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My Thesis

I investigated the microstructure of a wide variety of nano and microcrystalline Si (μc-Si:H) films

produced under different growth conditions using different characterization probes (spectroscopic

ellipsometry, Raman spectroscopy, atomic force microscopy and X-ray diffraction) at different stages of film growth.

In microstructural studies, I applied a novel modeling method for deconvolution of Raman

spectra of the μc-Si:H films and elucidated schematic growth models for the SiF4 based single phase μc-Si:H material.

I carried out studies on the optoelectronic properties of these microstructurally different

films using dark and photo- conductivity as functions of several discerning parameters. The results of these studies led me to expound a novel way of classifying the wide range of materials into three types based on microstructural attributes and correlative optoelectronic properties. My electrical transport

studies have uncovered some new aspects of the carrier conduction routes and mechanisms in the single phase μc-Si:H material. I have proposed the complete effective distributions of density of states (DOS) applicable to this wide microstructural range of μc-Si:H material based on the results of experimental and numerical simulation studies of the phototransport properties of the material.

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- 1. Influence of Microstructure on the Electronic Transport Behavior of Microcrystalline Silicon Films Sanjay K. Ram Dept. of Physics, Indian Institute of Technology Kanpur, INDIA
- 2. Outline Ch. I: Introduction Ch. II: Experimental Details Ch. III: Structural Investigation Ch. IV: Electrical Transport Properties 1: Dark conductivity Ch. V: Electrical Transport Properties 2: Photoconductivity Ch. VI: Numerical Modeling of Steady State Photoconductivity in µc-Si:H Ch. VII: Summary and Conclusions
- 3. Chapter-I INTRODUCTION
- 4. crystalline structure crystallites in a-Si random network Long-range order Medium-range order Short-range order
- 5. Role of Si thin films in large area microelectronics Thin film Poly Si Amorphous silicon (a-Si:H) Advantages: Advantages: Solid phase crystallization/LPCVD Possibility of low temperature Grain sizes of 10 nm to 1 μm are plasma deposition common Plays a dominant role in the Very high carrier mobility application of solar cells and TFTs Greater stability under electric field Good photosensitivity and light-induced stress Wide band gap Good for TFTs Issues: High doping efficiency Low carrier mobility (μn~1 cm2/V-s Issues: & μp~10-3 cm2/V-s Metastability High temperature deposition Poor doping efficiency Boundaries are not passivated
- 6. Why μc-Si:H thin films ?? Promising material for large area electronics Possibility of low temperature deposition Good carrier mobility Greater stability under electric field and light-induced stress Good doping efficiency Boundaries are passivated Further development requires proper understanding of carrier transport properties correlative with film microstructure
- 7. Why is a comprehensive description of optoelectronic properties of µc-Si:H difficult ??? 1. Complex microstructure 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.
- 8. Issues µc-Si:H is not a unique material. Electronic transport can be studied or understood after a proper structural characterization of the material. The quantitative analysis of microstructure of µc-Si:H is difficult and often ambiguous. Tools at different length scales required. Electrical transport properties are influenced by the constituent phases. The correlation between microstructure and electrical properties is unexplored.
- 9. 2. Non-availability of a complete DOS map of μc-Si:H system Difference between DOS map of c-Si and amorphous Silicon (a-Si:H)
- 10. Issues Smaller grains a-Si like properties Large grains c-Si like properties There is no unique effective DOS profile that can satisfy the whole range of materials included under the common name of microcrystalline Si, or explain all the transport processes.
- 11. Desired μc-Si:H material in TFTs (Staggered type) Need for BOTTOM Gate TFT Need for TOP Gate TFT Smooth Top layer of the film Crystallization should start at the beginning of the growth Bigger sizes of crystallite at the Top layer To reduce the amorphous incubation layer at the bottom Inverted pyramid shaped glass interface columnar crystallites are preferable
- 12. Approach In this work, we have studied the microstructure of µc-Si:H films having varying degrees of crystallinity and tried to identify the role of different deposition parameters on film microstructure and morphology. We have studied the optoelectronic properties of such well characterized films and attempted to correlate these properties to the film microstructure. Lastly, we have carried out an extensive numerical modeling study of phototransport properties of μc-Si:H system to understand the experimental findings.
- 13. Our Results Fully Crystallized plasma deposited μc-Si:H can be deposited and carrier transport in such films is different. Films with different microstructures lead to different effective density of states map that can be used to parameterize the electrical transport behavior.
- 14. Chapter-II EXPERIMENTAL DETAILS
- 15. Sample Preparation PECVD RF Parallel-plate glow discharge HH H Si H H N H H H H plasma deposition system H Si N Si N Si N μc-Si:H Substrate: Corning 1773 film High purity feed gases: Silane flow ratio SiF4 , Ar & H2 (R)= SiF4/H2 R=1/1 R=1/5 R=1/10 Rf frequency 13.56 MHz Ts=200 oC Thickness series
- 16. Film characterization Electrical Properties Structural Properties σd(T) measurement 15K≤T ≤ 450K X-ray Diffraction σPh(T,∅) measurement 15K≤T ≤ 325K Raman Scattering CPM measurement In-situ Spectroscopic Ellipsometry TRMC Atomic Force Microscopy
- 17. Chapter-III STRUCTURAL INVESTIGATION
- 18. Spectroscopic Ellipsometry Study 45 F0E31 40 Fit 35 a-Si:H 30 Top Layer (3.1 nm) c-Si Fcf = 15 %, Fcl= 62 %, Fv = 23 %, Fa=0 % 25 Upper Middle Layer (864 nm) <ε2> 20 Fcf = 9.8 %, Fcl = 90.2 %, Fv = 0 %, Fa=0 % 15 Lower Middle Layer (311 nm) Fcf = 86.2 %, Fcl= 0 %, Fv= 4.5 %, Fa=9.3 % 10 5 Bottom Interface Layer (27 nm) Fcf = 0%, Fcl = 0 %, Fv = 25 %, Fa= 75 % 0 -5 -10 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Energy (eV) Measured <ε2> spectrum for the µc-Si:H sample #E31 [deposition condition: R (SiF4/ H2)= 1/1, Ar flow = 25 sccm, TS = 200 °C, thickness = 1200 nm]. Peaks at about 3.5 and 4.2 eV are observed.
- 19. Bifacial Raman Study A deconvolution model that includes crystallite size distribution was employed for analysis of Raman data. 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 -1 Raman Shift (cm ) Raman Shift (cm ) collection collection Small grain (cd1) Large grain (cd2) a-Si:H Sample #E31 Fitting (1200 nm, Size (nm) XC1 Size (nm) XC2 Model excitation Xa (%) R=1/1) [σ (nm)] [σ (nm)] (%) (%) excitation cd1+cd2 6.1, [1.68] 20 72.7, [0] 80 0 Film side film glass cd1+cd2+a 6.6, [1.13] 8.4 97.7, [4.7] 52.4 39.2 Glass side glass film
- 20. Surface Morphology by AFM (b) 0.25 (a) B04 (t =950 nm; R=1/10) 0.20 Frequency (arb. unit) 0.15 (c) 0.10 0.05 0.00 0 100 200 300 400 Conglomerate surface grain size (nm) sample #B04 (thickness = 950 nm, R=1/10, roughness (σrms) = 5.26 nm)
- 21. Types of samples studied Fixed deposition parameters Plasma Power (W) 20 13.56 RF frequency (νrf) (MHz) Total Pressure (Torr) 1 SiF4 flow rate (sccm) 1 Ar flow rate (sccm) 25 R=SiF4/H2 = 1/1 Thickness series Thickness : R=SiF4/H2 = 1/5 50 nm to 1200 nm TS=200°C R=SiF4/H2 = 1/10 Set-A (thickness is ~ 50 nm) R series R: Set-B (thickness is ~ 400 nm) 1/1 to 1/20 TS=200°C Set-C (thickness is ~ 950 nm) TS series R=1/5 TS: 100 - 350°C
- 22. Effect of Film Growth E31 (R=1/1, t=1200nm) 30 Growth time 30 min (c) (b) 60 min 190 min 20 225 min 230 min < ε2 > 10 0 (a) (d) -10 2 3 4 5 Energy (eV) Film side R (SiF4 / H2) = 1/10 0.25 Intensity (arb. unit) thickness ----> thickness series of R =1/10 B04 (t=950 nm) B04 (t=950 nm) 0.20 Frequency (arb. unit) B11 (t=390 nm) B22 (t=170 nm) B23 (t=590 nm) F152 (t=52 nm) 0.15 D281 (t=422 nm) B11 (t=390 nm) 0.10 B22 (t=170 nm) 0.05 F152 (t=52 nm) 0.00 450 475 500 525 550 0 100 200 300 400 -1 Raman Shift (cm ) Conglomerate surface grain size (nm)
- 23. 30 F151 (R=1/1, t=62 nm) Effect of R (SiF4/H2) F152 (R=1/10, t=55 nm) H2 dilution F16 (R=1/20, t=58 nm) 25 20 SE: The film of higher value of R shows more void < ε2 > 15 fraction at the top layer, indicating more rough 10 surface compared to the films of lower value of R. 5 X-ray: Films deposited at highest R=SiF4/H2 flow 0 2.5 3.0 3.5 4.0 4.5 5.0 ratio 1/1 shows a preferred orientation of (400). Energy (eV) While films deposited at R=1/5 shows a preferred 4500 (111) (400) 4000 (220) orientation in (220) direction. (311) 1/1 3500 1.2 µm 3000 AFM: Films are rougher for higher values of R. Intensity (a.u.) 2500 1/5 1.1 µm Average grain size increases with the increase of R. 2000 1500 1000 1/10 0.95 µm 500 0 0.30 20 30 40 50 60 70 H2 dilution -----> Cu Kα 2θ (degrees) F16 (t=58 nm; R=1/20) F152 (t=55 nm; R=1/10) R =1/1 R =1/10 0.25 R =1/20 F151 (t=62 nm; R=1/1) Frequency (arb. unit) 0.20 (t ~ 55 nm) Set-A 0.15 0.10 0.05 0.00 0 40 80 120 160 Conglomerate surface grain size (nm)
- 24. Spectroscopic Ellipsometry Raman Scattering and AFM AFM: σrms = 0.9 nm Top Layer (0.98 nm) Fcf = 33 %, Fcl = 0 %, Fv = 67 %, Fa =0 % Outcome &validation RS from front side Bulk Layer (59.6 nm) Set-A XC1 = 35 %, Xa = 65 % Fcf = 73 %, Fcl = 0 %, Fv = 6 %, Fa = 21 % of analytical approach RS from glass side XC1= 26.8 %, Xa= 73.2 % AFM: σrms = 4.16 nm Top Layer (4.2 nm) Fcf = 43 %, Fcl = 32 %, Fv = 25 %, Fa =0 % Middle Bulk Layer (424 nm) RS from front side Characterization probes XC1 = 35 %, XC2= 65 %, Xa = 0 % Fcf = 58.7 %, Fcl= 37.6 %, Fv=3.7 %, Set-B Fa=0% operating at different RS from glass side Bottom Interface Layer (22 nm) XC1 = 17 %, Xa = 83 % Fcf = 0 %, Fcl= 0 %, Fv = 9.4 %, Fa =90.6 % length scales leads to a comprehensive picture of AFM: σrms = 5.2 nm Top Layer (5.1 nm) Fcf = 33 %, Fcl = 43 %, Fv = 24 %, Fa =0 % film microstructures. RS from front side Middle Bulk Layer (888 nm) XC1= 34 %, XC2= 66 %, Xa= 0 % Fcf = 51 %, Fcl = 45 %, Fv = 3 %, Fa =0 % RS from glass side Bottom Interface Layer (33 nm) A large number of μc-Si:H XC1 = 13.5 %, XC2 = 45.5 %, Xa = 41 % Fcf = 0 %, Fcl = 0 %, Fv = 32 %, Fa =68 % Set-C films can be classified into Top Layer (3.1 nm) Fcf = 15 %, Fcl = 62 %, Fv = 23 %, Fa=0 % RS from front side three different class of XC1 = 20 %, XC2= 80 %, Xa= 0 % Upper Middle Layer (864 nm) Fcf = 9.8 %, Fcl = 90.2 %, Fv = 0 %, Fa=0 % microstructures. Lower Middle Layer (311 nm) Fcf = 86.2 %, Fcl= 0 %, Fv= 4.5 %, Fa=9.3 % RS from glass side Bottom Interface Layer (27 nm) XC1 = 8.4 %, XC2 = 52.4 %, Xa = 39.2 % Fcf = 0%, Fcl = 0 %, Fv = 25 %, Fa= 75 %
- 25. Types of film growth 100 R=1/10 FV % 80 FCf % Fraction (%) FCl % 60 (a) Random Orientation 40 R =1/10 20 More Void fraction 0 Individual grains are bigger 0 200 400 600 800 1000 1200 Bulk Layer Thickness (nm) 100 80 Fraction (%) R=1/5 60 FV % (b) (220) orientation FCf % R =1/5 40 FCl % 20 0 0 200 400 600 800 1000 1200 Bulk Layer Thickness (nm) 100 (400) orientation 80 R =1/1 Tightly packed Fraction (%) 60 R=1/1 FV % (c) Smooth top layer 40 FCf % FCl % 20 Good crystallinity at bottom interface 0 0 200 400 600 800 1000 1200 Bulk Layer Thickness (nm)
- 26. Roughness Analysis and its correlation with film growth R=1/10 Roughness by AFM, σrms(nm) 7 R=1/5 R=1/1 6 5 10 4 Roughness by SE, σSE(nm) Roughness by AFM, σrms(nm) 6 average thickness ~ 55 nm, 3 SiF4 = 1 sccm, Ar =25 sccm, 8 o Ts = 200 C) 4 2 2 6 1 0 0 5 10 15 20 H2 dilution 0 0 200 400 600 800 1000 1200 4 Film thickness (nm) 10 2 Roughness by SE, σSE(nm) σSE= 0.85 σrms + 0.3nm 8 0 0 2 4 6 8 10 Roughness by AFM, σrms(nm) 6 4 R=1/10 guide line for R=1/10 2 R=1/5 guide line for R=1/5 R=1/1 guide line for R=1/1 0 0 200 400 600 800 1000 1200 Thickness (nm)
- 27. Summary of Structural Studies Fully crystallized microcrystalline silicon films having big grains have been deposited using standard 13.56 MHz PECVD at low substrate temperatures. Effective control of film orientation has been demonstrated by varying the SiF4 : H2 flow ratios in the feed gas. Tailing and asymmetry in the Raman spectrum on lower wave numbers need not be a contribution from amorphous silicon tissue, rather may indicate the contribution from smaller nanocrystallites. The roughness analysis by two different methods, SE and AFM shows no ambiguity in their results and are in good agreement with each other. “Surface roughness is an external mirror of the internal bulk processes”.
- 28. Chapter-IV Electrical Transport Properties-I: Dark conductivity Above room temperature (300 – 450 K) Below room temperature (15 – 300 K)
- 29. Above room temperature (300-450K) dark conductivity (σd) measurement Effect of film thickness on electrical properties R ( = SiF4/H2) =1/10 R (= SiF4/H2) =1/1 -3 -3 10 10 -4 10 -4 -1 10 σd (Ω.cm) -1 -5 σd (Ω.cm) 10 -5 10 -6 10 -6 -7 B04 (t=950 nm, Ea=0.33 eV) 10 10 B23 (t=590 nm, Ea=0.44 eV) E31 (t=1200 nm, Ea=0.2 eV)) B11 (t=390 nm, Ea=0.44 eV) F06 (t=920 nm, Ea=0.15 eV)) -8 10 B22 (t=170 nm, Ea=0.54 eV) -7 E30 (t=450 nm, Ea=0.55 eV)) 10 B21 (t=150 nm, Ea=0.54 eV) F05 (t=180 nm, Ea=0.57 eV)) F152 (t=55 nm, Ea=0.54 eV) -9 F151 (t=62 nm, Ea=0.58 eV)) 10 Fit Fit 2.0 2.5 3.0 3.5 2.0 2.5 3.0 3.5 -1 -1 1000/T (K ) 1000/T (K ) In thermally activated process dark electrical conductivity (σd) of disordered materials is given as: σd=σo e –Ea / kT
- 30. σd (R=1/10) -3 σd (R=1/5) 10 Classification from coplanar σd (R=1/1) σd (R=1/5, TS ) electrical transport point of view -1 σd (Ω.cm) -5 10 -7 10 High density of inter- grain & inter-columnar -9 10 0 200 400 600 800 1000 1200 Thickness (nm) boundaries 0.7 TYPE-A Zone-1 Small grains Zone-3 Zone-2 Thickness (50-250 nm) 0.6 0.5 Ea (eV) Marked variation in 0.4 morphology & moderate 0.3 Ea (R=1/10) 0.2 disordered phase in Ea (R=1/5) Ea (R=1/1) TYPE-B Ea (R=1/5, TS) 0.1 columnar boundary Thickness (300-600 nm) 0 200 400 600 800 1000 1200 Mixed grains Thickness (nm) Percentage of Large Grains (FCl %) 100 FCl % (R= 1/10) FCl % (R= 1/5) 80 Tightly packed FCl % (R= 1/1) columnar crystals 60 Less amorphous tissue 40 large grains TYPE-C 20 Thickness (900-1200 nm) 0 0 200 400 600 800 1000 1200 Bulk Layer Thickness (nm)
- 31. Activation Energy, Ea W = [ 2 E g / 3 + kTln ( N c / n)]N s / n qVd qVd Qd Vd = [2 Eg / 3 + kTln( N c / n)]2 N s2 q /(2nε s ) EC W W QS NS EF Energy band diagram at the grain boundaries •In Type-C samples-- material becomes relatively defect free (less Type-C Type-B Type-A traps at interface) with large grains (more free carriers)-- depletion width decreases --- Ea represents GB barrier height. •In Type-A samples-- depletion layers extend towards the center of crystallite--- Ea will represent The Grain Boundary Trapping (GBT) approximately the energy difference Model by Lecomber et al between the edges of the transport [J. Non-Cryst. Solids, 59-60, 795 (1983) ] bands and Ef
- 32. The significance of σ0 Correlation between σ0 and Ea In Type-A and Type-B materials According to Meyer-Neldel Rule (MNR) such correlation leads to Exp. data of type- A & B samples 4 Fit 10 σ0=σ00 eGEa σ0 (Ω cm ) -1 3 10 where G or EMN (1/G) and σ00 are −1 MNR parameters 2 10 −1 -1 σ00 = 0.014(Ω cm ) 1 -1 10 G = 19.7 eV 4 MNR 10 EMN= 51 meV anti MNR 0.3 0.4 0.5 0.6 0.7 3 10 Ea(eV) σ0 (Ω cm ) 2 -1 In Type-C materials 10 2 10 1 −1 10 −1 -1 σ00 = 86.8 (Ω cm ) -1 G = - 44.6 eV 1 10 0 10 EMN= - 22.5 meV σ0 (Ω cm ) -1 Anti MNR in type-C samples 0 MNR in type-A & B samples 10 -1 10 −1 MNR in a-Si:H Anti MNR in doped μc-Si:H -1 -2 10 10 Exp. data of type-C samples -2 10 Fit 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 data of doped μc-Si:H (Lucovsky et al) Ea(eV) 0.00 0.05 0.10 0.15 0.20 0.25 Ea(eV)
- 33. Below room temperature (15-300K) dark conductivity (σd) measurement -2 -2 10 10 E31 E31 E31 -3 E31 E25 10 E25 -4 10 E25 F06 E25 F06 σd ( Ω cm ) -1 B04 F06 B04 F06 B23 -4 -4 B23 B04 D26 10 -6 10 σd ( Ω cm ) B04 10 σd ( Ω cm ) −1 B11 D26 σd ( Ω cm ) -1 -5 B23 -1 B22 B23 10 -1 B11 B22 B11 -8 B22 10 Fits B22 −1 −1 -6 -6 −1 Fits 10 10 -7 -10 10 10 5 10 15 20 25 -1 1000/T (K ) -8 -8 -9 10 10 10 -11 -10 -10 10 10 10 10 20 30 40 50 25 30 35 40 6 8 10 12 14 -1 -1/4 -1/4 1000/T (K ) 100*T (K ) -1/2 -1/2 100*T (K ) T–½ dependence of σd(T) : tunneling of carriers between neighboring conducting crystals ~ granular metals? × ES hopping --unrealistically large Coulomb gap . σd(T): T–¼ dependence Diffusional model gives reasonable hopping parameter values. × Mott’s percolation-- unphysical parameters.
- 34. Summary of Dark Electrical Transport Studies Thermally activated carrier transport is found in above room temperature (300-450 K). Significant correlation between the observed electrical properties (σd and Ea) of the films with their microstructural properties is established. Classification of μc-Si:H films based on microstructural attributes that are well correlated to electrical transport properties The change in Ea with the film thickness is directly related to the density of localized states at the Fermi level in the grain boundary. The dependence of conductivity prefactor on the activation energy of type-A and type- B μc-Si:H films follows Meyer Neldel rule. Statistical shift of Fermi level as an origin of MNR in our samples. The grain boundary trapping model also supports the shift of Fermi level in changing the microstructure of the film. However, type-C μc-Si:H films show a signature of anti MNR
- 35. Chapter-V Electrical Transport Properties-II: Photoconductivity
- 36. Steady State Photoconductivity (SSPC) γ Light Intensity Dependence: σ ph ∝ GL In a disordered material: σph (T, φ)=e[μn(n-n0) + μp(p-p0)] where, GL = φ (1-R)[1-exp(-αd)]/d What is γ ? γ is a measure of characteristic width of tail states nearer to Ef Rose’s Model: γ = kTc/(kT+kTc) In amorphous semiconductor 0.5<γ <1.0 γ=0.5 => bimolecular recombination kinetics γ=1 => monomolecular recombination.
- 37. Steady State Photoconductivity: Experimental Results 1.0 Light intensity exponent (γ) σph (Ω cm ) -5 -1 -5 10 B22 10 −1 -6 10 σph (Ω cm ) 0.8 -1 -6 10 3 4 5 6 7 -7 -1 1.0 −1 1000 / T (K ) 10 B22 σd 0.8 -8 0.6 10 2 Φ (photons/cm sec) γ 0.6 17 1.2 x 10 16 8.4 x 10 -9 10 16 7.6 x 10 0.4 5 10 15 20 16 5.5 x 10 -1 1000/T (K ) 16 2.0 x 10 0.4 -10 15 1.6 x 10 10 0 10 20 30 40 50 60 70 10 20 30 40 50 -1 1000/T (K ) -1 1000 / T (K ) Type-A (#B22, t= 170 nm) 0.5 < γ < 1 with TQ effect
- 38. -4 10 1.0 Light intensity exponent (γ) Φ ( photons/cm -sec ) 2 B23 14 1x10 16 1x10 -6 16 10 5x10 17 10 σph (Ω cm ) 0.8 -1 −1 -8 10 σd 1.0 0.6 0.8 γ -10 10 0.6 B23 0.4 5 10 15 20 -1 1000/T (K ) 0.4 -12 10 10 20 30 40 50 60 70 5 10 15 20 -1 -1 1000/T (K ) 1000 / T (K ) Type-B (#B23, t=590 nm) 0.5 < γ < 1 with No TQ effect
- 39. -3 2 Φ ( photons/cm -sec ) 10 1.0 Light intensity exponent (γ) 17 1x10 F06 16 σph (Ω cm ) 8x10 -1 -4 -4 10 10 16 2x10 −1 0.8 15 7x10 -5 15 2x10 10 14 σph (Ω cm ) -6 6x10 10 -1 0.6 14 1x10 -6 10 3 4 5 6 1.0 −1 Light intensity exponent (γ) -1 1000 / T (K ) F06 -8 0.8 10 0.4 σd 0.6 0.4 0.2 -10 0.2 10 0.0 5 10 15 20 -1 1000 / T (K ) 0.0 -12 10 10 20 30 40 50 60 70 10 20 30 40 50 -1 1000 / T (K ) -1 1000 / T (K ) Type-C (#F06, t= 920 nm) 0.15 < γ < 1 with TQ effect
- 40. Photoconductivity Exponent: Applicability of Rose Model density of states (arb. unit) DOS of μc-Si:H (Type-B) 21 21 DOS of μc-Si:H (Type-A) 10 10 MPC-DOS of coplanar μc-Si:H (ICRS =0.5) ** [Ref. ] ! MPC-DOS of HWCVD μc-Si:H [Ref. ] 19 19 ! MPC-DOS of SPC μc-Si:H [Ref. ] 10 10 !! TOF-DOS of μc-Si:H [Ref. ] * SSPC-DOS of μc-Si:H [Ref. ] 17 17 10 10 15 15 10 10 13 13 10 10 0.0 0.2 0.4 0.6 EC- E (eV) DOS distribution obtained for SSPC measurement of type-A and B µc-Si:H are plotted along with DOS profiles of µc-Si:H suggested in literature from other experimental techniques.
- 41. QUALITATIVE ANALYSIS Phototransport properties of Type-A (TQ and 0.5< γ<1) This type of behavior is usually observed in typical a-Si:H Rose model works and width of CBT is deduced (kTc ~ 30 meV ) Possible explanation for “No TQ and 0.5< γ<1 “ as found in Type-B Usually observed in typical µc-Si:H Symmetric band tails Rose model works and width of CBT is deduced (kTc~25-28 meV) According to Balberg et al (Phys. Rev. B 69, 2004, 035203): a Gaussian type VBT responsible for such behavior Possible explanations for TQ behavior in Type-C material Rose model does not hold for Type-C material DBs unlikely to cause TQ Possibilities of asymmetric band tail states in this type of material lower DOS near the CB edge, i.e. a steeper CBT than VBT (supported by defect pool model) The CPM measurement supports the fact kTC<<kTV
- 42. Chapter-VI Numerical Modeling of Steady State Photoconductivity in µc-Si:H
- 43. Motivation Experimental results cannot discern the states where the recombination actually occurs S-R-H mechanism and Simmons-Taylor Statistics are extensively used to understand recombination mechanism in steady state process EC R9 R10 CBT R15 R4 R3 R1 R2 R16 GL DB U R13 R14 R6 R7 R8 R5 VBT R11 R12 EV DB 0 VBT CBT DB + DB - Schematics of different recombination processes taking place within the gap of a disordered material.
- 44. Charge neutrality equation [n − n0 ] − [ p − p0 ] + [QCT (n, p ) − QCT (n0 , p0 )] − [QVT (n, p ) − QVT (n0 , p0 )] + N DB (FDB + 2 FDB − FDB − 2 FDB ) = 0 − − 0 00 GL = U CT + U VT + U DB Recombination equation Steps in Numerical Simulation DOS distribution is first assumed Guess values of n and p are given Charge neutrality equation & recombination rates equation are simultaneously solved for a fixed value of T and GL S-R-H mechanism and Simmons-Taylor Statistics are applied Newton-Raphson method for finding roots of n and p Simpson’s method for numerical integration n and p are obtained We calculated σph (T, φ)=e[μn(n-n0) + μp(p-p0)] The corresponding γ values are obtained as in experimental case
- 45. Simulated Steady State Photoconductivity Results Type-A 21 21 10 10 Effective DOS (cm eV ) VBT1 CBT1 -1 19 19 10 10 -3 EC- EF=0.46 eV 17 17 10 10 DB 15 15 10 10 VBT2 CBT2 13 13 10 10 0.0 0.3 0.6 0.9 1.2 1.5 1.8 EC EV (E-EV) eV Light intensity exponent (γ) 1.0 -6 20 -3 -1 10 G=10 cm sec 19 -3 -1 G=10 cm sec 18 -3 -1 G=10 cm sec -7 10 σph (Ω cm ) 0.8 -1 17 -3 -1 G=10 cm sec -8 10 -1 -9 10 0.6 -10 10 0.4 -11 10 5 10 15 20 5 10 15 20 -1 -1 1000/T (K ) 1000/T (K )
- 46. Type-B 21 21 10 10 Effective DOS (cm eV ) CBT1 VBT1 -1 19 19 10 10 -3 EC- EF=0.42 eV 17 17 10 10 15 15 10 10 DB CBT2 VBT2 13 13 10 10 0.0 0.3 0.6 0.9 1.2 1.5 1.8 EC EV (E-EV) eV 1.0 Light intensity exponent (γ) 21 -3 -1 G = 10 cm sec 20 -3 -1 G = 10 cm sec -5 19 -3 -1 10 G = 10 cm sec 0.8 σph (Ω cm ) -1 -1 -6 10 0.6 -7 10 0.4 5 10 15 20 5 10 15 20 -1 -1 1000/T (K ) 1000/T (K )
- 47. Type-C 21 21 10 10 CBT1 Effective DOS (cm eV ) VBT1 -1 19 19 10 10 -3 EC- EF=0.34 eV 17 17 10 10 DB 15 15 VBT2 10 10 CBT2 13 13 10 10 0.0 0.3 0.6 0.9 1.2 1.5 1.8 EV EC (E-EV) eV 1.0 Light intensity exponent, γ 21 -3 -1 G=10 cm sec 20 -3 -1 0.8 G=10 cm sec -4 10 19 -3 -1 G=10 cm sec σph (Ω cm ) -1 0.6 -5 -1 10 0.4 -6 10 0.2 -7 0.0 10 5 10 15 20 5 10 15 20 -1 -1 1000/T (K ) 1000/T (K )
- 48. 20 Recombination rates (cm sec ) 10 -1 Recombination rates (cm sec ) Recombination rates (cm sec ) 19 -1 -1 10 18 10 -3 -3 -3 UVBT UVBT 16 15 1 10 1 10 UVBT 17 UVBT 10 2 2 UDB UDB UVBT UCBT UCBT 1 12 13 UVBT 1 10 10 1 UCBT UCBT 2 UDB 2 2 14 UCBT 10 1 9 UCBT 10 10 10 2 6 10 7 11 10 10 50 100 150 200 250 300 350 50 100 150 200 250 300 50 100 150 200 250 300 T (K) T (K) T (K) Summary of Phototransport Studies Morphological & Microstructural differences in the different types of such µc-Si:H material leads to totally different phototransport behavior. The results obtained by numerical modeling are found to be in good agreement with the experimental findings, esp. TQ, its onset in the photoconductivity and the γ values. The effective DOS distribution we have proposed differs for different microstructures of μc-Si:H thin films and successfully explains the different phototransport properties in the light of their microstructural properties as well.
- 49. Summary and Conclusions
- 50. Multi-pronged approach of characterization techniques used to provide a complete, quantitative and unambiguous µc-Si:H microstructural picture The µc-Si:H films produced by the PECVD of SiF4 based precursor are highly crystalline, even at substrate temperatures as low as ~100°C Achievement of preferential crystalline orientation Presence of CSD supported by RS, SE and AFM analysis A linear correlation is observed between the rms roughness measured by AFM and the top surface layer measured by SE Microstructural characterization shows three types of microstructures that leads to distinct electrical transport behavior
- 51. Simultaneous observation of MNR and anti-MNR in undoped µc-Si:H thin films Low temperature dark conductivity evinces a T-1/2 dependence supporting tunneling mechanism, and a T-1/4 dependence compatible with diffusional model. Different types of µc-Si:H films exhibit different phototransport behaviors, explained on the basis of Rose model in types A and B, whereas type-C material exhibits anomalous behavior, explained using an effective DOS consisting of two VBT slopes. Numerical modeling of phototransport properties is found to give results corroborative with the experimental results. We have proposed complete effective density of states distributions for different types of µc-Si:H thin films having different microstructures.
- 52. Thank you!

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