Evidence of Bimodal Crystallite Size
       Distribution in µc-Si:H Films

Sanjay K. Ram1,2, Md. Nazrul Islam3, Satyendra ...
Outline

• Introduction: motivation
• Experimental Details
• Microstructural Characterization
   – Spectroscopic ellipsome...
Complex microstructure of μc-Si:H
                  columnar boundaries
                                        grains   g...
Motivation
• Need for proper microstructural characterization
• Different microstructural tools: different length
  scales...
Sample preparation
                                                      PECVD
                                           ...
Film characterization


                                                Electrical Properties
Structural Properties


    ...
Spectroscopic Ellipsometry : measured imaginary part
              of the pseudo-dielectric function <ε2> spectra
     30 ...
Analyses of SE data: schematic view for two films
                       (initial and final growth stages)


             ...
X-ray diffraction analysis
                                                                                               ...
Surface morphology by AFM

                                                       σrms= 4 nm + 0.3 nm

                   ...
Presence of Size Distribution
Surface Morphology                                                                          ...
Bifacial Raman Study
                                 1.2                                                                 ...
Deconvolution of Raman Spectroscopy Data
• Conventionally: RS profiles are deconvoluted
  assuming:
   – a single mean cry...
Incorporation of CSD in Raman Analysis
According to our model, Φ(L) representing the CSD of an
  ensemble of spherical cry...
By putting Eq.(1) into Eq.(2) and then integrating the results over
the crystallite sizes L, and by restricting the disper...
RS Data Deconvolution : Our Model
            inclusion of crystallite size distribution
• In the absence of an explicit a...
RS analysis
                              fit model quot;cd1+cd2quot;
                                                    ...
Fractional composition of films: Qualitative
                                  agreement between RS and SE studies

      ...
Summary of variation in fractional compositions and roughness with film growth




                    Roughness by SE, σS...
Conclusions
• Microstructural characterization studies on plasma
  deposited highly crystalline µc-Si:H films to explore t...
Thanks
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Evidence Of Bimodal Crystallite Size Distribution In Microcrystalline Silicon Films

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It is known that there is a bimodal size distribution in microcrystalline silicon. How can the deconvolution of the Raman spectra be done with incorporation of a bimodal CSD to obtain more accurate and physical picture of the microstructure in this material?

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Evidence Of Bimodal Crystallite Size Distribution In Microcrystalline Silicon Films

  1. 1. Evidence of Bimodal Crystallite Size Distribution in µc-Si:H Films Sanjay K. Ram1,2, Md. Nazrul Islam3, Satyendra Kumar2 and P. Roca i Cabarrocas1 LPICM (UMR 7647 du CNRS ), Ecole Polytechnique, France 1 Dept. of Physics, I.I.T. Kanpur, India 2 QAED-SRG, Space Application Centre (ISRO), Ahmedabad – 380015, India 3
  2. 2. Outline • Introduction: motivation • Experimental Details • Microstructural Characterization – Spectroscopic ellipsometry – Atomic force microscopy – X-ray diffraction – Bifacial Raman spectroscopy • Conclusions
  3. 3. Complex microstructure of μc-Si:H columnar boundaries grains grain boundaries conglomerate crystallites surface roughness voids Film growth substrate Three main length scales for disorder: Local disorder: µc-Si:H contains a disordered amorphous phase Nanometrical disorder: nanocrystals consist of small crystalline (c-Si) grains of random orientation and a few tens of nanometres size. Micrometrical disorder: conglomerates are formed by a multitude of nanocrystals and generally acquire a pencil-like shape or inverted pyramid type shape.
  4. 4. Motivation • Need for proper microstructural characterization • Different microstructural tools: different length scales • Influence on carrier transport – Film morphology – compositional variation in constituent crystallites – crystallite size distribution (CSD) • Elucidation of CSD in single phase µc-Si:H as studied by different microstructural tools
  5. 5. Sample preparation PECVD RF Parallel-plate glow discharge HH H Si H H N H H plasma deposition system H H H Si N Si N Si N μc-Si:H Substrate: Corning 1773 film Flow ratio High purity feed gases: (R)= SiF4/H2 SiF4 , Ar & H2 R=1/1 R=1/5 R=1/10 Rf frequency 13.56 MHz Ts=200 oC Thickness series
  6. 6. Film characterization Electrical Properties Structural Properties σd(T) measurement 15K≤T ≤ 450K Xray Diffraction σPh(T,∅) measurement 15K≤T ≤ 325K Raman Scattering CPM measurement Spectroscopy Ellipsometry Hall effect TRMC Atomic Force Microscopy
  7. 7. Spectroscopic Ellipsometry : measured imaginary part of the pseudo-dielectric function <ε2> spectra 30 * Reference c-Si in BEMA model : E2 (4.2 eV) E1 (3.4 eV) LPCVD polysilicon with large 25 d=390 nm (pc-Si-l) and fine (pc-Si-f) grains d=170 nm d=590 nm 20 d=950 nm d=55 nm E2 (4.2 eV) 50 E1 (3.4 eV) < ε2 > 15 40 c-Si pc-Si-l 30 < ε2 > pc-Si-f 10 a-Si 20 μ c-Si:H 5 10 (d = 950 nm) 0 0 (a) -10 2 3 4 5 -5 Energy (eV) 2 3 4 5 Energy (eV) thickness series of R=1/10
  8. 8. Analyses of SE data: schematic view for two films (initial and final growth stages) TSL (8.3 nm) Fcf = 73.6 %, Fcl = 0 %, Fv = 26.4 %, Fa =0 % TSL (7.9 nm) MBL (918.9 nm) d = 950 nm Fcf = 32.3 %, Fcl = 0.6 %, Fcf = 50.4%, Fcl = 40.8%, Fv = 67.1%, Fa =0 % Fv=8.8 %, Fa=0% d = 55 nm BL (48.2 nm) BIL (27.7 nm) Fcf = 88.4 %, Fcl = 0 %, Fcf = 0 %, Fcl = 0 %, Fv = 10.1 %, Fa = 1.5 % Fv = 35.6 %, Fa =64.4 %
  9. 9. X-ray diffraction analysis Exp. XRD peak (400) Exp. XRD peak (111) Total Fit Total Fit Peak 1 (22.4 nm) Intensity (arb. unit) Peak 1 (14.8 nm) Intensity (arb. unit) Peak 2 (9 nm) Peak 2 (4.8 nm) 26 27 28 29 30 31 32 33 68.0 68.5 69.0 69.5 70.0 2θ (degree) 2θ (degree) Intensity (arb.unit) (400) (111) thickness ~ 1 µm (220) (311) 20 30 40 50 60 70 Cu Kα 2θ (degrees) Exp. XRD peak (311) Exp. XRD peak (220) Total Fit Total Fit (11.4 nm) Peak 1 (48 nm) Intensity (arb. unit) Intensity (arb. unit) Peak 2 (11.4 nm) 45 46 47 48 49 50 55 56 57 58 2θ (degree) 2θ (degree)
  10. 10. Surface morphology by AFM σrms= 4 nm + 0.3 nm d = 950 nm 10 Roughness by SE, σSE(nm) 8 σrms= 3.3 nm + 0.1 nm Frequency (arb. unit) d = 590 nm 6 σrms= 4.3 nm + 0.4 nm 4 d = 390 nm 2 σSE= 0.85 σrms + 0.3nm σrms= 7 nm + 0.1 nm 0 0 2 4 6 8 10 d = 180 nm Roughness by AFM, σrms(nm) σrms= 2.1 nm + 0.2 nm d = 55 nm 0 100 200 300 400 Conglomerate surface grain size (nm) thickness series of R=1/10
  11. 11. Presence of Size Distribution Surface Morphology X-ray diffraction by AFM Exp. XRD peak (220) Total Fit Intensity (arb. unit) Peak 1 Peak 2 46 47 48 49 50 Frequency (arb. unit) 2θ (degree) 0.2 Intensity (arb. unit) (111) (d) 0.1 (220) (311) (400) 0.0 0 100 200 300 400 20 30 40 50 60 70 Surface grain size (nm) Cu Kα 2θ (degrees)
  12. 12. Bifacial Raman Study 1.2 1.2 glass side exp. data of F0E31 film side exp. data of F0E31 cd1 cd1 cd2 cd2 Intensity (arb. unit) a Intensity (arb. unit) fit with - cd1cd2 fit with - cd1cd2a 0.9 0.9 0.6 0.6 0.3 0.3 0.0 0.0 400 425 450 475 500 525 550 450 475 500 525 550 -1 Raman Shift (cm ) -1 Raman Shift (cm ) collection collection excitation excitation Small grain (cd1) Large grain (cd2) a-Si:H Sample #E31 Fitting film (1200 nm, Size (nm) XC1 Size (nm) XC2 Model Xa (%) R=1/1) [σ (nm)] [σ (nm)] glass (%) (%) glass film Film side cd1+cd2 6.1, [1.68] 20 72.7, [0] 80 0 Glass side cd1+cd2+a 6.6, [1.13] 8.4 97.7, [4.7] 52.4 39.2
  13. 13. Deconvolution of Raman Spectroscopy Data • Conventionally: RS profiles are deconvoluted assuming: – a single mean crystallite size – a peak assigned to grain boundary material – an amorphous phase is included to account for the asymmetric tail • Samples in our study: – No a-Si:H phase – Presence of two (mean) sizes of crystallites • Previous efforts to include CSD in fitting of Raman Data – To achieve a more accurate mathematical fitting of the asymmetry observed in the RS profile as a result of CSD
  14. 14. Incorporation of CSD in Raman Analysis According to our model, Φ(L) representing the CSD of an ensemble of spherical crystallites, total Raman intensity profile for the whole ensemble of nanocrystallites becomes: (1) I (ω , L0 , σ ) = ∫ Φ (L )I (ω , L )dL ' For a normal CSD, Φ(L) is given as: ⎡ (L − L 0 )2 ⎤ Φ (L ) = 1 (2) exp ⎢− ⎥ 2σ 2πσ 2 2 ⎢ ⎥ ⎣ ⎦ where the mean crystallite size L0 and the standard deviation σ are the characteristics of the CSD. •Islam & Kumar, Appl. Phys. Lett. 78 (2001) 715. •Ram et al Thin Solid Films 515 (2007) 7619
  15. 15. By putting Eq.(1) into Eq.(2) and then integrating the results over the crystallite sizes L, and by restricting the dispersion parameter σ to be less than L0/3 one gets the modified Raman intensity profile as: (3) ⎧ q 2 L2 f 2 (q )⎫ f (q )q 2 exp⎨− 0 ⎬ 2α ⎭ ⎩ I (ω , L0 , σ ) ∝ {ω − ω (q )}2 + (Γ0 2)2 where the parameter ⎛ q 2σ 2 ⎞ f (q ) = 1 ⎜1 + ⎟ , ⎜ ⎟ α ⎝ ⎠ which incorporates the distribution broadening parameter σ into the Raman intensity profile. •Islam & Kumar, Appl. Phys. Lett. 78 (2001) 715. •Ram et al Thin Solid Films 515 (2007) 7619
  16. 16. RS Data Deconvolution : Our Model inclusion of crystallite size distribution • In the absence of an explicit amorphous hump, the asymmetry in the Raman lineshape of RS profiles, seen as a low energy tail, is attributed to the distribution of smaller sized crystallites • Incorporation of a bimodal CSD in the deconvolution of RS profiles: – avoids the overestimation of amorphous content while fitting the low frequency tail – Avoids the inaccuracies in the estimation of the total crystalline volume fraction in the fully crystalline µc-Si:H material. • RS(F) data bimodal CSD • RS(G) data bimodal CSD + an amorphous phase
  17. 17. RS analysis fit model quot;cd1+cd2quot; cd1 cd2 * deconvolution of d = 950 nm, RS(F) RS profiles using a Intensity (arb. unit) bimodal size cd1 fit model quot;cd1+cd2+aquot; distribution of cd2 d = 950 nm, RS(G) large crystallite a grains (LG ~70– 80nm) and small fit model quot;cd+aquot; cd crystallite grains d = 55 nm, RS(F) a (SG ~6–8nm) fit model quot;cd+aquot; cd a d = 55 nm, RS(G) 400 450 500 550 -1 Raman shift (cm )
  18. 18. Fractional composition of films: Qualitative agreement between RS and SE studies 100 Xc1 (%) 100 Fcf (b) (a) Fcl Fcf , Fcl , Fv (%) by SE Xc2 (%) Xa, Xc1, Xc2 (%) by RS 80 Xa (%) 80 Fv 60 60 40 40 20 20 0 0 200 400 600 800 1000 1200 200 400 600 800 1000 Film Thickness (nm) Film Thickness (nm) Samples belong to thickness series of R=1/10
  19. 19. Summary of variation in fractional compositions and roughness with film growth Roughness by SE, σSE (nm) Top surface layer (c) 6 thickness series of R=1/1 5 4 3 100 RS(F) (a) 2 80 1 60 Xc1 100 Fractional compositions by RS (%) (b) Bulk Layer Xc2 40 Xa 80 20 Fcf 60 Fractional compositions by SE (%) 0 Fcl 40 100 RS(G) (b) Fv Xc1 Fa 80 20 Xc2 Xa 60 0 100 40 Interface Layer (a) 80 20 60 0 0 200 400 600 800 1000 1200 40 Film Thickness (nm) 20 Fa Samples belong to thickness series of R=1/1 0 Fv 200 400 600 800 1000 1200 Film thickness (nm)
  20. 20. Conclusions • Microstructural characterization studies on plasma deposited highly crystalline µc-Si:H films to explore the distribution in the crystallite sizes • SE two types of crystallites having two distinct sizes • XRD two mean sizes of crystallites • Surface morphological images size distribution • Deconvolution of experimentally observed RS profiles using a bimodal size distribution of crystallites • In Raman spectra of single-phase µc-Si:H material: appearance of a strong and longer low-frequency tail, without any distinguishable amorphous hump, can be due to the presence of size distribution in nanocrystallites, instead of a contribution from disordered or amorphous phase.
  21. 21. Thanks

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