**Sixth Quarterly Status Report**

15 June 2003 to 14 September 2003

NREL Subcontract No. ADJ-2-30630-23

Principal Investigator: P. Craig Taylor

Organization: University of Utah

**Characterization of Amorphous Silicon Advanced Materials and PV
Devices**

Research results during the third quarter of Phase II of NREL
Subcontract ADJ-2-30630-23 are reported. During this quarter we have
continued collaborations with Penn State University on comparisons of
ESR and optical absorption and with NREL on tritiated a-Si::H. In this
quarter we have made the greatest progress on ESR measurements that
constitute the first observation of the Staebler-Wronski effect in
hydrogenated amorphous germanium (a-Ge:H). The appearance of this effect
in a-Ge:H has been controversial for many years, but the fact that it
exists now places serious restrictions on the various models that have
been proposed as explanations.

For over 25 years the Staebler-Wronski (SW) effect has persisted as the
fundamental metastability in hydrogenated amorphous silicon (a-Si:H).
This effect has also been observed in some hydrogenated amorphous
silicon-germanium alloys with low concentrations of germanium. The SW
effect was originally observed \[1\] as an optically induced, metastable
decrease in both the dark and photoconductivities, but soon afterward
this effect was tied to a concomitant increase in the electron spin
resonance signal \[2\] commonly attributed to silicon dangling bonds.
Over the years there has been some speculation concerning the existence
of the SW effect in hydrogenated amorphous germanium (a-Ge:H) but to
date no positive results have been reported. In 1991 Santos et al. \[3\]
observed a decrease in the dark conductivity and photoconductivity in
a-Ge:H, but the effect annealed at room temperature in the dark, which
was also confirmed by Bauer and Eberhardt in 1993 \[4\]. Several authors
\[5,6,7\] have suggested that below a specific optical gap energy, which
is greater than the optical gap energy of a-Ge:H, there is insufficient
recombination energy to lead to the creation of a metastable defect.

During this quarter we have studied an optically induced increase in the
electron spin resonance (ESR) spin density that can be partially
annealed (approximately 30 % of the increase remains) after one hour of
annealing at 170 C in a nitrogen environment. Even in the absence of
optical excitation, annealing above approximately 165 C produces a
small, but stable, increase in the dark spin density, presumably due to
hydrogen evolution from the sample. As in a-Si:H, some of the changes
are reversible (SW effect), but unlike a-Si:H the samples cannot be
recycled more than a few times before the irreversible increases
dominate. In comparison with the well-studied behavior in a-Si:H, a more
detailed study of the kinetics of the SW effect in a-Ge:H will be
important in testing possible microscopic models

Figure 1 shows the initial ESR derivative spectrum of a-Ge:H, the change
in the spectrum after 10 hours of irradiation at 2 W/cm^2^, and the
spectrum after annealing at 170 C for one hour. After two hours of
irradiation the increase in the spin density is approximately 50 % of
the initial

dark spin density. After ten hours irradiation, the increase is
approximately 100 %. However after the annealing approximately 70% of
the induced defects remain in the sample. These are irreversible defects
that do not anneal out at elevated temperatures even after longer
annealing times approaching several hours. The narrow signal near 3390
gauss in Fig. 1 is due to e΄ centers, which presumably occur at the
a-Ge:H/SiO~2~ interface. \[The e΄ center is an electron trapped in an
sp^3^ orbital at a silicon atom, which is bonded to three oxygen
atoms.\] The irradiation also produces a decrease in the e΄ center,
which is partially reversible on annealing. The mechanism producing the
changes in the e΄ center is unknown but may be due to hydrogen motion at
the interface.

![](media/image1.wmf)

> **Figure 1.** Derivative of the ESR absorption signals in a-Ge:H at 40
> K. The solid, dotted and dashed lines are the initial dark signal, the
> signal after ten hours of irradiation at 300 K with 1 W/cm^2^ of 1.17
> eV light, and the signal after subsequent annealing at 171 C for one
> hour. All traces were run at 40 K to improve the signal-to-noise
> ratios.

Figure 2 shows the growth of the total ESR signal in the stack of a-Ge:H
films under irradiation with 0.5 and 1 W/cm^2^. Both growth curves
depend on time as t^1/3^, which is similar to the kinetics observed in
a-Si:H \[10\] and is representative of a broad distribution in the time
constants. However, the data for a-Ge:H in Fig. 2 include all defects -
the initial dark spin density, the optically induced reversible (SW
effect) component, and the optically induced irreversible component.
Figure 3 shows the growth curves with the initial dark spin densities
subtracted. In this case, the growth appears to depart from a t^1/3^
dependence, especially at short times. The heating of the sample due to
the laser at the highest intensity will affect the details of the
kinetics but not the overall general features. The times indicated in
Figs. 2 and 3 are those that each orientation of the sample received.
Because four perpendicular orientations were employed, the total
irradiation times for all orientations are greater by a factor of four.

The maximum irradiation time employed at an intensity of 0.5W/cm^2^ is
30 hours, and the optically induced ESR intensity appears to saturate
after approximately 20 hours. At 1 W/cm^2^ the maximum irradiation time
is 26 hours, and saturation occurs after approximately 15 hours. At 2
W/cm^2^ the maximum irradiation time is 16 hours, and saturation occurs
after approximately 10 hours.

![](media/image2.wmf)

> **Figure 2.** Growth of the total (dark and light induced) ESR
> absorption signals in a-Ge:H after irradiation with 0.5 W/cm^2^ of
> 1.17 eV light (solid circles) and 2.0 W/cm^2^ of 1.17eV light (solid
> squares). All measurements were taken at 40 K. The dotted and solid
> lines are power law fits to the data with an exponent of 1/3 (t^1/3^).

The solid curves in Fig. 3 represent exponential rises to saturation
indicative of unique time constants. The dotted lines are stretched
exponential rises to saturation with a stretched exponential exponent of
α = 0.5. This value of α approximates the t^1/3^ dependence as shown in
Fig. 2 when the growth curve is not near saturation \[11\]. Although the
initial dark spin density is subtracted from all the data points in Fig.
3, these data do not represent the SW defects because there exists an
optically induced irreversible component that depends strongly on the
intensity of the exciting light. At an excitation intensity of 1 W/cm^2^
we have an estimate of the irreversible component at two irradiation
times after which the sample was annealed, 10 hours and 26 hours. From
these two data points, we estimate that the irreversible spin density
appears to have essentially the same time dependence as the reversible
spin density. If this is indeed the case, then subtracting the
irreversible spin density from the data points in Fig. 3 will merely
shift the graph downward on the log-log plot but will not change the
general shape. Therefore, we conclude that the curves shown in Fig. 3
are reasonable representations of the growth of the SW defects in
a-Ge:H, although the absolute spin densities are clearly not correct.

![](media/image3.wmf)

> **Figure 3.** Growth of LESR signals in a-Ge:H after irradiation with
> 1.17 eV light of intensity 0.5 W/cm^2^ (solid circles), 1.0 W/cm^2^
> (solid triangles), and 2.0 W/cm^2^ (solid squares). The solid and
> dotted lines are fits to the data assuming an exponential rise to
> saturation and a stretched exponential rise, respectively.

Using the saturated spin densities at the maximum times shown in Fig. 3,
after which we anneal the samples, we can estimate the fraction of
defects that is irreversible at these times. This fraction depends super
linearly on the generation rate, G, roughly as G^2^. This power
dependence indicates a two-photon process. The reversible SW defects, on
the other hand, are well fit to a G^2/3^ dependence, just as occurs in
a-Si:H. Therefore to maximize the influence of the irreversible defects
one should perform the experiments as the lowest excitation intensities
possible.

It is interesting to note that there is also a small irreversible
component to a-Si:H after many cycles of light irradiation and
annealing, but this effect has not been studied in detail \[12,13\]. It
is quite probable that this irreversible component is a two-photon
process too. In a-Ge:H this irreversible component is enhanced to such
an extent that multiple cycling of samples between irradiation and
annealing is very difficult. The irreversible defect density after 16
hours of irradiation at 2 W/cm^2^ is more than 70% of the total
optically induced defect density in the sample as measured by ESR. There
is also an irreversible component due to the annealing itself, which
gets larger at higher annealing temperatures. Fortunately, at the
annealing temperatures employed in this study, this thermally induced
component is very small compared to the light induced defect densities.

The range of practically accessible annealing temperatures in a-Ge:H
appears to be much narrower than in a-Si:H. We were able to observe
annealing of the SW defects between 150 and 170 C. Below 150 C the
annealing times were too long to obtain reliable data and above 170 C
the annealing itself produced an increase in the dark spin density that
was large enough to complicate the interpretation. The annealing is
probably thermally activated as it is in a-Si:H, but the primary
activation energy is difficult to quantify at this time. The value is
less than 1 eV and the distribution of activation energies is probably
narrower than in a-Si:H. This result, coupled with the growth curves,
which can be adequately fit by an exponential rise to saturation,
suggests that the time constants and activation energies may exhibit
narrower distributions in a-Ge:H than they do in a-Si:H.

The similarities in the kinetics for producing and annealing the
dangling-bond ESR signals between a-Ge:H and a-Si:H provide strong
evidence that we have observed the SW effect in a-Ge:H. However, it is
the differences between these two materials that will be important in
testing the various models proposed \[10,14\] to explain the SW effect.

## 

We have observed the Staebler-Wronski effect in a-Ge:H by measuring a
reversible increase in the ESR after optical excitation at 1.17 eV. In
general, the kinetics is similar to a-Si:H, but there may be a narrower
distribution of activation energies and time constants in a-Ge:H. The
kinetics of the SW effect is difficult to study in detail due to the
large percentage of irreversible defects, even at the lowest practical
excitation intensities. Future experiments at much lower irradiation
intensities are necessary to study the growth kinetics in detail. Also
longer annealing times at lower temperatures need to be investigated.

In addition to our studies in a-Si:H of the LESR decays over wide time
scales and at elevated temperatures, we are continuing our collaboration
with Penn State University on light induced defects where we currently
are re-evaluating samples that have been light soaked by the Penn State
group. The samples have been light-soaked for 10 hours and 100 hours and
the ESR increases are being compared with increases in the optical
absorption at low and high energies. Although these experiments are
proving to be very difficult because of the inability to obtain thick
samples, especially those grown with strong hydrogen dilution, this past
quarter we have employed our new ESR cavity to obtain results that are
very promising. The ESR increases appear to scale with only one of the
components of the optically induced absorption. Details will be reported
in a future progress report.

We are also continuing our collaborations with Weber State University
and MVSystems to examine the microcrystallinity of films made by pulsed
PECVD. Finally, we are continuing our collaborative studies with NREL on
tritiated amorphous silicon, where the decay of tritium naturally
produces defects because the tritium decays to helium, which is
contained in the sample interstitially.

We are still planning a new series of NMR experiments on a-Si:H films
with larger initial dark spin densities and therefore larger metastable
increases in the spin densities on light soaking. These samples will
probably be supplied by Penn State University because of the changes at
BP Solar. We eventually plan to examine the PL and ESR, in addition to
the NMR, in both light-soaked and annealed samples. The delivery of the
samples has been delayed due to the change in the supplier.

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