Download SOLID-STATE MATERIALS SYNTHESIS METHODS

Crystal pulling equipment - art or science?
GROWTH OF SINGLE CRYSTALS: VAPOR, LIQUID, SOLID
PHASE CRYSTALLIZATION
Useful for property measurements and fabrication of devices
GROWTH OF SINGLE CRYSTALS
MICRONS 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
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.
Crystal seed
Pulling direction of
seed on rod
Inert atmosphere under
pressure prevents
material loss and
unwanted reactions
Growing crystal
Layer of molten oxide
like B2O3 prevents
preferential
volatilization of one
component - precise
stoichiometry control
Counterclockwise
rotation of melt and
crystal being pulled
from melt, helps
maintain uniform T,
composition and
homogeneity of crystal
growth
Heater
CZOCHRALSKI
Melt just above mp
High viscosity low
vapor pressure
Crucible
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)
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 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 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
SILICON TO SILICON CHIPS
SILICON TO SILICON CHIP
PATTERNING Si WAFERS FOR CHIP
MANUFACTURING THE BILLION
DOLLAR MICROFABRICATION WAY
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
BRIDGMAN AND STOCKBARGER METHODS
Controlled Crystallization of a Stoichiometric Melt
T
Temperature gradient
STOCKBARGER fixed temperature
gradient - moving crystal
Tm
melt
crystal
Distance
T
T1
Tm
Crystallization of melt on seed as
crucible gradually displaced through
temperature gradient from hotter to
cooler end
BRIDGEMAN changing
temperature gradient - static crystal
T2
T3
Furnace gradually cooled and
crystallization begins on seed at
cooler end of crucible
Distance
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
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
ZONE MELTING CRYSTAL GROWTH AND
PURIFICATION OF SOLIDS
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
• 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
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
VERNEUIL FUSION FLAME METHOD
O2 + powdered precursor(s)
O2 + H2
Fusion flame
Liquid drops of molten precursor(s)
Growing crystal
Support for growing crystal
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
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
BASICS: RUBY RED TO SAPPHIRE BLUE
ELECTRONICALLY ISOLATED TO COUPLED Cr(3+) Oh CRYSTAL SITES
IN CORUNDUM LATTICE – Cr(3+) LOWER SYMMETRY HIGHER DOS
eg
t2g
Electronically
isolated Oh Cr(3+)
d3 CrO6
Electronically coupled
adjacent “Oh” Cr(3+)
d3 O5CrOCrO5
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
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?
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
phases that are unstable in a high temperature
preparation in the absence of water
HYDROTHERMAL AUTOCLAVE
Growth region
Crystal seeds
Separating baffle
Dissolving region
Source nutrient
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
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] 2fold axis) creating charge asymmetry and electrical charges
across opposite crystal faces that generates a V
• Berlinite alpha-AlPO4 more polar Al-O larger than alphaquartz Si-O with which it is isoelectronic and isostructural use 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
SiO2 powder nutrient dissolving region
400°C T2
Baffle allows passage of minerlized
species to quartz seed crystal
360°C T1
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-  SiO32- + 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
T1
• AlPO4 seed crystal at hot end T
2
a-AllPO4 powder
Baffle
H3PO4/H2O
mineralizer
a-AlPO4 seed
EMERALD CRYSTALS GROW IN
HYDROTHERMAL AUTOCLAVE
SiO2 powder nutrient at hot end
T2
T1
T2
Emerald - Cr(3+) doped beryl seed
crystal at cool center of hydrothermal
synthesis - crystal growth autoclave
Al2O3/Cr2O3/BeO powder nutrients at hot end
NH4Cl or HCl mineralizer
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
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-
T2
Metal Powder
Baffle
• Metal crystals grown include
T1
10MHI/I2
mineralizer
Metal seed
• Au, Ag, Pt, Co, Ni, Tl, As at 480-500oC
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?)
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?
HIGH PRESSURE ANVIL
SOLID STATE SYNTHESIS
HIGH PRESSURE POLYMORPHISM FOR
SOME SIMPLE SOLIDS
Solid
Normal structure
and coord. no.
Typical transformation
conditions P kbar, T °C
High P structure
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 3coordinate 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.
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