## NREL subcontract XDJ-2-30630-32

# Iowa State University

# Quarterly Progress report for April 14-July 15,2004

A.  [Changes in microstructure of nanocrystalline Si:H
    films]{.underline}

We previously reported in the annual report for Phase I of this contract
that the pressure during deposition affected the crystallinity of the
material, with higher pressures, for a given power level, leading to
amorphous films. We traced this phenomenon to the increase in ion flux
at lower pressures. We had also reported earlier that the films
contained both \<111\> and \<220\> orientations. We decided to
investigate this phenomenon in detail, because thermodynamically, the
preferred orientation should be \<220\> and not \<111\>. Upon further
research in this field, we discovered a very interesting phenomenon,
namely that the orientation of the grain itself, and the ratio of grain
sizes \[ \<220\> vs \<111\>\] itself depended upon pressure in the
reactor! This result is shown in Fig. 1, which shows that as pressure
increases, the grain orientation shifts to \<220\> from \<111\>. This is
a fascinating observation, since it implies that ion flux causes more
random nucleation and that leads to \<111\> grains. We are now
systematically exploring materials with different grain orientations,
and studying their PV related properties to see if they depend upon the
grain orientation.

![](media/image1.wmf){width="4.9in" height="3.3465277777777778in"}

[Fig. 1]{.underline} The dependence of different grain sizes in nc-Si:H
produced using different pressures during deposition in a VHF-plasma
reactor

Another benefit of being able to change the size of the different grains
in the [same]{.underline} material is that one can examine structural
details related to both grains \<111\> and \<220\> in the same film.
Structural details such as stress are obviously important, because a
film which is highly stressed is likely to flake off various substrates,
an observation which is unfortunately all too common. Also, too much
stress may lead to more amorphous structure, and potentially,
light-induced instability.

B.  [Measurement and influence of stress in the nanocrystalline Si:H
    films]{.underline}

If a film is stressed, the stress should show up as a satellite peak in
the x-ray spectrum. Such measurements are usually difficult to perform,
since the satellite peak is buried in the noise. Therefore, one needs to
do very long x-ray measurements, with long time constants, to increase
signal-to-noise ratio. When we did this (taking \~ 8-9 hours per
sample!), satellite peaks emerged in x-ray spectra of nanocrystalline
Si:H films. See Fig. 2. But what was most interesting was that the
peaks, corresponding to stress, were in different directions for the
\<111\> and \<220\> grains! The satellite peak for \<111\> indicated a
tensile stress, and for \<220\>, a compressive stress. The same film was
under both compression and tension. How can this be?

![](media/image2.wmf)

[Fig. 2]{.underline} x-ray diffraction spectra of a nanocrystalline Si
film with both \<111\> and \<220 oriented grains. The \<111\> grain has
a satellite peak showing tensile stress, and \<220\> grain has a
satellite peak showing compressive stress. Both stresses are
simultaneously present in the same film!

The answer lies in recognizing that for small grained films, there is a
significant amount of amorphous Si tissue at the grain boundaries. This
fact is clearly visible in the Raman spectra. The a-Si:H tissue puts the
\<111\> under tension and \<220\> under compression. Smaller the grain
size, more the strain. See Fig. 3.

Another implication of the results is that the stress should shift the
crystalline Raman peak to lower energies, and that the shift should
depend upon grain size. Smaller the grain size , larger the shift. That
is indeed what we found.

![](media/image4.wmf)

> [Fig. 3]{.underline} Influence of grain size on strain measured in the
> film.

Yet another consequence of the results is that it is futile to try to
gauge the degree of crystallinity by taking the areas under various
deconvoluted Raman curves, because the shift in the spectrum makes area
calculations virtually meaningless. People often use three peaks to
deconvolute the spectra. There is no physical basis for such triple
deconvolution. What is actually happening is that multiple orientations
and multiple grain sizes are shifting the Raman spectra. On the other
hand, the ratio of peaks is still a relatively good indication of the
degree of crystallinity.

These results are obviously very helpful for developing an understanding
of how the films grow, and how to achieve larger grains and better PV
properties in these materials, as well as providing quantitative
measures for quickly judging the structural quality of films.

Vikram L. Dalal, PI