of Thermodynamics crystal Growth for of Thierry DUFFAR Professor at Grenoble Institute of Technology SIMAP Laboratory France [email protected] 15th International Summer School on Crytsal Growth – ISSCG-15 Outline Introduction: thermodynamics OF crystal growth Thermodynamics FOR crystal growth 1) Minimization of energy the equilibrium shape of a crystal Chemical reactions 2) Equilibrium Phase diagrams Point defects 3) Out of equilibrium Driving force for phase change 4) Application Thermodynamics of epitaxy 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Josiah Willard Gibbs 1839-1903, New Haven, Connecticut Combination of 1st and 2nd principles of thermodynamics Introduction of Enthalpy and « Gibbs » free energy Chemical potential Multi-phase systems Variance, phase rule Nucleation theory Gibbs-Thomson equation (curved surfaces) Statistical physics: petit-, grand- and micro-canonical ensembles And many other things in mathematics (vector analysis) and physics (optics) 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Introduction Chemistry (species mixing) dU = TdS − PdV + ∑ µi dni i δQ, Heat Mechanical Intensive Extensive δW Work Variable variable Force F (N) l (m) Fdl Pressure P (Nm-2) V (m3) -PdV Elastic St (Nm-2) εV (m3) -StεdV Surface σ (N m-1) S (m2) -γdS Electric E (V) q (Cb) Edq Magnetic HB (Nm-2) V (m3) HBdV 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Introduction There are several thermodynamic functions: Enthalpy: H = U + PV dH = TdS + VdP + ∑ µi dni i Helmotz free energy: F = U − TS dF = − SdT − PdV + ∑ µi dni Gibbs free energy: G = H − TS i dG = − SdT + VdP + ∑ µi dni i Most crystal growth processes are under constant T and P: dG=0 Gibbs energy is generally the most convenient 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Introduction Thermodynamics OF crystal growth Crystal growth deals with the production of a crystal (solid) from another phase (liquid, gas …). When the crystal is in equilibrium with the fluid at the temperature TE: ∆G = ∆H − TE ∆S = 0 Or: ∆S = ∆H Energy of the transformation (bonding)t TE Disorder introduced by the transformation Crystal Growth is changing disorder into ordered bonding 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Exercise Thermodynamics OF crystal growth What is the entropy associated to: -Growth of Silicon from the melt by the Czochralski method Tmelt=1683K, ΔHmelt=50.7 103 J.mol-1 ΔS= 30 J.mol-1.K-1 - Growth of SiC from the vapor by the Lely method Tprocess=1683 K, ΔHsublimation= 1233 103 J.mol-1 ΔS= 731 J.mol-1.K-1 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Nomenclature a activity e thickness k Bolzmann constant n mole number r radius x molar fraction <…> solid A area E energy, or electric field D diffusion coefficient F Helmotz free energy G Gibbs free energy H enthalpy J flux K equilibrium constant N number of … (bonds, atoms, layers..) P pressure Q heat R perfect gas constant S entropy T temperature U internal energy V volume W work (mechanical energy) (…) liquid ((…)) liquid solution 15th International Summer School on Crystal Growth – ISSCG-15 γ activity coefficient δ lattice parameter ε electrical permittivity λ interaction parameter μ Chemical potential σ interfacial energy ω number of configurations Ω molar volume […] gas… LAST NAME, First Name – talk id 1) Minimization of energy Equilibrium shape of a crystal Good use of thermodynamics for chemical reaction studies 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Minimization of energy: Wulff plot dG = − SdT + VdP + σdA + ∑ µi dni i What is the shape of a crystal in equilibrium with its surrounding? Wulff theorem: dF = ∫ σ ( hkl )dA( hkl ) should be minimal The surface energy σ is function of orientation <hkl>: Si-vapor: σ(100)=2.13 J.m-2 σ(110)=1.51 J.m-2 σ(111)=1.23 J.m-2 R.J. Jaccodine, J. Electrochem. Soc. 110 (1963) 524-527. R. Drosd, J. Washburn, J. appl. Phys. 53 (1982) 397-403. 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Minimization of energy: Wulff plot Equivalent to: “distances of faces to the center are proportional to their surface energy” Wulff plot and construction: σ plot σ001 σ111 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Minimization of energy: Wulff plot Exercise What is the equilibrium shape of the crystal with this Wulff plot? σ(θ) σ plot R.F. Sekerka p. 57 in “Crystal growth-from Fundamentals to Technology”, Müller G., Métois J.J., Eds. (2004) Elsevier 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Minimization of energy: Wulff plot Don’t confuse the EQUILIBRIUM shape and the KINETIC shape (the survival faces are the slowest). (111) slowest (100) slowest 15th International Summer School on Crystal Growth – ISSCG-15 (110) slowest DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Minimization of energy: chemical reactions Thermochemistry allows computing the possibility of a chemical reaction: equilibrium is obtained when the Gibbs energy of the system is minimal (constant T and P). CORRECT use of Gibbs energy is mandatory Example: is it possible to melt Si in a sapphire (or possibly sintered alumina) crucible without pollution? Si droplets after solidification on sapphire silica substrates 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Minimization of energy: chemical reactions Common, but insufficient treatment: simple use of Ellingham diagram ∆G Al → Al O < ∆GSi → SiO 2 3 2 This means that alumina is more stable than silica It does NOT mean that Si cannot react with Al2O3! http://www.doitpoms.ac.uk/tlplib/ellingham_diagrams/interactive.php 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Minimization of energy: chemical reactions 1 3 Al2O3 ↔ (( Al ))Si + (( O ))Si 2 2 K Al = aAl a 3/ 2 O =e ∆G0Al O aAl = γ (( Al ))Si xAl ≈ γ aO = γ (( O ))Si xO ≈ γ 2 3 ∆G 0 2 RT Al2O3 ∞ (( Al ))Si xAl ∞ (( O ))Si O = −1128 10 3 J .mol −1 ( 1700 K ) γ ((∞ Al )) = 0.42 Si γ ((∞O )) = 8 10 −6 Si x 3xAl = 2 xO CONCLUSIONS 10-4 xAl=1.33 xO= 2 10-4 1)133 ppm Al pollution of Si melt 2) xO > O solubility limit in Si: SiO2 is formed! 3) Taking a=x gives xO= 1.36 10-7 Wetting of ceramics by molten silicon and silicon alloys: a review B. Drevet. N. Eustathopoulos, J. Mat. Sci. 47 (2012) 8247-8260. 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id 2) Equilibrium: phase diagrams Thermochemistry of solutions Phase equilibrium Manipulation of phase diagram Point defects 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: phase diagrams Thermochemistry of solutions Gibbs energy of ONE phase with two components A and B id xs G α = G 0 + ∆Gmix + ∆Gmix Before mixing Ideal solution Excess energy id id id id ∆ G mix = ∆ H mix − T ∆ S mix = −T ∆ S mix ∆S id mix = k ln ω ( n A + n B )! ω= n A ! nB ! id ∆ G mix = RT ( x αA ln x αA + x αB ln x αB ) 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: phase diagrams Thermochemistry of solutions REGULAR model of interactive mixing (liquid solution) ∆G xs mix = ∆H xs mix E AA + E BB = N AB ( E AB − ) = λx A x B 2 ln γ B = λ RT ( 1 − x B )2 DLP (δ Lattice Parameter) model (solid solution ex: GaxAl(1-x)As) ln γ B = λ RT ( 1 − x BC )2 2 δ AC + δ BC λ = 5.10 7 (δ AC − δ BC ) 2 − 4 .5 G.B. Stringfellow J. Crystal Growth 27 (1974) 21. Many mixing energetical models exist, which all end with γ function of various interaction parameters. In all cases: µ A = µ A0 + RT ln γ A x A µ B = µ B0 + RT ln γ B xB 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: phase diagrams Thermochemistry of solutions λ =0 λ < 0 A − B attraction λ > 0 A − B repulsion 15th International Summer School on Crystal Growth – ISSCG-15 λ >> 0 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: phase diagrams Equilibrium between phases α, β …: they don’t exchange energy, they are at the same energy level. G α = G β = ... dG = d ( G α + G β + ...) = 0 Constant T and P: α α β β µ dn + µ dn ∑ i i ∑ i i + ... = 0 i α i β µ = µi = ... i α G ∂ µ αi = ∂ni P ,T ,n j Each chemical component i should be in equilibrium between the various phases 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: phase diagrams In case of strong repulsion (λ>>0) Two phases appear (demixion) α1 α2 µB = µB ∂G α µ B = ∂nB α P ,T ,n A 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: phase diagrams Construction of a 3 solid phase binary phase diagram λ<<0 strong attraction: α+β C 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: phase diagrams Manipulation of phase diagram Langasite La3Ga5SiO14 µ A = µ A0 + RT ln γ A x A 500V.cm-1 Congruent growth from the melt! ∂ µ A = µ + RT ln γ A x A + ∂x A 0 A 1 2 Ω AεE S.Uda, X. Huang, S. Koh J. of Crystal Growth 281 (2005) 481–491 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects Intrinsic • Empty site : Vacancy, VSi • Misplaced atom : Interstitial (SiI) • Vacancy + interstitial = Frenkel Defect Extrinsic • Foreign atom: Impurity (Insertion or Substitution) Substitutional impurity Interstitial Impurity in Insertion Vacancy Frenkel defect 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects Vacancy 4 broken bonds 2 created bonds Interstitial 4 created bonds + lattice distorsion EV ~ Ecoh /2 EI high EV = 0.7 to 1 eV (cf. Ecoh) EI = 3 to 5 eV (strong distorsions) 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects Point defects are thermally activated: thermodynamic defects N0 atoms, NV vacancies . . . . . . . . . . EV = 1 eV N0 = 6 1023 mol-1 ∆G = NV EV + ∆H Mix – T ∆S Mix Ideal solution: ∆H Mix = 0 Boltzmann dist.: ∆S Mix = k ln( N 0 + NV )! N 0 ! NV ! N d∆G = 0 = EV – kT ln ( 0 ) dNV NV → NV = N 0 e - EV kT 1000 K : 6 1018 mol-1 300 K : 6 107 mol-1 In fact recombination, at Tmelt : [V]Si ≈1014 to 1015 mol-1 [I]Si≈ 0 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Vacancy concentration Equilibrium: point defects Temperature, K 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects µ-void precipitation E1 = N.EV (-TΔSMix) E2 = σsv4πR2 N < Ncrit = 9/16. π a6 (σsv/EV)3 < N 4 3 πR = NVV 3 15th International Summer School on Crystal Growth – ISSCG-15 for Si: VV = Vatom a3 = 8 a DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects µ-voids and interstitial precipitation Vacancies Interstitials 106 cm-3 1010 cm-3 Annealed under Ar, H2, > 1000°C As in GaAs, very stable 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects Exercise What is the density of VGa in GaAs at 1200°C? Will µ-voids precipitate? If yes, what will be their density, their diameter, their mean distance? Surface energy: 1,2 J.m-2 EV= 0,86 eV lattice parameter a=0,56 nm the crystallographic structure is the same than for Si, one atom over two being Ga and the other one As 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects Intrinsic Point defects in a compound: example of GaAs GaAs non-congruent -Excess of Ga or As - VGa, VAs, AsGa, GaAs -Precipitation of Ga, As or Vac. -Anionic and cationic Frenkel -Shottky (VGa+VAs) Wenzl H., Oates W.A., Mika K., in: Handbook of Crystal Growth, Vol 1a, Hurle D.T.J. (ed.) North Holland, Amsterdam, 1993 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects Intrinsic Point defects in a compound: example of GaAs 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects Intrinsic Point defects in a compound: example of GaAs Striations => stoichiometry=> dislocations 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects Intrinsic Point defects in a compound: example of GaAs E EL2 = As in a Ga site CB Important defect: fixes the Semi Insulating character of GaAs (109cm-3 vs. 1015cm-3 C) EC Gap 1.43 eV ++ As 0As + VGa− ↔ AsGa + VAs+ + 4e − xEL2 = x As ++ = K Ga xV − Ga EL2 Ev VB xV + N 4 As 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Equilibrium: point defects Intrinsic Point defects in a compound: example of GaAs As pressure control Lagowski J., Gatos H.C., Aoyama T., LIN D.G., Appl. Phys. Lett. 45 (1984) 680-682. . 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id 3) Out of equilibrium Driving force and supersaturation Various Crystal Growth processes 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Out of equilibrium No crystal growth at equilibrium! μ Liquid Δμ Crystal μLiquid=μCrystal= μ0 ΔT TMelt T Δμ = is the driving force for crystallization ΔT is the undercooling Growth rate depends directly on Δμ 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Out of equilibrium Growth from the melt ∆µ = µ Liquid - µCrystal = (µ Liquid - µ0 ) − (µCrystal - µ0 ) = T ∫ ∂µ Liquid ∂T TMelt ∆µ = TMelt ∫ ∆S Melt dT ≈ ∆S Melt ( Tmelt − T ) = ∆S Melt ∆T = ∆µ = T ∂µCrystal T ∫ dT - TMelt ∆H Melt Tmelt ∆T ∂T dT Undercooling Growth from vapour ∆µ = µVapor - µCrystal = ∆µ = P ∫ P0 ∂µ Liquid ∂P P dP - ∫ ∂µCrystal P0 Growth from solution 15th International Summer School on Crystal Growth – ISSCG-15 ∂P P dP = ∫ ( v vapor -vCrystal )dP P0 ∆µ = kT ln P P0 ∆µ = kT ln x x0 Supersaturation Supersaturation DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Out of equilibrium Mother phase e Growth by 2D nucleation ∆GNucleus = -πr 2 e ∂∆GNucleus =0 ∂r Crystal ∆µ + 2πreσ Ω σΩ r* = ∆µ ∆G * Nucleus πΩσ 2 e = ∆µ ∆T T0 ΔG or ∆P P0 Example of Si ∆G*Nucleus r* r P. Rudolph, in: Crystal growth Technology, H.J. Scheel and T. fukuda (eds.) Wiley (2003). 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id 4) Application to epitaxy “Far from equilibrium”? Driving force controls kinetics Stoichiometry and composition of layers: physical properties Layer structure 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Application to epitaxy ∆T T0 or ∆P P0 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Application to epitaxy Example of GaAs [AsH 3 ] + [( CH 3 )3 Ga] ↔ GaAs 1 [As4 ] + [Ga] ↔ GaAs 4 a GaAs K= PEq [Ga ]PEq1 /[4As4 ] P[Ga ]P[1As/ 44 ] 1 ∆µ = µ[Ga ] + µ[ As4 ] − µ GaAs = RT ln 4 PEq [Ga ]PEq1 /[4As4 ] 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Application to epitaxy Δμ* Example of GaAs kJ.mol-1 300 200 100 0 LPE VPE OMCVD MBE “Far from equilibrium” Interface kinetics >> diffusion Stringfellow G.B. in “Crystal growth-from Fundamentals to Technology”, Müller G., Métois J.J., Eds. (2004) Elsevier 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Application to epitaxy Generally thermodynamic equilibrium applies at the interface P[Ga]P[1As/ 44 ] >> PEq[Ga]PEq1/[4As4 ] The process runs with a strong excess of As: P[ As4 ] is constant: PEq[Ga] << P[Ga] << 4P[ As4 ] 1) The process is controlled by the diffusion of Ga: J = DGa 2) Layer stoichiometry has nothing to do with P[Ga] (P[ Ga] − PEq[Ga] RT P[ As4 ] 3) Changing the flow rate of [Ga] has no effect on layer stoichiometry 4) Example of GaxAl(1-x)As: x is totally controlled by 15th International Summer School on Crystal Growth – ISSCG-15 P[Ga] P[ Al] DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id ) Application to epitaxy Manipulation of phase diagram 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Application to epitaxy layer structure Gibbs energy per unit area G n σ Substrate atomic layers Substrate σ Substrate / layer + σ layer / vapor n Bu lk de po µ n −layer = > sit µbulk 15th International Summer School on Crystal Growth – ISSCG-15 ∂G ∂n Stable uniform layer ∂G = ∂n DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Application to epitaxy layer structure Gibbs energy per unit area G σ Substrate / layer lk Bu + σ layer / vapor si t po de Substrate γ Substrate n ∂G µbulk = ∂n > µ n −layer 15th International Summer School on Crystal Growth – ISSCG-15 ∂G = ∂n Unstable layer DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Application to epitaxy layer structure Gibbs energy per unit area G σ Substrate / layer lk Bu + σ layer / vapor nc si t po de Substrate nc σ Substrate n µbulk > µ n −layer µbulk < µ n −layer Stable thin layer for n<nc Unstable layer for n>nc 15th International Summer School on Crystal Growth – ISSCG-15 DUFFAR Thierry - Thermodynamics LAST NAME, First Name – talk id Conclusion Transport Kinetics How atoms are carried to the interface How atoms are piled up at the interface Thermodynamics apply because kinetic>>growth rate 15th International Summer School on Crystal Growth – ISSCG-15 LAST NAME, First Name – talk id Conclusion Thermodynamic is a powerful tool: -Is it is possible to grow a crystal? -By which technique? -Growth parameters? -Crystal composition? -Crystal thermodynamic defects? -Crystal quality (stable or not)? 15th International Summer School on Crystal Growth – ISSCG-15 LAST NAME, First Name – talk id Further reading I.V. Markov „Crystal Growth for beginners“ World Scientific (2003, 2nd edition) ISBN-13 978-981-238-245-0 W. Kurz, D.J. Fischer „Fundamentals of solidification“ Transtech Publication Ltd (1998, 4th edition) ISBN 0-87849-804-4 D.T.J. Hurle, editor „Handbook of Crystal Growth“, Vol. 1-a North Holland (1993) ISBN 0-444-88908 6 15th International Summer School on Crystal Growth – ISSCG-15 LAST NAME, First Name – talk id
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