Measurements

Chapter 6: Reversible and Irreversible Effects in µc-Si:H
instability effects already affected the material before the first measurement has
been performed. This is in agreement with the observed values of the spin density
N S , the electronic g-value, and the line width ∆H pp plotted in Fig. 6.2 (b,
c, d). Independently of the particular treatment one observes an increase of all
these three quantities. For all treatments (i) the spin density increases by up to
a factor of three, saturating at values of N S ≈ 6 × 10 16 cm −3 , (ii) the electronic
g-value shifts to higher values starting from g=2.0044 (2.0045 for H 2 O) and final
values of g=2.0046, and (iii) the line width increases from ∆H pp = 6.8 ± 0.1 G
(7.3 ± 0.1 G for H 2 O) to 8.7 ± 0.1 G. Quite surprisingly, the final values of N S ,g,
and ∆H pp are the same for all treatments.
This result clearly indicates that it is not the HCl etching step, which causes
the additional increase in N S , the shift of the electronic g-value, and the increased
∆H pp . In fact, the observed changes are essentially the same if the powder is just
treated in water and even a simple storage of the material in air leads to a similar
effect. The fact that the pre-treatment values of the ”H 2 O”-material are slightly
higher, compared to the material treated in HCl and air supports the assumption
that in this case instability effects already set in before the first measurement could
be performed.
The study of different treatments, described above, was performed on material
with a pronounced crystallinity. The effects of an increasing amorphous phase
is now discussed. For a better understanding of the results shown below, it is
important to note, that with increasing amorphous content of the films the film
structure may change as well as the film composition. While IC
RS = 0.82 material
shows a high degree of porosity, IC
RS = 0.71 material is still highly crystalline,
but compact. Finally IC
RS = 0.47 material is prepared at the transition between
microcrystalline and amorphous growth and shows a rather compact structure with
a high a-Si:H contribution (see section 2.1).
Fig. 6.3 (a) shows the ESR spectra of material with three different structure
compositions ranging from highly crystalline porous (IC
RS = 0.82) over highly
crystalline compact material (IC
RS = 0.71) to material at the threshold between
crystalline and amorphous growth (IC
RS = 0.47). The spectra were taken before
and after a H 2 O treatment and then normalized in order to show differences in the
line shape. Also indicated as vertical dotted lines are the resonances with g-values
at g=2.0043 and g=2.0052. Changes in the shape of the ESR spectra can be observed
for the IC
RS = 0.82 and IRS
C
= 0.71 material, while on the other hand the
line shape of the IC
RS = 0.47 spectra remains unchanged upon the H 2O treatment.
As for the IC
RS = 0.82 material, which was discussed above (see Fig. 6.2), in the
IC
RS = 0.71 samples the intensity at the high g-value site of the spectra has a larger
contribution to the total ESR intensity after the treatment. In Fig. 6.3 (b, c, d)
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