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On the Effect of the Film Hydrogen 

Content and Deposition Type on 

the Grain Nucleation and Grain 

Growth During Crystallization of 

a-Si:H Films 

Preprint 

A.H. Mahan, S.P. Ahrenkiel, and D.S. Ginley 

National Renewable Energy Laboratory 

B. Roy 

Colorado School of Mines 

R.E.I. Schropp and H. Li 

Utrecht University

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39901 

May 2006 
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ON THE EFFECT OF THE FILM HYDROGEN CONTENT AND 

DEPOSITION TYPE ON THE GRAIN NUCLEATION AND GRAIN 

GROWTH DURING CRYSTALLIZATION OF A-SI:H FILMS* 

A.H. Mahan1, S.P. Ahrenkiel1, B.Roy2, R.E.I. Schropp3, H. Li3, and D.S. Ginley1

1

National Renewable Energy Laboratory, Golden, CO 80401 2

Colorado School of Mines, Golden, CO 80401 3 Utrecht University, Faculty of Science, 3508 TA Utrecht, The Netherlands 

Tel: (303) 384 6697; Fax: (303) 384 6430; E-mail: harv_mahan@nrel.gov 

ABSTRACT 

We report the effect of the initial film hydrogen content 

(CH) on the crystallization kinetics, crystallite nucleation 

rate and grain growth rate when HWCVD and PECVD aSi:H films are crystallized by annealing at 600°C. For the 

HWCVD films, both the incubation time and crystallization 

time decrease, and the full width at half maximum 

(FWHM) of the XRD (111) peak decreases with 

decreasing film CH. However, other sources of XRD line 

broadening exist in such materials in addition to crystallite 

size, including the density of crystallite defects. To 

address these issues, TEM measurements have also 

been performed on a-Si:H films deposited directly onto 

TEM grids. Following the procedure of Iverson and Reif 

(J. Appl. Phys. 62 (1987) 1675), an examination of films 

with low grain density enables a determination of the 

crystallite nucleation rate as well as grain growth rate. We 

compare the results for HWCVD films of different film CH, 

and also for HWCVD and PECVD a-Si:H films containing 

the same initial film CH. We also perform Raman 

measurements on fully crystallized HWCVD films to 

explore whether film disorder plays a role in the different 

XRD FWHMs when different amounts of film hydrogen are 

evolved.

INTRODUCTION 

The crystallization of as deposited a-Si:H thin films is 

becoming increasingly important because of its potential 

use to produce higher mobility polycrystalline materials for 

use in solar cells and high performance thin film 

transistors. This process is believed, at relatively low 

temperatures (<1000C), to follow a classical model of 

nucleation and grain growth [1], where an amorphous 

incubation time, a steady state nucleation rate, grain 

growth of these nuclei, and a characteristic time of 

crystallization can be identified. In limited previous studies 

at an anneal temperature of 600C, all of these process 

steps were seen to depend on the film substrate 

temperature (TS) [2,3]. While there was agreement in 

these studies that lower TS films exhibited longer 

amorphous incubation times and larger grain sizes, the 

latter due primarily to the smaller nucleation rate, there 

was no general consensus as to why this occurred. One 

approach linked the trends in grain size with TS to 

differences in the Raman signatures of the as grown films, 

which were then related to differences in short range 

structural order [2,3]. However, these Raman analyses 

were either not rigorous [2] or were done with an 

interpretation no longer considered appropriate [3]. 

Further, recent results have shown that the crystallization 

time, for a given film CH, can depend upon the a-Si:H 

deposition method [4]. The present work thus reexamines a-Si:H crystallization [5], with an attempt to 

determine not only what factors limit grain size, but also to 

understand how H influences nucleation. 

EXPERIMENTAL 

A-Si:H films were deposited using the HWCVD and 

PECVD techniques, using deposition conditions described 

previously [6,7]. Two different thickness films were then 

analyzed. First, ~1 μm thick a-Si:H films were deposited 

on glass, and XRD and reflectivity were used to probe film 

crystallinity. Second, thinner (~0.1 μm) films were also 

grown directly on C-coated, 200 mesh Mo TEM grids 

which were mounted on the glass using colloidal graphite 

paste. The latter thicknesses were chosen because no 

sample thinning was required. TEM analysis was 

performed on a CM200 Scanning TEM using a Phillips 

single-slit holder and a Gatan Model 652 double-slit 

heating holder for in-situ annealing. TEM images were 

acquired in the conical dark-field mode with a 12-bit digital 

camera from Soft Imaging System, with the direct beam 

tilted by 0.70° and dynamically pressed about the optic 

axis of the microscope. To determine the crystalline 

volume fraction Xc from TEM images, we assume grain 

growth is two dimensional and use the ‘magic wand’ tool 

in Photoshop for boundary recognition to binarize the 

images, converting crystalline (amorphous) regions to 

black (gray) respectively. Histograms of the one-bit 

images were then computed in Digital Micrograph to 

——————————— 

*This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-

99GO10337 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for 

publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to

publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes. 

1
determine Xc. Direct grain counting was used to 

determine the areal grain number densities. Each image 

series was analyzed incrementally, starting at the early 

stages of crystallization, and keeping a running total while 

counting emerging nuclei in subsequent images. 

RESULTS AND DISCUSSION 

Figure I shows TEM images of partially crystallized 

HWCVD and PECVD a-Si:H films annealed at 600°C, with 

the annealing times indicated in the figures. Based upon 

infrared measurements of the Si-H wag mode, the film H 

contents (CHs) are 2.7, 12.5, and 11.5 at.% respectively 

for the low H HWCVD, high H HWCVD, and PECVD films. 

The Xc’s of these images are roughly similar. These 

images, and those taken in the respective image series, 

show clearly that crystallization occurs via grain 

 H2511_LH4: 67 min. @ 600°C 

L1596_P3:657 min. @ 600°C 

Figure 1. TEM images of the partially crystallized 2.5 at. % 

HWCVD film (top) and the 11.5 at.% PECVD film (bottom). 

nucleation, which occurs at random in the amorphous 

matrix, and grain growth into the surrounding amorphous 

material. From Figure 1, clear differences are seen not 

only in grain density but also in grain morphology between 

the different films. While a somewhat less distinct 

difference is seen between HWCVD films of different CH, 

the PECVD films by contrast show much lower grain 

densities and larger grains overall. These differences are 

tabulated in Table 1, which gives the measured 

crystallization parameters for the HWCVD and PECVD 

films annealed at 600°C. These include the crystallite 

nucleation rate (rn), the grain growth rate (sg), and the 

extrapolated final grain size (dg), the latter calculated 

following the methodology of Iverson and Reif [1]. 

Three comments can be made from these data. First, 

since the grain growth rates are roughly similar for all 

three films, the final extrapolated grain size is seen, to a 

first approximation, to be inversely proportional to the 

nucleation rate. Second, the film CH, for a particular film 

deposition type (i.e., HWCVD), does affect the 

crystallization kinetics, in that the high CH HWCVD film 

has a lower nucleation rate and a larger extrapolated final 

grain size than the low CH HWCVD film [8]. This trend is 

similar to that observed for PECVD films versus film CH

[2], but the magnitudes of the crystallization parameters 

are different. Finally, these results show that deposition 

type also has a major influence on crystallization, in that 

HWCVD and PECVD films containing approximately the 

same film CH have strikingly different crystallization 

Film Type HWCVD 

(low H) 

HWCVD 

(high H) 

PECVD 

(high H) 

rn (min.μm3)

-1 2.3 0.16 0.027 

sg (nm/min) 4.1 3.1 2.7 

dg (x) (μm) 0.31 0.66 1.20 

Table 1. Measured crystallization parameters for HWCVD 

and PECVD films annealed at 600°C. 

parameters. Significant differences in film incubation 

periods have been previously observed for HWCVD 

versus PECVD films [4,9]. A tentative model has been 

proposed to explain the very short incubation times for the 

HWCVD films [9]. 

Table 2 shows values of the apparent final grain size as 

determined from the XRD peak broadening using the 

Scherrer formula [5]. As can be seen, while the absolute 

magnitudes of the sizes for the HWCVD films are in 

general agreement with XRD literature values [10] for aSi:H, the trends are in the opposite direction compared to 

the dg values seen in Table 1 when film CH is considered 

[2]. However, other sources of XRD line broadening exist 

in such materials in addition to the crystallite size, such as 

the density of crystallite defects, in particular stacking 

faults and twins [5]. It is interesting to consider if the 

evolution of H, and the changes in structural ordering so 

produced when this H evolves, has any correlation to the 

XRD line broadening. Accordingly, Raman transverse 

optical mode half with at half maxima (HWHMs), which are 

traditionally used as a measure of short range order in aSi:H [11], are also presented in Table 2 for the HWCVD 

2
films, both in the as-grown state and which are also 

annealed long enough to evolve the vast majority of the 

film H but still remain amorphous. For the latter, the 

anneal times are still within the film ‘incubation periods’. 

Film Type HWCVD 

(low H)

HWCVD 

(high H)

PECVD 

(high H)

XRD ‘grain size’ 

(fully crystallized)

0.08 μm 0.052 μm 0.045 μm

Raman HWHM 

(as grown)

28 cm-1 28 cm-1

Raman HWHM 

(at end of incubation period, but 

still amprphous)

28 cm-1 33 cm-1

Table 2. XRD grain size and Raman TO HWHM for 

HWCVD and PECVD films as grown and annealed at 

600°C for indicated times. 

As can be seen, while the Raman HWHMs are similar for 

the as grown low and high CH HWCVD films, the film that 

evolves the largest amount of H (the high CH HWCVD film) 

exhibits Raman line broadening (a wider HWHM), 

indicating more film disorder, and this film also exhibits the 

smallest (largest) XRD (TEM) grain size. The present 

results for the HWCVD films therefore suggest that 

significant H evolution may translate into increased XRD 

line broadening, which may be an indication of i.e. an 

increased density of crystalline defects. In any case, the 

present results show that the XRD line broadening cannot 

be attributed to grain-size effects alone, but may be some 

signature of structural disorder due to inhomogeneous 

strain or intergranular defects, perhaps caused by the 

evolution of (significant) film H. Experiments are in 

progress to measure the Raman HWHMs for the as grown 

and annealed PECVD films, as well as determine the 

relative strains existing in the fully crystallized HWCVD 

and PECVD films [12]. Regarding the former, an 

increased Raman HWHM upon sample annealing was 

previously observed by Lee et al. in PECVD films 

containing varying hydrogen contents, with the broadening 

appearing larger for the film initially containing more 

bonded hydrogen [2], but this broadening was not 

quantified. 

CONCLUSIONS 

The crystallization parameters for HWCVD and PECVD 

films containing different initial film hydrogen contents 

have been determined. The film hydrogen content (CH) 

was found to affect grain growth for HWCVD films, with 

films containing more initial film hydrogen exhibiting lower 

film nucleation rates and larger extrapolated final grain 

sizes. In addition, the deposition type also was shown to 

play a major role in the crystallization process, in that a 

PECVD film exhibiting a film CH similar to that of a 

HWCVD film showed a much lower film nucleation rate 

and a larger extrapolated final grain size. The increased 

Raman short range disorder, occurring for the high CH

HWCVD film, was suggested to play a role in the 

crystallization process. 

ACKNOWLEDGMENTS 

The authors thank D. Young. P. Stradins, and Q. Wang for 

useful discussions. This work was supported by the 

United States DOE under subcontract number DE-AC36-

99-GO10337. 

REFERENCES 

 [1] R. B. Iverson and R. Reif, “Recrystallization of 

Amorphized Poly Si Films on SiO2: Temperature 

Dependence of the Crystallization Parameters”, J. 

Appl. Phys. 62, 1675 (1987). 

[2] J. N. Lee, B. J. Lee, D. G. Moon, and B. T. Ahn, 

“Effect of Deposition Temperature on the 

Crystallization Mechanism of Amorphous Silicon 

Films on Glass”, Jpn. J. Appl. Phys. 36, 6862 

(1997). 

[3] [K. Nakazawa, and K. Tanaka, “Effect of Substrate 

Temperature on Recrystallization of PECVD 

Amorphous Silicon Films”, J. Appl. Phys. 68, 1029 

(1990). 

[4] D. L. Young et al., “Solid Phase Crystallization of 

Hot-wire CVD Amorphous Silicon Films“, 

Proceedings 2005 Spring MRS Conf. (Symposium 

A), in press. 

[5] A.H. Mahan, B. Roy, R.C. Reedy Jr., D.W. 

Readey, and D.S. Ginley, “Rapid Thermal 

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the Film Hydrogen Content on the Crystallization 

Kinetics, Surface Morphology, and Grain Growth”, 

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[6] A.H. Mahan, J. Carapella, B.P. Nelson, R.S. 

Crandall, and I. Balberg, “Deposition of Device 

Quality, Low H Content Amorphous Silicon”, J. 

Appl. Phys. 69 (1991) 6728. 

[7] The substrate temperature for the PECVD 

deposition was 350°C, resulting in a film CH of ~ 12 

at.%. 

[8] S.P. Ahrenkiel, B. Roy, D.S. Ginley, and A.H. 

Mahan, “Grain Nucleation and Grain Growth during 

Crystallization of HWCVD a-Si:H Films”, 2006 

Spring MRS Conference (Symposium A), in press. 

[9] D.L. Young, D.L. Williamson, P. Stradins, Y. Xu, 

L.M. Gedvilas, R.C. Reedy, A.H. Mahan, H.M. 

Branz, and Q.Wang, “Rapid Solid-phase 

Crystallization in High-rate, Hot Wire Chemical 

Vapor Deposited Hydrogenated Amorphous 

Silicon”, submitted to Appl. Phys. Lett. 

[10] S. Hasegawa, T. Nakamura, and Y. Kurata, 

“Axially Controlled Solid-Phase Crystallization of 

Amorphous Silicon”, Jpn. J. Appl. Phys. 31, 161 

(1992). 

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[11] D. Beeman, R. Tsu, and M.F. Thorpe, “Structural 

Information from the Raman Spectrum of 

Amorphous Silicon”, Phys. Rev. B 32, 874 (1985). 

[12] X.-Z. Bo, N. Yao, S.R. Shieh, T.S. Duffy, and J.C. 

Sturm, “Large-grain Polycrystalline Silicon Films 

with Low Intragranular Defect Density by Low 

Temperature Solid-phase Crystallization without 

Underlying Oxide”, J. Appl. Phys. 91, 2910 (2002). 

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