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Thin Solid Films 516 (2

Structural determination of nanocrystalline Si films usingellipsometry and Raman spectroscopy

Sanjay K. Ram a,⁎, Md. N. Islam b, P. Roca i Cabarrocas c, Satyendra Kumar a

a Department of Physics & Samtel Centre for Display Technologies, Indian Institute of Technology Kanpur, Kanpur-208016, Indiab QAED-SRG, Space Application Centre (ISRO), Ahmedabad – 380015, India

c LPICM, UMR 7647-CNRS, Ecole Polytechnique, 91128 Palaiseau Cedex, France

Available online 23 December 2007

Abstract

Single phase nano and micro crystalline silicon films deposited using SiF4/H2 plasma at different H2 dilution levels were studied at initial andterminal stages of film growth with spectroscopic ellipsometry (SE), Raman scattering (RS) and atomic force microscopy (AFM). The analysis ofdata obtained from SE elucidates the microstructural evolution with film growth in terms of the changes in crystallite sizes and their volumefractions, crystallite conglomeration and film morphology. The effect of H2 dilution on film microstructure and morphology, and the corroborativefindings from AFM studies are discussed. Our SE results evince two distinct mean sizes of crystallites in the material after a certain stage of filmgrowth. The analysis of Raman scattering data for such films has been done using a bimodal size distribution of crystallite grains, which yieldsmore accurate and physically rational microstructural picture of the material.© 2008 Elsevier B.V. All rights reserved.

PACS: 68.55.-a; 68.55.Jk; 68.35.CtKeywords: Silicon; Thin films; Structural properties; Growth mechanism; Ellipsometry; Atomic force microscopy (AFM); Raman spectroscopy

1. Introduction

Recent demonstration of thin film solar cells produced fullyfrom nanocrystalline (or microcrystalline) silicon has generateda new wave of interest in undoped nanocrystalline silicon asthis material offers increased stability against light induceddegradation and has enhanced low-energy absorption in con-trast to amorphous silicon (a–Si:H) [1–4]. Hydrogenated nano-crystalline silicon (nc–Si:H) is also attractive for thinfilm transistors and other photovoltaic devices due to its easyfabrication technology allowing large area deposition, evenat low temperatures, on non-crystalline substrates [5,6]. Thestability and efficiency of the nc–Si:H films are important issuesfor these device applications and both these factors aredetermined by the microstructural characteristics of the film[7,8]. The microstructural characterization of the componentlayers are thus vital to the understanding and prediction of the

⁎ Corresponding author.E-mail addresses: skram@iitk.ac.in (S.K. Ram), satyen@iitk.ac.in (S.Kumar).

0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.12.113

device efficiency and properties. The structural evolution ofplasma deposited nc–Si:H films can be manipulated to yieldmaximum efficiency by altering the deposition parameters,mainly, hydrogen dilution. The knowledge of film microstruc-ture at various levels of film thicknesses and its relationshipwith different H2 dilutions provides a method to predict andassess the outcome of the controlled deposition techniques.

Microstructurally, plasma deposited nc–Si:H and micro-crystalline silicon (μc–Si:H) are heterogeneous materials con-sisting of Si nanocrystallites (≈2 to 20 nm) and aggregatesthereof (which may reach up to few hundred nm in μc–Si:H),dispersed in an amorphous Si matrix, and disorder in the form ofvoids and grain boundaries. In the absence of an amorphousphase in this material (which is then termed as single-phasenc–Si:H or μc–Si:H), the disordered phase consists of voids,density deficit and clusters of nanocrystallites. The hetero-geneous nature of nc–Si:H microstructure exists along thetimescale of growth as well, and with an increase in filmthickness, a distribution in the sizes of crystallite grains can beobserved, with variation in the fractional compositions of thedifferent sized constituent grains.

Fig. 1. (a) Measured imaginary part of bɛ2N spectra for nc–Si:H samples of earlystage of growth (d≈55 nm) prepared by different R values (1/1, 1/10 and 1/20);(b) schematic illustration of microstructures of the same films obtained fromSE data analysis; the parts (c), (d) and (e) show the 2D-AFM images(1000 nm×1000 nm) of the same films; (f) the histograms of size distribution ofconglomerate surface grains in these films.

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Spectroscopic ellipsometry (SE) and Raman spectroscopy(RS) are basic non-destructive optical tools routinely employedto study the microstructural properties of nc–Si:H films. In situSE provides important information about the film thickness,delineates film layers on the basis of their components andidentifies the volume fractions of these components as well. RSprovides valuable information regarding the film crystallinityand the nature of crystalline composition. It is apparent thatthe information derived from the SE and RS studies depends onthe relevance and accuracy of the data analysis method. Theconventional approach to the deconvolution of Raman spectra isthe same for films of any growth stage, and does not take intoaccount the presence of the size distribution [9–11]. This mayresult in some significant errors in the calculation of totalcrystallinity (may be underestimated) and the mean crystallitesize (may be overestimated) [12,13].

In this paper we present amicrostructural study of single-phasenc–Si:H thin films at the very early and terminal stages of filmgrowth, deposited at different H2 dilutions. Our contribution aimsto bring out the quantitative microstructural differences betweenfilms at different stages of growth, and the influence ofH2 dilutionon the film microstructure and morphology. We propose a needfor differential approach to microstructural data analysis based onthe stage of film growth. Our results evidence a significantpresence of size distribution in crystallite grains (CSD) in films,which varies with the stage of film growth. We have presented anapproach to the deconvolution of Raman spectra of nc–Si:Hbased on a model incorporating the CSD [12,13].

2. Experimental details

The undoped nc–Si:H films were deposited at low substratetemperature (≤200 °C) in a parallel-plate glow dischargeplasma enhanced chemical vapor deposition system operating ata standard rf frequency of 13.56 MHz, using high purity SiF4,Ar and H2 as feed gases. Different microstructural series of alarge number of samples were created by systematically vary-ing gas flow ratios (R=SiF4/H2) for samples having differentthicknesses (d≈50–1200 nm). Each nc–Si:H sample was stud-ied with different microstructural tools like RS, in-situ phasemodulated SE, X-ray diffraction (XRD), and atomic forcemicroscopy (AFM) to obtain comprehensive and complemen-tary microstructural information [14]. Bifacial Raman scatteringmeasurements were carried out by focusing the light probe (He–Ne laser,≈632.8 nm) from the top film surface [denoted here asRS(F)], as well as from the bottom surface [through the glasssubstrate, denoted here as RS(G)] on the samples to obtaininformation about the film at different stages of growth. In thisreport, the samples belong to two sets, the first set has films withthickness in the range of 55 nm, and the second set has filmshaving thickness ≈950 nm, to ensure comparability of thegrowth stage between the samples of a particular set.

3. Results and discussion

An important indication of the nucleation processes oc-curring at the initial stage of growth, and a fair prediction of the

microstructural growth at higher film thicknesses can beobtained from the in-situ spectroscopic study of films at veryearly stages of growth. Fig. 1(a) shows the imaginary part of thepseudo-dielectric function bɛ2N spectra measured by SE onsuch samples at initial stages of growth (average d≈55 nm),deposited under different R values. Energy positions at≈3.4 eV(E1) and at ≈4.2 eV (E2) are marked in this graph to denote thetwo prominent structures of the optical absorption in crystallinesilicon (c-Si), well predicted by the theoretical electronic bandstructure, and particularly sensitive to imperfect crystallinity. Incontrast, such peaks are not seen in the bɛ2N spectrum of a–Si:H, rather a broad distribution with a maximum at around E1 isseen. In the case of poly or nano or microcrystalline silicon,broad shoulders at the energy positions E1 and E2 are seen. TheE2 peak in Fig. 1 is completely absent from the spectra of the

Fig. 2. (a) Measured imaginary part of bɛ2N spectra for nc–Si:H samples ofterminal stage of growth (d≈950 nm) prepared by different R values (1/1, 1/5and 1/10); (b) schematic illustration of microstructures of the same filmsobtained from SE data analysis; the parts (c), (d) and (e) show the 2D-AFMimages (2000 nm×2000 nm) of the same films.

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film deposited at R=1/1, while a faint indication of such a peakcan be seen in the other two films (R=1/10 and 1/20). Thus,with increasing H2 dilution, there is an improvement in filmcrystallinity. High H2 dilution is known to etch out the weak andunsaturated bonds more effectively, resulting in a bettercrystalline material. The amplitude of the bɛ2N spectrum ofthe R=1/20 film is very low compared to the other two filmsbecause such a high H2 dilution is known to result in aformation of larger crystallites or crystallite aggregates, with anassociated increase in the amount of voids, thereby decreasingthe overall crystalline volume fraction.

Experimental SE data was fitted using standard Bruggemaneffective medium approximation. A well-chosen reference c-Siis essential for obtaining a good fit and a physically rationaldescription of the film structure. We have used publisheddielectric functions for low-pressure chemical vapor depositedpolysilicon with large (pc-Si-l) and fine (pc-Si-f) grains [15–17].The schematic view of themicrostructural layers of the same filmsobtained from modeling of the SE data is presented in Fig. 1(b),where the fractional composition of constituent components andthe layer thicknesses (dl) are mentioned. The percentage volumefractions of fine grains, large grains, voids and amorphous contentare denoted by Fcf, Fc1, Fv and Fa respectively. In this modeldiagram, we can see that with rise in H2 dilution amorphouscontent in the bulk layer (BL) decreases, resulting in a materialtotally free of any amorphous content at the dilution level ofR=1/20. However, the initial increase in Fcf in the BL seen onincreasing the R from 1/1 to 1/10, denoting improved crystal-lization, drops sharply with a further increase in R (1/20), while asimultaneous increase in Fv takes place. The R=1/20 film showsincreased void fraction due to the known effect of highH2 dilutionthat results in less nucleation site generation and increasedetching.

The top surface layer (TSL) in Fig. 1(b) demonstrates wellthe influence of H2 dilution on the microstructural remodelingof the nc–Si:H films. The film deposited at R=1/1 has the leasttop surface roughness layer thickness and contains only smallcrystallite grains. With an increase in the H2 dilution, a con-tinuous rise in the roughness layer is observed, which reachesthe highest value for the highest H2 dilution case. Simulta-neously, higher H2 dilution favors the formation of large crys-tallite grains, and large grains make an appearance in the filmsdeposited at R=1/20. These findings of surface layer aresubstantiated by the results of AFM study as well. The 2-DAFM images of the same three films are presented in Fig. 1(c),(d) and (e). Here, we see that on increasing the H2 dilution, thesize of surface grains increases [Fig. 1(d) and (e)]. These filmshaving d≈55 nm can be assumed to denote the beginning of thegrowth process, here the grain density is found to be higher at alower value of H2 dilution. A corroborative increase in surfaceroughness is also seen with increasing H2 dilution at this stageof film growth, as mentioned in each figure. The histograms ofthe size distribution of surface grains of these samples areshown in Fig. 1(f), where a shift in the distribution towardsbigger size is observed as H2 dilution is increased.

Applying such known considerations about the process ofgrain growth in plasma deposited nc–Si:H, one can reasonably

predict the further growth patterns of these films from themicrostructural picture observed at the early growth stage.Thus, the film deposited at lower H2 dilution is expected to havetightly packed crystallites and conglomeration may assume theshape of straight columns. In contrast, for films deposited underhigher H2 dilution, more lateral growth of crystallites may beseen with the preservation of the individuality of the crystallites.Now let us verify these predictions by looking at the results offilms at the terminal stages of growth. To explore how the filmmicrostructure changes over the growth process, the imaginarypart of bɛ2N spectra measured by SE on samples having averaged≈950 nm, deposited under different R values (1/1, 1/5 and 1/10), is presented in Fig. 2(a). These films have reached a steadystate growth stage, and the associated changes in the bulk crys-talline material with increase in film thickness is reflected in theincrease in the intensity of the shoulder of E2 peak, indicatingexcellent crystallization. The schematic model diagram of themicrostructure of the same three films is shown in Fig. 2(b),where the microstructure is seen to have changed remarkablycompared to the initial stages. At this stage, three layers of thefilm can be distinguished, and the modeling of SE data has beendone using a three layer model: a top surface layer (TSL), amiddle bulk layer (MBL) and a bulk interface layer (BIL).

In Fig. 2(b), with an increase in H2 dilution, the incubationlayer or BIL thickness is seen to decrease. An initial rise in H2

dilution eliminates the amorphous content in the BIL with

Fig. 3. Bifacial Raman spectra [RS(F) and RS(G)] for nc–Si:H samples of twostages of growth (at average d≈55 and 950 nm) and deposited under differentR values.

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improvement in overall crystallinity, but a further rise to R=1/10results in a lowered crystalline fraction with an associated in-crease in Fv. The middle bulk layer is devoid of any amorphouscontent, having both small and large crystallite grains in varyingratios for the three films. The variation in the ratios of the

Fig. 4. Deconvolution results consisting of the total fit, and the deconvoluted peaksdeconvolution of RS(F) and RS(G) profiles of R=1/10 film having d≈55 nm is showprofiles of R=1/5 film having d≈950 nm is shown in parts (c) and (d) respectively

crystallite grains of two different mean sizes with film growththat is recognized by our SE analyses in these films wasidentified in all our samples of other intermediate thicknessesalso, which are not shown here. In Fig. 2(b), again a very high H2

dilution level is seen to result in a higher void fraction in all thelayers. The AFM images of these samples in Fig. 2 (c), (d) and(e) reveal that the conglomeration of the grains has taken place,resulting in formation of large crystallite grains. The conglom-erate grains of R=1/5 and R=1/1 films are found to cover awider area by collecting more grains into their domains, com-pared to the sample of R=1/10. The film of R=1/10 containsconglomerate grains having a cauliflower-like shape, whichimplies that the growth in lateral direction is more when the H2

dilution is increased, which results in a bigger size of con-glomerate grains. These observations are in good agreementwith our earlier predictions. The surface roughness values men-tioned in the respective AFM images indicate the same findingas in Fig. 2(b), i.e., the roughness layer increases with increasingH2 dilution.

Now we come to the results of the bifacial RS measurementsof nc–Si:H samples at different stages of growth (at averaged≈55 and 950 nm) and deposited under different R values.The RS(F) and RS(G) profiles of these samples are presented inFig. 3. For the set of samples having d≈950, we see that anexplicit amorphous hump (a broad peak at 480 cm−1) is absentin their RS(F) profiles. The amorphous hump can be seen in theRS(G) of all the samples of both the thickness sets, due to thepresence of amorphous content in the interfacial layer. All thesefindings are in good agreement with the SE results. However, inthe samples at early stages of growth, such an amorphous hump

for the RS profiles of one samples from each thickness set of different d. Then in parts (a) and (b) respectively, while the deconvolution of RS(F) and RS(G). The respective fitting models are mentioned in each figure.

Table 1Microstructural information such as mean crystallite sizes, their distribution [σ (nm)] and fractional compositions obtained from deconvolution of bifacial RS profilesof the samples

Sample BifacialRS

Fittingmodel

Small grain (cd1) Large grain (cd2) a–Si:H

Size (nm) [σ (nm)] Xc1 (%) Size (nm) [σ (nm)] Xc2 (%) Xa (%)

Average d≈950 nm R=1/1 RS(F) cd1+cd2 6.4, [1.87] 31 70.4, [0] 69 0RS(G) cd1+cd2+a 6.4, [1.52] 16.4 97.6, [0] 47.6 36

R=1/5 RS(F) cd1+cd2 7.3, [1.98] 29.5 73.8, [0.06] 70.5 0RS(G) cd1+cd2+a 7.7, [1.93] 17.3 103.3, [0] 50 32.7

R=1/10 RS(F) cd1+cd2 8.4, [2.26] 48.1 87, [0] 51.9 0RS(G) cd1+cd2+a 7.7, [1.59] 23.4 101, [0] 48 28.6

Average d≈55 nm R=1/1 RS(F) cd+a 7.9, [1.93] 44.7 – 0 55.3RS(G) cd+a 2.6, [0.64] 29.6 – 0 70.4

R=1/10 RS(F) cd+a 8.1, [1.65] 71 – 0 29RS(G) cd+a 2.6, [0.50] 32.7 – 0 67.3

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can be seen in the RS(F) profile. In the absence of an explic-it amorphous hump, the asymmetry in the Raman lineshape ofRS profiles of nc–Si:H samples, seen as a low energy tail, canbe attributed to the distribution of smaller sized crystallites[12,13].

The presence of two distinct sizes of crystallites and theirvarying percentage volume fractions at different stages of filmgrowth deduced from SE results indicate a need for taking intoconsideration this CSD while deconvoluting the RS profiles, fora better agreement and physical accuracy. Keeping these con-siderations in mind, we have used the deconvolution modeldescribed in Ref. [12,13] for obtaining the CSD in Si nano-structures, for the analysis of our RS data. The final decon-volution results consisting of the total fit, and the deconvolutedpeaks for the RS profiles of one samples from each thickness setare shown in Fig. 4. The RS(F) and RS(G) profiles of R=1/10film having d≈55 nm are shown in Fig. 4(a) and (b) re-spectively, while the RS(F) and RS(G) profiles of R=1/5 filmhaving d≈950 nm are shown in Fig. 4(c) and (d) respectively.

Taking the inputs from SE studies basically gives rise tothree different models which can be applied to the deconvolu-tion of the RS profiles of our nc–Si:H samples:

(i) Model “cd+a” having a unimodal CSD (mainly the con-tribution from small sized crystallites) with an amorphousbackground: Fig. 4(a) and (b).

(ii) Model “cd1+cd2” where a bimodal CSD [small (cd1)and large (cd2) crystallites] without an amorphous back-ground is considered: Fig. 4(c).

(iii) Model “cd1+cd2+a” incorporating a bimodal CSD alongwith an amorphous phase: Fig. 4 (d).

These fitting models were applied to all the RS profiles of thesamples, and the best fit is shown in these graphs. For assessmentof the goodness of fit, not only visual scrutiny and statisticalvalidation, but also the physical plausibility, the corresponding SEdata and the direction of RS measurement need to be taken intoaccount. The direction of RS measurement is important becausethe Raman collection depth for μc–Si:H material at 632.8 nm(He–Ne red laser) is ≈500 nm, which has to be kept in view,especially in the samples at lower thicknesses, to understand thedata collected by RS from each direction.

The microstructural composition of the films deduced fromthe deconvoluted RS data is presented in Table 1. The per-centage volume fractions of the small and large crystallite grainsand amorphous content are denoted by Xc1, Xc2 and Xa. Thetrend indicated by these values are in good agreement with theoutcome of SE results, although a strict similarity between theabsolute values deduced from these two different tools oper-ating on different principles and at different length scales isneither possible, nor reasonable to aim for. The mean sizes ofthe large and small crystallites obtained from RS analyses are≈70–80 nm and ≈6–8 nm respectively.

4. Summary

Single phase nano (or synonymously,micro) crystalline siliconfilms were studied with differentmicrostructural tools to elucidatethe film microstructure at different stages of growth and for filmsdeposited under different levels of H2 dilution. Our results showthe microstructural evolution with film growth for varying H2

dilutions in the context of the crystallite grain growth, aggregationand variation in the percentage volume fractions of the constituentgrains of different sizes. An initial increase in the H2 dilutionresults in reduction of defects, better crystallinity and crystallitecolumn formation. However, increasing the H2 dilution furtherresults in an increase in void fraction.

Our SE results demonstrate the presence of two distinct meansizes of crystallites in the films after a certain growth stage,depending on the H2 dilution level, which substantiates the ra-tionale of taking into account a crystallite size distribution whileanalyzing microstructural data. The deconvolution of experimen-tally observed RS profiles of such nano and microcrystallinesilicon films where a CSD is evident, has been done using abimodal size distribution of large and small crystallite grains. Thefractional compositional analysis of the films obtained by thismethodology is consistent with the findings of SE and AFMstudies.

Acknowledgement

Financial support from Swiss National Science Foundation(#20720-109486) and Department of Science and Technology,New Delhi is gratefully acknowledged.

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