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Can anti-MNR be possible in the dark conductivity behavior in undoped single phase microcrystalline silicon? What may be the origin of anti-MNR in such a case?

Can anti-MNR be possible in the dark conductivity behavior in undoped single phase microcrystalline silicon? What may be the origin of anti-MNR in such a case?

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- 1. ICANS-22, Colorado, U.S.A Normal and anti Meyer-Neldel rule in conductivity of highly crystallized undoped microcrystalline silicon films Sanjay K. Ram, Satyendra Kumar Samtel Centre for Display Technologies & Dept. of Physics, I.I.T.Kanpur, India & P. Roca i Cabarrocas, LPICM (UMR 7647 du CNRS), Ecole Polytechnique, France
- 2. Outline Introduction Experimental and characterization details Electrical transport behavior : classification of material Observation of Meyer Neldel rule (MNR) & Anti MNR in single phase undoped μc-Si:H MNR & Anti MNR in μc-Si:H in literature Conclusions
- 3. Meyer Neldel Rule (MNR) Observed in: Materials: Processes: Activated process: Annealing Phenomena Ionic Materials Y=A.exp (-B/X) Trapping in crystalline Chalcogenide glasses MNR A=A’.exp(GB) Semiconductors Organic thin films where G and A’ are Aging of insulating polymers Amorphous Silicon MNR parameters Biological death rates doped μc-Si:H Chemical reactions Electrical conduction microscopic origin of MNR & physical meaning of G ?? Statistical shift of Fermi level electrical transport in a-Si:H/ σ0=σ00 eGEa , σd=σ0.exp(-Ea/kT) MNR disordered semiconductor: where G or EMN (=1/G) and σ00 are MNR parameters
- 4. Anti Meyer Neldel Rule Correlation between σ0 and Ea appears to change sign – a negative value of MN energy (EMN) is seen Experimentally observed in: – Heavily doped μc-Si:H – Heterogeneous Si (het-Si) thin film transistor – Organic semiconductors Theoretically explained: In doped μc-Si:H Lucovsky and Overhof (LO): considering a degenerate case Ef moving deep into the band tail In a-Si:H (experimentally NOT observed) Statistical shift model
- 5. Statistical Shift Model According to Mott: σd(T) =σM exp(-(EC - EF)/kT)) EC(T ) = EC0 - γCT ; EF(T ) = EF0 - γFT Ea= EC0 - EF0, σd=σo exp (–Ea / kT ) σo=σM exp [(γC - γF) / k] σ0=σ00 exp (GEa) --- MNR
- 6. The reason for observed anti MNR According to LO model in a degenerate case Ef moves above Ec in the crystalline phase consequently Ef can move deeply into the tail states in the disordered region, giving rise to anti MNR behavior. Energy band diagram as proposed by Lucovsky et al, J.N.C.S. 164-166, 973 (1993)
- 7. Motivation Many complex issues/phenomena related to electrical transport properties were explained while searching for the origin of MNR in a-Si:H MNR has also been reported in doped μc-Si:H o with MNR parameters similar to those obtained in a Si:H o Explained in terms of statistical shift model analogous to a-Si:H
- 8. General observations: • Optical properties of μc-Si:H are governed by crystalline component • Electrical transport is still dominated by a-Si:H phase Issues: • μc-Si:H has complex and heterogeneous microstructure • Electronic transport in single phase μc-Si:H films??? – Non-varying high crystallinity and non-existent amorphous phase – Is it dominated by crystalline phase ??? or By interfacial regions between crystallites or grains???
- 9. Our Results We prepared large numbers of single phase μc-Si:H films having varying degree of microstructure and morphology Both MNR and anti-MNR can be observed in single phase μc-Si:H films, depending on film microstructure Objectives Search for both the origin and significance of these relationships as observed in single phase μc-Si:H material
- 10. Experimental layout Parallel-plate glow discharge plasma deposition system Substrate: High purity SiF4, Ar + Corning 1773 and H2 as feed gases AFM Rf frequency 13.56 MHz Ts=100-300 oC + X-ray μc-Si:H Opto-electronic Diffraction film transport + Raman measurement + Thermal evaporation of Al Spectroscopy Ellipsometry
- 11. Results: Microstructural Characterization • Total crystallinity >90% from beginning – No amorphous phase – Rest density deficit • Two sizes of crystallites, large and small: LG & SG • LG fraction (Fcl) increases with film growth Conglomeration ↑ with film growth • • Variable effect of H2 dilution at different growth stages
- 12. Classification of films Type-C material Type-B material Type-A material • Highest fraction of LG. • Rising fraction of • Small grains (SG) LG. • Well formed large • Low amount of columns • Marked conglomeration morphological (without column • Least amount of variation: column formation) disordered phase in the formation columnar boundaries. • High density of • Moderate amount intergrain boundary of disordered phase regions containing in the columnar disordered phase. boundaries.
- 13. Classification of films: electrical transport behavior and Fcl σ0 4 10 Ea 0.5 3 10 2 -1 10 0.4 σ0 (Ω cm) Ea (eV) 1 10 0.3 1 0.2 -1 10 -2 10 0.1 type-B type-A type-C 0 20 40 60 80 100 Fcl (%)
- 14. σ0 vs. Ea Findings σo and Ea is found to follow a linear relationship MNR parameters type-A for the Type-A and Type-B type-B -1 G=25.3 eV (EMN=39.5 meV) 4 10 type-C σ00=7.2x10 (Ωcm) -4 -1 samples. γf ~ 0 anti MNR parameters Type-A samples are -1 2 -1 G = -44.6 eV γf ~ γc 10 σ0 (Ω cm) having high values of Ea or EMN=-22.5 meV σ00= 87 (Ωcm) -1 and σ0 0 10 γF This shows is extremely small in Type-A samples due to its pinning -2 10 The values of MNR 0.8 parameters nearly the same 0.0 0.2 0.4 0.6 as found in a-Si:H. Ea (eV) Correlation between σo MNR & anti MNR in single phase μc-Si:H and Ea appears to change sign for type-C samples: anti-MNR
- 15. MNR: type-A μc-Si:H • Consists mainly of SG with an increased number of SG boundaries. – No question of formation of potential barrier (i.e., transport through crystallites) – transport will be governed by the band tail transport. • Ea saturates (≈ 0.55 eV) and σo ≈ 103 (Ωcm)-1. – EF is lying in the gap where the DOS does not vary much and there is a minimal movement of EF, or γF ≈ 0 • The initial data points for type-A have higher σo [≈ 104 (Ωcm)-1] and Ea (≈ 0.66 eV) – because of a shift in EC and/or a negative value of γF, as happens in a-Si:H for Ea towards the higher side.
- 16. MNR: type-B μc-Si:H The improvement in film microstructure delocalization of the tail states – EF moves towards the band edges, closer to the current path at EC. – The statistical shift γF, depends on the temperature and the initial position of EF, and when the EF is closer to any of the tail states and the tail states are steep, γF is rapid and marked. Transition between Type-A and Type-B materials – Nearly constant σo [70-90 (Ωcm)-1] with the fall in Ea (0.54-0.40 eV), – Indicating that the temperature shift of EF and that of the CB have become equal, canceling each other out (i.e., γF ≈ γC ) – In this case, the EF is pinned near the minimum of the DOS between the exponential CBT and the tail of the defect states (DB–) – With increasing crystallinity and/or improvement in the microstructure, the minimum shifts towards EC leading to a decrease of Ea.
- 17. Anti MNR: type-C μc-Si:H • The value of EMN = -22.5 meV is close to the value reported in heavily doped µc-Si:H (-20meV) • EB diagram as suggested by LO model seems inapplicable to our undoped µc-Si:H case – Calculated free electron concentrations do not suggest degenerate condition. – Consideration of equal band edge discontinuities at both ends of c-Si and a-Si:H interface Doubtful – Also, in a degenerate case, the conductivity behavior of polycrystalline material is found to exhibit a T 2 dependence of σd
- 18. Anti MNR: type-C μc-Si:H • Applying Statistical shift model – Considering transport through the encapsulating disordered tissue, a band tail transport is mandatory. – The large columnar microstructure in a long range ordering delocalizes an appreciable range of states in the tail state distribution. – In addition, higher density of available free carriers and low value of defect density can cause a large increase in DB– density together with a decrease in DB+ states in the gap a lower DOS near the CB edge possibility of a steeper CB tail. – In this situation, if EF is lying in the plateau region of the DOS, it may create an anti MNR situation.
- 19. Evidence of Anti MNR in μc-Si:H in Literature
- 20. Undoped µc-Si:H 5 10 #1 (rH=21) MNR line of types: A & B μc-Si:H #1 (rH=32) MNR line of a-Si:H #2 3 10 #3 (a-Si:H) -1 this work σ0 (Ω.cm) 1 10 -1 10 anti-MNR line of type-C μc-Si:H -3 10 0.0 0.2 0.4 0.6 0.8 #1undoped µc-Si:H Ea(eV) #2p-doped µc-Si:H
- 21. Doped µc-Si:H anti MNR line (#7) [heavily doped μc-Si:H] 3 10 -1 σ0 (Ω.cm) MNR line (#7) 1 10 [a-Si,C:H+μc-Si,C alloy] -1 10 #4 (thickness series) #4 (doped series) #5 dope series, p-nc-Si-SiC:H alloy #5 dilution series, p-nc-Si-SiC:H alloy #6 (Boron doped μc-Si:H) #7 -3 10 0.0 0.2 0.4 0.6 0.8 Ea (eV)
- 22. MNR parameters Anti MNR parameters σ00 σ00 EMN EMN G G -1 -1 -1 (eV-1) Samples (Ω.cm) (Ω.cm) (eV ) (meV) (meV) This work 7.2×10-4 Type-A&B 25.3 -- 39.5 -- -- Type-C -- -- -44.6 -- -22.5 87 Published Data 4×10-3 1.26×1010 Case#1 20.7 -97.7 48.4 -10.2 (rH=21) 3.2×10-6 Case#1 -- 36.6 -- 27.3 -- (rH=32) 1.7×10-4 Case#2 6 23.4 -32.5 42.7 -30.8 7.7×10-3 Case#3 -- 24 -- 41.6 -- Case#4 0.32 59 15.4 -66.1 65.1 -15.1 4.2×10-3 Case#5 21 15.3 -64.9 65.4 -15.4 3.2×10-6 Case#6 2.4 31.3 -39.9 31.9 -25.1 2.3 Case#7 309 8.5 -49.5 118.3 -20.2 0.5 Case#8 -- 11.8 -- 84.5 -- 7.2×10-3 Case#9 -- 20 -- 50 --
- 23. If one has a collection of G and σ00 then: a-Si,C:H alloy (#7) #1 (rH=21) σ00=σM exp [(γC- γF)/k –GEa] 0 #1 (rH=32) 10 Porous Si (#9) #2 #3 σ00=σM exp [(γC- γF)/k –G(EC0 –EF0)] a-Si:H (#3) #4 -1 σ00 (Ω.cm) #5 -2 10 At a position of EF in DOS where #6 #7 #8 p-nc-Si-SiC:H alloy (#5) γF(EC0-Emin)=0 #9 -4 10 this work -1 σM=100 (Ωcm) (at γf=γc) Fit σ00=σM exp [(γC/k) –GEmin] Emin=0.61 eV 3 -1 σ0=1.2x10 (Ωcm) (at γf=0) The quantity Emin is a measure for the -6 10 5 10 15 20 25 30 35 40 -1 G (eV ) position of the DOS minimum within the mobility gap. If γC is known then for such a value of σ00 where G=0, one can obtain σM
- 24. Conclusions •Both MNR and anti MNR can be seen in the dark conductivity behavior of highly crystalline single phase undoped µc-Si:H material, depending on the microstructure and the correlative DOS features. •A shift in the Fermi level of µc-Si:H material induced by any means (doping or any change in microstructure and the consequent DOS features) can give rise to an appearance of MNR behavior in the dc conductivity. •The statistical shift model can successfully explain both the MNR and anti MNR behavior in our material. •Corroborative evidence of similar electrical transport behavior of µc-Si:H in literature is present ------------------------------------------------------------------------------- “Influence of the statistical shift of Fermi level on the conductivity behavior in microcrystalline silicon” by Sanjay K. Ram, Satyendra Kumar, P. Roca i Cabarrocas; Physical Review B 77, 045212 (2008).
- 25. Appendix
- 26. MNR parameters • The value of MNR parameter G for a particular µc Si:H material is related to the microstructure and DOS characteristic of that material, although different sets of MNR parameters G and σ00 values can exist for the materials of the same µc Si:H system. • If the shift in band edges γc is known, then for such a value of σ00 where G=0 (derived by extrapolation), one can obtain the value of σM. This information can further provide those values of σ0 (from Eq. 6), where γf =0, and where γc = γf, both very important positions for providing simplified information about the nature of carrier transport in the material. The quantity Emin is a measure for the position of the DOS minimum within the mobility gap.
- 27. Electrical transport behavior, Size distribution of surface grains and Fcl with film growth Ea (eV) 0.1 0.2 0.3 0.4 0.5 0.6 1200 1200 d = 950 nm 1000 1000 Film Thickness (nm) Film Thickness (nm) d = 590 nm Frequency (arb. unit) 800 800 600 d = 390 nm 600 400 400 d = 180 nm 200 200 d = 55 nm 0 0 0 100 200 300 400 0 -7 -6 -5 -4 -3 -2 20 40 60 80 10 10 10 10 10 10 Fcl (%) Conglomerate surface grain size (nm) σd (Ω cm) -1
- 28. Summary of RS and SE studies on the fractional composition of films 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) ~50 nm ~400 nm ~900 nm

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