This talk will consider the applicability of modern topological constraint theory of glass developed by Boolchand, Thorp and Phillips (BTP) to IGZO and ask if it might explain the high (compared to spins) H content of a-Si:H.
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Ast, Dieter G.

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Engineering Conferences InternationalECI Digital ArchivesInternational Conference on SemiconductorTechnology for Ultra-Large Scale IntegratedCircuits and Thin Film Transistors VI (ULSIC vsTFT 6)Proceedings5-24-2017Equilibrium mobility in IGZO TFT: Existence ofthe intermediate boolchand phase?Dieter G. AstCornell University, USA, dga1@cornell.eduFollow this and additional works at: http://dc.engconfintl.org/ulsic_tft_6Part of the Engineering CommonsThis Abstract and Presentation is brought to you for free and open access by the Proceedings at ECI Digital Archives. It has been accepted for inclusionin International Conference on Semiconductor Technology for Ultra-Large Scale Integrated Circuits and Thin Film Transistors VI (ULSIC vs TFT 6)by an authorized administrator of ECI Digital Archives. For more information, please contact franco@bepress.com.Recommended CitationDieter G. Ast, "Equilibrium mobility in IGZO TFT: Existence of the intermediate boolchand phase?" in "International Conference onSemiconductor Technology for Ultra-Large Scale Integrated Circuits and Thin Film Transistors VI (ULSIC vs TFT 6)", Yue Kuo(Texas A&M University, USA) Olivier Bonnaud (University of Rennes I, France) Eds, ECI Symposium Series, (2017).http://dc.engconfintl.org/ulsic_tft_6/9IGZO and a-Si:H: A topological constraint theory viewD.G.AstCornell UIGZO is an oxide glass that is  replacing a-S-:H  in TFTsModern glass theory analyses glass network as truss networkTruss Network Glass  Network Nodes                                                      <->                                     AtomsRigid Nodes                                            <->                            Bond Bending Bars  under tension                               <->                           Bond stretchingAfter matthieu.micoulaut@upmc.fr It all goes back to the Eifel Tower and Maxwell…Glass forming is maximized when the system is optimally constraint.Such networks are also termed to be isostatic  Coordination  and rigidity The first statistical analysis of truss networks   was carried out by J. Clerk Maxwell  151 years agoFrom the publication in Phil MagOn the calculation of the equilibrium and stiffness of framesJ. Clerk Maxwell  FSRPhil Mag 27,  1864Thorpe, Boolchand,  Phillips  extension oftruss theory to atomThe average of constraints , n , is linked to  average valence <r > :In a 3 D network ,  n= 3 , and solving for  <r > yields Xi is the atomic fractions, and ri the valences Summary When beams are replaced with bonds and nodes with atoms, analysis shows that in three dimensions networks are optimally constrained at the average atomic coordination of r=2.4 , a value first derived by Phillips in 1979Numerical calculation using percolation theory to  locate the floppy to rigid transition  back up the topologically derived valueFraction of floppy modes  as function of  <r>  .  Red line :  Maxwell Theory,  black lines numerical simulation s using the pebble game, for a-Si and a-diamond  models    (from Thorpe,   Maxwell solution added)Generic Rigidity Percolation: The pebble game  D.J. Jacobs,  M.F. Thorpe  PRL 75,22, 4541 (1995)  Topological Constraint  Theory  quantitatively calculates glass properties as function of composition that agree with experiment.  From Mauro,  calculated with temperature dependent constraint theory Hardness Viscosity Example of predicted viscosity versus temperature curve for an alkaline earth aluminosilicate composition. The prediction was  made with the glass composition as the only input to the model.Mauro et al.,  Applied Glass Science , 6 December 2012Experimental and calculated  glass transition temperature in  Calcium Borate GlassIGZO SEEN AS  AN  OXIDE GLASS“Rigidity percolation is similar conceptually to the more familiar connectivity percolation except that instead of demanding a connected pathway across the sample, the more stringent condition that the connected pathway is also rigid is required”Thorpe MF, Stinchcombe RB. 2014 Two exactly soluble models of rigidity percolation. Phil. Trans. R. Soc. A 372: 20120038.Electrical conduction  in  non-oxide  glassEffective  Medium Percolation Theory  (EMPT)  first was used in 1974  to calculate the conductivity of  chalcogenide semiconductorsas a function of composition Resistivity of  Asx-Te1-x as function of compositionThe dashed line is the fit with effective medium theory  Ast Phys. Rev. Lett. 1974Variation of the room-temperature resistivity in the As-Se system-------- dashed line :  Random mixture EMPT_____   solid  line    :  EMPT mixture of  Se, + As2Se3 (As < 40%)  and                     As2Se3  + As4Se4  clusters  (As > 40%)  usingρ of Se, As2Se3, As4Se4 Shimakawa and Shoji Nitta   Phys. Rev. B        1978Extension to  As-Se glasses Historically, the EMPT   cluster models  to fit the electrical properties of chalcogenide glasses preceded  the network        models  but the two fields never linked up  From the semiconductor point of view  what matters is • Network Stress    (stressed bonds  <->  traps)• Network  stability    (stable electronic properties)To consider above ,  we need to move to the intermediate phase concept introduced by Boolchand The intermediate phase The ideal network  (r = 2.4,  nc = 3)  has an existence range Deviations from the ideal network are accommodated by “defects”Useful to think of intermediate phase as being “phase separated”“Covalent glass near the phase transition at a mean coordination r= 2.4 green bonds are hinges, the red bonds isostatic and the black bonds hyperstatic” From  Networks, Flexibility and Mobility   by Thorpe The blue, green and red clusters =>  can conceptually be seen as an incipient phase separation on the intermediateLength Scale of glassGreen: floppy              Red:  Stress free          Black : Over constrained The intermediate phase was discovered by Boolchand using  MDSCLong term stability in amorphous  semiconductors requires to operate in the intermediate phaseTemporal Stability of  Elastic Phases in AsxGe xSe1-2x glassesP.Boolchand, M.Micoulaut et al 2003Observed reversibility windows from modulated DSC experiments for various glass systems.  Δr is deviation from the average coordinate number.The existence range of IGZO is unknownIf as in the two oxide glasses, it might  be of order  0.05Time scale: Days Threshold recovery of   commercial display transistors   in a  TV              display after photoresist treatment (  15 devices )D. Lynch   - Cornell IGZO  Group Network modifiers lower  TgTg          SiO2 ~  1475 KTg           (SiO2) 0.98 Na2O.02 ~ 775 K  Glass view :   Alkali lower  TgAtomistic view   :  Na takes out highly strained bridging oxygen bonds,Extension to  IGZO   : H will act as a “super alkali”  lowering the local TgKinetics of return to equilibrium mobility in IGZO transistors               is compatible with the diffusion of hydrogen  How to account for the kinetics ?SUMMARY OF EXPERIMENTAL OBSERVATIONS• The electronic properties of  well prepared IGZOreturn,  after being  stressed, to their equilibrium value• The kinetics is compatible with the diffusion of hydrogen  and the stretch exponent  is  compatible with diffusion (ß = 3/5)  within the accuracy of the experimental data• The temperature dependence of the recovery is compatible with the diffusion of hydrogen Implications for IGZO :   Device quality IGZO needs to be in the Boolchand intermediate phase.Thus prepared, devices will exhibit equilibrium electronic propertiesHydrogen enables the network to return to equilibrium on a time scale of daysControl of hydrogen :H is required to allow the network to relax below Tg but excess H  will decrease network stability by “dialing down”  r  and move the  network to the floppy regimeLaser spike annealing (LSA) of IGZOStage MotionHigh power CW laserLinear motorStage~1200 µm~100 µmLSA introduces a temporary increase in the mobility close to the intrinsic band mobility of oxide semiconductors   that returns to the equilibrium value on a time scale of days with  a kinetic compatible with H diffusionCornell IGZO groupCorresponding  Photon flux induced  transition  have previously  been seen  in   GexSe1-x glassesRaman directly measures network stress !!! Network stress is lowest and constant in the intermediate phaseP. Boolchand, D. G. Georgiev, B. Goodman 2001 Effect of increasing photon flux  density  by 104(a) Macro RamanPhoton Flux Density   1 (normalized)(b)  Micro RamanPhoton Flux Density 10 000 “Photo- Melting”  collapses the intermediate phaseLSA  of  IGZO  as explained by topological  glass theory • Intermediate phase collapses into  a narrow compositional range, becoming  spatially homogenous on the intermediate length scale of glasses.• The Ec conduction band energy landscape becomes flat and the activation barrier to carrier transport disappears. *• Electron mobility increases to the intrinsic limit  for oxide semiconductors (~ 60 cm2/V s)• In time, the system “de-mixes” the intermediate phase occupies a range of local arrangements , and electron mobility returns to its “equilibrium ” value of the roughened energy landscape of  the intermediate phase • The energy landscape roughness model was first introduced by Fritzsche in 1983Process window  per topological  constraint theory• LSA needs to “melt” the intermediate phase without crystallizing the material  <->  narrow process window <-> avoid excess H  too lower viscosity too much• The “molten”  state must be “quenched” sufficiently fast  freeze in the homogenous structure created by LSA <-> narrow process window.• The two conditions above, plus having the correct r to collapse to a singularity  set a very tight process window• The experimental observation are in agreement with above.C-Axis aligned crystalline IGZO    (CAAC)CAAC    IGZO Cross-section and plan-view HRTEM images of the highly aligned CAAC IGZO (310 °C/10% O2) film ((a) and (b)) and the weakly aligned CAAC IGZO (310 °C/100% O2) film ((c) and (d)) with image FFTs inset.a) X – sectional  HRTEM of  highly aligned CAAC  IGZO  - wavy structure +/- 90 parallel to substrateb) buckling pattern formed on a PDMS film comprised of a stiffer top layer and more pliant  bulk .  Stress generated by solvation  (after Chan and Crosby ,  Soft Matter)c) The “wavy”  geometry suggest stress relief  by “wrinkling” during nucleationIGZO D.Lynch et al  308 • SID 2015 DIGEST    Polymer :Chan and . Crosby . (Communication) Soft Matter, 2006, 2, 324-328  Weakly aligned CAAC X-Sectional view of a weakly aligned  CAAC film highlighting the increasingly random orientation of the grains. The numbered FFT diffractogram spots correspond to planes labeled within the images.D.Lynch et al  J. Appl. Phys. 119, 145304 (2016); 10.1063/1.4945580 CAAC  nucleation proceeds via a 3 to 5 nm thick amorphous layer that subsequently transforms into  wavy CAACSIMS  of “optimal" CAAC IGZO film (10% O2, ~310C - same as in APL letter)Corning  Inc . SIMS groupA Ga rich layer is inserted between the CAAC and the thermally oxidized Si substrate.The "bumps" in the In/Ga/Zn/O signals are due to matrix effects from the local electronic structure changing during SIMS sputtering - but all four bumps should occur at the same point.  They do not.  Nucleation of CAAC  as explained  by topological    constraint theoryStress  relief  by “wrinkling”D. M. Lynch, Bin Zhu, B.D.A. Levin et alSID 2015 DIGEST • 309Classification of CAAC  by  X ray and HRTEM and corresponding r value Best X ray  CAAC ever of Cornell IGZO group                              r= 2.328Best highly aligned  CAAC ( by  HRTEM previous slide)             r =2.321Weakly aligned    CAAC   (HRTEM  previous slide )                     r =2.313Topological  Constraint Theory  View of Optimal CAAC  Formation Conditions:• Select a film (not necessarily target !) composition that moves IGZO to the into the “edge” of intermediate  phase range• Control hydrogen - it changes  Tg (classical glass view), dials down r (topological constraint theory view),  eliminates bridging oxygen bond  (atomic view)• Operate under conditions that favour heteroepitaxial growth of subsequent CAAC on the (0001)  ZnO template “floating” on the “phase separating “amorphous IGZO Substrate :We use high T thermal oxide on Si as substrate. Inserting a Si wafer into a sputter system, heating it to 300oC prior to deposition will result in the formation of a low quality porous oxide that will interact with the deposition in irreproducible ways.(Kawata et al 2016-SID Symposium Digest of Technical Papers )SummaryTo achieve optimal IGZO based devices select conditions that deposit IGZO in the Boolchand intermediate phase .In the Boolchand intermediate phase, electrical properties will recover after electrical, photon, and chemical stressIn processing IGZO into the Boolchand phase, the  two agents most likely to shift r out of the optimal phase range   are  Zn and HAmorphous Si:H Internal stress is not easily measured, and often ignored in semiconductor physics.J.C. Phillips Reversible thermally induced transitions in  Amorphous Si:Ha) Isothermal decay of quenched-in excess conductivityb) Test for second order kinetics. The dashed line is a least square fitc) Temperature dependence of the second rate order constant of various device structures, dashed intrinsic a-Si:H , solid P-doped a-Si:H Reversible electronic properties as f(cooling rate)  Brodsky and Ast, 1979 Network view of the  defect pool model• a-Si:H is a hybrid glass, in an intermediate phase• The presence of H “dials down” r and moves the network to a more stress free state   • Microscopically, the network consists of 5,6, and 7 membered rings• If sufficiently decoupled by H  to be treated as rings   MO theory tells us a rough energy landscape  (Even rings having higher HOMO  LUMO splits)• See as such  the  network is “phase separated” on the intermediate  length scale of glass - leading support to the quantum well model of  BrodskyBefore the audience concludes that I am too far out, lets have a look at a-CarbonAbstract“Structural and mechanical properties of amorphous and nanocomposite carbonare investigated using tight-binding molecular dynamics and Monte Carlo simulations. In the case of amorphous carbon, we show that the variation of sp3 fraction as a function of density is linear over the whole range of possible densities, and that the bulk moduli follow closely the power-law variation suggested by Thorpe.”http://arxiv.org/abs/cond-mat/0605381v1Structure, stability, and stress properties of amorphous and nanostructured carbon films M.G. Fyta , C. Mathioudakis, G. Kopidakis , and P.C. KeliresNetwork viewThorpe and collaborators suggested that the elastic moduli of bond-depleted crystalline diamond lattices and of bond-depleted “amorphous diamond” networks (WWW model) follow a power-law behavior c ∼ (¯z−z¯f ) ν , with the exponent taking the value 1.5 ± 0.2. TBMD simulations and the EDTB model  Letting all fitting parameters in Eq. (2) free, we obtain B0 = 361 GPa, which is exactly the computed value for WWW, ¯zf = 2.25, and ν = 1.6. (For a measure of the quality of the fit: R2 = 0.9907). If we fix ν to be 1.5 (R2 = 0.9906), we get 2.33 for ¯zf , and if we fix ¯zf to be 2.4 (R2 = 0.9904), we get 1.4 for ν. We thus conclude that the variation confirms the constraint-counting theory of Phillips and Thorpea-Si:HIts electronic properties have been most difficult to calculatedfrom first principles and still have difficulties to match glow discharge deposited a-Si:HAn early attempt to explain its bandgap was the QW model of Brodsky (IBM)*  First-principles study of hydrogenated amorphous silicon K. Jarolimek,      R. A. de Groot, G. A. de Wijs, and M. Zeman Phys. Rev. B 79, 155206 ,20 April   **  A first principles analysis of the effect of hydrogen concentration in hydrogenated amorphous silicon on the formation of strained Si-Si bonds and the optical and mobility gaps Merid Legesse, Michael Nolan, and Giorgos Fagas Citation: Journal of Applied Physics 115, 203711 (2014M.H. Brodsky SolidState Communications, QUANTUM WELL MODEL OF HYDROGENATED AMORPHOUS SILICONVol.36, pp.55—59.a)  Real space band structure of an a-SiH-bounded region in pure a-Si. b) Schematic density ofstates N(E) vs energy E averaged over many regions. Structurally, the  1980  Q.W.  Model of a-Si:H  was an early attempt to use network theory to account for the optical properties of a-Si:H.  It postulated QW of  spatial extend of ~ 4 ÅIt accounted for the optical properties and the DOSBut it’s structural underpinning remained mysterious Modern network view would see a-Si:H as a Hydride GlassGiven enough H,  the network might be  sufficiently  decoupled such that 5, 6, and 7 membered rings become  legitimate building block. If so,  quantum confinement will  increase the bandgap, with a rough energy landscape (even numbered rings having higher Homo-Lumo gaps than odd ones) If so, the material would be inhomogeneous  on the length scale of the diameter of rings  ,  4.4 Å  How much H would be required?  Using a Pareto guess that 20% of rings account for 80% of stress, back-of-the-envelope calculations are leading to reasonable number, but clearly more work is required to put this most speculative model onto a firmer footing.Ab initio calculations of  the bandgap of a-Si:H renders  a value 0.92eV *To analyze a-Si:H by network theory requires to identify the (most) relevant structural units,  units that result from the introduction of H that breaks up the  continuous network.    a-C  models are a start* First-principles study of hydrogenated amorphous silicon K. Jarolimek, R. A. de Groot, G. A. de Wijs, and M. ZemanPhys. Rev. B 79** Structure, stability, and stress properties of amorphous and nanostructured carbon films M.G. Fyta , C. Mathioudakis, G. Kopidakis , and P.C. Kelires   Thin Solid Films, 482, 56 (2005)Cluster formation in a-C network with a density of 2.99 gcm−3 and 79% sp3  Open atoms sp3 , dark atoms sp2 **To my coworkers at CornellChen  Yang  Chung  (now at  Intel)David Lynn  (soon at Intel)Michael Thompson ( now Assoc. Dean of Undergraduate Education)Bin Zhu (now at Corning Inc)        J.C. Phillips for most valuable encouragement and commentsFinancial support  by Corning IncA 512 atom model of a-Si:H constructed with Tersoff potentialsMD + DFTA first principles analysis of the effect of hydrogen concentration in hydrogenated amorphous silicon on the formation of strained Si-Si bonds and the optical and mobility gaps Merid Legesse, Michael Nolan, and Giorgos Fagas Citation: Journal of Applied Physics 115, 203711 (2014Historical NoteIt has long been known that the presence of alkali in oxide glasses enables stress relaxation at room temperature“Thermometer Effect”Structural  “relaxation” in alkali containing oxide glasses hundreds of degree Celsius  below Tg glasses has been  since 1873. In a-Si:H  hydrogen , the equivalent electronic model is the “defect pool” model.Elimination of Alkali greatly reduced the effect, but thermometers still need to be recalibrated2009http://ws680.nist.gov/publication/get_pdf.cfm?pub_id=900914

Equilibrium mobility in IGZO TFT:  Existence of the intermediate boolchand phase?

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Equilibrium mobility in IGZO TFT: Existence of the intermediate boolchand phase?

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