**Quarterly Status Report**

1 May 2005 to 31 July 2005

NREL Subcontract No. XXL-5-44205-09

Principal Investigator: P. Craig Taylor

Organization: University of Utah

**Innovative Characterization of Amorphous and Thin-Film Silicon for
Improved Module Performance**

Research results during the second quarter of Phase I of NREL
Subcontract XXL-5-44205-09 are reported. During this quarter we have
continued collaborations with United Solar Ovonics Corporation on
defects that contribute to the Staebler-Wronski effect in modules made
using a-Si:H and a-Si~x~Ge~1-x~:H intrinsic layers. We are continuing
our collaborations with NREL on defects generated in tritiated a-Si:H
and have submitted a paper for publication. In this quarter we have made
the greatest progress our ^1^H NMR studies (in collaboration with United
Solar Ovonics) of hydrogenated amorphous silicon (*a-Si:H*) with
\~10^16^ cm^-3^ defects grown by PECVD at a rate of 5 Å/s. This sample
shows the existence of a hydrogen doublet in the as-grown film. We
observe the doublet over the temperature range from 5 to 20 K in a
sample where no intended light soaking has occurred. The doublet line
shapes display no narrowing over this temperature range. Vibrational
modes characteristic of *SiH~2~* wagging and scissor modes are seen from
infrared spectroscopy. These results suggest that the doublet is due to
*SiH~2~* that occurs at a density of approximately 1 at. % in this
sample. As we demonstrate below, we estimate a lower limit of 1.8 Å for
the hydrogen-to-hydrogen separation for this SiH~2~ site .

Since the discovery of the Staebler Wronski effect (SWE), many studies
have contributed to our understanding of the role of hydrogen in
producing this metastable effect in hydrogenated amorphous silicon
(a-Si:H). Models of this metastability, such as those of Stutzmann et
al. \[1\] and Branz \[2\], have the common component that hydrogen must
play a role in forming or stabilizing a metastable site \[1,2\]. Our
recent ^1^H NMR studies of device quality *a-Si:H* have found a paired
hydrogen site (*H-H*) with an *H-H* distance of 2.3 ± 0.2 Å \[3\]. This
previous study indicates that the *H-H* site is probably metastable;
however, the study does not identify the specific *H-H* bonding
configuration. A natural candidate for the bonding configuration is
*SiH~2~*, which is predicted to have an *H-H* separation of 2.4 Å,
assuming a tetrahedral structure with a *Si-H* bond length of 1.5 Å.
Furthermore, a recent calculation by Tasfaye *et al.* demonstrates the
existence of *SiH~2~* in the amorphous network with an *H-H* separation
of approximately 2.4 Å \[4\]. Although these results are suggestive, no
conclusive measurements of the *H-H* distance for *SiH~2~* have been
performed in the *a-Si:H* system.

The difficulty with assuming that *SiH~2~* is the metastable site
derives from a stability argument. Annealing studies have shown little
change in *SiH~2~* concentrations for temperatures up to 600 K \[5\],
and yet it is known that annealing of metastable defects occurs at
temperatures of \~ 200° C. From these disparate temperature dependences
it seems unlikely that *SiH~2~* is created or destroyed during light
soaking and subsequent annealing. It is possible that the metastable
doublet observed by Su *et al.* somehow becomes observable in the NMR
experiments after light soaking, but there are some compelling arguments
to suggest that this is not the case \[3\].

These considerations have motivated the current work. In particular, by
combining NMR and far infrared spectroscopic evidence, we have
positively identified a specific NMR lineshape as due to *SiH~2~* and
presented the first detailed ^1^H NMR study of stable *SiH~2~* in
*a-Si:H*.

The sample used for this set of experiments was grown at United Solar
Ovonics using plasma enhanced chemical vapor deposition (PECVD) of
silane (*SiH~4~*). The film was grown on aluminum foil at 5 Å/s with a
substrate temperature of 200 ^o^C. This high growth rate typically
yields films with a high defect density \[6\]. Half the sample was then
light soaked, with one sun intensity, for 200 hours while the other half
remained "as grown". Electron spin resonance (ESR) measurements showed
the defect densities for silicon dangling bonds to be on the order of
*10^16^ cm^-3^* for the as-grown sample. For the ESR and NMR
measurements, the samples were prepared as powders using methods
discussed previously \[3\].

Since hydrogen exists in a variety of environments in *a-Si:H*, each
having a unique ^1^H NMR signature, it is difficult to separate
individual components in a typical spectrum. Therefore it is often
necessary to use a pulse sequence that can suppress the dominant line
shapes with a suitable choice of pulse parameters. The three-pulse
Jeener-Brockaert (JB) sequence provides such a filtering method \[7\].
At a time \~ *~2~* after the third pulse, a spin echo forms with the an
echo intensity given by

$I(\tau_{1},\tau_{2},\tau_{1d}) = I_{0}(1 - e^{- \frac{\tau_{1}}{T_{1}}})e^{- 2\frac{\tau_{2}}{T_{2}}}e^{- \left( \frac{\tau_{1d}}{T_{1d}} \right)^{\frac{1}{2}}}$,
(1)

where *I~o\ ~*is the initial maximum intensity, *T~1\ ~*is the
spin-lattice relaxation time, *T~2~* is the spin de-phasing time or
spin-spin relaxation time, and *T~1d~* is the dipolar spin relaxation
time. Henceforth the pulse separations will be denoted as *τ~2~* and
*τ~1d~*, and the time over which the three-pulse sequence is repeated
will be denoted as *τ~1~*. The utility of this sequence relies on the
fact that each hydrogen environment has its own *T~2\ ~*and *T~1d~*. For
our purposes, it is also necessary to have either relaxation parameter,
*T~2~* or *T~1d~*, longer than those for the more dominant hydrogen
environments, which are due to *SiH* that is either randomly placed or
clustered in the sample. Since *τ~1\ ~*characterizes a signal growth,
filtering with parameter *τ~1~* is effective for components with *T~1~*
shorter than those of the dominant sites.

Using the Jeener-Brockaert sequence with parameters {7s, 60s, 20ms} we
obtain in the as-grown sample a line shape displaying a doublet feature
with peak-to-peak splitting (Δν) of \~ 16 kHz as shown in Fig. 1. For
reference, this figure also contains the line shape found by *Su et
al**.*** \[3\], which shows a similar doublet feature having Δν \~ 14
kHz. The dashed lines, which serve as aides to the eye, show a
qualitative difference in the peak positions. Signatures of the
clustered and isolated hydrogen sites (existing as *SiH*) are also
present. Past studies have shown these clustered and isolated components
contribute to the line shape a Gaussian of \~ *25 kHz* full width half
max (FWHM) and a Lorentzian of \~ *4 kHz* FWHM, respectively \[8\].

Figure 2 shows the doublet line shape for temperatures between 5 and 20
K. Due to the temperature dependence of *T~1~*, we vary
*τ~1\ ~*accordingly to obtain a similar line shape for

![](media/image1.jpeg){width="3.290277777777778in"
height="2.3895833333333334in"}

**Figure 1.** Characteristic line shape for a doublet in two different
samples. Shown in the top trace is the as-grown, high-growth-rate
sample. The bottom trace is the light-soaked, low-defect-density sample
from ref. \[3\]. Vertical lines are aides to the eye.

each temperature. The parameter *τ~1\ ~*is chosen to be roughly one
*T~1~* for the line-shape components attributed to clustered and
isolated *SiH* sites.

Going to longer *τ~1d~* suppresses the Gaussian and Lorentzian
components thereby yielding a more accurate line shape for the doublet.
Figure 3 shows such a trace (bottom trace). In the top trace in Fig. 3
the doublet signal has decayed almost to the noise level, and the
remaining two components in the line shape are due to molecular hydrogen
(*H~2~*). Shoulders occurring at *90* kHz are characteristic of *H~2~*,
which is frozen into the lowest rotational state. The narrow central
peak is due to *H~2~*, which exists in thermally excited rotational
states \[9\].

Subtracting the two traces in Fig. 3 yields a good approximation of the
doublet line shape. Figure 4 shows the result of this subtraction
superimposed on a calculated Pake doublet powder pattern that is
broadened by Gaussian of 15 kHz FWHM. The Pake doublet powder pattern is
the line shape expected theoretically for an ensemble of paired hydrogen
sites that are randomly oriented with respect to the magnetic field.
Divergences at *14.1* kHz in the de-convoluted powder pattern (not
shown) are related to the dipolar coupling constant, which varies
inversely as the H-H separation cubed. From the divergences in this
particular case, we calculate an *H-H* separation of 1.8 Å.

We note here, that the *real* Fourier transforms of time domain data are
shown. Imperfect phase mixing in the quadrature detection scheme,
especially for weak signals, results in a *complex* transform that is
asymmetric. We stress that the doublet is observed from the *full*
transforms but that the lineshape is distorted in data sets that are not
sufficiently well phased mixed.

The original intent of this work was to investigate in greater detail
the metastable hydrogen doublet that accompanies the SWE using a sample,
which has a large defect density in

![](media/image2.jpeg){width="3.290277777777778in"
height="2.3895833333333334in"}

**Figure 2.** Line shape of ^1^H NMR at various temperatures. NMR
parameters are {1 *T~1~,* 60 s, 20 ms} for each trace. *T~1~* values are
*T~1~* = *10s, 8s, 2s, 1s* for *T = 5K , 8K, 12K, 20K*, respectively.

![](media/image3.jpeg){width="3.2895833333333333in"
height="2.390277777777778in"}

**Figure 3.** Bottom trace: NMR line shape for {7 s, 60 s, 45 ms}.
Bonded SiH has decayed leaving only the doublet and H~2~. Top trace: NMR
line shape for {7 s, 60 s, 90 ms}. Only the H~2~ signal remains.

the light-soaked state. In this case, the as-grown sample would provide
a benchmark line shape for comparison with the light soaked sample.
Unfortunately, because of the much smaller

density of the metastable doublet sites, we have to date not succeeded
in making this comparison. The major complicating fact is the occurrence
in the sample of a doublet at much higher densities, and therefore
comparisons of the as-grown and light-soaked samples (not shown in this
quarterly status report) show no difference in the line shapes. The
stable hydrogen doublet site persists in both samples. Therefore, we
have concentrated on identifying the hydrogen-hydrogen (H-H) arrangement
to determine the *H-H* separation in this site.

![](media/image4.jpeg){width="3.263888888888889in"
height="2.3604166666666666in"}

**Figure 4.** Doublet resulting from subtraction of the two traces in
Fig. 3. The smooth line is a calculated Pake doublet powder pattern
broadened by a Gaussian of approximately 15 kHz FWHM. See text for
details of this calculation.

# 

# 

Besides the qualitatively different peak doublet positions, Fig. 1
suggests that the high-growth-rate sample contains a large density of
*H-H* sites (large compared to 10^-17^ *cm^-3^*, the density of sites
seen by *Su* *et al.* \[2\]). To quantify this claim, we have used the
NMR methods discussed in \[9\] to estimate a density of at most 3 *at.
%* for this doublet site*.* Also, we infer a minimum density of \~ 1
*at. %* from vibrational modes observed in Fourier transform infrared
spectroscopy. Densities this large, \~10^21^, suggest that this *H-H*
sight is fundamentally different from that found only in light-soaked
samples in ref. \[3\].

To characterize further this different *H-H* site, we have investigated
the line shape as a function of the temperature, as shown in Fig. 2.
Line shape variations due to motional effects usually exhibit strong
temperature dependences due to thermal activation over barriers to the
motion. The doublet line shapes in Fig. 2 display no evidence of
motional narrowing. Hence we conclude that the doublet consists of
hydrogen bonded to silicon.

Definitive identification of the identity of this *H-H* site comes from
comparison with infrared spectra. This sample exhibits vibrational modes
at 900 cm^-1^ and 850 cm^-1^, which correspond, respectively, to
scissors and wagging modes of *SiH~2~*. Therefore, based on the density
of sites, their thermal stability, and their infrared vibrational modes,
we attribute the doublet line shape to *SiH~2~*.

In summary, we have made the first detailed NMR measurement of stable
*SiH~2~* in high defect density *a-Si:H*. By filtering out the bonded
*SiH* signals, and subtracting the remaining *H~2~* signal, we obtain
the *SiH~2~* signature: a "doublet" with peak-to-peak splitting of \~ 16
kHz. By considering an isolated *SiH~2~* with a well-defined *H-H*
separation, we calculate 1.8 Å for the *H-H* separation in *SiH~2~*.

In addition to our collaborative studies on hydrogen doublets with
United Solar Ovonics, we are continuing our studies on tritiated samples
with NREL. We hope eventually to perform measurements on a-Ge:H to
compare with the a-Si:H results, but because of poor signal-to-noise
ratios, we will start by investigating the quality of a-Ge:H samples
supplied by United Solar Ovonics Corporation using electron spin
resonance and photothermal deflection spectroscopy. Our initial
experiments have shown that the defect densities are too high to perform
the light soaking experiments so the growth conditions need to be
optimized before these experiments can be done. When these experiments
are successful, we will plan the NMR measurements on large-area samples.
In addition, we are examining additional ways of obtaining samples
purposely designed by United Solar Ovonics Corporation to have much
greater degradation than the device quality films.

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(1985).

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4\. Tesfaye A. Abtew, D. A. Drabold, and P. C. Taylor, Appl. Phys. Lett.
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