Crystal Growth Techniques

Ron Graham

October 31, 2006 Agenda

•Summarize current techniques •Discuss advantages/disadvantages •Propose hybrid method Basic Methods •Czochralski (CZ) •Bridgman (and variations) •Various floating zone methods •EB drip melting •Strain annealing •Other methods Czochralski

•Czochralski (CZ) typically used for Si •Can grow boules to 300 mm with 400 mm being introduced •Uses seed •“Pulls” out of the melt Czochralski Puller

•Resistance or RF heating Seed View- Crystal •Melt contained in quartz or port

Si3N4 crucible

•Chamber under Argon Heaters Melt •Si melts 1421°C Czochralski •Growth speed is 1–2 mm/min •Crucible introduces oxygen contamination •Feed material form is unconstrained •Axial resistivity uniformity is poor •Heat up/cool down times are long

•Materials of construction are issue Nb Tm = 2477°C •Ingot weight can reach 400 kg Czochralski •Modification is a “Tri-arc” furnace •Melting accomplished by 3 arcs •Rotating, water-cooled Cu crucible •Melt conducted under vacuum •Reportedly can melt to 3000°C Bridgman Technique •Vertical or horizontal •Uses a crucible •Requires •Directional solidification •Precise temperature gradient required Bridgman Technique

Molten Furnace tube zone Heater Polycrystal

Pul l Molten Pull Crystal

Seed Crystal Seed Bridgman Technique

solid-liquid interface Carefully controlled temperature gradient required.

Temperature TM Bridgman Technique

•Growth rates of about 1 mm/hr •Crucibles usually used one time •Used for small Nb 10 x 40–60 mm •Requires only tip of seed to be molten •Can reach 200 mm for Si and GaAs crystals Floating Zone Techniques

•Electron beam floating zone (EBFZ) •RF floating zone EB Floating Zone •Actual experience with refractory alloys including Nb, Ta, Mo, Re, and W •Vacuum melting chamber, annular EB gun •Crystal rotator and translator •No crucible •0.5–50 mm/min growth rates EB Floating Zone Melt stock (anode)

W filament cathode

Liquid metal Focusing electrodes EB Floating Zone

•Zone is added benefit •Diameters up to 110 mm reported for Nb •Diameters limited by surface tension/runout •EB heating penetration limited •Does not seem practical for 300 mm EB Floating Zone

O<0.03 C<0.3 Impurity concentration N<50 of Nb as reported by H<0.1 Si <0.03 Giebovsky and Semenov Al <0.03 K<0.03 Ca <0.3 Na <0.03 P<0.03 S<0.1 ppm EB Floating Zone •Modified pedestal technique reported for Nb •Used annular EB gun •Nb feedstock is rotating pedestal •Melt top of pedestal and touch seed to it •Pull non-rotating seed up off the pedestal •1.5 x 30 - 50 mm length •After Naramota and Kamada Floating Zone RF

Melt Melt Stock Stock RF Coil RF Coil

Melt Seed Offset

Single Crystal Floating Zone RF •No practical advantage over EB heating •Diameter of Xtal can be made larger by off- setting bottom pull rod from melt stock •Requires multiple passes to achieve crystal •Molten zone stability critical •Surface tension •Cohesion •Levitation EB Vertical Drip Melting •Well known technology •Can readily make large-grain ingots to 400 mm •Rotating melt-stock, vertically oriented above water-cooled copper crucible •Multiple EB guns at 30° axis to melt stock •Bottom withdrawal of ingot •Excellent refining and purification EB Vertical Drip Melting

•“Single grain” (with surrounding equiaxed grains) demonstrated on small diameter •Large grains 150 x 220 mm possible •Not a “robust” process at this time •Limited by perturbations such as thermal gradients, vibrations, fluid flow, nucleation off crucible wall EB Vertical Drip Melting EB Vertical Drip Melting

•A reminder of how refractory metals solidify •These are the nuclei for new grains •Dendrites are easily disturbed and broken off •If they don’t re-dissolve they form new grains •There can only be one dendrite in a Single Crystal Turbine Blades

Radiation Heating •Uses columnar seed grain •Single crystal selector (pigtail)

Molten Metal •Ceramic mold maintained at ~Tm •Directional solidification from chill

Radiation to top of blade Cooling Ceramic Mold •Side entry gate/runner Single Crystal Selector •15 Kg is considered a large pour Columnar Grain Water Cooled Seed Crystal Chill Strain Annealing •Relies on principal of critical grain growth •Low strains = low dislocation density •Insufficient nucleation sites for new grains •Strain to ~ 3–5%, anneal •Results in large grains •Single grains to 5 mm2 •Impractical for our purposes Other methods

•Epitaxial growth - thin film only, very slow growth rate •Variations of Bridgman technique using IR heat lamps (so called image or mirror furnaces) •Levitation melting One Proposal •EBFZ on tubular melt stock •May be able to produce a single crystal tube •Thin wall contains molten zone •Surface tension may be able to support molten metal column •Benefits of zone refining •Tube could be hydroformed to cavity shape EB Floating Zone on Tube Tubular meltstock References 1. Handbook of Technology, W.C. O’Mara, R. B. Herring, L. P. Hunt, Noyes Publications, Norwich, NY, (1980). 2. Moment, R. L., J. Nucl. Mater. 20, (1966), pp 341. 3. Schulze, K. K. “Preparation and Characterization of Ultra-High Purity Niobium”, JOM, May, 1981, pp 33–4. 4. Giebovsky, V.G., Semenov, V.N., “Growing Single Crystals of High-Purity Refractory Metals by Electron-Beam Zone Melting”, High Temp. Materials and Processes, V. 14, No. 2, (1995) pp. 121–130. 5. Yudin, I.A., Elotin, A.V., “Usage of EB Floating Zone Melting for Production of Rhenium Alloys Wire”, Rhenium and Rhenium Alloys, ed. By B. D. Bryskin, TMS, (1997), pp. 805 808. 6. Liu, J., Zee, R.H. “Growth of molybdenum-based alloy single crystals using electron beam zone melting, J. of , 163 (1996) pp. 259–265. 7. Naramoto, H., Kamada, K., “Growth of Niobium Single Crystals by a Pedestal Method”, J. of Crystal Growth, 24/25, (1974), pp. 531-536. References

8. Chen, H. et. Al., “Growth of lead molybdate crystals by vertical Bridgman method”, Bull. Mater. Sci, Vol. 28, No. 6, Indian Academy of Sciences, (2005), pp. 555-560. 9. Singh, J., Electronic and Optoelectronic Properties of Semiconductor Structures, Cambridge University Press, 0521182379X, Chapter 1, Structural Properties of , Cambridge, UK, (2003). 10. Lawley, A., “Crystal Growing”, Vacuum Metallurgy, ed. By O. Winkler, R. Bakish, Elsevier Publishing Co., Amsterdam, (1971), pp 633-642. 11. Yang, X.L., Lee, P.D., D’Souza, N., “Stray Grain Formation in the Seed Region of Single-Crystal Turbine Blades”, JOM, (May, 2005), pp. 40-44. 12. Ford, T., “Single Crystal Blades”, Aircraft Engr. & Aerospace Tech., V. 69, No. 6, (1997), pp. 564-566. 13. M. Gell, D. N. Duhl, and A. F. Giamel, “The Development of Single Crystal Superalloy Turbine Blades”, Superalloys 1980: Proceedings of the Fourth International Symposium on Superalloys, edited by J. K. Tien, AIME/ASM, Metals Park, Ohio, 1980, pp 205-214.