.

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337 

H.R. Moutinho, C.-S. Jiang, Y. Xu, B. To, 

K.M. Jones, C.W. Teplin, and M.M. Al-Jassim 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37345 

Mechanisms of Growth of 

Nanocrystalline Silicon 

Deposited by Hot-Wire 

Chemical Vapor Deposition 
NOTICE 

The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a 

contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US 

Government and MRI 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/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste 
MECHANISMS OF GROWTH OF NANOCRYSTALLINE SILICON DEPOSITED 

BY HOT-WIRE CHEMICAL VAPOR DEPOSITION 

H.R. Moutinho, C.-S. Jiang, Y. Xu, B. To, K.M. Jones, C.W. Teplin and M.M. Al-Jassim 

National Renewable Energy Laboratory, Golden, CO 80401 

ABSTRACT

We have studied the growth of silicon thin films by

hot-wire chemical vapor deposition under different

conditions of filament temperature (Tf) and hydrogen

dilution ratio (R). We found that these two parameters 

have a similar effect on the properties of the deposited

films and show how they interact to control the growth

dynamics. For relatively low values of Tf and/or R, the

films are amorphous. An increase in the value of these

parameters results in the appearance of a new phase,

characterized by heavily faulted (220)-oriented columnar

grains; for even higher values, a randomly oriented

nanocrystalline phase appears. In general, there is more 

than one phase present in the film, and the microstructure

varies as we go from the bottom to the top of the film. 

Although Tf and R have similar effects on the physical

properties, they affect the deposition rate in a different

way.

INTRODUCTION 

Nanocrystalline silicon (nc-Si) is cheaper to produce

than crystalline silicon (c-Si), and thinner layers can be

used to produce solar cells. In addition, it is a stable

material and does not show degradation with light soaking

[1], as observed with amorphous silicon (a-Si). Moreover,

due to the differences in their bandgaps, it can be used in

tandem solar cells with a-Si [2]. For this reason, the

research on nc-Si has been increasing over the last

several years. A convenient method to grow nc-Si is by

using hot-wire chemical vapor deposition (HWCVD) [3].

This technique is able to produce high-quality films and 

provides higher deposition rates than plasma-enhanced

chemical vapor deposition (PECVD) because of the 

efficient decomposition of silane (SiH4) into atomic species

at the filament [4]. Nevertheless, at present, much work is

still needed to correlate the several growth parameters

with the quality of the produced films, and to create a

unified model on the mechanisms of film growth. 

This work complements previous research [5], in

which we studied the effects of R on the structural 

properties of nc-Si films. Now we correlate the evolution

of the microstructure, morphology, and crystalline

properties of Si films with changes in the temperature of 

the filament used for film growth. We also study the

interdependence of Tf with R, and explain how the film

growth, and consequent film structure, is determined by

these parameters. 

EXPERIMENTAL PROCEDURE 

The films were deposited using a double W filament,

with the following values for the current in each filament: 

12 A (≈1780°C), 13 A (≈1880°C), 14 A (≈1980°C) and 15

A (≈2080°C). For currents below 12 A there was no

deposition of film on the substrate, probably a result of the

formation of silicides on the filament surface at these

relatively low temperatures, which prevented the

decomposition of silane [6]. We also produced films with

the following values of R: 14, 25, and 50. These values

are the ratio between the H2 and SiH4 flow rates. In our 

experiment, we keep the H2 flow rate at 252 sccm, and 

vary the flow of silane. The implication for this procedure

is that, as we vary the silane flow rate, the relative number 

of silicon and hydrogen atoms produced at the filament 

changes. We used three kinds of substrates: glass, c-Si, 

and SnO2, for comparison purposes, and because certain

analyses require a specific type of substrate. 

The crystalline structure of the films was investigated 

with X-ray diffraction (XRD), in a Scintag X1 system, using

the Bragg-Brentano configuration. The relative amounts

of amorphous and crystalline phases were investigated

using Raman spectroscopy, with a single-grating Spex

270M spectrometer, using both 532 (green) and 633 nm

(red) lasers to obtain information from different depths of

the material. We studied the microstructure of the films 

with transmission electron microscopy (TEM) in a Philips 

CM30 TEM. To study the surface topography, we used

atomic force microscopy (AFM), with Si tips, in a Digital

Instruments Dimension 3100 scanning probe microscope

(SPM). Finally, to investigate the electrical properties of

the films, we used conductive-AFM (C-AFM), also in a

Dimension 3100 SPM, using conductive diamond tips. 

RESULTS 

We did not notice any major differences in the results 

of AFM and XRD analyses performed in films deposited

on different substrates but with the same deposition

parameters. For the other analyses, we used films

deposited on specific substrates: Si substrate, for Raman

and TEM, and conductive SnO2, for C-AFM.

Deposition Rate

High deposition rate is a feature that distinguishes

HWCVD from other techniques, such as PECVD, in the

growth of Si thin films. The reason for this is the efficient

dissociation of the silane and hydrogen molecules on the

1
heated filament. For instance, it has been reported that,

for filament temperatures above 1500°C, every silane

molecule that reacts with the filament is completely

dissociated [4,7]. Filament temperature and dilution ratio

are probably the two most important parameters 

controlling the deposition rate. The influence of these

parameters is shown in Fig. 1. It is clear that their effect 

on deposition rate is opposite, which is explained by the

fact that an increase in Tf increases the amount of atomic

silicon available for film growth, consequently increasing

the deposition rate. On the contrary, an increase in the

dilution ratio, obtained by decreasing the flow rate of

silane, results in fewer Si atoms and, consequently, in a

decrease of the deposition rate. As we will see below, an

independent increase in both Tf and R increases the

amount of the nanocrystalline phase in the film. This will

impose a compromise on the best value for R, but not on 

the value for Tf. An upper limit on the value of Tf may 

come from the fact that the nc-Si films grown at higher

temperatures (above 1930°C) have been observed to

have a porous microstructure, and minimal photoresponse

[8]. 

X-Ray Diffraction 

The diffraction peaks for all the samples

corresponded to the cubic structure of silicon (JCPDS card

number 27-1402). The only exception was a peak located

at 26.9°, observed for most samples. We have observed 

this peak in the XRD data presented by other authors [6,9]

and, to our knowledge, it has not been positively identified

yet. Because the results were the same for films

deposited in different substrates, including the relative

peak intensity, for simplicity we present the results for

films deposited on glass, because this substrate does not

produce peaks in the diffraction pattern. 

The XRD analysis showed that there is a close

relationship between filament temperature and dilution

ratio. Furthermore, an increase in each of these

parameters separately has the same effect on the films. 

In Fig. 2a, we show the evolution of the crystallinity with Tf

for a value of R equal to 14. A low value of Tf results in 

mostly amorphous films. As Tf increases, a new (220)-

oriented phase appears. For higher values of Tf, a 

randomly oriented phase appears. This phase has been

shown to be nanocrystalline [5]. The relative amount of

each phase also varies as Tf is increased, with films

deposited with higher filament temperatures being more

nanocrystalline. For the conditions used in this work, most 

films were composed of more than one phase. The

evolution of this process is also a function of R, as shown

in Fig. 2. It is clear that the effect of varying Tf is strongly

affected by the value of R. Similarly, the effect of varying

R depends on the value of Tf, as can be observed if we

compare curves from Figs. 2a and 2b with the same value

of Tf. Because of this interrelationship, there is not a

unique value of R or Tf separately that will guarantee that

the film has a given crystalline structure. 

Raman Spectroscopy

In Fig. 3a and 3b we show the Raman spectra of the

same samples analyzed by XRD (Fig. 2), using a 633 nm

laser beam. Raman provides a measure of the relative

 (a) 

 (b) 

Fig. 2. X-ray diffraction patterns from silicon thin films 

deposited on glass and grown with different filament 

temperatures. (a) R=14; (b) R=25. 

Fig. 1. Deposition rate as a function of filament

current (directly related to Tf) and dilution ratio. 

2
amounts of crystalline and amorphous phases in the films, 

but cannot distinguish between the different crystalline

phases. Crystalline material shows a sharp Raman peak

at 520 cm-1, while amorphous material has a broader peak

at lower wavenumber. The Raman data is consistent with

the XRD results in that both increasing Tf and increasing R

result in more crystalline films (Fig. 3). In Fig. 3b, we see

that R is large enough to produce mostly crystalline films,

even at the lowest filament temperature. Raman spectra

acquired with a 532 nm laser beam show a similar trend

as the data in Fig. 3, but the amorphous peaks are

generally smaller in comparison to the crystalline peaks.

Because the optical penetration depth in Si at 532 nm is

smaller than at 633 nm, this suggests that the films are

more crystalline near to the surface and more amorphous

at the film/substrate interface.

Transmission Electron Microscopy

Four phases were identified in the TEM analysis:

epitaxial Si, amorphous and nanocrystalline phases, and

(220)-oriented columnar grains. The relative amounts of

these phases depend on the values of Tf and R, and more

than one phase is present in most films. Because of

sample preparation issues, we could not precisely follow

the evolution of film microstructure with Tf and R.

Nevertheless, the TEM results fit the Raman and XRD

results. For Tf equal to 1880°C and R equal to 14, the film

is composed of columnar grains, forming close to the

interface with the substrate and growing all the way to the

film surface, surrounded by an amorphous phase. These 

grains are heavily faulted, and electron diffraction showed

that they are the (220) grains observed in XRD. For

higher values of Tf and R, the amount of the amorphous

phase decreases, and the films are mostly formed by

columnar grains surrounded by nc-Si. In the conditions

used in this work, even for the higher values of Tf and R,

the columnar grains are still present. Nevertheless, in

films grown previously, with R equal to 100 and Tf equal to

1880°C, we produced films with some epitaxial growth and

nanocrystalline phase only [5]. Figure 4 shows the

microstructure of a film with R equal to 14 and Tf equal to

2080°C. Few columnar grains are visible. Nevertheless,

for these grains to be visible, the sample has to be

positioned on the right diffraction condition; so not all the

columnar grains are visible at once. An electron diffraction

pattern of this film is seen in the inset of Fig. 4, where the

presence of both phases is clear. The rings are

characteristics of an area with small randomly oriented

grains, whereas the spots are typical of the oriented

material. 

(a) 

(b) 

Fig. 3. Raman spectra of silicon thin films grown at 

different filament temperatures. (a) R=14; (b) R=25. 

Atomic Force Microscopy

Due to the nature of the Si surface, even the

amorphous material looks to be composed by small

grains. The parameter that differentiates the amorphous

phase from the nanocrystalline phase is the roughness, 

which is about ten times higher for the nanocrystalline

phase. Also, the (220) columnar grains seemed to be

formed by small grains at the surface, which makes it 

difficult to isolate the nanocrystalline and the (220) phases

at the surface unambiguously. For R equal to 14 and Tf

equal to 1780°C, the surface of the film is formed by

elongated grains (the columnar grains observed in TEM), 

surrounded by a relatively smooth amorphous phase. As

we increased R or Tf, the amorphous phase disappeared

(in the Raman analysis, we observed that the material is

more crystalline at the surface), the number of elongated

grains increased, and a nanocrystalline structure

appeared (grain size varying mostly from 30 to 80 nm). 

For further increase in R, the number of elongated grains 

decreased and the amount of the nanocrystalline phase

increased. To a lesser extent, the same effect seemed to 

be present for further variation in Tf. 

Conductive-Atomic Force Microscopy

The analysis confirmed that the crystalline phase is

significantly more conductive than the amorphous phase.

Nevertheless, it was not possible to make a correlation 

between the current and the amounts of the

nanocrystalline and the (220)-oriented phases. The

technique proved to be especially useful for detecting the

(220) columnar grains from the amorphous phase. On the

AFM images, it is difficult to distinguish the (220) grains 

3
from grain-like structures that we believe to be amorphous

material. However, on the C-AFM image, the (220)-

oriented grains produce a higher current and are easily 

distinguishable. Due to lack of space, these results will be

presented in more detail in a future publication. 

DISCUSSION

By increasing the hydrogen dilution ratio in the

chamber, we increase the relative amount between atomic

hydrogen and silicon. This results in more etching of weak

bonds during film growth, resulting in more crystalline

films. Similarly, an increase in the filament temperature 

also produces more atomic hydrogen, increasing the

etching process. For this reason, a variation of Tf and R

produces similar results, as we have observed. Our 

results also demonstrate that the growth process of 

nanocrystalline Si is very complex, and it is very difficult to 

produce the material with a single phase, unless we use

high values of Tf and/or R. The growth of the columnar

grains for relatively low concentration of H indicates that

this phase grows faster than the nanocrystalline phase, 

but the bonds may be relatively weak. We believe that, for 

higher concentrations of atomic hydrogen, weak bonds in

the (220) phase are etched way, resulting in the

preferential growth of the nanocrystalline phase. 

CONCLUSIONS

We have shown that the growth of nanocrystalline

silicon films is complex, and, in general, results in a 

material with more than one phase. The filament

temperature and the hydrogen dilution ratio have a similar

effect on the properties of the films, and there is

interdependence between these two parameters, which is

related to the relative amounts of silicon and hydrogen

atoms available during film growth. For lower values of Tf

and R, the film is mostly amorphous, with the development

of a (220)-oriented columnar grain structure, and, 

subsequently, a nanocrystalline structure, as the values of

these parameters are increased. On the contrary, 

although an increase in R decreases the deposition rate,

an increase in Tf promotes a faster growth of the films. 

 

Fig. 4. Transmission electron microscopy of a Si film

grown with R=14 and Tf=2080°C. The inset shows an

electron diffraction pattern of the same film. 

ACKNOWLEDGES

We would like to thank Brent Nelson for valuable

discussions on the growth mechanisms of nanocrystalline

silicon. This work was supported by the U.S. Department

of Energy under Contract number DE-AC36-99GO10337. 

REFERENCES

[1] J.P. Kleider, C. Longeaud, R. Bruggemann, and F.

Houze, “Electronic and Topographic Properties of

Amorphous and Microcrystalline Silicon Thin Films,” Thin 

Solid Films 383, 2001, pp. 57-60. 

[2] Y. Hamakawa and H. Takakura, "Key Issues for the 

Efficiency Improvement of Silicon Basis Stacked Solar

Cells," Proc. Twenty-Eighth IEEE PVSC, 2000, pp. 766-

771. 

[3] M. Konagai, T. Tsushima, Y. Ide, K, Asakusa, T.

Jujisaki, M.K. Kim, Y. Wakita, and A. Yamada, "High Rate

Deposition of Silicon Thin Films by Hot Wire Cell Method

for Solar Cell Applications," Proc. Twenty-Eighth IEEE

PVSC, 2000, pp. 788-791. 

[4] R.E.I. Schropp, "Present Status of Hot Wire Chemical

Vapor Deposition Technology," Mat. Res. Soc. Symp.

Proc. 762, 2003, pp. A13.1.1-A13.1.12. 

[5] H.R. Moutinho, C.-S. Jiang, J. Perkins, Y. Xu, B.P.

Nelson, K.M. Jones, M.J. Romero, and M.M. Al-Jassim, 

"Effects of Dilution Ratio and Seed Layer on the

Crystallinity of Microcrystalline Silicon Thin Films 

Deposited by Hot-Wire Chemical Vapor Deposition," Thin

Solid Films 430, 2003, pp. 135-140. 

[6] C.H.M. van der Werf, P.A.T.T. van Veenendaal, M.K. 

van Veen, A.J. Hardeman, M.Y.S. Rushche, J.K. Rath, 

and R.E.I. Schropp, "The Influence of the Filament 

Temperature on the Sturcture of Hot-Wire Deposited 

Silicon," Thin Sold Films 430, 2003, pp. 46-49. 

[7] J. Doyle, R. Robertson, G.H. Lin, M.Z. He, and A.

Gallagher, "Production of High-Quality Amorphous Silicon

Films by Evaporative Silane Surface Decomposition," J. 

Appl. Phys. 64, 1988, pp. 3215-3223. 

[8] R.E.I. Schropp, Y. Xu, E. Iwaniczko, G.A. Zaharias,

and A. Mahan, "Microcrystalline Silicon for Solar Cells at

High Deposition Rates by Hot Wire CVD," Mat. Res. Soc.

Symp. Proc. 715, 2002, pp. A26.3.1-A26.3.6.

[9] J.K. Rath, A.J. Hardeman, C.H.M. van der Werf, 

P.A.T.T. van Veenendaal, M.Y.S. Rusche, and R.E.I. 

Schropp, "Deposition of HWCVD Poly-Si Films at a High

Growth Rate," Thin Sold Films 430, 2003, pp. 67-72.

4
F1147-E(12/2004) 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

February 2005 

2. REPORT TYPE

Conference Paper 

3. DATES COVERED (From - To)

3-7 January 2005 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Mechanisms of Growth of Nanocrystalline Silicon Deposited by HotWire Chemical Vapor Deposition 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-520-37345 

5e. TASK NUMBER

PVA53002 

6. AUTHOR(S) 

H.R. Moutinho, C.-S. Jiang, Y. Xu, B. To, K.M. Jones, C.W. Teplin, 

and M.M. Al-Jassim 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-520-37345 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

We have studied the growth of silicon thin films by hot-wire chemical vapor deposition under different conditions of 

filament temperature (Tf) and hydrogen dilution ratio (R). We found that these two parameters have a similar effect 

on the properties of the deposited films and show how they interact to control the growth dynamics. For relatively low 

values of Tf and/or R, the films are amorphous. An increase in the value of these parameters results in the 

appearance of a new phase, characterized by heavily faulted (220)-oriented columnar grains; for even higher values, 

a randomly oriented nanocrystalline phase appears. In general, there is more than one phase present in the film, 

and the microstructure varies as we go from the bottom to the top of the film. Although Tf and R have similar effects 

on the physical properties, they affect the deposition rate in a different way. 

15. SUBJECT TERMS

PV; nanocrystalline silicon; thin films; hot-wire chemical vapor deposition (HWCVD); filament temperature (Tf); 

hydrogen dilution ratio (R); 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18