Structural Determination of Nanocrystalline Si Films Using Ellipsometry and Raman Spectroscopy

Size: px

Start display at page:

Download "Structural Determination of Nanocrystalline Si Films Using Ellipsometry and Raman Spectroscopy"

Frederick Cross
5 years ago
Views:

1 Structurl Determintion of Nnocrystlline Si Films Using Ellipsometry nd Rmn Spectroscopy Snjy K. Rm *, Md. N. Islm b, P. Roc i Cbrrocs c nd Styendr Kumr Deprtment of Physics & Smtel Center for Disply Technology, Indin Institute of Technology Knpur, Knpur-2816, Indi b QAED-SRG, Spce Appliction Centre (ISRO), Ahmedbd 381, Indi c LPICM, UMR 7647-CNRS, Ecole Polytechnique, Pliseu Cedex, Frnce Single phse nno nd micro crystlline silicon films deposited using SiF 4 / H 2 plsm t different H 2 dilution levels were studied t initil nd terminl stges of film growth with spectroscopic ellipsometry (SE), Rmn scttering (RS) nd tomic force microscopy (AFM). The nlysis of dt obtined from SE elucidtes the microstructurl evolution with film growth in terms of the chnges in crystllite sizes nd their volume frctions, crystllite conglomertion nd film morphology. The effect of H 2 dilution on film microstructure nd morphology, nd the corrobortive findings from AFM studies re discussed. Our SE results evince two distinct men sizes of crystllites in the mteril fter certin stge of film growth. The nlysis of Rmn scttering dt for such films hs been done using bimodl size distribution of crystllite grins, which yields more ccurte nd physiclly rtionl microstructurl picture of the mteril. PACS numbers: 68..-, 68..Jk, 68.3.Ct Keywords: Silicon; Thin films; Structurl properties; Growth mechnism; Ellipsometry; Atomic force microscopy (AFM); Rmn spectroscopy. 1. INTRODUCTION * corresponding uthor. E-mil: skrm@iitk.c.in, snjyk.rm@gmil.com styen@iitk.c.in Recent demonstrtion of thin film solr cells produced fully from nnocrystlline (or microcrystlline) silicon hs generted new wve of interest in undoped nnocrystlline silicon s this mteril offers incresed stbility ginst light induced degrdtion nd hs enhnced low-energy bsorption in contrst to morphous silicon (-Si:H) [1,2,3]. Hydrogented nnocrystlline silicon (nc-si:h) is lso ttrctive for thin film trnsistors nd other photovoltic devices due to its esy fbriction technology llowing lrge re deposition even t low tempertures, on non-crystlline substrtes [4,]. The stbility nd efficiency of the nc-si:h films re importnt issues for these device pplictions nd both these fctors re determined by the microstructurl chrcteristics of the film [6,7]. The microstructurl chrcteriztion of the component lyers re thus vitl to the understnding nd prediction of the device efficiency nd properties. The structurl evolution of plsm deposited nc-si:h films cn be mnipulted to yield mximum efficiency by ltering the deposition prmeters, minly, hydrogen dilution. The knowledge of film microstructure t vrious levels of film thicknesses nd its reltionship with different H 2 dilutions provides method to predict nd ssess the outcome of the controlled deposition techniques. Microstructurlly, plsm deposited nc-si:h nd microcrystlline silicon (µc-si:h) re heterogeneous mterils consisting of Si nnocrystllites ( 2 to 2 nm) nd ggregtes thereof (which my rech up to few hundred nm in µc-si:h), dispersed in n morphous Si mtrix, nd disorder in the form of voids nd grin boundries. In the bsence of n morphous phse in this mteril (which is then termed s single-phse nc-si:h or µc-si:h), the disordered phse consists of voids, density deficit nd clusters of nnocrystllites. The heterogeneous nture of nc-si:h microstructure exists long the timescle of growth s well, nd with n increse in film thickness, distribution in the sizes of crystllite grins cn be observed, with vrition in the frctionl compositions of the different sized constituent grins. Spectroscopic ellipsometry (SE) nd Rmn spectroscopy (RS) re bsic non-destructive opticl tools routinely employed to study the microstructurl properties of nc-si:h films. In situ SE provides importnt informtion bout the film thickness, delinetes film lyers on the bsis of their components nd identifies the volume frctions of these components s well. RS provides vluble informtion regrding the film crystllinity nd the nture of crystlline composition. It is pprent, tht the informtion derived from the SE nd RS study depends on the relevnce nd ccurcy of the dt nlysis method. The conventionl pproch to the deconvolution of Rmn spectr is the sme for films of ny growth stge, nd does not tke into ccount the presence of the size distribution [8]. This my result in some significnt errors in the clcultion of totl crystllinity (my be underestimted) nd the men crystllite size (my be overestimted) [9]. 1

In this pper we present microstructurl study of single-phse nc-si:h thin films t the very erly nd terminl stges of film growth, deposited t different H 2 dilutions.

We propose need for differentil pproch to microstructurl dt nlysis bsed on the stge of film growth.

We hve presented n pproch to the deconvolution of Rmn spectr of nc-si:h bsed on model incorporting the CSD [9]. 2.

2 In this pper we present microstructurl study of single-phse nc-si:h thin films t the very erly nd terminl stges of film growth, deposited t different H 2 dilutions. Our contribution ims to bring out the quntittive microstructurl differences between films t different stges of growth, nd the influence of H 2 dilution on the film microstructure nd morphology. We propose need for differentil pproch to microstructurl dt nlysis bsed on the stge of film growth. Our results evidence significnt presence of size distribution in crystllite grins (CSD) in films, which vries with the stge of film growth. We hve presented n pproch to the deconvolution of Rmn spectr of nc-si:h bsed on model incorporting the CSD [9]. 2. EXPERIMENTAL DETAILS The undoped nc-si:h films were deposited t low substrte temperture ( 2 C) in prllel-plte glow dischrge plsm enhnced chemicl vpor deposition system operting t stndrd rf frequency of 13.6 MHz, using high purity SiF 4, Ar nd H 2 s feed gses. Different microstructurl series of lrge number of smples were creted by systemticlly vrying gs flow rtios (R = SiF 4 /H 2 ) for smples hving different thicknesses (d -12nm). Ech nc-si:h smple ws studied with different microstructurl tools like RS, in-situ phse modulted SE, X-ry diffrction (XRD), nd tomic force microscopy (AFM) to obtin comprehensive nd consistent microstructurl informtion [1]. Bifcil Rmn scttering mesurements were crried out by focusing the light probe (He-Ne lser, 632.8nm) from the top film surfce [denoted here s ], s well s from the bottom surfce [through the glss substrte, denoted here s ] on the smples to obtin informtion bout the film t different stges of growth. In this report, the smples belong to two sets, the first set hs films with thickness in the rnge of nm, nd the second set hs films hving thickness 9 nm, to ensure comprbility of the growth stge between the smples of prticulr set. 3. RESULTS AND DISCUSSION An importnt indiction of the nucletion processes occurring t the initil stge of growth, nd fir prediction of the microstructurl growth t higher film thicknesses cn be obtined from the in-situ spectroscopic study of films t very erly stges of growth. Fig. 1 shows the imginry prt of the pseudo-dielectric function <ε 2 > spectr mesured by SE on such smples t initil stges of growth (verge d nm), deposited under different R vlues. Energy positions t 3.4 ev (E 1 ) nd t 4.2 ev (E 2 ) re mrked in this grph to denote the two prominent structures of the opticl bsorption in c-si, well predicted by the theoreticl electronic bnd structure, nd prticulrly sensitive to imperfect crystllinity. In contrst, such peks re not seen in the <ε 2 > spectrum of -Si:H, rther brod distribution with mximum t round E 1 is seen. In the cse of poly or nno or microcrystlline silicon, brod shoulders t the energy positions E 1 nd E 2 re seen. The E 2 pek in Fig. 1 is completely bsent from the spectr of the film deposited t R = 1/1, while fint indiction of such pek cn be seen in the other two films (R < ε 2 > = 1/1 nd 1/2). Thus, with incresing H 2 dilution, there is n improvement in film crystllinity. High H 2 dilution is known to etch out the wek nd unsturted bonds more effectively, resulting in better crystlline mteril. The mplitude of the <ε 2 > spectrum of the R= 1/2 film is very low compred to the other two films becuse such high H 2 dilution is known to result in formtion of lrger crystllites or crystllite ggregtes, with n ssocited increse in the mount of voids, thereby decresing the overll crystlline volume frction. Experimentl SE dt ws fitted using stndrd Bruggemn effective medium pproximtion. A well-chosen reference crystlline silicon (c-si) is essentil for obtining 3 E 3 2 (4.2 ev) E 1 (3.4 ev) R=1/2 1 verge d ~ nm Energy (ev) Frequency (rb. unit) {d l (nm); F cf, F c1, F v, F (%)} TSL {.98; 33.2,, 66.8, } {7.9; 67.1,.6, 32.3, } R=1/2 {17.9; 24.3, 24, 1.7, } BL {9.6; 73,, 6, 21} {48.2; 88.4,, 1.1, 1.} R=1/2 {4.4; 69,, 31, } R=1/2 σ rms =.9 nm σ rms = 2.4 nm (e) R=1/2 σ rms = 6 nm Conglomerte surfce grin size (nm) FIG. 1. Mesured imginry prt of <ε 2 > spectr for nc-si:h smples of erly stge of growth (d nm) prepred by different R vlues (1/1, 1/1 nd 1/2); schemtic illustrtion of microstructures of the sme films obtined from SE dt nlysis; the prts, nd (e) show the 2D-AFM imges (1 nm 1 nm) of the sme films; (f) the histogrms of size distribution of conglomerte surfce grins in these films. (f) 2

good fit nd physiclly rtionl description of the film structure.

The schemtic view of the microstructures of the sme films obtined from modeling of the SE dt is presented in Fig.

The percentge volume frctions of fine grins, lrge grins, voids nd morphous content re denoted by F cf, F c1, F v nd F respectively.

3 good fit nd physiclly rtionl description of the film structure. We hve used published dielectric functions for low-pressure chemicl vpor deposited polysilicon with lrge (pc-si-l) nd fine (pc-si-f) grins [11]. The schemtic view of the microstructures of the sme films obtined from modeling of the SE dt is presented in Fig. 1, where the frctionl composition of constituent components nd the lyer thicknesses (d l ) re mentioned. The percentge volume frctions of fine grins, lrge grins, voids nd morphous content re denoted by F cf, F c1, F v nd F respectively. In this model digrm, we cn see tht with rise in H 2 dilution morphous content in the bulk lyer (BL) decreses, resulting in mteril totlly free of ny morphous content t the dilution level of R = 1/2. However, the initil increse in F cf in the BL seen on incresing the R from 1/1 to 1/1, denoting improved crystlliztion, drops shrply with further increse in R (1/2), while simultneous increse in F v tkes plce. The R = 1/2 film shows incresed void frction due to the known effect of high H 2 dilution tht results in less nucletion site genertion nd incresed etching. The top surfce lyer (TSL) in Fig. 1 demonstrtes well the influence of H 2 dilution on the microstructurl remodeling of the nc-si:h films. The film deposited t R= 1/1 hs the lest top surfce roughness lyer thickness nd contins only smll crystllite grins. With n increse in the H 2 dilution, continuous rise in the roughness lyer is observed, which reches the highest vlue for the highest H 2 dilution cse. Simultneously, higher H 2 dilution fvors the formtion of lrge crystllite grins, nd lrge grins mke n ppernce in the films deposited under R= 1/2. These findings of surfce lyer re substntited by the results of AFM study s well. The 2-D AFM imges of the sme three films re presented in Fig. 1, nd (e). Here, we see tht on incresing the H 2 dilution, the size of surfce grins increses [Fig. 1 nd (e)]. These films hving d nm cn be ssumed to denote the beginning of the growth process, here the grin density is found to be higher t lower vlue of H 2 dilution. A corrobortive increse in surfce roughness is lso seen with incresing H 2 dilution t this stge of film growth, s mentioned in ech figure. The histogrms of the size distribution of surfce grins of these smples re shown in Fig. 1(f), where shift in the distribution towrds bigger size is observed s H 2 dilution is incresed. Applying such known considertions bout the process of grin growth in plsm deposited nc-si:h, one cn resonbly predict the further growth ptterns of these films from the microstructurl picture observed t the erly growth stge. Thus, the film deposited t lower H 2 dilution is expected to hve tightly pcked crystllites nd conglomertion my ssume the shpe of stright columns. In contrst, for films deposited under higher H 2 dilution, more lterl growth of crystllites my be seen with the preservtion of the individulity of the crystllites. Now let us verify these predictions by looking t the results of films t the terminl stges of growth. To explore how the film microstructure chnges over the growth process, the imginry prt of <ε 2 > spectr mesured by SE on smples hving verge d 9 nm, deposited under different R vlues (1/1, 1/ nd 1/1), is presented in Fig. 2. These films hve reched stedy stte growth stge, nd the ssocited chnges in the bulk crystlline mteril with increse in film thickness is reflected in the increse in the intensity of the shoulder of E 2 pek, indicting excellent crystlliztion. The schemtic model digrm of the microstructure of the sme three films is shown in Fig. 2, where the microstructure is seen to hve chnged remrkbly compred to the initil stges. At this stge, three lyers of the film cn be distinguished, nd the modeling of SE dt hs been done using three lyer model: top surfce lyer (TSL), middle bulk lyer (MBL) nd bulk interfce lyer (BIL). In Fig. 2, with n increse in H 2 dilution, the incubtion lyer or BIL thickness is seen to decrese. An initil rise in H 2 dilution elimintes the morphous content in the BIL with improvement in overll crystllinity, but further rise to R = 1/1 results in lowered crystlline frction with n ssocited increse in F v. The middle bulk lyer is devoid of ny morphous content, hving both smll nd lrge crystllite grins in vrying rtios for the three films. The vrition in the rtios of the crystllite grins of two different men sizes with film growth tht is recognized by our SE nlyses in these films ws identified in ll our smples of other intermedite thicknesses lso, which re not shown here. In Fig. 2, gin very high H 2 dilution level is seen to result in higher void frction in ll the lyers. The AFM imges of these smples in Fig. 2, nd (e) revel tht the con- <ε 2 > 3 E E 2 (4.2 ev) 3 1 (3.4 ev) verge d 9 nm Energy (ev) {d l (nm); F cf, F c1, F v, F (%)} TSL {.1; 33, 43, 24, } {4.3; 1, 9, 26, } {8.3; 73.6,, 26.4, } MBL {888.1; 1, 4, 3, } {986.2; 3., 67, 2.6, } {918.9;.4, 4.8, 8.8, } BIL {32.8;,, 32, 68} {32.6; 79.,, 2., } {27.7;,, 3.6, 64.4} σ rms =.2 nm σ rms =.3 nm (e) σ rms = 6 nm FIG. 2. Mesured imginry prt of <ε 2 > spectr for nc-si:h smples of terminl stge of growth (d 9 nm) prepred by different R vlues (1/1, 1/ nd 1/1); schemtic illustrtion of microstructures of the sme films obtined from SE dt nlysis; the prts, nd (e) show the 2D-AFM imges (2 nm 2 nm) of the sme films. 3

4 Intensity (rb. unit) verge d ~ 9 nm verge d ~ nm FIG. 3. Bifcil Rmn spectr [ nd ] for nc-si:h smples of two stges of growth (t verge d nd 9 nm) nd deposited under different R vlues. glomertion of the grins hs tken plce, resulting in formtion of lrge crystllite grins. The conglomerte grins of nd films re found to cover wider re by collecting more grins into their domins, compred to the smple of. The film of R= 1/1 contins conglomerte grins hving culiflower-like shpe, which implies tht the growth in lterl direction is more when the H 2 dilution is incresed, which results in bigger size of conglomerte grins. These observtions re in good greement with our erlier predictions. The surfce roughness vlues mentioned in the respective AFM imges indicte the sme finding s in Fig. 2, i.e., the roughness lyer increses with incresing H 2 dilution. Now we come to the results of the bifcil RS mesurements of nc-si:h smples t different stges of growth (t verge d nd 9 nm) nd deposited under different R vlues. The nd profiles of these smples re presented in Fig. 3. For the set of smples hving d 9, we see tht n explicit morphous hump ( brod pek t 48 cm -1 ) is bsent in their profiles. The morphous hump cn be seen in the of ll the smples of both the thickness sets, due to the presence of morphous content in the interfcil lyer. All these findings re in good greement with the SE results. However, in the smples t erly stges of growth, such n morphous hump cn be seen in the profile. In the bsence of n explicit morphous hump, the symmetry in the Rmn lineshpe of RS profiles of nc-si:h smples, seen s low energy til, cn be ttributed to the distribution of smller sized crystllites. The presence of two distinct sizes of crystllites nd their vrying percentge volume frctions t different stges of film growth deduced from SE results indicte need for tking into considertion this CSD while deconvoluting the RS profiles, for better greement nd physicl ccurcy. Keeping these considertions in mind, we hve used the deconvolution model described in Ref. [9] for obtining the CSD in Si nnostructures, for the nlysis of our RS dt. The finl deconvolution results consisting of the totl fit, nd the deconvoluted peks for the RS profiles of one smples from ech thickness set re shown in Fig. 4. The nd profiles of R = 1/1 film hving d nm re shown in Fig. 4 nd respectively, while the nd profiles of R = 1/ film hving d 9 nm re shown in Fig. 4 nd respectively. Tking the inputs from SE studies bsiclly gives rise to three different models which cn be pplied to the deconvolution of the RS profiles of our nc-si:h smples: (i) Model hving only one type of CSD (minly the contribution from smll sized crystllites) with n morphous bckground: Fig. 4 nd. (ii) Model where bimodl CSD [smll (cd1) nd lrge (cd2) crystllites] without n morphous bckground is considered: Fig. 4. (iii) Model incorporting bimodl CSD long with n morphous phse: Fig. 4. These fitting models were pplied to ll the RS profiles of the smples, nd the best fit is shown in these grphs. For ssessment of the goodness of fit, not only re the visul scrutiny nd sttisticl vlidtion importnt, but the physicl plusibility, the corresponding SE dt nd the direction of RS mesurement lso needed to be tken into ccount. The direction of RS mesurement is importnt becuse the Rmn collection depth for µc-si:h mteril t nm (He- Ne red lser) is nm, which hs to be kept in view, especilly in the smples t lower thicknesses, to understnd the dt collected by RS from ech direction. Intensity (rb. unit) verge d ~ nm R = 1/1 cd Model "" cd Model "" verge d ~ 9 nm R = 1/ cd1 cd2 Model "" cd1 cd2 Model "" FIG. 4. Deconvolution results consisting of the totl fit, nd the deconvoluted peks for the RS profiles of one smples from ech thickness set of different d. The deconvolution of nd profiles of R = 1/1 film hving d nm is shown in prts nd respectively, while the deconvolution of nd profiles of R = 1/ film hving d 9 nm is shown in prts nd respectively. The respective fitting models re mentioned in ech figure

5 Tble 1. Microstructurl informtion such s men crystllite sizes, their distribution [σ (nm)] nd frctionl compositions obtined from deconvolution of bifcil RS profiles of the smples. Smple d 9nm d nm Bifcil RS Fitting Model Smll grin Lrge grin -Si:H (cd1) (cd2) Size, Size, X [σ ] c1 X [σ ] c2 X (%) (%) (%) (nm) (nm) 6.4, [1.87] 6.4, [1.2] 7.3, [1.98] 7.7, [1.93] 8.4, [2.26] 7.7, [1.9] 7.9, [1.93] 2.6, [.64] 8.1, [1.6] 2.6, [.] The microstructurl composition of the films deduced from the deconvoluted RS dt is presented in Tble 1. The percentge volume frctions of the smll nd lrge crystllite grins nd morphous content re denoted by X c1, X c2 nd X. The trend indicted by these vlues re in good greement with the outcome of SE results, lthough strict similrity between the bsolute vlues deduced from these two different tools operting on different principles nd t different length scles is neither possible, nor resonble to im for. 4. SUMMARY Single phse nno (or synonymously, micro) crystlline silicon films were studied with different microstructurl tools to elucidte the film microstructure t different stges of growth nd for films deposited under different levels of H 2 dilution. Our results show the microstructurl evolution with film growth for vrying H 2 dilutions in the context of the crystllite grin growth, ggregtion nd vrition in the percentge volume frctions of the constituent grins of different sizes. An initil increse in the H 2 dilution results in reduction of defects, better crystllinity nd crystllite column formtion. However, incresing the H 2 dilution further results in n increse in void frction. Our SE results demonstrte the presence of two distinct men sizes of crystllites in the films fter certin growth stge, depending on the H 2 dilution level, which substntites the rtionle of tking into ccount crystllite size distribution while nlyzing microstructurl dt. The deconvolution of experimentlly observed RS profiles of such nno , [] 97.6, [] 73.8, [.6] 13.3, [] 87, [] 11, [] nd microcrystlline silicon films where CSD is evident, hs been done using bimodl size distribution of lrge crystllite grins ( 7-8nm) nd smll crystllite grins ( 6-7nm), nd the frctionl compositionl nlysis of the films obtined by this methodology is strongly corrobortive with the findings of SE nd AFM studies. ACKNOWLEDGEMENTS Finncil support from Swiss Ntionl Science Foundtion (# ) nd Deprtment of Science nd Technology, New Delhi is grtefully cknowledged. [1] R.W. Collins, A.S. Ferluto, G.M. Ferreir, C. Chen, J. Koh, R.J. Kovl, Y. Lee, J.M. Perce, nd C.R. Wronski, Sol. Energy Mt.& Sol. Cells 78 (23) 143, nd references therein. [2] T. Toym nd H. Okmoto, Sol. Energy 8, 68 (26) ; F. Meillud, A. Shh, C. Droz, E. Vllt-Suvin, C. Mizz, Sol. Energy Mt. & Sol. Cells 9 (26) 292. [3] O. Vetterl, F. Finger, R. Crius, P. Hpke, L. Houben, O. Kluth, A. Lmbertz, A. Mück, B. Rech nd H. Wgner, Sol. Energy Mt. & Solr Cells 62 (2) 97. [4] I. Chun Cheng nd S. Wgner, Appl. Phys. Lett. 8 (22) 44. [] C.H. Lee, A. Szonov, A. Nthn, nd J. Robertson, Appl. Phys. Lett. 89 (26) [6] M. Goerlitzer et l., Mt. Res. Soc. Symp. Proc. 467 (1997) 31. [7] B. Yn et l., Appl. Phys. Lett. 8 (24)192. [8] E. Vllt-Suvin, C. Droz, F. Meillud, J. Bilt, A. Shh, C. Bllif, J. Non-Cryst. Solids 32 (26)12; D. Hn, J. D. Lorentzen, J. Weinberg-Wolf, L. E. McNeil, Q. Wng, J. Appl. Phys. 94 (23) 293; T. Knenko, M. Wkgi, K. Oniswe, nd T. Minemur, Appl. Phys. Lett. 64 (1994) 186. [9] Md. N. Islm nd S. Kumr, Appl. Phys. Lett. 78 (21) 71; Md. N. Islm, A. Prdhn nd S. Kumr, J. Appl. Phys. 98 (2) [1] S.K. Rm, Ph.D. thesis, I.I.T. Knpur, Indi, (26). [11] S. Kumr, B. Drevillon nd C. Godet, J. Appl. Phys. 6 (1986) 142; B. Drévillon, Prog. Cryst. Growth Chrct. Mter. 27 (1993) 1; G.E. Jellison Jr., M.F. Chisholm nd S.M. Gorbtkin, Appl. Phys. Lett. 62 (1993) 3348.

References and Resources:

References and Resources: Surfce nd Interfce Science Physics 627; Chemistry 542 Lectures 4 Feb 3, 2013 Determining Surfce Structure Diffrction methods: LEED; RHEED Rel Spce: STEM References nd Resources: Woodruff nd Delchr (2 nd