected by the substrate. As an another example if you want to use stress assisted technique, silicon nitride layer, due to several reasons such as high temperature deposition methods and residual stress and strain, can highly affect the crystalline nature of the achieved NWs .

There are two roles: adaptive and barrier. The first one is a structural role. If you would like to promote a growth with orientation in plane of gallium nitride, that is hexagonal (wurtzite type crystal), adding the silicon nitride (Si3N4) layer (hexagonal too) enables you to control the orientation of GaN crystals on the surface. The barrier role is lied to the peculiar characteristic of silicon nitride lattice. The dense structure of silicon nitride does not provide the open channels found in other structures; thus, nitride is widely employed as a barrier material. Even hydrogen diffuses slowly in a densified nitride film. Since oxygen diffuses very slowly through nitride, it can prevent oxidation of underlying silicon.

I'd love to examine these thoughts using Bragg XRD Microscopy non-destructively. This non-contact, in situ "Microcrystallography" method would certainly reveal the answer. This method allows the quantification of crystallographic orientation and dislocation density/distribution with spatial resolution in the micrometer range presently. The spatial resolution limit would be the wavelength of the X-rays used. However, detector technology currently limits this to 1-5um range. Using the X-ray Rocking Curve Technique we are able to measure variations in the FEMTOMETER range!

Here are some examples:

1. ZnSe (224) Bragg XRD Micrographs compared with conventional XRD Topography:

http://www.flickr.com/photos/85210325@N04/8493232400/in/photostream

2. YouTube Video play list for a variety of materials:

http://www.flickr.com/photos/85210325@N04/8493232400/in/photostream

Please feel free to join "X-ray Diffraction Imaging for Materials Microstructural QC" group in LinkedIn:http://www.youtube.com/watch?v=IU0m4yI7D-k&list=PL7032E2DAF1F3941F

We would love to benefit from your opinion as well.

Our objective is to implement/deploy the powerful XRD methods for use in modern R&D and manufacturing environments for routine nano-structural QC. Real time feed-back control!

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