**Quarterly Status Report**

15 June 2004 to 14 September 2004

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 III 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 the growth of
tritium-induced defects in tritium doped hydrogenated amorphous silicon
(a-Si:H,T) as measured by electron spin resonance (ESR) and photothermal
deflection spectroscopy (PDS). These measurements allow us to examine
the production of defects in a-Si:H,T where the defect production
mechanism is known (tritium decay to He). In addition, we have examined
the annealing kinetics of the tritium-decay-induced metastable defects.
These measurements may eventually lead to a better understanding of the
defect creation and annealing in amorphous silicon as well as the
kinetics of hydrogen motion.

# The appearance of optically or electrically induced defects in hydrogenated amorphous silicon (a-Si:H), especially those that contribute to the Staebler-Wronski (SW) effect, has been the topic of numerous studies \[1\], yet the mechanism of defect creation and annealing is far from clarified. We have investigated the intrinsic growth of defects in tritiated hydrogenated amorphous silicon (a-Si:H,T) as an alternative approach to inducing defects optically. Tritium decays to He^3^, emitting a beta particle (average energy of 5.7 keV) and an antineutrino. This reaction has a half-life of 12.5 years; therefore, in these tritium-doped samples each beta decay can, in principle, create a defect by converting a bonded tritium to an interstitial helium, leaving behind a silicon dangling bond. 

We have tracked these defects through electron spin resonance (ESR) and
photothermal deflection spectroscopy (PDS). Like the optically induced
defects, the defects created by tritium decay also anneal out at
approximately 150 C, confirming the earlier findings from luminescence
\[3\]. However, if each tritium decay produces a silicon dangling bond,
then the observed increase in defect density is several orders of
magnitude smaller than one would expect for samples that contain
approximately 6 at. % of tritium. After one year, the decay of tritium
to helium would be expected to yield defect densities of \~10^20^cm^-3^
without saturation, but the increases observed by ESR or PDS are much
smaller. There might be, of course, an additional effect due to the beta
particles passing through the lattice, but such an effect could only
increase the number of defects beyond the tritium decay limit.

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# At room temperature the defect densities reach saturated values of about 10^18^cm^-3^ within 1 to 4 months. At higher temperatures the saturated densities decrease rapidly. Therefore, a very efficient annealing process must exist in the tritiated samples. 

![](media/image1.png)The a-Si:H,T samples were first measured after 7
years of uninterrupted tritium decay at 300 K. These initial spin
densities in all three samples were orders of magnitude lower than would
be expected if each tritium decay produced a silicon dangling bond.
Figure 1a and b shows the annealing behavior of the initial spin
densities of two samples of a-Si:H,T. Figure 1a shows the annealing
behavior of the sample deposited at 250 C, and Fig. 1b shows the
annealing of the sample deposited at 225 C. It is interesting to note
that the defects anneal in a manner that is very similar to the
annealing of light induced defects in the non-tritiated a-Si:H. The
initial spin densities of the 250 C and the 225 C samples were 1.6 x
10^18^ spins/cm^3^ and 1.2 x 10^17^ spins/cm^3^, respectively. The final
spin densities after annealing of the 250 C and the 225 C samples were 7
x 10^17^ spins/cm^3^ and 2.7 x 10^16^ spins/cm^3^, respectively. The
annealed spin densities did not decrease further with increased
annealing time at 150 C.

> **Figure 1.** (a) Annealing behavior in a-Si:H,T made at a substrate
> temperature of 150 C. (b) Annealing behavior in a-Si:H,T made at a
> substrate temperature of 250 C.

The annealing can be understood, in principle. First, it is well known
that the light-induced defects anneal out at temperatures below about
200 C. In addition, the number of defects in films from which H has
evolved is much less than the number of H atoms that have left the
films. If hydrogen is mediating the annealing of light-induced defects,
which seems quite certain, then it must come from a reservoir such that
the new dangling bonds created by evolving H will be healed by some
structural rearrangements. Even if the sample is annealed at
temperatures low enough such that no hydrogen leaves the sample,
optically induced defects can still be annealed. In one specific model,
H comes from a metastable complex containing paired hydrogen atoms (as
suggested by Branz) \[4\].

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# Figure 2 (a, b and c) shows the growth of defects as a function of time after annealing as measured by ESR. The log of the spin density is plotted against the log of the time. Figure 2a shows a-Si:H,T deposited at 150 C; Figure 2b shows a-Si:H,T deposited at 225 C; and Figure 2c shows a-Si:H,T deposited at 250 C. The initial spin density was measured immediately after one hour of annealing at 150 C. We measured the spin density as a function of waiting time at room temperature (circles). The spin densities increase rapidly at first and then saturate at the long-time (7-year) values. The final data point in each graph is the initial spin density of the sample before the annealing process, seven years after deposition. The saturated spin densities are all much smaller than those one would predict if each tritium decay produced a neutral silicon dangling bond. The small densities of defects were first suspected to be the result of the production of charged dangling bonds in the sample, which are not detectable by ESR. However, the growth of defects as measured by PDS (triangles) is in one-to-one agreement with the ESR proving that most of the dangling bonds are uncharged. 

ESR only measures the uncharged dangling bond, and therefore a
relatively small defect density might be the result of charged dangling
bonds in the samples. However, PDS measures both charged and uncharged
defects. The triangles in Fig. 2a, b and c represent the growth of the
absorption coefficient as measured by PDS at 1 eV. The growth of the
absorption coefficient and the ESR follow the same trends within the
noise. The magnitude of the PDS absorption coefficient when scaled in
the standard fashion \[5\] agrees quantitatively with the ESR signal,
implying that all the dangling bond defects in the a-Si:H,T are
uncharged.

![](media/image2.wmf)

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# **Figure 2a**. The circles represent the growth of the spin densities as measured by ESR, and the triangles represent the growth of the PDS signal at 1 eV for the sample made at 150 C (log-log plot). 

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# **Figure 2b.** The circles represent the growth of the spin densities as measured by ESR, and the triangles represent the growth of the PDS signal at 1 eV for the samples made at a 225 C (log-log plot). 

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# **Figure 2c.** The circles represent the growth of the spin densities as measured by ESR, and the triangles represent the growth of the PDS signal at 1 eV for the sample grown at 250 C (log-log plot).

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These growth curves from the ESR and PDS experiments seem to be governed
by some very efficient defect annealing process. Some annealing process
at room temperature counterbalances the rapid creation of spins, such
that the spin density saturates at much lower values than predicted by
tritium decay. In contrast to the light- or electron-beam-induced
defects, in these tritium experiments we know exactly the rate at which
defects are initially created.

In addition to our collaborative studies on tritiated samples with NREL,
we are continuing our ^1^H NMR studies of the paired hydrogen sites that
appear to stabilize the silicon dangling bonds created during the
Staebler-Wronski effect. These experiments are very difficult because of
the low signal-to-noise ratios but the results are promising. We have
tried to perform measurements on a-Ge:H to compare with the a-Si:H
results, but the initial results are ambiguous because of poor
signal-to-noise ratios. In addition, we plan to make measurements on
samples purposely designed by United Solar Ovonics Corp to have much
greater degradation than the device quality films. These films will
improve our signal-to-noise ratios and therefore allow for more detailed
studies of the kinetics. We are also finishing our collaborations with
Weber State University and MVSystems to examine the microcrystallinity
of films made by pulsed PECVD.

REFERENCES

1.  D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. **31**, 292
    (1977).

2.  H. Fritzsche, Ann. Rev. Mater. Res. **31**, 47 (2001).

3.  S. Zukotynski, F. Gaspari, N. Kherani, T. Kosteski, K. Law, W.T.
    Shmayda, C.M. Tan, J. Non-Cryst. Solids **299-302,** 476 (2002).

4.  H. M. Branz, Phys. Rev. B **59**, 5498 (1999).

5.  W. B. Jackson and N. B. Amer, Phys. Rev. B. **25**, 5559 (1982).