Bimodal CSD in Microcrystalline Silicon

Crystallite Size Distribution in µc-Si:H

Plasma deposition techniques of microcrystalline silicon (µc-Si:H) fabrication inevitably lead to inhomogeneities in the microstructure of the material, as the contents of the constituent phases of mc-Si:H are influenced by the processing history. These inhomogeneities exist at different length scales and require to be studied with the help of different microstructural characterization tools acting at different length scales. An inhomogeneity in the form of a distribution in the sizes of the crystallites (CSD) is one such well-known feature present in plasma deposited µc-Si:H.



Significance


Electrical transport in µc-Si:H has been conventionally linked to and studied in the context of changing film crystallinity. In the fully crystalline single-phase µc-Si:H material, there is no appreciable change in crystallinity with film growth. In the absence of an amorphous phase, the CSD and conglomeration of grains in such material may have a significant influence on the electrical transport properties and mechanisms. A detailed knowledge about these microstructural properties would allow a better understanding of the anisotropic nature of electrical transport, which results from the influence of crystalline orientation and the location of grain boundaries on the transport path. With this knowledge, it would be possible to predict the transport behavior in a particular direction, i.e., planar or perpendicular configuration, which is essential for optimization of device performance.



Issues

Conventionally, Raman spectroscopy profiles are deconvoluted assuming a single mean crystallite size and a peak assigned to grain boundary material, and to account for the asymmetric tail, an amorphous phase is included. However, these assumptions could be erroneous for a single phase µc-Si:H material, in which the presence of a CSD is demonstrated by other microstructural characterization studies. The presence of CSD should be accounted for in the analyses of Raman spectra for more physically accurate results and picture of the material structure.






Measured imaginary part of <E2> spectrum for µc-Si:H sample; and reference spectra of µc-Si, amorphous silicon, and of low-pressure chemical vapor deposited polysilicon with large (pc-Si- l) and fine (pc-Si-f) grains.












What we have done


In our microstructural studies on µc-Si:H, we employed different microstructural characterization tools (spectroscopic ellipsometry, Raman spectroscopy, X-ray diffraction, and atomic force microscopy) to study the presence of CSD.
We acquired quantitative information about the mean crystallite sizes and their volume fractions in highly crystalline µc-Si:H with spectroscopic ellipsometry. Then we determined the actual crystallite size distribution using Raman spectroscopy with the help of a modification in the modeling method.




























Corroboration of results from different studies


The results of spectroscopic ellipsometry, X-ray diffraction, and atomic force microscopy demonstrate the presence of a distribution in the sizes of crystallites.

The modeling of spectroscopic ellipsometry results using two types of crystallites having two distinct sizes is corroborated with the deconvolution of experimentally observed RS profiles using a bimodal size distribution of crystallites having two mean sizes, large and small.

The fractional compositional analyses of the films obtained by this methodology are found to be in qualitative agreement with the findings of spectroscopic ellipsometry.


Important Result

Our study shows that the appearance of a strong and longer low-frequency tail in Raman profiles measured from film side of single-phase µc-Si:H material, without any distinguishable amorphous hump, can be due to the presence of size distribution in nanocrystallites, instead of a contribution from disordered or amorphous phase.


REFERENCES

Read more about these in these papers:

1. “Evidence of Bimodal Crystallite Size Distribution in µc-Si:H Films”, Sanjay K. Ram, Md. N. Islam, S. Kumar and P. Roca i Cabarrocas, Materials Science and Engineering B (in press, doi: 10.1016/j.mseb.2008.11.048)


2. “Influence of the statistical shift of Fermi level on the conductivity behavior in microcrystalline silicon”, Sanjay K. Ram, P. Roca i Cabarrocas and S. Kumar, Phys. Rev. B. 77, 045212 (2008).

3. “Structural determination of nanocrystalline Si films using ellipsometry and Raman spectroscopy”, Sanjay K. Ram, Md. N. Islam, S. Kumar, and P. Roca i Cabarrocas, Thin Solid Films 516 (2008) 6863.

4. "Influence of crystallite size distribution on the micro-Raman analysis of porous Si", M.N. Islam, S. Kumar, Appl. Phys. Lett. 78 (2001) 715.

5. "Effects of crystallite size distribution on the Raman-scattering profiles of silicon nanostructures", M.N. Islam, A. Pradhan, S. Kumar, J. Appl. Phys. 98 (2005) 024309.