GROWTH OF SINGLE CRYSTALS: VAPOR, LIQUID, SOLID PHASE CRYSTALLIZATION Useful for property measurements and fabrication of devices Crystal pulling equipment

GROWTH OF SINGLE CRYSTALS: VAPOR, LIQUID, SOLID PHASE CRYSTALLIZATION

Useful for property measurements and fabrication of devices

Crystal pulling equipment - art or science?

GROWTH OF SINGLE CRYSTALS MICRONS TO METERSMICRONS 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 Single crystals important for fabrication of devices, like silicon chips, yttrium aluminum garnet solid state lasers, beta-beryllium chips, yttrium aluminum garnet solid state lasers, beta-beryllium borate for doubling and tripling the frequency of CW or pulsed borate for doubling and tripling the frequency of CW or pulsed laser light, lithium niobate optoelectronic switch for guiding laser light, lithium niobate optoelectronic switch for guiding light in miniature optical circuits, quartz crystal oscillators for light in miniature optical circuits, quartz crystal oscillators for ultra-sensitive nanogram mass monitorsultra-sensitive nanogram mass monitors

LET'S GROW CRYSTALSLET'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.

Pulling direction of seed on rod

Heater

CZOCHRALSKICZOCHRALSKI

Crucible

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

Melt just above mpHigh viscosity low vapor pressure

Growing crystal

Crystal seed

Counterclockwise rotation of melt and crystal being pulled from melt, helps maintain uniform T, composition and homogeneity of crystal growth

CZOCHRALSKI METHODCZOCHRALSKI 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 highest viscosity, lowest vapor pressure favors crystal growthviscosity, 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)

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

GROWING BIMETALLIC SINGLE CRYSTALSBIMETALLIC 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

EXAMPLES OF CZOCHRALSKI GROWN SCs

SOLIDIFICATION OF STOICHIOMETRIC MELTSOLIDIFICATION 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 switchelectrooptical switch

• SrTiO3 - Perovskite substrate – used for epitaxial growth of high Tc defect Perovskite - YBa2Cu3O7 superconducting films - SQUIDSSQUIDS

• GaAlInP - quaternary alloy semiconductor - near IR diode lasersnear IR diode lasers

• GaAs wafers – red laser diodesred laser diodes - photonic crystal devicesphotonic crystal devices

• NdxY3-xAl5O12 – neodynium YAG - NIR solid state lasersNIR solid state lasers - 1.06 microns

• Si - microelectronic chipsmicroelectronic chips, Ge - semiconductor higher electron mobility faster electronics than Si

SILICON TO SILICON CHIPS

SILICON TO SILICON CHIP

PATTERNING Si WAFERS FOR CHIP MANUFACTURING THE BILLION

DOLLAR MICROFABRICATION WAY

Single crystal LiNbOSingle crystal LiNbO33 electrooptical switch electrooptical switch

Ferroelectric Perovskite in tetragonal form below Tc

Ti channel diffused into LiNbO3 as Ti(4+): LiTixNbO3

Ti(4+) > Ti(4+) > (Li+) so higher RI channel (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

BRIDGMAN AND STOCKBARGER METHODSControlled Crystallization of a Stoichiometric MeltControlled Crystallization of a Stoichiometric Melt

STOCKBARGER fixed temperature STOCKBARGER fixed temperature gradient - gradient - moving crystalmoving crystal

BRIDGEMAN changing BRIDGEMAN changing temperature gradient - temperature gradient - static crystalstatic crystal

T

T Distance

Distance

Crystallization of melt on seed as crucible gradually displaced through temperature gradient from hotter to cooler end

melt crystal

Furnace gradually cooled and crystallization begins on seed at cooler end of crucible

Tm

Tm

T1

T2

T3

Temperature gradient

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 in a temperature gradienttemperature gradient

BRIDGMAN AND STOCKBARGER METHODS

• Stockbarger and Bridgman methods both involve controlled solidification of a stoichiometric meltcontrolled solidification of a stoichiometric melt of the material to be crystallized in a temperature gradientin 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

T

Distance

Crystal or powder

Localized melt region - impurities concentrated in melt – energetic benefit

Crystal growing from seed

Temperature profile furnce

Pulling direction

Tm

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 Boat containing sample pulled at a controlled velocity through the thermal profile furnacethrough the thermal profile furnace

• Zone of material meltedZone of material melted, hence the name of the method - oriented solidification of crystal occurs on the seed - simultaneously more of the charge melts


• Partitioning of impurities occurs between melt and crystalPartitioning 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 where structure-energy constraints of crystal sites more severe than meltsevere than melt - impurities swept out of crystal by moving the liquid zone

• Used for purifying materials like W, Si, Ge, Au, Pt to ppbpurifying materials like W, Si, Ge, Au, Pt to ppb level of impurities, often required for device applications

O2 + powdered precursor(s)

O2 + H2

Fusion flame

Liquid drops of molten precursor(s)

Growing crystal

Support for growing crystal

VERNEUIL FUSION FLAME METHOD

VERNEUIL FUSION FLAME METHOD

• 1904 first recorded use of the method, useful for growing useful for growing crystals of extremely high melting and refractory metal crystals of extremely high melting and refractory metal oxidesoxides, examples include:

• Ruby red from Cr3+/Al2O3 powder, sapphire blue from Cr2

6+/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

RUBY - CRYSTAL PRESSURE SENSOR?

• [Cr(3+)] d3 determines Oh monatomic Cr(3+) or diatomic Oh monatomic Cr(3+) or diatomic Oh (Cr(3+)-O-Cr(3+))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 sapphirered 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

Electronically isolated Oh Cr(3+)

d3 CrO6d3 CrO6

Electronically coupled adjacent “Oh” Cr(3+)

d3 O5CrOCrO5d3 O5CrOCrO5

BASICS: RUBY RED TO SAPPHIRE BLUE

ELECTRONICALLY ISOLATED TO COUPLED Cr(3+) Oh CRYSTAL SITES ELECTRONICALLY ISOLATED TO COUPLED Cr(3+) Oh CRYSTAL SITES

IN CORUNDUM LATTICE – Cr(3+) LOWER SYMMETRY HIGHER DOSIN CORUNDUM LATTICE – Cr(3+) LOWER SYMMETRY HIGHER DOS

eg

t2g

Electronically isolated Oh Cr(3+)

d3 CrO6

Shorter Cr-O bonds - larger crystal field splitting of Oh

Cr(3+) d3 CrO6

BASICS: RUBY PRESSURE SENSOR

PRESSURE CAUSES SHORTENING OF Cr-O BOND LENGTHS AROUND PRESSURE CAUSES SHORTENING OF Cr-O BOND LENGTHS AROUND Cr(3+) Oh CRYSTAL SITES IN CORUNDUM LATTICE WITH INCREASE Cr(3+) Oh CRYSTAL SITES IN CORUNDUM LATTICE WITH INCREASE

IN CFSE AND ACCOMPANYING BLUE SPECTRAL SHIFTIN CFSE AND ACCOMPANYING BLUE SPECTRAL SHIFT

eg

t2g

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 QUALITYCRYSTAL 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?what does one use for large ones?

HYDROTHERMAL CRYSTAL GROWTH

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 synthesis and crystal growth of phases that are unstablephases that are unstable in a high temperature preparation in the absence of water

HYDROTHERMAL AUTOCLAVE

Growth region

Dissolving region

Crystal seeds

Separating baffle

Source nutrient


• 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!!!counterintuitive!!!

• Good example is -AlPO4 known as Berlinite, isoelectronic and isostructural with Quartz, important for its high piezoelectric coefficientpiezoelectric coefficient - application of pressure to a crystal of Quartz or Berlinite creates a distortion of structure, electrical polarization of the lattice and associated voltage


• Ability of certain non-centrosymmetric crystals like quartz to generate a voltage in response to applied mechanical stress - Greek piezeinGreek piezein - squeeze or press

• Effect reversiblereversible - 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 chargescharge 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 monitoruse as a high frequency oscillator and mass monitor

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

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

QUARTZ CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE

400°C T2

360°C T1

SiO2 powder nutrient dissolving region

Baffle allows passage of minerlized species to quartz seed crystal

NaOH/H2O mineralizer

SiO2 seed

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

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- SiO3

2- + H2O

• NaOH creates growth rates about 2x greater than with Na2CO3 because of different concentrations of hydroxide mineralizer

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

T1

T2

AllPO4 powder

Baffle

H3PO4/H2O mineralizer

AlPO4 seed

EMERALD CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE

T2

T1

T2

SiO2 powder nutrient at hot end

Al2O3/Cr2O3/BeO powder nutrients at hot end

Emerald - Cr(3+) doped beryl seed crystal at cool center of hydrothermal synthesis - crystal growth autoclave

NH4Cl or HCl mineralizer


• 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


• 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

T2

T1

Metal Powder

Baffle

10MHI/I2 mineralizer

Metalseed

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 Methods of obtaining high pressures: anvils, diamond tetrahedral and octahedral geometry pressure transmission, tetrahedral and octahedral geometry pressure transmission, shock waves, explosionsshock waves, explosions

• Go to another planet, recall hydrogen is metallic at 100 Gbars (explain why this is so?)

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 Often high pressure phases have a higher density, higher coordination numberdensity, higher coordination number

• In fact ruby is used for calibrating a high pressure diamond anvil – see earlier notes for how this method works?

HIGH PRESSURE ANVIL SOLID STATE SYNTHESIS

HIGH PRESSURE POLYMORPHISM FOR HIGH PRESSURE POLYMORPHISM FOR SOME SIMPLE SOLIDSSOME SIMPLE SOLIDS

Solid Normal structure Typical transformation High P structureand 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

• SiO2 Quartz 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

DIAMONDS ARE FOREVER

P-T PHASE DIAGRAM OF CARBON

RELATIVE STABILITY OF GRAPHITE AND DIAMOND

Graphite sp2 Diamond sp3

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???

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 earlier sounds like we have the perfect way to make a robust non-stick frying pan!!!

CHIMIE DOUCE WITH DIAMOND SYNTHESIS

P > 20 GPa

R.T.

Superabrasives (200 t/year) Gemstones Heat sinks for microelectronics Mechanical bearings Surgical knives Coatings - frying pans Semiconductors - wide band gap

APPLICATIONS OF SUPERHARD DIAMOND MATERIALS - CRYSTAL, POWDER, FILM

Documents

GROWTH OF SINGLE CRYSTALS: VAPOR, LIQUID, SOLID PHASE CRYSTALLIZATION Useful for property measurements and fabrication of devices Crystal pulling equipment