CALIFORNIA INSTITUTE OF TECHNOLOGY

Thomas J. Watson Laboratory of Applied Physics

Pasadena, California 91125

Harry A. Atwater haa@daedalus.caltech.edu

Howard Hughes Professor and Professor of tel: (626) 395-2197

Applied Physics and Materials Science FAX: (626) 844-9320

Dr. Bolko von Roedern

National Center for Photovoltaics

MS 3212

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401-3393

May 31, 2004

Dear Bolko,

I am writing to brief you on our Q4, 2003 activities associated with our
research contract "Thin Film Silicon Cells on Low-Cost Substrates"
(DE-AC36-99GO10337) under the Thin Film Photovoltaic Partnership
Program. I am sorry that this report is overdue.

##### During the fourth quarter, the major technical results consisted of 

-   Observation of **[long minority carrier lifetimes]{.underline}** in
    1-15 micron thick Si films grown by HWCVD on n^+^ large-grained
    polysilicon epitaxial templates on glass and also on Si (001).
    Minority carrier lifetimes ranging from 5-20 microseconds were
    measured using rf photoconductive decay measurement techniques,
    suggesting that minority carrier diffusion lengths in excess of the
    pn cell base thickness can be achieved for HWCVD polysilicon films
    up to 15 microns thick. These results also suggest that **[hydrogen
    passivation of grain boundaries may be very effective in HWCVD
    poly-Si*.*]{.underline}**

# 

# 

# **Minority Carrier Lifetime Measurements in HWCVD Poly-Si**

We have determined the minority carrier lifetimes of nearly-intrinsic
poly-Si films 1.5-15 *µ*m thick grown by HWCVD at 300 C on Si(100) and
SNSPE templates via resonant-coupled photoconductive decay (RCPCD)
measurements. Although the microstructure of these films is mostly
polycrystalline, the lifetimes for films on Si (100) range from 5.7 to
7.5 *µ*s while those for films on SNSPE templates range from 5.9 to 19.3
*µ*s. Since the surface recombination velocity is unknown for these
films, the lifetime measured by RCPCD is a lower bound on the bulk
lifetime in the poly-Si films. 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/image1.png){width="3.647222222222222in"
height="2.6777777777777776in"}

**Growth and Lifetime Characterization Experiments**

HWCVD films of 1.5, 3.5, 11.5 and 15 *µ*m thicknesses were grown at 300
C on Si(100) and SNSPE templates. Growth of a given film thickness on
Si(100) and SNSPE templates was performed in the same experiment. The
Si(100) substrates were p-type with resistivity 100 Ω·cm; the SNSPE
templates were degenerately-doped n-type (*n* = 10^20^cm^-3^). Films
were nearly intrinsic (*p*=10^12^cm^-3^) as determined by spreading
resistance analysis. Since HWCVD epitaxial growth at 300 C persists to a
thickness of between 1 and 2 *µ*m, the microstructure of thick films was
primarily polycrystalline for films \> 1.5 μm thickness. A lower wire
temperature of 1750 C was used to avoid W incorporation into the films.
SIMS analysis indicated bulk W levels of *\<*1×10^17^ cm^-3^ in films
grown at 1800 C and *\<*1×10^16^ cm^-3^ in films grown at 1750 C. The
minority carrier lifetimes of these films were measured by RCPCD, using
a 532-nm laser as the excitation source. The lowest possible excitation
power was used and was too low to be measured; an upper limit on the
power is 1 *µ*W that corresponds to approximately 1/10 mJ per 10 ms
pulse in a 5 mm spot size. Based on these estimates we can estimate an
upper bound on ∆*p* of approximately 1×10^15^ cm^-2^. No effort was made
to passivate the film surfaces. The absorption coefficient of Si at 532
nm is approximately 9×10^3^ cm^-1^, which gives a penetration depth of
approximately 1.1 *µ*m, so we can be confident that carrier generation
occurs in the films and not in the Si (100) substrate. The lifetime of a
Si (100) wafer with no film was also measured for comparison. Because
the SNSPE templates are only 100 nm thick, the lifetime of the template
layer alone cannot be reliably measured by RCPCD at 532 nm.
![](media/image2.png){width="3.9118055555555555in"
height="2.6791666666666667in"}

![](media/image3.png){width="3.9097222222222223in"
height="3.165277777777778in"}

The RCPCD voltage signal as a function of time for each film can be
found in Figures 2.1 and 2.2. One or more exponential decays were fit to
the curves from which the minority carrier lifetimes for each sample
under HLI and LLI conditions were derived. The measured minority carrier
lifetime is actually an effective lifetime; because the surface
recombination velocity is not known, this effective lifetime is a lower
bound on the true minority carrier lifetime in the material. The
curvature of the data indicates the dominant type of recombination
center in each film. In Figure 2.1(a), the RCPCD data for a 1.5 *µ*m
HWCVD film on Si is typical of that for bulk Si, with the HLI lifetime
greater than the LLI lifetime. This is consistent with an epitaxial film
on Si(100). Under illumination, the charged midgap recombination centers
fill up quickly and the rest of the excess carriers fill neutral
centers, so that under HLI conditions the dominant process is hole
capture, which is slower than electron capture. The LLI lifetime then
reflects the release of carriers from the midgap recombination centers
in the material. In 3.5 *µ*m and 11.5 *µ*m-thick films on Si(100)
(Figure 2.1(b) and (c)), the HLI lifetime is lower than the LLI
lifetime. Such a decay pattern is indicative of a shallow recombination
center. Since the microstructure of these thicker films is mostly
polycrystalline, this suggests that ***[hydrogen passivation of grain
boundaries may be very effective in HWCVD]{.underline}***. For a 15-*µ*m
thick film on Si(100) (Figure 2.1(d)), a single decay is observed,
corresponding to the LLI lifetime. The decay patterns of HWCVD films on
SNSPE templates, seen in Figure 2.2, have a fast component at the
beginning of the illumination. This is likely due to a surface roughness
effect; for samples with very rough surfaces, the surface recombination
velocity component of the minority carrier lifetime is dominant in the
early stages of illumination before the carriers diffuse into the bulk.
This feature is less pronounced for the thicker films, which may
indicate a decrease in roughness with film thickness. The decay for the
1.5 *µ*m-thick film (Figure 2.2(a)) indicates a single lifetime, while
the decays for 3.5 *µ*m and 11.5 *µ*m-thick films (Figure 2.2(b) and
(c)) are characteristic of a deep-level recombination center. This
center may arise from the diffusion of residual nickel from the SNSPE
templates into the films. The 15 *µ*m-thick film on SNSPE templates is
also characterized by a single lifetime. The factor of two discrepancy
in lifetime values may be due to non-uniformity in the films, i.e. it
may depend on whether a "good" or "bad" region of each film was
illuminated. Although nickel is a known lifetime killer even in small
concentrations, the lifetimes of films on SNSPE templates are comparable
to the lifetimes of films on
Si(100).![](media/image4.png){width="3.8875in"
height="3.1979166666666665in"}

The minority carrier results are summarized in Figures 2.3 and 2.4 for
films on Si(100) and SNSPE templates, respectively. Under LLI
conditions, the minority carrier lifetimes for films on Si(100) range
from 5.7 to 7.5 *µ*s, and the minority carrier lifetimes for films on
SNSPE templates range from 5.9 to 19.3 *µ*s. Polycrystalline films grown
by HWCVD have been used in the fabrication of 1.5 *µ*m-thick thin-film
transistors with channel mobilities of 4.7 cm2/V·s on glass substrates.
Using the Einstein relation, we can estimate that, if the mobilities in
our films were comparable, the minority carrier diffusion coeffcient
would be 0.1175 cm2/s. From this value and the minority carrier lifetime
of 7 *µ*s measured by RCPCD in a 1.5 *µ*-thick film on Si(100), we
obtained a value for the minority carrier diffusion length of
approximately 9 *µ*m, which is comparable to the thicknesses of the
active layers for thin-film photovoltaics (1-30 *µ*m). The minority
carrier lifetimes of films on SNSPE templates are comparable, making it
possible that the growth of epitaxial films by HWCVD on large-grained
SNSPE templates is a viable strategy for the fabrication of thin-film
photovoltaics. If the mobilities in these films are also high, it is
possible that HWCVD epitaxy on large-grained SNSPE templates will be a
viable strategy for the fabrication of thin-film photovoltaics.

#### Publications under Subcontract

1.  Maribeth Mason, Ph.D. Thesis, Caltech 2004.

#### Personnel Associated with Subcontract

1.  Maribeth Swiatek, graduate student

2.  Christine Richardson, graduate student

3.  Harry A. Atwater, principal investigator

> Sincerely yours,
>
> ![](media/image5.jpeg){width="2.348611111111111in"
> height="0.5833333333333334in"}