Amorphous-Nanocrystalline Transition in Silicon Thin Films Obtained by Argon Diluted Silane PECVD

doi:10.4236/csta.2012.13011

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Crystal Structure Theory and Applications, 2012, 1, 57-61

http://dx.doi.org/10.4236/csta.2012.13011 Published Online December 2012 (http://www.SciRP.org/journal/csta)

Amorphous-Nanocrystalline Transition in Silicon Thin

Films Obtained by Argon Diluted Silane PECVD

Rachid Amrani1,2*, Frederic Pichot1, Larbi Chahed2, Yvan Cuminal1

1Université Montpellier II, Place Eugène Bataillon Montpellier, Montpellier, France

2Département de Physique, Université d’Oran ES-Sénia, Oran, Algérie

Email: *[email protected], [email protected]

Received October 4, 2012; revised November 8, 2012; accepted November 22, 2012

ABSTRACT

The Plasma-Enhanced Chemical Vapor Deposition (PECVD) method is widely used compared to other methods to de-

posit hydrogenated silicon Si:H. In this work, a systematic variation of deposition parameters was done to study the

sensitivities and the effects of these parameters on the intrinsic layer material properties. Samples were deposited with

13.56 MHZ PECVD through decomposition of silane diluted with argon. Undoped samples depositions were made in

this experiment in order to obtain the transition from the amorphous to nanocrystalline phase materials. The substrate

temperature was fixed at 200˚C. The influence of depositions parameters on the optical proprieties of the thin films was

studied by UV-Vis-NIR spectroscopy. The structural evolution was also studied by Raman spectroscopy and X-ray dif-

fraction (XRD). The structural evolution studies show that beyond 200 W radio frequency power value, we observed an

amorphous-nanocrystalline transition, with an increase in crystalline fraction by increasing RF power and working

pressure. The deposition rates are found in the range 6 - 10 Å/s. A correlation between structural and optical properties

has been found and discussed.

Keywords: Silicon Thin Film; PECVD; Amorphous-Nanocrystalline Transition; Deposition Rate; Powders; Argon

1. Introduction

The hydrogenated nanocrystalline silicon (nc-Si:H) has

gained much attention over amorphous silicon (a-Si:H).

The light-induced degradation of a-Si:H, also named Sta-

bler-Wronski effect [1,2], has been a major bottleneck

since it was observed in practical device applications

such as thin film solar cells. Several deposition methods

have been used to prepare device quality films. These in-

clude Hot Wire-CVD (chemical vapor deposition) [3],

Electron Cyclotron Resonance-CVD [4], conventional

Plasma-Enhanced-CVD (PECVD) [5], Very High Fre-

quency Plasma Enhanced-CVD [6], Microwave-CVD [7]

and Radio-Frequency Magnetron Sputtering of crystal-

line silicon target [8]. Among these, only PECVD has

been established for industrial applications.

However, the device quality nc-Si:H films prepared by

PECVD method with silane hydrogen mixture, at opti-

mized deposition parameters show lower deposition rate

(0.4 - 3 Å/s) .

Another problem with this kind of method is the pow-

ders formation during the film growth. A general study

of powders formation has been performed by Dorier et al.

[9-11] as a function of silane dilution in argon, helium or

hydrogen in PECVD system. Dorier [12] compared the

powder appearance time value ta versus gas used for si-

lane dilution; argon, helium or hydrogen. As shown in Fig-

ure 1, it is interesting to note that a strong difference is

evidenced when comparing between these three situa-

tions. ta decreases when silane is diluted in argon while

helium and hydrogen dilution lead to the inverse effect.

The probability that initial clusters have a chance to form

small particles being stored in the plasma is enhanced for

argon dilution and reduced for helium or hydrogen dilu-

tion.

Figure 1. Particle appearance time as a function of silane

partial pressure in silane mixtures with argon, helium or

hydrogen [12].

*Corresponding author.

R. AMRANI ET AL.

When silane is diluted with hydrogen, it is well known

that growth rate is reduced, with simultaneous trends to

form nanocrystalline structure in place of a-Si:H. The

lower deposition rates increase the process operation

time and hence the production cost.

In this work, the Si:H thin films were deposited in a

conventional PECVD system, using argon silane mixture.

By Raman and XRD spectroscopy, the structural evolu-

tion was studied. The optical characterization for these

films was also appraised by UV-Visible-NIR spectropho-

tometer.

2. Experimental Procedure

Samples were deposited in a Radio-Frequency (RF)

13.56 (MHZ) PECVD chamber. The electrode area was

350 cm2 and the distance between electrodes 1.2 cm. The

mixed precursor gases were ultrahigh purity silane (SiH4)

and argon (Ar). The dilution ratio (the argon to silane

flow ratio) was fixed at 25 during all the processes. The

deposition pressure varied from 400 mTorr to 1400

mTorr, at various RF powers (50, 100, 200 and 250 W)

and substrate temperature was fixed at 200˚C.

Before the deposition, all glass substrates were cleaned

strictly following a standard procedure to ensure a good

adhesion between the substrate and the Si:H layer. Ra-

man spectra were recorded with micro-Raman spectros-

copy (Jobin Yvon Horibra LABRAM-HR). The spec-

trometer has backscattering geometry for detection of

Raman spectrum with the resolution of 1 cm–1. The exci-

tation source was 473.5 nm line of Argon laser. The

power of the Raman laser was kept about 2 mW to avoid

laser induced crystallization on the films. Low angle

X-ray diffraction pattern was obtained by X-ray dif-

fractometer using CuKα line (λ = 1.54056 Å). Thickness

and refractive index were determined by UV-Vis-NIR

spectroscopy using the envelope method [13,14]. The

average band gap EM was estimated using the procedure

followed by Wemple et al. [15].

3. Results and discussion

The variation of deposition rate plotted as a function of

process pressure is shown in Figure 2 . It shows that, the

increase of RF power increases the deposition rate. It is

seen also that the deposition rate increases from ~ 6 Å/s

to ~7.5 Å/s, when the process pressure increases from

400 mTorr to 1000 mTorr. With further increase in pro-

cess pressure to 1400 mTorr, the deposition rate de-

creases. Thus, with increase in process pressure the im-

pingement rate of silane on the plasma increases. As a

result, the number of film-forming radicals and hence the

deposition rate increases. With further increase in pro-

cess pressure the supply of film-forming radicals also

increases. However, the powders electrostatically trapped

in the plasma prevent the films from growth. So, in order

to obtain a deposition without dust trapped in the plasma,

the process pressure must be below 1000 mTorr.

The crystallinity of the films was studied by low angle

X-ray diffraction (XRD). XRD patterns of two typical

samples at the same pressure (1400 mTorr) with different

RF power are shown in Figure 3. At 100 W RF power,

no crystal grains are detected in this sample; there is no

apparent structural evolution in the thin films.

When RF power reaches 200 W, diffraction peaks

arise. With the increase of RF power, there appear three

peaks symbolizing three different silicon crystalline ori-

entations. Also, the growth of grains in the thin films is

multi-oriented. The average crystallite size (dXRD) can be

estimated using the classical Scherrer’s formula [16].

The estimated average crystallite size dXRD obtained for

the films deposited at RF power 200 W and at working

pressure of 800, 1000, 1200 and 1400 mTorr are 7.1, 9.5,

12.2 and 15.1 nm, respectively.

Micro-Raman spectroscopy has been widely used as a

powerful technique to characterize deposited thin layers.

It can elucidate the amorphous and crystalline phase.

Figure 4 shows Raman spectra of Si:H films deposited at

various process pressure and RF power.

Figure 2. Variation of deposition rate as a function of pro-

cess pressure and RF power for Si:H films deposited by

PECVD.

Figure 3. Low angle X-ray diffraction pattern of two typical

samples deposited at the same pressure (1400 mTorr) with

different RF power.

R. AMRANI ET AL. 59

For samples deposited below 200 W RF power, the

TO band is composed by only a broad peak located

around 480 cm–1, characteristic of a completely amor-

phous structure. Beyond 200 W RF power and beyond

800 mTorr pressure deposition, the TO band can be cor-

rectly fitted using three Gaussian components centered

around 480, 510 and 520 cm–1, suggesting the presence

in these films a mixture of amorphous as well as crystal-

line structure with different grain sizes. The estimated

crystalline volume fraction Fc [17,18] and crystallite size

dRaman [19] in the films are also indicated in Figure 4. For

nanocrystalline materials, with increase process pressure

and RF power, both, Fc and dRaman in the films increase.

For samples deposited at 200 W, Fc increases from 21%

to 90% and dRaman increases from 6.4 nm to 16.2 nm,

when the process pressure increases from 800 mTorr to

1400 mTorr.

Film particle sizes measured by XRD method turned

out significant difference with those measured by Raman

method. The difference can be due to the different detec-

tion sensitivity of characterization techniques. However,

it is important to note that the crystallite sizes determined

by both techniques at various process pressure show

same trend. Therefore, for nanocrystalline samples, with

increase in process pressure, the grain size increases.

To achieve a better understanding of the optical pro-

prieties of Si:H, the optical transmission of films was

Figure 4. Typical Raman spectra obtained in the TO-like

mode, for films deposited (a): with deposition pressure of

1000 mTorr and varying RF power (50, 100, 200 and 250

W); and (b): RF power 200 W and varying pressure (600,

800, 1000 mTorr).

measured by UV-Vis-NIR spectrophotometer. The films

thickness and the refraction indexes were determined

using the method proposed by Swanepoel [13].

Figure 5 shows the variation of static refractive index

as a function of deposition parameters. The refractive

index increases when the working pressure increases.

Also, the refractive index decreases when RF power in-

creases.

The refraction index is an important parameter, since it

could be linked to the material density [8,20]. Indeed, a

good material is compact, i.e. it presents a minimum of

micro-cavities.

Detailed analysis of the refractive index spectra were

performed using the model suggested by Wemple and

Didomenico [15]. At energies below than of the optical

bandgap, the refractive index is related to the square of

the photon energy (ħω)2 by:

 









The plot of 1/[n2(ħω) − 1] versus (ħω)2 allows the de-

termination of the average gap EM, and the dispersion

energy ED.

Figure 6 shows that all samples deposited at 100 W

RF power have a high average gap value EM, which exhi-

bits a completely amorphous structure. But for films grown at

Figure 5. Variation of static refractive index for samples

deposited at different process pressure and RF power.

2.0

1.5

Figure 6. Variation of average band gap for Si:H films de-

posited by PECVD as a function of process pr essure and RF

power.

R. AMRANI ET AL.

200 W and beyond 800 mTorr, EM decreases considera-

bly. This confirms once again, that beyond these deposi-

tion parameters, an amorphous-nanocrystalline transition

is observed.

The RF bias voltage is an important deposition pa-

rameter of nc-Si:H films. During the growth, the RF bias

has a decisive effect on the energy and natures of (SiHn)

species [19]. The appropriate bias may control the reac-

tion rate at the gas-substrate interface, so as to promote

the nucleation rate, and enhance the densification of

nanocrystallites in the growing films. He et al. demon-

strate also, that bias voltage controls the crystalline frac-

tion for nc-Si:H samples [19]. In this work, and at certain

deposition parameters, the bias varies. For power more

than 200 W and pressure below 800 mTorr, RF bias in-

creases from 3 V to 80 V, after a few minutes for the

beginning of the deposition process. The reflect RF

power is also an important parameter. After a specific

deposition time (~10 min), the reflect power increases,

and in consequence the real power deposition decreases.

To resolve the effects of RF bias and RF power reflect

values, samples were deposited with layer by layer (LBL)

deposition method.

4. Conclusion

As conclusion, we have proved that the formation of

powders in Silane-Argon plasma, initially considered as

a drawback, can be exploited to produce nanocrystalline

silicon particles. We have shown that hydrogenated

nanocrystalline silicon (nc-Si:H) films can be prepared

from silane argon mixture without hydrogen dilution at

high deposition rates (~10 Å/s) and at low substrate tem-

perature (200˚C) using PECVD method. The structural

evolution studies show that beyond 200 W RF power and

beyond 800 mTorr pressure deposition conditions, we

observed an amorphous-nanocrystalline transition. From

the present study it has been concluded that the process

pressure and RF power are the key process parameters to

induce the crystallinity in the Si:H films grown by

PECVD method. From materials and devices application,

especially, photovoltaic devices application, nanocrystal-

line silicon produced at low temperature in silane plasma,

open the way to new applications, like the deposition on

flexible substrate.

5. Acknowledgements

The authors would like to thank D. Bourgogne for Ra-

man experiments assistance and B. Fraisse for the X-ray

diffraction measurements. This work is kindly supported

by Averroes Program.

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