**Quarterly Status Report**

1 February 2006 to 30 April 2006

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 first quarter of Phase II 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. Specifically, we
have received another set of device-quality samples, in an attempt to
minimize the starting defect densities. We need to minimize these
densities to have any chance of improving on our previous results in
measuring changes in the NMR on light soaking. We are starting our NMR
experiments on these new samples. We are also finishing our NMR studies
of samples from United Solar Ovonics, which were purposely made to
contain large defect densities. In these samples, using both NMR and
FTIR measurements, we have positively identified a hydrogen doublet as
due to dihydride bonding sites. In addition, we are continuing our
studies of light soaking at 77 K to determine if the Staebler-Wronski
effect saturates at this temperature or grows continuously as occurs in
the case of the tritium decay in the tritiated samples. We are also
preparing a manuscript on the tritiated samples. In this quarter we have
made the greatest progress on identifying the hydrogen-hydrogen
separation at silicon dihydride bonding sites in a-Si:H.

The implication of hydrogen in the Staebler-Wronski effect (SWE) in
hydrogenated amorphous silicon (a-Si:H) has generated enormous interest
in understanding the local bonding environments of the hydrogen. In
particular, recent proton (^1^H) nuclear magnetic resonance (NMR)
experiments have linked a specific metastable, "paired" hydrogen site,
in which the hydrogen pair is separated by 2.3 ± 0.2 Å, to the SWE
\[1\]. Silicon dihydride (SiH~2~) is a possible candidate for this site
since calculations using the conventional tetrahedral silicon structure
give 2.4 Å as the hydrogen separation \[2\].

Hydrogen environments can be probed using ^1^H NMR techniques because
the hydrogen nucleus communicates with the environment through a variety
of spin-mediated interactions. In a-Si:H, the dipole-dipole interaction
between two protons dominates. For these magnetic dipole interactions,
the strength depends strongly on the proton-proton separation. Exciting
the system with radiation corresponding to the interaction strength
causes the proton system to resonate thereby producing a detectable time
varying signal. The resulting Fourier-transformed line shape yields
details of the local hydrogen bonding configurations. For instance, by
analysis of dipole broadened line shapes, it has been determined that in
a-Si:H bonded hydrogen in the silicon hydride (SiH) configuration exists
in both clustered and essentially randomly isolated environments \[3\].

The SiH~2~ line shape exhibits a well-known form resulting from a dipole
interaction between two protons. An analytic expression of the intensity
of this so-called "Pake doublet" line shape can be expressed in terms of
the coupling strength, and hence the proton-proton separation, *r.* It
is therefore possible to extract the proton-proton separation by fitting
simulated line shapes to the experimental data. This would suffice if
the SiH~2~ existed as an isolated species with a well-defined *r*. In
the case where modulations to the local field exist, due to nearby
hydrogen nuclei, the pure Pake doublet is broadened by an amount
corresponding to the proximity and number of the nearby nuclei. This
"isotropic" broadening effect, which is Guassian for clustered arrays
\[3\], can be taken as a fitting parameter in subsequent simulations.

Broadening of the pure Pake doublet may also result from a distribution
of proton-proton separations. It is known from scattering experiments
that the nearest neighbor, Si-Si bond angles vary by 10 \[4\]. Thus it
is natural to expect some distribution in *r* that will reflect the
amorphous system. The distribution can also be used as a fitting
parameter whereby the resulting broadened spectrum is a summation of
Pake doublets.

Observation of SiH~2~ in low defect density a-Si:H has eluded ^1^H NMR
experiments primarily for two reasons. The first reason is obvious: in
device quality films the SiH~2~ concentrations are usually below the
threshold detection limit of conventional ^1^H NMR (\~ 10 ^16^ cm^-3^).
Moreover, SiH occurs in concentrations \~ 10 ^21^ cm^-3^ and therefore
dominates the line shape in standard free induction (FID) decay
experiments. This fact alone suggests that the FID experiments are
insufficient for observing the low concentrations of SiH~2~.

A more subtle reason for the lack of experimental observation of SiH~2~
in a-Si:H derives from the line shape broadening effects discussed
above. Isotropic broadening becomes a problem when it is of the same the
magnitude as the dipolar coupling. In this case the Pake doublet is
unresolved since its defining features smear to form a nearly Guassian
line shape. For instance, the un-broadened proton dipolar coupling in
SiH~2~ with *r* = 2.4 Å is 13.4 kHz \[3\]. The typical clustered
hydrogen environments yields a Gaussian with \~ 25 kHz full width at
half maximum (denoted henceforth as ). Thus there will be no resolved
SiH~2~ line shape if it exists in an environment that resembles the
standard clustered array.

In the present study, we have used ^1^H NMR to obtain the stable SiH~2~
Pake doublet. Simulations of the Pake doublet for various choices of
broadening effects were performed. We discuss the simulation details and
relevant fitting parameters needed to replicate the experimental data.

## 

## The a-Si:H sample used in this study was made at United Solar Ovonics Corporation at high growth rate (5 Å/s). The film was grown on aluminum foil at a substrate temperature of 200 C. Such samples contain higher defect densities (≥ 10^17^ cm^-3^) and are generally not suitable for device applications. Fourier transform infrared spectroscopy (FTIR) measurements show IR active modes at 900 cm^-1^ and 850 cm^-1^, indicative of the scissor and wagging SiH~2~ modes respectively. The presence of these modes in the FTIR spectrum implies an SiH~2~ concentration on the order of 10^21^ cm^-3^ -- well above the observation threshold for NMR. For subsequent NMR measurements, the sample was prepared as a powder according to methods previously discussed \[1\]. 

For spectral simulations, we numerically compute the "powder pattern"
using a method similar to that described by Slitcher \[5\]. The program
was built in C++ using the pre-written functions of the GAMMA libraries
\[6\]. For simulations requiring a distribution of *r*, we chose a
Gaussian distribution where and mean separation, *R~o~*, are the fitting
parameters. Thus the full spectrum resulting from a distribution in *r*
is the summation of individual powder patterns (whose weight depends on
the distribution parameters).

Figure 1 shows the frequency domain spectrum of the experimentally
obtained Pake doublet with two simulations superimposed. We obtain the
experimental Pake doublet by Fourier transforming the time domain signal
and subtracting the remaining molecular hydrogen signal. The Zeeman
frequency (147.417 MHz) has been subtracted so that the spectral center
occurs at 0 Hz. The solid line represents the experimental Pake doublet.
For comparison we present the simulated Pake doublets for *r* = 1.8 Å
(peaks at 15 kHz) and 2.4 Å (peaks at 7 kHz). These simulations are
broadened by 1 kHz, but they do not include a distribution of distances.
The areas have been normalized to equal that of the experimental data.

Figure 2 demonstrates, in frequency space, the separate effects of
isotropic broadening by clustering and by a distribution in *r.* The top
trace represents the unbroadened powder pattern for *r =* 1.8 Å. Below
it, we present a spectrum broadened by 15 kHz (approximately 72% of the
dipolar coupling). The bottom trace represents the powder pattern for a
Gaussian distribution of *r* characterized by the parameters *R*~o~ =
1.8 Å, = 0.52 Å (30% of *R*~o~). The bottom trace has been further
broadened by 1 kHz. Similarities between the broadened spectra suggest
that the set of simulation parameters replicating the data is not
unique.

![](media/image1.wmf){width="3.3125in" height="3.0659722222222223in"}

Figure 1. Frequency domain spectrum of the experimental Pake doublet
with simulations for *r =* 1.8 Å (shoulder peaks at 15 kHz ) and *r* =
2.4 Å (shoulder peaks at 7 kHz). The experimental data was obtained via
a Fourier transform of the time domain data and subsequent subtraction
of the molecular hydrogen signal. The Zeeman frequency (147.47 MHz) has
been subtracted out so that the spectrum center occurs at 0 Hz.

![](media/image2.wmf)

Figure 2. Broadening effects on the simulated Pake doublet for *r =* 1.8
Å. The top trace is Pake broadened by 1 kHz. The middle trace is the
Pake doublet for *r =* 1.8 Å (no distribution) with 15 kHz isotropic
broadening. The bottom trace represents the Pake doublet simulation for
*R*~o~ = 1. 8 Å, = 0.54 Å, and 1 kHz isotropic broadening.

Figure 3 depicts the simulated spectral fits to the experimental data.
The top trace is the simulated Pake doublet for *r* = 1.8 Å, broadened
by 15 kHz superimposed on the experimental data. The bottom trace shows
the Pake doublet for *R*~o~ = 1.8 Å, = 0.52 Å broadened by 1 kHz also
superimposed on the experimental data. Dashed lines denote the simulated
spectra.

The experimental data of Fig 1 yield important clues for judicious
choices of the fitting parameters. Of the three fitting parameters, the
fit is most sensitive to *R*~o~ since it varies as *r*^-3^. Therefore we
first choose *R*~o~ according to the qualitative agreement between the
unbroadened spectra and experimental data. For *r* = 2.4 Å, the standard
result for *r* in SiH~2~ according to theoretical calculations \[2\],
there is a clear discrepancy in the shoulder peaks. Since subsequent
broadening will only decrease the separation between peaks, it is clear
that larger coupling constants (smaller *r*) are favored. With *r* = 1.8
Å, the shoulder peaks occur just outside of the experimental Pake
doublet and serve as a good candidate for *R*~o~.

![](media/image3.wmf){width="3.420138888888889in"
height="3.140972222222222in"}

Figure 3. Simulated Pake doublets superimposed on the experimental data.
The dashed line in the top trace shows the simulation fit *r =* 1.8 Å
(no distribution) with 15 kHz isotropic broadening. The dashed line in
the bottom trace shows the simulation fit for *R*~o~ = 1. 8 Å, = 0.54 Å,
and 1 kHz isotropic broadening.

In accounting for the broadening effects, we note that the resolved
peaks in the data require the total broadening to be less than the
coupling constant---20.6 kHz for *r* = 1.8 Å. Two broadening mechanisms
are available and hence any combination of the two satisfying the
overall broadening requirement will yield a suitable fit. The NMR data
are insufficient to answer the question of uniqueness. However, by
mimicking the clustered and isolated environments known to exist for SiH
(see for instance \[3\]), we deduce limits on the range of acceptable
parameters.

We obtain the first limiting case by assuming that SiH~2~ exists in
hydrogen-clustered regions of the amorphous network. We also place the
constraint that there exists a well-defined *r* = 1.8 Å for the SiH~2~
sites. This seems unreasonable in light of the known nearest neighbor
bond angle variation \[4\]. In the clustered region, the SiH~2~ is
isotropically broadened. The effect of 15 kHz of isotropic broadening is
shown as the middle trace in Fig. 2. We present the fit to the data as
the dashed line in the top trace of Fig. 3. The resulting spectrum sits
outside the shoulders in the data---a discrepancy that may be accounted
for by minor adjustments in *R*~o~ and the amount of isotropic
broadening.

By imitating the isolated environment of SiH, we obtain the second
limiting case. It is known, from hole burning experiments, that isolated
SiH sees \~ 1 kHz broadening due to the surrounding hydrogen nuclei
\[3\]. Thus we take this as our isotropic broadening parameter. To
complete the broadening, we include a Gaussian distribution of *r*. The
resulting spectrum with parameters *R*~o~ = 1.8 Å and = 0.54 Å is shown
as the bottom trace of Fig. 2. The bottom trace of Fig. 3 shows the near
perfect fit of these simulations to the data. Simulations with *R*~o~ =
1.9 Å and 1.7 Å have yielded inferior fits to those presented.

As demonstrated in Fig. 3 the true SiH~2~ environment cannot be inferred
directly from the NMR results. In either limiting case, *R*~o~ = 1.8 Å
is the essential ingredient. Thus in light of prior ^1^H NMR results
linking a paired hydrogen site with *r* = 2.4 Å to the SWE, it is
unlikely that an "average" SiH~2~ site is involved. However, if the true
*r* distribution includes this value, we cannot rule out SiH~2~ as a
candidate. Therefore, if the metastable, paired hydrogen site that
stabilizes the SWE is indeed a SiH~2~ site, then it must be a rare
embodiment that is not indicative of the average site.

In summary, we have fitted the experimental Pake doublet for stable
SiH~2~ with line shape simulations whose fitting parameters reflect the
local environment and variation in proton-proton separations. Accurate
fits of the data are available for fitting parameters that imitate the
clustered and isolated SiH environments. We take these sets of fitting
parameters as the limiting cases whereby the true SiH~2~ environment is
a mixture of the two. We find for both extremes that an accurate fit is
obtainable only when the proton-proton separation in SiH~2~ is on
average 1.8 Å. Hence a distribution of proton-proton separations is
necessary if SiH~2~ is to be involved with the SWE.

In addition to our studies on hydrogen doublets with United Solar
Ovonics, we are continuing our collaborative studies on tritiated
samples with NREL. We still hope eventually to perform measurements on
a-Ge:H in collaboration with United Solar Ovonics, but our efforts are
currently limited by the quality of the samples. In spite of efforts to
optimize sample quality our experiments so far have shown that the
defect densities are too high to perform the light soaking experiments.
In addition, we are continuing to examine additional ways of obtaining
samples purposely designed by United Solar Ovonics Corporation to have
much greater degradation than the device quality films. Finally, we are
continuing our irradiation of device quality samples of a-Si:H at 77 K
for long periods of time to test for saturation.

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