Final Technical Report to

**Midwest Research Institute**

## **National Renewable Energy Laboratory**

> **Subcontract (XDJ-2-30630-25)**

### "Thin Film Silicon Cells on Low-Cost Substrates"

![](media/image1.wmf){width="1.09375in" height="1.09375in"}

Attention:

Dr. Bolko von Roedern

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401-3393

Principal Investigator

Harry A. Atwater

Thomas J. Watson Laboratory of Applied Physics

California Institute of Technology

Pasadena, CA 91125

tel: (626) 395-2197

FAX: (626) 844-9320

email: haa@daedalus.caltech.edu

Period 7/11/2002 --6/30/2004

[Executive Summary]{.underline}

Project Goals:

The overall project goal was to synthesize polycrystalline Si thin films
for thin film solar cells at low temperature (T \< 600 ^o^C) on low-cost
(e.g., glass) substrates. This understanding was be used to delineate
the path to break through existing barriers to high-rate synthesis of
high-quality thin films for polycrystalline silicon photovoltaics
applications. In this context, high quality refers to large grain size
(\> film thickness) and long (\> film thickness) minority carrier
diffusion length. The program in Tasks I and II was successful against
these goals; Task III was cancelled by NREL.

**Project Approach:**

In developing the approach to high rate synthesis of large-grained films
at low temperatures, W e investigated the fabrication of large-grained
polycrystalline silicon by hot-wire chemical vapor deposition (HWCVD)
and its suitability for thin-film photovoltaic applications.

Early in the project we focused on direct growth of polycrystalline
silicon on SiO~2~ by HWCVD. We used atomic force microscopy (AFM) to
characterize fully continuous polycrystalline silicon films grown by
HWCVD on SiO~2~, as well as the nucleation density of silicon islands
formed in the early stages of HWCVD growth, as a function of temperature
and hydrogen dilution (H~2~:SiH~4~). Our observations of the nucleation
kinetics of Si on SiO~2~ can be explained by a rate-equation
pair-binding model, from which we derive an estimate for the prefactor
and activation energy for surface diffusion of Si on SiO2 and assess the
viability of this method for the rapid growth of large-grained
polycrystalline silicon on SiO~2~.

After this initial focus we devised a two-fold strategy for the fast,
low-temperature growth of thin, large-grained polycrystalline silicon
films on glass substrates. The two step process involves:

1.  ***Selective Nucleation and Solid Phase Ep*itaxy** (SNSPE) growth to
    form a thin (\< 1 μm thick) large-grained polycrystalline silicon
    template film from amorphous silicon starting material. The rate of
    solid phase crystallization is greatly enhanced relative to that for
    pure amorphous silicon via dopant-enhanced or silicide-enhanced
    crystallization, enabling large grain structures to be achieved at
    \< 600 ^o^C in reasonable times.

2.  ***High Rate Epitaxial Growth By Hot-Wire Chemical Vapor
    Deposition*** (HWCVD) of a thicker (1-30 μm thick) layer of silicon
    on the large-grained polycrystalline silicon template film. Hot-wire
    chemical vapor deposition is a promising technique for deposition of
    thin film amorphous, polycrystalline, and epitaxial silicon films
    for photovoltaic applications. We developed large-grained
    (\~100*μ*m) polycrystalline silicon layers fabricated by selective
    nucleation and solid-phase epitaxy (SNSPE) on SiO~2~ substrates as
    templates for epitaxial growth by HWCVD.

**Accomplishments under Project:**

While researching the SNSPE/HWCVD process described above, the following
achievements were made:

-   Using reflection high energy electron diffraction (RHEED) and
    transmission electron microscopy (TEM), we have **[derived a phase
    diagram for Si epitaxy on Si(100)]{.underline}** consisting of
    epitaxial, twinned epitaxial, mixed epitaxial/polycrystalline, and
    polycrystalline phases of growth on Si(100) in the 50 nm--2 *μ*m
    thickness regime.

-   **[Epitaxial Si growth was found on SNSPE templates]{.underline}**,
    which use nickel nanoparticles as nucleation sites for the
    solid-phase crystallization of phosphorus-doped amorphous silicon on
    SiO~2~. Residual nickel present in the SNSPE templates does not
    significantly affect the lifetime of films grown on SNSPE templates,
    making the growth of epitaxial layers by HWCVD on SNSPE templates a
    possible strategy for the fabrication of thin-film photovoltaics.

-   **[Minority carrier lifetimes for films on Si(100)]{.underline}**,
    as measured by resonant-coupled photoconductive decay experiments,
    ranged from **[5.7 to 7.5 *μ*s]{.underline}** while those for
    **[films on]{.underline}** **[SNSPE templates range from 5.9 to 19.3
    *μ*s]{.underline}.**

**Technical Results:**

HWCVD epitaxial growth on large-grained templates is one strategy for
the fast low-temperature growth of large-grained films with
hydrogen-passivated low-angle grain boundaries.

We have proposed a structure by which a template with grains on the
order of 10--100 µm is fabricated on ITO-coated glass or another low
cost conductive substrate by a solid-phase crystallization process
called SNSPE\[1\]. This layer serves as the n+ back surface field, and
the n and p+ layers are grown epitaxially on this template by HWCVD,
using phosphine and tri-methyl boron as dopants. In order to achieve
this goal, investigation of the phase diagrams that lead to epitaxial
growth in HWCVD are necessary. Preliminary work towards the fabrication
of this cell and the necessary intermediate goals are discussed.

# EXPERIMENTAL Approach

**Dilute Silane Growth**

In order to promote crystalline growth under dilute silane conditions,
we used a high hydrogen partial pressure dilution ratio of 50:1, using a
mixture of 4% SiH~4~ in He at a partial pressure of 25 mTorr and H~2~ at
a partial pressure of 50 mTorr. The 0.5mm diameter tungsten wire was
positioned 2.5 cm from the substrate for a growth rate of 1Å/s. The wire
temperature was set to 1800^º^C as measured by optical pyrometry, and
substrate temperatures ranged from 300^º^C to 475^º^C.

Silicon (100) substrates and large-grained polycrystalline layers formed
by SNSPE were used as templates for epitaxial growth. The formation of
the large-grained templates is described elsewhere\[1\]. The resulting
polycrystalline templates have grain sizes on the order of 10 to 100 μm
with low-angle grain boundaries. Before growth, both surfaces were
cleaned with UV ozone for 10 minutes followed by an HF dip. Once in the
chamber, they are heated at 200^º^C at a pressure of less than 10^--6^
Torr to desorb any residual hydrocarbons.

**Pure Silane Growth**

Using pure silane, the influence of dilution ratio was studied. The
substrate temperature was held constant at 380 ^º^C while graphite
filaments set to 2100 ^º^C by optical pyrometry were placed 3.5 cm from
the substrate. The H~2~ to SiH~4~ ratio ranged from 10 to 50, total
pressure also had to be adjusted for the higher H~2~ flow rates and
ranged from 20 to 50 mTorr. Substrates were cleaned as described above.

**Dilute Silane Growth - Substrate temperature effects**

We used TEM and RHEED to characterize the crystallinity of films grown
at 50:1 hydrogen dilution and temperatures between 300-475^0^C in the 50
nm to 2 μm thickness regime and observed four phases of growth. A
completely defect-free epitaxial phase was observable by TEM at
thicknesses below 50nm; the twinned epitaxial, mixed and polycrystalline
phases were observable by TEM and RHEED\[2\]. Twinned epitaxial silicon
denotes silicon material containing twin boundaries, but no other
dislocation or grain boundary defects. From this data, we derived the
phase diagram in Figure 1. At 300^0^C, the predominant phases are
epitaxial and twinned, with a transition to mixed phase or
polycrystalline growth occurring somewhere between 1-2 μm of growth. As
temperature increases, the epitaxial and twinned phases no longer
persist and the transition to mixed phase or polycrystalline growth
occurs at smaller film thicknesses. This is in direct contrast to work
done by Thiesen \[3\] and Watahiki \[4\] where epitaxial thickness
increased with substrate temperature. We believe that the decrease in
epitaxial thickness with the increase in substrate temperature is due to
an interplay between surface hydrogenation at low temperatures and
surface oxidation at high temperatures that reduces the epitaxial
thickness. This is possibly related to the higher hydrogen content in
HWCVD as compared to PECVD or MBE along with oxygen contamination in the
deposition chamber\[5\].

![](media/image2.png){width="3.459722222222222in"
height="2.5055555555555555in"}

**Figure 1:** Phase diagram of HWCVD films grown at 50:1 hydrogen
dilution with 4% SiH~4~ in He.

Silicon films 300 nm thick grown on SNSPE templates under the same
conditions showed epitaxial growth results consistent with local low
temperature epitaxy on each of the 100 μm grains in the SNSPE templates
\[Fig. 2\]. Epitaxial breakdown is observed in the diffraction pattern
of the HWCVD film, but some of the underlying low-order diffraction
spots are visible. The underlying film therefore likely has morphology
similar to that of the HWCVD films on Si(100). The effect of the
orientation of the underlying grain structure of the SNSPE template on
the morphology of the HWCVD film is shown in Figure 2.

Cross-sectional analysis of these films reveals some areas of epitaxial
growth as well as some areas of columnar growth. Before HWCVD growth,
the SNSPE templates were cleaned in a solution of 3:7 HNO~3~: H~2~O,
which has been shown by Auger spectroscopy to remove elemental Ni from
the template surface\[6\]. The lack of epitaxy in some areas is thus
more likely to have been caused by the presence of ubiquitous surface
contaminants, such as carbon and oxygen, than by nickel nanoparticles,
since a similar microstructure can be seen in deposition on Si(100)
substrates as well.

**Pure Silane Growth -- Hydrogen dilution effects**

We used TEM and Raman Spectroscopy to determine the structure of Si thin
films grown at 380^º^C with graphite wires at 2100^º^C. We found that
the crystallinity did not increase with hydrogen dilution as
expected\[7\]. Instead, an amorphous/protocrystalline structure is
observed at dilution ratios of 20 to 30, while at both the 40 and 10
H~2~/SiH~4~ dilution ratios we observe epitaxial growth with
polycrystalline breakdown. This suggests a complex interaction between
pressure and dilution, which will be discussed later.

![](media/image3.png){width="2.7291666666666665in"
height="2.060416666666667in"}

![](media/image4.wmf)

**Figure 2:** Plan-view TEM of HWCVD epitaxial film (T=300^0^C) on SNSPE
template. (a) Selected area diffraction pattern from underlying SNSPE
template. (b) Selected area diffraction pattern from HWCVD film on SNSPE
template. (c) Bright-field image indicating selected area diffraction
regions. Inset: diffraction from entire area. (d) Bright-field image of
HWCVD film (T=300^0^C) on SNSPE template showing selected area
diffraction region. Inset: selected area diffraction pattern showing
areas of large-grained polycrystalline growth.

+-----------+-----------+------------+----------------+---------------+
| Hydrogen  | Pressure  | Deposition | Raman          | Phase         |
| Dilution  |           | Rate       | Crystallinity  |               |
| Ratio     | (mTorr)   | (nm/min)   | %              |               |
+-----------+-----------+------------+----------------+---------------+
| 50:1      | 50        | 4          | 83             | Poly          |
+-----------+-----------+------------+----------------+---------------+
| 40:1      | 27        | 5          | 84             | Epi/poly      |
+-----------+-----------+------------+----------------+---------------+
| 30:1      | 20        | 3          | 9              | a-Si/Proto    |
+-----------+-----------+------------+----------------+---------------+
| 20:1      | 20        | 6          | 10             | a-Si/Proto    |
+-----------+-----------+------------+----------------+---------------+
| 10:1      | 20        | 9          | 96             | Epi/poly      |
+-----------+-----------+------------+----------------+---------------+

**Table 1:** Results of HWCVD growth with pure silane at 380 ^º^C
substrate temperature and at 2100 ^º^C graphite filament temperature
with 3.5cm spacing.

# Minority Carrier Lifetimes of HWCVD Films 

The minority carrier lifetimes of 1.5--15 μm thick films grown at
300^ °^C on Si(100) and SNSPE templates were determined through
resonant-coupled photoconductive decay (RCPCD) measurements\[8\].

![](media/image5.png){width="3.7368055555555557in" height="3.0125in"}

**Figure 3:** LLI and HLI minority carrier lifetimes of HWCVD films on
Si(100) as measured by RCPCD. The dashed and dotted lines represent the
LLI and HLI lifetimes, respectively, of the bulk Si(100) substrate.

These films began with epitaxial growth which broke down to
microcrystalline growth at thicknesses between 1 -- 2 μm. Although the
microstructure of these films is mostly microcrystalline, the lifetimes
are microseconds long. The lifetimes for films on Si(100) range from 5.7
to 14.8 μs while those for films on SNSPE templates range from 5.9 to
19.3 μs under low level injection conditions, as summarized in Figures 3
and 4. Residual nickel present in the SNSPE templates does not
significantly affect the lifetime of films grown on SNSPE templates,
making the growth of epitaxial layers by HWCVD on SNSPE templates a
viable strategy for the fabrication of thin film photovoltaics.

![](media/image6.png){width="4.361805555555556in"
height="3.6069444444444443in"}

**Figure 4:** LLI and HLI minority carrier lifetimes on SNSPE templates.

**Hot Wire CVD -- A simple model**

The epitaxial growth and growth breakdown trends described above are
consistent with a simple model correlating epitaxial growth breakdown
with surface oxidation. Starting with an initial hydrogen surface
coverage dependent only on the substrate temperature determined from TPD
data\[9\], the model determines the steady-state surface hydrogen
coverage by balancing thermal desorption of surface hydrogen with
adsorption and abstraction of surface hydrogen by atomic hydrogen
produced by the hot wire. Oxygen atom can be incorporated into the film
at any empty sites. We used the model to determine the amount of oxygen
deposited during the growth of the first monolayer of silicon for a
given growth temperature as a function of dilution ratio R
(R=H~2~/SiH~4~) at constant pressure, assuming that all silicon atoms
incident on the substrate contribute to growth.\[5\]

Figure 5 shows that the maximum silicon to oxygen ratio decreases with
temperature for between 311°C and 520°C at a hydrogen dilution ratio of
50:1. This may explain the decrease in epitaxial thickness with
temperature (Fig. 1).

![](media/image7.wmf)

**Figure 5**: Silicon to oxygen ratio in the first monolayer of growth
as a function of substrate temperature for dilute silane growth at 50:1
hydrogen dilution and epitaxial thickness.

The dependence of the epitaxial thickness on the silicon to oxygen ratio
is difficult to quantify. However, it is known that, during MBE crystal
growth, impurities at the growing interface can lead to surface
roughening and subsequent epitaxial breakdown through the formation of
voids which may lead to twinning and surface facets\[10\]. For our
dilute silane experiments, a decrease in the maximum silicon to oxygen
deposition ratio with temperature, as predicted by the model, may
explain the observed decrease in epitaxial thickness with temperature.

Figure 6 may also give some qualitative insight into the
amorphous/protocrystalline peak at intermediate dilution for growth with
pure silane. In this simulation the deposition pressure is changed along
with the dilution ratio in order to match the deposition conditions in
Table 1. The Si/O ratio is highest at the two extremes of the deposition
conditions: low dilution ratio, low pressure; and high dilution ratio,
high pressure. The high Si/O ratio correlates with a higher degree of
crystallinity. A low Si/O ratio leads to the amorphous and ultimately
protocrystalline film. Through these experiments and simulations we are
working toward determining the optimal deposition parameters for
epitaxial growth with polycrystalline breakdown.

![](media/image8.wmf)

**Figure 6**: Silicon to oxygen ratio in the first monolayer of growth
as a function of R for pure silane growth using the deposition
conditions in Table 1 along with the Raman crystalline %.

**SUMMARY OF RESEARCH UNDER SUBCONTRACT**

Under the Thin Film Partnership Program subcontract, we explored the
phase space of HWCVD growth with dilute and pure silane at various
substrate temperatures and dilution ratios. The complex interaction
between deposition parameters can be qualitatively explained by an
oxidation model. Twinned epitaxial growth of Si to thicknesses over 1 µm
is observed with polycrystalline breakdown under dilute conditions.

The minority carrier lifetimes of nearly-intrinsic
epitaxial/microcrystalline films grown on Si (100) by HWCVD range from
5.7 to 7.5 *µ*s. The lifetimes of films grown under the same conditions
on SNSPE templates range from 5.9 to 19.3 *µ*s, making them suitable for
incorporation into photovoltaic devices. In particular, residual nickel
from the SNSPE templates does not appear to be significantly detrimental
to the lifetime of films grown on these templates. If the mobilities in
these films are also high, it is possible that HWCVD epitaxy on
large-grained SNSPE templates could be a viable strategy for the
fabrication of thin-film photovoltaics.

**REFERENCES**

\[1\] C. M. Chen, \"Polycrystalline Silicon Thin Films for
Photovoltaics,\" California Institute of Technology, 2001.

\[2\] M. S. Mason, Christine E. Richardson and Harry A. Atwater, MRS
Proceedings A8.11, San Francisco, 2004.

\[3\] J. Thiesen, E. Iwaniczko, K.M. Jones, A. Mahan, and R. Crandall,
*Appl. Phys. Letters*, vol. 75, pp. 992, 1999.

\[4\] T. Watahiki, A. Yamasa, and M. Konagai, *J. Crys. Grwth*, vol.
209, pp. 335, 2000.

\[5\] Christine E. Richardson, M. S. Mason, and Harry A. Atwater *Thin
Solid Films*, to be published.

\[6\] R.A. Puglisi, H. Tanabe, C.M. Chen, and Harry A. Atwater, *Mat.
Sci. Eng. B*, vol. 73, pp. 212, 2000.

\[7\] M. S. Mason, \"Synthesis of Large-Grained Polycrystalline Silicon
by Hot-Wire Chemical Vapor Deposition for Thin Film Photovoltaic
Applications,\" California Institute of Technology, 2004.

\[8\] R. K. Ahrenkiel and S. Johnston, *Sol. Energy Mat. Sol. Cells*,
vol. 55, pp. 59, 1998.

\[9\] M.C. Flowers, N. B. H. Jonathon, Y. Liu, and A. Morris, *Journal
of Chemical Physics*, vol. 99, pp. 7038, 1993.

\[10\] D. J. Eaglesham, *Journal of Applied Physics*, vol. 77, pp. 3597,
1995.

# Publications Under Subcontract:

# \"*Comparison Of Nanorod And Planar Solar Cells: Basic Device Physics Principles And Application To Silicon\"* B.M. Kayes, N.S.Lewis, H.A. Atwater, presented at 2004 Spring Materials Research Society Spring Meeting, April 12-16, 2004, San Francisco CA. paper S7.2; in Mat. Res. Soc. Symp. Proc.

# *"A Phase Diagram For Morphology And Properties Of Low Temperature Depositedpolycrystalline Silicon Grown By Hot-Wire Chemical Vapor Deposition",* Christine E. Richardson, Maribeth S. Mason And Harry A. Atwater, presented at 2004 Spring Materials Research Society Spring Meeting, April 12-16, 2004, San Francisco CA. paper A1.5; in Mat. Res. Soc. Symp. Proc.

# \"*The Aging Of Tungsten Filaments And Its Effect On Wire Surface Kinetics In Hot-Wire Chemical Vapor Deposition*" Holt JK, Swiatek M, Goodwin DG, and H.A Atwater, J.Appl. Phys. 92 (8): 4803-4808 (2002).

1.  **"*Quantitative Modelling of Nucleation Kinetics in Experiments for
    poly-Si Growth on SiO~2~ by Hot Wire Chemical Vapor Deposition***"
    Mason MS, Holt JK, Atwater HA\
    Thin Solid Films **458** (1-2): 67-70 (2004).

2.  ***"Hot-Wire CVD-Grown Epitaxial Si Films on Si(100) Substrates and
    a Model of** **Epitaxial Breakdown"*****,** Christine E. Richardson,
    M.S. Mason and Harry A. Atwater, presented at the *3^rd^
    International Conference on Hot Wire Chemical Vapor Deposition
    (CAT-CVD), August 2004, Utrecht, Netherlands;* Thin Solid Films, to
    be published.

# *"Solid Phase Crystallization For Polycrystalline Si And Ge Films*", H.A. Atwater And Claudine Chen, Chapter 4 In Growth, Characterization And Electronic Applications Of Si-Based Thin Films, Isbn: 81-7736-108-2, Editor:Ralf B. Bergmann Research Signpost Publications, New Delhi India, 2002.

3.  "***Hot-wire chemical vapor deposition for grained polycrystalline
    epitaxial silicon growth on large-silicon templates"*** Mason MS,
    Chen CM, Atwater HA\
    Thin Solid Films **430** (1-2): 54-57 (2003)

Ph.D. Theses under Subcontract:

# "*Synthesis Of Large-Grained Polycrystalline Silicon By Hot-Wire Chemical Vapor Deposition For Thin Film Photovoltaic Applications*" Ph.D. Thesis By Maribeth Swiatek Mason, California Institute Of Technology, Pasadena, California, 2004

Awards/Honors Under Subcontract:

# Christine Richardson, \'Best Early Stage Researcher\'s Talk\' At The 3^rd^ International Conference on Hot-Wire CVD (CAT-CVD), Utrecht, Netherlands, August 23^th^ , 2004.

1.  Harry A. Atwater, Howard Hughes Chair, Applied Physics and Materials
    Science, Caltech.

#### Personnel Associated with Subcontract

1.  Maribeth Swiatek, graduate student (currently staff member,
    Aerospace Corporation)

2.  Christine Richardson, graduate student

3.  Brendan M. Kayes, graduate student

4.  Harry A. Atwater, principal investigator

#### Collaborators under Subcontract

1.  Pronob Bhardhan, Corning, Inc.

2.  Richard Ahrenkiel, NREL

3.  Eric Schiff, Syracuse University

#### Unobligated Funds

None