Tufts Rpic Crystal
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Tufts Rpic Crystal






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Tufts Rpic Crystal Presentation Transcript

  • 1. Crystal Growth
      • Melt growth, solution growth and vapor growth.
    • 2.) Process for crystal growth from melt :
      • 2.1 Directional solidification/Bridgman process.
      • 2.2 Zone melting and floating zone.
      • 2.3 Czochralski method.
      • 2.4 Liquid encapsulated Czochralski.
    • 3) Convection and segregation
  • 2. 1) Introduction
    • Motivation for growth of single crystals
      • Research (physics/materials):
        • Properties of solids are obscured by grain boundaries (to understand solids we must understand crystals).
          • Metals
          • Semiconductors
          • Superconductors
          • Protein crystals
  • 3. Applications
        • Uniform properties on microscopic level
          • Micro-devices: electronic, optical, and mechanical
        • No creep, fatigue…
        • Beautiful objects
  • 4. Methods for Crystal Growth
    • Directional solidification from the melt ~ cm/hr
    • Solution growth (supersaturation) ~ mm/day
    • Vapor growth (sublimation-condensation) ~ µm/hr
    • Thin layers
    • Boules
  • 5. a.) Growth from the melt:
    • Conditions:
      • Material must melt congruently (no change in composition during melting) e.g. Yttrium iron garnet (YIG) is grown from solutions because it does not melt congruently.
      • Material must not decompose before melting. e.g. SiC is grown from vapor phase (sublimation-condensation) because it decomposes before melting.
      • Material must not undergo a solid state phase transformation between melting point and room temperature. e.g. SiO 2 is grown from solution (hydrothermal growth) because of a α-β quartz transition at 583°C.
  • 6. Advantages of solidification:
    • Fast (~ cm/hr ); growth rate depends on heat transfer (not on mass transfer).
    • Variety of techniques developed (e.g. crystal pulling and directional and zone solidification).
  • 7. b.) Growth from solution:
    • For materials that:
      • (i) melt non congruently or
      • (ii) decompose before melting or
      • (iii) undergo a solid state phase transformation before melting or
      • (iv) have very high melting point.
    • Classification is based on the solvent type.
    • Key requirement: High purity solvent which is insoluble in the crystal.
  • 8. b.1) Molten salt (flux) growth:
    • Common solvents: PbO, PbF 2 , B 2 O 3 , KF.
    • Used for oxides with very high melting points (or melt congruently, decompose or undergo a solid phase transformation).
    • e.g. Yttrium iron garnet (YIG) is grown from solutions because it does not melt congruently.
      • Advantages: carried on at much lower temperatures than melt growth.
      • Limitations: very slow; borderline purity, platinum crucibles, stoichiometry is hard to control.
  • 9. b.2) Metallic solution growth:
    • Liquid phase Epitaxy – for high quality epitaxial layers of III-V compounds and boules;
      • GaAs from Ga solution (melt with > 50% Ga).
      • GaSb from Ga solution (melt with > 50% Ga).
      • Terary III-V compounds (solid solutions of III-V compounds): Ga 1-x ln x As, GaAs x P 1-x .
        • Advantages: growth at lower temperatures than melt growth yields high quality.
        • Limitations: very slow = small crystals or thin layers.
  • 10. b.3) Hydrothermal growth:
    • Aqueous solution at high temperature and pressure (e.g. SiO 2 is grown by hydrothermal growth at 2000 bars and 400 °C because of α-β quartz transition at 583°C).
  • 11. c.) Growth from the vapor phase:
    • Boule growth: only when other methods are not useful (SiC, AlN sublimation-condensation).
    • Thin layers, i.e., vapor phase epitaxy: extensively used (chemical vapor deposition, sputtering). E.g. SiC is grown from vapor phase (sublimation-condensation) because it decomposes before melting.
  • 12. 2) Processes for crystal growth from the melt :
    • 2.1 Directional solidification, i.e. Bridgman process
    • 2.2 Czochralski Method (CZ) and LEC
    • 2.3 Zone melting and floating zone (FZ)
  • 13.  
  • 14. It’s a Boy!! Born May 8, 2001 at 10:35 p.m. Weight: 14 lbs, 9 oz Length: 15 inches Crystal growth furnace for SUBSA investigation, destined for Space Station Alpha in May 2002.
  • 15. Directional Solidification, i.e. Vertical Bridgman Growth
    • Charge and the seed are placed into the crucible
    • Conservative process: no material is added or removed from either solid or liquid phase, except by crystallization (R.A. Laudise).
    • Axial temperature gradient is imposed along the crucible.
  • 16.
    • Growth: Interface is advanced by moving the container or the gradient (furnace/ heat source).
    • Seeding: part of the seed is molten
  • 17. Advantages of the Bridgman Process:
      • Simple: in confined growth, the shape of the crystal is defined by the container.
      • Radial temperature gradients are not needed to control the crystal shape.
      • Low thermal stresses result in low level of stress-induced dislocations.
      • Crystals may be grown in sealed ampules (stoichiometry of melts with volatile constitutes is easy to control).
      • Relatively low level of natural convection; Melt exposed to stabilizing temperature gradients (VB only).
      • Process requires little attention (maintenance).
  • 18.
    • Drawbacks
      • Confined growth: container pressure on the crystal during cooling.
      • Hard to observe the seeding process and growing crystal.
      • Level of natural convection changes as the melt is depleted, forced convection is hard to impose.
      • Ampule and seed preparation, sealing, etc., does not lend itself to high throughput production.
    • Applications:
      • Melts with volatile constituents: III-V (GaAs, lnP, GaSb) and II-VI compounds (CdTe).
      • Ternary compounds (Ga1-lnxAs, Ga1-xlnxSb, Hg1-xCdxTe).
  • 19. Liquid Encapsulation Advantages: Properties of a good encapsulant - Prevents contact between the crystal and the melt - Reduced nucleation - Thermal stresses are reduced - Reduced evaporation - Melting temperature lower than the crystal - Low vapor pressure - Density lower than the density of the melt - No reaction with the melt or the crucible Best encapsulans: - B 2 O 3 - LiCl, KCl, CaCl2, NaCl Crucible Encapsulant Melt Crystal
  • 20. Bridgman growth with the Submerged Baffle
    • H ~ 1 cm
    • low dT/dr
    • no free surface
    • forced convection
    • H(t) ~10 cm
    • large dT/dr
    • free surface
    Gr  g     T  H 3  2
  • 21. 2.2 Czochralski Method (CZ):
    • Conservative process: no material is added or removed from either solid or liquid phase, except by crystallization.
    • Charge is held at temperature slightly above melting point.
    • Seed is dipped into the melt and slowly withdrawn.
    • Crystal grows as the atoms from the melt adhere themselves to the seed.
  • 22. Advantages:
      • Growth from free surface (accommodates volume change).
      • Crystal can be observed.
      • Forced convection easy to impose.
      • High throughput; large crystals can be obtained.
      • High crystalline perfection can be achieved.
      • Good radial homogeneity.
  • 23. Drawbacks:
    • Materials with high vapor pressure can not be grown.
    • Batch process; hard to adapt for continuous growth; result: axial segregation.
    • The crystal has to be rotated; rotation of the crucible is desirable.
    • Process requires continuous attention (seeding, necking) and sophisticated control.
  • 24. Drawbacks (continued):
    • Melt is thermally upside down.
    • Temperature gradients are high to control the crystal diameter.
    • High thermal stresses.
    • Shape and size of the crystal is hard to control if temperature gradients are low.
  • 25. Liquid encapsulated Czochralski method (LEC)
    • Advantages:
      • Materials with high vapor pressure can be grown.
      • Retains most of CZ advantages: growth from a free surface, etc.
      • B2O3 prevents reaction between melt and crucible: prevents reaction between melt and ambient; dissloves oxides (eg. Ga2O3).
  • 26. Drawbacks:
      • Some loss of volatile constituent.
      • “ Contamination” by B 2 O 3 .
      • B 2 O 3 is too viscous below 1000 °C.
      • Encapsulant becomes opaque towards the end of growth.
  • 27. 2.3 Zone melting and floating zone:
      • Nonconservative process: material is added to molten region.
      • Only a small part of th charge is molten (except the seed).
      • Axial temperature gradient is imposed along the crucible
      • Molten zone (the interface) is advanced by moving the charge or the gradient.
  • 28. Advantages :
      • Charge is purified by repeated passage of the zone (zone refining).
      • Crystals may be grown in sealed ampules or without containers (floating zone).
      • Steady-state growth possible.
      • Zone leveling is possible; can lead to superior axial homogeneity.
      • Process requires little attention (maintenance).
      • Simple: no need to control the shape of the crystal.
      • Radial temperature gradients are high.
  • 29. Drawbacks :
      • Confined growth (except in floating zone).
      • Hard to observe the seeding process and the growing crystal.
      • Forced convection is hard to impose (except in floating zone).
      • In floating zone, materials with high vapor pressure can not be grown.
  • 30. 3) Convection and segregation
  • 31. Enclosure Heated from Below
  • 32. Natural buoyancy forces moving boundary less difficult predict (at S/L interface) hard to predict, model and control Magnitude: Features: V ~ w L unsteady: Gr > 5,000 turbulent: Driving mech. Forced Convection L and  T = f (time)  ° f (time) Growth process: all CZ, FZ Comparison of Natural and Forced Convection
  • 33. c) Impose forced convection - Accelerated Crucible Rotation Technique - Coupled Vibrational Stirring - Rotating Baffle Control of Crystal Homogeneity a) Reduce natural convection: - Reduced gravity (µg) - Magnetic fields - Submerged baffle b) Enhance natural convection - centrifuges
  • 34. No motion of phase boundary Beginning of motion Mass Transfer: Solid-Liquid Interface
  • 35. Diffusion-controlled segregation Tiller et al.
  • 36. Perfect Mixing Scheil (1942), Pfann(1952) ∆ f S = change in solid fraction Solidified Fraction, f S • no steady state • axial inhomogeneity (k<<1)
  • 37. Burton, Prim and Slichter’s BPS Model • assumption: 1-D flow (?) • Stagnant solute layer, at y = 0, v=0 C L = C 0 at x =  BPS at x = 0
  • 38. Burton, Prim and Slichter’s BPS Model, cont. Levich: Kodera (1953): measurement of D [cm2/s] Levich soulution  -Czochralski only -crucible = finite melt -natural convection, -couterrotation -turbulence
  • 39. Solute Conservation in CV: Ostrogorsky & Müller: Integral CV approach
  • 40. Ostrogorsky and Müller: Integral control-volume approach (cont.)
    •  D and V D are real physical parameter; analytical solutions exist.
    • laminar and turbulent flow
    a = 1/6 Table 1  D and V ∞ for several important melt growth techniques (CZ=Czochralski, FZ = Floating Zone, Gr = Grashof number) Driving Mechanism Growth Method  D V ∞ Crystal rotation Cz, FZ V∞  L Natural Convection Bridgman V ∞ ~Gr (  /L) Weak natural convection in microgravity Bridgman V ∞ ~Gr(  /L) 1/2
  • 41. • Cochran's ∞ rotating disc: J.Appl.Phy. 27(1956)686 • Levich (Sparrow and Gregg) Model of Ostrogorsky and Müller and Data of Bridges k eff versus growth rate R and  for Czochralski grown crystals.
  • 42. Microscopic Inhomogeneity (1  m to 1 mm)
    • Caused by unsteady conditions:
      • Unsteady (turbulent) flow, temperature, composition
      • Crystal rotation
      • Vibrations
  • 43. Bridgman growth with the Submerged Baffle
    • H ~ 1 cm
    • low dT/dr
    • no free surface
    • forced convection
    • H(t) ~10 cm
    • large dT/dr
    • free surface
    Gr  g     T  H 3  2
  • 44. Micro-segregation (a) Bridgman and (b) Baffle Spreading Resistance in 6 cm diameter Ga-doped Ge-2%Si alloy Measurements conducted by M. Lichtensteiger at NASA-MSFC [9]