Anomalous Behavior Of SSPC In Highly Crystallized Undoped Microcrystalline Si Films

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Microcrystalline silicon is a heterogenous material. We show that different effective DOS distribution can be possible for micro-structurally different μc--Si:H thin films

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Anomalous Behavior Of SSPC In Highly Crystallized Undoped Microcrystalline Si Films

  1. 1. Study Of Anomalous Behavior Of Steady State Photoconductivity In Highly Crystallized Undoped Microcrystalline Si Films Sanjay K. Ram Dept. of Physics, Indian Institute of Technology Kanpur, INDIA
  2. 2. Outline Motivation Sample preparation & structural characterization Steady state photoconductivity (SSPC) measurements Qualitative analysis Numerical simulation of SSPC Conclusion
  3. 3. MOTIVATION μc-Si:H thin films Promising material for large area electronics Good carrier mobility Greater stability under electric field and light-induced stress Good doping efficiency Possibility of low temperature deposition Further development requires proper understanding of carrier transport properties correlative with film microstructure
  4. 4. ISSUES Why is comprehensive description of its opto-electronic properties difficult ??? Complex microstructure & inhomogeneity in the growth direction columnar boundaries grain grains conglomerate crystallites boundaries surface roughness voids Film growth substrate
  5. 5. ISSUES Non-availability of complete density of state (DOS) map of µc-Si:H system Difference between Density of States (DOS) map of c-Si and amorphous Silicon (a-Si:H)
  6. 6. ISSUES Electrical transport ??? Is it dominated by crystalline phase ??? or By interfacial regions between crystallites or grains??? A large number of studies claim that electronic transport in μc-Si:H films is analogous to that observed in a-Si:H films GOAL To study the opto-electronic properties of well characterized μc-Si:H films Identify the role of microstructure in determining the electrical transport behavior
  7. 7. 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 High purity feed gases: Silane flow ratio (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
  8. 8. Film characterization Electrical Properties Structural Properties σd(T) measurement 15K≤T ≤ 450K Xray Diffraction σPh(T,∅) measurement 15K≤T ≤ 325K Raman Scattering CPM measurement In-situ Spectroscopy Ellipsometry Hall effect TRMC Atomic Force Microscopy
  9. 9. Raman Scattering 3.2 Layer side R (SiF4 / H2) = 1/10 μc-Si (X, SiF4) μc-Si Std. 2.8 Intensity (arb. unit) FB04GF t=950 nm 2.4 t=590 nm FB23GF 2.0 t=422 nm Intensity (a.u.) F281GF 1.6 t=390 nm FB11GF 1.2 t=170 nm FB22GF 0.8 0.4 t=52 nm F152GB 450 475 500 525 550 -1 Raman Shift (cm ) c-Si 400 450 500 550 600 Effect of thickness variation -1 Raman shift (cm )
  10. 10. Spectroscopy ellipsometery study B23 (R=10, t=590 nm) 45 B11 (R=10, t=390 nm) 25 F0E31 40 B22 (R=10, t=170 nm) F152(R=10, t=55 nm) Fit 35 F16 (R=20, t=35 nm) a-Si:H 20 30 c-Si 25 <ε2> < ε2 > 20 15 15 10 10 5 0 5 -5 -10 0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 2.5 3.0 3.5 4.0 4.5 5.0 Energy (eV) Energy (eV) Fig. Measured <ε2> spectrum for the µc-Si:H samples. The sample name, thickness and its 1/R value are shown in the graph.
  11. 11. X-ray diffraction study XRD study to see the effect of R (H2/SiF4) 4500 Film deposited at SiF4/H2 (111) (400) 4000 (220) flow ratio 1/1 shows a preferred (311) 1/1 3500 orientation of (400). 1.2 µm 3000 Intensity (a.u.) film deposited at SiF4/H2 2500 R 1/5 1.1 µm flow ratio of 1/5 shows a 2000 preferred orientation in (220) 1500 direction. 1000 1/10 0.95 µm 500 These results demonstrate the 0 20 30 40 50 60 70 effectiveness of using fluorine Cu Kα 2θ (degrees) based precursors in controlling the orientation of polycrystalline films on insulating glass substrates.
  12. 12. Structural Findings Random Orientation More Void fraction R =1/10 Individual grains are bigger (220) orientation R =1/5 (400) orientation Tightly packed Smooth top layer R =1/1 Good crystallinity at bottom interface
  13. 13. Classification from coplanar electrical transport point of view TYPE-A More amorphous tissue Small grains Thickness (50-250 nm) TYPE-B Moderate amorphous tissue Thickness Small grains (300-600 nm) TYPE-C Tightly packed columnar crystals Less amorphous tissue Thickness (900-1200 nm) Big grains
  14. 14. We have measured temperature and light intensity dependent steady state photoconductivity (SSPC) for the samples of different microstructure SSPC Process absorption of photons recombination of transport of and generation of free excess free electrons mobile carriers electron-hole pairs and holes through recombination centers
  15. 15. In a disordered material: σph (T, φ)=e[μn(n-n0) + μp(p-p0)] γ σ ph ∝ GL Light Intensity dependence: where, GL = φ (1-R)[1-exp(-αd)]/d Significance of γ γ is a measure of characteristic width of tail states nearer to Ef According to the Rose model: the exponentially distributed tail state shows: γ = kTC/(kT+kTC) In amorphous semiconductor 0.5<γ <1.0 γ=0.5 bimolecular recombination kinetics γ=1 monomolecular recombination
  16. 16. Experimental Results [σph(φ , T)] of sample #B22 of Type-A -5 10 -5 -5 10 10 σph (Ω cm ) -1 −1 -6 10 -6 10 σph (Ω cm ) σph (Ω cm ) -1 -6 -1 10 3 4 5 6 7 -7 10 -1 1000 / T (K ) −1 −1 -7 10 -8 σd 10 -8 10 310 K 275 K -9 10 17 250 K 1.2 x 10 (100%) 225 K 16 8.4 x 10 (75.4%) -9 10 175 K 16 7.6 x 10 (65%) 125 K 16 -10 5.5 x 10 (49%) 80 K 10 16 50 K 2.0 x 10 (15%) 30 K -10 15 1.6 x 10 (1.25%) 10 14 15 16 17 10 10 10 10 0 10 20 30 40 50 2 -1 Intensity F (photons/cm . sec) 1000 / T (K ) σph(T) vs φ σph(φ ) vs T Note: σPh (T) shows thermal quenching (TQ) with an onset at ~ 225K
  17. 17. Experimental Results [σph(φ , T)] of sample #B23 of Type-B -4 10 2 -5 Φ ( photons/cm -sec ) 10 14 1x10 16 1x10 -6 10 σph (Ω cm ) 16 5x10 -1 -7 σph (Ω cm ) 10 17 10 -1 −1 -8 −1 10 324 K 300 K -9 σd 10 275 K 250 K 225 K 200 K 175 K -10 153 K 10 128 K -11 101 K 10 72 K 60 K 50 K 25 K -12 10 12 13 14 15 16 17 10 10 10 10 10 10 4 8 12 16 20 2 Φ (Photons/cm -sec) -1 1000 / T (K ) σph(T) vs φ σph(φ ) vs T Note: σPh (T) shows NO TQ
  18. 18. Experimental Results [σph(φ , T)] of sample #F06 of Type-C 2 Φ ( photons/cm -sec )10-3 -4 10 17 15K 1x10 20K 16 -4 8x10 10 σph (Ω cm ) -4 -1 10 30K 16 2x10 40K −1 15 7x10 σph (Ω cm ) 50K -1 -5 -6 10 σph (Ω cm ) 15 10 2x10 60K -1 -6 10 14 70K 6x10 80K 14 1x10 -6 −1 σd 10 90K 3 4 5 6 −1 100K -1 1000 / T (K ) -8 -8 10 10 150K 200K 250K 300K -10 10 -10 10 -12 10 0 10 20 30 40 50 14 15 16 17 10 10 10 10 -1 2 1000 / T (K ) Φ (photons/cm . sec) σph(T) vs φ σph(φ ) vs T Note: σPh (T) shows TQ with an onset at 225 K
  19. 19. Comparison of phototransport properties of all the three types of samples Findings: TQ and 0.5<γ <1 : as 1.0 found in Type-A: 0.8 NO TQ and 0.5<γ <1 : as 0.6 found in Type-B: γ 0.4 γ TQ and value B22 F06 0.2 B23 approaches to a lowest value of 0.14 at 225 K: as 0 10 20 30 40 50 60 70 -1 found in Type-C: 1000/T (K ) temperature dependencies of light intensity exponent (γ)
  20. 20. DISCUSSION Qualitative analysis Causes of TQ : The transformation of the recombination traffic from VBT states to DB The asymmetry in band tails in the gap. Low value of defect densities or increasing n-type doping level may shift the onset of TQ to higher T. Causes of sublinear behavior of γ (<0.5) The saturation of recombination centers The shift of EF towards band edges in doped material.
  21. 21. 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 Symmetric band tails Usually observed in typical µc-Si:H 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 to be responsible for such behavior
  22. 22. Qualitative analysis Phototransport properties of Type-C (TQ and γ<0.5) 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
  23. 23. Qualitative analysis Possible explanation for sublinear behavior of γ (<0.5) in Type-C In Type-C material, EF is found to be very close to Ec (EC-EF ~ 0.34 eV) δn ≈ n0 then Rose model In doped a-Si:H when kTc << kTv doesn’t hold (by C. Main ….) γ=T/Tv for low excitation γ= Tc/Tv at high excitation According to Polycrystalline Si model two different VBT is also possible; A sharper shallow tail near the edge-> originating from grain boundary defects A less steeper deeper tail associated with the defects in columnar boundary regions. Capture cross section for the deeper tail is smaller than the shallower one.
  24. 24. Numerical Simulation Motivation Experimental results cannot discern the states where the recombination actually occurs S-R-H mechanism and Simmons-Tylor Statistics are extensively used to understand recombination mechanism in steady state process EC R9 R10 CBT R15 R4 R3 R1 R2 GL R16 U R13 R14 R6 R7 R8 R5 VBT R11 R12 EV DB 0 VBT CBT DB + DB - Schematic illustration of DOS in amorphous semiconductor and representation of electron (solid lines) and hole transitions (dotted lines)
  25. 25. 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 EC QCT = ∫ N CT (E )FCT (E )dE = QCT (n, p ) EV EC QVT = ∫ NVT (E )[1 − FVT (E )]dE = QVT (n, p ) EV ( ) QDB (n, p ) − QDB (n0 , p0 ) = N DB FDB + 2 FDB − FDB − 2 FDB − − 0 00 Recombination equation S n n + S CT p ' CT GL = U CT + U VT + U DB FCT (E ) = p ( ) ( ) S n n + n ' + S CT p + p ' CT p [( )]dE )( EC U DB = ∫ N DB (E ) n FDB S n + FDB S n − FDBε n + FDBε n + + −− 0 0 0 0 EV S n n ' + S VT p VT FVT (E ) = p ( ) ( ) S n n + n ' + S VT p + p ' VT p
  26. 26. ⎡ − ((Ec − E ) − Etc1 )⎤ ⎤ ⎡ ⎢ ⎥⎥ exp ⎢ ⎡ ( Ec − E ) ⎤ ⎢ ⎣ ⎦⎥ kTc 2 N ct1 = N ct1 × exp ⎢− × 0 ⎥ CBT ⎡ − ((Ec − E ) − Etc1 ) ⎤ ⎥ kTc1 ⎦ ⎢ ⎣ ⎢1 + exp ⎢ ⎥⎥ ⎢ ⎦⎥ ⎣ kTc 2 ⎣ ⎦ ⎡ (− (E − E v ) + Etv1 ) ⎤ ⎤ ⎡ ⎢ ⎥⎥ exp ⎢ ⎡ (E − E v ) ⎤ ⎢ ⎣ ⎦⎥ kTv 2 × = N vt1 × exp ⎢− 0 VBT ⎥ N vt1 ⎡ (− (E − E v ) + Etv1 ) ⎤ ⎥ kTv1 ⎦ ⎢ ⎣ ⎢1 + exp ⎢ ⎥⎥ ⎢ ⎦⎥ ⎣ kTv 2 ⎣ ⎦ ⎡ (E − EDB )2 ⎤ ND N DB ( E ) = DB exp ⎢ ⎥ (2π ) W ⎣ 2W 2 ⎦ 1/ 2
  27. 27. 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-Tylor 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
  28. 28. Simulation results for Type-C material (ex. #F06) 21 10 μn = 10 cm2V-1s-1 -4 19 -3 -1 CBT G=10 cm sec 10 VBT1 20 -3 -1 G=10 cm sec 19 10 21 -3 -1 G=10 cm sec μp = 0.5 cm2V-1s-1 DOS (cm eV ) σph (Ω cm ) -1 -1 -3 -5 -1 10 17 10 EC- EF=0.34 eV 15 10 VBT2 -6 10 DB 13 10 0.0 0.3 0.6 0.9 1.2 1.5 1.8 4 6 8 10 EV -1 EC (E-EV) eV 1000/T (K ) 0.6 Recombination rates (cm sec ) -1 19 10 γ -3 Uct1 17 10 0.5 Uvt1 Uvt2 UDB 15 10 γ 0.4 13 10 11 10 0.3 100 150 200 250 300 4 6 8 10 T (K) -1 1000/T (K )
  29. 29. Simulation results for Type-B material (#B23) 21 10 μn = 10 cm2V-1s-1 -5 CBT1 10 VBT1 19 DOS (cm eV ) 10 -1 μp = 0.5 cm2V-1s-1 σph (Ω cm ) -1 EC- EF=0.42 eV -6 -3 10 17 10 -1 -7 10 CBT2 15 10 18 -3 -1 G=10 cm sec DB 19 -3 -1 G=10 cm sec VBT2 20 -3 -1 G=10 cm sec 21 -3 -1 G=10 cm sec -8 13 10 10 4 6 8 10 0.0 0.3 0.6 0.9 1.2 1.5 1.8 -1 (E-EV) eV EC EV 1000/T (K ) 0.9 Recombination rates (cm sec ) -1 19 10 γ Uct1 -3 Uct2 0.8 17 10 Uvt1 Uvt2 UDB 15 0.7 10 γ 13 10 0.6 11 10 0.5 100 150 200 250 300 4 8 12 16 20 T (K) -1 1000/T (K )
  30. 30. Summary The qualitative as well as quantitative analysis of the study of our phototransport properties of undoped µc- Si:H thin films are in good agreement Micro-structural differences leads to totally different phototransport behavior. The recombination rate of deeper valence band tail is higher in percolated grains than in unpercolated grains We propose different effective DOS distribution for micro-structurally different μc-Si:H thin films

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