Transcript CHM 434F/1206F SOLID STATE MATERIALS CHEMISTRY

PHOTOEPITAXY
Making atomically perfect thin films under milder and more controlled
conditions
• Mullin and Tunnicliffe 1984
• Et2Te + Hg (pool) + H2 (h, 200oC)  HgTe + 2C2H6
• MOCVD preparation requires 500oC using Me2Te +
Me2Hg
• Advantages of photo-epitaxy
• Lower temperature operation, multi-layer formation,
less damage of layers - ternaries HgxCd1-xTe, n- and pdoping, Te and Hg/Cd rich, diodes, IR detectors, multilayers, quantum size effect devices HgxCd1-xTe-HgTe-
PHOTOEPITAXY
Making atomically perfect thin films under milder and more controlled
conditions
• Lower interlayer diffusion, easy to fabricate
• Abrupt boundaries, less defects, strain,
irregularities at interfaces
• Note that H2 gas window in apparatus prevents
deposition of HgTe on observation port
• CdTe can be deposited onto GaAs at 200-250oC
even with a 14% lattice mismatch
• GaAs is susceptible to damage under MOCVD
conditions 650-750oC
MERCURY PHOTOSENSITIZATION IDEA FOR
PHOTOEPIXIAL FORMATION OF HgTe
• Hg(6s2), 1S/(H2) (weakly 6s-s bonded VDWs GS) (h)
 Hg*(6s16p1), 1P/(H2) (strongly 6p-p* bonded ES) 
HHgH (insertion transient - mercury dihydride)
• HHgH + Et2Te  [HHgH•••Et2Te] (4-center TS) 
HHgTeEt + C2H6
• HHgTeEt  HgTe + C2H6
elimination)
(reductive
EXTENSIONS OF PHOTOLYTIC DEPOSITION
METHODS, LASER WRITING AND LASER ETCHING
• Laser writing:
• Substrate GaAs
• Me3Al or Me2Zn adsorbed layer or gas phase
• Focussed UV laser on film
• Photodissociation of organometallic precursor:
• Me3Al or Me2Zn  Al or Zn + C2H6
• Creates sub-micron lines of Al or Zn
EXTENSIONS OF PHOTOLYTIC DEPOSITION
METHODS, LASER WRITING AND LASER ETCHING
• Laser photoetching:
• GaAs substrate, gaseous or adsorbed layer of CH3Br
• Focussed UV laser, creates reactive Br atoms
• CH3Br(g) (h)  CH3(g) + Br(g)
• Br(g) + GaAs(s)  GaAs…Brn(ad)
• GaAs…Brn(ad)  GaBrn(g) + AsBrn(g)
• Adsorbed reactive Br erode surface regions irradiated with
laser, vaporization of volatile GaBrn and AsBrm from surface,
creates sub-micron etched line
High P 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 vital for meaningful property
measurements of materials
• 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 and betaberyllium borate for doubling and tripling the
frequency of CW or pulsed laser light, quartz crystal
oscillators for 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,
size of crystal desired, stability in air, morphology
or crystal habit required, doping, defects,
impurities and so forth
• So let's proceed to look at some case histories.
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
Counterclockwise
rotation of melt and
crystal being pulled
from melt, helps
unifomity of
temperature and
homogeneity of crystal
growth
Growing crystal
Heater
Melt just above mp
CZOCHRALSKI
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
• 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
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 invaluable for
growing many large single crystals as a rod, to be cut into
wafers and polished for various applications
• 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 - refractive index control - electrooptical switch
•
SrTiO3 - perovskite substrate - epitaxial growth of high Tc defect
perovskite YBa2Cu3O7 superconducting films - fabrication of
SQUIDS
•
GaAlInP - quaternary alloy semiconductor - near IR diode lasers
•
GaAs wafers - laser diodes, Lincoln log photonic crystal switch
•
NdxY3-xAl5O12 - near IR slab lasers - 1.06 microns
•
Si - microelectronic chips, Ge - semiconductor high electron
mobility faster electronics than Si
SAND TO SILICON CHIPS
SAND 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(Nb5+) > a(Li+) so higher RI channel
Light coupled from external optical fiber to LiTixNbO3
higher RI channel surrounded by lower RI LiNbO3
causes waveguiding of light in channel by TIR
Light waveguides along LiTixNbO3 channel - voltage off
Voltage on - E-field between LiTixNbO3 channels causes
polarizability of LiNbO3 region between channels to
increase and light in LiTixNbO3 channel no longer
confined and switches to other LiTixNbO3 channel
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
BRIDGMAN AND STOCKBARGER METHODS
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 and Bridgman methods both involve
controlled solidification of a stoichiometric melt of
the material to be crystallized
• Enables oriented solidification
• Melt passes through a temperature gradient
• Crystallization occurs at the cooler end
• Both methods benefit from seed crystals and
controlled atmospheres
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
T
Temperature profile
Tm
Pulling direction
Distance
Crystal or powder
Crystal growing from seed
Localized melt region - impurities
concentrated in melt
ZONE MELTING CRYSTAL GROWTH AND
PURIFICATION OF SOLIDS
• Partitioning of impurities occurs between melt and
crystal
• This is the basis of the zone refining methods for
purifying solids
• Impurities concentrate in liquid more than the solid
phase where structure-energy constraints of crystal
sites more severe, 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
VERNEUIL FUSION FLAME METHOD
• 1904 first recorded use of the method, useful for
growing crystals of extremely high melting metal
oxides, examples include:
• Ruby red from Cr3+/Al2O3 powder, sapphire blue
from Cr26+/Al2O3 powder, luminescent host CaO
powder
• Starting material fine powder, passed through
O2/H2 flame or plasma torch
• Melting of the powder occurs in the flame, molten
droplets fall onto the surface of a seed or growing
crystal, leads to controlled crystal growth
VERNEUIL FUSION FLAME METHOD
O2 + powdered precursor(s)
O2 + H2
Fusion flame
Liquid drops of molten precursor(s)
Growing crystal
Support for growing crystal
RUBY - CRYSTAL PRESSURE SENSOR?
• [Cr(3+)] 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 - red to blue color of ruby and
sapphire
• t26 to eg transitions sensitive to Cr-O distance pressure decreases these distances and increases
CF splitting causing blue shifts proportional to
pressure - hence senses pressure
CRYSTAL GROWING METHODS
COCHRALSKI, 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, mosaicity,
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
include: confocal optical microscope, polarization optical
microscope birefringence, Raman microscope, spatially
resolved XRD, TEM, ED, EDX, AFM
HYDROTHERMAL CRYSTAL GROWTH
HYDROTHERMAL SYNTHESIS AND GROWTH OF
SINGLE CRYSTALS
• Basic methodology, water medium and high
temperature growth, above normal boiling point,
water acts as a pressure transmitting agent
• Water functions as solublizing phase, often
mineralizing agent added to enable 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
Dissolving region
Separating baffle
Source nutrient
HYDROTHERMAL SYNTHESIS AND GROWTH OF
SINGLE CRYSTALS
• Temperature gradient reactor - dissolution of
reactants at one end - transport with help of
mineralizer to seed at the other end - crystallization
at the other end
• Because some materials have negative solubility
coefficients, crystals can grow at the hotter end in
a temperature gradient hydrothermal reactor,
counterintuitive
• Good example is alpha-AlPO4 known as Berlinite,
important for its high piezoelectric coefficient larger than alpha-quartz with which it is
isoelectronic - and use as a high frequency
HYDROTHERMAL GROWTH OF
QUARTZ SINGLE CRYSTALS
• Water medium - Nutrients 400oC - Seed 360oC
• Pressure 1.7 Kbar - Mineralizer 1M NaOH
• 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
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 (not too much) and transported to the
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
• SiO2 + 2OH-  SiO32- + H2O
• CO32- + H2O  HCO3- + OH• NaOH creates growth rates about 2x greater than
with Na2CO3 because of different concentrations of
hydroxide mineralizer
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
EXAMPLES OF HYDROTHERMAL CRYSTAL GROWTH
AND MINERALIZERS
• Berlinite alpha-AlPO4 - larger piezoelectric
coefficient than quartz
• Powdered AlPO4 cool end of reactor, negative
solubility coefficient T2 > T1
a-AllPO
T1
Baffle
• H3PO4/H2O mineralizer
4
• AlPO4 seed crystal at hot end
T2
powder
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 mixture 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
Metal Powder
T
crystal:
2
Baffle
• Au + 3/2I2 + I-  AuI4-
T1
10MHI/I2
mineralizer
Metal seed
• Metal crystals grown include