Spectroscopic studies on nanocrystalline silicon thin films preparedfrom H2-diluted SiH4-plasma in inductively coupled low pressureRF PECVD

Mahua Chakraborty, Amit Banerjee, Debajyoti Das n

Nano-Science Group, Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

H I G H L I G H T S

� nc-Si films of �84–67% crystallinityprepared by ICP-CVD at 300 1C.

� When nanocrystallinity decreasesthe ultra-nc component increases athigher SiH4 flow.

� Reduced porosity and surface rough-ness caused by increased ultra-nccomponent.

G R A P H I C A L A B S T R A C T

Structural evolution of nanocrystalline silicon thin films exhibiting enhanced ultra-nc component duringreduction of crystallinity at higher SiH4 flow, as evident from UV-ellipsometry and Raman studies.

a r t i c l e i n f o

Article history:Received 23 December 2013Accepted 17 March 2014Available online 25 March 2014

Keywords:Spectroscopic ellipsometryNano-crystalline silicon thin filmH2-diluted SiH4 plasmaICP-CVDOptical constantDielectric function

a b s t r a c t

A comprehensive analysis on the evolution of the microstructure as well as optical constants anddielectric functions of intrinsic hydrogenated nano-crystalline silicon thin films prepared by highly H2

diluted SiH4 plasma in a planar inductively coupled RF plasma chemical vapor deposition (ICP-CVD)reactor has been performed by spectroscopic ellipsometry. Films are assumed to have a three-layerstructure, with a thin incubation layer at substrate/bulk interface, the bulk layer and a thin growth zoneand surface roughness layer. Individual composition and the thickness of each layer have been estimatedfrom the simulation of the ellipsometry data using Bruggeman effective medium approximation (BEMA).The ellipsometry results are correlated with atomic force microscopy and micro-Raman data of thesefilms. The effect of the flow rate of SiH4 and the key role of hydrogen dilution on growth dynamics,optical constants and dielectric functions of highly crystalline nanosilicon films is discussed elaborately.The bulk crystalline volume fraction of the deposited films varies considerably (�67–84%) with thechange in flow rate of SiH4. With increasing SiH4 flow rate the overall bulk crystallinity reduces; howeverthe ultra-nanocrystalline component (Xunc) enhances substantially that helps reducing the porosity andsurface roughness.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

In the past few decades hydrogenated micro-crystalline andnano-crystalline silicon (mc/nc-Si:H) thin films are receiving con-siderable attention in view of their extensive utilization in thefabrication of optoelectronic as well as electronic devices such as

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Physica E

http://dx.doi.org/10.1016/j.physe.2014.03.0161386-9477/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ91 33 2473 4971; fax: þ91 33 2473 2805.E-mail address: erdd@iacs.res.in (D. Das).

Physica E 61 (2014) 95–100

solar cells, image sensors, thin film transistors (TFT), and manyother memory and display devices [1–3]. Both mc-Si:H and nc-Si:Hfilms exhibit higher doping efficiency, higher mobility as well asconductivity, enhanced low-energy optical absorption, tunableband gap and greater stability against light induced degradationcompared to their amorphous counterparts, leading to impressiveimprovement in efficiencies, in the case of single- or multi-junction solar cells [4].

The nc-Si:H can be described as a mixed phase materialconsisting of Si nanocrystallites (E2 to 20 nm) and its aggregatesembedded in an amorphous Si network, with disorder in the formof voids and grain boundaries. The material property of the nc-Si:H absorbing layer depends strongly on deposition parameters e.g.,plasma frequency, flow rate and ratio of individual gases, gaspressure in the plasma, RF power, substrate temperature and H2-dilution of SiH4 in PECVD [5,6]. Capacitively coupled plasma (CCP)systems have been widely used over the past few decades.However, high density plasma (HDP) sources e.g., inductivelycoupled RF plasma chemical vapor deposition (ICP-CVD) has beengaining increasing attention. Some of the unique characteristicse.g., (i) high plasma density, in particular high electron density(�1012 cm�3), (ii) low plasma sheath potential leading to reducedion bombardment on the deposited films, (iii) low electrontemperatures (� few eV), (iv) reduced ion bombardment due toremote plasma generation and (v) good uniformity of the plasmaparameters in the radial and axial directions, make ICP-CVDadvantageous over CCP systems [7–10]. Recently, optical proper-ties of Si nanoparticles of SiO2 based nanostructures have beenreported, using spectroscopic ellipsometry [11–13]. Although fewreports are available on dielectric functions of nc-Si with a few nmgrain sizes [14], a detailed experimental investigation on theoptical properties of nc-Si films synthesized by ICP-CVD, inparticular, is still lacking.

The present spectroscopic investigation, primarily by ex-situUV ellipsometry, deals with the evolution of intrinsic hydroge-nated nano-crystalline silicon network at a low substrate tem-perature in atomic hydrogen rich ensemble in ICP-CVD. The opticalconstants, i.e. refractive index, extinction coefficient and dielectricfunctions of nanocrystalline silicon, embedded in amorphoussilicon matrix accompanied by voids, are determined. Micro-Raman spectroscopy and atomic force microscopy (AFM) data areused to corroborate the inferences drawn from ellipsometry.

2. Experimental procedure

In a planar radio frequency inductively coupled plasma chemi-cal vapor deposition (ICP-CVD) reactor, a set of intrinsic nc-Si:Hfilms were deposited on Cornings Eagle2000™ glass substrates ata moderately low temperature of 300 1C. Films were prepared bysystematically varying SiH4 flow rate, F(SiH4), from 1.5 to 7.5 sccm,while maintaining a fixed H2 flow of 50 sccm that introduces avariation of H2-dilution, R(H2)¼[H2/(SiH4þH2)], from �97% to�87% in the plasma. The corresponding RF power was kept fixedat 500 W and a constant pressure was maintained at 50 mTorr,during deposition. The detail description of the ICP-CVD reactorhas been presented elsewhere [10]. The structural composition ofthe nc-Si:H films from surface to film-substrate interface wasinvestigated by using a fixed angle of incidence (701) spectroscopicellipsometer (Horiba Jobin-Yvon, Model: HR 460 FUV AGAS),operating within a span of energy from 1.5 to 6.0 eV. Micro-Raman measurements was carried out by a Renishaw inVia micro-Raman spectrophotometer (Sl. No. 12W143), at room temperaturein a backscattering geometry, using 514 nm Arþ laser as anexcitation source. Laser light was focused on the sample throughthe 50� magnification Leica microscope objective that provides a

laser spot size in the range �3 μm. The laser power was kept at3 mW/cm2 to prevent undue heating of the sample. The surfaceroughness was estimated by an atomic force microscope (AFM;Bruker AXS, Mannheim, Germany).

3. Spectroscopic ellipsometry and simulation

Spectroscopic ellipsometry (SE) is a very precise, non-destructiveand non-contact optical method, widely used to study the evolutionof optical constants of solid, its thickness and composition as well.It measures the change in polarization state of light that is obliquelyreflected from the surface of a sample. The polarization change isdescribed by an amplitude ratio tan (Ψ) and phase difference Δ,between light oriented in the parallel (p-) and perpendicular (s-)directions relative to the sample surface shown as follows:

ρ¼ tan ψeiΔ ¼ Rp

Rsð1Þ

where Rp and Rs represent complex Fresnel reflection coefficientsfor p- and s-polarized light respectively [15]. The pseudo-dielectricfunction is calculated from ρ using the following relation [16]:

⟨ε1⟩þ i⟨ε2⟩ ¼ sin 2φ 1þ 1�ρ

1þρ

� �2

tan 2φ

" #ð2Þ

where φ is the angle of incidence.The dielectric function obtained from signals detected by

ellipsometry on a sample-on-substrate system is termed as the‘pseudo-dielectric function’ ⟨ε⟩ that usually contains informationnot only of the film, but the whole system including the substrate,bulk of the film, interfaces and surface over-layers. For properellipsometric data interpretation comparison of the experimentaldata to a calculated dielectric function based on a specific opticalmodel of the sample-on-substrate system is done. In the presentoptical model used to fit the experimental data we assume the filmas a three-layer structure consisting of (i) completely amorphousthin incubation layer immediately on the substrate surface, (ii) thebulk nano-crystalline layer which is a mixture of variable compo-sition of amorphous, crystalline and void components and (iii) athin layer identifying the growth zone and surface roughness,having higher fraction of void compared to other two components[17]. A classical model calculation is based on the effectivemedium approximation that describes the dielectric response ofa heterogeneous system as a combination of the dielectric func-tions of its different components. The nc-Si film can be schema-tized as an effective medium, where a-Si is the host matrix and Sinanocrystals are embedded in it accompanied by voids, repre-sented by Bruggeman effective medium approximation (BEMA)[18].

f nc�Si⟨εnc�Si⟩� ⟨εeff ⟩

⟨εnc�Si⟩þ2⟨εeff ⟩þ f a�Si

⟨εa�Si⟩� ⟨εeff ⟩

⟨εa�Si⟩þ2⟨εeff ⟩

þ f void⟨εvoid⟩�⟨εeff ⟩

⟨εvoid⟩þ2⟨εeff ⟩¼ 0;

f nc�Siþ f a�Siþ f void ¼ 1 ð3Þwhere fnc-Si, fa-Si and fvoid are the volume fractions of nc-Si, a-Si andvoids respectively; ⟨εnc-Si⟩, ⟨εa-Si⟩, ⟨εvoid⟩ and ⟨εeff⟩ are pseudo-dielectric functions of nc-Si, a-Si, void and the effective medium,respectively. Here the fit parameters are individual layer thick-nesses and volume fractions of components constituting theeffective medium. As a reference to the BEMA model, the opticalconstants of amorphous silicon given by Tauc–Lorentz [19,20] andthose of nanocrystalline silicon given by Jellison's fine grainpolysilicon model [21] are used. The simulations have beenperformed by iteratively minimizing the weighted unbiased esti-mator χ [11] between the measured and the calculated values; χ2 is

M. Chakraborty et al. / Physica E 61 (2014) 95–10096

estimated using linear regression method as

χ2 ¼ 12N�M

∑N

i ¼ 1ð tan ψ c

i � tan ψmi Þ2�ð cos Δc

i � cos Δmi Þ2

h ið4Þ

where N and M are numbers of measured and calculated wave-lengths respectively; c and m are calculated and measured valuesof Ψ and Δ respectively.

4. Results and discussion

The distribution of the imaginary part of the pseudo-dielectricfunction ⟨ε2⟩, extracted from the spectroscopic ellipsometry data ofthe samples prepared at different SiH4 flow rates, has beendisplayed in Fig. 1. The oscillations of ⟨ε2⟩ at low energies below3.0 eV are related to the nature of the substrate, the film thicknessand the composition of the incubation layer, whereas the magni-tude of ⟨ε2⟩ at high energies above 3.0 eV is influenced by the bulkcomposition and the surface roughness. For every sample thedistribution of the ⟨ε2⟩ spectrum has two distinct shoulders, S1and S2 around 3.41 eV and 4.2 eV respectively, corresponding to E1and E2 transitions of crystalline silicon and hence signifies theexistence of crystallinity in the prepared samples [22–24]. WhenSiH4 flow rate varies from 1.5 sccm to 3 sccm, the ⟨ε2⟩ distributionsof the corresponding samples almost overlap with each other, witha very little increase in the intensity difference between the twoshoulders. Further increase in SiH4 flow rate to 4.5 sccm inducessignificant enhancement in the amplitude of ⟨ε2⟩ over the entireenergy band. The intensity difference between S1 and S2 ismarkedly increased. The increasing nature of the amplitude of ⟨ε2⟩and the intensity difference between the two shoulders S1 and S2continue with increase in SiH4 flow rate up to 7.5 sccm. Fig. 2 showsa typical simulation result presented by the continuous line plotwhich is in good agreement with the experimental data. Theschematic illustration of the three-layer optical model has beenpresented by the inset of Fig. 2. χ2 varies in between 0.01 and 0.07in the present study.

The fitting parameters obtained from the simulation of theellipsometry data for the nc-Si:H films prepared at various SiH4

flow rates in ICP-CVD plasma have been displayed in subsequentfigures. Fig. 3 depicts variations in thicknesses of the growth zoneand surface roughness layer and the incubation layer with changesin SiH4 flow rate. The incubation layer thickness of the nc-Si:Hnetwork has been found to increase progressively from 46 Å to128 Å with increase in SiH4 flow rate. The thicknesses of the

growth zone and surface roughness layer reduce monotonicallywith increase in SiH4 flow rate. From the simulation of theellipsometry data it is observed that the bulk layer of nc-Si:H filmdeposited at lowest SiH4 flow rate (1.5 sccm) corresponding to R(H2)¼97%, contains �84% c-Si component, and 16% voids with notrace of a-Si component (Fig. 4(a)). As SiH4 flow rate is increased insteps to 7.5 sccm corresponding to R(H2)¼87%, the bulk crystal-linity gradually drops to�67% at the cost of similar gradualenhancement of the a-Si component to �23%. The void fractionin the bulk layer reduces gradually to �10% when SiH4 flow rate isincreased to its maximum value. The increase in SiH4 flow rate,however, induces almost similar lowering in the void fraction aswell as the nanocrystalline component in the growth zone andsurface roughness layer (Fig. 4(b)). The corresponding amorphouscomponent increases gradually with the rise in SiH4 flow rate.Fig. 5(a) and (b) displays the Gaussian deconvolutions of theRaman spectra for the nc-Si:H films for the two extreme SiH4

flow rate conditions, into three satellite components correspond-ing to amorphous, ultra-nanocrystalline (Xunc) and nanocrystalline(Xnc) volume fractions. Considering ultra-nanocrystalline compo-nent as a part and portion of crystallinity, the overall crystallinityin the bulk has been found to vary from �84% to 67% onincreasing the SiH4 flow rate from 1.5 to 7.5 sccm, as shown inFig. 5(c) wherein an excellent matching with similar bulk crystal-linity obtained from ellipsometry has been demonstrated.The fraction of the ultra-nanocrystalline component in the bulk,

Fig. 1. Distribution of the imaginary part of pseudo-dielectric function ⟨ε2⟩

extracted from the spectroscopic ellipsometry data, for nc-Si:H films prepared byICP-CVD at various flow rates of SiH4 to the plasma.

Fig. 2. Typical fitting curve from the results obtained by BEMA optical modelingwith the ellipsometry data, exhibiting an excellent match with the experimentallyobtained data points. The inset presents the schematic illustration of the three-layer optical model used to fit the ellipsometry data for nc-Si:H film prepared onglass substrate.

Fig. 3. Variations in individual thickness for the incubation layer and growth zoneand surface roughness with changes in SiH4 flow rate.

M. Chakraborty et al. / Physica E 61 (2014) 95–100 97

Xunc/(XuncþXnc), has been identified to increase almost linearlyfrom 18% to 32% at higher SiH4 flow rate when the overallcrystallinity reduces. The AFM morphology images of the depos-ited films for the minimum (1.5 sccm) and maximum (7.5 sccm)SiH4 flow rate are shown in Fig. 6(a) and (b), respectively. The rmsroughness on the sample surface obtained from AFM measure-ment is plotted in Fig. 6(c) which shows very close nature ofvariation of the surface roughness estimated from the ellipsometrydata.

The optical properties of the nc-Si:H layer in the visible andultraviolet regions largely depend on its microstructures [14,25–27]. The absolute optical constants and the dielectric functions ofthe film are simulated utilizing the composition of the bulk layerobtained from the BEMA fitting of the ellipsometry data, consider-ing the system as a single layer component i.e., only bulk materialof extended thickness and are presented in Figs. 7 and 8 respec-tively. The optical constants and dielectric functions of bulk c-Si(dotted curves) are also included in these figures for comparison. Ithas been identified that the bulk component of all the depositedfilms exhibits a significant lowering in the refractive index,extinction coefficient and dielectric functions compared to bulkc-Si. The sharp peaks observed in the ε2 spectrum of c-Si shown inFig. 8(b) are attributed to direct interband transitions related to E1and E2 critical points (CPs) at different parts of the Brillouin zone(BZ) [28]. The E1 and E2 peaks of the bulk nc-Si films are grosslydifferent from those of c-Si, in terms of sharpness of peaks. The E2peak position of ε2 spectra also redshifted with the higher flowrate of SiH4. Broader ε2 peaks of films indicate smaller size of thegrains compared to c-Si as the half-width of the E1 and E2transition peaks is inversely proportional to the grain size of Sicrystals [29]. It can be observed from Figs. 7(a) and 8(a) that n

Fig. 4. Variation of the structural composition of (a) bulk layer and (b) growth zoneand surface roughness layer of nc-Si:H films prepared at different flow rates of SiH4.

Fig. 5. (a, b) Raman spectra for the nc-Si:H films for SiH4 flow rates of 1.5 and7.5 sccm, respectively and their Gaussian deconvolution into three satellite com-ponents, corresponding to amorphous, ultra-nanocrystalline and nanocrystallinevolume fractions. (c) Variation of the overall crystalline volume fraction in the bulkof the nc-Si:H films with SiH4 flow rates, as estimated from the Raman data, andthe comparison on similar variation separately obtained from ellipsometricsimulations. Raman data separately identifies an increasing fraction of the ultra-nanocrystalline component in the overall crystallinity, at increasing SiH4 flow rate.

Fig. 6. (a, b) AFM images demonstrating the surface morphology of samplesprepared at SiH4 flow rates of 1.5 and 7.5 sccm, respectively. (c) Comparison ofthe thickness of growth zone and surface roughness layer estimated separatelyfrom AFM studies and spectroscopic ellipsometry.

M. Chakraborty et al. / Physica E 61 (2014) 95–10098

value as well as ε1 found from the simulation results reduces withdecrease in the flow rate of SiH4 at Eo3.4 eV and at E44.3 eV.With decrease in SiH4 flow rate the magnitude of ε2 at Eo3.4 eValso decreases.

Undoped hydrogenated nanocrystalline silicon (nc-Si:H) filmsare deposited at the low temperature ICP-CVD reactor when alarge quantity of hydrogen is added to the process gas SiH4 so thata large flux of atomic hydrogen remains available on the growthsurface. The growth of nc-Si:H induced by the presence ofhydrogen in the plasma could be interpreted in terms of severalmechanisms [30] e.g., hydrogen-enhanced surface diffusion ofadsorbed precursors [31,32], preferential etching of noncrystallinematerial by hydrogen [33,34] and subsurface restructuring, termedas ‘hydrogen chemical annealing’ [35,36]. The structural networkof a thin film is specifically controlled by energy relaxation processof adsorbed precursors on the growing surface as well as by theirsurface diffusion lengths. A highly ordered and dense siliconnetwork is produced when the precursors possess a larger surfacediffusion coefficient (Ds) or a longer staying time (τs). SiH3 happensto be the main precursor responsible for the film growth in SiH4/H2 gas mixture [37,38] due to its long reaction lifetime in SiH4

plasma [39] while other lower hydride precursors like SiHx

(1rxr2) and ions play a crucial role in controlling the propertiesof the material, as they are more reactive compared to SiH3.Similar to the effect of atomic H induced control of growth, siliconnetwork has also been shown to be modulated through energytransfer by de-excitation at the growth zone from the excitedstates of Ar* and He* in the noble gas diluted SiH4 plasma inPECVD [17,40,41].

In ICP-CVD a relatively high plasma density and low plasmapotential is generated as compared to CCP which leads to twoimportant effects. Firstly, high electron density accelerates theprocess of dissociation and ionization of SiH4 precursor throughelastic/inelastic interactions between electrons and SiH4 moleculesand produces more free SiHx (x¼1, 2, 3) radicals. Secondly largeamount of atomic hydrogen produced during the dissociation ofSiH4 molecules in high density plasma leads to a high surfacecoverage that ultimately increases the surface diffusion length of

SiHx (x¼1, 2, 3) radicals due to lower surface diffusion activationenergy [31]. ICP-CVD also reduces the degree of ion-bombardmenton the growing surface due to remote plasma generation.

During the deposition of nc-Si:H film on insulator surface atlow substrate temperature, a-Si dominated incubation phase isinitially formed owing to the poor migration of the depositionprecursor, followed by a crystallization phase in which nucleationand growth of the crystallites take place [42,43]. A thick incuba-tion layer in the nc-Si:H film deteriorates the performance ofdevices [44]. With increasing hydrogen dilution, by reducing SiH4

flow rate systematically, a usual improvement in bulk crystallinityis observed, leading to the decrease in the amorphous incubationlayer thickness. Increasing hydrogen radical density wouldenhance the surface diffusion of the precursors (SiH3) and producelocal heating through hydrogen-exchanging reactions on thegrowing surface, thereby enabling precursors to reach energeti-cally favorable sites, resulting in the formation of a crystallinenucleus [45,46]. In our study, maximum H2 dilution ratio of�97%is used to fabricate the nc-Si:H films; a minimum thickness ofincubation layer of 36 Å has been achieved. The higher crystallinityof the material is accompanied by an increase in surface roughnessas well as the void content in the layer. The increase in H2 dilutionresults in the formation of larger crystallites or crystallite aggre-gate [47] which causes an increase in the void content (Fig. 4) aswell as surface roughness of the film (Fig. 3). A continuous increasein the thickness of surface roughness layer is observed with theenhancement in H2 dilution. At maximum H2 dilution in thepresent work (i.e. R(H2)¼97%) the material becomes almost freeof any bulk amorphous component. However, with reduction in H2

dilution i.e., increase in SiH4 flow rate, the overall crystallinityreduces whereas the thickness as well as void content of thegrowth zone and surface roughness layer decreases that may becorrelated also with the higher ultra-nanocrystalline component(Xunc) calculated from Gaussian deconvolution of Raman spectrashown in Fig. 5. The high magnitude of Xunc even at lower H2

dilution could be the consequence of high electron density avail-able in the ICP-CVD reactor. Voids are usually generated in order torelease the stress formed between the long range ordered c-Si and

Fig. 7. Variation of (a) refractive index, n, and (b) extinction coefficient, k, of thebulk of nc-Si:H films deposited with different SiH4 flow rates. The dotted linerepresents the corresponding variation for c-Si.

Fig. 8. Variation of the (a) real part, ε1, and (b) imaginary part, ε2, of dielectricfunction of the bulk of nc-Si:H films deposited with different SiH4 flow rates. Thedotted line represents the corresponding variation for c-Si wherein E1 and E2critical points are separately identified.

M. Chakraborty et al. / Physica E 61 (2014) 95–100 99

short range ordered a-Si phase in nc-Si:H films. At enhanced SiH4

flow rate, ultra-nanocrystalline grains formed in larger proportionresult in less void generation in the deposited film and lowersurface roughness.

5. Conclusion

The evolution of growth of intrinsic hydrogenated nano-crystalline silicon thin films have been studied by spectroscopicellipsometry using highly H2 diluted SiH4 plasma at a moderatelylow temperature of 300 1C in a planar inductively coupled RFplasma chemical vapor deposition (ICP-CVD) reactor. The thicknessand the composition of each layer are precisely determined usingBEMA fitting to the ellipsometry data considering a three layeroptical model of the film. By varying the SiH4 flow rate from 1.5 to7.5 sccm, corresponding to H2 dilution of highest 97% to lowest87%, nc-Si:H films with high crystalline volume fraction (�84–67%) are developed. From ellipsometric simulations it has beenobserved that at lower SiH4 flow rate the material quality isdeteriorated in terms of porosity and surface roughness in spiteof having higher overall crystallinity and thinner incubation layer.On the contrary the film becomes more dense and smooth athigher SiH4 flow rate, with reasonably high crystalline volumefraction, a major contribution of which arises from its ultra-nanocrystalline component, Xunc.

Acknowledgments

The work has been done under nano-silicon projects funded bythe Department of Science and Technology (Nano-Mission Pro-gram) and the Council of Scientific and Industrial Research,Government of India.

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