structural studies on the microcrystallization of si:h network developed by hot-wire cvd

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Solar Energy Materials & Solar Cells 90 (2006) 849–863

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Structural studies on the microcrystallization ofSi:H network developed by hot-wire CVD

Koyel Chakraborty, Debajyoti Das�

Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

Received 11 April 2005

Available online 20 June 2005

Abstract

Effect of H2-dilution to the SiH4 plasma, R(H2), on the microcrystallization of Si:H

network at a low substrate temperature (180 1C) has been studied by using hot-wire CVD.

Structural characterization of the films has been performed by micro-Raman, ellipsometry,

infrared absorption and X-ray diffraction studies. A dramatic structural transformation from

amorphous to microcrystalline phase has been identified at an H2-dilution beyond 92.0%,

induced by high atomic H density in the plasma. A virtual saturation in overall crystallinity

has been attained for H2-dilution in the range 92.75pR(H2) (%)p93.75, contributing

crystalline volume fraction changing between 60% and 64%, the average crystalline grain size

varying between 150 and 200 A and bonded hydrogen content maintaining between 3.3 and

2.6 at%. A crystalline volume fraction of 86.6% was obtained along with a low bonded H-

content of 1.76 at% at R(H2) ¼ 98.0%. However, at such extremely high H2-dilution, overall

crystallization is hindered due to enormous polyhydrogenation and formation of lesser dense

network full of voids. Hence, microcrystallization in Si-network can be easily obtained in

HWCVD, at a relatively low hydrogen dilution and low substrate temperature, without

compromising much with the deposition rate arising out of those two stringent factors

affecting in the conventional technique; and thereby, enhancing the technological acceptability

of the deposition process presently dealt with.

r 2005 Elsevier B.V. All rights reserved.

Keywords: Microcrystallization; H2-dilution; Hot-wire CVD; Ellipsometry; Micro-Raman

see front matter r 2005 Elsevier B.V. All rights reserved.

.solmat.2005.05.004

nding author. Tel.: +9133 24 73 6612; fax: +91 33 24 73 2805.

dress: [email protected] (D. Das).

www.elsevier.com/locate/solmat

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K. Chakraborty, D. Das / Solar Energy Materials & Solar Cells 90 (2006) 849–863850

1. Introduction

The plasma-enhanced chemical vapor deposition (PECVD) has been most widelyused in low-temperature processing of silicon (Si) thin films for fabrication of deviceslike solar cells and thin film transistors, as it produces high-quality materialsuniformly on reasonably large area substrates. However, PECVD has some inherentlimitations. The growing surface of the films could suffer from plasma damage orcharge-induced damage. The design of the apparatus becomes complicated so as tominimize the electrical potential around the sample holder. In addition, since usuallyradio frequency (RF) plasma is used, the enlargement of the deposition area islimited due to standing wave of plasma.

As an interesting alternative to PECVD, hot-wire chemical vapor deposition(HWCVD) or catalytic chemical vapor deposition (Cat-CVD) has drawn consider-able attention of materials scientists in recent years. In HWCVD, source gasesundergo pyrolytic dissociation as well as catalytic cracking reactions on resistivelyheated filaments (catalysers) facing the substrate. The species are transported to thesubstrate and contribute to the film growth at low temperatures in the range1501–400 1C, making the process compatible with the use of low-cost glasssubstrates. Relatively simpler design and cheaper set up may thus be used forHWCVD. This technique emphasizes soft reactions on the growing surface, freefrom ion bombardment. The deposition area could be expanded arbitrarily byenlarging the spanning area of the catalyser filaments. In HWCVD the gas utilizationefficiency is 5–10 fold higher, thereby contributing to higher growth rate of thematerial than in PECVD. The HWCVD thus provides enormous promise for bettertechnological feasibility towards commercial production of large area semiconductordevices, in general [1].

Hydrogenated microcrystalline silicon (mc-Si:H) is a mixed phase material with acomposition of crystalline grains embedded in an amorphous matrix. Because of thepresence of both the components and their combined effects therein, mc-Si:Hpossesses properties like doping efficiency, carrier mobility and optical absorptionwhich are of intermediate magnitude compared to those of their individuals. In viewof possessing a favorable combination of higher doping efficiency, enhanced carriermobility and reduced optical absorption compared to its amorphous counterparts,mc-Si:H attains enormous importance in amorphous Si-related thin-film technolo-gies. Apart from its use as a component layer in photovoltaic devices and thin-filmtransistors, solar cells comprising entirely of mc-Si:H layers have also been realizedwith marked improvements in terms of light-induced degradation and near-infraredabsorption of solar radiation.

High H2-dilution and high level of electrical excitation to the SiH4 plasma are thetwo critical parameters facilitating the growth of mc-Si:H network in PECVD [2].Both the parameters increase the atomic H density in the plasma, while the atomicHydrogen-induced growth modulation demonstrates wide structural changes fromamorphous to micro- and nano-crystalline configurations in Si:H network [3,4].However, high H2-dilution retards the film growth rate and high electrical powercauses surface damage of other component layers in the device. The combination of

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K. Chakraborty, D. Das / Solar Energy Materials & Solar Cells 90 (2006) 849–863 851

stringent parameters required for its growth limits the use of this material in devicefabrication in PECVD.

In HWCVD, it has been reported that atomic Si is one of the major products inthe cracking reaction of SiH4 on heated tungsten surface [5], while the directproduction of SiHn radicals are minor [6,7]. However, atomic H is producedefficiently from H2 on the heated catalyser surface [8,9] and can be obtained indensity much higher than that available in typical PECVD process [10]. Underpractical conditions, therefore, chemical reactions of atomic H with SiH4 in the gasphase produce SiHn precursors [11]. In view of the availability of a significantlyhigher atomic H density in HWCVD, microcrystallinity in Si:H should be achievableat a relatively lower substrate temperature and at lower H2-dilution to SiH4 as well,thereby increasing the film growth rate and enhancing the technological acceptabilityof the deposition process involved. However, there are many issues in theoptimization of the microcrystalline growth. In the present paper, a detail structuralcharacterization of the material has been performed using micro-Raman, ultravioletellipsometry, infrared absorption and X-ray diffraction studies on mc-Si:H filmsprepared at different H2-dilution of SiH4 in HWCVD.

2. Experimental

Heated tungsten (W) wire was used for the catalytic reactions in HWCVD becauseof its high melting point with a high evaporation temperature. However, an optimumtemperature of 1500 1C was maintained on the heated filament, using an electricpower of 200W, so as to avoid the W-silicide formation at lower temperature, toincrease the precursor density including the atomic H density at higher temperatureand to minimize the W-contamination probability of the deposited samplesoccurring at a much higher temperature. Tungsten wire with a purity of 99.98%and of diameter 0.5mm has been used in two straight filaments with a length of13.5 cm for each, placed at 6 cm apart and connected in series. Samples wereprepared on Corning 1737F glass and single-crystal silicon substrates placed at 6 cmaway from the filaments. The substrates were heated by a heater placed inside thesubstrate holder and/or by radiation from the filament. A constant temperature of180 1C on the substrate surface was maintained during the film growth. SiH4 wasused as the source gas and H2 as a diluent; and the gaseous mixture within thestainless-steel reactor was maintained at a pressure of �0.17mbar during deposition.A set of films were prepared by varying the H2-dilution in the ensemble, defined asR(H2) ¼ [H2/(SiH4+H2)], from 92.0% to 98.0% and that was obtained by reducingthe SiH4 flow from 6.4 to 1.5 sccm, while maintaining a fixed H2 flow of 73 sccm inthe plasma.

The Raman spectra of the films were obtained at room temperature, in a backscattering geometry, by a micro-Raman spectrophotometer (Jobin Yvon). The laserbeam of 632.8 nm wavelength from a He–Ne source was focused through an opticalmicroscope (Olympus) objective, providing a power density of �7.5mW/cm2 ontothe sample surface. Ellipsometry studies were performed using a fixed angle of

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incidence spectroscopic ellipsometer (JobinYvon). The light source consisted of axenon lamp and a monochromator, which allowed optical measurement in theUV–visible range of wavelength corresponding to a span of energy from 1.5–5.0 eV.The hydrogen bonding structure of the films was investigated by using FTIRspectrometer (Nicolet Magna-IR 750). The X-ray diffraction analysis was carriedout using a conventional CuKa X-ray radiation (l ¼ 1:5418 (A) source and a Braggdiffraction set-up (Seifert 3000P).

3. Results

3.1. Raman studies

The Raman backscattering spectra of the films prepared at different hydrogendilution, R(H2), to the SiH4-plasma has been presented in Fig. 1. Spectrum-acorresponding to the sample with R(H2) ¼ 92.0% exhibited a Gaussian distributionwith a Raman peak at �480 cm�1, assigned to the transverse optical (TO) mode of a-Si and that demonstrated a complete amorphous-like structure of the material. For asmall increase in R(H2) to 92.5%, the associated Raman signal of the samplechanged radically as shown by spectrum-b. The Raman signal consisted of the mainpeak at around 520 cm�1, associated with the TO vibrational mode of c-Si, alongwith its broad distribution at lower wave numbers. A sharp change in the materialstructure from purely amorphous to a gross composition of crystalline andamorphous components was clearly identified and that happened due to a minoraddition of H2-dilution to the SiH4 plasma at its arbitrarily chosen rather criticalparametric condition. With the further increase in H2-dilution, the sharpness of theRaman spectrum increased and the contribution from the lower wave numberdistribution gradually reduced, leading to an almost amorphous-free structure of Simaterials at R(H2) ¼ 98%, as demonstrated by spectrum-f in Fig. 1.

The crystalline volume fraction (Fc) of the films was estimated from the Gaussiandeconvolution of the Raman spectra into three satellite peaks corresponding to theamorphous component (am) at �480 cm�1, the micro-crystalline component (mc) at�520 cm�1 and an intermediate component (nc) at �510 cm�1 arising due to bonddilation at grain boundary zone [12]. The intermediate component in the Ramanspectra was associated, in some earlier works, to the thermodynamically stable nano-crystalline grains of dimension in the range p3 nm [13,14]. The deconvolutedRaman spectra with all the three satellite components for two microcrystallinesamples prepared at two extreme H2-dilution conditions, R(H2) ¼ 92.5% and 98.0%have been presented in Fig. 2.

Considering the intermediate component as the part and portion of crystallinity,the crystalline volume fraction (Fc) was estimated [15] as

F c ¼ ðInc þ ImcÞ=ðbIam þ Inc þ ImcÞ;

where Iam, Inc, and Imc are the integrated intensities of the amorphous component,the intermediate component and the microcrystalline component, respectively. The b

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400 420 440 460 480 500 520 540 560 580

f : 98.00

e : 95.00

d : 93.25

c : 92.75

b : 92.50

a : 92.00

R(H2)=

Ram

an In

tens

ity (

a.u)

Raman Shift (cm-1)

Fig. 1. Raman spectra of mc-Si:H films prepared at different hydrogen dilution, R(H2) ¼ [H2/(SiH4+H2)],

in the plasma.


is the ratio of the cross section of the amorphous phase to the crystalline phase, andis given by

bðDÞ ¼ 0:1þ exp ð�D=250Þ,

where D is the grain size in nm [16]. When the films were amorphous, dominated bythe 480 cm–1 peak in the Raman spectra, the crystallites were extremely small(o10 nm). Hence, the b value was nearly unity. When the films becamemicrocrystalline, b was in the range of 0.8–0.6. For most of these microcrystallinefilms, the integrated intensity Iam of the 480 cm�1 components in the Raman spectrawas small compared to Inc and Imc. The pre-factor in such cases had a minor influenceon Fc, which is expressed as a percentage. In view of simple qualitative discussion bhas been assumed to be unity for present calculations.

Fig. 3 represents the distribution of Fc in Si:H films with the variation of H2-dilution to the SiH4-plasma. It demonstrates that at the chosen parametriccondition, the material changes radically from its purely amorphous state to amixture of amorphous, micro- and nano-crystalline mixed phase structure for slightincrease in H2-dilution beyond R(H2) ¼ 92.0%. A virtually saturated crystallinevolume fraction, F c460% is obtained at a narrow range of hydrogen dilution,

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420 440 460 480 500 520 540 560

R(H2) = 98.0

R(H2) = 92.5

µc

ncam

Ram

an In

tens

ity (

a.u)

Raman Shift (cm-1)

Fig. 2. Deconvoluted Raman spectra with three individual satellite components for two mc-Si:H films

prepared at two extreme hydrogen dilution, R(H2) ¼ [H2/(SiH4+H2)], in the plasma.

92 93 94 95 96 97 98

0

20

40

60

80

100

Cry

stal

line

volu

me

frac

tion

(%)

R(H2) (%)

Fig. 3. Variation of crystalline volume fraction in Si:H network prepared at different hydrogen dilution,

R(H2), to the SiH4 plasma.


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92.75%pR(H2)p93.75%, however, further increase in H2-dilution contributesincreasing crystallinity monotonically, leading to F c�86:6% at R(H2) ¼ 98.0%.

3.2. Ellipsometry

Spectroscopic ellipsometry is a sensitive and non-destructive probe to determinethe optical properties of solids. It deals with the measurement and interpretation ofthe changes in the state of polarized light undergoing oblique reflection from asample surface. Signals detected by ellipsometry consist of contributions fromdifferent parts of the sample–substrate system. The measured dielectric function,called ‘pseudo-dielectric function’, h�i, usually contains information about the wholesystem, including the substrate, the bulk of the film, the interfaces and surfaceoverlayers.

Fig. 4 represents the typical feature of the spectrum of the imaginary part ofpseudo-dielectric function, h�2i, extracted from the spectroscopic ellipsometry data,for mc-Si:H films prepared by HWCVD. At low energies, the penetration depth ofphotons could be larger than the film thickness and that leads to interference fringesdue to the interaction between the beams reflected at the film surface and thesubstrate–film interface. At higher photon energies, the penetration depth decreasesuntil the film is opaque and the reflection at the substrate does not play anymore. So,the oscillations of h�2i at low energies o3.0 eV are related to the nature of thesubstrate, the film thickness and the composition of the first incubation layer,whereas the magnitude of h�2i at high energies is controlled by the bulk compositionand the surface roughness.

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0-15

-10

-5

0

5

10

15

20

25 data points

Pse

udo

< ε

2 >

Energy (eV)

Fig. 4. Typical feature of the spectrum of the imaginary part of pseudo-dielectric function, /e2S,

extracted from the ellipsometry data, for mc-Si:H films prepared by HWCVD.

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3.0 3.5 4.0 4.5 5.0

4

6

8

10

R(H2)=

a : 92.0b : 92.5c : 92.75d : 93.75e : 95.5f : 96.5g : 98.0

S2

S1

g

f

e

d

cb

a

Pse

udo

<ε 2

>

Energy (eV)

Fig. 5. Comparison in the nature of distribution in the imaginary part of pseudo-dielectric function, /e2S,

for mc-Si:H films prepared at various R(H2).


In view of obtaining a good comparison of the bulk properties among varioussamples, the distribution of the imaginary part of pseudo-dielectric function, h�2i, atphoton energies above 3.0 eV have been presented in Fig. 5. Spectrum-a presents thedistribution of h�2i for the sample prepared at R(H2) ¼ 92.0% and that demonstratesa broad hump at around 3.58 eV, signifying an amorphous dominated structure of thematerial. On increase in R(H2) to 92.50%, the nature of h�2i distribution changedradically (spectrum-b) exhibiting two distinct shoulders, S1 and S2 around 3.50 and4.18 eV, respectively. The appearance of the dual shoulder is the signature of thepresence of crystallinity in the film structure [17,18]. On further increase in R(H2) to92.75%, h�2i increased in magnitude while the intensity difference between S1 and S2increased. Higher magnitude of h�2i is an indication of higher density of the network.In addition, the intensity difference between these two shoulders correspondsqualitatively to the amount of crystallinity in the network; higher intensity differencesignifies better crystallinity [19]. However, when R(H2) was increased systematicallybeyond 92.75%, a continuous reduction in the magnitude of h�2i was evident from theellipsometry data [20] and that indicated the formation of lesser dense network full ofvoids [21–23]. In addition, the separation between two shoulders was enhanced by S1shifting towards lower energy and S2 towards higher energy, at elevated H2-dilution.

3.3. Infrared studies

The silicon–hydrogen bonding configurations in the network were investigatedfrom infrared absorption studies using a FTIR spectrophotometer. Fig. 6 represents

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1800 1900 2000 2100 22000

200

400

600

800

f

ed

c

b

a

R(H2) =

a : 92.0b : 92.5c : 92.75d : 93.75e : 96.0f : 98.0

Abs

orpt

ion

co-e

ffici

ent (

cm-1

)

Wavenumber (cm-1)

Fig. 6. Absorption co-efficient spectra of Si:H stretching bands for mc-Si:H films prepared at different

R(H2).


the Si–H stretching mode absorption for Si:H films prepared at various hydrogendilution to the SiH4 plasma. At the lowest R(H2) ¼ 92.0%, the Si:H bondingstructure of the film was mostly of monohydride configuration, contributingstretching mode absorption mainly around 2010 cm�1 as presented by spectrum-a,although a small dihydride (SiH2) or polyhydride (SiHn, n42) component stillappeared at around 2080–2100 cm�1. At R(H2) ¼ 92.5% the relative intensity of themonohydride bonding component reduced, however, that maintained its dominantcontribution over the entire absorption spectrum, in consistent with the amorphousdominated structure of the material. However, the nature of the absorption spectrum(curve-c) changed radically at R(H2) ¼ 92.75% while the major absorption wascontributed by the higher wave number component. Increasing dihydride andpolyhydride bonding contribution in silicon–hydrogen bonding configurations forincrease in R(H2)X92.75% is consistent with the increasing crystallization of thenetwork. At an extreme hydrogen dilution, R(H2) ¼ 98%, huge polyhydrogenationof the network was identified by the solo 2100 cm�1 Si–H stretching absorption band(curve-f).

The wagging mode absorption spectra of the series of samples have been presentedin Fig. 7. The absorption band was quite extended for R(H2) ¼ 92.0% and 92.5%,while the intensity reduced for higher R(H2). However, for R(H2)X92.75%, thenature of the spectrum changed radically with a quite narrow energy spread, whilethe intensity reducing gradually with increasing R(H2), as before. The observedchanges in the nature of the Si–H absorption spectra in both stretching and wagging

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560 580 600 620 640 6600

500

1000

1500

2000

2500

3000R(H2) =

a : 92.0b : 92.5c : 92.75d : 93.75e : 96.0f : 98.0f

e

d

c

b

a

Abs

orpt

ion

Co-

effic

ient

(cm

-1)

Wavenumber (cm-1)

Fig. 7. Absorption co-efficient spectra of Si:H wagging bands for mc-Si:H films prepared at various R(H2).


mode vibrations demonstrate definite structural transformation, e.g., fromamorphous to crystalline phase, occurring in the Si:H network and controlled byH2-dilution to the SiH4 plasma at around R(H2)�92.75%.

The bonded hydrogen content (CH) in the films was estimated from the integratedintensity of IR absorption in wagging mode vibration, using [24]

CH ¼ ðAo=NSiÞ

Zða do=oÞ � 100 at%,

where Ao ¼ 1.6� 1019 cm�2 is the Si–H wagging mode oscillator strength andNSi ¼ 5� 1022 cm�3 is the atomic density of crystalline silicon. Fig. 8 exhibitsthe nature of variation of bonded hydrogen content in the Si:H network preparedat different H2-dilution to the SiH4 plasma. CH reduced sharply from 6.2 to3.3 at% for increase in R(H2) from 92.0% to 92.75% where a structural transitionfrom amorphous to microcrystalline network occurred. However, once themicrocrystalline network was formed, further increase in R(H2) to a ratherwider range from 92.75% to 98.0%, reduced the hydrogen content only marginallyto 1.8 at%.

3.4. X-ray diffraction

The X-ray diffractograms for a scattering angle between 201 and 701 obtainedfrom the set of Si:H films prepared at various H2-dilution to the SiH4 plasma havebeen presented in Fig. 9. A broad band centered at around 251 that was consistentlypresent within the diffraction pattern of all the samples was identified as the

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92 93 94 95 96 97 981

2

3

4

5

6

7

Bon

ded

H-c

onte

nt (

at %

)

R(H2) (%)

Fig. 8. Variation of the bonded hydrogen content in the Si:H network prepared at different R(H2).


contribution from the common amorphous glass substrates. Spectrum-a in Fig. 9does not show any additional signal that may arise from the sample and, thereby,signifies an almost amorphous nature of the Si:H film prepared at R(H2) ¼ 92.0%.However, a minor increase in R(H2) to 92.5% gave rise to very small additionalsignals at around 28.41 and 47.31 which correspond to /1 1 1S and /2 2 0S latticeplane reflections from crystalline silicon, as evident in spectrum-b. The XRD signalcorresponding to 28.41 became prominent at slightly higher R(H2) to 92.75%and continued to maintain almost similar intensity up to R(H2) ¼ 93.75%,thereby, conclusively identifying a definite fraction of crystallinity already beendeveloped within the Si:H network. At R(H2)493.75%, /1 1 1S crystalline peaksharply increased in intensity. The mc-Si:H films of predominant /1 1 1S crystal-lographic orientation were obtained at R(H2)493.75%, however, at R(H2) ¼ 98.0%/1 1 1S peak intensity, in particular, reduced in intensity along with simultaneousevolution of another small peak around 56.11 corresponding to /3 1 1S orientationof c-Si.

During the increase in H2-dilution to the SiH4 plasma, mc-Si:H films ofcertain volume fraction of crystallinity with /1 1 1S preferential crystallographicorientation was formed easily at a low R(H2)X92.5% in HWCVD. However,very high H2-dilution may hinder the crystallization process. The average grainsize as calculated from Debye–Sherrer formula was estimated to vary between150 and 200 A.

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20 30 40 50 60 70

<311><220>

<111>

g

f

e

d

c

b

a

R(H2) =

g : 98.0

f : 96.5

e : 95.0

d : 93.75

c : 92.75

b : 92.5

a : 92.0

Inte

nsity

(a.

u)

2θ (degree)

Fig. 9. X-ray diffraction pattern for various mc-Si:H films prepared at different hydrogen dilution, R(H2),

to the SiH4 plasma.


4. Summary and discussion

By analyzing various experimental results obtained so far, it has been conclusivelyidentified that the Si:H film prepared by HWCVD at a hydrogen dilutionR(H2) ¼ 92.0% in the plasma, was of purely amorphous structure. By increasingR(H2) to only 92.5%, a rather mixed phase material was obtained and that wasidentified to be more towards amorphous by IR studies, however, more towardscrystalline by ellipsometry. The Fc increased from 0% to 42.1% as estimated byRaman studies and bonded H-content reduced from 6.2 to 5.0 at%. AtR(H2) ¼ 92.75%, the Si:H material became predominantly microcrystalline innature with its mainly SiHn (nX2) type of bonding configurations and �3.3 at% ofbonded H-content, along with �60% of Fc. A virtual saturation in overallcrystallinity was maintained for H2-dilution in the range 92.75pR(H2) (%)p93.75,contributing Fc changing between 60% and 64%, CH between 3.3 and 2.6 at% andexhibiting identical characteristics in the Raman, ellipsometry, IR and XRD spectra.The crystallinity within the Si:H network again started increasing gradually forR(H2) beyond 93.75%, in every aspect. A Fc of 86.6% was obtained while thebonded H-content was estimated to be 1.76 at% at R(H2) ¼ 98.0%. However, at anextremely high H2-dilution, e.g., R(H2) ¼ 98.0%, enormous polyhydrogenation ofthe network as demonstrated by the solo 2100 cm�1 Si–H stretching absorption

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band, indicate the generation of a lot of voids and structural imperfections.Continuous reduction in the magnitude of h�2i, as evident from the ellipsometry datais in conformity with the effect of formation of lesser dense network full of voids athigher R(H2). Thereby, crystallization is hindered at very high H2-dilution andconsequently, the /1 1 1S peak in the XRD spectra reduced in intensity.

In PECVD it has been found, in general, that high atomic H density in H2-dilutedSiH4 at high level of electrical excitation have the most determining contributions infacilitating the microcrystalline Si growth. Abstraction of hydrogen bonded to Si,breaking of weak Si–Si bonds, chemical etching by SiH4 formation, recombination,formation of Si–H bonds by rehydrogenation of surface dangling bonds, anddiffusion of atomic H into the Si-matrix are the various processes those occur on thegrowing network during film deposition [25,26]. These reactions are competitive onthe surface induced by atomic H of the plasma and results in the final film structureguided by the parametric conditions [3].

In case of hot-wire CVD, atomic H is produced efficiently from H2 on the heatedcatalyser surface [8,9]. In HWCVD atomic H density in the H2-diluted SiH4

ensemble is very high, about one to two orders of magnitude higher than thatobtained in typical PECVD [10]. A large number of atomic H generated in theplasma have been considered to trigger mc-Si nucleation within the a-Si:H network ata relatively lower R(H2). While at increasing H2 dilution, in addition to otherassociated effects, atomic H induced etching of the growing network becomes verymuch prominent, particularly at lower growth temperature [27]. Atomic H inducedetching of the crystalline component in the network at R(H2)493.75% creates a lotof broken Si bonds, which remain unsaturated as the structural reorientation of thenetwork happens to be inadequate due to lower mobility at the rather lowtemperature (180 1C) of film growth on the substrate surface, and partially due to arelatively faster deposition rate in hot-wire CVD, providing insufficient time forproper network modulation. The obvious result is the inevitable enhancement of thedensity of voids [28] grown by virtue of strain minimization in the mc-Si:H network.

In HWCVD a dramatic structural transformation from amorphous to microcrystal-line phase happens to occur at an H2-dilution beyond 92.0%. A reasonably good mc-Si:Hnetwork has been attained at a relatively low H2-dilution, R(H2) ¼ 92.75% in HWCVD,because of its associated very high atomic H density in the plasma compared to that inPECVD [29]. Although high atomic H density is very much needed in facilitating themicrocrystalline growth, it cannot be increased arbitrarily in the development ofimproved microcrystalline network, depending upon the associated parametriccondition, e.g., at lower substrate temperature and high deposition rate. Enhancedatomic H induced etching at the growth zone promotes porosity in the mc-Si:H networkat an elevated H2 dilution to SiH4, in general, and hinders the crystallization process.

5. Conclusion

A set of mc-Si:H films have been prepared by changing the H2-dilution to the SiH4

plasma in HWCVD at a low substrate temperature of 180 1C. A detail structural

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investigation of the films has been performed by Raman, ellipsometry, IR and X-raydiffraction studies. A dramatic structural transformation from amorphous tomicrocrystalline phase happens to occur at an H2-dilution beyond 92.0%. Areasonably good mc-Si:H network has been attained at a relatively low H2-dilution,R(H2) ¼ 92.75% at this low temperature, because of the associated very high atomicH density in the plasma compared to that in PECVD. A virtual saturation in overallcrystallinity was maintained for H2-dilution in the range 92.75pR(H2) (%)p93.75,contributing crystalline volume fraction changing between 60% and 64%, theaverage crystalline grain size varying between 150 and 200 A, bonded hydrogencontent maintaining between 3.3 and 2.6 at% and exhibiting identical characteristicsin the Raman, ellipsometry, IR and XRD spectra. A Fc of 86.6% was obtained alongwith a low bonded H-content of 1.76 at% at R(H2) ¼ 98.0%. However, at suchextremely high H2-dilution, overall crystallization is hindered due to enormouspolyhydrogenation and formation of lesser dense network full of voids.

Hence, microcrystallization in Si-network can be easily obtained in HWCVD, at arelatively low hydrogen dilution and low substrate temperature, without compro-mising much with the deposition rate arising out of those two stringent factorsaffecting in the conventional technique and thereby, enhancing the technologicalacceptability of the deposition process presently dealt with.

Acknowledgements

Experimental support from Laboratoire de Physique des Interfaces et des CouchesMinces, Ecole Polytechnique, France, is gratefully acknowledged.

References

[1] H. Matsumura, H. Umemoto, A. Izumi, A. Matsuda, Thin Solid Films 430 (2003) 7.

[2] D. Das, Solid State Phenomena, Special Volume on Hydrogenated Amorphous Silicon, 44–46, Scitec

Publication, Switzerland, 1995, p. 227.

[3] D. Das, Phys. Rev. B 51 (1995) 10729.

[4] D. Das, J. Phys. D 36 (2003) 2335.

[5] J. Doyle, R. Robertson, G.H. Lin, M.Z. He, A. Gallagher, J. Appl. Phys. 64 (1988) 3215.

[6] Y. Nozaki, M. Kitazoe, K. Horii, H. Umemoto, A. Masuda, H. Matsumura, Thin Solid Films 395

(2001) 47.

[7] H.L. Duan, G.A. Zaharias, S.F. Bent, Thin Solid Films 395 (2001) 36.

[8] S.A. Redman, C. Chung, K.N. Rosser, M.N.R. Ashfold, Phys. Chem. Chem. Phys. 1 (1999) 1415.

[9] A. Gallagher, Thin Solid Films 395 (2001) 25.

[10] H. Umemoto, K. Ohara, D. Morita, Y. Nozaki, A. Masuda, H. Matsumura, J. Appl. Phys. 91 (2002)

1650.

[11] N.L. Arthur, L.A. Miles, Chem. Phys. Lett. 282 (1998) 192.

[12] S. Veprek, F.A. Sarott, Z. Iqbal, Phys. Rev. B 36 (1987) 3344.

[13] H.S. Mavi, A.K. Shukla, S.C. Abbi, K.P. Jain, J. Appl. Phys. 66 (1989) 5322.

[14] Y. He, Y. Wei, G. Zheng, M. Yu, M. Liu, J. Appl. Phys. 82 (1997) 3408.

[15] G. Yue, J.D. Lorentzen, J. Lin, D. Han, Q. Wang, Appl. Phys. Lett. 75 (1999) 492.

[16] E. Bustarret, M.A. Hachicha, M. Brunel, Appl. Phys. Lett. 52 (1988) 1675.

ARTICLE IN PRESS


[17] S. Kumar, B. Drivillon, C. Godet, J. Appl. Phys. 60 (1986) 1542.

[18] D. Das, Thin Solid Films 476 (2005) 237.

[19] D. Das, J. Appl. Phys. 93 (2003) 2528.

[20] C. Niikura, R. Brenot, J. Guillet, J.E. Bouree, J.P. Kleider, R. Bruggemann, C. Longeaud, Sol.

Energy Mater. Sol. Cells 66 (2001) 421.

[21] E.A. Hamers, A. Fontcuberta i Morral, C. Niikura, R. Brenot, P. Roca i Cabarrocas, J. Appl. Phys.

88 (2000) 3674.

[22] S. Hamma, P. Roca i Cabarrokas, J. Non-Cryst. Solids. 227–230 (1998) 852.

[23] D. Das, Solid State Commun. 128 (2003) 397.

[24] H. Shanks, C.J. Fang, L. Ley, M. Cardona, F.J. Demond, S. Kalbitzer, Phys. Status. Solidi. B 100

(1980) 43.

[25] R. Robertson, A. Gallagher, J. Chem. Phys. 85 (1986) 3623.

[26] S. Veprek, F.A. Sarrott, Phys. Rev. B 36 (1987) 3344.

[27] H. Matsumura, K. Kamesaki, A. Masuda, A. Izumi, Jpn. J. Appl. Phys. 40 (Part 2) (2001) L289.

[28] P. Danesh, B. Pantchev, D. Grambole, B. Schmidt, Appl. Phys. Lett. 80 (2002) 2463.

[29] A. Matsuda, J. Non-Cryst. Solids. 338 (2004) 1.

structural studies on the microcrystallization of si:h network developed by hot-wire cvd

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