NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Epitaxial Crystal Silicon 

Absorber Layers and Solar Cells 

Grown at 1.8 Microns per Minute

Preprint

David C. Bobela, Charles W. Teplin, 

David L. Young, Howard M. Branz, 

and Paul Stradins

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50708

July 2011
NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC 

(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US 

Government and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes.

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/help/ordermethods.aspx

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
1 

EPITAXIAL CRYSTAL SILICON ABSORBER LAYERS AND SOLAR CELLS GROWN 

AT 1.8 MICRONS PER MINUTE

David C. Bobela, Charles W. Teplin, David L. Young, Howard M. Branz, and Paul Stradins

National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO, 80401, U.S.A

ABSTRACT

We have grown device-quality epitaxial silicon thin films at 

growth rates up to 1.8 μm/min, using hot-wire chemical 

vapor deposition from silane at substrate temperatures 

below 750°C. At these rates, which are more than 30 

times faster than those used by the amorphous and 

nanocrystalline Si industry, capital costs for large-scale 

solar cell production would be dramatically reduced, even 

for cell absorber layers up to 10 μm thick. We achieved 

high growth rates by optimizing the three key parameters: 

silane flow, depletion, and filament geometry, based on 

our model developed earlier. Hydrogen coverage of the 

filament surface likely limits silane decomposition and 

growth rate at high system pressures. No considerable 

deterioration in PV device performance is observed when 

grown at high rate, provided that the epitaxial growth is 

initiated at low rate. A simple mesa device structure 

(wafer/epi Si/a-Si(i)/a-Si:H(p)/ITO) with a 2.3-μm epitaxial 

silicon absorber layer was grown at 700 nm/min. The 

finished device had an open-circuit voltage of 0.424 V 

without hydrogenation treatment.

INTRODUCTION AND THE GROWTH RATE THEORY

Due to the potential to rapidly deposit high quality film 

silicon on inexpensive substrates coated with crystalline 

seed layers [1], hot wire chemical vapor deposition 

(HWCVD) of silicon shows great promise for costeffective, large-scale production of film silicon photovoltaic 

devices by silicon epitaxy. Inexpensive substrates such as 

display glass or metallic foils [2-4], generally require 

absorber layer growth temperatures below 750°C. 

HWCVD epitaxy satisfies this requirement and thus can 

provide a means to circumvent the expensive processing 

steps used in wafer silicon technologies, where nearly 

one-half of module cost comes from the wafer itself.

To keep the capital costs of HWCVD epitaxy low, 

deposition rates must be high (of order micron/minute), 

without sacrificing the epitaxial silicon quality. High growth 

rates relax the requirements for light trapping by growing a 

thicker layer quickly, and can also allow creation of 

micron-scale surface texture by sacrificing part of the fastgrown, inexpensive absorber layer to etching.

Achieving fast epitaxial growth rates without compromising 

the layer quality can be challenging due to possibility of 

powder formation, higher defect concentration in the films, 

etc. Similarly, increasing the HWCVD growth rate itself 

can be done in various semi-empirical ways far from 

optimum conditions. Generally, the higher the silane 

molecular flux to the filament, the higher the growth rate.

This flux, being proportional to the silane concentration in 

gas phase can be increased by the system pressure. 

However, the depletion of silane at low flow typically 

results in high hydrogen dilution and relatively low growth 

rate. On the other hand, increasing the silane flow leads to 

higher growth rate but poor silane utilization. In the 

present work, we examine and experimentally apply the 

optimized growth parameters (silane flow, system 

pressure, filament geometry) that lead to high epitaxial 

HWCVD growth rate, and reveal the rate-limiting factors. 

We base our approach on our earlier model for silicon 

epitaxy growth rate (GR) by Martin et al. [5] which 

quantitatively predicts GR values for wide range of silane 

flows into the HWCVD reactor, at moderate reactor 

pressures, and examine the limitaions that occur at higher 

pressures. The incoming silane decomposes on a hot W 

wire (filament) mostly into silicon atoms and hydrogen 

molecules, with relatively small and pressure-dependent 

fraction of atomic H [6]. The Si atoms then react with 

surrounding silane molecules to produce other simple 

radicals such as Si2H2, as well as higher silanes [6]. As 

long as the substrate temperature is above 650 o

C, most 

radicals (both Si Si2H2) contribute to high quality epitaxial 

growth [5].

The deposition chemistry starts at the filament, and its 

geometry is critical for achieving high GR. Its surface area, 

Afil\, determines the rate of useful radical production, while 

the radical flux pattern away from it determines how many 

radicals impinge on the substrate. Under our typical 

HWCVD epitaxy conditions, the filament decomposes 

about 30% of the impinging silane, each outgoing Si atom 

reacts with SiH4 to create Si2H2, all Si2H2 radicals travel to 

the substrate, and then, with the probability, s ≈1, stick to it 

[5]. Martin et al. showed that the growth rate Rrad due to 

the radical flux from the filament is a product of silane flow, 

depletion, radical sticking coefficient, and a factor that 

accounts for filament geometry:

Rrad = G 1

dLfil

 

 

  

 

 

  × f × D pAfilα

f

 

  

 

  × s (1) 

where the units of Rrad are nm/minute. Here, G (nm/cm3) is 

a geometrical factor that depends on the length of the 

filament Lfil, and on filament-substrate distance d because 

of radical conservation in the cylindrical flux pattern 

emanating from the wire filament [5]. Incoming silane flow 

is denoted by f. The silane depletion D defined by the 

relative decrease in the silane molecule concentration with 

filament on and off, (noff – non)/noff and is bound by 0 ≤ D ≤ 

1, where 1 represents the case where all silane is 

decomposed. Experimentally, this depletion of silane, D, is 

a function of f, system pressure p, and filament surface 
2 

area Afil. Here, f and p control the quantity of new silane 

entering the chamber, and the dominant radical species 

impinging the substrate, respectively. The silane 

decomposition probability α is about 0.3 at low pressure 

[6]. Finally, s is the radical sticking probability to the 

growing surface, which for our epitaxial growth 

temperatures >650 C is constant and close to unity [5]. 

Thus, to increase growth rates one should: 1) decrease 

substrate-filament distance d and thereby increase the 

radical flux delivered to the substrate, 2) increase both 

silane flow f and system pressure p while maintaining a 

high depletion D of silane, 3) increase the filament area Afil

to increase the number of silane decomposition sites and 

thus depletion D. 

In this work we detail how the total GR increases by 

implementing the high rate conditions 1-3. Our 

measurements show that the predicted 1/d relationship for 

a cylindrical flux pattern holds except at very high 

pressures and small d, where the approximations break 

down. Moreover, at high p, the experimentally measured 

depletion decreases from the predicted value. These 

observations suggest that the reactions at the filament and 

gas phase chemistry very near the filament ultimately limit 

the GR, though we have not yet reached that limit. The 

absorber layer quality produced at high GR is comparable 

to absorber layers grown at low rates; our mesa structure 

devices with absorber layers deposited at 700 nm/min 

perform similarly to control devices with absorber layers 

grown at 50 nm/min. 

EXPERIMENT 

Epitaxial silicon films were grown on (100) oriented silicon 

wafers in HWCVD reactor. Before loading into the 

chamber, the surface oxide was stripped by bathing the 

wafer in dilute 4% HF for 30 seconds. Baseline pressures 

with filament on were below 10-6 Torr. After heating the 

wafer to about 750 o

C, silane gas was flown into the 

reactor at a fixed flow f. We controlled the chamber 

pressure p over 2 – 100 mTorr range using a throttle valve 

on the turbo-pump. Silane gas was decomposed using a 

0.5 mm diameter tungsten wire, wound 11 times 

(“standard filament”) or 24 times (“compact filament”) to 

make a coil with inner diameter of about 3 mm. We used 

16 A current to heat the filament. The filament was 

attached to a translation arm allowing its distance to the 

substrate d to vary from 1 to 5 cm. The GR and material 

quality were determined by in-situ spectroscopic 

ellipsometry (SE). We measured D using a residual gas 

analyzer (RGA) attached to the chamber, by comparing 

the silane partial pressure during deposition to the silane 

partial pressure present with the filaments turned off. 

To demonstrate the cell absorber layer quality in a 

high-growth rate epitaxial solar cell, we deposited a 

2.3-μm epitaxial silicon absorber layer at 700 nm/min on a 

RCA-cleaned, (100) oriented, n+

 (Si:As) silicon wafer. A 

mesa-isolated silicon heterojunction structure (wafer/epiSi/a-Si(i)/a-Si:H(p)/ITO) was processed into the solar cell 

by wet and dry etching techniques [7]. We compared its 

performance against a control device with absorber layer 

deposited at 150 nm/min using our standard conditions of f

= 20 sccm, p = 10 mTorr, d = 5 cm, and standard filament. 

RESULTS AND DISCUSSION 

Figure 1 demonstrates the total GR we achieved after 

implementing each of the three considerations detailed 

below Eq. 1. We first measured s by plotting the 

dependence of Rrad as function of product f*D following the 

approach of [5] and found that at these substrate 

temperatures, s ≈ 1 for all values of f and p. Deposition 

conditions for the leftmost data point of Fig. 1 represent 

our standard operating conditions (standard filament, 10 

mTorr system pressure, 20 sccm silane flow). Here, the 

measured GRs of ~ 0.170 μm/min which is far below our 

estimated threshold GR of ~ 1 μm/min where module 

capital costs become largely GR independent for film c-Si 

PV. Under these standard conditions, silane depletion is 

moderate, (D = 0.78) indicating that about 22% of the 

silane leaves the system unreacted and therefore wasted. 

We now apply our three factors that lead to growth rate 

increse. 

Figure 1. Growth rate for several experimental 

conditions. High rates are achieved left to right, by 

decreasing substrate-to-filament distance, increasing 

silane flow and pressure, and increasing filament 

area. 

1) When we decrease d from 5 to 1 cm, while keeping all 

other parameters constant, GR more than doubles. The 

roughly 20 o

C increase in substrate temperature due to the 

close proximity of the hot filament may slightly change s

and the (small) contribution of thermal epitaxy, but this 

change is too small to account for the nearly 40% smaller 

GR as compared to factor 5 increase expected from just 

the radical flux geometry.
3 

2) When p and f are both doubled from 10 mTorr to 20 

mTorr and 20 sccm to 40 sccm (keeping d = 1cm) we 

achieve a GR of about 1.1 μm/min. Depletion D remained 

approximately unchanged, since the combined effects of 

doubling both f and p approximately cancel each other. 

We tried increasing D by increasing p further to 60 mTorr, 

but found that D actually decreased (Fig. 2), along with 

GR indicating a departure from the simple model 

predictions of Eq. 1. 

3) The final increases in GR to 1.8 μm/min and above the 

target threshold of ~1 μm/min, were obtained by 

implementing the compact filament (upper right in Fig. 1). 

Also, a modest gain in D was obtained: the near doubling 

of Afil increased D to 0.85, indicating that the near doubling 

of GR was most likely due to the additional filament length. 

Although we have demonstrated GR above the ~1 μm/min 

threshold, further increases in GR should be possible if we 

more deeply understand the physics limiting growth at 

small d and high p. In Fig. 2 the measured GR is plotted 

as function of d. 

Figure 2. Growth rate versus substrate-to-filament 

distance for 40 sccm, 20 mTorr, and using the 

compact filament. Dotted line is the predicted 1/d 

dependence. The data deviate from the expected 1/d 

dependence (dotted line) for d < 2 cm. 

At d > 2 cm, GR decreases according to 1/d dependence 

expected for the cylindrical flux pattern emanating from the 

filament. [2]. Below d = 2 cm, we observe a departure from 

the 1/d dependence. This is likely due to the reduction in 

the number of collisions between the Si atoms from the 

filament and SiH4. Several collisions are needed to initiate 

reaction between them to produce Si2H2. As the number of 

collisions drop with decreasing d, growth is likely due to Si 

atoms (1 Si into the film per SiH4 decomposition on the 

filament) instead of Si2H2 (2 Si per decomposition), 

resulting in a 2X decrease in the growth rate. As 

mentioned above, at lower 10 mTorr pressure the 

decrease in d from 5 to 1cm lead only to doubling of the 

GR. At higher pressures, this effect becomes less 

pronounced as the number of collisions even at small d is 

relatively large. However, at still higher pressures above 

40 mTorr (not shown) we observe an increase in the 

disilane partial pressure with the RGA, find silicon dust in 

the chamber after growth, and achieve no further increase 

in the growth rate. The optimal high growth rate conditions 

are therefore at small d and at pressures below the p at 

which higher silanes and particle form. 

In conjunction with the formation of higher order silane 

radicals at high p, we also observe a decrease in the 

filament’s ability to decompose silane. In Fig. 3, we plot D

as p is increased while maintaining 20 sccm the flow at 20 

sccm. At p < 40 mTorr, the data fit to the predicted value 

(dotted line, Ref. 5), but then deviate from the model 

prediction, to smaller values at higher pressures. This 

observation strongly indicates that a change occurs in the 

gas-filament interactions. Notably, the H2 partial pressure 

is nearly 10X greater than the silane partial pressure for p 

> 40 mTorr (depletion around 0.8) and thus most of the 

high system pressure is due to molecular hydrogen. These 

hydrogens interact with filament surface (kept near 2100 

o

C), dissociate and temporarily bind to it [6] until 

recombining at with other bound H and leaving as 

hydrogen molecule. The large concentration of H bound to 

W filament surface might partially saturate the dissociation 

sites, which could affect the dissociation of incoming 

silane [8]. We are currently testing this hypothesis. 

Figure 3. Depletion dependence on chamber pressure. 

The data deviate from the predicted depletion (dotted 

line) from pressure > 40 mTorr. 

As a preliminary test of epitaxial Si quality, we deposited 

absorber layers for solar cells using the high rate 

conditions. The layer quality is highly sensitive to the initial 

moments of growth. When growth is initiated at high rates, 

the two-peak ε2 SE signal at photon energy range 3 – 5eV 

shows a reduction from the strong wafer silicon peaks and 

never subsequently recovers to the wafer value; there are 
4 

also strong interference fringes in the near infrared region. 

This indicates polycrystalline silicon growth (confirmed by 

XRD) and a poor silicon-epitaxial silicon interface. We 

therefore adopted a slow-to-fast growth technique, 

whereby we initialize epitaxial growth at standard 

conditions (10 mTorr, 20 sccm, d = 5 cm) to obtain GR ~ 

150 nm/min. The SE signal remains close to that of the Si 

wafer, suggesting good-quality starting epitaxial layer near 

the wafer interface. We then switch to high GR (for 

example 700 nm/min) at conditions of 20 mTorr, 40 sccm, 

and d = 2 cm.

Our preliminary results suggest the high rate absorber 

layer is of comparable quality to those layers grown under 

standard conditions. Table 1 shows the solar cell 

performance for devices with standard and high GR 

absorber layers, without hydrogen passivation. Here the 

absorber layers are epitaxial Si grown on heavily-doped 

“dead” n-type c-Si wafers.

Table 1. Comparison of standard and high rate solar 

cell performance.

Solar Cell Growth Rate 

(nm/min)

Voc

(mV)

FF Efficiency

Standard ~150 470 63 3.1

High Rate ~700 420 61.7 2.2

The high rate device has 50 mV less open circuit voltage, 

but has nearly the same fill factor as the standard device. 

These preliminary results give us confidence that high GR, 

high quality material is possible with some optimization of 

the growth conditions. These growth rates exceed by more 

than 10X those used in current a-Si:H and nc-Si:H 

manufacturing. Our highest epitaxial growth rates are 

nearly three times higher.

CONCLUSIONS

We have theoretically and experimentally examined the 

conditions necessary to obtain the high growth rate of the 

HWCVD epitaxial film silicon above ~ 1 micron/minute. By 

decreasing filament-substrate distance, maximizing silane 

flow and depletion product, and increasing the filament 

area we have achieved growth rate of 1.8 micron/minute. 

The device performance for high growth rate is 

comparable to that grown at low rate provided that the 

growth is initiated at low rate. The increase in growth rate 

at high system pressure is likely limited by high hydrogen 

concentration near and at the filament.

ACKNOWLEDGEMENTS

This work was supported by the U.S. DOE Solar Energy 

Technology Program under Contract No. DE-AC36-

08GO28308. We thank Lorenzo Roybal, our late colleague 

Eugene Iwaniczko, and Carolyn Beall for assistance with 

device fabrication, and Maxim Shub for technical support.

REFERENCES

[1] H.M. Branz, C. W. Teplin, M. J. Romero, I. T. Martin, Q. 

Wang, K. Alberi, D. L. Young, and P. Stradins, Thin 

Solid Films (2011), doi:10.1016/j.tsf.2011.01.335

[2] C. E. Richardson, K. Langeland, and H. A. Atwater, 

Thin Solid Films, 516 (2008).

[3] R. B. Bergmann and J. H. Werner, Thin Solid Films 

403, 162 (2002). 

[4] C. W. Teplin, D. S. Ginley, and H. M. Branz, J. NonCryst. Solids 352, 984 (2006). 

[5] I. T. Martin, C. W. Teplin, J. R. Doyle, H. M. Branz, and 

P. Stradins. J. Appl. Phys. 107, 054906 (2010).

[6] W. Zheng, A. Gallagher, Thin Solid Films 516 (2008) 

929.

[7] K. Alberi et al., Appl. Phys. Lett. 96, 073502 (2010).

[8] J.R. Doyle, Y. Xu, R. Reedy, H.M. Branz, A.H. Mahan, 

Thin Solid Films 516 (2008) 526.