Role of hydrogen in controlling the growth of μc- Si:H films from argon diluted SiH 4plasmaMadhusudan Jana, Debajyoti Das, and A. K. Barua Citation: Journal of Applied Physics 91, 5442 (2002); doi: 10.1063/1.1454201 View online: http://dx.doi.org/10.1063/1.1454201 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/91/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Growth mechanism of polycrystalline silicon films from hydrogen-diluted SiCl 4 at low temperature J. Appl. Phys. 98, 034907 (2005); 10.1063/1.1996828 Silicon deposition from BCl 3 / SiH 4 mixtures: Effect of very high boron concentration on microstructure J. Vac. Sci. Technol. B 22, 1 (2004); 10.1116/1.1631292 Atomic-scale analysis of deposition and characterization of a-Si:H thin films grown from SiH radical precursor J. Appl. Phys. 92, 842 (2002); 10.1063/1.1483920 Film growth precursors in a remote SiH 4 plasma used for high-rate deposition of hydrogenated amorphoussilicon J. Vac. Sci. Technol. A 18, 2153 (2000); 10.1116/1.1289541 Role of the surface roughness in laser induced crystallization of nanostructured silicon films J. Vac. Sci. Technol. A 18, 529 (2000); 10.1116/1.582252

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Role of hydrogen in controlling the growth of mc -Si:H films from argondiluted SiH 4 plasma

Madhusudan Jana, Debajyoti Das, and A. K. Baruaa)

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

~Received 15 October 2001; accepted for publication 2 January 2002!

Hydrogenated microcrystalline silicon thin films have been prepared by the rf glow dischargemethod using argon as a diluent of SiH4 to achieve a high growth rate.mc-Si:H film havingconductivity;1025 S cm21 was achieved at a deposition rate of 36 Å/min at a moderate powerdensity of 90 mW/cm2, without hydrogen dilution. Micrograins were identified with several welldefined crystallographic orientations. Inhomogeneity and porosity at the grain boundary zone havea significant effect on the electrical properties of the films due to adsorption when exposed toatmosphere. However, by adding hydrogen to the Ar-diluted SiH4 plasma, a homogeneous andimproved network structure without having any effect of adsorption was obtained at a reduceddeposition rate. Highly conducting (sD;1023 S cm21) undopedmc-Si:H film was prepared at adeposition rate of 15 Å/min having 90% crystalline volume fraction. The energy released by thede-excitation of Ar* in the plasma initiates rapid nucleation in the Si network and atomic hydrogenin the plasma helps in the defect elimination, structural reorientation, and grain growth. ©2002American Institute of Physics.@DOI: 10.1063/1.1454201#

I. INTRODUCTION

Hydrogenated microcrystalline silicon thin films havebeen a subject of extensive studies over the last decade al-though it was prepared for the first time in 1968 applying theplasma enhanced chemical vapor deposition~PE-CVD!method by Veprek and Marecek.1 Interest in the microcrys-talline silicon materials processing has been increasing sig-nificantly because of its potential use in thin film technolo-gies such as solar cells and thin film transistors.Microcrystalline silicon films have higher doping efficiency,higher carrier mobility, and lower optical absorption com-pared to their amorphous counterparts. Because of the favor-able combination of all these properties,mc-Si:H films arebeing used as a window layer and also at the tunnel junctionin a-Si based solar cells. They are also being used as acarrier injection layer in thin film transistors.2 Doped micro-crystalline silicon films asp or n layers can increase the opencircuit voltage of amorphousp- i -n solar cells.3 Fabricationof silicon solar cells with entirely microcrystalline compo-nents have also been reported with marked improvement inthe near-infrared absorption of the solar spectrum and reduc-tion of light induced degradation.4,5

Several methods like rf sputtering,6 solid phasecrystallization,7 photo-CVD,8 electron cyclotronresonance-CVD,9 and MW-CVD10 have been applied to pre-pare large area uniform device qualitymc-Si:H films. Everymethod has several advantages and disadvantages too. ThePE-CVD technique, however, appears to be a promising onein the area of thin film technology. In preparingmc-Si:Hfilms using SiH4 as a source gas in rf PE-CVD, the dilution

level of silane in hydrogen is one of the key parameters.11,12

Applied rf power to the plasma and substrate temperature arealso very important parameters in controlling microcrystalli-zation of the Si network. Increasing rf power is beneficial upto a certain level, beyond which it creates damage to thesurface of the film due to highly energetic ionicbombardment.13 Sometimes powder formation may also hap-pen at higher rf power. The formation ofmc-Si:H will firstbe enhanced by increasing temperature and then suppressedabove 400 °C due to the escape of hydrogen atoms from thegrowing surface.14 Very high hydrogen dilution though it fa-cilitates the microcrystallization of silicon thin film at mod-erate rf power and substrate temperature, it also reduces thedeposition rate drastically. Low deposition rate is a seriousbarrier toward its cost effective utilization in devices. Noblegas dilution to the SiH4 , on the other hand, produces highdensity plasma at a low level of electrical excitation. Wepreparedmc-Si:H films using argon as a diluent of SiH4 atdeposition rate as high as 30 Å/min.15 The material preparedfrom (Ar1SiH4) plasma had columnar structure. Such co-lumnar material exhibits rapid oxidation when exposed to air,indicating that the column surfaces have a very large openstructure that allows enhanced diffusion of oxygen. The ef-fect of oxygen adsorption on the electronic properties of thematerial has been discussed in the previous article.15 Thedark and photoconductivity change over a few orders ofmagnitude before and after annealing the samples at 150 °Cfor 1 h was great evidence of the effects of adsorption. In thepresent work, we have tried to eliminate the adsorption effectby changing the growth pattern by introducing hydrogen tothe Ar-diluted SiH4 plasma. Changes of optoelectronic andstructural properties with the change in the degree of addi-tion of hydrogen to the plasma have been studied thoroughly.

a!Author to whom correspondence should be addressed; electronic mail:erakb@mahendra.iacs.res.in

JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 8 15 APRIL 2002

54420021-8979/2002/91(8)/5442/7/$19.00 © 2002 American Institute of Physics

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II. EXPERIMENT

Microcrystalline hydrogenated silicon thin films havebeen prepared in an indigenously fabricated conventional ca-pacitively coupled rf PE-CVD~13.56 MHz! system. A set ofsamples were prepared from (SiH41Ar1H2) plasma vary-ing hydrogen flow rate (H f) from 0 to 40 sccm in steps. Thedeposition conditions for the preparation ofmc-Si:H filmshave been summarized in Table I. The applied rf power den-sity was kept fixed at 90 mW/cm2 and gas pressure in thereaction chamber was maintained at 0.8 Torr. The films werecharacterized by electrical, optical, and structural studies.The dark and photoconductivity of the films were recorded atroom temperature after thermal annealing of the samples for1 h at 150 °C at a vacuum of;1025 Torr to avoid the ad-sorbed effect. The optical density data of the samples wereobtained from the transmission and reflection measurementsin the ultraviolet ~UV!-visible ~VIS! region, by a doublebeam spectrophotometer~Hitachi, 330!. The structural char-acterization was done using infrared vibrational spectroscopyon films deposited on polished surface of single crystal Siwafers. Samples deposited on carbon coated copper micro-scope grids were used for transmission electron microscope~TEM! studies to identify the crystallinity of the films. Ra-man study was performed to estimate the degree of crystal-linity.

III. RESULTS AND DISCUSSION

Figure 1 shows the variation of dark conductivitysD ,activation energyDE, and the increase~as a ratio! in theroom temperature dark conductivityDsD due to thermal an-

nealing of the films, plotted as a function of hydrogen flowrate in sccmH f . At H f50, i.e., for the film prepared fromonly Ar-diluted SiH4 plasma, dark conductivitysD was;1025 S cm21 with its activation energyDE50.443 eV,which altogether indicated the microcrystalline nature of thefilm. Before annealing thesD was;1028 S cm21, which is3 orders of magnitude lower than that after annealing. Ad-sorption of atmospheric oxygen within the microvoids re-maining in the Si:H network was assumed to be responsiblefor such lowering in electrical conductivity.15,16 Adsorbedoxygen is likely to behave as an electron trap and reduce thecarrier density in the material.

When we introduced only 5 sccm of hydrogen~i.e. atH f55! in the plasma the film exhibited asD value almostthe same as that of the nonhydrogen diluted case, with aslight lowering inDE. However, it was observed that therewas a significant reduction inDsD value after annealing.When hydrogen flow rate was increased to 10 sccm,sD

increased slightly with associated further reduction inDE.The DsD value was sharply reduced to almost unity. Therethe adsorption effect was minimized due to the introductionof hydrogen as another diluent of the source gas silane inaddition to argon. This indicated that the inhomogeneity andporosity were reduced due to the change in growth morphol-ogy. By increasingH f to 20, sD increased sharply to 1.331023 S cm21 without having any effect of adsorption inthe electrical properties.DE sharply reduced to a minimumvalue of 0.163 eV. Hence, a structural improvement toward ahigher order of microcrystallinity was obtained as a result ofhydrogen-assisted modulation of the network. BeyondH f

520, sD decreased slowly with a corresponding increase ofDE. This was an indication of deviation from the highestorder of crystallinity. However no significant change inDsD

was observed.The optical density data were obtained from absorption

spectra of the film, using UV-VIS near infrared double beamspectrophotometer. Figure 2~left axis! shows the variation of

TABLE I. Typical deposition parameters used formc-Si:H growth.

Substrate temperature: 200 °Crf power applied to the electrodes: 90 mW/cm2

Gas pressure in the plasma: 0.8 TorrSiH4 flow rate: 1 sccmAr flow rate: 10 sccmH2 flow rate: 0–40 sccm

FIG. 1. Variation of dark conductivitysD , activation energyDE, and in-crement insD DsD of mc-Si:H films with hydrogen flow rateH f in theplasma.

FIG. 2. Changes of optical absorption coefficient and optical gap with H2

dilution H f .

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optical absorption coefficienta at a particular photon energyof 2.5 eV, with the change in the H2 flow rate in theplasma.15 A systematic decrease ofa was observed with in-crease ofH f from 0 to 20 sccm. At higher hydrogen flow ratereduction of the disordered amorphous portion resulted inlower optical absorption. However, the material has lowestlight absorption forH f520, at which highest electrical con-ductivity was attained. This indicated the formation of themost rigid Si network structure atH f520. BeyondH f.20,however,a increased slowly as a result of deviation fromsharp crystallinity.

Bonded hydrogen content of the film was estimated fromthe infrared wagging mode absorption around 630 cm21 andusing the oscillator strengthAw51.631019 cm22 in

CH5AwE @a~v!/v#dv.

In Fig. 2 ~right axis! changes ofCH in the film have beenplotted as a function of hydrogen flow rate. AtH f50, i.e.,for the film prepared without hydrogen dilutionCH was 15.4at. %. A gradual reduction ofCH up toH f510, was observed

and at H f520, CH reduced sharply to 8.5 at. %. ForH f

.20, CH increased gradually due to the appearance of theamorphous portion in the material as seen from Raman study~discussed in a later section!. It should be noted that thelowest values ofa and CH were obtained atH f520 wherethe highestsD was achieved.

The degree of crystallinity was estimated from Ramanbackscattering spectra of the films. Figure 3 shows the Ra-man spectra of the films prepared at various hydrogen flowrates. The spectra were deconvoluted into three satellitespectra~as shown in inset of Fig. 3 as an example! in the430–570 cm21 region. Among the three, the most flat Gauss-ian component had a peak distribution around 475–480cm21 and was assigned to the transverse optical~TO! modeof amorphous silicon. The corresponding integrated area wasidentified asI (a). The most intense component was a sharpLorentzian fit with a peak distribution around 518–521cm21. This component was assigned to the TO vibrationalmode of crystalline silicon and the associated integrated areawas identified asI~c!. The intermediate component was abroad Gaussian fit with peak distribution around 504–510cm21. According to Vepreket al.17 this intermediate compo-nent arose due to bond dilation at grain boundaries. Thisgrain boundary component systematically decreased on themonotonically increasing crystalline fraction as demon-strated by Yueet al.18 However, to estimate the crystallinevolume fraction, they considered the grain boundary compo-nent as the inherent part and portion of the crystallinity. Maviet al.19 and Heet al.20 associated this intermediate compo-nent in the Raman spectra to the thermodynamically stablemicrograins of size in the range< 30 Å. Accordingly, weassign this intermediate component to the TO vibrationalmode of Si microcrystals. The volume fraction of crystallin-ity has been estimated from the integrated area of the com-ponents assigned asI~a!, I~c!, and I~b! for amorphous, crys-talline, and grain boundary components, respectively. Peakcenters along with % volume fraction of each component andfull width at half maximum~FWHM! of crystalline compo-nent have been summarized in Table II. The volume fractionf C has been estimated following the empirical formula:

f C5@ I ~b!1I ~c!#/@ I ~a!1I ~b!1I ~c!#.

It was observed thatf C at H f50, was 76.6% i.e., microcrys-tallinity at such high degree can be achieved without hydro-gen dilution. With the introduction of 5 sccm of hydrogenf C

FIG. 3. Raman backscattering spectra of the Si:H films prepared at variousH f .

TABLE II. Summary of Raman deconvoluted spectra of themc-Si:H films prepared at variousH f .

Hydrogenflow rate

(H f) in sccm

Amorphouscomponent

Grainboundary

componentCrystallinecomponent

FWHM ofcrystallinecomponent

~cm21!

Crystallinevolume

fraction (f C)~%!

Peakcenter~cm21!

Xa

~%!

Peakcenter~cm21!

Xb

~%!

Peakcenter~cm21!

XC

~%!

0 478 23.4 507 20.1 518 56.5 12.98 76.65 478 21.0 510 24.0 519 55.0 13.30 79.020 477 10.3 504 3.6 520 86.1 12.44 89.740 476 19.1 505 15.9 521 65.0 13.23 80.9

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increased to 79%. AtH f520 where highestsD was attained,f C has its highest magnitude of 89.7%, however, it decreasedto 80.9% atH f540. Deviation from higher order crystallin-ity as indicated by the lowering in the magnitude ofsD atH f540 was supported by the Raman study. In these samplescontribution of 520 cm21 component (XC) varied from56.5% to 86.1% of the total integrated area of the TO peak.At H f520, where the highest conductivity was achieved, thegrain boundary component (Xb) reduced sharply to 3.6%along with the minimization of the amorphous component(Xa) to a volume fraction of 10.3%. At higherH f , sayH f

540, growth of crystallites was hampered and this is evidentfrom the increase of the amorphous fraction (Xa) from10.3% to 19.1%. The FWHM of the 520 cm21 componentattained its lowest value for the film prepared atH f520 andincreased for both higher and lower H2 flow rate. In spite ofthe increase off C from 76.6% to 79.0% due to the additionof 5 sccm of H2 to the purely Ar-diluted SiH4 plasma, theFWHM of the crystalline component increased by 0.32 cm21

accompanied by the decrease ofXC by 1.5% and increase ofXb by 3.9%. Such increase ofXb due to the introduction ofhydrogen to the plasma may be attributed to the developmentof additional silicon microcrystallites having a size< 30 Åat the periphery of larger grains.19 If we look at the samplesprepared withH f50 andH f520, the FWHM of the crystal-line component reduced from 12.98 to 12.44 cm21 only. Theamorphous component (Xa), however, reduced significantlyfrom 23.4% to 10.3%. The crystalline component (XC) in-creased remarkably from 56.5% to 86.1% along with effi-cient elimination of the contribution from the grain bound-ary. The net crystalline volume fraction of the materialincreased from 76.6% to 89.7%.

Figure 4 represents the variation of the line shape in thesecond order 2TO Raman spectra with different hydrogendilutions. The two-phonon Raman spectra correspond tozone-edge phonons~ZEPs! between 870 and 1060 cm21.21

The spectrumH f50 represents the same for the sample pre-pared from Ar-diluted SiH4 plasma without hydrogen dilu-tion. The spectra forH f55, 20, and 40 were also shown inFig. 4. The maxima of these broadband spectra along withthe FWHM have been summarized in Table III. It was seenthat the maxima position shifted to higher frequency when20 sccm of hydrogen was introduced to the Ar-diluted SiH4

plasma and where the highest degree of crystallinity wasattained. Simultaneously, FWHM of the band exhibited thelowest value for this sample. Both observations led to theconclusion that the highest degree of crystallinity and homo-geneity was achieved at this parametric condition. When hy-drogen flow rate increased to 40, the FWHM suddenly in-creased from 65.80 to 78.67 cm21, though the maximashifted to a lower position only by 1 cm21. This result is incorrelation with the observed decrease ofsD and f C .

Figure 5 shows the variation of deposition rateRd as afunction ofH f . At H f50, Rd was as high as 36 Å/min, i.e.,mc-Si:H films were obtained at a high deposition rate with-out hydrogen in the plasma. The addition of 5 sccm of hy-drogen reduced theRd slightly, but it was prominent atH f

510, when it sharply reduced to 24.0 Å/min. AtH f520,

where the highest degree of crystallinity was attained,Rd

was 14.7 Å/min, which is almost two times higher than thatof the mc-Si:H films obtained from purely hydrogen dilutedsilane plasma. Such sharp lowering ofRd due to the additionof hydrogen in the plasma and simultaneous increase of crys-tallinity as confirmed from Raman study, was attributed toH-radical assisted etching of the disordered amorphous por-tion of the material. With further increase ofH f the rate oflowering in the deposition rate became much slower, because

FIG. 4. Variation of the line shape in the 2TO Raman spectra with differentH f .

TABLE III. The maxima position and FWHM of the band of two-phononRaman spectra corresponding to~ZEPs! between 870 and 1060 cm21 as afunction of H f .

Hydrogen flowrate (H f) in sccm

Maxima positionof band~cm21!

FWHM ofthe band~cm21!

0 961 77.265 961 75.6020 968 65.8040 967 78.67

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of the deviation from the sharp crystallinity and appearanceof a more amorphous portion in the material.

TEM studies were done to identify the crystallinity ofthe films. Figure 6~a! shows the TEM photograph ofmc-Si:H films prepared from purely Ar-diluted silaneplasma. The basic matrix appeared to be a very specific nettype like a mosaic pattern with a large number of separateand randomly distributed areas enclosed by well-definedboundaries. A large number of micrograins, with dimensionsvarying from 100 to 350 Å were developed within the en-closed areas, called islands. The clearly visible boundaryzone that maintained the continuity in the growth is normallyporous and the center of electronically active defects havinga large internal surface area that contributed to the adsorbedeffects on the electrical properties. One interesting feature tobe mentioned here again is that the microcrystalline filmsprepared from only Ar-diluted silane plasma exhibited anenhancement in the room temperature conductivities on ther-

mal annealing and that arose from the outdiffusion of ad-sorbed gases, probably the oxygen, trapped within the micro-voids remaining in the Si:H network.15,16 Themicrocrystallization of the network was conclusively identi-fied by sharp crystallographic rings observed in the electrondiffraction pattern as depicted in Fig. 6~b!. The most promi-nent crystallographic planes were~111!, ~220!, and~311! ofc-Si. Other less prominent planes like~440!, ~331!, and~422! were also observed. Figure 6~c! shows the TEM pho-tographs of the films prepared withH f520. One most inter-esting fact was that the supernetwork observed in the TEMof the films prepared without hydrogen completely disap-peared. A large number of micrograins having dimensions50–200 Å appeared in the homogeneous matrix ofa-Si:H. Anumber of sharp crystallographic rings as appeared in Fig.6~d! identified the development of Si microcrystallites insimilar planes as mentioned above.

In our previous article,15 we described the formation pro-cess of microcrystalline silicon from Ar-diluted SiH4 plasma.We have found that the metastable state of Ar designated asAr* (3P0,2) plays a crucial role in transferring energy to thegrowth zone. The process of nucleation and development ofcrystallites has been explained. The energy released fromde-excitation of Ar* is utilized in breaking up of strainedweak Si–Si bonds and remaking of strong Si–Si bonds at theboundary ofc-Si nucleation centers and the amorphous ma-trix. Thus the network is relaxed creating a more compactand hydrogen free zone. Formation of large-size crystalliteswas promoted by such heterogeneous structures containinglocalized hydrogen depleted regions. The heterogeneity is,however, removed due to the addition of hydrogen to theplasma through hydrogen coverage of the growth surface.The size of the compact hydrogen-free region decreased, re-sulting in the decrease in grain size. Another important as-pect of preparation ofmc-Si:H from Ar-diluted SiH4 plasmais its high-growth rate. Apart from electron impact dissocia-tion of SiH4 , Ar* would directly lead to dissociation of SiH4by utilizing its de-excitation energy. Some of the Ar* reactswith SiH4 molecules to form radicals such as SiH3 , SiH2 ,etc., which contribute to the high rate of deposition of thefilms. Columnar growth morphology is an interesting featureof mc-Si:H prepared from Ar-diluted SiH4 plasma. In thecase of hydrogen diluted SiH4 plasma, SiH3 is the main filmforming precursor having a long diffusion length andtraverses a long distance under the high H-covered surfaceuntil it encounters a surface dangling band. This is the gen-eral feature of CVD-like growth which results in homoge-neous film. For Ar-diluted SiH4 plasma, SiH2 radical densityincreased linearly with increasing Ar dilution.22 Again Rob-ertson and Gallagher23 observed that among the monosiliconradicals SiH has the largest concentration in the plasma,which increase sharply with increasing Ar dilution to theSiH4 plasma in rf glow discharge. Low mobility lower hy-dride, SiHn (0<n<2) contribute significantly to the filmgrowth. Since their diffusion lengths are lower, they can beeasily inserted into the Si–H bond on a H-covered surface ofthe film. In this case growth of the material takes place asisolated columns. Hence microvoids develop in significantnumbers, generating porosity at the grain boundary/column

FIG. 5. Deposition rate as a function ofH f .

FIG. 6. ~a! TEM micrograph and~b! electron diffraction pattern of themc-Si:H film prepared without hydrogen dilution;~c! TEM micrograph and~d! electron diffraction pattern of themc-Si:H film prepared atH f520.

5446 J. Appl. Phys., Vol. 91, No. 8, 15 April 2002 Jana, Das, and Barua

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boundary as the intercolumnar space is poorly filled up. Si-multaneously, inhomogeneity in the network structure ap-pears to be inevitable. Such type of growth is termed physi-cal vapor deposited ~PVD!-like growth. The TEMphotograph presented in Fig. 6~a! shows the supernetworkappeared due to density fluctuation and columnar agglomera-tion of the grains. The connecting areas inbetween the col-umns indicated by the white contrast in the micrograph werelikely to be the amorphous connecting tissues associatedwith columnar growth morphology and probably the charac-teristic feature contributed by Ar-diluted plasma.24 SimilarTEM features have been reported to be observed inmc-Si:Hfilms prepared by reactive sputtering25 or from Ar-dilutedglow discharge plasma26 and are characteristics of columnarstructure.27 A consequence of the columnar growth is thatsuch samples can have a very large internal surface area.When such samples are exposed to atmosphere postdeposi-tion oxidation was observed. Due to adsorbed oxygen in thematerial, the sD value before thermal annealing was;1028 S cm21 for the sample without hydrogen dilution.But after thermal annealing when the adsorbed gases areeliminated the conductivity increases to;1025 S cm21. Theeffect of such postdeposition oxidation on the electrical prop-erties has been observed by Street and Knights.16 However,this effect was not present when we preparedmc-Si:H byintroducing an optimized amount of hydrogen to the Ar-diluted SiH4 plasma. No trace of bonded oxygen was de-tected from infrared spectra so there is a change in growthmorphology due to the addition of hydrogen in the plasma.

We shall now discuss the role of atomic hydrogen in theimprovement/modification of network structure. Before go-ing to our Ar-diluted case, we summarize here the multiplerole of atomic hydrogen in the formation ofmc-Si:H, as:

~i! The hydrogen coverage of the growing surface en-hances the diffusion of the adsorbed radicals such asSiH3 , SiH2 , or SiH.14

~ii ! Hydrogen radicals act as an etchant of Si atoms fromthe weak Si–Si bonds at the growing surface and mi-crocrystallization takes place when chemical equilib-rium between deposition and etching is attained.28

~iii ! Energetic atomic hydrogen penetrates into severallayers below the top surface and promotes the net-work propagation reaction29 by ~a! dangling bondcompensation,~b! weak bond breaking and recon-struction of strong Si–Si bonds,~c! strain relaxation,and ~d! giving chemical potential to the growing sur-face by breaking the Si–H bond which is an exother-mic process, thereby leading to structural reorienta-tion for attaining energetically favorable configurationand hence microcrystallization.

In the Ar-diluted case addition of hydrogen improves theSi network within the columns by increasing the hydrogencoverage factor at the column surface.30 The lower hydrideshaving lower mobility take an active role in nucleation andgrain growth. The hydrogen coverage on the surface en-hances the diffusion of the film forming precursors and helpsto reach the energetically favorable sites. Hence the resultingfilm is homogeneous containing less active defects. This nor-

mally happens in the case of CVD-like growth. So, due tothe addition of an optimum amount of hydrogen the growthmechanism changes from PVD like to CVD like. Besidesthis hydrogen radical assisted etching of the amorphous part,which is confirmed by sharp lowering of the deposition rateas shown in Fig. 5 also helps in the formation of microcrys-tallites. Formation of porosity and rough surface were elimi-nated through hydrogen assisted modulation@as mentionedin ~iii !# of the films prepared with a criticalH f520, as indi-cated by the unity value ofDsD and the TEM feature. Thedefects produced in the material by the heavy Ar atom werecompletely offset by the H atom through structural reorien-tation and modulation of the network. In this regard, additionof a critical amount of hydrogen is beneficial. At this para-metric condition chemical equilibrium between depositionand etching was established.28 For H f55, the amount of hy-drogen atom in the plasma was not sufficient to completelyoffset the defects created by argon, which was indicated by asignificantly large value ofDsD . Again at H f510, theamount of hydrogen was at such a level that this was suffi-cient to offset the defects as seen from the almost unity valueof DsD , but was not enough for the improvement in micro-crystallinity. As a result, thesD value did not increase sig-nificantly, whereasH f520 provided the critical amount ofhydrogen over the columnar growth surface that improvedthe microcrystallinity within the columns and modulated theboundary of the column surface. At hydrogen flow rateshigher than the optimum value, there is deviation fromchemical equilibrium which adversely affects the crystallin-ity. Raman study confirmed the existence of the more amor-phous part in the material at this higher hydrogen dilutionlevel. Increase in the amorphous component was seen fromthe increase inCH and absorption coefficient forH f.20. AtH f50, the hydrogen content and optical absorption werecomparatively higher than that of themc-Si:H films pre-pared from purely hydrogen diluted SiH4 plasma. This is aconsequence of structural heterogeneity in the material. Theintercolumnar space was filled with voids containing largequantities of bonded hydrogen.

IV. CONCLUSIONS

Using Ar dilution to the SiH4 plasma, undopedmc-Si:Hfilms were prepared with a dark conductivity;1025 S cm21

and its corresponding activation energy of 0.443 eV. This isobtained at a high deposition rate of 36 Å/min using a mod-erate rf power density of 90 mW/cm2. A large number ofmicrograins with dimension 100–350 Å were developed andseveral well defined crystallographic planes with dominanceof ~111!, ~220!, and~311! orientations were identified. How-ever, the grain boundary area was quite extended and thatcontained a large amount of defects probably in the form ofvoids. This contributed to the change of electrical propertiesof the films due to adsorption effects when exposed to atmo-sphere.

When hydrogen was introduced in increasing steps, theadsorption effects were completely eliminated at a certainlevel of hydrogen in the plasma. The network structurechanged from inhomogeneous porous to a homogeneous,

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nonporous, rigid one. Highly conducting as well as highlycrystalline mc-Si:H film was obtained at a hydrogen flowrate of 20 sccm only, havingsD;1023 S cm21 and a crys-talline volume fraction of 89.7%. An optimum amount ofhydrogen was necessary to get a highest order of crystallin-ity. The presence of hydrogen more than the optimum valueled to a deviation from chemical equilibrium between depo-sition and etching. The improvement in network structure aswell as degree of crystallinity due to the addition of theproper amount of hydrogen in Ar-diluted silane plasma wasalso supported from lower FWHM of the 2TO Raman band;970 cm21. We have obtained good qualitymc-Si:H at adeposition rate of;15 Å/min, which is almost two timeshigher than that obtained from only H2-diluted SiH4 plasma.We have been able to combine the beneficial effects of usingAr as well as H2 as diluents to SiH4 plasma to obtain highquality mc-Si:H film at an increased deposition rate.

ACKNOWLEDGMENTS

The work was carried out under a project funded jointlyby the Department of Science and Technology and the Min-istry of Non-Conventional Energy Sources, Government ofIndia.

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