Journal of Non-Crystalline Solids 227–230 Ž1998. 857–860

Control of crystallinity of microcrystalline silicon film grown on
insulating glass substrates
Jiang-Huai Zhou a , Kazuyuki Ikuta b, Tetsuji Yasuda b, Takahide Umeda b,
Satoshi Yamasaki b,) , Kazunobu Tanaka bbaJoint Research Center for Atom Technologyr Angstrom Technology Partnership, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan
Joint Research Center for Atom Technologyr National Institute for AdÕanced Interdisciplinary Research, 1-1-4 Higashi, Tsukuba,Ibaraki 305, JapanAbstract
The amorphous incubation layer commonly found in microcrystalline silicon Ž m c-Si:H. grown on insulating glass
substrates by plasma-enhanced chemical vapor deposition has been successfully eliminated simply by dilution of the SiH 4 toa SiH 4rH 2 flow ratio - 1r99. Using an amorphous layer free m c-Si:H as a seed an epitaxial-like growth of m c-Si:H wasobtained. It is shown that, with the seeded growth scheme, one can grow amorphous layer free thin m c-Si:H films andcontrol the crystallinity of the films with relative ease. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: Microcrystalline silicon; Plasma-enhanced chemical vapor deposition

1. Introduction
Microcrystalline silicon Ž m c-Si:H. was first prepared at low temperatures Ž- 400 8C. by plasma-enhanced chemical vapor deposition ŽPECVD. nearly
20 years ago w1x. However, it has been a challenge tocontrol the crystallinity of very thin Ž- 50 nm. Si
films grown on electrically insulating glass substrates such as Corning 7059 glass and fused quartzŽsilica.. The difficulty arises from the fact that alayer of the film right on top of the glass substrate isamorphous even though the rest of the film may
exhibit a high crystallinity w2–5x. The reported values for the thickness of this initial amorphous layer)

Corresponding author. Tel.: q81-298-542732; fax: q81-298542786; e-mail: satoshi@jrcat.or.jp.

Žalso sometimes termed the incubation layer. are) 10 nm. In this paper, we report on the successful
elimination of the amorphous incubation layer inPECVD growth of m c-Si:H. We show that, with atriode PECVD system w6x, H 2 dilution of the SiH 4
gas provides a simple and effective way to control
the structure of very thin Si:H films on insulating
glass substrates.

2. Experimental details
The triode PECVD system used in this work wasoperated at 13.56 MHz. The substrate holder wasgrounded. The substrates were fused quartz and werec le a n e d u s in g th e S C -1 s o lu tio nŽH 2 O:31%H 2 O 2 :28%NH 4 OH s 5:1:1, 808C.. The
reactant gas was SiH 4 diluted in H 2 . The flow rates

0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 9 9 - 9

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J.-H. Zhou et al.r Journal of Non-Crystalline Solids 227–230 (1998) 857–860

of SiH 4 and H 2 were such that the total flow rate
was ; 100 sccm, and the SiH 4rH 2 flow ratio wasvaried down to 0.33r99. A Pd-film H 2 purifier wasused to purify the H 2 gas. The ratio of the SiH 4
partial pressure to that of H 2 in the chamber waslarger than the SiH 4rH 2 flow ratio, due to a slower
evacuation speed for SiH 4 of the pumping system.
The deposition parameters are as follows. Substratetemperature: 2508C; rf power density: ; 1 Wrcm2 ;chamber pressure: 6.65 Pa; mesh bias: y100 V. Thebase pressure of the chamber was ; 4 = 10y6 Pa atroom temperature. A shutter was placed right above
the substrate during the adjustment of the plasma
following ignition. The crystallinity of the films wasdetermined using Raman back scattering excited witha He–Ne laser.

3. Results
Apparently, the key to controlling the crystallinityof very thin films is to eliminate the amorphousincubation layer. To this end, we tried depositions
with SiH 4rH 2 flow ratios Ž R . down to 0.33r99. It
is found that with decreasing R, the crystallinity of
the film improved w7x. In particular, we find that with
R s 0.33r99, the film is of high crystallinity evenwhen the thickness is ; 10 nm. Fig. 1 shows theRaman spectra for two films grown with R s
0.33r99. The thicknesses of the two films were notdirectly measured and were estimated from the deposition rate for thicker films, but confirmed by trans-Fig. 1. Raman spectra for two Si:H films deposited at a SiH 4 rH 2
flow ratio of Rs 0.33r99. The film thicknesses are given in thefigure.

mission electron microscopic ŽTEM. measurement.
As one can see from Fig. 1, the crystallinity remains
high Ž; 90%. at 12 nm. At a film thickness of 6 nm,
however, the broad shoulder Žat ; 480 cmy1 . in theRaman spectrum increased, indicative of an increased amorphous phase fraction. Nevertheless, thespectrum is clearly still dominated by the crystal
band at ; 520 cmy1 . Unfortunately, for films thinner than 6 nm, a reliable Raman measurement was
no longer possible, due to an increased influence ofthe signal from the substrate. However, the highresolution TEM cross-sectional view of a related
sample reveals that at a SiH 4rH 2 flow ratio of
0.33r99 under the present deposition conditions,crystal grains start to form essentially right from thesubstrate, with an incubation layer of less than 1 nm.
Therefore, we have effectively eliminated the initialamorphous layer in m c-Si:H growth simply by using
high H 2 dilution of SiH 4 .
One drawback of high H 2 dilution is that it results
in a lower deposition rate. To speed up the deposition, low H 2 dilution is desirable. An interestingquestion to ask is, if one increases R after thedeposition of an ultrathin layer with a much smaller
R Žsay 0.33r99. will the structure of this ultrathin
layer be carried on during the growth that follows?
The answer to the above question is important not
only from the viewpoint of applications but from theviewpoint of material science in general. Such agrowth scheme is termed seeded growth and theinitial ultrathin layer is called the seed layer w7x. The
effect of the seed layer on the crystallinity of thefilm is shown in Fig. 2. The seed layer in Fig. 2 wasdeposited with R s 0.33r99 and the gas flow ratio
for the subsequent growth was either R s 1.2r99
Žlower two spectra. or R s 3.2r97 Župper two spectra.. The thickness of the seed layer is given in theparentheses and the total film thickness is ; 40 nm
for R s 1.2r99 and 53 nm for R s 3.2r97. One cansee that, for both gas flow ratios the use of a seedlayer improves the crystallinity of the film. In fact,even with a 2 nm thick seed layer improvement of
the crystallinity has been obtained w7x.
Thus far, we have implicitly assumed that theseed layer is a proper layer, i.e., having a welldefined thickness. However, this assumption is notguaranteed for an ultrathin layer. In the worst case,the ‘layer’ would consist of spatially well-separated

J.-H. Zhou et al.r Journal of Non-Crystalline Solids 227–230 (1998) 857–860

859Fig. 2. Raman spectra for Si:H films grown with or without a seed
˚layer ŽSL.. The lower two films are for Rs1.2r99 and are 400 A˚thick. The upper two films are for Rs 3.2r97 and are 530 A
thick. The seed layer was deposited with Rs 0.33r99 and its
thickness is given in the parentheses.

pillars whose heights are in excess of the nominal
thickness of the layer. To gain more insights into theseed layer, we carried out atomic force microscopic
ŽAFM. measurement on two 2 nm thick films deposited, respectively, with R s 0.33r99 ŽFilm 1.
and R s 1.2r99 ŽFilm 2.. Fig. 3 shows the topographies of the two films. Clearly, Film 1 ŽFig. 3a. isrougher than Film 2 ŽFig. 3b.. For the entire scannedarea Ž400 = 400 nm2 ., the root-mean-square ŽRMS.
surface roughness is 0.56 nm for Film 1 and 0.35 nm
for Film 2. On the other hand, we do not see muchdifference between the feature Žisland. densities ofthe two films and the feature density is ; 2 = 10 11
cmy2 . Judging from the crystallinity of thicker films
deposited with R s 1.2r99, Film 2 is most certainly
amorphous. Thus, the larger surface roughness ofFilm 1 indicates that this film contains sufficient
fraction of crystal grains, which is consistent withthe TEM result. It is important to note, however,
that, even without taking into account the contribution from the roughness of the substrate Žwhich is0.28 nm., the average peak-valley vertical distancefor Film 1 is only 1 nm Žhalf of the nominal film
thickness.. These results suggest that the seed layer

˚ thick films. Ža. Deposited withFig. 3. Topographies of two 20 A
Rs 0.33r99. Žb. Deposited with Rs1.2r99. The height of thevertical bar is 4 nm.

Ždeposited with R s 0.33r99. is reasonably flat andthat, perhaps, coalescence has already started at athickness of 2 nm.
4. Discussion
We have seen that H 2 dilution is essential for
eliminating the amorphous incubation layer. Thus
one might think that nucleation of crystallites on
glass is controlled only by the gas phase. While thegas phase chemistry is undoubtedly important, westress here that the physical and chemical propertiesof the substrate surface also play a crucial role indetermining the structure of very thin films. We havefound that the crystallinity of the film depends on thechemical treatment of the substrate.
Next, we discuss how the seed layer can serve toimprove the crystallinity of the film. There are two
possibilities: Ža. quasi-epitaxy and Žb. secondary nucleation of crystallites. In Ža., the structure of the860

J.-H. Zhou et al.r Journal of Non-Crystalline Solids 227–230 (1998) 857–860

seed layer is important and, ideally, is exactly followed during the growth. For each grain, regardlessof the orientation, the growth is epitaxial in the usualsense. In Žb., however, the structure of the seed layer
is not copied in the growth. Rather, the seed layer
only provides a growth surface that enhances crystal
nucleation. The TEM measurement of a film grownat R s 1.2r99 with a seed layer of 6 nm shows that
there is no clear interface between the seed layer andthe rest of the film; crystal grains formed in the seedlayer continue to grow after the gas flow ratio isincreased by nearly four times. These results indicatethat quasi-epitaxial growth indeed takes place under
the deposition conditions employed. This is not say,however, that secondary nucleation does not occur
throughout the growth. In fact, as the growth proceeds, some of the grains will be terminated andsecondary nucleation of crystallites must occur. Evidence for epitaxial-like growth has also been reported for polycrystalline Si w8x.
Obviously, a key requirement on the seed layer isthat it must be stable against the film growth conditions. In this sense, it is relevant to mention that asimilar growth scheme has also been tried by Nguyen
et al. w9x on Cr coated glass substrates, but with nosuccess. Nguyen et al. prepared a seed layer 1 nm
thick by atomic H etching of amorphous Si. However, they found that the seed layer was etched awaywhen exposed to a SiH 4rH 2 plasma used in PECVD.
In our approach, however, the seed is planted using
the same growth mode as for the film growth but at alower SiH 4rH 2 gas flow ratio Žhigher H 2 dilution..
This ensures that the seed layer is not altered when
exposed to the growth plasma. Another importantfeature of our approach is that the seed layer isrelatively flat, in the sense that there are no featureson the surface whose heights are larger than thenominal layer thickness. This suggests that the seeded
growth scheme would find a wide range of applications in devices where very thin microcrystalline
layers of well defined thicknesses are required.

5. Conclusions
The amorphous incubation layer commonly foundin m c-Si:H grown on insulating glass substrates by

PECVD has been successfully eliminated simply by
H 2 dilution of the SiH 4 to a SiH 4rH 2 flow ratio
well below 1r99. Using an ultrathin amorphouslayer free m c-Si:H as a seed layer quasi-epitaxial
growth of m c-Si:H was obtained. It is shown that,with the seeded growth scheme, one can grow amorphous layer free thin m c-Si:H films and control thecrystallinity of the films with relative ease. The AFM
observation of the seed layer shows that the surfaceof the seed layer is relative flat, indicating that theseeded growth scheme is well suited for applications
in devices that require very thin microcrystalline
layers of well-defined thicknesses.

Acknowledgements
The authors are indebted to Dr A. Matsuda of theElectrotechnical Laboratory for allowing them to use
the Raman Spectrometer in his laboratory. This work,
partly supported by NEDO ŽJapan., was performedat the Joint Research Center for Atom Technology
under the joint research agreement between the National Institute for Advanced Interdisciplinary Research and the Angstrom Technology Partnership.

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