GROWTH OF SINGLE CRYSTALS: VAPOR, LIQUID, SOLID PHASE CRYSTALLIZATION Useful for property measurements and fabrication o

1. Crystal pulling equipment - art or science? GROWTH OF SINGLE CRYSTALS: VAPOR, LIQUID, SOLID PHASE CRYSTALLIZATION Useful for property measurements and fabrication of devices

2. GROWTH OF SINGLE CRYSTALSMICRONS TO METERS • Vapor, liquid, solid phase crystallization techniques • Single crystals - meaningful materials property measurements • Single crystals allow measurement of anisotropic phenomena in crystals with symmetry lower than cubic (isotropic) • Single crystals important for fabrication of devices, like silicon chips, yttrium aluminum garnet solid state lasers, beta-beryllium borate for doubling and tripling the frequency of CW or pulsed laser light, lithium niobate optoelectronic switch for guiding light in miniature optical circuits, quartz crystal oscillators for ultra-sensitive nanogram mass monitors

3. LET'S GROW CRYSTALS • Key point to remember when learning how to be a crystal grower (incidentally, an exceptionally rare profession and extraordinarily well paid) • Many different techniques exist, hence one must think very carefully as to which method is the most appropriate for the material under consideration • Think also about size of crystal desired, stability in air, morphology or crystal habit required, orientation, doping, defects, impurities • So let's proceed to look at some case histories.

4. CZOCHRALSKI Pulling direction of seed on rod Crystal seed Inert atmosphere under pressure prevents material loss and unwanted reactions Layer of molten oxide like B2O3 prevents preferential volatilization of one component - precise stoichiometry control Growing crystal Heater Melt just above mp High viscosity low vapor pressure Counterclockwise rotation of melt and crystal being pulled from melt, helps maintain uniform T, composition and homogeneity of crystal growth Crucible

5. CZOCHRALSKI METHOD • Interesting crystal pulling technique (but can you pronounce and spell the name!) • Single crystal growth from the melt precursor(s) • Crystal seed of material to be grown placed in contact with surface of melt • Temperature of melt held just above melting point, highest viscosity, lowest vapor pressure favors crystal growth • Seed gradually pulled out of the melt (not with your hands of course, special crystal pulling equipment is used)

6. CZOCHRALSKI METHOD • Seed gradually pulled out of the melt (not with your hands of course, special crystal pulling equipment is used) • Melt solidifies on surface of seed • Melt and seed usually rotated counterclockwise with respect to each other to maintain constant temperature and to facilitate uniformity of the melt during crystal growth, produces higher quality crystals, less defects • Inert atmosphere, often under pressure around growing crystal and melt to prevent any materials loss and undesirable reactions like oxidation, nitridation etc

7. GROWING BIMETALLIC SINGLE CRYSTALS LIKE GaAs REQUIRES A MODIFICATION OF THE CZOCHRALSKI METHOD • Layer of molten inert oxide like B2O3 spread on top of the molten feed material to prevent preferential volatilization of the more volatile component of the bimetal melt • Critical for maintaining precise stoichiometry, e.g., Ga1+xAs and GaAs1+x when made rich in Ga and As, become p- and n-doped!!! • The Czochralski crystal pulling technique is invaluable for growing many large single crystals as a rod, to be cut into wafers and polished for various applications like silicon, germanium, lithium niobate • Utility of some single crystals made by Czochralski listed below

8. EXAMPLES OF CZOCHRALSKI GROWN SCs SOLIDIFICATION OF STOICHIOMETRIC MELT • LiNbO3- NLO material - Perovskite - temperature dependent tetragonal-cubic-ferroelectric-paraelectric phase transition at Curie T – electrical control of refractive index – use electrooptical switch • SrTiO3 - Perovskite substrate – used for epitaxial growth of high Tc defect Perovskite - YBa2Cu3O7 superconducting films - SQUIDS • GaAlInP - quaternary alloy semiconductor - near IR diode lasers • GaAs wafers – red laser diodes - photonic crystal devices • NdxY3-xAl5O12 – neodynium YAG - NIR solid state lasers - 1.06 microns • Si - microelectronic chips, Ge - semiconductor higher electron mobility faster electronics than Si

9. SILICON TO SILICON CHIPS

10. SILICON TO SILICON CHIP

11. PATTERNING Si WAFERS FOR CHIP MANUFACTURING THE BILLION DOLLAR MICROFABRICATION WAY

12. Single crystal LiNbO3 electrooptical switch Ferroelectric Perovskite in tetragonal form below Tc Ti channel diffused into LiNbO3 as Ti(4+): LiTixNbO3 aTi(4+) > a(Li+) so higher RI channel Light coupled from external optical fiber to RHS LiTixNbO3 higher RI channel cladded by lower RI LiNbO3 causes wave-guiding of light in channel by TIR Light waveguides along LiTixNbO3 channel - voltage off Voltage on - E-field between LiTixNbO3 channels causes polarizability-RI of LiNbO3 region around channels to increase and light in LiTixNbO3 channel no longer confined and switches to other LiTixNbO3 channel

13. T Temperature gradient STOCKBARGER fixed temperature gradient - moving crystal Tm Crystallization of melt on seed as crucible gradually displaced through temperature gradient from hotter to cooler end melt crystal Distance T BRIDGEMAN changing temperature gradient - static crystal T1 Tm T2 T3 Furnace gradually cooled and crystallization begins on seed at cooler end of crucible Distance BRIDGMAN AND STOCKBARGER METHODSControlled Crystallization of a Stoichiometric Melt

14. BRIDGMAN AND STOCKBARGER METHODS • Stockbarger method is based on a crystal growing from the melt, involves the relative displacement of melt and a temperature gradient furnace, fixed gradient and a moving melt/crystal • Bridgman method is again based on crystal growth from a melt, but now a temperature gradient furnace is gradually lowered and crystallization begins at the cooler end, fixed crystal and changing temperature gradient • Both methods are founded on the controlled solidification of a stoichiometric melt of the material to be crystallized in a temperature gradient

15. BRIDGMAN AND STOCKBARGER METHODS • Stockbarger and Bridgman methods both involve controlled solidification of a stoichiometric melt of the material to be crystallized in a temperature gradient • Enables oriented solidification • Melt passes through a temperature gradient • Crystallization occurs at the cooler end • Both methods benefit from seed crystals, predetermined orientation and controlled atmospheres

16. T Temperature profile furnce Tm Pulling direction Distance Crystal or powder Crystal growing from seed Localized melt region - impurities concentrated in melt – energetic benefit ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF SOLIDS

17. ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF SOLIDS • Method related to the Stockbarger technique - thermal profile furnace employed - material contained in a boat • Only a small region of the charge is melted at any one time - initially part of the melt is in contact with the seed • Boat containing sample pulled at a controlled velocity through the thermal profile furnace • Zone of material melted, hence the name of the method - oriented solidification of crystal occurs on the seed - simultaneously more of the charge melts

18. ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF SOLIDS • Partitioning of impurities occurs between melt and crystal • Basis of the zone refining methods for purifying solids • Impurities concentrate in melt more than the solid phase where structure-energy constraints of crystal sites more severe than melt -impurities swept out of crystal by moving the liquid zone • Used for purifying materials like W, Si, Ge, Au, Pt to ppb level of impurities, often required for device applications

19. O2 + powdered precursor(s) O2 + H2 Fusion flame Liquid drops of molten precursor(s) Growing crystal Support for growing crystal VERNEUIL FUSION FLAME METHOD

20. VERNEUIL FUSION FLAME METHOD • 1904 first recorded use of the method, useful for growing crystals of extremely high melting and refractory metal oxides, examples include: • Ruby red from Cr3+/Al2O3 powder, sapphire blue from Cr26+/Al2O3 powder, luminescent host CaO powder • Starting material fine powder form, passed through O2/H2 flame or plasma torch • Melting of the powder occurs in the flame, molten microdroplets fall onto the surface of a seed or growing crystal, leads to controlled crystal growth

21. RUBY - CRYSTAL PRESSURE SENSOR? • [Cr(3+)] d3 determines Oh monatomic Cr(3+) or diatomic Oh (Cr(3+)-O-Cr(3+)) sites in Al2O3 corundum lattice • t2g to eg d-d electronic transition red shifts with concentration – increase in d orbital DOS and narrowing of CF splitting - red to blue color of ruby and sapphire • t26 to eg d-d transitions sensitive to Cr-O distance – increase in pressure decreases these distances and increases CF splitting causing blue shifts proportional to pressure – • hence senses pressure - useful for in situ high pressure diamond cell materials synthesis, spectroscopic and diffraction studies

22. BASICS: RUBY RED TO SAPPHIRE BLUEELECTRONICALLY ISOLATED TO COUPLED Cr(3+) Oh CRYSTAL SITES IN CORUNDUM LATTICE – Cr(3+) LOWER SYMMETRY HIGHER DOS eg t2g Electronically coupled adjacent “Oh” Cr(3+) d3 O5CrOCrO5 Electronically isolated Oh Cr(3+) d3 CrO6

23. BASICS: RUBY PRESSURE SENSOR PRESSURE CAUSES SHORTENING OF Cr-O BOND LENGTHS AROUND Cr(3+) Oh CRYSTAL SITES IN CORUNDUM LATTICE WITH INCREASE IN CFSE AND ACCOMPANYING BLUE SPECTRAL SHIFT eg t2g Electronically isolated Oh Cr(3+) d3 CrO6 Shorter Cr-O bonds - larger crystal field splitting of Oh Cr(3+) d3 CrO6

24. CRYSTAL GROWING METHODS CZOCHRALSKI, BRIDGMAN, STOCKBARGER, ZONE MELTING, VERNEUIL • All methods have the advantage of rapid growth rates of large crystals required for many advanced device applications • BUT the CRYSTAL QUALITY obtained by all of these techniques must be checked for inhomogeneities in surface and bulk composition and structure, gradients, domains, impurities, point-line-planar defects, twins, grain boundaries • THINK how you might go about checking this if you were confronted with a 12"x12"x12" crystal - useful methods for small crystals include: confocal optical microscope, polarization optical microscope birefringence, Raman microscope, spatially resolved OM, XRD, TEM, ED, EDX, AFM – what does one use for large ones?

25. HYDROTHERMAL CRYSTAL GROWTH

26. HYDROTHERMAL SYNTHESIS AND GROWTH OF SINGLE CRYSTALS • Basic methodology, water medium and high temperature growth, above normal boiling point • Water functions as solublizing phase, pressure transmitting agent, often mineralizing agent added to enhance dissolution, transport of reactants and crystal growth, speeds up chemical reactions between solids • Useful technique for the synthesis and crystal growth of phases that are unstable in a high temperature preparation in the absence of water

27. HYDROTHERMAL AUTOCLAVE Growth region Crystal seeds Separating baffle Dissolving region Source nutrient

28. HYDROTHERMAL SYNTHESIS AND GROWTH OF SINGLE CRYSTALS • Temperature gradient reactor - dissolution of reactants at one end - with help of mineralizer transport to seed at the other end - crystallization at seeded end • Because some materials have negative solubility coefficients, nutrients dissolve at cooler end and crystals grow at the hotter end in a temperature gradient hydrothermal reactor, counterintuitive!!! • Good example is a-AlPO4 known as Berlinite, isoelectronic and isostructural with Quartz, important for its high piezoelectric coefficient - application of pressure to a crystal of Quartz or Berlinite creates a distortion of structure, electrical polarization of the lattice and associated voltage

29. HYDROTHERMAL SYNTHESIS AND GROWTH OF SINGLE CRYSTALS • Ability of certain non-centrosymmetric crystals like quartz to generate a voltage in response to applied mechanical stress - Greek piezein - squeeze or press • Effect reversible - piezoelectric crystals, subject to an externally applied voltage, change shape by a small amount • Compressive stress along [100] disturbs crystal symmetry distorting SiO4 tetrahedra along 3-fold axis (not for [001] 2-fold axis) creating charge asymmetry and electrical charges across opposite crystal faces that generates a V • Berlinite alpha-AlPO4 more polar Al-O larger than alpha-quartz Si-O with which it is isoelectronic and isostructural - use as a high frequency oscillator and mass monitor

30. HYDROTHERMAL GROWTH OF QUARTZ SINGLE CRYSTALS • Water medium - Nutrients 400oC - Seed 360oC • Pressure 1.7 Kbar - Mineralizer 1M NaOH dissolves silica • Uses of single crystal quartz: radar, sonar, piezoelectric transducers, mass monitors • Annual global production hundreds of tons of quartz crystals, amazing

31. HYDROTHERMAL METHODS SUITABLE FOR GROWING MANY TYPES OF SINGLE CRYSTALS • Ruby: Cr2O3/Al2O3 Cr3+/Al2O3 and sapphire: Cr26+/Al2O3 • Chromium dioxide: Cr2O3 + CrO3 3CrO2 • Yttrium aluminum garnet: 3Y2O3 + 5Al2O3 Y3Al5O12 • Corundum: alpha-Al2O3 • Zeolites: Al2O3.3H2O + Na2SiO3.9H2O + NaOH/H2O  Na12(AlO2)12(SiO2)12.27H2O • Emerald: 6SiO2 + (Al/Cr)2O3 + 3BeO  Be3Al(Cr)2Si6O18 • Berlinite: alpha-AlPO4 • Metals: Au, Ag, Pt, Co, Ni, Tl, As

32. QUARTZ CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE SiO2 powder nutrient dissolving region 400°C T2 Baffle allows passage of minerlized species to quartz seed crystal NaOH/H2O mineralizer 360°C T1 SiO2 seed

33. ROLE OF THE MINERALIZER IN HYDROTHERMAL SYNTHESIS AND CRYSTAL GROWTH • Consider growth of quartz crystals - control of crystal growth rate, through mineralizer, temperature pressure • Solubility of quartz in water is important • SiO2 + 2H2O  Si(OH)4 • Solubility about 0.3 wt% even at supercritical temperatures >374oC • A mineralizer is a complexing agent (not too stable) for the reactants/precursors, which have to be solublized (dissolved not too quickly) and transported to the growing crystal

34. ROLE OF THE MINERALIZER IN HYDROTHERMAL SYNTHESIS AND CRYSTAL GROWTH • NaOH mineralizer, dissolving reaction, 1.3-2.0 KBar • 3SiO2 + 6OH- Si3O96- + 3H2O • Na2CO3 mineralizer, dissolving reaction, 0.7-1.3 KBar • CO32- + H2O  HCO3- + OH- • SiO2 + 2OH- SiO32- + H2O • NaOH creates growth rates about 2x greater than with Na2CO3 because of different concentrations of hydroxide mineralizer

35. a-AllPO4 powder T1 Baffle H3PO4/H2O mineralizer T2 a-AlPO4 seed EXAMPLES OF HYDROTHERMAL CRYSTAL GROWTH AND MINERALIZERS • Berlinite alpha-AlPO4 - larger piezoelectric coefficient than quartz – polarity effect Al-O > Si-O • Powdered AlPO4 cool end of reactor, negative solubility coefficient T2 > T1 - try to explain this effect • H3PO4/H2O mineralizer • AlPO4 seed crystal at hot end

36. SiO2 powder nutrient at hot end T2 Emerald - Cr(3+) doped beryl seed crystal at cool center of hydrothermal synthesis - crystal growth autoclave T1 T2 Al2O3/Cr2O3/BeO powder nutrients at hot end NH4Cl or HCl mineralizer EMERALD CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE

37. EXAMPLES OF HYDROTHERMAL CRYSTAL GROWTH AND MINERALIZERS • Emeralds Be3Al(Cr)2Si6O18 Beryl contains Si6O1812- six rings • SiO2 powder at hot end 600oC • NH4Cl or HCl/H2O mineralizer, 0.7-1.4 Kbar • Cool central region for seed, 500oC • Al2O3/BeO/Cr3+ dopant powder mix at other hot end 600oC • 6SiO2 + Al(Cr)2O3 + 3BeO  Be3Al(Cr)2Si6O18

38. Metal Powder T2 Baffle 10MHI/I2 mineralizer T1 Metalseed EXAMPLES OF HYDROTHERMAL CRYSTAL GROWTH AND MINERALIZERS • Metal crystals - metal powder at hot end 500oC • Mineralizer 10M HI/I2 - metal seed at cool end 480oC • Dissolving reaction transports Au to the seed crystal: • Au + 3/2I2 + I- AuI4- • Metal crystals grown include • Au, Ag, Pt, Co, Ni, Tl, As at 480-500oC

39. DRY HIGH PRESSURE METHODS OF SOLID STATE SYNTHESIS • Pressures up to Gbars accessible, at high T with in situ observations by diffraction and spectroscopy - can probe chemical reactions, structural transformations, crystallization, amorphization, phase transitions - kinetics and mechanism of solid state transformation - think about this? Nucleation and growth of one phase within another!!! • Methods of obtaining high pressures: anvils, diamond tetrahedral and octahedral geometry pressure transmission, shock waves, explosions • Go to another planet, recall hydrogen is metallic at 100 Gbars (explain why this is so?)

40. DRY HIGH PRESURE METHODS OF SOLID STATE SYNTHESIS • Pressure techniques useful for synthesis of unusual structures, metastable materials yet stable when pressure released (explain why?) • Often high pressure phases have a higher density, higher coordination number • In fact ruby is used for calibrating a high pressure diamond anvil – see earlier notes for how this method works?

41. HIGH PRESSURE ANVIL SOLID STATE SYNTHESIS

42. HIGH PRESSURE POLYMORPHISM FOR SOME SIMPLE SOLIDS Solid Normal structure Typical transformation High P structure and coord. no. conditions P kbar, T °C and coord. no. • C Graphite 3 130 3000 Diamond 4 • CdS Wurtzite 4:4 30 20 Rock salt 6:6 • KCl Rock salt 6:6 20 20 CsCl 8:8 • SiO2Quartz 4:2 120 1200 Rutile 6:3 • Li2MoO4 Phenacite 4:4:3 10 400 Spinel 6:4:4 • NaAlO2 Wurtzite 4:4:4 40 400 Rock salt 6:6:6

43. DIAMONDS ARE FOREVER

44. P-T PHASE DIAGRAM OF CARBON

45. RELATIVE STABILITY OF GRAPHITE AND DIAMOND Graphite sp2 Diamond sp3

46. SO WHY IS IT SO DIFFICULT TO TRANSFORM GRAPHITE INTO DIAMOND? • Industrial diamonds made from graphite around 3000oC and 15 GPa – extreme conditions and slow!!! • Problem is activation energy required for a sp2 3-coordinate to a sp3 4-coordinate structural transformation is very high, requires extreme conditions • Ways of getting round the difficulty???

47. SO WHY IS IT SO DIFFICULT TO TRANSFORM GRAPHITE INTO DIAMOND? • Ways of getting round the difficulty??? • Squeezing C60 at RT whose carbons are already intermediate between sp2-3. In the case of C60 diamond anvil, 20 GPa instantaneous transformation to bulk crystalline diamond, highly efficient process, fast kinetics – why not CNs??? • Using 1% CH4/H2 microwave discharges to create reactive atomic carbon whose orbitals are more-or-less free to form sp3 diamond, in the presence of atomic hydrogen • This is the method of choice for making CVD diamond films – very hard, hydrophobic, high thermal conductivity, large area thin films can be deposited on a range of substrates and made at low cost – as we said earliersounds like we have the perfect way to make a robust non-stick frying pan!!!

48. P > 20 GPa R.T. CHIMIE DOUCE WITH DIAMOND SYNTHESIS

49. APPLICATIONS OF SUPERHARD DIAMOND MATERIALS - CRYSTAL, POWDER, FILM  Superabrasives (200 t/year)  Gemstones  Heat sinks for microelectronics  Mechanical bearings  Surgical knives  Coatings - frying pans  Semiconductors - wide band gap